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Polymer 55 (2014) 5095e5101

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Polymer

journal homepage: www.elsevier .com/locate/polymer

Transparent, conductive polystyrene in three dimensionalconfigurations

Alexander Trachtenberg a, T.P. Vinod a, b, Raz Jelinek a, b, *

a Department of Chemistry, Ben Gurion University of The Negev, Beer Sheva 84105, Israelb Ilse Katz Institute for Nanoscale Science and Technology, Ben Gurion University of The Negev, Beer Sheva 84105, Israel

a r t i c l e i n f o

Article history:Received 3 June 2014Received in revised form4 August 2014Accepted 9 August 2014Available online 20 August 2014

Keywords:Flexible electronicsPolystyreneAu nano-structures

* Corresponding author. Ilse Katz Institute for NanoBen Gurion University of The Negev, Beer Sheva 84105

E-mail address: razj@bgu.ac.il (R. Jelinek).

http://dx.doi.org/10.1016/j.polymer.2014.08.0260032-3861/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Development of technologies for constructing three-dimensional (i.e. non-planar) transparent conduc-tive electrodes from polymeric materials is a major goal in diverse applications, including optoelectronicdevices, flexible electronics, photovoltaics, and others. We present a facile new strategy for creatingconductive, transparent gold layering on polystyrene, a widely-used polymer, in different shapes andsurface morphologies. The approach is based upon amine functionalization of the polystyrene surfacefollowed by incubation in an aqueous solution of Au(SCN)4� and brief plasma treatment. We show thatthis simple deposition process resulted in a homogeneous, transparent, and highly conductive crystallineAu coating. Importantly, electrical conductivity was attained for long distances, even in highly non-planar surfaces containing physical barriers. We further show that the approach can be employed forfabrication of conductive hollow tubes using electrospun polystyrene fibers as templates. The newsynthesis scheme might make possible varied applications in polymer-based electronic and photonicdevices.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer-based electronics exhibit broad potential for variedapplications ranging from printed-polymer electrical circuits,flexible electronics, “intelligent labels”, large area displays, andsolar panels [1]. A principal advantage of polymer electronics is thatlarge, flexible and low cost substrates can be used. In addition,polymers exhibit great variety of physical properties such asmolding capabilities, surface roughness and morphology, thermalexpansion, and barrier properties, which can be readily modulatedand exploited as substrates for electronic and photonic devices [1].Indeed, the rapid development of this field in recent years couldradically alter the electronics market by offering lighter, flexible,and more cost-effective alternatives to conventional materials andproducts [1].

A particular draw of polymers is the feasibility for constructingcurved, flexible, or stretchable substrates for conductive surfaces[2e4]. Conventional “top-down” technologies, such as vapordeposition and lithography, are limited in their applicability to non-

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planar surfaces, as they exhibit less control of spatial dimensionsand result in inadequate uniformity of deposited films [5]. “Bottom-up” strategies relying on molecular self-assembly have recentlyemerged as viable alternatives for the construction of conductivesurfaces, providing simple, inexpensive, and environmentallybenign solution-based synthetic routes [6e8].

We have recently developed a generic, single step solution-based “bottom-up” approach for assembling conductive, trans-parent nano-structured gold layers on varied surfaces [9e11]. Thetechnology relies upon spontaneous crystallization/reduction ofgold thiocyanate [Au(SCN)4�] in aqueous solutions, without co-addition of reducing or stabilizing agents [9]. While previousstudies have demonstrated conductive/transparent film formationsupon glass [9], silicon [9], b-sheet peptide domains assembled atthe air/water interface [10], and organo-silicon polymer substrates[11], implementation of the technology for coating organic polymerssuch as polystyrene (PS) would be significant, as this class ofpolymers has been widely used in diverse applications, and areconsidered promising electro-optic materials [12e14].

