Approach to Control, Protect and Switch Charge Transport Through Molecular Junctions and Atomic Contact Yong Ai

Approach to control, protect and switch charge transport through molecular junctions and atomic contact Yong Ai

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Yong Ai. Approach to control, protect and switch charge transport through molecular junctions and atomic contact. Theoretical and/or physical chemistry. Université Sorbonne Paris Cité, 2016. English. NNT : 2016USPCC125. tel-01542780

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UNIVERSITE SORBONNE PARIS CITE

UNIVERSITE PARIS DIDEROT

Ecole Doctorale de Chimie Physique et de Chimie Analytique de Paris Centre (ED 388) Interfaces, Traitements, Organisation et Dynamique des Systèmes (ITODYS) DOCTORAT SPECIALITY: Surface, Interfaces, Matériaux Fonctionnel

Approach to Control, Protect and Switch Charge Transport through Molecular Junctions and Atomic Contact

Ohm’s law Classical regime Atomic contact

S S S

Quantum regime S S S

Molecular junction

Présenté par : Yong AI Directeur de thèsis : Jean Christophe LACROIX

Soutenance le 11 Octobre 2016

Composition Du Jury:

M. Richard L. McCreery University of Alberta Examinateur M. Dominique VUILLAUME Université Lille 1 Examinateur Mme. Anna PROUST Université Pierre et Marie Curie Rapporteur M. Frédéric CHERIOUX FEMTO-ST Rapporteur M. Jean Christophe LACROIX Université Paris Diderot Directeur de thèse M. Jalal GHILANE Université Paris Diderot Co-directeur de thèse

Acknowledgement

This work was performed in the laboratory interfaces, Treatment, and Organization Dynamics Systems (ITODYS) in Nanoelectrochemistry (NEC) team. This thesis was conducted under the direction of Jean-Christophe Lacroix, Professor at the University Paris Diderot. First and foremost, I would like to give the deepest gratitude to my supervisors, Prof. Jean Christophe LACROIX, for giving me the opportunity to do Ph.D. research. I deeply appreciate him for the guidance on scientific research, including writing, giving a scientific presentation and so on. I have obtained so many valuable advice as well as the knowledge and skills from the discussions with him. He is a nice master with enthusiasm and positive attitude in the research work. This spirit inspired me a lot during the past three years. And I am pretty sure I will learn such spirit from him in future. I am also grateful to Dr. Jalal GHILANE. He gives me the guidance on the manipulation of SECM. I am thankful for all dairy discussion with him during my Ph.D. My Ph.D work would not be easily complete without his guidance and help. Besides, I would like to thank all the members of jury of my thesis: M. Richard L. McCreery (University of Alberta, Canada), M. Dominique VUILLAUME (Université Lille 1), Mme. Anna PROUST (Université Paris 6), M. Frédéric CHERIOUX (FEMTO-ST) for reviewing my thesis. I would like to thank Prof. Pierre Camille LACAZE for the correction on chapter 5. I appreciate him for the discussion and the guidance on plasmons research. His rigorous scientific thinking inspired me very much. I would like to thank Frédéric LAFOLET for offering me the chemical compound used in chapter 4. I am also thankful to other permanent staffs in our group, Pascal MARTIN, Delphine SCHAMING, Hyacinthe RANDRIAMAHAZAKA for all the kindness during the daily working. I would also express my gratitude to Prof. John LOMAS for the English correction for the whole thesis. I am thankful to Aymeric Noël, Brigitte EFTASSIOU, and Jean-Claude PERTAYS and Alexandre CHEVILLOT for their technical help. I would like to thank Dr. Nguyen QUYNH, Hassiba SMIDA and Mickaël Sfez for their help on my PhD research. I would like to thank Prof. Dr. Chang-Zhi DONG and Dr. Xiao-Nan SUN for their advice and helps. I would like to thank my dear friends in ITODYS: Andrés LOMBANA, I

Vitor BRASILIENSE, Alexandra TIBALDI, Dr. Jonathan FOUINEAU, Eswaran MURUGASEN, Dr. Xue-Feng WANG, Dr. Deng-Jun WANG, Dr. Zhao-Jun SHENG, Dr. Jun HAI, Dr. Shihui SHI, Van Quyen NGUYEN, Thuan Nguyen PHAM TRUONG, Thi Hong Lien HAN, Dr. Thi Tuyet Van BUI and so on for their creating a friendly and positive atmosphere for working and study. I would like to thank Prof. Dr. Haoli ZHANG (my master advisor), who has ever given me the guidance on science research. I am grateful to my Chinese friends: Lingling WU, Zihan QU, Tiancai ZHANG, Chengshuo SHEN, Jianjun WU, Junying FANG, Qi LI, Songhui XIN, Yangjunjie XU, Min RUAN, Jiangchao LIU, Yuming XING, Ning XIE, Xian DONG, Yuying YAO, Mengye SUN, Lijiao HUA, Ruohong ZHANG, Huan YU, Fang ZHANG, Gen LI, Chao NIU and Dongli WU, who have ever supported me and shared the beautiful days in Paris. I would thank my family who support me and help me in countless ways in spite of the distance, for the trust on me. Yong AI Paris, 1st September 2016

Table of Contents

Acknowledgement ··································································· I

ABSTRACT ······································································· VII

RESUME ············································································ IX

General introduction ····························································· XI

Chapter 1. Introduction ···························································· 1

1.1 Background of molecular electronics ····································· 1

1.2 Molecular wires, molecular switches, and molecular rectifiers ······· 7 1.2.1 Molecular wires ···································································· 8 1.2.2 Molecular switches ································································ 8 1.2.3 molecular rectifiers ······························································· 12

1.3 Molecular junctions and atomic contact test-beds ···················· 13

1.3.1 Scanning Tunneling Microscope break junction（STM-BJ）············· 14

1.2.2 Mechanically controlled break junction (MCBJ) ···························· 16 1.2.3 Single-wall Nanotubes (SWNT) ················································ 17 1.2.4 Conducting probe atomic force microscopy (CP-AFM) ···················· 18

1.4 Scanning electrochemical microscopy (SECM) ······················ 20 1.4.1 Background ········································································ 20 1.4.2 SECM principle ··································································· 20 1.4.2.1 Theoretical tip approach ······················································· 23 1.4.2.2 Experimental tip approach ···················································· 23 1.4.2.3 Molecular junctions by SECM ··············································· 23

1.5 Summary ··································································· 24

REFERENCES ································································· 24 Chapter 2. Approaching the Frontier between Fiber Devices and

Single-Molecule Devices in Redox-Gated Junctions ························ 31

2.1 Electropolymerization of conducting polymers on microelectrodes 31

2.2 Molecular junctions generated by SECM set-up ······················ 41 2.2.1 Filling the SECM gap with conducting polymer ····························· 41 III

2.2.2 Characterization of conducting polymer junctions ··························· 43 2.3 Approaching the Frontier between Fiber Devices and Single

Molecule Devices in Redox Gated Junction ································ 47 2.3.1 Introduction ········································································ 47 2.3.2 how to fabricate PBT and PEDOT molecular junctions by SECM ········ 50 2.3.3 PBT molecular junctions························································· 51 2.3.4 PEDOT molecular junctions ···················································· 53 2.3.5 The frontier between fiber devices and single molecular devices ········· 57 2.3.6 summary ············································································ 61 2.3.7 Supporting information ·························································· 62

REFERENCES ································································· 65 Chapter 3. Controllable construction of redox-gated polymer junctions by SECM ············································································· 71 3.1 Conducting Polymer Nano Junctions Fabricated with a

Self-terminated Electrochemical Method ··································· 71 3.1.1 PANI junction fabricated with Self-terminated method ····················· 71 3.1.2 Observe the formation of PANI junction ······································ 79 3.1.3 Other molecular junctions controlled by self-terminated method ········· 81 3.1.4 Conclusion ········································································· 85 3.2 Conducting polymer molecular junctions generated by differential

scan voltammetry (DSV) ······················································ 86 3.2.1 PANI and PEDOT junctions generated by DSV ····························· 86 3.2.2 PEDOT junction created on PEDOT-modified substrate by DSV ········ 89 3.2.3 Combining self-terminated with DSV ········································· 91 3.2.4 Conclusion ········································································· 92 3.3 Extending the Capability of SECM for Fabrication of Single

molecular Junction: a break junction Strategy ···························· 93

3.3.1 How to construct molecular junctions by SECM-BJ? ·············· 93 3.3.2 PEDOT junctions generated by SECM-BJ ···································· 95 3.3.2.1 SECM-BJ in Z direction ······················································· 97 3.3.2.2 SECM-BJ in X direction ···················································· 100

3.3.3 Crossing the frontier between fiber devices and single-molecule devices ···························································································· 102 3.3.3.1 SECM-BJ in Z direction ····················································· 102 3.3.3.2 SECM-BJ in X direction ···················································· 104 3.3.4 PEDOT junctions formed repeatedly on a large Pt electrode ············ 106 3.3.5 PANI junctions formed by SECM-BJ ······································· 107

3.4 Conclusion ································································ 109

REFERENCES ································································ 110

Appendix ······································································· 112 Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions ············································································ 115

4.1 Introduction ······························································· 115

4.2 n-doped Rh-Rh chain molecular junctions ···························· 116 4.2.1 Electrochemical fabrication of Rh-Rh chain polymer thin films ········ 116 4.2.2 Molecular junctions based on Rh-Rh chain polymer ······················ 118

4.3 Ambipolar type conducting polymer molecular junctions ·········· 122

4.3.1 Electrochemical fabrication of PFTQ and PFETQ films ·················· 122 4.3.2 Charge transport through both n- and p-channel in PFTQ and PFETQ · 124

4.4 Summary ·································································· 135

REFERENCE ································································· 135

Appendix ······································································· 137 Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions ································································ 139

5.1 Introduction ······························································· 139 5.1.1 What is localized surface plasmon resonance (LSPR)? ··················· 139 5.1.2 Hot electron decay ······························································ 141

5.2 Preparing AuNPs on an ITO surface ·································· 143

5.3 PEDOT molecular junction switched by a plasmonic effect ······· 144 5.3.1 Blank test: PEDOT junction on bare ITO or on an Au plate ············· 145 5.3.2 Photo-induced switching of a PEDOT molecular junction connected to an AuNPs@ITO ··········································································· 147 V

5.3.3 PEDOT molecular junctions switched by filtered light ··················· 150 5.3.4 The mechanism of photo-induced switching of PEDOT molecular junctions ···························································································· 154 5.3.4.1. PEDOT junctions destroyed by illumination ? ·························· 155

5.3.4.2 Conductivity variations of a PEDOT junction vs VG and alternately submitted to illumination ····························································· 157 5.3.4.3. Interpretation of the switching mechanism ······························ 160

5.4 PANI molecular junctions switched by plasmonic effect ·········· 164

5.5 Summary ·································································· 167

REFERENCES ······························································· 168

Chapter 6 Protect and Switch Copper Atomic Contact ·················· 171

6.1 Introduction ······························································· 171

6.2 Atomic contact generated by self-terminated methodology ········ 173

6.3 Atomic contact protected by mesoporous silica on ITO ············ 176 6.3.1 Metallic Cu nanowire on ITO substrate ····································· 176 6.3.2 Metallic Cu nanowire on/through mesoporous silica thin films ITO substrate ················································································· 179 6.3.3 Mesoporous silica protect effect on copper atomic contact ··············· 182

6.4 switching copper atomic point contact towards: memory devices 183 6.4.1 Electrochemistry triggered destroy-reconformation based copper atomic switch ···················································································· 183 6.4.2 Redox-gated PEDOT-induced copper atomic switch ······················ 185

6.4 Conclusions ······························································· 190

References ····································································· 191

GENERAL CONCLUSION ················································ 195

ABSTRACT

Molecular electronics has attracted increasing interest in the past decades. Constructing metal/molecules/metal junctions is a basic step towards the investigation of molecular electronics. We have witnessed significant development in both experiment and theory in molecular junctions. This thesis focuses mainly on the study of charge transport through molecular junctions. Conducting polymers and copper filaments were electrochemically deposited with a scanning electrochemical microscope (SECM) configuration between a tip and a substrate electrode. In doing so, we have developed a new way to fabricate atomic contact and molecular junctions, and we have explored the possibility to control, protect and switch these systems.

Firstly, SECM, where two microelectrodes are located face-to-face separated by a micrometric gap, has been successfully used for the fabrication of redox-gated conducting polymers junctions, such as PEDOT and PBT. Highly stable and reversible redox-gated nano-junctions were obtained with conductance in the 10-7-10-8 S range in their conducting states. These results, associated with the wire-like growth of the polymer, suggest that the conductance of the entire junction in the conductive state is governed by less than 20 to 100 oligomers.

Secondly, to obtain the nano-junctions in a controllable way, a break junction strategy combined with the SECM set up is adopted. A nano-junction could be acquired by pulling the tip away from its initial position. And conductance traces showed that PEDOT junctions can be broken step by step before complete breakdown. Similarly as STM-BJ conductance steps were observed on a PEDOT molecular junction before break down by using SECM-BJ. SECM break junction technique proved to be an efficient way of molecular junction fabrication studies, especially for redox gated polymer molecular junctions. Moreover, a self-terminated strategy is found to be another way to obtain nano-junctions. An external resistance connected to the electrode plays an important role in controlling the size of conducting polymer junctions. PFTQ and PFETQ molecular junctions exhibit well-defined ambipolar transport properties. However, an unbalanced charge transport properties in n- and p- channel for these two polymer junctions was observed when the junctions are in the fiber device scale. In contrast, when molecular junction changes into nano-junction, a balanced n- and p-

VII

channel transport property is acquired. We propose that such effect is due to charge transport mechanism changing from diffusive (ohm’s law) to ballistic (quantum theory) when the junction size is reduced from fiber devices to nanodevices.

High stable Au NPs/ITO electrodes exhibit a well localized surface plasmon (LSP) behavior. These plasmonic substrates have been successfully used to trigger switching of molecular junctions under light irradiation, demonstrating that surface plasmon resonance can induce electrochemical reduction. Such conductance reduction can be attributed to the hot electrons plasmonically generated from gold nanoparticles trapped into the PEDOT junction, resulting in PEDOT being reduced and changed to an insulating state.

Finally, copper metallic nanowires were generated using an electrochemical self-terminated method based on SECM configuration. The presence of a few atoms that control the electron transport highlights the formation of metallic nanowires between the asymmetric electrodes. Furthermore, a similar study was performed on mesoporous silica film on ITO used as a substrate electrode. The mesoporous silica films have vertically aligned channels with a diameter of about 3 nm and a thickness of 115 nm, which play a crucial role in protecting the copper filament.

VIII

RESUME

Ces dernières décennies, l'électronique moléculaire a suscité un intérêt croissant. La construction de jonctions métal / molécules / métal est une étape fondamentale dans la compréhension de ce domaine. Nous avons été témoins d’avancées importantes concernant les jonctions moléculaires tant sur le plan théorique que sur le plan expérimental. Cette thèse se concentre principalement sur l'étude du transport de charge à travers les jonctions moléculaires. Des polymères conducteurs et des filaments de cuivre ont été déposés, par électrochimie avec un microscope électrochimique à balayage (SECM), entre une pointe et une électrode substrat. Ainsi, nous avons développé une nouvelle façon de réaliser des contacts atomiques et des jonctions moléculaires permettant de contrôler, d’activer et de protéger ces systèmes.

La fabrication de jonctions à grille redox de polymères conducteurs, tel que le PEDOT et le PBT, a été effectuée dans l’intervalle micrométrique séparant les deux électrodes du SECM. Ces nano-jonctions, hautement stables et réversibles, ont montré des conductances de 10-7-10-8 S dans leur état conducteur. Ces résultats, liés à la croissance du polymère, donnent à penser que la conductance de l'ensemble de la jonction est régie par 20 à 100 oligomères.

Afin d’obtenir des nano-jonctions de manière contrôlée, une méthode combinant la stratégie dite « Break Junction » (BJ) et le SECM a été mise en place. Une nano-jonction peut être obtenue en éloignant la pointe de sa position initiale. Les variations de conductance obtenues ont montré que des jonctions moléculaires au PEDOT peuvent être brisées par paliers. Des paliers de conductance ont été mesurés par SECM-BJ, et sont comparables à ceux observés par des approches STM-BJ classiques. La technique SECM-BJ s’est avérée efficace pour la fabrication et l’étude de jonctions moléculaires de polymères à grille redox. Le SECM permet également de réaliser des nano-jonctions en utilisant une stratégie d'auto-terminaison. La croissance du polymère peut être arrêtée dès que quelques brins de polymère relient les deux électrodes initialement séparées. La taille de la jonction peut donc être contrôlée par cette méthode.

Les jonctions au PTFQ et PFETQ ont montré des propriétés de transport ambipolaires. Lorsque les jonctions sont constituées de plusieurs fibres, un déséquilibre dans le transport est observé entre canaux de type p- et n-. Au contraire, un équilibre est mis en évidence lorsque les jonctions atteignent une taille nanométrique. Nous attribuons cet effet à un mécanisme de transport qui passe d’un régime diffusif (loi d’Ohm) à un régime balistique (quantique) lorsque les dimensions du dispositif deviennent nanométriques.

Par ailleurs, le comportement d’électrodes d’ITO avec des nanoparticules d’or (Au NPs/ITO) dénote la présence de plasmons localisés de surface (LSP). Ces substrats ont été utilisés, sous irradiation lumineuse, pour activer la jonction démontrant ainsi que la résonance plasmon peut induire une réduction électrochimique. La diminution de conductance observée peut être attribuée à des électrons chauds générés par les plasmons sur les nanoparticules d’Au piégées dans la jonction de PEDOT, réduisant celui-ci en un état isolant.

Enfin, des nano-fils de cuivre ont été élaborés par SECM en utilisant un procédé électrochimique. L’étude du transport a permis de suivre la formation de ces fils entre des électrodes asymétriques. Une étude similaire a été conduite sur une électrode constituée d’un film de silice mésoporeuse sur ITO. Les films ont une épaisseur de 115 nm et les filaments de cuivre sont protégés par encapsulation dans des canaux poreux verticaux d’environ 3 nm de diamètre.

General introduction

Molecular electronics is an active research area utilized molecules and their assemblies as components to achieve the electronic functionalities. Understanding charge transport through those molecules is the essential task in this field. Sandwich molecules between electrodes, forming metal/molecule/metal junctions, are the basic idea toward molecular electronics study. Organic molecules have been employed as fundamental electronic components such as wires, transistors, memory cells, and logic elements. Conducting polymers (CPs) has been applied in organic electronics because of their remarkable electronic properties, chemical stability and low cost. Recently outstanding progress has been achieved in the CPs based molecular electronics. Considering as a candidate for molecular electronics, CPs has unique properties, such as high conductivity and the flexibility. Such properties enable CPs to act as molecular wires, molecular switches and molecular rectifiers with desirable electronic functions. This thesis covers several scientific issues, as depicted in the following scheme, which will be discussed in the following chapters.

Scheme. Schematic illustration of the main goals of this thesis

In the first chapter, I will present the bibliographic study of the development of molecular electronics and a few relevant concepts. I will introduce the basic principle of the SECM set-up. In chapter 2, we will introduce the investigation of electrodeposition of conducting polymer on the UME by SECM set-up. Subsequently, we will discuss both microscopic junctions and nanojunctions based on conducting polymers. Chapter 3 aims to control the size of conducting polymer molecular junctions using the SECM set-up. Three kinds of methods, namely, the self-terminated strategy, differential scan voltammetry, and the SECM break junction approach, will be presented to generate nanojunctions.

Chapter 4 reports the investigation of n-type and ambipolar type conducting polymer molecular junctions. Both n-channel and p-channel charge transport characteristics will be discussed in this section. Chapter 5 discusses whether molecular junctions can be switched by an external input. To do so, an Au NP/ITO electrode is selected as the substrate electrode. Plasmonic effects on conducting polymer molecular junctions will be studied. Chapter 6 is concerned with the protection of atomic contacts. Copper metallic nanowires will be generated using an electrochemical self-terminated method based on the SECM configuration. Nano probes will be introduced for the sake of protecting atomic contact. In addition, atomic switches will also be investigated in this section.

XII

Chapter 1. Introduction

1.1 Background of molecular electronics

Electronic devices are one of the greatest inventions of the 20th century. Their emergence and development have played a vital role in promoting human society. Microelectronic devices are widely used in various fields and have a strong influence on our lives. Since their invention, electronic devices have undergone the periods of the vacuum tube, the transistor and integrated circuits.1-3 Over the past 40 years, the development of large-scale integrated circuits on chips has led to the shrinkage of the dimensions of electronic devices. According to the development trend of electronic devices, Moore in 1965 predicted an annual doubling in the number of components per integrated circuit. In the 1980s, he revised the forecast to doubling every two years.4,5 This is the famous "Moore's law".6 The size of a single component on a chip has been decreasing during the past decades. In 2010, a chip on CPU made up of 700 million transistors, where the dimension of a single component was only 40 nm, was created by the Intel company. As can be seen in figure 1.1, the dimension of the components on the chip decreased from 180 nm in 2002 to 16 nm in 2015. Even though shrinkage no longer cut transistor costs from 2012, it is clear that devices were minimized to the nanoscale. With Skylake, the codename used by Intel for a microarchitecture processor, Intel will have virtually minimized the scale to 14 nm by the end of 2016.

Figure 1.1 Moore’s law and shrinking chips lead to smaller devices.

Chapter 1. Introduction

Recent decades have witnessed a significant development of techniques for the fabrication of nanostructures. The requirement for small devices stimulates the development of nanotechnologies. These technologies can be classified into two categories: top-down and bottom-up methods. Top-down methods, such as patterning and lithography, 7,8 are based on a large-scale preparation reducing the dimensions to the nanoscale. An example of a simplified nanofabrication method used to construct 1D nanochannels is presented in reference 9 and shown in figure 1.2. The processes are significantly simplified and sub-50 nm-deep fluidic channels are formed after bonding the nanopatterned wafers with silicon or borofloat-glass wafers.9 On the other hand, bottom-up methods begin with atoms or molecules to build up nanostructures, in some cases through the smart use of self-organization.7 By starting from an atom or a single molecule to complete the integration of the device, nanoscale molecular devices may be realized. This concept evolved from the Feynman's famous statement: "There’s plenty of room at the bottom".10 He put forward the idea of "assemble from individual molecules or atoms". Then a new subject is known as "molecular electronics" emerged.

Figure 1.2. The basic fabrication process of a nanochannel.

When device size is reduced to below 10 nm, quantum effects will be detected.11 Charge transport properties in the component may change from the classical regime to the quantum regime. What are the classical and the quantum regimes? As illustrated in figure 1.3, when the length (L) and width (W) of the wire are much larger than the electron mean free path (l), electron transport in the nanowire involves collisions and is in the classical regime. Macroscopic conductors are characterized by Ohm’s law: G = σS/L,12 where σ is the

Chapter 1. Introduction intrinsic conductivity of the wire, S is the cross-sectional area, and L is the length of the wire.

Figure 1.3. The classical limit and quantum limit of electron transport through a wire.11

When the mean free path, l, is greater than L, the electron moves ballistically without collision through the nanowire. Furthermore, when the fermi wavelength of the electrons

(λF) becomes comparable or larger than the contact diameter, quantum effects will govern the electric transport properties.13 This is the so-called quantum regime. In these cases the metallic nanowire is also named “atomic point contact”. The electrical conductance through such a metallic nanowire is quantized and is expressed by the Landauer formula14,15:

2푒2 G = ∑푁 푇 , ℎ 𝑖=1 𝑖 where e is the electron charge, h is the Planck constant and Ti is the transmission probability the ith channel. The transmission probability of the conductance channel depends on the chemical valency of the metallic atoms and the precise atomic arrangement of the contact.16,17 It has been frequently observed to be nearly 100% in many cases.13,18-20 As a consequence, the conductance tends to be quantized in near integer multiples of the 2 conductance quantum, G0 = 2e /h=77.4 µS. According to the I(V) characteristic (Figure 21 1.4), the atomic contact has ohmic behavior with a slope corresponding to 1G0.

Chapter 1. Introduction

Figure 1.4. Schematic drawing of atomic contact and the current through copper atomic contact is linearly dependent on the voltage.

Charge transport through metal/molecules/metal (MMM) junctions is expressed by a similar formula:

2푒2 G = × 푇 × 푇 × 푇 , ℎ 푙 푟 푀 where e is the electron charge, h is the Planck constant, Tl, Tr, and Tm is the transmission probabilities through the left electrode, right electrode, and the molecule, respectively. Unlike atomic contacts, the conductance for a single molecular junction range from 10-5 to -1 22 10 G0. Charge transport through molecular junctions is also described by the I-V characteristics, as shown in Figure 1.5.23 I-V curves show a non-linear relation between the voltage and the current.

Chapter 1. Introduction

Figure 1.5 . Schematic illustration of a single molecular junction and typical I-Vbias curves for oligo(phenylene ethynylene) recorded at a bias voltage below 2 V.23

The mechanism of charge transport through a molecule is described by tunneling24 and hoping theory25. In most cases, where the molecule length is as short as a few nanometers, charge transport properties are described by tunneling theory and the conductance (G) through a single molecule decreases exponentially with the length of the molecule (l). It follows the general relation: −훽푙 G = 퐺퐶푒 , where GC is the contact conductance, β is known as the attenuation factor, referring to the efficiency of charge transport through the molecules. When the molecular length become longer and exceeds a certain threshold, the contribution of tunneling is gradually replaced by a hopping mechanism. Hopping is believed to be responsible for charge transport along long molecules. The conductance (G) is dependent on the temperature (T) and follows an Arrhenius relation given by: ∆퐸 G ∝ 푒 ⁄푘푇, Where ΔE is the hopping activation energy, T refers to temperature, and k is the Boltzmann constant.

Chapter 1. Introduction

Tunneling is observed in molecular junctions, occurs with small molecules (usually l < 5 nm) and is independent of temperature.26 It was found that the β value differs from alkyl chain molecules to conjugated molecules.27 Hopping is operative in thicker films or long molecular systems and depends on the temperature. The distinction between hopping and direct tunneling transport is important and depends on the transport distances. If it is greater than a certain value (around 6 nm), conduction will be temperature-dependent.

Figure 1.6 Measurements of molecular wire resistance with CP-AFM. A gold-coated tip was brought into contact with an OPI monolayer on a gold substrate. (A) Semilog plot of R versus L for the gold/wire/gold junctions. Each data point is the average differential resistance obtained from 10 I-V traces in the range –0.3 to +0.3 V. Error bars, 1 SD. Straight lines are linear fits to the data according to Eq. 1. (Inset) A linear plot of R versus L, demonstrating linear scaling of resistance with length for the long OPI wires. (B) Arrhenius plot for OPI 4, OPI 6, and OPI 10. Reproduced from reference 28.

Figure 1.6 shows an example from Frisbie et al.28 A series of oligophenyleneimine molecules with lengths from 1.5 to 10 nm was investigated using a conducting probe atomic force microscopy (CP-AFM) as the top contact. It was found that the mechanism of direct-current transport changed from tunneling to hopping as a function of the systematically controlled wire length. As depicted in figure 1.6a, the β value changed from 3 nm-1 for d < 5 nm to 0.9 nm-1 for d > 5 nm, and the observed resistance of the molecular layer increased linearly with length for the longer molecules. The region where β = 0.9 nm-1 was temperature-dependent and tunneling were a negligible contributor, as can be seen in figure 1.6b. Yamada et al.29 investigated temperature-dependent charge transport of oligothiophene molecules. Temperature has no influence on the thiophene molecules with a length of 2.2 nm, where charge transport is governed by tunneling theory. When the molecule length is 6.7 nm, molecular conductance apparently decreases as the temperature drops. The transition between tunneling and hopping mechanisms occurs when the

Chapter 1. Introduction molecular length is around 6 nm. Lacroix and McCreery30 found three distinct transport mechanisms for 4.5–22-nm-thick oligo(thiophene) based layers between carbon contacts. Tunneling operates when d < 8 nm. Activated hopping is observed when d > 16 nm for high temperatures and low bias. A third unique activationless transport mechanism, consistent with field-induced ionization of highest occupied molecular orbitals, operates when d = 8– 22 nm. This phenomenon has also been proved by other investigations.31-33

1.2 Molecular wires, molecular switches, and molecular rectifiers

The size of the components in a microelectronic circuit will be reduced to the scale of a few molecules or even a single molecule. Molecules act as a basic component performing special functions in digital electronic devices.34-37 Functional organic molecules were selected to perform as molecular wires, molecular switches, molecular rectifiers, etc. (Figure 1.7). Those molecules with specific functions are designed in order to integrate molecular functionalities into electrical circuits.

Figure 1.7. a) Typical molecular wires; b) molecular switches; c) molecular rectifiers.38

Chapter 1. Introduction

1.2.1 Molecular wires

Molecular wires work as a component connecting other functional devices in the molecular electrical nanocircuit. Organic molecules with conjugated structures are widely investigated as molecular wires (Figure 1.7). Effective molecular wires must satisfy the following criteria: first, suitable conductance; second, suitable molecular length; third, suitable linker group at the end of the molecules; fourth, encapsulation to avoid arbitrary electron transfer. In general, molecular wires are structurally classified into two parts: the linker group that connects the electrodes and molecular backbones that offer the electron pathway. The interface between the molecules and the electrodes, including the coupling, contact geometries, and the energy alignment has a strong influence on the conductance of the wires. Both conjugated chains and alkane chains are widely investigated. McCreery et al26 summarized conductance of molecular wires of aliphatic and aromatic molecules (Figure 1.8). It is found that the decay factor β differ from alkane chain to conjugated molecules.

Figure 1.8. Attenuation plots for aromatic and aliphatic molecules in ensemble molecular junctions. NAB = nitroazobenzene, ONI=oligonaphthalenefluoreneimine.26

1.2.2 Molecular switches

Another important subject in molecular electronics is the construction of reliable molecular switches for memory and logic devices. Molecular switches involve molecules which can change their conductance with external inputs, such as the change of pH,39 redox reactions,40 illuminations,41 etc. The organic molecule used as memory elements are promising to replace the traditional silicon-based transistor in electrical logic circuits. 8

Chapter 1. Introduction

Kim et al.42 reported charge transport through photoswitching molecules. The most common used in photoswitching are sulfur based diarylethene molecules. However, in Kim’s work, sulfur-free diarylethene molecules were developed and studied via electrical and optical measurements as well as density functional theory calculations. The molecules are switched to the open and closed isomers forms using UV/Vis illumination (Figure 1.9). These molecules show different conductance for the open and the closed forms.

Figure 1.9. Switching sulfur-free diarylethene molecules using MCBJ device.

Among those switches, redox-active molecules are often chosen for the design of switch systems with hopefully high on/off ratios. These molecules exhibit different conductivities when switched between their oxidized and reduced forms.28,43-48 It allows one to alter the conductance of individual molecules by controlling their redox states. Besides organic molecules, conducting polymers (CPs) and oligomers have also attracted much attention for applications in molecular electronic devices. The performances of CPs used as switchable molecular wires are quite encouraging due to their flexible and low-cost characteristics. They are one of the most promising candidates for the replacement of traditional silicon-based materials used in metal oxide semiconductor field-effect transistors (MOSFET). Wrighton et al.49 pioneered a redox polymer as a microelectrochemical transistor to measure the electrical properties between source-drain electrodes by controlling the gate potential. Tao’s Group contributed to this topic in a series of works. The diagram of the experimental setup is shown in Figure 1.10.46 Polyaniline (PANI) is deposited electrochemically on the tip of an STM. PANI junction is formed when it connect a gold surface filling the electrodes gap with a distance of 20 nm to 100 nm. The STM tip is then pulled up, while the current flowing through the junction is recorded. The conductance

Chapter 1. Introduction curves recorded during the elongation process of the junction are shown in Figure 1.11. It increases at the beginning and then decreases gradually, and finally, the polymer thread breaks (Figure 1.11a). The conductance levels of 60nS (Figure 1.11b) are due to transitions between stable configurations of the polymer chains induced by the elongation of the junction.

Figure 1.10 Schematic drawing of the nanowire was controlled with respect to a reference electrode. A counter electrode was used as in a standard electrochemical setup. In comparison to a field effect transistor, the RE, WE1, and WE2 electrodes are analogous to the gate, source and drain electrodes.46

Figure 1.11 (a) conductance of a PANI fiber during elongation process, (b) decrease the zoom increments conductance. Potential of the tip and substrate are now at 0.45 V and 0.5 V respectively.46 10

Chapter 1. Introduction

Another experiment forms the same group described the generation of a PANI junction formed between two nano-electrodes separated by a gap in a size of 20 to 80 nm fabricated by lithography with an electron beam.50 When PANI connects the second electrode, the current flowing between the two electrodes suddenly increases. The deposit process is then stopped. Scanning the gate potential (Vg) and applying a fixed bias between the two electrodes, the current through the junctions progressively increases with Vg, and reaches a maximum value at 0.4 V, where it is in the conductive form, as shown in Figure 1.12 a.

Figure 1.12 (a, b, c) of the electrochemical potential function of current for nanojunctions PANI between two gold electrodes separated by about 50 nm (a) and 1 nm (b) and (c). (d) the characteristic I / V of a nanojunction PANI in the conductive state.50

When the junction is reduced to a few nanometers (Figure 1.11 b and c), instead of a progressive conductance change, an abrupt jump or several abrupt jumps in conductance are observed. At potential less than 0.15V, PANI is in the reduced form (insulating state), no current flows between the electrodes. Increasing the potential to above 0.15 V, the current increases suddenly of several orders of magnitude up to 30 nA (for a bias of 20 mV). The characteristic I / V of nanojunction in conductive form reveals an ohmic behavior and the slope gives the conductance of approximately 400 ns (Figure 1.12d). Another PANI junctions were studied by Tao et al,51 as shown in Figure 1.13. When the PANI is maintained between 0.05V and 0.15V, the current signal switches drastically between zero and a few tens of nanoamperes. The on/off switching can be controlled by the redox state of the polymer with the potential of the nanoelectrodes. The frequency of the 11

Chapter 1. Introduction on/off switching is small below 0.1V, and increases as the potential increase, then decrease above 0.15V. At high potentials, the polymer predominately stays in the on state with only occasionally fluctuations to the off state then returning to the on the state.

Figure 1.13 (a) On-off random switching in the conductance of polyaniline ~potential 0.2 V. (b) The dependence of the on-off switch on the potential of the nanoelectrodes. The inset shows more clearly the on-off switching.

1.2.3 molecular rectifiers

Since 1940, n-doped and p-doped Si has been used as inorganic p-n junction rectifiers on chips. However, flexible materials with high on/off ratios are required for rectifier devices in electric nanocircuits. In 1974 Aviram and Ratner43 proposed the first molecular rectifier model. They believed that molecules with donor (D)-σ-acceptor (A) structure would exhibit rectification properties. Since then the interest in molecular rectification has spread. Because of the difficulties of organic synthesis methodologies, a molecular rectifier using a single molecule with D−σ−A structure (Aviram-Ratner model) has not yet been reported. However, rectifying molecules with D−π−A has been widely investigated. Molecular rectification is characterized by following the I-V curves. As shown in Figure 1.14, Tao et al.52 used STM break junctions to study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules. These nonsymmetric molecules(Figure 1.14b) exhibit rectification behavior (rectifying

Chapter 1. Introduction ratio of 5) as compared to symmetric molecules (Figure 1.14a). note that the rectifying ratio remains small for the real application.

Figure 1.14 Current-voltage (I–V) curves for the symmetric (a) and nonsymmetric molecules(b).52

In fact, besides the Aviram−Ratner model, several other factors may also lead to molecular rectification phenomena, such as two electrodes of different work functions involved in MMM junctions,53-55 different anchoring groups of the molecules resulting in asymmetric interfacial coupling,56,57 and the contact geometry in the electrode-molecule interface.58,59

1.3 Molecular junctions and atomic contact test-beds

To investigate charge transport through molecular electronic devices, one needs to find a way to wire molecules between the electrodes. Constructing the Metal/Molecule/Metal (MMM) junctions is the basic concept in molecular electronics investigations. In general, the conductivity of the metallic electrode is much higher than the molecules of the MMM junction. As a consequence, the conductance of the entire MMM junction depends on the molecule itself and on the interface between the molecule and the electrode.60 Usually, inert metals such as gold and platinum are used as electrodes, due to their chemical stability in ambient conditions. Molecules containing sulfur and nitrogen as terminal groups anchor the gold and platinum electrodes. Although it is a long way to achieving commercial molecular electronic devices, many ways have been discovered to study molecular electronics. Recently using bottom-up strategy or sophisticated top-down fabrication has been witnessed. Each MMM fabrication technique has its own unique advantages but still is far from perfect.61 Here we will recall

Chapter 1. Introduction some of the most general techniques and summarize the basic principles in the fabrication of MMM junctions.

1.3.1 Scanning Tunneling Microscope break junction（STM-BJ）

Scanning Tunneling Microscope break junction (STM-BJ）is a technique used for the in situ formation of an MMM junction. It starts with a metallic STM tip located a few nanometers above the substrate electrode. The tip movement is controlled by the piezo traveling toward the substrate electrode. Due to the precise control in the z-direction of the piezo, the gap distance between the tip and the substrate electrodes can be tuned within a few nanometers. As shown in Figure 1.15a,62 the STM tip repeatedly contacts the substrate. The molecules may bridge both the tip and the substrate electrodes during tip movement. Current versus tip travel time (I-t) curves are recorded (Figure 1.15b). Plateaus are often observed in the I-t curves. Such a plateau represents the transport current value of the molecules involved in the MMM junction.25 The conductance of the single molecule is determined by the current plateau as well as the conductance histogram (Figure 1.15c) which is constructed from a large number of I-t curves.

Figure 1.15. STM-BJ measurements: (a) Schematic depiction of break junction system used for measuring conductance; (b) Several example conductance versus time traces; (c) Conductance histogram with a peak at the most probable conductance value for a single-molecule junction.62

Tao’s group pioneered the development of STM-BJ for conductance measurement of molecular junctions.24 Figures 1.15a and b show values of conductance quantum recorded 14

Chapter 1. Introduction when the tip electrode is pulled away from the substrate. It represents the typical conductance traces of Au atomic contacts. Conductance decreases in a stepwise fashion with each step occurring at integer values of G0. The corresponding conductance histogram

(Figure 1.16b) shows a well-defined peak at N G0, indicating the formation of a clean Au atomic contact. Next figures 1.15c and d correspond to molecular junction breaking during tip retraction. When the contact shown in (a) is completely broken, corresponding to the collapse of the last quantum step, a new series of conductance steps appears if molecules such as 4,4′- bipyridine are present in the solution. These steps are due to the formation of the molecular junction between the tip and the substrate electrode. Conductance below 1 G0 was observed in Figure 1.16 c. Such conductance values are attributed to charge transport through the molecular junction. The corresponding conductance histogram shows dominant conductance peaks (see Figure 1.16d). Various conductance peaks represent different numbers of molecules involved in the junctions. In the case where no molecules are present in the solution, none of these peaks of low conductance value is observed (Figures 1.15e and f).

Figure 1.16 (A) Conductance of a gold contact formed between a STM gold tip and a gold substrate; (B) Corresponding conductance histogram constructed from 1000 conductance curves as shown in (A) shows well-defined peaks near 1 G0, 2 G0, and 3 G0 due to conductance quantization; (C) New series of steps that appear after the gold contact is broken, if molecules such as bipyridine are present in solution; (D) Histogram of conductance measurements obtained for 1000, corresponding to conductance curves of the same type as in (C); (E) and (F) Conductance curve and histogram when there are no molecules in solution. 24

This technique has been used by many groups. It can also be used in three-terminal nanoelectronic devices controlling charge transport in a redox-active channel.47,48,63 Leary et al. have shown that conductance switching is obtained in a redox-gated single-molecule junction.64 Venkataraman et al.65 also reported that electrochemical gating directly 15

Chapter 1. Introduction modulates the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the single junction transport properties. Zhou et al.66 studied three molecules with various redox centers and found on/off ratios of 2 to 10 when the redox state of the molecules was varied.

1.2.2 Mechanically controlled break junction (MCBJ)

Mechanically controlled break junctions (MCBJ) were first developed by Moreland and Ekin in 1985 to investigate tunneling characteristics.67,68 Then Muller and van Ruitenbeek developed MCBJ to study atomic contacts.69 It was first used for the measurement of single-molecule conductance by Reed et al. in 1997.70

Figure 1.17. The principle of the mechanically controlled break junction (MCBJ) measurement and the formation of a metal-molecule-metal bridge during the breaking process.71

Figure 1.17 illustrates the basic principle of MCBJ.71 It starts with a notched wire over a bendable substrate, a counter-rod to support the substrate, and a push-rod to break the wire. The substrate is bent by a piezo-controlled push-rod within micrometer precision in the z-direction, while the counter-rod support is kept at the fixed position. After breaking the metal wire, two clean nanoelectrodes are generated face-to-face. The distance between the electrodes for both the opened and the closed directions is controlled by the bending and relaxing of the substrate, respectively.72 In the presence of individual molecules, an MMM junction is probably generated during the mechanical movement. Current versus displacement (I-S) curves are simultaneously recorded. Molecular conductance is determined by the plateaus in the I-S curves as well as the conductance histogram constructed from a large number of individual I-S events, which is similar to that 16

Chapter 1. Introduction determined by STM-BJ. As a technique for generating MMM junctions, MCBJ has been employed in the measurement of the conductance of a single molecule73,74 as well as the conductance quantum of atomic contacts.75,76 Even though it is difficult to use MCBJ, because it remains a rather sophisticated system compared to STM-BJ, it provides the possibility of combination with other 77 techniques. For example, as shown in Figure 1.18, Tian et al reported a combined SERS and MCBJ method to measure SERS signals of molecules located inside the nanogap between two electrodes on a Si chip. The distance between electrodes is controlled within several nanometers by MCBJ. Meanwhile, in-situ molecular assembly in the gap is observed by SERS. Such a combination of MCBJ and SERS makes it possible to detect a single or a small number of molecules between two probe electrodes.

Figure 1.18 Study of electrical and optical properties of probe molecule by using mechanically controlled break junction (MCBJ) combined with in-situ surface enhanced Raman spectroscopy (SERS).

