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Synthetic Metals 161 (2011) 481–488

Contents lists available at ScienceDirect

Synthetic Metals

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

lectronic properties of soluble functionalized polyaniline (polyanthraniliccid)-multiwalled carbon nanotube nanocomposites: Influence of synthesisethods

run Kumar Singh, Leela Joshi, Bhavana Gupta, Ashish Kumar, Rajiv Prakash ∗

chool of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India

r t i c l e i n f o

rticle history:eceived 27 July 2010eceived in revised form5 November 2010ccepted 24 December 2010vailable online 31 January 2011

eywords:olyanthranilic acid

a b s t r a c t

Processible nanocomposites of carboxyl functionalized conducting polymer “polyanthranilic acid”(PANA) with multiwalled carbon nanotubes (MWNTs) are prepared using two different synthesis routesviz. single phase and two phase polymerization. The novel nanocomposite materials are characterizedusing X-ray diffraction (XRD), thermogravimetric (TGA), electrochemical impedance (EI), scanning elec-tron microscope (SEM) and high resolution transmission electron microscope (HRTEM) techniques fortheir structural, thermal, electrochemical and surface morphological properties. SEM and HRTEM imagesare confirmed that nanotubes are dispersed uniformly in polymer matrix and polymer chains wrap aroundthe nanotubes walls. The interaction between MWNTs and PANA is analyzed by Raman and Fourier trans-

ulti walled carbon nanotubeanocompositeonducting polymerchottky diodeetal-polymer junction property

form infrared (FTIR) spectroscopy. UV–vis spectroscopic technique is used to obtain the optical bandgap ofnanocomposites. PANA-MWNTs nanocomposites are used for the first time for fabrication of sandwichtype devices with a configuration of metal Al/PANA-MWNTs nanocomposite/indium tin oxide coatedglass (ITO). The current density–voltage (J–V) and capacitance–voltage (C–V) characteristics of the Schot-tky diode are subsequently used for extracting electronic parameters of the devices. These measurementsrevealed that the junction electrical parameters depend strongly on the synthesis route for preparation

of nanocomposites.

. Introduction

Conducting polymers and their derivatives are already wellnown for application in various semiconducting devices suchs Schottky diodes, field effect transistors, light emitting diode,emory devices and solar cells. Conducting polymers got muchore attention for electronic devices because of their unique elec-

ronic and optical properties, processibility and low cost, abilityo be chemically tuned, and most importantly their lightweightnd foldable mechanical properties [1–10]. The interfaces consist-ng of conducting polymer/metal are necessary in these devicesherefore, the electronic phenomenon at the interface is criticallymportant to the performance and function of such devices. Thelectrical and electronic properties of the devices are affected bynterfacial properties, and it is necessary to understand not only

he properties of conducting polymers itself, but also interfaciallectronic phenomena. Among the conducting polymers, polyani-ine has attracted much attention due to advantages over otheronducting polymers because of its preparation from cheap mate-

∗ Corresponding author. Tel.: +91 542 2307047; fax: +91 542 2368707.E-mail address: rajivprakash12@yahoo.com (R. Prakash).

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.12.031

© 2011 Elsevier B.V. All rights reserved.

rials, well behaved electrochemistry [11,12], superior stability [13],and controllable electrical conductivity by doping. However, itsinsolubility in most of the common solvents and poor mechani-cal property are restricted its applications in devices. A number ofattempts have been made to improve the processibility of polyani-line by polymerization of aniline derivatives with alkyl, sulfonicacid group, and carboxyl group substitution [14]. The carboxylfunctionalized polyaniline “polyanthranilic acid” (PANA) has over-come this problem due to its solubility in common organic solvents.Recently, we synthesized PANA with different oxidizing agents andalso used for processible composites formation [15].

Currently carbon nanotubes (CNTs) are considered as promisingcandidates to serve as building blocks for several device architec-tures in chemical sensors, hydrogen energy storage, field emissionmaterials and electronic devices [16–20], due to their unique elec-trical and mechanical properties combined with chemical stability.CNTs can be divided two main categories [21]: single walled carbonnanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs).

For the application in organic electronic and optoelectronic devices,MWNTs can provide better carrier transport than SWNTs, sincesemiconducting tubes usually dominate SWNT product. MWNTalso offers a better mechanical strength and can be produced inlarge quantity with a much lower cost [22]. However, the major

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82 A.K. Singh et al. / Synthet

rawback of CNTs is their poor processibility, since they are prac-ically insoluble in commonly used solvents. The insolubility ofNT has been limited its practical application in microelectronics.owever, today’s most efficient devices in the field of electronicnd optoelectronics are based on spin coating or solution cost-ng techniques [23,24]. Functionalization or incorporation of CNTs

ith polymer backbone or the formation of composite materialss the most common way to overcome this problem. The polymer

rapped onto the walls of CNTs through non-covalent interactionsike, �–� interaction, can improve the dispersity of CNTs in com-osites [25,26].