PS, in particular, is amenable for three dimensional applications.This polymer can be injection-molded and processed into thin filmsand fibers [15]. Thus, PS has been widely employed in the auto-motive, electrical and electronics, and consumer product areas [15].PS is optically transparent, which has opened possibilities for opto-

A. Trachtenberg et al. / Polymer 55 (2014) 5095e51015096

electronic and photonic applications [12e14,16]. PS spheres, forexample, have been particularly useful for the formation of pho-tonic crystals [16]. Their high degree of shape and size uniformityaids their assembly into colloidal crystals, and the relatively highrefractive index contrast between the spheres and air-filled voidswithin them allows for enhanced confinement of light [16,17].

Fabrication of hybrid gold/PS materials has been reported,mostly based upon synthesis of Au nanoparticles (NPs) or Au col-loids, which are subsequently adsorbed onto the PS surface [18].Some studies have described simultaneous reduction and crystal-lization of gold salt precursors in the presence of the PS substratetargeted for deposition [19,20]. In general, almost all synthesisschemes developed thus far require addition of reducing agents aswell as careful control of Au colloid structures and surface func-tionalization essential for directing Au NP adsorption. Moreover,the propensity of gold ions to assemble into colloids and nano-particles in solution often impedes effective Au layering and cor-responding electrical conductance. Electroless plating deposition ofgold upon PS substrates has been also reported [21]. Electrolessdeposition processes employ only one electrode and no externalsource of electric current, however, similar to Au colloid deposition,co-addition of reducing agents are required [22]. While electrolessplating has certain advantages, including direct synthesis of Auseeds upon the PS surface and better control of Au layer thickness[21], the process is generally slow, and limited in scope for surfacepatterning and coating of three-dimensional surfaces [22].

Here we present a generic and facile strategy for fabricatingnon-planar conductive surfaces on amorphous PS in varied mor-phologies. The approach consists of a two-step process performedin aqueous solutions, combining display of amine residues upon thePS surface, followed by spontaneous assembly of a nano-structuredgold layer through incubation of the amine-functionalized PS withthe gold complex Au(SCN)4�. We show that this process yieldeduniform gold films upon the PS surface that were optically trans-parent in a broad spectral range, and exhibited excellent conduc-tivity, even across significant physical barriers.

2. Materials and methods

2.1. Materials

96-well plate lids with condensation rings made of Polystyrenewere purchased from Greiner Bio-One (Germany). HAuCl4$3H2O,KSCN, Polystyrene (PS) with average Mn ~280,000 by GPC,branched polyethylenimine (b-PEI) with average Mw ~25,000 by LSand average Mn ~10,000 by GPC, 2-(N-morpholino)ethane sulfonicacid (MES), N-Hydroxysuccinimide (NHS), Na2HPO4, NaH2PO4 andTriton X-100 were purchased from SigmaeAldrich (USA). KMnO4was purchased from Solufix (Israel). 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) 98% was purchased from Alfa Aesar (USA).Conductive silver paint was purchased from SPI Suppliers (USA).Dimethylformamide (DMF) 99.8% was purchased from Fisher Sci-entific (USA). H2SO4, HCl, NaOH and Toluene were purchased fromBio Lab (Israel). All above listed reagents were used as received. Thewater used in the experiments were doubly purified by a BarnsteadD7382water purification system (Barnstead Thermolyne, Dubuque,IA), at 18.3 MU cm resistivity.

2.2. Preparation of PS templates

1. Rectangular samples with dimensions of 1 cm � 1 cm(referred to in the article as “plate samples”) were cut out from thePS well plate lids, afterwards the surface of the side that containsthe condensation rings was chemically modified (as specifiedbelow). 2. Preparation of PS micro-fibers through electrospinning

was based upon a published protocol, using an electrospinningapparatus schematically shown in Fig. 1,SI [23]. Specifically, 30%(w/v) PS solution was prepared by dissolving PS in DMF at roomtemperature, after stirring for 5 h, a homogeneous and clear solu-tion was obtained. The polymer solution was placed in a 5 ml sy-ringe fitted with a metallic needle, of 0.483 mm of inner diameter,that was controlled by a syringe pump (PHD 22/2000 Infusion only,Harvard Apparatus). The electrode of the high voltage power sup-ply (Gamma High Voltage Supply, PS/EL30P01.5-22) was clampedto the metal needle tip. The flow rate of polymer solutionwas 1 ml/h and the applied voltage was 15 kV. We collected randomly ori-ented nonwoven meshes of PS micro-fibers on a grounded sta-tionary rectangular metal collector (15 cm � 15 cm) covered by apiece of aluminum foil, placed 10 cm below the tip of the needle(tip-to-collector distance). Then, a 0.5 cm long mesh of electrospunPS fibers was transferred into water. Since PS is highly hydrophobicmaterial, the mesh of fibers turned into a tightly entangled bundle.This bundle sample was chemically modified (as specified below).

