Elsevier

Thin Solid Films

Volume 519, Issue 3, 30 November 2010, Pages 1003-1006
Thin Solid Films

Experimental investigations of semi-crystalline plasma polymerized poly(3-octyl thiophene)

https://doi.org/10.1016/j.tsf.2010.08.031Get rights and content

Abstract

Plasma polymerized thin film of conducting poly(3-octylthiophene) was deposited at room temperature by plasma enhanced chemical vapor deposition method using (3-octylthiophene) monomer as precursor. The radio frequency (RF: 13.56 MHz, power supply: 30 W) was applied at constant argon gas pressure for the formation of plasma. Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), X-ray diffractometry (XRD) and high resolution transmission electron microscopy (HRTEM) have been done for the as grown films. As reported in literature polythiophenes prepared by rf plasma polymerization are highly crosslinked and amorphous. However, in present investigations, well defined crystalline regions have been observed by HRTEM investigations and have been correlated with X-ray diffraction data. The observed crystallinity is attributed to controlling the parameters of the synthesis.

Introduction

The processing of polymeric materials for their specified applications has become one of the most active areas of research in material science. Amongst these polymeric materials conducting polymers are of main interest due to their applicability in electronic devices. In this respect, plasma polymerization is a novel technique of synthesis, which produces thin films of polymers that may differ slightly from same polymers prepared by other conventional routes, in terms of their structure and morphology. However, by controlling the synthesis parameters, the majority of properties [1], [2], [3], [4] can be retained. In plasma polymerization, the overall power input is used for the fragmentation of monomer and for creating plasma. Therefore it includes an all-dry, one-step process, as the films deposit directly on the surfaces of substrates. This is relatively a rapid process that provides adhesive, pinhole free, thin films of uniform thickness [5]. Plasma polymerized thin conjugated polymeric films are used as optical coatings, corrosion resistant films, dielectrics, reverse osmosis membranes, optical fibres, metal/plasma polymer/semiconductor used as photovoltaic energy converters, photodiodes, etc. [6], [7].

In plasma polymerization it is possible to alter the film properties for different applications by changing the deposition parameters such as pressure, power, gas flow, and chamber geometry [8], [9], [10], [11]. At the present time, many investigators support this process, as proposed by Yasuda et al. [12], [13], [14], [15]. For example, low dielectric constant polyaniline films have been prepared by rf plasma polymerization [16]. On the other hand, plasma polymerized polypyrrole has been used as a good dielectric material by Kumar et al. [17]. Highly crosslinked and amorphous polythiophene films have been prepared by Bhatt et al. [18]. Decrease in band gap of poly (3-methyl thiophene) from 2.69 to 1.86 eV has been observed, with increasing the rf plasma power from 30 to 120 W [19]. Low temperature fuel cell has been prepared by A. Ennajdaoui et al. using plasma enhanced chemical vapor deposition technique [20]. Plasma polymerized polymer (polypyrrole) has been deposited on nano materials [21], nano particles [22] and carbon nano tubes [23]. Plasma polymerized conducting polymer thin films are efficiently used for sensor applications. In recent years, optochemical methods have become more popular for gas detection [24]. Their main advantages, compared with conventional electrical methods, are their fast response time, insensitivity to electro-magnetic interferences, and possible use in most toxic environment. Several new approaches have been reported for the optical chemical sensor elaboration such as: change of absorption spectra [24], measurement of reflectance changes [25], variation of the intensity of luminescence [26], inter-ferometric method [27], and methods based on the evanescent wave analysis [28]. Waveguide-based chemical sensors with thin polymer films have been widely studied in many applications such as ion [9], flow-through [29], [30] and gas detection systems [31], [32]. These waveguide sensors open the opportunity to create complete optical remote sensing systems [33]. Airoudj et al. have designed and developed a new integrated polyaniline optical sensor for ammonia detection using plasma polymerization technique [34]. Hosono et al. has prepared 4-ethylenebenzenesulfonic acid doped polypyrrole films as gas sensor for volatile organic compounds by RF plasma polymerization [35]. Poly (3-alkylthiophenes) prepared by plasma polymerization show sensitivity to alcohol vapors [36].

In the present investigation, we have synthesized thin, transparent, pinhole free, semi-crystalline films of poly(3-octylthiophene) by rf plasma polymerization using (3-octylthiophene) monomer vapor in the Argon (Ar) plasma zone of rf sputtering setup. The thickness of the film has been measured by ellipsometry, and characterized by various techniques, such as Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and X-ray diffraction (XRD). The formation of crystalline regions in plasma polymerized pure conducting polymer and the determination of the inter planer separation by HRTEM to support XRD results have been discussed in detail. These semi-crystalline plasma polymerized poly (3-octylthiophene) polymeric films with room temperature conductivity 7.321 × 10 10 1 cm 1) can be used for sensor applications [36], [37].

Section snippets

Experimental

Plasma polymerization has been carried out in the setup which consists of stainless steel chamber of 20 cm diameter coupled with monomer feed through attachment, rf power supply and vacuum system. One ON/OFF valve and one needle valve have been given in the monomer feed through attachment. The ON/OFF valve is to create a quick vacuum in a monomer container whereas the needle valve regulates the flow of monomer vapors into the chamber. As the vacuum of 10 5 Torr is achieved, the monomer is feed to

Results and discussion

Poly(3-octyl)thiophene films obtained by plasma polymerization are morphologically uniform, highly reproducible, semi-crystalline, and environmentally stable with the room temperature conductivity 7.321 × 10 10 1 cm 1).

The principal IR absorption bands observed for plasma polymerized films were compared with those reported for chemically synthesized P3OT films [38], [39] and are given in Table 1. The infrared spectra (Fig. 1) of plasma polymerized P3OT are similar to chemically synthesized P3OT

Conclusions

Thin films (100 nm) of poly(3-octylthiophene) conducting polymer has been prepared by plasma polymerization using 30 W RF power supply. The formation of polymer from the monomer has been confirmed by comparing the results of FTIR and AFM with that of chemically synthesized P3OT. These films are morphologically uniform, highly reproducible, semi-crystalline, and environmentally stable. The crystalline phases formed in the film have been analyzed by XRD and confirmed by HRTEM.

Acknowledgments

Vice-chancellor, University of Delhi, is gratefully acknowledged for funding. Financial assistance under IUAC sponsored project (UFUP 42313) is also acknowledged. Thanks are extended to Dr. S. K. Dhawan of National Physical Laboratory (NPL), New Delhi, for providing FT-IR facility.

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