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Letter

Macroscopic electrical wires from vapour deposited poly(3,4-ethylenedioxythiophene) Lukas Koch, Anna Polek, Sam Rudd, and Drew R. Evans ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14727 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Applied Materials & Interfaces

Macroscopic electrical wires from vapour deposited poly(3,4-ethylenedioxythiophene) Lukas Koch1,2, Anna Polek1,2, Sam Rudd2, Drew Evans2* 1. Department of Materials, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093, Zürich, Switzerland. 2. Thin Film Coatings Group, Future Industries Institute, University of South Australia, Mawson Lakes, South Australia, 5095 Australia

Keywords: Conducting polymers, Memristor, Organic wires, Plasticizer, poly(3,4ethylenedioxythiophene), Tensile strength

ABSTRACT: Conducting polymers represent a field of materials innovation that bridges the properties of metals (electrical conduction) with those of traditional polymers (mechanical flexibility). While electronic properties have been studied, minimal attention is given to their mechanical properties such as tensile strength. macroscopic

wires

made

from

the

vapour

phase

This study presents

polymerisation

of

ethylenedioxythiophene) using triblock copolymers as a molecular template.

poly(3,4These

macroscopic wires are conductive (up to 5x104 S/m), and possess tensile properties (Young’s modulus ~ 1.1GPa; tensile strength ~ 90 MPa) comparable to commercially available polymers (Nylon 6 and poly(methylmethacrylate)), without need for non-conductive mechanical fillers.

ACS Paragon Plus Environment


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As new technology permeates through society, from smart phones to sensors in the internet of things, there is a growing need for materials that are not only electrically and/or optically active, but also mechanically robust.

Conducting polymers are an emerging

material for future device fabrication, primarily arising from their potential electronic properties from semiconducting to metallic, with recent observations made of semimetallic behaviour1. Since initial reports of highly conductive polyacetylene2 and the first air-stable conducting polymer polyaniline3, much attention has been devoted to understanding the fundamental electrical transport properties of these materials1,

4, 5

.

The prototypical

conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is commonly used in investigations aimed at understanding the charge transport in polymeric materials. Such properties are of interest when considering the miniaturisation of the conducting polymer structures to create micropatterns6 and nanowires7 for ultimate use in complex electrical circuits. Turning attention to macroscopic structures, organic conductors from carbon nanotubes to graphene to conducting polymers have been employed to produce yarns or fibres8-11. Recently attempts have been made to enhance the mechanical strength of large area conducting polymer thin films by introducing fillers, such as nanofibrillated cellulose (NFC)12 and 2D nanomaterials13. The enhanced mechanical strength allows for macroscopic structures to be fabricated thus opening the door to deploy them in modern electrochemical devices, such as batteries14 and supercapacitors15. In such cases devices which are not only functional, but also stretchable can be achieved thus taking advantage of the polymeric nature of the materials used. Bao and co-workers demonstrated the use of PEDOT on stretchable substrates that remain conductive up to strains of almost 200%16. Through use of chemicals to plasticize PEDOT doped with polystyrenesulfonate, Lipomi and co-workers robust and macroscopic conductive materials for use in strain sensors and organic solar cells17. While


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this form of PEDOT is solution processed, Park et al. demonstrated oxidative chemical vapour deposition to fabricate the hole transport layer from PEDOT doped with Cl.18 In the study herein, we present the fabrication of macroscopic wires from the vapour phase polymerisation (VPP) of PEDOT doped with tosylate (Tos) using a non-ionic triblock copolymer as the molecular template. These VPP-PEDOT:Tos wires display mechanical strength comparable to commercial Nylon 6 or poly(methylmethacrylate) (PMMA), beyond what has been achieve thus far for other conducting polymer macrostructures, without need for fillers. The electrical conductivity is maintained, being comparable to that achieved for other macroscopic structures of organic conductors (carbonaceous materials or PEDOT-based polymers), although less than that observed for thin films and nanostructures of PEDOT19. The wires were produced using PEDOT thin films doped with Tos that were fabricated with VPP1,

