Si

JSC

a

ARR1A

KNPGLA

1

imantcdaptmtwtmmtdt

0d

Materials Science and Engineering B 157 (2009) 87–92

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

elf-assembly of conducting polymer nanowires at air–water interface andts application for gas sensors

ianhua Xu, Yadong Jiang, Yajie Yang ∗, Junsheng Yutate Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC),hengdu 610054, PR China

r t i c l e i n f o

rticle history:eceived 13 June 2008eceived in revised form3 November 2008ccepted 19 December 2008

eywords:

a b s t r a c t

Conducting polymer poly (3, 4-ethylenedioxythiophene) (PEDOT) nanowires were prepared by wet-ting Al2O3 membrane (AAO) template method, which could be well dispersed in organic solvent withultrasonic treatment. In order to obtain highly ordered structure of nanowires, the self-assembly filmof nanowires at air/water interface was investigated by Langmuir–Blodgett (LB) technique. The resultsshowed that PEDOT nanowire-surfactant complex at air/water interface had well self-assembly capa-bility, and the stable float layer was formed with collapse pressure more than 50 mN/m. This well

anowiresEDOTas sensitivityB techniqueir/water interface

arranged nanowire film was transferred onto interdigitated electrode successfully as novel gas sens-ing layer through a vertical dipping method. The as-prepared PEDOT nanowire gas sensor was appliedto the precise detection of NH3 and HCl gas, especially for low gas concentration (lower than 5 ppm),and showed higher gas sensitivity than conventional nanowire gas sensor. The chemical sensors basedon ordered PEDOT nanowires presented good reversibility and reproducibility in response. Notably, ourwork presents an appropriate methodology for fabricating ordered conducting polymer nanomaterial for

licatio

gas sensor and other app

. Introduction

Conducting polymer nanostructure has been a subject of grow-ng interest in recent years for their promising application in

icroelectronics, sensor, solar cell, etc. [1–4]. As for sensingpplication, 1D conducting polymer nanostructures, includinganowires, nanofibers, and nanotubes have a larger surface areahan their conventional bulk counterparts. Therefore, they have theapability of offering amplified sensitivity and real-time responseue to the enhanced interaction between conducting polymer andnalyte [5–8]. To date, some works have been carried out on thereparation of 1D conducting polymer nanostructure and the inves-igation of gas sensitivity of these nanostructures. Those preparing

ethods include hard (soft) template method and different otheremplates, such as porous alumina membrane and reversed micelle,ere used to prepare different 1D conducting polymer struc-

ures [9–12]. Conducting polymer nanowires have been fabricatedainly using hard template such as alumina/polymer membranes,

esoporous silicas and organic/organic nanofibers. The use of hard

emplates is of advantage in tailoring the diameter and length ofesirable nanomaterials as their dimension is defined by the usedemplate [13]. However, such approaches have produced 1D nano-

∗ Corresponding author. Tel.: +86 28 83207157; fax: +86 28 83206123.E-mail address: jj eagle@163.com (Y. Yang).

921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2008.12.015

ns.© 2008 Elsevier B.V. All rights reserved.

materials in non-ordered configuration and these nanomaterialsshow disadvantage in terms of obtained high performance gas sens-ing materials and gas sensor, mainly due to drawback such as thenon-ordered structure of sensing layer for adsorption and des-orption of gas analyte. To overcome these problems, a novel andeffective route to fabricated ordered structure materials as sensinglayer of gas sensor is required.

As for amphiphilic molecules that can be floated on the surfaceof water, Langmuir–Blodgett (LB) technique has been demonstratedlargely successful in preparing their monolayers. It has been usedextensively in the preparation of monolayer for molecular electron-ics, and more recently to create nanocrystalline monolayer withtunable properties. Furthermore, LB technique has been provedequally powerful for the assembly of nanowires with larger aspectratios [14,15]. In this case, suspension of nanowires (mixed withsurfactants) is dispersed on the water surface of a LB trough, theinteraction between surfactants and the nanowires causing thenanowires to float on the water surface. Then the floating nanowiresare compressed to higher density on the surface, with computer-controlled trough barriers mimicking the banks of logging river.Many nanowires reorient themselves and align paralleling to the

trough barrier, finally forming a closely packed monolayer [16].There are several important features of such assemblies. First, thepitch of the nanowires pattern can be controlled at nanometric ormicrometric scales through compression process, which is criti-cal for building ultra-high-density microelectronics. Second, it is