Recently, conducting polymer-carbon nanotubes nanocompos-tes have attracted much more attention for their applications inlectronic and optoelectronic devices because the charge trans-er between the components imports new electronic properties. Inddition, the introduction of CNTs in polymer matrix improves theechanical properties of original polymer matrix [27–29]. It has

een also suggested that uniformly distributed nanotubes in theolymer matrix act as nanometric heat sinks, preventing the build-p of large thermal effects and thus reducing material and deviceegradation [26]. Various methods have been developed for theynthesis of conducting polymer-CNTs nanocomposites. Howeveronducting polymers are generally insoluble in common solvents,o it is difficult to prepared homogenous and stable conductingolymer-CNTs nanocomposites by conventional blending or mix-

ng techniques. Thus the most appropriate method for preparingonducting polymer-CNTs nanocomposites is polymerization ofonomer in situ (single phase) in the presence of CNTs. Even

nder this condition the uniform dispersion of CNTs is still a chal-enge. Another method for formation of homogenous and stableanocomposite is may be the interfacial (two phase) polymeriza-ion. This method is explored due to its advantage of synthesis withery low rate of reaction, which enables the controllable entrap-ent of nanostructures such as MWNTs in this case, quantum dots,etallic nanoparticles and subsequently their better dispersion.cid functionalized polyanthranilic acid-MWNTs nanocompositeas overcome the problem of both polyaniline and CNTs due to

ts solubility. Although many papers have been published in thiseld and electrical properties of conducting polymers nanocom-osites were studied [27–29]. However, no any paper has discussedhe effect of synthesis routes on electronic and junction proper-ies of conducting polymers-MWNTs nanocomposites. In this papere report the synthesis and characterizations of PANA-MWNTsanocomposites prepared by two different methods (single phaseolymerization and two phase polymerization) and also the effectf synthesis methods on electronic and junction properties ofANA-MWNTs nanocomposite Schottky contact with Al metal.

. Experimental

.1. Synthesis of nanocomposite by two phase polymerization

Ammonium peroxodisulphate, acetonitrile (HPLC grade) andulphuric acid were obtained from Merck, India. Potassiumermanganate was obtained from S.D. Fine Chemicals, India.nthranilic acid monomer was obtained from Rolex India Ltd.,

ndia. All other chemicals used were of analytical grade. MWNTbtained from Aldrich, USA was functionalized by acid treatments discussed earlier [30]. Acid functionalized multi walled carbonanotubes (c-MWNT) purified and oxidizing agent (KMnO4) was

oated as per our earlier work [30]. The nanocomposite is syn-hesized by two different methods viz. two phase polymerizationnd single phase or in situ polymerization. In two phase polymer-zation PANA-c-MWNT nanocomposite formed as following steps:10 mg of KMnO4 coated MWNTs (MWNT is 10 wt.% to total con-

als 161 (2011) 481–488

tent) taken in 8 ml of 0.2 N H2SO4 (0.08 M). c-MWNT dispersed inaqueous phase by ultrasonication for 15 min and this solution wasreferred as A. Anthranilic acid monomer 175 mg was dissolved in8 ml chloroform solution (0.16 M) and this solution was referred toas solution B. Solution A was added in the solution B gently thoughthe wall of the flask and allowed for polymerization with mild stir-ring below 20 ◦C for 3 h. After overnight incubation upper aqueousphase was filled with nano composite. Aqueous and non aqueousphase separated by separating funnel. Precipitate was collected bycentrifugation, washed with cold 0.5 N H2SO4 for two times anddried under vacuum at 50 ◦C. Yields of the nanocomposite was100 mg (including c-MWNT 10 wt.%).

2.2. Synthesis of nanocomposite by single phase polymerization

Nanocomposite was prepared by dispersing 5 wt.% of c-MWNT(of monomer) in 0.2 M HCl (10 ml) followed by sonication and stir-ring for complete dispersion of c-MWNT. Monomer of anthranilicacid was dissolved in above solution. To this solution, a solutionof 0.2 M ammonium peroxodisulphate (used as oxidizing agent)0.2 M (1:2 molar ratio of monomer and oxidizing agent ratio) in10 ml 0.2 M HCl, was added drop wise with constant stirring. Aftercomplete addition of oxidizing agent this solution was kept for stir-ring for further 1 h. The resulting solution was kept at 26 ± 1 ◦Covernight for complete polymerization. Precipitate was collectedby centrifugation followed by washing with 0.2 M HCl and finallywith water and dried under vacuum oven at 50 ◦C.

