Vacancies In Clusters Research Articles

QMLMaterial─A Quantum Machine Learning Software for Material Design and Discovery.

Structural elucidation of chemical compounds is challenging experimentally, and theoretical chemistry methods have added important insight into molecules, nanoparticles, alloys, and materials geometries and properties. However, finding the optimum structures is a bottleneck due to the huge search space, and global search algorithms have been used successfully for this purpose. In this work, we present the quantum machine learning software/agent for materials design and discovery (QMLMaterial), intended for automatic structural determination in silico for several chemical systems: atomic clusters, atomic clusters and the spin multiplicity together, doping in clusters or solids, vacancies in clusters or solids, adsorption of molecules or adsorbents on surfaces, and finally atomic clusters on solid surfaces/materials or encapsulated in porous materials. QMLMaterial is an artificial intelligence (AI) software based on the active learning method, which uses machine learning regression algorithms and their uncertainties for decision making on the next unexplored structures to be computed, increasing the probability of finding the global minimum with few calculations as more data is obtained. The software has different acquisition functions for decision making (e.g., expected improvement and lower confidence bound). Also, the Gaussian process is available in the AI framework for regression, where the uncertainty is obtained analytically from Bayesian statistics. For the artificial neural network and support vector regressor algorithms, the uncertainty can be obtained by K-fold cross-validation or nonparametric bootstrap resampling methods. The software is interfaced with several quantum chemistry codes and atomic descriptors, such as the many-body tensor representation. QMLMaterial's capabilities are highlighted in the current work by its applications in the following systems: Na20, Mo6C3 (where the spin multiplicity was considered), H2O@CeNi3O5, Mg8@graphene, Na3Mg3@CNT (carbon nanotube).

One-pot solvothermal synthesis of In-doped amino-functionalized UiO-66 Zr-MOFs with enhanced ligand-to-metal charge transfer for efficient visible-light-driven CO2 reduction

Metal organic frameworks (MOFs) with high porosity and highly tunable physical/chemical properties can serve as heterogeneous catalysts for CO2 photoreduction, but the application is hindered by the large band gap (Eg) and insufficient ligand-to-metal charge transfer (LMCT). In this study, a simple one-pot solvothermal strategy is proposed to prepare an amino-functionalized MOF (aU(Zr/In)) featuring an amino-functionalizing ligand linker and In-doped Zr-oxo clusters, which enables efficient CO2 reduction driven with visible light. The amino functionalization leads to a significant reduction of Eg as well as a charge redistribution of the framework, allowing the absorption of visible light and the efficient separation of photogenerated carriers. Furthermore, the incorporation of In not only promotes the LMCT process by creating oxygen vacancies in Zr-oxo clusters, but also greatly lowers the energy barrier of the intermediates for CO2-to-CO conversion. With the synergistic effects of the amino groups and the In dopants, the optimized aU(Zr/In) exhibits a CO production rate of 37.58 ± 1.06 μmol g-1 h−1, outperforming the isostructural University of Oslo-66- and Material of Institute Lavoisier-125-based photocatalysts. Our work demonstrates the potential of modifying MOFs with ligands and heteroatom dopants in metal-oxo clusters for solar energy conversion.

Modelling the primary damage in Fe and W: influence of the short-range interactions on the cascade properties: Part 2 – multivariate multiple linear regression analysis of displacement cascades

Neutrons and high energy ions create displacement cascades in materials, which have been simulated using Molecular Dynamics, for many decades now. The breakthrough of this work is to explore in a large statistics of more than 7000 cascades the relation between early cascade morphology and the final primary damage using a multivariate multiple regression analysis. For two energies in Fe and W, the total number of defects, the number of SIA and vacancy clusters and their full size distributions have been characterized using a multivariate multiple linear regression analysis based on 7 descriptors of the primary damage and 3 morphology descriptors. We find that the combination of the volume and the sphericity is significant. This analysis highlights several cascade properties, among them, that the large and spherical cascades create less defects and in particular, less mono defects than small and fragmented ones. 13 interatomic potentials differing either by their equilibrium part or the way they were hardened have been included in this study and the multivariate analysis shows that the choice of potential has a limited influence on the total number of defects but a large one on the number of mono vacancies. On average, soft potentials create cascades of larger volume, smaller sphericity and producing more defects than hard potentials. Finally, the formation of vacancy clusters is different in Fe than in W. In Fe, the fraction of vacancies in clusters is larger than that of SIAs and larger vacancy clusters are created than SIA clusters. In W, it is the opposite. The reasons are the differences of stopping power and threshold displacement energies, which result in different spatial distributions of open volumes that form during the expansion stage of the cascade.

