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Materials Science and Engineering B 168 (2010) 45–54

Contents lists available at ScienceDirect

Materials Science and Engineering B

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

ormation of polythiophene multilayers on solid surfaces byovalent molecular assembly

undaramurthy Jayaraman, Dharmarajan Rajarathnam, M.P. Srinivasan ∗

epartment of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

r t i c l e i n f o

rticle history:eceived 4 January 2010eceived in revised form 25 January 2010

a b s t r a c t

In this work, we demonstrate the deposition of multilayers of conducting polythiophene films on differentsubstrates with substrate-film and layer-to-layer links established by covalent bonding. The films were

ccepted 27 January 2010

eywords:ovalent molecular assemblyolythiophene

characterized by UV–Visible spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopyand ellipsometry. Thickness of the films and UV–Visible absorption intensity increased linearly withincrease in the number of layers. The covalently bonded nanostructure films possessed uniformity, goodthermal and chemical stabilities. Electrochemical impedance spectroscopy confirmed uniform coverageof the substrate by the films and improvement of the electrical characteristics of the films after doping.

form.

lectrochemical impedance spectroscopy The technique facilitatesphotovoltaic applications

. Introduction

Organic functionalization of semiconducting surfaces is emerg-ng as an important area in the development of new semiconductor-ased materials and devices. Among organic materials, �-onjugated polymers have attracted most attention for potentialpplications in electronic devices because of their advantagesn electrical and optical properties [1–3]. Polythiophenes are anmportant class of �-conjugated polymers with a wide range oflectronic and optical properties. Because of their robust thermalnd mechanical nature, they have found potential applications innumber of areas and devices such as light emitting diodes, solar

ells, memory devices, thin film transistors and chemical sensors4–9].

Molecular assembly techniques such as Langmuir–Blodgettssembly and electrostatic assembly have been employed to formelf-assembled monolayers (SAMs) of polythiophene films on var-ous substrates [10–14]. Though polythiophenes assembled bybove methods have found many applications, issues such as lackf film stability, orientation, conjugation and uniformity need to beddressed. The method of covalent molecular assembly providesn alternate approach where the monomers/polymers can be cova-

ently anchored to the substrates. Covalently bonded films are veryobust and they possess good chemical, mechanical and thermaltabilities [15]. Depending on the needs and application, the endroup and functionality of the monomers/polymers can tailored

∗ Corresponding author. Tel.: +65 65162171; fax: +65 67791936.E-mail address: chesmp@nus.edu.sg (M.P. Srinivasan).

921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2010.01.052

ation of robust, functional ultrathin films for use in chemical sensing and

© 2010 Elsevier B.V. All rights reserved.

and multilayer structures of covalently linked molecular layers canbe produced. Several researchers showed the effectiveness of thecovalent binding and addressed some of the issues listed above.Covalently bonded films have found many applications in sensors,tribiology, memory devices, light emitting diodes and bio-imaging[16].

Various research groups have adopted different approaches tocovalently bind the polythiophene films to silicon. Okawa et al.deposited a monomer thiophene derivative containing a thiol groupon a gold wafer and the molecules were adsorbed at the thiolgroup with end-on configuration [17]. Fabre et al. functionalizedsilicon surfaces with alkyl chains possessing thienyl units that weresubsequently polymerized electrochemically on the silicon surface[18]. Fikus et al. performed a two step procedure to covalentlybind polythiophene on silicon surface. In the first step, a thienylmolecule possessing trichlorosilane was deposited by sponta-neous self-assembly. Later, the covalently bonded thienyl moleculewas chemically polymerized [19]. These methods employedmonomers as the starting materials and the films formed werenot homogenous. Polymerization is effected on the substrate andis accompanied by the possibility of residual contamination andlack of control over film thickness. Further, steric hindrance, regio-regularity and random orientation of the films impose additionalconstraints that affect the functional properties. To avoid suchissues and to maintain the electronic and other characteristic prop-erties, synthesized polythiophenes have been widely used. Apart

from silicon, gold, indium-tin oxide (ITO) and other metal oxides arealso employed as substrates for polythiophene deposition [20,21].