2.3. Characterization of nanocomposite

The PANA-MWNTs nanocomposites were characterized foroptical and electrical properties. XRD characterization was donewith 18 kW rotating anode powder X-ray diffractometer Rigku,Japan. UV–visible study was done using Lamda 25 spectrophotome-ter of PerkinElmer, Germany. Raman spectra were recorded witha Renishaw system RM 1000 (UK) using an Argon ion laser oper-ating at 514.5 nm. The spectrometer was equipped with a 2400grooves/mm grating. Fourier transform infrared (FTIR) study wascarried out with a FTIR spectrometer (model 8400S, SHIMADZU,Japan) using KBr pellet. Thermogravimetric analysis (TGA) was car-ried out using METTER TULEDO (model-TGA/DSC1, Switzerland)from ambient temperature to 700 ◦C at 10 ◦C min−1 rate in nitro-gen atmosphere. Electrochemical impedance (EI) of PANA andits MWNTs nanocomposites were recorded using ElectrochemicalWork Station (CH Instrument Inc., USA) at open circuit potential(OCP) in the frequency range of 10 mHz to 10 KHz at an ampli-tude of 5 mV. For EI, three electrodes cell assembly having PANAor its MWNTs-nanocomposites coated Pt disc electrode as work-ing electrode, Pt plate (of area nearly 10× of working electrode)as counter electrode and Ag/AgCl as reference electrode was usedfor impedance analysis in 0.5 M H2SO4 solution. Scanning elec-tron microscope (SEM) images were obtained at operating voltage15–20 kV using SHIMADZU SS-550 SUPER SCAN, Japan. High reso-lution transmission electron microscope images were obtained byHRTEM model Tecnai 30 G2 S-Twin electron microscope, operatedat 300 kV accelerating voltage.

2.4. Fabrication of devices and measurements

PANA-MWNTs nanocomposites were dissolved in THF (tetrahydrofuran) to get solution of concentration of 10 mg/ml. PANA-

MWNTs nanocomposites were spin-coated on ITO glass substrate(with surface resistance of 12 �/cm2) by spin coating (at 1200 rpm)technique (Spin coater-SHINU MST Co. Ltd., made in Korea) anddried in vacuum. Al metal was deposited with area of 16 mm2

using mask on different samples of PANA-MWNTs/ITO by vac-

A.K. Singh et al. / Synthetic Metals 161 (2011) 481–488 483

Fpp

uHwioAfi(AEHd

3

3

ppmMgfFphpptnsnisipigp

stretching vibration and at about 1246 cm is interpreted as a C–Nstretching vibration in the polaron structure of PANA [31]. Due tocoating of MWNTs with PANA, the spectrum of PANA dominatesin nanocomposite samples. The Raman spectra of nanocompositesdo not exhibit any positional shift (<2 cm−1) as compare to pure

ig. 1. XRD pattern of (a) MWNTs, (b) PANA, (c) PANA-MWNTs nanocompositesrepared by single phase polymerization and (d) PANA-MWNTs nanocompositesrepared by two phase polymerization method.

um evaporation method by using vacuum coating system of HINDIVAC (Model no.12A4D), India. The devices Al/PANA-MWNTs/ITOere not sealed from attack of moisture and oxygen, but kept

n vacuum dessicator for further characterization. The thicknessf nanocomposite was of the order of 400 nm as estimated fromFM measurement and thickness of metal on top of the polymerlm was pre-adjusted to ∼90 nm in each case. Current–voltageI–V) and capacitance–voltage (C–V) measurements of devicesl/PANA-MWNTs/ITO were carried out with Keithley Model 6517Alectrometer, USA and LCR Meter from Hewlett Packard, Model-P4284A, respectively, at room temperature (26 ± 1 ◦C) in air underark condition.

