CC BY
20A. Choukourov, I. Gordeev, D. Arzhakov, A. Serov, P. Solar, M. Drabik, O. Polonskyi, A. Artemenko,
J. Kousal, O. Kylian, D. Slavinska, H. Biederman
NANOSTRUCTURED PLASMA POLYMERS AND THEIR NANOCOMPOSITES
(Charles University in Prague, Faculty of Mathematics and Physics, Department of Macromolecular Physics)
e -mail : choukourov@kmf.troj a.mff.cuni.cz
The methods of structuring ofplasma polymers at nano-level are reviewed with special attention paid to deposition by magnetron sputtering, both in normal and in glancing angle configuration. Possible applications of such materials are discussed.
Key words: nano-particles, magnetron sputtering, plasma polymerization
А. Шукуров, И. Гордеев, Д. Аржаков, А. Серов, П. Солар, М. Драбик, О. Полонский, А. Артеменко, Д. Коусал, О. Килиан, Д. Славинска, Х. Бидерман
НАНОСТРУКТУРИРОВАННЫЕ ПЛАЗМЕННЫЕ ПОЛИМЕРЫ И ИХ НАНОКОМПОЗИТЫ
Дается обзор методов структурирования плазменных полимеров на наноуровне. Особое внимание уделяется магнетронному распылению как при нормльномых, так и скользящих углах. Обсуждаются возможные применения таких материалов
Ключевые слова: наночастицы, магнетронное распыление, плазменная полимеризация
INTRODUCTION
Formation of quazi-polymeric thin films in electrical discharges in organic gases has been known for a long time, however a systematic research on such materials, also called plasma polymers, started in the 1960s [1]. Since then, a wide variety of plasma polymers have been synthesized and studied.
It is generally recognized that plasma polymerization processes are based on formation of free radicals from the molecules of organic precursors by electron impact and/or UV radiation followed by their random poly-recombination. The power delivered to the discharge, the pressure, the flow rate and the molar mass of precursor are the main external parameters that influence the properties of deposited films. If the pressure is sufficiently low (units to tens of Pa) then the probability of gas phase radical recombination is low and plasma polymer is preferentially synthesized heterogeneously on the surfaces adjacent to the discharge in the form of a thin film. At higher pressures, gas phase plasma polymerization becomes dominating and this may lead to the formation and deposition of powders. Such dusty plasmas emerged as a separate field of research in 1970s [2]. At that time, the main focus was set on the characterization of the parameters of plasma itself influenced by a cloud of the charged particulates whereas the resultant deposits stayed on periphery of scientific interest or, at least, they were not given as thorough consideration.
Nevertheless, in the field of surface modification micro- and nano-particles may play a crucial role as they may be used as building blocks for structuring
of the surface. Micro- and nano-structured surfaces, especially those structured in a well-defined manner, exhibit unprecedented physical, chemical and biological properties. The ability of plasma polymerization methods to synthesize nano-particles with controlled size and chemical composition and to deliberately deposit them onto solid substrates is still a challenging task.
This work is not an exhaustive review of the plasma-based methods of fabrication of nano-structures. It rather represents an overall insight into the problematics and gives specific examples obtained by the group of plasma polymer physics at the Faculty of Mathematics and Physics, Charles University in Prague.
RESULTS AND DISCUSSION
As it was mentioned above, formation of particles during plasma polymerization at increased pressures has been studied in the field of dusty plasmas. Recently, a simple gas aggregation source (GAS) was used for fabrication of the nano-particles from hexane vapours with directional deposition of the beam of the nano-particles onto substrates. The deposition of a monolayer of nano-particles of a hydrocarbon plasma polymer (pp-C:H) has been shown [3]. Rf magnetron equipped with a graphite target was mounted inside the GAS to deliver power to the discharge operated in a mixture of Ar and hexane. Graphite was chosen as a material with a low sputtering yield to avoid the pollution of the gas phase with the products of the magnetron sputtering. The outlet of the GAS was equipped with a nozzle which separated the GAS
from the main deposition chamber. The pressure inside the GAS was adjusted by changing the flow rate of the gas mixture and by changing the diameter of the orifice. Optimal experimental parameters were found which allowed effective formation of the pp-C:H nano-particles in the gas phase, their transport through the nozzle with the flowing gas and deposition in the main chamber.
