Organic electronics on micro and nano fibers from e-textiles to biomolecular nanoelectronics

Mahiar Hamedi

Linköping 2008

Cover art: “Tartan” Design by Koshi Hamedi © 2008, Koshi Hamedi

Organic electronics on micro and nano fibers from e-textiles to biomolecular nanoelectronics Mahiar Hamedi

Serie: Linköping studies in science and technology. Dissertations, No. 1224 ©, 2008, Mahiar Hamedi, unless otherwise noted Printed by LiU-Tryck, Linköping, Sweden 2008 ISBN: 978-91-7393-763-4 ISSN: 0345-7524

“Systems of theories are tested by deducing from them statements of a lesser level of universality. These statements in their turn, since they are inter-subjectively testable, must be testable in a like manner – and so ad-infinitum”

-Karl Popper (1959) “The logic of scientific discovery”

Abstract

Research in the field of conjugated polymers (CPs) has led to the emergence of a number of interesting research areas and commercial applications, including solar cells, flexible displays, printed electronics, biosensors, e- textiles and more.

Some of the advantages of organic electronics materials, as compared to their inorganic counterparts, include high elasticity, and mechanical flexibility, which allows for a natural integration of CPs into fabrics, making them ideal for e-texile. In this thesis, a novel approach for creating transistors is presented, through the construction of electrolyte gated transistors, directly embedded on functional textile fibers. Furthermore theoretical and experimental results of the integration of functional woven devices based on these transistors are shown. The realization of woven digital logic and design schemes for devices that can be placed inside living tissue, for applications such as neural communication, are demonstrated.

Reducing feature sizes in organic electronics is necessity just as in conventional microelectronics, where Moore's law has been the most impressive demonstration of this over the past decades. Here the scheme of self-assembly (SA) of biomolecular/CP hybrid nano-structures is used for creating nano electronics. It is demonstrated that proteins in the form of amyloid fibrils can be coated with the highly conducting polythiophene derivative (PEDOT-S) through molecular self-assembly in water, to form conducting nanowire networks and nanodevices at molecular dimensions. In a second SA scheme, large area patterning of connected micro-nano lines and nano transistors from the conducting polymer PEDOT-S is demonstrated through assembly of these from fluids using soft lithography. Thereby the problems of large area nano patterning, and nano registration are solved for organic electronics. The construction of functional nanoscopic materials and components through molecular self-assembly has the potential to deliver totally new concepts, and may eventually allow cheap mass production of complex three dimensional nano electronic materials and devices.

Populärvetenskaplig sammanfattning

Ordet polymer kommer från grekiskans “poly” som betyder många, och “mer” som betyder del, och syftar till väldigt långa molekylkedjor som kan bestå av tiotusentals sammanlänkade atomer.

De polymerer som bokstavligen står oss närmast är livets polymerer. Proteiner och DNA är exempel på polymerer som ingår i allt levande och är känt för de flesta idag. En DNA molekyl kan exempelvis bestå av en kedja av miljoner sammanlänkade atomer.

En annan klass av allmänt kända polymerer är de konstgjorda polymerer som vi känner till som plaster. Det tillverkas idag genom petrokemiska processer hundratals miljoner ton plast i världen årligen. Dessa otroliga mängder av material har många användningsområden. Plaster kan bli formgjutna, exempelvis till läskflaskor, möbler, förpackningar och elektronikhöljen, eller formade till texilfibrer som exempevis nylon. De kan också appliceras från lösning på ytor i form av färger.

Livets och dagens petrokemiska framställda polymerer är inte elektroniska material. Vi är istället bekanta med metaller som strömförare och lyselement, som till exempel i elkablar och glödlampor. Vi känner också till att vanliga elektroniska kretsar består av kisel och att det finns andra hårda material som är elektriskt aktiva material.

Elektronik baserad på metaller och kiselbaserade kretsar har förändrat samhället radikalt under det senaste århundradet. Plast har också haft stor påverkan på samhället som ett viktigt material i många produkter. Elektroniken och plastens värld har dock varit separerade under många år. Men år 1977 gjorde en grupp forskare i USA en spännande upptäckt som bröt denna separation när de hittade en klass av plaster/polymerer som kunde leda ström. Dessa plaster har sedan dess utvecklats, diversifierats och tillförts förbättrade funktioner såsom elektroniska lyselement i alla färger, relativt hög ledningsförmåga, och bättre stabilitet- och processegenskaper. I och med denna utveckling börjar nu den tidigare döda plasten i vissa av våra vardagsprodukter att få liv. Vi börjar idag se helt nya produkter och prototyper baserade på ledande plaster, såsom nästa generations platta

skärmar från SONY, lysande plastfilmer från General Electrics, elektroniska papper från Plastic Logic, och plastsolceller från Konarka.

I första delen av denna avhandling har ledande plaster undersökts i samband med textila mikrofibrer. Det visas att man kan bygga transistorer och därmed digitala kretsar enbart med hjälp av vanliga textilfibrer, genom att kombinera dessa med ledande plaster. Detta är ett steg mot elektronisk textil, där man slutligen ska kunna väva textiler från nästa generations syntetiska klädfibrer, och forma avancerad elektronik som är helt inbäddad i själva tyget.

I och med att ledande plaster nu börjar att användas till att bygga alltmer avancerade kretsar ställs man också inför samma utmaning som den kiselbaserade elektroniken, nämligen att skapa väldigt många komponenter på väldigt små ytor. Ett chip i din dator har idag nästan 1 miljard transistorer på en area av någon kvadratcentimeter. För att göra detta krävs det att man utvecklar metoder för att mönstra plaster på nanoskalan (1 nanometer = 1 miljondels millimeter). I denna avhandling har en metod att skapa nanomönster i en ledande plast demonstrerats. Detta görs genom att en lösning av plasten formas i en slags gjutform med mikro- och nanometer stora kanaler. Metoden visar möjligheter hur man att på ett enkelt och effektivt sätt kan framställa delar av framtidens plastnanokretsar.

Ännu än metod som i denna avhandling utvärderar ledande plaster på nanometer skalan, bygger på att man förenar en av livets byggstenar, i form av en lång proteinkedja, med en ledande plast. Resultatet är att den ledande plasten fastnar på proteinkedjorna och formar ett elektrisk ledande skal runt dessa. Effekten är att man skapar ledande nanofibrer som är tiotusentals gånger mindre än exempelvis textilfibrer. Denna demonstration visar också på de enorma möjligheter som uppstår i föreningen av biologi och ledande plaster.

Included Papers

Paper 1 “Towards woven logic from organic electronic fibres” Hamedi M, Forchheimer R, Inganäs O. Nature Materials 6 (2007) 357

Paper 2 “Electrochemical devices made from conducting nanowire networks self- assembled from amyloid fibrils and alkoxysulfonate PEDOT” Hamedi M, Herland A, Karlsson RH, Inganäs O. Nano Letters 8 (2008) 1736

Paper 3 “Fiber embedded electrolyte-gated organic TFTs for e-textile” Hamedi M, Herlogsson L, Marcilla R, Crispin X, Berggren M, Inganäs O. Advanced Materials, Accepted

Paper 4 “Bridging dimensions in organic electronics: assembly of electroactive polymer nanodevices from fluids” Hamedi M, Tvingstedt K, Karlsson RH, Åsberg P, Inganäs O. Nano letters, Submitted (2008)

Paper 5 “Construction of wire electrodes and 3D woven logic as a potential technology for neuroprosthetic implants” Asplund M, Hamedi M, Forchheimer R, Inganäs O, Von Holst H, IEEE Transactions on Biomedical Engineering, Submitted (2008)

Related work not included

Papers “Limits to nanopatterning of fluids on surfaces in soft lithography” Wigenius, J.A., Hamedi, M., Inganäs, O., Advanced Functional Materials 18 (17), 2563-2571 (2008).

“Iron Catalysed Polymerization of Alkoxysulfonate-Functionalized EDOT gives Water-soluble PEDOT of High Conductivity” Karlsson, R. H., Herland, A., Hamedi, M., Wigenius, J., Åslund, A., Inganäs, O., Konradsson, P. Chemistry of Materials (Submitted) (2008).

Patents “Micro and nano structures in elastomeric material“ Hamedi, M., Tvinstedt, K., Åsberg, P., & Inganäs, O. WO/2006/096123 (2006).

“Electronic circuitry integrated in fabrics” Hamedi, M. Forcheimer, R. Asplund, M. Inganäs, O. WO/2008/066458 (2008)

Highlights in the news and media “Designer Logic Comes to E-Textiles” Service, R. F. ScienceNOW 3 April 2007: 1

“Electronic textiles: A logical step” De Rossi, D. Nature Materials 6 (5), 328-329 (2007).

“NextWorld: Intelligence" http://dsc.discovery.com/tv/next-world/next-world.html Discovery Channel “Nextworld” series

Tackord

Denna höst gulnar de vackra löven för femte gången sedan jag började som doktorand hos Olle Inganäs. Doktorandperioden har inneburit en otrolig personlig utveckling för mig. Jag har under dessa år blivit skickligare på att omsätta ideer till verklighet i både akademisk och kommersiell form, och jag har kommit att fördjupa min förståelse för vetenskap och för den oändliga process i vilken vetenskapen utvecklas. I denna utvecklingsprocess finns det ingenting som fortsätter att fascinera mig mer än den vackra dynamik som finns i de möten och band som uppstår mellan oss människor, och mellan människa och natur. Allting som jag har åstadkommit här är ett resultat av dessa möten och band, mellan mig och så många vackra personer som jag för evigt är bunden till och har allt att tacka för.

Olle Inganäs är en person med enorm dynamik och djup i sina tankar, och en person som verkligen tror på, och generöst stödjer individens frihet. Jag har verkligen haft tur att få jobba med en person med sådana sällsynta kvalitéer.

Mina kollegor i vår forskningsgrupp har alla varit fantastiska människor som jag har haft många bra stunder med. Ni har alla bidragit direkt och indirekt till utvecklingen av detta arbete. Jag vill tacka Per Björk, Anna Herland, Mattias Andersson, Kristofer Tvingstedt, Maria Asplund, Wan-yu Lin, Fengling Zhang, Manoj M, Xiangjun Wang, Abay Gadisa, Peter Åsberg, Peter Nilsson, Roger Karlsson, Tomas Johansson, Nils-Krister Persson, Jens Wigenius, Sophie Barrau, Bekele, Shimelis, Wataru, Viktor Andersson.

Mina kollegor och vänner i Norrköping har jobbat med närbesläktade forskningsområden och både inspirerat och bidragit till utvecklingen av mitt arbete. Jag vill tacka Payman Tehrani, David Nilsson, Peter Andersson, Fredrik Jakobsson, Joakim Isaksson, Elias Said, Lars Herlogsson, Nathaniel Robinson, Xavier Crispin, och Magnus Berggren

Robert Forchheimer, har varit min ”inofficiella” biträdande handledare. Ditt stöd har betytt mycket för mig.

Jag tackar Jens Birch, har varit min mentor och kommit med flera goda råd längs min resa.

Hans Von Holst för att du tror på organisk elektronik som ett medicinsk tverktyg.

Jones Alami, tack för att du trodde på mig som forskare innan jag själv gjorde det.

Personalen på IFM, som hjälpt till med lab och pappersarbeten, som möjliggjort min forskning. Jag vill främst tacka, Stefan Klintström, Agneta Askendal , Ann-Marie Holm, Mikael Amle´, och Bo Thunér.

Mina kollegor på vårt företag Donya Labs. Jag vill tacka Martin Ekdal, Gustaf Johansson, Ulrik Lindahl, Koshi och Matt Connors för att ha delat en vision med mig och hjälpt till att förverkliga den genom sitt fulla engemang med start och drift av bolaget.

Ett tack til mina nära vänner som alltid uppmuntrar mig i mina vägval, Amir, Toni, Murat, Ali, Said, Ulrik.

Min nära släkt, som alltid har uppmuntrat och inspirerat min nyfikenhet. Daryoush var den första som lärde mig vad en transistor var, redan när jag var ett barn, jag har dig att tacka mycket för i detta arbete. Azita och Azar har alltid lyssnat på mina tankar och teorier som ett litet barn och som vuxen. Göran, och Anders för att alltid visa sitt genuina intresse för mitt arbete. Mina kusiner som är mig så nära Farzad, Dornoosh, Danesh, och Hanna.

Utan min familjs stöd skulle jag inte ha klarat min doktorandtjänst. Koshiar du har hjälpt mig med att koppla min vetenskap till konstens värld, och jag skulle inte ha kunnat publicera några av mina artiklar eller denna bok utan dig. Maziar jag ha haft många djupa diskussioner med dig om mina arbeten, du inspirerar mig till att vara open-minded i ordets sanna bemärkelse. Mamma och Pappa, allt jag är och kommer att vara är tack vare er. Älskar er.

Min kärlek Anna Herland. Du har varit och är min stora förebild, både i forskningen och privat. Vi har många vackra stunder kvar att dela.

Contents

1 GENERAL INTRODUCTION ...... 1

2 CONDUCTING POLYMERS (CP) ...... 3 2.1 Molecular electronics and CPs...... 3 2.2 Electronic structure...... 5 2.3 Charge carriers...... 7 2.4 Electrochemistry in CPs...... 8 2.5 Ionic transport in polymers and CPs ...... 10 2.5.1 Ionic transport in polymers ...... 10 2.5.1 Ionic transport in CPs ...... 12 2.6 Electron charge transport theory...... 12 2.6.1 Towards a unified theory for disordered CP films ...... 12 2.6.2 Metallic and truly metallic conduction in CP films ...... 15 2.6.3 Charge transport in cp nanofibers ...... 17 2.7 Common CPs, PEDOT and P3HT...... 19

3 SELF-ASSEMBLY OF CP MICRO AND NANOFIBERS ...... 23 3.1 CP decorated biomolecular nanowires ...... 23 3.1.2 PEDOT:amyloid nanowires ...... 25 3.2 Soft-lithography patterning ...... 28 3.2.1 Patterning of PEDOT in bridged micro- and nanowire arrays ...... 29

4 ELECTROLYTE GATED TRANSISTORS ...... 33 4.1 Electrolyte materials ...... 33 4.2 Electrochemical PEDOT transistors (ECT) ...... 35 4.2.1 Theory of operation ...... 35 4.2.2 Some ECT based applications ...... 38 4.2.3 Microfiber ECTs ...... 40 4.3 PEDOT nanofiber ECTs ...... 42 4.3.1 Molecular ECTs ...... 42 4.3.2 Nano ECTs assembled on large area micro/nano arrays ...... 45 4.4 Microfiber electrolyte-gated OFETs ...... 46 4.4.1 Electric double-layer capacitance (EDLC) OFETs ...... 46 4.4.2 electrochemical enhancement mode transistors ...... 50 4.5 EDLC OFET and ECT regions ...... 50 4.6 Speed of electrolyte gated transistors ...... 53 4.6.1 EDLC mode ...... 53 4.6.2 ECT mode ...... 55

5 E-TEXTILE: WOVEN CP DEVICES ...... 58 5.1 CP functionalized textile microfibers ...... 58 5.2 Woven electrolyte based devices ...... 62 5.2.1 Design rules for woven circuits ...... 63 5.2.2 Woven digital addressing devices ...... 65

6 NANO CROSSBARS FOR SCALING OF ORGANIC NANOELECTRONICS. . 71 6.1 Beyond CMOS ...... 71 6.1.2 CMOS based defect-tolerant computers towards the crossbar ...... 72 6.2 The nano crossbar ...... 73 6.2.1 Conducting nanolines in crossbars ...... 75 6.2.2 Memristors, transistors and diodes as crossbar components ...... 76 6.3 Architectures: logic, memory, and hybrids ...... 81 6.3.1 Demultiplexing crossbars ...... 82 6.4 Towards all organic nano crossbars ...... 85 6.5 Demultiplexers on PEDOT crossbars ...... 87

7 SUMMARY AND FUTURE OUTLOOK ...... 90

REFERENCES ...... 93

PAPERS I-V

Chapter 1 General introduction

Polymers consist of large molecules i.e. macromolecules. The word has Greek origins, like many other western scientific words, where, “poly” means many and “mer” means part. The entanglement of these long polymer chains, give polymeric materials a wide range of useful mechanical properties. The most widely known man made organic polymers are petrochemical plastics, which are currently used in various everyday products, such as plastic bags, insulating bodies of electronic products and even in awkward products such as plastic flowers, which indeed have a common factor with real flowers, as polymers are also the building blocks of all life on earth. Proteins and DNA are for example polymers of life, and well known to almost anyone.

Polymers are currently not perceived as electronic materials by general public. Instead metals are generally known as conductors and seen in for example cables and light bulbs. In the microelectronics industry the mix of conducting metals and semi-conducting silicon based materials and other inorganic semi-conductor crystals are the main materials in all microchips. Silicon chips have changed the face of our modern world. These are embedded in almost every electrical product today, and the entire computer industry is currently very much connected with the word silicon, and still dominated by the hardware and software companies located in the infamous “silicon valley”.

This psychological, linguistic and commercial distinction between polymers and electronic materials are however on the verge of change, and they are becoming blended. This interesting change has its roots in the development of molecular electronics, and has taken off seriously since an important discovery was made in 1977. This year a group of researchers at Pennsylvania state university USA, discovered that a class of polymers could conduct high electrical currents [1]. The combination of the electro-optical and processability properties, being similar to plastics, has given rise to a number of fundamental discoveries and inventions since then, and only three decades later a number of commercial products are already emerging on the market, where color displays (e.g.

