Pulsed DC discharge for synthesis of conjugated plasma polymerized aniline thin film

Applied Surface Science 259 (2012) 691–697

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Pulsed DC discharge for synthesis of conjugated plasma polymerized aniline thin film Tapan Barman, Arup R. Pal ∗ Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Guwahati 781035, India

a r t i c l e

i n f o

Article history: Received 16 May 2012 Received in revised form 18 July 2012 Accepted 19 July 2012 Available online 27 July 2012 PACS: 52.77.Dq 52.80.Pi 81.07.Pr

a b s t r a c t The polymerization of aniline in pulsed dc plasma is studied and the effects of variation of pressure, power, frequency and duty cycle on the chemical structure of the obtained film are examined. During the film deposition optical emission spectroscopy is used to investigate the molecular dissociation of aniline. The chemical structure of the films is characterized using Fourier transform infra-red spectroscopy. The surface morphology is studied using atomic force microscopy. Results show the retention of polyaniline like structure having conjugated nature at some particular discharge conditions. Moreover, it is observed that a strong dependence of film chemistry is obvious on the discharge power, reactor pressure, pulse repetition frequency and duty cycle. The advantages of the pulsed dc for deposition of conjugated plasma polymerizes thin film have been highlighted. © 2012 Elsevier B.V. All rights reserved.

Keywords: Pulsed DC plasma deposition Conjugated polymer Glow discharge Thin film

1. Introduction Conjugated polymers are the backbone of organic electronics and polyaniline (PAni) is the centre of attraction among all conducting polymers due to its unique properties [1–5]. Depending on the synthesis method and process control parameters, polyaniline exists in different chemical structures corresponding to different oxidation states [2,3]. The half oxidized state, emeraldine base is the conducting form of polyaniline when it is protonated using a protonic acid or halogen doping [6–9]. A huge number of conducting polymer based devices are being developed now a days using thin films of polyaniline mostly prepared by chemical or electrochemical routes and subsequent deposition of the polymer in the form of a thin film by spin casting or screen printing [8–13]. Direct synthesis of the polymer in the form of a thin film may reduce number of steps involved in the process. Plasma polymerization is an alternative for direct synthesis of polymer films on many different kinds of substrates. It is a versatile technique that has a number of inherent advantages, such as wide range of monomers can be used, cost effective, less time consuming, synthesis does not require the use of solvents and oxidants and thus gives a product with less contamination and excellent stability

∗ Corresponding author. Tel.: +91 361 2912073; fax: +91 361 2279909. E-mail address: arup [email protected] (A.R. Pal). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.099

[14–16]. Plasma initiation corresponds to the ionization of gaseous molecules through electronic impacts which forms radical cations that are responsible for formation of the polymer [10,11]. Also, plasma generated ultraviolet radiation has a strong influence in oxidation as well as enhanced cross linking of the deposited material [16,17]. In recent years a significant amount of research work has been carried out on the synthesis of conducting polymers by plasma polymerization process [18–24]. These works are carried out mostly using radio frequency plasma and in few cases by DC as well as by low frequency AC plasma. In DC discharge it is difficult to sustain the discharge after few monolayer of film deposition since the deposited film remains in the insulated regime prior to the protonation of the film. Radiofrequency (RF) discharge is so far established to be the only option for synthesis of such films. A good quality conjugated polymer film can be deposited if deposition is performed in energy deficient regime with high flow rate and low power input. Pulsed RF discharge shows better result in terms of structure retention [23,25]. However, in case of plasma polymerization with RF source, the impedance matching is in many cases a tiresome task. Particularly, after one set of deposition the chamber wall gets coated with polymer film, which results in the inefficient grounding of the chamber. Due to this, impedance matching becomes difficult. In the RF plasma polymerization the entire inner wall of the plasma reactor gets coated hence after each run of deposition it is necessary to clean the entire chamber. Discharge cleaning

