Aromaticity of planar Si6 rings in silicon–lithium clusters

Ipsocentric calculations of current density at the B3LYP/6-311++G(2d,2p) level show that the planar Si₆ ring supports a diatropic π ring current of about half the strength of the equivalent π current in benzene, both in the presumed global optimum geometry of Si₆Li₆ and in geometries occupying higher-energy local minima, corroborating the attribution of aromaticity to this silicon analogue of benzene.     

Electron–acoustic phonon interaction and mobility in stressed rectangular silicon nanowires

We investigate the effects of pre-stress and surface tension on the electron–acoustic phonon scattering rate and the mobility of rectangular silicon nanowires.With the elastic theory and the interaction Hamiltonian for the deformation potential,which considers both the surface energy and the acoustoelastic effects,the phonon dispersion relation for a stressed nanowire under spatial confinement is derived.The subsequent analysis indicates that both surface tension and pre-stress can dramatically change the electron–acoustic phonon interaction.Under a negative(positive)surface tension and a tensile(compressive)pre-stress,the electron mobility is reduced(enhanced)due to the decrease(increase)of the phonon energy as well as the deformation-potential scattering rate.This study suggests an alternative approach based on the strain engineering to tune the speed and the drive current of low-dimensional electronic devices.     

Measurement of large aspheric surfaces by annular subaperture stitching interferometry

A new method for testing aspheric surfaces by annular subaperture stitching interferometry is introduced. It can test large-aperture and large-relative-aperture aspheric surfaces at high resolution,low cost,and high efficiency without auxiliary null optics.The basic principle of the method is described,the synthetical optimization stitching model and effective algorithm are established based on simultaneous least-square fitting.A hyperboloid with an aperture of 350 mm is tested by this method.The obtained peak-to-valley (PV) and root-mean-square (RMS) values of the surface error after stitching are 0.433λand 0.052λ(λis 632.8 nm),respectively.The reconstructed surface map is coincide with the entire surface map from null test,and the difference of PV and RMS errors between them are 0.031λand 0.005λ,respectively. This stitching model provides another quantitive method for testing large aspheric surfaces besides null compensation.     

Optimal magnetization in liquids,generated by triplet—doublet interaction

This study is an extension of a recent theoretical treatment for calculating the magnetization generated by the encounters of radicals and photoexcited triplets in solution. Whereas the previous study employed a restricted analytical approach to the problem, the present study takes into account a general numerical formulation for the solution of the stochastic Liouville equation to calculate the electron spin polarization generated in the radical, following its encounter with the triplet. This method considers the efficiency of triplet quenching by the radical, which is an important factor in determining the radical polarization and the triplet lifetime in the solution. In addition, numerical calculation of the diffusion process is used to obtain the overall magnetization of the radical and its time dependence. The theory presented here complies with the experimental results and allows for efficient optimization of the magnetization in terms of magnitude and overall lifetime. Such an optimization is accomplished by the proper choice of the chemical system, which is exposed to light excitation, solvent properties and temperature. The ultimate goal of this study is to achieve photo-controlled high magnetization, which can be used in a variety of novel microwave applications.     

Simultaneous Measurement of Concentration and Temperature in Methane–Hydrogen Flames by Laser-Induced Fluorescence

Journal of Russian Laser Research - We analyze the structure of methane–air and methane–hydrogen–air flames stabilized over the porous plate burner, using the method of...     

Chain structures and clusters of particles with the mixed dipole–quadrupole interaction in smectic freely suspended nanofilms

The formation of unusual chain structures and clusters of particles with the mixed dipole–quadrupole interaction has been found in smectic nanofilms. Unlike topological dipoles and quadrupoles, the interaction between which leads to the formation of structures with finite interparticle distances, the particles with the mixed interaction touch each other and form stable chains and two-dimensional clusters. The orientation of particles in chains is intermediate between dipole and quadrupole chains. The variation of the interparticle distance and orientation of chains is explained qualitatively on the basis of the calculation of the с-director (field lines) near particles and the mutual arrangement of particles providing the minimum distortion of field lines.     

