Porous silicon stabilization

A. Valance. Porous silicon formation Stability analysis of the silicon-electrolyte interface. Phys Rev B 52 8323, 1995. 

The SERS-active solid substrates on the basis of silver-coated porous silicon (Ag-PS) can readily be prepared by the immersion plating [2], They offer high sensitivity, stability in air and so they are promising for ultrasensitive chemical analysis. Recently, we have shown that the tetrapyrrolic molecules, in particularly, photosensitizer chlorin can be studied at trace amounts by SERS on Ag-coated meso-PS [3]. It should be noted that in the case of meso-PS major Ag deposition occurs on the top of the porous layer occluding the pores. In order to use the vast surface area of PS for the generation of SERS signal we have... 

Y. Xiao, M. J. Heben, J. M. McCullough, Y. S. Tsuo, J. I. Pankove, and S. K. Deb, Enhancement and stabilization of porous silicon photoluminescence by oxygen incorporation with a remote-plasma treatment, Appl. Phys. Lett. 62(10), 1152, 1993. 

S. La Monica, P. Jaguiro, and A. Ferrari, A thermodynamical explanation for pore growing stability in porous silicon, Electrochem. Soc. Proc. 9tJ T), 140, 1997. 

T. Dubois, F. Ozanam, and J.-N. Chazalviel, Stabilization of the porous silicon surface by grafting of organic groups Direct electrochemical methylation, Electrochem. Soc. Proc. 97(7), 296, 1997. 

Hydrothermal synthesis is one of the important methods for producing fine powders of oxides. A hydrothermal system is usually maintained at a temperature beyond 100 °C and the autogenous pressure of water exceeds the ambient pressure, which is favorable for the crystallization of products. Recent research indicates that the hydrothermal method is also a practical means for preparing chal-cogenide and phosphide nanomaterials, and hydrothermal treatment is an effective method for passivating porous silicons. Similar to hydrothermal synthesis, in a solvothermal process, a non-aqueous solvent, which is sealed in an autoclave and maintained in its superheated state, is the reaction medium, where the reactants and products are prevented effectively from oxidation and volatilization and the reaction and crystallization can be realized simultaneously. Furthermore, organic solvents may be favorable for the dispersion of non-oxide nanocrystallites and may stabilize some metastable phases. 

Ongoing investigations into the chemistry of porous silicon surfaces seek to develop methods for the preparation of chemically functional interfaces that protect the underlying silicon nanocrystallites from degradation without changing or annihilating their intrinsic behavior. The native, hydride-terminated surface is only metastable under ambient conditions and oxidation of freshly prepared porous silicon commences within minutes when exposed to air. While surface oxide can suitably passivate the nanocrystalline silicon and stabilize its photoluminescence, the electrically insulating and structurally defective character of this oxide layer... 

RAPID THERMAL PROCESSING OF POROUS SILICON FOR THE STRUCTURE STABILIZATION... 

Porous silicon (PS) is one of the nanoscale modifications of silicon. There are various approaches to PS producing that are now in use. The technique most generally employed today is known as wet anodization of a crystalline silicon. With this technique, yield parameters of porous material (porosity, pore size and shape, interpore distance) may be readily varied by anodization regimes. However, it is well known the problem of the PS stability influencing the physical properties of the PS layers. P S instability is c onditioned b y very large specific surface area of the porous material. 

A number of studies has been attempted to stabilize porous silicon low-temperature oxidation in a controlled way [1-3], surface modification of silicon nanocrystallites by chemical [4] or electrochemical [5] procedures etc. Rapid thermal processing (RTP) is thought to be a shortcut method of the PS stabilization for a number of purposes. However, there is no data about RTP influence on the PS structure. Therefore, the study of lattice deformations of PS layers after RTP is of great interest. In the present work. X-ray double-crystal diffractometry was used to measure lattice deformations of PS after RTP of millisecond and second durations. 