2.3. Amine modification of the PS surfaces

In order to perform surface amine modification of the twodifferent PS templates, we adapted and optimized a methoddescribed in a published protocol [24]. Specifically, the followingsteps were performed: carboxylation of PS surfaces was achievedby immersion of PS templates in a solution of KMnO4 in 1.2 N H2SO4(5 g/100 ml) for 1 h at 60 �C under shaking. Afterwards the tem-plates were washed with 6 N HCl (4 � 5 min, 23 �C, shaking) toremove the brown manganese oxide and rinsed with water (10times). Carboxyl activation and amine grafting were thereaftercarried out at the same time (one-step procedure), under thefollowing experimental procedure: the PS templates wereimmersed in a solution of 1.4 gr b-PEI in 20 ml of 0.1 M MES buffer,pH¼ 6, containing 105 mM EDC and 100mg of NHS for 2 h at 23 �C,under shaking. Afterwards the templates were washed with 0.1 MMES buffer pH ¼ 6 (3 � 5 min, 23 �C, with continuous shaking),followed by a solution of Triton X-100 in 10 mM Phosphate bufferpH ¼ 7.3 (100 ml/100 ml) (5 min, 23 �C, with continuous shaking.This was repeated 5 times), and finally distilled water (2 � 5 min,23 �C, with shaking).

2.4. Synthesis of the KAu(SCN)4 complex

Synthesis was based upon a published protocol [9]. Briefly, 1 mlof an aqueous solution of HAuCl4$3H2O (24 mg/ml) was added to1 ml of a solution of KSCN in water (60 mg/ml). The precipitateformed was separated by centrifugation at 4000 g for 20 min. Thesupernatant, containing KCl and excess of KSCN, was decanted, andthe residue was dried at room temperature.

2.5. Growth of Au films on amine-modified PS surfaces

An aqueous solution of Au(SCN)4� (0.7 mg/ml) was prepared indoubly distilled water. The amine-modified PS templates werevertically immersed in the solution and kept at 4 �C for 3 days. Afterthe gold growth had reached completion, the templates were takenout of the growth solution and washed thoroughly with water toremove non-reacted materials and subsequently dried at roomtemperature.

2.6. Scanning electron microscopy (SEM)

SEM Images were recorded on a JEOL JSM-7400F (JEOL LTD,Tokyo, Japan) scanning electron microscope, at an accelerationvoltage of 2 kV.

A. Trachtenberg et al. / Polymer 55 (2014) 5095e5101 5097

2.7. Atomic force microscopy (AFM)

Average cross-section measurements were conducted bygenerating two perpendicular scratches on gold film, grown on PSplate sample, using soft tweezers. An AFM height measurementnear the edge of the scratch was then carried out. (The perpen-dicular scratch was aimed to ascertain that the basis from whichheight measurement was carried out is indeed the PS substrate).AFM images were recorded at ambient conditions in tapping modeusing a Dimension 3100 SPM instrument from Digital Instruments(Veeco, NY) mounted on a passive anti-vibration table.

2.8. X-ray photoelectron spectroscopy (XPS)

Analysis was conducted using a Thermo Fisher ESCALAB 250instrument with a basic pressure of 2 � 10�9 mbar. The platesamples were irradiated in two different areas using mono-chromatic Al Ka, 1486.6 eV X-rays, using a beam size of 500 mm.

2.9. Powder X-ray diffraction (XRD)

Measurements were taken using a Panalytical Empyrean pow-der diffractometer equippedwith a parabolic mirror on the incidentbeam providing quasi-monochromatic Cu Ka radiation(l ¼ 1.54059 Å) and an X'Celerator linear detector.

