Hydrogen Dissociation Catalyzed by Carbon‐Coated Nickel Nanoparticles: Experiment and Theory

Based on the combination of experimental measurements and first‐principles calculations we report a novel carbon‐based catalytic material and describe significant acceleration of the hydrogenation of magnesium at room temperature in the presence of nickel nanoparticles wrapped in multilayer graphene. The increase in rate of magnesium hydrogenation in contrast to a mix of graphite and nickel nanoparticles evidences intrinsic catalytic properties of the nanocomposites explored. The results from simulation demonstrate that doping of the metal substrate and the presence of Stone–Wales defects turn multilayer graphene from being chemically inert to chemically active. The role of the size of the nanoparticles and temperature are also discussed.     

Role of decay processes in the propagation of slightly nonequilibrium thermal-pulse phonons in YAlO3:Lu crystals

Data on the propagation of slightly nonequilibrium phonons, ΔT≪T _b (T _b is the cryostat bath temperature) in Y_0.9Lu_0.1AlO₃ crystals with effective elastic scattering by rare-earth metal atoms are analyzed. The experimental results are interpreted on the basis of the “quasidiffusion” model, where three-phonon inelastic decay processes play a decisive role in the phonon kinetics. Monte Carlo calculations with these processes taken into account yield good quantitative agreement with the experimental data. Zh. éksp. Teor. Fiz. 112, 325–331 (July 1997)     

Tensor approach to mixed high-order moments of absorbing Markov chains

The moments of the number of the visits in an absorbing Markov chain are considered. The first moments and the non-mixed second moments of the number of the visits are calculated in classical textbooks such as the book of J. Kemeny and J. Snell “Finite Markov Chains”. The first moments and the non-mixed second moments can be easily expressed in a matrix form using the fundamental matrix of the absorbing Markov chain. Since the representation of the mixed moments of higher orders in a matrix form is not straightforward, if ever possible, they were not calculated. The gap is filled now. A tensor approach to the mixed high-order moments is proposed and compact closed-form expressions for the moments are discover.     

Cinnamal Sensing and Luminescence Color Tuning in a Series of Rare-Earth Metal−Organic Frameworks with Trans-1,4-cyclohexanedicarboxylate

Three isostructural metal–organic frameworks ([Ln₂(phen)₂(NO₃)₂(chdc)₂]·2DMF (Ln³⁺ = Y³⁺ for 1, Eu³⁺ for 2 or Tb³⁺ for 3; phen = 1,10-phenanthroline; H₂chdc = trans-1,4-cyclohexanedicarboxylic acid) were synthesized and characterized. The compounds are based on a binuclear block {M₂(phen)₂(NO₃)₂(OOCR)₄} assembled into a two-dime nsional square-grid network containing tetragonal channels with 26% total solvent-accessible volume. Yttrium (1)-, europium (2)- and terbium (3)-based structures emit in the blue, red and green regions, respectively, representing the basic colors of the standard RGB matrix. A doping of Eu³⁺ and/or Tb³⁺ centers into the Y³⁺-based phase led to mixed-metal compositions with tunable emission color and high quantum yields (QY) up to 84%. The bright luminescence of a suspension of microcrystalline 3 in DMF (QY = 78%) is effectively quenched by diluted cinnamaldehyde (cinnamal) solutions at millimolar concentrations, suggesting a convenient and analytically viable sensing method for this important chemical.     

Investigation of X-ray diffraction limitations upon the analysis of tellurium-atom injection into GaAs epitaxial layers

GaAs lattice “superdilation” caused by an introduced tellurium impurity, which is well known in publications, is experimentally studied. This phenomenon consists in the fact that the GaAs-lattice dilation can be more than 10 times greater than expansion that would appear upon the replacement of arsenic atoms with tellurium atoms if calculations are performed using the current-carrier concentration and Vegard’s law. The given phenomenon has already been observed at n _Te > 3 × 10¹⁸ cm^–3. A series of GaAs epitaxial layers heavily doped with tellurium and grown via metal-organic chemical vapor deposition are investigated using high-resolution X-ray diffractometry (HRXRD), secondary-ion mass spectrometry (SIMS), and the Hall effect. It is demonstrated that, despite a high Te concentration (10²⁰?10²¹ cm^–3) in the layer and variations in the growth conditions, the concentration estimates based on HRXRD data depend linearly on the results of elemental analysis performed by means of SIMS. The GaAs lattice expands even somewhat slighter as compared to the case where arsenic atoms are replaced with all Te atoms injected into the layer. At the same time, the Hall carrier concentration decreases sharply beginning at 2 × 10²⁰ cm^–3. In accordance with the obtained results, the examined phenomenon can be interpreted as the strong compensation of donor and acceptor carriers rather than as superdilation.     

Facile Access to Optically Active 2,6‐Dialkyl‐1,5‐Diazacyclooctanes

The chiral substituted 1,5‐diazacyclooctane (1,5‐DACO) is of considerable importance and has attracted attention from a wide range of fields due to their unique chemical and biological properties. Despite the application potential, further study has not been optimized due to difficulties in their synthetic accessibility. Here, we report that the 1,5‐DACO bearing a chiral auxiliary obtained from the formal [4+4] cycloaddition of N‐alkyl‐α,β‐unsaturated imines can be further derivatized by nucleophilic alkylation to give various chiral substituted 1,5‐DACO derivatives. The removal of the chiral auxiliary was effectively carried out using hydrogenation over Pearlman's catalyst. This methodology allows the production of a broad range of unprecedented optically active 2,6‐dialkyl‐1,5‐DACO, which could not be accessed by other methods.     

Optical monitoring of technological parameters in metal-organic vapor-phase epitaxy

The possibility of applying low-coherent tandem interferometry to optical monitoring of the temperature of a semiconductor substrate and the thickness of a deposited layer in metal-organic vapor-phase epitaxy (MOVPE) is demonstrated for the first time. The absolute accuracy in the temperature measurements of Si, GaAs, and sapphire substrates under MOVPE conditions is limited by the calibration accuracy and is ±1°C. The accuracy in the measurement of the deposited layer thickness is 2 nm. A considerable (10–100°C) deviation of the temperature measured by a thermocouple placed inside a susceptor from the actual substrate temperature is found. A significant temperature gradient along the susceptor depending on the gas flow rate and other factors is revealed. It is shown that, owing to the high heating efficiency of sapphire substrates, there is no need to coat their reverse with absorbing layers upon heating up to 300°C or in the presence of hydrogen pressure of higher than 100 mbar.     

Isoreticular homochiral porous metal-organic structures with tunable pore sizes

Chiral-layered building motifs of zinc(II) camphorate are linked through linear N-donor ligands, forming series of three-dimensional isoreticular porous homochiral frameworks. The lengths of these linear ligands control the pore sizes and free accessible volumes of the homochiral metal-organic structures.