In this work, we have used covalent binding to immobilizemolecular layers of pre-synthesized polythiophene on to sili-con/quartz and indium-tin oxide surfaces. The side chains of the

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olythiophene were functionalized to ensure reactivity and bind-ng was secured with the surface via secondary amine bonds.urther, multilayers of the polythiophene films were formed onhe substrates by use of a bifunctional intermediate to covalentlyind adjacent polythiophene layers. The covalent binding was con-rmed by X-ray photoelectron spectroscopy (XPS) and UV–Visiblepectroscopy. The surface coverage and conductive behavior ofhe polythiophene film was studied by electrochemical impedancepectroscopy. The electrical characteristics of the polythiophenelms improved after doping and subsequently decreased partiallyfter de-doping. The covalent molecular assembly technique pro-ides a facile way to fabricate robust ultrathin films for a varietyf sensing/electronic/photonic/photovoltaic applications. In thisork, we further demonstrate the retention of the desirable elec-

rical characteristics of conducting polythiophene films subsequento covalent molecular assembly.

. Experimental details

.1. Materials

Regio-regular poly (3-(2-methoxyethoxy)ethoxymethylthiophe,5-diyl (PMEEMT), (MW—11,000) and 33% hydrogen bromide

n acetic acid were purchased from Sigma–Aldrich. p-minophenyltrimethoxysilane (APhS) and 4,4′-diaminodiphenylther (DDE) were purchased from Gelest and Fluka, respectively,nd stored in a desiccator. Their molecular structures are shown inig. 1. Solvents chloroform, dimethyl acetamide (DMAc), methanolnd acetone of HPLC grade and nitromethane of synthesis gradeere purchased from Merck. Ferric chloride (anhydrous) was pur-

hased from Merck. All chemicals were used as received. Siliconafers (Engage Electronics Pte Ltd., Singapore) used were 0.6 mm

hick, p-doped, polished on one side and with a native oxideayer. Indium-tin oxide coated glass slides with sheet resistance of0 � cm, were purchased from Anhui Bengbu Huayi Conductiveilm Glass Co., Ltd. (China). Quartz slides were purchased fromchema Co., Singapore.

.2. Substrate preparation

Silicon wafers and quartz slides were treated with a 7:3 (v/v)iranha solution mixture of concentrated sulfuric acid and 30%ydrogen peroxide at 70 ◦C for 45 min. Caution: piranha solution

s highly oxidative, wear proper personal protective equipment (PPE)hen handling it. The wafers were then rinsed with de-ionized (DI)ater and acetone several times and blown dry with nitrogen. ITO

ubstrates were treated with a 5:1:1 (v/v) solution of DI water,0% ammonia solution and 30% hydrogen peroxide at 80 ◦C for 1 h.he substrates were then rinsed with DI water and acetone severalimes and blown dry with nitrogen.

Fig. 1. Molecular structures of materials.

d Engineering B 168 (2010) 45–54

2.3. Polymer modification

1:2 stoichiometric ratio of 2 mM PMEEMT and hydrogen bro-mide was taken in a round bottom flask and refluxed for 1 h at50 ◦C. After the reaction, the reaction mixture was cooled andthe bromine-modified polythiophene precipitated in methanoland washed multiple times with methanol to ensure thatresiduals were completely removed. Later, the precipitated poly (3-(2-bromoethoxy)ethoxymethylthiophene-2,5-diyl (PBrEEMT) wasfiltered from the methanol solvent by a nylon filter membrane(0.2 �m pore size, Whatman filters) and dried in vacuum overnight.

2.4. Film fabrication

The procedure of multilayer assembly of polythiophene on sili-con is as follows:

Step 1: Substrate preparation: The piranha-treated surface rich inhydroxyls was immersed in a 3 mM APhS solution in toluene for2.5 h at room temperature. After immersion, the substrates wererinsed copiously with toluene and followed with 10 min sonica-tion in toluene to remove loosely bound APhS, again rinsed withtoluene and methanol in succession and blown dry with nitrogen.Step 2: Deposition of the first polythiophene layer: The APhS-derivatized substrates were immersed in 1 mM PBrEEMT inchloroform for 3 days at room temperature. Subsequently, thewafers were rinsed with copious amounts of chloroform, soni-cated for 10 min, again rinsed with chloroform and blown dry withnitrogen.Step 3: Linker diamine deposition: The polythiophene containingsubstrates were immersed in 1% (w/v) DDE in DMAc for 3 daysat room temperature. After immersion, the substrates were rinsedcopiously with DMAc and sonicated for 10 min, rinsed successivelywith DMAc and methanol and blown dry with nitrogen.Step 4: Second layer polythiophene deposition: The step 2 wasrepeated.

The reaction schematics of the multilayer deposition are shownin Fig. 2.

