Thermal Oxidation Of Porous Silicon Research Articles

Thermal Oxidation for Controlling Protein Interactions with Porous Silicon

Thermal oxidation of porous silicon (pSi) has been used to control interactions with three proteins; lysozyme, papain, and human serum albumin (HSA) enabling the influences of protein structure, molecular weight, and charge to be elucidated. Adsorption behavior was assessed via adsorption isotherms while the structures of adsorbed proteins were investigated using a bioactivity assay, FTIR, and zeta potential. Time-of-flight secondary ion mass spectrometry was used to examine protein pore penetration. High protein adsorption onto unoxidized pSi (240-610 microg/m(2)) was attributed to predominately hydrophobic interactions which resulted in structural changes of the adsorbed proteins and significant loss of bioactivity. Thermal oxidation at 400 and 800 degrees C significantly reduced protein adsorption (80-485 microg/m(2)) by reducing hydrophobicity. Oxidation of pSi modified the protein adsorption mechanisms to solely electrostatic attraction for positively charged proteins and structural rearrangement for negatively charged proteins. Adsorption via electrostatic attraction preserved protein bioactivity and zeta potential, thus inferring a retention of their native structure. In contrast, the negative charge and globular structure of HSA resulted in a loss of structure. We have demonstrated that thermal oxidation of pSi can be used to control protein interactions, adsorbed structure, and bioactivity.

Aqueous and Thermal Oxidation of Porous Silicon Microparticles: Implications on Molecular Interactions

Links between the mechanisms and kinetics of aqueous and dry thermal oxidation of porous silicon (pSi) microparticles have been investigated and the influence on molecular interaction established. zeta potential measurements have established the interplay between the dry oxidation state of pSi microparticles and their interfacial chemistry in aqueous solution, and Fourier transform infrared spectroscopy has demonstrated the effect of immersion time and oxidation temperature on surface chemistry. The influence of aqueous and thermal oxidation on molecular interactions and loading was investigated using methylene blue as a probe molecule. Aqueous immersion of pSi microparticles results in an initial increase in OySiH (y = 1-3) species with increasing immersion times, reducing O2SiH concentration, while O3SiH concentration remained constant. Thermal oxidation from 473 to 1073 K causes the gradual transition from SiySiHx to OySiH and finally OySiOH species. Both aqueous and thermal oxidations had an effect on the zeta potentials of pSi microparticles. Methylene blue discoloration occurred due to its reduction by the SiSiHx-terminated surface thereby demonstrating the reactivity of such species. Aqueous and thermal oxidations modify pSi microparticle surface chemistry, which has therefore shown to influence molecular interactions. Understanding the aqueous oxidation of pSi is crucial when loading pSi from aqueous solution due to its impact on molecular interactions. These molecular interactions play an important role in the loading of pSi since they dictate the attraction of the molecule toward the surface and therefore ultimately the loading level.

Thermal conductivity and nanoindentation hardness of as-prepared and oxidized porous silicon layers

Porous silicon (PS) offers promising possibilities to be applied as thermal insulating material in thermal effect microsystems for its thermal conductivity (TC) is up to two orders smaller than that of bulk silicon. In order to find a compromise between efficient thermal isolation and good mechanical stability of PS, thermal oxidation of PS is commonly used to tune the mechanical and thermal properties of PS. Both TC and the hardness of as-prepared and oxidized PS have been thoroughly investigated. TC and the hardness of as-prepared and oxidized PS were measured using micro-Raman scattering and nanoindentation, respectively. Experimental results revealed that TC and the hardness of as-prepared PS, exhibiting a strong dependence on the preparing conditions, decrease with increasing porosities. After oxidization at different temperatures, TC of oxidized PS decreases with increasing oxidation temperatures, whereas the hardness increases a lot. PS with a moderate porosity of 73.4% oxidized at 600 °C has a compromise between low TC [2.100 W/(m K)] and high hardness (∼1.160 GPa). So this process finalizes this kind of oxidized PS to be used as a suitable thermal insulation substrate in thermal effect microsystems.