MCBJ is one of the most widely adopted methods in the measurement of molecular electronics. However, MCBJ remains difficult to use and to form stable molecular junctions. This limitation has been recently overcome by Van der Zant et al. They showed that a single molecular junction with high time endurance can be generated by MCBJ.78

1.2.3 Single-wall Nanotubes (SWNT)

A recent method for the fabrication of a molecular junction is based on the so-called single-wall nanotubes (SWNT).79 SWNT have a special structure in chirality and diameter and a simple composition of carbon. Since their discovery more than twenty years ago, nanotubes have stimulated a large amount of research. They can be used as the electrode due to their high conductivity and richness of electrical diversity. There are several methods for creating SWNT nanogap electrodes, including focused-ion-beam etching (FIB), electron beam, electrical breakdown, and lithography-defined oxidative cutting.80-83 As shown in Figure 1.19, an electrode gap was fabricated by cutting the SWNTs using an

Chapter 1. Introduction electron beam.84 A molecular junction was created by amide bond formation between the amines and the carboxylic acid groups that terminate the electrodes. It provides the chemical attachment to span the gap. In this work, it was found that the photo-switching can cycle between the open and closed states.

Figure 1.19. (A) Molecular bridges between the ends of an individual single-walled carbon nanotube (SWNT) electrode; (B) Switching between conjugated and non-conjugated molecular structures.

Due to their unique physical and chemical properties, such as the nano-size, mechanical strength, toughness, and excellent electrical conductivity, SWNTs have unique advantages in the study of single-molecule devices. It also has an advantage for building horizontal molecular devices with high stability. However, preparing an SWNT electrode is complicated. The yield for the formation of such molecular junctions is only ∼3%.61

1.2.4 Conducting probe atomic force microscopy (CP-AFM)

Conducting probe atomic force microscopy (CP-AFM) is another technique widely used in the investigation of MMM junctions. Frisbie et al.85,86 pioneered the development of this technology. In CP-AFM, as shown in Figure 1.20, an AFM tip, coated with a conductive metal layer (usually Au), is positioned by piezo using feedback based on van der Waals force.87 A nanometric gap is created by approaching the tip to the substrate. MMM junctions are established when the tip contacts a self-assembled molecular monolayer deposited on the substrate. Then charge transport through the junction is followed by applying a bias between the tip and the substrate.

Chapter 1. Introduction

Figure 1.20. Typical point-contact CP-AFM experiment in which an Au-coated AFM tip is used to probe the resistance of a thin crystal of sexithiophene.85

CP-AFM is an extension of the standard AFM set-up. Its major advantage is the ability to control tip-sample separation, providing a tip-sample contact in order to measure the conductance of the SAMs. It gives an MMM junction with high stability. It has been successfully employed in the investigation of large-area molecular junctions by conductance measurements on alkane chains88,89 and conjugated molecules.90,91 However, the tip size of AFM is usually larger than that of STM. The number of molecules involved in the MMM junctions using CP-AFM is usually hard to control. If the substrate electrode is smaller than the AFM tip, the number of molecules involved in the MMM junctions will be limited. Dominique et al92 reported that conductance of alkylthiol molecules was measured by CP-AFM. In this work, a large array of sub-10 nm single crystal Au nanodots electrodes was used as a substrate electrode, which allowing to measure the conductance of up to a million of junctions in a single CP-AFM image. In this section, we have reviewed some molecular electronics test-beds in the literature. There are still other techniques used in the research of single-molecule junctions and large-area molecular junctions, such as liquid metal contact,93,94 electromigration breakdown junction,95 on-wire lithography,96-98 etc. Comparing those techniques, the basic principles for the fabrication of molecular junctions are quite similar. These techniques share a common structure: an electrode gap that fits the molecular length should be generated, a reliable connection between molecules and the electrode. Single or more molecules are thus sandwiched inside the gap of the electrodes.

Chapter 1. Introduction

1.4 Scanning electrochemical microscopy (SECM)

In our group, we plan to investigate charge transport properties through conducting polymer junctions and atomic contacts using scanning electrochemical microscopy (SECM). To do so, a gap between two electrodes is required in order to generate MMM junctions.

1.4.1 Background

Dating back to 1986, Engstrom et al.99 pioneered to approach an ultramicroelectrode (UME) (10 µm in diameter) on a conventional millimeter electrode at a distance of 5 μm. They were able to probe the amperometric response in the diffusion layer of the electrode. In the same year, Bard et al.100 developed a similar technique to study the electrochemical response of a redox probe. In 1989, Bard proposed the name of this technique: Scanning Electrochemical Microscopy (SECM) and published several papers describing the technique and the theory.101,102 Since then, the number of studies employing this technique continues to increase.103-106

1.4.2 SECM principle

Piezo Potential Controller Programmer

x Piezo y Tip potential Bipotentiostat z Positioner Reference Electrode

Counter Electrode

Substrate Potential

Figure 1.21. Schematic principle of SECM

The SECM device is schematically shown in Figure 1.21. This device consists of: - A system with 3 or 4 electrodes: a working electrode (UME), a counter electrode, a reference electrode and optionally a second working electrode that is used as the substrate;

Chapter 1. Introduction

- A piezo-controlled positioning system permitting movement of the UME tip relative to the substrate in three dimensions; - A potentiostat or bipotentiostat to monitor or measure the potential and current. The monitored signal of the SECM is the electrochemical current on the microelectrode tip. When the tip is moved close to the surface of a sample, disruption of current provides information about the nature and electrochemical properties of the substrate. When a UME (disk shape) is immersed in a supporting electrolyte containing a redox system, the current through the UME during a potential sweep gives the signal, as shown in Figure 1.22.

Figure 1.22. Distribution of species and their electrochemical reaction on the UME and cyclic voltammetry response of a redox system on UME.

The diffusion-controlled current on the UME is given by the formula:

It, inf = 4nFDCa, - Where n is the number of electrons exchanged during the electrochemical reaction (Ox + ne = R), F is Faraday's constant, D is the diffusion coefficient of the redox species in solution, C is the concentration of the redox system, and a is the radius of the UME. This expression is valid when the UME is far from the surface and nothing disrupts diffusion of the redox species to its surface. As the tip get close to the substrate the electrochemical current change. The current variation depends on the distance between the tip and the substrate. The shape of the approach curve gives information on the reactivity of the substrate.

Chapter 1. Introduction

a -e- R O d R O -e- Conductor

Figure 1.23 (a)Regeneration of the mediator on a conductive substrate and (b) the approach curve.

Figure 1.23 shows the schematic approach of a UME to a conductive substrate and the corresponding approach curve. When the UME approaches a conductive surface, the oxidized redox mediator to the tip broadcasts to the conductive substrate, where it is reduced, and then diffused again in the vicinity of the UME. The mediator is thus regenerated close to the probe, causing an increase in the flow of the reactive species arriving on the UME. Therefore, the oxidation current at the tip increases as the UME approaches the surface, as shown in Figure 1.23, this is called "positive feedback". Figure 1.24 shows a schematic approach of a UME to an insulating substrate and the corresponding approach curve. In this case, the surface is insulating and chemically inert. When the approach distance is reduced, the diffusion of the mediator to the UME is particularly hampered by the insulating body of the surface. The current at the UME is reduced as the tip approaches the surface. As shown in Figure 1.24, this is called "negative feedback". a

-e- d R O × Insulator

Figure 1.24. (a)Regeneration of the mediator on an insulating substrate and (b) the approach curve.

Chapter 1. Introduction

1.4.2.1 Theoretical tip approach

The theory of the regeneration mode proposed by Kwak and Bard102 is based on the numerical solution of diffusion equations. In many cases, analytical approximations allow a much simpler solution. The following theoretical results are for a disc-shaped electrode. The theoretical approach curves were obtained by simulating the approach of the UME to the substrate, and reflect the evolution of the current iT depending on the distance d separating from the substrate. These curves are characteristic of the nature of the substrate and redox processes occurring there. They are usually displayed in their normalized form, with IT = iT/iT,inf and L = d/a. In this way, the approach curves are independent of the electrode radius a, the diffusion coefficient D and the mediator concentration C. The analytical expression of the approach curve on an insulating substrate is given by:

IT,insulating(L) = 1/{0.15 + (1.5358/L) + 0.58exp(-1.14/L) + 0.0908exp [(L-6.3)/(1.017L)]}, and the expression for the approach curve on a conductive substrate is given by:

IT,conducting (L) = 0.68 + (0.78377/L) + 0.3315exp(-1.0672/L).

1.4.2.2 Experimental tip approach

After the UME is immersed in a solution containing a reversible redox couple, the current at the UME is recorded as it approaches the surface. Depending on the nature of the substrate, the current increases (decreases) when the UME comes close to the conductive (insulating) substrate surface. The approach is then automatically stopped at a set value. Efforts have been focused on the fabrication of smaller tips, with radius of the order of a few nanometres, with the aim of achieving a close approach, even at L = 0.2.107,108 It has been reported by Bard et al. that tip-to-substrate gaps of d = 110 nm can be achieved.107 In the context of this Ph.D, a controlled size gap can be easily formed between a UME and another electrode by recording the approach curve in the presence of a redox mediator.

1.4.2.3 Molecular junctions by SECM

Since a controlled size gap can be easily formed between UME and another electrode, metal or a conductive polymer can be deposited to connect the probe and the substrate. The SECM setup seems to favor the manufacturer of junctions or metallic contact. In the previous work in our group, it has been clearly observed that a separate charge transfer and charge transport processes occur in a redox gated polyaniline (PANI) 23

Chapter 1. Introduction nanojunction.109 And even a PANI junction where charge transport is controlled by a single oligoaniline strand was obtained. SECM, is also proposed as a tool for the fabrication of copper atomic point contact.110 The formation of atomic contacts is supported by the ohmic behavior of the I–V curve. It depends neither on the UME tip radius nor on the initial gap size between tip and substrate. The redox gated conducting polymers molecular junctions and atomic point contacts studied in this thesis will be created by electrochemical method using SECM.

1.5 Summary

In this chapter, we briefly discussed the historical development of molecular electronics and a few relevant concepts, as well as the experimental techniques developed over the past decades. We recalled the basic theory of electron transport in the classical regime and quantum regime. Furthermore, we mentioned the techniques to create molecular junctions. We introduced the basic principle of the SECM set-up. This thesis is based on the expertise of the Nano Electrochemistry’s (NEC) group in nanoscale electrochemistry. In my Ph.D. work, SECM will be used to generate MMM junctions. The thesis focuses on several scientific issues which have been expressed in the general introduction and will be presented in details in the next chapters.

REFERENCES

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90 Xie, Z., Bâldea, I., Smith, C. E., Wu, Y. & Frisbie, C. D. Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function. ACS nano 9, 8022-8036 (2015). 91 Sangeeth, C. S. et al. Comparison of DC and AC Transport in 1.5-7.5 nm Oligophenylene Imine Molecular Wires across Two Junction Platforms: Eutectic In-Ga versus Conducting Probe Atomic Force Microscope Junctions. Journal of the American Chemical Society (2016). 92 Smaali, K., Clément, N., Patriarche, G. & Vuillaume, D. Conductance statistics from a large array of sub-10 nm molecular junctions. ACS nano 6, 4639-4647 (2012). 93 Race, H. & Reynolds, S. Electrical properties of multimolecular films. Journal of the American Chemical Society 61, 1425-1432 (1939). 94 LI, J.-C., WU, J.-Z., ZHOU, C. & GONG, X. Latest Studies on Metal-Molecule-Metal Junctions. Acta Physico-Chimica Sinica 29, 1123-1144 (2013). 95 Van der Zant, H., Osorio, E., Poot, M. & O'Neill, K. Electromigrated molecular junctions. physica status solidi (b) 243, 3408-3412 (2006). 96 Qin, L., Park, S., Huang, L. & Mirkin, C. A. On-wire lithography. Science 309, 113-115 (2005). 97 Banholzer, M. J., Qin, L., Millstone, J. E., Osberg, K. D. & Mirkin, C. A. On-wire lithography: synthesis, encoding and biological applications. Nature protocols 4, 838-848 (2009). 98 Mbindyo, J. K. et al. Template synthesis of metal nanowires containing monolayer molecular junctions. Journal of the American Chemical Society 124, 4020-4026 (2002). 99 Engstrom, R. C., Weber, M., Wunder, D. J., Burgess, R. & Winquist, S. Measurements within the diffusion layer using a microelectrode probe. Anal. Chem. 58, 844-848 (1986). 100 Bard, A. J., Crayston, J. A., Kittlesen, G. P., Varco Shea, T. & Wrighton, M. S. Digital simulation of the measured electrochemical response of reversible redox couples at microelectrode arrays: consequences arising from closely spaced ultramicroelectrodes. Anal. Chem. 58, 2321-2331 (1986). 101 Bard, A. J., Fan, F. R. F., Kwak, J. & Lev, O. Scanning electrochemical microscopy. Introduction and principles. Anal. Chem. 61, 132-138 (1989). 102 Kwak, J. & Bard, A. J. Scanning electrochemical microscopy. Theory of the feedback mode. Anal. Chem. 61, 1221-1227 (1989). 103 Wipf, D. O. Initiation and study of localized corrosion by scanning electrochemical microscopy. Colloids and Surfaces A: Physicochemical and Engineering Aspects 93, 251-261 (1994). 104 Heinze, J. Ultramicroelectrodes in electrochemistry. Angewandte Chemie International Edition in English 32, 1268-1288 (1993). 105 Leroux, Y., Schaming, D., Ruhlmann, L. & Hapiot, P. SECM investigations of immobilized porphyrins films. Langmuir : the ACS journal of surfaces and colloids 26, 14983-14989 (2010). 106 Nyffenegger, R. & Penner, R. Nanometer-scale surface modification using the scanning probe microscope: progress since 1991. Chem. Rev. 97, 1195-1230 (1997). 107 Shen, M., Arroyo-Currás, N. & Bard, A. J. Achieving Nanometer Scale Tip-to-Substrate Gaps with Micrometer-Size Ultramicroelectrodes in Scanning 29

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Electrochemical Microscopy. Anal. Chem. 83, 9082-9085, doi:10.1021/ac2021294 (2011). 108 Sun, P. & Mirkin, M. V. Kinetics of electron-transfer reactions at nanoelectrodes. Anal. Chem. 78, 6526-6534 (2006). 109 Janin, M., Ghilane, J. & Lacroix, J.-C. When electron transfer meets electron transport in redox-active molecular nanojunctions. Journal of the American Chemical Society 135, 2108-2111 (2013). 110 Janin, M., Ghilane, J. & Lacroix, J.-C. Scanning electrochemical microscopy for the fabrication of copper nanowires: Atomic contacts with quantized conductance, and molecular adsorption effect. Electrochimica Acta 83, 7-12 (2012).

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

It is well known that electrochemical oxidation of monomers is a simple and effective tool for the fabrication of conducting polymers. EDOT, aniline, and pyrrole (scheme 2.1) have been intensively investigated in the past decades due to the high conductivity of the corresponding polymers.1-3 Conducting polymers are widely used in photovoltaic devices,4-7 sensors,8-11 energy storage,12-14 biomedical applications,15-17 etc.

O O electropolymerization O O

S * S n* EDOT PEDOT

electropolymerization

N * N n* H H pyrrole Ppy

electropolymerization H H2N * N * n aniline PANI Scheme 2.1 synthesis PEDOT, Ppy, and PANI by electrochemical polymerization.

In this chapter, a conducting polymer will be wired between an ultra-micro electrode (UME) tip and a substrate electrode in an SECM configuration, aiming to generate molecular junctions. The growth of polymer on millimeter-sized electrodes and UMEs will first be investigated. A metal/molecule/metal junction will be created by filling the SECM electrode gap with conducting polymer. The charge-transport current through the molecular junctions is characterized by following the I/V curves. The results obtained will be discussed in the next part of this chapter.

2.1 Electropolymerization of conducting polymers on microelectrodes

EDOT electropolymerization on a millimeter Pt electrode was firstly studied. It was carried out by cyclic voltammetry (CV) in an acetonitrile solution containing 20 mM EDOT and 0.1 M TBAPF6 as the electrolyte. From the first cycle (figure 2.1a, red line), the oxidation of EDOT started at nearly 1.2 V/SCE, and an oxidation peak is observed at 1.4 31

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions V/SCE. The peak current is 20 times the oxidation peak current of 1 mM Fc (figure 2.1a, black line). This is in good agreement with the concentration ratio of EDOT to Fc, which suggests that the peak current is approximately limited by EDOT diffusion.a The formation of the corresponding polymers is given in Figure 2.1b. As the number of cycles increases, an increase of the capacitive-like current of the electrode (in figure 2.1b) is observed. This indicates that the amount of electropolymerized polymer on the working electrode increases. Figure 2.1c shows the CV response in monomer-free acetonitrile of the PEDOT film created. The broad cathodic and anodic peaks indicate that the polymers are well deposited on the working electrode surface. The polymer film could be oxidized (anodic peak potential at 0.1 V, Ea) and reduced (cathodic peak potential at -0.5 V, Ec) reversibly.

CV of 1mMFc 800 25 400 CV of 1st cycle of PEDOT polymerization 13

600 19

)

2 A

A 300 10

2  a  b 400 13

200 6 J(mA/cm

J(mA/cm 200 6 Current/ 100 3 Current/

0 0 0 0

-100 -3 -200 -6 0.0 0.4 0.8 1.2 1.6 0.0 0.4 0.8 1.2 1.6 Potential/V(SCE) Potential/V(SCE)

80 2.5 bare electrode 60 CV of PEDOT film 1.9

40 1.3 )

c 2 A

 20 0.6 0 0.0

-20 -0.6

J(mA/cm Current/ -40 -1.3 -60 -1.9 -80 -2.5 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE)

Figure 2.1. (a) The first cycle of 20 mM EDOT oxidized (red) in a solution of 0.1 M TBAPF6 in acetonitrile and CVs of 1 mM ferrocene (black) in a solution of 0.1 M TBAPF6 in acetonitrile on 2 mm-diameter Pt electrode. (b) CVs of electrochemical PEDOT deposit on Pt electrode, scan rate 0.1 V s-1. (c) CV response of PEDOT films (red line) and blank Pt electrode (black line) in acetonitrile containing 0.1 M TBAPF6.

Then we studied EDOT electropolymerization on a 10 μm-diameter UME in an aAs the EDOT oxidation peak is irreversible and the PEDOT film generated is easier to oxidize than EDOT, the current under this peak cannot be only attributed to EDOT oxidation. It is not entirely correct to state that the current is limited by EDOT diffusion. 32

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions acetonitrile solution of 20 mM EDOT and 0.1 M TBAPF6. Figure 2.2a shows the CV response of EDOT on such a UME. Its oxidation start at around 1.3 V. During the back scan a peak is observed at 1.4 V. In fact, the peak current is a nucleation loop with the current in the back scan becoming higher than in the forward scan. Such an effect is not observed on a millimeter electrode (see the arrows in figure 2.1a and figure 2.2a). The oxidation current density of EDOT reaches a value as high as 4 A cm-2 and 10.5 A cm-2 in the first and second cycles, respectively, whereas the oxidized current density of 1 mM Fc reaches 9.5 mA cm-2 (see figure 2.2b) on the same UME. The EDOT oxidation current value is almost 1000 times that of Fc. The current values do not correspond anymore to the calculated concentration ratio of 20 between EDOT and Fc. Clearly, the EDOT oxidation current on the UME is above the diffusion limits, and PEDOT growth on the UME seems extremely fast.

10 13 8 10 8 10

) 2 A 6 b 8

 6 8

) 2

4 5 4 5

J(mA/cm

Current/nA J(A/cm Current/ 2 3 2 3

0 0 0 0 -2 -3 0.0 0.4 0.8 1.2 1.6 0.0 0.4 0.8 Potential/V(SCE) Potential/V(SCE)

Figure 2.2. (a) CVs of 20 mM EDOT and 0.1 M TBAPF6 in acetonitrile on 10 μm-diameter -1 UME; scan rate 0.1 V s . (b) CVs of 1 mM ferrocene in 0.1 M TBAPF6 in acetonitrile on 10 μm-diameter UME.

In order to further understand how fast the electropolymerization of EDOT on the UME is, we calculated the thickness of the PEDOT deposited. To do so, we assume that polymer growth induces a cylindrical film of area S and thickness d on the UME. The CV of EDOT oxidation on a 10 μm UME was recorded (figure 2.3a) in acetonitrile containing

20 mM EDOT and 0.1 M TBAPF6. The PEDOT film was characterized in a monomer-free solution of 0.1 M TBAPF6 in acetonitrile (figure 2.3b).

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

10 13 50 1st 64 1st 2nd a 2nd b 40 51

3rd 5 3rd 6

)

A 

30 38 4th ) 2 4th  5th 5th 20 25

0 0

J(A/cm

J(A/cm Current/

10 13 Current/

0 0 -5 -6

-10 -13 0.0 0.4 0.8 1.2 1.6 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE)

Figure 2.3. (a) CVs of solution of 20 mM EDOT and 0.1 M TBAPF6 in acetonitrile on 10 μm-diameter UME; scan rate 0.1 V s-1. (b) CVs of PEDOT film in a solution of 0.1 M -1 TBAPF6 in acetonitrile on a UME; scan rate 0.1 V s .

The amounts of PEDOT deposited can be estimated by calculation.18 In the ideal case, the deposition yield is assumed to be 100%. The thickness of the polymer film on the electrode is given by the formula: (M  0.2M )Q d  EDOT counterions deposition 2.2FS the doping yield of counter-ions in the conducting polymer is usually taken to be 20-30%.19,20 Since PEDOT is generated in its oxidized state, the effective number of electrons associated with PEDOT growth per EDOT monomer should be taken as 2.2.18,21

Here, F is the Faraday constant, and Qdeposition is the charge used for polymerization. The polymer occupies a volume V equal to S*d. Assuming that linear growth leads to a cylindrical PEDOT deposit on the UME, this volume is also equal to n*M/ρ where M is the molecular weight of EDOT, and ρ is the polymer density (reported to be 1.5 g/cm3).21 22 It has been reported that Qdeposition is 10 times Qdoping in the ideal case. In our case,

Qdeposition is more than 10 times Qdoping (table 2.1). To be more precise, we tend to use Qdoping to calculate the thickness. In this way, all the charge used for calculation is determined by the PEDOT which has been created on the UME. Then the thickness is given by: (M  0.2M )10  Q d  EDOT counterions dopping 2.2FS According to this calculation, the thickness of PEDOT can reach 15.4 μm after just one electropolymerization cycle, and increases to 1.2 mm after 5 electropolymerization cycles, which shows extremely fast growth of PEDOT film on the UME, and that the hypothesis that growth is cylindrical is not correct.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

1st 2nd 3rd 4th 5th

Qdoping 280 nC 450 nC 5.1 μC 11 μC 19.5 μC

Q 9 μC 49 μC 146 μC 323 μC 608 μC deposit

Film thickness 16.9 μm 27.2 μm 307 μm 663 μm 1175 μm Table 2.1 PEDOT thickness after each CV polymerization on 10 μm UME

To further prove that the growth of PEDOT on the UME is fast, we have taken a picture of the Pt UME tip, bare and modified with PEDOT film. (figure 2.4) The bare UME has a clean surface with a dark spot corresponding to 10 μm of platinum in the center of the glass (figure 2.4). After five EDOT electropolymerization cycles, PEDOT covers the platinum electrode and the glass. Such a phenomenon of PEDOT growing extremely rapidly on an electrode has rarely been reported.

Figure 2.4. Image of bare platinum UME(a) and PEDOT-modified electrode (b).

The growth rate for PEDOT electrodeposition on a Pt electrode is different when the diameter of the electrode is changed from 1.5 mm to 10 μm. It is much faster on the UME than on the millimeter electrode. To understand this fast growth on the UME, we assume that the conductive PEDOT may behave as the metallic material. Ohmic drops are negligible and the potential applied to the Pt UME tip is transmitted to the PEDOT/solution interface. When PEDOT is deposited on the electrode surface, it acts as the electrode; consequently, the electrochemical surface is no longer the platinum but the surface between PEDOT and the solution. The increasing amount of PEDOT on the electrode increases the real electrochemical active area. As illustrated in figure 2.5, on the

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions large platinum surface (diameter 1.5 mm), the additional area of PEDOT is small. However, when the size of the electrode is reduced to the micrometer scale, PEDOT deposition on the UME may create a hemispherical or a dendritic mass of conducting polymer. The surface area of such a PEDOT hemisphere is much bigger than that of the initial platinum electrode, as shown in figure 2.5. If the ohmic drop is negligible, the potential applied on the UME is now on the PEDOT. Thus, the active Pt surface is replaced by the PEDOT surface generated. As a consequence, the extremely rapid growth of PEDOT on the UME, observed here when the applied potential reaches 1.3 V/SCE and above, can be attributed to a displacement of the electrochemical interface and to an increase in the effective area of the PEDOT/solution interface. It is very important to note at this point, that the potential is now applied to this new interface, not to the Pt (UME)/PEDOT interface anymore.

Figure 2.5. Cross-section of conducting polymer deposited on 1.5 mm-diameter Pt electrode (uniform growth) and on 10 μm-diameter UME (hemispherical or dendritic growth).

The flux of monomers during polymerization can be affected by the electrolyte and the monomer concentration.23 Therefore, in order to see whether the type of electrolyte has an influence on PEDOT deposition on UME, electropolymerization was performed in acetonitrile containing 20 mM EDOT but using 0.1 M LiClO4 as the electrolyte.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

500 CV of 1mMFc 16 12 15 CV of PEDOT polymerization 20mM EDOT 10 13 400 13

) 8 10

A a b

)

 2

300 10  6 8

200 6 4 5

J(A/cm

current/ J(mA/cm 100 3 current/ 2 3 0 0 0 0 -2 -3 -100 -3 0.0 0.4 0.8 1.2 1.6 0.0 0.4 0.8 1.2 1.6 Potential/V(SCE) Potential/V(SCE) Figure 2.6. (a) First cycle of EDOT electropolymerization (red) of 20 mM EDOT and 0.1 M LiClO4 in acetonitrile and CVs of 1 mM ferrocene (black) and 0.1 M TBAPF6 in acetonitrile on 1.5 mm-diameter Pt electrode. (b) CVs of PEDOT deposition on UME (10 μm); scan rate 0.1 V s-1.

Figure 2.6a shows the first cycle of CV responses of EDOT electropolymerized on a 1.5 mm Pt electrode. Here again the EDOT oxidation current value is close to 20 times that of Fc (black line). Then EDOT was electropolymerized on the UME. In figure 2.6b, the EDOT oxidation current is again 1000 times higher than that of Fc, which indicates fast growth of PEDOT on the UME. The growth of PEDOT on the Pt electrode is quite similar to that shown in figure 2.1a with TBAPF6 as the electrolyte. The fast growth of PEDOT on UME does not seem to depend on the electrolyte type. Then EDOT was electropolymerized on the UME with a monomer concentration of 10 mM and 1 mM. As can be seen in figure 2.7a, the EDOT oxidation current in the first cycle decreases by 50% compared with that in figure 2.6b. The forward scan shows the EDOT oxidation current is still above the diffusion limits. When the concentration of EDOT changed to 1 mM (figure 2.7b), the oxidation current in the first cycle became close to that of 1 mM Fc (black line). EDOT started to be oxidized at 1.0 V. The oxidation current seems to depend strongly on the applied potential, with two different regimes (below and above 1.25 V). At low potential (below 1.25 V) PEDOT was slowly electrodeposited on the UME. Fast electrodeposition is obtained when the potential is above 1.25 V. The EDOT oxidation current density saturated at 50 mA cm-2 (1 mM EDOT used in figure 2.7b) instead of 3 A cm-2 (10 mM EDOT used in figure 2.7a). This current value is now only 5 times that obtained with Fc, which indicates that electropolymerization is not fast in 1 mM EDOT solution. Figure 2.7c shows the current of PEDOT growth on UME in a 1 mM EDOT solution. The EDOT oxidation current gradually increased but much less than observed at higher monomer concentration. This experiment shows that the monomer concentration influence PEDOT deposition on the 37

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions UME.

12 12

10 CV of PEDOT polymerization 10

8 a 8

) 2  6 6

4 4

A/cm (

2 2 J Current/ 0 0

-2 -2

0.0 0.4 0.8 1.2 1.6 Potential/V(SCE)

CV of Fc 40 CV of PEDOT polymerization 51

30 38 ) b 2

20 25 J(mA/cm

Current/nA 10 13

0 0

0.0 0.4 0.8 1.2 1.6 Potential/V(SCE) 300 382

250 318

200 c 255 ) 2 150 191

100 127 J(mA/cm Current/nA 50 64

0 0

-50 -64 0.0 0.4 0.8 1.2 1.6 Potential/V(SCE) Figure 2.7. CVs of (a) 10 mM EDOT (b) first cycle of EDOT electropolymerization (red) with 1 mM EDOT and CVs of 1 mM ferrocene (black) and LiClO4 in acetonitrile on UME; -1 scan rate 0.1 V s . (c) 1 mM EDOT and 0.1 M LiClO4 in acetonitrile on UME; scan rate 0.1 V s-1.

In order to reduce the rate of EDOT electropolymerization and achieve better control of PEDOT growth, we stopped the sweep potential at 1.25 V/SCE. As shown in figure 2.8, the EDOT oxidation current stayed on the nano-ampere scale. The thickness of the 38

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions PEDOT film generated on the UME was calculated. From table 2.2 it can be seen that 10 nm of the film was created on the UME after one electropolymerization cycle and only 250 nm is generated after 5 cycles. The growth of PEDOT on the UME is now much slower with a low oxidation potential.

0.6 0.8 10 1st 13 1st 2nd 0.4 2nd 0.5 8 3rd a 10 3rd b 4th

4th ) ) 2 5th 0.2 0.3 2 6 5th 8 0.0 0.0

4 5

Current/nA

J(mA/cm Current/nA 2 3 -0.2 -0.3 J(mA/cm

0 0 -0.4 -0.5

-2 -3 -0.6 -0.8 0.0 0.4 0.8 1.2 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE)

Figure 2.8. (a) CVs of 20 mM EDOT and 0.1 M TBAPF6 in acetonitrile on 10 μm-diameter -1 UME; scan rate 0.1 V s . (b) CVs of PEDOT film in 0.1 M LiClO4 in acetonitrile on UME; scan rate 0.1 V s-1.

1st 2nd 3rd 4th 5th

Qdoping 0.2 nC 0.55 nC 1.4 nC 2.4 nC 4.2 nC

Q 7.5 nC 14.7 nC 28.3 nC 51.7 nC 87.6 nC deposit

Film thickness 12 nm 33 nm 81 nm 141 nm 250 nm Table 2.2. PEDOT thickness after each CV polymerization on 10 μm UME

It is also interesting to understand the growth of other conducting polymers on UMEs, such as polyaniline (PANI) and polypyrrole (Ppy). To avoid fast polymer growth, we have also studied aniline and pyrrole electropolymerization on the UME under low-potential conditions. Figure 2.9a shows a typical CV of aniline (ANI) electropolymerized on a platinum microelectrode at a scan rate of 100 mV s-1 from 1 M aniline and 2 M H2SO4 in water. The oxidation current of aniline is a few nanoamperes during 5 cycles when the potential window is set at 0 to 0.85 V. This value is close to that observed in figure 2.8 and shows that the growth of PANI is similar to that of PEDOT at a low potential. The ultrafast growth of PANI, as that observed with PEDOT at a high potential, can thus be avoided. Figure 2.9b shows the typical characteristics of PANI film on the UME. The increase in electroactivity current of PANI indicates that the film is growing. The low growth rate of PANI has also been demonstrated by calculating the

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions amount of PANI on the UME (table 2.3). PANI film a few hundreds of nanometers thick is obtained after 5 cycles. Note that Qdeposit is 10 times Qdoping, indicating that the deposition yield is close to 100%.

25 32 4 5 1st 1st 2nd 20 2st 25 3 4 3st 3rd b

a ) 4st 2 4th 3 2 15 19 5st ) 5th 2 1 1 10 13

0 0

J(mA/cm Current/nA

5 6 J(mA/cm Current/nA -1 -1

0 0 -2 -3

-5 -6 -3 -4 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 Potential/V(SCE) Potential/V(SCE)

Figure 2.9. (a) CVs of 0.5M ANI in H2O containing 2 M H2SO4 on 10 μm-diameter UME; scan rate 0.1 V s-1. (b) CVs of PANI film on UME; scan rate 0.1 V s-1.

1st 2nd 3rd 4th 5th

Qdoping 0.8 nc 2.1 nC 3.8 nC 6.7 nC 11.7 nC 10 nC 24 nC 44 nC 74 nC 118 nC Q deposit

Film thickness 48 nm 126 nm 228 nm 400 nm 700 nm

Table 2.3. PANI thickness after each polymerization CV on 10 μm UME

The electropolymerization of another monomer, pyrrole (py), was investigated. It started in a solution containing 20 mM pyrrole and 0.1 M TBAPF6 as an electrolyte in acetonitrile. Figure 2.10a shows that pyrrole is oxidized above 1.1 V/SCE. The peaks due to the oxidation and reduction (0.1 V and -0.1 V, respectively) of the film increase in intensity as the film grows. The thickness is also calculated (table 2.4). After one cycle, 100 nm of Ppy film was deposited on UME. The thickness of Ppy created on the UME was calculated to be 2.4 μm after 5 CV electropolymerization cycles.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

350 446 30 38 1st 1st 300 382 2nd 2nd 20 25 3rd 250 3rd a 318 b 4th

4th )

) 2 200 5th 255 2 10 5th 13 150 191 0 0

100 127 Current/nA

J(mA/cm

J(mA/cm Current/nA 50 64 -10 -13

0 0 -20 -25 -50 -64 -0.4 0.0 0.4 0.8 1.2 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE)

Figure 2.10. (a) CVs of 20 mM pyrrole and 0.1 M TBAPF6 in ACN on 10 μm-diameter UME; scan rate 0.1 V s-1. (b) CVs of Ppy film on UME; scan rate 0.1 V s-1.

1st 2nd 3rd 4th 5th

Qdoping 4 nC 13 nC 27 nC 43 nC 95 nC

Q 130 nC 250 nC 414 nC 687 nC 1200 nC deposit

Film thickness 0.1 μm 0.33 μm 0.7 μm 1.1 μm 2.4 μm Table 2.4 Polypyrrole thickness after each CV polymerization on 10 μm UME

The thickness of conducting polymer deposited on the UME is strongly influenced by the applied potential. Fortunately, slow film growth on the UME has been achieved by choosing the proper monomer concentration and controlling the applied potential. These conditions for electrodepositing conducting polymers on UMEs, where ultrafast growth is avoided, will be adopted for generating molecular junctions between tip and substrate in an SECM set-up.

2.2 Molecular junctions generated by SECM set-up

2.2.1 Filling the SECM gap with conducting polymer

The fabrication of conducting polymer molecular junctions can be divided into two steps. Initially, a millimeter platinum electrode is used as the substrate, and the platinum tip with a radius of 5 to 12.5 μm is positioned upon the substrate at a large distance. An electrode gap distance is created by running the approach curve in acetonitrile containing

1M Fc and 0.1 M TBAPF6. As shown in figure 2.11, the current on the UME is recorded while it is brought up to the surface by the piezo-electric motors. The current increases when the UME comes close to the platinum substrate (conducting surface-positive 41

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions feedback). When the UME is close to the substrate, the approach is automatically stopped when the recorded current reaches a setting value. As described in chapter 1, the gap distance can be controlled and is given by the formula 24:

IT(L)=0.78377/L +0.3315exp(-1.0672/L)+0.68 Here L = d/a, where d is the gap distance between tip and substrate and a is the UME radius.

50 approach process i-d 45

Current/nA 30

20 0 10 20 30 40 50 60 70 80 Distance/m Figure 2.11 Positioning the UME at a controlled distance from the substrate in redox probe (ferrocene) solution.

After positioning the UME at about 5-10 μm from the substrate, the redox probe solution used to position the UME is removed, and acetonitrile containing 20 mM EDOT and 0.1 M TBAPF6 is introduced into the electrochemical cell. To create conductive polymer junctions, we deposit PEDOT on the UME by chronoamperometry. To avoid ultrafast PEDOT growth on the platinum UME and the gap being filled in less than one second, we set a constant potential of 1.1 V for electrodeposition. A potential of 0 V/SCE is applied to the substrate. At this potential, EDOT oxidation occurs on the tip and no PEDOT is deposited on the substrate. As shown in figure 2.12b, the current increases smoothly for the first 22 s. Such a current corresponds to the electrochemical polymerization of EDOT on the tip. At t = 22 s, the current increases suddenly and saturates. The most plausible explanation is that PEDOT has reached the substrate and that a connection between the tip and the substrate has been established. Since the PEDOT is in its conductive form, the current can suddenly go through this organic junction, and a current jump is observed. Such a current jump, which is mainly due to charge transport through PEDOT junctions, is the signal for their formation. This hypothesis will be discussed further in this chapter.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

1200

1000

800

600

Current/nA 400

200

0 0 5 10 15 20 25 Time/s Figure 2.12. Filling the gap by EDOT electropolymerization on the tip: Creation of the junction.

2.2.2 Characterization of conducting polymer junctions

The junctions obtained were characterized by following the current through the polymer as a function of its electrochemical potential, measured versus a reference electrode acting as the gate electrode of a solid-state transistor. Using such analogy, the tip can be seen as the source, the substrate as the drain and reference + counter electrodes as the gate. As illustrated in scheme 2.2, the tip and the substrate are connected by the polymer generated by chronoamperometry. The potential difference between the tip and the substrate is equivalent to VSD, and the current through the junction is described as ISD. The conductivity of the junction depends on the sweep potential VG (VG = Vtip - Vref), controlled relative to the reference electrode, which is equated with the gate of a field-effect transistor. The conductance of the junction can thus be modulated by varying the oxidation state of the polymer.

Scheme 2.2. Schematic illustration of polymer junction acting as a field-effect transistor (FET) device.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions As previously investigated in our group,25 a PANI junction was created by chronoamperometric electrodeposition in the same way as described in part 2.2.1. PANI film deposited on the UME is firstly characterized by cyclic voltammetry. In figure 2.13, the gray line represents the electrochemical response of the PANI film with a 0 V bias between source and drain. A dominant electron transfer process of PANI is observed with an oxidation peak at 0.2 V/SCE and a reduction peak at 0.05 V/SCE.

Figure 2.13. (Black) Itip current as a function of the gate voltage using −20 mV bias; (gray) cyclic voltammogram at the tip when the substrate is unbiased.

We will now proceed to describe the characterization of the PANI junctions obtained, allowing us to observe the evolution of the conductance of junctions depending on the potential VG. The variation of the source-drain current ISD through the junction of the polymer is recorded as a function of VG, that is to say, the potential of the tip is scanned relative to the potential of the reference while a fixed potential difference VSD is maintained between the source (tip) and drain (substrate). When 20 mV was used as the bias, as shown in figure 2.13 (black line), the shape of the curve differs from a classical voltammogram (gray line in figure 2.13). Obtaining this response is a signal that a PANI junction is formed, and the current measured is current through the junction. At potentials below 0.15 V/SCE, the current ISD through the junction is zero, because PANI is still in its reduced form. The possible leakage current is less than 10 pA. Above 0.15 V, PANI is oxidized and becomes conductive, inducing an increase in current. In figure 2.13, the black line shows that the electron-transfer and electron-transport processes are observed in the same experiment and in the same current range.

To further identify the transport current of PANI molecular junctions, the bias VSD was varied. We can see that how the response of the junction is sensitive to the VSD.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

Figure 2.14. Source-drain current vs. gate voltage at by using various bias. Scan rate: 100 mV/s.

The current ISD in figure 2.14 is initially small and suddenly starts to increase, then reaches a maximum value at 0.3 V/SCE. The electron-transfer current of PANI is negligible compared to the electron-transport current when PANI is polarized in the conductive form. Increase the bias between the electrodes increases the current through the junction. This depends on the VSD bias between the tip and the substrate, and the sign of the transport current through PEDOT junction reversed when the polarity of the junction is changed. The currents flowing in one direction or the other with the same bias are approximately symmetric. In order to further characterize the PANI junctions formed, another type of I(V) curve was recorded. The current ISD through the junction is measured with constant potential fixed on the substrate (ES) while the potential of the tip (ET) is swept by ET=ES ± 50 mV. Figure 2.15 shows one of these curves, with the substrate potential set at 0.3 V/SCE.