1 “XEL-1” OLED TV from Sony), light emitting plastic sheets (e.g. roll-to- roll manufactured plastic light-emitters from General Electrics) and plastic solar cells (e.g. from Konarka) are among the first big products.

If conducting plastics are regarded as the fourth generation of plastics we can think of using these for embedding electronics into the dead plastics that we have in our current products, and bring these to life. In this thesis this possibility is explored for textiles, where synthetic plastic microfibers such as nylon/polyester can be enhanced with conducting plastics to achieve electronic functions.

Since plastics are today mainly used for their mechanical properties at large scales, not much interest has been given to the nanopatterning of plastics, as compared to nanopatterning of many other materials. This will however be necessary as plastic electronics will undergo the same miniaturization process as conventional electronics has done, even if Moore’s law is not defined for plastics. In this thesis the possibility of nanopatterning of conducting polymers from fluids is demonstrated.

If conducting polymers are regarded as organic polymers structurally identical to the molecules of life, we can instead think of using them for embedding/interacting electronically with the building blocks of life. This intriguing possibility is also explored in this thesis by demonstrating protein based structures that can conduct electricity.

Although significant advance has been made during this 30 years, organic/plastic/polymer electronics is still at it infancy.

2

Chapter 2 Conducting polymers (CP)

2.1 Molecular electronics and CPs The physics of conducting polymer electronics is based on the physics of the more general concept molecular electronics, where micro- or macromolecules (polymers) are the electron conductors. These molecules, and specially the macromolecules, are mainly carbon based, and therefore the word organic electronics is used .

In order to put conducting polymers into historical context, here a chronological overview on the history of molecular electronics is given, partly based on a summary by Hush. [2]. This section can be skipped or revisited after studying of the other chapters.

One of the initial ideas behind molecular electronics dates back to the work of Hund and Mulliken who together outlined the molecular orbital (MO) theory around 1930. The work on MO theory continued through Lennard- Jones who introduced the linear combination of atomic orbitals (LCAO) and was further completed by Mulliken who introduced self consistent field theory (SCF). The ideas take their modern form through the density functional theory of Kohn and the semi-empirical theories of Pople. Mulliken also studied donor-acceptor charge transfer complexes, defining supra molecular ground states consisting of linear combinations of the orbitals of a donor- and an acceptor. [3] . The electron donor acceptor (EDA) ideas initially led to the study of solid state EDA complexes such as semiconductors. In 1973 a metallic complex of (TCNQ) and (TTF) molecules was described [4], and later the first EDA superconductor was described [5].

An important contribution to the field of molecular electronics came from Albert Szent-Gyorgi, who in 1941 [6] suggested that if electrons in a crystal would be delocalized, then, theoretically, there is a possibility that common energy levels could exist in bimolecular systems where many molecules form one structural unit, for example in the chlorophyll system.

3 Later work by Evan and Gergely predicted bandgaps of 2-4 eV in an infinite hydrogen bonded protein, the work sparked off experimental work where electronic measurements through bio molecules were performed, with the conclusion that these were very poor semi-semiconductors (insulators).

Another major advance in the field was the insight into the electronic structure of transition metal ions, from the development of the ligand field theory (1950- onwards), with the basic insight that the electronic degeneracy of ions could be lifted by the electric field of surrounding ligands. Leading on from that work the fifth major advance in molecular electronics was the development of the theory of the kinetics of homogeneous electron transfer reactions. The basic theories underlying the kinetics of thermal and optical electron transfer are now well established, where R.A Marcus was awarded the Nobel prize 1992 for his work in this area. The main concept of this theory is to depict the complex electron-phonon (vibronic) coupling processes in terms of simpler and more tangible chemical properties.

The limits of the “Marcus theory” are coherent vibrationless through- molecule electronic transport and conduction via coherent geometric disturbances, such as solitons and polarons. A further advance that led to the Nobel prize in this area was awarded to Heeger, Shirakawa, and McDiarmid for the synthesis of conducting/conjugated polymers, and demonstration of high conduction in these materials. This led to the development of the theory of soliton transport in polyacetylene [7]. This theory facilitated the development of modern applications in molecular electronics based on conducting polymers (CPs). The modern theories of coherent and incoherent exciton conduction also fall under the general electron-transfer theory, (for example polaron-type hopping). The focus of this thesis is on systems utilizing conducting polymers, and a more detailed theoretical description is given in the following section mainly focusing on the modern theories of hopping transport in CPs.

The sixth major advance in molecular electronics could be the Aviram- Ratner proposal [8] that a single DBA molecule could act as a rectifier. It was shown that a molecule would be an insulator for low applied voltages and at a critical point suddenly switch on. The landmark proposal builds on the Mulliken donor-acceptor concept, the idea of conduction through a bridge and the theory of electron transfer.

Although the focus of this thesis is on micro and nano electronics based on conducting polymers, one might keep in mind that conducting small

4 molecules and CPs are already being used together in devices such as organic light emitting displays. Furthermore as nano electronics circuits based on CPs shrink, the combination of CPs with molecular components, such as those first suggested by Aviram-Ratner, can become interesting. The work by Akkerman, Blom et. al. demonstrates this beautifully by combining the conjugated polymer PEDOT and single molecule of alkanethiols [9]. The remaining part of this chapter is focused on conducting polymers.

2.2 Electronic structure The structure of a CP is a chain of a carbon-based polymer, having conjugated units with alternating single and double bonds. The conjugation of this organic polymer is what distinguish them from regular plastics, which consists of only single bonds, and the conjugation leads to electronic properties. The words conjugated polymers and conducting polymers are therefore used interchangeably throughout the thesis and abbreviated by “CP”.

Every carbon atom in the conjugated chain is sp2-hybridised. Three of four sp2 orbitals lie in one plane with 120 degrees separation forming strongly localised σ-bonds, and the remaining fourth pz orbital forms a π-bond. The formations of bonding and anti-bonding orbitals from the π-orbitals give rise to a bandgap with energy states in Lowest Occopied Molecular Orbitals (LUMO) band corresponding to the conduction band (π*-band), and Highest Occupied Molecular Orbitals (HOMO) corresponding to the valence band (π-band).

5 Figure 2.1 Schematic picture of the splitting of HOMO and LUMO levels with increasing lengths of an acethylene molecule, towards the formation of bands for polyacethylene.

Figure 2.1 shows how HOMO LUMO levels split up with increasing CP chain lengths, to form bands.

A simple model for understanding the bandgap in CPs comes from Kuhns [10] description, which is based on the electron in a box potential. In this model the π -electrons are considered to be in a box with boundaries corresponding to the length of the molecule L.

2 2 ! ! 2 E = n n 2mL2

With N electrons the energy difference distance between the highest filled n=(N/2) electron level defining the HOMO and the first unoccupied energy level LUMO would then define the bandgap Eg of the molecule. i.e

C Eg = E " E ! 1 n+1 n n

The important result of this model is that the band gap is lowered when making the CP longer. A correction to this model which prevents the band gap from going to zero as n becomes large is given by the Peierls distortion

6 [11]. Since all carbon-carbon lengths are not equal in a CP, the bonds are dimerized (Peierls Distortion). The dimerization would increase the periodicity from L to 2L and add an additional term to the equation, i.e.

C Eg = 1 + C n 2

This formula is more successful in describing the band gap of oligomers and some polymers.

2.3 Charge carriers The linear chain of the CP backbone confines the electrons in the π-orbitals to one dimension. Furthermore the described band gap of the polymer gives a low density of free electrons in the conduction band, hence the CP can be regarded as a 1-D semiconductor (with the exception of broadening). The CPs thus have a conductivity which span from close to insulators to semiconductors. Upon doping of the polymers however, the conductivity can be increased by several orders of magnitude (see figure 2.2). Doping is achieved either by withdrawing electrons (p-doping) or by adding electrons (n-doping), to the polymer backbone. p-doped polymers are usually more stable and mainly used in CP applications today.

Figure 2.2 Conductivity of doped and undoped CPs PEDOT and PA, versus other materials.

The considerable change in conductivity is due to the fact that new electronic species/quasi particles are introduced in the polymer upon doping, which act

7 as charge carriers. These charge carriers are transported through the π - bonded polymer chain, and can be either solitons for degenerate polymers and polarons or bipolarons for non-degenerate polymers. The solitons and polarons are quasi-particles with strong electron-phonon coupling, and these give rise to new energy states within the bandgap of the CP resulting in lowered bandgap and also changed color, a phenomenon described as electrochromism. Figure 2.3 shows schematically the formation of the more common bi-polaron band upon p-doping. Upon doping the number of charge carriers are increased and the charge carrier mobility is also increased due to formation of new band states and so is the conductivity σ= neµ, where n is the concentration of charge carriers, e is the electron charge and µ is the charge carrier mobility. The mobility of the charge carriers in CPs is mainly described by hopping transport. This is described in more detail in section 2.6.

Figure 2.3 Polaron / bi-polaron energies in the band gap of a CP.

2.4 Electrochemistry in CPs The most convenient way of dynamically altering the doping state of a polymer is through electrochemical redox reactions. Here the CP is brought into contact with an electrolyte and the doping/de doping of the polymer is made by applying a potential between the CP and a counter-electrode in

8 contact with the electrolyte. A reference electrode can also be used for more exact potential measurements such as in cyclic voltammograms. The electrochemical doping of a CP can occur not just at the surface but throughout the bulk of a CP film because the polymer acts as a mixed ion and electron conductor. This mixed conduction property is described in more detail in the next section. The more common process of oxidation of a neutral polymer P0 to oxidized state P+, through insertion of an anion A- and withdrawal of an electron e-can be described by

P0 → P+(A-) + e-

In other forms, the oxidation can occur from a polymer-ion complex form, described by

P0(M+A-) → P+(A-) + M+ + e-

The n-doping reaction can be described by a similar chemical reaction with cations instead of anions.

The doping and de-doping of CPs is not a symmetric process, which is reflected by the asymmetric voltammogram illustrated in figure 2.4. One explanation for this is that the system undergoes conformational changes during doping [12,13]

Figure 2.4 shows a schematic complete CV where both n-doping and p- doping can occur in an electrochemical setup with a bithiophene [14].

9 Figure 2.4 CV of both p-doping and n-doping.

This kind of CV measurements can be useful for characterization of polymer films and measurements of parameters such as electrochemical bandgap [15]. Usually in these measurements classical electrochemical setups are used comprising liquid electrolytes. The dynamic and reversible redox response of polymers in electrochemical setups are however also interesting for the construction of electronic components. Two types of electrolyte based components in the form of transistors, are at focus in this thesis, and will be described in detail in chapter 3.

2.5 Ionic Transport in polymers and CPs

2.5.1 Ionic transport in polymers As already described, the important process of CP doping requires a mixture of electronic and ionic charge transport, and they are therefore closely connected in these materials. The conduction of ions in polymers, and use of

10 polymer electrolytes (PE) was discovered about a decade earlier than the discovery of electronic conduction in polymers. The understanding of ionic conduction in CPs could therefore be based on the understanding of ionic conduction in polymers.

Figure 2.5 Schematics ions dissolved inside a polymer matrix.

One of the most common PEs is polyethylene oxide (PEO). PEO can dissociate low molecular weight salts into ions that carry the charge (seen schematically in figure 2.5) The high ion transport properties in amorphous PEO are associated with transport by hopping between positions, which are dynamically created by the slower dynamics of polymer chains. In weak electrolytes, the ionic conductivity is the sum of partial conductivities of all ionic species. If an amorphous phase is assumed, then the thermal motion of polymer chains cause the movement of ions and the ionic conductivity is similar in behavior to the mechanical properties of the polymer. The ionic conductivity of PEs can be related to the glass temperature Tg through

$ ' #a(T # Tg ) "(T) = exp& ) %& T0 + (T # Tg )()

The glass temperature is connected to the free volume of the polymer, which is a quantity that directly affects the thermal motion of polymer chains in ! inter-intra chain reactions. Furthermore the ionic species in polymer electrolytes are expected to reside in the free volume. The physic of ionic

11 charge transport is therefore intertwined with the packing, and the free volume between the polymer chains.

2.5.1 Ionic transport in CPs In CPs just as in regular polymer electrolytes the free volume defines the ionic conductivity. In electrochemical reactions however the structure of the CP can change radically and in extreme cases such as in so called micro muscles [16], where the polymer can increase in volume as ions move in and out of the polymer film . When using larger ions and specially anions, the result can be difficulties in moving ions through remaining free volume after an oxidation or electro- polymerization reactions. An effect of structural changes in regard to CVs has already been shortly discussed.

What therefore differentiates CPs that have ionic conductivity as compared to pure PEs is the coupling between electric fields, ions and electrons, during electrochemistry. The analysis of ionic diffusion and field-driven ionic movement is therefore more difficult in CPs. In most cases it can be assumed that the electronic drift and diffusion is faster, (especially when the polymer is doped), than the ionic movement, and therefore ionic movement should be the main limiting factor in fast processes.

The intertwined ionic and electronic mechanism during electrochemical reactions is of great importance for understanding of the operation of electrolyte based devices such as electrochemical transistors, and electrochromic windows, where bulk doping is dominant, and in electrolyte gated OFETs where the formation of double layers and doping at the interface of CPs and polymer electrolytes is dominant. Electrolyte-gated transistors will be discussed in more detail in chapter 3.

2.6 Electron charge transport theory

2.6.1 Towards a unified theory for disordered CP films The dominant mechanism of electron charge transport in classical in-organic electronics such as metals and in-organic crystalline semiconductors is based on band like motion of charges, where charges are delocalized through the crystal. The delocalization is due to the good electron overlap between adjacent atoms in the crystal. Here the electron mobility is limited only by

12 deviations in the lattice such as impurities and dislocations that cause electron scattering. The result is a high mobility, which decreases with increasing temperature, as higher temperature increases scattering events. The first description of such transport was presented more than a century ago by Drude [17], and therefore referred to as Drude transport. Sommerfeld further developed the Drude model to include quantum mechanical and statistical physics in the free electron gas model.

In CPs film, however, the high disorder of the material and the weak van der Waals interaction between the molecules lead to localized sites/states for the charges, where the charge is mainly transported via thermal hopping between the sites. Here the energy between the sites is disordered both in the energetic and the spatial domain. This, for CPs common type of transport, is only apparent in extreme cases for amorphous in-organics, with very high impurities. Low temperature transport in such in-organic systems was first described in the 1930s, before the discovery of conducting polymers, by Sir Neville Mott using variable range hopping (VRH). The VRH conductivity is gives by

* 1/(d +1)- $ T0 ' " = " 0 exp,# & ) / + % T ( .

where d is the dimension of the system. This formula correctly describes the dependence of conductivity on temperature also for CPs. The correct ! understanding of charge transfer in molecular systems such as CPs however should be based on charge transfer, where the Marcus theory can be applied for the charge carriers (i.e polarons).

13 Figure 2.6 Gaussian density of energy states.

In pioneering work by Bässler [18], a model was proposed that disregards the exact chemical nature of the molecules, and instead assumes a Gaussian DOS for the transport sites (see figure 2.6)

1 g(") = exp %" /2$ 2 2#$ ( )

The effect of polarons is further neglected in the model as σ for the site distribution is assumed to be larger than the polaron binding energy. As a ! result the hopping probability is here described by using the more simple Miller-Abraham [19] hopping probability, as compared to Marcus Theory. The formula used is

' % #% )exp( #2$R # j i ) , % & % ij j i " ij = " 0 ( KBT ) * exp(#2$Rij ) , % j < %i

Where υij is the jump rate between site i to j. !

14 One disadvantage of this model is that there exist no simple analytical solutions, however simulations with Monte-Carlo methods by Novikov et. al. using 2D- 3D sites have shown that this empirical formula [20] holds for disordered CP films:

$ $ ' 2 $$ ' 3 / 2 ' ' & 3# # eaE ) µ(E,T) = µ0 exp "& ) " C0&& ) " *) & 5k T & k T ) ) % % B ( %% B ( ( # (

This formula is based on a slightly more sophisticated correlated Gaussian disorder model GDM, and explains the mobility over wide range of fields ! and temperatures. The types of formulas based on the GDM and Miller-Abrahams hopping master equations are the most successful formulas to this date for describing mobility in disordered CPs. The Bässler model has been successful in unifying mobility description for many types of CP based devices, and shown good consistency to experimental data for mobility in disordered CPs, over a wide range of electric fields, temperatures and charge carrier densities. The work by Blom et. al. describes for example a unified mobility for both FETs and LEDs devices based on disordered CPs [21].

Hulea et.al have studied the DOS of the Poly(p-Phenylene Vinylene) over a wide energy range [22], by using electrochemical transistors (described in more detail in chapter 4). The interesting results here is that the DOS can be fitted quite well with Gaussian Distributions and that the only effect that the induced ions in the CP have for these devices, is a broadening of the DOS, as suggested by Arkhipov et. al. [23] for PHT based devices. This means that charge transport even in electrolyte gated devices, which is the device in focus in this thesis, can be explained within the framework of the Bässler hopping model, with a Gaussian or exponential energy distribution.