2.10. Transmittance measurements

UVeVis and IR transmittance measurements in the range of300 nme2000 nmwere carried out in Caria 5000, Varian AnalyticalInstruments, Melbourne.

2.11. Attenuated total reflectance e Fourier transform infrared(ATR-FTIR)

FTIR measurements were carried out in a Nicolet 8700 FTIRspectrometer fitted with a MCTA detector. All single-beam spectrawere measured against a background recorded from clean Dia-mond crystal. The spectra were recorded in the range from 4000 to650 cm�1, at 8 e resolution and 36 scans with optical velocity e 1.4

Fig. 1. Experimental schemes. A) Au coating of PS multi-well plate. The top photographs shodeposition (note the red-brownish appearance indicating Au coating). Scale bar in top left imof electrospun PS fibers, followed by creating hollow Au tubes through dissolution of the PSthe references to color in this figure legend, the reader is referred to the web version of th

and aperturee 20. The FTIR datawere collected and analyzed usingthe OMNIC software.

2.12. Conductivity measurements

Room temperature conductivitymeasurements were performedon a Keithley 2400 sourcemeter in a two-probe configuration. Twodifferentmethodswere used to prepare the two different templatesfor conductivity measurements: 1. Gold coated PS plate sampleswere mounted in a thermal evaporator with a stainless steelshadow mask attached. Cr (10 nm) and Au (90 nm) were evapo-rated onto the substrates to obtain the electrode patches at a pre-defined spacing (50 mm,100 mm, 500 mm and 1 mm). 2. Gold coatedPS bundle sample was attached, using silver paint, between twovertically evaporated gold electrodes with a predefined spacing of2 mm.

Prior to the thermal evaporation or bundle attachment, bothkinds of samples were inserted to a plasma cleaner, PDC-32G,Harrick plasma, and were exposed twice to plasma for 0.5 minunder vacuum of 2 � 10�3 mbar at high RF (radio frequencies) andpower of 18W.

3. Results and discussion

This study describes the construction of transparent, conductivefilm coatings upon polystyrene (PS) substrates exhibiting variedthree-dimensional configurations. The thrust of the syntheticapproach is a self-assembly process in which nano-structured Aufilms spontaneously form through anchoring of water-solubleAu(SCN)4� onto amine moieties displayed upon the target surface[9]. Fig. 1 outlines the experimental scheme. We examined two PSconfigurations as substrates for gold coating: commercially-available “multi-well plates” (Fig. 1A), and long PS wires pro-duced through electrospinning [23] (Fig. 1B). In both cases, the PSsubstrates were amine-functionalized through oxidation of the PSsurface to produce carboxylic moieties followed by grafting ofbranched polyethylenimine (b-PEI). [24] The amine-displaying PSsurfaces were then immersed in an aqueous Au(SCN)4� solutionresulting in deposition of a thin Au layer upon the PS substrates.After drying and brief plasma treatment [25] the Au films exhibitedtransparency and electrical conductivity.

w the plates employed: images on the left e before Au deposition, right image: after Auage is 5 mm; scale bars in the two magnified images correspond to 1 mm. B) Au coatingfibers using toluene. Scale bar in left SEM corresponds to 10 mm. (For interpretation ofis article.)

A. Trachtenberg et al. / Polymer 55 (2014) 5095e51015098

Figs. 2 and 3 present microscopic and spectroscopic analyses ofthe Au films deposited upon the PS templates. Fig. 2 depicts a smallregion of the PS well-plate after gold deposition (Fig. 2AeB), andgold-coated PS fibers (Fig. 2C). As apparent in the scanning electronmicroscopy (SEM) image in Fig. 2A, the Au-coated PS plate displays adense gold layer. Homogeneous Au coatingwas observed both upontheplanararea of theplate (Fig. 2A, topmagnifieddomain), aswell ason the surface of the protruding well-barriers (the curved feature inFig. 2A andmagnified area in the bottom; note that the lack of focusin parts of the SEM image in the bottom magnification of Fig. 2Areflects the non-planar topography of the Au-coated PS surface). Theatomic force microscopy (AFM) analysis in Fig. 2B, depicting the“edge” of a Au layer deposited upon PS, indicates that the Au film isfairly uniform, exhibiting an average thickness of around 65 nm.

