1,721,049 research outputs found
Computational study of graphene growth on copper by first-principles and kinetic Monte Carlo calculations
No full textInner shell photoionization and non-radiative decay processes in molecules: theory and calculations
No full textA Bird’s Eye View on the Concept of Multichannel Scattering with Applications to Materials Science, Condensed Matter, and Nuclear Astrophysics
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Enabling Materials By Dimensionality: From 0D to 3D Carbon-Based Nanostructures
No full textThis chapter is aimed at analysing the influence that dimensional scaling exerts on the electronic, optical, transport and mechanical properties of materials using both experiments and computer simulations. In particular, to climb the “dimensional ladder” from 0D to 3D, we analyse a specific set of all-carbon allotropes, making the best use of the versatility of this element to combine in different bonding schemes, such as sp2 and sp3, resulting in architectures as diverse as fullerenes, nanotubes, graphene, and diamond. Owing to the central role of carbon in future emerging technologies, we will discuss a variety of physical observables to show how novel characteristics emerge by increasing or decreasing the dimensional space in which particles can move, ranging from the charge transport in semiconductor (diamond) and semimetallic (graphite) samples to the stress-strain characteristics of several 2D carbon-based materials, to the gas absorption and selectivity in pillared structures and to the thermal diffusion in foams. In this respect, our analysis uses ab initio, multiscale and Monte Carlo (MC) methods to deal with the complexity of physical phenomena at different scales. In particular, the response of the systems to external electromagnetic fields is described using the effective dielectric model of the plasma losses within a Monte Carlo framework, while pressure fields are dealt with the ab initio simulation of the stress-strain relationships. Moreover, in this chapter we present recent theoretical and experimental investigations aimed at producing graphene and other carbon-based materials using supersonic molecular beam epitaxy on inorganic surfaces, starting from fullerene precursors. We mostly focus on the computational techniques used to model various stages of the process on multiple length and time scales, from the breaking of the fullerene cage upon impact to the rearrangement of atoms on the metal surface used to catalyse graphene formation. The insights obtained by our computational modelling of the impact and of the following chemical-physical processes underlying the materials growth have been successfully used to set up an experimental procedure that ended up in the production of graphene flakes by C60 impact on copper surfaces
Multiscale Investigation of Oxygen Vacancies in TiO2 Anatase and Their Role in Memristor’s Behavior
No full textThe structure, energetics, and transport properties of TiO2 anatase with different densities of oxygen vacancies are studied by computer simulations using a variety of techniques, ranging from first-principles to Monte Carlo methods, to span different time scales. This work is motivated by the recent development of memristive electronic devices, usually made of metal oxides in which arrays of defects control the resistance switching mechanism. Anatase, in particular, emerged as one of the most promising candidates for memristor design. However, the microscopic behavior of these multivacancy systems is not yet entirely understood. In this regard, electronic and transport properties of TiO2 anatase containing neutral and charged oxygen vacancies are investigated within density functional theory (DFT) by adding a Hubbard-like term to the generalized gradient approximation of the electron density (GGA+U). Calculated observables are the formation energy of oxygen defects, the cohesion energy of multivacancy systems, and the energy profiles of oxygen diffusion pathways, computed through the nudged-elastic band (NEB) approach. Furthermore, a kinetic Monte Carlo model (KMC) of the conductive channel formation in bulk anatase, based on the corresponding diffusion rates, is discussed. Finally, to demonstrate the relation between energetically stable structures and the conductive phase of memristors, we study electron transport within a tight-binding approximation to DFT, using the nonequilibrium Green’s function (NEGF) formalism
Modeling flexibility in Metal-Organic Frameworks: comparison between Density-Functional Tight-Binding and Universal Force Field approaches for bonded interactions
No full textIn this work we use Density-Functional Tight-Binding (DFTB) to investigate dynamical processes dependent
on the flexibility in metal–organic frameworks (MOFs). We show that one can perform DFTB simulations
on systems having unit cells of several hundreds atoms at a modest computational cost. This
approach is validated by calculating the barriers to diffusion for small molecules (hydrogen, carbon dioxide,
and methane) crossing the windows connecting the pores of ZIF-7 and ZIF-8, two prototypical materials
that have been the subject of many experimental and theoretical investigations. Additionally, we
calculate the vibrational densities of states for MOF-5 and ZIF-8.
These calculations are compared with simulations using the bonded and non-bonded part of the Universal
Force Field (UFF). The results show that UFF’s description of the bonded interactions has a quality
comparable to DFTB’s, at an even smaller computational cost
Advancements in secondary and backscattered electron energy spectra and yields analysis: From theory to applications
No full textOver the past decade, experimental microscopy and spectroscopy have made significant progress in the study of the morphological, optical, electronic and transport properties of materials. These developments include higher spatial resolution, shorter acquisition times, more efficient monochromators and electron analysers, improved contrast imaging and advancements in sample preparation techniques. These advances have driven the need for more accurate theoretical descriptions and predictions of material properties. Computer simulations based on first principles and Monte Carlo methods have emerged as a rapidly growing field for modelling the interaction of charged particles, such as electron, proton and ion beams, with various systems, such as slabs, nanostructures and crystals. This report delves into the theoretical and computational approaches to modelling the physico-chemical mechanisms that occur when charged beams interact with a medium. These mechanisms encompass single and collective electronic excitation, ionisation of the target atoms and the generation of a secondary electron cascade that deposits energy into the irradiated material. We show that the combined application of ab initio methods, which are able to model the dynamics of interacting many-fermion systems, and Monte Carlo methods, which capture statistical fluctuations in energy loss mechanisms by random sampling, proves to be an optimal strategy for the accurate description of charge transport in solids. This joint quantitative approach enables the theoretical interpretation of excitation, loss and secondary electron spectra, the analysis of the chemical composition and dielectric properties of solids and contributes to our understanding of irradiation-induced damage in materials, including those of biological significance
A wave packet method for treating nuclear dynamics on complex potentials
No full textA general time-dependent description of the dissociative attachment of a triatomic molecule is presented. The approach presented works within the Feshbach projection operator formalism and gives an algorithm for solving the nuclear motion problem which reduces the computational effort required. The method uses a complex potential energy surface to characterize the formation and decay of resonances as modified by the coupling to the nuclear motion which are treated using multidimensional complex wave packets. A basis-independent wave packet method is developed and used to treat the propagation of a wave packet on a complex three-dimensional potential appropriate for resonance states of the water anion. A complete derivation of the system of linear equations used in the time iteration is presented. The method can be applied to resonant vibrational excitation, for which the electron impact vibrational excitation of water is considered
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