8 nd En

pacfptaqobts

asoaeasrtPwoad

2

2

Aco4n

2

piwapqttow

rn(i

2

dopsr

8 J. Xu et al. / Materials Science a

ossible to transfer monolayers layer by layer, to form parallelingnd crossed-nanowire structure that could serve as optoelectronicomponents [17,18]. Well arranged monolayer can then be trans-erred onto any substrate or devices. This ordered monolayer showsromising application for sensing material due to its well orienta-ion structure for well adsorption and desorption for gas analyte,nd then high gas sensitivity of sensors can be obtained conse-uently. There have been several reports on the preparation ofrdered nanowire structure via LB technique. However, there haseen a lack of preparing ordered conducting polymer nanowireshrough LB technique, and few reports have been made on the gasensitivity investigation of this ordered nanowire structure.

As a promising conducting polymer, PEDOT is suitable for sensorpplication due to its high conductivity, environmental stability andensitivity for the detection of ammonia, nitrogen dioxide and otherrganic toxic vapors [19–22]. Some research works have also beenttempted to prepare gas sensitive 1D PEDOT nanomaterials. How-ver, there is no report about the self-assembly of PEDOT nanowirest the air/water interface by LB technique and the study of their gasensitivity. Therefore, the aim of this work is to investigate the fab-ication of ordered nanowire structure through LB technique andheir application for gas sensor. Herein, the preparation of dopedEDOT nanowires which can be well dispersed in organic solventith ultrasonic treatment was carried out. The self-assembly ability

f nanowires at air–water interface was systematically investigated,nd the application of PEDOT nanowires as chemical sensors foretecting NH3 and HCl vapors was also studied.

. Experimental

.1. Materials

3,4-Ethylene dioxythiophene (EDOT) was purchased from BayerG. Toluene-p-sulfonic acid (Tpsa), FeCl3, stearic acid (SA) and otherhemical reagents were purchased from Aldrich and used with-ut further purification. Al2O3 membrane (AAO, Whatman Anodisc7, pore size 300 nm, thickness 60 �m) was used as template foranowires preparation.

.2. Fabrication and characterization of PEDOT nanowires

PEDOT nanowires were prepared in membrane pore. Prior to theolymerization, AAO template was firstly ultrasonicated in deion-

zed water and methanol. Then the AAO membrane was filledith aqueous oxidizing/doping (FeCl3/Tpsa) solution under neg-

tive pressure, under which the solution can be injected into AAOores by pressure, and then the oxidizer-filled AAO membrane wasuickly transferred to an EDOT ambience to initiate the polymeriza-ion. After polymerization, PEDOT nanowires were separated fromhe AAO membrane by dissolving AAO into a 25 vol.% NaOH aque-us solution and washing the residue with excess NaOH, deionizedater, methanol three times and centrifuged sequentially.

For nanowires characterization, UV–vis absorption spectra wereecorded with a UV-1700 spectrometer. Surface morphology of theanowires was investigated using scanning electron microscopySEM) model S-2400 from Hitachi. FT-IR spectrum was character-zed with a WGH-30 analysis instrument.

.3. Preparation of gas sensor based on PEDOT nanowires

Self-assembly ability of nanowires at air/water interface and the

eposition of nanowires Langmuir film on gas sensor were carriedut in KSV-5000 LB system (Finland). The subphase solution wasrepared using ultrapure water (resistance >18 M�, pH 5.1). For thepreading solution, 0.5 mg/ml nanowire-surfactant (SA) (7:1)/chlo-oform solution was first pretreated in an ultrasonic bath for 15 min,

gineering B 157 (2009) 87–92

and then carefully spread on an aqueous subphase. After the thor-ough evaporation of the solvent for 20 min, the floating layer onthe subphase was compressed at a rate of 1 mm/min and the �–Aisotherm was recorded simultaneously. After an appropriate sur-face pressure was obtained, the monolayer was compressed for30 min under an oscillation mode with a speed of 0.5 mm/min.Then it was easy to transfer nanowire Langmuir film onto thesubstrate or devices using vertical dipping mode. The nanowiresfloat at interface was transferred on hydrophilic-treating interdig-ital electrode (30 �m), and the nanowires was build easily acrossthe electrode because of large area of nanowire film at the air/waterinterface, and a gas sensor covered with nanowire LB film was fab-ricated.

