Plasma Polymerized Polyaniline Thin Films by Double Discharge Technique
H. Goktas1, T. Gunes1, Z. Demircioglu1, D. Mansuroglu2, I. Kaya3
1
Canakkale Onsekiz Mart University, Physics Department, 17020, Canakkale-TURKEY
2
Middle East Technical University, Physics Department, 06531, Ankara-TURKEY
3
Canakkale Onsekiz Mart University, Chemistry Department, 17020, Canakkale-TURKEY
Abstract: We report herein the characterizations of polyaniline thin films synthesized by using double
discharge technique which has a fast filamentary discharge formed from the superposition of an ordinary lowpressure dc glow discharge and high-current pulsed one. Quartz glass substrates were coated at a pressure of 0.8
mbar, 19 kV pulsed and 2 kV dc potential. The substrates were located at different regions in the reactor to
evaluate the influence of the position on the molecular structure of the obtained thin films.
The molecular structure of the thin films was investigated by XPS, UV-visible, and the morphological studies
carried out by SEM. The XPS, and UV-visible results reveal that the molecular structures of the synthesized thin
films due to the fragmentation of the monomer during the film formation at plasma processes are different from
that of the one produced via conventional techniques. The optical energy band gap values of the as-grown
samples demonstrated that these materials would have potential applications at semiconductor devices. The
morphology of the synthesized thin films has granular structures with different size depending on the location of
the substrate.
Keywords: Plasma polymerization, aniline, filamentary discharge
1. Introduction
Conductive polymers have played a major
role in plastic electronics and photonics due to their
low cost and ease preparation. Polyaniline (PANi)
and its derivatives, probably the most common
conducting polymer, have been studied extensively
due to their good conductivity, electrochromic
properties and environmental stability. [1].
Plasma polymerization is a dry processing
method which is a solvent-free and cost effective
technique for synthesizing organic thin films of
varying thickness on a variety of substrates from
almost any organic vapors. The chemical and
physical structure of the plasma polymerized thin
films are different from the one synthesized via wet
processing method, such as the chemical and
electrochemical one. The parameters which strongly
affect the molecular structure and properties of the
plasma polymer are the monomer itself, gas phase
composition, monomer flow rate, reactor pressure,
plasma power and the geometry of the reactor [2].
Those parameters provide to tailor the polymer thin
films with the desired properties.
The studies presented herein are the
production of PANi thin films by superposing
simultaneously a continuous and pulsed discharge
and the characterization of these samples which is
produced at different location at the plasma reactor.
Plasma polymerized PANi thin films were either
produced via continuous wave [3-5], or pulsed
plasma [6]. The aims are to study the molecular
structure and the morphology of the plasma
polymerized PANi thin films by double discharge
technique employing simultaneously the continuous
wave and the pulsed plasma. The molecular and
morphological structures of the samples were
investigated by X-ray photoelectron spectrometer
(XPS), UV-visible (UV-vis) and scanning electron
microscope (SEM).
Fig. 1. R1, R2, and R3 are the locations of the quartz glass substrates at the experimental set-up.
2. Experimental details
The details of the experimental setup of the
plasma system are given in [7-9]. A part of the
system, shown in Fig.1, consists of three cylindrical
hollow electrodes K1,2, A1, A2 and two quartz tubes
with 100 mm length and 30 mm internal diameter in
between them. The first electrode K1,2 acts as
cathode for both dc and pulsed discharges and the
others act as anodes. While an ordinary low-pressure
glow discharge is operating between hollow cathode,
K1,2 and A1, a high pulsed voltage with a 5 Hz
repetition rate is applied to K1,2 and A2. For specific
values of current and pressure, a filamentary pulsed
discharge with 2 mm diameter is formed along the
symmetrical axis of the tube [7]. And, the duration
of the filamentary discharge is shorter than 0.5 μs
having a few centimeters in length.
The quartz glass substrates are placed at
three different regions at the reactor: one of them is
located inside the hollow cathode (R1) where glow
discharge regime is present, and the others are
located in between K1,2 – A1 (R2), and inside A1
(R3) to determine the effect of the glow and
filamentary discharges. The main discharge
considered as hollow cathode discharge, similar to a
pseudo-spark one, where the hollow cathode effect
(HCE) [10, 11] is observed. Hence, a higher plasma
densities and energies were established at region R1
(see Figure 1).
The thin films were produced at fixed 1.5
kV dc and 19 kV pulsed voltages, with a 5 Hz
repetition rate for a 20 min deposition time at 0.8
mbar operating pressure. The aniline monomer (Alfa
Aesar, A Johnson Matthey Company) was
evaporated at constant temperature, 180 ºC and was
fed to the reactor at the “Gas in” part.
The UV-visible spectrum was recorded by
Analytikjena Specord S600 spectrometer. The
morphology of the films was investigated by FEI
Quanta 400F SEM system, equipped with field
emission gun. The XPS analysis carried out by a
Specs EA 200 system; the measurement performed
by using 279 W Mg Kα X-ray source and SPECS
EA-200 Energy analyzer at a vacuum of 1 x 10−7 Pa
equipped with a hemispherical electron analyzer
operated with a focusing lens at a spot size of 250
μm and at a take-off angle of 90°.
3. Results and discussions
The UV-vis spectra of the liquid aniline
monomer and the PANi thin films are shown in Fig.