Fig. 1 shows the AFM height image of the pp-C:H nano-particles deposited on the silicon substrate. Monodisperse spherical particles with diameter of 100 nm form a closely packed monolayer. At extended times of deposition, multi-layered thin films composed of such particles can be obtained. The XPS analysis revealed the availability of carbon and oxygen, the latter being present in amount of several percent. The FTIR analysis showed the abundance of the hydrocarbon species with slight contribution of hy-droxyl- and carbonyl-based species. The XPS and FTIR results are typical for those obtained on conventional thin films of hydrocarbon plasma polymers. Furthermore, other trends characteristic for plasma polymer deposition, e. g. enhancement of unsaturation with increasing power of discharge, are fulfilled for the pp-C:H nano-particle deposition as well.
Such nano-particles are purposed to be used for fabrication of surfaces with controllable nano-roughness which are extremely important, for example, in biological appications for protein adsorption and cell adhesion tests.
Fig. 1. The AFM height image of the particles of hydrocarbon plasma polymer produced by the GAS (Ar/hexane 4:1, total pressure 160 Pa, total flow rate 11.5 sccm, discharge power 100 W) Рис. 1. АФМ вид высоты частиц плазменного углеводородного полимера (Ar/гексан 4:1, общее давление 160 Па, расход 11.5 см3/с, мощность разряда 100 Вт)
The magnetron in the experiments described above was used merely to initiate and maintain the plasma and special precautions were undertaken to avoid sputtering. However, it has been shown that magnetron sputtering can also be implemented for
deposition of plasma polymers provided that the target is fabricated from a classical polymer [4]. In this case, positive ions of the working gas (usually argon) are accelerated by the negative self-bias of the magnetron and bombard the surface of the target. As a consequence, severe cleavage of the macromolecular chains occurs with emission of the low-molar mass fragments into the gas phase. These may serve as precursors for further plasma polymerization processes. The obvious drawback of this method is that the composition of the precursors' mixture can be very complex. It is strongly related to the processes happening on the surface of the target whereas in conventional plasma polymerization precursors are introduced to the discharge zone as vapours of a single type species. The advantage of magnetron sputtering is in its technological feasibility as there are numerous industrial technologies that successfully mastered this method for various applications. Furthermore, environmental issues are solved easier in this case where no hazardous liquid chemicals are involved.
The GAS similar to the described above was used for the fabrication of nano-particles of fluoro-carbon plasma polymers with the only exception that the graphite target was replaced with the one made of poly(tetrafluoroethylene). The parameters of the experiments (discharge power, Ar pressure and flow rate) were optimized to obtain stable formation of the na-no-particles. The deposits were observed both on the substrates placed in the main deposition chamber and within the chamber of the GAS. The SEM analysis shows that sub-micron size particles can be prepared (Fig. 2). It is worth noting that two types of the particles are formed: the largest population is represented by the spherical particles with smooth surface while the smaller amount of the "cauliflower"-type particles with developed topography can also be observed.
After optimization of the deposition parameters, the particles of much smaller size could be obtained. Their diameter averaged around 70 nm and only smooth nano-particles were observed.
Expectedly, the deposits were found to consist of carbon and fluorine, the ratio between both depending on the discharge power and Ar pressure. XPS also showed that bonding environment of carbon, i.e. the contribution of the -CF, -CF2 and -CF3 species, depended on the mentioned parameters as well. Under optimal conditions, the nano-particles of fluorocarbon plasma polymer with up to 90% retention of the -CF2 groups were obtained. Thin films grown from these nano-particles exhibit super-hydrophobic behavior with the water contact angle approaching 180°. Such surfaces are of importance in various applications where non-wetting, slippery behaviour is required.
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Fig. 2. The SEM images of the particles of fluorocarbon plasma polymer produced by magnetron sputtering of PTFE (Ar pressure
100 Pa, discharge power 60 W) Рис. 2. СЭМ фото частиц фторуглеродного плазменного полимера, полученного магнетронным распылением политетрафторэтилена (давление Ar 100 Па, мощность разряда 60 Вт)
Deposition of nano-particles is obviously not the only route to structuring of the surface. In the last decade, a so-called Glancing Angle Deposition (GLAD) was intensively investigated [5], however the scientific basis for this method had been described much earlier [6, 7]. In GLAD, substrates are placed at a large angle (>75°) to a source of a depositing material. In such configuration, growth of nano-columns is possible provided that several conditions are fulfilled. First, the initial stages of the film formation should proceed by an island-growth mechanism which requires that depositing atoms interact stronger with each other than with the substrate and that surface diffusion is restricted. This is usually fulfilled by a proper choice of materials and by cooling the substrates. The second condition requires the use of col-limated fluxes. In such case, a self-shadowing effect occurs when the substrate areas behind the growing nuclei are screened from the incoming flux by the nuclei themselves. Directional fluxes of species are achieved by performing the deposition under high-and ultra-high vacuum to minimize their scattering on the atoms/molecules of residual gases. Long-throw
arrangement of substrates with collimators is frequently used as well.
