Surface Characterization of Organic and Bioinspired Nanoscale Devices

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SURFACE CHARACTERIZATION OF ORGANIC AND BIOINSPIRED

NANOSCALE DEVICES

Kuo-Yao Lin

APPROVED BY SUPERVISORY COMMITTEE:

______Dr. Jason D. Slinker, Chair

______Dr. Mark Lee

______Dr. Majid Minary-Jolandan

______Dr. Anvar A. Zakhidov

Kuo-Yao Lin

To my parents and Whity

SURFACE CHARACTERIZATION OF ORGANIC AND BIOINSPIRED

NANOSCALE DEVICES

KUO-YAO LIN, BS, MS, ME

DISSERTATION

Presented to the Faculty of

The University of Texas at Dallas

in Partial Fulfillment

of the Requirements

for the Degree of

DOCTOR OF PHILOSOPHY IN

PHYSICS

THE UNIVERSITY OF TEXAS AT DALLAS

May 2018

ACKNOWLEDGMENTS

I really appreciate the help and guidance from my advisor, Dr. Jason Slinker for the past five years.

He always provides useful insight during the most tough moments, and gives me a clear path and direction when I take the wrong turn in my research. He spends extra time and effort to assist me in completing my paper and dissertation to make my graduation possible. I am also very grateful to my PhD committee, Dr. Mark Lee, Dr. Anvar Zakhidov, Dr. Fan Zhang, and Dr. Majid Minary, they have given me valuable opinions to advance my research works.

I also want to thank to former lab members, Chris Wohlgamuth, Marc McWilliams, and Lyndon

D. Bastatas. Chris and Marc assisted me to get started in the cleanroom for chip fabrication. With their help and knowledge, I quickly adapted to the cleanroom work environment. Lyndon helped me to prepare the samples to allow for the completion of my first publication.

I also would like to show my gratitude to current lab members Dimithree Kahanda, Kelly Jackson and Nolan King. Dimithree spends a considerable amount of time on the material I need for the research. With her help, all the samples are ready in time for experiments. Nolan provides his software and hardware skills to enable the automation measurements in my second project.

I also would like to thank to the collaborators from UTD and other schools for providing their resources to make my research work go smoothly. Finally, I want to thank to all the professors and faculty at UTD for help me to go through this journey.

December 2017

SURFACE CHARACTERIZATION OF ORGANIC AND BIOINSPIRED

NANOSCALE DEVICES

Kuo-Yao Lin, PhD The University of Texas at Dallas, 2018

ABSTRACT

Supervising Professor: Jason D. Slinker

For the past few decades, the research and industrial application of organic semiconducting materials has been very active. Compared to traditional semiconducting materials, facile chemical modification and processing are advantageous properties of organic electronics. This dissertation focuses on two classes of organic semiconducting devices: light-emitting electrochemical cells

(LEECs) and bioinspired nanowires. Organic light-emitting diodes (OLEDs) have emerged in display applications, but not lighting due to high fabrication costs. To achieve high OLED performance at low cost, efforts have focused on light-emitting electrochemical cells (LEECs).

LEECs, and particularly iridium LEECs, exhibit substantial efficiency, high luminance, and long lifetime in a simple, solution processable device architecture. Performance is facilitated by the redistribution of ions that assists charge injection. However, the physics of iridium LEECs has not been fully explored, particularly brightness enhancement with lithium additives. Scanning Kelvin

Probe Microscopy (SKPM) was used to reveal the surface potential profile of iridium LEEC devices and clarify the effect of lithium addition. We found that ions do not pack densely at the

cathode in pristine iridium LEECs devices. Li[PF6] addition produced a doubling of the peak

electric field at the cathode from an increase of ionic space charge. This work was the first to clarify the nature of iridium device performance and enhancement from lithium salt additives: the additional mobile cations improves space charge accumulation for improved electron injection.

The second class of devices concerns nanowires, specifically, the DNA-inspired self-assembly of nanoscale electronic devices. There is a need to fabricate nanoscale electronics with high yield and high purity. We created devices based on 20 nm long DNA nanowires incorporating a perylene-

3,4,9,10-tetracarboxylic diimide (PTCDI) derivative, an organic semiconductor with dimensions similar to two DNA bases. We synthesized these nanowires by automated DNA phosphoramidite chemistry and purified these wires by high performance liquid chromatography, thus achieving high control and purity of a nanoscale electronic element. We patterned gold nanogap electrodes and assembled the nanowires by gold-thiol self-assembly. Current voltage characterization revealed that the current of perylene nanowires was enhanced 4.4 fold over conventional DNA nanowires. Temperature dependence revealed that the current increased from room temperature up to 35 °C for each type of wire, and then lowered rapidly, consistent with DNA melting. We performed atomic force microscopy imaging studies to observe an instance of a single nanowire spanning a nanogap. This research provides a new approach to fabricate nanoscale devices with lower cost and high yield. Chapter 1 will serve as an introduction to organic semiconductor fundamentals and applications. Chapter 2 will discuss the state-of-the-art lithographic fabrication methods and the progress of molecular electronics, including research with DNA nanowires.

Chapter 3 describes our research in performing surface characterization of iridium light-emitting electrochemical cells and the effect of lithium additives. Finally, the construction of bioinspired

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nanowire devices with the organic semiconductor perylene and associated electronic, thermal, and surface characterization is reported in Chapter 4.

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TABLE OF CONTENTS

ACKNOWLEDGMENTS………………………………………………………………………...v

ABSTRACT……………………………………………………………………………….……..vi

LIST OF FIGURES……………………………………………………………………….……...xi

CHAPTER 1 INTRODUCTION ...... 1 1.1 Preface ...... 1 1.2 Energy state of organic materials ...... 2 1.3 Polymer structure and charge transport ...... 5 1.4 Applications-Organic light emitting diodes ...... 5 1.5 Organic Field effect transistors ...... 8 1.6 Organic semiconductor material process ...... 10 1.7 Conclusion and dissertation outline ...... 11

CHAPTER 2 MOLECULAR ELECTRONICS AND DNA- BRIDGED NANOGAP JUNCTIONS ……………………………………………………………………………………12 2.1 Preface ...... 12 2.2 Current silicon technology and limitation ...... 12 2.2.1 Electron beam lithography ...... 13 2.2.2 Lithography limitations and next step ...... 15 2.3 Molecular electronics ...... 16 2.3.1 Current development of molecular electronics ...... 16 2.3.2 Molecular junctions ...... 17 2.3.3 Single and ensemble junction ...... 18 2.3.4 Type of single molecule devices ...... 19 2.3.5 Type of ensemble molecule devices ...... 21 2.3.6 Interfaces and bonding ...... 21 2.4 DNA based nanowire MMM junction ...... 22 2.4.1 DNA nanowire device experiments ...... 25 2.5 Conclusions ...... 26

CHAPTER 3 THE INFLUENCE OF LITHIUM ADDITIVES IN SMALL MOLECULE LIGHT-EMITTING ELECTROCHEMICAL CELLS ...... 27 3.1 Preface ...... 28 3.2 Introduction ...... 28 3.3 Scanning kelvin probe microscopy ...... 31 3.4 Devices for surface probe study ...... 32 3.5 Potential distribution of steady state ...... 34 3.6 Relaxation dynamics ...... 38 3.7 SKPM of LEECs at high salt concentrations ...... 39 3.8 Discussion ...... 41 3.9 Conclusion ...... 42 3.10 Experimental section ...... 43 3.10.1 [Ir(ppy)2(bpy)] complex synthesis...... 43 3.10.2 Planar device fabrication...... 44 3.10.3 Scanning kelvin probe microscopy measurement...... 45

CHAPTER 4 BIOINSPIRED NANOWIRE DEVICES UTILIZING PERYLENE DIIMIDES .46 4.1 Preface ...... 47 4.2 Introduction ...... 47 4.3 Nanowire concept and construction ...... 49 4.4 Devices for nanowire study ...... 51 4.5 Current-voltage characteristics of nanowires ...... 53 4.6 Temperature dependence of device characteristics ...... 55 4.7 AFM characterization of nanowire devices ...... 56 4.8 Discussion ...... 57 4.9 Conclusion ...... 59 4.10 Experimental section ...... 60

REFERENCES ...... 64

BIOGRAPHICAL SKETCH ...... 77

CURRICULUM VITAE ...... 78

LIST OF FIGURES

Figure 1.1. Top of the figure shows several frequent use conjugated material, include TPD, Alq3, Ir(ppy)3, Anthracene, PPV, P3HT, and Pentacene. The bottom of the figure visualizes the p bonds delocalize from the backbone which is constructed by s bonds [1]. Reproduced from Physics today volume 58, issue 5, start pg 53 (2005), with the permission of the American Institute of Physics. 3

Figure 1.2. Organic light emitting diode as display. Red, green, and blue are the typical colors of each pixel in an OLED matrix. Reproduced from Wikimedia Commons licensed under [CC BY 3.0]...... 6

Figure 1.3. On the left is the conventional OLED device with multi-layer material, on the right is iTMC OLED device with simple 3 layers structure...... 7

Figure 2.4. Perylene as building block for both p-type and n-type monomers. A) the core- unsubstituted perylene-3,4,9,10-tetracarboxylic acid diimide (red) and B) the tetrachloro- substituted perylene-3,4,9,10-tetracarboxylic acid diimide (blue)...... 25

Figure 3.1. Conceptual drawing of the effect of lithium ionic additives on small molecule LEECs − under bias. In pristine devices, [PF6] anions redistribute freely due to their smaller size (3.2 Å + radius) and lead to high space charge near the anode, but the larger [Ir(ppy)2(bpy)] cations (12.6 Å radius) do not move appreciably and cannot pack as densely, leading to smaller space charge + near the cathode. Addition of Li[PF6] contributes small Li cations (0.76 Å radius) [80], leading to more balanced double layer formation for balanced carrier injection and improved device performance...... 30

Figure 3.2. (A) Photolithographic steps for patterning with parylene to restrict the iTMC active layer between two electrodes. (B) A photograph of the interdigitated device used in this study. The overall chip is approximately 1 cm by 1 cm. (C) A fluorescence microscopy image of one gap of the interdigitated device, showing yellow fluorescence from the iridium layer between the two electrodes. (The photoluminescence is enhanced near the contacts.) ...... 33

Figure 3.3. Steady state potential distribution of planar iTMC-LEECs at 9 V operation. Scanning Kelvin probe microscopy data (voltage versus position) of pristine (upper graph, green) Au/[Ir(ppy)2(bpy)][PF6]/Au devices and devices with lithium ionic additive (lower graph, red), Au/[Ir(ppy)2(bpy)][PF6] + 0.5%/wt Li[PF6]/Au under a 9 V bias. Devices were also subjected to a 50 V prebias for 30 min. Dashed lines represent the approximate location of the gold electrode edges. The inset is the overlap range from 7-21 µm positions on the graphs...... 34

Figure 3.4. Voltage profile versus position for a pristine Au/[Ir(ppy)2(bpy)][PF6]/Au device under 9 V steady state operation, focused on the region between the electrodes. The black dotted line is a fit to the data of the form shown in the inset equation, consistent with the exponential trend of a diffuse outer Helmholtz layer of positive ions. The decay constant κ was found to be 0.165 µm-1. R2 = 0.9946...... 36

Figure 3.5. Steady state electric field distribution of iTMC-LEECs under 9 V bias. Electric field versus position of pristine (green, open bars) Au/[Ir(ppy)2(bpy)][PF6]/Au devices and devices with lithium ionic additive (red, closed bars), Au/[Ir(ppy)2(bpy)][PF6] + 0.5%/wt Li[PF6]/Au...... 37

