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Synthetic Metals 159 (2009) 2430–2436

Contents lists available at ScienceDirect

Synthetic Metals

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

nhanced field emission from polyaniline nano-porous thin filmsn PET substrate

. Goswamia, M.K. Mitrab, K.K. Chattopadhyaya,b,∗

Thin Film & Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, IndiaCentre for Nanoscience and Technology, Jadavpur University, Kolkata 700032, India

r t i c l e i n f o

rticle history:eceived 25 August 2008eceived in revised form 31 July 2009ccepted 7 August 2009vailable online 2 September 2009

a b s t r a c t

Conducting polyaniline (PANI) nano-porous film synthesized on PET substrate by vapor depositiontechnique has shown excellent field emission behaviour and the emission was further enhanced by con-ditioning with vacuum electric discharge. Detailed study on the field emission for PANI films synthesizedwith different oxidant concentration was performed. Measurement was also carried out for differentanode-sample separation. Turn-on field as low as 0.5 V/�m and emission current density as high as

eywords:olyanilineapor phase polymerizationET substrateorphology

20.96 mA/cm2 was observed. The field enhancement factors were found to be in the range of 1192–3782.The films were characterized with X-ray diffraction, UV–vis spectroscopy, Fourier transformed infraredspectroscopy, scanning electron microscopy and also atomic force microscopy. The synthesized PANIfilms may be a promising material for field emission devices and also for plastic display industry.

© 2009 Elsevier B.V. All rights reserved.

lectric dischargeield emission properties

. Introduction

For the past few years conducting polymers are drawing hugettention as an interesting field of research due to their vast range ofpplicability. Polyaniline (PANI) is definitely the most investigatedember of this class because of its ease of synthesis, excellent envi-

onmental stability and many interesting chemical, electrical andptical properties. These properties are promising for wide rangef applications in electronic, optical and magnetic materials such asnformation storage, rechargeable batteries, electrochromic displayevices, sensors and nonlinear optics [1–7]. However, there existany challenging problems regarding their structures, properties

nd efficient means of modification.In recent days, while the role of conducting polymers in the field

mission display (FED) is being extensively studied, mostly in theiranostructure form, thin films of these polymers are comparatively

ess investigated. For example, field emission behaviour of elec-rosynthesized amorphous polypyrrole nanowires (turn-on electriceld 1.2 V/�m and current density 200 �A/cm2 at 2.6 V/�m) [8],

emplate-synthesized PANI nano-fibril array membrane (thresh-ld electric field 5–6 V/�m and maximum emission current densitymA/cm2) [9] have been reported earlier. Musa et al. has previously

eported the ultra-low threshold field emission from conjugated

∗ Corresponding author at: Thin Film & Nanoscience Laboratory, Department ofhysics, Jadavpur University, Kolkata 700032, India. Fax: +91 33 2414 6007.

E-mail address: kalyan chattopadhyay@yahoo.com (K.K. Chattopadhyay).

379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2009.08.007

polymers [10,11]. A recent study has established that PANI-CSAfilm on bare glass or ITO glass substrate, conditioned by electricdischarge can be used as the emitter in field emission devices[12]. However, there are few reports on the thin films of polyani-line (PANI) and their field emission properties. In this study, wehave investigated the field emission property of PANI thin film onpolyethylene terephthalate (PET) substrate prepared by a simplevapor phase deposition technique (VDP). As far as our knowledgegoes, there is only one report on the vapor phase polymerizationof aniline monomer [13] though it may be considered as a sim-ple and cost effective process for synthesizing PANI films. Here toimprove the applicability we have chosen PET as a substrate whichhas already become a useful name in plastic display industry andorganic electronic devices due to its robustness, light weight, thin-ness, flexibility, longevity, and easy scaling-up to large format forlarge volume roll-to-roll production [14,15]. A huge enhancementin field emission property has been also occurred due to the condi-tioning of these films by vacuum electric discharge. Therefore theconditioned PANI films on PET substrate may have a tremendouslyeffective application in future field emission devices.