2.6.2 Metallic and truly metallic conduction in CP films In more rare cases at higher conductivities in CPs, it is possible to find temperature ranges where this thermal activation of conductivity is changed into a metallic type of temperature dependence, i.e where the conductivity does not increase but decreases with temperature. The highly conducting polymers have been described as disordered metals. Here the conductivity is described as a tunneling between metallic islands separated by non- conducting regions (barriers). These regions could consist of other materials that exist in blends or they could be regions with bad doping or very high 15 disorder. The conduction can in this case be described with a Fluctuation Induced Tunneling FIT model [24]. For small metallic islands FIT predicts

ln" #$T$1/ 2

This is similar to the 1D case for the VRH model. In 2006 Lee and Heeger, et. al. [25] showed that a form of the CP ! polyaniline possesses behavior that fully resembles a metal where both the conductivity vs. temperature is Drude type in the full temperature range. Furthermore the reflection of the polymer shows a plasma frequency in the infrared region, and correct conductivity values could be estimated from these reflectivity measurements using the connection between reflectivity and conduction in the Drude model. One proposed explanation for the deviation from localized hopping transport is that conduction electrons screen local potentials from polarons and structural disorders [26]. The conductivity of these metallic polyaniline films are around 1000 S/cm at room temperature. It is noteworthy that this is the same as the highest conductivities measured in the polymer PEDOT that is extensively used in this thesis. One of the important results of this model in the context of this thesis, is that although high conductivity is observed in CP films at larger dimensions, the presence of non conducting islands could lead to a scenario where the paths for hopping are broken. It is believed that this scenario occurs in PEDOT:PSS nano sized structures, where the non-conducting parts between the islands comprise the large counter ion PSS (see figure 2.7). A solution to this problem is to use highly conducting polymers that do not have any big counter-ions. In papers 2,4, the introduction of PEDOT-S as such a nano compatible materials solves this problem, and enables creation of metallic nanowires with large aspect ratios. The physical phenomenon that underlie the miniaturization of conducting nano fibers are discussed in more detail in the next section where deviations from the VRH for nanofibers are discussed.

16 Figure 2.7 Schematic demonstration of conduction in disordered metals by hopping between metallic islands. The break of conductivity for small / nano structures is demonstrated due to the distance between the islands.

2.6.3 Charge transport in CP nanofibers As each CP is inherently a quasi 1D nanostructure, the use of CPs as components in future organic nano-electronics is very interesting. The advances in nanopattering methods such as self-assembly, nano imprinting, soft-lithography and more, have resulted in construction and electrical characterization of a number of highly conducting CP nanostructures in materials such as Polyaniline, PEDOT and polypyrrole.

The general conclusion from measurements on these nano fibers so far is that there is a very strong power law behaviour for both I(V) and G(T), with power exponents between 2-7 for G(T). An argument of Aleshin et. al [24] based on the exponents in these power laws is that none of the nano systems can be described by either VRH models nor other models for 1D systems. By assuming that parts of the CP nano fibers can be described as one-

17 dimensional Fermi liquids with high repulsive short range electron-electron interaction (Luttinger Liquids LL) [27], it is suggested that CP nanofibers can be seen as quasi 1D systems composed of several LLs connected in series. Some interesting results that further point in these directions are magnetotransport studies that have suggested that conductivity in 3D bulk materials in highly doped Polyaniline is controlled by interfibrillar point contacts between nanofibers with high conductance.

Conclusions that can be drawn from these results is that the inherent 1D nature of conjugated polymers could be manifested in nano fibers that are carefully designed at both molecular level and mesoscopic level. In this thesis the demonstration self-assembled PEDOT-S nanowires through molecular self-assembly has enabled yet another material class and tool for the analysis of the physics of transport in highly conducting polymers [28,29].

18 2.7 Common CPs: PEDOT and P3HT The structure and composition of the polymer repeating unit gives rise to a variety of different physical properties, such as optical, electrical and chemical. For example the polyfluorenes and poly(para-phenylene-vinylene) (PPV) are highly luminescent and are therefore serving as active materials in LEDs.

A class of high conductive polymers that can be produced at low cost in large amounts are based on polyaniline, however certain disadvantages such as the formation of hazardous compounds upon degradation has limited the use of polyaniline.

The first synthesis of polythiophene was reported by Yamamoto and Lin in the 1980s. Since then a variety of different synthesis routes based on the polythiophene have been reported and many applications have been demonstrated with these derivatives such as OLEDs, FETs, and Solar Cells.

The most successful forms of the derivatives of the polythiophenes are: (i) The polythiophene polymer where a side-group is added to the 3- position to give solubility. (ii) Poly(3,4-ethylenedioxythiophene) PEDOT and derivatives thereof. These classes of polymers, especially the PEDOT polymer, are the ones used most extensively in the thesis, and will be described in more details below.

Poly(3-hexylthiophene (P3HT) One of the successful polymers of Polythiophene is Poly(3-hexylthiophene) P3HT with an alkane chain containing six carbons on the side chain (Figure 2.8). The pure and regioregular form of P3HT is today commercially available. This CP has good solubility and the ability to form regions with good packing (crystalline domains). The level of regio-regularity can be altered in P3HT to give different levels of crystallinity.

P3HT is semi-conducting (low conductive) in its pristine state. With a bandgap of around 2 eV P3HT can be used in photovoltaics for charge separation and hole transport, in OLEDs as electroluminescent material, and as channel material in OFETs or electrochemical transistors.

19 Panzer et. al. [30] showed that field effect mobilities of P3HT could reach as high values as 0.7 cm2V-1s-1, with 1022 charges percm3 and a room temperature conductivity of 1000 Scm-1, by using an electrolyte gated doping. (see more details of electrolyte-gated devices in chapter 4). Although one should be careful with the interpretation of these results as pure FET, these are the highest reported values. In this thesis P3HT is used as channel material for the creation of electrolyte gated OFETs on fibers (see section 4.4). Electrochemically doped P3HT conductivity values of around 20Scm-2 are reported by Jiang et al. [31].

The highest efficiencies of CP based solar cells to date are reported by Kim, Heeger et.al. for tandem solar cell that are partly based on P3HT [32].

PEDOT-analogues The most commercially successful polythiophene derivative is poly(3.4- ethylenedioxythiophene) PEDOT [33,34], developed in the 1980 by Bayer AG Germany. Bayer developed a water soluble form of this polymer having a charge balancing counter-ion poly(styrene sulfonate) PSS. The result is a water soluble dispersion called PEDOT:PSS, where PSS polymers that are significantly longer than the PEDOT polymers and present in excess contribute both to solvation, and to the formation a polyelectrolyte system. This dispersion is highly p-doped in the pristine state, resulting in a material with high conductivity. The clever introduction of a dioxythiophene on the thiophene, results in a very air-stable polymer although the system is p- doped and electron rich.

The highest conductivity achieved in PEDOT is reported for a controlled form of vapor phase polymerized films with a conductivity of 1000 S cm-1 [13]. The more interesting form of high conductivity variants of PEDOT has however been achieved by further development of PEDOT:PSS. This is done by adding various additives such as di-ethylene glycol and DMSO to create a phase separation between PEDOT and PSS in films [35]. Conductivities as high as 500 Scm-1 are today achieved for commercial PEDOT:PSS in films made by solution processing.

The doped state of PEDOT is almost transparent as the polaron bands absorb in the IR region. The de-doped form of PEDOT is dark blue with an absorption maximum located slightly above 600 nm.

20 The combinations of commercial availability of PEDOT:PSS in large quantities, the high air stability and high conductivity of this CP in pristine state, have resulted in numerous studies and demonstrated application, too large to be mentioned within the scope of this text. The most widespread applications to date include hole injection layers in OLEDs, anti-static coatings, and electrochromic windows [33].

As PEDOT is highly conducting in pristine state it can however not be used as channel material in depletion mode OFETs. Instead the good ion mobility of PEDOT:PSS and other PEDOT analogues, makes this material ideal for use in electrochemical devices. These will be discussed in more detail in section 4.1.

One of the weaknesses of PEDOT:PSS is that the excess of the large PSS- ions create dispersions of micellar structures resulting in domains with non conducting islands [36]. As already discussed in section 2.6.3, these high conducting blends can cause problems in the nano domain due to the character of hopping. A solution to this problem it to create a PEDOT based material with only one component. Attempts to do this were initiated through the modification of the PEDOT monomer EDOT. Reynold’s group reported an alkane substituted EDOT, that could be polymerized into a chloroform-solube PEDOT, and derivatives thereof called PPproDOT [37]. An important chemical step was the synthesis of EDOT with a more polar group, hydroxymethyl, resulting in EDOT-M [37]. EDOT-M is a good starting material for the formation of a number of truly water soluble derivatives. One of these is alkoxsysulfonate EDOT-S where the S group very much resembles the sulfonate in PEDOT:PSS. We reported on a fully water soluble form of PEDOT-S through oxidative polymerization of EDOT-S [29]. This CP has a conductivity of above 1Scm-1 to date. The interesting character of PEDOT-S is that it is believed to be partially self-doped by its ionic side groups, and that the formation of micellar islands cannot occur here. In papers 2,4 PEDOT-S is used to create functional nano electronic structures at dimensions that are not realizable with PEDOT:PSS.

Another positive side effect of PEDOT-S is that this polymer is a polyelectrolyte, (i.e a CP carrying permanent ionic charge), as compared to PEDOT. The polyelectrolyte character of PEDOT-S contributes to the possibility of interaction with biomolecules which often carry net charges [38]. In paper 2 self-assembly of PEDOT-S onto protein molecules is demonstrated for the creation molecular conducting nano wires.

21

These results indicate that PEDOT-S like classes of polymers may eventually fully replace PEDOT:PSS and become the next generation of commercial highly conducting stable CPs.

Figure 2.8 Chemical structure of P3HT, PEDOT, and PEDOT-S

22 Chapter 3 Self-assembly of CP micro and nanofibers

Micro and nano scaling of patterns in CPs is a necessity in the continued developments of all application areas based on CPs. In organic electronics for example creation of patterns in micro and nano dimensions is important for increasing performance of individual devices and for increasing device density in circuits, just as for inorganic micro- electronics [39]. However the conventional methods that are used for pattering of in-organic materials are usually not compatible with organic electronics. Furthermore the character of CPs gives rise to completely new ways for creating micro and nano patterns. One of the most promising and successful methods for creating patterns in CPs are those that take advantage of solvated CPs for the creation of micro and nano patterns through different forms of self-assembly.

In this thesis the two concept of self-assembly from fluids are used. These are self-assembly of CP onto biomolecular nano fibers, and fluidic patterning with soft lithography for making arrays of micro and nano wires. These methods are presented here in detail, and the results of patterning of PEDOT from paper 2,4 are presented. In the coming chapters the use of these self-assembled micro and nano fibers/wires for making transistors and devices is discussed.

3.1 CP decorated biomolecular nanowires All living systems constitute building blocks that are hierarchically self- assembled. Whiteside categorizes [40] two main types of self-assembly: - Static self-assembly involving systems that are at equilibrium and do not dissipate energy (e.g. crystals, and folded proteins). The formation of such structures can require energy to form, but once formed they are stable. Current research is mainly based on this type of self-assembly. - Dynamic self-assembly occurs the interactions responsible for the formation of structures or patterns requires dissipation of energy from the system, such as in all life-forms. The understanding of the processes that 23 involve dynamic self-assembly in life is still very rudimentary. However biological systems provide us with many biomolecular building blocks that can be mimicked, modified or directly used for the purpose of bottom up construction of mesoscale structures. One of the interesting SA geometries involves semi-one-dimensional structures such as nanowires and nanotubes. Functional SA nanowires can be used as building blocks in next generation nano-electronic circuits, were they can be used as transistor channels, emitters, nanowires in crossbars and more (see chapters 4,6 for more detail). In the field of inorganic nano electronics the functional self-assembled nanowires: carbon nanotubes, and semi- conducting crystalline nanowires [41] are the most extensively studied materials.

Biomolecules can in the same manner be self-assembled into biomolecular nanowires. The most well-known biomolecular nanowire is the DNA molecule. DNA can have very high aspect ratios of several thousands, and its unique functionality has enabled DNA molecules to be used as nanowires as well as in SA of quite complex 2D and 3D nanostructures [42]. Peptides and proteins are another class of biomolecules that can generate 1D nanowires. These are referred to as fibrils, filaments or protein fibers, and mainly self-assemble from other smaller protein polymers or from peptide monomers. In this thesis the focus has been on amyloid fibrils. These are biomolecules with a diameter of approximately 10nm [43], and lengths extending up to 10 µm. The structure of amyloid fibrils is based on extended β-sheets [44]. Furthermore these fibrils are very stable and can easily be functionalized. As biomolecules are not easily electronically utilized, CPs offer a great opportunity for making biomolecules functional, where the biomolecule acts as a nanotemplate that is functionalized/coated with an electroactive CP. In previous works, DNA has been used as template together with conducting CPs. Polypyrrole has been chemically polymerized on DNA to form self- assembled nanowires [45], polypyrrole and poly(3,4- ethylenedioxythiophene) have been electrochemically polymerized on DNA to form chiral CP structures [46], and polyaniline nanowires have been formed on immobilized DNA on surfaces using chemical polymerization routes [47]. There are however problems with such polymerization methods such as degradation of the biomolecules, and poor CP quality. This is probably the reason for why none of the previously demonstrated polymerizations of highly conducting polymer have resulted in highly conducting nanowires.

24 A much more simple and natural route involves direct SA of CPs onto biomolecular nano templates in the fluid phase. This powerful method of combining the functionality of both biomolecules and CPs have been demonstrated in schemes based on combining the molecular recognition capabilities of DNA together with the optical properties of CPs to self- assemble supramolecular systems. Systems with optical properties that mimic logic operations [48,49] have been demonstrated. The research group affiliated with this thesis work, have previously demonstrated that luminescent conjugated poly-electrolytes can be self- assembled onto amyloid fibrils [38,50], and also onto DNA [51] to form optically functional nanowires. None of these CP:biomolecular nanowires however exhibit measurable electrical characteristics. In paper 2 in this thesis we show highly conducting biomolecular nanowires, further described in the next chapter.

3.1.2 PEDOT:amyloid nanowires The successful self-assembly of luminescent conjugated-poly-electrolytes [50] onto DNA and protein fibrils can not be demonstrated for the highly conducting polymer PEDOT:PSS. The reason is probably that PEDOT in PEDOT:PSS lacks ionic side chains and that the presence of the large and non-conducting macromolecule PSS prevents biomolecules to be used as nano templates. In paper 2 the newly developed form of PEDOT-S [29] is used. It is believed that the CP backbone in PEDOT-S is partly self-doped by the alkoxysulfonate ionic group, and that the rest of the ionic groups make the polymer water soluble (dispersible). The ionic interactions of the ionic side groups can also together with other weak interaction forces, (such as hydrogen binding/hydrophobic interactions), be the mechanisms of binding between this CP and positively charged biomolecules.

The beauty of the SA process is the ease under which PEDOT-S binds onto amyloid fibrils directly in water without the need of any heat, and in a matter of a few minutes. One of the difficulties in making conducting nanowires, is however that the CP has to coat the entire fibrils so that no lack of PEDOT-S along the fibers would allow for conduction breaks. This is solved by using the CP in excess during self-assembly. It is believed that PEDOT-S chain assemble around amyloid fibrils until all assembly sites are filled. The remaining PEDOT-S is then filtered away since the size of these is smaller than the size of the coated fibrils. This is demonstrated schematically in figure 3.1.

25 It should be noted that this types of self-assembly with CP being in excess has not been done in the previous studies where purely luminescent CPs are used. One reason is that luminescence is seen even if the fibers are not fully coated. Instead a smaller amount of CPs relative to biomolecules is used in order to avoid any filtering of the supernatant solution to remove excess of unbound CP. AFM and TEM images of PEDOT-S:amyloid reveals nanowire geometries with very high aspect ratio and relatively high stiffness. Networks of these fibers are used as channel material in electrochemical transistors, for analysis of the electrical character of the nanowires. This is described in more detail in section 4.3.1.

Figure 3.1 Schematic figure of self-assembly of the CP, PEDOT-S, onto biomolecular fibrills, and the filtering of residual PEDOT-S

The SA method of PEDOT-S:amyloid can most probably be applied for a number of other biomolecules as well. Figure 3.2 shows AFM and TEM pictures of SA PEDOT-S:biomolecule fibers where also a peptide NW coated with PEDOT-S is shown.

26 Figure 3.2 a) TEM picture of peptide NW network coated with PEDOT-S b) TEM picture of amyloid fibrillar networks coated with PEDOT-S c) AFM figure of free standing amyloid fibrlls coated with PEDOT-S, amyloid network, and two PEDOT-S coated d) TEM picture of pure amyloid fibrillar network

27 3.2 Soft-lithography patterning Soft lithography is a non-photolithographic set of micro/nano fabrication methods, where all of its members use a patterned elastomer as a stamp, mold or mask, instead of a rigid photo mask as in conventional photo lithography. The method was popularized in the beginning of 90s by George Whitesides et. al. [52]. The elastomers can be used to generate two- and three-dimensional patterns through a different set of techniques, which have been developed during the years. The smallest features that are demonstrated with SL are as small as around 1 nm [53], which means that SL can remain a nano patterning tool for the future. The powerful capabilities of different techniques and the experimental simplicity of SL allows for the use of these in a very wide range of applications.

The possibility of templated self-assembly [40] of a diversity of molecules and materials with elastomer templates, has been exploited in this thesis for the patterning of CPs. The techniques of molding, especially molding in capillaries, and soft embossing / soft nanoimprint lithography are the main techniques that has been used for micro and nano patterning of CPs.