Helium self-trapping and diffusion behaviors in deformed 316L stainless steel exposed to high flux and low energy helium plasma

A large number of dislocation networks were introduced in to 316L stainless steel by cold rolling. Subsequently, low energy (40 eV) helium ions were implanted by exposing the steel to helium plasma. Thermal desorption and positron annihilation spectroscopy were used to study the behavior of helium in the presence of dislocations, with emphasis on helium self-trapping and migration behaviors. Helium desorption behaviour from different helium trapping states was measured by the thermal desorption spectroscopy. Most of the helium desorbed from the HemVn clusters, and the corresponding desorption peak is located at ~650 K. The desorption peak from helium-dislocation clusters (HemD) is at approximately 805 K. The effect of annealing on the defect evolution was investigated by positron annihilation spectroscopy. For the specimen exposed to helium plasma without displacement damage, the increment of S parameter meant the existence of helium self-trapping behavior (HemVn). Helium atoms could diffuse two to three orders of magnitude deeper than the implantation depth calculated by SRIM. The diffusing helium atoms were gradually trapped by dislocation lines and formed HemD. Elevated temperatures enhance the self-trapping behavior and cause helium atoms to dissociate/desorb from the HemVn clusters, increasing the S parameters at 473–673 K. The gradual recovery of vacancies in the HemVn clusters decreased the S parameter above 673 K.

Competition of inelastic electron scattering and Interatomic Coulombic Decay in Ne clusters

The creation of slow electrons due to Interatomic Coulombic Decay of 2s vacancies in Ne clusters is quantitatively compared to the slow electron yield by intracluster electron impact ionization. Using electron–electron coincidence spectroscopy, both channels can be separated experimentally. Two cluster size distributions, corresponding to two and five to six filled solvation shells, are probed.

Reprint of Measuring the efficiency of interatomic coulombic decay in Ne clusters

The efficiency of interatomic coulombic decay (ICD) in Neon clusters with a mean size of 〈N〉=480 is measured directly. By detecting the photoelectrons and the ICD electrons in coincidence and normalizing their ratio using the detection probability of the respective electrons we show that the relaxation of Ne 2s vacancies in Ne clusters by ICD has an efficiency of unity.

Measuring the efficiency of interatomic coulombic decay in Ne clusters

The efficiency of interatomic coulombic decay (ICD) in Neon clusters with a mean size of 〈N〉=480 is measured directly. By detecting the photoelectrons and the ICD electrons in coincidence and normalizing their ratio using the detection probability of the respective electrons we show that the relaxation of Ne 2s vacancies in Ne clusters by ICD has an efficiency of unity.

Quantum-chemical modeling of defects in PbTe crystals doped with VTe2 and V3Te4

Quantum-chemical modeling of defects in PbTe crystals doped with VTe2 and V3Te4 (PbTe(V)) has been performed. It has been shown that, at the B3LYP/LanL2DZP level, the effect of single defects (like vanadium atoms or lead vacancies) in the charges and electrostatic potentials is small and does not exceed 0.1e and 0.3 eV, respectively. Calculations predict that the location of a lead vacancy in the vicinity of a vanadium atom is energetically favorable. Fragments of chains of tellurium atoms formed upon association of vacancies can serve as active annihilation centers (AACs). In PbTe clusters, Te-Te chains are most abundant, Te-Te-Te-Te chains are less abundant, and Te-Te-Te chains are even less frequently encountered. AACs form beginning with association of three vacancies in PbTe clusters and beginning with association of two vacancies in PbTe(V) clusters. This fact that a smaller number of vacancies are required for formation of AACs in the PbTe(V) cluster can be responsible for that the number of annihilation centers experimentally found by the positron annihilation method in PbTe(V) is ∼1.2–1.4 times as large as the number of AACs in undoped PbTe.