2. Experimental

2.1. Synthesis of polyaniline (PANI) films

Firstly, we have ultrasonicated the PET substrates (2 cm × 2 cm)with ethanol for 1 h to make them well cleaned. After ultrasoni-

c Metals 159 (2009) 2430–2436 2431

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3.2. Spectroscopic studies

We have studied the spectroscopic characteristics of the filmsby UV–vis spectroscopy and infrared spectroscopy. Fig. 2 shows

Fig. 2. UV–vis-NIR absorption spectrum of PANI film on PET substrate.

S. Goswami et al. / Syntheti

ation ethanol was allowed to evaporate in open atmosphere foraking the substrates dry. Then we have spin-coated each of these

ubstrates with 10% (w/w) ferric chloride (FeCl3) (Merck, Germany)olution in methanol (Merck, Germany), in ambient conditions.fter the solvent (methanol) was evaporated, the substrate with

he oxidant (FeCl3) film was introduced in the reaction chamber.niline monomer (Merck, Germany) was taken in a beaker andas placed in the reaction chamber for 10 min at 60 ◦C. The vaporressure of aniline at 60 ◦C is ∼10 mmHg. Hence during this timeeriod the generated aniline monomer vapor comes into contactith the ferric chloride coated substrate and gets polymerized to

orm a film. All these were done in a closed reaction chamber undertmospheric pressure.

After the above mentioned time duration the films have beenaken out of the chamber and washed several times with methanolo remove the by-product ferrous chloride and the unreacted ferrichloride and aniline monomer. Finally, they were dried out in ovent 60 ◦C for 5 min. In this self-assembly process pure PANI filmsnano-porous) with high conductivity and moderate transparency40–50%) are easily obtained. The thickness and the conductivity ofhe films can be optimized by judiciously varying the oxidant con-entration or the deposition time or the evaporation temperature ofniline. Same conditions have been maintained while preparing thelms on PET substrates with different concentration of the oxidant11%) for field emission study and on silicon (Si) substrates for XRDnd IR studies. The required amount of ferric chloride was mea-ured by a microbalance for preparing the solution of the oxidantn methanol in a particular weight/weight percentage. Later, weave conditioned the PANI films by treating the surface of the filmsith vacuum electric discharge to study if there comes any change

n the field emission characteristics of the films. During this treat-ent, a voltage of 2500 V was applied to the samples at a pressure

f ∼10−4 mbar for 2 min.

.2. Characterization

The formation of polyaniline films were confirmed by (a) X-ay diffraction (performed by Bruker D8 Advanced Diffractometer,perating at 40 kV and 40 mA, by using CuK� radiation with wave-ength � = 0.15406 nm, in the ‘2�’ range of 15–35◦) pattern, (b)ptical transmittance spectra obtained from UV–vis-NIR spec-rophotometer (Shimadzu-UV-3101-PC) in the wavelength rangef 350–1500 nm (taking similar bare substrates as reference andence the spectra gives transmittance of the films only) at roomemperature and (c) infrared absorption spectra obtained from FTIRpectrophotometer (IR Prestige, Shimadzu) in the wavenumberange of 750 to 3000 cm−1. The surface morphology of the filmsas studied by a scanning electron microscope (SEM, JEOL-JSM-

360) and an atomic force microscope (AFM, NT MDT, Solver pro).he average thickness of the PANI films was measured by AFM andas found to be 1.8 �m. We have performed all the field emis-

ion measurements in a home made high vacuum chamber at aase pressure of about 7 × 10−7 mbar with a stainless steel conicalhaped tip (with 1 mm tip radius) as the anode and as-preparedhin film of PANI on PET substrate as the cathode. The current haseen measured by a Keithley Electrometer (model 6514).

