A. S. Vorontsov

Silicon Nanocrystals As Photosensitizers of Active Oxygen for Biomedical Applications

Silicon nanocrystals dispersed in water have been used to photosensitize the generation of active oxygen. The photosensitizing efficiency has been estimated through the quenching of the exciton photoluminescence of silicon nanocrystals. Experiments have revealed a strong (up to 80%) decrease in the number of cancer mouse fibroblast cells when they come into contact with photoexcited silicon nanocrystals. The obtained results show that the use of silicon nanocrystals for biomedical applications, in particular, for photodynamic therapy of cancer, is feasible.

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I2 iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I2 molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ∼1018−1019 cm−3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, Pb centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I2 molecules.

Paramagnetic properties of carbon-doped titanium dioxide

This paper reports the experimental results on paramagnetic properties of carbon-doped titanium dioxide. The electron paramagnetic resonance study of the samples has been carried out both in dark and under illumination. The nature of defects and their dynamics under illumination of carbon-doped TiO2 samples is discussed.

Silicon nanocrystals as efficient photosensitizer of singlet oxygen for biomedical applications

Luminescent silicon nanocrystals (nc-Si) are shown to be efficient photosensitizers of singlet oxygen (SO) generation. Photoluminescence (PL) spectroscopy method is used to study the mechanism and efficiency of the SO photosensitization in gaseous and aqueous ambiences. In vitro experiments demonstrated that the SO, photosensitized by nc-Si dispersed in nutrient solutions, could kill cancer cells. This finding opens a broad opportunity for biomedical applications of nc-Si, e.g. for the photodynamic therapy of cancer or antibacterial treatments.

Effect of adsorption of the donor and acceptor molecules at the surface of porous silicon on the recombination properties of silicon nanocrystals

The effect of adsorption of the donor and acceptor molecules on the spectra of photoluminescence and electron spin resonance (ESR) of microporous silicon is studied. It is found that photoluminescence of microporous silicon is quenched, the photoluminescence peak shifts to shorter wavelengths, and the intensity of the ESR signal increases after adsorption of molecules of nitrogen dioxide and pyridine. The results obtained are interpreted using a model of radiative excitonic recombination in porous silicon that takes into account the formation of both the charged (NO2)− and (C5H5N)+ complexes and defects (e.g., dangling bonds at the silicon surface) at the surface of silicon nanocrystals.

Voltage effect on the sensitivity of nanocrystalline indium oxide to nitrogen dioxide under ultraviolet irradiation

The influence of the applied voltage on the sensitivity of nanocrystalline indium oxide to nitrogen dioxide under ultraviolet irradiation at room temperature was studied. The sensitivity of nanocrystalline indium oxide to nitrogen dioxide increases with increasing applied voltage. This effect is presumably explained by the influence of the voltage on the bending of energy levels existing on the sample surface.

Mobility of charge carriers in porous silicon layers

The (conduction) mobility of majority charge carriers in porous silicon layers of the n and p types is estimated by joint measurements of electrical conductivity and free charge carrier concentration, which is determined from IR absorption spectra. Adsorption of donor and acceptor molecules leading to a change in local electric fields in the structure is used to identify the processes controlling the mobility in porous silicon. It is found that adsorption of acceptor and donor molecules at porous silicon of the p and n types, respectively, leads to a strong increase in electrical conductivity, which is associated with an increase in the concentration of free carrier as well as in their mobility. The increase in the mobility of charge carriers as a result of adsorption indicates the key role of potential barriers at the boundaries of silicon nanocrystals and may be due to a decrease in the barrier height as a result of adsorption.

Influence of Pyridine Molecule Adsorption on Concentrations of Free Carriers and Paramagnetic Centers in Porous Silicon Layers

The influence of adsorption of donor pyridine (C5H5N) molecules on free-hole and defect concentrations in porous silicon layers differing in the morphology of composing nanocrystals and pores, as well as in the boron doping concentration, was studied using infrared and electron spin resonance spectroscopy. It was shown that the dependence of the hole concentration on the pyridine vapor pressure is controlled by the initial boron doping level of porous silicon, while the number of defects, i.e., dangling silicon bonds, is almost unchanged during adsorption for all sample types. For samples on substrates with a boron concentration of ∼1020 cm−3, a decrease in the number of holes at low pyridine pressures is observed and is attributed to hole capture by surface states of adsorbed C5H5N molecules. At pyridine pressures close to the saturated vapor pressure, the hole concentration in porous silicon layers increases, which is associated with hole “trap” depopulation due to an increase in the permittivity of the silicon nanocrystal neighborhood under conditions of C5H5N vapor condensations in sample pores.

The influence of an air atmosphere on the electrical properties of two-phase films of hydrogenated silicon

Amorphous and microcrystalline hydrogenated silicon is an important material in modern thinfilm electronics. In some cases, the electrical parameters of silicon film samples depend on the ambient environment, in particular, the air. In this work we studied the effect of air exposure on the electrical properties of two-phase silicon films with a volume fraction of the crystalline phase of 0 to 80%. It was shown that the change in the conductivity of the two-phase films that were exposed to the air atmosphere depends on the proportion between the amorphous and microcrystalline phases that form the film microstructure. The use of two differing methods for film manufacturing allowed us to qualitatively evaluate how the conductivity of the films with a nonuniform structure across the thickness is affected by gas adsorption from air onto the film surface. Air exposure of the samples with a small amount of the crystalline phase located near the film surface leads to the specific features of temperature dependence of their conductivity.

Conductivity of structures with silicon nanocrystals in oxide matrix

The current-voltage characteristics and temperature dependences of conductivity (in the temperature range from 210 to 330 K) of the structures containing silicon nanocrystals in the SiO2 matrix have been measured. Samples with various numbers of layers and nanocrystal sizes have been investigated. Based on the results, possible mechanisms of the charge carrier transfer in the studied structures at different temperatures have been analyzed.