Recently, Wolkin et al. observed an upper limit of the PL emission energy of 2.1 eV in oxidized porous silicon even if the nanocrystal size became smaller than 2 nm. This behavior, which seems to contradict quantum confinement, was explained in terms of the formation of stabilized electronic states on Si=0 bonds at the surface. For nanocrystals with diameters smaller than 2.8 nm, the widening of the band gap due to quantum confinement makes them appear as inner band gap states. Including the results of Wolkin et al. in our model calculations, we obtained nice agreement with the experimental data. " ... 

Salem MS, Sailor MJ, Hanaz FA, Sakka T, Ogata YH (2006) Electrochemical stabilization of porous silicon muUilayeis for seising various chemical compounds. J. Appl. Phys. 100 083250-7... 

The present paper discusses the preparation and properties of high surface area silicon carbide and oxynitride with respect to possible application in catalysis. The synthetic work includes new routes to high surface area forms of these materials. Regarding properties, an important aspect is stability. This refers both to the stability of a pore system in a non-oxide material, on which there is very little information available, and to the stability of the surface composition in the case of the latter, oxidation to the oxide will be thermodynamically preferred in most cases. We report data on the textural stability of porous silicon carbide and on the surface stability of high surface area silicon oxynitrides. Some of the work reported in the present paper has been described at recent conferences (7,8) and in a communication (9). 

Silicon carbide has been manufactured commercially since 1891 and the current world market is about 500 000 tons. This material is dense and crystalline. It is only recently, however, that a porous form has been reported. These two forms can be regarded as the analogues of quartz (dense, crystalline silicon oxide) and silica gel (porous, amorphous silicon oxide). We were interested in the properties of the porous silicon carbide, and in particular its stability. It is not improbable that this be higher than that of silica in view of the four-fold coordination of carbon compared to the two—fold coordination of oxygen. Although data on the stabilities of dense forms are ayailable, the information is not necessarily relevant to the properties of porous forms. 

Furthermore, results obtained with respect to the thermal stability of the pore structure in porous silicon carbide and the stability towards air, hydrogen or steam of the surface of a silicon oxynitride powder indicate that the stability of high surface area non-oxidic materials can be promising with respect to potential application in catalysis. 

A new preparation method is described to synthesize porous silicon carbide. It comprises the catalytic conversion of preformed activated carbon (extrudates or granulates) by reacting it with hydrogen and silicon tetrachloride. The influence of crucial convoaion parameters on support properties is discussed for the SiC synthesis in a ftxed bed and fluidized bed chemical vapour deposition reactor. The surface area of the obtained SiC ranges ftiom 30 to 80 m /g. The metal support interaction (MSI) and metal support stability (MSS) of Ni/SiC catalysts are compared with that of conventional catalyst supports by temperature programmed reduction. It is shown that a Ni/SiC catalyst shows a considnable Iowa- MSI than Ni/Si(>2- and Ni/Al203-catalysts. A substantially improved MSS is observed an easily reducible nickel species is retained on the SiC surface after calcination at 1273 K. 

Jugdaohsingh R, Anderson SH, Tucker KL, Elliott H, Kiel DP, Thompson RP, Powell JJ (2002) Dietary silicon intake and absorption. Am J Clin Nutr 75(5) 887-893 Li HL, Zhu Y, Xu D, Wan Y, Xia L, Zhao X (2009) Vapour-phase silanization of oxidised porous silicon for stabilizing composition and photoluminescence. J Appl Phys 105 114-307 Mclnnes SJP, Thissen H, Choudbury NR, Voelcker NH (2009) New biodegradable materials produced by ring opening polymerisation of poly(L-lactide) on porous silicon substrates. J Coll Interf Sci 332 336-344... 

Arguably, the simplest method to stabilize the porous silicon surface is oxidation. A popular technique is to use ozone to rapidly generate a Si-OH capped surface with a thin oxide layer. Alternatively, thermal treatment in air (400-800 °C) is used to generate thicker oxide layers (Pap et al. 2004). Surface hydroxyl groups can be further reacted with silanes, which can further stabilize the surface against hydrolytic attack, as well as provide a means of attaching functional groups to the... 

Salonen J, Lehto V, Bjorkqvist M, Laine E, Niinisto L (2001) Chemical stability of thermally-carbonized porous silicon. Mat Res Soc Symp Proc FI4.19. 638 11-16... 

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