. Results and discussion

.1. XRD study

Fig. 1 shows a typical XRD pattern of PANI film on Si substrate.ere we have obtained a broad peak centered about 2� = 25.94◦.his broad peak is the characteristic peak of PANI and also indicateshe amorphous structure of the film [16,17]. Another sharp peak

Fig. 1. XRD pattern of the as-synthesized polyaniline film on Si substrate.

obtained in the diffraction pattern at a ‘2�’ value of 28.59◦ is due tocubic Si (JCPDS card number-03-0544).

Fig. 3. FTIR spectra of the as-synthesized PANI film.

2432 S. Goswami et al. / Synthetic Metals 159 (2009) 2430–2436

repare

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Fig. 4. SEM micrographs of porous polyaniline films (on PET substrates) p

he typical UV–vis-NIR absorption spectrum of the oxidized PANIn the emeraldine salt form. There are three prominent peaks at70 nm, 434 nm and 850 nm in the spectrum. The peak at 370 nm

s due to the �–�* transition between quinoid and benzenoid ringf PANI. The other two optical absorption peaks at 434 nm and50 nm may be interpreted as the result of excitations of valencelectrons to the polaron band during the formation of the conduct-ng emeraldine salt form of PANI. Specially, the appearance of atrong peak at 850 nm firmly supports the formation of the PANI

lm having emeraldine salt structure [13,18]. The transmittancef the as-prepared PANI films is between 50 and 60%. But it can beontrolled (i.e. more or less than 50–60%) by changing the thicknessf the film or the evaporation time of aniline monomer.

ig. 5. The SEM images of the as-prepared polyaniline film: before conditioning underagnification.

d using (a) 10% oxidant concentration and (b) 11% oxidant concentration.

Fourier transmission infrared (FTIR) spectroscopy has been usedto confirm that as-synthesized nano-porous films are that of PANI.For this characterization here we have used the films preparedunder the same condition on Si substrate. From Fig. 3 we can see thecharacteristic absorption peaks of the as-prepared PANI film at 807,1122, 1244, 1304, 1481, 1578 cm−1 which also in turn confirms thesuccessful synthesis of PANI. The peaks at 1578 and 1481 cm−1 aredue to C C stretching of quinoid and benzenoid rings; the peaks at1304 and 1244 cm−1 are attributed to C–N stretching modes of the

benzenoid ring; the peak at 1122 cm−1 indicates the presence ofvibrational modes of –N Q N– (Q stands for quinoid) and anotherpeak at 807 cm−1 represents C–H stretching vibration mode of ben-zene circle of PANI [19].

(a) low and (b) high magnification; after conditioning under (c) low and (d) high

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S. Goswami et al. / Syntheti

.3. Morphological studies

The morphology of the as-synthesized products has been exam-ned by scanning electron microscope (SEM), before and afterreating with electric discharge. Fig. 4a and b shows the SEM micro-raphs of two representative PANI films synthesized with 10%nd 11% oxidant concentrations, respectively. It can be seen fromhe micrographs that the films thus produced are nano-porous inature having a network of elongated clusters of polyaniline. TheEM images shown in Fig. 5a–d clearly reveal the fact that there isdifference between the film before and after the treatment with

lectric discharge (images for 10% oxidant only shown here). Thelm has been ruptured after the conditioning (i.e. vacuum electricischarge) and the clusters have been separated in some regionsthe distance among these regions are clearly visible from the high

agnification SEM image as shown in Fig. 5d).We have also taken the AFM images of the PANI films to study

heir surface morphologies. The PANI films prepared using differ-nt oxidant concentrations (10% and 11%) have almost the sameind of surface morphologies as shown in Fig. 6a (for 10% oxidantoncentration) and 6b (for 11% oxidant concentration). But the sur-ace morphology changed significantly after treating with vacuum

ig. 6. AFM images of the PANI films: (a) for unconditioned film prepared with 10% oxidantnd (c) for conditioned film (arrows clearly showing the effected sites).

ls 159 (2009) 2430–2436 2433

electric discharge as can be seen from Fig. 6a and c. The surface ofthus conditioned film has a number of sites affected by the afore-said treatment, some of which can be easily identified from Fig. 6c(deep-hole-like morphology as shown by the arrows in the figure).