Figure 3.6. Relaxation dynamics of iTMC-LEECs at 0 V bias following 9 V operation. Scanning Kelvin probe microscopy data (voltage versus position) of pristine (upper graph, green) Au/[Ir(ppy)2(bpy)][PF6]/Au devices and devices with lithium ionic additive (lower graph, red), Au/[Ir(ppy)2(bpy)][PF6] + 0.5%/wt Li[PF6]/Au. Solid traces were taken upon initial application of 0 V, and dotted traces were recorded 30 minutes later. Dashed vertical lines represent the approximate location of the gold electrode edges...... 38

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Figure 3.7. (A) Steady state potential distribution of planar iTMC-LEECs at 9 V operation. Scanning Kelvin probe microscopy data (voltage versus position) of Au/[Ir(ppy)2(bpy)][PF6] + Li[PF6]/Au devices with various concentration of Li[PF6] under a 9 V bias. Devices were also subjected to a 50 V prebias for 30 min. Dashed lines represent the approximate location of the gold electrode edges for the 5.0%/wt device. (B) Relaxation dynamics of a high salt iTMC device at 0V bias following 9V operation. Scanning Kelvin Probe Microscopy data (voltage versus position) of Au/[Ir(ppy)2(bpy)][PF6] + 5.0%/wt Li[PF6]/Au devices. Solid traces were taken upon initial application of 0V, and dotted traces were recorded 30 minutes later. Dashed vertical lines represent the approximate location of the gold electrode edges...... 40

Figure 4.1. Nanogap device concept and sequence composition. (Upper) Nanoscale electrodes are patterned, and nanowires incorporating perylenediimide moieties are assembled in the electrode gaps. (Middle) The structure of perylene-3,4,9,10-tetracarboxylic diimide (PTCDI). (Lower) Three DNA sequences utilized in this study. The perylene (P) duplex incorporates 58 well matched DNA base pairs and two PTCDI molecules in the center. The perylene mismatch (P-MM) duplex is similar to P but incorporates two CA mismatches (each noted by C). The DNA sequence is a pure, well-matched 60 mer...... 50

Figure 4.2. Fabrication process and microscopy images of nanogap electrodes for nanowire study. Substrates are spin cast with electron beam resist and patterned by exposure to a focused electron beam and resist development. Metal evaporation is followed by lift off of the remaining resist. Optical microscopy image showing one overall nanogap device, with 50 µm contact pads on the periphery. The scanning electron microscopy image reveals that a ~20 nm nanogap is formed from the process...... 52

Figure 4.3. A) Average current from 10 nanowire devices from perylene, perylene mismatch and DNA samples. B) Average current and current enhancement factor (ratio of nanowire current over device average open circuit current, normalized to DNA nanowire) at 0.4V for perylene, perylene mismatch and DNA samples. Asterisks represent t-test p-values as follows: *p ≤ 0.05; **p ≤ 0.01...... 53

Figure 4.4. Temperature dependence of P and DNA nanowires. A) Temperature dependence of current/voltage characteristics for a P nanowire. B) Temperature dependence of current/voltage characteristics for a DNA nanowire. C) Current at 0.75 V versus temperature for a P nanowire. D) Current at 0.75 V versus temperature for a DNA nanowire. E) UV-Visible absorbance at 260 nm versus temperature for P and DNA nanowires, the conventional melting temperature analysis. 55

Figure 4.5. Atomic force microscopy images of a nanogap device before assembly (left), and a higher magnification image of the device after assembly...... 57

Figure 4.6. Energy band level diagram for DNA inspired nanowires incorporating PTCDI suspended between gold electrodes...... 58

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CHAPTER 1

INTRODUCTION

1.1 Preface

Organic semiconducting materials have been an active area of research for the past few decades. One of the first examples of this research occurred in 1964 [1], when Martin Pope published the paper “Electroluminescence and Band Gap in Anthracene”[2], which opened a new gate for organic semiconductor research. The first widespread organic semiconductor application was xerography, which was implemented in industry in the 1980s. Organic semiconductor materials were deemed to be suitable for laser printers and laser copiers due to the high charge transport capabilities, and the xerography technology has been widely used since then [3]. Ongoing research with organic materials will provide knowledge to advance current technologies and to build up theoretical models of operation.

Organic electronics development today is cooperating with current silicon technology to produce devices with smaller dimensions. Concurrently, organic light-emitting diodes (OLED) have emerged in commercial applications, particularly in lighting and displays due to low power consumption and high performance. The success of OLEDs has strongly encouraged the electronics industry to move forward to develop more applications with organic semiconductors.

Processing and modification flexibility are important properties that make organic electronics unique in comparison to other semiconducting materials. For example, in OLEDs, the wavelength of light emission can be tuned from material synthesis. Similarly, OLED electrical properties, such as energy levels and free carrier concentrations, can be modified by materials

synthesis to fit specific purpose. Concerning processing, the organic material can be coated on large scales to create arrayed devices. The overall process cost has the potential to be considerably lower than current silicon technology.

The beginning of this chapter will focus on the fundamental knowledge of organic semiconductors. Next, the chapter will address the theories behind the OLED operation and application. Finally, the chapter will conclude with discussion of organic field effect transistors

(OFET).

1.2 Energy state of organic materials

Organic semiconductors are carbon-based materials, and what makes these semiconductors unique is their bonding structure. The molecular structure of an organic semiconductor often displays two different carbon bonds, alternating between a single carbon-carbon bond and a double carbon-carbon bond. When carbon atoms bond with sp2 hybrid orbitals, three covalent s bonds will be formed in the same plane, but a remaining, nonhybridized pz orbital is not in the same plane. Nearby pz orbitals overlap with each other and become p orbitals. The electrons of these p orbitals are completely delocalized over the molecule, and this structure is called a conjugated system. With this p orbital delocalization, two electronic states are formed due to Pauli exclusion.

One is the valence state, which is comprised of filled p bonding orbitals, and the other is unfilled p* antibonding orbitals known as the conduction state (Figure 1.1). The organic material bonding is based on sp2 hybrid orbitals with s single bonds. Compared to p bonds, s bonds are significantly stronger, which helps to hold the structure together to order the p orbital [1, 4]. A useful conjugated material for semiconductor devices needs to have ordered, interconnected delocalized conduction

bands and valence bands. One of the most famous conjugated materials is graphite, the conduction bands and valence bands in the graphite have many of the properties described above; however, part of the conduction bands overlap with the valence bands, which makes graphite become semimetal material [4].

The charge transport in conjugated materials defines the electrical properties, which relies on proper p orbital overlap between nearby molecules. The parameter which determines the charge transport property is transfer integral. In small molecules, chemical bonding makes the whole

structure in the same plane, assisting the p orbital overlap in the molecule and yielding a ~0.1 eV transfer integral [1]. In polymers, the structure of the bonding pushes the p orbital away from the backbone. As a result, the band gap between p* conduction band and p valence band is bigger, and the transfer integral is around 1 to 3eV.

The excited state of molecular semiconductors is usually explained in two ways: one theory specifies the excitation state is located at one individual molecule, and the other theory incorporates the wavefunction to explain the excited state as energy band behavior. Organic semiconductor behavior is generally a mixture of these two theories, complicating the overall interpretation. When the excited state is localized to a single molecule, there will be a big energy difference between spin triplet excitons and spin singlet excitons. Due to the lack of spin-orbit coupling, there won’t be any radiation from the triplet state for a conventional OLED [1, 2, 5], which limits the overall emission efficiency. In the polymer, both singlet and triplet excitons are distributed evenly cross the material, but singlet excitons could be delocalized away from the chain by as much as 10nm, and the triplet excitons are considerably more localized due to their antisymetric wavefunctions.

In the localized excitation, the interaction between an electron and the lattice could affect the energy states. This happens when an electron moves from valence band to conduction band and the rest of the electrons in the valence band rearrange to adapt the condition. The rearrangement affects the bandgap size, the bandgap change can be observed through light emission or absorption from the device [6, 7].

1.3 Polymer structure and charge transport

Charge transport occurs when charges hop between molecules [3]. The charge mobility in an organic semiconductor based on a conjugated material is typically around 10-6 to 10-1 cm2/Vs, and can be higher for crystalline organic semiconductors [1-3]. The applied electric filed through the device and the disorder in the material can impact charge mobility. Random orientation and position of each molecule also affect the charge mobility, which makes carriers tend to localize on an individual molecule. The Gaussian disorder model can be used to explain the charge transport in amorphous organic materials, where each localized energy state has the Gaussian density of states, and the charges hop across each localized state in the material [1, 7].

One important behavior in organic semiconductors is the electron-lattice interaction, or polaron formation [8], by which the electronic excited state affects the structure of lattice. When the structure is viewed symmetrically, the bond lengths in the material could rearrange to help the polaron state to overlap with the polymer structure. The other factor that influences this effect is the disorder, as disordered ions have a strong Coulombic potential to keep the wavefunction of the charge-carrier on the polymer structure, and the Coulombic potential also assists the polymer restructure.

1.4 Applications-Organic light emitting diodes

Organic light emitting diodes (OLED) have been in commercial products for years. OLEDs can be comprised of multi-layers of organic and inorganic materials [9], which are usually deposited by spin-coating or direct printing [5]. The color and functionality can be tuned based on

different materials for either lighting or display applications (Figure 1.2). Emission in OLED is based on electron and hole recombination, in which opposite electrodes inject electrons and holes.

Figure 1.2. Organic light emitting diode as display. Red, green, and blue are the typical colors of each pixel in an OLED matrix. Reproduced from Wikimedia Commons licensed under [CC BY 3.0]. These carriers recombine with each other in the bulk of device as bound excitons to emit light. To facilitate the device efficiency, low work function metals are used for electron injection, and conversely a high work function metal assists hole injection [5, 9]. An organic coating can be added to the electrodes to match energy levels and improve injection. The boundaries between the organic materials and electrodes is an interesting area for research. Electrochemistry at the metal- organic interface could influence device performance by affecting hole and electron injections. and techniques such as Scanning Kelvin Probe Microscopy(SKPM) can be used to understand the boundary potential distribution [10, 11].

In order to have better control over electron-hole recombination in the bulk, a heterojunction is often designed by incorporating two different band gap semiconducting organic materials in the device. The exciton ionization near the heterojunction affects the efficiency of the

OLED device, and optimal organic materials have a higher exciton binding energy than the

ionization energy to keep the exciton stable near heterojunction [6]. For example, the charge-

transfer exciton can be formed by an electron from one side of heterojunction and a hole from the

other side [1, 6]. The heterojunction can trap the exciton by lowering exciton’s energy, which

significantly improve the device life time.

As discussed above, there are two types of excitons, singlets and triplets. Triplet excitons

typically have lower energies than singlet excitons, and can be difficult to radiate. However, the

electron hole recombination process only creates one singlet out of every four recombination,

which leads to very low efficiency OLED devices that do not make use of triplet emission. In ionic

transition metal complexes (iTMCs), however, both singlet and triplet excitons can radiate due to

spin-orbit coupling in the material [12]. Also, conventional OLED devices require multiple layers

of material to reach desired efficiency, but iTMC materials such as iridium complexes have

demonstrated high efficiency in a simple, single layer device (Figure 1.3). This simplicity of

fabrication with an efficient device makes iTMCs an interesting area for research.

Figure 1.3. On the left is the conventional OLED device with multi-layer material, on the right is iTMC OLED device with simple 3 layers structure.

1.5 Organic Field effect transistors

One of the most important applications in semiconductor technology is the transistor. The semiconductor industry and research labs are working hard to develop the next generation transistor. One area of interest is the switch speed, as a transistor with high on/off switch speed allows fast process power. Another is the efficiency, facilitated by low leakage current when the

Figure 1.4. Metal–oxide–semiconductor field-effect transistor (MOSFET). It consists of four components: S is the source, D is the drain, B is bulk or substrate, G is the gate, and the purple layer is the dielectric. Reproduced from Wikimedia Commons licensed under [CC BY-SA 3.0]. transistor is off, and low power consumptions when transistor is on. An additional area of interest is the cost of manufacturing. As the length of the channel gets smaller, the fabrication expense increases due to increased processing steps [13]. The simplicity of fabrication and compatibility with flexible substrates of organic field effect transistors [14, 15] make them an attractive area of interest for future transistor applications. Compared to traditional metal–oxide–semiconductor field-effect transistors (MOSFET) (Figure 1.4), organic field effect transistors (Figure 1.5) have simpler structures. As an example, an organic FET can be formed from one thin layer of organic material between two sources and drain electrodes. A doped silicon substrate is used as a gate,

and silicon dioxide is the dielectric layer. When a voltage is applied on the gate, the corresponding electric field in the dielectric will induce surface charge density between the dielectric and the organic material. The surface charge density change in the organic material establishes a pathway between the source and drain electrodes for current injection. This simple structure with the field effect transistor property provides variety of possibilities for organic semiconductor applications.