. Results and discussion

.1. Structural and morphological properties of nanocomposites

Polyanthranilic acid/multi walled carbon nanotubes nanocom-osites were synthesized by single phase and interfacialolymerization techniques and studied for their structural andorphological properties. Powder X-ray diffraction pattern ofWNTs, PANA, PANA-MWNTs nanocomposites prepared by sin-

le phase and by two phase polymerization technique as recordedrom 2� = 5◦ to 60◦ with scan rate 3◦ per minute are shown inig. 1. From the figure it is apparent that MWNTs has diffractioneak at 2� = 26◦ for the 0 0 2 reflection of graphite, however PANAas a broad peak at 2� = 25◦ shows the amorphous nature of theolymer. The X-ray diffraction pattern of PANA-MWNTs nanocom-osite prepared by single phase polymerization method showshe semi crystalline nature of polymer, however PANA-MWNTsanocomposite prepared by two phase polymerization techniquehows the amorphous nature as similar to pure PANA. Thereforeo crystallinity or chain order is introduced into the nanocompos-

te prepared by two phase polymerization technique; however thetructure of PANA is much affected by MWNTs in nanocompos-te prepared by single phase polymerization method. The addition

eaks at about 2� = 17.6◦, 19.4◦, 23.2◦, and 31.01◦ of relatively week

ntensity were appeared in case of nanocomposite prepared by sin-le phase polymerization method probably due to arrangement ofolymer chains over the surface of MWNTs.

Fig. 2. Raman spectra of (a) MWNTS, (b) PANA, (c) PANA-MWNTs nanocompos-ites prepared by two phase polymerization and (d) PANA-MWNTs nanocompositesprepared by single phase polymerization method.

Raman spectra of MWNTs, PANA, PANA-MWNTs nanocom-posites prepared by single phase and two phase polymerizationtechniques are shown in Fig. 2. MWNTs are identified by theircharacteristics peaks as the D band (disordered-induced band)at 1352 cm−1, its second-order harmonic D′-band at 2701 cm−1

and strong G band at 1580 cm−1 (Raman allowed high frequencymode). The spectrum of neat PANA shows the peaks at 1246 cm−1,1392 cm−1 and 1580 cm−1. The spectrum of PANA exhibits at about1392 cm−1 probably corresponding to the hydrogen-bonded C–N+

−1 +

Fig. 3. FTIR spectra of (a) MWNTS, (b) PANA, (c) PANA-MWNTs nanocompositesprepared by single phase polymerization and (d) PANA-MWNTs nanocompositesprepared by two phase polymerization method.

484 A.K. Singh et al. / Synthetic Metals 161 (2011) 481–488

F polymi

PtRtMPea

p

FP

ig. 4. SEM images of (a) PANA-MWNTs nanocomposites prepared by single phasezation method.

ANA. Many researchers were reported such type of Raman spec-ra in the case of polymer-MWNTs nanocomposites [31–33]. Theaman spectra of PANA in the nanocomposite are much broaderhan the pure PANA and are not as intense as the ones due to the

WNTs. These changes may be due to the interaction between

ANA and MWNTs [32]. This Raman spectra only indicate the pres-nce of polymer over the MWNTs which are also supported by SEMnd TEM images.

FTIR spectra of MWNTs, PANA, PANA-MWNTs nanocompositesrepared by single phase and two phase polymerization techniques

ig. 5. HRTEM images of (a) PANA-MWNTs nanocomposites prepared by single phase pANA-MWNTs nanocomposites prepared by two phase polymerization method, Inset sho

erization and (b) PANA-MWNTs nanocomposites prepared by two phase polymer-

are shown in Fig. 3. Raman active D- and G-band are inactive in IRspectra, because they are absent due to the symmetry of the car-bon network. The peak at 1743 cm−1 is confirmed the carboxylicgroup at surface of MWNTs (similar FTIR spectra is reported ear-lier) [34,35]. The FTIR spectra for PANA show strong bands for

the C O stretching at 1694 cm−l. The C C stretching frequencyof quinoid and benzenoid rings is observed at 1568 cm−1 and1512 cm−1, respectively [15]. The band appearing at 754 cm−l cor-responds to the C–H out-of-plane bending vibration of the benzenerings. Another absorption peak at 1250 cm−1 which is mainly due

olymerization, Inset shows multiwalls of CNT and uniform coating of polymer. (b)ws bundles of CNTs coated with polymer.

A.K. Singh et al. / Synthetic Metals 161 (2011) 481–488 485

tom1ptbc

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tTr

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wlceF

after a plateau for PANA and PANA–MWNTs nanocomposite. Thesecond weight loss for the PANA-MWNTs nanocomposite preparedby single phase method, PANA-MWNTs nanocomposite preparedby two phase method and pure PANA are started from 240 ◦C,200 ◦C and 153 ◦C, respectively, which may be assigned for struc-

Fig. 6. UV–vis spectrums of PANA and PANA-MWNTs nanocomposite.

o C–N stretching of secondary amine [36] got broadening in casef nanocomposite prepared by single phase and two phase poly-erization techniques. The increase in intensity of the peak at

120 cm−1 appears in FTIR spectra of PANA-MWNTs nanocom-osites prepared by single phase and two phase polymerizationechniques is probably indicating charge transfer from PANA chainsy carboxylated MWNT. This facilitates the effective degree of delo-alization and increase in conductivity [37].

