P. M. Lytvyn

Microscopic and optical investigation of Ge nanoislands on silicon substrates

We investigate self-assembled nanoislands in heteroepitaxial GeSi systems by means of atomic force microscopy and micro-Raman scattering techniques. We show that the surface diffusion of Si atoms from the substrate to the islands is strongly enhanced when the temperature increases, giving rise to a wider stability range of pyramid-shaped volumes.

Gigantic uphill diffusion during self-assembled growth of Ge quantum dots on strained SiGe sublayers

Raman spectroscopy and atomic-force microscopy were applied to study the morphology of nanoislands grown on strained Si1−xGex sublayers. It was shown that the growth of nanoislands on strained Si1−xGex sublayer not only induces the effect of their spatial ordering but also enhances the role of interdiffusion processes. Unusual high island volume increase during the epitaxy is explained by anomalous strong material diffusion from the sublayer into the islands, induced by nonuniform field of elastic strains.

Preparation and optical properties of highly luminescent colloidal single-layer carbon nitride

Thermal treatment of graphitic carbon nitride (g-CN) in aqueous solutions of tetraethyl ammonium hydroxide at ∼100 °C yields transparent colloidal solutions retaining stability at a CN concentration of up to 50 g L−1 and upon dilution by a factor of 103. Atomic force microscopy showed that the major part of the CN particles in diluted colloidal solutions are characterized by a lateral size of around 30–50 nm and a thickness of 0.3–0.4 nm typical of a single CN layer. Besides, a small fraction of multi-layer 2–5 nm thick g-CN particles with a size of ∼60 nm is present in the solutions. Photoexcitation of the colloidal CN near the edge of the absorption band results in emission of strong broad-band photoluminescence with a maximum at 460–470 nm and a quantum yield of 45–50%.

Au-TiBx-n-6H-SiC Schottky barrier diodes: Specific features of charge transport in rectifying and nonrectifying contacts

Mechanism of charge transport in a diode of a silicon carbide’s Schottky barrier formed by a quasi-amorphous interstitial phase TiBx on the surface of n-6H-SiC (0001) single crystals with an uncompensated donor (nitrogen) concentration of ∼1018 cm−3 and dislocation density of ∼(106–108) cm−2 has been studied. It is demonstrated that, at temperatures T ≲ 400 K, the charge transport is governed by the tunneling current along dislocations intersecting the space charge region. At T > 400 K, the mechanism of charge transport changes to a thermionic mechanism with a barrier height of ∼0.64 eV and ideality factor close to 1.3.

Two-dimensional ordering of (In,Ga)As quantum dots in vertical multilayers grown on GaAs(100) and (n11)

We have investigated lateral self-assembling in In0.4Ga0.6As∕GaAs quantum dot (QD) multilayers, which were grown by molecular beam epitaxy on GaAs(100) and (n11)B substrates with n=9,8,7,5,4,3. The lateral self-assembling and the QD size distribution have been studied by atomic force microscopy depending on substrate orientation and the number of periods within the multilayers. The observed two-dimensional ordering can be described by a centered rectangular surface unit cell. Derived autocorrelation functions exhibit the most pronounced lateral QD assembling along the elastically soft directions [1¯n0]. This can be attributed to elastic interaction, the particular elastic anisotropy of the high index substrates, and the minimization of the strain energy.

The role of beneficial bacteria wall elasticity in regulating innate immune response.