Soft nano imprint lithography Nanoimprint lithography NIL, refers to patterning methods where printing is done by pressing a pre-generated nano relief structure into a meltable material. If an elastomer is used as the relief structure and the fusible structure is UV curable resist (which is usually the case), the method can be called soft UV NIL [54]. Soft UV NIL has been used in order to pattern the passive structures in a number of CP based devices such as LED and TFTs 39. The use of NIL for direct patterning of CPs is more rarely occurring, but has for example been done for polyaniline [55]. In this thesis soft NIL is for patterning of passive resists, in order to make three-dimensional masks (see coming section 3.2.1).

Capillary molding patterning The method of capillary molding comprises laminating an elastomer against a substrate to form micro or nano fluidic channels, into which liquid materials such as CPs in solutions can be filled. The filling process can be driven by pure capillary force (often referred to as micromolding in capillaries, MIMIC) or external pressure. When the liquid has filled channels

28 it is solidified through solvent evaporation or curing, and the solid micro/nano objects or molecules that were carried by the liquid are delivered to the substrate surface forming a negative of the channel. The mold is finally removed to complete the fabrication leaving only the patterns of active material.

Molding is an interesting SL technique since it allows direct patterning of organic electronic materials directly from fluids 56, without any additional step that could degrade the material. Molding allows patterning in both micro and nano dimension, with good feature size control especially in the planar directions, allowing for construction of dense nano structures such as dense arrays of nano lines.

Direct patterning of organic electronic materials by capillary molding in previous works include patterning of conducting carbon micro electrodes [57], and patterning of micro and nano structures in polyaniline [58].

3.2.1 Patterning of PEDOT in bridged micro- and nanowire arrays In the continued development of organic electronics, scaling of patterns sizes is an important issue. Two main challenges can be pointed out here:

1) Methods for scaling patterns (to sub micro, nano and molecular scales), that match the requirement of cheap and large area patterning, since this is required for organic electronics. Previous examples that try to deal with this issues include self-aligned printing [59,60] where nano gaps are easily created between inkjet printed lines by dewetting from hydrophobic surfaces. However these schemes do not provide sub micron features throughout the pattern as the thickness of the lines that create the nano gaps still is in the ten micrometer regime.

2) The second challenge is alignment and registration of printed nano electronic devices over large areas. The reason is that large area flexible substrates such as plastics are structurally instable, meaning that alignment between several nano patterning steps is very difficult [39]. Furthermore alignment at submicron scales is not compatible with printing techniques such as ink-jetting.

29 One of the purposes of Paper 4 in this thesis is to show a method that deals with both of these issues by demonstrating large area patterning of seamlessly connected micro-nano patterns the CP PEDOT. The idea here is to take advantage of the water solubility of PEDOT polymers, (which makes the polymer solutions compatible with elastomer materials such as PDMS and derivatives of PDMS), and create PEDOT patterns through molding in capillaries. This technique is used for patterning connected PEDOT wires that range from nano sizes and up to tens of micrometers thereby fulfilling the requirement of large area cheap nano patterning. The connection between the nano-micro features would further mean that the larger micro parts could be used as pads and connected to other structures, which in turn are aligned and patterned on-top using macro scale patterning techniques such as roll to roll printing (100 µm). In this way the issue of nano alignment for the PEDOT layer would also be automatically eliminated. The reason for choosing PEDOT is that the material is multifunctional, and can act as many parts in organic electronics devices such as OLED backplane layer, conduction lines, crossbar lines, source-drain contacts, transistor channel material, and more.

The presented idea of capillary molding of connected features ranging from nanometers and up to micrometers is however not straightforward. One reason is that the softness of the elastomer in SL limits the possible aspect ratios a (a=height/width) of the relief that can be used, when the relief in the stamp has one constant height, as is usually the case. If a is too low, the elastomeric material will deform/sag defining a lower limit al. If a is too high the stamp will instead collapse under its own weight defining an upper limit ah The lower and upper limits of the aspect ratio define a working range w=ah / al for the elastomer (see Figure 3.3). The most commonly used material in soft lithography, poly(dimethylsiloxane) (PDMS), has a working range of w~10 based on calculations and experimental findings [61]. The idea of one step patterning of features in a range of some 100 nm, to some 100 µm requires working ranges in the order of at least 1000. In this thesis this is achieved by creating a three-dimensional hybrid templates with two height levels, where level one contains the nano structures with one height and level two the higher micro structures. These templates were produced using relatively cheap production methods (for more information, see methods paper 4, and patent application [62]).

30 Figure 3.3 (top) The sagging and collapse problem of soft lithographical stamps. (Bottom) Schematics of a 3D stamp with connected micro and nano channels, where sagging/collapse does not occur.

When using these templates for capillary molding, the channels are filled from the micro side by aqueous PEDOT through capillary forces. As the fluid enters the nano channels the capillary forces decrease and the overall speed of the filling becomes limited by the speed in the nano channels. The general fluidics principles are similar in both micro and nano dimensions and this allows for continuous flow until all the parts of a channel are completely filled and the PEDOT is deposited as the solvent evaporates. Figure 3.4 shows SEM pictures of such patterned PEDOT structures. Interestingly the pattern is truly connected at the bridge between the micro and nano channels. One possible reason for this could be that the pressure difference between the connected micro/nano channels assures for continuous flow of material to this critical part, so that no cracks can occur during the drying process. This continuous flow and deposition process during evaporation might also explain why the very long channels (width/length>1000) can be molded without considerable breaks along the patterned PEDOT.

31 In paper 4 the electrical character of the PEDOT micro-nano arrays is described and the concept of using these in nano electronics is further discussed in chapter 6.

Figure 3.4 SEM image of PEDOT-S structures created through capillary molding in PDMS channels replicated from a micro-nano template.

32

Chapter 4 Electrolyte gated transistors

As discussed in section 2.4 CPs can change intrinsic properties, such as electrical, mechanical and optical properties, by electrochemical redox reactions. CPs are therefore very well suited for devices that are based on electrochemical (EC) cells. In these types of EC devices typically the CP is partly or fully in contact with an electrolyte to create possibilities for doping and de-doping.

Some electrochemical devices are light emitting electrochemical cells, micro muscles, ion pumps, and the electrolyte-gated transistors described here.

In this thesis electrolyte based transistors and transistor devices are demonstrated at the micro and nano scales, where the techniques of micro fiber weaving and SA are used for making these devices.

This chapter contains a brief overview of electrolyte material, a description of the transistor devices that are used in the thesis, and discussions about transistor performance due to scaling to micro and nano dimensions.

4.1 Electrolyte materials The devices in this thesis are demonstrated using both liquid electrolytes (paper 2), and solid polymer electrolytes (papers 1,3,4), and these types of electrolytes are described here.

Liquid electrolytes Classical liquid electrolytes (LE) comprise salts that are dissolved in water (aqueous electrolytes) or other solvents (non-aqueous electrolytes). As water has a narrow electrochemical window it is more common to use other solvents such as acetonitrile for controlled EC experiments. LE based devices are operated either by immersing the device into the LE, as done in paper 2, or by confining the electrolyte to a desired region over the active part of the device, using methods such as micro fluidics [63].

33 The advantage of using LE is that the mobility of the ions can be very high and that a large number of salts can be used.

Solid polymer electrolytes The use of liquid electrolytes for CP electrolyte devices is very limited since in device applications it is desirable to form stable and well defined patterns of electrolytes. Therefore solid forms of electrolytes are needed. The subject of solid polymer electrolyte SPE was already discussed in section 2.5.1, where one of the most common solid polymer electrolytes, PEO, was mentioned. PEO has been used in numerous CP based applications such as electrolyte based transistors [30] and other electrochromic devices, with LiClO4 as a commonly used salt. In this thesis a solid polymer electrolyte based on poly(styrene sulphonate)acid PSSH was used for making devices with PEDOT in papers 1,4. One of the reasons for choosing such systems is that this electrolyte should have similar ion mobility conditions as PEDOT:PSS in which ion conduction is dominated by PSS. The PSSH electrolyte has protons as cations and in aqueous environments these can also be exchanged with other cations such as Na+ or Li+. One disadvantage of using the ionic conductivity of PEO and similar SPEs is a relatively low ionic conductivity that is in the range of 10-4-10-5 Scm-1. It is desirable to increase the ionic mobility in solid polymer electrolytes to enhance device performance.

Both PEO and PSS based SPEs are hygroscopic and the ions are normally solvated in water, as these polymers hydrate. Most CP (except for PEDOT) are however sensitive to degredation in the presence of water, and the use of SPE with little or no water content is therefore also desirable.

One strategy for achieving both of these properties is to use solid polymer ionic liquids. These are solid electrolytes that are based on ionic liquids ILs, which are organic salts that are molten at low temperatures. Two strategies exist for creating solidstate electrolytes based on ILs. The first strategy involves mixing of conventional PEs with ILs, and the second involves designing functional polymers that present some of the characteristics of ionic liquids. In this thesis a solid polymer IL is used which is a hybrid of the two mentioned solid ILs. These consist of a mixture 1-butyl-3- methylimidazolium a- [bmim][a-] and a corresponding polymer IL poly(1- vinyl-3-methylimidazolium a- poly[ViEtIm][a-] [64], where a_ denotes an - - - - anion that can be for example Br , TF2N , PF6 , or BF4 . These types of

34 imidazolium based PILs have high ionic conductivity in the range of 10-3 Scm-1 [64], and have been demonstrated by others in both electrochromic devices [64] and electrolyte gated transistors [65,66].

As both stability [67] and ionic mobility of ILs are better than conventional PEs it is conceivable that PIL will be the first material of choice for in the future of electrolyte based CP devices.

Figure 4.1 Chemical structure of three polymer electrolytes: [bmim][TFsi], Poly Styrene sulfonate (PSSH), and polyethylene oxide (PEO).

4.2 Electrochemical PEDOT transistors

4.2.1 Theory of operation As described in section 2.4, doping of conjugated polymers can change the electronic conductivity of the polymer by several orders of magnitude. In 1984-85, Thackeray, White et al. [68] reported polypyrrole based transistors with a three-electrode configuration, where the channel was modulated by electrochemical doping/de-doping. These transistors are called electrochemical transistors ECTs, and the most common ECTs today comprise those based on PEDOT. The redox process of the previously described forms of PEDOT, PEDOT:PSS and PEDOT-S, can described with the following reactions

PEDOT + : PSS" + C + + e" # PEDOT 0 (C + : PSS" )

PEDOT +S" + C + + e" # PEDOT 0S" : C + ! 35 ! where C+ denotes the cation used in the electrochemical redox setup. The reaction described for PEDOT-S here assumes that the sulfonate group in PEDOT-S self dopes the polymer, which might not be the case for all the PEDOT-S units in a polymer film. In other forms of PEDOT material, such as PEDOT tosylate, a similar doping occurs where the PSS is instead replaced with the anion of that particular system.

The reversibility of the redox of PEDOT together with the big dynamic range in conductivity (~4 orders of magnitude) and electronic conductivity in all states has made PEDOT an ideal material in the construction of PEDOT ECTs. The idea behind such a transistor structure is to control the current between a drain and source electrode through modulations of the oxidation state of the channel. Nilsson et. al. first demonstration of patterned PEDOT:PSS transistors with a more complicated four-terminal structure [69] . In later demonstrations these terminals were reduced to three to resemble a normal transistor with a Source (S), Drain (D) and Gate (G) [70]. A schematic of this simple structure is seen in figure 4.2. This ECTs is realized by two patterned layers, where one layer consists of the PEDOT, and the second layer consists of patterned electrolyte that is in ionic contact with the PEDOT.

Figure 4.2 Schematics of PEDOT ECT, with a 3D view and side views, showing the reduced and oxidized parts of the transistor under operation.

36

The mechanism of operation is that as gate potential is applied electrochemical reactions de-dopes the channel through the two terminal electrochemical cell comprising the channel bridged ionically to the gate. The transistor channel will go from an initial conducting state, as PEDOT is doped in its pristine state, to a less conducting de-doped state, resulting in a depletion mode operation since the channel is turned off as the gate voltage increases. The potential difference between S-D and gate have to be positive, meaning that negative voltage is applied to the drain and positive voltage to the gate, so the transistor operates in the third quadrant. As the PEDOT materials used here can be highly conducting polymers (500 S/cm to date can be used), the same PEDOT layer that forms the channel also serves as both Source and Drain contacts. This means that no additional patterning of S-D contacts is necessary, which is a huge advantage of this transistor structure. Furthermore as the entire film in the channel is conducting holes (as compared to FETs where only a thin layer is conducting), the transistor can deliver very large current (around 100 Ω/square in the transistor on-state for best PEDOT materials today).

The operation voltage of any electrochemical transistor is a function of the electrochemical potential for redox reactions, rather than the geometry of the device, in contrast to FETs. This means that the PEDOT ECT operates between approximately 0-1 Volt. The pinch-off mechanism in the ECT is also different from FETs, in which current pinch off occurs due to space charge limitations. Figure 4.3 shows an transistor curve for a PEDOT:PSS ECT. The reason for this behaviour is believed to be that electrochemistry also occurs for the channel structure, being simply a single PEDOT layer with an electrolyte on top (figure 4.2), although it is not a cell with two electrodes. The operation of this structure has been analyzed by Robinson et. al. [71] where they show that a gradient of electrochemical potential is distributed along the film. The result is electrochemistry with oxidation at the positively biased and reduction at the negatively biased electrode. Redox reactions occur until the electrochemical potential is uniform and equilibrium is reached. At equilibrium the current is only transported by holes, the current has a linear dependence on the applied voltage at low voltages, and pinches off at voltages above a critical pinch-off level. It is interesting that this saturation behaviour gives rises to curves very similar to those observed for FET structures; however the physics underlying

37 is totally different, as space charge is not limiting in the ECT case, since the conductivity of the material is high. These similarities can give rise to misinterpretations in other forms of devices such as electrolyte gated OFETs. This subject is discussed in more detail in section 4.5.

Figure 4.3 Electrical characteristics of an ECT, with the pinch off, depletion mode, both being described as electrochemical mechanisms.

In this thesis, PEDOT based ECTs are shown for non-planar systems and at nano dimensions, and these will be discussed in more details in section 4.3.

4.2.2 Some ECT based applications The true advantage of ECT PEDOT transistors lies in the fact that PEDOT is a multi functional material that can act as resistors, gate, source-drain, transistor channel, gate, electro chromic pixels, transducers, and more. As discussed previously, electrochemical operation alleviates needs for Source, Drain patterning and furthermore no patterning of thin dielectric material is needed, as in the case for FETs. In fact the gate can be in the 38 same plane as the channel, which is not at all possible for FETs on micro scales. The properties have been used in a number of different applications based on patterning PEDOT and electrolytes on planar substrates. The first demonstration of printed logic circuits with PEDOT ECTs, was demonstrated by Nilsson et. al [70], where only two layers (PEDOT and solid electrolyte) was used, and where PEDOT resistors were utilized. Active matrix displays in only 3 layers, have also been demonstrated where the active matrix transistors and pixels are in the same PEDOT material [72]. The very small number of layers in devices based on PEDOT ECTs make roll to roll printing very advantageous since printing of many layers can not be done easily at high speeds [73]. Roll-to-roll printed electronics based on PEDOT ECTs is currently being commercialized at ACREO in Norrköping (www.acreo.se).

Other interesting applications based in PEDOT ECTs comprise various kinds of sensors [74]. These are based on the fact that changes in the ionic conductivity and character of the electrolyte will change the S-D current. Thereby the transistor works as transducer, that can convert any physical quantity that affect ion concentration and mobilities in the electrolyte. Nilsson et. al has demonstrated PEDOT ECT humidity sensors [69], with the use of an electrolyte Nafion in which ionic conductivity is a strong function of humidity. Even more interesting classes of sensors include enzymatic sensors. The most widely used enzyme in sensors, glucose oxidase (GOx) has been used by the research group of Malliaras for the detection of glucose. This is done by using an electrolyte that contains GOx, as glucose is added to the electrolyte GOx catalyzes the reaction of glucose in the presence of oxygen and produces hydrogen peroxide (H2O2) and gluconic acid. The H2O2 is oxidized at the gate as voltage is applied and the ECT channel is reduced, resulting in modulation of the channel current.

All of the previous results of ECTs have been based on planar substrates, and usually with larger patterns (>100 µm) in order to match cheap printing technologies. In paper 1 the advantages of ECTs is use to take them from planar substrates onto micro fibers, and we have furthermore showed that ECTs also can be weaved. This will be discussed in more detail in the next section.

39 4.2.3 Microfiber ECTs As already discussed, the mechanism of operation in an ECT is similar to an electrochemical cell. In these cells the potential of the anode and cathode control the half side reactions on each electrode at each doping site, and the actual field distribution between the anode and cathode is not important for the modulation of the transistor, as the case is for FETs. The only necessity for modulation to occur is that free ions are present at the doping sites, and this is fulfilled by the choice of an electrolyte with sufficient ion concentration, and by the fact that the entire bulk of the PEDOT channel also transports ions as PEDOT itself is solid polymer electrolyte.