One of the challenges of organic materials in transistors is the contact effect [14], which happens due to the polarization at the interface between organic material and metal electrodes. By using SKPM to understand these interface effects [16], the effect can be fully studied and avoided by multilayer organic coating or different electrode materials [17]. Another challenge is the charge

Figure 1.5. Organic field effect transistor (OFET), the voltage will apply on gate, drain, and source for devcie to work, the isolator works as dielectric which will have induced electric field corresponding to gate voltage. Reproduced from Wikimedia Commons licensed under [CC BY- SA 3.0]. mobility. When the induced charge between the organic and dielectric are not aligned properly, the conduction path is not ideal for mobility. Diffusing this induced interface charge helps to improve this issue. One approach is to coat a thin layer of alkyl material [18], which has been used to enhance the charge mobility in different applications. For a high charge mobility polymer, the behavior cannot be explained by the model that assumes charges are hopping across each localized

state. In this case, the polaron model gives a better overview of the high charge mobility with its impact [8]. When the voltage applied to the device, polaron formation will change the lattice structure, and the conduction band and valence band will shift according the lattice deformation.

This will affect the size of the bandgap. By measuring the optical absorption of the organic field effect transistor, polaron formation can be observed from the change of the bandgap size.

While the charge mobility limit in organic semiconductors is still under study, currently most organic materials have a lower charge mobility than silicon based devices, particularly those based on crystalline. Devices based on rubrene have reached a charge mobility of 15 cm2/Vs [19], competitive with amorphous silicon. However, the device fabrication process and scaling of the material for commercial production still requires more research to support its application.

1.6 Organic semiconductor material process

Compared to silicon and other traditional crystalline semiconductor materials, organic materials are often soft and soluble in solvents, the way to process organic material has to be different. Soft, soluble materials are generally easier to fabricate, particularly advantageous for building up large arrays of devices; however, solubility doesn’t always ensure that a uniform film will be deposited, as the organic material may crystalize in certain areas.

Vacuum sublimation has been proven to be a useful method to improve the thin film uniformity and stability. Researchers at the University of Bonn in Germany have used vacuum sublimation to coat quinacridone on an organic field effect transistor [20], which shows promising charge mobility and long-term stability. For conventional organic lighting emitting diodes, it is important to have a heterojunction of two uniform layers of organic material, and vacuum sublimation has been widely applied in OLED fabrication for better layer adhesion. However, this

does lead to greater time and expense in processing versus solution based techniques. Another known method for organic material processing is ink-jet printing [14, 21], which has a lower cost compared to vacuum sublimation., and it also allows large area processing. Different colored pixels can be accurately printed in the desired location for OLED panel fabrication. The same technique can also be used on organic transistor and other organic circuits.

1.7 Conclusion and dissertation outline

This chapter has explained the theoretical models behind the properties of organic semiconductors, introduced the applications of OLEDs and OFETs, and showed the different material processing techniques. Semiconducting organic materials have been widely used now, and possibly using will increase in consumer electronics in the near future. However, fundamental knowledge is still needed, requiring study from both theoretical and experimental aspects to make organic electronics viable for more applications.

This dissertation is going to focus on two different types of organic semiconducting devices.

One class is iridium-based light-emitting electrochemical cells (LEECs), and chapter 3 will show the research of the influence of lithium additives in small molecule light-emitting electrochemical cells. The other class of devices concerns nanowire applications, and the specific example is the

DNA-inspired self-assembly of nanoscale electronic devices. Chapter 2 will introduce the molecule devices and DNA nanowire. The detail of DNA nanowire research will be presented in chapter 4.

CHAPTER 2

MOLECULAR ELECTRONICS AND DNA- BRIDGED NANOGAP JUNCTIONS

2.1 Preface

The semiconductor industry has advanced quickly for the past few decades, and the key for technology to continue to advance is to minimize the device size. Smaller devices give us higher device density in the same area, which leads to more functions and low power consumption.

Currently, the semiconductor industry has encountered challenges in trying to shrink the device size while maintaining low cost. The cost for device processing increases as the device size is reduced, and simultaneously the yield of the devices also decreases due to the high density patterned features. It would be valuable for current technology to develop a new method to fabricate devices with atomic precision in high yield at low cost. This chapter will start by illustrating the current silicon technology limitation, and then explain how molecular electronics work. The final sections will focus on DNA properties and DNA nanowires, and how to use DNA as a molecular electronic device.

2.2 Current silicon technology and limitation

For the past 50 years, the semiconductor industry has followed Moore’s law prediction: the number density of transistors doubles every two years. The semiconductor manufacturing processes include photolithography, etching, metal deposition and other detailed techniques. In order to follow Moore’s law [22], all processes are required to coordinate with each other, and the industry has been able to follow the trend so far. However, the overall process requires more precise control now that the critical dimensions have reached the nanometer scale in recent years.

The first issue that industry now faces is photolithography. Modern photolithography tools shine photons on masks, which have alternating section of dark and transparent features to transfer a pattern. The photons passing through the mask expose the photoresist with the desired pattern from the mask. Based on different type of photoresists, the exposed part of photoresist can be washed away or rendered to stay on the wafer for further etching or film deposition processing.

The advantage of photolithography is high throughput compare to direct writing electron beam lithography. One layer of sophisticated patterning takes seconds to several minutes for photolithography to write.

The key to shrink the device size is the photon wavelength; the wavelength could be from visible light to ultraviolet (UV) or even extreme ultraviolet (EUV) for small features under 20nm.

When the device size is reduced, additional procedures need to be added to the wafer process, such as multi-patterning to reduce the pitch width and extra etching and cleaning steps to ensure the wafer quality. To increase the resolution of photolithography, multiple enhancements in the tool need to be made, improvements that include fine-tuned optics, integrated stepper function, and the immersion technique. These improvements provide the industry with most high resolution photolithography tools. However, the advanced tools bring significant cost to the semiconductor industry, and methods to inexpensively shrink the size of devices are of high interest.

2.2.1 Electron beam lithography

Besides photolithography, another lithography method is electron beam lithography (EBL).

Compared to photolithography, there are several fundamental differences between EBL and photolithography (Figure 2.1). EBL uses electrons to direct write on the EBL resist to create a pattern, while photolithography requires a prefabricated mask. Instead of traditional optics in

photolithography, EBL uses electron optics, which are basically electrostatic and magnetic forces.

The interaction between photoresist and photons is also different from the interaction between electrons and the EBL resist.

Current EBL resolution can reach less than 10nm. EBL is a proper tool for research and high precision nanometer scale fabrication. Since there is no mask needed for EBL, the time from

Figure 2.1. Lithography tool comparison between Photolithography and Electron beam lithography. Photolithography requires mask to define the pattern, Electron beam lithography can directly write pattern on the sample. pattern design to a completed prototype device may only take a few days. For mass production, the mask can be created with EBL, and then photolithography or nanoimprint lithography can be used to achieve rapid production. Throughput is the greatest weakness, as a 500um by 500um area of a pattern would take a typical EBL writer five hours to complete, while the same pattern size would only take minutes for photolithography.

2.2.2 Lithography limitations and next step

Besides the cost, the major issue for photolithography is a technological limitation.

Immersion lithography is very common for sub-100nm dimension devices. To increase the resolution of immersion tool, liquids with high refractive indices are used; however, these fluids can form bubbles, and exhibit unstable temperature and pressure, all of which compromise the quality of pattern transfer.

To get down to smaller features, multiple patterning is also common with photolithography, but complicated mask design and accurate overlay control make this approach expensive. EBL has higher resolution, and while it is convenient for prototype pattern design and research purposes, the low throughput is a drawback for implementation in industry. Significant resources have been invested into silicon technology, enabling the current pace of advancement with Moore’s law.

However, when the size of the device reaches the molecular scale, each molecule has to be accurately located to guarantee functional devices. As silicon based devices are apparently reaching the limits of photolithographic patterning, molecular scale patterning becomes difficult for industry to follow.

Traditional top-down processes have faced a variety of issues to meet the need for sub-

10nm technology [22]. The next step for semiconductor device development is to maintain high throughput with precise control at the nanometer scale. Molecular electronics could be the next generation electronic components, following 50 years of silicon based technology. The following sections will summarize the properties and the fabrication processes of various types molecular electronics.

2.3 Molecular electronics

Modern semiconductor devices use doping or ion implantation to give silicon n-type or p- type conductivity. The implanted ion number density is significantly smaller than the host number density in the targeted material to maintain the original crystal structure. When the size of the device shrinks down to near one thousand atoms, the whole implantation technique no longer provides predictable results, as quantum tunneling and difficult to predict effects dominate. Ion implantation requires that the desired doped area be opened for exposure by photolithography.

When the device size is reduced, photolithography requires additional steps and expensive tools to be compatible with ion implantation, which raises the complexity of the process and leads to additional cooperative problems.

To solve the above issues, molecular building blocks can serve as a new fabrication approach. Molecular building blocks provide a predictable structure and precise number of atoms.

Knowledge of the underlying molecular dynamics and stereochemistry allows control of the manufacturing process at the atomic level. Molecular devices could exhibit behavior close to traditional semiconductor devices, or the quantum effects in the devices may dominate and offer unique possibilities for the future electronics.

2.3.1 Current development of molecular electronics

The success of the silicon semiconductor industry has limited a full investment in molecular electronics for the past few decades, but the molecular limit of devices may finally create a paradigm shift. The growing quality and quantity of molecular electronics research provides hope for this shift, and current study is focused on understanding the underlying device

physics. Focusing on molecular junctions, most of the work shows reveals the charge transport behavior in various molecular crystals and molecules.

The structure of a molecular junction is similar to traditional electrical junctions. Either single molecules or several molecules bridge the gap between two electrical contacts [23-25]. At this stage, no research and study has successfully put molecular devices into interconnected circuits, due to the extreme small dimension and complexity. Thus, considerable progress needs to be made for molecular devices to thrive in consumer electronics.

2.3.2 Molecular junctions

The most common molecular junction is the metal-molecule-metal (MMM) junction. The

MMM junction is not strong, and it is usually sensitive due to the metal-molecule bonding and the molecular conformation [26]. The boundary condition between the metal and the molecule becomes an important topic in MMM junction, and study is based on the number of molecules between the junction. There are two types of MMM junctions, single molecule junctions and ensemble junctions. A single molecule junction simply places one molecule between two metal electrodes.

For ensemble junctions, multiple molecules stack between the electrodes, similar to a sandwich structure with one electrode on the top and the other one at bottom. Single molecule devices could be used to investigate the electrical properties of one molecule directly. Ensemble molecular devices could involve electronic coupling from adjacent molecules, and additional cooperative behavior between each molecule could affect the measurement.

Measurement of single molecule behavior can be performed by scanning probe microscopy

(SPM) [27, 28]. The single molecule image and atomic level metal bonding information can be

extracted from SPM measurement. Other experiments can also be done to understand how single molecules react to various temperature or humidity conditions, as well as surroundings gas, liquid, or vacuum. Another measurement of interest for molecular devices is inelastic tunneling spectroscopy IETS [29]. By using IETS at extremely low temperature, the molecular vibrational spectrum, conductance, and current-voltage IV curve data can be extracted.