In-situ monitoring and control of hydrogenated amorphous silicon–germanium band-gap profiling during plasma deposition process

In-situ germanium content monitoring and its characteristics in SiH₄/GeH₄/H₂ plasmas was studied during hydrogenated amorphous silicon–germanium (a-SiGe:H) film depositions. Since an appropriate band-gap profiling in a-SiGe:H deposition is very important to achieve high efficiency solar cell, the accurate monitoring and control of Ge contents are required. In this work, we found the spectral intensity ratio of silicon atom (288.2 nm) and germanium atom (303.9 nm) emission has strong relation with Ge content in plasmas. In typical, band-gap energy of films was decreased with the increasing of gas flow ratio GeH₄/SiH₄. However, at different total flow rate of GeH₄, the band-gap was different for same gas flow ratio cases because the Ge content in plasmas was changed due to the changes of electron temperature by hydrogen dilution. On the other hand, the emission intensity ratio Ge/Si detected the band-gap variation. Using this method, therefore, we measured and control Ge/Si to make a U-shape band-gap profile which was proved by an ellipsometer and Auger electron spectroscopy depth profile analysis.     

Investigation of muon states in silicon and germanium by field-quenching and RFμSR

The temperature dependence of the three states of positive muons in the semiconductors with diamond structure ( ⁺ in diamagnetic states ^d and paramagnetic muonium Mu and Mu^*) have been investigated on six Si (pure, B and P doped) and four Ge (ultrapure, CZ-grown undoped, Ga and Sb doped) single crystals by longitudinal field-quenching and radio-frequency ⁺SR. Clear evidence for the transition Mu^{* d} is found. The influence of light-induced charge-carriers is shown to be quite different in p- and n-type material.The work has been supported by the Bundesministerium für Forschung und Technologie in Bonn, Germany, under contract no. 03-SE3STU.     

Symmetry and structure of N–O shallow donor complexes in silicon

Shallow donors in silicon related to nitrogen–oxygen complexes have been investigated by piezospectroscopy of their hydrogenic transitions in the far infrared. Complete stress dependences up to 0.25 GPa were obtained for the 1s→2p₀ and 1s→2p_± transitions of the most prominent members of the (N, O)-family, N–O-3 and N–O-5. Very unusual for shallow donors in silicon, the symmetry of the ground state wave function is T₂-like. The lifting of orientational degeneracy for stress in the 〈1 0 0〉, 〈1 1 1〉, and 〈1 1 0〉 directions is compatible with a C_2v defect symmetry. Data from the other species of the (N, O)-family are indicative for the same symmetry. The microscopic structure of these centers, in part contradictory to present theoretical models, is discussed.     

Adsorption of nitrogen and ammonia by polycrystalline iron surfaces in the temperature range 80–290 K studied by electron spectroscopy

X-ray and uv induced photoelectron spectroscopy have provided information on the various molecular states of nitrogen formed on polycrystalline iron surfaces from dinitrogen and ammonia. At 85 K two distinct states are observed with N₂(g) which have N(1s) binding energy values of 405.3 and 400.2 eV. These are in equilibrium with N₂(g), are weakly held, and are desorbed on warming to 290 K leaving a nitrogen free surface. The two states are assigned to a molecularly adsorbed

and linear

species the former characterised by an N(1s) value of 400.2 eV and the latter by 405.3 eV. At 290 K nitrogen is adsorbed with a very low sticking probability (?10^?6) giving rise to an N(1s) value of 397.2 eV. This is undoubtedly the dissociatively chemisorbed

species. At a nitrogen pressure of l Torr adsorption is “instantaneous” and the N(1s) value is 397 eV. No evidence for the unstable bridged and linear forms of nitrogen is obtained at 290 K although they may well be precursors to the formation of the strongly chemisorbed nitrogen species. Shifts in the N(1s) binding energy induced by subsequent oxygen adsorption are discussed briefly. At 85 K ammonia adsorbs largely in the molecular form with a broad N(1s) peak centred at about 400 eV but on warming to 290 K this splits to give two peaks one at 397 eV and the other at 400 eV. Interaction at 290 K leads to a dominant peak at 397.2 eV and a subidiary one at 400 eV. Helium (1) spectra support the assignment of the 397.2 eV peak to dissociated species (N, NH) and the 400 eV peak to molecular adsorption. The conclusions with N₂ and NH₃ are substantiated further by comparing the data with results for nitric oxide. The concentration of nitrogen adatom species formed from NO at 290 K and 10^?6 Torr is some ten times that formed from N₂ at 1 Torr and three times that from NH₃ at 10^?6 Torr and the same temperature.     

Growth of polymer–metal nanocomposites by pulsed laser deposition

Complex polymer–metal nanocomposites have a wide range of applications, e.g. as flexible displays and packaging materials. Pulsed laser deposition was applied to form nanostructured materials consisting of metal clusters (Ag, Au, Pd and Cu) embedded in a polymer (polycarbonate, PC) matrix. The size and amount of the metal clusters are controlled by the number of laser pulses hitting the respective targets. For Cu and Pd, smaller clusters and higher cluster densities are obtained as in the cases of Ag and Au due to a stronger reactivity with the polymers and thus a lower diffusivity. Implantation effects, differences in metal diffusivity and reactivity on the polymer surfaces, and the coalescence properties are discussed with respect to the observed microstructures on PC and compared to the metal growth on poly (methyl methacrylate), PMMA.     

