696
Power Beams And Materials Processing - 2002
Structural and Optical Properties of Poly Aniline Thin Films Deposited by R.F. and AC Plasma Polymerization U.S.Sajeev, C.Joseph Mathai, S.Saravanan, S.Venkatachalam *, M.R.Anantharaman§ Department of Physics, Cochin University of Science and Technology, Cochin - 682 022, Kerala, * PCM Division, Vikram Sarabhai Space Centre Thiruvananthapuram-695 022, Kerala, India § Author for Correspondence:
[email protected]
Abstract Polyaniline in its bulk and thin film forms are being investigated widely because of their excellent optical and electrical properties. Preparation of pure and doped polyaniline thin films using rf and ac plasma polymerisation techniques and their characterisation are reported here. The effect of insitu doping of iodine and Camphor Sulphonic Acid (CSA) on the optical band gap of polyaniline thin films prepared by ac and rf are also studied. The structural properties of these films were carried out by FTIR spectroscopy (Fourier Transform InfraRed spectroscopy) and the optical band gap was evaluated from Uv-Vis-Nir measurements. Comparative studies on the optical and structural properties of ac and rf polymerization are presented here. It has been found that the optical band gap of the polyaniline thin films prepared by rf and ac plasma polymerisation techniques differs considerably and the band gap is further reduced by insitu doping of Iodine and CSA. The results are compared and correlated and have been explained with respect to the different structures adopted under these two preparation techniques. Key Words : conducting polymers, plasma polymerization, optical bandgap, polyaniline, ac and rf plasma polymerisation, doped polyaniline.
1. Introduction Plasma polymerization or glow discharge polymerization includes ac, rf and dc techniques. It is an excellent and inexpensive tool for growing uniform, ultra thin, amorphous, and pinhole free polymer layers on different substrates. These films are insoluble in organic solvents and resistant to high temperatures. These properties are due to the highly cross-linked structure of polymer thin films evolved under plasma polymerization [1]. They find extensive application in various devices [2]. Among the various conducting polymers synthesised, polyaniline occupies a prime position, because of its unique characteristics like inexpensiveness of the monomer, ease of processing and excellent stability. It is widely investigated in both thin film and bulk forms. Its electronic and photonic properties are interesting. Polyaniline in its pure and doped states find extensive applications in making devices like Light Emitting Diodes, photovoltaic, sensors, high-density storage batteries and super capacitors. [3]. The low dielectric constant ac plasma polymerised polyaniline has particular use in the microelectronics industry [4]. It has been reported by various researchers that doping of polyaniline both in the bulk and in the thin film forms enhance the electrical conducting properties. Dopants like HCl, Iodine, ClO4, CSA, have been reported to be incorporated in the polyaniline backbone to produce novel materials for different applications [5,6]. However reports on the modification of electrical and optical properties of plasma polymerised polyaniline by the incorporation of dopants is less
Structural and Optical Properties of Poly Aniline Thin Films Deposited by R.F. and...
697
abundant in the literature. It is in this context a systematic study on the preparation of pure and doped polyaniline thin films by employing both rf and ac plasma polymerisation techniques is undertaken. The optical and electrical properties of these films are compared and correlated. An attempt is made to explain the observed change in the structural and optical properties with respect to the structure adopted during polymerisation.