4, 20-23

. Firstly, glass slides of 20 cm length and 5 cm width were

cleaned using ethanol, water and compressed air. The cleaned glass slides were subsequently air-plasma treated (NANO, Diener electronic). In parallel, an oxidant solution was prepared using by weight 6 parts ethanol, 3 parts non-ionic PEG-PPG-PEG triblock copolymer (P123, Mw 5800, Sigma Aldrich) and 4 parts of a 40 wt% iron(III) tosylate solution in n-butanol (CB-40, Heraeus). The additives were added to a reagent bottle with screw cap and heated at 70 °C for 1 h until the PEG-PPG-PEG was visually observed to dissolve into solution. Taking the glass slides and oxidant solution, the pre-treated glass slides were spin coated (WS-65015, Laurell) with the oxidant solution at 500 rpm. The spin coated glass slides were then placed on a hot plate at 40°C for 5 min to evaporate any excess ethanol and n-butanol, and subsequently placed in a vacuum oven at 35 °C (115L Series VD, Binder). A few drops of the EDOT monomer were placed in a Petri dish in the vacuum oven on a heating element at 40 °C. The oven was then evacuated to a reduced pressure of 40 mbar. EDOT monomer evaporates and then recondenses on the oxidant-vapour interface, thus undergoing oxidative



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polymerisation at the surface20, 24. After the polymerisation process of 30 min the chamber was flooded with air. The process until here is the typical VPP process prior to the washing step. The samples of VPP PEDOT:Tos on the glass slides were then submerged into an ethanol bath and the film was peeled off the glass slides using tweezers. One end of the floating PEDOT film was then placed upon a glass slide and slowly pulled out of the ethanol bath (see Figure 1a). Through the capillary forces of the draining ethanol away from the film into the bulk of the solvent bath, a wire like structure was formed. The wire shaped PEDOT film was then placed on a lint-free paper towel to absorb any excess ethanol. The PEDOT wire was then rolled into a denser wire with gloved hands and was hung using clamps (Figure 1b). To prevent the wire from rolling up a second clamp was placed at the other end of the wire in order to slightly stretch the wire. The wires were left to dry overnight at 23 °C and 35 ± 5%RH, although after 30 min of drying no change in appearance of the wires was distinguishable.


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Figure 1. Fabrication of macroscopic wires of PEDOT. The wires are produced via VPP of PEDOT:Tos as thin films, which are then drawn from a bath of ‘washing agent’. (a) This drawing process results in formation of wires due to capillary action of the draining liquid. (b) Wires are then hung under tension to dry in ambient conditions (23 °C and 35 ± 5%RH). The washing agents of (c) methanol, (d) ethanol, and (e) 1-butanol were used to produce wires of comparable diameter. Note the differing scale bars on (c) – (e).

As an alternative to the ethanol bath for separating the PEDOT film from the glass slide 1-butanol, methanol and tap water were independently used for the VPP-PEDOT:Tos wire production. The solvent for the peeling off and washing of the PEDOT films will henceforth be referred to as the ‘washing agent’. Scanning Electron Microscopy (SEM) was used to inspect the fabricated VPP-PEDOT:Tos wires. A comparison of the diameters of the



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VPP-PEDOT:Tos wires for different washing agents is given in Table 1. The standard deviation is provided for each of the variants of wires. This value comes from the variation along the length of any one wire, as the formation of the wires along its length from the precursor thin film is not perfectly controlled. It is noted that subsequent wire diameters fall within this range of values depending on the thickness of the starting thin film (nominally 300 nm) and the washing agent used. Table 1. The diameter of VPP PEDOT:Tos wire prepared using different washing agents, as determined by SEM imaging. This diameter is then converted into an approximate cross sectional area. Washing agent

Diameter [µm]

Average Area [mm2]

Tap water

420 ± 40

0.139

Methanol

210 ± 20

0.035

Ethanol

200 ± 20

0.031

Butanol

250 ± 20

0.049

The diameter of the PEDOT wires produced with ethanol as the washing agent could be reduced to 200 µm (Figure 1d). The longer chain of 1-butanol increased the diameter of the PEDOT wire (Figure 1e) whereas the short methanol molecule did not result in a decrease of the diameter (Figure 1c). Conversely, when water was used as the washing agent the diameter of the VPP-PEDOT:Tos wire was significantly larger (Figure 2a and b). The impact of the ‘washing agent’ and the mechanism by which it acts will be discussed later. In all cases using the alcohol washing agents, there appears to be smaller fibre structure on the surface of the macroscopic wire. Cross-sectional SEM imaging of these wires (not shown)


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highlight that these are wrinkles in the original PEDOT thin film. It is unlikely that these wrinkles play a significant role in defining the ensuing properties of the macroscopic wires.

