Nanocomposite Films with Plasma Polymer Matrix Prepared Using a Gas Aggregation Cluster Source

Transcription

1 WDS'13 Proceedings of Contributed Papers, Part III, , ISBN MATFYZPRESS Nanocomposite Films with Plasma Polymer Matrix Prepared Using a Gas Aggregation Cluster Source A. Shelemin, A. Choukourov, O. Kylian, J. Hanus, J. Kousal, D. Slavinska, H. Biederman Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. Basics of plasma polymerization processes are introduced. Principles of the gas aggregation cluster source (GAS) are briefly reviewed including recent improvements. Deposition and characterization of nanocomposite films are described. Introduction Strong interest in the investigation of plasma polymerization processes appeared in the second half of the 20 th century. Possibility of different potential applications of plasma polymers was the main reason for such interest. Nanocomposite metal/plasma polymer films based on plasma polymer matrix with metallic inclusions have shown unusual electrical and optical properties which have not been detected before in any individual component. For instance, Kay investigated nanocomposite Au/ plasma polymer C 3 F 8 and observed that with increasing of gold content, resistivity is decreased from about 10 6 Ω cm up to 10 6 Ω cm [1]. The main reason for this was influence of filling factor f (is a metal volume in a unit volume of composite) on the conductivity. It also has been noticed that electrical properties of such composite are defined not only by the metal volume fraction but also by cluster microstructure. The investigation of optical properties has shown strong absorption maximum due to collective resonant oscillations of electrons in Au, i.e., particle plasmon resonance [2].The color of the coating deposited on glass in transmitted light changed for samples with different filling factor (pink (f=0.01), red (0.06), violet (0.24), blue (0.37)). Wielonski and Beale proposed to use these properties for producing of decorative coatings [3]. Biederman suggested making optical filters of large area based on Au/fluorocarbon plasma polymer [4]. Further research of plasma polymerization process showed that fabrication of plasma polymer-based nanocomposites gives the opportunity to get novel types of materials with improved properties and to extend the range of their applications. Many combinations of plasma polymer-based nanocomposites were developed, the most studied being plasma polymer/metal [5] and polymer/metal oxide [6]. The main attention in this review has been paid to various technologies of production of nanocomposite metal/plasma polymer films using gas aggregation cluster sources (GAS). A few distinct systems of GAS are described and basic characteristics of nanocomposite thin films are discussed. Plasma polymer Plasma polymer is understood as a material (usually in a form of thin film) that forms as a result of a passage of an organic gas or vapor through the glow discharge [7]. There are several models of plasma polymerization [7 9]. The model [10] that describes large amount of plasma polymerization processes is based on the idea that energetic electrons in the discharge volume interact with monomer molecules and convert them into free radicals. These diffuse to the substrate where they react with adsorbed (un-activated) monomer molecules. On the substrate surface polymer chains grow and terminate with a lot of branching and crosslinking and thus creating disordered and rigid network of a plasma polymer. Structurally, plasma polymer consists of short chains that are randomly branched and cross-linked. The degree of cross-linking depends on energetic parameters of the plasma. Depending on chemical composition, several types of plasma polymers can be distinguished: hydrocarbon, fluorocarbon, nitrogen-containing, oxygen-containing, silicon-containing plasma polymers and the nanocomposites there of. The experimental systems used for deposition of plasma polymers can be generally divided into the three types: bell-jar type reactors with internal parallel plate electrodes, tubular type reactors with external ring electrodes, and electrodeless microwave reactors [7, 8]. The type of the reactor plays an important role in choosing the parameters of deposition. It has been also shown [9] that geometrical configuration of the reactor, in addition to pressure and flow rate of the precursor s vapor, power and 139

