Porous silicon stabilization oxidation

Temporal Stabilization of Porous Silicon Through Oxidation... 

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... 

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. " ... 

The template removal step, needed to achieve porous materials, is one of the most critical points. In contrast to silica, other compositions are usually more sensitive to thermal treatments and calcination can result in breakdown of the mesostructures. Hydrolysis, redox reactions, or phase transfonnarions to the thermodynamically preferred denser crystalline phases account for this lower thermal stability. Many of the transition metal-based mesostruetured materials synthesized in the presence of cationic surfactants collapse during thermal treatments. The poor thermal stability observed could be due to the different 0x0 chemistry of the metals compared to silicon. Several oxidation states of the metal centers may be responsible for oxidation and/or reduction during calcination. In addition, incomplete condensation of the framewoik is possible. 

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. 

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... 

The LED top electrode (usually deposited directly onto porous silicon), also the optical window for EL output, is also a source of stabiUty concerns. Most devices use either ultrathin gold (a few nanometer thick in order to have a reasonable transparency to visible light) or indium tin oxide (ITO) electrodes. ITO shows better stabiUty than gold (Simons et al. 1997), in particular due to better air permeabiUty and better mechanical stabUity (ITO electrodes can be much thicker than gold ones). Fluorinated tin oxide electrodes exhibited much better stability (three orders of magnitude in... 

Gelloz B, Bsiesy A, Herino R (2003a) Electrically induced luminescence quenching in p(+)-type and anodieally oxidized n-type wet porous silicon. J Appl Phys 94(4) 2381-2389 Gelloz B, Sano H, Boukherroub R, Wayner DDM, Lockwood DJ, Koshida N (2003b) Stabilization of porous sihcon electroluminescence by surface passivation with controlled covalent bonds. Appl Phys Lett 83(12) 2342-2344... 

Since the 2002 discovery of nano-explosive devices using solid-state oxidants in porous silicon at room temperature, the technology has reached the stage where several applications are considered. Issues of interest are the cost-effectiveness of fabrication, future integration with CMOS technology, the long-term stability, and the sensitivity to electrostatic discharge. 

Petrova-Koch V, Muschik T, Kux A et al (1992) Rapid-thermal-oxidized porous Si-the superior photoluminescent Si. Appl Phys Lett 61 943-945 Porter LA, Choi HC, Ribbe AE et al (2002) Controlled electroless deposition of noble metal nanoparticle films on germanium surfaces. Nano Lett 2 1067-1071 Rabinal MK, Mulimani BG (2007) Transport properties of molecularly stabilized porous silicon schottky junctions. New J Phys 9 440-448... 

While most data relate to porous silicon layers, recently a lot of effort was devoted to the fabrication of porous silicon powders, in particular for bio-imaging purposes. Some powders were obtained by breaking down anodized porous layers followed by various types of oxidation steps used to tune the emission spectra and improve the efficiency and stability (Park et al. 2009 Xia et al. 2012 Tu et al. 2012). 

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