The mechanism of vapor phase polymerization of PANI can bedescribed as suggested by others [20–23]. Firstly, the oxidation ofaniline monomer into a radical cation form (C6H5NH2

•+) by the oxi-dant takes place, then occurs the formation of the intermediatelyoxidized nitrenium cation (C6H5NH+) and finally the continuouscombination of these intermediates results in the formation of PANI(in emeraldine salt form having Cl− as counter-anion).

Chemical composition of the as-synthesized PANI salt, i.e. C,H, N were determined by combustion analysis method (Eager200 model, C, H, N, O). Cl was determined using Hg2+ (mer-curiometric determination) after Pt-catalyzed combustion of thesample in an oxygen atmosphere. The weight percentages of thedifferent elements obtained are as follows: C (wt%): 64.77, H

(wt%): 5.67, N (wt%): 10.32, Cl (wt%): 12.17; Cl/N (molar ratio):0.47.

We have not observed any peak of Fe or its by-products inthe XRD spectra. Moreover, in the FTIR spectra also we havenot observed any absorption peak due to Fe related compounds.

concentration, (b) for unconditioned film prepared with 11% oxidant concentration

2 ic Metals 159 (2009) 2430–2436

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434 S. Goswami et al. / Synthet

herefore, it can be concluded that no residual Fe was present inhe PANI films.

.4. Field emission measurement

Figs. 7–9 show the emission current vs. applied field character-stics of as-prepared PANI films for different conditions. One set of

easurements have been carried out with different anode–cathodeistance for a particular film and another set for films synthesizedith different oxidant concentration. Field emission was also mea-

ured after conditioning the films with electric discharge. The roomemperature sheet resistance of as-prepared PANI films on PETubstrate was 12 k�/square. After discharge the sheet resistancencreased to ∼1.4 M�/square. The change of sheet resistance of thelms could be explained by the fact that after discharge some island

ormation in the films occurred (physically disconnected regions).his was supported by the AFM observation shown in Fig. 6c. TheV–vis spectra of the films measured after discharge (not shownere) were similar to that of films before discharge as shown inig. 2. Hence it can be concluded that there was no change of oxi-ation state of PANI due to discharge and the change of surfaceesistivity was due to the disruption of the films as discussed above.

In order to understand the field emission behaviour of our

ample, we have used the most familiar Fowler-Nordheim (F-N)quation:

=(

˛Aˇ2E2

˚

)exp[−B˚3/2(ˇE)−1] (1)

ig. 8. (a) Comparative I–E plot of field emission from as-synthesized PANI film (10% oxi50 �m. (b and c) Corresponding F-N plots of unconditioned and conditioned film.

Fig. 7. The I–E plot of field emission from the polyaniline films on PET substrate (oxi-dant concentration used during synthesis, 10%) for three different anode–cathodedistances. Inset: corresponding F-N plot.

where I is the emission current, E is the macroscopic field which

is calculated from the external applied voltage (V) divided byanode-sample spacing (d), ˛ is the area of emission, ˚ is the workfunction of the sample material, and ˇ is the field enhancementfactor. A and B are constants having values 1.56 × 10−10 A eV V−2

and 6.83 × 103 V �m−1 eV−3/2 respectively. Fig. 7 shows I–E plot of

dant concentration used) before and after conditioning at an electrode distance of

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oxidant concentration) with an initial emission current of 1.65 �Aand Fig. 10b shows the result for conditioned PANI film (preparedwith 10% oxidant concentration) with an initial emission current of658 �A (current density 20.96 mA/cm2). As we can see from these