For conventional PEDOT nanowire gas sensor preparation,1.5 wt% nanowires were dissolved into ethanol and pretreated inan ultrasonic bath for 15 min, and then 0.5 ml of this solution wasdropped onto interdigital electrode to prepare sensitive layer.

2.4. Gas sensitive characterization of PEDOT nanowires

The gas sensitivity measurement was carried out in a home-made chamber with a volume of 2.5 L, and NH3 (HCl) gas withvarious concentrations was continuously introduced into the air-tight cell. Prior to the measurements, the interdigitated electrodeswere blown with a flow of dry N2, after which the test vapor (NH3 orHCl gas) was injected into the gas chamber. The resistance change ofthe PEDOT nanowires was monitored with a computer-controlledsource meter (Keithley 2400). All the measurements were per-formed at ambient temperature.

3. Results and discussion

Fig. 1 shows the image of scanning electron microscopy(SEM), which clearly indicates the successful formation of PEDOTnanowires with an average diameter of ca. 350 nm and a lengthover 10 �m. As shown in the inset of Fig. 1, the fluctuated sur-face structure of nanowires indicates the figuration of obtainednanowires tuned by the AAO template. It also has been found thatthe diameter of nanowires was larger than the size of the AAO pore(300 nm), which may be ascribed to the swelling effect of poly-mer nanowires after removing the AAO template. The obtainedPEDOT nanowires can be dispersed very well in organic solvent,for example chloroform etc, under ultrasonic treatment. Interest-ingly, we found that the dispersion ability of PEDOT nanowirescan be improved dramatically by adding toluene-p-sulfonic acidinto oxidizer solution prior to polymerization, and the obtainedPEDOT nanowires can be dispersed more easily in organic solution.It is unclear for this phenomenon and we postulate this may bedue to the effective doping of toluene-p-sulfonic acid into PEDOTnanowires, which changed the charged state of nanowires surface,and adjacent PEDOT nanowires dispel each other resulting in welldispersion of nanowires in solution.

Ultraviolet–visible–near IR (UV–vis–NIR) spectra indicate anabsorption peak at 880 nm for the nanowire film as shown inFig. 2(a), corresponding to the polarons and bipolarons adsorp-tion in PEDOT, which indicates PEDOT nanowires have been dopedduring the polymerization process. In our study, FeCl3 and toluene-p-sulfonic acid were simultaneously injected into AAO templatepores, followed by the initiation polymerization of EDOT monomerinto this oxidizing/doping system. Therefore, the obtained PEDOT

nanowires may be doped by Cl− and toluene-p-sulfonic acidcounter-ions simultaneously. FT-IR spectra (shown in Fig. 2(b)) alsoconfirmed the formation of PEDOT nanowires as evidenced withthe presence of the peaks at 1507 and 1322 cm−1 (C C or C Cstretching of thiophene ring), 1189, 1073, and 1044 cm−1 (C O C

J. Xu et al. / Materials Science and Engineering B 157 (2009) 87–92 89

Fig. 1. SEM image of (a) undispersed PEDOT nanowires prepared by AAO template method, the inset is the single nanowires and (b) single nanowires with higher resolution.

and

b6

tsntLicspaa

Fcm

Fig. 2. (a) UV–vis adsorption spectrum

ond stretching in ethylene oxide group), and 1640, 983, 825, and88 cm−1 (C S bond stretching in the thiophene ring).

To obtain highly ordered arrangement PEDOT nanowires struc-ure, the self-assembly ability of pure nanowires and nanowire-urfactant complex at air/water interface were investigated. Theanowire/chloroform and nanowire-surfactant/chloroform solu-ion were simultaneously spread at air/water interface of doubleB trough, and the surface pressure–mean molecule area (�–A)sotherm curve were recorded (as shown in Fig. 3). From Fig. 3 wean see that the pure PEDOT nanowires exhibited relatively low

elf-assembly ability at air/water interface. With continuous com-ressing, pure nanowires float layer exhibits higher compressibility,nd the surface pressure increases slowly with further compressing,nd the film collapsed at comparatively low pressure (<32 mN/m).

ig. 3. �–A isotherm curve of PEDOT nanowires (dashed) and PEDOT-surfactantomplex at air/water interface, and a schematic illustration of nanowires arrange-ent at interface during compressing process.

(b) FT-IR spectra of PEDOT nanowires.