Microwave-hydrothermal preparation of alkaline-earth-metal tungstates

Microwave-hydrothermal processing was employed to synthesize nanostructured alkaline-earth-metal tungstate compounds in environmentally friendly conditions not yet found in the literature: 110 °C for times ranging from 5 to 20 min. X-ray diffraction showed that crystalline, single-phase materials were produced, while electron microscopy evidenced nanostructured particles with different morphologies. Raman spectroscopy was used to probe short-range ordering and also to obtain a reliable set of spectra containing all the Raman-active bands predicted by group-theory calculations. The Raman spectra showed no extra feature, indicating that our method gave well-ordered materials without important tetrahedral distortions. Sintered ceramics were submitted to polarized Raman scattering in parallel- and cross-polarized configurations. The Eg modes were totally evidenced in a cross-polarized configuration, while Ag and Bg modes appeared in the parallel arrangement, in complete agreement with the theoretical predictions. Photoluminescence measurements showed broad, complex emission bands, which varied from the blue to the red visible light regions. These results are related to shallow and deep defects into the band gap, through the presence of oxygen vacancies in the complex clusters with metal cations and tungsten atoms.

Bond-order potential for point and extended defect simulations in tungsten

A reactive interatomic bond-order potential for bcc tungsten is presented. Special attention in the potential development was given for obtaining accurate formation and migration energies for point defects, making the potential useful in atomic scale simulations of point and extended defects. The potential was used to calculate binding energies and trapping distances for vacancies in vacancy clusters and the recombination radius for self-interstitial atom and monovacancy.

Collective Interatomic Decay of Multiple Vacancies in Clusters

We predict that inner-shell ionization of more than one atom (or molecule) in a cluster, e.g., by intense free-electron laser radiation, can lead to an interatomic decay process in which the formed vacancy states decay simultaneously, while a neighboring neutral species is ionized. This collective decay phenomenon can be regarded as a transfer of two or more virtual photons from the ionized cluster units to a neutral one. Simulations of collective decay in (4s-1, 4s-1) (Kr+)2Ar show that the two-virtual-photon process can be competitive with the dissociative nuclear dynamics of the doubly ionized cluster. Generality of the collective interatomic decay is discussed.

Ab initio calculation of interatomic decay rates of excited doubly ionized states in clusters

Recently, a computational technique for ab initio calculation of the interatomic and intermolecular nonradiative decay processes has been developed [V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005)]. It combines the Fano formalism with the Green's function method known as the algebraic diagrammatic construction. The problem of normalization of continuum wave functions stemming from the use of the Gaussian basis sets is solved by using the Stieltjes imaging technique. In the present paper, the methodology is extended in order to describe the interatomic decay of excited doubly ionized states of clusters. The new computational scheme is applied to compute the interatomic decay rates of doubly ionized states formed by Auger relaxation of core vacancies in NeAr and MgNe van der Waals clusters.

Calculation of interatomic decay widths of vacancy states delocalized due to inversion symmetry

In a recent publication [J. Chem. Phys. 123, 204107 (2005)], we have introduced a new ab initio approach for the calculation of the widths of interatomic electronic decay of inner-shell vacancies in clusters. The new technique is based on the configuration interaction formalism first introduced for the description of resonance states by Fano [Phys. Rev. 124, 1866 (1961)] and on a Green function method for the description of the many-electron states involved in the electronic decay. Central to the new method is the selection of the physical excitation operators for the construction of the initial and final states of the interatomic decay. The previously described selection procedure has been formulated for localized vacancy states and runs into difficulties when applied to the decay of vacancy states delocalized due to inversion symmetry, e.g., (2s(-1)) 2Sigma(g,u)+ states of Ne2+. Here we present a modified computational scheme suitable for interatomic decay of the energy-split gerade and ungerade states and apply it to the interatomic Coulombic decay in two homonuclear diatomic clusters: Ne2 and Ca2.

The efficiency of Interatomic Coulombic Decay in Ne clusters

In this article, we demonstrate that Interatomic Coulombic Decay (ICD) is the dominant relaxation channel of Ne 2s inner valence vacancies in free Ne clusters, with an efficiency close to 100%. ICD designates a novel autoionization process of a vacancy in a weakly bonded atomic or molecular cluster. Its main characteristic is the release of an electron from a site different than the original vacancy, which is mediated by ultrafast energy transfer. Results are shown for cluster sizes between approx. 50–600 atoms. A trend towards apparently increased efficiency for larger clusters may result from inelastic scattering processes inside the cluster.