Figure 2.15. I(V) characterization of a PANI junction obtained by scanning the potential between 0.25V/SCE and 0.35 V/SCE while a constant potential of 0.3 V/SCE is applied to the substrate.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions PANI has a maximum conductivity between 0.2 V/SCE and 0.4 V/SCE. In figure 2.15, the substrate is fixed at 0.3 V/SCE, and the tip potential is scanned from 0.25 V/SCE to 0.35 V/SCE. In this potential range, the PANI remains at its maximum conductivity. Since the potential of the peak is not scanned over a wide range, the conductivity changes within the junction are negligible. Consequently, PANI is in its conductive form, and the I(V) curve obtained is linear. The slope of the I(V) curve gives us the conductance value. It depends on the potential applied to the substrate. At potential less than 0.1 V/SCE PANI becomes insulating, the slope of the I(V) curves thus decreases strongly. The variations of the slope of the I(V) curves will be discussed later in this chapter. These early results demonstrate the possibility of using the SECM as a starting configuration to manufacture switchable polymer junctions. In addition, the repeated oxidation and reduction processes indicate that stable conducting polymer junctions can be generated. In the next work, we will introduce molecular junctions based on PEDOT and polybithiophene (PBT). We aim to reduce the conductance of the polymer junction, and in particular to reduce the number of strands governing the charge transport through the junctions. The work was published in the Journal of Physical Chemistry C and will be described in the following section.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions 2.3 Approaching the Frontier between Fiber Devices and Single Molecule Devices in Redox Gated Junction

2.3.1 Introduction

In nanoelectronics and nanoelectronic devices with surfaces below 20×20 nm, i.e. 400 nm2 and thicknesses in the 5 to 20 nm range are the next frontiers. As a consequence, understanding and studying charge transport across a small number (10 to 400) of π-conjugated molecules or π-conjugated oligomers are of heightened interest.26-29 One first wishes to electrically wire a few molecules between two electrodes and to fabricate a metal/molecules/metal junction. To do so, various methods have been proposed,30-38 including scanned-probe techniques,30 electrical or mechanical break junctions, mercury drop electrodes, sandwich electrodes,34 and top contact on self-assembled monolayer, CMOS compatible processes. Besides that, controlling and triggering charge transport across a metal/molecules/metal junction by an external input is the desired task. When redox active molecules are used, the switching of their redox states using an electron transfer process, to or from a gate, is proposed for such control.27,28,39-43 Indeed, as the electronic states of a molecule in its oxidized and reduced forms are different, conductance

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions variation between the two states can be observed, on/off ratios measured, and logic or memory devices designed.44,45 In three-terminal solid-state devices, a gate electrode, separated from the channel by a dielectric, is used to control the doping level of the channel.46,47 An electrochemical gate can also be used in three-terminal nanoelectronic devices to control charge transport, in a redox active channel.25,42,43,48-55 Such systems make it possible to drastically reduce the gate voltages, thanks to double-layer effects, and pave the path towards reduced energy consumption in logic and memory devices. Several studies have focused on redox-gated single-molecule junctions using the electrochemically gated STM-BJ method. Xiao et al. have reported the conductance variations upon redox switching through molecules of various lengths containing one ferrocene unit.44 The single molecule conductance was reported to be 23 nS in the reduced state whereas it increased to 150−200 nS in the oxidized state (on/off ratio of 6−8). Zhou et al. studied three molecules with various redox centers, and found on/off ratios of 2−10.56 Mayor’s group reported conductance around 2 nS for a series of alkylviologenes and on/off ratio below 2.57 Single hepta-aniline conductance was found to switch from 0.32 to 5.3 nS leading to an on/off ratio of 16.53 Benzodifuran single molecule redox switches were also studied showing that depending on the molecule/electrode coupling unit, on/off ratio 2−8 can be obtained.58 More recently, tunable charge transport in redox inactive single molecule junction via electrolytic gating was also reported, and changing the gate potential by 4 V yielded to conductance variations of 2−8 without any redox events involved in the process.59 In all these experiments, the distance separating the two metallic electrodes is way below 5 nm. The main mechanism for charge transport is coherent tunneling through the molecule. Charge transport is mainly controlled by the distance between the two metallic electrodes and by the type of groups anchoring the molecule to the metal but does not strongly depend on the molecular backbone of the systems nor on its redox state. As a consequence, the measured on/off ratio cannot be very high in such single molecule junctions. Many redox-gated devices were also fabricated with conducting polymer fibers grown between two metallic electrodes separated by a large distance. Most of them show a high on/off ratio, thanks to the large conductivity variation upon redox electrochemical doping/dedoping reactions, the close-to-zero direct background tunneling current across the fibers (when the two metallic electrodes are separated by a distance of 5 nm, direct tunneling is not the main transport mechanism anymore, and activated hopping of charge carriers between redox sites dominates), and the many molecules involved in the polymer 48

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions fibers. Wrighton et al. have pioneered a conducting polymer microelectrochemical transistor to measure the electrical properties of polymers between two microelectrodes.55,60 Alam et al. have reported electrolyte-gated transistors based on many conducting polymer nanowire junction arrays with a large on/off ratio of 1000. Tao et al. were also among the first to study polyaniline fibers grown between two nanoelectrodes, and they observed an abrupt switch between two states.61 Individual fibers of conjugated polymer were also studied. To do so, fibers were generated in various membrane templates and, after the dissolution of the membrane, deposited on a surface. Once located using SEM techniques, lithography was used in order to contact an individual fiber by two or four metallic electrodes. Using this set-up the transport properties of individual fibers of various diameters were studied. With this method, confinement effects on the transport properties of conjugated polymer were studied, and tuning of their electrical properties by diameter was demonstrated, with the conductivity of PEDOT nanowires increasing from 10 to 500 S/cm at room temperature when the fiber diameter decreases from 200 to 25 nm.62-74 This behavior appeared general and most conductive polymer fibers reported in the literature exhibit conductivity increase with a decrease in the fiber diameter. In this paper, we wish to close the gap between redox-gated single-molecule devices and fiber devices involving many molecules. We report here the fabrication and characterization of redox-active nanojunctions based on poly(bithiophene) and poly(ethylene dioxythiophene) wires. This technique uses a scanning electrochemical microscope (SECM)75,76 with two micrometric tips (tip1 and tip2) face-to-face separated by an initial gap, which is way beyond the tunneling distance used in the STM-BJ method. This initial gap is varied from 300 nm to 5 μm, and the tip size is varied from 10 to 1 μm. Starting from these configurations, PBT and PEDOT nanowires are grown electrochemically from one tip, until they bridge the two microelectrodes. The junctions were investigated by following the variation of the current at the tips or through the wires as a function of the gate potential at fixed tip1/tip2 bias voltage. We show that, independent of the initial gap separating the two microelectrodes, a stable nanojunction in which charge transport within the whole micrometric gap is controlled by a limited number of strands can be easily generated. Furthermore, it is possible to reach a situation in which electron transport, through the wires, can be easily and reversibly studied around various gate voltages. Moreover, this knowledge is used to study and compare steady state and dynamic on/off ratios of the nanojunctions.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions 2.3.2 how to fabricate PBT and PEDOT molecular junctions by SECM

Chemicals: Ferrocene (Acros) was used as redox couple and tetrabutylammonium tetrafluoroborate (Acros) as supporting electrolyte. EDOT and BT monomer (Sigma-Aldrich) was used as received. Acetonitrile (ACN) was supplied by Prolabo. Electrodes: Disk microelectrodes (UMEs) were made by sealing platinum wires (Goodfellow) in soft glass tubes using a laser puller. The radius of tip1 used for this investigation was varied between 5 µm and 340 nm whereas that of tip2 is 12.5 µm. The SECM tips (tip1) were conically polished with an RG ratio (glass radius over the platinum radius) equal to 10 for the 5 µm radius tip while the substrate (tip2) has an RG around 103 which enable the positioning of the tip1 over tip2. Prior to use, the UME was polished using diamond pastes of decreasing sizes. A silver reference electrode was used but all potentials were referenced to an SCE. A platinum wire (1 mm diameter) was used as the counter-electrode. Electrochemical and transport Measurements: Electrochemical and transport measurements were performed using a commercial scanning electrochemical microscope (SECM), CHI 900B (CH Instrument, Austin, TX). A four-electrode set-up was employed. Two UMEs were used as the working electrode (labeled tip1) and the substrate (labeled tip2). When a junction connects tip1 and tip2, as for a solid-state transistor, the variation of the tip1-tip2 (source-drain) current versus the gate (Etip1 − Eref = Vg) potential was recorded.

The two tip potentials are scanned simultaneously while a fixed bias (Etip2 − Etip1 = VSD) is maintained between the two electrodes. Negative bias corresponds to a situation where tip2 is at a lower electrochemical potential than tip 1. Electrode positioning and Polymer deposition: The SECM approach curve and the SECM images performed in an electrolyte solution containing reversible redox probe (ferrocene) permit the positioning of the two UMEs. The procedure to align and approach the electrodes consists in using the feedback mode followed by SECM image at a constant distance (Figure 1b). The bright spot observed in the SECM image corresponds to the platinum area of tip2. Next, by lateral displacement, tip1 is brought face-to-face with tip2. This procedure has been described in detail in the literature.75,76 At this stage, tip1 and tip2 are positioned face-to-face separated by a gap that can be adjusted depending on the positive feedback observed when tip 1 approaches tip 2. In the present work the initial gap, deduced from the approach curves, was varied between 340 nm and 5 µm.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

a) b) Tip 1

x Tip 2

Figure 1. (a) Positioning of the two UMEs face-to-face (tip1 is the SECM UME while tip2 is the substrate); (b) SECM image (100*100 μm) of tip2 (diameter 25 µm). PEDOT and PBT deposition was performed on tip1 at 1 V/SCE constant potential while monitoring the current at tip 1 and tip 2. Tip2 was polarized at 0 V/SCE during PEDOT or PBT deposition at tip 1. When some polymer strands contacted tip2, a sudden jump of the current was observed and polymerization was stopped immediately. The amount of polymer deposited on tip 1 was estimated through the charge density used in the polymerization process using a well-known procedure.77 The estimated volume of deposited polymer is always much below the volume between tip1 and tip 2 (cylinder of radius equal to the radius of tip 1 and a length L equal to the gap between tip1 and tip2). This indicates that the growth is not uniform and that dendrites or nanowires are generated. Contact occurs when one of these nanowires connects tip1 to tip2. PEDOT was generated using tetrabutylammonium tetrafluoroborate and tetrabutylammonium perchlorate salts as supporting electrolytes. PEDOT films grown in such media are reported to have bulk conductivities between 120 and 400 S cm-1, i.e. above 100 S cm-1.63 We have measured the conductivities of the bulk PEDOT generated in this study (2 µm-thick films grown on ITO electrodes) using a four-band set-up applied on both sides of the free-standing film, and found bulk PEDOT conductivities above 100 S cm-1.

2.3.3 PBT molecular junctions

Bithiophene was electrochemically polymerized in acetonitrile at tip1 while tip2 was kept at a negative potential to avoid spreading of the polymer when contact is made. After contact is established the variation of the tip1 and tip2 currents as a function of the gate potential using -100 mV bias was recorded. Figure 2 shows the response of one PBT junction. (Figure SI1 shows two other examples.)

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

/nA 0

tip I -10

-20

0.6 0.8 1.0 1.2 1.4 V /V(SCE) g

Figure 2. Itip as a function of the gate voltage using -100 mV interelectrode bias and 10 mV s-1 scan rate. Lines: black, tip1; red, tip2. PBT junctions were fabricated in ACN solution containing 0.01 M BT and 0.1 M tetrabutylammonium hexafluorophosphate. The curves (black and red) show that during the forward scan, below 1.2 V/SCE the junction is in an insulating state and no current (or leakage current in the sub-nanoampere range) is flowing between the two electrodes. Next, the current starts to increase, when PBT starts to be oxidized and reaches a plateau value at 1.3 V. The curves recorded at tip1 (black line) and tip2 (red line) are almost symmetric, with Itip1 = −Itip2. (See Figure S1 for fully symmetric curves.) This clearly shows that the current measured is mainly due to charge transport across the PBT junction and that the junction behaves as a transistor (source−drain current ISD = Itip1 = −Itip2, Vg = Etip1 − Eref triggering the transport properties). In this case, the current due to electron transfer used for doping the polymer is at least one order of magnitude smaller than that due to electron transport across the junction, and Igate is thus 25,54 almost negligible compared to ISD. During the back scan the PBT junction is reversibly switched from the conducting to the insulating state at a potential close to 1.1 V. Backward curves show a small hysteresis induced by structural relaxation of the junction during the scan.

The same junction was studied at constant Etip2 values while the potential of tip1 was swept (Vg = Etip1 − Eref) by 100 mV around various Etip2 values. Such experiments make it possible to record the ISD versus VSD characteristics of the device for various doping levels of the conjugated polymer strands. Figure 3 shows the response of the PBT nanojunction for various Vg values. I(V) characteristics of the device clearly depend on Vg, i.e. on the redox states of the PBT strands. When Vg is below 1 V, PBT strands are in their reduced insulating states. As a consequence, the current across the junction is very small (a few hundreds of picoamperes and corresponds to electrochemical leakage current). The slope of the I(V) curve is between 2 and 20 nS, which is the upper limit of the conductance of the 52

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

PBT nanojunction in this potential range (non-conducting state). In the region between Vg =

1.1 and 1.2 V conductance switching starts to occur, and above Vg = 1.3 V, the I(V) curve is almost ohmic. The slope of this I(V) curve is much higher than those observed below 1 V. The junction has now reached its maximum conductance value. From the slope of this curve, one can estimate that the conductance of the device is around 270 nS whereas that of Figure 2 reaches a value of 220 nS for the same junction. Similar results on other junctions are displayed in Figure S2.

25 15 20

/nA

tip I 10 270nS 10 5

0 0.4 0.6 0.8 1.0 1.2 1.4 165 V /V(SCE) 5 g 95 45 13 17 22

0 2.3

/nA tip I -5

-10

-15

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 V /V(SCE) g Figure 3. Charge-transport current of PBT junction as a function of the VSD bias around various gate voltages. Inset: charge-transport current as a function of the gate voltage at -100 mV bias. At this point, it is interesting to compare these conductances with those reported for the many nanofiber devices generated with various conducting polymers and that of single-molecule devices. Conductance values for a single strand of doped conjugated oligomers, measured by an STM-BJ set-up, are reported to be around 3 to 5 nS,53,78 depending on their length and anchoring groups, whereas fiber devices have conductance of several µS despite diameters as small as 25 nm and distances between the metallic contacts close to 2 µm.64,68,71,74 In the present case, 200-300 nS conductance values appear to be an intermediate value between values reported in the literature for fiber and single-molecule devices. This key point will be discussed further in the following.

2.3.4 PEDOT molecular junctions

Next PEDOT junctions were created using a strategy similar to that described above and characterized; Figure 4 presents tip1 and tip2 currents (black and red curves) as a function of the gate potential using −100 mV bias. It also compares the current observed in a PEDOT junction with those observed in a PBT junction. In figure 4, the curves recorded 53

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions at tip1 and tip2 are close to symmetric, with Itip1 = −Itip2 and ΔItip1 = −ΔItip2. This clearly shows that these two currents are due to charge transport across the PEDOT junction. Here again the current due to electron transfer used for doping the polymer is at least one order of magnitude smaller than that due to electron transport across the junction, and Igate is thus almost negligible compared to ISD. The overall shape of the Itips versus Vg curve is similar to that observed with PBT. However, PEDOT conductance switch occurs at a much lower potential (0.2 V versus SCE in the forward scan and −0.1 V in the back scan) as expected from the bulk redox potentials of these two electroactive materials. A conductance plateau is reached at 0.4 V. Hysteresis between forward and back scans is again seen. Fluctuations with ΔItip1 = −ΔItip2 are observed and are larger than those observed for PBT nano-junctions. Such conductance fluctuations have been already observed with single conducting polymer strands, such as polyaniline and polypyrrole.61,79

45 PEDOT tip 1 PEDOT tip 2 PBT tip 1 30 PBT tip 2

/nA

tip 0 I

-15

-30

-0.4 0.0 0.4 0.8 1.2 1.6 V /V(SCE) g Figure 4. Itip as a function of the gate voltage using -100 mV interelectrode bias and 10 mV s-1 scan rate of PEDOT and PBT junction: Lines: black, PEDOT tip1; red, PEDOT tip2; blue, PBT tip 1; green, PBT tip2.

Such transport curves have been obtained at different bias and are shown in Figure S3. As expected, the variation of the bias voltage affects the transport current: (i) increase in the bias value enhances the current through the PEDOT junction; (ii) reversal of the bias sign induces a reversal of the current flow; (iii) symmetry of the current measured through tip1 and through tip2 in terms of current intensity and fluctuations is maintained (not shown in Figure S3).

As was already done with PBT, a PEDOT nanojunction was studied at constant Etip2 values while the potential of tip1 was swept (Vg = Etip1 − Eref) by 200 mV. Such experiments make it possible to record the ISD versus VSD characteristics of the device for the various

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions doping levels of the conjugated polymer strands. Figure 5 shows the response of a PEDOT nanojunction for various Vg values during the forward scan.

8 12 10 8 6 6

/nA 4 tip I 2 0 54nS 4 -2 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Vg/V(SCE)

2 23nS

/nA 12nS

4nS 5nS 9nS tip I 0

-2

-4

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 V /V(SCE) g Figure 5. Charge-transport current as a function of the VSD bias for various gate voltages (scan rate 100 mV s-1) Inset: Charge-transport current as a function of the gate voltage -1 using a fixed –100 mV VSD bias and a scan rate of 100 mV s

The general shapes of these curves are close to those obtained with PBT nanojunctions (see Figure 3), but the switching potential is around 0 V/SCE instead of 1.2 V. Despite steplike fluctuations seen in the inset, the PEDOT nanojunction appears sufficiently stable for the same junction to be studied using successive potential sweeps around several constant Etip2 values. The I(V) characteristics of the device are reproducible and clearly depend on Vg, i.e. on the redox states of the PEDOT strands. When Vg is above 0 V, PEDOT strands are in their oxidized conductive state, and the slope of the I(V) curve is high and reaches 54 nS. Below 0 V, PEDOT is in its reduced state; the slope is smaller and reaches 4 nS in the present case. Apparent on/off ratios between oxidized and reduced states of 14 are thus obtained here. This value is smaller than that obtained with PBT nanojunctions and is due to the currents being higher when PEDOT is reduced. Indeed, the inset of Figure 5 shows a higher electrochemical leakage current with the present PEDOT junction than the inset of Figure 3. Such a difference is probably due to oxygen or hydrogen evolution in this potential range, since platinum micro-electrodes are used, and due to the capacitive behavior of the electrode response when the potential is swept at 100 mV s−1. (See Figure S4 for similar curves with forward and reverse scan emphasizing the capacitive contribution of the electrodes.)

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

2.5 Tip 0.4V Tip 0.3V 2.0 Tip 0.2V 150 Tip 0.1V Tip -0.1V -3 1.5

100

X10

0 1.0

50 G/nS G/G 0.5

0.0 0

0 50 100 150 200 250 300 Time/s Figure 6. Conductance versus time for a PEDOT junction at various gate potentials (VSD bias of 100 mV, static measurements). In order to decrease the electrochemical background contribution of the electrode to the overall current measured on the tips when PEDOT is reduced, we have measured the time-dependent conductance of the nanojunctions at various fixed Vg values with a constant

VSD bias of 100 mV. These curves are widely used when dealing with metallic nanowires or atomic contacts in order to judge the stability of such systems.80-82 Figure 6 shows the measured conductance values of the junctions using two different scales (conductance and normalized conductance G/G0, where G0 is the quantum of conductance). One can see that the PEDOT nanojunction is stable over long periods, and a conductance plateau can be observed for more than 1000 seconds (shown here for 300 s). Small conductance variations during the time scale of the measurement are observed, suggesting either structural relaxation of the polymer or additional oligomer that connects strands (increasing the conductance) and disconnects (decreasing the conductance) to the substrate. These fluctuations seem to be related to the oxidation state of the oligomer strands. Indeed, the intensity of the fluctuations decrease when the potential applied to the nanojunction is close to the reducing state. More interestingly, the conductance is −3 redox-gated with a conductance of 100 nS or 1.3 × 10 G0 (currents around 10 nA) in the −6 oxidized state, whereas the conductance is 0.1 nS or 1.3× 10 G0 (current is only of 10 pA) in the reduced state. Thus, the apparent on/off ratios measured through this static procedure are in the range of 1000, which is well above that found by sweeping the gate potential around a fixed tip2 potential in the dynamic mode. Such on/off values are of interest in nanoelectronic and nanoelectronic devices. More importantly, the conductances are again intermediate values between those reported for single nanofiber devices generated with various conducting polymers (several

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions µS despite diameters as small as 25 nm)64,68 and that of single-molecule devices (single strand of doped conjugated oligomers are reported to be around 3 to 5 nS depending on their length and anchoring groups).53,78 From the nanofiber device viewpoint, it suggests that the fiber is not very conducting and has a smaller diameter than those already reported in previous studies, i.e. below 25 nm. From the single-molecule viewpoint, and despite the large gap between the two metallic electrodes, values of 100-400 nS appear to be only 100 times that of a single strand. In both cases, such values may indicate that the conductance of the junction is controlled by a small part of the fiber, that corresponding to its smallest diameter and that this diameter is so small that it only involves a limited number of strands.

2.3.5 The frontier between fiber devices and single molecular devices

In order to address this point, we first varied the gap size and the radius of the tip where PEDOT is generated (tip1). The tip1 radius of each electrode was measured using the ferrocene limiting current far away from the substrate, and the gap size was measured using the positive feedback current observed when tip1 approaches tip2 using ferrocene as a mediator in solution. The tip1 radius were varied from 5 to 1 μm, and the gap size from 10 μm to 340 nm (see Table 1). Figure 7 and Figure S5 present tip1 and tip2 currents as a function of the gate potential, using −100 mV bias for the 6 nanojunctions of Table 1. In all the cases, it was possible to generate PEDOT junctions with conductance values in the oxidized state between 200 and 400 nS. The results reported here are thus independent of the tip1 radius, and the distance between the two metallic electrodes (within the range investigated). In most cases, switching between insulating and conducting states takes place in several abrupt steps. While the process is reversible, the hysteresis can be very small, with some junctions showing almost no significant hysteresis, whereas others show hysteresis as great as that of the bulk material.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

60 Tip 1 Tip 2

/nA 0

tip I

-30

-60 -0.4 -0.2 0.0 0.2 0.4 V /V(SCE) g Figure 7. Itip as a function of the gate voltage using -100 mV interelectrode bias and 10 mV s-1 scan rate of PEDOT. Lines: black, tip1; red, tip2. Initial gap size 380 nm and tip1 radius 1.9 μm.

Then, we calculated the PEDOT conductivity assuming that a cylindrical PEDOT wire whose length L is the gap size and the radius, r, that of tip1 has been generated (area A = r2). To do so we used the hypothesis that the observed conductance is governed by a classical diffusive regime (main transport mechanism assumed to be activated hopping of charge carriers between redox sites) where G = A/L. With this hypothesis, the calculated conductivity of the PEDOT material generated between the two tips is only 10-4 S cm-1, i.e. 6 orders of magnitude smaller than that measured for bulk PEDOT, which is clearly not plausible, as most studies performed on fiber devices have shown that conductivity increases when the fiber diameter decreases. Secondly we calculated the conductance of a PEDOT wire whose length L is the gap size and the area, A, calculated if the radius of the wire is the same as that of tip1. To do so we again used the hypothesis that the observed conductance is governed by a classical diffusive regime, and assumed that the conductivity of the fibers is the same as that of the bulk material (σ = 100 S cm−1). These results are summarized in Table 1 for various junctions. As can be seen, there is complete disagreement between the measured conductances and those calculated using these hypotheses, indicating that the effective radius of the nanowires are far below the radius of tip1 (the length of the wire cannot be less than the tip1-tip2 distance, and the conductivity of PEDOT fibers at 0.4 V cannot be less than the bulk conductivity of PEDOT).

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions Example Tip radius Measured gap Measured calcd calcd

(µm) conductance at a b Distance (L in 0.4 V conductance radius µm) (ns) (nm)

1 5 10 350 78 500 000 11 2 5 3 450 262 000 000 6 3 2.2 0.630 400 253 000 000 3 4 1.9 0.380 450 298 000 000 2 5 1.3 7 300 17 000 000 6 6 1.1 0.34 400 112 000 000 2 a If σ = 100 S cm−1 in the classical regime. b Over the whole gap length if σ = 100 S cm−1 in the classical regime.

Table 1. Measured conductances of several PEDOT nanojunctions generated on tips of various radius and with various gap distances and calculated conductance and nanowire radius, assuming classical diffusive transport regime and conductivity of 100 S cm-1. Finally, we have also calculated the radius of each PEDOT wire whose conductance is that measured, and whose length L is the gap size, assuming that the observed conductance is governed by a classical regime with  = 100 S cm-1 (that of bulk PEDOT). With this hypothesis, the calculated radius of the wire would be between 2 and 11 nm with aspect ratio, defined as L/r, between 150 and 1000. Even though these values are plausible, it is difficult to imagine here that a nanowire 10 µm long with a uniform radius of 11 nm has been created in experiment n° 1. It is also hard to believe that a nanowire 380 nm long with a uniform radius of 2 nm has been generated in example 4. 11 nm and 2 nm are thus the upper limits of the smallest diameter of the generated wires in these two examples, with larger diameters obtained near tip 1 where PEDOT starts to grow but lower diameter in part of the fibers connecting tip1 to tip2. Consequently, we believe that the limit of the classical regime has been reached here. The conductance of the nanowire is no longer controlled by its full length and diameter but is controlled by a small part of the wire, that corresponding to its smallest diameter. Following these findings, and the calculated radius of a uniform wire in the classical regime, whose conductance is 200-400 nS, and whose length L is the gap size (between 300 nm and 10 µm in the present case) it appears that the smallest diameter of the PEDOT wire obtained in this set of samples must be below 4 nm. This diameter is so small that it can only involve

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions a limited number of strands. Indeed, considering the geometric size of an EDOT unit (0.54 nm between CH2 groups and sulfur) and the π-stacking distance between two adjacent EDOT moieties that has been measured in ordered PEDOT mesostructures (0.5 nm),83 one strand has a diameter of 0.5 nm and a 4 nm-diameter fiber can only accommodate a few strands (fewer than 100). The measured conductance can also be compared with conductance values for single oligomers measured by STM-BJ. Let us recall that the main mechanism for charge transport in such devices is coherent tunneling through the molecule. Charge transport is ballistic and mainly controlled by the distance between the two metallic electrodes. The conductance of a single strand of a doped conjugated oligomer wired in between two metallic electrodes is reported to be around 3−5 nS, depending on their length and anchoring groups. Even though, in the present case, the distances between the two metallic tips are above 300 nm, if we assume that, within a small part of the wire, that corresponding to its smallest diameter, the conductances of the junctions are governed by ballistic transport, a conductance of 100−400 nS points again to nanowires with fewer than 100 strands controlling the whole transport of the junction. Overall, analyzing the conductance of the nanojunctions obtained with this SECM set-up, it being assumed that transport is governed by a classical regime where G = σA/L, or comparing the conductance with that of single-molecule devices, leads to the same conclusion: it is possible to generate PEDOT and PBT devices with transport controlled by a limited number of strands (below 100). As a consequence, the redox-gated nanojunctions obtained here seem to be at the frontier between fiber devices and single-molecule devices, and are particularly well adapted for studying and understanding charge transport across a small number (10 to 400) of π-conjugated molecules, which is of considerable interest for developing nanoelectronic and nanoelectronic devices with surface areas below 20 nm × 20 nm, i.e. 400 nm2. The situation is close to that obtained in the context of metallic nanowires. In such systems, when the diameter of the wire is below the electron wavelength at the Fermi level and the length of the wire is below the electron mean free path (20 nm for copper), a transition between the classical diffusion regime and a quantum regime is observed. The conductance of such a metallic nanowire is quantized and controlled by a small part of the wire, that corresponding to its smallest diameter where transport is ballistic. If this part of the wire consists of only a few atoms, G is quantized, and is given by the Landauer formula, and for some metals (Cu, Ag, Au), G is an integer value of G0. As G is independent of the 60

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions entire wire length, an important characteristic of such systems is that conductance quantization can be observed with wires as long as several micrometers provided the smallest diameter of the wire involves only a small number of atoms. As a consequence, atomic contacts can be easily generated using an SECM set-up and electrochemical deposition of metal between two tips, despite initial gaps between the two tips in the micron range.80-82 When metallic nanowires are replaced by conductive polymer nanowires, as depicted in figure 8, it is likely that a transition, similar to that observed when the size of a metallic nanowire is reduced, will be obtained. Such a transition from fiber device behavior (diffusive transport controlled by hopping between redox sites) to ballistic transport over a length close to the mean free path of the carrier in the polymer strands (coherent tunneling) is thus likely to occur.

Figure 8. Analogy between atomic contacts and molecular junctions with two different transport mechanisms along the nanowires. Transition between coherent tunneling and hopping has been investigated by many groups in large-area molecular junctions and has been found to occur for thicknesses between 5 and 8 nm.37,40 Besides, the size of a polaron in oxidized polythiophene materials is also close to 8 nm29,84 and the localization length of electron for conducting polymer close to the metal-insulator transition has been reported to be 20 nm. Therefore, generating nanojunctions whose smallest diameter is below 4 nm and whose length is close to the size of a polaron, or its localization length, makes it possible to reach the frontier between fiber devices and single-molecule devices.

2.3.6 summary

In summary, scanning electrochemical microscopy (SECM), where two microelectrodes are located face-to-face separated by a micrometric gap, has been 61

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions successfully used for the fabrication of PBT and PEDOT redox-gated junctions. Highly stable and reversible redox-gated nanojunctions were obtained with conductance in the 10-7 S range in their conducting state. These results are independent of the tip radius and the initial gap size, and suggest that the conductance of the entire junction in the conductive state is governed by fewer than 100 oligomers. As a consequence, the redox-gated nanojunctions obtained here seem to be at the frontier between fiber devices and single-molecule devices, and are particularly well adapted for studying and understanding charge transport across a small number (10 to 400) of π-conjugated molecules. The switching potential of the nanojunction depends on polymer type, with values around 0 and 1.2 V for PEDOT and PBT, respectively. Dynamic measurements give apparent on/off ratios of 10 to 100, whereas, static measurements lead to apparent on/off ratios above 1000. The difference between the two measurement methods is attributed to the higher electrochemical background currents measured in the dynamic mode when the nanojunction is in its reduced insulating state. Overall, this study shows that PEDOT and PBT nanowires consisting of a small number of π-conjugated molecules (10 to 400) and integrated in nanoelectronic devices, with surface areas below 400 nm2 and thickness above the direct tunneling limit between the two metallic electrodes, may be of interest for low energy consumption and relatively large on/off ratios in logic and memory devices.

2.3.7 Supporting information

80 Tip 1 Tip 1 60 Tip 2 Tip 2 100 40 50

20 /nA

/nA

tip

I tip I -20 -50 -40 -100 -60

0.6 0.8 1.0 1.2 1.4 0.4 0.6 0.8 1.0 1.2 1.4 1.6 V /V(SCE) V /V(SCE) g g

Figure SI 1. Itip as a function of the gate voltage using -100 mV interelectrode bias and 10 mV s-1 scan rate for two different redox-gated PBT junctions. Lines: black, tip1; red, tip2. PBT junctions were fabricated in ACN containing 0.01 M BT and 0.1 M tetrabutylammonium hexafluorophosphate.

Figure SI 1 shows other typical source-drain current of two different PBT junctions under different gate voltages with a fixed interelectrode bias, Etip2 − Etip1 = −100 mV. The

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions black and red curves show that below 1.2 V/SCE the junction is in the insulating state and no current flows between the two electrodes. Above 1.2 V the polymer starts to be oxidized, PBT switches to its conducting state, and the current reaches a maximum value at 1.4 V. The current of PBT junctions exhibit fully symmetric properties, which indicate that the current in the oxidized state is due to electron transport while electron transfer is negligible.

10 234nS

5 122nS

35nS

/nA 1.3nS 1.7nS 1.8nS 2.3nS 8.2nS

tip 0 I

-5

-10 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 V /V(SCE) g

720nS 20 425nS 154nS 0.7nS 2.3nS 6.2nS 17nS 50nS

/nA

tip I -20

-40

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 V /V(SCE) g Figure SI 2 shows the I(V) characteristics of PBT strands in different redox states. In the present case, conductance values of 234 and 720 nS indicate that the transport properties of the junction are probably controlled by a limited number of PBT strands. On/off ratios are 180 and 1000 for these two PBT junctions.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

/nA tip 90 I 40

0 60 -150 -100 -50 0 50 100 150 Bias/mV -40

30 -80

/nA tip

I -30

-60

-90

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 V /V(SCE) g Figure SI-3. Source-drain current vs. gate voltage at 100 mV s-1 scan rate by using different bias. Bias: (-)-150 mv; (-)-100 mv; (-) -50 mv; (-) 50 mv; (-) 100 mv; (-) 150 mv. Inset: plot of charge transport current of PEDOT junctions at different bias for Vg = 0.4 V.

10 94nS 58nS 5 45nS 31nS

13nS

/nA 5nS tip

I 0

-5

-10 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 V /V(SCE) g

Figure SI 4. Charge-transport current as a function of the VSD bias around various gate -1 voltages (scan rate, 100 mV s ) of PEDOT junctions. The two curves around each Vg value correspond to forward and reverses CV scans.

Figure SI 4 shows the charge-transport current response of the PBT nanojunction for -1 various Vg (scan rate, 100 mV s ). In this case a conductance of 94 nS indicates that the transport properties of the junction are probably controlled by a limited number of PEDOT strands and an on/off ratio of 20 can be estimated. The capacitive contribution of the currents, which can be observed during the forward and reverse scans, decreases the on/off ratio.

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions

40 Tip 1 Tip 1 Tip 2 a 45 Tip 2 b 20 30

/nA /nA

0 0

tip

tip I -15

-20 -30

-45 -40 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 V /V(SCE) V /V(SCE) g g

60 Tip 1 60 c Tip 1 40 Tip 2 Tip 2 d

30 20

/nA 0

/nA

tip

tip I -20 -30 -40

-60 -60 -0.4 -0.2 0.0 0.2 0.4 -0.4 -0.2 0.0 0.2 0.4 V /V(SCE) V /V(SCE) g g

45 30 Tip 1 Tip 1 Tip 2 e Tip 2 f 30 15

15 /nA /nA 0

tip

I I -15 -15 -30 -30 -45 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 V /V(SCE) V /V(SCE) g g -1 Figure SI 5. Itip as a function of the gate voltage using -100 mV bias and 50 mV s scan rate. Figures a-f correspond to examples 1-6, respectively, in Table 1.

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Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions 22 LaCroix, J. C. & Diaz, A. Electrolyte effects on the switching reaction of polyaniline. Journal of the Electrochemical Society 135, 1457-1463 (1988). 23 Xiao, R., Cho, S. I., Liu, R. & Lee, S. B. Controlled electrochemical synthesis of conductive polymer nanotube structures. J. Am. Chem. Soc. 129, 4483-4489 (2007). 24 Mirkin, M. V., Fan, F.-R. F. & Bard, A. J. Scanning electrochemical microscopy part 13. Evaluation of the tip shapes of nanometer size microelectrodes. J. Electroanal. Chem. 328, 47-62 (1992). 25 Janin, M., Ghilane, J., Randriamahazaka, H. & Lacroix, J.-C. Electrochemical fabrication of highly stable redox-active nanojunctions. Anal. Chem. 83, 9709-9714 (2011). 26 Yaliraki, S. N., Kemp, M. & Ratner, M. A. Conductance of Molecular Wires: Influence of Molecule−Electrode Binding. J. Am. Chem. Soc. 121, 3428-3434, doi:10.1021/ja982918k (1999). 27 Aviram, A. & Ratner, M. A. Molecular rectifiers. Chem. Phys. Lett. 29, 277-283 (1974). 28 Ratner, M. A. et al. Molecular wires: Charge transport, mechanisms, and control. Ann. N. Y. Acad. Sci. 852, 22-37 (1998). 29 Lacroix, J. C., Chane-Ching, K. I., Maquere, F. & Maurel, F. Intrachain electron transfer in conducting oligomers and polymers: The mixed valence approach. J. Am. Chem. Soc. 128, 7264-7276 (2006). 30 Xu, B. & Tao, N. J. Measurement of single-molecule resistance by repeated formation of molecular junctions. Science 301, 1221-1223 (2003). 31 Kergueris, C. et al. Electron transport through a metal-molecule-metal junction. Phys. Rev. B 59, 12505 (1999). 32 Reed, M. A., Zhou, C., Muller, C., Burgin, T. & Tour, J. Conductance of a molecular junction. Science 278, 252-254 (1997). 33 Haag, R., Rampi, M. A., Holmlin, R. E. & Whitesides, G. M. Electrical breakdown of aliphatic and aromatic self-assembled monolayers used as nanometer-thick organic dielectrics. J. Am. Chem. Soc. 121, 7895-7906 (1999). 34 Kushmerick, J., Naciri, J., Yang, J. & Shashidhar, R. Conductance scaling of molecular wires in parallel. Nano Lett. 3, 897-900 (2003). 35 Chabinyc, M. L. et al. Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers. J. Am. Chem. Soc. 124, 11730-11736 (2002). 36 McCreery, R. et al. Molecular rectification and conductance switching in carbon-based molecular junctions by structural rearrangement accompanying electron injection. J. Am. Chem. Soc. 125, 10748-10758 (2003). 37 Yan, H. et al. Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions. PANS 110, 5326-5330 (2013). 38 Martin, P., Della Rocca, M. L., Anthore, A., Lafarge, P. & Lacroix, J.-C. Organic electrodes based on grafted oligothiophene units in ultrathin, large-area molecular junctions. J. Am. Chem. Soc. 134, 154-157 (2011). 39 Nitzan, A. & Ratner, M. A. Electron transport in molecular wire junctions. Science 300, 1384-1389 (2003). 40 Choi, S. H., Kim, B. & Frisbie, C. D. Electrical resistance of long conjugated molecular wires. Science 320, 1482-1486 (2008). 41 He, H., Li, C. & Tao, N. Conductance of polymer nanowires fabricated by a combined electrodeposition and mechanical break junction method. Appl. Phys. Lett. 78, 811-813 (2001). 42 Lindsay, S. M. & Ratner, M. A. Molecular transport junctions: Clearing mists. Adv. Mater. 19, 23-31 (2007). 67

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions 43 Ulgut, B. & Abruna, H. D. Electron transfer through molecules and assemblies at electrode surfaces. Chem. Rev. 108, 2721-2736 (2008). 44 Xiao, X. et al. Redox-gated electron transport in electrically wired ferrocene molecules. Chem. Phys. 326, 138-143 (2006). 45 Chen, F. & Tao, N. Electron transport in single molecules: from benzene to graphene. Acc. Chem. Res. 42, 429-438 (2009). 46 Di Ventra, M., Pantelides, S. & Lang, N. The benzene molecule as a molecular resonant-tunneling transistor. Appl. Phys. Lett. 76, 3448-3450 (2000). 47 Damle, P., Rakshit, T., Paulsson, M. & Datta, S. Current-voltage characteristics of molecular conductors: two versus three terminal. IEEE Trans. Nanotech. 1, 145-153 (2002). 48 Paul, E. W., Ricco, A. J. & Wrighton, M. S. Resistance of polyaniline films as a function of electrochemical potential and the fabrication of polyaniline-based microelectronic devices. J. Phys. Chem. 89, 1441-1447 (1985). 49 Rosenblatt, S. et al. High performance electrolyte gated carbon nanotube transistors. Nano Lett. 2, 869-872 (2002). 50 Tao, N. Probing potential-tuned resonant tunneling through redox molecules with scanning tunneling microscopy. Phys. Rev. Lett. 76, 4066 (1996). 51 Gittins, D. I., Bethell, D., Schiffrin, D. J. & Nichols, R. J. A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups. Nature 408, 67-69 (2000). 52 Xu, B., Xiao, X., Yang, X., Zang, L. & Tao, N. Large gate modulation in the current of a room temperature single molecule transistor. J. Am. Chem. Soc. 127, 2386-2387 (2005). 53 Chen, F. et al. A molecular switch based on potential-induced changes of oxidation state. Nano Lett. 5, 503-506 (2005). 54 Janin, M., Ghilane, J. & Lacroix, J.-C. When electron transfer meets electron transport in redox-active molecular nanojunctions. J. Am. Chem. Soc. 135, 2108-2111 (2013). 55 White, H. S., Kittlesen, G. P. & Wrighton, M. S. Chemical derivatization of an array of three gold microelectrodes with polypyrrole: fabrication of a molecule-based transistor. J. Am. Chem. Soc. 106, 5375-5377 (1984). 56 Zhou, X.-S. et al. Do molecular conductances correlate with electrochemical rate constants? Experimental insights. J. Am. Chem. Soc. 133, 7509-7516 (2011). 57 Li, Z. et al. Conductance of redox-active single molecular junctions: an electrochemical approach. Nanotechnol. 18, 044018 (2006). 58 Li, Z. et al. Regulating a benzodifuran single molecule redox switch via electrochemical gating and optimization of molecule/electrode coupling. J. Am. Chem. Soc. 136, 8867-8870 (2014). 59 Capozzi, B. et al. Tunable charge transport in single-molecule junctions via electrolytic gating. Nano Lett. 14, 1400-1404 (2014). 60 Jones, E. T. T., Chyan, O. M. & Wrighton, M. S. Preparation and characterization of molecule-based transistors with a 50-nanometer source-drain separation with use of shadow deposition techniques. Toward faster, more sensitive molecule-based devices. J. Am. Chem. Soc. 109, 5526-5528 (1987). 61 He, H. et al. A conducting polymer nanojunction switch. J. Am. Chem. Soc. 123, 7730-7731 (2001). 62 Morvant, M. C. & Reynolds, J. R. In situ conductivity studies of poly (3, 4-ethylenedioxythiophene). Synthetic metals 92, 57-61 (1998).