2.3.3 Single and ensemble junction

One of the biggest challenges for single molecule measurements is the inconsistency of results between different measurements of the same molecule or measurements of similar systems between different laboratories. This issue impedes the progress of molecular device development.

Issues could arise from the sophisticated metal-molecule junction, molecular conformation changes, and multifarious environmental conditions between labs and measurements.

Understanding the junction details under an applied bias is the key to ensure consistency of the measurements. Characterization of the structure of the molecule during the measurement has proven challenging for most traditional characterization tools. However, characterization technology has advanced, and new theoretical models assist the understanding of single molecule devices.

Compared to single molecule junctions, ensemble junctions have multiple parallel molecules bridged between two contacts. In this case, the structure is visible by current characterization tools during electrical measurement. The ensemble molecular structure can be monitored right after the device fabricated, and structure changes under an applied bias can be observed. Ultraviolet visible absorption spectroscopy and Fourier transform infrared radiation spectroscopy have been used for in situ measurements of molecular ensemble devices. Another

advantage for the ensemble experiments is the fabrication process. The fabrication can be done with widely used lithography tools on silicon wafers. This advantage allows ensemble junction devices to more easily interface with current silicon devices, and more applications may be discovered due to this feature. Even though single molecule junction devices are not well suited for interface with current silicon technology, progressing lithography resolution and advancing silicon processing techniques could help single molecule devices become more practical in the near future.

One issue with ensemble device measurements is the interpreting of results. The measurement data could be from one molecule or multiple molecules, and the precise number of molecules between two electrodes needs to be determined for interpreting the data correctly.

Besides the numbers of molecules, the surface area of electrodes and conformation changes of each molecule also affects the electrical data in the ensemble device.

2.3.4 Type of single molecule devices

One of the processes to fabricate the single molecule junction is the break junction method

[30]. The gap between the junction can be patterned under one nanometer or even mechanically controlled to a picometer. One break junction method starts with a flexible substrate, electron beam lithography patterns a metal wire on the substrate, and the gap is formed by bending the substrate.

The gap size is based on the degree of bending of the substrate. This method is widely used with various metals with a variety of substrates. Alternatively, a high current can be passed through a thinly patterned wire until electromigration of the wire atoms opens a gap.

Another technique comes from the scanning probe microscopy (SPM). Haiss has developed several SPM methods for forming and characterization molecular junctions [27, 28].

These methods require a SPM tip to use the same material with the desired electrode metal, and the electrode metal is in the solution. The SPM tip is dipped into the solution and then pulled away from the substrate, and the nanogap will be formed between the tip and substrate metal. All methods of junction fabrication allows a large amount of devices to be manufactured and characterized, providing statistical data to eliminate the variables from electrode geometries and conformational changes of molecules. However, obtaining single molecule information is still difficult, but the statistical data from mass characterizations will provide significant resources for overall estimation.

The assembly process for break junction and SPM methods are also different. For the break junction method, the molecule is in the solution, and after the junction is formed the single molecule attaches to both electrodes. In the SPM process, when SPM tips gets close to the metal substrate, a single molecule will bridge the gap between the tip and substrate. This allows the SPM method to measure a single molecule within a film, a thin layer of molecules, or a large distribution of molecules on the substrate.

By applying the SPM measurement principle, other tools can serve a similar purpose for single molecule characterization. Scanning tunneling microscopy (STM) [31] and atomic force microscopy (AFM) [32] with a conductive tip can obtain conductance information from single molecules. The advantage of the SPM method allows the measurement of inhomogeneous junctions, while the data from break junctions are most easily interpreted when restricted to homogeneous junctions.

2.3.5 Type of ensemble molecule devices

Depending on the number of molecules between the junction and the contact method, there are several different techniques for constructing ensemble devices. One of the most famous approaches is to form a Langmuir-Blodgett film [33]. This approach, utilizing repetitive dipping of a substrate into a solution of component molecules, allows the thickness of the single molecule film to be uniformly controlled. Another method, the self-assembled monolayer approach [34], forms a layer of molecules on the substrate, with one side of the molecule bonded to the substrate by surface-specific chemistry.

One of the issue for ensemble device is the top electrode deposition after the molecule film formed on the substrate. A thin molecular film is usually easily broken and damaged, and the top electrode metal deposition has to be done precisely to prevent a top electrode short with the substrate.

Besides traditional metal evaporation, electrochemical metal deposition [35] is also feasible. The metal stamp method [34] directly imprints a thin layer of metal on the thin layer of molecules. The top and bottom electrode size, uniformity, and material properties affect the ensemble device measurement. To avoid electrode defects and other issues, a small electrode area is preferred to enable more accurate experiment control.

2.3.6 Interfaces and bonding

One of the common interfaces between a metal and a molecule is the gold-sulfur bond, which is easily self-assembled and provides stable bonding. The problems of the metal-molecule- metal device are the unknown contact geometry, numbers of molecules between junctions, and

molecule conformation during the measurement [32, 34, 36]. Current metal-molecule-metal junction research focuses on device fabrication and characterization to obtain statistical data of each molecule. There are three types of metal-molecule interfaces, and they are classified by the bond strength. Langmuir-Blodgett films have the lowest strength linkers [33]; the bonding energy is from van dear Waals forces and is usually less than 0.5 eV. The bonds made from thiols and amines are classified to intermediate bonding linkers [37, 38] with bonding energy around 1.9eV, and they usually bond well with common used noble metals. Carbon based bonds show the highest bonding energy [38] around 3.5eV to 4eV. Typical carbon-carbon bonds and silicon-carbon bonds are widely used in daily applications due to their excellent electronic coupling property. Each bonding strength has its own pro and cons, as the lower bond strength molecule devices have worse stability but easier self-assembly for highly ordered films. Higher bond strength devices require more processing and cost for self-assembly, but it creates stronger and more reliable bonding. The intermediate linkers are widely used due to the balanced performance for both self-assembly and coupling. Linkers of this type involve amines (-NH2), thiols (-S), carbodithioates (-NS), carboxylic acid (-COOH), selenols (-Se), sulfoxides (-SO), sulfinates (-SO2), sulfonates (-SO3), cyanides (-

CN), diisothiocyanates (-NCS), and isocyanides (-NC) [25]. The unique coupling properties among these linkers allow different applications. In particular, the metal selectivity of each linker makes the fabrication of multi-linker devices possible.

2.4 DNA based nanowire MMM junction

There are several advantages for using DNA as a molecular building block due to the unique properties of DNA. The cost is lower than other similar materials, yields for automated synthesis are high, and it also has better adaptability for both structure and size. The DNA

electrochemistry and charge transport property has been used in enzymatic sensing [39-41], and the gold nanodot arrays found in some of these sensing devices has proven that DNA can bridge a

Figure 2.2. Perylene-3,4,9,10-tetracarboxylic diimides PTCDI monomers integrate to nanowire through automated synthesis process. Blue monomers represent n-type semiconductor, red monomers represent p-type semiconductor. The nanowire can be formed by different number and type of monomers. gap on the nanometer scale. There are several different linkers that allow DNA to connect to metals or semiconductor materials. The conductivity of DNA has been proved by using DNA as sensing devices [41] and other molecular applications [42].

A DNA-based nanowire could serve the purpose of future electronic elements, as it demonstrates stable structure and self-assembly [39, 40, 43-45]. To enable electrical modification of the DNA nanowire, molecules based on perylene diimides [46-51] could be utilized as surrogate bases. Perylene diimides can be modified to exhibit both n-type and p-type semiconductor property modifications. Importantly, perylene diimides naturally π-stack with each other at a distance of

~3.4 Ǻ, similar to the spacing between bases along the DNA axis. For DNA assembly and

characterization, the electrode patterns can be fabricated by electron beam lithography, which allows high accuracy nanometer scale devices for DNA nanowires. The DNA technology today in bioconjugate chemistry enables phosphoramidite synthesis to covalently attach a large number of moieties and molecules at specific locations in a sequence with high yields. DNA origami with perylene-3,4,9,10-tetracarboxylic diimides PTCDI monomers (Figure 2.2) provides the possibility to construct complex and functional nanowires. Coating a complete monolayer of DNA on a device requires only a nanogram of DNA, which makes the cost of synthesis very low. The wide options of linkers between DNA and metals also gives a DNA-silicon fusion device a promising future.

Currently, a fully functional electronic DNA chip is still far from full implementation, but a DNA molecular device combined with current silicon technology is a feasible option for future electronics. Based on the various experimental results concerning the conduction of DNA, there has been an ongoing debate about the ability of DNA to support charge transport. Based on variety of approaches, and notably, the variable environmental control of each experiment, DNA could be

Figure 2.3. Nanowire based diode and field effect transistors (FETs). Blue monomers represent n-type semiconductors, and red monomers represent p-type semiconductors. A) DNA nanowire PN diode device, B) DNA nanowire N-Type FET device, C) DNA nanowire P-Type FET insulating, semiconducting, and some even suggest it could be superconducting. Once the environmental conditions (DNA must be maintained in a biologically relevant condition to be fully

base paired) and electronic coupling are considered, there is little debate that DNA supports some appreciable degree of conductivity. The semiconducting property of DNA is based on DNA p- stack integrity, temperature, humidity, and solvent conditions [40, 43-45].

2.4.1 DNA nanowire device experiments

The DNA nanowire device can be designed to be 2-terminal as a diode and 3-terminal as a field effect transistor (Figure 2.3). In order to have the device control between p-type and n-type

(Figure 2.4), two base pairs in the DNA p-stack will be substituted by perylene-3,4,9,10- tetracarboxylic diimides (PTCDIs). Perylene has been utilized as an organic semiconductor, which serves as active elements in the DNA to allow electrically tunable for the DNA based device. To fabricate the DNA nanowire device, the first challenge is to have the controllable nanoscale gap to match the desired DNA length for successfully self-assembly.

The electron beam lithography can provide precise high resolution from 2nm to 20nm, this will define the nanogap between 2 electrodes. To enable the device to be electrically tested, the test pads need to be at least 50um square for probes to reach, the large array of test pads can be

done with traditional photolithography, then use electron beam lithography to process the nanogap between test pads.

There are several characterizations that can be done with current technology for better understanding of the DNA nanowire device. Atomic force microscopy (AFM) can observe the topography of device to demonstrate whether it is a single molecule device or ensemble device.

An AFM with a conductive tip can perform Scanning Kelvin Probe Microscopy (SKPM) to identify the surface potential distribution. The current-voltage (I-V) curve measurement from either the DNA diode or DNA FET device provides information for future electronics applications.

2.5 Conclusions

The limitation of silicon technology will not stop the development of next generation electronics. Instead of silicon semiconductor devices, a molecular device could provide similar or even better applications with lower fabrication cost. The combination of silicon device process technology and molecular device knowledge opens a new path for device physics study. DNA has demonstrated semiconducting ability from previous research, and a DNA nanowire can be tuned as n-type or p-type semiconductor with chemical modification and then self-assembled on a large array of devices. All these advantages allow DNA nanowire devices to be functional electronic components with potential for future commercial products.

CHAPTER 3

THE INFLUENCE OF LITHIUM ADDITIVES IN SMALL MOLECULE LIGHT-

EMITTING ELECTROCHEMICAL CELLS1

Authors: Kuo-Yao Lin1, Lyndon D. Bastatas1, Kristin J. Suhr2, Matthew D. Moore2, Bradley J.

Holliday2, Majid Minary-Jolandan3,4, and Jason D. Slinker1*

1Department of Physics, The University of Texas at Dallas, 800 W. Campbell Rd., PHY 36,

Richardson, TX 75080-3020, USA

2Department of Chemistry, The University of Texas at Austin, 105 E. 24th St., Stop A5300,

Austin, TX 78712-0165, USA

3Department of Mechanical Engineering, The University of Texas at Dallas, 800 W. Campbell

Rd., EC 38, Richardson, TX 75080-3020, USA

4Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas, 800 W. Campbell

Rd, Richardson, TX 75080-3020, USA

1 The following chapter is reproduced with permission from Lin, Kuo-Yao, et al. "Influence of Lithium Additives in Small Molecule Light-Emitting Electrochemical Cells." ACS applied materials & interfaces 8.26 (2016): 16776-16782. Copyright 2016 American Chemical Society.

