Monocrystalline Quartz ICP Etching: Road to High-Temperature Dry Etching

Plasma Chemistry and Plasma Processing - In this study, the dry plasma-chemical etching process of monocrystalline SiO2 (quartz) in a fluoride-based (sulfur hexafluoride, SF6) inductively coupled...     

Observation of Spin and Valley Splitting of Landau Levels under Magnetic Tunneling in Graphene/Boron Nitride/Graphene Structures

Resonance magnetic tunneling in heterostructures formed by graphene single sheets separated by a hexagonal boron nitride barrier and bounded by two gates has been investigated in a strong magnetic field, which has allowed observing transitions between spin- and valley-split Landau levels with various indices belonging to different graphene sheets. An unexpected increase with the temperature in the interlayer tunneling conductance owing to transitions between the Landau levels in strong magnetic fields cannot be explained by existing theories.     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

Vibrational spectrum of Li2B4O7 crystals

Polarized IR reflection spectra of Li₂B₄O₇ crystals are studied in a spectral range of 80–1600 cm^?1 and compared with their Raman spectra. Based on the results of the dispersion analysis of the spectra, the frequencies, damping constants, and oscillator strengths of all vibrations are determined. The inversion of frequencies of the longitudinal and transverse vibrations of the A ₁ and E symmetry in a range of 900–1150 cm^?1 is found. Based on the data thus obtained, the effective charges are calculated and the types of chemical bonds are analyzed for structural groups of the Li₂B₄O₇ crystal.     

Vibrational spectrum of Li2B4O7 crystals

The polarized infrared reflection spectra of Li₂B₄O₇ were studied in the spectral range 80–1600 cm^?1 and compared with Raman spectra. From the spectrum dispersion analysis, the frequencies, damping, and dielectric oscillator strengths were determined for all vibrational modes observed. A calculation of the effective charges and an analysis of the chemical-bond types of the Li₂B₄O₇ crystal structural units were carried out on the basis of the obtained data.     

The half-life of the 1437 keV 11/2− state in97Mo

Using the (,n) reaction, the half-life of the first 11/2^– level in⁹⁷Mo has been determined as T_1/2(1437.0 keV)=2.5±0.3 ns by means of the pulsed beam method. The experimentally obtainedB(M2) value between theh _11/2 andg _7/2 configurations in⁹⁷Mo has been satisfactorily described within the quasiparticle-phonon model usingg _s ^eff =0.6g _s ^free . Similar calculations and comparisons with experminental data have been performed also for^95,99Mo as well as for⁹⁹Ru and¹⁰¹Pd.This work is partly supported by the Bulgarian National Research Foundation     

Solubility and Raman Spectroscopic Study of As(III) Speciation in Organic Compound-Water Solutions. A Hydration Approach for Aqueous Arsenic in Complex Solutions

Solubilities of arsenolite (As₂O₃, cub.) were measured from 22 to 90°C in water–acetone, water–acetic acid, and water—formic acid solutions of compositions ranging from the pure organic compound to pure water. Raman spectra were obtained at ambient temperature on As-containing water–acetic acid and water–acetone solutions. Results show that arsenic solvation by these organic compounds is negligible and hydroxide species dominate As speciation over a wide range of water activity (a_{H 2 O}> 0.01). The solubility data were analyzed using an approach based on stoichiometric hydration reactions. Results show that As₂O₃ solubility can be described as a function of water activity, independently of the nature of the organic compound, by involving two neutral As hydroxide complexes: As(OH)₃ and As(OH)₃·4H₂O. Stability constants derived for these species indicate that hydration weakens with increasing temperature. Calculations using these constants show that at low temperatures the tetrahydrate As(OH)₃·4H₂O is dominant in water-rich solutions; by contrast, in high-temperature crustal fluids, As(OH)₃ becomes the major As species. The proposed hydration model can be used to analyze solubility of arsenic-bearing minerals and arsenic transport in complex H₂O–CO₂—electrolyte solutions encountered in natural and industrial environments.     

Pseudoisocyanine J-aggregate to optical waveguiding crystallite transition: microscopic and microspectroscopic exploration

Using fluorescent microscopy and microspectroscopy, optical properties and morphology transformations in individual pseudoisocyanine (PIC) J-aggregates in aqueous electrolyte solutions have been explored. A stringlike structure of J-aggregates with a string diameter much less than 1 microm has been observed. Photodestruction of the strings under short-wavelength excitation has been revealed. Rodlike PIC crystallites, about 1 microm in diameter, have been observed with time. The fluorescence spectrum of rodlike crystallites has been found to differ from that of stringlike J-aggregate and from PIC crystal powder spectra. The crystallites are very stable, and their photodestruction has not been observed under any excitation conditions. It has been found that rodlike crystallites in contrast to stringlike J-aggregates possess optical waveguide properties. The luminescence of crystallites can be observed only at the excitation spot and at butt ends located up to hundreds of micrometers from the excitation spot.     

Cathodes based on (La,Sr)MnO<Subscript>3</Subscript> modified with PrO<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>

The electrochemical characteristics of composite cathodes made of (La, Sr) MnO₃-(Zr, Sc)O₂ (LSM-SSZ), modified with PrO_{2 − x} additive, and designed for application in solid oxide fuel cells at moderately high temperatures were studied. The relationship between activity of catalytically modified composite LSM-SSZ cathodes and dispersity of electrocatalyst was revealed. The boundaries of the temperature range with the maximum dispersity of electrocatalyst and electrochemical activity of cathodes were found. The composite LSM-SSZ cathodes modified with PrO_{2 − x} were shown inert with respect to oxidation reactions of hydrocarbon fuel (methane) and highly active electrochemically with respect to oxygen reaction in non-equilibrium gas mixture of CH₄ and O₂. In cells with (Ce, Sm)O₂ (SDC) and (Zr, Y)O₂ (YSZ) electrolytes, their overvoltage is below 80 mV at the current density about 0.5 A/cm² and temperature of 600°C. These electrodes can be used as cathodes in single-chamber fuel cells. Long-term experiments were carried out to study time stability of characteristics of the said composite cathodes. The studied electrodes show parabolic or damped exponential time curves of polarization resistance if contacting with YSZ or SDC electrolyte, respectively. According to the forecast based on the experimental regularities, the polarization resistance of LSM-SSZ cathodes in 10,000 h will not exceed 0.4 or 0.13 Ohm cm², respectively, if YSZ or SDC electrolyte is used.     

Charge-exchange transitions in hot nuclei

A formalism based on the thermo field dynamics and allowing one to treat thermal effects on the strength distribution of charge-exchange transitions in hot nuclei is developed. The strength distributions of the allowed and first-forbidden p → n transitions are calculated for the neutron-rich nucleus ⁸⁰Ge at different temperatures. Then the electron capture rates on the same nucleus are calculated at temperatures and densities corresponding to an advanced stage of stellar evolution. Published in Russian in Yadernaya Fizika, 2009, Vol. 72, No. 8, pp. 1373–1384. The text was translated by the authors.