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions 63 Aubert, P.-H. et al. In situ conductivity measurements on polyethylenedioxythiophene derivatives with different counter ions. Synthetic metals 126, 193-198 (2002). 64 Duvail, J. L., Long, Y., Cuenot, S., Chen, Z. & Gu, C. Tuning electrical properties of conjugated polymer nanowires with the diameter. Appl. Phys. Lett. 90, 102114 (2007). 65 Duvail, J. et al. Transport and vibrational properties of poly (3, 4-ethylenedioxythiophene) nanofibers. Synthetic metals 131, 123-128 (2002). 66 Long, Y. et al. Electrical conductivity of a single Au/polyaniline microfiber. Appl. Phys. Lett. 88, 162113 (2006). 67 Duvail, J. et al. Effects of the confined synthesis on conjugated polymer transport properties. The Journal of Physical Chemistry B 108, 18552-18556 (2004). 68 Long, Y., Duvail, J., Chen, Z., Jin, A. & Gu, C. Electrical properties of isolated poly (3, 4‐ethylenedioxythiophene) nanowires prepared by template synthesis. Polymers for Advanced Technologies 20, 541-544 (2009). 69 Yun-Ze, L., Jean-Luc, D., Zhao-Jia, C., Ai-Zi, J. & Chang-Zhi, G. Electrical conductivity and current–voltage characteristics of individual conducting polymer PEDOT nanowires. Chinese Physics Letters 25, 3474 (2008). 70 Delvaux, M., Duchet, J., Stavaux, P.-Y., Legras, R. & Demoustier-Champagne, S. Chemical and electrochemical synthesis of polyaniline micro-and nano-tubules. Synthetic Metals 113, 275-280 (2000). 71 Long, Y. Z. et al. Electrical conductivity studies on individual conjugated polymer nanowires: two-probe and four-probe results. Nanoscale research letters 5, 237-242 (2010). 72 Long, Y.-Z. et al. Recent advances in synthesis, physical properties and applications of conducting polymer nanotubes and nanofibers. Progress in Polymer Science 36, 1415-1442 (2011). 73 Yun-Ze, L. et al. Current-voltage characteristics of individual conducting polymer nanotubes and nanowires. Chinese Physics B 18, 2514 (2009). 74 Duvail, J. et al. Physical properties of conducting polymer nanofibers. Synthetic metals 135, 329-330 (2003). 75 Bard, A. J. et al. Chemical imaging of surfaces with the scanning electrochemical microscope. Science 254, 68-74 (1991). 76 Bard, A. J. M., M. V., Eds.; . Scanning Electrochemical Micros-copy; Marcel Dekker: New York, (2001). 77 Louet, C. et al. A comprehensive study of infrared reflectivity of poly (3, 4-ethylenedioxythiophene) model layers with different morphologies and conductivities. Solar Energy Materials and Solar Cells 143, 141-151 (2015). 78 Capozzi, B. et al. Length-dependent conductance of oligothiophenes. J. Am. Chem. Soc. 136, 10486-10492 (2014). 79 He, H. et al. Discrete conductance switching in conducting polymer wires. Phys. Rev. B 68, 045302 (2003). 80 Li, C., He, H. & Tao, N. Quantized tunneling current in the metallic nanogaps formed by electrodeposition and etching. Appl. Phys. Lett. 77, 3995-3997 (2000). 81 Janin, M., Ghilane, J. & Lacroix, J.-C. Scanning electrochemical microscopy for the fabrication of copper nanowires: Atomic contacts with quantized conductance, and molecular adsorption effect. Electrochim. Acta 83, 7-12 (2012). 82 Leroux, Y. R., Fave, C., Zigah, D., Trippe-Allard, G. & Lacroix, J. C. Atomic contacts via electrochemistry in water/cyclodextrin media: a step toward protected atomic contacts. J. Am. Chem. Soc. 130, 13465-13470 (2008). 69

Chapter 2. Approaching the Frontier between Fiber Devices and Single-Molecule Devices in Redox-Gated Junctions 83 Cho, B. et al. Single-crystal poly (3, 4-ethylenedioxythiophene) nanowires with ultrahigh conductivity. Nano Lett. 14, 3321-3327 (2014). 84 Long, Y. et al. Electronic transport in single polyaniline and polypyrrole microtubes. Phys. Rev. B 71, 165412 (2005).

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

In chapter 2, conducting polymer junctions on the nanoscale were created by chronoamperometry. Such redox-gated polymer junctions have been proved to lie between fiber devices and single-molecule devices. However, the size of polymer junctions formed by chronoamperometry, deduced from the conductance values, was difficult to control. Mostly the conductance value was high in the micro Siemens range, which implies that the molecular junctions generated are fiber devices. Nanojunctions in chapter 2, where charge transport through the molecular junctions is at the frontier between the classical and ballistic regimes, were not easily reproduced. How to control the molecular junction’s size is the main goal of this chapter. Can we generate nanojunctions in a reproducible way? Three kinds of methods will be developed to generate controllable conducting polymer junctions.

3.1 Conducting Polymer Nano Junctions Fabricated with a Self-terminated Electrochemical Method

3.1.1 PANI junction fabricated with Self-terminated method

The key point for the construction of conducting polymer junctions is to control the potential applied on the UME. In order to fabricate polymer junctions on the nanoscale and in a controllable way, the polymer growth process must be terminated once the desired junction is established. Therefore, it is very important to decrease the applied potential on the UME tip after the junction has been established. A self-terminated method was found to be effective for fabricating atomic contacts between two metal electrodes by means of an electrochemical process. It was firstly developed by Tao.1 As shown in scheme 3.1, it starts with a pair of microelectrodes separated by a relatively large gap (20 μm). One of the electrodes is wired to an external resistor (Rext). When a bias voltage is applied between the two electrodes, copper atoms are etched off the anode and dissolved, then deposited onto the cathode. Consequently, the gap is gradually filled by the nucleation of the copper filament. The applied potential V0 is divided into two parts. One is Vgap and the other is Vext. Vgap is given by:

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

Vgap = Rgap V0/(Rgap+Rext), where Vgap is the gap bias, Rgap is the gap resistance between the two electrodes, Rext is the resistance of the external resistor and V0 is the total applied bias voltage. The gap is initially large; Rgap is much higher than Rext, and Vgap is close to V0. The entire applied voltage is used for copper filament growth. As the gap narrows, once a contact between the electrodes is formed, Rgap rapidly decreases. As a result, a sudden drop of Vgap leads to the termination of copper filament growth. The self-terminated method was widely developed in the investigation of atomic contacts.2-5 However, it has been seldom reported in the context of molecular junctions. In this work, it is introduced to control the size of conducting polymer junctions.

Scheme 3.1. (a) Schematic drawing of atomic contact formed by self-terminated method by Tao’s group. (b) Snapshots of experiment that started with two 25 μm Cu electrodes separated by 20 μm gap.1

The principle of self-terminated deposition of a polymer junction by the SECM set-up is shown in scheme 3.2. It starts with one UME tip connected to the potentiostat through an external resistor. Consequently, the effective potential applied to the UME (VUME) is different from that applied on the WE1 (VWE1) channel of the potentiostat. The substrate electrode is directly wired to the WE2 channel. The two electrodes (UME and substrate) are

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM located face-to-face and are separated by a micrometric gap in the SECM configuration.6,7A potential of 0 V is applied to the substrate to avoid electropolymerization. A positive potential is applied to WE1, leading to polymer electrodeposition. The voltage across the gap (Vgap = VUME-VWE2) which is responsible for polymer deposition is given by the same formula as used in atomic contacts:

Vgap = RgapV0/(Rgap+Rext)

Here Rgap is the resistance of the gap between the UME tip and WE2, and V0 is the bias between the two WE (V0 = VWE1-VWE2) channels. Before contact occurs (scheme 3.2 left figure), Rgap is determined by ionic conduction between the two electrodes and by electropolymerization at the electrodes. The gap is initially set at a distance of a few micrometers. Therefore, Rgap >> Rext, then Vgap ≈ V0. The potential applied to WE1 is almost equal to the potential on the UME tip where polymer grows. Consequently, this potential leads to the continuous growth of PEDOT on the tip electrode. As the gap becomes narrower, Rgap decreases. When the polymer contacts WE2, Rgap now represents the resistance of the polymer wire between the tip and the substrate; Rgap becomes comparable to Rext. Then V0 is divided into two parts, namely Vext and Vgap. The decrease in

Vgap causes the potential applied to the UME tip electrode to decrease. If the real potential applied to the UME tip is no longer enough for polymer deposition, polymer growth will terminate.

WE 1 WE 1

V Rext V Rext ext RE ext RE

V0 CE V0 CE UME tip UME tip

Polymer Polymer

Vgap Vgap

substrate substrate

WE 2 WE 2

Before contact After contact Scheme 3.2. Schematic drawing of conducting polymer junctions before and after contact, fabricated by self-terminated strategy with SECM set-up. As the gap is filled by the polymer, the gap resistance decreases; consequently, a fall of Vgap results in termination of the deposition process.

Finding a proper resistor is crucial for controlling polymer molecular junctions. A set of resistors was chosen to investigate self-terminating polymer deposition. 73

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

Figure 3.1 shows the current vs. time plot of PANI generation by a self-terminating electrochemical method with an external resistor of 0 Ω, 10 KΩ, 50 KΩ, 100 KΩ, 1 MΩ and 10 MΩ, with an applied potential of 0.85 V/SCE on the WE1 and 0 V/SCE on the substrate (WE2).

25 0.30 (a) no R (b)R=10K 0.64V 0.25 20

0.20 15

 /mA

/ 0.73V

0.15

tip

tip I I 10 0.76V 0.10 0.79V 5 0.05 0.82V 0.00 0 0.84V

-20 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 160

Time/s Time/s 5 10 (d) R=100K (c) R=50K 0.47V 4 8 0.52V

A 3

A 6



0.6V tip 0.6V

I tip I 4 2

2 1

0 0.6V 0 0.85V

-20 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 Time/s Time/s

400 60 (e) R=1M 0.54V (f) R=10M 50 300 40

/nA 0.53V

/nA

tip 200 30 I

0.55V tip I 20 100 10

0 0.85V 0 0.83V

0 50 100 150 200 0 100 200 300 400 500 Time/s Time/s Figure 3.1. PANI molecular junctions fabricated by chronoamperometric electropolymerization by self-terminated deposition with various external resistors wired onto the UME in water containing 0.5 M aniline and 2 M H2SO4. Tip: 0.85 V/SCE; substrate: 0 V/SCE. External resistor: (a) no resistor; (b) 10 KΩ; (c) 50 KΩ; (d) 100 KΩ; (e) 1 MΩ; (f) 10 MΩ.

During the first 40 s (Figure 3.1a), only an electropolymerization current in the nano-ampere range is observed at the tip electrode. The current increases gradually and reach a value on the milli-ampere scale. This is mainly due to charge transport through PANI junctions. It increases with fluctuations from 40 s to 140 s, indicating that the PANI

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM has started to bridge the tip and the substrate electrode. Moreover, the continuous increase in the transport current implies that the size of PANI junction is increasing. Then PANI was deposited using 10 KΩ as external resistor. As shown in figure 3.1b, the current at the first 70 s is mainly due to aniline oxidation on the tip electrode. When the contact occurs, the current with is progressively increasing. To better understand the self-terminated method, we calculated the real potential applied to the tip electrode, based on the formula: Vgap = RgapV0/(Rgap+Rext). When the transport current increases to 3 μA, the real potential on the tip is 0.82 V. At this potential ANI can still be oxidized, so PANI growth continues. The conductance of the junctions and the transport current increase. As illustrated in figure 3.1b, the real potentials applied on the tip were calculated to be 0.79 V, 0.76 V and 0.73 V when the transport current reaches 6 μA, 9 μA and 12 μA, respectively. Obviously, as the PANI junction grows, the transport current increases, which leads to a fall in the real potential applied to the tip. When the current finally stabilizes at 20 μA the real potential on the tip has decreased to 0.64 V, which is now insufficient for ANI electropolymerization; PANI growth is stopped but the junctions generated here are very large. Next, an external resistor of 50 KΩ was used. As can be seen in figure 3.1c, the current during the first 38 s is mainly due to aniline oxidation at the tip electrode. Then a sudden current jump is observed, which indicates that the junction has been created. Based on the formula Vgap=RgapV0/(Rgap+Rext), When the transport current increases to 5 μA at 38 s, the real potential on the tip is 0.6 V. When the transport current increases to 8 μA the potential applied to the tip is only 0.47 V. At this potential no ANI electropolymerization occurs; PANI growth is terminated and the junction generated is smaller than in the previous example.

When Rext is changed to 100 KΩ, an abrupt current jump is again observed after 30 s (figure 3.2d). This sudden increase in the current is indicative of charge transport through a molecular junction. The transport current levels off at a value of 3 μA where the real potential applied to the tip is only 0.52 V, which insufficient to oxidize ANI. Subsequently, 1 MΩ and 10 MΩ resistors were used. A dramatic transport current jump is observed after a few seconds (figures 3.1 e and f), which suggests that a molecular junction is established. Furthermore, the final real potential is calculated to be 0.54 V and 0.53V for figure 3.1 e and figure 3.1f, respectively. These potentials are again insufficient for electropolymerization. After contact occurs the current stabilizes at a level which is

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM much less than that observed when a 50 KΩ resistor was used. This suggests that PANI growth was stopped more rapidly and that the wires generated are now much smaller. The experiments with an external resistor show clearly that before contact a tiny decrease occurs in the applied potential at the tip. However, the potential decreases drastically after the junctions are established. These results indicate that the real potentials applied at the tip are no longer enough for polymerization after contact occurs. To summarize this investigation, we compare the real potential applied at the tip during the chronoamperometric deposition before and after contact occurs, based on the current versus time curves (figure 3.1). In the absence of an external resistor, PANI growth did not stop. The potential applied to the tip is always 0.85 V before and after contact, despite the current changing from a few nano-amperes to 0.25 mA. The current before contact is taken as 10 nA, which is the value observed in most cases.b As shown in table 3.1, the real potential applied at the gap drops slightly (with respect to the initial value of 0.85 V) before contact is established. Such small drops in potential are not enough to stop electropolymerization. However, after contact, the real potential drops to a value which is insufficient for electropolymerization.

External R Ibefore contact I after contact Vgap before contact Vgap after contact no R 10 nA 0.26 mA 0.85 V 0.85 V 10 KΩ 10 nA 21 µA 0.848 V 0.64 V 50 KΩ 5.6 nA 7.6 µA 0.847 V 0.47 V 100 KΩ 8n A 3.34 µA 0.849 V 0.52 V 1 MΩ 5 nA 305 nA 0.845 V 0.55 V 10 MΩ 2.5 nA 32 nA 0.825 V 0.53 V Table 3.1. Measured current of PANI junction fabrication and calculated potential applied to the tip electrode during chronoamperometric deposition before and after contact. In order to measure the conductance of the PANI junctions formed by the self-terminated method with various external resistors, we have measured the transport current of the junctions as the applied gate voltage is varied for fixed bias in between the UME and the substrate. The external resistor used for deposition was thus removed during a In fact the current is 150 μA with no Rext and 195 nA with 10 KΩ before contact. Such unusual currents are mainly due to the low current sensitivity we chose forn the CHI instrument, in order to measure the current after contact. 76

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM the characterization. Figure 3.2a shows the variation of the tip current versus the gate potential using a fixed bias of 100 mV between the tip and the substrate. At potentials below 0.1 V/SCE, PANI is reduced and stays in the insulating state. Above 0.1 V/SCE it starts to be oxidized and becomes conductive. The observed current is due to the charge flowing across the junctions. In this case the transport current of PANI junctions was 25 μA in the oxidized state, which gives a conductance of 250 μS. Clearly such a conductance indicates that charge transport is governed by Ohm’s law.

25 (a) no R (b) R= 10K 1.5

A A

 15

 /

/ 1.0

tip

tip I 10 I 0.5 5

0 0.0

-0.2 0.0 0.2 0.4 0.6 -0.2 0.0 0.2 0.4 0.6 V /V(SCE) g V /V(SCE) g

1.5 (c) R= 50K (d) R= 100K 600

1.0

A 400

/nA



tip

tip I 0.5 200

0.0 0

-0.2 0.0 0.2 0.4 0.6 -0.2 0.0 0.2 0.4 0.6 V /V(SCE) V /V(SCE) g g

70 300 (e) R=1 M (f) R=10 M 60 50 200

/nA 40

/nA

tip

tip I

I 30 100 20 10 0 0 -10 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 V /V(SCE) V /V(SCE) g g Figure 3.2. Characterization of PANI junctions created by self-terminated deposition with various external resistors wired onto the microelectrode in a quasi-solution containing 0.5M ANI and 2 M H2SO4. External resistor: (a) no resistor; (b) 10 KΩ; (c) 50 KΩ; (d) 100 KΩ; (e) 1 MΩ; (f) 10 MΩ.

Following this, we examined the conductance of junctions formed with different Rext.

It is found that the conductance of PANI decreases as Rext, used during the junction

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM formation, increases. Indeed, when the external resistor is changed from 10 KΩ to 50 KΩ, 100 KΩ, 1 MΩ and 10 MΩ, the current at 0.4 V (figure 3.2) decreases from 1.6 μA to 1.4 μA, 600 nA, 300 nA and 60 nA, respectively. When the external resistor increases to 1 MΩ and 10 MΩ (figures 3.3e and f), the transport current of PANI junctions decrease to 300 and 60 nA at 0.4 V, respectively. Such transport current of 60 nA corresponds to a conductance of 600 nS and suggests that the junctions are now approaching the frontier between fiber devices and single-molecule devices with roughly 100 molecular strands governing charge transport through the junction. To confirm the reproducibility, we made a statistical study of the transport current of junctions formed by the self-terminated method with 10 kΩ and 10 MΩ external resistors. Figure 3.3 plots the histogram of the result. One can clearly see that the conductance of the junctions generated with a 10 MΩ resistor is different and smaller from that when the 10 KΩ resistor is used. Overall, the transport current depends strongly on the external resistor. Increasing the external resistor lowers the conductance, which means that the junctions are smaller. Thanks to the self-terminated method, the size of the PANI junctions seems to be controllable. 12 R=10K 10 R=10M

4 Occurrence

0 1E-7 1E-6 1E-5 Current/A Figure 3.3 Histogram of the terminal transport current where 100 mV bias was used with Rext preset at 10 KΩ and 10 MΩ.

We have attempted to use a Rext of 25 MΩ in order to further decrease the size of the junctions, but in this case, no junction was generated. This can be compared with the results 1 of atomic contact in Tao’s work. When Rext is too big, atomic contact is not obtained, as the

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM electrochemical deposition of copper stops in the tunneling regime before contact occurs. Here, a similar phenomenon is observed on PANI junctions generated by the self-terminated method. When a 25 MΩ resistor is used the current stops at the tunneling level before contact occurs. Note that this is the first time that the self-terminated method is used to generate conducting polymer junctions. The self-terminated method has proved to be an effective way of controlling PANI junctions. By varying the external resistor one can control the size of the junctions.

3.1.2 Observe the formation of PANI junction

To further confirm self-terminated deposition, we made a video of the formation of a PANI junction. Figure 3.4 shows PANI electrodeposited on WE1 in a 4-electrode set-up. The starting electrodes in this experiment were prepared from a 25 μm Au wire (99.999%) isolated by glue. The initial gap between the electrodes was 50-100 μm. It was created by carefully cutting the gold wires. The gap, which is larger than that used in the SECM set-up, was rapidly filled by electrochemical deposition of PANI from an aqueous solution of 0.5 M aniline and 1 M H2SO4. A 100 KΩ external resistor was connected to the side where PANI grows. In order to avoid electropolymerization on the other working electrode, a potential of 0 V was applied. When a 0.85 V voltage (V0) is applied to the circuit, a dendritic filament is deposited on the UME (figure 3.4). Once electrodeposition starts, it gradually increases and is highly directional, with a sharp growth front pointing towards the other electrode wire.

Figure 3.4. Picture of dendritic PANI growth on UME wire with 1 MΩ resistor wired onto the electrode.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

T=35s

T=0s T=50s

25μm

tip 1 25μm 25μm 600 tip 2 400 200 0

-200 Current/nA -400 T=120s -600 T=240s 50 100 150 200 250 Potential/V(Ag wire)

T=180s

25μm 25μm

25μm

Figure 3.5. The real-time growth of PANI by self-terminated process with 1 MΩ resistor wired onto one electrode.

Figure 3.5 shows another real-time example of PANI junctions formed by self-terminated. In the video, one can see that PANI fills up half the gap in 50 s. The growth front reaches the anode within 120 s. No further increase in the polymer on the electrode is observed after contact occurs. In other words, the deposition process, which is controlled by Rext, terminates itself. Furthermore, the current (figure 3.5a) shows an abrupt jump at 120 s when contact is made, which is in good agreement with growth termination observed in the video. Obviously, PANI growth is controlled by self-terminated.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

3.1.3 Other molecular junctions controlled by self-terminated method

In order to make sure that the self-terminated method is a general way of controlling the size of the conducting polymer junctions in the SECM set-up. PEDOT and Ppy junctions were also produced by the self-terminated strategy. Figure 3.6 shows the current vs. time plots for PEDOT junctions formed by self-terminated with 10 KΩ, 100 KΩ, 1 MΩ and 10 MΩ resistors, with an applied potential of 1.15 V/SCE.

30 8 (a) R=10K 7 (b) R=50K 25 0.9V 6 0.81V

A 20 A  0.98V 5  0.95V 15 1.0V 4 1V 1.02V 3 10

Current/ 2 Current/ 5 1

0 0 -1 0 20 40 60 80 100 120 0 20 40 60 80 Time/s Time/s

3.5 400 (c) R=100 K 350 (d) R=1 M 3.0 0.9V 300 2.5 A 0.85V  250 2.0 200 1.5

150 Current/nA Current/ 1.0 100 0.5 50 0.0 0 0 20 40 60 80 0 20 40 60 80 100 120 140 Time/s Time/s Figure 3.6. PEDOT junctions fabricated by chronoamperometric electropolymerization by self-terminated with various external resistors wired onto the UME in acetonitrile containing 20 mM EDOT and 0.1 M LiClO4. External resistor: 10 KΩ; 50 KΩ; 100 KΩ, where Tip: 1.15 V/SCE; substrate: 0 V/SCE and external resistor 1 MΩ where Tip: 1.25 V/SCE; substrate: 0 V/SCE.

As can be seen in figure 3.6a (Rext = 10 KΩ), during the first 17 s the only current is that due to EDOT oxidation at the tip electrode. After PEDOT bridges the tip and the substrate electrodes, the current increases drastically. We calculated the real potential applied to the tip: in the first 20 s the potential on the tip decreased to 1.02 V, which still sustains electropolymerization even though the contact is already generated (figure 3.6a). Then the potential on the tip continued to decrease as the transport current increased. When it decreased to a rather low value (i.e. 0.9 V) polymerization was stopped.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

When using a 50 KΩ as an external resistor (Figure 3.6b), the potential on the tip decreased to 1 V after 13s with a current jump. The transport current increased, which led to the potential decrease. The transport current reached a plateau of 6 μA and the effective potential applied to the tip dropped to 0.81 V. Growth of the molecular junction stopped at this potential, which is insufficient to sustain electropolymerization. Next external resistors of 100 KΩ and 1 MΩ were used to generate PEDOT junctions. As shown in figures 3.6c and 3.6d, the abrupt current jump is the signal that the molecular junctions are established. The potential applied to the tip electrode during chronoamperometric deposition before and after contact occurs is calculated from the current vs. time curves (table 3.2). Before contact occurs the currents are on the nano-ampere scale, which is due to electropolymerization of EDOT on the microelectrode.

The potential applied to the tip before contact is close to 1.15 V. However, Vgap decreases to rather low values when the transport current stabilizes after contact occurs. Such a low potential, below 1V, applied to the tip is insufficient for electropolymerization to continue. It is thus possible by wiring an appropriate external resistor to stop PEDOT growth when contact occurs.

I before contact I after contact Vgap before contact Vgap after contact 10 KΩ 36 nA 25 µA 1.15 V 0.9 V 50 KΩ 23 nA 6.8 µA 1.149 V 0.81 V 100 KΩ 28 nA 2.5 µA 1.147 V 0.9 V 1 MΩ 20 nA 225nA 1.24 V 0.85 V Table 3.2. Measured current of PEDOT junction fabrication and calculated potential applied at UME tip electrode during chronoamperometric deposition before and after contact. PEDOT junctions created by self-terminated with various resistors were also characterized by examining the transport current. Figure 3.7a shows the variation of the tip current with the potential, using a fixed bias of 100 mV (Rext is removed in these experiments). The transport current increases to the maximum of 10 μA at 0.4 V in this case. Taking 5 nS as the conductance of a single molecular strand, the current (10KΩ as an external resistor) indicates that molecular junctions are fiber devices where charge transport obeys Ohm’s law. Similar to self-terminated on PANI junctions, the final transport current of PEDOT junctions created by self-terminated (figure 3.7) strongly depends on the external resistor. Increasing the resistance lowers the transport current.

When Rext went from 10 kΩ to 1 MΩ, nanojunctions of PEDOT were obtained. The

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM transport current of junctions created with a 1 MΩ external resistor was only 10 nA, which implies that the device is at the frontier between the fiber devices and single-molecule devices.

10 6 (a) R=10 K (b) R=50 K 5

A 

 4 6 3 4

Current/ 2 Current/ 2 1

0 0

-0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE) 2.5 14 (c) R=100 K (d) R=1 M 2.0 12

A 10

1.5  8

1.0 6 Current/nA Current/ 0.5 4 2 0.0 0 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE) Figure 3.7. Characterization of PEDOT junctions created by self-terminated deposition with various external resistors wired onto the UME in acetonitrile containing 20 mM EDOT and 0.1 M LiClO4. External resistor: 10 KΩ; 50 KΩ; 100KΩ; 1MΩ.

We made a statistical study of the transport current of PEDOT junctions formed by c self-terminated with 50 kΩ and Rext > 1 MΩ external resistors. Figure 3.8 plots the histogram of the result. It is clearly that increasing the external resistor smaller transport current has been acquired, which means the smaller size of junctions is obtained. Overall the size of the PEDOT junctions can be controlled by the self-terminated method.

c In fact, 1 MΩ and 10 MΩ are used as Rext, and the bias was 1.15 V or 1.25 V. 83

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

6 R=50K 5 R>1M

2 Occurrence

0 1E-9 1E-8 1E-7 1E-6 1E-5 Current/A Figure 3.8. Histograms of the terminal transport current where 100 mV bias was used with Rext preset at 50 KΩ and above 1 MΩ.

The self-terminated method was also used for the formation of Ppy molecular junctions. Figure 3.9 shows the current vs. time plot for junctions formed by self-terminated with 10 KΩ, 50 KΩ,100 KΩ and 1MΩ, with an applied potential of 1.1 V/SCE. Here again, the current jumps abruptly and stabilizes at certain values for each deposition with different resistors, which suggests that electropolymerization is self-terminated. Table 3.3 lists the calculated potentials at the tip electrode during chronoamperometric deposition before and after contact occurs. The current, which is on the nano-ampere scale before jumps, is mainly due to pyrrole oxidation on the UME. Vgap suddenly decreases to a rather low value when contact occurs. For example, for the 10 KΩ resistor the potential falls to 0.5 V when contact is made. It is obvious that the real potential applied to the gap is no longer enough to sustain polymerization. Increasing Rext leads to the smaller molecular junctions. As already observed with PANI and PEDOT, it is thus possible to control the size of a polypyrrole junction. Self-terminated appears to be a general method for controlling conducting polymer junctions by the SECM set-up.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

70 5 60 (a) R=10 K (b) R=50 K 4 50

3 A

A 40



tip tip

30 I I 2 20 1 10

0 0

0 20 40 60 80 100 120 140 0 10 20 30 40 Time/s Time/s 5 30 (c) R= 100 K (d) R= 1M 4 20



/nA

tip

tip I 2 I 10

0 0

0 10 20 30 40 50 10 20 30 Time/s Time/s Figure 3.9. Ppy molecular junctions fabricated by chronoamperometric electropolymerization by self-terminated with various external resistors wired onto the UME in acetonitrile containing 20 mM pyrrole and 0.1 M TBAPF6. Tip: 1.1V/SCE; substrate: 0V/SCE. External resistor: (a) 10 KΩ; (b) 50 KΩ; (c) 100 KΩ; (d) 1MΩ.

I before contact I after contact Vgap before contact Vgap after contact 10 KΩ 41 nA 57 µA 1.0996 V 0.53 V 50 KΩ 27 nA 3.3 µA 1.0999 V 0.94 V 100 KΩ 16 nA 3.3 µA 1.098 V 0.77 V 1 MΩ 0.45 nA 10 nA 1.099 V 0.90V Table 3.3. Measured current of Ppy junction fabrication and calculated potential applied to tip electrode during chronoamperometric deposition before and after contact.

3.1.4 Conclusion

In summary, scanning electrochemical microscopy (SECM), where one UME tip is located face-to-face with a substrate electrode separated by a micrometric gap, has been used successfully for the fabrication redox-gated conducting polymer junctions. A self-terminated strategy was used to control the size of the junctions generated. It is based on introducing an external resistor in the set-up, which makes the effective potential applied to the UME tip fall as soon as the contact is established between the electrodes. By 85

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM comparing the calculated potential applied to the gap before and after contact, it is found that the real potential applied to the tip is no longer enough to sustain electropolymerization after contact. The real-time formation of PANI junctions between two electrodes was observed under a microscope and recorded on a video. It proves that polymer growth suddenly stops by self-terminated once contact is established. The external resistance wired onto the tip plays an important role in terminating polymer deposition, in that the size of the molecular junctions is controlled by this resistance. Increasing Rext reduces the size of the junctions. The method is general; PANI, PEDOT and Ppy gave similar results. We are now able to generate conducting polymer nanojunctions at the frontier between fiber devices and single-molecular device in a reproducible and controllable way.

3.2 Conducting polymer molecular junctions generated by differential scan voltammetry (DSV)

3.2.1 PANI and PEDOT junctions generated by DSV

By using differential scan voltammetry (DSV), one can sweep the potential on both the tip and substrate electrodes while a constant positive or negative bias is maintained between them. Oxidation of ANI occurs only above 0.75 V/SCE while the switching potential of PANI is below 0.6 V/SCE. If the potential of the tip is swept up to 0.85 V and the differential voltage is kept at 0.2 V, the potential on the substrate is 0.2 V lower than that of the tip. It is thus possible for electropolymerization to occur only on the tip electrode and not on the substrate electrode during DSV. Contact between tip and substrate through PANI wires can be detected by a sudden jump in the current during the DSV experiment. Such a procedure, a possible way of controlling the size of the junctions, will be investigated in this part.

Figure 3.10 shows ANI (0.5 M) oxidized in water containing 1 M H2SO4 by using DSV. In Figure 3.10a, typical electrochemical growth reveals monomer oxidation and formation of PANI. The increase of the capacitive current indicates that the amount of polymer increases on the UME. It grows on the UME tip towards the substrate electrode during 20 cycles, and a current peak at 0.15 V/SCE is the signal of oxidized PANI. When there is no contact between the tip and the substrate only the electrochemical current of the polymer can be seen. In the 21st cycle, (figure 3.10b), the current abruptly increases. This current indicates that a PANI junction has been established between the electrodes.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

20 120 a b 15 100 80 10 60

5 40 Current/nA

Current/nA 20 0 0

-5 -20 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 Potential/V(SCE) Potential/V(SCE)

60 c 50

20 Current/nA 10

-10 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 Potential/V(SCE) Figure 3.10. PANI molecular junctions by DSV with a 0.2 V difference between electrodes and 0.1 V s-1 scan rate. (a) First 20 cycles of the DSV, red zone part of (b) 20 cycles and the 21st cycle of DSV with transport current appearing in the 21st cycle. (c) I(V) curves of PANI junctions with 0.1 V bias and 0.1 V s-1 scan rate.

The PANI junctions created by DSV were examined by measuring their I/V characteristics. Figure 3.10c shows the variation of the tip current with the potential using a fixed bias (100 mV) and a scan rate of 100 mV/s. At a potential below 0.1 V/SCE, PANI was reduced to the insulating state. Above 0.1 V/SCE it started to be oxidized to a conductive state; the transport current increases to a maximum value of 55 nA at 0.4 V. The conductance of 500 nS (50 nA under 0.1 V bias) with a gate potential of 0.4 V/SCE is in good agreement with the transport current (100 nA with 0.2 V bias) observed during DSV at 0.4 V (figure 3.10b). As described in part 3.1, the potential applied to the WE1 channel was kept constant during electropolymerization by the self-terminated technique. When contact occurs, the potential applied to the tip becomes insufficient for electropolymerization, and deposition stops. Unlike the self-terminated strategy, electropolymerization does not stop automatically when contact occurs by DSV but is stopped when the potential of the tip is swept to values where monomers are not oxidized. Moreover, further polymer growth can be avoided by stopping the experiment during the cycle that shows the current jump. 87

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

In order to prove that DSV is a general method for the construction of other conducting polymer molecular junctions, PEDOT junctions were investigated by this strategy. As shown in figure 3.11, EDOT is electropolymerized by DSV. EDOT (20 mM) was electropolymerized with 0.1 M TBAPF6 as electrolyte in acetonitrile. Figure 3.11a shows the last cycle of DSV used. The potential for electropolymerization on the tip was swept between 0 and 1.2 V/Ag wire while that on the substrate was kept 0.2 V lower than the tip to avoid deposition on the substrate electrode. Monomer oxidation started at 1.0 V and led to PEDOT deposition on the UME in the first cycle. Once the gap was filled with PEDOT, a current jump was observed in the last cycle, and electropolymerization was immediately stopped. The fluctuation of the curves in Figure 3.11a represents the transport current through the PEDOT junction.

140 14 120 a 12 b 100 10 80 8

60 6 Current/nA Current/nA 40 4 2 20 0 0 -2 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 Potential/V(Ag wire) Potential/V(Ag wire) Figure 3.11. PEDOT molecular junctions fabricated by DSV with a 0.2 V difference between electrodes and 0.1 V s-1 scan rate. (a) DSV of PEDOT (b) I/V curves of PEDOT molecular junctions with 0.1 V bias and 0.1 V s-1 scan rate.

We examined the PEDOT molecular junctions by following their I/V characteristics. Surprisingly, for a fixed bias of 0.1 V the transport current in the conducting state was 10 nA, corresponding to a conductance of only 100 nS (figure 3.11b). During DSV in figure 3.11a, the transport current at 0.4 V is 100 nA with a bias of 0.2 V. However, the current observed during the I/V characterization, 10 nA, is not of the same magnitude. Probably some PEDOT strands disconnect during the change from DSV to I/V characterization. To prove this, we compared the conductance of the junction during the DSV experiment with that deduced from the I/V characterization after the DSV experiment. Several events are listed in table 3.4. It is found that the conductance by I/V characterization does not agree with that calculated from DSV. This suggests that some PEDOT strands involved in the initial contact do not remain connected to the substrate during the characterization.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

Events Conductance from DSV Conductance from I/V Bias: 0.2 V Bias: 0.1 V 1 600 nS No junction 2 10 nS 1 nS 3 600 nS 12 nS 4 600 nS 100 nS 5 60 µS 4 µS 6 200 nS 20 nS Table 3.4. The conductance of PEDOT by DSV and by I/V characteristics.

3.2.2 PEDOT junction created on PEDOT-modified substrate by DSV

Note that the PEDOT nanojunctions with the conductance of 100 nS created by chronoamperometry are highly stable, as described in chapter 2. PEDOT junctions fabricated by DSV were on the nanoscale but had random conductance values. Polymer deposited by sweeping the potential may lead to weak coupling with the Pt substrate (WE2). Is it possible to make PEDOT junctions with strong connections by using DSV? To solve this problem, we plan to use PEDOT functionalized Pt as the substrate. As illustrated in scheme 3.3, by using the PEDOT-modified substrate as WE2, more strongly coupled junctions may be achieved.

WE 1 WE 1

RE CE RE CE

UME tip UME tip PEDOT-Pt Weak coupling PEDOT-PEDOT Strong coupling

PEDOT PEDOT substrate substrate

WE 2 WE 2 Scheme 3.3. PEDOT molecular junctions created on bare Pt and PEDOT-modified substrate.

To test this idea, PEDOT was electropolymerized by CV on a Pt substrate (WE2). Figure 3.12 shows the CV of the film with an oxidation peak at 0 V and a reduction peak at -0.5 V. The thickness of the PEDOT film was calculated to be 135 nm. This PEDOT-functionalized Pt electrode was used as the substrate for further studies.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

A 

0 Current/ -10

-20

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Figure 3.12. CV of PEDOT film on Pt substrate in 0.1M TBAPF6 acetonitrile solution.

Next, a PEDOT junction was formed on this modified platinum electrode by DSV. Figure 3.13 shows EDOT (20 mM) electropolymerization in acetonitrile containing 0.1 M

TBAPF6. The typical growth of PEDOT was observed before contact occurs (figure 3.13a). Electropolymerization was stopped when transport current was observed. PEDOT junctions were examined by measuring the transport current. As shown in figure 3.13b, the current is in good agreement with that in figure 3.13a. The contact conductance during DSV and the conductance during I/V are now similar.

a 1000 800 b 800

600 400

400

Current/nA Current/nA 200

0 0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE) Figure 3.13. (a) CV of EDOT electropolymerization by DSV. (b) Characterization of PEDOT junctions. Tip: 10 μm; UME substrate: 2 mm Pt electrode modified by PEDOT layer.

Figure 3.14 shows the histogram of conductance values of PEDOT junctions, which are determined by DSV and I/V and are constructed from 10 events. The conductance of the junctions created on a PEDOT-modified substrate is in the 5-10 μS range. The contact conductance during DSV and the stabilized conductance during I/V are now close to each

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM other. However, the size of the junctions based on conductance values is far greater than that of nanojunctions.

4 4 By DSV By I(V) 3 3

2 2 Occurrences 1 Occurrences 1

0 0 1E-7 1E-6 1E-5 1E-4 1E-6 1E-5 1E-4 Conductance/S Conductance/S Figure 3.14. Histogram of transport current of PEDOT molecular junctions determined by DSV (0.2 V bias) and I/V (0.1 V bias).

Overall, DSV allowed us to demonstrate that substrate functionalization has an important impact on the stability of the junctions.

3.2.3 Combining self-terminated with DSV

PEDOT junctions formed on a PEDOT-modified substrate by DSV are on the fiber devices scale, which conductance is usually a few micro Siemens. Obtaining stable small junctions is the next step. Given that self-terminated is a useful way of controlling the size of junctions, we expect that stable controllable PEDOT junctions could be formed on the nanoscale by combining self-terminated and DSV. Figure 3.15 shows PEDOT junctions formed by DSV with an external 1 MΩ resistor on the tip electrode. The deposition was stopped when a transport current was observed. The junctions were checked by following the I/V characteristics using a fixed bias of 0.1V. A current of 50 nA is observed during DSV by using a 0.2 V bias (figure 3.15a). Figure 3.15b shows that the transport current is 32 nA in the conducting state. Such a value implies that the PEDOT junctions are on the nanoscale. Figures 3.15c and 3.15d show histograms of transport currents from DSV events and I/V characterization events, respectively. Both histograms show that the transport current is of the order of a few tens of nanoamperes. This result implies that the junctions are at the frontier between fiber devices (classical regime) and single-molecule devices (ballistic regime).

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

60 35 50 a 30 b 40 25 30 20

20 15 10 Current/nA 10 Current/nA 5 0 0 -10 -5 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE) 4 4 occurence of PEDOT transport current from DSV occurrence of PEDOT transport current from I(V)

3 c 3 d

2 2 Occurence

1 Occurrence 1

0 0 1.00E-08 1.00E-07 1.00E-06 1E-8 1E-7 1E-6 Current/A Current/A

Figure 3.15 (a) CV of PEDOT electropolymerization by combining self-terminated with DSV. (b) Characterization of PEDOT junctions. Tip: 10 μm; UME substrate: 2 mm Pt electrode modified by PEDOT layer. (c) Histogram of transport current of PEDOT junctions created by combining self-terminated with DSV. Rext = 1 MΩ; (c) from DSV; (d) from I/V characteristics.

3.2.4 Conclusion

In summary, DSV has been successfully used for the fabrication of conducting polymer junctions. More importantly, it has allowed us to realize that when the substrate is not functionalized there is a difference between the contact conductance and the stabilized conductance observed by I/V characterization. This difference is not observed when a functionalized substrate is used, suggesting that oligomer wires can bind covalently to the polymer previously deposited on the substrate. Finally, by combining DSV and self-terminated, it is possible to generate small controllable PEDOT junctions.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

3.3 Extending the Capability of SECM for Fabrication of Single molecular Junction: a break junction Strategy

3.3.1 How to construct molecular junctions by SECM-BJ?

The strategy of molecular conductance measurement is based on wiring molecules to the metal electrode, forming a metal/molecule/metal (MMM) junction to the circuit. Research on charge transport through MMM junctions have been focused on both experimental8-11 and theoretical simulation studies,12-15 which stimulated the development of measurement techniques. In recent years, scanning tunneling microscopy (STM), conducting atomic force microscopy (CAFM), mechanically controlled break junctions (MCBJ) and cross-wire junctions, have been widely used to construct MMM junctions. As mentioned in chapter 1, the break junction strategy is the most used method for making MMM junctions. This procedure is posted on performing repeatedly molecular junctions thousands of times, followed by histogram representation illustrating the distribution of the conductance.

Scheme 3.3. STM-BJ technique used to form metal/molecule/metal junctions. The graph shows I(s) curves recorded for the 6-[10-(6-mercapto-hexyl)-[4,40]bipyridinium]hexane-1-thiol. The corresponding state of molecular formation of B, C and D are marked on the decay curve.

As illustrated in scheme 3.3,16 by repeatedly moving a tip in and out of contact with the substrate when molecules are present in the solvent or already adsorbed on the electrode surface, an MMM junctions can be generated in situ by STM-BJ.17 This tip movement is precisely controlled by a piezo-motor. When the tip is close to the substrate, the molecules with the anchoring group may connect the electrodes and an MMM is

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM created. When the tip is moved away from the substrate, the MMM junctions are broken. The transport current of the MMM junction versus distance is recorded during tip retraction. Molecular conductance is usually determined from conductance traces and conductance histograms constructed from a large number of individual conductance trace events.9,18-21 The conductance steps give information about the number of molecules involved in the junctions. One can get the conductance value of a single molecule from the last step in the I(distance) curves, which represent the variation of the current with the stretching distance (figure 3.16a).22 Here the traces show distinct steps at multiples of 0.4 nA. A histogram of the currents recorded for hundreds of break junctions (figure 3.16b) shows a series of peak currents which is in good agreement with the current steps observed in figure 3.16a. These current values (0.4 nA, 0.8 nA and 1.2 nA) (figure3.16) correspond to one, two and three molecules involved in the gap.11 From these results, Lindsay et al. deduced that the conductance of a single oligophenylene wire containing 5 conjugated phenylene units is 4 nS.

Figure 3.16. (a) Examples of current vs. stretching distance data at a bias of 0.2 V. (b) Histogram of recorded currents shows peaks at ca. 0.4, 0.8, and 1.2 nA. The slope of a plot of peak value vs. peak number (inset) gives the current per molecule.

In the same way as for the STM-BJ technique mentioned above, we have in this chapter developed a novel break-junction technique, which is based on an SECM set-up, to investigate molecular junctions. The SECM tip, controlled by a motor, was manipulated in the X, Y or Z direction. The current of the molecular junction versus distance is recorded. Similar to other break-junction techniques, conducting polymer junctions can be dragged in the X, Y or Z direction (scheme 3.4). The tip is driven to move the UME tip at a rate of 250 nm/s d. Contrary to other break-junction techniques, d Many experiments indicated that the best tip rupture rate was 250 nm/s. 94

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM our approach starts with polymer junctions which have already been created between the gaps. Some of the molecular strands may disconnect during the mechanical movement. Consequently, it is possible to observe that the junctions become smaller and smaller during tip retraction and to control the size of the junctions by this movement. Ultimately we may observe single-molecule junction behavior during the break-junction experiments in a way similar to that in STM-BJ and MCBJ experiments. Thanks to the four electrodes used in an SECM set-up, where UMEs can be used as working electrodes, molecular junctions are characterized by measuring the transport current. This will make it possible to check whether the junction is not completely broken and if the junction is smaller.