3.1 Preface

Light-emitting electrochemical cells (LEECs) utilizing small molecule emitters such as iridium complexes have great potential as low cost emissive devices. In these devices, ions rearrange during operation to facilitate carrier injection, bringing about efficient operation from simple, single layer devices. Recent work has shown that the luminance, efficiency, and responsiveness of iridium-based LEECs are greatly enhanced by the inclusion of small amounts of lithium salts (≤ 0.5%/wt) into the active layer. However, the origin of this enhancement has yet to be demonstrated experimentally. Furthermore, although iridium-based devices have been the longstanding leader among small molecule LEECs, fundamental understanding of the ionic distribution in these devices under operation is lacking. Herein, we use scanning Kelvin probe microscopy to measure the in situ potential profiles and electric field distributions of planar iridium-based LEECs and clarify the role of ionic lithium additives. In pristine devices, it is found that ions do not pack densely at the cathode, and ionic redistribution is slow. Inclusion of small amounts of Li[PF6] greatly increases ionic space charge near the cathode that doubles the peak electric fields and enhances electronic injection relative to pristine devices. This study confirms and clarifies a number of longstanding hypotheses regarding iridium iTMC-LEECs and recent postulates concerning optimization of their operation.

3.2 Introduction

Light-emitting electrochemical cells (LEECs) have great potential in passive lighting applications as low-cost emissive devices [52-54]. LEECs may be based on polymeric [55-59] or small molecule emitters [52, 53, 60-65]. Among small molecule LEECs, devices based on ionic

transition metal complexes (iTMCs) containing iridium metal centers [52, 54, 66-71] have emerged as attractive device compositions as the iridium iTMCs are highly efficient triplet emitters offering superior brightness and efficiency [52, 53, 61, 62]. These iTMC-LEECs exhibit efficient performance from a single, solution-processable mixed conductor layer between two electrodes

[66, 68, 71, 72]. The simplistic operation of iTMC-LEECs stems from their identity as mixed conductors — materials that possess both ionic and electronic conductivity[69, 73, 74]. Upon application of a bias, ions in the active layer redistribute, leading to double layer formation at the contacts which facilitates injection of electrons and holes. These devices have shown promising operation from laminated fabrication [75] and in fault-tolerant, scalable lighting architectures [76,

77].

Recent advances have shown that simple modification of the active layer greatly improves the maximum luminance and response times of iridium-based iTMC-LEECs without negatively impacting the device lifetime. In particular, this was achieved by blending small amounts of

Li[PF6] into iTMC films. Lithium salt additives in [Ir(ppy)2(bpy)][PF6] and another specifically designed iridium-based emitter films improved the maximum luminance to over 5000 cd/m2 while utilizing simple low-voltage, constant current driving [78, 79]. Additionally, these additives were also found to improve turn-on response times from two days to a matter of seconds [78, 79].

Indeed, salt blending improved all efficiency metrics, suggesting that ionic blending balanced carrier injection. It has been postulated that the small radius of Li+ compensates for the low ionic mobility of Ir(III)+ in the pristine iTMC films, enabling a greater and more rapid accumulation of space charge at the cathode for improved electron injection (Figure 3.1) [79]. However, no direct measurement of the effect of lithium salts on the active layer of LEECs has been demonstrated to

date. Thus, both the factors governing the limitations of the performance of iridium-based iTMC-

LEECs and the impact of ionic additives on the kinematic ion motion and equilibrium ion distribution need to be clarified.

Here, we utilize scanning Kelvin probe microscopy (SKPM) of iTMC-LEECs to discern the in-situ potential and electric fields within the active layers. Planar devices were prepared with a parylene fabrication strategy to restrict the mixed conductor between gold electrodes, representing the blocking electrodes of thin-film, sandwich-structure devices. Mixed conducting

films of pristine [Ir(ppy)2(bpy)][PF6] and films with Li[PF6] additive were measured to obtain comparative potential profiles in these device compositions. This approach enabled the direct study of the steady state and transient implications of ion additives to iTMC-LEEC devices.

3.3 Scanning kelvin probe microscopy

Scanning Kelvin probe microscopy (SKPM) is a modified atomic force microscopy technique that utilizes a conducting tip to discern the potential difference between the tip and sample [10]. The surface potential obtained from these studies represents the combination of the work function difference between the tip and the sample, the trapped charge, and the applied voltage difference between the tip and the sample. Overall, these measurements provide a direct and accurate determination of potential differences on surfaces [10].

SKPM utilizes DC and AC potentials applied between the conducting tip and the sample to produce an electrical force on the cantilever. Modeling the tip-sample interface as a capacitor, of capacitance C, this force F depends on the voltage difference V and tip-sample distance z as:

1 ¶C F = V 2 . (Equation 1) 2 ¶z

This potential difference V depends on the sum of the surface potential difference Vsp, the applied DC potential VDC, and the applied AC potential VAC of angular frequency ω:

V = Vsp +VDC +VAC sin(wt). (Equation 2)

Combining these equations, it can be seen that the force on the tip separates in to three terms, one constant term, one term linear in ω, and one term depending on ω2:

1 ¶C ìé 2 1 2 ù é1 2 ùü F = V -V + V + 2 V -V V sin wt - V cos 2wt íê( DC sp ) AC ú [( DC sp ) AC ( )] ê AC ( )úý .(Equation 3) 2 ¶z îë 2 û ë2 ûþ

For SKPM, the term linear in ω is key, as the amplitude depends simply on the difference between the DC applied potential VDC and the surface potential Vsp. Thus, SKPM provides a straightforward means of obtaining spatial profiles of surface potentials [10].

3.4 Devices for surface probe study

Typical iridium-based iTMC-LEECs are prepared in a sandwich structure format with a

~100 nm thick iTMC layer between two electrodes [52-54]. However, it is difficult to use surface scanning probe methods on sandwich structure devices due to the confined and small geometry.

Thus, it is beneficial to prepare devices in a planar geometry that reproduces the features of the sandwich structure architecture. In particular, the electrode interfaces of conventional devices are generally ionically blocking — ions can migrate to the edge of the electrodes but typically not into or through them. Thus, simple spin coating of films onto electrodes or evaporating electrodes on top of planar films is insufficient to capture this feature, as ions can migrate from the region between the electrodes due to fringing fields and diffusion. Previous electric force microscopy studies of iTMC-LEECs clearly demonstrated this important effect [73]. However, preparing organic films to address this concern is complicated by the common solvents used with typical photoresists and iTMCs [81-83].

To fabricate our devices to reproduce blocking electrodes, we utilized a parylene patterning technique. Parylene (or poly(p-xylylene)), a common encapsulating agent, can be utilized as a sacrificial lift-off layer in the photopatterning of thin films through a method developed by Ilic et al. and DeFranco et al [81-84]. Parylene is a self-initiated chemical-vapor deposited polymer that offers conformal, pinhole-free coatings that adhere weakly to many surfaces, including freshly cleaned gold. Planar devices were prepared using a modified form of this process as illustrated in

Figure 3.2A. A silicon wafer with a native surface oxide serves as the substrate. A 100 nm thick gold film and a 2.5 µm thick parylene layer are each thermally evaporated onto the surface.

Photoresist is spin cast on top of this stack and patterned by photolithography. Parylene is then etched by oxygen plasma, which generally also removes most of the remaining photoresist by the end of the process. The parylene then acts as a chemical mask against a wet gold etch. The iTMC layer, either pristine [Ir(ppy)2(bpy)][PF6] or this material with a small fraction of Li[PF6], is then spin coated from solution on top. The remaining parylene is then removed by mechanical peeling.

The advantage of this patterning technique is that the film is patterned strictly between the electrodes and well aligned within the structure. Figure 3.2 also shows (B) the interdigitated device used for this study and (C) a fluorescence image demonstrating the film patterned between the electrodes as a result of parylene processing.

3.5 Potential distribution of steady state

Patterned interdigitated devices were then characterized by SKPM with a detection limit of 10 V under nitrogen atmosphere to limit degradation. Devices subject to only the 10 V bias did not demonstrate appreciable changes over the course of several hours. While this would be sufficient bias to turn on a thin film sandwich structure device of ~100 nm thickness, these planar devices on the order of 10 µm thickness needed higher applied bias to achieve comparable electric fields and promote redistribution of ions leading to observed light emission. Thus, devices were first prebiased at 50 V for 30 minutes, apart from the SKPM apparatus, to promote ionic redistribution. This is near the voltage that was observed to produce light emission from these ionic additive planar structure iTMC-LEECs. Then devices were biased at 9 V for 1 hr under SKPM to achieve a steady state measurement after ionic redistribution. Figure 3.3 shows the resulting SKPM surface voltage versus position data for both a pristine planar device of structure

Au/[Ir(ppy)2(bpy)][PF6]/Au as well as a device enhanced with an ionic additive,

Au/[Ir(ppy)2(bpy)][PF6] + 0.5%/wt Li[PF6]/Au, a concentration of Li[PF6] previously shown to dramatically enhance performance [78, 79]. The pristine device shows a voltage distribution resembling a simple resistor — a nearly linear drop between the electrodes. However, upon closer inspection, there is a small potential drop near the anode (~9 µm position). The linear potential drop in the voltage profile near the anode is consistent with an inner Helmholtz layer of densely

− packed [PF6] . There is also a curvature to the overall potential distribution. We present a closer view of this voltage profile in the region between the electrodes in Figure 3.4. The curvature from positions 19 to 8 µm can be modeled as an exponential dependence on position (~e−κ(x)), consistent with a broad outer Helmholtz layer of diffuse cations beginning from the cathode [85]. This

+ demonstrates that the [Ir(ppy)2(bpy)] cations do not accumulate in high density near the cathode

in a pristine device and suggests that there is limited benefit from ionic redistribution on electron injection under these conditions. This overall view of the pristine device contrasts with the voltage distribution previously obtained from pristine [Ru(bpy)3][PF6]2 devices [73], which clearly show large linear potential gradients at both of the contacts consistent with dense packing of ions at both electrodes. On the contrary, the distribution of the pristine device shown in Figures 3.3 and 3.4 confirms the overall poor ionic redistribution of pristine [Ir(ppy)2(bpy)][PF6], particularly cationic accumulation at the cathode. This is also consistent with the longer turn-on times (times to maximum luminance) associated with iridium-based LEECs relative to ruthenium-based LEECs

[86]. The addition of Li[PF6] changes the observed voltage distribution considerably. Linear gradients in voltage are observed near both the anode and cathode consistent with double layer formation. This is particularly clear at the cathode, where there is a ~4 V drop within its vicinity.

From the overlap of distributions in Figure 3.3, the structures of the two profiles clearly differ in the linear behavior. Clearly, the addition of Li+ has a significant impact on the device dynamics near the cathode. The larger potential gradient near the cathode suggests dense packing of Li+ at the electrode, promoting electron injection.

To further evaluate the differences between pristine and ionic additive devices, a plot of electric field versus position is provided in Figure 3.5. Both devices show similar field strengths

− near the anode. This would be consistent with similar accumulation of [PF6] in each case.

However, a much stronger electric field is observed in the ionic additive device near the cathode, with a peak electric field over two times higher than that of the pristine device. This again highlights the dramatic influence of densely packed Li+ near the cathode. Furthermore, the electric field is considerably repressed in the region from 12 to 16 µm in the ionic additive device relative to the pristine device, another effect of ideal LEECs operating below device turn-on [87-89]. In ideal LEECs, ions redistribute in response to the applied electric field and accumulate at the contacts until the applied electric field is effectively cancelled out by the ionic space charge. In

+ practical devices, however, uncompensated low mobility ions, such as [Ir(ppy)2(bpy)] in the pristine device case, may persist in the bulk away from the contacts even with a substantial electric

field in the bulk, and the electric field remains. Addition of an appropriate concentration of Li[PF6] contributes Li+ ions that redistribute efficiently and cancel out this electric field. For the current study, in the region from 12-16 µm, representing the bulk of the device, the electric field is lowered by a factor of 2.7 with the ionic additive. Overall, the electric fields reveal substantial differences between the pristine and ionic additive devices in the cathodic and bulk regions.