For the ‘soft’ PEDOT nanowires, the nanowires at interface wouldinteract or overlap each other more easily at higher surface pres-sure, and early collapse of single nanowires layer would occur. Asfor surfactant-nanowire floating layer, the film shows low com-pressibility and a fast increase of surface pressure occurred withcontinuous compressing, and the float layer changes to solid statemore quickly. This complex nanowires film collapsed at relativelyhigh pressure (>50 mN/m), which indicates that the stable LB filmis formed at the air/water interface with the assistance of sur-factant. Due to the mixing of PEDOT nanowires and surfactant,nanowires show high performance as a LB film material. The inter-action between surfactants and nanowires causes the nanowiresto float stably on water surface until the floating nanowires arecompressed to higher density on surface. Nanowires reorient them-selves and align paralleling to LB trough barrier, finally forminga closely packed monolayer (as shown in Fig. 3). This compress-ing process could be continued to a surface pressure more than50 mN/m. When the film collapsed, some wire-like longitudinallyoriented long folds could be observed, indicating the formation ofmultilayer film. It was also found that, during the expansion pro-cess, some rigid blue-black patches could be observed at air/waterinterface, which exhibits strong attractive interaction between thePEDOT nanowires.

The surfactant-nanowire LB film could be transferred verticallyonto a hydrophilic surface of solid substrates such as interdigi-tal electrode, and then a conducting polymer nanowires coveredgas sensor could be fabricated. Fig. 4 shows SEM image of singlelayer PEDOT nanowires deposited on interdigital electrode, andthe deposition surface pressure was at a relatively high value of40 mN/m. It indicates that a compact and ordered arrangement

nanowire structure was constructed across the electrode and dif-ferent arrangement of nanowires can be achieved by adjustingself-assembly process. Hence, it is possible to obtain ultrahighdensity nanowires array at higher surface pressure, which showspromising application on organic optoelectronic and electronic

90 J. Xu et al. / Materials Science and Engineering B 157 (2009) 87–92

Fig. 4. A schematic illustration of LB nanowires film covered interdigitated electrode and SEM image of ordered nanowires building across the electrode.

rdigit

dnhn

pntadfenwac

wtna

crrr

may result in more negatively charged counter-ions incorporatedinto the polymer to compensate the positive of polarons or bipo-larons, and leading to the conductive enhancement. It can beseen that the response and recovery time of LB film covered gas

Fig. 5. A schematic illustration of conventional nanowires covered inte

evices. For conventional PEDOT nanowires shown in Fig. 5,anowires entangle each other and this non-ordered structureampers gas molecules from fast diffusing in and out from PEDOTanowires.

As conjugated conductive polymer material, PEDOT exhibitsotential application for gas sensor and the nanometer size PEDOTanowires will show more promising as gas sensing material due toheir larger surface-to-volume ratio. Furthermore, this single layernd ordered nanowires film may exhibit excellent sensitivity for gasetection due to its well orientation structure, which could be easyor gas analyte to diffuse in and out from sensing film. In order tovaluate the sensor performance of single layer and ordered PEDOTanowires film, the film was deposited on interdigitated electrodesith 30 �m interval and tested in a gas chamber. The real-time

nd initial resistance was recorded, and the normalized resistancehange could be described as Eq. (1)

�R

RI= (R − RI)

RI(1)

here R and RI are the real-time resistance and initial resis-ance, respectively. For comparison, gas sensitivity of conventionalanowires film fabricated by conventional dropping method waslso studied.

Fig. 6 displays gas sensitivity of single layer nanowires LB filmovered sensor to 20 ppm HCl gas (defined as the normalizedesistance change measured after a 20 s vapor exposure, and theesponse and recovery times are respectively defined as the timesequired for sensor signals to reach 90% of saturation and original

ated electrode and SEM image of nanowires building on the substrate.

values). The dashed line is the sensitive characteristics of con-ventional PEDOT nanowire gas sensor. After the exposure of twokinds of PEDOT film to HCl gas, the enhancement of conductiv-ity of both films was observed. The exposure to HCl atmosphere

Fig. 6. Response of PEDOT nanowires exposed to HCl vapor (20 ppm).