Morphology of PANA-MWNTs nanocomposites is studied usingEM and TEM as shown in Figs. 4 and 5, respectively. From SEMmages it is clearly seen that the nanotubes were distributed uni-ormly in the PANA matrix. The growth of polymer over MWNTs is

uch uniform in the case of PANA-MWNTs nanocomposite pre-ared by single phase polymerization method in comparison toANA-MWNTs nanocomposite prepared by two phase polymeriza-ion method. This is also confirmed from HRTEM images as shownn Fig. 5. PANA much uniformly coated over MWNTs of thicknessbout 10 nm in single phase polymerization, however thickness ofolymer over MWNTs in the two phase polymerization methodaries from 20 nm to 50 nm.

.2. UV–vis spectra

The UV–vis spectra of PANA and PANA-MWNTs nanocompositesrepared by both methods are shown in Fig. 6. Pure PANA showsbsorption band at 273 nm corresponding to the �–�* transitionf its conjugated segments. After addition of MWNTs in PANA, thisand shifts towards the higher wavelength region in both cases,ecause of the interaction between PANA and MWNTs, which maye due to uncoil the PANA chains and increasing its conjugation

ength. The absorption band seen at about 480 nm assigned to be–�* transition in PANA, this band is red shifted in PANA-MWNTsanocomposites, which may be due to functionalized MWNTs act-

ng like a dopant.The bandgaps were evaluated from the absorbance spectra of

he PANA and PANA-MWNTs nanocomposites taken in 0.5 N HCl.he optical bandgap of polymer was estimated by fundamentalelation given by [38].

h� = B(h� − Eg)n (1)

here ˛ is the absorption coefficient, hv is the energy of absorbedight, n = 1/2 for direct allowed transition and B is proportionalityonstant. Energy gap (Eg) was obtained by plotting (˛hv)2 vs. h� andxtrapolating the linear portion of (˛hv)2 vs. h� to zero, as shown inig. 7 The bandgap of PANA-MWNTs nanocomposites prepared by

Fig. 7. (�h�)2 versus h� plot for bandgap estimation of PANA-MWNTs nanocom-posites along with bandgap estimation of PANA as shown in inset.

single phase polymerization and two phase polymerization meth-ods were estimated to be 2.45 eV and 2.85 eV, respectively, by usingthis method, however bandgap of PANA is found to be 3.4 eV shownin inset of Fig. 7.

3.3. Thermogravimetric analysis (TGA)

Thermal analysis (cf. Fig. 8) clear showed that there are twomajor stages of weight loss for the both the polymers and theirnanocomposites prepared by two different methods. The firstweight loss (about 5%) at the lower temperature results frommoisture evaporation and perhaps out gassing of unknown smallmolecules. There appeared obvious difference for the second stage

Fig. 8. TGA of PANA and its nanocomposite.

486 A.K. Singh et al. / Synthetic Metals 161 (2011) 481–488

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na

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ig. 9. Electrochemical impedance of PANA and PANA-MWNTs nanocompositelong with electrochemical impedance of PANA-MWNTs (single phase) nanocom-osite as shown in inset.

ural decomposition. The TGA study showed only 24.28% weightoss up to 300 ◦C for the PANA-MWNTs nanocomposite preparedy single phase method and 26.86% weight loss up to 300 ◦C forhe PANA-MWNTs nanocomposite prepared by two phase method,owever, PANA showed 44.3% weight loss up to 300 ◦C. The thermaltability of PANA in the composites is somewhat better com-are with neat PANA, which is due to interaction between chainsf PANA with walls of MWNTs. The better thermal stability ofanocomposite prepared by single phase method in comparison toanocomposite prepared by two phase method is probably due toetter dispersion of MWNTs and uniform coating of polymer overWNTs.

