Strain effect and intermixing at the Si surface: A hybrid quantum and molecular mechanics study

We investigate Ge mixing at the Si(001) surface and characterize the $2\times N$ Si(001) reconstruction by means of hybrid quantum and molecular mechanics calculations (QM/MM). Avoiding fake elastic dampening, this scheme allows to correctly take into account long range deformation induced by reconstruted and defective surfaces. We focus in particular on the dimer vacancy line (DVL) and its interaction with Ge adatoms. We first show that calculated formation energies for these defects are highly dependent on the choice of chemical potential and that the latter must be chosen carefully. Characterizing the effect of the DVL on the deformation field, we also find that the DVL favors Ge segregation in the fourth layer close to the DVL. Using the activation-relaxation technique (ART nouveau) and QM/MM, we show that a complex diffusion path permits the substitution of the Ge atom in the fourth layer, with barriers compatible with mixing observed at intermediate temperature.Comment: 11 pages, 7 figures, 3 table

Tunable magnetic states in h-BN sheets

Magnetism in 2D atomic sheets has attracted considerable interest as its existence could allow the development of electronic and spintronic devices. The existence of magnetism is not sufficient for devices, however, as states must be addressable and modifiable through the application of an external drive. We show that defects in hexagonal boron nitride present a strong interplay between the the N-N distance in the edge and the magnetic moments of the defects. By stress-induced geometry modifications, we change the ground state magnetic moment of the defects. This control is made possible by the triangular shape of the defects as well as the strong spin localisation in the magnetic state.Comment: 10 pages, 3 figures, published in AP

Degenerate epitaxy-driven defects in monolayer silicon oxide onto ruthenium

The structure of the ultimately-thin crystalline allotrope of silicon oxide, prepared onto a ruthenium surface, is unveiled down to atomic scale with chemical sensitivity, thanks to high resolution scanning tunneling microscopy and first principle calculations. An ordered oxygen lattice is imaged which coexists with the two-dimensional monolayer oxide. This coexistence signals a displacive transformation from an oxygen reconstructed-Ru(0001) to silicon oxide, along which latterally-shifted domains form, each with equivalent and degenerate epitaxial relationships with the substrate. The unavoidable character of defects at boundaries between these domains appeals for the development of alternative methods capable of producing single-crystalline two-dimensional oxides

The Activation-Relaxation Technique : ART nouveau and kinetic ART

The evolution of many systems is dominated by rare activated events that occur on timescale ranging from nanoseconds to the hour or more. For such systems, simulations must leave aside the full thermal description to focus specifically on mechanisms that generate a configurational change. We present here the activation relaxation technique (ART), an open-ended saddle point search algorithm, and a series of recent improvements to ART nouveau and kinetic ART, an ART-based on-the-fly off-lattice self-learning kinetic Monte Carlo method

First principles calculations of surfaces and layered materials

Descripció del recurs: el 21-08-2008Consultable des del TDXTítol obtingut de la portada digitalitzadaEn este trabajo se han realizado cálculos de primeros principios para estudiar las propiedades físicas de superficies y materiales laminares. Los cálculos se basan en la obtención de las propiedades electrónicas por medio de la Teoría del Funcional de la Densidad, con la que se obtienen la energía y fuerzas atómicas para cada sistema estudiado. De esta forma, se realizaron cálculos de optimización estructural y de dinámica molecular, que proporcionan información sobre las estructuras de equilibrio y la dinámica atómica en función de las condiciones externas (tales como presión y temperatura). Los cálculos realizados se han centrado en diferentes sistemas con interés experimental, siempre en estrecha colaboración con distintos grupos experimentales. En el campo de superficies, se han estudiado problemas relacionados con el crecimiento de láminas delgadas de Cobre para metalización de circuitos microelectrónicos, la adsorción de fullerenos sobre superficies de Germanio, y la medición de ondas de densidad de carga mediante microscopía de efecto túnel en bronces azules. En materiales laminares, el trabajo se ha centrado en comprender el efecto de la presión sobre las propiedades estructurales y electrónicas de semiconductores laminares III-VI, así como en explicar la aparición de nuevas fases no-laminares para varios de estos sistemas a altas presiones.In this work we have made first-principles calculations to study the physical properties of surfaces and layered materials. The calculations are based on obtaining the electronic properties through the Density Functional Theory, with which the energy and atomic forces for each system studied are derived. Thus, structural optimization calculations and molecular dynamics were carried out. They provide information about the equilibrium structure and the atomic dynamic as a function of external conditions (such as temperature and pressure). Calculations have focused on different systems with experimental interest, always in strong collaboration with different experimental groups. In the field of surfaces we have been studied problems associated with the growth of thin films of copper for metallization of microelectronic circuits, the adsorption of fullerenes on Germanium surfaces, and the measurement of charge density waves by scanning tunneling microscopy in bronze blue. In layered materials, the work has focused on understanding the effect of pressure on the structural and electronic properties of layered semiconductor III-VI, as well as explaining the emergence of new no-layered phases for several of these systems at high pressures

Order and disorder at the C-face of SiC: A hybrid surface reconstruction

International audienceIn this letter, we explore the potential energy surface (PES) of the 3×3 C-face of SiC by means of density functional theory. Following an extensive and intuitive exploration, we propose a model for this surface reconstruction based on an all-silicon over-layer forming an ordered honeycomb-kagome network. This model is compared to the available experimental Scanning Tunneling Microscope (STM) topographies and conductance maps. Our STM simulations reproduce the three main characteristics observed in the measurements, revealing the underlying complex and hybrid passivation scheme. Indeed, below the ordered over-layer, the competition between two incompatible properties of silicon induces a strong disorder in the charge transfer between unpassivated dangling bonds of different chemistry. This effect, in conjunction with the glassy-like character of the PES, explain why it took decades to explain this structure

Revisiting the domain model for lithium intercalated graphite

International audienc

Capturing Centimeter-Scale Local Variations in Paper Pore Space via μ -CT: A Benchmark Study Using Calendered Paper

A two-step framework to analyze local microstructure variations of paper sheets based on 3D image data is presented. First, a multi-stage workflow efficiently acquires a large set of highly resolved tomographic image data, which enables—in combination with statistical image analysis—the quantification of local variations and pairwise correlations of morphological microstructure characteristics on length scales ranging from micrometers to centimeters. Secondly, the microstructure is analyzed in terms of the local behavior of porosity, thickness, and further descriptors related to transportation paths. The power of the presented framework is demonstrated, showing that it allows one (i) to quantitatively reveal the difference in terms of local structural variations between a model paper before and after unidirectional compression via hard-nip calendering and that (ii) the field of view which is required to reliably compute the probability distributions of the considered local microstructure characteristics is at least 20 mm. The results elucidate structural differences related to local densification. In particular, it is shown how calendering transforms local variations in sheet thickness into marked local mass density variations. The obtained results are in line with experimental measurements of macroscopic properties (basis weight, Bekk smoothness parameters, thickness, and Gurley retention times)