J. Xu et al. / Materials Science and Engineering B 157 (2009) 87–92 91

snowwtLrCiambtaninrvwtoortsfisg

Fig. 7. Response of PEDOT nanowires exposed to NH3 vapor (20 ppm).

ensor is ca. 10 s, which is faster than conventional PEDOTanowires deposited sensor at ca. 25 s. The larger frequency shiftf ordered nanowire film covered gas sensor was observed in Fig. 6,hich means the nanowires LB film exhibited higher sensitivity. It isell known that nanowires can offer larger surface-to-volume ratio

o benefit for the optimum sensitivity of gas sensor. As a result, theB nanowire film should show high performance as sensing mate-ial due to the larger surface-to-volume ratio and ordered structure.ompared to conventional PEDOT nanowire film, the main mer-

ts of nanowire LB film were obtained from self-assembly processt air/water interface, and the adjacent PEDOT nanowires can beanipulated to avoid snarling, and the well ordered structure can

e obtained through dynamic self-assembly process. It also needso be mentioned that effective sensing layer can be constructed wellcross the electrode of gas sensor at high surface pressure, and everyanowire may connect well to others to obtain effective conduct-

ng channel through mutual connection. So, the response of everyanowire to analyte gas can be detected and collected, which mayesult in a sensitivity enhancement of gas sensor. However, by a con-entional dropping method, isolated nanowires or nanowire clusteras formed between electrode and this structure has no contribu-

ion to the response of analyte gas because of a non-connection tother nanowires. Then the falling of sensitivity of gas sensor mayccur especially at low gas concentration due to the reduction inesponse signal of nanowires to analyte gas. It has been found thathe deposition pressure had some influence on the gas sensitivity of

ensors. The optimized pressure for the fabrication of nanowire LBlm is 25–35 mN/m, and the ordered nanowires covered gas sen-ors exhibited ca. 10 s of response and recovery time to 20 ppm HClas.

Fig. 9. Sensitivity change of PEDOT nanowires LB film as a f

Fig. 8. Response of PEDOT nanowires exposed to HCl vapor (5 ppm).

Fig. 7 shows gas sensitivity of nanowires LB film covered sensorand conventional PEDOT nanowires to 20 ppm NH3 gas. A reductionin conductivity of both films was observed obviously. The introduc-tion of NH3 into PEDOT nanowires led to the formation of a neutralpolymer and a decrease in charge carriers, which induced a decreasein conductivity. Kim et al. also investigated gas response of seriesPEDOT derivate LB films and it has been found that these derivateLB films showed sensitive to NH3 gas [23]. From Fig. 7 we can seethat the nanowires LB film also had higher sensitivity than con-ventional one and Kim’s films, which directly arises from the orderarrangement of nanowires.

Fig. 8 shows gas sensitivity of two kind of nanowires film to5 ppm HCl gas. At this lower concentration, it has been found thatthe nanowires LB film shows excellent gas sensitivity than conven-tional one. The response and recovery time of nanowires LB filmbecame longer at lower gas concentration, as it needs longer timefor gas molecules to adsorb on the nanowires surface and diffuseout from the nanowires film. The PEDOT nanowires LB film obtainedsaturation adsorption state and recovered to original state quicklydue to its well ordered structure, which satisfies optimum condi-tion for gas molecules to fast diffuse in and out from the sensingfilm. The results also indicate that the PEDOT nanowires LB film ismore suitable for detecting low concentration HCl gas and high gassensor performance can be achieved.

The single layer PEDOT LB nanowires response as a function ofdifferent HCl and NH3 gas concentrations is shown in Fig. 9. Forboth analyte gases, it can be seen that the frequency shift increases

with the increasing gas concentration. This indicates that moreanalyte molecules adsorbed on the nanowires surface at higherconcentration. A nonlinear response of PEDOT nanowires LB filmwith the increasing gas concentration was also confirmed, which

unction of (a) HCl gas and (b) NH3 gas concentration.

92 J. Xu et al. / Materials Science and Engineering B 157 (2009) 87–92

e to 20

ig

fiHc2wfloTase2cs

4

nnnPoscTrvp

[

Fig. 10. Responses of PEDOT nanowires LB film upon cyclic exposur

ndicates high sensitivity of nanowires LB film to HCl and NH3ases.