2. Experimental RF Plasma Polymerization: Plasma polymerised thin films of aniline on ultrasonically cleaned glass substrates were obtained by polymerising the aniline monomer under radio frequency plasma discharge in a home built set up. The details of the experimental set up is shown in fig 2.1.The rf plasma polymerization unit consists of a deposition cell made up of borosilicate glass tube of about 0 .5m in length and a diameter of about 0.05m.The monomer is injected into the chamber by means of a needle valve. Power from a radio frequency oscillator is capacitively coupled to the deposition chamber by means of aluminium foils wrapped around the glass chamber. The thickness of the films were measured by utilising a home built set up based on the Tolansky multiple beam interferrometry [7]. The conditions of polymerisation have been standardised and optimised and it has been found that good quality films were deposited under a monomer vapour pressure of 0.1 Torr and a current of 60-80 mA in a frequency range 9 MHz to 11.5MHz. The time of deposition for a film of about 150nm thickness is nearly 5 minutes. AC Plasma Polymerization: The experimental set up for ac plasma polymerization unit (also home built) is depicted in fig 2.2. It consists of two parallel stainless steel electrodes, each of diameter 0.23 m and placed 0.05 m apart. Ultrasonically cleaned glass substrates were placed on the lower electrode for the polymer thin film deposition. The ac plasma polymerization chamber was evacuated using a rotary pump. Monomer aniline is injected into the glass chamber between the electrodes by means of a glass sprayer at a monomer vapour pressure of 0.2 Torr. Plasma discharge was obtained in the chamber by applying a potential of 500-800V between the electrodes at an current in the range 40-70 mA. The time of deposition for growing a film 150nm thickness is around I hour under optimum condition. Doping of The Thin Film Samples: Iodine doping of plasma polymerized aniline thin films was effected by introducing iodine vapour in to the plasma polymerization chamber along with the monomer vapour. The doping of CSA was achieved by dissolving appropriate amount of CSA in the monomer aniline and subjecting it to plasma discharge under similar conditions as described in sections 2.1 and 2.2. Uv-Vis -Nir and FTIR Spectroscopy: The FTIR spectra of pure and doped RF and ac polyaniline thin film samples were recorded by Nicolet Avatar 360 FTIR Spectrophotometer in the wavelength range of 400 cm-1-4000cm-1 under identical conditions. The Uv-Vis-Nir absorption spectra of the samples were recorded using a Hitachi U 3410 Uv-Vis-Nir spectrophotometer in the wavelength region of 300 nm to 2600 nm. The optical band gap of these samples was evaluated from the energy-absorption plot.
698
Power Beams And Materials Processing - 2002
3. Results and Discussion FTIR Studies: Figure 3.1 shows the FTIR spectrum of monomer aniline and polyaniline prepared under ac and rf conditions. Analysis of the FTIR spectra of these thin film samples indicate the presence of aromatic ring stretching (1600 cm-1), C-C stretched vibration (1500 cm-1) persisted even after polymerisation. It can also be seen that the aromatic ring is intact in post plasma polymerised films. It is evident from the spectra (fig 3.1) that the aromatic CN bonding around 1240 cm-1 which is principal to all polymerised samples is found to be blue shifted with respect to the monomer. A quick comparison between the spectra of ac and rf thin films indicate marked structural difference between the polymers evolved by both polymerisation routes namely rf and ac. NH stretching (3220 cm-1) is not very prominent in the ac plasma polymerised samples but its presence is indicated in the rf samples. A peak at 3038 cm-1 corresponds to CH stretching is retained in the polymerised samples. The peak at around 2960 cm -1 corresponding to NH asymmetric stretching in the monomer is found in 2928 cm -1 in ac and 2970 cm-1 in rf samples. The CH in-plane deformation (1175cm-1) is found to be retained in the ac samples but shifted to 1105 cm-1 in rf samples. The FTIR studies and analysis establishes the structural difference in the ac and rf plasma polymerised aniline thin films samples [8]. It can also be seen that doping with iodine modifies the bond length and facilitate the shifting of functional groups. Uv-Vis-Nir Studies: The photon absorption in many amorphous semiconductors is observed to obey the Tauc relation [9] ahg = B (hn-Eopt)n
(1)
Where, a is the absorption coefficient, hn is the Photon Energy, B is a Constant and n =1/2 for direct transition. From the Uv-Vis-Nir spectra the absorption coefficient is plotted in the Y-axis and energy hg in the x-axis and the optical band gap of pure and doped polyaniline thin films are determined from these plots (Fig 3.3.1). The optical band gap corresponding to the films prepared under various deposition conditions is given in table 3.3.1. It can be seen that the optical band gap of rf pani films is considerably less with respect to its ac counterpart. It is also evident from the table that the iodine doping decreases the optical band gap from 3.7 eV to 3.4 eV and CSA doping reduces the band gap to 3.3 eV in the case of ac plasma polymerised thin films. In rf polymerisation a reduction of 0.3 eV in the band gap is noticed with respect to pure and doped samples. However in the case of plasma polymerised thin film polyaniline samples, the optical band gap has been considerably reduced, by around 1.4 eV. The reduction in the optical band gap is probably due to the modification of the polymer structure. DC conductivity studies needs to be carried out along with Nuclear Magnetic Resonance (NMR) for a clear elucidation of the mechanism of reduction in band gap in polyaniline films. The reduction of band gap in the rf polymerised samples arises out of extended conjugated structure evolved in the rf polymerisation mechanism [10]. Doping induces a structural ordering of the polymers due to the incorporation of the charged species. There are signatures supporting these changes in the Uv-Vis-Nir and FTIR spectra.