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The flow of aniline vapour is controlled and recorded by a Vapour Source Mass Flow Controller (1150, MKS Instruments, USA). The pulsed plasma is generated by a pulsed DC generator (Pinnacle Plus, Advanced energy, USA). The glow discharge is initiated in aniline vapour at pulsed DC power of 20–50 W, pulse repetition frequency of 50–150 kHz and duty cycle of 55–85% for a total pressure of 0.05 to 0.15 mbar. Monomer vapour flow at these conditions is 9 to 32 sccm. During polymerization, the vapour pressure is monitored by a capacitance manometer (Baratron, MKS Instruments, USA). The films are deposited on silicon, quartz and microscopic glass substrates which are kept at the powered electrode and polymerization time is fixed for 20 min. Powered electrode is kept cooled by circulating chilled water at 10◦ centigrade. During the deposition process diverse species formed in the plasma are analyzed using Optical Emission Spectroscopy (Andor Technology, UK, Shamrock SR-303i spectrometer). Fig. 1. Block diagram of the plasma reactor.

is not sufficient in most of the cases; hence mechanical etching is the only way to practically avoid this problem. Pulsed DC could be an alternative to RF discharge for synthesis of conducting polymer films. In general, pulsed DC is popularly used for deposition of thin films by plasma based sputtering process. Report on the synthesis of any polymer film by pulsed DC is scarce and to the best of author’s knowledge no report is available in literature on the deposition of conducting polymer by pulsed DC plasma. It has been observed in this experiment that desired conjugated polymer structure can be achieved using pulsed DC as well. Due to the difficulties involved in RF plasma polymerization and cost effectiveness of pulsed DC power supply it could be a better choice for plasma polymerization of conjugated polymers. Results of a detailed investigation on the applicability of pulsed DC discharge for synthesis of plasma polymerized aniline (PPAni) thin film is presented in this report. 2. Experimental set up and procedure The plasma reactor (Hindhivac, India) used in this work is a stainless steel (SS) cylindrical chamber of 40 cm in length and 30 cm in diameter (Fig. 1). The reactor is equipped with a capacitively coupled planar stainless steel electrode of 10 cm in diameter for the synthesis of films by plasma polymerization processes. The system is evacuated to a base pressure of 1.5 × 10−5 mbar by using a diffusion pump in combination with a rotary pump. Guaranteed Reagent (G. R.) grade aniline (Merck, India) without further purification is introduced into the reactor from a glass container in vapour form through a set of needle valve and stop valve.

2.1. Characterization of plasma polymerized aniline (PPAni) The thickness of the film is measured by a surface profiler (Dektak 150, Veeco). One part of the substrate is covered by a micro-slide during the film deposition, which results in the formation of a step due to film deposition. Using a profillometer by scanning the height versus position on the sample surface the step height can be found out. The height of the step is a measure of film thickness. By this procedure the final film thickness has been measured. It has been found that deposition rate is low at a pressure of 0.05 mbar, which varies from 6.5 nm/min to 30 nm/min, having film thickness from 130 nm to 600 nm, with frequency variation from 50 kHz to 150 kHz at a fixed duty cycle of 55%. With increasing the working pressure the deposition rate is found to increase. At a pressure of 0.1 mbar the deposition rate it varies from 23.3 nm/min to 44.3 nm/min and corresponding film thickness variation from 466 nm to 886 nm, with frequency variation from 50 kHz to 150 kHz at a fixed duty cycle of 55%. With the variation of duty cycle from 55% to 85% the deposition rate varies from 46.8 nm/min to 110.1 nm/min and corresponding film thickness variation from 936 nm to 2202 nm at a pressure of 0.15 mbar and frequency of 125 kHz. Table 1 lists the different deposition conditions and corresponding discharge parameters. The plasma-polyaniline films have been characterized with Fourier transform infra-red (FTIR), spectroscopy in order to study its chemical structure. FTIR measurements were performed on a Bruker Vector 22 FT-IR spectrometer in transmission mode using a DTGS detector over the range of 400–4000 cm−1 at 2 cm−1 resolution averaged over 32 scans. For FTIR analysis, PPAni is deposited directly onto low doped silicon substrate.