.4. Electrochemical properties

Electrochemical impedance (EI) of PANA and its MWNTsanocomposites are carried out at OCP in a three electrodes cellssembly using polymer/nanocomposites coated platinum (Pt) disc

ig. 11. 1/C2 versus V characteristics of devices of configuration (a) Al/PANA-MWNTs (sinV) as shown in inset and (b) Al/PANA-MWNTs (two phase)/ITO along with variation of d

Fig. 10. J–V characteristics of Al/PANA-MWNTs/ITO devices in dark condition anddevice configuration as shown in inset of the figure.

electrode as working, Pt plate as counter and Ag/AgCl as a referenceelectrode. Nyquist plots of PANA and its MWNTs nanocompositesare shown in Fig. 9. In Nyquist plot for high frequency inter-cept and low frequency intercept shows solution and sum of thesolution plus charge transfer resistance, respectively. It is clearlyobserved from Fig. 9 that the charge transfer resistance of PANA-MWNTs nanocomposite is much lower in compare to pure PANAalone. This may be due to the presence of MWNTs, and closeinteraction increased the conductivity. The charge transfer resis-tance of PANA-MWNTs nanocomposite prepared by single phasemethod is much lower in compare to PANA-MWNTs nanocom-posite prepared by two phase method, this may be due to better

chains alignment, uniform dispersion and homogenous coating ofMWNTs in the case of PANA-MWNTs nanocomposite prepared bysingle phase method, which was also confirmed by TEM and XRDresults.

gle phase)/ITO along with variation of depletion width (Wd) versus applied voltageepletion width (Wd) versus applied voltage (V) as shown in inset.

A.K. Singh et al. / Synthetic Metals 161 (2011) 481–488 487

Table 1Device electrical parameters.

Devices � �B (eV) J0 (A/cm2) Vbi (V) NA (cm−3) Wd (nm) for appliedvoltage of

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Al/PANA-MWNTs (single phase)/ITO 1.43 0.70Al/PANA-MWNTs (two phase)/ITO 1.48 0.82

.5. Junction properties

The J–V characteristics under dark condition of a samplel/PANA-MWNTs (single phase)/ITO and Al/PANA-MWNTs (twohase)/ITO devices taken from the three sets of devices fabricated

n the laboratory is illustrated in Fig. 10. The vertical lines indi-ate the departure in the measured value of the current for theorresponding voltages in terms of the standard deviation in theeasured value from samples taken from three sets as shown in

ig. 10.The schematic cross sectional view of fabricated devicesTO/PANA-MWNTs/Al Schottky junction is shown in inset of Fig. 10.he rectifying nature of the devices is confirmed by the measuredurrent density–voltage (J–V) characteristics. The electrical char-cteristics of ITO/PANA-MWNTs/Al junction have been analyzed byssuming the standard emission–diffusion theory. According to thisheory, the J–V relationship is expressed as [39].

= J0

[exp

(qV

�kT

)− 1

](2)

here J (=I/A) is current per unit area, q is the electronic charge, Vs the applied voltage, T is the absolute temperature, J0 the satura-ion current density in absence of external bias, � is diode qualityactor (ideality factor) and k is the Boltzmann constant. Further J0s related to the Schottky barrier height, �B as

0 = A∗T2 exp(−q�B

kT

)(3)

here A* is the effective Richardson constant usually taken as20 A cm−2 K−2 for free electron [40], �B can be evaluated from Jo.he value of �B for Al/PANA-MWNTs/ITO based Schottky diode cane evaluated from

B = kT

qln

[A∗T2

J0

](4)

onsidering the forward J–V characteristics for V > kT/q, the Eq. (2)an be approximated as

= J0exp(

qV

�kT

)(5)

he value of ideality factor � of the devices Al/PANA-MWNTs/ITOre determined from the slope of the plot ln(J) vs. V at 27 ◦C andiven in Table 1.The obtained ideality factor greater than unityhows the deviation of Schottky diode characteristic from idealehavior and it may be due to the barrier inhomogeneity. Theeverse saturation current density are determined by the inter-ept of ln(J) vs. V at V = 0 and found to be 1.36 × 10−5 A/cm2 inevice Al/PANA-MWNTs (single phase)/ITO and 1.32 × 10−7 A/cm2

n device Al/PANA-MWNTs (two phase)/ITO. The barrier heightB of diodes Al/PANA-MWNTs (single phase)/ITO and Al/PANA-WNTs (two phase)/ITO are calculated by using J0 at room

emperature from Eq. (4) and found to be 0.70 eV and 0.82 eV,espectively. The larger reverse saturation current density (J0) in the

evice Al/PANA-MWNTs (single phase)/ITO accounted for smallerepletion width (as confirmed from C–V measurements discussed

ater in this section), which increase the possibility of tunneling athe interface between Al and PANA-MWNTs nanocomposite. Thedeality factor of the devices Al/PANA-MWNTs (single phase)/ITO

0 V −3 V

× 10−5 0.63 6.85 × 1017 22.5 54× 10−7 0.79 1.3 × 1016 182 404

and Al/PANA-MWNTs (two phase)/ITO is not significantly changedby methods of synthesis, however the lower ideality factor of thedevice Al/PANA-MWNTs (single phase)/ITO may due to the uniformcoating of MWNTs. The ideality factor of the devices Al/PANA-MWNTs/ITO is found to be better than our previously reported [41]device Al/PANA/ITO, which may be due to the MWNTs in polymermatrix enhancing the charge transport across the interface.