Figure 2. The response of PEDOT wires to water. When water is used as the washing agent, different VPP-PEDOT:Tos wires are formed. (a) The wires are subsequently thicker, and possess a folded structure (SEM in b) compared to the dense packing from the alcohol washed wires. (c) subsequent soaking of the wires in water leads to swelling and unfolding of the water washed wires, while the alcohol washed wires remain densely packed. (d) When a VPP-PEDOT:Tos wire is prepared using water as the washing agent, soaked in water, and then dried, it densifies to a similar diameter as the as-prepared VPP-PEDOT:Tos wires using an alcohol washing agent.

The holes and gaps created by the folding structure contribute towards the larger diameter of the wires produced with tap water but cannot alone explain the entire increase in



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diameter. Mechanical profilometry (Dektak XT, Bruker) of a VPP-PEDOT:Tos thin films treated with tap water or ethanol respectively showed that the water treated film was up to a factor of 5 times thicker than the ethanol treated film. When VPP-PEDOT:Tos wires using ethanol or water as the washing agent are further subjected to soaking in water, the compact nature of the ethanol prepared wire is maintained whereas the wires produced with tap water swell within minutes (see Figure 2c). The swelling behaviour is hypothesised to in part originate from the presence of non-ionic triblock copolymer within, and its hygroscopic behaviour leading to the uptake of water. More specifically, it is the PEG moiety in the triblock copolymer that interacts with the water25, 26. Berthier et al. state that ethanol is a good solvent for both PEG and PPG27. This changing solvation for the triblock copolymer is hypothesized to yield different formed structures and morphology within the PEDOT wires. For VPP-PEDOT:Tos wires produced using water as the washing agent, owing to the folded structure of the wires, soaking in water leads to partial unfolding hence an increase in the perceived diameter of the wire. However, the dense VPP-PEDOT:Tos wire achieved through ethanol as the washing agent limits the diffusion of water into the structure, thus inhibiting the swelling, and providing dimensional stability to the wire in aqueous environments. When the VPP-PEDOT:Tos wires produced with tap water are dried again after soaking they become much denser and exhibit a much smaller diameter (see Figure 2d). This result indicates that the presence of triblock copolymer alone is not the only consideration, with the doping anions within the wire having an impact on the wire geometry. As will be discussed below, prolonged exposure of the wires to a large reservoir of water leads to leaching of the doping anions out of the wire, as interpreted from the loss in electrical conductivity. Of primary interest is the mechanical properties of the various VPP-PEDOT:Tos wires. Tensile testing was conducted on the wires (ethanol Figure 3a and water Figure 3b),


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with Table 2 showing that those produced with organic solvents as the washing agent have a Young’s modulus and ultimate tensile strength 1 to 2 orders of magnitude greater than those using water as the washing agent. This enhancement may relate to the ethanol plasticizing the PEDOT material, in the same manner as observed by Savagtrup et al. when plasticizing PEDOT:PSS films17. Such plasticizing by ethanol ties back to its good solvent properties for the PEG-PPG-PEG triblock copolymer discussed earlier27. The ultimate tensile strength of the VPP-PEDOT:Tos wires produced with an organic solvent as washing agent is comparable to that of Nylon 6 (~ 70MPa28) or PMMA (~ 30MPa29), albeit falling short of strength’s achieved for carbon nanotube yarns (> 400MPa30). Similarly the Young’s modulus is comparable to commercial polymers while being an order of magnitude below the carbon nanotube yarns. The behaviour of the VPP-PEDOT:Tos wires during the tensile test is indicative of a semi-crystalline polymer below its glass transition temperature.

Such

semicrystalline structures have been observed by Gleason and co-workers in their study of oxidative chemical vapour deposition of PEDOT31.



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Figure 3. Tensile properties of PEDOT wires. The tensile properties of the VPPPEDOT:Tos wires depend greatly on the washing agent used. (b) Relatively poor strength is observed when water is the washing agent. (a) conversely, ethanol as the washing agent results in wires that have comparable mechanical strength to Nylon 6 and PMMA. Table 2. Summary of the tensile testing of the VPP PEDOT:Tos wires, giving the maximum force and strain applied to the wires before failure, and the resultant mechanical properties (Young’s modulus and Ultimate tensile strength) and the measured electrical conductivity. Ultimate Young’s Washing

Max.