One of the major implications of this mechanism is that the actual shape of neither the gate or the source and drain is critical for the operation, in fact in classical electrochemical setups usually rod shaped metallic electrodes are used. However this quality is not utilized in any previous ECT applications where the PEDOT structures used have all been patterned thin film on planar substrates. In this thesis in paper 1, it was first demonstrated that PEDOT ECTs can be manufactured using cylindrical micro fibers that are coated with a thin film of PEDOT. The PEDOT micro fibers could be weaved to form crossings also termed junctions, and by placing an electrolyte at such junctions an ECT was formed. Figure 4.4 shows schematically one such wire ECT WECT, and a schematic of how the ion distributions in the gate and channel, showing that the entire channel of the transistor is active around the cylinder.

40 Figure 4.4 Schematics of a microfiber junction ECT. (left) 3D picture. (Left) Illustration of the ionic distribution due to bulk doping of the film around the fiber.

The electrical character of the WECTs is very similar to the planar ECTs, with same on-off ratios, electrochemical pinch-off and low operation voltages. This means that the assumption the electrochemical reactions occurs around the entire cylindrical PEDOT film is correct. One argument is that if a part of the channel would be in-active in WECT the transistor would have had a lowered on-off ratio.

Many of the advantages of the PEDOT ECT are demonstrated much more efficiently in these wire structures. The most striking effect is that there is no patterning step involved in the making of the WECT. Instead, the shape of the fiber itself and the self-assembled electrolyte drops form the transistor. It is also noteworthy that fibers as small as 10µm have been demonstrated in WECTs, meaning that the smallest WECTs are smaller in diameter than the width of any planar PEDOT ECT that can be made with conventional printing.

The demonstration of all plastic, low voltage, transistors that can easily be embedded on textile micro fibers, has naturally large implications for the field of electronic textiles. The use of WECTs and similar devices in e- textiles will therefore be described in more detail in chapter 5.

41 4.3 PEDOT Nanofiber ECTs As described in chapter 3 one focus of this thesis has been nano patterning of CPs with self-assembly methods, which resulted in successful demonstration of PEDOT nano structures at molecular scale with biomolecular templates and in predefined large area arrays with MIMIC.

These structures allow for the construction of nano ECTs, where the channel can consist of molecular scale structures or well-ordered nanopatterns such as nanowire arrays of PEDOT. These two types of nano ECTs are further discussed in this chapter.

4.3.1 Molecular ECTs A few examples can be found in the literature where PEDOT nano structures in electrochemical setups have been evaluated. Alam et. al [75] demonstrated PEDOT nanowire sponge like structures that were electrochemically grown with varying nanofibers line widths between 100-200 nm. Abidian et. al. [76] has shown electrochemical activity in PEDOT nanotubes with similar structures and dimensions. In these demonstrations the conducting film is 3D network of nanostructures with varying sizes and very short length to width ratios, all of the structures are also inherently connected as a result of the bottom up assembly through growth. This means that the channels in these ECTs really are just bulk films that are more porous.

In this thesis a totally different scheme is used, where self assembled PEDOT-S coated amyloid fibers are used as active nano fiber materials for making nano ECTs. The background to assembly methods of these PEDOT nanofibers are described in more detail in chapter 3.

Conceptually PEDOT-S coated amyloid fibrils can be seen as similar building blocks as the coated textile micro fibers, but much smaller and at the molecular scale (at least 1000 times smaller in diameter). With this analogy in mind, a number of features of these fibers as building blocks can be mentioned: i) The amyloid fibrils act as strong nano templates with high width/length ratio, exceeding 1000 in some cases. The hydrogen bonds in the beta sheets of the amyloid fibrils are very strong and they do not break when used in bottom up assembly processes.

42 ii) Each amyloid fibril is coated with a number of PEDOT-S polymers building a shell around the fibril (the electrical nature of this is discussed in more detail below) iii) The diameter of the coated fibrils is in the range of 10-20 nm, which is on the border of the limit of true molecular electronics. iv) Coated fibrils can be assembled from aqueous solution in structures where the fibrils retain their high aspect ratio and can be seen as 2D long lines. The figure 1 in paper 2 clearly demonstrates this feature by showing a complex 2D figure on the micro scale, which is randomly assembled from only a few coated fibrils.

Figure 4.5 (left) Schematic picture of PEDOT-S decorated amyloid beta sheet nanowires (right) Schematic 3D representation of an electrochemical nanowire based on networks of the decorated nanowires.

These points imply that complex dense molecular devices can theoretically be made by bottom up assembly with such fibrils, a truly remarkable feature of this scheme.

In paper 2 assembled nano fibrils are coated onto inert Pt electrodes. The structure formed is a 2D nanowire network connected to S-D contacts. These are immersed in an electrolyte and an additional Pt electrode, also immersed, is used as gate to form an ECT (see figure 4.5). The resulting transistor characteristics of the ECTs reveal a number of interesting statistical data about the electrical character of the 2D fiber networks.

43 The conductivity of the nano fibers ECT in the linear regime is shown to be very high and very similar to the conductivity that we would have in a spin- coated thin film of PEDOT-S, if we extrapolated sheet resistances to films with the same height as the height of the coated PEDOT-S fibril, and with a similar coverage fraction as the nanowire network (see paper 2 supporting details). This means that the major part of the nano fibers in the network conduct as well as PEDOT-S does in a film. This result alone is remarkable since it shows that the coated fibers have almost no breaks along the long fibrillar path, even though the diameter of the total fibril is only around 15nm. The explanation for this can be that the fibers are very well coated with PEDOT-S. PEDOT-S is used in excess during the SA (see section 3.1), meaning that these CPs can assemble around the fibrils filling as many binding sites as possible until no more available binding sites are present. Furthermore, the fact that PEDOT-S is a mono- component polymer and not blended with any additional polymer means that no non-conducting islands can be formed along the molecular nanowires and break conduction paths. It is possible that the charge transport character in single coated fibrils is indeed quasi 1D. This is an interesting subject for future studies in device structures that allow probing of single wires.

The nano network ECTs also show electrochemical pinch-off and reversible modulation, meaning that the networks are electrochemically active and that switching does not destroy the structure or electrical behaviour of the coated fibrils. This means that the strength of the binding forces between the PEDOT-S and fibrils are not much affected by the ions during the redox reaction. The nature of this binding is probably a mixture of hydrophobic interactions, van-der Waals interactions and ionic binding between a fraction of the alkoxysulfonate groups on the chains and the biomolecule.

These results have shown that highly conducting multifunctional CP materials work down to the limit of molecular scales. However there is still some work to be done in order to reach a full bottom-up assembly of large area nano devices from these nano fibrillar building blocks, where the devices are assembled in pre-defined geometries. The most probable route for doing this is to further take advantage of the diverse structural bottom up assembly that biomolecules can offer. The great advancement in the field of bottom up assembly from DNA is a perfect example of this [42].

Another route for making predefined nano-structures through directed self- assembly is based on nanofluidics. This has been used, as described, to create pre-defined patterns in the PEDOT-S material. In the next chapter we

44 describe how these are used as the base for making another type of ECTs at the nano scale.

4.3.2 Nano ECTs assembled on large area micro/nano arrays In section 3.2 the patterning of very long fibers of PEDOT-S using bridged micro nano soft lithography is described. The resulting patterns are dense arrays of separated nano wires with a width to length ratio exceeding 1000.

In paper 4 the unique properties of PEDOT-S ECTs are used in order to create ECTs along these nano lines without the need of any alignment step at the nano scale.

In this demonstration we create millions of ECTs by simply placing a micro scale gate at an arbitrary position along the nano wires (see figure 4.6). The beauty of this crossbar structure is that no patterning and alignment of pads are needed. The reason is that, at the nano scale source and drain are, as described previously, defined automatically at the sides of the channel, these are in turn bridged via the same PEDOT material to larger micro pads that can be probed easily.

Figure 4.6 Schematics of nano ECTs on bridged micro-nano lines, where the gate is at micro dimension and the source and drain are also at the micro dimension and connected to the channel via a micro-nano bridge.

The electrical conduction through 500 nm wide and 5 mm long PEDOT-S wire is an amazing demonstration of the fact that fluids can efficiently transport polymers and assemble them into connected structures in a very

45 wide range of aspect ratios, both in width/length and micro-width/nano- width.

It is noteworthy that PEDOT:PSS also was tested in these patterns but did not result in electrically connected structures. This is a nice demonstration of the fact that nano scaling of cheap large area electronics is as much a matter of patterning technique, as it is a matter of material design and synthesis.

The important implication of the demonstration of working ECTs in paper 4 is that very large-scale dense devices can be made by simple fluidic patterning and printing methods on flexible substrates.

This is a step towards making of very cheap large area nano devices. In chapter 6 these nano ECTs are revisited in the context of scaling of organic nano electronics and crossbar devices.

4.4 Microfiber electrolyte-gated OFETs In this chapter electrolyte-gated transistors that are based on intrinsically semi-conducting/ low-conducting polymers are discussed. These types of transistors can be both electrochemical and field-effect operated, and they have been explored for the first time in this thesis for the purpose of making field-effect transistors on micro fibers, as a complement to the previously discussed PEDOT fiber ECT. As a result of these experiments it is discovered that actually both FETs and ECT can co-exist, which is discussed in more detail in the “dual mode” chapter.

4.4.1 Electric Double-Layer Capacitance (EDLC) OFET It is interesting that the very first experiments of John Bardeen and Walter Brattain on semi-conducting transistors utilized a water electrolyte to change surface properties of the semiconductor. This was in a sense an electrolyte based device. These devices were finally perfected by Shockley to p-n junction field-effect transistors. Electrolyte gating of in-organic semi-conductors was later reported. The most studied systems here have been on the inorganic semiconductor-based ion-sensitive field-effect transistors (ISFETs) [77].

The organic counterpart of the field-effect transistor, the OFET, is an FET which has a (thin) active layer of a CP as channel material. The first OFET

46 [78] was demonstrated in 1986 using polythiophene as the active CP layer. Like the FET the OFET has basically a plate capacitor where one plate is the gate electrode and the other is the organic semiconductor film. When a voltage is applied between the source and gate, majority carriers are generated at the insulator/semiconductor interface, leading to the formation of a conducting channel between source and drain. An OFET is called n- channel transistor if the organic semiconductor conducts electrons and p- channel is it conducts holes. The output character of the transistor shows a linear source-drain I-V character for low gate voltages (linear regime) and a saturating I-V curve for higher gate voltages (saturation regime). OFETs are believed to be one of the key components in future organic electronic devices. Many results have therefore been published on the subject of device realizations, where example of interesting developments involve those that deploy non-conventional patterning methods to realize sub micrometer OFET structures [60].

It is however noteworthy that the OFETs transistors were reported later than the first electrochemical CP based ECT which was reported in 1984-85 by Thackeray et. al. [68]. One explanation might be that the conduction in CP was discovered by chemical doping.

In recent years the hybrid combinations of electrochemical and field-effect transistors have given rise to devices that are believed to work by the field- effect caused by the electric double layer that is formed at the interface of the electrolyte/semi conductor. Panzer et. al. reported P3HT based transistors in 2006 [30], where it is claimed that the very capacitance in the double layer gives rise to high charge carrier concentrations through electrostatic charge injection. These types of devices have also been called Electric Double Layer Capacitance EDLC OFETs by Said [79] and Herlogsson et. al., where they have further analyzed and refined the EDLC OFET by increasing its speed through use of new electrolytes and reduction of the channel length to nano dimensions [80,81, 82].

47 Figure 4.7 (left) Schematic picture of the electrostatic field distribution in planar FETs with a thin film dielectric, (middle) micro cylindrical FET with a micro thick dielectric (right) electrolyte-gated microfiber FET.

The most notable advantages that until now has been demonstrated for EDLC devices are - The exact positioning of gate is not necessary [80] - The distance of the parallel plates (gate-channel) does not determine the operation voltage, and therefore the operation voltage is very low (around 1V) - The field’s very concentrated field-distribution at the double layer means that short channel effects are suppressed in nano transistors, as was recently reported by Herlogsson et. al. [81]

In paper 3 similar concepts as those utilized for fiber based PEDOT ECTs are used for making a fiber based OFET transistor. In these fiber OFETs yet another advantage of the EDLC is utilized, being that the actual geometry of gate and channel do not affect to character of the electrostatic field at the interface. Figure 4.7 demonstrates this by schematically showing how the electric field is distributed in a planar regular OFET, and how it would be distributed if we changed from planar to cylindrical micro fiber geometries, where the gate and channel are on separate fibers. It is clear that the field at the interface can only be strong at the nearest contact points between the fibers. The field is then rapidly decreased as we move away from these contact points along the circumference of the fiber. If we for example assumed that the nearest points between the fibers were distanced 100 nm (which in itself is difficult

48 to achieve with textile production methods), and further assumed a 100µm fiber diameter, then the capacitance would be 100µm/100nm=1000 times smaller at some distance away from the nearest point between the fibers, whereupon no modulation would be achieved. When the dielectric is replaced with an electrolyte, the field is instead fully located at the electrolyte/semiconductor interface, where we can assume a very high capacitance due to the small thickness of the EDL.

Figure 4.8 Schematic of a Fiber based electrolyte-gated OFET.

In paper 3 such fiber based EDLC OFET geometries are made with methods that are compatible with weaving. The main construction difference between these fiber based transistors and the PEDOT fiber ECTs is that a metallic source and drain contact has to be placed along the fiber. This was solved by evaporating gold on the fibers and using other fibers as masks to create S-D gaps. Figure 4.8 shows the schematic of a fiber OFET from paper 3, where the CP material P3HT is used together with a solid polymer ionic liquid (see chapter “Electrolyte materials”). A fast transient measurement reveals that these fiber transistors turn on with an on-off ratio of around 100 at a speed of <10 ms. These measurement were however difficult to make because of a very large parasitic EDLC charging current between the source and the gate. The reason for this is that the electrolyte in these particular devices cover a relatively large part of the source-drain area as a result of how they are assembled at the fiber junctions.

49 This parasitic current could be partly removed by subtracting the pure parasitic charging curve, which is measured by grounding both the source and drain. Furthermore the parasitic current complicate design of digital systems, and in future developments these currents could and should be minimized. Because of the high ion concentration in the polymer IL and the ability for the anions to penetrate the P3HT the devices also showed electrochemical doping, which is discussed in the next section.

4.4.2 Electrochemical enhancement mode transistors As already discussed the first transistors based on CPs were electrochemical polythiophene based transistors. It is therefore no surprise that the fiber OFET in paper 3 also showed ECT operation. The use of IL liquids however allowed for a quite fast ECT response where the transistor channel is turned on and saturation current is reached already at around 1 second. The main difference between this mode of operation and the EDLC is that much higher current can be passed through the channel as the entire doped bulk of the CP conducts. In these experiments around 100 times more current could pass through the channel. This is however at the expense of speed. The parasitic capacitances are of no problem in the ECT operation mode, because these currents are small as compared to the on current of the ECT and because the stray capacitance current peak saturates at much faster speeds. In paper 3 the fiber transistors were operated in ECT mode for the demonstration of logic functions. The experimental results show both ECT and EDLC operation occurring in different time domains. This has not been shown experimentally before, and is therefore discussed in some more detail in the next section.

4.5 EDLC OFET and ECT regions In paper 3 transistor transient currents are measured at both millisecond and second domains directly after a fast gate voltage step. The choice of ionic liquids allows for the existence of both a stable ECT and EDLC modes within the time span of only few seconds. As the results of these measurements, that spans over at least 3 orders of magnitude in time, are depicted in a log time vs. log ID current, we see results that are similar to the curves schematically depicted in figure 4.9.

50 Figure 4.9 Schematic diagram showing capacitive charging peak, quasi plateau EDLC, and ECT mode in different time domains, and corresponding schematics of how ions are distributed in each domain.

In these curves 3 different regions can be characterized, after the transistor is turned on 1) A stray capacitance-charging region. Where the charging of the parasitic capacitors, defined by the overlap between the gate, source and drain electrodes [65] results in an RC type current. (more on speed in section 4.6) 2) An EDLC OFET region where the current reaches a quasi-stable value after both the EDLC is formed and the stray capacitance

51 current has vanished. The formation of the EDLC can be limited either by electron or ion mobility. Usually for micro scale devices the speed is not affected by electron mobility, and the full modulation current should be seen in the curve. 3) The electrochemical doping region, where the ions start to penetrate into the bulk and open the channel even more.

It should be noted that the ECT operation of region 3 the character of the transistor curves show both saturation and modulation, very similar to FET curves, (but with different mechanism based on EC as discussed previously). This shows that care should be taken in interpretation of the modes of operation in electrolyte gated transistors, when output curves are measured in the Hz regimes [83, 84].

It is interesting to evaluate ring oscillators based on these dual mode devices, wherein the stray capacitance of regions 1 is suppressed by geometrical design. The question is whether or not the ring oscillator can be designed to operate at two resonance frequencies, a higher EDLC mode frequency, and lower ECT frequency?

52 4.6 Speed of electrolyte gated transistors One of the most important factors for transistors is the switching speed. For organic field-effect transistors the simplest model is to take the time a charge carrier needs to cross the transistor channel from the source to the drain electrode.

An estimate for this time is given by

L2 " = µVd

where, L, µ, and Vd are the channel length, the field-effect mobility, and the source–drain voltage. Other models based on the time for the dynamics of ! the formation of accumulation layers 85 give similar L2/µ results. However in electrolyte gated transistors the dynamics can be very different. The reports on the transient behaviour in electrolyte gated devices are very few. Here a short non-exhaustive discussion is given about the subject, wherein the two previously described modes of operation are discussed separately.

