The evolution of the aging process of glassy materials quenched from temperatures above their glass transition temperature $T_g$, when plotting the relaxed enthalpy versus the decrease in volume, leads to a slope comparable to the isothermal compressibility close to $T_g$. This empirical result was explained in an earlier publication (V. Katsika-Tsigourakou, G. E. Zardas J. Non-Cryst. Solids 356 (2010) 179-180) by means of a thermodynamical model. Here, we show that the same model enables the explanation of the rare cases of negative defect activation in solids.
We present a first-principles study of native defects in NaAlH$_4$. Our analysis indicates that the structure and energetics of these defects can be interpreted in terms of elementary building blocks, which include $V_{\rm{AlH_4}}^+$, $V_{\rm{Na}}^-$, $V_{\rm{H}}^+$, H$_i^-$, and (H$_2$)$_i$. We also calculate migration barriers for several key defects, in order to compare enthalpies of diffusion to experimentally measured activation energies of desorption. From this, we estimate activation energies for diffusion of defects and defect pairs. We suggest that $V_{\rm{AlH_4}}^+$ and H$_i^-$, or $…
Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used to accurately determine actual activation energies that can be directly compared with DLTS. Our analysis is illustrated with hybrid functional calculations for two important defects in GaN that have s…
The stability of magnetic skyrmions against thermal fluctuations and external perturbations is investigated within the framework of harmonic transition state theory for magnetic degrees of freedom. The influence of confined geometry and atomic scale non-magnetic defects on the skyrmion lifetime is estimated. It is shown that a skyrmion on a track has lower activation energy for annihilation and higher energy for nucleation if the size of the skyrmion is comparable with the width of the track. Two mechanisms of skyrmion annihilation are considered: inside the track and escape through the bounda…
Annealing Mg-implanted homoepitaxial GaN at temperatures at or above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects in the form of inversion domains contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple axis X-ray data show that the implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that the strain-inducing point defects formed during implantation…
The preparation of mixed crystals of NaBr and KBr from melt as well as their physical characterization have been reported. Electrical measurements were carried out at various temperatures, which showed that their conductivity increases with the increase in temperature and varies nonlinearly with the bulk composition. We find that for most of the compositions studied, the activation energy deduced from the temperature variation of their conductivity obeys a thermodynamical model that interrelates the defect Gibbs energy with the bulk elastic and expansivity data.
We propose a di-interstitial model for the P6 center commonly observed in ion implanted silicon. The di-interstitial structure and transition paths between different defect orientations can explain the thermally activated transition of the P6 center from low-temperature C1h to room-temperature D2d symmetry. The activation energy for the defect reorientation determined by ab initio calculations is 0.5 eV in agreement with the experiment. Our di-interstitial model establishes a link between point defects and extended defects, di-interstitials providing the nuclei for the growth.
An optimized direct arylation polycondensation (DAP) protocol for the synthesis of a novel naphthalene diimide (NDI) 2,2'-bithiazole (2-BTz) copolymer (PNDI-2-BTz) is presented. The regioselective C-H activation of 2-BTz at the 5-positions allows for the synthesis of fully regioregular and homocoupling-free PNDI-2-BTz of high molecular weight in less than 1 h in quantitative yield. Complete end group assignment shows functionalities according to monomer structures or to nucleophilic substitution, and allows for the reliable determination of absolute molecular weight. Compared to the well-known…
At low temperatures, dynamics in amorphous silicon occurs through a sequence of discrete activated events that locally reorganize the topological network. Using the activation-relaxation technique, a data base containing over 8000 such events is generated, and the events are analyzed with respect to their energy barrier and asymmetry, displacement and volume expansion/contraction. Special attention is paid to those events corresponding to diffusing coordination defects. The energetics is not clearly correlated with the displacement, nor with the defect density in well relaxed configurations. W…
The ohmic conductivity of the sol-gel derived Pb1+y(Zr0.3Ti0.7)O3 thin films (with the excess lead y=0.0 to 0.4) are investigated using low frequency small signal alternate current (AC) and direct current (DC) methods. Its temperature dependence shows two activation energies of 0.26 and 0.12 eV depending on temperature range and excess Pb levels. The former is associated with Pb3+ acceptor centers, while the latter could be due to a different defect level yet to be identified.
This letter introduces ab initio study of the full activation-volume tensor of crystalline defects as a means to make contact with mechanical response experiments. We present a theoretical framework for prediction of the internal friction associated with divacancy defects and give the first ab initio value for this quantity in silicon. Finally, making connection with defect alignment studies, we give the first unambiguous resolution of the debate surrounding ab initio verification of the ground-state structure of the defect.
Thermal quenching is one of the essential factors in reducing the efficiency of radiative processes in luminophores of various nature. The emission activity of low dimensional structures is influenced also by multiplicity of parameters that are related to synthesis processes, treatment regimes, etc. In the present work, we have investigated the temperature dependence of photoluminescence caused by exciton and defect-related transitions in ensembles of biocompatible InP/ZnS core/shell nanocrystals with an average size of 2.1 and 2.3 nm. The spread in the positions of energy levels is shown to b…
Defect-controlled twin activation in crystallographically equivalent magnesium micropillars
RWTH Aachen University（亚琛工业大学）；CNRS, Université de Lorraine, Arts et Métiers Institute of Technology（法国国家科学研究中心，洛林大学，国立工艺学院）；Helmholtz Zentrum Hereon（海姆霍兹HEREON中心）
The influence of implantation conditions on dopant activation in Al-implanted 4H-SiC: A MD study applying an Al potential fitted to DFT barriers
CDL for Multi-Scale Process Modeling of Semiconductor Devices and Sensors, at the Institute for Microelectronics, TU Wien（维也纳工业大学微电子研究所多尺度半导体器件与传感器工艺建模中心）；Silvaco Europe Ltd.（Silvaco欧洲有限公司）
We describe a defect in pentacene single crystals that is created by bias stress and persists at room temperature for an hour in the dark but only seconds with 420nm illumination. The defect gives rise to a hole trap at Ev + 0.38eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67eV activation energy with a small (108 s-1) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.