3.6 Relaxation dynamics

Some details of device kinetics and ionic space charge effects can be extracted from the measure of the relaxation of devices once turned off. Thus, we probed our devices under 0 V bias immediately following the 9 V steady state operation noted above (Figure 3.6). Data was recorded immediately after turn-off (solid lines) and after 30 minutes of relaxation (dotted lines). For the pristine device, a linear drop across the device is observed, consistent with a linear gradient of

− [PF6] throughout the film. Surprisingly, this gradient remains virtually unchanged after half an hour with shorted electrodes. This speaks to the poor ionic conductivity of pristine

[Ir(ppy)2(bpy)][PF6] films and corroborates the extremely long turn-on times of hours to days associated with their operation. The ionic additive devices, however, are considerably more dynamic. Initially, the devices show a large negative potential throughout the film, potentially indicative of residual electrons in the device. Unlike the pristine device, the space charge in this device dramatically changes with time. In short, this further affirms the ability of the ionic additive device to facilitate injection of electrons by facile space charge redistribution.

3.7 SKPM of LEECs at high salt concentrations

We have observed that conventional sandwich structure device performance degrades quickly (lower current, radiant flux, and external quantum efficiency) when salt concentrations higher than 0.5%/wt Li[PF6] are used, an effect also seen in polymer LEECs.40 To explore the drop in device performance with higher salt concentration, we performed SKPM of planar

[Ir(ppy)2(bpy)][PF6] films with 5.0% Li[PF6]. The result is presented in Figure 3.7A and directly compared to devices with 0.5% Li[PF6]. As above, devices were subject to a 50 V prebias to encourage ionic redistribution across the large interelectrode gap. Clearly, the 0.5% Li[PF6] device

produces well-defined potential drops at the contacts, with a larger drop at the cathode, which is consistent with the large potential barrier for electron injection with the Au contact. In the case of the 5.0% device, the potential profile changes dramatically, with a very small cathodic potential

drop and a substantially larger anodic drop. This distribution no longer correlates with the response anticipated to compensate for the resistance arising from the electronic barriers at the gold electrodes. Measurement of the residual charge upon removing the bias (Figure 3.7B) shows high negative space charge throughout the bulk which efficiently relaxes away with time, suggestive of electronic charge. It is possible, then, that highly efficient electron injection occurs at high Li[PF6] levels and overcomes the available hole concentration, compromising radiant flux and external quantum efficiency. In any case, this study clearly reveals an optimal Li[PF6] concentration at lower weight percentages produces a more idealized potential profile that can be correlated with the higher device performance. In short, it appears that the beneficial balanced EDLs of the 0.5%

Li[PF6] device are lost at higher Li[PF6] concentrations.

3.8 Discussion

This study confirms and clarifies a number of longstanding hypotheses regarding iTMC devices and recent postulates concerning optimization of their operation. It was realized with the first report of Ir(III) iTMC-LEECs [68] and several subsequent reports [67, 70, 90] that device radiant flux and efficiency depended strongly on the metal electrode work functions, which pointed to poor electron injection. At the time, this was surprising as ruthenium iTMC-LEECs showed identical radiant flux and efficiency in the steady state when corrected for differences in electrode reflections [91]. This current effort clearly shows that double layer formation is significantly frustrated in pristine iridium-based LEECs. The long turn-on times (time to maximum radiant flux) of these devices were postulated to arise from poor ionic mobility. Fundamentally, this is supported by these SKPM studies with the lack of ionic space charge redistribution under 0 V bias in the relaxation study and by the minimal departure from a simple resistor voltage distribution under

steady state 9 V operation. It was postulated that lithium ionic additives enhanced cationic accumulation at the cathode [78, 79], and this is confirmed by the potential and electric field distributions revealed by SKPM.

It should be noted that emission is not observed in the model devices used in this study during SKPM testing due to the relatively low driving voltages utilized and the ~10 µm channel length of the devices. (The devices were verified to exhibit electroluminescence clearly visible to the eye at higher voltages.) The aim of this work is to clarify the effects of double layer formation and the influence of ionic additives, and thus not intended to address light emission modes or distinguish between operational mechanisms. Two predominant mechanisms have been proposed, termed the electrochemical and electrodynamic mechanisms. Following the seminal surface probe work of Slinker et al. on small molecule based devices and subsequent efforts on polymeric devices

[11, 73, 92-94] proponents of both models generally agree that double layer formation occurs in these devices and is key to their operation. Beyond this, work is still needed to clarify the details of the emission zone in small molecule LEECs, particularly iridium iTMC devices.

3.9 Conclusion

Scanning Kelvin probe microscopy under steady state and dynamic (relaxation) conditions revealed the potential profiles of iridium iTMC-LEECs both with and without ionic additives. At low steady state voltages, pristine devices show a linear potential drop near the anode consistent

− with dense [PF6] anion packing, but a gradual exponential drop from the cathode consistent with a diffuse outer Helmholtz layer. This demonstrates that Ir(III) complex cations do not pack densely near the cathode, and that electron injection is likely limited in pristine devices. A transient relaxation study also reveals that active layers of the pristine emitter have very low inherent ionic

mobility. Under steady state operation, Li[PF6]-enhanced iridium iTMC-LEECs show similar behavior to pristine devices near the anode but substantially different behavior in the bulk and at the cathode. Large linear potential drops indicative of dense Li+ packing leads to electric fields over two times higher at the cathode and 2.7 times lower in the bulk. The analogous relaxation study of the ionic additive devices indicates significant electron injection in these structures, which correlates with recent reports of improved luminance, efficiency, and responsiveness of lithium enhanced Ir(III) iTMC-LEECs [78, 79].

3.10 Experimental section

3.10.1 [Ir(ppy)2(bpy)] complex synthesis.

The phosphorescent, cationic, heteroleptic iridium(III) complex, [Ir(ppy)2(bpy)][PF6], was synthesized according to slight modifications of literature procedures [52-54]. All chemicals, including the ligands 2-phenylpyridine, 2,2’-bipyridine, and the Li[PF6] salt, were used as received from commercial suppliers. Iridium(III) chloride hydrate was refluxed overnight with 2- phenylpyridine in 3:1 2-methoxyethanol / DI water. The resulting yellow precipitate was isolated by vacuum filtration, rinsed with DI water and diethyl ether, and allowed to dry, yielding the

[Ir(ppy)2-µ-Cl]2 dimer. Formation of the complex was achieved by cracking the dimer with 2,2’- bipyridine in refluxing ethylene glycol to give the Cl- salt. This salt was then exchanged with the

− [PF6] anion through simple metathesis in water with K[PF6]. The isolated yellow powder was then subjected to column chromatography using 2% acetone in dichloromethane. Once eluted the complex was dried by rotary evaporation and recrystallized 3x with dichloromethane / diethyl ether, then acetonitrile / diethyl ether, and finally dichloromethane / pentane to give single

crystalline material. The composition and purity were confirmed by 1H NMR spectroscopy, ESI mass spectrometry, and combustion analysis with the measured values showing excellent agreement with either those previously reported (NMR) [95] or expected theoretical values (MS and EA). Additionally, the photophysical properties of the complex were recorded and carefully compared to literature values. This included a determination of the UV-Vis absorption profile, molar absorptivity (ε), and emission profile both in solution and in the solid state.

3.10.2 Planar device fabrication.

Si wafers (1 mm thick) with 10,000 Å oxide were purchased from Silicon Quest. To fabricate devices, 100 nm of gold (with a 50 Å Cr adhesion layer) was deposited with an electron beam evaporator. The gold surface was treated with a surfactant (Micro 90 cleaning solution) to discourage irreversible adhesion of the parylene layer. Approximately 2.5 µm of parylene C was deposited on the wafer in a Specialty Coating Systems PDS 2010 Labcoater 2 instrument.

Microchem SU8 2002 photoresist was spun on top of the parylene layer at 2000 rpm and pre-baked at 95 ºC for 120 s. The device patterns were exposed on a contact aligner and developed after a post-exposure bake at the same time and temperature as the pre-bake. The parylene layer was etched in an oxygen plasma etcher at 200 mTorr O2 and 150 W for 20 min, removing it completely to expose gold in the resist patterned areas. The exposed gold was then etched with liquid gold etchant for 40 s and rinsed thoroughly with DI water. To remove the adhesion layer, the wafer was further etched with Cr etch for 5 s, rinsed, and dried. The resulting patterned devices had interdigitated electrodes of gold with parylene on top (Figure 3.2B). These devices were then hand cleaved from the wafers. Subsequently, they were treated with a UV Ozone cleaner for one minute.

A dilute solution of pristine [Ir(ppy)2(bpy)][PF6] (24 mg in 1 ml of acetonitrile) or solutions

enhanced with 0.5%/wt or 5.0%/wt Li[PF6] were passed through a 0.2 µm nylon filter and then spin-cast in a nitrogen glove box onto individual devices at 8000 rpm, to make a layer approximately 100 nm thick in the channels. The parylene was peeled from the gold, leaving the iTMC layer in the channel, but not on top of the electrodes, as shown for one electrode gap in

Figure 3.2C. Patterning was confirmed with fluorescence microscopy. Devices for measurement were transported directly from the glove box to the SKPM in a vacuum desiccator to minimize exposure to ambient conditions, where they were maintained under nitrogen atmosphere after mounting. Electrical contact for SKPM operation was made by connecting wire leads to the devices with spring clips, during which time devices were exposed to ambient conditions for approximately 20 min.

3.10.3 Scanning kelvin probe microscopy measurement.

Scanning Kelvin probe microscopy was recorded on an Asylum MFP-3D Classic. SKPM was obtained using a 1 V drive amplitude, a 149 kHz drive frequency, a 800 mV set point and a delta height of 5.0 nm. Tips (HQ:NSC14/Cr-Au-15, force constant 5 N/m, 160 kHz nominal resonant frequency) were obtained from MikroMasch. Devices were biased with a Keithley 2400

Source Measure Unit. All findings were repeated over at least three devices.

CHAPTER 4

BIOINSPIRED NANOWIRE DEVICES UTILIZING PERYLENE DIIMIDES

Authors: Kuo-Yao Lin1, Andrew Bartlett2, Nolan B. King1, Dimithree Kahanda1, Marc A.

McWilliams1, Majid Minary3, Alon A. Gorodetsky2, and Jason D. Slinker1*

1Department of Physics, The University of Texas at Dallas, 800 W. Campbell Rd., PHY 36,

Richardson, TX 75080, United States

2Department of Chemical Engineering and Materials Science, University of California, Irvine,

Irvine, CA 92697

3Department of Mechanical Engineering, The University of Texas at Dallas, 800 W. Campbell

Rd., EC 38, Richardson, TX 75080-3020, USA

4.1 Preface

Herein, we demonstrate the creation of 20 nm long nanowires incorporating a perylene-

3,4,9,10-tetracarboxylic diimide (PTCDI) derivative and measurements of their electrical properties in nanoscale devices. We synthesized 60 mer DNA duplexes incorporating two perylene diimide moieties in the center of the wire as surrogate bases by phosphoramidite chemistry and purified these wires by high performance liquid chromatography. We patterned gold nanogap electrodes by electron beam lithography and assembled the nanowires in the gaps by gold-thiol self-assembly. We characterized these devices with current-voltage (I-V) measurements and followed these measurements with temperature. Current voltage characterization revealed absolute currents of fully well matched duplexes incorporating perylene units were enhanced by twofold over such nanowires incorporating two C-A mismatches and 4.4 fold over conventional DNA nanowires. Temperature dependence revealed that the current increased up to 35 °C for each type of wire, and then lowered rapidly consistent with the dissociation of the modular duplex. We performed atomic force microscopy imaging studies to understand the nature of nanowire assembly, observing an instance of a single nanowire spanning a nanogap. Overall, this demonstrates a high fidelity and scalable method for forming modular nanoscale devices.

