Biomedical subjects

D Kovalev

Publications and source records attributed to D Kovalev.

12 recordsLinked to original sources

PubMed2004

Capillary condensation in linear mesopores of different shape.

The hysteresis and kinetics of capillary condensation of N2 and Ar in linear mesopores, produced by etching of Si wafers, have been studied for different pore shapes, including the ink bottle geometry. Pore blocking has been observed in the solid state of the pore fillings, but not in the liquid state. We conclude that individual local geometries such as the pore mouth, a blind end, or a single constriction have no effect on the shape of sorption isotherms, that the pore space should be regarded as a statistical ensemble of pore segments with a lot of quenched disorder.

Journal Article↗

PubMed2004

Planar silicon-based light polarizers.

Silicon-based thin-film polarizers operating in the visible and near-infraed spectral range are fabricated by electrochemical etching of bulk silicon wafers. Anisotropically etched (110) porous silicon layers exhibit a strong in-plane anisotropy of the refractive index. Stackes of alternating layers with different mean refractive indices and thicknesses act as dichroic Bragg reflectors or microcavities, respectively. Both structures have two distinct reflection and transmission bands depending on the polarization of the incident linearly polarized light. Planar polarizers are realized through the combination, in one structure, of a dichroic reflector with either a second reflector or a microcavity with different spectral responses.

Journal Article↗

PubMed2002

Stimulated light emission in dense fog confined inside a porous glass matrix.

We report on light amplification through stimulated emission in a dielectrically disordered medium. Liquid fragments confined in the solid matrix of porous quartz layers result in a random fluctuation of the dielectric function, and dye molecules embedded in the voids yield optical gain. The level of opacity is tunable by the ambient vapor pressure of the dielectric substance. In the multiple scattering regime, a strong intensity enhancement of the dye emission accompanied by significant spectral narrowing is observed above the threshold for a layer being in the opalescence state.

Journal Article↗

PubMed2002

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules.

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.

Journal Article↗

PubMed2001

Strong explosive interaction of hydrogenated porous silicon with oxygen at cryogenic temperatures.

We report new types of heterogeneous hydrogen-oxygen and silicon-oxygen branched chain reactions which have been found to proceed explosively after the filling of pores of hydrogen-terminated porous silicon (Si) by condensed or liquid oxygen in the temperature range of 4.2-90 K. Infrared vibrational absorption spectroscopy shows that, while initially Si nanocrystals assembling the layers have hydrogen-terminated surfaces, the final products of the reaction are SiO2 and H2O. Time-resolved optical experiments show that the explosive reaction develops in a time scale of 10(-6) s. We emphasize the remarkable structural properties of porous Si layers which are crucial for the strong explosive interaction.

Journal Article↗

PubMed2000

Ordering and self-organization in nanocrystalline silicon

The spontaneous formation of organized nanocrystals in semiconductors has been observed during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 A into brick-shaped crystallites oriented along the (111) crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.

Journal Article↗

PubMed2000

[Isolated reversible unilateral paresis of hypoglossal nerve in tonsillitis--case report].

We report on a 30-year-old patient with isolated, left-sided hypoglossal nerve palsy after uncomplicated, presumably streptococci-induced tonsillitis. Needle electromyography (EMG) of the tongue showed denervation changes in the muscles supplied by the left hypoglossal nerve. Cranial CT and MRI, CSF examination, Doppler sonography, visual evoked potential (VEP), and auditory evoked potential (AEP) showed no abnormalities and, in particular, no signs of the carotid artery dissection or brainstem lesion. The symptoms and signs resolved within a few weeks after penicillin V treatment. We suppose that the hypoglossal affection of the uncomplicated tonsillitis in this case was due to an aberrant position of the nerve.

Adult↗

PubMed1996