Fig. 2C presents SEM images of PS fibers grown through electro-spinning after amine functionalization and incubation with

Fig. 2. Au-coated PS templates. A) SEM images of a small region containing part of the cir100 mm. Top magnification: flat region at the side of the well-barrier showing the dense Ashowing complete Au coverage. Scale bars in the two magnified images correspond to 1 mm.uniform nature of the Au layer. Scale bar (blue color) corresponds to 1 mm. C) SEM imagesresulting in formation of hollow Au tubes (right). Scale bars in top images correspond to 10color in this figure legend, the reader is referred to the web version of this article.)

Au(SCN)4� (The electrospinning apparatus is schematically depictedin Fig. 1,SI). Similar to the gold-treated PS plates (shown in Fig. 2A),the PS fibers appear encased by a dense and uniform Au layer.Importantly, the chemical reactions of the PS template fibers(amine functionalization and gold deposition) did not adverselyaffect the fibers' morphology, lengths, and dispersion (Fig. 2C).Particularly remarkable are the results obtained after treating theAu-coated PS fibers with toluene, an organic solvent known todissolve PS (Fig. 2C) [26]. Indeed, incubating the Au/PS fibers intoluene for 2 h generated hollow tubes, comprising only the goldcoating after dissolution of the inner PS tubing. This was confirmedby Fourier transform infrared (FTIR) spectroscopy which demon-strated disappearance of FTIR peaks ascribed to PS following tolu-ent treatment (Fig. 2,SI).

Spectroscopic characterization of the Au-coated PS samples inFig. 3 illuminates the compositional and crystalline properties of

cular barrier within a multi-well plate after gold deposition, scale bar corresponds tou film; bottom magnification: surface of the protruding barrier of the individual wellB) AFM image (top) and an average cross-section (bottom) pointing to the thickness andof Au-coated electrospun PS fibers (left), and the fibers after PS dissolution by toluene,mm, magnified areas (bottom images) e 1 mm. (For interpretation of the references to

Fig. 3. Spectroscopic characterization. A) XPS spectra of the Au-coated PS substrate before (bottom spectrum) and after (top spectrum) plasma treatment. B) Powder XRD spectra ofthe Au-coated PS substrate before (bottom spectrum) and after (top spectrum) plasma treatment.

Fig. 4. Physical properties of Au-coated PS multi-well plate. A) Optical transmittance ofuntreated PS plate (broken curve) and optical transmittance of PS plate after golddeposition and plasma treatment (solid curve). B) Electrical conductance. The picturein the left depicts the array of square-shaped electrodes (white spots) evaporated uponthe plate surface; the arrows indicate the electrodes between which voltage wasapplied and electrical current was measured (scale bar corresponds to 1 mm). Rightgraph: I/V curve corresponding to the electrode-spacing indicated by the arrows (notethat the electrode spacing encompasses two well barriers).

A. Trachtenberg et al. / Polymer 55 (2014) 5095e5101 5099

the Au films, and provides insight into the gold deposition process(the results presented in Fig. 3 were recorded for the Au-coated PSfibers; similar spectroscopic data were obtained in case of the Au-coated PS plates, shown in Fig. 3,SI). Fig. 3A depicts the x-rayphotoelectron spectroscopy (XPS) data acquired for the as-synthesized Au-coated PS fibers (Fig. 3A, bottom spectrum), andafter subsequent plasma treatment (Fig. 3A, top). Notably, the XPSspectrum recorded right after incubation of the amine-displayingPS with Au(SCN)4� reveals that AuI is the main constituent withinthe gold layer (giving rise to the Au 4f7/2 and 4f5/2 peaks shown inblue (in the web version) at 85.9 eV and 89.6 eV, respectively,Fig. 3A e bottom spectrum).