The requirement of ultra-high vacuum significantly narrows the field of the methods which can be used for production of the fluxes of materials to be deposited in GLAD. In fact, majority of the papers published deal with evaporative Physical Vapour Deposition of metals and a number of inorganic compounds. A limited number of authors studied the applicability of magnetron sputtering in GLAD configuration [8, 9]. The use of magnetrons is hindered by relatively high pressure (units of Pa) required for their normal operation which cannot satisfy the requirement of collisionless transport of emitted species.
Fig. 3. The SEM images of the Ti/pp-C:H nano-columns grown in GLAD configuration by magnetron sputtering (total pressure 0.15 Pa, DC 0.45 A, deposition angle 75°) Рис. 3. СЭМ фото Ti/pp-C:H наноколонн, выращенных при скользящей конфигурации магнетронным распылением (общее давление 0.15 Па, постоянный ток 0.45 А, угол осаждения 75°)
Our group has developed a planar magnetron with an enhanced magnetic field which could be operated at pressures as low as 0.06 Pa. Under such conditions, a zig-zag columnar structure of molybdenum was formed on glass by GLAD magnetron sputtering. The further research was focused on the implementation of reactive magnetron sputtering for fabrication of structured nano-composite thin films [10]. Sputtering of titanium was performed in mix-
tures of Ar and hexane with different concentration of hexane, other parameters (the total pressure, the magnetron current, the deposition angle, the substratetarget distance) being fixed. The resultant films were imaged in cross-section by the SEM and the results are shown in Fig. 3. In the entire range of hexane concentrations, porous columnar structure was obtained. At concentrations of hexane >5% the discharge collapsed because of the target poisoning effects.
Elemental composition assessed by XPS showed gradual increase of the carbon and decrease of the titanium content with increasing concentration of hexane. Oxygen also appeared in the spectra as a result of post-deposition oxidation reactions. Decon-volution of the high-resolution spectra revealed that oxygen is bound mainly to titanium in the form of TiO2 and sub-stoichiometric oxides. Metallic titanium as well as titanium carbide was detected as well. It was argued that the film formation in the case of reactive magnetron sputtering is a complex interplay between the anisotropic growth of titanium columns (governed by the shadowing effects) and isotropic plasma polymerization processes governed by surface diffusion of hydrocarbon species. The incident flux of titanium atoms should significantly exceed the deposition rate of the plasma polymer to retain the columnar growth. Otherwise, continuous films will be formed.
The combination of titanium and hydrocarbon plasma polymers is an attractive growth support for osteoblast-like and endothelial cells. The biological response of cells may be further controlled by surface morphology and roughness of thus structured Ti/ hydrocarbon plasma polymer films.
Last but not least, formation of polymeric na-no-structures can be achieved during the initial stages of plasma polymer growth, especially when higher molar mass precursors are used. Fig. 4 shows an AFM image of a dendrite structure formed during the first moments of deposition of poly(ethylene)-like (PE) plasma polymer over poly(ethylene oxide)-like (PEO) plasma polymer. The growth of the islands is governed by a Diffusion Limited Aggregation (DLA) mechanism in this case. The long PE oligomers per se have limited surface mobility, which is further hindered by the energetic and steric barriers occurring when diffusing the hydrophobic macromolecular chains over hydrophilic polymeric network. As a result, the dendrites are formed which expand laterally with the deposition time but maintain their average height at 5-6 nm. Such structure comprising of PEO and PE areas can be attractive for biomedical research where nanoscale separation of the protein-resistant and protein-adhesive domains is required.
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Fig. 4. The AFM image of the dendrite structure observed during the initial stages of growth of PE-like plasma polymers (the substrate is PEO-like plasma polymer on Si) Рис. 4. АСМ вид дендритной структуры, наблюдаемой на начальных стадиях роста полиэтилена как полимеров плазмы (подложка - оксид полиэтилена как полимер плазмы на Si)
CONCLUSIONS
The magnetron-based methods in combination with plasma polymerization processes are very perspective for fabrication of nano-structured surfaces. Deposition of nano-particles with controllable size distribution, chemical and physical properties by gas aggregation sources as well as sculpturing of the films under glancing angle configuration and by polymeric nanophase separation offer numerous possibilities in various technological and biological applications.
Acknowledgements. This work was supported by the grant agency of the Academy of Sciences of the Czech Republic under contract KAN101120701 and by the Charles University in Prague through the grant SVV-2011-263305.
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