Plasma Polymerized Polyaniline Thin Films by

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

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