2.5. Polymer doping

The polythiophene film-laden substrates were immersed in0.1 M of FeCl3 in nitromethane for 5 min and were rinsed thor-oughly in nitromethane subsequent to the immersion. De-dopingwas carried out by thermal treatment under vacuum and at 100 ◦Cfor 1 h.

2.6. Characterization

The 1H Nuclear Magnetic Resonance (NMR) spectrum wasobtained from Bruker Avance 300 NMR at 300 MHz and with CDCl3as solvent. UV–Visible absorption spectra were recorded on a Shi-madzu UV-1601 PC scanning spectrophotometer operating at aresolution of 1 nm. A bare quartz slide was used as the background.

X-ray photoelectron spectroscopy (XPS) was performed toverify the atomic concentrations of species. XPS measurementswere made on a Kratos Analytical AXIS HSi spectrometer with amonochromatized Al K� X-ray source (1486.71-eV photons) at aconstant dwell time of 100 ms and pass energy of 40 eV. The core-

level signals were obtained at a photoelectron takeoff angle of 90◦

(with respect to the sample surface). The X-ray source was run at areduced power of 150 W. The pressure in the analysis chamber wasmaintained at 7.5 × 10−9 Torr or lower during each measurement.All binding energies (BEs) were referenced to the C 1s hydrocarbon

S. Jayaraman et al. / Materials Science and Engineering B 168 (2010) 45–54 47

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Fig. 2. Schematic of the layer-by-layer covale

eak at 284.6 eV. In peak analysis, the line width (full width at half-aximum) for the Gaussian peaks was maintained constant for all

omponents in a particular spectrum.Surface morphologies were investigated using a Nanoscope III

tomic force microscope (AFM). All images were collected in airsing the tapping mode and a monolithic silicon tip. The drive fre-uency was 330 ± 50 kHz, and the voltage was between 3.0 and.0 V. The drive amplitude was about 300 mV and the scan rate was.5–1.0 Hz.

Thickness characterization was performed by ellipsometryWVASE 32, J.A. Woollam Co. Inc.). Scanning spectra were acquiredver the wavelength range of 600–1000 nm at three different inci-ence angles, 65◦, 70◦ and 75◦. The modeling for the thin filmeasurement used silicon (Si·MAT) as the base layer with 0.6 mm

hickness, the silicon dioxide (SiO2·MAT) as the next layer with a

hickness of 2.5–3 nm and the cauchy package (CAUCHY·MAT) forhe polymer films. Multiple readings were taken and the readingsere averaged.

The impedance measurements were performed using a modeleference 600 Potentiostat electrochemical impedance spec-

lecular assembly of polythiophene on silicon.

troscopy (EIS) instrument (Gamry Instruments) interfaced to apersonal computer. The measurement were carried out in a glasscell equipped with a gold-coated silicon counter electrode andan Ag/AgCl as reference electrode containing an aqueous solu-tion of 0.1 M Na2SO4, 1 mM K3Fe(CN)6 and 1 mM K4Fe(CN)6·3H2O.Polythiophene films coated on an ITO substrate acted as work-ing electrode. Impedance data were obtained by applying a 10 mVAC potential with frequency ranging from 5 mHz to 20 kHz. Allimpedance curves were plotted in semi-logarithmic scale. Quan-titative estimates of the film resistance values were determinedby fitting the EIS data with an equivalent circuit using nonlinearleast-square fitting routines of the software package Gamry EchemAnalyst.

3. Results and discussion

3.1. Amine bond formation

The amine derivatized surfaces were prepared by deposition ofAPhS on the hydroxyl-covered silicon surface. APhS has trimethoxy

48 S. Jayaraman et al. / Materials Science an

Fig. 3. 1H NMR spectra for (a) pure PMEEMT and (b) modified PBrEEMT.

Fig. 4. UV–Vis spectra of layer-by-layer deposition of polythiophenes on quartzsurface.

d Engineering B 168 (2010) 45–54

silane end groups which easily hydrolyze on the hydroxyl-coveredsilicon surface forming a tripod [22]. The bromine functional groupin the PBrEEMT side chain facilitated the formation of secondaryamine bond with the amine-covered silicon surface via nucleophilicdisplacement reaction [23–25]. Due to regio-regularity of the poly-thiophene, unreacted bromine would also be available for furtherreactions to form multilayers. The reaction scheme could be reusedto form multilayers linked with secondary amine bonds by using adiamine such as DDE as the linking agent.