S. Goswami et al. / Syntheti

nconditioned PANI film (10% oxidant concentration has been useduring synthesis of this film) for different anode-sample separa-ion (d), and the inset shows the corresponding ln(I/E2)–1/E plotF-N plot). The turn-on field (Et), which we define as the macro-copic field needed to obtain an emission current I = 0.034 �A,re 6.30, 7.04 and 12.56 V/�m for the ‘d’ values of 120, 150 and80 �m respectively. So the turn-on field value is increasing withhe increasing value of anode-sample spacing. By determining thelope of the corresponding F-N plot (Fig. 7, inset) using the workunction value of PANI (4.8 eV) [24], the field enhancement factor,, has been calculated and its values are 1192, 2425 and 3782 forifferent d values. Here the value of ˇ is increasing with increas-

ng ‘d’ values that can be described as follows: according to somerevious report [25],

local = ˇE =(

1 + sd

r

)E (2)

here Elocal is the local electric field nearby the emitter tip, E is theacroscopic field (applied field), r is the radius of the emitter and

is a parameter which describes the degree of the screening effect.herefore for a particular film (s and r being fixed) at a fixed appliedeld (E) if d increases then ˇ will also increase. It is expected thathe result obtained in our case has followed the above relation (2).t may be mentioned here that although the intercept term of F-

plot also contains the enhancement factor, but it is difficult toetermine that from the intercept, because it contains ‘effectivemission area’, which is not precisely known and hence large errorill be introduced in such determination.

Now, in Fig. 8a we have shown the comparative I–E plot for theeld emission from unconditioned and conditioned PANI film at annode-sample separation of 150 �m. A huge enhancement in themission current can be easily observed for the film after condition-ng. The turn-on field has been reduced to a value as low as 0.5 V/�mnd the field enhancement factor ˇ (calculated from F-N plot ofig. 8b and c) has been increased to a very high value like 3642,hich was previously 2425 (before conditioning of the film). These

alues are far better than some previously reported data [12]. Actu-lly, due to conditioning with vacuum electric discharge, the wholeorous network of the unconditioned film has been separated inome regions. The interspacing among these regions being quitearge, the electrostatic screening effect has been greatly reduced

hich finally enhanced the field emission and thus the ˇ value islso increased.

Fig. 9 shows the I–E plots of field emission from the PANIlms synthesized using different concentrations of the oxidantFeCl3) (anode-sample separation during all these measurementas 150 �m) and the inset shows the corresponding F-N plots.

rom the I–E plot we can infer that the emission current has beenncreased for the case of larger oxidant concentration. The turn-oneld and the field enhancement factor as calculated for 10% and 11%xidant concentrations are 7.04 V/�m, 3.31 V/�m and 2374, 1975espectively. With the increase of oxidant concentration duringynthesis, number of nucleation centers increases and eventuallyhe number of emitter sites also increases, i.e. the film becomesenser. Due to the increase in emitter site number the emission cur-ent has been increased for larger oxidant concentration. However,he enhancement factor has been decreased for larger concen-ration of the oxidant and the probable reason may be that thelm when becoming denser enhances the electrostatic screeningffect.

For practical applications as an emitter material, the emission

urrent stability of all the films has been tested under high vac-um condition for a long period of time. Fig. 10 shows the resultf emission current vs. time for a period over 1 h at a pressuref 7 × 10−7 mbar for an applied field fixed at E = 20 V/�m. Fig. 10ahows the result for unconditioned PANI film (prepared with 11%

Fig. 9. Emission current (I) vs. macroscopic field (E) curves of PANI films on PETsubstrate for different concentration of oxidant (ferric chloride) during synthesis(at an electrode distance of 150 �m). Inset: corresponding F-N plot.

Fig. 10. Field emission current stability of the as-prepared PANI film under anapplied field of 20 V/�m at 7 × 10−7 mbar: (a) for unconditioned film, synthesizedwith 11% oxidant concentration and (b) for conditioned film, synthesized with 10%oxidant concentration.

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gures, the fluctuation of the field emission current is very small.ll the above results indicate that as-synthesized PANI film is aromising field emission material.