Table 1 Discharge parameters for different deposition conditions. Pressure (mbar)

Power (W)

Frequency (kHz)

Reverse time (␮s)

Duty cycle (%)

Discharge voltage (V)

Discharge current (mA)

0.05

50 25

50 50 50 75 100 125 150

9 9 9 6 4.5 3.6 2.9

55 55 55 55 55 55 55

340 239 206 213 201 190 180

150 100 100 90 100 100 110

0.10

20

50 75 100 125 150

9 6 4.5 3.6 2.9

55 55 55 55 55

210 210 201 189 181

100 100 100 110 110

125

1.20 2.40 3.60

85 70 55

369 260 189

50 80 110

0.15

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Table 2 Identified spectral lines/bands of wavelength spectra. Wavelength (nm) 337.09 357.52 370.83 375.18 380.42 385.79 394.28 399.72 405.96 419.43 586.88 590.38 606.16 738.70 750.32 762.53 385.54 386.22 387.54 390.83 391.15 394.49 399.46 402.86 336.63 402.59 387.13 471.22 563.90 605.80 611.24

Plasma species

N2

Electronic transition

0,0 0,1 2,4 1,3 0,2 4,7 2,5 1,4 0,3 2,6

C 3 − B 3

10,6 9,5 6,2 6,4 4,2 3,1

N2 B3 –A3 

–2 , ground state

CN

2

NH

A3 –X3 , ground state 2

–2 , ground state

CH C2

3

–3 , ground state

Surface roughness of the composite film deposited on silicon substrate is measured by an Atomic Force Microscope (NTMDT, Russia Model: Solver Pro 47). The root mean square (rms) surface roughness, Rq is determined using the software supplied with the AFM instrument.

Vibrational transition

3,3 2,2 1,1 10,10 11,11 12,12 13,13 14,14

References

[27–32]

[32]

[27,32]

[27] 1,1 0,0 2,1 0,1 2,4 1,3

[32]

[32]

disappearance of atomic lines and drop in the emission intensity implies that benzenoid ring opening has been reduced in the high pressure process at 0.15 mbar since the electron temperature decreases significantly [26]. Hence, it may be inferred that higher working pressure is favourable for structure retention in the polymer film.

3. Results and discussion 3.1. Structure of the pulsed DC plasma deposited PPAni films The aniline molecule has the ionization energy of 7.72 eV, electrons having energy above this value may lead to the formation of radical cations. The radical cations drift to the cathode which is at a negative voltage at about 200 V or more, reaction among the radical cations ultimately results in the formation of polymer layer on the substrate surface. Optical emission spectroscopy (OES) provides information about the different species formed in the plasma during the deposition process. For formation of a good quality conducting polymer films dissociation of monomers should be very low. The bands from OES spectra are determined according to the records available in literature [27–32]. Table 2 lists the different species formed during plasma deposition of PPAni film and the corresponding spectra are presented in Fig. 2. While depositing PPAni several molecular bands are appearing in the optical emission spectra e.g. N2 molecular bands in the ranges of 350–358 nm, 371–406 nm and 727–775 nm. A very strong emission corresponding to C2 appears in the range of 592–612 nm. CH and CN emission bands also emerge in the ranges of 410–440 nm and 385–389 nm respectively. The appearance of carbon containing species e.g. C2 , CH and CN signifies that the benzenoid ring has been ruptured partially due to interaction with energetic electrons in the plasma to release carbon from the structure of the monomer. At lower pressure of 0.05 mbar the dissociation of different functional groups are significant and hence there appears some atomic lines in the emission spectra. The

Retention of conjugated structure is the necessary condition for a polymer film to be conducting. In case of polyaniline like structure presence of benzenoid di-amine and quinoid di-imine in equal amount signifies the retention of conjugated structure. Fourier

Fig. 2. Optical emission spectroscopic data recorded during deposition at power of 20 W, pulse repetition frequency of 125 kHz and duty cycle of 55% for three different pressures.