3.2. Polymer modification

Due to the limited reactivity of ether bonds, the ether functionalgroup at the end of the side chain in PMEEMT was converted to abromine terminated functional group since hydrogen halides havethe tendency to break the ether bond. The reactivity of the hydro-gen bromide depends on the length of the ether chain. Therefore,the stoichiometric ratio of hydrogen bromide to polythiophene wasmaintained so as to ensure that only the end ether group is cleavedand no other intermediates or byproducts formed [26–28].

Conversion of the methoxy end group to bromine terminated

functional end group was confirmed by 1H NMR spectroscopy.Fig. 3a and b show the 1H NMR spectra of pure PMEEMT and con-verted PBrEEMT. The respective peak positions of all the protonsfor both polythiophenes are reported below [29–31].

Fig. 5. XPS spectra of (a) C 1s and (b) N 1s of APhS deposition on silicon surface.

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S. Jayaraman et al. / Materials Scien

PMEEMT: 1H-NMR, (CDCl3, (ppm), ı, 7.27 (s, 1H), 4.66 (s, 2H),.66–3.75 (m, 6H), 3.54 (m, 2H), 3.36 (s, 3H).

PBrEEMT: 1H-NMR, (CDCl3, (ppm), ı, 7.27 (s, 1H), 4.66 (s, 2H),.56–3.75 (m, 4H), 1.59 (m, 2H), 3.36 (t, 2H).

The main differences between the two spectra were the peakositions of triplet proton of the methyl group at position ‘e’ssigned to a new value from 3.66 ppm to 1.59 ppm. The methyl

Fig. 6. XPS spectra for (a) C 1s, (b) N 1s, (c) S 2p, (d) S 2s, (e) Br wides

d Engineering B 168 (2010) 45–54 49

triplet proton at peak position ‘g’ was converted to bromide afterthe nucleophilic displacement reaction. Upon conversion of the

methoxy group to bromide, the triplet proton peak at position ‘f’shifted from 3.54 ppm to 3.36 ppm. In addition, increase in elec-tronegativity due to bromide in the chain caused the triplet protonat position ‘e’ to shift to a lower ppm value [32]. Both observationsconfirmed the successful conversion of PMEEMT to PBrEEMT.

can comparison and (f) Br 3d for the first polythiophene layer.

5 ce and Engineering B 168 (2010) 45–54

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Table 1C/N ratio and N/Br ratios after successive depositions.

APhS 1rst layer PBrEEMT DDE 2nd layer PBrEEMT

C/N 1.76 3.4 3.3 6.6N/Br – 2.33 7.5 3.65

0 S. Jayaraman et al. / Materials Scien

.3. UV–Visible absorption

The displacement reactions and binding of polythiophene onmine surfaces were investigated using UV–Visible and X-ray pho-oelectron spectroscopies. Fig. 4 shows the UV–Visible spectra

f layer-by-layer deposition of polythiophenes on quartz surface.–�* conjugation due to phenyl ring in APhS was observed at wave-

ength (�max) 243 nm.The peak for the phenyl ring in APhS was red shifted to 258 nm

hich confirmed the change in the basicity of the phenyl ring due

Fig. 7. AFM images of (a) hydroxyl surface, (b) APhS surface, (c) 1st poly

thiophene layer, (d) DDE surface and (e) 2nd polythiophene layer.

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3.4.4. Comparison of C/N and N/Br ratiosThe C/N ratio and N/Br ratios were calculated from XPS data

at the end of each deposition step (Table 1). The C/N ratio forthe first polythiophene layer (step 2) layer increased due to the

Fig. 9. Equivalent electrical circuit for polythiophene films on ITO surface.

S. Jayaraman et al. / Materials Scien

o the formation of secondary amine bond. The �–�* conjugationf the thiophene backbone corresponding to �max at 380 nm waslso observed. Both the �max at 258 nm and 380 nm confirmed theuccessful deposition of the first layer. The covalent binding of DDEith PBrEEMT was confirmed again by the presence and increase in

max at 253 nm. Even though the basicity of the amine was changedue to the second secondary amine bond, free amine attached tohe phenyl ring caused a small shift in the �max from the 258 nm to53 nm. Deposition of second polythiophene layer was confirmedy the shifting of phenyl ring �–�* conjugation from 253 nm to56 nm as well as the increase in the absorbance of polythiophenet 380 nm.