Mechanism of Interatomic Coulombic Decay in Clusters

Interatomic (or intermolecular) Coulombic decay is a general, very efficient mode of decay of inner valence vacancies in clusters. The physically appealing interpretation of such decays as a transfer of a virtual photon between two cluster units rests on the neglect of the orbital overlap between them. We show that even in loosely bound van der Waals clusters the orbital overlap is a crucial factor. At the equilibrium geometry of a cluster, the overlap effect can bring about an enhancement of the decay rate by 2-3 orders of magnitude, making the process dramatically more efficient than implied by the simple estimations.

Depth profiling of vacancy clusters in MeV-implanted Si using Au labeling

A technique for profiling the clustered-vacancy region produced by high-energy ion implantation into silicon is described and tested. This technique takes advantage of the fact that metal impurities, such as Au, are trapped in the region of excess vacancies produced by MeV Si implants into silicon. In this work, the clustered-vacancy regions produced by 1-, 2-, and 8-MeV Si implants into silicon have been labeled with Au diffused in from the front surface at 750 °C. The trapped Au was profiled with Rutherford backscattering spectrometry. The dynamics of the clustered-vacancy region were monitored for isochronal annealing at 750–1000 °C, and for isothermal annealing at 950 °C, for 10–600 s. Cross-sectional transmission electron microscopy analysis revealed that after the drive-in anneal, the Au in the region of vacancy clusters is in the form of precipitates. The results demonstrate that the Au-labeling technique offers a convenient and potentially quantitative tool for depth profiling vacancies in clusters.

Positron annihilation in 14 MeV neutron irradiated nickel and nickel containing 25 at% iron

The defect structure in fusion neutron irradiated Ni and Ni+25 at% Fe has been studied by positron lifetime spectroscopy. The positron trapping is used as a defect indicator. Two lifetime components τ 1 and τ 2 are resolved. It is found that the number of vacancies in vacancy clusters is larger than 11. The recovery stage in Ni+25 at% Fe is shifted from 750 to 850 K.

Periodic {001} Walls of Defects in Proton-Irradiated Cu and Ni

ABSTRACTThe evolution of the defect structure in 3 MeV-proton irradiated Cu and Ni has been investigated by transmission electron microscopy and by differential dilatometry. The proton irradiations were performed at T≦100°C up to irradiation doses of 2 dpa. An efficient loss of selfinterstitial atoms at dislocations and a consequently high average concentration of vacancies in clusters is observed starting from rather low fluences. In addition an ordering of the defects in the form of periodic {001} walls with a typical periodicity length of ≈ 60 nm is observed for all equivalent {001} planes. The walls consist of high local concentrations of dislocations, dislocation loops and stacking-fault tetrahedra. The observed formation of periodic arraysof defect walls is considered as an example for a possibly general microstructural phenomenon in metals under irradiation.

Positron trapping rate into vacancy clusters

The trapping rate of positrons into vacancy clusters in metals has been calculated. It increases with the trap size and binding energy and approximately scales with the number of vacancies in small clusters. The phonon-mediated contribution to the trapping rate is small. The temperature dependence of the trapping process is discussed.

Nucleation and Growth of Voids in Quenched Aluminum

The nucleation and growth process of voids in quenched aluminum has been studied by observing the changes in the stability of vacancy clusters with time and temperature. The nucleation is found to occur after quenching and to finish in a period very short as compared with the time required for the annihilation of quenched-in vacancies. The nucleation time is much shorter than that for dislocation loops. It is concluded that there exists a small definite size of the stable nucleus of a void. The cessation of the nucleation is performed by the retention of mobile vacancies in unstable vacancy clusters whose sizes are comparable to or larger than the void nucleus. The formation of dislocation loops and the escape of vacancies to pre-existing sinks are discussed in relation to the nucleation and growth of voids and dislocation loops. The annihilation process of vacancies during a slow quenching is interpreted in terms of the secondary defect formation.