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Fig. 3. FTIR transmittance spectra (a) at three different powers of 20 W, 25 W and 50 W, (b) with frequency variation from 50 kHz to 150 kHz, (c) with pressure variation from 0.05 mbar to 0.15 mbar and (d) at three different pulse reversal times (duty cycles) of 1.2 ␮s (85%), 2.4 ␮s (70%) and 3.6 ␮s (55%).

transform infra-red spectroscopy is a very useful tool for study of such structure retention which indicates the presence of benzenoid and quinoid units by showing strong absorption in the wave numbers around 1500 cm−1 and 1600 cm−1 . The FTIR measurement depicts that under certain experimental conditions pulsed DC plasma deposited PPAni retains most of the characteristic bands of linear polyaniline. Depending upon the plasma control parameters e.g. power, pressure, pulse repetition frequency and duty cycle the chemical structure of the film changes from fully oxidized to half oxidized states. Moreover, cross linking in the deposited film structure is obvious which is found to be strongly dependent on the experimental parameters.

3.1.1. Dependence of film properties on pulsed DC power Fig. 3(a) shows the IR spectra of the plasma polymerized aniline at different discharge powers from 20 to 50 W at a pressure of 0.05 mbar, frequency of 50 kHz, pulse reversal time of 9 ␮s and duty cycle of 55%. The FTIR band assignment is given in Table 3. A very strong band corresponding to the quinoid unit appears at 1602 cm−1 and a weak band corresponding to the benzenoid unit appears at 1497 cm−1 at a power of 20 W. The decrease of the band at 1497 cm−1 and increase of that at 1602 cm−1 with increasing power has also been observed. As presented in Table 1, discharge voltage goes on increasing significantly from 206 V to 340 V with an increase in power from 20 W to 50 W, which in turn leads to an increase in the energy of the ions bombarding the substrate. This energetic ion bombardment may be the reason for reduction of benzenoid units at higher powers.

The appearance of the band at 2930 cm−1 assigned to saturated C H stretch shows the partial loss of aromaticity during plasma polymerization. This intensity of 1602 cm−1 increases with the power and the band at 2930 cm−1 increases whereas that at 3030 cm−1 decreases. Therefore, the loss of aromaticity is more at high powers, whatever the plasma conditions are. The more the input power is important, the more the band at 1497 cm−1 decreases. The aromatic ring retention in the plasma polymer can be analysed by the evolution of two IR bands: at 2930 cm−1 assigned to saturated C H stretch vibration, and at 1497 cm−1 attributed to aromatic C C stretch vibration of benzenoid unit. The loss of aromaticity by ␲ bond scission and aromatic ring opening is minimized at 20 W before which it is difficult to obtain glow discharge. At high discharge powers the IR bands become broader meaning

Table 3 Assignment of different peaks observed in FTIR analysis. Wave number (cm−1 )

Assignment

3379–3365 3052–3028 2949–2921 2212 1602 1497 1385 1313–1308 835 758 696

Secondary amine N H stretch Unsaturated (aromatic and alkenes) C H stretch Saturated C H stretch Nitrile C N stretch C C stretch of quinoid unit C C stretch of benzenoid unit C N stretch of QBQ C N stretch (QBQ, QBB, BBQ) 1,4 di-substituted aromatic ring (para) 1,2 di-substituted aromatic ring (ortho) 1,3 di-substituted aromatic ring (meta)

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that the polymer is highly disordered without discernable repeating units.

Fig. 4. Relative intensity (area under the curve) of Quinoid and Benzenoid units, estimated from FTIR spectra for different deposition conditions.

3.1.2. Dependence of film properties on pulse repetition frequency The results of FTIR spectroscopic study on the films deposited at a power of 20 W, pressure of 0.1 mbar and duty cycle of 55% with frequency variation from 50 to 150 kHz is presented in Fig. 3(b). It shows slight increase of the saturated CH band at 2930 cm−1 , which implies small increase in the aliphatic content of the film material. Moreover, both the bands at 1497 cm−1 and 1602 cm−1 corresponding to the benzenoid and quinoid unit become prominent when pulse repetition frequency increases. The IR bands at around 758 cm−1 , due to ortho di-substituted aromatic ring, and at around 696 cm−1 assigned to meta disubstituted aromatic ring. These IR bands give evidence of chain scission and cross-linking of PPAni. The plasma polymer crosslinking can be evaluated by the relative intensities of the peaks at 696, 758 and 835 cm−1 . The weak intensity of this last peak compared to the meta and ortho di-substitutions is the sign of a branched and cross-linked polymeric structure, which is

Fig. 5. (a) Atomic force microscopic image of the film deposited at a power of 20 W, frequency of 125 kHz, duty cycle of 55% and pressure of 0.15 mbar, (b) AFM phase diagram and (c) Histogram of the brightness distribution of roughness height.