2. The maximum absorption wavelength (λmax)
which is attributed to π - π* transition are observed
at around 285 nm for the monomer (Fig. 3 a) and
380, 330 and 280 nm for the thin films produced at
region R1, R2, and R3 (Fig. 2 b), respectively. It’s
known that the length of conjugation directly effects
the observed energy of the π - π* transition, which
appears as the maximum absorption [12]. The
shifting to higher wavelength indicates the increase
of the conjugation length. Although the λmax of the
R3 sample decreases a few nanometers in
wavelength with respect to the monomers, the
absorption edge extend to almost to 450 nm which
also an indication of the increase in conjugation
length.
The UV-visible spectra provide to calculate
an optical band gap by using the equation
Eg=1242/λonset, where the λonset is calculated from the
absorption edges [13]. The obtained Eg value of the
aniline monomer is 3.9 ± 0.2 eV that can be
considered as non-conductive material. However,
the calculated Eg values of the as-grown PANi thin
films for R1, R2 and R3 regions are 2.4 ± 0.1, 2.9 ±
0.1 and 3.2 ± 0.2 eV, respectively. Those values
indicate that except the R3 sample, the thin films
have semi-conducting properties and R1 having the
longest conjugation chain among the others [14].
content towards region R3 (see Table 1) shows that
the N is evacuated from region R1 through R3, and
is included in the volatile products formed in the
plasma.
Fig. 2. The UV–visible absorption spectra of; (a) aniline
monomer, (b) the plasma polymerized PANi at R1, R2, and R3.
The atomic composition at the surface of the
plasma polymerized PANi thin films was
determined with XPS. The results are shown in
Table 1, where xi indicates the percentage of i
elements and the error given for the XPS
measurements is 2.5 %. The stoichiometry of the
monomer (C6H5NH2), carbon to nitrogen ratio of
6:1, almost preserved at the regions of R2 and R3.
The highest XC /XN ratio at R1 indicate that the
dissociation of the monomer takes place due to
hollow cathode effect, and the increase of the N
Fig. 3. SEM images of the R1, R2 and R3 PANi thin films.
Table 1. Atomic composition of the PANi thin films from XPS
R1 (%)
R2 (%)
R3 (%)
energy where the hollow cathode effect observed is
much smoother and more uniform than that of the
one obtained at the filamentary region.
XC
82.3
76.5
77.6
XN
8.8
11.5
12.7
References
XO
8.9
12.0
9.7
XC / XN
9.3
6.6
6.1
[1] X. G. Li, M. R. Huang, and W. Duan, Chem.
Rev. 2002, 102, 2925-3030
[2] H. Biederman, Plasma Polymer Films, Imperial
College Press, London, 2004.
Although oxygen doesn’t exist in the
monomer structure and wasn’t fed to the reactor
during polymerization process, the oxygen content
measured at the surface of the thin films can be
ascribed as when plasma polymer is exposed to the
open atmosphere, the trapped long-lived radicals
with in the network react with the atmospheric gases
and water vapor [2].
The SEM images for the three regions are
given in Figure 3 and the regions are labeled at the
right up corner at each image. Due to the higher
plasma temperature and energy present at R1 and R2
regions, a relatively smooth topography is obtained
there with respect to the other region. The R3 image
reveals that there are assemblies of submicron
particles formed by a process of gas phase
polymerization where the filamentary discharge
takes place.
3. Conclusions
We presented the characterizations of the
plasma polymerized polyaniline thin films produced
by double discharges technique on quartz glass
substrates at different locations in the reactor. The
optical energy band gap values of the as-grown
samples demonstrated that these materials would
have potential applications at semiconductor
devices. The XPS result revealed that comparing
with the UV-vis results, a higher concentration of N
gives a lower optical band gap, Eg. For a concrete
explanation, XPS depth analysis underneath the
surface of the thin films and FTIR analysis is under
investigations. The SEM results revealed that the
films produced at the higher plasma temperature and
[3] G.B.V.S. Lakshmi, A. Dhillon, A. M. Siddiqui,
M. Zulfequar, D.K. Avasthi, European Polymer
Journal, 45 (2009) 2873–2877
[4] S. Saravanan, et., al., Synthetic Metals, 155
(2005) 311–315
[5] C. J. Mathai, et., al., J. Phys. D: Appl. Phys., 35
(2002) 240–245
[6] P. A. Tamirisa, K. C. Liddell, P. D. Pedrow, M.
A. Osman, Journal of Applied Polymer Science, 93,
(2004) 1317–1325
[7] H. Goktas, M. Udrea, G. Oke, A. Alacakir, A.
Demir, J. Loureiro, J. Phys. D: Applied Physics, 38
(2005) 2793
[8] H. Goktas, E. Kacar, A. Demir, Spectroscopy
Letters, 41 (2008) 189–192
[9] H. Goktas, F.G. Ince, A. Iscan, I. Yildiz, M.
Kurt, I. Kaya, Synthetic Metals, 159, (2009) 20012008
[10] G. Schaefer and K.H. Schoenbach (Eds.), Basic
Mechanism Contributing to the Hollow Cathode
Effect, NATO ASI Series B: Physics vol 219, New
York: Plenum, 1989.
[11] V. I. Kolobov and L. D. Tsendin, Plasma
Sources Sci. Technol., 4 (1995) 551
[12] Z. Qi, W. Feng, Y. Sun, D. Yan, Y. He, J. Yu; J
Mater Sci: Mater Electron, 18, (2007) 869–875
[13] K. Colladet, M. Nicolas, L. Goris, L. Lutsen, D.
Vanderzande, Thin Solid Films, 451, (2004) 7–11
[14] J. B. Lambert, et. al., Organic Structural
Spectroscopy, Prentice-Hall, Inc., New Jersy 1998.