.4. X-ray photoelectron spectroscopy

.4.1. Deposition of APhSFig. 5 shows the XPS spectra of C 1s and N 1s peaks after APhS

eposition on silicon. In the C 1s spectra (Fig. 5a), the peak at84.6 eV is due to the C C bond in the phenyl ring and the peakt 286 eV is due to the C N bond. The core-level N 1s peak (Fig. 5b)t 399.2 eV corresponds to amine group and the peak at 400.8 eVorresponds to the protonated amine [33]. Both the C 1s spectrand N 1s spectra confirmed successful deposition of APhS.

.4.2. First polythiophene layerFig. 6 shows the XPS spectra for C 1s, N 1s, S 2s, S 2p, Br widescan

omparison and Br 3d. Successful deposition of polythiophene wasonfirmed by the presence of C S peak and C O C peak at85.1 eV and 286.4 eV, respectively, in addition to the presence of

C peak at 284.6 eV and C N peak at 286 eV (Fig. 6a). The N 1spectra show the presence of free amine along with the secondarymine peak at 399.2 eV and the protonated amine peaks (Fig. 6b).2s, S 2p3/2 and S 2p1/2 peaks at 228.2 eV, 164.2 eV and 165.6 eVere also observed (Fig. 6c and d). The absence of the bromine

n the widescan for the APhS layer and its presence after modifi-ation confirmed the successful deposition of the polythiopheneayer (Fig. 6f). The core-level bromine peaks 3d3/2 and 3d5/2 werebserved at 71.5 eV and 70.4 eV, respectively (Fig. 6e). The presencef S and Br core-level peaks and Br widescan spectra confirmed

eposition of PBrEEMT.

.4.3. DDE depositionDDE was anchored to the previously deposited PBrEEMT by

he second secondary amine bond with one NH2 group of DDE

Fig. 8. Average thickness of layer-by-layer structures on silicon surface.

d Engineering B 168 (2010) 45–54 51

reacting with the available bromine of the polythiophene. As dis-cussed in the above section, the characteristic peaks for C N,

C S, C O C , NH2, S 2p and S 2s were observed (XPS scansnot shown). The presence of bromine after DDE deposition sug-gested that some were unreacted due to the steric hindrance of theside chains of polythiophene obstructing secondary amine bondformation.

Fig. 10. (a) and (b) Impedance and Nyquist spectra for polythiophene on ITO.

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inding of polythiophene with the amine from APhS. Both nitro-en and carbon contents increased simultaneously after depositionf DDE (step 3). After the deposition of the second polythiopheneayer (step 4) the carbon content increased as expected. The N/Bratio observed after deposition of the first polythiophene layer sug-ested that all the amines were not completely converted intoecondary amines. Nonetheless, the ratio increased after step 3ue to increased nitrogen content and reduced bromine contentecause of secondary amine bond formation between DDE andBrEEMT. The ratio decreased after deposition of the second layerf polythiophene due to increase in bromine content. Thus, thehanges in the C/N and N/Br ratios were consistent with the suc-essful deposition of the multilayers of polythiophene.

.5. Morphology study

Fig. 7 shows the sequence of AFM images of the layer-by-layertructures. Firstly, the smooth hydroxyl-covered, piranha-treated

ilicon surface (Fig. 7a) was modified to show well-defined featuresf the densely packed amine-covered surface after APhS deposi-ion (Fig. 7b). The first polythiophene layer (Fig. 7c) resulted in ameared surface with sharper features. The sharpness was reduced

ig. 11. (a) and (b) Comparison of impedance spectra and Nyquist spectra of firstayer polythiophene film after doping and de-doping.

d Engineering B 168 (2010) 45–54

after deposition of DDE (Fig. 7d) on first polythiophene layer result-ing in a smoother surface with amine functional group exposedsimilar to that of the APhS layer. The second polythiophene layer(Fig. 7e) re-created topography similar to that observed after thedeposition of the first polythiophene layer. The above observationswere consistent with the measured surface roughness values. Theroughness value of 0.11 nm for the hydroxyl-covered silicon sur-face increased to 0.18 nm after deposition of APhS. Deposition ofsubsequent layers on APhS resulted in changes in surface rough-ness (0.34 nm after the first polythiophene layer, 0.27 nm after DDEand 0.29 nm after the second polythiophene layer). The reduction inroughness after deposition of DDE suggests that DDE was randomlyoriented. This randomness may have persisted after the depositionof the second polythiophene layer.