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prominent at higher frequencies. The band at 2212 cm−1 corresponding to nitrile vibration and is associated to a possible chain termination.

3.1.3. Dependence of film properties on working pressure Fig. 3(c) shows the IR spectra at power of 20 W, frequency of 125 kHz and duty cycle of 55% with pressure variation from 0.05 to 0.15 mbar. The variation in the polymer structure is most significant in case of change in working pressure. It has been observed that benzenoid unit retention is poor at a pressure of 0.05 mbar. At lower pressures electron temperature is higher, which leads to effective dissociation of monomer units and hence the film becomes quinoid dominant [26]. With increasing the working pressure the structure of the polymer becomes better and at a pressure of 0.15 mbar a fairly good conjugated structure is obtained which is having almost equal amount of benzenoid and quinoid units. This observation is supported by the optical emission spectroscopic results, which shows enhanced dissociation of monomer units at lower pressure and retention of structure at higher pressures.

3.2. Surface morphology of the film The surface morphology of the PPAni film has been studied using atomic force microscopy. Image presented in Fig. 5(a) show almost uniform surface morphology for the film deposited at the best deposition condition as mentioned above. Formation of some ripple like structure is clearly observed on the film surface, the exact reason for the formation of such structures is not obvious. AFM phase image is presented in Fig. 5(b). A uniform single phase film formation is observed except some nanoscale inclusions, which may be due to some unreacted monomer or some powder formation in the bulk plasma which may be incorporated into the film. The histogram of brightness distribution of roughness height of the film surface is shown in Fig. 5(c). It shows that the complete height variation is having a narrow distribution with height variation up to 4.5 nm with rms roughness of 3.18 nm. Slightly higher roughness of the film may be related to the higher ion energy in the pulsed discharge environment. However, the roughness level is low enough for most of the electronic or optoelectronic applications. 4. Conclusion

3.1.4. Dependence of film properties on pulse duty cycle Fig. 3(d) shows the plot of IR spectra for different pulse reversal times (duty cycles). The intensity of the band at 1497 cm−1 decreases with increase in duty cycle. It shows that retention of conjugated structure is better at higher pulse reversal time (lower duty cycles). This is due to the fact that during the pulse on-time, radicals and ions are initiated; while only less energetic radicals are retained during the plasma off-time and can react with new monomer molecules. Thus, the use of a pulsed discharge creates polymer compositions favourable for retention of conjugated structure necessary for electrical conduction. Fig. 4 represents the relative intensity of the IR absorption bands of benzenoid (1497 cm−1 ) and quinoid (and 1602 cm−1 ) units for different deposition conditions. It summarizes the outcome of this experimental study. It is obvious that the film quality is worst with negligible amount of benzenoid units at a pressure of 0.05 mbar at higher powers from 25 to 50 W. Also, the film adhesion to the substrate is very poor and film peeled out from the substrate after few hours of deposition, the reason why the thickness of the film could not be measured for the films deposited higher powers. As shown in Table 1, the discharge voltage varies from 239 to 340 V for this power range. Since pressure is not so high, the effect of collision between ions accelerated towards the substrates and the neutrals are not very significant. Hence, the poor quality of the film may be due to energetic ion bombardment at the substrate during the growth process. The result is slightly better at a power of 20 W when discharge voltage reduces to 206 V. In the present study a highly conjugated structure is obtained at input power 20 W, reactor pressure 0.15 mbar, frequency 125 kHz and pulse repetition frequency 3.6 ␮s. Film structure can be associated to the W/FM parameter (where W is the power, F the monomer flow rate and M the molecular mass of the monomer) as introduced by Yasuda [16]. Low values of W/FM, means at low discharge powers, for the same monomer and flow rates, the plasma phase is energy deficient. Then fragmentation of the monomer is low, leading to high retention of the monomer structure. On the other hand, high values of W/FM, means at high discharge powers, plasma phase is monomer deficient inducing the dissociation monomeric bonds up to a greater extent. The best quality conjugated polymer obtained at input power 20 W, reactor pressure 0.15 mbar, frequency 125 kHz and pulse repetition frequency 3.6 ␮s (duty cycle of 55%) is supported by the fact that at this particular condition the value of W/FM parameter is the lowest under present experimental conditions.