. Conclusion

PANI films synthesized by vapor phase polymerization on PETubstrates can be used as a good and stable electron field emitter.he emission current is greatly enhanced (current density obtainedp to 20.96 mA/cm2 at an applied field of 20 V/�m which is muchreater than that of previously reported data) due to the condition-ng of the film with vacuum electric discharge and the film remainsstable emitter even after the conditioning. The oxidant concen-

ration for synthesis also has immense effect on the field emission.esides, the use of PET as a substrate would widen its applicabilityspecially in field emission display. Moreover, the PANI films syn-hesized in this process have a porous network of elongated clustersnd particle like structures, which may be a good enough reasono think of their application in sensors due to their large surfacerea.

cknowledgments

SG wishes to thank the University Grants Commission (UGC), theovernment of India for providing her a junior research fellowshipuring the execution of the work and for financial support underhe University with ‘potential for excellence’ scheme.

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als 159 (2009) 2430–2436

References

[1] A.G. MacDiarmid, Angew. Chem. Int. Ed. 40 (2001) 2581.[2] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269 (1995) 1086.[3] O. Kalinina, E. Kumacheva, Macromolecules 32 (1999) 4122.[4] P. Alexander, O. Nikolay, K. Alexander, S. Galina, Prog. Polym. Sci. 28 (2003)

1701.[5] G.R. Pedro, Adv. Mater. 13 (2001) 163.[6] Y. Cao, P. Smith, A.J. Heeger, U.S. Patent 5,232,631 (1993).[7] S. Ding, D. Chao, M. Zhang, W. Zhang, J. Appl. Polym. Sci. 107 (2008) 3408.[8] H. Yan, L. Zhang, J. Shen, Z. Chen, G. Shi, B. Zhang, Nanotechnology 17 (2006)

3446.[9] C. Wang, Z. Wang, M. Li, H. Li, Chem. Phys. Lett. 341 (2001) 431.10] I. Musa, D.A.I. Munindrasdasa, G.A.J. Amaratunga, W. Eccleston, Nature 395

(1998) 362.11] I. Musa, W. Eccleston, G.A.J. Amaratunga, IEEE 98 (1998) 867.12] G. Lai, Z. Li, L. Cheng, J. Peng, J. Mater. Sci. Technol. 22 (2006) 677.13] J.Y. Kim, J.H. Lee, S.J. Kwon, Synth. Met. 157 (2007) 336.14] A.N. Banerjee, C.K. Ghosh, K.K. Chattopadhyay, H. Minoura, A.K. Sarkar, A. Akiba,

A. Kamiya, T. Endo, Thin Solid Films 496 (2006) 112.15] H. Lee, S. Hong, K. Yang, K. Choi, Appl. Phys. Lett. 88 (2006) 143112.16] S.C. Raghavendra, S. Khasim, M. Revanasiddappa, M.V.N.A. Prasad, A.B. Kulkarni,

Bull. Mater. Sci. 26 (2003) 733.17] L. Li, J. Jiang, F. Xu, Mater. Lett. 61 (2007) 1091.18] J. Chen, B.W. Jensen, Y. Pornputtkul, K. West, L.K. Maquire, G.G. Wallacea, Elec-

trochem. Solid-State Lett. 9 (2006) C9.19] K.R. Reddy, K.P. Lee, A.G. Iyengar, J. Appl. Polym. Sci. 104 (2007) 4127.20] F. Lux, Polymer 35 (1994) 2915.

22] N. Gospodinova, L. Terlemezyan, L. Mokreva, K. Kossev, Polymer 34 (1993) 2434.23] E.M. Genies, M. Lapkowski, J. Electroanal. Chem. 236 (1987) 189.24] D.P. Halliday, J.M. Eggleston, P.N. Adams, I.A. Pentland, A.P. Monkman, Synth.

Met. 85 (1997) 1245.25] Q. Zhao, J. Xu, X.Y. Xu, Z. Wang, D.P. Yu, Appl. Phys. Lett. 85 (2004) 5331.