Z X

O Less O S Less O O O S O S O S O S O O S O O O S O O S O O S O O O strands S strands S O S O O S O O O O O S S O O O O O S S S O S O O O O O O connect connect O S S O S S O O O O

Scheme 3.4. Schematic process of SECM break junctions applied to PEDOT junctions in the Z and X directions.

3.3.2 PEDOT junctions generated by SECM-BJ

Figure 3.17a shows the typical transport current characteristics of PEDOT junctions formed by chronoamperometric deposition, as used in chapter 2. With the two UMEs facing each other, it is possible to observe simultaneously the current at both tips. The PEDOT junction was under a fixed interelectrode bias with Etip2 − Etip1 = 0.1 V. As the sweep potential increased above 0.1 V/SCE, PEDOT was oxidized to the conducting state and the transport current of the junction reached a maximum of 90 nA at 0.4 V. A symmetric current flowing in both electrodes is observed, which is indicative of charge transport through the junctions. The PEDOT was gradually reduced to the insulating state by sweeping the potential back, with a hysteresis which is due to structural relaxation of the polymer.

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

Before the SECM-BJ experiment, we first ran blank experiments. A bias of 0.1 V was applied to the electrode gap by the set potential of 0.4 V/SCE on the UME tip and 0.3 V/SCE on the UME substrate. Thus, PEDOT was polarized in the conducting state. As shown in figure 3.17b, symmetric transport currents with a value of 120 nA are observed on both electrodes. One can see that the junction is high stable and a conductance plateau can be observed for more than 1000 seconds (shown here for 300 s).

100 a UME 1 b UME 1 0.4V UME 2 200 UME 2 0.3V 50

100

/nA

tip

I 0 0

Current/nA -100 -50

-200 -100 -0.4 -0.2 0.0 0.2 0.4 0 50 100 150 200 250 300 V /V(SCE) g Time/s

180 c UME 1 0.4V UME 2 0.3V 150

120

Current/pA 60

0 0 20 40 60 80 Distance/m Figure 3.17. (a) Transport current measurement of PEDOT junctions with 0.1 V bias and 0.1 V s-1 scan rate. Blank test of SECM-BJ: (b) current on both UMEs versus time of PEDOT junctions; (c) current on both UMEs versus retract distance when there is no connection between the electrodes.

Then we did another blank test by pulling the tip which was modified by PEDOT film (figure 3.17c). Another UME was used as the substrate electrode. No molecular junction is established between the two UMEs in the blank experiment. With an applied potential of 0.5 V on UME 1, PEDOT was in the conducting state. A stable current at 110 pA (black line), which is due to electrochemical leakage of PEDOT on tip 1, was observed. The current of less than 20 pA (red line) is due to the background value on the

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM bare UME substrate. As expected, no current change is observed when the tip is retracted from the substrate toward X direction (figure 3.17c).

3.3.2.1 SECM-BJ in Z direction

120 30 a b 100

80 20

/nA

tip

I I

40 10

0 0 0 5 10 15 20 0 2 4 6 8 10

Distance/m Distance/m

15 c

/nA

tip 5 I

0 10 20 30 40 50 60 Distance/m Figure 3.18. Current traces on UME tip acquired when the PEDOT junction was pulled in the Z direction at a rate of 0.25 μm s-1 in the presence of 20 mM EDOT and 0.1 M TBAPF6. UME tip was at applied potential 0.5 V and Pt substrate (1.5 mm in diameter) at 0.4 V.

After the molecular junction was established, the UME tip was retracted in the Z direction. Figure 3.18a shows the tip current traces obtained when the junction was pulled -1 at a rate of 0.25 μm s in the presence of 20 mM EDOT and 0.1 M TBAPF6 in acetonitrile. The transport current started at the saturated value because of the CHI instrument sensitivity, which was set to 10 nA so that a small current could be measured. With this sensitivity a transport current above 120 nA saturates the operating amplifier of the instrument. After pulling to 2 μm, the transport current of the junction dropped drastically to 40 nA. This abrupt drop associated with tip stretching suggests a decrease in the number of molecule strands bridging the tip-substrate gap. Subsequently, the junctions are broken down in a stepwise fashion, with transport current values falling to 37, 20, 5, and finally -0.3 nA. The presence of multiple transport current values is ascribed to the 97

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM different sizes of the molecular junctions binding the tip to the substrate. The value of -0.3 nA (negative current, while a positive potential is applied to the tip), suggests that the junctions were finally broken. Comparison of the current trace in figure 3.18a with that in figures 3.17 b and c (blank test) suggests that the stepwise current drops in figure 3.18a are probably due to the fact that the number of molecular strands involved in the junctions is decreased. In the case of STM-BJ, the average stretching distance of a single junction is typically of the order of a few nanometers at room temperature.23-25 In contrast, the retraction length of the conducting polymer wire can be as long as a few micrometers. This is due to the flexibility of the polymer. The retraction distance should not be taken as the length of a polymer chain. In fact, it refers to the stretching length of a bundle of polymer chains involved in the junction. The high stability of molecular junctions come from the great length of the polymer chains26, which is controlled on the scale of a few micrometers by the SECM gap.27 Other examples of SECM-BJ (figures 3.18b and 3.18c) exhibit similar behaviors as shown in figure 3.18c. The transport currents observed in the last few steps were a few nanoamperes, which suggests that the junctions are at the frontier between fiber devices and single-molecule devices. Each current plateau observed in figures 3.18b and c can be treated as charge transport through a certain number of PEDOT strands. We attribute such current drops to some strands gradually disconnecting when the tip moves in the Z direction. The current of the junction finally drops to zero or the background level after these steps. It has been reported that it is easy to hold the polymer nanowires at a fixed conductance step.8 The decrease in current during tip retraction indicates that it is possible to reduce the size of the molecular junctions by pulling tip moving in the Z direction. To further confirm that, the transport current of PEDOT junctions was characterized after tip movement had been stopped at a certain transport current value. Figure 3.19a shows a typical junction; SECM-BJ started with a transport current of 4.7 μA with 0.1 V bias. The tip movement was stopped at a plateau with a transport current of 700 nA after going through several current plateaus of 4, 2.3, 2.15 and 1.7 μA with rupture distances of 1.5, 1, 0.4 and 0.25 μm, respectively. The junctions were examined by following the I/V curves. Figure 3.19b presents their characterization just after the retraction to a transport current of 700 nA (figure 3.19a). The tip potential was swept using a fixed tip-substrate bias of 0.1V. PEDOT exhibits redox-gated properties switched by the potential. Above 98

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

0.1 V/SCE PEDOT is oxidized to a conductive state, and the transport current increased to the maximum value of 700 nA above 0.4 V. The current value of 700 nA is in good agreement with the last plateau, as shown in figure 3.19a. It is evident that the PEDOT junction still bridges the two electrodes.

800 5000 a 700 b 600 4000 500 3000 400 stop 300

Current/nA 2000 Current/nA 200

1000 100 0 0 -100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -0.4 -0.2 0.0 0.2 0.4 0.6 Distance/m Potential/V(SCE) 900 350 300 d 800 c 250 700 200 600 150 stop

500 100

Current/nA Current/nA 400 50

300 0 -50 200 0 1 2 3 4 5 6 7 8 -0.4 -0.2 0.0 0.2 0.4 0.6 Distance/m Potential/V(SCE)

350 e 30 300 f 20 250 200 10

150 stop 0 Current/nA

100 Current/nA -10 50 0 -20

-50 -30 0 1 2 3 4 5 -0.4 -0.2 0.0 0.2 0.4 0.6 Distance/m Potential/V(SCE) Figure 3.19. (a)(c)(e) SECM-BJ current traces acquired when PEDOT junction was pulled -1 in the Z direction at 0.25 μm s in the presence of 20 mM EDOT and 0.1 M TBAPF6 acetonitrile solution. UME tip was at an applied potential of 0.5 V and Pt substrate (1.5 mm in diameter) at 0.4 V. (b)(d)(f) I/V characterization of junctions after tip movement stopped at a certain current plateau with 0.1 V bias and 0.1 V s-1 scan rate.

The retraction of PEDOT junctions was continued and stopped at a current plateau of 300 nA (figure 3.19c). Figure 3.19d shows the characterization of the junctions after

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM retraction. The observed transport current of 300 nA is also in good agreement with the last current plateau of figure 3.19c. The junction continued to be ruptured. Figure 3.19e shows the plateau current drastically dropped from 350 nA to 0.4 nA, which is comparable with the background value. Characterization of the junctions is shown in figure 3.18f; no transport current is now observed, indicating that the junctions were totally broken after SECM-BJ. The current plateaus signify that PEDOT bridges the electrodes with a certain number of molecular strands. Smaller junctions can be obtained by SECM-BJ from larger junctions. The size of the junctions can be controlled by retracting the tip in the Z direction.

3.3.2.2 SECM-BJ in X direction

Another way to reduce the connection of PEDOT strands by SECM-BJ is to pull the tip in the X direction. Figure 3.20 shows two examples of current traces recorded when the junction was pulled in this way at 0.25 μm s-1 in the presence of 20 mM EDOT and

0.1 M TBAPF6 acetonitrile solution. A potential of 0.5 V/Ag was applied to the UME tip and 0.4 V/Ag to the millimetric Pt substrate electrode. Thus, PEDOT was polarized in the conducting state. After moving the tip 5 μm in the X direction, the transport current dropped sharply from 180 nA to the background value with two current steps at 140 nA and 90 nA (figure 3.20). The abrupt current drop associated with tip moving suggests a decrease in the number of strands bridging the tip-substrate gap. The presence of multiple current plateaus is ascribed to the different numbers of strands involved in the junctions.

200

140nA

/nA

tip I 100 90nA

0 0 3 6 9 Distance/m Figure 3.20. SECM-BJ current traces acquired when PEDOT junction was pulled in -1 the X direction at 0.25 μm s in the presence of 20 mM EDOT and 0.1 M TBAPF6 solution. UME tip 1 was at applied potential 0.5 V and UME substrate at 0.4 V.

100

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

It is very difficult to control the size of the polymer junctions from large to nanoscale. Indeed, when molecular junctions are started with transport currents in the μA range the last step is usually observed at a few hundred nanoamperes. Next, we investigated SECM-BJ in the X direction on junctions of initial small size (initial conductance below 200 nS). Figure 3.21a shows the I/V characterization of PEDOT junctions before the SECM-BJ experiment. These transport current curves were obtained at a bias of 0.1 V (black line) and a reverse bias of -0.1V (red line). As expected, reversal of the bias sign induces a reversal of the current flow. It is evident that the current flows shown in the black and red lines are due to transport, as compared to the current when the junctions were completely broken by over pulling (blue line).

12 CV of PEDOT junc bias 0.1V 10 CV of PEDOT junc bias -0.1V a 20.0 b 8 CV of PEDOT lose connection 17.5 6 15.0 4 12.5

/nA 10.0 tip

0 /nA

I tip -2 I 7.5 -4 5.0 -6 2.5 -8 0.0 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0 2 4 6 8 10 V /V(SCE) g Distance/m

10.0 100 0.6 CV of PEDOT junc bias 0.1V c CV of PEDOT junc bias -0.1V d CV of PEDOT lose connection 0.4 7.5 75

0.2

/nA

0.0

tip 5.0

/nA 50

tip G/nS I -0.2

-0.4 2.5 25

-0.6

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 6.0 6.5 7.0 7.5 8.0 8.5 9.0 V /V(SCE) g Distance/m Figure 3.21. I/V characterization of PEDOT junctions (a ) before and (c) after tip movement, stopping at a certain current plateau, with 0.1 V bias and 0.1 V s-1 scan rate. (b) SECM-BJ current traces acquired when the junction was pulled in the X direction at -1 0.25 μm s in the presence of 20 mM EDOT and 0.1 M TBAPF6 acetonitrile solution. UME tip 1 was at an applied potential of 0.5 V and substrate at 0.4 V (d) zoom of (b).

Figure 3.21b shows the SECM-BJ in the X direction of PEDOT junctions whose characterization is shown in figure 3.21a. The experiment started with a transport current of 16 nA at a bias of 0.1 V, corresponding to a conductance of 160 nS, which indicates that the junctions are between fiber devices and single-molecule devices. The fluctuations observed in figure 3.20b are mainly due to PEDOT strands repeatedly connecting to and 101

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM disconnecting from the substrate. As can be seen in figure 3.21d, which is a zoom of figure 3.21b, the transport current of the junctions goes through several plateaus and finally stops at 0.6 nA. To be sure that when the current stops at the 0.6 nA plateau it is due to charge transport through the junctions, we examined their I/V characteristics. As shown in figure 3.21c, reversal of the bias sign reverses the current flow, which indicates that PEDOT is still bridging the two electrodes. The conductance of 6 nS suggests that very few molecular strands or even a single strand are involved in the junctions. It is thus possible to make a small PEDOT junction from a relatively large one by SECM-BJ.

3.3.3 Crossing the frontier between fiber devices and single-molecule devices

The last conductance plateau of a PEDOT junction in figure 3.21d indicates that the frontier between fiber devices and single-molecule devices has been crossed. Can we control molecular nanojunctions at a certain conductance step before breaking them? Can we observe single-molecule behavior? In this part, we will further discuss this point. As we proved above, the current value for a molecular junction drops stepwise in the SECM-BJ experiment. The last conductance steps indicate that the junctions involve only a few molecular strands. The final conductance step seems to be close to that observed in STM-BJ. Single-molecule strand behavior may be observed by SECM-BJ.

3.3.3.1 SECM-BJ in Z direction

To address this point, we investigated the transport current vs. distance traces of PEDOT junctions (bias 0.1 V, room temperature) using SECM-BJ in the Z direction. The stretching of the PEDOT junctions is shown in figure 3.22. Figure 3.22a shows conductance traces obtained during the breaking process; several conductance plateaus at 29, 19, 14, and 9 nS were observed before the contact was broken. Another example of SECM-BJ performed on these junctions is shown in figure 3.22c; conductance plateaus were observed at 24, 19, 14, and 5 nS before the junctions broke down. The conductance difference between steps tends to be a multiple of 5 nS, which is in good agreement with that of a single oligoEDOT conductance, as mentioned in chapter 2. The conductance difference between two plateaus suggests that fewer than three oxidized oligoEDOT

102

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM strands of unknown length are broken during movement at this potential. In addition, the final conductance step at 0.09 nS implies that the junctions are completely broken. No more steps are observed when the PEDOT is completely broken after being pulled a certain distance. The conductance drop between the plateaus suggests that oligoEDOT strands are disconnected from the substrate during the SECM-BJ movement and that single-molecule conductance can be measured.

4.0 40 5 50 3.5 a 35 b 3.0 29nS 30 4 40 2.5 25 3 30 2.0 20

19nS 24nS

G/nS G/nS

1.5 14nS 15 2 19nS 20

Current/nA Current/nA 1.0 9nS 10 14nS 1 10 0.5 5 5nS 14nS 0.09nS 0.0 0 0 0 0 10 20 30 40 0 10 20 30 40 Distance/m Distance/m 750 c

500

Counts 250

0 0 5 10 15 20 25 30 35 40 Conductance/nS Figure 3.22 (a) and (b) examples of SECM-BJ current traces acquired from nanojunctions when the tip was pulled in the Z direction at 0.25 μm s-1 in the presence of 20 mM EDOT and 0.1 M TBAPF6 acetonitrile solution. UME Tip: 0.5 V; Pt substrate (1.5 mm in diameter): 0.4 V. (c) Conductance histogram constructed by summing all the points of conductance traces that showed plateaus.

To further confirm the single-molecule strand behavior, conductance histograms were constructed from a set of individual SECM-BJ traces. The statistical study was performed with a Matlab program.28 Figure 3.22c shows pronounced conductance peaks at different values. The peaks separation implies that the conductance of a single oligoEDOT strand with unknown length is roughly 3 to 5 nS. Such a difference is in good agreement with the conductance steps. Note that figures 3.22a and b and histogram of figure 3.22c are close to those obtained by Lindsay et al. using SEM-BJ (figure 3.16); the current values deduced (0.4 nA) are very similar. 103

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

3.3.3.2 SECM-BJ in X direction

3 30 a 2 G=8nS 20 G=8nS 1 10

G=12nS /nA

tip 0 0

I G/nS

-1 -10

-2 -20

5 10 15 20 25 Distance/ m

4 b 40 G=16nS 2 20 G=8nS

G=12nS /nA

0 0

tip

G/nS I

-2 -20

-4 -40

5 10 15 20 25 Distance/ m

2000 c

1500

1000 Counts

500

0 0 5 10 15 20 25 G/nS Figure 3.23 (a) (b) examples of SECM-BJ current traces acquired from nanojunctions when PEDOT junction was pulled in the X direction at 0.25 μm s-1 in the presence of 20 mM EDOT and 0.1 M TBAPF6 acetonitrile solution. (c) Conductance histogram constructs by summing all the points of conductance traces that showed plateaus.

Next SECM-BJ in the X direction was also performed on the PEDOT nanojunctions to see if single molecular strand behavior can be observed.e Figure 3.23 shows two

e Two UMEs are used as the working electrodes. 104

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM examples of the variation of the transport current with distance, obtained as the tip moves in the X direction. The symmetric current, recorded on both UMEs, demonstrates that current here is due to charge transport through the PEDOT junctions. Figure 3.23a shows an example of SECM-BJ experiment, starting with a transport current of 1.2 nA. After the tip is moved 2.5 μm in the X direction, the current jumps abruptly, which can be attributed to rearrangement of the PEDOT junctions. After several connecting and disconnecting arrangements of the junctions on the substrate, the current drops to the initial value when the tip travels 10 μm and then the junction breaks. The conductance differences between two plateaus are 8 nS and 12 nS for the last two steps. Another example of SECM-BJ in the X direction is shown in figure 3.23b; conductance plateaus are again observed at a few nano-Siemens. This can be attributed to junctions involving a few oligoEDOT strands. The conductance difference between plateaus tends to be a multiple of 4 nS (figure 3.23), which clearly implies that the conductance of a single molecular strand is close to 4 nS. To further confirm the single molecular strand behavior, conductance histograms were constructed from a set of individual SECM-BJ traces in the X direction. Figure 3.23c shows the histograms for PEDOT junctions at different values; three significant conductance peaks were recorded. The difference is conductance between two peaks is roughly 4 nS. Comparison of the histogram in figure 3.23c with that in figure 3.22c, indicates that the conductance of the single molecular strand is roughly the same and is independent of the rupture direction.

N Au PEDOT tip tip

O S

O O N diffusive regime S O

ballistic regime

STM-BJ SECM-BJ

Figure 3.24. The analogy between STM-BJ and SECM-BJ with two different transport mechanisms along MMM junctions.

105

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

This single molecular junction event is quite similar to that observed by STM-BJ. As depicted in figure 3.24, it is likely that the conductance of the MMM junctions is controlled by a limited number of conducting polymer strands. Ballistic transport over a length close to the mean free path of the carrier in these strands is thus likely. Similarly to STM-BJ, with SECM-BJ it is possible to regenerate PEDOT junctions repeatedly.

3.3.4 PEDOT junctions formed repeatedly on a large Pt electrode

As we know, a molecular junction can be repeatedly destroyed and rebuilt during tip movement in an STM-BJ experiment. The number of individual molecules involved in MMM junctions is determined by the conductance plateau. The conductance of a single molecule is determined by the current level of the last plateau in the current versus distance curve. Here, SECM-BJ in the X direction may lead to the rearrangement of PEDOT connecting and disconnecting with the substrate electrode. If SECM-BJ is performed using a substrate electrode much larger than the UME tip, there may be a chance to observe molecular junctions created on the substrate repeatedly.

25 120 a b 20 100

15 80

/nA

tip

/nA

10 tip 60 I

5 40

0 20

-5 0 0 50 100 150 200 0 50 100 150 200 Distance/m Distance/m

6 60

5 50 5 50

4 40 4 c 40 d 3 30

/nA 3 30

tip I

/nA 2 20

tip

G/nS G/nS 2 20 I 1 10 1 10 0 0 0 0 -1 -10

20 30 40 50 60 70 20 30 40 50 60 70 80 90 Distance/m Distance/m Figure 3.25 (a) (b) examples of SECM-BJ current traces acquired when the PEDOT junction was pulled in the X direction at 0.25 μm s-1 in the presence of 20 mM EDOT and 0.1 M TBAPF6 acetonitrile solution; (c) zoom (a); (d) zoom (b).

106

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

As shown in figure 3.25a, typical SECM-BJ traces were recorded when the tips moved in the X direction and the applied potential was 0.5 V on the tip and 0.4 V on the substrate. The current of the junction dropped abruptly to the background value after a tip displacement of 8.5 µm, which suggests that the junction is broken. As shown in figure 3.25c, representing a zoom of the red part of figure 3.25a, an abrupt current jump at a distance of 18 µm is observed. This jump represents a reconstruction of the junction. Then the junction breaks after undergoing several junction rearrangements. Figure 3.25b shows another example of a junction regenerated by SECM-BJ. Figure 3.25d is a zoom of the red part of figure 3.25b. The current abruptly drops and jumps, representing disconnection and reconnection of the PEDOT junctions, respectively. The transport current is less than 5 nA, corresponding to a conductance between 10 nS and 50 nS, which means that only a few oligomer strands govern charge transport across the junction. A junction reconnected each time should be treated as a new junction involving a different number of oligoEDOT strands. In figure 3.25, the abrupt current jumps and drops represent the rearrangement of PEDOT junctions with a certain number of oligoEDOT strands. This phenomenon is the same as that observed in STM-BJ. Here a similar regeneration of molecular junctions is observed.

3.3.5 PANI junctions formed by SECM-BJ

In order to prove that SECM-BJ is a general way of controlling the size of conducting polymer junctions, PANI junctions were formed and studied by this method.f Experiments were performed on junctions with conductances in the 200-400 nS range. Figure 3.26a shows the transport current characteristics typical of a junction formed by DSV deposition. As the sweep potential increases above 0.1 V/SCE, PANI is oxidized to the conducting state and the transport current through the junction starts to increase at 0.25 V and reached a maximum of 25 nA at 0.4 V. Following that, it is gradually reduced to the insulating state with a hysteresis which is due to structural relaxation of the polymer.

f In this expetiment, a UME was used as tip electrode and a millimeter size Pt was used as substrate electrode. 107

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

40 a

/nA 20

tip I 10

-10 -0.1 0.0 0.1 0.2 0.3 0.4 V /V(SCE) g

35 50 30 c 40 b 25

30 20

15 20 10

Current/nA 10 Current/nA 5

0 0

0 5 10 15 20 0 2 4 6 8 10 12 14 16 18 20 Distance/m Distance/m

7 70

6 60 6 60

5 50

4 40 4 40

3 30 G=12nS G/nS G=20nS G/nS

2 20 G=12nS Current/nA

1 G=8nS 10 Current/nA 2 G=4nS 20

0 0 G=12nS -1 -10 8 10 12 14 10 12 14 Distance/m Distance/m Figure 3.26. (a) Transport current measurement of PANI junctions at 0.1 V bias and 0.1 V s-1 scan rate. (b) (c) SECM-BJ current traces acquired when the junction was pulled -1 in the Z direction at 0.25 μm s in the presence of 0.5 M aniline and 2 M H2SO4 in water. UME tip 1 was at an applied potential of 0.4 V and Pt substrate (1.5 mm in diameter) at 0.3 V. SECM-BJ curves were obtained by retracting the UME tip after the junctions were established and polarized in the conducting state (potential of 0.5 V on the tip and 0.4 V on the substrate). As shown in figure 3.26b, after a retraction of 4 μm, the transport current, where the bias between the two electrodes was 0.1 V, drops from 40 to 35 nA. The abrupt drop associated with tip stretching suggests a decrease in the number of strands bridging the tip-substrate gap. Subsequently, the junctions were broken down step by step with transport current plateaus of 15, 3.2, 1.2, and 0.5 nA. The existence of multiple transport current values is ascribed to junctions with different numbers of strands 108

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM connected to the substrate. The conductance differences between the last few plateaus are only 20 nS and 8 nS. Another example of SECM-BJ (figure 3.26c) exhibits similar properties of transport current decrease. In this case, the transport current goes through several steps. The conductance difference between the last few plateaus is 12, 12, 4 and 12 nS, respectively. This phenomenon suggests that the conductances of the strands are again multiples of 4 nS. The transport current of the junction finally dropped to zero or the background level after going through several steps. Such a background value, which is on the pico-ampere scale, indicates that the junctions are completely broken. Clearly, the size of the junctions was successfully controlled by SECM-BJ.

3.4 Conclusion

In this chapter, we have investigated three techniques to control the size of conducting polymers junctions, namely self-terminated, DSV and SECM-BJ. The main results of these techniques are listed as follows: By using the self-terminated strategy, the growth of conducting polymers was automatically terminated because of the external resistance connected to the tip electrode. Due to this resistor, the real potential applied at the tip where the conducting polymer deposits are no longer enough to sustain electropolymerization when contact occurs. A statistical study of the transport current of conducting polymer junctions, by using various external resistors, showed clearly that increasing the external resistor lowers the transport current, which means that smaller molecular junctions were obtained. The size of molecular junctions can be controlled by the self-terminated method. Nanojunctions of PANI and PEDOT were easily obtained using this method. DSV makes it possible to simultaneously observe the electropolymerization current and the transport current across the junctions. Polymer deposition can be stopped once the transport current is observed. Consequently, further growth is avoided once the junctions are established. Stable PEDOT junctions can be obtained by generating molecular junctions on a substrate which is functionalized with PEDOT film. Finally, by combing DSV and self-terminated, it is possible to generate controllable PEDOT junctions at the frontier between fiber devices and single-molecule devices. Charge transport through conducting polymer junctions (PEDOT and PANI) was monitored during the mechanical displacement of the tip by SECM-BJ. Conductance traces

109

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM showed that the size of junctions can be controlled. New junctions involved fewer molecular strands were created by pulling the tip away from the initial position. As in STM-BJ, conductance steps were observed in SECM-BJ experiments. The frontier between fiber devices and single-molecule devices has clearly been crossed by starting SECM-BJ from nanoscale molecular junctions. Single-molecule junction behavior is clearly observed and the conductance of a single oligomer strand is determined by the difference in conductance between plateaus in the conductance traces. Here, we find a clear single molecular strand signature with conductance values tending to be multiples of 4 nS. This value is close to that obtained by STM-BJ on similar conjugated molecular strands, which clearly suggests that one part of the polymer behaves as the metal tip in STM-BJ and that the part of the junction with the smallest diameter controls the whole device. In this part of the junction, charge transport is probably ballistic. Finally, PEDOT junctions can be repeatedly regenerated on the large platinum substrate by pulling the tip in the X direction. SECM-BJ enables the systematics of conductance to be investigated across a range of conducting polymers for molecular electronics. Overall, we have demonstrated that the size of molecular junctions can be controlled by these three techniques.

REFERENCES

1 Boussaad, S. & Tao, N. Atom-size gaps and contacts between electrodes fabricated with a self-terminated electrochemical method. Appl. Phys. Lett. 80, 2398-2400 (2002). 2 Chopra, H. D., Sullivan, M. R., Armstrong, J. N. & Hua, S. Z. The quantum spin-valve in cobalt atomic point contacts. Nat. Mater. 4, 832-837 (2005). 3 Xiang, J., Liu, B., Liu, B., Ren, B. & Tian, Z.-Q. A self-terminated electrochemical fabrication of electrode pairs with angstrom-sized gaps. Electrochemistry communications 8, 577-580 (2006). 4 Molet, P. et al. Electrochemical generation of stable copper nanowires with quantized conductance in DNA media. Electrochemistry Communications 13, 272-274 (2011). 5 Leroux, Y. R., Fave, C., Zigah, D., Trippe-Allard, G. & Lacroix, J. C. Atomic contacts via electrochemistry in water/cyclodextrin media: a step toward protected atomic contacts. J. Am. Chem. Soc. 130, 13465-13470 (2008). 6 Shen, M., Arroyo-Currás, N. & Bard, A. J. Achieving nanometer scale tip-to-substrate gaps with micrometer-size ultramicroelectrodes in scanning electrochemical microscopy. Anal. Chem. 83, 9082-9085 (2011). 7 Janin, M., Ghilane, J. & Lacroix, J.-C. Scanning electrochemical microscopy for the fabrication of copper nanowires: Atomic contacts with quantized conductance, and molecular adsorption effect. Electrochim. Acta 83, 7-12 (2012).

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Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

8 He, H., Li, C. & Tao, N. Conductance of polymer nanowires fabricated by a combined electrodeposition and mechanical break junction method. Appl. Phys. Lett. 78, 811-813 (2001). 9 He, J. et al. Measuring single molecule conductance with break junctions. Faraday discussions 131, 145-154 (2006). 10 Xia, J., Diez-Perez, I. & Tao, N. Electron transport in single molecules measured by a distance-modulation assisted break junction method. Nano Lett. 8, 1960-1964 (2008). 11 Xu, B. & Tao, N. J. Measurement of single-molecule resistance by repeated formation of molecular junctions. Science 301, 1221-1223 (2003). 12 Nitzan, A. & Ratner, M. A. Electron transport in molecular wire junctions. Science 300, 1384-1389 (2003). 13 Qian, X., Li, J., Lin, X. & Yip, S. Time-dependent density functional theory with ultrasoft pseudopotentials: Real-time electron propagation across a molecular junction. Phys. Rev. B 73, 035408 (2006). 14 Basch, H., Cohen, R. & Ratner, M. A. Interface geometry and molecular junction conductance: Geometric fluctuation and stochastic switching. Nano Lett. 5, 1668-1675 (2005). 15 Cuniberti, G., Fagas, G. & Richter, K. Introducing molecular electronics: A brief overview. (Springer, 2006). 16 Nichols, R. J. et al. The experimental determination of the conductance of single molecules. Physical Chemistry Chemical Physics 12, 2801-2815 (2010). 17 Xiang, D., Wang, X., Jia, C., Lee, T. & Guo, X. Molecular-Scale Electronics: From Concept to Function. Chem. Rev. 116, 4318-4440 (2016). 18 Zhou, X.-S. et al. Extending the capability of STM break junction for conductance measurement of atomic-size nanowires: an electrochemical strategy. J. Am. Chem. Soc. 130, 13228-13230 (2008). 19 Díez-Pérez, I. et al. Rectification and stability of a single molecular diode with controlled orientation. Nature chemistry 1, 635-641 (2009). 20 Martin, C. A., Ding, D., van der Zant, H. S. & van Ruitenbeek, J. M. Lithographic mechanical break junctions for single-molecule measurements in vacuum: possibilities and limitations. New Journal of Physics 10, 065008 (2008). 21 Chen, F., Hihath, J., Huang, Z., Li, X. & Tao, N. Measurement of single-molecule conductance. Annu. Rev. Phys. Chem. 58, 535-564 (2007). 22 He, J. et al. Electronic decay constant of carotenoid polyenes from single-molecule measurements. J. Am. Chem. Soc. 127, 1384-1385 (2005). 23 Park, Y. S. et al. Contact chemistry and single-molecule conductance: a comparison of phosphines, methyl sulfides, and amines. J. Am. Chem. Soc. 129, 15768-15769 (2007). 24 Martin, S., Haiss, W., Higgins, S. J. & Nichols, R. J. The Impact of E− Z Photo-Isomerization on Single Molecular Conductance. Nano Lett. 10, 2019-2023 (2010). 25 Chen, W. et al. Highly conducting π-conjugated molecular junctions covalently bonded to gold electrodes. J. Am. Chem. Soc. 133, 17160-17163 (2011). 26 Kanatzidis, M. G. Conductive polymers. Chemical & engineering news 68 (1990). 27 Bard, A. J., Denuault, G., Friesner, R. A., Dornblaser, B. C. & Tuckerman, L. S. Scanning electrochemical microscopy: theory and application of the transient (chronoamperometric) SECM response. Anal. Chem. 63, 1282-1288 (1991). 28 Martinez, W. L. & Martinez, A. R. Computational statistics handbook with MATLAB. Vol. 22 (CRC press, 2007). 111

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

Appendix

Example 1: PEDOT self-t

7 (a) R=10K 1000 1000 (c) R=100K (b) R=50K 6 800 800

A 5  4 600 600

3 400 400

Current/nA Current/nA

Current/ 2 200 200 1 0 0 0 -1 0 50 100 150 0 30 60 0 50 100 150 200 Time/s Time/s Time/s 18 250 (d) R=1M 40  (e) R=10M 16 (f) R=25M 14 200 30 12 150 10 20 8

100 Current/nA Current/nA

Current/nA 6 10 50 4 2 0 0 0 0 100 200 300 400 500 0 50 100 150 200 250 300 -20 0 20 40 60 80 100 120 140 160 Time/s Time/s Time/s Figure A 3.1 PEDOT molecular junctions fabricated by chronoamperometric electropolymerization by self-terminated method with various external resistors wired onto the tip electrode in acetonitrile containing 20 mM EDOT and 0.1 M LiClO4. Tip: 1.25 V/SCE; substrate: 0 V/SCE. External resistor: (a) 10 KΩ; (b) 100 KΩ; (c) 1MΩ; (d) 10MΩ; (e) 25MΩ.

External R I before contact I after contact Vgap before contact Vgap after contact 10K Ω 7.1nA 6µA 1.24993V 1.19V 100KΩ 10.8nA 804nA 1.2499V 1.1V 1MΩ 7nA 225nA 1.243V 1.0V 10MΩ 7.2nA 20nA 1.178V 1.0V 25MΩ 2.5nA 12nA 1.188V 0.95v

Table A 3.1. Measured current of PEDOT junction fabrication and calculated potential applied on the UME tip electrode during chronoamperometric deposition before and after contact.

112

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

1.0 150 40 a b c 0.8 30 100 0.6 20 0.4

Current/mA 50 Current/nA Current/nA 10 0.2

0.0 0 0

-0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Potential/V(SCE) Potential/V(SCE) 20 80 100 d e 15 f 80 60

60 10 40

40 Current/nA 20 Current/nA 5 Current/nA 20 0 0 0

-20 -20 -5 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential/V(SCE) Time/s Time/s Figure A3.2 Characterization of PEDOT molecular junctions created by self-terminating deposition with various external resistors wired onto the tip electrode in acetonitrile containing 20 mM EDOT and 0.1 M LiClO4. External resistor: (a) 10 KΩ; (b) 50 KΩ; (c) 100 KΩ; (d) 1MΩ; (e) 10MΩ; (f) 25MΩ. Example 2: PEDOT self-t

30 (a) R=10 K 10 25 (b) R=50 K

20 8 A

 6 /

15 A

tip



I /

10 tip 4 I

5 2

0 0

0 5 10 15 20 25 -5 0 5 10 15 20 25 30 35 Time/s Time/s

400 1.6 (C) R= 100K (d) R=1 M 300

A 1.2

 /

tip 200

I 0.8

/nA

tip I 0.4 100

0.0 0 0 20 40 60 80 100 0 20 40 60 Time/s Time/s

Figure A3.3 PEDOT molecular junctions fabricated by chronoamperometric electropolymerization by self-terminated method with various external resistors wired onto the tip electrode in acetonitrile containing 20 mM EDOT and 0.1 M LiClO4. Tip: 1.3 V/SCE; substrate: 0 V/SCE. External resistor: (a) 10 KΩ; (b) 50 KΩ; (c) 100 KΩ; (d) 1MΩ.

113

Chapter 3. Controllable construction of redox-gated polymer junctions by SECM

1.2 1.0 (a) R= 300 (b) R=50 K 0.8 0.6

200

/nA 

/ 0.4

tip

I tip I 0.2 100 0.0 -0.2 0 -0.4 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 V /V(SCE) V /V(SCE) g g 100 4

80 (c) R= (d) R= 3

/nA /nA

tip tip I I 40 1 20

0 0

-0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 V /V(SCE) V /V(SCE) g g Figure A 3.4 Characterization of PEDOT molecular junctions created by self-terminating deposition with various external resistors wired onto the tip electrode in acetonitrile containing 20 mM EDOT and 0.1 M LiClO4. External resistor: (a) 10 KΩ; (b) 50 KΩ; (c) 100 KΩ; (d) 1MΩ.

I before contact I after contact Vgap before contact Vgap after contact 10KΩ 27nA 26µA 1.2997V 1.04V 50KΩ 7.7nA 9.9µA 1.2996V 0.81V 100KΩ 5.4nA 1.7µA 1.2995V 1.13V 1MΩ 4.6nA 286nA 1.2954V 1.01V

Table A 3.2 Measured current of PANI junction fabrication and calculated potential applied to the tip electrode during chronoamperometric deposition before and after contact.

114

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

In the previous chapter, we have shown that conducting polymer based nanowires are fabricated by several kinds of methods. The conducting polymers (CPs) studied in chapter 2 and chapter 3 are p-type where holes act as charge carriers. In this chapter, we will introduce molecular junctions based on n-type as well as the ambipolar type of CPs.

4.1 Introduction

Due to the potential for low cost portable electronic and photovoltaic devices such as DSSC, OLED,OFETs,1-3 organic semiconductors have received remarkable attention all over the world since the first OFETs invented by Tsumura and coworkers in 1986.4 Most of the reports about organic semiconductors exhibit p-type transport properties. The developments of n-type organic semiconductors are not so fruitful than their p-type counterpart because of their lower mobility, instability under ambient conditions, etc. Ambipolar type of materials makes it possible to avoid deposit p- and n- type materials separately in the logic circuit. An ideal ambipolar type material which works both as anode and cathode materials is required to have a balance on the both n- and p- channel performance. In principle, most of the CPs, such as PEDOT,5,6 are intrinsically ambipolar and are capable of transporting both holes and electrons. Reversible n- and p- doped behavior is observed in CPs.7-11 However, obstacles remain in the observation of the electron-transporting properties in OFETs. The main reason is the vulnerability of electron charge carriers to atmospheric constituents, such as O2 and H2O, which induce the degradation of charge transport.12,13 From the molecular design point of view, those molecules with low band gap and/or with strong electron-withdrawing and electron-donating units have high performance in ambipolar transport properties.14,15 Introducing the electron-withdrawing group is an effective approach to obtain high-performance n-doped materials.16,17 Indeed n-type organic molecules can also be obtained from known p-type building block by adding electron-withdrawing groups to reduce the LUMO levels for electron transport.12

115

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

Is it possible to create molecular junctions based on a molecule that can be easily n-doped? Lafolet et al18 reported redox-active molecular wires based on the extended Rh-Rh bonded chains. The metal-metal building blocks exhibit well-defined n-doped properties. Such conducting polymer with Rh-Rh chain as a center and organic ligands as encapsulation side group was used to investigate the n-channel conduction properties in this chapter. Is it possible to have junctions behaving as n- and p-doped switches using a low band gap polymer? What is the difference of the current density of the molecule when it is n and p doped? What is the effect of the junction size on the molecular conductance of the junctions? To answer these questions, low band gap molecules are needed. It is reported that the 2,3-di(2-furyl) quinoxaline combine with the thiophene donors was used as a low band gap materials, showing both n-doping and low switching times.19 In this chapter, two CPs were electrochemically synthesized. They are poly[2,3-di(2-furyl)-5,8-bis(2-(3,4-ethylenedioxythiophene)) quinoxaline] (PFETQ) and poly[2,3-di(2-furyl)-5,8-bis(2-thienyl) quinoxaline] (PFTQ). Both will be used to fabricate molecular junctions by SECM, in order to investigate charge transport properties through the n- and p-channel.

4.2 n-doped Rh-Rh chain molecular junctions

4.2.1 Electrochemical fabrication of Rh-Rh chain polymer thin films

The rhodium(II) binuclear complex monomer ( [Rh2(lOOCCH3)2(Phen)2(X)2], phen = 1,10-phenanthroline) has been synthesized by a member of our group, Frédéric Lafolet, as reported in the literature.20 Molecular wires with Rh–Rh chains center can be chemically and also electrochemically prepared.18,21-23 In terms of electrochemical method, the complexes with various phen ligands (L) are prepared by reducing the complex through a reversible one-electron process, as shown in scheme 4.1.

N N

II II Reduction n H2O Rh Rh OH2

O O O O

Scheme 4.1 Schematic drawing of electrochemical synthesis of Infinite coordinated Rh-Rh wires. 116

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

10 60

A 0

A 30



-10 0

Current/ -30

-20 Current/ -60 -30 -1.2 -0.8 -0.4 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 V /V (SCE) V /V(SCE) g g Figure 4.1 (a) first cycle and (b)successive CVs curves of Rh-Rh chain polymer electrochemical deposit on the Pt electrode (diameter 1.5mm) in an acetonitrile solution containing 1mM monomer and 0.1M TBAP, scan rate 100mV/s.

Figure 4.1 shows the electropolymerization CVs of 1mM monomer and 0.1M TBAP in acetonitrile solution on a Pt electrode with a diameter of 1.5mm and scan rate of 0.1V/s. The solution was prior degased in order to remove oxygen from the electrochemical cell.

It has been reported that the monomer complexes [Rh2(l-OOCCH3)2(L)2(CH3CN)2](BF4)2 18 are reduced by an irreversible one-electron process with 1 mol electron per Rh2 unit. Rh complex starts to be reduced from −0.6V/SCE. The electrochemical curve revealing the formation of the corresponding polymers is given in figure 4.1. The increasing electroactive current observed at -1.3V is due to the formation of a polymer film deposited on the working electrode (WE) surface, in accordance with the following reaction: 2+ − n [RhII,II 2(l-OOCCH3)2(phen)2(X)2] + ne →[RhI,II,II,I 4(l-OOCCH3)4(phen)4] 2+ − n/2+2n X . the oxidation and reduction peaks(-1.25V and -1.35V) observed in figure 4.1 are attributed to the electroactivity of a tetra-nuclear cores of rhodium molecular center switching from RhI-RhII–RhII-RhII to RhII-RhII-RhII-RhII.