A detailed experimental investigation has been carried out on the appropriateness of the pulsed DC plasma for synthesis of conducting polymer thin film. Results show that conjugated structure retention is possible in the pulsed DC plasma, particularly at higher working pressures i.e. higher flow rates. The structure retention is similar in case of pulsed DC prepared film as compared to the film deposited by RF plasma as reported in literature. It may be inferred that pulsed DC plasma could be an alternative to RF plasma because of the simplicity in operation and cost effectiveness involved in the process. Acknowledgements Financial support from the Department of Science & Technology, Govt. of India (Grant No. SR/FTP/PS–41/2008) is thankfully acknowledged. Authors thank Department of Physics, Indian Institute of Technology, Guwahati and Birla Institute of Technology, Mesra for sample characterization. References [1] J.-C. Chiang, A.G. MacDiarmid, Synthetic Metals 13 (1986) 193. [2] W.-S. Huang, B.D. Humphrey, A.G. MacDiarmid, Journal of the Chemical Society, Faraday Transactions 1 (82) (1986) 2385. [3] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer 30 (1989) 2305. [4] S. Quillard, G. Louarn, S. Lefrant, A.G. MacDiarmid, Physical Review B 50 (1994) 12496. [5] C.-G. Wu, T. Bein, Science 264 (1994) 1757. [6] H. Shirakawa, E.J. Louis, A.G. Macdiarmid, C.K. Chiang, A.J. Hegger, Journal of the Chemical Society, Chemical Communications 16 (1977) 578. [7] C.K. Chiang, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G. Macdiarmid, Physical Review Letters 39 (1977) 1098. [8] J. Stejkal, R.G. Gilbert, Pure and Applied Chemistry 74 (2002) 857. [9] J. Stejkal, I. Sapurina, Pure and Applied Chemistry 77 (2005) 515. [10] L. Chvatalova, R. Cermak, A. Mracek, O. Grulich, A. Vesel, P. Ponizil, A. Minarik, U. Cvelbar, L. Benicek, P. Sajdl, European Polymer Journal 48 (2012) 866. [11] A. Mracek, M. Lehocky, P. Smolka, O. Grulich, V. Velebny, Fibers and Polymers 11 (2010) 1106. [12] A. Tsumura, H. Koezuka, T. Ando, Applied Physics Letters 49 (1986) 1210. [13] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, ACS Nano 4 (2010) 1963. [14] Y.-H. Kim, M. Kim, S. Oh, H. Jung, Y. Kim, T.-S. Yoon, Y.-S. Kim, H.H. Lee, Applied Physics Letters 100 (2012) 163301. [15] S. Sharma, A.R. Pal, J. Chutia, H. Bailung, N.S. Sarma, N.N. Dass, D. Patil, Applied Surface Science 258 (2012) 7897. [16] H. Yasuda, Plasma Polymerization, Academic Press, New York, 1985. [17] A.J. Choudhury, J. Chutia, S.A. Barve, H. Kakati, A.R. Pal, J.N. Mithal, R. Kishore, M. Pandey, D.S. Patil, Progress in Organic Coatings 70 (2011) 75. [18] Yu.M. Yablokov, A.B. Gil’man, N.M. Surin, I.V. Semenov, A.A. Kuznetsov, I.A. Chmutin, High Energy Chemistry 44 (2010) 431. [19] N.V. Bhat, N.V. Joshi, Plasma Chemistry and Plasma Processing 14 (1994) 151.

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