BackgroundProbiotics have great potential to contribute to development of healthy dietary regimes, preventive care, and an integrated approach to immunity-related disease management. The bacterial wall is a dynamic entity, depending on many components and playing an essential role in modulating immune response. The impact of cell wall elasticity on the beneficial effects of probiotic strains has not been sufficiently studied.The aim was to investigate the effect of lactic acid bacteria (LAB) and bifidobacteria strains on phagocytic system cells (macrophages) as related to bacterial wall elasticity, estimated using atomic force microscopy (AFM).MethodsWe conducted studies on Balb/c line mice 18–20 g in weight using lyophilized strains of LAB—Lactobacillus acidophilus IMV B-7279, Lactobacillus casei IMV B-7280, Lactobacillus delbrueckii subsp. bulgaricus IMV B-7281, and bifidobacteria—Bifidobacterium animalis VKL and Bifidobacterium animalis VKB. We cultivated the macrophages obtained from the peritoneal cavity of mice individually with the strains of LAB and bifidobacteria and evaluated their effect on macrophages, oxygen-dependent bactericidal activity, nitric oxide production, and immunoregulatory cytokines. We used AFM scanning to estimate bacterial cell wall elasticity.ResultsAll strains had a stimulating effect on the functional activity of macrophages and ability to produce NO/NO2 in vitro. Lactobacilli strains increased the production of IL-12 and IFN-γ in vitro. The AFM demonstrated different cell wall elasticity levels in various strains of LAB and bifidobacteria. The rigidity of the cell walls among lactobacilli was distributed as follows: Lactobacillus acidophilus IMV B-7279 > Lactobacillus casei IMV B-7280 > Lactobacillus delbrueckii subsp. bulgaricus IMV B-7281; among the strains of bifidobacteria: B. animalis VKB > B. animalis VKL. Probiotic strain survival in the macrophages depended on the bacterial cell wall elasticity and on the time of their joint cultivation.ConclusionLAB and bifidobacteria strains stimulate immune-modulatory cytokines and active oxygen and nitrogen oxide compound production in macrophages. Strains with a more elastic cell wall according to AFM data demonstrated higher resistance to intracellular digestion in macrophages and higher level of their activation.AFM might be considered as a fast and accurate method to assess parameters of probiotic strain cell wall to predict their immune-modulatory properties.

Influence of template type and buffer strain on structural properties of GaN multilayer quantum wells grown by PAMBE, an x-ray study

The influence of template type and residual strain of the buffer layer on the structural properties of GaN/AlN superlattices (SLs) was studied using high resolution x-ray diffraction. Using sapphire substrates, an effective thinning of the GaN quantum wells and the corresponding thickening of the AlN barriers were observed in SL structures grown on thin, strained AlN templates as compared with SL structures grown on thick, relaxed GaN templates. Moreover, a bimodal strain relaxation of SL structures in dependence of template type was observed. The SLs grown on AlN templates relax predominantly by the formation of misfit dislocations, while the SLs grown on GaN templates relax predominantly by cracking of the layers. We explain these effects by the influence of residual strain in the buffer/template systems used for the growth processes of SL layers. A correlation is made between the strain state of the system and the cracking processes, the dislocation density, the radius of curvature and the layer thickness.

Interface roughness scattering in laterally coupled InGaAs quantum wires

Conductivity of In0.38Ga0.62As quantum wires grown on the (311)A plane of GaAs was studied using temperature dependent Hall effect. Detailed analysis of the scattering phenomena which control electron mobility along the wires indicates that the most significant scattering mechanism which limits electron mobility at low temperatures is scattering from interface roughness. This finding contributes to the general understanding of the potential to use self-assembled semiconductor quantum wires in the fabrication of thermoelectric devices where one-dimensionality and enhanced scattering at rough boundaries are important.

Effect of dimensionality and morphology on polarized photoluminescence in quantum dot-chain structures

Change of the photoluminescence (PL) polarization is studied by changing the excitation intensity and temperature for aligned In(Ga)As quantum dot (QD) structures with varying inter-dot distances grown by molecular beam epitaxy on semi-insulating GaAs (100) substrates. An unusual increase of the polarization ratio is observed by increasing the temperature and/or excitation intensity throughout a low temperature (T < 70 K) and low intensity (Iex < 1 W/cm2) range. This increase as well as the general behavior of the polarized PL are the results of the exciton dynamics and the peculiarities of the system morphology. They are due to the varying inter-dot distances which change the system from zero-dimensional comprised of isolated QDs to one-dimensional comprised of wire-like structures.

The influence of TiB2-thin film thickness on metal–GaAs structural characteristics

The X-ray diffraction and Atomic Force Microscope investigations of TiB 2/GaAs as-produced and annealed device structures has been carried out. The samples were obtained by magnetron sputtering on previously photon cleaned Czochralski-grown (001) GaAs substrates doped by Te up to concentration 10 18 cm -3 . The magnetron sputtering was carried out in the argon atmosphere at pressure in the chamber 5·10 -3 torr. The currents of sputtering were 0.3 and 0.4A, and thickness of TiB 2-films was from 10 nm up to 50 nm.. The samples annealing was carried out in a stream of hydrogen in the furnace at temperatures 400, 600, and 800 °C during 1 minute with heating velocity of 1800°C/min. It was shown that film thickness and magnetron sputtering current determines the film surface and interface structural parameters as well as a fe atures of processes of structural relaxation under short-term thermal annealing.