117

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

Rh-Rh chain polymer 150 blank 100 insulating

A 50

 conductive

-50

Current/ -100

-150

-200 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 V /v (SCE) g Figure 4.2 CV of Rh-Rh chain polymer film recorded in 0.1M TBAP acetonitrile monomer free solution on a Pt electrode (diameter 1.5mm), scan rate 100mV/s (black line) and the blank test of bare Pt electrode in the monomer free solution (red line).

Figure 4.2 shows the CV response of the created Rh-Rh chain film in 0.1M TBAP acetonitrile monomer free solution. The cathodic and anodic peaks indicate that a polymer with an excellent redox activity was deposited on the working electrode surface. The film is essentially characterized by one well-defined peak systems at −1.3 and −1.35 V, corresponding respectively to the reduction and oxidation of the polymer deposited as a thin film on the WE surface. According to this electroactivity, the polymer is expected to be insulating above -1.3V, whereas it is conductive below -1.3V. Thanks to the mixed valence obtained when RhII-RhII–RhII-RhII chains are reduced to RhI-RhII–RhII-RhII, this process can be treated as an n-doping of the polymer chain.

4.2.2 Molecular junctions based on Rh-Rh chain polymer

The aim of generating molecular wire based on Rh–Rh chain polymer is to investigate the charge transport properties through n-doped conducting polymer. ‘Rh-Rh’ chain molecular junctions were electrochemically created between the tip and substrate in the SECM 4-electrodes configuration. To do so, the ‘Rh-Rh’ chains were electrochemically deposited on the tip applying a constant potential (-1.0V) at the tip1, the substrate was kept at 0V to avoid spreading of the polymer when contact occurs.

118

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

-200

-400

-600

Current/nA -800

-1000

-1200 0 200 400 600 800 1000 Time/s

Figure 4.4 of Rh-Rh chain polymer junctions generated by chronoamperometry technique, substrate potential: 0V; tip potential: -1.0V.

As shown in figure 4.4, the current are mainly due to the electrochemical reduction of monomers on the UME tip during the polymerization (here before 850s). Subsequently, an abrupt current jump is observed at 850s, which indicates the formation of the molecular junctions. Finally, the current saturated at -1000nA, indicate that a large wire has been created and that the used sensitivity of the CHI instruments is not capable of measuring the transport current through this large junctions. The molecular junctions were characterized by recording the variation of current on the UME tip as a function of the gate potential using -100 mV bias. The curves in figure 4.5a show that, during the forward scan, from -0.8 to -1.25V the junction is in an insulating state and no current (or leakage current in the sub-nano ampere range) is flowing between the two electrodes. Next, the current starts to increase from -1.25V to more negative potential (see figure 4.5) with fluctuations. We ascribe such current at UME tip1 to charge transport across the polymer junction, which is now conductive. The first important result is that it seems possible to generate redox gated molecular junctions using this type of molecules that become conductive through an n-doping process.

119

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

300 100

250 a 80 b

200 60

150

100 Current/nA Current/nA 20 50 0 0 -1,8 -1,6 -1,4 -1,2 -1,0 -0,8 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 V /v (SCE) V /V (SCE) g g Figure 4.5. Characterizations of two Rh-Rh chain junctions using -100mV interelectrode bias and 100mV.s-1 scan rate in monomer free solution with TBAP as the electrolyte.

Figure 4.5b shows another example of a Rh-Rh chain molecular junctions. In this case, the transport current starts to increase as polymer starts to be reduced into conducting state at -1.2V and reaches the maximum value at -1.45 V. The polymer junction shows well-defined n-doped electron transport properties with conduction in the reduced state. Furthermore the electron transfer current used for doping the polymer is much smaller than electron transport current across the junction. Igate is thus almost negligible compared to ISD. During the backward scan, the junction is reversibly switched from the conducting to the insulating state. Backward curves show again a small hysteresis, which is induced by structural relaxation of the junction during the scan. As we have already described with p-type materials, such charge transport curves, obtained in figure 4.5, are strongly affected by the variation of the bias voltage: (i) increase in the bias value enhances the current through the junction. (ii) Reversal of the bias sign induces a reversal of the current flow. To further confirm that the current observed in figure 4.5 are due to the charge transport across the Rh-Rh chain molecular junctions, experiment at various bias voltages was prepared. As shown in figure 4.6, increasing the bias voltage induces the transport current in the junctions to increase when the polymer is in its n doped conducting state. Reversing the bias sign reverses the current flow between the electrodes. The conductance value of the generated molecular junctions in figure 4.6 is calculated to be 4μS. Such conductance value is relatively high, which indicates that a lot of molecular strands bridge the two electrodes.

120

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

800 50mV 600 100mV 150mV 400 -50mV -100mV 200 -150mV

/nA 0

tip

I -200 -400 -600 -800 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 Potential/V(SCE) Figure 4.6. Source-drain current vs gate voltage at 100mV/s scan rate by using various bias.

A smaller ‘Rh-Rh’ nanojunction was generated. In figure 4.7a, the current recorded on the UME tip shows a strong dependence on the bias indicating that the current is due to charge transport through the junctions. The transport current is 10nA using 100mV bias, which is corresponding to a conductance value of 100nS. As shown in figure 4.7b, increasing the scan rate from 50 to 150mV/s, no effect on the charge transport current is observed. These experiments confirm that the current observed in the conducting state are due to the charge transport through the molecular junctions. Note that, despite the low conductance shown here, no fluctuation is observed in the curve in figure 4.7.

12 20 150mV 100mV 10 50mV/s 50mV 100mV/s -50mV 150mV/s -100mV 8 -150mV

6 /nA

tip

/nA

tip 4 I

-20 0

-2 -1,7 -1,6 -1,5 -1,4 -1,3 -1,2 -1,1 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 V /V(SCE) V /V(SCE) g g Figure 4.7. (a) Source-drain current vs gate voltage at 100mV/s scan rate by using various bias. (b) Source-drain current vs gate voltage at 100mV bias by using various scan.

II,II It shows clearly that the electrochemical reduction of Rh 2(l-OOCCH3)2(phen)2 complex center causes the formation of a Rh-Rh chain polymer film on the WE. Rh-Rh

121

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions chain redox-gated junctions were created by SECM. The polymer junctions exhibit a well-defined n-type charge transport properties with conduction in the reduced state. Highly stable and reversible junctions were obtained with an unknown number of oligomer strands. This kind of redox gated polymer junctions with metallic wire center may open a new view for future n-type molecular design and preparation of strategic redox molecules and supramolecule systems.

4.3 Ambipolar type conducting polymer molecular junctions

4.3.1 Electrochemical fabrication of PFTQ and PFETQ films

Two molecules, known in the literature to give the ambipolar polymers, were synthesized by Frédéric Lafolet according to the procedure described in the literature.19 The first one is called 2,3-di(2-furyl)-5,8-bis(2-thienyl) quinoxaline (FTQ) and the second one is called 2,3-di(2-furyl)-5,8-bis(2-(3,4-ethylenedioxythiophene)) quinoxaline (FETQ). They have a central quinoxaline unit substituted by furan groups and lateral thiophene(FTQ) or EDOT (FETQ) group. Electrochemical polymerizations of FTQ and FETQ (scheme 4.2) were carried out by CV in an acetonitrile (ACN)/dichloromethane (DCM) (1:1, by volume) solvent containing 0.1M TBAPF6 as the supporting electrolyte and 5mM monomers. The solution was prior degassed and kept in the argon atmosphere in order to remove oxygen from the electrochemical cell.

O O O O ox N N N N

S S * S S n

FTQ PFTQ

O O O O ox O N N O O N N O O O O O

S S * S S n FETQ PFETQ Scheme 4.2. Schematic drawing of electrosynthesis of PFTQ and PFETQ.

122

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

Figure 4.8 shows the monomers electropolymerization by successive CV curves. As can be seen from the first cycle of the CV, the potential corresponding to the irreversible monomer oxidation on the Pt electrode is observed at 1.1 V/SCE for FTQ (insert figure). Comparing the oxidation potential of monomers, FETQ was much easier to be oxidized than FTQ thanks to the EDOT moieties. As the cyclic scans continue, electroactivity of the polymer at a potential below the oxidation potential of the monomer develops. This indicates that a polymer is deposited on the electrode surface.

60 20 80 30

A 25 15

 A

20 a  10 b

15 60 40 5

Current/ 10 Current/ 5 0

0 -5 A A 40 -0.8 -0.4 0.0 0.4 0.8 1.2

-5   E(V vs. SCE)

0.2 0.4 0.6 0.8 1.0 1.2 20

20 E(V vs. SCE)

0 0

Current/ Current/

-20 -20 -40 0.2 0.4 0.6 0.8 1.0 1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 E(V vs. SCE) E(V vs. SCE)

Figure 4.8. Repeated CVs electropolymerization of 5mM (a) FTQ (b) FETQ at 100mV/s in 0.1M TBAPF6/ACN/DCM on a platinum wire electrode (with a diameter of 1.5 mm). Insert: polymerization route of (a) FTQ and (b) FETQ.

It is well known that conducting polymer which contains donor-acceptor functional group exhibit ambipolar properties.24-28 During the p-doped process, the conjugated chain loses electrons and anions move into the polymer film. During the n-doped process, the conjugated chain gains electrons and cations move into the polymer film. The electrochemical behavior of PFTQ and PFETQ modified electrodes was characterized in

CH3CN + 0.1M TBAPF6, as shown in figure 4.9. Two CPs were electrochemically deposited on the WE and exhibit a well-defined ambipolar property with distinct n-doped/dedoped and p-doped/dedoped process. In the cathodic area (n-doping), quasi-reversible E1/2 = -1.48 V/SCE for PFTQ and E1/2 = -1.39 V/SCE for PFETQ are observed, indicating that these polymers can be n-doped. In the anodic region, a broad system is recorded at Epa = 0.6V/SCE for PFTQ and Epa = -0.2V/SCE for PFETQ, indicating they are p-doped.

123

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

200 200 (b) CV of PFETQ film on Pt (a) CV of PFTQ Film on Pt 150 150

100 100 A

A 50 

50 

0 0

-50 Current/

-50 Current/ -100

-100 -150 -200 -150 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 E(V vs. SCE) E(V vs. SCE)

250 200 (d) CV of PFETQ film on Pt (c) CV of PFTQ Film on Pt 200 150

150 100

A 

100 A 50



50 0 0 -50

Current/ -50 Current/ -100 -100 -150 -150 -200 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 E(V vs. SCE) -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 E(V vs. SCE) Figure 4.9. CV curves of the PFTQ (a) and PFETQ (b) film on a platinum electrode at scan rate of 100mV/s in the monomer-free 0.1MTBAPF6/ACN/DCM solution. Measurements of the stability of PFTQ (c) and PFETQ (d) film.

The stability of PFTQ during the n-doped process was poor as can be seen in figure 4.9c, 80% of electroactivity is lost after 5 cycles. While the p-doped shows a well-defined stability with 95% electroactivity retained after 5 cycles. The reversible peaks of PFETQ n-doped/dedoped process are at -1.5 V and -1.3 V (figure 4.9d). The PFETQ films almost completely retain their electroactivity in both the n- and p-doped processes during 5 cycles.

4.3.2 Charge transport through both n- and p-channel in PFTQ and PFETQ

4.3.2.1 PFTQ and PFETQ molecular junctions by DSV Figure 4.10 shows PFTQ and PFETQ electropolymerization in 5mM monomer and

0.1M TBAPF6/ACN/DCM using DSV method as described in chapter 3. As can be seen in Figure 4.10, when there is a contact between the tip and the substrate electrode, transport current (distinguished from electrochemistry current) is observed. Such current is the signal that the polymer molecular junctions have been established between the electrodes.

124

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

Increasing CV cycles, more polymers are deposited on the electrode, which results in transport current increasing under p-doped potential. Transport current is dominant during the DSV.

0.6 150 a b

100 0.4

/nA

tip

I I 50 0.2

0 0.0

0.4 0.6 0.8 1.0 1.2 1.4 0.2 0.4 0.6 0.8 Potential/V Ag wire Potential/V Ag wire Figure 4.10 first and the second cycle of (a) FTQ and (b) FETQ electropolymerization by -1 DSVs at 100mV s in the 5mM monomer and 0.1M TBAPF6/ACN/DCM on a platinum UME (5μm in radius).

After the contact was established the PFTQ junctions were characterized by recording the variation of the tip current as a function of the gate potential using 100mV or 200 mV bias. Figure 4.11 shows Source-drain current vs gate voltage response of two different PFTQ molecular junctions. In figure 4.11a, potential sweep starts from 0.25V. When it is below 0.5 V the PFTQ is in the insulating state and no transport current is flowing between the two electrodes. A peak current, observed at 0.6V, is due to the electroactivity of PFTQ, which starts to be oxidized in the conductive state. Following that, when the tip potential reached 1V, a new current through the junction starts to be observed. This current increases and reaches a maximum current value of 700nA at 1.3 V. It starts to decrease in the backward scan. This current can be attributed to the charge transport through the junctions. As the potential is swept to a negative regime where the PFTQ get reduced in an n-doped conductive state, a current increase is observed and it reaches a maximum value at -1.1V. Note that a bell shape transport current is observed when PFTQ is n-doped. Then the bias was reversed. Current observed in figure 4.11b changed from positive to negative when the bias was reversed from 200mV(red line) to -200mV(blue line), indicating that the current in the n- and p- doped regime are mainly due to the charge transport through the PFTQ molecular junctions. The small hysteresis of the backward curves results from the molecular structural relaxation. Higher transport current is clearly 125

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions observed when the PFTQ is p-doped compared to n-doped, suggesting large conductivity difference for electron and hole as charge carriers. At that point, it is possible to draw an intermediate conclusion: a) PFTQ exhibits a well-defined ambipolar charge transport properties. It is possible to have n- and p- doing switches using such polymer. b) The conductance of p-doped PFTQ is higher than that of n-doped polymer junctions. c) the shape of the curve indicates a different type of conductance in the p- and n-doped regime.

bare UME in FTQ solution 2000 CV of PFTQ junc 200mV 600 CV of PFTQ junc -200mV 1000

400 /nA

tip

/nA

tip I -1000 200 -2000

0 -3000

-1.0 -0.5 0.0 0.5 1.0 1.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Potential/V Ag wire Potential/V Ag wire

Figure 4.11 Source-drain current of PFTQ molecular junctions vs gate voltage using 100mV/s scan rate under (a) 100mV interelectrode bias and (b) 200mV(red line) and -200mV bias (blue line).

Similarly, PFETQ molecular junctions have been generated by using DSV method. It started in the 5mM monomer and 0.1M TBAPF6/ACN/DCM solution which was prior degassed and kept in the argon atmosphere in order to remove oxygen in the electrochemical cell. Figure 4.12 presents tip currents as a function of the gate potential using 100 mV (black line) and -100mV (red line) bias. Current starts to increase from 0.3V and reaches a maximum value of 20μA at 0.8V under 100mV bias (black line p-doped regime). Such current values suggest that the generated molecular junctions are fiber devices. A bell shape current with a value of the only 30nA is observed in the PFETQ n-doped regime at potential around -1.35V. Here again, the variation of the bias voltage affects the transport current: reversal of the bias sign induces a reversal of the current flow.

126

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

40 60 100mV 40

20 -100mV

/nA

tip

I 0 20 -20 -40

A -1.6 -1.4 -1.2 -1.0

Potential/V Ag wire



tip I 0

-20

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential/V Ag wire Figure 4.12. Source-drain current of PFETQ vs gate voltage at 100mV/s scan rate by using 100mV and -100mV bias in the monomer free TBAPF6 acetonitrile solution. Insert: transport current curves of PFETQ n-channel performance.

We can thus conclude that: a) PFETQ exhibits ambipolar charge transport properties. b) A bell shape transport current is only observed in the n-doped potential range. c) The conductance of PFETQ in the p-channel is 1000 times than that in n-channel. The huge difference implies again that the conductivity between the p- and n- the channels through such CPs junctions are quite different. d) These results are close to those obtained with PFTQ. Another example of PFETQ molecular junctions is shown in figure 4.13 but focused more in the n doping regime. The sensitivity of the SECM set up is set in a high value so that transport current in the n doped regime can be more clearly observed. Various bias voltages are applied. As expected, higher bias value enhances the current through the junction. Changing the bias sign changes the direction of the current flow. It is clear that: (a) these two currents are due to charge transport in the n- and p- doped regime. (b) The transport current values are high and suggest that the PFETQ molecular junctions are fiber device. (c) The conductivity in PFETQ n- and p- doped are not the same which indicate that electrons and holes have different mobilities in such macroscopic molecular junctions.

127

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

1.2 bias -200mV bias -100mV 0.8 bias 100mV bias 200mV

0.4

A 

0.0 I/ -0.4

-0.8

-1.2

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential/V (Ag wire) Figure 4.13. Source-drain current of PFETQ molecular junctions vs gate voltage at 100mV/s scan rate by using various bias in the 0.1M TBAPF6 acetonitrile solution.

At this point, it is interesting to compare such phenomenon to other ambipolar semiconductors. It is reported that ambipolar materials could have different p- and n- channel mobilities.29-31 For instance, dihexanoylquaterthiophene gave good ambipolar transport behavior, with hole mobility and electron mobility values of ∼0.01 and 0.1 cm2/V·s, respectively.32 An ideal ambipolar device is required to give a balanced performance in both n-channel and p-channel transport; therefore, it is strongly desired that the ambipolar materials have similar n- and p-channel performance. Here PFTQ and PQETQ exhibit well defined ambipolar transport properties. However, at the macroscopic level the conductivity of these two polymer in the p-doped state is higher than the n-doped state.

4.3.2.2 Effect of junction size on the p and n conductance Note that the conductance PFTQ and PFETQ molecular junctions in the p-doped region are in the micro Siemens, which indicates that the junctions are fiber devices. What will happen in the PFTQ and PFETQ nanojunctions, where the charge transport properties are ballistic? It is possible that an effect of the junctions size on the molecular conductance of the junctions will be observed. As far as we know, there are more chances to acquire nanojunction of CPs using UME as substrate electrode based on the DSV method. CPs bridged two platinum UMEs using DSV method were thus studied. It started from FETQ electrochemically polymerizing in the 5mM monomer ACN DCM (V=1:1) solution on the tip electrode. The potential applied to both working electrodes was kept with a difference of 0.2V so that polymer only 128

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions deposited on the UME tip1 and no polymerization occurs on the UME tip 2 (substrate). When there is no contact between two UMEs, as can be seen in figure 4.16 a, electroactivity current of the polymer is only on UME tip (tip 1, black line not on UME substrate (tip 2, red line). Once the polymer molecular junctions are established, as can be seen in figure 4.16b from the 21st cycle of DSV, another type of current with fluctuation are recorded on both UMEs. The symmetric currents on both UMEs in figure 4.16 b are the signal of charge transport through the PFETQ molecular junctions. As the DSV cycles continuing the symmetric current increase on both electrodes. The polymer deposition was stopped when the transport current is observed at a certain value so that further growth of molecular junctions is avoided. In this way, the size of the molecular junctions is somehow controlled.

600 tip 1 a tip 1 80 tip 2 tip 2 400 b

200

0 0 Current/nA

Current/nA -200

-80 -400

-600 0,3 0,6 0,9 0.0 0.2 0.4 0.6 0.8 1.0 Potential/V(Ag wire) Potential/V(Ag wire)

Figure 4.16. PFETQ molecular junctions generated by DSV at 100mV s-1 in the 5mM monomer and 0.1M TBAPF6 ACN/DCM solution on a platinum UME (5μm in radius) with a 0.2V difference between electrodes and 0.1V/s scan rate. (a) First 20 cycles of the DSV, red zone part of (b) DSV with transport current occurring at the 21st cycle.

PFETQ junctions were characterized by following the variation of the UME tip currents as a function of the gate potential. As shown in figure 4.17, the shape of curves recorded on both UMEs are symmetric, with Itip1 =−Itip2 and ΔItip1 =−ΔItip2 when the PFETQ is polarized into conducting state. Similarly, as other CPs, such as PEDOT, PANI, PFTQ, the spike current (black curve) observed at 0.15V are due to the electroactivity of PFETQ on the tip electrode. Such spike current is not observed on the UME substrate (red line in figure 4.17), which induced asymmetric current recorded on both UMEs. To eliminate the disturbance of PFETQ electroactivity current, the substrate currents, which are mainly transport current across the junction, are selected for further discussion.

129

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

300 tip 1 200 tip 2

100

0 Current/nA

-100

-200 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential/V(Ag wire)

Figure 4.17. Source-drain current on both UMEs of PFETQ molecular junctions vs gate voltage at 100mV/s scan rate by using 200mV in the acetonitrile monomer free solution

containing 0.1M TBAPF6.

25 a 10 b 20

5 Current/nA

-5 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential/V Ag wire Potential/V(Ag wire) Figure 4.18. (a) and (b) Source-drain current UME substrate of PFETQ molecular junctions vs gate voltage at 100mV/s scan rate by using -200mV in the acetonitrile monomer free solution containing 0.1M TBAPF6.

In figure 4.18a, sweeping potential starts from -0.3V to positive region. PFETQ stay in the insulating state in a potential range between -1.0 and 0.2 V/Ag. Above 0.2V the PFETQ started to be p-doped and become conductive. The observed increasing current from 0.2V is due to the charge transport the junctions. It reaches 5nA at the oxidized state in this case, which gives a conductance value of 25nS (Gp,max). Clearly, such conductance indicates that charge transport is governed by a few molecular strands. Small hysteresis is observed in the backward scan. PFETQ started to be n-doped and become conductive from -1.0V to the more negative region. The transport current reaches the maximum value of 15nA in the n-doped region in the forward scan, which gives a conductance value of 75nS (Gn,max). PFETQ exhibited obvious ambipolar transport properties. Figure 4.18b present the characterization of another PFETQ molecular junction with 7nA observed in n- channel and 5nA in p-channel transport current. Here again 5nA

130

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

transport current values gives the conductance value of 25nS (Gp,max), which implied that only several oligomer strands governed the charge transport. Interestingly, a balanced p- and n- channel performance are now observed in electrochemically gated PFETQ.

10 a

4 Current/nA 2

-2 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Potential/V. Ag wire

40 b 30

20 Current/nA 10

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Potential/V. Ag wire

10 c 8

4 Current/nA 2

-2 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Potential/V Ag wire Figure 4.19. Source-drain current of PFTQ molecular junctions vs gate voltage at 100mV/s scan rate by using 200mV bias in monomer-free solution.

Next PFTQ nanojunctions are created using a similar DSV strategy as described above. To avoid the electroactivity current of PFTQ, current recorded on the UME substrate are 131

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions selected for analysis. Figure 4.19 presents three examples of the transport current versus gate voltage response. Figure 4.19a shows a PFTQ molecular junction with 7nA observed in both p-and n- channel transport current. Figure 4.19b shows a transport current of 35 nA in the n channel and 8 nA in the p channel. Figure 4.19c shows that a balanced transport current of 5nA is observed in both p-and n-doped state. The transport current is in the nanoampere scale, which suggests that charge transport is governed by only a few molecular oligomer strands. In these nanojunctions, p-channel transport current becomes comparable with n-channel transport current. Here again a balanced charge transport through n- and p- channel is observed. This phenomenon is quite similar as that observed in PFETQ molecular junctions. In order to address this point, we have made the comparison of conductance between PFETQ and PFTQ molecular junction created on the macro size substrate and micro size substrate. The p- or n-channel conductance (Gp,max or Gn,max ) are recorded when the polymers are polarized (p- or n-doped) to conducting state. Transport current in p- and n- channel of the molecular junctions are listed in table 4.1 of PFTQ and table 4.2 of PFETQ.

As shown in figure 4.20, we plot the ratio of Gp,max/Gn,max versus the Gp,max values of each individual transport current curve of PFETQ and PFTQ molecular junctions. The black dot represents the conductance of polymer junctions formed either on the 1.5mm platinum or 4mm ITO, whereas the red dot represents that formed on the UME substrate.

100 large substrate large substrate 4 (b) 10 UME substrate (a) UME substrate

103

n,max 10

n,max /G

p,max p,max 1 G G 10 1

100

0 1 2 3 4 5 10 10 10 10 10 10 102 103 104 G /nS G /nS p,max p,max

Figure 4.20. comparison of the ratio of Gp,max/Gn,max of (a) PFETQ and (b) PFTQ molecular junctions when the UME (red dot) and millimeter size Pt as substrate (black dot).

132

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

polymer/substrate(diameter) Gn,max/nS Gp,max/nS Gp,max/Gn,max PFTQ/Pt(1.5mm) 400 2000 5 PFTQ/Pt(1.5mm) 600 3350 5.6 PFTQ/Pt(1.5mm) 408 3500 8.6 PFTQ/Pt(1.5mm) 1100 11500 10.5 PFTQ/Pt(1.5mm) 400 22500 56 PFTQ/Pt(1.5mm) 365 35000 96 PFTQ/UME(10μm) 130 110 0.85 PFTQ/UME(10μm) 530 754 1.4 PFTQ/UME(10μm) 341 740 2.2 PFTQ/UME(10μm) 200 700 3.5 PFTQ/UME(10μm) 550 4100 7. 5 PFTQ/UME(10μm) 16.5 255 15.5

Table 4.1. Measured p- and n- channel conductances and the ratio of Gp,max/Gn,max of several PFTQ and PFETQ junctions generated on the macro size and micro size substrates.

polymer/substrate(diameter) Gn,max/nS Gp,max/nS Gp,max/Gn,max PFETQ/ITOAuNPs (4mm) 225 950 4 PFETQ/ITO(4mm) 12.5 183 15 PFETQ/ITOAuNPs (4mm) 150 2250 15 PFETQ/ITOAuNPs (4mm) 200 3800 19 PFETQ/Pt(1.5mm) 5.5 108 20 PFETQ/Pt(1.5mm) 8.5 170 20 PFETQ/Pt(1.5mm) 3.7 75 20 PFETQ/Pt(1.5mm) 7 175 25 PFETQ/Pt(1.5mm) 82.5 2150 26 PFETQ/ITOAuNPs(1.5mm) 270 8000 30 PFETQ/Pt(1.5mm) 27.5 850 31 PFETQ/Pt(1.5mm) 180 6250 35 PFETQ/ITOAuNPs (4mm) 165 5800 35 PFETQ/Pt(1.5mm) 232 11000 47 PFETQ/GC(1.5mm) 500 60000 120 PFETQ/ITOAuNPs(1.5mm) 50 10500 210

133

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

PFETQ/ITO(1.5mm) 35 15000 430 PFETQ/ITOAuNPs(1.5mm) 100 50000 500 PFETQ/ITO(1.5mm) 6.2 9350 1508 PFETQ/UME(10μm) 200 80 0.5 PFETQ/UME(10μm) 70 30 0.4 PFETQ/UME(10μm) 30 15 0.5 PFETQ/UME(10μm) 50 25 0.5 PFETQ/UME(10μm) 10 10 1 PFETQ/UME(10μm) 12 20 2 PFETQ/UME(10μm) 75 250 3 PFETQ/UME(10μm) 50 380 7.5 PFETQ/UME(10μm) 50 400 8 PFETQ/UME(10μm) 35 350 10 PFETQ/UME(10μm) 75 750 10 PFETQ/UME(10μm) 100 1250 12 PFETQ/UME(10μm) 90 2500 28 PFETQ/UME(10μm) 25 1100 44 PFETQ/UME(10μm) 200 80 0.4

Table 4.2. Measured p- and n- channel conductance and the ratio of Gp,max/Gn,max of several PFETQ junctions generated on the macro size and micro size substrates.

It is interesting to find that there is more chance to have nanojunction when using a smaller UME than using a macro substrate. Figure 4.20a shows the result for PFETQ molecular junctions. The black dot shows that the p-channel conductance is found to be in the micro Siemens scale when millimeter size electrode is used as the substrate. Such conductance values suggest that charge transport is diffusive and the molecular junctions should be treated as fiber devices. However, the conductance is only a few nano-Siemens when UME is used as the substrate. Charge transport through such device is ballistic and controlled by the limited smallest amount of molecular strands. Moreover, unbalanced charge transport between n- and p-channel are observed when the molecular junctions are fiber devices with Gp,max above 1μS. The black dot in figure 4.20a shows that the Gp,max are larger than the Gn,max, which implies that conductivity of PFETQ is higher in p-channel than that in n-channel. As ambipolar materials, PFETQ exhibits unbalanced transport properties when fiber devices of this material are generated between two electrodes.

134

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

However, Gp,max of PFETQ nanojunction are comparable with that of Gn,max when UMEs are used as substrate (red dot in figure 4.20a). Figure 4.20b shows the same result than that for PFTQ molecular junctions. We proposed that ballistic charge transport through nanojunctions gives a balanced performance in both negative and positive voltage regimes.

4.4 Summary

In this chapter, we have demonstrated that the electrochemical reduction of Rh-Rh complex leads to the electrodeposition of an n-doped polymer. N-type conducting polymer junctions based on Rh-Rh complex chain were generated using SECM configuration. The bias dependence shows that the current in the n-doped regime are due to electron transport through n-channel in the Rh-Rh chains. It is thus possible to create molecular junctions based on Rh-Rh chain molecule that can be easily n-doped. Two kinds of conducting polymer junctions were also investigated by SECM. PETQ and PFETQ molecular junctions exhibit well-defined ambipolar transport properties. However charge transport properties in n- and p- channel of these two polymer junctions are unbalanced when the junctions are in the fiber device scale. When a nanojunction was created, a balanced n- and p- channel transport properties is observed. This evolution is highly reproducible and is observed with both polymers. We propose that such effect is due to charge transport mechanism changing from diffusive (ohm’s law) to ballistic (quantum theory) when the junction size minimizes from fiber devices to nanodevices.

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Appendix

300 -200mV 0

200

-5

/nA

tip

/nA

100 I

tip -10 I

0 -15

-100 -20 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Potential/V. Ag wire Potential/V Ag wire Figure A 4.1. (a) Source-drain current of PFTQ molecular junctions vs gate voltage at 100mV/s scan rate by using 200mV bias in monomer-free solution. Black: tip 1; red: tip 2. (b) Other examples of PFTQ molecular junctions of substrate UME current.

137

Chapter 4 n-type Rh-Rh chain junction and ambipolar type polymer junctions

300 300 250 tip 1 tip 1 tip 2 tip 2 200 200 150 100 100

50 0 Current/nA 0 Current/nA -50 -100 -100 -200 -1.5 -1.0 -0.5 0.0 0.5 1.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential/V(Ag wire) Potential/V(Ag wire)

300 tip 1 60 200 tip 2

100 40 0

-100 20

Current/nA Current/nA -200 0 -300

-400 -1.5 -1.0 -0.5 0.0 0.5 1.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential/V(Ag wire) Potential/V(Ag wire) Figure A 4.2. Source-drain current of PFETQ molecular junctions vs gate voltage at 100mV/s scan rate by using (a-b)200mV and -200mV(c) bias in monomer-free solution. Black: tip 1; red: tip 2. (d-f) Other examples of PFETQ molecular junctions of substrate UME current.

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Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

Molecular junctions which can be tuned in two states with significantly different resistance are of great importance in molecular switching devices. Various conductance switching mechanisms involving an external input have been recently investigated1-3. In this chapter, we describe a way to elaborate plasmonic substrates which could be used as external inputs for switching molecular junctions. Can we use the hot electron, generated by localized surface plasmon resonance, to trigger molecular switching? Electrochemical deposition of AuNPs on indium tin oxide (ITO) appeared suitable and will be further used to investigate plasmonic response behavior of electroactive polymer junctions.

5.1 Introduction

In 1957, Ritchie predicted that free electrons on a metal surface would be excited by external electromagnetic fields, leading to free electron oscillation.4 This phenomenon was experimentally confirmed by Powell5 who measured the electron energy loss of Al foils. Thereafter, surface plasmons have attracted more and more attention from scientists, and this field has seen tremendous development. Plasmonics, related to the collective oscillation of conduction electrons on a metal surface excited by electromagnetic radiation, has wide applications in sensor devices,6 photocatalysis,7-9 medical therapy,10,11 photovoltaic devices and information storage areas.12,13 The plasmonic effect is effective in coupling with adsorbates and could be used in the field of molecular photonic sensor devices.14,15

5.1.1 What is localized surface plasmon resonance (LSPR)?

Irradiation of a noble metal by light induces the collective oscillation of the free electrons. When the frequency of the electromagnetic field is tuned with the oscillation frequency of electrons, a strong resonance occurs, leading to the extinction of light, corresponding to absorption and scattering. In the case of a one-dimensional nanostructure such as a gold nanowire, incident light will excite the free electrons and thus create a propagating surface plasmon or

139

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions surface plasmon polariton (SPP) that can travel along the surface of the metal nanostructure (figure 5.1a). On the contrary, in the case of nanostructures such as nanospheres, electron oscillations are localized at the surface, and are named localized surface plasmons (LSP). When the resonance occurs between electron oscillations and radiation (LSPR), the electric field near the surface of the nanoparticle is strongly enhanced (figure 5.1b).

Figure 5.1 Scheme of the two types of plasmonic nanostructures excited by the electric field of incident light. (a) Propagating plasmon or surface plasmon polariton (SPP) travels along the surface of the metal nanostructure. (b) localized surface plasmon resonance (LSPR) of metal nanoparticles (MNPs) exposed to an electric field. Adapted from Ref.16

The excitation of SP occurs usually with materials with high free-electron mobility such as Ag, Au, Al or Cu. In the case of Ag22, Au23, and Cu24 this resonance energy occurs in the visible region. Gold is the most popular material because of its chemical stability in the ambient environment and its high inertia to oxidation. These two types of surface plasmon resonances (SPRs) are shown in figure 5.1: (a) a surface plasmon which propagates on the continuous metal surface (SPP) and (b) a localized surface plasmon resonance (LSPR) which occurs inside the metal nanostructure. In LSPRs, metal nanoparticles collect and transfer light into localized electric fields and, simultaneously, hot electrons with high energy are generated. The study of LSPR has been the subject of numerous studies and their properties have been widely described in several review papers.16-18 In particular, Jain, El-Sayed et al.16,17,19 have clearly shown that absorption is predominant when the NP diameter is small (between 5 and 50 nm), while light scattering grows significantly for NPs beyond 50 nm. Moreover, comparing different forms of NPs (nanospheres, Au-SiO2 nanocore-shells,

140

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions nanorods), and according to Mie theory, they show that the plasmon resonance is strongly red-shifted in the case of nanorod structures.16,17 Therefore, these nanostructures appear to be well adapted for the realization of biological sensors.18 Another interesting property of the LSP is the high intensity of light scattering near the surface, with cross-sections several orders of magnitude greater than those of very good fluorophores.16,20 This property is particularly important for the spectroscopic detection of traces of chemical compounds in solution, and is the basis of Surface-Enhanced Raman spectroscopy (SERS), developed in the late 70s by Van Duyne et al.21,22

5.1.2 Hot electron decay

Figure 5.2 Schematic of the three decay mechanisms of a LSPR produced with an initial dephasing of 5-100 fs.(1) Elastic re-radiation, hv; (2) Formation of an e-/h+ pair (Landau damping); (3) Chemical interface damping (CID) resulting from absorption of light by an adsorbate or transfer of an electron/hole to the adsorbate. According to Ref.17

Plasmons can be easily activated by light radiation when the photons match the oscillation frequency of surface conduction electrons, submitted to the restoring force of the positively charged surface nuclei.17 Energy is thus collected by the AuNPs. Then, three main plasmon decay modes are likely to occur: (1) elastic radiative re-emission of photons; (2) non-radiative Landau damping, resulting in the generation of energetic electrons and holes in the metal particle within a few femtoseconds; (3) interaction of excited surface plasmons with unpopulated adsorbate acceptor states. This latter process, called chemical interface damping (CID) (figure 5.2), corresponds to direct electron injection into the adsorbate acceptor states.18,20 Generally, process (1) is the radiative decay, and processes (2) and (3) constitute non-radiative decay. Process (1) results in the enhanced scattering effect. In process (2), plasmons decay by converting photon energy to a single electron/hole (e-/h+) pair after initial plasmon excitation.20 These energetic charge carriers will decay by

141

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions electron-electron collisions within a few hundreds of femtoseconds and further electron-phonon interaction within a few picoseconds, resulting in heating of the nanoparticle.23 In process (3), energetic electrons are directly injected into the adsorbate. It is known that at the resonant frequency of LSP, the local electric field enhancement is the largest. This results in the creation of a great number of “charge carriers” (electron-hole pairs) named “hot electrons and hot holes”. They are generated and confined at the surface of the metallic nanoparticle or in its immediate near field. The electrons/holes are considered “hot” because their energies are larger than those of thermal excitations at ambient temperature.24 CID is associated with hot electron injection into the unpopulated state of adsorbates, and results in a reduction reaction. Recently, trapping of surface plasmons (SPs) by an adsorbed molecule has been described as an efficient way for studying charge transfer between nanoparticles and molecules.25 Vadai et al.3 studied the effect of plasmons using a break junction method. They find that conductance enhancements are due to the plasmon oscillating field within the nanoscale metal gap of the molecular junctions. Shin et al.26 claim that plasmon-induced hot electrons from Ag nanoparticles can reduce 4-nitrobenzenethiol (4-NBT) to 4-aminobenzenethiol (4-ABT). Holleitner et al.27 demonstrated that resonant photoconductance is sublinear in laser intensity, which suggests that trap state dynamics of the optically excited charge carriers dominate the optoelectronic response. These investigations, as well as other theoretical simulations,28 imply that plasmons may have a strong effect on the current transport properties of molecular junctions. Plasmons may change the conductive state of a semiconductor. This plasmonic reducing power is frequently reported in the literature29-32 and has been used recently in our group to reduce diazonium salts on an AuNP surface. In this work, we investigate the plasmonic effect on electroactive polymer junctions, which have the particularity of switching between insulating and conducting states when their redox state is changed from neutral to oxidized. The plasmonic nanostructures are Au nanoparticles (AuNPs) which are deposited on a commercial ITO surface by electrochemical reduction of an AuCl3 solution. A gap of a few micrometers between the two electrodes of a Scanning Electrochemical Microscope (SECM), a Pt UME tip (diameter 10 μm) and the AuNPs, is created by running the approach curve in a redox probe solution (1 mM ferrocene). As described in the previous chapters, starting from this SECM configuration, conducting polymer wires are electrochemically generated from the UME

142

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions tip until they bridge the two electrodes. PEDOT and PANI were chosen for fabrication of the junctions because they can be switched easily between conducting and insulating states through oxidation and reduction. To achieve our target, there remain a few questions: Is it possible to generate the CP junctions in their conductive state on the AuNP surface? Can we generate plasmons by irradiating the AuNPs? Is it possible that these plasmons could trigger the switch of the polymer junction?

5.2 Preparing AuNPs on an ITO surface

In the SECM configuration, the second electrode, which faces the tip, is a gold NP on ITO (AuNPs@ITO). These gold NPs were prepared by electroreduction of an aqueous auric solution (2 mM KAuCl4 and 0.25 M Na2CO3) by applying a potential of -0.8 V/SCE at the ITO and delivering a controlled total charge current density of 20 mC cm-2.33 Prior to electrolysis, the ITO was sonicated for 20 minutes in dilute alkaline Extran solution, then carefully rinsed with distilled water and ethanol and stored in ultrapure Milli-Q water. All manipulations were performed under argon atmosphere.

0.20

a 100nm b 0.15

0.10

0.05 Optical Density/ a.u. Density/ Optical

0.00 400 500 600 700 800 900 1000 Wavelength/nm Figure 5.3. (a) SEM image of AuNPs on ITO, (b) Optical extinction spectra of AuNPs on ITO electrodes obtained from the electrochemical reduction of Au3+ at -0.8 V/SCE and with a total current charge density, Q = 20 mC cm-2.

SEM pictures of the AuNPs@ITO (Figure 5.3a) shows that the ITO surface is coated with AuNPs which are randomly distributed, with diameters between 50 nm and 120 nm. This type of AuNPs@ITO substrate exhibits a sharp LSPR maximum at 580 nm (figure 5.3b). This LSPR compares well with those obtained with plasmonic substrates generated by e-beam lithography.34

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Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

100 AuNPs on ITO bare ITO

A 0



-100 Current/ -200

-300 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Potential/V(SCE) Figure 5.4. CVs of bare ITO (red curve) and AuNPs@ITO (black curve) in 0.05 M H2SO4; scan rate: 100 mV s-1.

Interestingly, in H2SO4 solution, the cyclic voltammetric curves of the ITO electrode before and after modification with AuNPs clearly show that the NPs are intimately bound to the ITO substrate (figure 5.4). In this medium, bare ITO is not electroactive between 0 and 1.6 V (red curve), while ITO modified by adsorbed AuNPs leads to a voltammetric curve, characteristic of the formation and reduction of gold oxide (black curve).35

5.3 PEDOT molecular junction switched by a plasmonic effect

The whole set-up consists of a 4-electrode system comprising a classical 3-electrode system used to synthesize the PEDOT junction and control its oxidation state, and a 2-electrode circuit (bias circuit) used to measure the conductance of the junction. The Pt tip of the SECM is used as the working electrode and as one electrode of the bias circuit.

A potential VG is applied to the Pt tip versus an Ag pseudo-reference, considered as a gate electrode, and the electrochemical current is measured between the working electrode and an auxiliary electrode (Pt). The substrate (ITO plate and AuNPs) constitutes the second contact of the bias circuit. This set-up allows the formation of an oxidized PEDOT junction (conductive), obtained by growing PEDOT from the gold tip until it connects with the conductive substrate (AuNPs@ITO). A small bias (100 mV), applied between the tip (similar to a

Source) and the substrate (similar to a Drain), allows to follow the ISD transport current of the junction and its conductance (G = ISD/VSD) when submitted to different inputs (potential and light). Such experiments have been described in chapter 2 and 3. Note that we do not use in this chapter the self-termination method described in chapter 3. ITO was chosen as a substrate because it is an electrically conductive and optical transparent material, which has been widely used in photovoltaic devices,36,37 144

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions Dye-Sensitized Solar Cells (DSSCs)35, Organic Light-Emitting Diodes (OLEDs),38 and other devices, but with no plasmonic resonance in visible light.