4.2 Introduction

DNA is an intriguing and versatile biomolecule for electronic applications. The well- ordered arrangement of stacked, conjugated bases resembles stacked layers of graphite [96], facilitating charge transfer reactions over long distances [97-99]. potentially even in living systems

[100, 101]. Exogenously, DNA has been directly leveraged for such wide ranging uses as a

nanoscale circuit element [43, 102-105], biological sensor [106-111], piezoelectric material [112,

113], spin valve [114-116], and photovoltaic component [117, 118]. Recent efforts have sought to evoke enhanced properties of DNA through alternative DNA structures [119, 120] and hybrid compositions. Concerning hybrid DNA structures, altered or enhanced conduction has been seen through modified or surrogate bases [121-124], alternative backbones and linkers [125, 126], and metalation [127, 128]. These diversified approaches endow DNA with an exciting landscape for the construction of bioinspired electronics.

A particularly attractive class of surrogate bases for use in DNA is given by perylene-

3,4,9,10-tetracarboxylic diimides (PTCDIs) [129-133]. PTCDIs have attracted interest in organic optoelectronic [134-136] and transistor applications [137-139], with experimentally reported mobilities on the order of ~ 1 to 10 cm2 V-1 s-1 (or greater) (Scheme 1) [129-131, 137-139]. PTCDIs can be efficiently incorporated into DNA, naturally π-stacking with each other and within the DNA helix [129, 140, 141]. Modification of PTCDIs at the imide positions facilitates coupling to the

DNA backbone for precise control over sequence location [133, 140, 142, 143]. Core- unsubstituted PTCDIs are established p-type semiconductors and tetrachloro-substituted PTCDIs are established n-type semiconductors [129-131, 137-139]. Such inherent chemical modularity, coupled with established excellent performance in organic electronics, makes PTCDIs particularly attractive as building blocks for nanoscale circuit active elements. In particular, PTCDI incorporation into DNA has great promise for enhanced conductivity through a precisely-coupled base pair surrogate.

Previously, we have demonstrated the construction of nanowire elements incorporating

PTCDIs utilizing the advantages of DNA bioconjugate chemistry [142, 144-146]. Herein, we

demonstrate the creation of 20 nm long nanowires incorporating a PTCDI derivative and measurements of their electrical properties in nanoscale devices. We synthesized 60 mer DNA duplexes incorporating two perylene diimide moieties in the center of the wire by phosphoramidite chemistry and purified these wires by high performance liquid chromatography. We patterned gold nanogap electrodes by electron beam lithography and assembled the nanowires in the gaps by gold- thiol self-assembly. We characterized these devices with current-voltage (I-V) measurements and followed these measurements with temperature. Finally, we performed atomic force microscopy imaging studies to understand the nature of nanowire assembly within the gaps.

4.3 Nanowire concept and construction

Figure 4.1 illustrates the overall concept of creating nanowire devices incorporating

PTCDIs. Nanogap electrodes are fashioned by lithography, and bioinspired nanowires are assembled between the electrodes by gold-thiol self-assembly. The PTCDI molecule located in the center of the duplex effectively replaces a base-pair and naturally π-stacks with itself and within the duplex. Close inspection of its structure reveals the overall conjugation of the molecule and approximate diameter of approximately 2 nm [129, 131, 140, 141], similar to the natural width of two conventional hydrogen bonded DNA bases. Thus, in this context the PTCDI molecule functions as a surrogate base pair to endow the nanowire with high conductivity.

Three nanowires were created for this study in order to understand the electrical dependence of the perylene moiety within the duplex. First, an effective 60-mer duplex noted P incorporated 58 well matched DNA base pairs and two PTCDI molecules in the center was created.

This standard sequence couples two well-ordered DNA duplex segments bridged by two highly

conductive perylene molecules. This construct is anticipated then to support high conductivity given previous understanding of perylene and DNA duplexes in nanogap devices [102, 105, 144,

145]. The perylene mismatch (P-MM) duplex is similar to P but incorporates two CA mismatches

(each noted by C). This highly disrupts the base pairing of the DNA on one side of the perylene

and is anticipated to lower charge transport [142]. The DNA sequence is a pure, well-matched 60 mer and serves as an inherent reference between a purely biological nanowire and a hybrid, bioinspired nanowire incorporating perylene units. Each of these nanowires were actually constructed from five individual strands (see methods section) to improve overall yield (important for thiol and perylene modified nanowires) and permit straightforward interchange of strands to modularly create each nanowire.

Overall, this method of creating nanowires benefits from the many advantages afforded by

DNA bioconjugate chemistry. Each base or base surrogate can be precisely incorporated within the sequence with automation via a DNA synthesizer. Furthermore, the molecular resolution is remarkable, with each base a mere 3.4 Å from the next. These nanowires were purified by HLPC and the precise molecular weights of each component verified by mass spectrometry. This degree of precision, purity, and verification is generally not achievable in a modular nanoscale system.

Overall, this should have implications for the fidelity and reproducibility of the resulting electronic devices formed.

4.4 Devices for nanowire study

Devices for nanowire study were fabricated by electron beam lithography and characterized by optical and scanning electron microscopy. The fabrication steps and resulting microscopy images of devices are shown in Figure 4.2. First, silicon substrates with a 10,000 Ǻ thick oxide layer were coated with electron beam resist by spin casting, patterned with an electron beam, and developed to expose portions of the wafer. A 50 Å titanium adhesion layer and 250 nm of gold were then evaporated by electron beam deposition. The remaining photoresist was then removed by immersion in acetone solvent yielding the patterned devices. As seen in the optical

microscopy image, these devices were fabricated with 50 µm square pads with tapering electrodes in between. The square pads enabled contact by micromanipulator probes. Imaging of the nanogap by scanning electron microscopy reveals that a ~20 nm gap is created by this process. Consistent asymmetry of the electrodes at the nanoscale was noted, likely due to the directional patterning of the electron beam writer at these features.

4.5 Current-voltage characteristics of nanowires

Nanowire devices were made by self-assembling the P, P-MM or DNA samples in the nanogap devices described in Figure 4.2. Current was measured in the range from −0.80 V to 0.80

V, and connected nanowire devices were discerned by observing currents above the noise threshold (~1 fA) and below that of an apparent short (1 µA). The average over 10 apparent connected nanowire devices of each type were recorded and are shown in Figure 4.3 A. The voltage limits of the IV measurements were set by the limits of the gold-thiol bonds used to connect the nanowires.

Clear picoamp-level current trends were observed from each sample, such that, in general

P>P-MM>DNA. Notably, a clear asymmetry is noted in the current, a somewhat surprising feature given that both electrodes are made of gold and that the nanowires presumably can flip orientations freely due to these electrodes. It is believed that this asymmetry comes about due to the consistent asymmetry in the fabrication, which produced one electrode larger than the other. This likely leads to more favorable charge injection from the larger electrode.

Figure 4.3B shows a plot of the average current over 10 devices at 0.4 V for the P, P-

MM or DNA nanowires. The P nanowire, composed of two perylene units and fully well matched DNA, yielded a 16.8 pA current. The P-MM nanowire average current was 8.6 pA, approximately a factor of two lower. In spite of the two perylene units incorporated in P-MM, similar to P, the current through this nanowire was presumably lowered due to the two disruptive

CA mismatches within the DNA that tends to break multiple bonds near the perylenes. The DNA average current was 3.86 pA, lower than P-MM and a factor of 4.4 lower than P. Statistical comparison of the data by a t-test reveals statistically significant difference between these sets

(Figure 4.3B). Interestingly, the incorporation of the perylene units endows the P nanowire with significantly higher conductivity than the two AT base pairs in the center of the pure DNA nanowire.

The current through each nanowire is also strongly influenced by the actual space between each nanowire, which may vary by a few nanometers from device to device. To account for differences in electrode geometries, we calculated the average open current from −0.80 V to 0.80

V for each device. We found the ratio of the nanowire current with this average open current factor, and then normalized to the current found through the DNA nanowire. From this it was found that,

once the device open circuit characteristics were accounted for, that the current was enhanced by

22.6 fold for the P nanowire and 5.5 fold for the P-MM nanowire. Again, t-test comparison of the data reveals statistically significant differences between these sets (Figure 4.3B). This speaks to a significant enhancement of the current upon incorporation of PTCDI units in the nanowires and the dependence on well ordering of the flanking DNA structure.

4.6 Temperature dependence of device characteristics

The temperature dependence of the P and DNA nanowires were recorded in detail (Figure

4.4 A-D) to follow the scaling of current and to confirm that the current follows from the nanowire itself. Devices were slowly heated from room temperature through 40 °C. For both the P and DNA nanowires, current increased up through 35 °C, but rapidly dropped at 40 °C. The thermal enhancement of over two orders for the P nanowire was much more pronounced than that from the DNA nanowire. The enhanced of each current could follow from thermally assisted hopping, thermally assisted injection, or, in the case of the P nanowire, thermal promotion across a perylene/DNA interface. The large drop of current at 40 °C from each followed from the melting of each nanowire—the thermal dissociation of each duplex into the composite individual strands.

This can be confirmed by viewing the 260 nm UV-Visible absorbance versus temperature graph of Figure 4.4 E. The absorbance increases as the nanowire splits into individual strands due to hyperchromicity of the DNA components of each strand. Note that a noticeable increase of absorbance initiates near 40 °C for each strand, indicating the temperature-induced dissociation of the nanowire. This observation of nanowire current loss at the onset of the absorbance change due to melting is consistent with multiple observations of this phenomenon in monolayer DNA devices

[39, 40, 142, 147]. Overall, temperature dependence thus demonstrates that the device current signal is tied to the stability of each nanowire, and thus strongly argues for the presence of the nanowire in the nanogap.

4.7 AFM characterization of nanowire devices

To investigate the number and nature of the nanowires spanning the electrode gap, we performed atomic force microscopy imaging of nanogap devices before and after assembly of the nanowire (Figure 4.5). Such measurements are challenging given that the nanowire draws

considerable salt and water to the surface of the device. Nonetheless, we were able to image an example of a nanowire spanning a gap. Close inspection of this nanowire reveals a diameter of

~2nm, the diameter anticipated for DNA. Interestingly, close inspection of this and further features reveal wires of ~40 nm, approximately double that of a synthesized nanowire. This may indicate that our nanowires tend to couple by forming dithiol bonds between them. Nonetheless, this reveals at least one example where it appears a single nanowire is spanning the gap. Given the relatively low variation between the current measurements of the nanowires of this study, it appears that we are generally measuring single nanowires, or at the very least, only a few nanowires at each gap.

Figure 4.5. Atomic force microscopy images of a nanogap device before assembly (left), and a higher magnification image of the device after assembly.

4.8 Discussion

Finally, the origin of the significant current enhancement upon incorporation of the P nanowires as compared to the DNA nanowires. In Figure 4.6, we present the band level diagram for P nanowires, given prior measurements of energy levels for DNA [148, 149] and PTCDI [144,

150] units. The HOMO energy levels of DNA are considerably more favorably aligned with the gold electrodes than the LUMO levels.

Figure 4.6. Energy band level diagram for DNA inspired nanowires incorporating PTCDI suspended between gold electrodes.