Max.

tensile

Conductivity

strength

[x104 S/m]

Modulus agent

Force [N]

Strain [%] [MPa]

[MPa] Tap water

11.3 ± 1.1

19.1 ± 5.0

2.0 ± 0.2

1.25 ± 0.14

Methanol

0.28 ± 0.04 2.56 ± 0.2

25.5 ± 0.7

405.5 ± 91.2

71.1 ± 5.5

2.60 ± 0.47

Ethanol

2.93 ± 0.4

22.4 ± 2.9

1157 ± 133

93.2 ± 12.5

5.86 ± 1.04

Butanol

2.78 ± 0.1

14.8 ± 8.8

1097 ± 45

85.5 ± 1.7

2.02 ± 0.09

The VPP-PEDOT:Tos wires were then soaked in water for 24h to assess their change in mechanical properties. Firstly, with water as the washing agent there was no significant change in properties upon soaking. Conversely, the ethanol washed wires greatly reduced in mechanical strength (Young’s modulus ca. 220 MPa, Ultimate tensile strength ca. 50 MPa). These results support the hypothesis about plasticizing, with soaking removing the plasticizer level in the ethanol washed wires (removal of ethanol) while the water washed wires had no or very little plasticizer to begin with.


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The Electrical conductivity was determined from I-V curves where the current flowing along the wire was measured as a function of the applied voltage by cycling the voltage between -3 and +3 V. The Ohmic behaviour (linear response) arising from the voltage cycling can then be used to determine the resistance of the wire, and when coupled with the geometry of the wire the electrical conductivity can be determined. The measured conductivity of the VPP-PEDOT:Tos wires was the highest for the wires produced with ethanol (5.86 x104 S/m, Figure 4a) as washing agent (Table 2). This is 4 to 5 times greater than that achieved when water is used as the washing agent (Figure 4b). Similarly, the reported value using ethanol as the washing agent is comparable to that of PEDOT:PSS-NFC (4.2 x104 S/m

12

). This magnitude of electrical conduction also compares favourably with

those reported for different carbon nanotube yarns fabricated by a variety of means32-34. While the electrical properties were not tested during tensile testing, it was observed that the electrical conduction was the same before and after stretching, provided the VPPPEDOT:Tos wires were not elongated until breakage.



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Figure 4. Electrical characterisation of PEDOT wires from cyclic voltammetry. The electrical conductivity of the wires can be determined from the Ohmic response of the I-V curves upon voltage cycling, where (a) ethanol washed wires show the highest conductivity when compared to (b) the wires prepared using water as the washing agent. (c) After soaking the prepared wires in water for extended periods of time, the electrical conductivity is greatly reduced; more so for water washing agent (red) compared to ethanol (blue). The green arrows in (a), (b), (d) and (f) indicate the onset of electrical conditioning in the VPP-PEDOT:Tos wires. Prior to this the wires are deemed almost non-conductive, until a threshold is reached and conduction occurs. This conditioning is reversible, with (d) the first cycle of an asprepared wire showing the conditioning step, (e) no conditioning after resting the wires for 2 min (red) and 30 min (green) respectively, and (f) the return of the conditioning step after 90 min resting.

When the wires are subsequently soaked in water for an extended period of time, the influence of the washing agent becomes apparent. Not only are there possible changes in geometry, as discussed earlier, the electrical conductivity is impacted. Wires produced using the washing agents of ethanol or water decrease in electrical conductivity after water soaking to approximately 2 x104 S/m and < 0.2 x104 S/m respectively (Figure 4c). This impact of soaking is different from the mechanical property change (or lack thereof).

Using the

hypothesis of plasticizing by the washing agent to explain the mechanical response does not hold for the electrical response. That is, the electrical response is hypothesized to be related to the structure/morphology within the wires and the doping level. The washing agent plays a role in defining this, but the presence or otherwise of the washing agent (plasticizer) within the PEDOT does not partake in charge transport.


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Interestingly the VPP-PEDOT:Tos wires show an interesting “conditioning” behaviour when interrogated by the I-V curves. The first cycling of the applied voltage yields very little current flow, until a critical voltage of ca. -1 V is reached (marked by green arrows in Figure 4) after which the wires display Ohmic behaviour. Noting here that -1 V is the relative voltage of one end of the wire to the other, and owing to symmetry should equally occur at +1 V. Inspection of Figure 4b, d and f at +1 V shows the presence of increased current flow as well. Herein analogies are drawn with the electroforming process in Memristors, where defects within the material are transformed from a random configuration to varying degrees of order or correlation35. In the case of the VPP-PEDOT:Tos wires, the defects may be electrical, chemical and/or morphological in nature, as opposed to the oxygen vacancies within the inorganic metal oxide materials used in the studies of Memristors thus far36. In fact the mechanism appears somewhat different from that observed and proposed in other systems, given the lack of a field effect arising from the use of an electrolyte and gate electrode37. The metal cations in the electrolyte are proposed as the key charge transport entity in organic Memristors. For the study herein there are no metal cations present. This does not rule out the dissolution of the metal contacts into the VPP-PEDOT:Tos wires. However, the increasing current density with increasing voltage cycles herein is opposite to that seen for Metal/PEDOT:PSS/Metal configurations (PSS = polystryenesulfonate)38,

39

.

