Probing high‐energy ion‐implanted silicon by micro‐Raman spectroscopy

The effect of ion implantation (4 MeV¹²C²⁺, 5 MeV¹⁶O²⁺, and 8 MeV²⁸Si²⁺) on [110] silicon wafers in channeling and random orientation is investigated by micro‐Raman spectroscopy. The profiles were measured using Scanning Electron Microscope (SEM) showing that the ions were penetrating deeper inside the wafer in the channeling case creating a 1–2 µm wide strongly modified region and agreeing with the d‐nuclear reaction analysis measurements. Micro‐Raman spectroscopy was employed for the assessment of the lattice damage, probing the side surface of the cleaved wafers at submicron step. The phonon modifications show strong lattice distortions in zones parallel to the front surface of the wafers and at depths, which agree with the results of the characterization techniques. In these strongly damaged zones, there is a substantial reduction in the phonon intensity, a small shift in wavenumber position, and a large increase in the phonon width. On the basis of a modification of the phonon confinement model that takes under consideration the laser beam profile, the reduction in intensity of scattered light, and the nanocrystallite size distribution from the simulation of the lattice displacements, the main characteristics of the Raman spectra could be reproduced for the random C and O implantations. The results indicate that at a critical doping level, the induced defects and lattice distortions relax by breaking the silicon single crystal into nanocrystallites, thus creating the observed zones of strongly distorted lattice. Copyright © 2014 John Wiley & Sons, Ltd.     

J-modulation effects in DOSY experiments and their suppression: the Oneshot45 experiment

Diffusion-ordered spectroscopy (DOSY) is a powerful NMR method for identifying compounds in mixtures. DOSY experiments are very demanding of spectral quality; even small deviations from expected behaviour in NMR signals can cause significant distortions in the diffusion domain. This is a particular problem when signals overlap, so it is very important to be able to acquire clean data with as little overlap as possible. DOSY experiments all suffer to a greater or lesser extent from multiplet phase distortions caused by J-modulation, requiring a trade-off between such distortions and gradient pulse width. Multiplet distortions increase spectral overlap and may cause unexpected and misleading apparent diffusion coefficients in DOSY spectra. These effects are described here and a simple and effective remedy, the addition of a 45° purging pulse immediately before the onset of acquisition to remove the unwanted anti-phase terms, is demonstrated. As well as affording significantly cleaner results, the new method allows much longer diffusion-encoding pulses to be used without problems from J-modulation, and hence greatly increases the range of molecular sizes that can be studied for coupled spin systems. The sensitivity loss is negligible and the added phase cycling is modest. The new method is illustrated for a widely-used general purpose DOSY pulse sequence, Oneshot.     

Representations and descriptors unifying the study of molecular and bulk systems

Establishing a unified framework for describing the structures of molecular and periodic systems is a long-standing challenge in physics, chemistry, and material science. With the rise of machine learning methods in these fields, there is a growing need for such a method. This perspective aims to discuss the development and use of three promising approaches—topological, atom-density, and symmetry-based—for the prediction and rationalization of physical, chemical, and mechanical properties of atomistic systems across different scales and compositions.     

Numerical distortion and effects of thermostat in molecular dynamics simulations of single-walled carbon nanotubes

In this paper, single-walled carbon nanotubes （SWCNTs） are studied through molecular dynamics （MD） simulation. The simulations are performed at temperatures of 1 and 300K separately, with atomic interactions characterized by the second Reactive Empirical Bond Order （REBO） potential, and temperature controlled by a certain thermostat, i.e. by separately using the velocity scaling, the Berendsen scheme, the Nose-Hoover scheme, and the generalized Langevin scheme. Results for a （5,5） SWCNT with a length of 24.5 nm show apparent distortions in nanotube configuration, which can further enter into periodic vibrations, except in simulations using the generalized Langevin thermostat, which is ascribed to periodic boundary conditions used in simulation. The periodic boundary conditions may implicitly be applied in the form of an inconsistent constraint along the axis of the nanotube. The combination of the inconsistent constraint with the cumulative errors in calculation causes the distortions of nanotubes. When the generalized Langevin thermostat is applied, inconsistently distributed errors are dispersed by the random forces, and so the distortions and vibrations disappear. This speculation is confirmed by simulation in the case without periodic boundary conditions, where no apparent distortion and vibration occur. It is also revealed that numerically induced distortions and vibrations occur only in simulation of nanotubes with a small diameter and a large length-to-diameter ratio. When MD simulation is applied to a system with a particular geometry, attention should be paid to avoiding the numerical distortion and the result infidelity.     

On the Volterra's Distortions Theory

This paper has been spurred by the great influence of the Volterra's distortions theory on the modern theory of dislocations and particularly of the ones which study the crystals, aelotropic materials. Volterra developed his theory, as well as the explicit solutions, for elastic, homogeneous, isotropic, and right hollow cylinders. Here we are subduing the assumptions developing the theory for transversely isotropic hollow cylinders.     

Density functional studies of LaMnO3 under uniaxial strain

We study the electronic and magnetic properties of tetragonal LaMnO₃ (LMO) under uniaxial strain, appropriate for epitaxially grown LMO heterostructures, from density functional calculations. The optimized tetragonal structure without strain has volume, magnetic order, and Jahn-Teller distortions similar to the bulk LMO, which forms in the orthorhombic structure. Strain affects the relative magnitudes of these distortions and changes the magnetic and conduction properties. While unstrained LMO is a type A antiferromagnet and insulating, we find that it changes to a ferromagnetic metal under tensile strain condition (c-axis stretched). The latter is the result of a diminishing magnitude of the Jahn-Teller distortion with strain, which in turn reduces the splitting of the Mn-e_g orbitals, eventually leading to a metallic state, and finally to a ferromagnet due to the double exchange interaction. The calculated Poisson's ratio from geometry optimization agrees with the experimental values for the bulk.     

Structural distortions of semiconducting silicon crystals caused by constant electric field

Structural distortions in nonpolar, particularly, in semiconducting silicon crystals, caused by constant electric field have been disclosed by means of X‐ray interferometry. It was shown that in the field both the direction of moiré fringes and the frequency (period) are changed, and the moiré patterns disappear at values of potential difference in excess of 1.5 kV. The moiré pattern obtained under the action of electrostatic field is independent of the direction (polarity) of the field, the pictures for both the polarities being completely identical. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Detection of Heme-Distortions in Ferri- and Ferrocyto-Chrome C by Resonance Raman Scattering

Abstract

We have measured the depolarization ratio and polarized intensities of the oxidation marker A_1g-Raman line at 1363 cm^?1 and 1375 cm^?1 in ferri- and ferrocytochrome c respectively. From these data we derive symmetry classified distortions of the heme group from its ideal D_4h-symmetry. In ferrocytochrome c these distortions are independent of the pH-value of the solution, indicating that the heme environment is stable with respect to pH. In ferricytochrome c we observe pH-dependent changes of the distortions, indicating the well known conformational change with pK = 9.05, where the methionine 80 ligand is replaced by lysine-79. The data indicate that in this conformation due to opening of the heme crevice the heme group has a higher effective symmetry.