5.3.1 Blank test: PEDOT junction on bare ITO or on an Au plate

In order to evidence the role of plasmons on the variations of conductance induced by illumination of the PEDOT junctions, two kinds of junctions were synthesized: one was directly created on the bare ITO electrode (blank test), the second on an AuNPs@ITO substrate. A typical well defined redox-gated PEDOT molecular junction was created on ITO by using the procedure described in 2.2.1 (polarization of the tip at 1.1 V/ SCE for about 20 s in a 10 mM EDOT + 0.1 M TBAPF6/acetonitrile solution). This PEDOT junction can be switched from insulating to conducting through the gate potential (scanned from -0.3 V to 0.6 V). Although all the curves are obtained in the presence of EDOT, it is important to note that in this range of potential, EDOT is not oxidized (PEDOT formation occurs at above 1 V) and the voltammetric curves of figure 5a are related to oxidation/reduction of PEDOT only. Figure 5.5a shows the transport current of a typical PEDOT junction connecting ITO to the SECM tip. The transport current of the junction starts to increase at 0 V/Ag and reaches a maximum of 400 nA at 0.3 V/Ag. The backward curves show a hysteresis induced by structural relaxation of the junction, as described in chapter 2.

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Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

400 a 3 b

300 A

 2

200

/nA

tip I

100 Current/ 1

0 -0.4 -0.2 0.0 0.2 0.4 0.6 0 200 400 600 800 1000 V /V(Ag) g Time/s 3.0 9 8 c 2.5 d 7

6 2.0

A 

5 A



1.5 4

3 1.0 Current/

2 Current/ 1 0.5 0 0.0 0 150 300 450 600 750 900 0 150 300 450 600 750 900 Time/s Time/s

Figure 5.5. (a) CV curves for redox-gated PEDOT junction on AuNPs@ITO. Itip = f(VG (scan rate of 100 mV s-1); (b) Tip/ITO transport current of an oxidized PEDOT junction versus time: bias = 100 mV. White light is switched on/off every 100 s (tip: 0.4 V, ITO: 0.3 V); (c) Same conditions as (b), but light switched on/off every 150 s; (d) ITO replaced by a flat gold substrate and light switched on/off every 150 s. The same bias of 100 mV is applied between the tip (0.6 V) and the substrate (0.5 V). PEDOT molecular junctions are characterized in 10 mM EDOT + 0.1 M TBAPF6/acetonitrile solution. Yellow rectangles correspond to irradiation.

The stability of the nanojunction in its conductive state was examined when it was irradiated or not every 100 s. A fixed VG value of 0.4 V/Ag, which maintains the nanojunction in its conductive state, was applied with a constant VSD bias of 100 mV between tip and substrate. From figure 5.5b, it can be seen that the transport current ISD does not depend on light and is stabile at around 2 µA for more than 15 min. Changing the light switching delay (150 s instead of 100 s) does not qualitatively modify the previous results (figure 5.5c). The PEDOT junction exhibits good time stability, and a conductance plateau can be observed for more than 15 min with a transport current of 6 µA (This value is greater than the previous one and could be due to the fact that the morphology differs from one junction to another.). A similar blank experiment was also performed on a bare gold electrode instead of the previous ITO electrode. As shown in figure 5.5d, the transport current of the junction 146

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions remains stable at 1.7 µA for 900 s in spite of an on/off incident light radiation switched every 150 s. This blank test shows clearly that light has no influence on the conductance of an oxidized PEDOT junction when ITO and bulk gold are used as substrates.

5.3.2 Photo-induced switching of a PEDOT molecular junction connected to an AuNPs@ITO

In this case, instead of flat ITO and gold substrates, we used a modified ITO substrate (AuNPs@ITO) on which Au NPs have been deposited in order to permit the formation of localized plasmons, but also, with help of the SECM, to form and localize the PEDOT junction between the tip and one Au nanoparticle.

1 10 1 a 10 b

0 10 0

A 10



-1

10 10-1

Current/ Current/ 10-2 10-2

-3 10-3 10 0 150 300 450 600 750 900 0 150 300 450 600 750 900 Time/s Time/s Figure 5.6. Current transport in Pt(tip)/PEDOT/AuNPs@ITO junctions. (a) and (b) are two PEDOT junctions switched on/off every 150 s in 10 mM EDOT + 0.1 M TBAPF6/acetonitrile solution; tip: 0.5 V; substrate: 0.4 V. Yellow rectangles correspond to irradiation.

When this PEDOT junction is maintained in its oxidized state (tip potential (VG) at 0.5 V) the transport current under a bias of 100 mV is about 2.5 µA and remains at this value for 150 s until the light is turned on (Figure 5.6a). The effect is immediate: the current in the junction drops abruptly and stabilizes to a background value (a few nano-amperes). Then, after a delay of 150 s, the light is turned off again and, as expected, the transport current across the junction increases to the same value of 2 µA. When the light is switched several times with the same junction ISD is reproducible, but from one junction to another one, the transport current varies, probably resulting from some lack of reproducibility in the morphology of the junctions (In figure 5.6a, ISD = 2.5 µA without light, whereas in another junction (figure 5.6b) ISD = 3.5 µA.).

147

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions Compared with the blank experiment, where the junctions were synthesized on bare ITO or bare gold electrodes, this clearly demonstrates that such transport current switching on PEDOT junctions is related to the AuNPs coated on the electrode surface and, therefore, suggests that it is due to plasmon-induced phenomena. It is important to note that the PEDOT junction has the properties of a conducting polymer and thus must behave like a metal in its conducting state. This is confirmed by the ohmic behavior of the junction when the tip electrode is polarized at 0.4-0.6 V (see figure 5 in chapter 2). The sign of the transport current ISD is reversed when the polarity of the VSD bias is changed from 100 to -100 mV (figure 5.7).

12 0.2 b 10 a 0.0

8 -0.2

A 

6 A -0.4



4 -0.6

Current/ 2 -0.8 Current/

0 -1.0

-2 -1.2 0 150 300 450 600 750 900 0 150 300 450 600 750 900 Time/s Time/s Figure 5.7. Transport current in PEDOT junction submitted to on/off light switching every 150 s in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution. (a) VSD bias 100 mV (tip: 0.5 V, substrate: 0.4 V) (b) VSD bias -100 mV (tip: 0.4 V, substrate: 0.5 V). Yellow rectangles correspond to irradiation. .

The transport current in a junction can be an indication of its structure (nano or bulk) as well as the nature of its connection with the substrate (ITO or AuNP). When nanojunctions are generated, there may be a difference in the photoconductance. The two examples shown in figure 5.8 are an illustration of this diversity. In figure 5.8a, the PEDOT junction has all the characteristics of a nanojunction, as attested by a very low value of the transport current in the dark (25 nA with a bias of 100 mV) which drops abruptly to 0.2 nA when the light is on. Figure 5.8b shows an example with a transport current of 30 nA in the dark: the junction seems at first identical to the nanojunction of figure 5.8a. In fact, it differs significantly, because switching does not change the transport current, which remains roughly constant at about 30 nA for 900 s despite the switching. The two types of switching observed in figures 5.8a and b can be explained by different structures of the PEDOT junctions, and assuming that they consist of many oligomer strands

148

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions connecting the Pt tip of the SECM to the substrate. Two possibilities should be taken into account the PEDOT junction is connected to the AuNPs (figure 5.8c) or to ITO only (figure 5.8d). In the first case, plasmon-induced switching should be observed. This is the most common phenomenon observed in our system. 95% of the junctions behave as in figures 5.8a and c, whereas the behavior illutrated in figure 5.8b and d is rarely observed. In the second case, no switching should be observed, because ITO does not give a plasmonic effect.

102 100 a b

101

100

Current/nA Current/nA

10-1 1 0 150 300 450 600 750 900 0 150 300 450 600 750 900 Time/s Time/s c d

illumination illumination

e------+ + - - + + - + + - - + + + + - + + - + + - + + - + + - - + + - - + + - - + + - - + + - - + + - - + + -

Figure 5.8. PEDOT junctions switched by light every 150 s in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution. Two types of light switching: (a) transport current drops to background value; (b) no current drop during illumination. Yellow rectangles correspond to irradiation.. Two possibilities for irradiation of PEDOT molecular junctions: (c) connection occurs on the surface of an AuNP; (d) connection occurs on ITO between AuNPs.

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Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

a b

A 

Current/ illumination

e------0.1 + + + + + + - + + - - + + - - + + - 0 150 300 450 600 750 900 - + + - - + + - - + + - Time/s

Figure 5.9. (a) PEDOT junctions switched by light every 150 s in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution. (b) Illustration of a junction connected to both Au and ITO.

In contrast with the previous case, the transport current of figure 5.9a appears to be characteristic of a bulk PEDOT junction. In the dark it is much higher (4 µA) than that of the nanojunction of figure 5.8a (25 nA) and after illumination it also decreases, but remains still high at about 1 µA with a conductance ratio of 4 between dark and light; this differs dramatically from that observed in figure 5.8a, which is about 125. This indicates that the reduction of the PEDOT junction by light is poor, and could be due to a mixed junction connecting both ITO and Au, as illustrated in figure 5.9b. These experiments are in good agreement with our assumption that the PEDOT junction in contact with the substrate consists of a few oligomer strands. This transport current switching appears to be plasmon-induced. The most plausible mechanism is the following. When contact occurs on the AuNPs (figures 5.8a and 5.8c) light induces a surface plasmon at the NP, and its decay produces hot electrons which can reduce a small fraction of the PEDOT junction and, therefore, induce an insulating state. On the contrary, when contact is made with the bare ITO (figures 5.8b and 5.8d), far from any Au NP, illumination does not produce any plasmonic effect in ITO and, therefore, the junction remains in its conductive state. The case of figure 5.9a could correspond to a mixed junction, with connections to both AuNP and ITO (figure 5.9b); a strand connected to ITO could explain why the residual transport current under illumination is high.

5.3.3 PEDOT molecular junctions switched by filtered light

150

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions These experiments were carried out with the view to providing further proof concerning the light switching effect and to supporting the hypothesis that the partial reduction of the PEDOT junction is induced by hot electrons resulting from the formation of localized plasmons at the surface of the AuNPs.25,39 Hot electrons, resulting from the plasmon resonance of the surface electrons and generated by illumination of gold nanoparticles, only occur for a specific wavelength range. It is expected that the resonant or non-resonant interaction between photons and surface electrons (production or not of hot electrons) should have a large effect on the redox state of the junction. Thus, by using filtered incident light, the plasmon could be excited or not excited and, therefore, it should be possible to establish whether or not switching is due to the formation of localized plasmons on the AuNPs.

1.2 0.25 blue a orange b 1.0 0.20 black 0.8 0.15

0.6 0.10 0.4

0.05 Transmission

0.2 Density/a.u. Optical 0.00 0.0

400 600 800 1000 400 500 600 700 800 900 1000 Wavelength/nm Wavelength/nm Figure 5.10. Transmission spectra of black, orange, and blue filters and optical extinction spectra of AuNPs on ITO electrodes used for filtered radiation experiments.

Figure 5.10a shows the transmission spectra obtained with three filters (black, orange and blue), used to cut a particular wavelength range. It clearly shows that black and orange filters absorb all the light at wavelengths below 700 nm and 520 nm, respectively, whereas the blue filter absorbs light at wavelengths beyond 600 nm. As the absorption peak of the AuNPs@ITO substrate ranges between 500 and 700 nm with a maximum at 600 nm (figure 5.10b), it is obvious that the use of the black or blue filter will not induce the localized plasmon resonance in contrast to the orange filter. Indeed, when the black and blue filters are used, there is not enough light at the right wavelength to excite the localized surface plasmon resonance (LSPR) on the AuNPs, in contrast to the orange filter which matches the wavelength range for possible excitation of the LSPR.

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Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

103 100 a) white light

102 10

G/nS 1

10 1 Current/nA

100 0.1 0 150 300 450 600 750 900 Time/s

103 100 b) white light + blue filter

10 10

G/nS Current/nA

101 1 0 150 300 450 600 750 900 Time/s c) white light +orange filter 103 100

102 10

G/nS

101 1 Current/nA

100 0.1 0 150 300 450 600 750 900 Time/s

Figure 5.11. Transport current ISD versus time of PEDOT molecular junctions under light switching with different filters (on/off switching every 150 s in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution: tip: 0.5 V; substrate: 0.4 V). (a) white light; (b) blue-filtered light; (c) orange-filtered light. Colored rectangles correspond to irradiation.

Figure 5.11 shows the variations vs. time of the conductance (or transport current) of PEDOT junctions when filtered light is switched on and off. The transport current drops 152

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions abruptly when the junctions are irradiated by white light (figure 5.11a). No significant switching occurs when the wavelength range of the transmitted light is below 550 nm (blue filter, figure 5.11 b). On the contrary, the current drops abruptly again when the junction is irradiated with orange-filtered light, whose wavelength range lies above 520 nm (figure 5.11c). It is worth noting that the on/off switching of charge transport is highly reversible and that the junction recovers its initial conductivity every time the light is switched off. A similar experiment was carried out on the same junction, irradiated or not (on/off switching) through different filters, and polarized with a constant bias of 100 mV (figure 5.12).

102

101

Blue filter Blue

Black filter Black

White light White

Orange filter Orange 

100

10-1 Current/

10-2

10-3 0 200 400 600 800 1000 Time/s Figure 5.12. Transport current vs time of PEDOT molecular junctions under resonant and non-resonant excitation induced by different light filters in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution: tip: 0.6 V; substrate: 0.5 V. Colored rectangles correspond to irradiation.

The first 100 s of the I/t curve correspond to the transport current of the PEDOT junction without irradiation, which gradually stabilizes at 1.2 µA despite a few spikes. During the following 100 s the junction is illuminated with white light, and immediately the current drops to a near-background value (2 nA), indicating the reduction of the PEDOT and its change to an insulating state. Then, in the following 200-300 s, where the light is switched off, the current gradually increases and recovers its initial value after about 80 s. This delay in the transport current recovery may be due to the low rate of the counter-ion doping process.

153

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions Subsequently, the junctions are illuminated through blue, orange and black filters. Clearly, no significant current change appears with blue- and black-filtered light. However, the transport current drops slowly to the background value (2 nA) when the white light is orange-filtered. These latter experiments confirm without ambiguity that PEDOT junctions are switched to an insulating state, only when the resonant plasmonic excitation of an AuNP connected to the junction occurs.

5.3.4 The mechanism of photo-induced switching of PEDOT molecular junctions

Recently, it has been shown that hot carriers (electrons and holes) generated from plasmonic nanostructures could have an important catalytic effect in some reactions. In particular, the use of plasmonic metal-semiconductor nanostructures has been found to enhance the efficiency of solar water-splitting.40-42 As described in part 5.1, among the different plasmon decay mechanisms, the first one corresponds to an elastic radiative re-emission of photons and leads to field enhancement in the gap. This field effect cannot be at the origin of the observed switching, as it should increase the number of charge carriers and increase the conductance. The second one, corresponding to non-radiative Landau damping with formation of electron-hole pairs, occurs independently. It is responsible for a thermal effect but, on single nanoparticles with a diameter of 100 nm and irradiation below 104 W cm-2, the local temperature increase cannot be more than 10 K.23 Thermal effects are therefore not responsible for conductance switching in our system. However, the presence of an additional adsorbate on the surface of the nanoparticles can lead to an ultrafast dephasing pathway on a time scale of ∼5-100 fs. This process, named Chemical Interface Damping (CID),17,43-46 must be taken into account in our system. Indeed, the switching of PEDOT junctions from conductive to insulating states could be attributed to hot electrons undergoing the CID process along with direct injection into unpopulated states of the adsorbate. Hot electrons generated from AuNP plasmons are trapped by the PEDOT, which results in an insulating state corresponding to the neutral form. To make sure that switching is due only to hot electron injection into the junction, it is necessary to rule out another

154

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions processes whereby the junction is possibly broken by hot electrons or by the localized thermal effect. This can be elucidated by studying the conductivity variations of a junction submitted to different gate potentials with or without illumination of the plasmonic nanostructures. The reversibility of the switching process is also proof that the junction is not broken.

5.3.4.1. PEDOT junctions destroyed by illumination ?

Figure 5.13a shows the I/t curve of PEDOT junctions with a constant VSD bias of 100 mV and with incident light switched every 150 s. For a potential of 0.5 V and 0.4 V applied to the tip and substrate, respectively, the transport current switches from around 10 µA to a near-background value when the NPs are in the dark or are illuminated, respectively. This clearly indicates that the junction is not broken by light and that it recovers its initial conducting state in the dark, due to the oxidizing potential of the tip. The fact that the insulating state is observed under illumination can be attributed to electron transfer from the gold to the junction.

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Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

102 a

101

A 

10 Current/ 10-1

10-2 0 150 300 450 600 750 900 Time/s 5 light on b 4 light off



tip 2 I

-0.4 -0.2 0.0 0.2 0.4 0.6 V /V(Ag) g

80 light on c 60

A 40



tip I 20 light off light on

0 light off -0.2 0.0 0.2 0.4 0.6 V /V(Ag) g Figure 5.13. (a) Conductance variations of PEDOT junctions with and without illumination. Light is switched on/off every 150 s in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution;. tip: 0.5 V; substrate: 0.4 V. (b) Conductance variation of the junction vs VG measured from its insulating initial state. VG is scanned between -0.4 and 0.6 V with a VSD bias of 100 mV. Black line: light on; red line: light off. (c) ISD as a function of the gate voltage, using -100 -1 mV bias and 5 mV s scan rate in monomer-free 0.1 M TBAPF6/acetonitrile solution. Forward scan: light on at 0.3 V, off at 0.5 V. Backward scan: light on at 0.5 V, off at 0.4 V.

156

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions Another test to prove the stability of the junction under repeated light switching was carried out by scanning the tip voltage. As can be seen in figure 5.13b, the transport current is approximately nil when the junction is illuminated and the tip potential is scanned from -0.4 V to 0.6 V (black line). On the contrary, with the same range of potentials but without light, the junction gradually shifts to the conducting state as VG increases (red line), and reaches the highest transport current of 4 μA at 0.6 V. All these experiments clearly confirm that the PEDOT junction is stable; it does not break when illuminated but only changes its conductivity.

Figure 5.13c shows the ISD current (ITip) with 100 mV bias (VSD) of another PEDOT junction vs. gate potential VG. In the forward scan, the transport current starts to increase at 0.27 V and abruptly drops at 0.3 V when the light is switched on. Then, the light is switched off at 0.5 V, and after a few seconds the PEDOT starts to be oxidized and returns to the fully conducting state, with the transport current at its highest value (70 µA at 0.6 V/Ag). During the backward scan, the light is switched on from 0.5 V/Ag to 0.4 V/Ag and the transport current immediately falls to the low value. Gradually, PEDOT returns to its conducting state after it is switched off. The assumption that the junction breaks and reconnects could indeed explain how the variation of tip current is suppressed. However, since there is no monomer in the medium, self-healing of the junction is impossible. Besides, self-healing at this applied potential (below 0.6 V) is impossible. The conclusion that the junction is not broken agrees also with the fact that, when it is connected to bare ITO (figure 5.8b), light has no effect on its conductive state. Moreover, current recovery to the same level every time the radiation is removed is a further proof that the junction is stable (figures 5.6 and 5.11). Thus, the assumption that the junction is broken by light and reconnected with the same number of molecular strands after light is removed is highly unlikely. We can also conclude that light switching does not change the number of strands involved in the junctions.

5.3.4.2 Conductivity variations of a PEDOT junction vs VG and alternately submitted to illumination

It has been reported that oxidized PEDOT exhibits well defined quasi-metallic 47,48 49 behavior. The Fermi level of PEDOT:PSS is 5 - 5.2 eV below the vacuum level EVL

157

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions (Scheme 5.1a) and corresponds to the energy level of the polaron (P+), which is aligned with the Fermi level of Au.50 The Work Function (WF) of gold is generally assumed to be 4.5 ± 0.1 eV in ambient atmosphere rather than 5.2 eV (in vacuum).51 Thus, as shown in scheme 5.1b, when light irradiates the junction, the plasmonic gold NPs provide an electron + excess at the EF level; electron transfer from gold toward P will occur easily through direct resonant tunneling between the hot electron level and the P+ state of PEDOT, and therefore will induce the reduction of a small part of the polymer in contact with gold. When the PEDOT is reduced, its Fermi level rises to the middle of the HOMO-LUMO gap, and then electron transfer is stopped (scheme 5.1c).

a In dark b Under light c Under light

EVAC EVAC EVAC

5~5.2 eV 5~5.2 eV

4.5 eV 4.5 eV 4.5 eV LUMO LUMO LUMO P- P- + 1.2~1.7 eV eV P e- P+ 1.2~1.7 EF EF 1.2~1.7 eV + h EF HOMO HOMO

HOMO AuNPs oxidized AuNPs oxidized AuNPs PEDOT PEDOT Reduced PEDOT Scheme 5.1 Energy alignment of gold nanoparticle with PEDOT (a) in dark and under light (b) upon and (c) after hot electron injection into PEDOT.

However, no metallic behavior is observed in reduced PEDOT. Therefore, the reducing power of hot electrons may have no effect on the insulating PEDOT. To confirm this, we measured the conductivity variations of a PEDOT junction vs VG when it was submitted to on/off illumination. Figure 5.14 shows the I/t curve of the junction (already shown in figure 5.13).

However, various fixed VG potentials with a constant VSD bias of 100 mV are used in figure 5.14. Similar effects are observed as in the previous illumination experiments, when the junctions were submitted to smaller fixed tip potentials (VG) leading to slight oxidation of the PEDOT. As depicted in figure 5.14a, with a gate potential of 0.3 V applied to the tip (and 0.2 V at the substrate) PEDOT is only slightly oxidized, and this induces a less conductive state which gives a current of 1.6 µA, much lower than that observed at 0.6 V (≈ 10 µA, figure 5.13a). Besides, when the light is switched on and

158

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions off, the transport current oscillates between 0 and 1.6 µA, confirming again the stability of the junction as a switching material.

1 Tip 0.3V 10 a 4 light on Substrate 0.2V b light off

100 3



-1 / 2

10 tip I

Current/ 1 10-2 0

-3 10 -0.4 -0.2 0.0 0.2 0.4 0.6 0 150 300 450 600 750 900 V /V(Ag) Time/s g

Tip -0.2V 4 light on c d -2 Substrate -0.3V 10 light off

A 

-3 A



10 / 2

tip I

Current/ 1 10-4

0 10-5 0 150 300 450 600 750 900 -0.4 -0.2 0.0 0.2 0.4 0.6 Time/s V /V(Ag) g Figure 5.14. Conductance variations of polarized PEDOT junctions with and without illumination in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution. Light is switched on/off every 150 s and the junction goes from insulating to conducting, respectively. (a) tip at 0.3 V and (c) -0.3 V. (b ) and (d): Conductance variation of the junction vs VG measured from its insulating initial state. VG is scanned between -0.4 and 0.6 V with a VSD bias of 100 mV. Black line: light on; red line: light off. Yellow rectangles correspond to irradiation.

Interestingly, when the tip is maintained at -0.2 V (and substrate at -0.3 V), a potential corresponding to the fully reduced state of PEDOT, the transport current is near zero and light has no effect on the conductivity (figure 5.14c). This agrees with the fact that illumination produces an additional reduction by hot electrons, which contributes to maintaining the junction in its insulating state.

When VG is scanned between -0.4 and 0.6 V with or without light (figures 5.14 b and d), similar conductance variations are found as previously (figure 5.13b). When

159

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions illuminated, the junctions are switched to the insulating state, and there is no transport current but only electrochemical current on the tip (far from the nanoparticle) (black line in figures 5.14b and d). When the light is switched off transport current is observed again (red line in figures 5.14 b and d). This phenomenon will be further discussed in part 5.3.4.3.

In conclusion, as reflected in all later experiments, the transport current ISD of a PEDOT junction remains at a rather low value under illumination despite the oxidative gate potential applied to the tip. Then, when the light is off, ISD returns to a high value, characteristic of the oxidized state. All these results argue in favor of a very stable junction and confirm the fact that the plasmons induced by illumination of the AuNPs generate hot electrons capable of partially reducing the part of the junction in contact with gold. The junction does not break when illuminated, and its insulating state results from the reduction of the part of the junction in contact with one AuNP. This latter result means also that, under our conditions of illumination (intensity 260 W/m2), the reduction of oxidized PEDOT by hot electrons is faster than its electrochemical re-oxidation at a gate potential of 0.6 V.

5.3.4.3. Interpretation of the switching mechanism

Figure 5.15 Schematic drawing of the mechanism of PEDOT junction switching by plasmonic hot electrons.

The PEDOT junction between the tip and the substrate fills a gap of about 10 μm. As illustrated in figure 5.15, most of the PEDOT connected to the tip probably remains in the conducting state upon illumination, whereas the small part of the PEDOT which is connected to the AuNP is reduced to the insulating state. The reduction of such a small 160

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions part of the polymer by hot electrons is enough to determine charge transport through the junction. It constitutes a barrier which switches the transport current to a value close to zero. One way to confirm this point is to examine the electroactivity of the junction when it is illuminated. The I(V) characteristics are examined by using a 100 mV bias and scanning -1 the tip potential (VG) at 100 mV s (It must be recalled that ITip is the addition of

IElectrochemical + ISD).

30 80 a b 20 60

/nA

tip

tip I I 20 0 0 -10 -20 -20 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 V /V(Ag) V /V(Ag) g g

Figure 5.16. Characterization of PEDOT junction on AuNPs/ITO in 0.1 M TBAPF6 + 10 mM EDOT/acetonitrile solution. Scan rate: 100 mV s-1; bias: 100 mV. a) white light irradiation; b) shutting off the light.

As depicted in figure 5.16a, no transport current occurs under irradiation, and there is only PEDOT redox electrochemical current, with a significant oxidation peak at 0.25 V. Clearly, the electrochemical curve of the junction (5 reproducible cycles) shows that PEDOT is successively oxidized and reduced according to the potential applied to the tip, but no transport current flows through the entire junction. This apparent contradiction implies that there is a small part of the junction in contact with the substrate which remains in the neutral state and is, therefore, insulating.

Then, when the light is cut off, transport current ISD is again observed and ITip stabilizes at 80 nA at 0.6 V (figure 5.16b). These results are in good agreement with the assumption (illustrated in figure 5.15) that a small fraction of the junction in contact with the AuNPs@ITO surface is reduced by hot electrons and is enough to switch the whole junction to the insulating state. The light intensity has also a strong impact on the excitation of plasmons and the production of hot electrons on the NP surface,17,52,53 and may affect the reduction rate of oxidized junctions. The experiment of figure 5.16 was repeated, but with various light

161

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions intensities ranging from 4 W/m2 to 260 W/m2 (figure 5.17). These experiments show that PEDOT junctions are switched to the insulating state when the light has a minimal intensity 2 of 260 W/m , for which the maximum ITip intensity is limited to 200 nA. On the contrary, 2 under ambient illumination or under an irradiation of 4 W/m , ITip reaches a maximum value of 1200 nA, which indicates that the whole junction is conductive. This also means that the flux of hot electrons is too weak to induce partial reduction of the oxidized PEDOT. A mixed situation occurs with an irradiation of 220 W/m2, where there are some oscillations of ITip at 600 and 800 nA, indicating that the hot electron flux is not high enough to compensate the oxidation occurring at the tip electrode at the same time. The quite good reproducibility of these experiments can be appreciated by comparing the curves in figures 5.17a and 5.17e.

162

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

300 1000 2 (a)260W/m 2 200 800 (b)220W/m

100 600

/nA /nA 400

tip 0

tip

I I 200 -100 0 -200 -200 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 V /V(Ag) g V /V(Ag) g 1600 1600 2 (c)120W/m2 (d)4W/m 1200 1200

800 800

/nA

tip

I I 400 400

0 0

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 V /V(Ag) V /V(Ag) g g

300 0.25 (e)260W/m2 0.20 f 200 0.15

100 0.10

/nA tip I 0 0.05 0.00

-100 OpticalDensity/a.u. -0.05

-200 -0.10 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 400 500 600 700 800 900 1000 V /V(Ag) Wavelength/nm g Figure 5.17. Successive characterizations of PEDOT junctions on AuNPs/ITO with different incident light intensities: (a) 260 W/m2 (b) 220 W/m2 (c) 120 W/m2 (d) 4 W/m2 (e) 260 W/m2. f) Optical extinction spectra of light through AuNPs/ITO. Scan rate: 0.1 V s-1; bias: 100 mV.

In conclusion, molecular switching is attributed to the direct injection of plasmon relaxation-induced hot electrons from AuNPs to PEDOT by resonant tunneling. These results suggest that the switching of PEDOT junctions by the plasmonic effect depends on the flux of photons which irradiate the AuNPs, a fact which seems to indicate that switching between insulating and conductive states results from a competition between the rate of 163

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions production of hot electrons, the rate of plasmon-induced hot electron injection and the rate of the electrochemical oxidation of the PEDOT.

5.4 PANI molecular junctions switched by plasmonic effect

In order to prove that plasmon-induced conductance switching is a general property in conducting polymer junctions, PANI junctions were selected for further investigation. As previously, a blank experiment was carried out on a bare ITO electrode as substrate. Pt/PANI/AuNPs@ITO molecular junctions were fabricated by the chronoamperometric technique, as described previously (PANI was polarized in the conducting state in an aqueous solution of 0.5 M ANI and 2 M H2SO4). Figure 5.18a shows the current vs. time response of PANI junctions in the presence of aniline, but at potentials (0.4 V) where no polymerization can occur. As with a PEDOT junction and for a period of 10 min, the transport current is stable at around 500 nA, regardless of the illumination (on and off every 100 s) of the junction. This establishes that with an ITO substrate light has no effect on the conductivity of the PANI junction.

0.8 a b 600

0.6 A 400 

0.4

Current/nA Current/ 200 0.2

0 0.0 0 200 400 600 0 150 300 450 600 750 900 Time/s Time/s Figure 5.18. Tip current vs time curves of PANI junction maintained in oxidized state and irradiated by white light in 0.5 M aniline + 2 M H2SO4/water. Bias 100 mV; tip: 0.4 V; substrate: 0.3 V. (a) Irradiation on Pt(tip)/PANI/ITO junctions (blank test): switched on/off every 100 s. (b) Irradiation on Pt(tip)/PANI/AuNPs@ITO junctions: switched on/off every 150 s. Yellow rectangles correspond to irradiation.

A drastic change is observed when the PANI junction is connected to an AuNP adsorbed on the ITO substrate (figure 5.18b). In its initial oxidized state (tip potential at 0.4 V/SCE and substrate at 0.3 V/SCE), the transport current is stabilized at 0.6 µA. Then (after 150 s without illumination) the light is switched on and, as with the PEDOT junction, the PANI junction current drops drastically to 0.2 µA. The process can be reversed and the junction recovers partially its maximum conductance when the light is 164

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions off (figure 5.18b). However, we note that the recovery of the initial conductance is slower than with PEDOT. Besides, the conductance of PANI junctions is not switched to a background value, which evidently indicates that the PANI junction is not broken during the irradiation. In order to confirm that the switching properties of PANI junctions are due to the same mechanism as with PEDOT, namely, hot electrons plasmonically generated from gold nanoparticles, PANI junctions were illuminated with the same light filters as in the previous experiments.

a) white light b) white light+blue filter 300 300

200 200

Current/nA Current/nA 100 100

0 0 0 300 600 900 0 300 600 900 Time/s Time/s

c)white light+orange filter d) white light +black filter 300 300

200

Current/nA 150

Current/nA 100

0 0 0 300 600 900 0 300 600 900 Time/s Time/s Figure 5.19. Transport current versus time curves of PANI molecular junctions. The colored parts correspond to illumination with (a) white light; (b) blue-filtered light; (c) orange-filtered light; (d) black-filtered light. Switched on/off every 150 s in 0.5 M aniline + 2 M H2SO4/water: tip: 0.4 V; substrate: 0.3 V.

As can be seen in figures 5.19a and 5.19c, the same drastic current decrease is observed when the PANI junctions are irradiated by either white or orange-filtered light. The conductance is partially recovered when the light is off, and the switching process appears to be quite reproducible. In figure 5.19b, no significant switching is observed when the cell is illuminated with a blue filter, a result which agrees with the fact that plasmons are not be excited by the light transmitted by this filter (λ below 500 nm). Only a small switching effect was observed

165

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions when the black filter was used (figure 5.19d), indicating that the filtered light (λ > 700 nm) can still partially match the plasmon resonance of gold NPs, which ranges between 550 and 700 nm. In conclusion, all these results clearly indicate that the switching of PANI junctions can be attributed to plasmon-induced hot electrons, which operate in the same manner as with PEDOT.

40 80 a b 30 60

20 40 10

Current/nA Current/nA 0 0 -10 -20 -20 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 V/V (SCE) V /V (SCE) g g Figure 5.20. Characterization of a PANI junction on AuNPs/ITO with scan rate: 0.1 V s-1; bias: 100 mV. a) white light irradiation; b) shutting off the light after the second cycle.

In order to confirm the stability of the PANI junction and to exclude its breaking during illumination, it was submitted to cyclic polarization while a bias of 100 mV was maintained between the tip and the substrate, and VG was scanned from -0.2 V to 0.4 V at a rate of 100 mV s-1. Figure 5.20a shows the tip current when the substrate is illuminated. The electrochemical redox I/V curve is typical of PANI and there is no transport current. Clearly, this means that the polymer strand consists of two different parts: one part in contact with the tip, which can be oxidized and reduced electrochemically, and another part in contact with the substrate, which is reduced by the hot electrons generated by the light and which remains insulating, and therefore makes the transport current through the junction close to zero. On the contrary, the same experiment carried out with the same junctions when the light is switched off shows that the transport current gradually increases and stabilizes at 80 nA at 0.4 V (figure 5.20b), in contrast to the dominant redox current peak current observed in figure 5.20a. These results indicate that with PANI, as with PEDOT, only the part of the filament connected to the top of the AuNPs@ITO surface is reduced to the insulating state during illumination, as depicted in scheme 5.2.

166

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions

RE CE RE CE

Reduced Oxidized PEDOT/PANI PEDOT/PANI Insulating Conducting

RE CE RE CE

oxidized Reduced Conducting PEDOT/PANI PEDOT/PANI Insulating radiation radiation Insulating

- - - e - - - - e- + + + + + + ------+ + - - + + - - + + - + + + + + + + + - + - - + + - - + + - - + + - - + - - + - + - + + - - + + - - + + -

Scheme 5.2 Illustration of the mechanism of PEDOT or PANI molecular junction switching by plasmonic hot electrons.

5.5 Summary

Microscopic PEDOT and PANI junctions were electrochemically synthesized between the tip of an SECM and AuNPs adsorbed on ITO. The AuNPs are between 50 and 120 nm in diameter and are characterized by plasmonic resonances centered at 600 nm with a Full-Width Half-Maximum (FWHM) of about 100 nm. All these polymer junctions exhibit highly reproducible behavior when they are submitted to redox variations as well as various periods of light and dark. Under the resonant excitation of the AuNPs, PEDOT and PANI junctions switch to an insulating state, which results from the reduction to the neutral state of a small part of the junction in contact with the AuNPs and leads to a negligible transport current. This transport current switching is due to the plasmonic effect generated on the Au NPs, and can be attributed to the injection of hot electrons. To our knowledge, this is the first time that conductance-reducing phenomena induced by a plasmonic effect have been observed in an electroactive polymer junction. This work may provide a new understanding of the plasmonic effect on the transport current through molecular devices. 167

Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions Moreover, it must be underlined that these results were obtained with AuNPs of great dispersivity, very easily synthesized.

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Chapter 5 Plasmon-induced conductance switching in conducting polymer junctions 37 Kymakis, E. & Amaratunga, G. Single-wall carbon nanotube/conjugated polymer photovoltaic devices. Appl. Phys. Lett. 80, 112-114 (2002). 38 Nguyen, T. P., Le Rendu, P., Dinh, N., Fourmigue, M. & Meziere, C. Thermal and chemical treatment of ITO substrates for improvement of OLED performance. Synthetic Metals 138, 229-232 (2003). 39 Manjavacas, A., Liu, J. G., Kulkarni, V. & Nordlander, P. Plasmon-induced hot carriers in metallic nanoparticles. ACS nano 8, 7630-7638 (2014). 40 Zhang, P., Wang, T. & Gong, J. Mechanistic understanding of the plasmonic enhancement for solar water splitting. Adv. Mater. 27, 5328-5342 (2015). 41 Zhang, L., Herrmann, L. O. & Baumberg, J. J. Size dependent plasmonic effect on BiVO4 photoanodes for solar water splitting. Scientific reports 5 (2015). 42 Dutta, S. K., Mehetor, S. K. & Pradhan, N. Metal semiconductor heterostructures for photocatalytic conversion of light energy. The journal of physical chemistry letters 6, 936-944 (2015). 43 Hendrich, C. et al. Chemical interface damping of surface plasmon excitation in metal nanoparticles: a study by persistent spectral hole burning. Applied Physics B 76, 869-875 (2003). 44 Persson, B. Polarizability of small spherical metal particles: influence of the matrix environment. Surface Science 281, 153-162 (1993). 45 Charlé, K. P., Frank, F. & Schulze, W. The optical properties of silver microcrystallites in dependence on size and the influence of the matrix environment. Berichte der Bunsengesellschaft für physikalische Chemie 88, 350-354 (1984). 46 Chen, X.-J., Cabello, G., Wu, D.-Y. & Tian, Z.-Q. Surface-enhanced Raman spectroscopy toward application in plasmonic photocatalysis on metal nanostructures. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 21, 54-80 (2014). 47 Randriamahazaka, H., Noël, V. & Chevrot, C. Nucleation and growth of poly (3, 4-ethylenedioxythiophene) in acetonitrile on platinum under potentiostatic conditions. Journal of Electroanalytical Chemistry 472, 103-111 (1999). 48 Fahlman, M. et al. Electronic structure of hybrid interfaces for polymer-based electronics. Journal of Physics: Condensed Matter 19, 183202 (2007). 49 Shrotriya, V., Li, G., Yao, Y., Chu, C.-W. & Yang, Y. Transition metal oxides as the buffer layer for polymer photovoltaic cells. Applied Physics Letters 88, 073508 (2006). 50 Braun, S., Salaneck, W. R. & Fahlman, M. Energy‐level alignment at organic/metal and organic/organic interfaces. Advanced Materials 21, 1450-1472 (2009). 51 Osikowicz, W. et al. Energetics at Au top and bottom contacts on conjugated polymers. Applied physics letters 88, 193504 (2006). 52 Christopher, P., Xin, H. & Linic, S. Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures. Nature chemistry 3, 467-472 (2011). 53 Govorov, A. O. & Richardson, H. H. Generating heat with metal nanoparticles. Nano today 2, 30-38 (2007).

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Chapter 6 Protect and Switch Copper Atomic Contact

In this chapter, we will focus on the investigation of copper atomic contacts. A copper filament is generated by SECM using the electrochemical method. Atomic contacts are sensitive to their chemical environment due to adsorption. Remarkable differences are observed between atomic contacts generated in water and in the presence of added molecules. Therefore, it is necessary to coat contacts permanently to avoid nonspecific adsorption. Can we find ways to protect copper atomic contacts? This is one of the main goals in this chapter. Atomic switch systems behave with two distinct conductive states, and are considered as a promising non-volatile memory devices for the future. Here, two kinds of atomic switch devices, namely destroy-reconformation-based electrochemically triggered switches and externally switchable molecular-induced switches, will be discussed. The switching of a copper atomic contact is obtained via an electrochemical reaction, which causes a change in resistance. Can we build atomic switches whose conductance changes are detectable? Can we build atomic switches using external inputs, such as a redox-gated conducting polymers? This is the second goal of this chapter.

6.1 Introduction

Important advances in the fabrication of nanostructured materials and devices have been achieved during the last decade. Future integrated chips and nanoelectronic systems could change from a planar array to a vertical configuration in order to increase device density.1,2 In this context, generating and studying electron transport in metallic nanowires is an important topic in nanotechnology, and has attracted much attention.3-6 This interest is driven by the fact that these systems display several exciting physical phenomena,7-11 and may also be used in the next generation of two-terminal memory devices.12-18 Metallic nanowires presenting quantum point contact properties exhibit two striking features, namely their atomic-scale dimensions and quantum electron transport. Indeed, when the cross-section of a metal wire is reduced to a few atoms, its diameter becomes comparable to the Fermi wavelength of the electrons, and quantum effects govern the electron transport properties.7 Moreover, if the length of the wire is smaller than the mean free path of the electron, transport is ballistic within this part of the nanowire. Its conductance G is then

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Chapter 6 Protect and Switch Copper Atomic Contact quantized and expressed by the Landauer formula,19,20 where e is the electron charge, h is 2 Planck's constant, G0 is the conductance quantum: G0 = 2e /h ≈ 77 µS, Ti is the transmission probability of each conductive channel and N is the number of parallel channels. 2e2 N G  Ti h i1 The transmission probability of each channel depends on the chemical valency of the metal,21 on the precise atomic arrangement at the constriction and the environment of the contact.22 For several metals at room temperature (gold, copper, silver), it is close to unity.23,24 As a consequence, their conductances often vary in stepwise multiples of the conductance quantum. Previous work has also shown that the conductance of an atomic contact can be modified by its chemical environment, by local roughness or by impurities. Indeed, when molecules adsorb on and/or near an atomic contact they act as individual local 22 scatterers, and Ti decreases significantly below 1, so each quantum step is not exactly an 25-31 integer value of G0, and a conductance below 1 G0 can be observed. A transmission probability of 1 is found only in the absence of defects in the region close to the central atom. Advances in experimental techniques have made it possible to investigate the transport properties of metallic atomic contacts.8 Experimentally, two different approaches are available for the fabrication of such systems: mechanically controlled deformation of thin metallic junctions,8, 32-36 and electrochemical techniques.37-39 From an experimental point of view, despite a few Scanning Transmission Electron Microscopy (TEM) visualizations of atomic point contacts,3, 40 evidence for contact involving a single metal atom or a few atoms is based essentially on transport measurement and statistical analysis of a large number of events. Additionally, the I(V) characteristics of contacts showing ohmic behavior have been used to demonstrate their metallic character.41 Metallic nanowires are easily generated electrochemically.41-45 A gap between two electrodes is filled with metals by means of an electrochemical process until contact is reached. The electrochemical route has several advantages: control of the applied potential, thanks to the use of a reference electrode; absence of mechanical deformation; the possibility of planar or vertical configuration; various possible metal electrode materials. Recently, scanning electrochemical microscopy, SECM, which combines an electrochemical process with mechanical movement of the SECM tip, has been

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Chapter 6 Protect and Switch Copper Atomic Contact successfully employed for creating metallic copper nanowires exhibiting the conductance quantum.46 The SECM set-up has also proved useful for studying small numbers of molecules in solution trapped between the tip and the substrate,47-49 and provides a vertical configuration where the two electrodes are located face-to-face and separated by a micrometric gap.50 In the present chapter, metallic Cu nanowires were generated between a Pt microelectrode and an Indium Tin Oxide (ITO) electrode using the SECM configuration. ITO is an electrically conductive and optically transparent material which is widely used in organic light-emitting diodes and photovoltaic devices. To the best of our knowledge, nanowires with quantized conductance, bridging a metallic electrode and a transparent semiconducting electrode such as ITO, have not yet been reported. Another type of ITO electrode, covered by a highly ordered mesoporous silica film with an unusual vertical orientation of small pore channels (diameter, 3 nm),51, 52 was also used as the substrate for atomic contact formation. Vertically-aligned and ordered mesoporous silica thin films are not easy to prepare,53 but they can be generated on ITO electrodes, as closely packed mesochannels of hexagonal geometry, by electro-assisted self-assembly.54, 55 These oriented and nanostructured substrates have perfect molecular sieving properties56 and are promising hard templates for nanocasting.57, 58 Here, we wish to generate, through such a nanomembrane, encapsulated and protected metallic nanowires with quantum conductance, and in a further step to separate parallel nanowires for crossbar circuitry in next-generation memory devices.