Furthermore, the proposed electroforming process appears to be reversible, as the ohmic behaviour is lost when the VPP-PEDOT:Tos wires are left to rest for extended periods of time (Figures 4d to f). After 30 min of resting in ambient conditions (Figure 4e), the wires continue to display ohmic behaviour for all I-V cycles, whereas the initial electroforming step returns for wires rested for 90 min (Figure 4f). Such relaxation in the electrical properties of the wire indicates the mechanism(s) driving the property change is itself reversible. Again, this is hypothesized to be related to molecular (re)ordering in the PEDOT:Tos structure



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and/or electrochemical reactions with water absorbed from the surrounding environment. Much more detailed studies are required to ascertain the origins of the conditioning and relaxation process. Macroscopic organic electrical wires have been successfully fabricated using the vapour phase polymerisation of PEDOT:Tos. Through appropriate choice of the washing agent (preferably ethanol) the wires possess good electrical conductivity as well as mechanical properties comparable to commercial grade polymers.

With enhanced

mechanical strength the VPP-PEDOT:Tos wires show promise for application in a wide range of devices, owing to the inherent properties of conducting polymers (electroactive, electrochromic, thermally conductive, etc).

Beyond this, investigation of a potential

electroforming process within the wires provides hints to the VPP-PEDOT:Tos being suitable for a range of new technologies. AUTHOR INFORMATION Corresponding author *Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge scientific discussions with Xavier Crispin and Abdellah Malti. DRE acknowledges the support of the Australian Research Council through the Future Fellowship scheme (FT160100300). REFERENCES


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The wires are produced via VPP of PEDOT:Tos as thin films, which are then drawn from a bath of ‘washing agent’. (a) This drawing process results in formation of wires due to capillary action of the draining liquid. (b) Wires are then hung under tension to dry in ambient conditions (23 °C and 35 ± 5%RH). The washing agents of (c) methanol, (d) ethanol, and (e) 1-butanol were used to produce wires of comparable diameter. Note the differing scale bars on (c) – (e). Figure 1 705x521mm (72 x 72 DPI)



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When water is used as the washing agent, different VPP-PEDOT:Tos wires are formed. (a) The wires are subsequently thicker, and possess a folded structure (SEM in b) compared to the dense packing from the alcohol washed wires. (c) subsequent soaking of the wires in water leads to swelling and unfolding of the water washed wires, while the alcohol washed wires remain densely packed. (d) When a VPP-PEDOT:Tos wire is prepared using water as the washing agent, soaked in water, and then dried, it densifies to a similar diameter as the as-prepared VPP-PEDOT:Tos wires using an alcohol washing agent. Figure 2 419x381mm (72 x 72 DPI)


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The tensile properties of the VPP-PEDOT:Tos wires depend greatly on the washing agent used. (b) Relatively poor strength is observed when water is the washing agent. (a) conversely, ethanol as the washing agent results in wires that have comparable mechanical strength to Nylon 6 and PMMA. Figure 3 415x341mm (72 x 72 DPI)



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The electrical conductivity of the wires can be determined from the Ohmic response of the I-V curves, where (a) ethanol washed wires show the highest conductivity when compared to (b) the wires prepared using water as the washing agent. (c) After soaking the prepared wires in water for extended periods of time, the electrical conductivity is greatly reduced; more so for water washing agent (red) compared to ethanol (blue). The green arrows in (a), (b), (d) and (f) indicate the onset of electrical conditioning in the VPPPEDOT:Tos wires. Prior to this the wires are deemed almost non-conductive, until a threshold is reached and conduction occurs. This conditioning is reversible, with (d) the first cycle of an as-prepared wire showing the conditioning step, (e) no conditioning after resting the wires for 2 min (red) and 30 min (green) respectively, and (f) the return of the conditioning step after 90 min resting. Figure 4 741x403mm (72 x 72 DPI)


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Table of Contents Graphic 510x406mm (72 x 72 DPI)


Macroscopic Electrical Wires from Vapor ... - ACS Publications

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