4.6.1 EDLC mode In a recent elegant experimental study by Herlogsson et. al. [81] it is demonstrated that for planar EDLC OFET with a thin polymer electrolyte layer, the speed for channel lengths shorter than 10µm, is largely limited by the formation of the EDL, and for lengths above 10µm the response is similar to a regular FET with approximately L2/µ behaviour.

The upper limit for the speed of an electrolyte gated transistor in EDLC mode, (if we neglect stray capacitive RC responses), is therefore the speed at which the EDL can be formed in such a device. This speed should be a function of a number of parameters, including ionic concentration, ion mobility, diffusion and electrical fields.

Speed difference between planar and micro fibers The speed for forming EDL depends on the characteristic time constant for ionic transport in the electrolyte τi. Using Gouy–Chapman theory for double layer and other approximations the time constant is given by

53 l " # ionic C

where l is distance between the channel and gate electrode and C is the ionic concentration [86]. In planar devices [80,79] l is constant and around 100nm. ! In the micro fibers however, the length varies and is in the same dimension as the fiber diameter of around 100µm. This indicates that EDL switching should be much slower and limit the switch speed even for a channel length of 100µm, which is used in paper 3. But instead switching speeds of <3 ms are observed, (after exclusion of the stray capacitive response) for the micro fibers. This is not so different from the P3HT field-effect response with L2/µ dependence, meaning that EDL formation is not limiting the speed for the micro fiber devices with ionic liquids. One explanation could be that the total ion concentration, which is also higher here because of the use of ionic liquids and because of the much thicker electrolyte.

It should also be noted that the character of EDLs in ionic liquids are up to date not fully understood and it is believed that they do not follow the classical Gouy-Chapman equation [87]. It is also noteworthy that the picture of an electric double layer at the interface of a metal/electrolyte cannot be fully applied for any disordered CP/polymer-electrolyte interface. The reason is of course that both of the materials conduct ions and the interface is actually much more diffuse.

Finally the main speed limiting issue with the micro fiber transistors with the geometry presented in paper 3 should be the stray capacitance, which has to be lowered in future developments.

Nano devices If the formation speed of EDL can be increased by choice of materials (for example ILs as illustrated in paper 3), it should be possible to increase the speed of EDLC OFETs as the channel length is decreased below the current 10µm limit demonstrated by Herlogsson et. al. [81], down to the nano domain. If it is further assumed that EDL can give rise to higher mobilities, as compared to regular FET mobilities, because of the very large capacitances, one could eventually achieve higher switching speeds with nano EDLC OFETs as compared to nano OFETs.

54 4.6.2 ECT mode In the ECT mode the entire bulk of the channel has to be considered as a capacitance instead of just the interface of the film as in the case for EDLCs. The description of the transients for ECTs is much more complex because of the coupling between the electrical and ionic parameters that affect each other.

A successful and simple way of describing the mechanisms of doping is based on using pure electrical circuit models where variable resistances are used to describe the change of resistance in CPs during doping and the character of the electrolyte is described by using an RC circuit.

These types of models have been used to describe the movement of redox fronts in geometries that correspond to an ECT, where the drain is floating and voltage is applied only between the end of the channel and the gate. Johansson et. al. has described redox fronts using partly electrochemical equations [88], and Warren et. al. [89] has described this using purely electrical models. Warren’s and Johansson’s models are based on an approximation which assumes that the time scales for front movement is greater than the time that it takes to dope the film laterally, resulting in a circuit depicted in figure 4.10. Since the CP films (in for example PEDOT) are usually around 100nm, micrometer ECTs should fit into this approximation.

The speed limit for an ECT within the results of this model should be set by speed of the redox front. This speed would be the equivalent of electron mobility in OFETs, but always lower. The redox front speed should also always be limited to the case when the channel is turned from off to on. The models of Warren does not provide analytical solutions for redox front speed. However the model shows that an increase in both ion and electron conductivity inside the CP film should increase this speed.

55 Figure 4.10 (top) A transmission line model of a CP film immersed in electrolyte and contacted at one end with a working electrode and gated via a gate electrode through an electrolyte. The model assumes a number of segments along the CP film, where for each segment the top branch is the electron flow, in series through the CP resistance Rcp. The bottom branch is the ion flow, where the capacitance of each segment is charged like an RC circuit, so that the charge qn is accumulated at segment n. Rs is the series resistance in the circuit. (bottom) Schematic of the CP based electrolyte gated device

In a recent study by Bernards et. al. [86] a similar approach of circuit models is used to model an entire depletion mode ECT, (e.g. PEDOT ECT). This model assumes that the gate and source-drain are of metal, and the limiting factor for speed here is believed to be the double layer charging of the metallic gate. However in the devices where gate and source drain also comprise PEDOT, as for the micro fiber devices reported in paper 1 and other applications [72], the transient model of Bernards is no longer valid. Furthermore in these devices additional phenomenon such as the de-doping

56 of the PEDOT a bit outside of the channel can severely limit the speed of the transistor.

In summary circuit models can empirically explain the speeds of ideal macro/micro ECTs, and suggest that an increase in both ionic and electronic conductivity Of CP films should increase ECT speeds. This result should be applicable for both planar and microfiber based ECTs. There is however still no exact formula that describes the speed of ECTs for micro dimensions, and for channel lengths in the nano dimensions the mentioned models probably do not apply at all.

57 Chapter 5 E-textile: Woven CP devices

One just has to look at how the synthetic polymer nylon first changed the textile industry in order to realize that conducting polymers will also open completely new avenues in the future development of the textile industry [90].

The demonstration of transistors and woven logic in papers 1,3 based on textile micro fibers functionalized with CPs, is one step towards showing how CP based fibers can add completely new functions embedded into textiles, and open a route for making the electronic-textiles (e-textiles) of the future.

The development of organic e-textile, relies partly on the development of its basic building blocks, the electronically functionalized microfibers. These will be used just as regular fibers in weaving and knitting processes, to create truly functional e-textile. In this chapter a short overview of some current functional microfibers is given, and further the use of these fibers for integration of textile embedded circuits, based on electrolyte gated devices, is discussed.

5.1 CP functionalized textile microfibers The industrial manufacturing of textile microfibers is primarily based on fiber spinning, through the processes of melt spinning or solution spinning. There are three main strategies for integrating CPs in spun microfibers, and these will be described in more detail here.

58 Figure 5.1, (top) Schematics of heterogeneous fibers having blends of conducting (dark islands) and non conducting material, and coated microfibers having a thin film coating of a conducting material around a non conducting fiber. (Bottom) SEM picture of polymerized PEDOT on polyester. Micrograph of PEDOT coated polyester microfiber bundles, sewn into in a non conducting textile.

Homogeneous microfibers Homogeneous fibers are fibers that contain only one material. The non CP based conducting homogeneous microfibers usually comprise purely metallic fibers that have quite poor mechanical properties. CP based homogeneous fibers instead has a core that conducts through the CP, without any metal at all. The main process for producing these fibers is based on solution spinning, and the reason for this is that most of the CPs cannot be melt-processed Here the fibers is extruded through a highly concentrated solution (10-30 wt%) and coagulated in a bath. The main issues in solution spinning of the fibers is 59 that high concentration of melted CPs are difficult to make due to their low solubility and rapid gelation [91]. The most successful route for making high conducting homogeneous fibers is based on high molecular weight polyaniline. Among the best results are reported by Pomfret et. al. where one step spinning of polyaniline fibers are reported with a conductivity of 600 Scm-1 and a high tensile strength of around 100 MPa [92], which makes these fibers strong enough to be compatible with weaving processing. Polypyrrole has also been melt-spun [93], with conductivities of around 1Scm-1 and low tensile strengths of 0.025 GPa. Results of PEDOT:PSS solution-spun microfibers have also been reported by Takahashi et. al. however these are very thin fibers (around 10 µm), and around 15 MPa in tensile strength and do not really qualify as textile microfibers.

The only real candidates to date for these types of CP fibers matching textile applications are polyaniline microfibers. These are however not commercialized yet because of difficulties in up scaling. These fibers can probably not act as a channel material for making the electrolyte-based transistors of papers 1,3. The reason is that CPs with high pristine state doping cannot work as a depletion material in FET. Furthermore homogeneous/heterogeneous microfibers cannot work efficiently as an ECT channel since the entire bulk of the microfiber has to be doped. Instead these fibers have to work as purely conducting material were they compete with the already commercial carbon black fibers that have conductivities of 100 S/cm. Hence there is still some work to be done before CPs can entirely replace plastics in homogeneous microfibers.

Heterogeneous microfibers Heterogeneous microfibers are blends of an insulating conventional microfiber material and a conductive filler (figure 5.1). If the amount of the filler is above a certain percolation threshold we can have conductivity throughout the fiber. The heterogeneous fibers however have lower mechanical properties due to the high concentration of fillers that have to be added. A common filler is carbon black, but also CP materials have been used to make heterogeneous fibers. The main advantage with this technique is that microfibers can be melt-spun easier, however the conductivity of the blends become much lower.

60 PANI has for example been blended with isotactic polypropylene, and nylon [94], and melt-spun. The conductivity of such fibers are however very low, about 10-8 Scm-1. In more recent developments of blended systems, di-block copolymers between CPs and plastics are used, and this will most probably be the best alternative for achieving high conductivity melt-spun microfibers from blends in the near future, recent examples of mechanically tough P3HT di- block copolymer films by Muller et. al. [95] represents a very good example. These types of fibers could probably be used as channel material for field effect transistors in the devices in paper 3.

Coated microfibers Coated microfibers are regular microfibers that have thin submicron CP coating as the functionalized layer (figure 5.1). The thin coating does not affect the mechanical properties of the fiber at all, so the strength is retained, however as the core is not conducting, the fiber will have higher resistance, and furthermore become more sensitive to surface damage. Previously a wide variety of processes have been used to coat fibers with metals such as evaporation (also used as method for gold coating in paper 3) and electroless deposition. Coating of polypyrrole was first reported by Kuhn et. al using in situ polymerization methods [96], and surface resistances of 5Ω/square was achieved. In-situ polymerization of PANI 97 (20Scm-1 for 150 nm film) and PEDOT [98] (100 Scm-1 for 100nm films) has also been reported, with film conductivities being the same as the conductivity of regular thin films of these polymers. The process of in-situ polymerization can be done using standard textile dyeing equipment, and therefore coated CP fibers is the only type of fibers that have been commercialized to date. Milliken introduced a range of conducting polypyrrole-coated fiber under the trade name contex.

The microfiber ECTs in paper 1 rely on PEDOT coated microfibers, where both in-situ polymerization and direct coating from solution, results in coated fibers that work for making microfiber ECTs.

The different types of fibers are summarized in table 5.1. The combination of CP functionalized microfibers is a very promising route for making woven e-textile, where the combinations of different fibers and coatings can

61 result in advanced woven electronic devices. This will be discussed more in the rest of this chapter.

Fibers Process route Type Tensile Scm-1 Ωcm-1 Ref Material strength (100µm diameter Nylon (ref) Melt spin Homogeneous >500 MPa - CarbonBlack Melt spin Heterogeneous 100 Scm-1 100 Ω 99 PANI Solution spin Homogeneous 100 MPa 600 Scm-1 15 Ω 92 PANI Melt spin Heterogeneous <500 MPa 10-6 Scm-1 10 GΩ PANI Polymerization Coating >500 Mpa 20 Scm-1 100 KΩ 97 PEDOT:PSS Melt spin Homogeneous 10 KPa 1 Scm-1 100 KΩ 100 PEDOT Polymerization Coating >500 MPa 1 Scm-1 2 MΩ 98 PEDOT:PSS SolutionCoating Coating >500 MPa 100 Scm-1 20 KΩ 101 Polypyrrole polymerization Coating >500 MPa 100 KΩ

Table 5.1 Summary of properties for different organic electronically functionalized fibers

5.2 Woven electrolyte based devices The concept of embedding a large number of components into textile can add complete new functions to textiles. The only commercially available technique for embedding large number of components today is based on attaching conventional off the shelf electronics onto cloth. However, in order to make really integrated e-textile the large number of components has to be embedded on the textile microfibers themselves. These textile microfibers must in turn be compatible with textile manufacturing; meaning that they must be available from rolls, so that they can be weaved to form a textile wherein a large number of the components form a device. In a weave there are a huge numbers of fiber junctions / crossings, and the most natural way of embedding a large number of components would be to place these at these junctions. The junctions can further be used as interconnects.

The electrolyte gated transistors presented in papers 1,3 is among the few, if not only example, of advanced electronic components, which fulfill the requirement that they can be embedded in junctions of microfibers and at the same time compatible with textile manufacturing. Figure 5.2 shows a schematic of these types of components placed at woven microfiber junctions, with the addition of other simple but fundamental components such as interconnects (fibers in contact at junctions), resistors (fibers of different length and conductivity) and electrical breaks along fibers. With the combination of all these components it is possible to design a variety of

62 circuits and in particular digital devices (as these basically only transistors and resistors). Naturally such woven devices will have somewhat different design rules as compared to planar thin film devices, and this will be discussed in some more detail in the next chapter.

Figure 5.2 Micrograph of fiber embedded junction components, and corresponding electrical symbols.

5.2.1 Design rules for woven circuits A good way to look at the design rules for woven circuits is to look at similarities and differences between these and planar circuits. - In woven circuits no micro patterning of thin films is required, as the thin films or bulk CPs are already carried by the fibers, and the dimensions of the fibers is the micro feature size. - In planar devices, several layers of thin films are patterned, aligned and stacked on top of each other on a substrate. This is a 2.5-dimensional structure. The woven devices instead are truly 3-dimensional as the films are not stacked on top of each other, as the fiber that carries the film can be weaved in any direction.

63 - The inherent mechanical properties that textile have as compared to planar substrates are different, such as more holes and more flexibility, which could define other design, or at least application rules.

By making two assumptions it is possible to constrain the design parameters even more so they fit industrial weaving with warps and wefts. These assumptions are that:

1. Any type of fiber can be chosen as warp or weft, so that combinations of conventional plastic and metallic fibers and all of the CP functionalized functions described above, are possible. 2. Any component (including electrical breaks) can be placed at any junction in the weave by patterning (alternatively by other methods such as weaving and melting). Note that the only alignment steps necessary is if several patterns have to be made to place several types of devices, at the junctions. No alignment of the fibers is done except placing them by weaving.

In the most common case of weaves with only one layer warp and weft, the weave resembles a kind of micro crossbar. The concept of nano crossbars (discussed in chapter 6) is similar because it is based on the fact that molecular devices are formed between junctions of crossbars and that no alignment of the crossbars is needed. Woven micro crossbars in the same way actually need no alignment, and they can have a density of 1/F2, where F is the distance between two fiber junctions.

Digital woven circuits The use of electrolyte gated transistors for making woven digital logic circuits have a number of noteworthy points:

- The on/off ratio of transistors should be high enough. For the PEDOT microfiber ECTs in paper 1 this is >1000 ECT and for the depletion mode transistors in paper 3 on/off ratios can exceed 10,000. These values are sufficient for making many types of digital devices.

- In digital logic design usually the resistors have to have values that are at least as high as the transistor channels resistance in off-state. The best way to achieve this is to use the high resistive fibers as resistors, and always have electrical connections between fibers that are low resistive so that these do

64 not constitute a part of the resistor. The resistance of fibers is very stable in all types of the discussed fibers meaning that the length of a fiber can really be proportional to the resistance. In this way the accuracy in resistance values in the woven circuit become equal to the accuracy in which the length of fibers between two junctions can be controlled. If we for example choose polymerized PEDOT fibers then the resistances in the circuit should be 2GΩ/cm, i.e 1000 times lower than 2MΩ/cm (table 5.1), in this case melt spun PANI fibers could be an alternative for resistors with 10GΩ/cm.

- Several transistors can be placed on one fiber where the source of one transistor is the drain of the neighbor, and also one fiber can act as a common gate for several transistors crossing this gate fiber.

- The maximum current that can be delivered through a transistor, with PEDOT ECTs is 1V / Ron = 1/20KΩ = 5mA (corresponding to PEDOT:PSS solution coated fibers). This is based on the assumption that the transistor length in total is 1 cm, (channel + cables that run to source and drain), before it is connected to more a low resistance cable through interconnects at S-D. These values are high enough for many applications. In paper 5 it is proposed that woven digital devices that can deliver mA current through single fibers should be able to stimulate neuron cells in vivo.

- The speed of digital circuits based on electrolyte gated transistors is already discussed in section 4.6. In summary ECTs have around 1000 times lower speed than EDLC FETs. However the EDLC FETs cannot reach full speed without removing stray capacitances. In paper 1 PEDOT fiber ECTs operate in Hz regimes and the EDLC fiber OFETs in around 1 kHz.

The design of circuits in weaves is a new concept and in order to fully understand and exploit these circuits simulations tools that integrate the design rules of textiles and electronics will be needed.