The presence of AuI as the predominant gold species is indica-tive of partial reduction of AuIII within the complex, and stabiliza-tion of the amine-anchored AuI

film through aurophilic interactions[9]. Formation of surface-attached AuI after incubation of amine-displaying surfaces with Au(SCN)4� was indeed previously re-ported [9]. Following plasma treatment of the Au-coated PS,virtually all the gold is converted into Au0, reflected in the XPSpeaks at 83.9 eV and 87.6 eV (Fig. 3A, top spectrum). Plasmatreatment of Au films is known to induce efficient reduction of themetal [25].

Powder x-ray diffraction (XRD) data in Fig. 3B corroborate theXPS analysis and provide further information upon the crystallineproperties of the Au films. Following incubation of the amine-functionalized PS fibers with Au(SCN)4�, the XRD pattern inFig. 3B, bottom, shows small peaks corresponding to crystallinegold, as well as three peaks corresponding to 5.12 Å, 3.0 Å and 2.6 Å,respectively, reflecting the prominence of the aurophilic organiza-tion of AuI [27,28]. Aurophilic interactions between closed-shell AuI

atoms have been discerned in varied supramolecular systemscomprising gold complexes, giving rise to diverse morphologies,including nanowires [29,30], and sheet-like structures [31,32].

Consistent with the XPS data in Fig. 3A, plasma treatment of theAu-coated PS transformed the gold layer into fully crystalline Au0

assembly (Fig. 3B, top spectrum). Indeed, the XRD pattern in Fig. 3B(top) clearly displays the (111) and (200) lattice planes of crystallinegold [33,34]. Solid substrates have been previously shown to inti-mately participate in shaping growth of crystalline gold films,usually through surface patterns attracting Au colloids [35,36]. Inthe system presented here, binding of the Au thiocyanate complexto the amine residues provides the spatial framework for long-range crystalline organization through aurophilic interactions be-tween the AuI centres [9].

One of the major goals of this work is to demonstrate possiblepractical applications of the Au-coating technology. Figs. 4 and 5examine functionalities of the Au/PS assemblies, particularly opti-cal transparency and electrical conductivity properties recorded for

both the multi-well plate and the fiber constructs. Fig. 4 presentslight transmittance and conductivity data acquired after golddeposition and plasma treatment of the multi-well PS plate. Fig. 4Aunderscores the transparency of the Au/PS material. Around 60%optical transmittance was retained both in the visible spectral re-gion (above 450 nm) and well into the IR range (up to around1650 nm) e indicating that the thin Au layer hasn't stronglyaffected the intrinsic transparency of PS (depicted as the brokencurve in Fig. 4A).

Fig. 4B demonstrates an excellent electric conductivity achievedin the Au-coated PS plate. The experimental setup is depicted in theimage in Fig. 4B. Specifically, an array of gold electrodes at a

Fig. 5. Au-coated PS fibers. A) Photographs of the Au-coated PS fibers employed for electrical current measurements. Bottom picture: a bundle of fibers placed between two silver-paint electrodes, scale bar corresponds to 200 mm; Top picture: magnified area within the bundle showing the individual fibers. Scale bar corresponds to 20 mm. B) I/V curve recordedfor the setup shown in A.

A. Trachtenberg et al. / Polymer 55 (2014) 5095e51015100

predefined spacing of 1 mmwas evaporated onto the Au-coated PSsurface through a stainless steel shadow mask attached to the Au-coated PS plate. Electrical current was subsequently measured atspecific distances through placing electrical contacts and applyingvoltage between selected evaporated electrodes.

The current/voltage (I/V) curve in Fig. 4B was recorded at thedistance indicated by the arrows (around 10 mm). Significantly, thelinear appearance of the I/V curves and the high current valuesrecorded (corresponding to sheet resistance of approximately1.0 U/square) attest to excellent electron transport at the inter-electrode spacing examined. Particularly striking is the fact thatthe pronounced conductivity was measured even across the twoprotruding barriers enclosing the well depressions within the platee confirming that effective Au film deposition was accomplishedupon the entire three-dimensional PS substrate (themulti-well plate).

