10 research outputs found
Role of structure of C-terminated 4H-SiC(000) surface in growth of graphene layers - transmission electron microscopy and density functional theory studies
Principal structural defects in graphene layers, synthesized on a
carbon-terminated face, i.e. the SiC(000) face of a 4H-SiC substrate, are
investigated using microscopic methods. Results of high-resolution transmission
electron microscopy (HRTEM) reveal their atomic arrangement. Mechanism of such
defects creation, directly related to the underlying crystallographic structure
of the SiC substrate, is elucidated. The connection between the 4H-SiC(000)
surface morphology, including the presence of the single atomic steps, the
sequences of atomic steps, and also the macrosteps, and the corresponding
emergence of planar defective structure (discontinuities of carbon layers and
wrinkles) is revealed. It is shown that disappearance of the multistep island
leads to the creation of wrinkles in the graphene layers. The density
functional theory (DFT) calculation results show that the diffusion of both
silicon and carbon atoms is possible on a Si-terminated SiC surface at a high
temperature close to 1600{\deg}C. The creation of buffer layer at the
Si-terminated surface effectively blocks horizontal diffusion, preventing
growth of thick graphene layer at this face. At the carbon terminated SiC
surface, the buffer layer is absent leaving space for effective horizontal
diffusion of both silicon and carbon atoms. DFT results show that excess carbon
atoms converts a topmost carbon layer to sp2 bonded configuration, liberating
Si atoms in barrierless process. The silicon atoms escape through the channels
created at the bending layers defects, while the carbon atoms are incorporated
into the growing graphene layers. These results explain growth of thick
graphene underneath existing graphene cover and also the creation of the
principal defects at the C-terminated SiC(0001) surfaceComment: 20 pages,11 figure
A comparative DFT study of electronic properties of 2H-, 4H- and 6H-SiC(0001) and SiC(000-1) clean surfaces: Significance of the surface Stark effect
Electric field, uniform within the slab, emerging due to Fermi level pinning
at its both sides is analyzed using DFT simulations of the SiC surface slabs of
different thickness. It is shown that for thicker slab the field is nonuniform
and this fact is related to the surface state charge. Using the electron
density and potential profiles it is proved that for high precision simulations
it is necessary to take into account enough number of the Si-C layers. We show
that using 12 diatomic layers leads to satisfactory results. It is also
demonstrated that the change of the opposite side slab termination, both by
different type of atoms or by their location, can be used to adjust electric
field within the slab, creating a tool for simulation of surface properties,
depending on the doping in the bulk of semiconductor. Using these simulations
it was found that, depending on the electric field, the energy of the surface
states changes in a different way than energy of the bulk states. This
criterion can be used to distinguish Shockley and Tamm surface states. The
electronic properties, i.e. energy and type of surface states of the three
clean surfaces: 2H-, 4H-, 6H-SiC(0001), and SiC() are analyzed and
compared using field dependent DFT simulations.Comment: 18 pages, 10 figures, 4 table
Processed Fruiting Bodies of Lentinus edodes as a Source of Biologically Active Polysaccharides
Water soluble polysaccharides (WSP) were isolated from Lentinus edodes fruiting bodies. The mushrooms were previously subjected to various processing techniques which included blanching, boiling, and fermenting with lactic acid bacteria. Therefore, the impact of processing on the content and biological activities of WSP was established. Non-processed fruiting bodies contained 10.70 ± 0.09 mg/g fw. Boiling caused ~12% decrease in the amount of WSP, while blanched and fermented mushrooms showed ~6% decline. Fourier transform infrared spectroscopy analysis (FTIR) confirmed the presence of β-glycosidic links, whereas due to size exclusion chromatography 216 kDa and 11 kDa molecules were detected. WSP exhibited antioxidant potential in FRAP (ferric ion reducing antioxidant power) and ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assays. Cytotoxic properties were determined on MCF-7 and T47D human breast cell lines using MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) test. Both biological activities decreased as the result of boiling and fermenting
Density functional study of GaN(0001)/AlN(0001) high electron mobility transistor structures
Preparation, Characterization of Granulated Sulfur Fertilizers and Their Effects on a Sandy Soils
There is a potential for using sulfur waste in agriculture. The main objective of this study was to design a granular fertilizer based on waste elemental sulfur. Humic acids and halloysite were used to improve the properties and their influence on soil properties. This is the first report on the use of proposed materials for fertilizer production. The following granular fertilizers were prepared (the percentage share of component weight is given in brackets): fertilizer A (waste sulfur (95%) + halloysite (5%)), fertilizer B (waste sulfur (81%) + halloysite (5%) + humic acids (14%)), fertilizer C (waste sulfur (50%) + halloysite (50%)) and fertilizer D (waste sulfur (46%) + halloysite (46%) + humic acids (8%)). Basic properties of the obtained granulates were determined. Furthermore, the effect of the addition of the prepared fertilizers on soil pH, electrolytic conductivity, and sulfate content was examined in a 90-day incubation experiment. Enrichment with humic acids and the higher amount of halloysite increased the fertilizer properties (especially the share of larger granules and bulk density). In addition, it stabilized soil pH and increased the sulfur content (extracted with 0.01 mol·L−1 CaCl2 and Mehlich 3) in the soil