2 frequency of the discharge, temperature and size of substrates, may significantly influence the resultant properties of the plasma polymers. Gas aggregation cluster source The first sources capable of producing the beams of nanoclusters with different sizes were developed about 30 years ago by K. Sattler and co-workers [11]. Their operation is based on evaporation of the material into a quite high pressure of He. The atoms are effused from the evaporation cell into the cooling condensation chamber where they condense by homogeneous nucleation to form clusters. The clusters are then drawn by the gas flow into the deposition chamber with lower pressure. Further modification of the GAS was suggested by Haberland [12]. In his system, magnetron sputtering under high pressure of Ar was used as a source of metallic atoms to be condensed into the clusters. Utilization of the magnetron offers several advantages as compared to conventional evaporation. The clusters passing the discharge zone get partially ionized and therefore their manipulation with electrostatic field becomes possible. In the deposition chamber of the whole the system several diaphragms are used to extract the clusters from the GAS, to separate them by mass and to deposit them onto the substrates. Electric positive potential of the substrate allow to accelerate negatively charged clusters to allow fast landing. However, deposition rate is very low. More details of the GAS can be found in [13]. Based on the Haberland s concept, the group of plasma polymer physics of DMF MFF UK developed a simplified and compact GAS [14]. It consists of the cylindrical aluminium aggregation chamber of 100 mm inner diameter. The chamber has a 3 mm long nozzle of variable diameter (from 1.5 to 3 mm). Similar to the Haberland s GAS, planar magnetron is used for sputtering. The walls of the aggregation chamber are cooled with water. The system uses all clusters that allow achieving high deposition rate as compared to the Haberland s source. The GAS can be easily used in combination with other high vacuum systems. Investigation of the parameters of formation cluster in this source gives the opportunity to control the size or amount of the clusters on surface. Nanocomposite metal/plasma polymer Polonskiy and co-workers investigated the factors that influence the deposition of nanocomposite Ag cluster/plasma polymer thin films [15]. Silver nanoclusters and plasma polymer matrix were deposited from the two independent sources. The plasma polymer was deposited from the mixture of argon and n-hexane. A rotating substrate with frequency of rotation of 5Hz was placed between the magnetron and the cluster source. absorbance [a. u.] 40 ma 50 ma 60 ma 70 ma 100 ma 150 ma OH CHx C=C CHx C=O wavenumber [cm-1] Figure 1. FTIR RAS spectra of the Ag clusters/plasma polymer samples deposited by DC sputtering with different applied currents. Adopted from [15] Polonskyi et al., Thin solid films 520, ,

3 The distance from the sample holder to both the magnetron and the cluster source nozzle was 5 cm. The size of the Ag clusters depends strongly on the operational parameters. The first parameter that influences the dimension of the clusters is their residence time in the aggregation chamber. The residence time is dependent on the length of the aggregation chamber, diameter of the output orifice of the cluster source, pressure and gas flow of argon. Another important parameter that affects the size of the clusters is the magnetron current. The size of the clusters increases slightly with increasing magnetron current. The UV-vis measurements revealed anomalous absorption around 450 nm with intensity of absorption increasing with magnetron current. Some interesting changes were found in the FTIR spectra as well (see Fig. 1 adopted from [15]). The peaks from the carbonyl and carboxyl groups in vicinity of the Ag clusters that were observed previously for Ag/C:H nanocomposites [16] are not detected here. It has been suggested that oxidation of Ag nanoclusters which are formed in the cluster source is less effective than in the case of silver deposited on the substrate in atomic form. Solar and co-workers prepared nanocomposite C:H/Ti double layers [17]. The main purpose of their work was to adjust the roughness of the surface by controlling the amount of C:H particles under the titanium thin film. The principal difference to previous experiments was in the use of the plasma polymer particles as the substrate for deposition of titanium. A scheme of the experimental equipment for two-step deposition of thin film can be seen in figure 2. All depositions were done without breaking vacuum. Gas aggregation source with RF excitation was used for production of C:H plasma polymer particles. Titanium was DC magnetron sputtered under pressure of 0.15 Pa and magnetron current of 0.45 A. Investigation of morphology showed that the roughness of the deposits depends strongly on the amount of the C:H particles on the surface which was given by the deposition time. The biggest values of the roughness were observed in the first 10 minutes of the deposition of the plasma polymer particles. On the other hand, thickness of titanium capping layer did not result in noticeable alteration of the morphology of the nanocomposite. Roughened surfaces exhibited super-hydrophilicity with the water contact angle (WCA) being well below 5. One has to note that super-hydrophilicity was maintained for significantly longer time than that of the flat titanium surface. Kylian O. and collaborators also observed a significant dependence of roughness on the deposition time in the case of Ti particles covered with the nylon-sputtered or n-hexane plasma polymerized thin films [18]. Using two different plasma polymers provided the method to investigate how roughness influences the wettability in two opposite cases. With the nylon-sputtered overcoating, the static WCA decreased from 42 to 16 with increasing roughness. In the case of n-hexane plasma polymer, the increase of the surface roughness led to increase of the static WCA from 90 to 130. Figure 2. The schematic view of the deposition chamber used for deposition of Ti/C:H nanocomposites. Adopted from [17] Solar et al., Surface & Coating Technology 206, ,