Fig. 11 shows the 1/C2 versus V characteristics of deviceAl/PANA-MWNTs/ITO measured at 10 kHz by LCR meter.Capacitance–voltage (C–V) measurements are carried out fordetermination of carrier concentration in depletion layer ofdiode, built-in potential Vbi and depletion width Wd [39]. Thecapacitance–voltage (C–V) relationship of the Schottky junctionunder bias can be expressed as

1C2

= 2Vbi

qε0εsNA− 2V

qε0εsNA(6)

where C is the junction capacitance of the Schottky diode per unitarea, V is the applied voltage, ε0 is the free space permittivity, Vbi isthe built-in potential, εs is the dielectric constant of materials andNA is the acceptor concentration in the depletion layer. Further, thedepletion layer width is expressed as

Wd =[

2ε0εs(Vbi − V)qNA

]1/2

(7)

The built-in potential in the devices Al/PANA-MWNTs (singlephase)/ITO and Al/PANA-MWNTs (two phase)/ITO are determinedby extrapolating the linear region of 1/C2 versus V to cut the V-axis and found to be 0.62 V and 0.79 V, respectively as shown inFig. 11. The acceptor concentration are calculated from slope of 1/C2

versus V and given in Table 1. The variation of the depletion layerwidth with the applied voltage as estimated on the basis of Eq. (7)is shown in inset of Fig. 11.The larger depletion width of the deviceAl/PANA-MWNTs (single phase)/ITO is due to smaller conductiv-ity of PANA-MWNTs nanocomposite prepared from single phasepolymerization.

4. Conclusion

Processible nanocomposites of carboxyl functionalized con-ducting polymer “polyanthranilic acid” (PANA) with multi walledcarbon nanotubes (MWNTs) are formed using two differentsynthesis routes (single phase polymerization and two phasepolymerization) and used for the first time for fabrication of sand-wich type device with a configuration metal Al/PANA-MWNTsnanocomposite/indium tin oxide coated glass (ITO). I–V and C–Vcharacteristics are measured and performance parameters likebarrier height, ideality factor, reverse saturation current, carrierconcentration, built-in potential, and depletion width are calcu-lated for nanocomposite devices. The observed parameter clearly

showed that the electrical and junction properties were muchinfluenced by method of preparation of nanocomposites. Thenanocomposites formed rectifying contacts with cheaper metal likeAl and may be used for construction of cost effective devices. Theoptical characterization of the devices is currently underway.

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88 A.K. Singh et al. / Synthet

cknowledgements

Authors are thankful to Prof. D. Pandey of School of Materialscience and Technology Institute of Technology, BHU, India, Prof.vinash Pandey, Department of Physics, Allahabad University, Indiand Prof. K. Kaneto of KIT, Japan, for their help and supports duringhe experiments. Authors are also thankful to Prof. B.P. Asthana,epartment of Physics, BHU, India for providing the Raman spec-

ra of our samples. A.K. Singh acknowledges CSIR, New Delhi forranting senior research fellowship.

eferences

[1] T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds, Hand book of Conducting Poly-mer, 2nd ed, Publ. Marcel Dekker, New York, 1998.

[2] A.K. Singh, A.D.D. Dwivedi, P. Chakrabarti, R. Prakash, J. Appl. Phys. 105 (2009)114506–114511.

[3] F. Yakuphanoglu, B.F. Senkal, J. Phys. Chem. C 111 (2007) 1840.[4] K. Kaneto, M. Yano, M. Shibao, T. Morita, W. Takashima, Jpn. J. Appl. Phys. 46

(2007) 1736.[5] H. Dong, H. Li, E. Wang, S. Yan, J. Zhang, C. Yang, I. Takahashi, H. Nakashima, T.

