Chemical stability studies of thermally-carbonized porous silicon
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Chemical stability studies of thermally-carbonized porous silicon J. Salonen, V-P. Lehto, M. Björkqvist, E. Laine, and L. Niinistö 1 Department of Physics, University of Turku, FIN-20014 Turku, Finland. 1 Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02015 Espoo, Finland. ABSTRACT We have studied chemical stability of thermally-carbonized porous silicon (PS). The initial hydrogen termination of PS has been replaced by carbon using thermal dissociation of acetelyne molecules. This kind of carbonized surface has been found to be at least as stable in humid atmosphere as a thermally-oxidized PS surface. It is also found to be stable in an aqueous KOH and HF. In-vitro studies of tissue compatible in simulated human fluid indicate improved stability and that the carbonized surface could be bioactive. INTRODUCTION During the first few years after the report of efficient photoluminescence from porous silicon (PS)[1], the main efforts in studies of PS were focused on optical and optoelectronic applications, but recently several other interesting applications have been introduced. One of these applications is a use of PS as a biocompatible material [2]. Together with other recently introduced biological applications which include, different kind of biosensors [3], enzyme microreactors [4], micronutrients and therapeutic elements delivery [5], a new field of interest in PS has been awakened. In some of these applications, the instability of PS can be even utilized, for example, using PS as a biodegradable material [5]. In the “older” field of applications, chemical sensing, a progress can also be seen. An increasing number of novel methods and applications have been reported during the last two years. An electronic artificial nose [6], NOx [7] and very sensitive ethanol sensors [8], demonstrated among other novel sensing applications, have been extended the prospect of the future applications in the field of sensing. Also the continuous attempt to stabilize the PS surface has given new results. New methods to improve the stability of PS by white light-promoted or by the Lewis acid catalyzed hydrosilylation and by a cathodic electrografting reactions [9], have been quickly employed in the other studies [10]. In our previous paper we introduced a simple method to stabilized the PS surface [11]. The method employs an interesting property of acetylene molecule (C2H2). At room temperature adsorbed acetylene molecules remain on the surface and undergo
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dissociation as temperature increases. At 600 oC all the hydrogen have left on the surface and carbon atoms bind to silicon atoms. Because of the fast and easy diffusion of the small acetylene molecules, a complete carbonization of the PS surface by can be achieved. Unfortunately, the luminescence properties of PS does not remain after the thermal carbonization process. However, there is a number of interesting applications which does not take the advantage of the luminescence. Instead, improved thermal and electrical conductivity
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