Structural and Optical Properties of Poly Aniline Thin Films Deposited by R.F. and...
699
Table 3.2.1: Comparison of bandgap of polyaniline samples prepared by rf And ac Plasma Polymerisation Sample polyaniline
Ac plasma Polymerisation (eV)
rf plasma polymerisation (eV)
Pure
3.7
2.25
Iodine doped
3.4
2.0
CSA doped
3.3
2.0
4. Conclusion Employment of rf plasma polymerisation techniques result in low band gap polymeric thinfilms. Incorporation of iodine and CSA reduces the bandgap by around 0.4 eV in both rf and ac. The reduction in the optical band gap in the rf polyaniline films can be attributed to the conjugated structure evolved during the rf plasma polymerisation. However this requires further substantiation.
5. Acknowledgements MRA thanks the Department of Space for financial Assistance received in the form of a project under ISRO-RESPOND, Government of India (File. No. 10/03/354 dtd.23-02-1999)
6. References. [1] H.Yasuda, Plasma Polymerization (Academic Press. Inc (1985)) [2] Xaoyi Gang, Liming Doy, Albert W.H. Mau, Hans J.Griesser, J.of Poly. Sci.: Polymer Chem. Vol 36, 633-643 (1998) [3] C. Joseph Mathai, S.Saravanan, M.R. Anantharaman, S. Venkatachalam, S. Jayalekshmi, J. Phys, D. Appl. Phys.35, 240-2457 (2002) [4] Terja A. Skotheim (Editor), Hand Book of Conducting polymers, (Marcel Dekker, New York) [5] A.G. Macdiarmid, J.C.chiang and A.F.Ritcher, synthetic Metals, 18, 285-290, (1987) [6] A.J.Macdiarmid, Current Appl Phys., 269-279, (2001) [7] Tolansky, Multiple Beam Interferrometry of Surface thin films, Dover, (New York, (1970)) [8] L.J. Bellami, Infrared spectrum of Complex Molecules, John Willy and Sons, (New York (1962) [9] A.Goswami, Thin films Fundamentals, New age Inter National Publishers, (New Delhi (1996)) [10] A.B.P.Lever, Adv.Inorg.Chem.Radio Chem.7, 44 (1965)
700
Power Beams And Materials Processing - 2002
Fig. 2.1. RF plasma polymerisation set up
Fig. 2.2. AC plasma polymerisation set up
T r a n s m itta n c e %
A n ilin e m o n o m e r
R F I o d in e d o p e d
R F p u re
a c io d in e d o p e d
ac -p u re
500
1000
1500
2000
2500
3000
W avenum ber cm
3500
4000
-1
Fig. 3.1.1. FTIR spectrum of plasma polymerised polyaniline thin films
Polyaniline ac polymerisation(pure)
UV-Vis-Nir spectra of Polyaniline ac pure 2.0 2.0
1.5
Absorption
Absorption
1.5
1.0
0.5
1.0
0.5
0.0
0.0
300
350
400
450
500
Wavelength (nm)
Fig. 3.2.1.a Wavelength-absorption graph of ac plasma
Band gap Eg = 3.7 eV 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
photon energy hn (eV)
Fig. 3.2.1.b Calculation of band gap in ac plasma polymerised aniline thin films polymerised aniline thin films