Keiichi, W. Hu, J. Phys. Chem. B 113 (2009) 4176.[6] K.H.Ho. Peter, J.H. Ji-Seon Kim, H. Burroughes, F.Y. Becker, T.M. Li Sam, F. Brown,

R.H. Cacialli, Friend, Nature 404 (2000) 481.[7] K. Kaneto, K. Mori, T. Morita, W. Takashima, Jpn. J. Appl. Phys. 47 (2008) 1382.[8] R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano Lett. 5 (2005) 1077.[9] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15.10] C. Shi, Y. Yao, Y. Yang, Q. Pei, J. Am. Chem. Soc. 128 (2006) 8980.

11] R. Prakash, J. Appl. Polym. Sci. 83 (2002) 378.12] N.S. Sariciftci, H. Kuzamany, H. Neugebauer, A. Neckel, J. Chem. Phys. 92 (1990)

4530.13] J.L. Camalet, J.C. Lacroix, S. Aeiyach, K. Chane-ching, P.C. Lacaze, Synth. Met. 93

(1998) 133.14] M.T. Nguyen, A.F. Diaz, Macromolecules 28 (1995) 3411.

[[[

[[

als 161 (2011) 481–488

15] B. Gupta, R. Prakash, Polym. Adv. Technol. 21 (2010) 1.16] X. Dong, D. Fu, Y. Xu, J. Wei, Y. Shi, P. Chen, L. Li, J. Phys. Chem. C 112 (2008)

9891.17] C.T. Hsieh, Y.W. Chu, J.Y. Lin, Int. J. Hydrogen Energy 32 (2007) 3457.18] S. Lee, W. Sung, W. Kim, J. Ok, Y. Kim, Jpn. J. Appl. Phys. 47 (2008) 2339.19] N. Rajalakshmi, H. Ryu, M.M. Shaijumon, S. Ramaprabhu, J. Power Sources 140

(2005) 250.20] Y. He, J. Zhang, S. Hou, Y. Wang, Z. Yu, Appl. Phys. Lett. 94 (2009) 093107–93111.21] S. Iijima, Nature 354 (1991) 56.22] E.M. Byrne, M.A. McCarthy, Z. Xia, W.A. Curtin, Phys. Rev. Lett. 103 (2009)

045502–45511.23] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4

(2005) 864.24] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617.25] V. Saini, Z. Li, S. Bourdo, E. Dervishi, Y. Xu, X. Ma, V.P. Kunets, G.J. Salamo, T.

Viswanathan, A.R. Biris, D. Saini, A.S. Biris, J. Phys. Chem. C 113 (2009) 8023.26] M. Baibarac, P. Gómez-Romero, J. Nanosci. Nanotechnol. 6 (2006) 289.27] A.J. Miller, R.A. Hatton, S.R.P. Silva, Appl. Phys. Lett. 89 (2006) 123115–123121.28] W. Cheung, P.L. Chiu, R.R. Parajuli, Y. Ma, S.R. Ali, H. He, J. Mater. Chem. 19

(2009) 6465.29] H. Yoon, S. Ko, J. Jang, J. Phys. Chem. B 112 (2008) 9992.30] R.K. Srivastava, A. Srivastava, R. Prakash, V.N. Singh, B.R. Mehata, J. Nanosci.

Nanotechnol. 9 (2009) 1.31] E.N. Konyushenko, J. Stejskal, M. Trchova, J. Hradil, J. Kova’rˇova, J. Prokes, M.

Cieslar, J.Y. Hwang, K.H. Chen, I. Sapurina, Polymer 47 (2006) 5715.32] D. Zhang, M.A. Kandadai, J. Cech, S. Roth, S.A. Curran, J. Phys. Chem. B 110 (2006)

12910.33] T.M. Wu, Y.W. Lin, C.S. Liao, Carbon 43 (2005) 734.34] L. Joshi, B. Gupta, R. Prakash, Thin Solid Films 519 (2010) 218.35] Y.T. Shieh, G.L. Liu, H.H. Wu, C.C. Lee, Carbon 45 (2007) 1880.36] F. Yakuphanoglu, E. Basaran, B.F. Senkal, E. Sezer, J. Phys. Chem. B 110 (2006)

16908.

37] S. Quillard, G. Louarn, S. Lefrant, A.G. Macdiarmid, Phys. Rev. B 50 (1994) 12496.38] J. Tauc, Amorphous and Liquid Semiconductors, Plenum Press, London, 1974.39] S.M. Sze, Physics of Semiconductor Devices, 2nd ed, John Wiley & Sons, USA,

1981.40] R. Sahingoz, H. Kanbur,.M. Voigt, C. Soykan, Synth. Met. 158 (2008) 727.41] A.K. Singh, L. Joshi, R. Prakash, K. Kaneto, Jpn. J. Appl. Phys. 49 (01 2010) AD06.