To further examine potential applicability of the Au-coated/PStechnology, we calculated the Figure of Merit (FoM) e a widelyemployed parameter for assessing the performance of transparentconductive electrodes (TCEs) or transparent conductive films (TCFs)[37]. FoM is expressed as the ratio between DC conductivity andoptical conductivity (sDC/sOP), and is commonly used for filmthicknesses of less than the wavelength of light [38] (which is alsothe case here). A FoM greater than 35 is considered sufficient forindustrial applications [39]. However, for many TCEs exhibitingsheet resistance that is smaller than 10 U/square coupled with atransmittance of 85% the FoM is greater than 220.We calculated theFoM for the Au/PS system depicted in the manuscript (sheetresistance of 1 U/square and transmittance of 60% in the visibleregion) using the equation [38]:

T ¼�1þ Z0

2RssOP

sDC

��2

in which Z0, T, and Rs correspond to the impedance of free space(377 U), the transmittance, and sheet resistance, respectively. Thecalculated FoM value was around 650 confirming that the Au-coating technology might be successfully implemented for fabri-cating TCEs and electro-optical devices.

Electrical conductivity was also evaluated in case of the Au-coated PS fibers (Fig. 5). As depicted in Fig. 5A, a bundle of Au-coated fibers was placed between two electrodes using silver-paint contacts (black circularly-shaped areas on both sides of thebundle, Fig. 5A). The linear I/V curve recorded between the twoelectrodes and corresponding current values (Fig. 5B) confirm thatexcellent electron transport occurred through the Au-coated PSfibers. This result, similar to the conductivity measurements for theplate sample depicted in Fig. 4, is ascribed to the dense, continuousAu coating.

4. Conclusions

We present a new approach for creating transparent conductivepolymers in different shapes and morphologies through surfacecoating with a nano-structured Au layer. The process was demon-strated for polystyrene (PS), a widely-used transparent polymer. Thetechnique is based on amine functionalization of the PS surface andincubation of the amine-modified PS with Au(SCN)4� in water. Thestrategy depicted here exhibits notable advantages in comparisonwith currently-applied metal deposition technologies. Specifically,the Au coating technique can be directly implemented on differentpolymer morphologies (i.e. surfaces with “steps” and three-dimensional protrusions, fiber-like structures), and does not requirespecific templates. Importantly, the gold coating demonstrated hereinvolves single-step incubation of the target PS surfacewith the goldcomplex, without co-addition of reducing agents (or other reagents).

The Au-coated PS substrates were subjected to plasma treatmentand displayed attractive properties, particularly optical transparencyand excellent electrical conductivity. Importantly, the conductivityprofiles attest to efficient electron transport even across surface“barriers” (e.g. the steep edges of the wells), indicating that contin-uous Au coatingof the PS substrateswas attained even in non-planarconfigurations. Good conductivity was also recorded for the Au-PSfibers. Furthermore, immersing the Au-coated fibers in toluene,which dissolves the PSfiber template, gave rise to remarkable hollowgold tubes. In conclusion, this work demonstrates a facile, versatilegold coating technology, employed for fabricating PS substratesencased with transparent and conductive gold layers. The technol-ogy can be easily expanded to include other types of polymers, sincethe only chemical requirement is amine surface display, which canbe easily carried out through varied synthetic routes.

Acknowledgments

We are grateful to the Ministry of Trade, the Kamin Grant Pro-gram, for partial financial support. We thank Ms. Roxana Golan forAFM analysis, Mr. Jurgen Jopp for electrical current measurementsof gold coated PS bundle sample, Dr. Dimitry Mogiliansky for XRDmeasurements, Dr. Natalya Froumin for XPS measurements, Dr.Sofiya Kolusheva for ATR-FTIR measurements, and Mr. MishaVolokh for transmittance measurements. We also thankMs. SharonFleischer and Dr. Assaf Shapira from Dr. Tal Dvir lab (TAU, Israel) forintroducing us to the electrospinning technique.

Appendix A. Supplementary data

Supplementary data related to this article can be found online athttp://dx.doi.org/10.1016/j.polymer.2014.08.026.

A. Trachtenberg et al. / Polymer 55 (2014) 5095e5101 5101

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