Role of surface texturization on the gas-sensing properties of nanostructured porous silicon films

In this work, by means of high-resolution X-ray diffraction, excitation λ-dependent photoluminescence (PL) and Fourier transform infrared studies, it has been demonstrated that hydrogen-passivated porous silicon (PS) films with high PL and stability can be obtained on p-type Si(100) substrate by simple texturization process rather than by resorting to any anodic, chemical or thermal oxidation of PS [T. Karacali, B. Cakmak, H. Efeoglu, Opt. Express 11 (2003) 1237]. PS formed on textured substrates is superior to PS formed on polished silicon substrates at the same current density and time of anodization. The application of PS films formed on textured substrate as a gas sensor has been demonstrated and it shows higher sensitivity values upon exposure to ethanol as compared with polished PS specimens of similar porosity. The improved properties are attributed to the formation of highly porous vertical layers separating macroscopic domains of nanoporous silicon.

Some preliminary observations of the rapid thermal oxidation of porous silicon, and the rapid thermal nitriding of oxidized porous silicon

Porous silicon, produced by the anodic oxidation of p-type single crystal wafers of silicon in hydrofluoric acid/ethanol electrolytes has been subjected to rapid thermal oxidation. For comparison purposes, and also to generate porous oxide for subsequent nitriding, other samples of porous silicon were subjected to conventional furnace oxidation. By TEM, EDS, and ESCA it was shown that the structures and compositions of the oxides produced by the two routes were similar. It was further demonstrated that oxidized porous silicon could be nitrided by rapid thermal processing. The resulting structure, although still porous, showed a much diminished etching rate in hydrofluoric acid, compared to the original oxide.

Partial oxidation of porous silicon by thermal process: study of structure and electronic defects

We present a preliminary study on thermal oxidation of porous silicon with the aim to form Si single crystal nanostructures, with controlled sizes. Porous silicon layers, prepared by electrochemical etching of (1 0 0) p + material, were oxidized at high temperature (1000°C), low oxygen pressure (12 mbar) during varying time (0–10 min) in a set-up allowing treatments in a controlled atmosphere. The composition of the layers and their oxygen content have been determined using nuclear reaction analysis and Rutherford backscattering spectrometry. From oxygen measurements, the lateral size of the Si wires has been estimated as a function of the oxidation time. According to our estimations, thermal oxidation during 10 min should lead to silicon wires of 5 nm lateral size about. Photoluminescence and electron paramagnetic resonance measurements have been performed on these samples. The PL integrated signal increases as a function of the oxidation time but is weaker than the one observed in p + porous layer of high porosity obtained directly by electrochemical etching. This was correlated to a decrease of the (1 1 1) P b defects density which, however, remains high. The weak PL signal could also be due to the relatively high size of the Si nanostructures.

Microcrystallites in Oxidized Porous Silicon

ABSTRACTRapid thermal oxidation of porous silicon leads to desorbtion of hydrogen from the inner surface and formation of a thin oxide layer. Despite this dramatic change in the chemical composition oxidized microporous silicon (miPS) shows photoluminescence (PL) in the visible region. This is contradictory to the idea that the observed PL originates from chemical compounds like siloxene or polysilane, which would require a certain stoichiometry. If silicon microcrystallites are still present in the oxidized miPS, the observed luminescence can be explained in terms of a quantum confinement effect. It is the purpose of this work to prove the existence of microcrystallites in oxidized miPS using electron microscopy and X-ray diffraction.

Effects of Surface Treatment on Light Emission from Porous Silicon

ABSTRACTVisible light emission from porous silicon has recently been demonstrated by several groups. We have found that annealing of porous silicon samples in an inert atmosphere over 250C reduces the emission intensity, almost fully quenching it by 450C. We believe that this is due to the removal of passivating hydrogen, based on measurements of infrared absorption due to Si-H vibrational modes. We have also found that we can partially recover the emission intensity in these annealed porous silicon layers by passivation in a hydrogen plasma. We have therefore demonstrated that hydrogen passivation is necessary for the observation of visible emission from porous silicon. We have also shown that thermal oxidation of porous silicon over 200C reduces the PL emission. We propose that this reduction is also due to the loss of hydrogen passivation during the oxidation.