3.6. Thickness measurements

The thickness measurements gave physical evidence of layer-by-layer deposition of polythiophene on silicon surfaces. Fig. 8

shows the gradual increase in thickness with layer-by-layer assem-bly. The thickness of the APhS layer was 0.8 nm. The thickness ofthe first polythiophene layer was around 1.4 nm suggesting thatthe polythiophene was not orienting vertically to the surface [34].A similar inference can be made for the DDE films whose thickness

Fig. 12. (a) and (b) Comparison of impedance spectra and Nyquist spectra of linkingDDE layer after doping and de-doping.

ce and Engineering B 168 (2010) 45–54 53

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S. Jayaraman et al. / Materials Scien

as 0.7 nm [35]. Finally, the thickness of the second polythiopheneayer was around 1.3 nm [36,37].

.7. Electrochemical impedance analysis

Due to the high resistance in the silicon substrate and the com-lexity in modeling, an ITO substrate was preferred because of

ts high electron density and to facilitate easier AC impedanceeasurements [38]. Multilayered polythiophene films on APhS-

unctionalized ITO surfaces were prepared similar to those onilicon substrates. The behavior of polythiophene films has beenepresented as electrical circuit elements. The Constant Phase Ele-ent (CPE) with diffusion model has been used to calculate the

esistance of the films. In the CPE model, the circuit element consistsf the electrolyte solution resistance, Ru, polarization resistance, Rp,constant phase element and the Warburg diffusion impedance,d [39,40]. Fig. 9 shows the equivalent electrical circuit. Due to the

on-ideal behavior of the polythiophene films, we have used the

PE and Warburg diffusion impedance to calculate the resistance.urther details about the CPE and Warburg diffusion impedancean be obtained from various sources [41–43].

Fig. 10a shows the impedance of each layer as a function of fre-uency. Fig. 10b shows the Nyquist spectra of each layer. Increase

ig. 13. (a) and (b) Comparison of impedance spectra and Nyquist spectra of secondayer polythiophene film after doping and de-doping.

Fig. 14. Computed values of the film resistances (Rp) using equivalent circuit anal-ysis.

in film thickness resulted in increase in the impedance of the films[44]. Since ITO is a good conductor, further addition of the organicfilms on the substrate reduced the contact between the electrolytesolution and the ITO substrate. The impedance spectra consist ofa semicircle at higher frequency representing the electron transferlimited process, whereas linear part at lower frequency representsthe limited electron transfer due to diffusion.

3.8. Doping and de-doping of layer-by-layer polythiophene films

Fig. 11a and b show the impedance and Nyquist spectra of thefirst layer polythiophene before and after doping, and after ther-mal treatment. The impedance of the first layer polythiophenefilm decreased after doping because of the injection of Cl3− ionsinto the polythiophene backbone [45–48]. Upon thermal treat-ment, the films get partially oxidized which subsequently increasedthe impedance of the film [49]. Similar behavior of the decreasedimpedance after doping and increase in impedance after thermaltreatment was observed for the linking DDE layer as well as secondpolythiophene layer (Fig. 12a and b, Fig. 13a and b).

Fig. 14 shows the computed values of the film resistances (Rp)before and after doping, and after de-doping of the polythiophenestructures. The film resistance decreased after doping and subse-quently increased after thermal treatment. The film resistance ofthe first layer and second layer polythiophene structure decreasedby about 50% after doping whereas the film resistance decreased byaround 32% for the linking DDE layer. The difference in the decreasein resistance is expected due to the amount of dopants incorporatedas well as discontinuity in the DDE film when compared with thepolythiophene films. Successful deposition of subsequent layers ofpolythiophene films and uniform film coverage were confirmed bythe gradual increase in the Rp and decrease in Rp upon doping, thusimproving the conducting behavior of polythiophene.

4. Conclusions

Multilayers of robust, bromine-modified polythiophene filmswas successfully deposited on amine terminated surfaces byformation of secondary amine bonds. The secondary amine bonds

were formed by SN2 nucleophilic displacement reaction betweenamine and bromide. The covalent binding was confirmed byUV–Visible and XPS spectroscopies. Electrochemical impedancespectra confirmed the layer-by-layer polythiophene structure anduniform coverage of the films. Expected changes in resistance upon

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oping and de-doping of the polythiophene structures confirmedhe retention of their electrical characteristics.

cknowledgement

The authors thank the National University of Singapore for pro-iding financial support for this project and a research scholarshipor Sundaramurthy Jayaraman.

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