Computational atomic-scale methods continue to provide new information about geometry, energetics, and transition states for interstitial elements in crystalline lattices. This data can be used to determine the diffusivity of interstitials by finding steady-state solutions to the master equation. In addition, atomic-scale computations can provide not just the site energy, but also the stress in the cell due to the introduction of the defect to compute the elastic dipole. We derive a general expression for the fully anistropic diffusivity tensor from site and transition state energies, and thre…
We report modifications of the ferroelectric and electrocaloric properties of BaTiO$_3$ by defects. For this purpose, we have combined \textit{ab initio}-based molecular dynamics simulations with a simple model for defects. We find that different kinds of defects modify the ferroelectric transition temperatures and polarization, reduce the thermal hysteresis of the transition and are no obstacle for a large caloric response. For a locally reduced polarization the ferroelectric transition temperature and the adiabatic response are slightly reduced. For polar defects an intriguing picture emerge…
This work proposes an effective thermal activation method with low technical effort for industrially produced titanium-iron-manganese powders (TiFeMn) for hydrogen storage. In this context, the influence of temperature and particle size of TiFeMn on the activation process is systematically studied. The results obtained from this investigation suggest that the activation of the TiFeMn material at temperatures as low as 50 °C is already possible and that an increase to 90 °C strongly reduces the incubation time for activation, i.e. the incubation time of the 90 °C/90 °C sample is about 0.84 h wh…
The appealing connection between non-Euclidean geometries and defects in solids is brought forth in this article. Drawing a correspondence between the nature of a defect and a specific geometric property of the material space not only illuminates the underlying structure of defects in solids but also provides an unambiguous way to represent defect densities within a physical theory. We present a rigorous background of the relevant concepts from classical differential geometry, as well as illustrations of isolated defects in a lattice, to motivate the relationship between continuous defect dens…
Negative defect activation volumes are extremely rare in solids. Here, we report for the first time that this holds in a couple of hydrated rocks for dielectric relaxation by exploring the complex impedance spectra at various pressures and temperatures. The present findings mean that the relaxation time of the relevant relaxation mechanisms decreases upon increasing pressure, thus it may become too short at higher pressure and hence lead to the emission of transient electric signals before fracture. This may constitute the long-standing laboratory confirmation for the explanation of the genera…
We present an extensive first-principles study of the pressure dependence of the formation enthalpies of all the know vacancy and self-interstitial configurations in silicon, in each charge state from -2 through +2. The neutral vacancy is found to have a formation volume that varies markedly with pressure, leading to a remarkably large negative value (-0.68 atomic volumes) for the zero-pressure formation volume of a Frenkel pair (V + I). The interaction of volume and charge was examined, leading to pressure--Fermi level stability diagrams of the defects. Finally, we quantify the anisotropic na…
Silica is known as the archetypal strong liquid, exhibiting an Arrhenius viscosity curve with a high glass transition temperature and constant activation energy. However, given the ideally isostatic nature of the silica network, the presence of even a small concentration of defects can lead to a significant decrease in both the glass transition temperature and activation energy for viscous flow. To understand the impact of trace level dopants on the viscosity of silica, we measure the viscosity-temperature curves for seven silica glass samples having different impurities, including four natura…
First-principles density functional theory studies have been carried out for native defects and transition-metal (Ti and Ni) impurities in lithium alanate (LiAlH$_{4}$), a potential material for hydrogen storage. On the basis of our detailed analysis of the structure, energetics, and migration of lithium-, aluminum-, and hydrogen-related defects, we propose a specific atomistic mechanism for the decomposition and dehydrogenation of LiAlH$_{4}$ that involves mass transport mediated by native point defects. We also discuss how Ti and Ni impurities alter the Fermi-level position with respect to t…
Within a nanoscale volume, an acoustic wave interacts with radiation defects in ionic solids. The radiation-induced optical absorption in ionic crystals is remarkably removed by a room-temperature ultrasonic treatment of the crystals. It is shown that the effect can be explained by defect migration processes occurring in ultrasonic fields. The inter-ion charge exchange, termed as acoustically-stimulated chemical reaction, is furthermore suggested to occur affecting the defect migration. This new method of a cold annealing of radiation defects in solids can be regarded as nanoacoustic defect ma…
Two models have been proposed for the interconnection of the defect Gibbs energy g^i with bulk properties almost 60 and 30 years ago, respectively. The one, proposed by Zener, assumes that g^i can be accounted for the work that goes into straining the lattice and hence it is proportional to the shear modulus of the solid. The other, considers that, since g^i corresponds to an isobaric and isothermal process, it should be proportional to the isothermal bulk modulus and the mean volume per atom. The results of these two models are compared for different processes (defect formation, self-diffusio…
We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident p…
Surface defects in carbon-doped hexagonal boron nitride for negative-contrast direct laser writing
National University of Singapore（新加坡国立大学）；Institute for Functional Intelligent Materials, National University of Singapore（新加坡国立大学功能智能材料研究所）；National Institute of Material Science（日本物质材料研究机构）；Research Center for Functional Materials, National Institute of Material Science（日本物质材料研究机构功能材料研究中心）；International Center for Materials Nanoarchitectonics, National Institute of Material Science（日本物质材料研究机构国际纳米材料中心）