4 Other types of metal/plasma polymer nanocomposites have been also prepared [19]. Al nanoclusters were deposited simultaneously with the plasma polymer. Formation of the nanoclusters was carried out in the GAS by DC sputtering of the aluminium target in the atmosphere of Ar. The RF planar magnetron was used for ignition the discharge in Ar/n-hexane mixture as a source for deposition of the plasma polymer. It has been shown that Al clusters alone have weak adhesion to any kind of the substrates. Fixation of the nanoclusters on the surface improved in the case of their simultaneous deposition with the plasma polymer. As a consequence, nanocomposite films were grown. XPS was applied to determine the elemental composition of the nanocomposites. Since the aluminium clusters are covered with the plasma polymer only 2 % of Al were detected. The analysis of the high resolution spectra showed that 30 % of aluminium is in the metallic form. From exponential attenuation of the XPS signal, it was assumed that the amount of Al is much bigger than it was detected. Most of the clusters are slightly oxidized. This supports the fact that clusters forms in the GAS are less prone to oxidation in comparison with usual deposition methods. Conclusions A huge amount of publications and research works in the field of plasma polymerization appeared in the last 50 years. A great interest in plasma polymers can be explained by numerous advantages they may bring as compared to conventional materials. The benefits include: the ability to produce thin organic coatings from precursors that do not polymerize by conventional chemical way; versatility of substrate material; possibility to fabricate pinhole free, high dense thin films; small amount of waste etc. In this review, the short characteristic of plasma polymer and plasma polymerization process has been done and the basics of the gas aggregation sources have been described. A few experiments with nanocomposite metal/plasma polymer thin films deposited by GAS were discussed. From all the above a short conclusion can be made: 1. The method of production of clusters using gas aggregation cluster source is simple, inexpensive and is characterized by very good reproducibility. 2. In spite of numerous studies, the future developments in this field are still needed. This is true mainly with regard to the investigation of mechanisms of nanoparticles formation and growth, their charging or transport towards a substrate. In addition, in case of production of nanocomposite films, important issues that need to be addressed are adhesion of nanoparticles to the surface as well as temporal and mechanical stability of produced coatings. Furthermore, possibility of production of core-shell nanoparticles receives increasing attention. References [1] Kay E. Synthesis and properties of metal clusters in polymeric matrices, Phys. D Atoms,Molecules and Clusters 3, , [2] Martinu L., Biederman H., Plasma deposited composite polymer-metal thin films and their optical characteristics, Vacuum 36, , [3] Wielonsky R., Beale H., Colored polymeric coatings by plasma polymerization, Thin Solid Films 84, , [4] Biederman H., Metal doped polymer films prepared by plasma polymerization and their potential applications, Vacuum vol. 34, , l984. [5] Körner E., Aguirre M.H., Fortunato G., Ritter A., Ruhe J., Hegemann D., Plasma Process. Polym. 7, 619, [6] Drabik M, Hanus J., Kousal J., Choukourov A., Biederman H., Slavinska D., Mackova A., Pesicka J., Plasma Process. Polym , [7] Biederman H., Plasma polymer films, Imperial College Press, London, pp , [8] Yasuda H., Plasma Polymerization, Academic Press, Orlando, [9] Biederman H., Osada Y., Plasma polymerization process, Elseiver, Amsterdam, [10] Lam D.K., Baddour R.F. and Stancell A.F., in M. Shen (ed.), Plasma Chemistry of Polymers, Marcel Dekker, New York, [11] Sattler K., Muehlbach J., Recknagel E., Phys. Rev. Lett. 45, 821, [12] Haberland H., Mall M., Mosseler M., Qiang Y., Rainers T., Turner Y., J. Vac. Sci. Technol. A12, 2925, [13] K.Wegner, P. Piseri, H. Vahedi Tafreshi, P.Milani, J. Phys. D:Appl. Phys. 39,

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