5.2.2 Woven Digital addressing devices Woven devices, like any other large area micro electronic devices, will have many wires and components that have to be individually addressed. In order to do this, addressing devices have to be implemented. In paper 1 addressing is demonstrated with a binary tree demultiplexer (demux) based in PEDOT microfiber ECT. In paper 5 another type of 3D matrix based addressing device is proposed and simulated, also based on

65 PEDOT microfiber ECTs. These two woven devices are discussed in more detail in this section.

3D matrix addressing In paper 5 an addressing device is proposed which is based on matrix addressing. The conventional concept of matrix addressing is that a point in a 2D crossbar matrix can be addressed by choosing a row (x position) and a column (y position) that corresponds to the crossing point of the column and row or the x,y position. In these devices x+y address lines are needed in order to address x⋅y positions. LCD displays represent a good example of a so called active matrix addressing where each pixel is addressed by addressing a transistor (which makes the pixel active), that is located at the x,y position.

The proposed matrix addressing is also an active one, where instead two transistors are addressed at each x,y position, where each transistors gate is the x respectively y fiber as seen in Figure 5.3. The transistor channels then run out of the x,y weave plane into the z direction forming a kind of mat of fibers where each fiber sticking out of the mat can be addressed uniquely according to the circuit in figure 5.3. The design is based in the fact that the z fiber is connected to ground if any of the two transistors are on and only when both transistors are off (the fiber is addressed), can the signal current pass through the fiber.

This design really takes advantage of the 3D nature of weaving and gives rise to a design, which would be difficult to achieve with 2.5-dimensional electronic circuits.

The proposed idea of paper 4 is to use the addressable matrix, (fiber matt), for delivery of currents to a given x,y position in order to locally stimulate neuronal cells in vivo, as textiles are suitable materials for in vivo implants. The currents that can be delivered depend, as described in section 5.2.1, on the on-state of the PEDOT ECTs. These ECTs can deliver quite high currents in the range of some milliAmperes for each 100 µm fiber. This is enough for the stimulation of many types of neural cells, as described in more detail in paper 5.

This 3D active matrix woven device is only simulated in paper 5 and no actual implementation of such circuits is done. The construction of these

66 devices should however not be difficult since no patterning of the transistors is required as these are present at all junction where x,y,z fibers meet.

Figure 5.3 Schematic design of a 3D woven matrix addressing device, and corresponding electrical circuit.

67 2D matrix addressing (binary tree demux) In conventional logic circuits, a digital demux can be designed using a number of AND or OR gates. However one of the advantages of fiber ECTs is the possibility of creating many transistors along one wire. Therefore the most straightforward way to design a digital demux using the previously described design rules for woven logic devices is to construct a demux on a 2-dimensional crossbar (weave in this case). This design is called a binary tree demux. The binary tree demux needs 2log2N wires in order to address N wires, which really makes these devices much more useful than matrix addressing devices as N grows. Figure 5.4 shows such a binary tree demux with 32 channels lines and 2log232=10 control lines. The lines in the figure, represent conducting fibers and the dots represent electrolyte-gated transistors that form the demux when placed in this pattern. In order to address a channel in this device all the transistors along each channel line have to be in the on-state, and this happens only for a unique set of input combinations on the control lines. The transistors can be of either depletion mode or enhancement mode, and the only difference between the two is that their address input values are inverted.

68 Figure 5.4 Circuit design of a binary tree demultiplexer, where the channel lines are addressed uniquely with addressed on the control lines. A device can be constructed along the channel lines after the demultiplexer.

In paper 1 a simple binary tree demux is demonstrated using 4 channels and PEDOT:PSS coated fibers and PEDOT ECTs as transistors. A micrograph of such a device is seen in figure 5.5. One of the limiting factors of these demux devices like any other crossbar based device is the resistance drop across channel lines. In figure 5.4 the resistance along one channel is shown consisting of the demux resistance Rdemux and the channel resistance Rchannel in series. In real devices with large number of channels the demux will not occupy a large portion of each channel line and therefore Rchannel>> Rdemux. The resistance drop across the control lines is not a problem since these are the gate fibers to the transistors and carry very small currents. However the current through the channels lead to a voltage drop along the channel, depending on the resistance of the fiber. In the case of very long channels,

69 decreasing the resistance under the 20KΩ/square (which is the value for the PEDOT:PSS microfibers in paper 1), could be required.

Figure 5.5 Micrograph picture of a binary tree multiplexer made with PEDOT:PSS coated nylon fibers (100µm) and fiber ECTs.

The issue of addressing of fibers in crossbars is also important in the field of nanoelectronics where the crossbars instead consist of ultra high-density nanowire crossbars. In the coming chapter 6, the binary tree demux is revisited in the context of organic nanoelectronics, and some system design issues at the nanoscale are discussed in more detail.

70

Chapter 6 Nano crossbars for scaling of organic nanoelectronics

Downscaling of organic electronic devices is today of great interest just as the downscaling of inorganic conventional CMOS based devices have been during the last decades. However most of the problems and concepts in the developments of bottom-down construction of CMOS cannot be applied to the field of organic electronics. Instead it is instructive to look at the so called revolutionary approaches that are discussed in the field of downscaling of conventional electronics, and find relevant ways of applying and developing these in the field of organic nano electronics. One such approach is the idea of bottom up self-assembly of nano crossbars from the molecular level. In this chapter relevant previous concepts and experimental work is discussed, where nano crossbars are put in a larger context. The works of the papers in this thesis is discussed with emphasis on their relevance towards organic nano electronics and especially towards organic nano crossbar architectures.

6.1 Beyond CMOS The current computer hardware utilizes CMOS technology. During the last decades an enormous demand on for higher computational efficiency and increased memory storage has been pushing the packing density of CMOS circuitry to increase exponentially, as predicted by Moore and postulated by Moore’s Law. The consequences of this miniaturization are mainly processing limitation and the scaling problem of the MOSFET channel length [102]. There are also many other details that can make further miniaturization of CMOS difficult in the future. The result is that both evolutionary and revolutionary approaches are being considered for replacing conventional CMOS. The evolutionary changes are defined as changes that retain the basic CMOS technology, whereas the revolutionary approaches are entirely new approaches to computer hardware. There are also hybrids between conventional and non-conventional CMOS.

71 6.1.2 CMOS based defect-tolerant computers towards the crossbar Researchers at HP demonstrated in the mid 90s that a machine could be developed based on reconfigurable defective components. The machine was built using semi-conductor programmable logic (Filed Programmable Gate Arrays FPGA), and called teramac (tera multiple architecture computer). The logic units in the teramac are connected in a fat tree (a tree with a high degree of connectivity between the nodes), so that the machine can identify chip defects and reconfigure the computational pathways. Teramac illustrated that:

1. It is possible to build a powerful computer that contains a high level of defects. 2. A high degree of connectivity is more important than regularity in a computing machine. 3. The most essential components for a nano scale computer are the switching and the interconnect machines. 4. A new algorithm for computer manufacturing was introduced: build the computer, find and map the defects, configure the resources with software, compile and execute the program.

These points are very interesting as configurable computing could naturally be extended to molecular systems. In other words if one uses the advantages of molecular design with self reproduction and self organization, to design a highly complex system, then the machine does not have to be fully designed to begin with; only have enough interconnections between computing elements, and methods for switching these interconnects. Based on these outcomes, the researchers (mainly Keuekes, Heath, Snider, Williams, De Hon), proposed in an 1998 article [103] that the defect tolerant computing is of general interest for future nano electronics. One of the most interesting realizations of the HP group is that the simple crossbar geometry is the ultimate abstraction of the Teramac configuration, and therefore a lot of attention has been given to the crossbar architecture with some pioneering results.

The concept of crossbars is therefore very important in the field of all revolutionary electronics. This is discussed in the next chapter as a possible method for designing very high-density nanocircuits. The concept of using molecular/organic computing in crossbar architectures is discussed where point 2 is fulfilled, and the concept of addressing nanowires in crossbars is

72 discussed in detail as switching and interconnections are the most essential parts of these devices according to point 3. Finally the work of paper 4 is put into the crossbar context as a step towards CP based nanoelectronics.

6.2 The nano crossbar The simple three-layer architecture of a crossbar is realized by

• First designing an array of highly packed conducting nano lines • On top of these self assemble a molecular layer / thin film active layer • Last place a third layer of a line array (perpendicular to the first array) forming a crossbar geometry, so that the thin film at every junction of the crossbar forms a single device

A schematic of a nanowire crossbar is depicted in figure 6.1, and here a line of larger lines (microwires) is also depicted to show the possibility of mixing in microwires in the nanowire crossbar.

The most noteworthy points about the nano crossbars are

1. Ideally single molecules or small groups of molecules in the thin film/molecular layer embed the electronic function of a component, so theoretically the crossbar could be scaled so that the area of every junction almost reaches molecular scales, and the single junction components would still operate. This is different as compared to using conventional components such as FETs since the function of these is very much dependent on their size. 2. No alignment of the nanowires is required since two terminal components are automatically formed at the junctions, independent of alignment or even perfect angular matching. 3. Micro / nano lines can be mixed (see figure 6.1) 4. The device has a very high density, and is easy to construct since it only comprises 3 layers without nano alignment, 5. As mentioned the crossbar if the fundamental block of defect tolerant nano computing, resulting that a lot of theoretical framework has been developed around it.

73 Figure 6.1 Schematic of a nano crossbar, where a thin film / monolayer is sandwiched between dense nano crossbars. A microwire is also present on this crossbar.

The construction of crossbar architecture relies on making nano arrays in conducting materials, these have to be based on unconventional patterning methods that reach beyond regular CMOS feature sizes, and it also needs the making of the components (thin film/ molecular layers). The most interesting approach for achieving this is to use bottom-up self- assembly of building blocks for assembly of all layers of the crossbar, where both the functions and feature sizes are embedded in the nano building blocks.

Finally depending on the components different architectures can be built on crossbars. All these points are discussed in the next chapters.

74 6.2.1 Conducting nanolines in crossbars The demands on the arrays of nanolines in crossbars is that they should have a very small pitch (distance between the lines in the array), and a very high length/width ratio with minimum breaks along the lines. The nano lines should also have as low resistance as possible to minimize potential drop along the wires. Furthermore the patterning procedure of the wires should be such that the thin layer is not destroyed as the third layer is added.

In the case of implementation of three terminal devices, as compared to the two terminal devices (see next section), the nanolines should also have the possibility to locally change conductivity along the line at the junctions (to form a transistor). A number of different technologies have been used for creating nanowires in metals, inorganic semi-conductors, carbon nanotubes and conjugated polymers.

Some previous example of candidates for crossbar nanoline arrays include: - Grown highly doped silicon nanowires with a pitch of 34 nm and aspect ratios of 105 by Heath et. al. [104]. These can then be transfer printed onto surfaces using soft lithography.

- Grown aligned carbon nanotubes arrays with that are around 1nm in diameter with conductivities of 10000 Scm-1 and lengths of some 100 µm [105], by Kang, Rogers et. al.

-Nano imprint lithography NIL moulds with a pitch of 30nm have been produced by Yu, Williams et. al. [106]. These NIL should be able to pattern several perfect metallic nano arrays in additive steps as is requires for crossbars.

In the field of CP electronic not many examples have presented perfect arrays of nano lines that are sub CMOS in feature size. Some notable works includes roll-to-roll nano imprinting of the CP polyaniline nano arrays with a pitch of 1µm [55]. This method should be able to produce much smaller pitch if high resolution NIL is used. However these patterns are not separated nanolines, as there is a residual PANI film between the lines. The work in this thesis in paper 4 has demonstrated organic electronic nanowires arrays with a width of 500 nm in PEDOT-S (see section 3.2.1). These nanolines have high width/length ratio >1000 and can work as conducting nanoline arrays crossbars. Here the feature size of the conducting

75 lines is defined by the soft lithography template and the limits of nano fluidic and the function of the molecules as they become less in numbers, and it is believed that both the material and the patterning method for making PEDOT-S nano lines are scaleable to sub 100 nm dimensions In paper 4 the conducting PEDOT-S fibers could be used as building blocks, where instead the inherent dimension of the fiber would define the critical feature size of the crossbar. This approach is similar to the approach of CNTs and inorganically grown nanowires.

Many other examples on the construction of crossbars have been demonstrated, mainly in metals, inorganic semi conducting nanowires, and carbon nano tubes.

6.2.2 Memristors, transistors and diodes as crossbar components The components that can be embedded in a crossbar are either two- or three terminal devices. In the case of two-terminal devices the component is embedded at the junction of two conducting lines, and as described above these components need no alignment to be implemented. The important devices here comprise switch deivce / memristors. The three-terminal devices are crossed-transistor devices that can change conductivity along the line upon application of voltage along the other crossing wire (identical to the fiber ECTs presented in sections 4.3-4.4). These three components will be discussed here in more detail, with special emphasis on previous implementations and especially on those relevant for organic CP nano electronics

Diodes One of the approaches to molecular electronics, also called moletronics, is to use molecules in a computer architecture, where a few or even one molecule could integrate the elementary electronic functions and replace CMOS devices . [107]. This idea was initiates by Aviram and Ratner, where the first proposal was that a single molecule could embed a diode function with rectification [8]. Electronic transport through single molecules has been studied extensively with a number of different techniques including break junction, and metallic crossbars [108]. One of the popular ways of making such molecular layers in crossbars is to build a self-assembled monolayer SAM on metal and

76 sandwich this between the second metal forming a metal/SAM/metal crossbar. The most relevant development in this field with regard to CPs is the work of Akkerman, Blom et. al. [9]. Here they show that a thin layer of PEDOT ontop of the SAM helps protecting the SAM against evaporation of second metal. The metal/SAM/PEDOT/metal is furthermore, a very stable structure for implementation of molecular crossbar diodes. This is noteworthy as the work in paper 4 shows creation of PEDOT based nano arrays, and eventually a PEDOT/SAM/PEDOT structure would be enough for a crossbar.

The implementation of molecular diodes in crossbars is not the only choice, as any semiconductor between two metals with different work function should show rectification even if the junction area is very small.

Pure diodes however are not very interesting components for creating advanced architectures. The reason is that the implementation of high level architectures based on diodes would require patterning of the diodes with the same resolution as the nano junction resolution. This is however not possible since the whole concept of nano crossbars is to go below the limit of perfect patterning and alignment.

Instead the use of diodes that can be programmed would enable much more possibilities. This is discussed below.

Switch devices / Memristors Instead of showing pure rectification many of the examples of molecular junctions show behaviour of a rectifying device with a hysteresis or memory effect. One example is a rotaxane molecule deposited from LB monolayers and demonstrated in a crossbar architectures made with nano imprint lithography NIL [108]. This molecular device shows behaviour of a diode that can be switched on and off by the use of a high voltage. These types of behaviour have been observed in many devices that are based on organic [109] and inorganic thin films or monolayers [9]. The physics behind the behaviour of switching is still not fully worked out and the mechanisms probably also differ much between the different examples, so misinterpretation have and are being made in the explaining the switching of these devices.

On of the most relevant example of a switchable thin film based on CPs is demonstrated by Asadi, Blom et. al. [109] where a blend of P3HT and the

77 ferroelectric polymer P(VDF–TrFE) sandwiched in a crossbar between two metals show reversible switch behaviour. The integration of such devices in crossbars based on only CPs might be possible in future developments, where the metal is changed to metallic CPs patterned in arrays similar to the patterns in paper 4 in this thesis.

The most interesting aspect of switchable devices is that memories and logic devices can be implemented in crossbars using these and the programmable nature of such components removes the need for exact patterning of these (see more details in the next section).

In an attempt to unify these types of switch devices with inherent memories, which have lately been called many different names such as hysteric diodes, latches, configurable switches, switch diodes, non-volatile memory devices and more, Strukov, Williams et. al. at HP recognize all of these devices as memristors in a recent paper [110]. The memristor has been called the forth fundamental circuit element, and was found and named by Chua et. al. [111,112] already in 1971. Memristors are basically current/voltage controlled resistors with a memory. In the discussions of Strukov et. al., it is proposed that the mechanism of these devices are based on drift of charged dopants atoms in inorganic materials. This leads to a doped and undoped regions with a high resistor RON respectively low resistor ROFF in series. So that the memristance M has a function of charge q, and is described by

# µRON & M(q) = ROFF %1 " q(t)( $ D2 '

Where µ is the dopant mobility and D is the film thickness. This model is based on the fact that the entire bulk of the memristor film is active, and ! effects that are purely interfacial should not be covered. Although Strukovs model is not exhaustive, and applicable to all devices such as electromechanical CNT switches [113] or switches that do not need charge for switching [109], it is interesting to note that Sturkov’s proposed drifting doped region model is even better applicable for CP thin films, than inorganic thin films. The most simple CP based two-terminal memristor fitting the model of doping could be a CP/electrolyte blend sandwiched between two metals (see figure 6.2).

78

Figure 6.2 (left) Schematic picture of 2-terminal switch device with a metal/CP blend/metal (right) Electrical behaviour of a switch device / memristor.

Transistors The three terminal transistor devices in a crossbar require that a part of the nanowire is switchable at the junction (the transistor channel). The thin film would here instead act as a gate dielectric rather than a conductor as in the diode or memristor case. This means in practice that transistors and diodes/memristors cannot be patterned adjacent to each other since this would require top-down patterning of the thin film itself at the crossbar resolution, which is not compatible with the bottom-up assembly scheme of crossbars. Instead the different components can only occupy larger block. Different approaches have been deployed by the inorganic community for making crossbar compatible transistor.