Assuming then that holes are the dominant carriers, the PTCDI HOMO level is well aligned with the DNA HOMO levels. Along these lines, a recent report of an anthroquinone base pair surrogate within DNA monolayers resulted in either enhanced or diminished currents, as modulated by varying of HOMO energy levels by the redox state of the anthroquinone [124]. Our high currents from the P monolayers with energetically aligned PTCDI units would agree well with this interpretation from scanning tunneling microscopy measurements. Furthermore, evidence suggests that PTCDI also offer substantially better electronic coupling between units than standard DNA bases. Our recent DFT calculations of the energy levels of stacked PTCDI units revealed significant overlap of the HOMO (and LUMO) levels with adjacent molecules [150], implying a high degree of conjugation that is also confirmed by evolution of the optical and electrochemical properties [144, 145, 150]. This coupling of levels implies greater values in the elements of the coupling matrix HAB for hopping transport:

2p k = H2 (FC) (Equation 1) h AB

where k is the rate of charge transfer, h is Plank’s constant and FC is the Frank-Condon factor

[151]. Thus, the enhancement in current through perylene diimide incorporation can be rationalized through consideration of energy level alignment and enhanced electronic coupling.

4.9 Conclusion

We synthesized and purified 60 mer DNA-inspired duplexes incorporating two perylene diimide moieties in the center of the wire as surrogate bases by phosphoramidite chemistry and compared these nanowires to a standard 60 mer DNA sequence. We synthesized devices with a 20 nm nanogap between electrodes and assembled these nanowires within these gaps. We characterized these devices with current-voltage (I-V) measurements and followed these measurements with temperature. Current voltage characterization revealed absolute currents of fully well matched duplexes incorporating perylene units were enhanced by twofold over such nanowires incorporating two C-A mismatches and 4.4 fold over conventional DNA nanowires.

Temperature dependence revealed that current increased up to 35 °C for each type of wire, and then lowered rapidly consistent with the dissociation of the modular duplex. We performed atomic force microscopy imaging studies to understand the nature of nanowire assembly, observing an instance of a single nanowire spanning a nanogap. The enhancement in current through perylene diimide incorporation was rationalized through consideration of energy level alignment and enhanced electronic coupling. Overall, this demonstrates a high fidelity and scalable method for forming modular nanoscale devices.

4.10 Experimental section

Synthesis of perylene-3,4,9,10-tetracarboxylic diimide derivative. The perylenediimide phosphoramidites were prepared according to established literature protocols [133, 140, 143, 152].

Synthesis of DNA. Each double-stranded DNA-inspired nanowire was prepared from five sequences in order to maximize yield and maintain synthetic flexibility. The perylene P nanowire was created with the following sequences: Strand A) the 15 mer 3′ (3′-C3 thiol)-ATC AGT CTG

CGC CAT 5′, where (3′-C3 thiol) is the 3′ Thiol modifier C3 S-S modified from IDT DNA; Strand

B) a 30 mer 3′ TCA TGA CAT ACG TA PP CG CAG TAG GTG TCG 5’, were P is the PTCDI modifier; Strand C: a 15 mer 3′ TGG CAG GTC AGT CAT 5′; Strand D) a 29 mer 5′ TAG TCA

GAC GCG GTA AGT ACT GTA TGC AT 3′; Strand E) a 29 mer 5′ GC GTC ATC CAC AGC

ACC GTC CAG TCA GTA-(3′-C3 thiol) 3′. In this arrangement, strands A-C formed one 60 mer strand of each nanowire, and strands D-E provided the 58 complementary bases, with no complementary bases across from the perylenes. The perylene mismatch strand was formed by exchanging strand D with the 29 mer 5′ TAG TCA GAC GCG GTA AGT ACT GTA CGC AC where C locates the positions of C-A base pair mismatches when duplexed as noted above. The

DNA sequence was made by exchanging the D strand with the 30 mer 5′ TAG TCA GAC GCG

GTA AGT ACT GTA TGC ATT 3′ and the E strand with the 30 mer 5′ TGC GTC ATC CAC

AGC ACC GTC CAG TCA GTA- C6-SH 3′. Note in this way that several nicks were located in the sugar phosphate backbone, but these have little to no influence on the overall charge transport

[43, 153]. This method of construction does, however, have a significant impact on the stability of the duplex, as these strands will melt in accordance with the 14 mer and 15 mer sequence segments associated with the overlaps involving complete sugar phosphate backbones. All strands were

purchased from Trilink DNA. Perylene modified strands were synthesized in three steps. First, the

14 mer 3′ TCA TGA CAT ACG TA 5′ was synthesized by Trilink and shipped. Second, the perylene units were coupled and shipped back to Trilink. Finally, the remaining bases were coupled by Trilink. Perylene coupling was accomplished by methods described earlier [142].

Purification of DNA. All oligonucleotides were purified via two rounds of high performance liquid chromatography (HPLC) on a Shimadzu LC-20AD instrument outfitted with an SIL-20A autosampler and an SPD-M20A diode array detector. In the first purification round, DNA oligonucleotides with the 4,4′-dimethoxytrityl group on were eluted on a gradient that was evolved from 5% acetonitrile and 95% 50 mM ammonium acetate, pH = 8 buffer to 75% acetonitrile and

25% 50 mM ammonium acetate, pH = 8 buffer over 30 min. In the second purification round,

DNA oligonucleotides with the 4,4′-dimethoxytrityl group off were eluted on a gradient that was evolved from 5% acetonitrile and 95% 50 mM ammonium acetate, pH = 8 buffer to 15% acetonitrile and 85% 50 mM ammonium acetate, pH = 8 buffer over the first 35 min; from 15% acetonitrile and 85% 50 mM ammonium acetate, pH = 8 buffer to 50% acetonitrile and 50% 50 mM ammonium acetate, pH = 8 buffer over the next 5 min; and finally, held constant at 50% acetonitrile for another 5 min. The identity of the desired products was confirmed by matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) on a

Shimadzu Axima Confidence mass spectrometer.

Preparation of the double-stranded DNA. The oligonucleotides were quantified via UV-visible spectroscopy on a Beckman DU-800 UV-Visible spectrophotometer. This instrument was also used for melting temperature analysis. Duplex DNA was prepared by mixing equimolar amounts of complementary strands and annealing the solution to 95 °C, followed by slow cooling to room

temperature over a period of 90 min. The formation of duplex DNA was verified by temperature dependent absorbance measurements and melting temperature analysis (See Figures 3c and 3e).

Fabrication of nanogap devices. 1 mm thick Si wafers featuring a 10 000 Å thick oxide layer

(Silicon Quest, Inc.) were patterned via electron beam lithography, with gold electrodes deposited by a lift-off technique. Initially, the wafers were cleaned thoroughly by CO2 snow cleaning and acetone to remove organic impurities. PMMA 950 A4 electron beam photoresist was spin cast at

2000 rpm to create an approximately 300 nm thick resist layer. The resist was patterned with a

Raith 150-TWO high resolution electron beam writer. The resist was subsequently developed in

PMMA developer for 30 seconds, and post develop cleaned in IPA for 15 seconds. Electrodes were then deposited by a Temescal e-gun evaporator by evaporating a 50 Ǻ thick Ti adhesion layer and a 250 Ǻ thick gold electrode. The remaining photoresist was removed by immersing the electrodes in acetone for 24 h.

Before assembly of the devices, chips were immersed for 10 mins in sulfuric acid, rinsed in DI water, and then immersed for 5 mins in acetone three successive times. Chips were then plasma cleaned by a Jelight model 42 UV-Ozone cleaner for 5 min. The nanowires were self- assembled onto the gold electrode pads from a solution with 25 µM of the duplex DNA, 5 mM phosphate, 50 mM sodium chloride, pH = 7 buffer solution supplemented with 50 mM MgCl2 over a period of 12 h to 18 h under refrigeration, contained with a rubber gasket and plastic clamp.

Subsequently, the chips were rinsed in 0.5 mM phosphate, 5 mM sodium chloride, pH = 7 buffer solution ten times to remove excess salt from the surface.

Current voltage characterization of nanowires. Currents were recorded with a custom probe station outfitted with two micromanipulator 450 probe positioners and a Keithley 6430 sub-

femtoamp remote sourcemeter utilizing the remote preamp. An atmosphere of nitrogen with ~30% relative humidity was established over each chip by alternatively flowing nitrogen gas or nitrogen bubbled through water and maintained over the assembly with a plastic bag. Temperature control was accomplished by stacked peltier heaters and monitored with a thermocouple connected to a digital meter.

AFM Characterization. Atomic force microscopy was recorded on an Asylum MFP-3D Bio. The

AFM image was obtained using a 11.9mV, 119.48 kHz drive frequency with a 450mV set point.

Tips (n+ silicon, force constant 5-37 N/m, 96-175 kHz nominal resonant frequency) were obtained from MikroMasch.

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76 BIOGRAPHICAL SKETCH

Kuo-Yao Lin is from Taiwan, he is interested in device physics, and he has a bachelor's degree in physics and a master's degree in engineering physics. After several years of experience in the semiconductor industry, he entered the Physics PhD program at The University of Texas at Dallas in August 2012. In his PhD research, he focused on characterization with nano-scale devices incorporating with organic semiconducting material. His research has helped the understanding in light emitting electrochemical cells (LEECs), and has discovered a new possibility for future electronic devices with DNA nanowires.

CURRICULUM VITAE

Kuo-Yao Lin [email protected] EDUCATION The University of Texas at Dallas, Richardson, Texas * Ph.D. Candidate -Physics, Expected graduation fall 2017 * Master of Science -Physics, Dec 2013 Stevens Institute of Technology, Hoboken, New Jersey * Master of Engineering -Engineering Physics, May 2008 Fujen Catholic University, Taipei County, Taiwan * B.S. in Physics, June 2005

WORK EXPERIENCE The University of Texas at Dallas, Dallas, USA, Sep 2012 to present Research Assistant * Currently fabricating bioinspired nanowire devices with 10-20 nm electrode gaps by electron beam lithography and testing by current-voltage characterization with custom built hardware and software. * Used scanning Kelvin probe microscopy to demonstrate the physical origin of efficiency enhancement from lithium additives in organic light emitting devices (OLEDs). * Fabricated multiplexed electrodes for electrochemical characterization of bioinspired perylene nanowire monolayers. Texas Instruments, Dallas, USA, Jun 2016~Aug 2016 Process Engineering Intern * Tcad simulation and device analysis * 65nm device design and process development Taiwan Semiconductor Manufacturing Company (TSMC), Taiwan Sep 2009~Aug 2012 Process Integration Engineer * 40nm/28nm process integration and yield improvement. * MOS devices analysis. * Customer mask layout design check. * Cooperate with customers to enhance device performance. Industrial Technology Research Institute, Hsinchu, Taiwan Aug 2008~Sep 2009 Engineer * Solid state lighting products characterization and products failure investigation

PUBLICATIONS 1. L. D. Bastatas, K.-Y. Lin, M. D. Moore, K. J. Suhr, M. H. Bowler, Y. Shen, B. J. Holliday, and J. D. Slinker. Discerning the Impact of Salt Additives in Thin Film Light-Emitting Electrochemical Cells with Electrochemical Impedance Spectroscopy. Langmuir 32, 9468 (2016).

2. K.-Y. Lin, L. D. Bastatas, K. J. Suhr, M. D. Moore, B. J. Holliday, M. Minary-Jolandan, and J. D. Slinker. The Influence of Lithium Additives in Small Molecule Light-Emitting Electrochemical Cells. ACS Appl. Mater. Interfaces 8, 16776 (2016).

3. (Invited Article) C. H. Wohlgamuth, M. A. McWilliams, A. Mazaheripour, A. M. Burke, K.- Y. Lin, L. Doan, J. D. Slinker, and A. A. Gorodetsky, Electrochemistry of DNA Monolayers Modified With a Perylenediimide Base Surrogate. J. Phys. Chem. C. 18, 29084 (2014).

PRESENTATIONS 1. 2017 Spring MRS meeting, Phoenix, Arizona 2. 2016 Spring Texas Section APS Meeting, Lamar University, Lamar, TX. 3. 2015 Fall Texas Section APS Meeting Baylor, Waco, TX.