6.2 Atomic contact generated by self-terminated methodology

Copper atomic contacts are manufactured according to the so-called self-terminated method, described in chapter 3. Tao et al.30 have shown experimentally that metal is dissolved at the anode, and that metal ions are reduced mainly on the points or asperities of the cathode. This phenomenon results in a copper filament growing and filling the gap. A contact with conductance near a multiple of G0 is fabricated. Figure 6.1 shows the electrical circuit used for atomic contact fabrication in a 2-electrode set-up (figure 6.1a) and a 4-electrode set-up (figure 6.1b).20,34 The voltage applied between the two electrodes, where the copper is deposited, is calculated by the formula:

Vgap = V0 ×Rgap / (Rgap + Rext),

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Chapter 6 Protect and Switch Copper Atomic Contact

Initially, the distance between the electrodes is about twenty microns. The current is due to electrochemical reactions taking place at the electrodes, and the resistance between the two electrodes, Rgap, is determined by ionic conduction and depends on the size of the electrodes.

At the beginning, the Rgap resistance is much higher than Rext; therefore, the potential applied to the gap (Vgap) is practically equal to V0. The oxidation and reduction processes at the electrodes are then at their fastest. When the distance between the electrodes is no more than a few nanometers, a tunnel current is added to the electrochemical current, and the resistance between the electrodes decreases. Finally, when contact occurs, the transport current increases abruptly, because of the sudden decrease in Rgap. This then results in a lower potential Vgap and, therefore, terminates the electrochemical processes.

a b 10kΩ WE 1 V gap anode cathode Vext Pt RE CE

R Bias V R gap ext 0

Cu WE 2 Figure 6.1 schematic drawing of manufacture of copper atomic contacts by (a) Keithley 6487 set-up (2-electrode) and (b) SECM set up (4-electrode)

Despite the scale of the two copper electrodes, the contact point of the copper filament can be on the atomic scale if the external resistor is well chosen. If Rext is too big, then electrochemical growth stops before contact occurs. If Rext is too small, after contact occurs the electrochemical process is not fully stopped and the wire continues to grow. If Rext is around 10 KΩ, then electrochemical growth can be stopped just after contact and an atomic point contact can be achieved. Conclusions are based on a statistical analysis of a set of results. The curves in this chapter are examples from among the numerous experiments carried out, but they are selected to illustrate the typical behavior frequently obtained during the formation of atomic contacts.

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1.5 54 3 70 a b

1.0 44 2 61



G/G G/G 1 44

0.5 28

Current/ Current/

0 0 0.0 0 200 250 300 0 100 200 300 Time/s Time/s

400

200 Counts

0 0 1 2 3 4 5 6 G/G 0 Figure 6.2 (a) (b)Conductance vs. time plot of copper filaments growth in milliQ water in a SECM (4 electrodes) configuration where Cu electrode used as substrate and Pt UME used as tip.(c) Conductance histogram. of Cu nanowires generated by SECM between Pt UME and Cu substrate.

Figure 6.2 shows two examples of atomic contact generated between Pt UME and Cu substrate by a SECM set up. A positive potential (0.5V/Ag) was applied to the Cu substrate and a negative potential (-0.5V/Ag) was applied to the Pt UME. Metal atoms are etched from the anode and dissolved into the electrolyte as metal ions followed by deposition on the cathode. The gap is initially large (20μm) with Rgap >>Rext, Vext is thus equal to the applied voltage V0. The current in the circuit is limited by the electrochemical processes and the transport of ionic species to the electrodes. At t = 215s (figure 6.2a), the current suddenly increases and finally stabilize at a value of about 45μA, which indicates that both electrodes were connected by the copper filament. The curve representing the current versus time is then converted into curve conductance versus time. The current value of

45μA is corresponding to 1G0, and 60μA refers to 2G0. Figure 6.2b shows another example of atomic contact. After the electrochemical processes, a brief plateau at 45μA is observed at t=10s. The current increases sharply again to about 60 μA at t=220s, then remains relatively stable for 150s.

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Chapter 6 Protect and Switch Copper Atomic Contact

As shown in figure 6.2c, we have constructed a histogram displaying the results of about 100 conductance versus time curves. The histogram shows that the conductance located at around peaks at integral value of G0. Overall, the majority of the Cu nanowires, generated on Cu substrate, have a conductance of expected integer value of the conductance quantum.

6.3 Atomic contact protected by mesoporous silica on ITO

10kΩ WE 1 10kΩ b a WE 1 Pt RE CE Pt RE CE Cu Bias V0

2+ Cu Bias V0 Cu Cu2+

ITO

ITO substrate ITO substrate WE 2 WE 2

Figure 6.3 Atomic contact generated using SECM configuration. Simplified schematic drawing of SECM set-up for the formation of copper filaments from 10-2 M CuSO4 solution between Pt UME and (a) ITO substrate, (b) ITO/nanopores.

Figure 6.3 summarizes the SECM configuration used and the principle of Cu nanowire formation on ITO and ITO/nanopores. Following that, the electron transport properties of the nanowires were investigated. High-resolution electron micrographs of the silica nanopores after Cu growth were obtained and, finally, the protective effect of the nanopores was studied and atomic switch behaviour demonstrated.

6.3.1 Metallic Cu nanowire on ITO substrate

The principle and the procedure for the fabrication of copper nanofilaments are shown in figure 6.3. Briefly, a platinum UME (radius, 5 µm) was positioned above the ITO substrate by recording the classical approach curve in a 10-3 M ferrocenemethanol electrolyte solution in water. The approach (tip approaching the substrate using the piezo element) was deliberately stopped before contact, leading to a tip-substrate separation about the same as the tip radius, 5 µm. A copper filament was then electrochemically -2 generated on the Pt UME (cathode) from a 10 M solution of CuSO4 (copper source) in 176

Chapter 6 Protect and Switch Copper Atomic Contact water and finally, contact was made with the ITO substrate (anode). A bias voltage of 1 V between the tip and the substrate was applied, and the UME was connected to an external resistance of 10 kΩ, causing the electrochemical process to self-terminate after contact.34 Figure 6.4a displays, as a typical example, the time dependence of the current and the normalized conductance (G/G0), recorded at the two electrodes, during Cu deposition and after contact.

1.5 54 6000 1.0 a 44 b

0.5 28 A  0 4000

0.0 0 G/G

0.5 -28 Counts Current/ 2000

1.0 -44

1.5 -54 0 0 200 400 600 800 1000 1 2 3 4 G/G Time/s 0 Figure 6.4 Conductance of atomic copper contact on ITO. a) Current and conductance vs. time curves during Cu electrodeposition and after generation of filament between Pt -2 UME tip and ITO substrate in presence of 10 M CuSO4 with 10 kΩ resistance connected to Pt tip. red: ITO; black: Pt UME. Tip: -0.9 V, ITO: 0.1 V vs. SCE. b) Conductance histogram. During the first 150 s, a very small initial current, in the nA range, due to ionic conduction and electrochemical reactions, is observed. This is followed by a sudden current jump which occurs when a nanofilament has bridged the two electrodes. After this, the electrochemical deposition process terminates itself thanks to the control by the external resistance, and the current in both electrodes stabilizes at several µA, corresponding to that between the tip and the ITO substrate through the copper nanowire. The formation of copper filaments bridging the Pt UME and the ITO electrode is supported by the symmetric

I-T curves, with Itip = −IITO, the currents recorded on the tip (red line) and on the ITO (black line) electrodes, although their areas are different. Knowing the bias applied to the circuit, it is possible to measure the conductance of the nanowire between the two electrodes. In figure 6.4a, shown as an example, the conductance of the nanowire stabilizes at a value close to 0.7 G0 for 300 s then jumps to 1 G0, corresponding to the conductance quantum, for more than 800 s. Results similar to those shown in figure 6.4a were obtained when different tip sizes and/or tip–ITO gaps were used, highlighting the fact that the conductance of the

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Chapter 6 Protect and Switch Copper Atomic Contact nanowires is independent of their length, which confirms that transport in such wires is not governed by a classical regime.

It is well known that atomic contact experiments give disparate results.54-56 In order to overcome this difficulty, we constructed a histogram displaying the results of about 100 conductance vs. time curves (figure 6.4b). It shows that the conductance aggregates around a few well defined non-integer values of G0, with a major peak centered at 0.7 G0. Overall, the majority of the Cu nanowires, connected to the ITO substrate, have a conductance below the expected conductance quantum G0 or a non-integer value thereof. From previous studies, Cu nanowires generated between two metallic electrodes exhibit conductance steps close to integer values of G0 with a majority of the conductance around 1 G0, corresponding to mono-atomic point contacts. In our case, however, the conductance is no longer an integer value of G0. This deviation has been already observed and attributed to the variation of the transmission probabilities Ti which may take values between 0 and 1, depending on how strongly a particular state is backscattered at the quantum contact. Any partial reflection of the electron wave as a result, for instance, of dissymmetric configurations on the two sides of the constriction may alter the current and thus decrease the conductance. The transmission probabilities also change with the chemical environment15 and the metal studied. Indeed, the main conductance peak for iron 56 57 has been reported to be 0.89 G0 or between 0.5 and 0.7 G0 for iron adatom deposited on 56 58 Au(111), that of Pd around 0.9 G0 , that of cobalt close to 1 G0 and in another study close 57 to 0.8 G0, despite the fact that these atoms have more than one conduction channel per atom. The conductance of individual xenon atoms absorbed on a nickel surface and connected to a tungsten tip and a tip terminated by a xenon atom was studied; contact conductances are 0.2 G0 and 0.001 G0 for the single xenon atom and the two-atom chain, respectively.59 These conductances, much lower than that of an ideal one-dimensional conduction channel, were attributed to conduction through the tail of the xenon 6s resonance, which lies far above the Fermi level of the metal. In the present case, when the ITO substrate was replaced by a copper electrode under similar experimental conditions and set-up, the main conductance peak was close to 1 G0

(figure 6.2). This suggests that the observed non-integer values of G0 are probably due to the ITO substrate. More precisely, the Cu atoms that reach the ITO may contact indium, tin or oxygen of the oxide layer, causing changes in the transmission coefficient and, consequently, lead to values that deviate from the conductance quantum. Another factor 178

Chapter 6 Protect and Switch Copper Atomic Contact that could explain this deviation is the difference in the Fermi levels of Cu and ITO. The determination of the exact conductance of a mono-atomic contact between Cu and ITO requires further experimental and theoretical investigations which are not within the scope of this work. Overall, whatever the exact value of the conductance of such atomic point contacts, our experiments highlight the possibility of generating Cu nanowires between a Pt UME and an ITO substrate, where the Cu nanowire/ITO contact is in a quantum regime and electron transport between the two electrodes across the Cu nanowires is governed by a few atoms.

6.3.2 Metallic Cu nanowire on/through mesoporous silica thin films ITO substrate

Cu nanowires were next generated using the SECM configuration, under experimental conditions similar to those described above, but taking mesoporous silica thin films on ITO as the substrate. Oriented mesoporous silica films are formed by combining the concepts of electrochemically-driven cooperative self-assembly of surfactant micelles60 and silica formation resulting from electrogeneration of hydroxyl ions likely to catalyse the polycondensation of metal alkoxide precursors61 around spatially arranged amphiphilic molecules at the electrode/solution interface.49,50 The driving force of the process is the in-situ formation on the working electrode of metastable amphiphilic assemblies which are immediately captured by an inorganic deposit, resulting in the formation of new phases and new orientations of inorganic mesoporous structures.50,62,63 While these films can be generated in functionalized forms (i.e. with pendent organic functional groups attached to the internal surface of the mesopore channels64,65), the growth of nanowires in such very small mesopores (using either chemical or electrochemical deposition) remains highly challenging.48,52 Here we wish to evaluate the interest of SECM to achieve this goal on a local scale. The SECM tip was brought up to the silica substrate in an electrolyte solution containing ferrocene, creating a micrometric gap between the tip and the substrate. Next, aqueous CuSO4 solution was introduced into the cell and Cu wires were grown electrochemically from the tip side. Figure 6.5a and b illustrate two typical experiments representing the conductance variation as a function of time. During copper deposition, the current is in the nA range and Cu growth lasts approximately 100-200 s before contact with the substrate occurs. After 179

Chapter 6 Protect and Switch Copper Atomic Contact this, a large current jump is observed and the conductance of the wire stabilizes (figure

6.5a) at between 0.2 and 0.3 G0. In other examples (figure 6.5b), after a first current jump the conductance varies in stepwise fashion with a value that remains below 1 G0. In this case, a first plateau is observed at 0.1 G0, followed by successive steps at 0.25 and 0.5, and finally the conductance stabilizes at 0.75 G0. In all experiments, the curves recorded at the tip (black line) and at the substrate (red line) exhibit a perfect symmetry, indicating that the recorded current is due to charge transport through the copper filament junctions.

0.4 1.0 a b

0.2 0.5

0 0

0.0 0.0

G/G G/G

0.2 0.5

0.4 1.0 0 100 200 300 400 0 200 400 600 800 1000 Time/s Time/s

9000 c

6000 Counts 3000

0 0.5 1.0 1.5 G/G 0 Figure 6.5 Conductance of atomic copper contact on ITO/nanopores. (a) and (b) Conductance vs. time curves of Cu filament generated between Pt UME tip and ITO -2 modified by mesoporous silica film in presence of 10 M CuSO4 with 10 kΩ resistance connected to Pt tip. Red: ITO/nanopores; black: Pt UME. Tip: -0.9 V, ITO/nanopores: 0.1 V vs. SCE. (c) Conductance histogram of Cu nanowires. Figure 6.5c shows the histogram of the results of about 100 conductance vs. time traces. It shows that the conductance aggregates around a few well defined peaks with non-integer values of G0. There is now a major peak centered at 0.2 G0 and minor peaks at

0.7 G0 and 1 G0 All the Cu nanowires generated through the nanopores have conductances

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Chapter 6 Protect and Switch Copper Atomic Contact below the conductance quantum, lower than found for nanowires between a Pt tip and a bare ITO substrate. These results demonstrate that Cu nanowires are formed across the silica film with transport properties in the quantum regime, where only a few atoms control transport between the two electrodes. They also suggest that the impact of the nanopores on the nanowire conductance is similar to that of molecular adsorption.

Figure 6.6 Identification of atomic copper contact on ITO nanopores. STEM micrographs (top views) of: a) Mesoporous film generated from 100 mM TEOS before any copper growth. Film thickness, 115 ± 10 nm; lattice parameter, 4 nm; b) Film after successive generation and mechanical breaking of more than 100 copper nanowires. In the center, enlargement of one filled pore and some empty pores, showing clearly same diameter for Cu nanowire and silica mesopore (2 nm). Next, the substrates were characterized by Scanning Transmission Electron Microscopy (STEM) using a procedure already described in the literature:51 portions of the silica film were scratched from the surface, deposited on a STEM grid, and the top view of the nanopores image was recorded. Figure 4a shows the STEM view of the silica film on the ITO substrate used in this study. The nanopores appear as dark spots, because of the STEM contrast between heavy and light elements. This image confirms the existence of vertically aligned mesopores packed in a hexagonal arrangement with a diameter close to 2 nm (see enlargement in the bottom center of figure 6.6). Next, a film on which more than 100 nanowires had been successively generated and mechanically broken was also scratched from the surface and analyzed by STEM (figure 6.6b). The nanopores are still observed and, as expected, most of them are empty (dark spots). However, some individual pores are seen as bright spots, which indicate that they are filled by an element heavier than silicon. The size of these spots is that of one pore (compare enlargements in the center of figure 6.6) and in a 100 nm×100 nm area a few bright spots can be observed. We attribute 181

Chapter 6 Protect and Switch Copper Atomic Contact these spots to one of the experiments performed with the mesoporous substrate. One copper wire generated on the SECM tip reaches the upper surface of the silica film and is separated into several nanowires that enter the narrow channels. Nanowires grow in parallel but separate channels. Ultimately one of these nanowires reaches and contacts the underlying ITO substrate; this terminates the deposition process. Note that such bright spots were never seen before in the numerous STEM characterizations performed on mesoporous silica films;46-48 they are clearly due to the presence of Cu nanowires and not to any artefact of the experiment. Moreover, the presence of copper was confirmed by Energy Dispersive Spectroscopy (EDS) analysis of the sample corresponding to figure 6.6b.

6.3.3 Mesoporous silica protect effect on copper atomic contact

In order to test the protection of the Cu atomic contact by the mesoporous silica film, the effect of sodium salicylate on nanowire conductance was studied. Sodium salicylate is known to passivate copper very efficiently through copper dissolution to copper(II) and the precipitation of an insoluble layer consisting of a copper(II) salicylate complex. This is usually a good way to protect bulk copper but can destroy copper nanowires if their diameter is too small. Indeed, the introduction of sodium salicylate onto a Cu nanowire between two Cu electrodes results in the breakdown of the contact.20

0,3 a Salicylate injection 1,0 b 0,2 0,5

0,1 0

0 0,0 0,0

G/G Salicylate injection G/G 0,1 0,5 0,2 1,0 0,3 0 200 400 600 800 1000 0 200 400 600 800 Time/s Time/s Figure 6.7 Protection of copper atomic contact. Conductance vs. time curves of Cu filament generated between Pt tip and (a) ITO substrate; (b) ITO substrate modified by 115 nm-thick mesoporous silica film, and the effect of 10-1 M sodium salicylate on Cu nanowire. black: Pt UME; red: ITO or ITO/nanopores. Figure 6.7 shows the effect of sodium salicylate on contacts generated between a Pt tip and ITO (Figure 6.7a) or through a silica film supported by ITO (Figure 6.7b).

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Chapter 6 Protect and Switch Copper Atomic Contact

When ITO is used as substrate the conductance stabilizes at 0.75 G0 and, a few seconds after salicylate addition, it drops to 0 and the contact breaks down. In addition, the contact is not reformed, indicating that Cu wires no longer grow under these conditions. This behavior is reproducible and is independent on the initial conductance of the Cu nanowires, below or higher than the conductance quantum. In contrast, nanowires generated through the silica film supported on ITO appear to be unaffected by salicylate and the conductance of the Cu nanowires is unchanged. This suggests that the film protects the Cu contacts and inhibits copper dissolution in the region where their diameter is small. This protection is probably due to the structure of the film, which has pores no larger than 2 nm and 115 nm long. Indeed, this configuration prevents access of the salicylate ions to the Cu atom(s) making the contact, since the mesochannel is already filled with Cu. As a result, the Cu nanowires are protected from added molecules.

6.4 switching copper atomic point contact towards: memory devices

In principle, the challenge in single-atom memory devices is to build single-molecule switches with a conductance switching between GL (OFF state) and GH (ON state). From an engineering point of view, to create memory effects in an atomic switch, one way is to rely on the electrochemistry in a solid state electrolyte to destroy and/or reconstruct the atomic contact. Another way is to use an external trigger, such as the adsorption of switchable molecules.

6.4.1 Electrochemistry triggered destroy-reconformation based copper atomic switch

Nanowires were generated as explained in figures 6.3 and 6.4, and the tip potential was pulsed between 0 V (copper dissolution) and -0.9 V or -1.1 V (copper deposition) every 30 s while the substrate potential was kept at 0.1 V vs. SCE (no electrochemistry when contact occurs and copper dissolution when contact is broken). Figure 6.8 shows the result of these atomic switch experiments. When nanowires are generated between the Pt UME tip and ITO, the successive potential steps between 0 V and -0.9 V make it possible to reversibly break and generate them (figure 6.8a). The conductance of the overall device changes from a very small value (not measured here), when the nanowires are broken, to a value between 1 and 2 G0 when a

183

Chapter 6 Protect and Switch Copper Atomic Contact nanowire connects the two electrodes. Atomic switch behavior is thus obtained, despite the large distance between tip and substrate.

3 ITO ITO/nanopores UME a UME b 2 0.7

0 0

0 G/G G/G 0.0 -1

-2 -0.7 -3

0 60 120 180 240 300 0 60 120 180 240 300 Time/s Time/s

4 40 c d 2 20

A 0

 

-2

-20 Current/

Current/ -4

-40 -6

-60 -8 -1.2 -0.8 -0.4 0.0 0.4 -0.6 -0.4 -0.2 0.0 Potential/V Ag wire Potential/V Ag wire Figure 6.8 Atomic switch. (a and b) Variation of the conductance vs. time after successive potential steps: (a) Cu nanowires generated on ITO substrate, tip potential stepped between 0 and -0.9 V every 30 s; substrate potential, 0.1 V; (b) Cu nanowires generated on ITO/nanopores silica film, tip potential stepped between 0 and -1.1 V every 30 s; substrate potential, 0.1 V. (c and d) Variation of the conductance versus tip potential during successive potential sweeps at 100mV/s (c) Cu nanowires generated on ITO substrate, substrate potential, -0.2 V. (d) Cu nanowires generated on ITO/nanopores, substrate potential, -0.3 V. Changing ITO to a film-modified ITO has a marked impact on atomic switch experiments. The nanowires can still be broken when the tip potential is 0 V but are more difficult to rebuilt in 30 seconds when copper is deposited at -0.9 V. However, when the deposition potential is raised to -1.1 V vs. SCE, atomic switch behaviour is observed (Figure 6.8b). More importantly, the conductance of the system oscillates between a very low value, when nanowires are broken, to values between 0.2 and 0.7 G0, indicating that the connection between the Pt UME tip and ITO through the nanopores can be restored despite the film. Similar differences are observed when sweeping the tip potential between -1V and 0.4V at 100mV/s scan rate with a substrate potential fixed at -0.2 or -0.3V. Switching behaviour between a low conductance state and a high conductance state is repeatedly

184

Chapter 6 Protect and Switch Copper Atomic Contact observed independently of the used substrates (ITO or ITO/nanopores). Contacts are broken at -0.1 to 0.2 V and are rebuilt at negative potentials. The potential for breaking the nanowires seems to be more reproducible when ITO/nanopores are used. Moreover, when

ITO/nanopores are used, the successive conductances of the on states are below 1G0

(between 0.3 and 0.5 G0 in figure 6.8d) and are systematically smaller than those observed when contacts are made on ITO substrates (Figure 6.8c).

6.4.2 Redox-gated PEDOT-induced copper atomic switch

6.4.2.1 Atomic contact through PEDOT layer In order to achieve an atomic switch, we have designed an atomic contact through a PEDOT film. As illustrated in scheme 6.1, the formation and annihilation of a Cu atomic bridge is controlled by a redox reaction between two electrodes. Note that PEDOT is a redox-gated CP; it is thus expected that an atomic switch could be realized by switching the PEDOT redox state.

Cu atomic contact

PEDOT Pt substrate

Scheme 6.1 schematic drawing of Cu atomic contact through PEDOT

PEDOT deposition on the Pt electrode was conducted in a solution containing 20mM

EDOT, 0.1M SDS and 0.1M LiClO4 in water. Figure 6.9a shows that the potential is at 0.8 V during the entire deposition process. If the deposition yield is assumed to be 100%, the amount of polymer deposited, n, expressed as the number of EDOT units, is given by: n = Q/neF where Q = I.t is the charge used for polymerization; ne is the effective number of electrons associated with PEDOT growth per EDOT monomer (taken as 2.2, since PEDOT is generated in its oxidized state), F is the Faraday constant, and I is the constant current used. The polymer occupies a volume V equal to (Sd) where S is the electrode area and d the film thickness. This volume is also equal to nM/ρ where M is the molecular weight of EDOT plus 0.2 times that of the

185

Chapter 6 Protect and Switch Copper Atomic Contact counter-ion and ρ is the polymer density (reported to be 1.5). The film thickness is expressed by: (M  0.2M )Q d  EDOT counterions 2.2FS In the present case, the thickness of PEDOT film deposit on the Pt electrode was calculated to be 300nm. Figure 6.9b shows the CV response of the PEDOT film in the monomer free solution. The broad cathodic and anodic peaks (black line) indicate that the polymers are deposited on the working electrode surface.

1.0 a 15 PEDOT film b 0.8 10 blank 0.6

A 5 

0.4 0

0.2 -5 Current/ Potential/ V Potential/ -10 0.0 -15 -0.2 -20 0 5 10 15 20 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Time/s Potential/V (SCE)

Figure 6.9 (a) Deposition curve in 0.02 M EDOT, 0.1 M SDS and 0.1 M LiClO4 in water at constant 1 mA cm-2 current density. (b) CV of blank test (red line) and CV of polymer films (black line) electrodeposited using constant current density in 0.1 M LiClO4/water solution.

In figure 6.10a, for 90 s there is a very small relaxation current, due to ionic conduction and electrochemical reactions, on the substrate electrode (black line). Then a sharp current jump occurs, which indicates that a copper atomic contact has been obtained. After this, the electrochemical deposition process self-terminates thanks to the control by the external resistor. The formation of a copper atomic contact between the electrodes is supported by symmetric I-G curves, with Itip = −Isub, the currents recorded on the tip (red line) and on the substrate (black line). In figure 6.10a, the conductance of the nanowire stabilizes at a value close to 0.7 G0. A similar result is found in figure 6.10b with a final conductance value of

0.5 G0. Results similar to those shown in figure 6.10 were obtained when different tip sizes and gap sizes were used, highlighting the fact that the conductance of the nanowires is independent of their length, which confirms that transport in such wires is not governed by the classical ohm’s law. Figure 6.10c shows the histograms of the stabilized conductances of atomic contacts generated through the PEDOT film. The conductance histogram reveals well defined peaks at non-integer values of G0 and most of the contacts have conductances

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Chapter 6 Protect and Switch Copper Atomic Contact

below 1 G0. These results are in agreement with those conductance curves. Figure 6.10d shows a linear variation of the current versus the applied bias voltage. Based on the slope of this curve the conductance of the corresponding atomic contact is close to 0.5 G0.

0,8 1,0 a b

0,4

0,5 0

0,0 0 0,0

G/G G/G -0,5 -0,4

-1,0 -0,8 0 100 200 300 0 100 200 300 Time/s Time/s

20000 30 c 25 20 15

10000

A 10

 Couonts I/ 5 0 -5 0 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0.0 0.2 0.4 0.6 0.8 1.0 G/G Potential/V 0 Figure 6.10 Conductance of atomic copper contact on PEDOT/Pt. (a) and (b) Conductance vs. time curves of Cu filament generated between Pt UME tip and Pt substrate -2 modified by PEDOT film in presence of 10 M CuSO4 with 10 kΩ resistance connected to Pt tip. red: UME tip; black: PEDOT/Pt substrate. Tip: -0.9 V/ Ag wire, substrate: 0.1 V / Ag wire. (c) the I–V characteristic. (d) Histograms of stabilized conductances.

It is interesting to compare the conductance value of copper atomic contact generated on PEDOT/Pt to those generated on bare Cu or Pt substrate. Integer value of the conductance quantum is observed when bare Cu or Pt electrode is used as substrate, as shown in figure 6.2. Whereas a non-integer value of the conductance quantum (or below

1G0) is observed in figure 6.10. In the present case, when the Pt substrate was replaced by a PEDOT/Pt under similar experimental conditions and set-up, the main conductance value was below 1 G0. This suggests that the observed non-integer values of G0 are due to the substrate. These conductance values (figure 6.10) are in agreement with molecular adsorption on an atomic contact, which is known to decrease the conductance. These experiments prove the possibility of generating Cu filament through a PEDOT film, where the Cu PEDOT/Pt contact is in a quantum regime

187

Chapter 6 Protect and Switch Copper Atomic Contact

It is interesting to compare the conductance values of copper atomic contacts generated on PEDOT/Pt with those generated on bare Pt substrate. Integer values of the conductance quantum are observed when a bare Pt electrode is used as substrate, as shown in figure 6.2, whereas a non-integer value (or below 1 G0) is seen in figure 6.10. In the present case, when the Pt substrate was replaced by PEDOT/Pt under similar experimental conditions and set-up, the main conductance value was below 1 G0. This suggests that the non-integer values of G0 are due to the substrate. These conductance values (figure 6.10) can be attributed to molecular adsorption on an atomic contact, which is known to decrease the conductance. These experiments prove the possibility of generating Cu filaments through a PEDOT film, where the Cu PEDOT/Pt contact is in the quantum regime.

6.4.2.2 Copper atomic switch triggered by a conductive PEDOT

20 10 b a 5 10 0

-5 

 0 /

/ -10

tip

I I -10 -15 -20 -20 -25 -30 -30 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 Potential/V Ag wire Potential/V Ag wire Figure 6.11 Two examples of electrochemical reactions for formation and annihilation of Cu filament between Pt UME and PEDOT/Pt substrate. Variation of the current vs. tip potential during successive potential sweeps at 100 mV s-1 where PEDOT/Pt is used as substrate. The tip was at a constant potential of -0.2 V and the substrate potential was swept.

After achieving an atomic contact through the PEDOT film on a Pt substrate, we then switch it by sweeping the potential on the substrate while keeping the constant potential on the tip. Figure 6.11 shows two example of an atomic switch. The current drops to ‘zero’ when the potential is around 0 V/Ag. This drop indicates that the atomic contact is destroyed through copper oxidation. The current increases when the potential is swept to the negative regime. This increase indicates that the atomic contact is rebuild through the copper reduction. Depending on the slope, the conductance of the atomic contacts

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Chapter 6 Protect and Switch Copper Atomic Contact

generated are 0.7 G0 (figure 6.11a) and 0.9 G0 (figure 6.11b). A atomic contact system is repeatedly switched between its low-conductance and high-conductance states by a threshold voltage of only 1 V. This constitutes an atomic switch with bistable conductance. Another way to trigger the switch is using a redox-gated conducting polymer, such as PEDOT. As described in previous chapters, the conductive state of PEDOT depends on the gate potential. Note that there are strong adsorption effects of PEDOT on the conductance of atomic contacts generated through the PEDOT film, as shown in figure 6.10. Conducting and insulating PEDOTs may have different impacts on the adsorption. It is expected that this impact could be used to trigger the atomic switch.

0,8 0,7 a b 0,6 0,6

0,5

0 0 0,4

0,4

G/G G/G 0,3

0,2 0,2

0,1 0,0 0 60 120 180 240 300 0 60 120 180 240 Time/s Time/s Figure 6.12 Atomic switches. (a and b) Variation of the conductance vs. time after successive potential steps: Cu nanowires generated on PEDOT/Pt substrate, substrate potential stepped between 0.2 and -0.2V every 30 s; tip potential, -0.7 V.

We then conduct the atomic switch experiments. Figure 6.12 shows the variation of the conductance vs. time of two atomic contact generated on PEDOT/Pt substrate. Substrate potential was stepped between 0.2 and -0.2V every 30 s while tip potential was kept at a constant of -0.7 V. an abrupt conductance change is observed when the PEDOT film was switched between conducting and insulating state. When the 0.2V is applied on the substrate (figure 6.12a), conductance of the atomic contact is 0.45G0. As the potential is decreased to -0.2 V, an abrupt decrease in conductance is observed. This changes the device between a high conductivity OFF state and a low-conductivity ON state. Bistable conductance appears, with a reproducibility of nearly 100% despite the fact that the conductance is below the conductance quantum (1 G0). The switching amplitude

∆G = Gh - Gl (where the Gh and Gl are the high and the low conductance values, respectively) varies from one contact to another, but typical values are 0.1~0.4 G0. Note that devices 189

Chapter 6 Protect and Switch Copper Atomic Contact made with just copper atomic contact on a bare Pt electrode show no electronic switching, indicating that the PEDOT film is responsible for the switching. These result are in good agreement with our hypothesis. More precisely, atomic switches were realized by changing the conducting state of the adsorbate (PEDOT). The electrical bistability suggests that the copper filament atomic contact through PEDOT can be investigated for non-volatile memory devices.

6.4 Conclusions

In summary, metallic copper nanowires were generated using the electrochemical self-terminated method based on the SECM configuration. The method allows to connect asymmetric electrodes, a Pt SECM tip and a transparent semiconductor substrate, ITO, through Cu nanowires. The conductances of the nanowires are found to be non-integer values of G0 and in most cases below the conductance quantum. Transport is in the quantum regime and is controlled by a few atoms. Similar studies were performed using the same ITO substrate but covered with a vertically-oriented mesoporous silica film with a pore diameter of about 2 nm and a thickness of 115 nm. It is possible to generate nanowires on this transparent nanostructured substrate. The nanowires grow inside the pores and reach the ITO, making a contact between the two electrodes. The conductance values are below the conductance quantum and indicate that they are affected by the pores. Indirect proof of nanowires entering the narrow mesopore channels is obtained by adding sodium salicylate to the system. This results in breakdown of contacts generated on unmodified ITO, but there is no significant conductance change for those generated through the mesoporous silica film. STEM experiments also strongly support the idea that copper wires grow in parallel separated channels. Finally, atomic switch experiments on both substrates suggest that the nanopores/ITO can be used in atomic switch devices. Such a system may prove to be useful for future memory device applications and, thanks to the use of transparent substrate, we anticipate that this study could be a starting point for photo-induced atomic switches. Finally, atomic switch experiments using various substrates were successfully performed. Atomic switches have been achieved by construction-annihilation of Cu atomic contacts controlled by an electrochemical reaction. Another type of atomic switch is typically realized by switching the conductive state of a PEDOT film surrounding the copper nanowires. Such a system may be useful for future memory device applications.

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Chapter 6 Protect and Switch Copper Atomic Contact

Thanks to the use of a transparent substrate, we anticipate that this study could be a starting point for photo-induced atomic switches.

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57 Burtzlaff, A., Schneider, N. L., Weismann, A. & Berndt, R. Shot noise from single atom contacts in a scanning tunneling microscope. Surface Science 643, 10-12 (2016). 58 Kröger, J., Néel, N. & Limot, L. Contact to single atoms and molecules with the tip of a scanning tunnelling microscope. Journal of Physics: Condensed Matter 20, 223001 (2008). 59 Yazdani, A., Eigler, D. & Lang, N. Off-resonance conduction through atomic wires. Science 272, 1921 (1996). 60 Chen, M., Burgess, I. & Lipkowski, J. Potential controlled surface aggregation of surfactants at electrode surfaces–A molecular view. Surface Science 603, 1878-1891 (2009). 61 Shacham, R., Avnir, D. & Mandler, D. Electrodeposition of Methylated Sol‐Gel Films on Conducting Surfaces. Adv. Mater. 11, 384-388 (1999). 62 Choi, K. S., McFarland, E. W. & Stucky, G. D. Electrocatalytic Properties of Thin Mesoporous Platinum Films Synthesized Utilizing Potential‐Controlled Surfactant Assembly. Adv. Mater. 15, 2018-2021 (2003). 63 Guillemin, Y., Etienne, M., Aubert, E. & Walcarius, A. Electrogeneration of highly methylated mesoporous silica thin films with vertically-aligned mesochannels and electrochemical monitoring of mass transport issues. J. Mater. Chem. 20, 6799-6807 (2010). 64 Etienne, M., Goux, A., Sibottier, E. & Walcarius, A. Oriented mesoporous organosilica films on electrode: a new class of nanomaterials for sensing. Journal of nanoscience and nanotechnology 9, 2398-2406 (2009). 65 Vilà, N., Ghanbaja, J., Aubert, E. & Walcarius, A. Electrochemically Assisted Generation of Highly Ordered Azide‐Functionalized Mesoporous Silica for Oriented Hybrid Films. Angew. Chem., Int. Ed 53, 2945-2950 (2014). 66 Dos Santos, L. M. et al. Electrochemical synthesis of polypyrrole films on copper electrodes in acidic and neutral aqueous media. J. Electroanal. Chem. 587, 67-78 (2006).

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CONCLUSION

GENERAL CONCLUSION

In this thesis, we have developed several approaches to generate controllable CPs junctions. Several effective methods for generating nanojunctions are discussed. We have also presented some strategy to switch CPs molecular junctions as well as several approaches to protecting and switch copper atomic point contacts.

In chapter 1, we shortly discussed the development of molecular electronics and a few relevant concepts. We mentioned the fabrication techniques for molecular junctions, including STM-BJ, MCBJ, CP-AFM, SWNT and so on. Each technique has its own advantages and they share a common principle: molecules are sandwiched inside the electrode gap through a reliable connection between molecules and the electrode. In our group, scanning electrochemical microscopy (SECM), where two microelectrodes are located face-to-face separated by a gap with a distance from 100nm to 10μm, proved to be capable in the generation of CPs molecular junctions.

In chapter 2, PBT and PEDOT redox-gated junctions were investigated using SECM set up. Highly stable and reversible redox-gated nanojunctions were obtained with conductance in the nanosiemens range in their conducting state, which indicates the conductance of the entire junction is governed by a few oligomers. The redox-gated nanojunctions obtained here seem to be at the frontier between fiber devices and single-molecule devices. The switching potential of the nanojunction depends on polymer type. PEDOT and PBT nanowires, with surface areas below 400 nm2 and thickness above the direct tunneling limit between the two metallic electrodes, may be of interest for low energy consumption and relatively large on/off ratios in logic and memory devices. This study makes that SECM is a common technique to investigate CPs molecular junctions.

In chapter 3, three methods, namely self-terminated method, DSV and SECM-BJ, are developed to control the formation of the molecular junctions. A self-terminated strategy is found to be effective to obtain nano-junctions. Well organized PANI and PEDOT nanojunctions have been obtained by the self-terminated method. An external resistance plays the important role in controlling the size of conducting polymer junctions. Second, DSV is a way to observe the electropolymerization current and the transport current across

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CONCLUSION the junctions. It gives the chance to stop the polymer growth, therefore, the molecular junctions are somehow controlled in a small size. Finally, due to the piezo motor, it is possible to shrink the junctions by withdrawing the tip. This technique is called SECM-BJ. Similarly, as STM-BJ, conductance steps by SECM-BJ represent a number of molecules governed the charge transport through the molecular junctions. SECM break junction technique proved to be an efficient way for generating molecular nanojunction. Among these three methods, SECM-BJ is the most promising technique that can be widely used in the measurement of molecular junctions. The next step of SECM-BJ should be the investigation of single molecular junctions of small molecule systems.

Then, these methods have been used for the generation of n-type and ambipolar type conducting polymer junctions. In Chapter 4 we presented molecular junctions based on Rh-Rh chain polymers, which exhibit a well-defined n-type charge transport properties with conduction in the reduced state. This kind of redox gated polymer junctions with metallic wire center may open a new view for future n-type molecular design. Furthermore, the ligands, which encapsulated metallic chain center, may play a role in protecting the charge transport through the molecular junctions. In this chapter, we also introduced the study of PFTQ and PFETQ molecular junctions. They exhibit well-defined ambipolar charge transport properties. However, an unbalanced charge transport properties in the n- and p- channel were observed when the junctions are in the fiber device scale. When molecular junction change into nanojunction, a balanced n- and p- channel transport properties is observed. This phenomenon can be attributed to charge transport mechanism changing from diffusive (ohm’s law) to ballistic (quantum theory) when the junction size minimizes from fiber devices to nanodevices.

In chapter 5, we have demonstrated that plasmonic hot electron can act as an external input to trigger molecular switching. To our knowledge, it is the first time that a bistable resistive system induced by a plasmonic effect is observed with an electroactive polymer junction. This work may provide a new understanding of the plasmonic effect on the transport current through molecular devices and, besides, it must be underlined that these results were obtained with Au NPs of great dispersivity, very easily synthesized. In the future, with NPs of better dispersivity and of smaller size for which a stronger light absorption is expected and therefore a stronger plasmonic effect should be obtained. In

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CONCLUSION future, improvements should be acquired by using plasmonic hot hole injection triggered molecular switching system.

In chapter 6, copper atomic point contact was generated using an electrochemical self-terminated method based on SECM configuration. Charge transport is in the quantum regime and is controlled by a few atoms despite that the conductances of the nanowires are found to be non-integer values of G0 and in most cases below the conductance quantum. The same studies were performed using a mesoporous silica film coated ITO as a substrate electrode. Such silica film with a pore diameter of about 2 nm and a thickness of 115 nm plays a crucial role in protecting the atomic contact from the sodium salicylate. In this chapter, we also investigated the atomic switch. Atomic switch experiments using various substrates have been successfully demonstrated. It has been achieved by construction-annihilation of Cu filament controlled by an electrochemical reaction. Another type of atomic switch is typically triggered by a switchable redox gated PEDOT film surrounding the copper nanowires. In the future, improvements of the atomic switch should be obtained by using an ionic liquid as an electrolyte in a solid device system towards memory devices.

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