The Study of Aggregation Processes in Colloidal Solutions of Magnetite–Silica Nanoparticles by NMR Relaxometry,AFM, and UV–Vis-Spectroscopy

Colloidal solutions of magnetic nanoparticles were studied as a promising magnetic resonance imaging (MRI) contrast agent. The problem of aggregative stability of solutions is considered. Sol-gel synthesis of magnetite colloidal solutions stabilized by silica is described. Transmittance spectra were measured to analyze sedimentation of nanoparticles in magnetite–silica solutions of different compositions and concentrations. It is shown that the synthesized nanoparticles can be used as MRI contrast agents. The surface morphology and particle size of Fe₃O₄/SiO₂ layers were estimated by atomic force mictroscopy (AFM) technique. The mechanism of magnetic-field-induced aggregation of Fe₃O₄/SiO₂ nanoparticles into chain-like and fractal structures is described.     

Substrate and pretreatment dependence of Cu nucleation by metal–organic chemical vapor deposition

The nucleation of copper (Cu) with (hfac)Cu(VTMS) organometallic precursor is investigated for Si, SiO₂, TiN, and W₂N substrates. As the deposition temperature is increased, the dominant growth mechanism is observed to change from the nucleation of Cu particles to the clustering of Cu nuclei around 180 °C independent of the employed substrates. It is also observed that the cleaning of substrate surfaces with the diluted HF solution improves the selectivity of Cu nucleation between TiN and SiO₂ substrates. Dimethyldichlorosilane treatment is found to passivate the surface of TiN substrate, contrary to the generally accepted belief, when the TiN substrate is cleaned by H₂O₂ solution before the treatment.     

Effect of anisotropy and dipole interaction on long-range order magnetic structures generated by Dzyaloshinskii–Moriya interaction

Self-organized long-range order structures, such as stripe domains and magnetic skyrmion lattices, are formed by the competition between ferromagnetic exchange interaction and Dzyaloshinskii–Moriya (DM) interaction. We investigated the properties of the magnetic structures generated by a DM interaction under the influence of anisotropy or magnetic dipole interaction, by performing Monte-Carlo simulated annealing. We constructed phase maps in external-field and anisotropy space to study the effect of anisotropy or dipole interaction on the phase boundaries between the magnetic structures. The simulation results show that the phase boundaries are sensitive to perpendicular anisotropy and that the skyrmion lattice region in phase space is extended under easy-plane anisotropy. The effect of the long-range dipole interaction was studied and was found to stabilize the skyrmion lattice phase and reduce the size of the magnetic structures.     

Measurement of temperature and temperature gradient in?millimeter samples by chlorine NQR

A mini-thermometer based on the ³⁵Cl nuclear quadrupole resonance (NQR) frequency temperature dependence in the chlorates KClO₃ and NaClO₃ was built and successfully tested by measuring temperature and temperature gradient at 77 K and higher in about 100 mm³ active volume of a mini Joule–Thomson refrigerator. In the design of the tank-circuit coil, an array of small coils connected in series enabled us (a) to achieve a suitable ratio of inductance to capacity in the NQR spectrometer input tank circuit, (b) to use a single crystal of KClO₃ or NaClO₃ (of 1–2 mm³ size) in one coil as a mini-thermometer with a resolution of 0.03 K and (c) to construct a system for measuring temperature gradients when the spatial coordinates of each chlorate single crystal within an individual coil are known.     

Percolation and the electron–electron interaction in an array of antidots

A square lattice of microcontacts with a period of 1 μm in a dense low-mobility two-dimensional electron gas is studied experimentally and numerically. At the variation of the gate voltage V _g , the conductivity of the array varies by five orders of magnitude in the temperature range T from 1.4 to 77 K in good agreement with the formula σ(V _g ) = (V _g ?V _g ^* (T))^β with β = 4. The saturation of σ(T) at low temperatures is absent because of the electron–electron interaction. A random-lattice model with a phenomenological potential in microcontacts reproduces the dependence σ(T, V _g ) and makes it possible to determine the fraction of microcontacts x(V _g , T) with conductances higher than σ. It is found that the dependence x(V _g ) is nonlinear and the critical exponent in the formula σ ∝ ? (x - 1/2) ^t in the range 1.3 < t(T, V _g ) < β.