The first approaches are those that enable patterning of transistors on only parts of the junctions and does not require that all junction have the same transistor. One of the schemes here is based on changing the doping profile of the nanowires (axial modulation) during growth. Yang, Lieber et. al. [114] have demonstrated controlled growth of silicon nanowires, where the doping profile along the fiber is varied between n, and n+ doping, with modulation dimension of 50nm, so that the 50nm long n-doped region can be placed under gate wires of larger dimensions, to form transistors. Another scheme comprises changing the doping profile by top down methods. Beckman, Heath et. al. [115], have demonstrated that highly doped silicon nanowire arrays can be locally etched along the wires to become more sensitive to gating.

79 The possibilities that these two schemes represent is that transistors can be placed at only certain points in the crossbar and thus in a sense patterned. However it is not possible to have nanowires as gates since this would require that the axially doped region is perfectly aligned under the gate wire for axially doped NWs, or that the local etching/doping is done with top- down patterning with the same resolution as the crossbar (see figure 6.3). The result is that the gate wire must be made much larger than the transistor channel nano wires, meaning that the density of the crossbar increases substantially in one dimension. (This is however still acceptable for some device architectures such as a demux which is discussed in coming chapters).

The second approach is to use NWs as both gate and channel. The Lieber group showed in 2001 top down self assembly of p-n junction made by crossing two inorganic nano wires, such as p-type silicon grown nanowire and an n-type gallium nitride by Huang et. al. [116], and indium phosphide p- and n-doped grown nanowires by Duan et. al. [117]. Huang demonstrated that the n-p junction could create depletion mode FETs with low operation voltage. The advantage of these methods is that full density can be achieved, however no patterning alternatives for these types of crossed NW transistors have been suggested to date that match the resolution of nano crossbars.

In paper 4 it is demonstrated that depletion mode transistors could be made at the junctions of PEDOT nano wires in an arrays. These transistors have micrometer gates and therefore resemble the type of geometries that was described as the first approach. However since PEDOT ECTs do not require predefined axial doping it is conceivable that these types of devices could be constructed using two crossed nano wires. The use of either nano fluidic assembly of the CP from channels (paper 4), or biomolecular NW templates (paper 2) should enable demonstration of these all NW devices in future developments.

80 Figure 6.3 (left) Schematic picture of micro Gate/nano channel crossbar transistors, where the channels are aligned under the channel. (right) Nano gate/ nano channel transistors in crossbar, where the alignment problem between gate and channel is shown.

6.3 Architectures: logic, memory, and hybrids A large number of theoretical work has been presented that analyze integrated devices in crossbars based on the presented components. The HP group with Snider et. al. have summarized these results [118], where it is for example shown that logic can be integrated [119] using only memristors In combination with resistor/diode logic gates, this results is interesting since memristors also store memory so in principle the two terminal devices should be enough for enabling universal computing for crossbar circuits. Any type of transistor can also be used to enable logic and computation. Eventhough transistors can enable easier architectures it should be remembered that their implementation with full resolution (both nano channel and nano gate) is more difficult.

Further, the real interesting part of any of these theoretical results is those that analyze the defect tolerance of devices. The reason is as explained previously that no schemes today exist for creating exact patterns of predefined components on the crossbar. Instead one has to assume that the components are placed stochastically, or in patterns that has a very high degree of defects (figure 6.4)

81 The most important part of any integrated device in crossbars is a circuit that can address a specific junction. These addressing devices or demultiplexers (demux) are discussed in detail in the next section.

Figure 6.4 Circuit symbols of the 3 main crossbar components, and schematics of how each of these can occupy separate blocks/regions of a crossbar.

6.3.1 Demultiplexing crossbars Mathematically, a digital addressing device can address uniquely 2n wires using a minimum of n control wires. This means that for nano crossbars 2 demuxes can through a very low number of control wires O(n) access a very large number of crossbar components 22n.

One solution for addressing each junction in a crossbar is the proposal of hybrids that move the addressing and routing problem to a CMOS layer. The most mature theoretical studies here are driven by Likharev et. al. and known as CMOS/nanowire/molecular hybrid (CMOL) [120]. In CMOL the idea is that n2 crossbar junctions are connected to, and addressed by 2n CMOS wires. The crossbar thus allows n2/2n=n/2 times more devices to be implemented as compared to what the pure CMOS layer would allow. However no practical demonstrations of any CMOL structures exist today. Furthermore hybrids are not an attractive approach for making total bottom up assembled structures. These types of hybrids are not the only choice for addressing, since crossbars themselves offer possibilities for addressing by direct implementation of demuxes on crossbars.

82 The demuxing of microfibers in a crossbar geometry was already mentioned in section 5.2.2 for e-textile applications. Here the binary tree multiplexer was introduced which is the most efficient device for addressing 2n crossbar wires, using 2n control wires. The implementation of such a device is possible using both transistors [121] or diodes [122,123]. However the use of transistors gives more favorable design possibilities. As discussed previously the integration of transistors is much easier if the gate is at micro dimension. In the case of a demux this is not a problem since the gate is the control wire of the demux and must be in larger dimensions anyway for probing reasons. Furthermore the relatively low number of control wires needed do not consume much space on the crossbar (see more calculation in next section).

The most relevant experimental results of multiplexers on crossbars to date include: • The work of Zhong, Lieber et. al [124], based on the silicon nanowire junction transistors described previously. Here a simple 2x2 crossbar is demonstrated where each transistor at the NW junctions is specifically patterned with photolithography to form the binary tree pattern. The implemented scheme of top-down patterning would however fail here, if the distance between the nanowires would be decreased to a pitch close to the nanowire diameters. Alternative schemes therefore have to be presented for exact patterning of transistors at specific junctions using this scheme. • Beckman, Heath et. al. 115 demonstrated demultiplexing of a high density NW array made with SNAP (see section 6.2.1). This NW array has a pitch beyond CMOS dimensions (sub 30nm). This method uses an exact binary tree, which is created with e-beam, by etching the NWs at specific junctions. The e-beam allows registration of single NWs. This is the only real demonstration to date of demuxing of a high density NW array and represents a large step forward. However the patterning of transistors with the top-down sequential e-beam is against the concept of bottom up self assembly, and even here bottom up methods have to be deployed for patterning the transistors in future developments.

These two experiments clearly demonstrate the possibilities, but also the limitation of implementing devices in crossbars, which is always limited by the difficulty of patterning components in a predefined pattern with the same resolution as the crossbar, and more specifically a binary tree pattern in the case of a demux. Due to this limitation a number of theoretical proposals for defect tolerant nano crossbar demuxes have been presented.

83 Hogg, Kuekes et. al. [121] were among the first to propose a solution to the problem of demuxing unique nanowires in a nano array, without the need of placing components with perfect resolution. It was proposed that the components should be self-assembled between the NWs in a fashion so that only a random pattern is created with a certain percentage (e.g. 50%) of the junctions integrating a component. Interestingly it was shown that such random/stochastic patterns still allow unique addressing of each NW. The only difference is that the number of control wires has to be increased. Even more interestingly the extra number of control wires do not need to be more than about 5 times the minimum number of 2log2N. Figure 6.5 shows a schematic of a random transistor based demux with micro control wires, and a schematic of how several such devices (two for each crossbar) can be used to address several different blocks of nano crossbars, where each block can contain for example memory or logic. This concept of stochastic demuxes has been analyzed in a number of later studies, where the concept is refined by analyzing in more detail how the components are constructed at the nanowire/ microwir. A recent paper by Savage, Dehon, et. al. [125] summarizes all the relevant results. The main finding of all the results is still the same in all these studies, i.e. that a maximum of ~5 times more control wires is enough to build a purely stochastic demux.

The stochastic distribution of components should be the most simple to achieve with bottom up methods, as basically any bottom up assembly of components with a certain fraction of defects would lead to such a device. Once blocks of demuxes are created, the most straightforward device to implement at the crossbar junctions that are demultiplexed would probably be a memory, based for example on an arbitrary and stable thin film switch diode/memristor.

84 Figure 6.5 (left) Schematics of a demux based on stochastic transistors with micro gates and nano channels. (right) Several Demux blocks each addressing memory/logic

In the next section concepts towards an all organic CP based nano crossbars is discussed, with consideration to the results of papers 2,4.

6.4 Towards all organic nano crossbars As described in previous chapters memristors and diodes are crossbar elements that were conceived as single molecular components from the beginning and have since then been implemented in various all-organic, single and multi molecular schemes. The addition of transistors and conducting crossed organic nanowire arrays will complete the number of necessary all organic elements needed for all organic nano crossbars.

Fluidics could also be a powerful tool for patterning crossbars with CP coated biomolecular nanowires (e.g. paper 2). In fact fluidic patterning is already used for creating carbon nanotubes crossbars and crossbars of

85 inorganic nanowires, such as in the work of Lieber’s group, described previously, for distributing single nanowires in junctions from fluids using PDMS [126], and the distribution of CNT in micro-junctions from fluidic microcontact printing [127]. Fluidics has also been used for fluidic alignment in one layer of CNTs inside PDMS channels, by Park, Rogers et. al. [128]. Figure 6.6 shows a schematic of a crossbar that could be made using fluidics pattering of biomolecules in two steps. These wires could for example be the PEDOT-S coated amyloid fibrils presented in paper 2, or other biomolecules coated with other CPs, and combinations of these.

Figure 6.6 schematics of (left) bridged arrays of micro-nano lines forming a crossbar, where the NW junctions can be directly probed using the bridged micro probes. (right) Aligned CP:biomolecular nanowires forming crossbars, where certain NW junctions are probed using pre-patterned microprobes.

Another method for making CP crossbars from fluids would be to repeat the molding in capillaries approach of paper 4 to create two nano arrays with an organic thin film in between, and form a PEDOT crossbar (see figure 6.6). Alternatively another highly conducing polymer such as PANI could be used. The demonstrated micro bridges would here allow easy direct probing the crossbar junctions (see figure 6.6). It should be noted that direct probing

86 of single nanowires in dense nanoarrays on flexible substrates is very difficult. The reason for this is that flexible large area substrates are not dimensionally stable at nano scales, so it is difficult to align bottom up assembled nano structures with top down patterned probes. The bridged micro-nano fluidics patterning in paper 4 solves this problem by simultaneously creating dense nanoarrays and probes in the same bottom up procedure. Figure 6.6 shows an example of a structure with bridged micro nanowires, where fluidic patterning creates 3x3 dense nanowire arrays, and simultaneously microprobes for probing these.

6.5 Demultiplexers on PEDOT crossbars The methods of direct probing of each nanowire using bridged micro-nano wires, as described above, could be useful for testing few single high density nano junctions, however the more interesting implication of the results from paper 4 are the crossbar transistors, which would allow construction of an all organic demux, to fully address the conceived all organic nano crossbars Figure 6.7 displays the schematics of such a crossbar architecture where the addressing of micro lines are handled by exact binary tree demux architectures. Each addressed microwire is then bridged to an array of nanowires, which in turn are addressed using a stochastic demux (as described previously). The making of the stochastic demux requires random patterning of PEDOT transistors at the micro/nano junctions, which should be possible to make as exact patterning of these transistors is already demonstrated in paper 4. It is noteworthy that all of the top down patterning steps are at the micro meter scale, steps, where exact patterning and alignment is possible. These micro structures could for example be at a size which is compatible with conventional large printing techniques such as inkjet (10-100 µm). The fluidic assembly of bridged micro-nano arrays is also compatible with large area patterning techniques, and it should be possible to integrate the nano patterning step with the rest of the fluidic patterning micro patterning techniques in an all integrated assembly process. The only penalty that is paid for bridging the dimensions between the micro and nano world without the need for exact nanopatterning, according to this scheme, is that a larger fraction of the nanowire array that has to be occupied by the control wires. It is therefore important that the aspect ratio (width/length) of the NW array is large enough to fit both the stochastic demux and the crossbar.

87 Figure 6.7 shows a graph where the total size of a stochastic demux is calculated based on Hoggs calculations on stochastic demuxes [121], as a function of the micro feature sizes and the width/pitch of the NW arrays. If we for example would have a micro patterning technique which would give access to 10µm precision, (which is quite straightforward with a number of different techniques), and a nanowire width/pitch of 100 nm we would need less than 103 µm space along the NWs to fit the Demux. This means that we should have an aspect ratio of <104 for each NW. This criteria is fulfilled for the NW arrays in paper 4, and even larger aspect ratios should be possible to obtain through future optimization of nano fluidic patterning techniques. Even smaller aspect ratios would of-course bee needed if the microwires are smaller (figure 6.7).

Figure 6.7 Graph showing aspect ratio versus size of the number of junctions that can be addressed with stochastic addressing.

Another issue that is of importance in nano crossbars is the resistance of the NWs. Since the high-density crossbars, that are demuxed, are passive matrix devices, there will be a voltage drop along the line meaning that the components at the end of the line do not experience the same voltage as the ones at the beginning.

88 Fredriksson et. al. has shown [129], (based on a simple crossbar resistor model), that around 103-104 junctions can be addressed along each NW, by assuming non linear memristors as nanojunction components, and further assuming a resistance of the NW is in the range of 1 KΩ/square. In paper 4 the resistance of the PEDOT-S NWs is in the range of 10 KΩ/square, this value has already been optimized in our lab by a factor of 10, and future optimization is expected. PEDOT-S should thus already work as NW material in high-density crossbars where at least some Mega bits of data (103*2) can be stored in each separate nano crossbar.

Figure 6.8 Schematics of an all-organic demultiplexer, integrated on bridged micro-nano PEDOT arrays.

89

7 Summary and future outlook 7.1 Summary work The focus of this thesis has been to examine and present possibilities for making organic electronics on micro and nanofibers.

Microfiber based electronics is especially suitable for electronic textiles (e- textiles), as most fabrics are weaved with microfibers such as nylon. In papers 1,3 a novel approach for embedding transistors and devices directly on textile microfibers is presented. These devices are based on electrolyte-gated transistors that have been presented for the first time on microfibers. The operation of the presented microfiber transistors is very insensitive to the shapes and positioning of microfibers and thus very suitable for integration with conventional textile manufacturing techniques such as weaving. The use of electronic textiles based on electrolyte-gated transistors is also analyzed for biomedical applications. In paper 5 a concept is presented, where the combined advantages of three-dimensional circuit design, flexibility and biocompatibility of organic materials are considered for in- vivo operation of e-textiles as neural communication.

Electronics based on fibers that are thousand of times smaller than textile microfibers, i.e. nanofibers/nanowires, is crucial for the future development of organic electronics. In paper 2 molecular self-assembly is carried out in pure water for making conducting nanowires that consists of amyloid fibrils nanofibers functionalized with a conducting polymer (PEDOT-S). These nanofibers have diameters of around only 10nm but still demonstrate electro-activity. This is demonstrated by showing electrochemical transistors in which the conductance is modulated by changes of the degree of doping of PEDOT-S, for the individual nanowires in the transistor channel. This work constitutes a step towards the fabrication of advanced self-assembled organic nanoscopic devices, with very small dimensions, where highly conducting polymer nanowires with the right properties, have until now been a missing puzzle.

In paper 4 large area patterning of bridged micro and nanowire arrays is demonstrated, by using micro/nano fluidic assisted patterning. These arrays

90 contain a large number of separated nanowires that are densely packed, where the nanowires can easily be accessed via the bridges to the larger microwires, by electrical probing of the microwires. Electrical measurements show conduction through individual groups of nanowires. In addition to electrical conduction, a crossbar geometry is used to create electrochemical transistors along the nanowires. In this paper the demonstration of - the material PEDOT-S as a new material suitable for nanoelectronics - cheap large area patterning of conducting dense nanowire arrays - bridging of micro and nano domains without alignment steps - and crossbar micro/nano transistors. The combination of these points, constitute a big step towards the demonstration of organic nano crossbars. These types of crossbars can be a potential platform for very high-density flexible organic nanoelectronics.

7.2 Future outlook In the field of nanoelectronics, the demonstration of PEDOT-S functionalized amyloids, can open possibilities for measuring conductivity on single molecular wires. The self-assembly of these molecular wires, together with their high aspect ratio should make probing of single NWs possible at micro dimensions and therefore quite straightforward. Analysis of conduction on single wires as a function of temperature, doping, length etc. can reveal interesting information, about 1D nature of conduction. More interesting use of CP decorated biomolecules can involve the bottom up self assembly of functional devices at the junctions of single NWs, analogue to what has been demonstrated in the field of inorganic nanoelectronics, by Lieber and others. Indeed the use of polymers for layer by layer bottom up self-assembly from liquids is much more compatible with organic polymer NWs than it is for inorganic crystalline NWs, as crystals are more stiff and insoluble. Combinations of conducting and semi-conducting CP decorated biomolecules can give rise to interesting components at junctions, and it might be possible to construct simple logic devices by placing several components along a single wire, like what has been demonstrated for single CNTs [130], and on grown crystalline nanowires [116]. All organic nano components could also allow demonstration of functions beyond logic, such as sensors with extremely enhanced transduction due to the high volume to area ratio and the bio-compatibility of organic nanostructures.

91 In the field of e-textiles other types of electrolyte-based devices should be possible to implement in a similar fashion as the presented electrolyte gated transistors. These devices can include light emitting electrochemical cells (LECs) and electrochromic fibers. The addition of these devices could allow for the design of active matrix displays integrated in textiles. The presented design rules for weaving of logic devices (section 5.2.1), could be implemented in future developments by using weaving machines together with different combinations of functionalized and regular microfibers, in order to create more advanced digital woven devices.

Furthermore the demonstration of EDLC and ECT behaviour in different time domains, in paper 3, could help to shed more light on the mechanisms of operation of electrolyte gated devices and should be analyzed further.

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