To further investigate the applicability of PEDOT nanowire LBlm sensors, nanowire LB film was exposed periodically uponCl/N2 and NH3/N2, respectively. Fig. 10(a) represents the electri-al response of PEDOT nanowire LB film upon cyclic exposure to0 ppm HCl gas. The resistance of nanowires dramatically decreasedhen exposed to HCl vapor. After the HCl flow was replaced by a N2ow, the resistance slowly recovered to the original level due to des-rption of HCl molecules from the ordered arrangement nanowires.he ordered nanowires allow the fast diffusion of HCl molecules innd out of the materials. Consequently, the nanowire LB film sen-ors showed good reversible and reproducible response with thevidence that the sensor almost recovered to initial frequency after0 test cycles. The PEDOT nanowire LB film also showed identicalharacterization to periodic exposure upon NH3 and the result ishown in Fig. 10(b).

. Conclusions

A facile and effective route to prepare ordered 1D PEDOTanowires was explored on the basis of LB technique. Stableanowire LB film can be obtained through mixing spreadinganowires with surfactant at air/water interface. Well arrangedEDOT nanowires at air/water interface could be transferred easilynto substrate as gas sensing material. These single layer nanowireshowed high sensitivity to HCl and NH3 gases and were espe-

ially suitable for gas detection at lower concentration (<5 ppm).he PEDOT nanowire LB film sensor showed good reversibility andeproducibility to analyte gas resulting from the larger surface-to-olume ratio and highly ordered arrangement of nanowires. Weerformed an effective attempt to fabricate conducting polymeric

[[[[

ppm analyte gas and N2 at room temperature (a) HCl and (b) NH3.

nanomaterial in large scale which shows highly potential applica-tion for sensors and organic electronics.

Acknowledgements

This work is partially supported by the Science and EngineeringFoundation of China through the Key Laboratory of Novel Transduc-ers, National Science Foundation of China via grant no. 60771044,and 863 Foundation of China via grant no. 2007AA03Z424.

References

[1] A. Basudam, Prog. Polym. Sci. 29 (2004) 699–766.[2] M.K. Ram, Sens. Actuators B 106 (2005) 750–757.[3] U. Sree, Y. Yamanoto, B. Deore, Synth. Met. 131 (2002) 161–165.[4] S.A. Wagjuley, S.M. Yenorkar, S.S. Yawale, S.P. Yawale, Sens. Actuators B 128

(2008) 366–373.[5] D.H. Zhang, Y.Y. Wang, Mater. Sci. Eng. B 134 (2006) 9–19.[6] J. Janata, M. Josowicz, Nat. Mater. 2 (2003) 19–24.[7] J. Huang, S. Viriji, B.H. Weiller, R.B. Kaner, Chem. Eur. J. 10 (2004) 1314–1319.[8] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, J. Am. Chem. Soc. 125 (2003) 314–315.[9] A.G. MacDiarmid, Synth. Met. 125 (2001) 11–22.

[10] J.N. Barisci, C. Conn, G.G. Wallace, Trends Polym. Sci. 4 (1996) 307–311.[11] J. Huang, R.B. Kaner, J. Am. Chem. Soc. 126 (2004) 851–855.12] R. Xiao, S. Cho, R. Liu, S.B. Lee, J. Am. Chem. Soc. 129 (2007) 4483–4489.

[13] G.H. Moon, S.H. Foulger, Chem. Commun. 1 (2005) 1–4.[14] A. Ulman, An Introduction To Ultrathin Organic Films: From Langmuir-Blodgett

To Self-Assembly, Academic, San Diego, 1991.[15] L. Caseli, F.N. Crespilho, T.M. Nobre, Colloids Surf. A 317 (2008) 618–624.[16] P.D. Yang, Nature 425 (2003) 243–244.[17] J. Cabaj, K. Idzik, J. Soloducho, A. Chyla, Thin Solid Films 516 (2008) 1171–1174.[18] C.W. Chen, J.H. Yeh, T.J. Liu, Thin Solid Films 515 (2007) 7299–7306.

[19] Y.J. Yang, Y.D. Jiang, J.H. Xu, J.S. Yu, Thin Solid Films 516 (2008) 2120–2124.20] J. Liu, M. Aganrwal, K. Varahramyan, Sens. Actuators B 129 (2008) 599–604.21] M.R. Abidian, D.H. Kim, D.C. Martin, Adv. Mater. 18 (2005) 405–409.22] J. Joo, S.K. Park, D.S. Seo, S.J. Lee, Adv. Funct. Mater. 15 (2005) 1465–1470.23] S.-R. Kim, S.-A. Choi, J.-D. Kim, K.J. Kim, C. Lee, S.B. Rhee, Synth. Met. 71 (1995)

2027–2033.