1,721,048 research outputs found
Initial stages of growth of copper on MgO(100): A first principles study
No full textWe have studied the energetical and structural properties of copper clusters (Cu-n, n = 2-6, 8, 9, 13, 14, 25, and 29) and films (with n(L) = 1, 2, and 3 layers) adsorbed on MgO(100) by means of first principles density functional calculations. We find that Cu-Cu interactions dominate over Cu-surface interactions, so that three dimensional (3D) structures are largely preferred with respect to two dimensional (2D) ones. This indicates a Volmer-Weber growth mode for Cu on MgO(100), in agreement with recent experimental observations
THEORY OF VACANCY-STABILIZED (SQUARE-ROOT-3 X SQUARE-ROOT-3) DISPLACIVE RECONSTRUCTION OF THE CLEAN SI(111) SURFACE
No full textWe show that a (square-root 3 x square-root 3) buckling distortion with one raised and two lowered surface atoms per unit cell lowers by as much as 0.18 eV/(surface atom) the energy of the clean, unreconstructed Si(111) surface. On the defect-free surface this state is not a true energy minimum, and is unstable against conversion into the lower energy (2 x 1) pi-bonded chain reconstruction. A low concentration of added surface vacancies, however, is found to yield a buckled (square-root 3 x square-root 3) state which is a stable local minimum. Predictions are made for possible scanning-tunneling-microscopy observation of this displacive reconstruction
SI(111), (2 X-1) RECONSTRUCTION AND SURFACE PHONONS FROM ABINITIO MOLECULAR-DYNAMICS
No full textWe present results of ab-initio Molecular Dynamics simulations of the Si(III) surface. Pandey's 2 x 1 chain-geometry is obtained by simulated annealing, starting directly from the ideal bulk-terminated surface. The dynamical path followed during the reconstruction is described. The resulting 2 x 1 structure agrees well with Low Energy Electron Diffraction and Medium Energy Ion Scattering results. The phonon spectrum which is calculated ab-initio for the first time shows well defined surface modes at frequencies close to the measured values. The atomic motions and the dynamic effective charges of these modes are also analyzed
Metallization and incomplete melting of a semiconductor surface at high temperature
No full textWe present a first microscopic study of the state of a semiconductor surface, clean Ge(111), close to the bulk melting temperature. Both electronic states and ionic motion are fully treated via first-principles molecular dynamics. Results indicate a clear dynamical disordering, confined, however, to the first atomic bilayer. This region acquires a liquidlike diffusion, and is metallic. Lack of melting of the second and deeper bilayers, found to be in good quantitative agreement with recent x-ray data, indicates an incomplete wetting of the semiconducting solid by its own metallic melt. Previously conflicting data on Ge(111) are also reconciled within this picture
Structure and Stability of TiO2-B Surfaces: A Density Functional Study
No full textWe investigate the structure and energetics of low-index surfaces of the TiO2-B polymorph by means of periodic density functional theory calculations within the generalized gradient approximation. The bulk structure contains two nonequivalent Ti ions, one of them exhibiting an octahedral coordination, while the other is square-pyramidal. When exposed at the surface, these two types of ions display different relaxation schemes, which ultimately tend to make them more similar. On the basis of the computed surface energies and of the Wulff construction, we predict for TiO2-B a pseudohexagonal prismatic equilibrium shape and an average surface energy practically identical to that of TiO2-anatase
Chemical pseudopotential and semiconductor surface states
No full textChemical pseudopotential calculation of silicon surface
Energetics of Atomic Hydrogen Diffusion on Si(100)
No full textWe present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100) 2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H2 recombinative desorption
Electronic structure of the (111) ideal and relaxed surfaces of silicon by the chemical pseudopotential method
No full textThe electronic energy structure for the (111)ideal and relaxed surfaces of silicon is calculated by the chemical pseudopotential method. We use a minimal basis set of localized orbitals and an atomic-like crystal potential to compute the interaction parameters and include self-consistency. Results are compared with other more involved theoretical calculations with satisfactory agreement
Methane Activation on Metal-Doped (111) and (100) Ceria Surfaces with Charge-Compensating Oxygen Vacancies
No full textReducing the temperature for methane activation is an important objective that would benefit many technological applications. In this work, we explore the possibility to achieve this goal using single-atom catalysts (SACs) on ceria surfaces. We focus on Ag SACs, which have recently been suggested to be promising catalysts for both H2 and CH4 oxidation. Using first-principles calculations, we investigate methane activation on CeO2(111) and CeO2(100), two frequently exposed surfaces on ceria nanoparticles. The presence of Ag is found to reduce the activation energy for methane dissociation on both surfaces. On Ag-doped CeO2(111), the formation of methanol via the Mars-van Krevelen mechanism has a slightly lower energy barrier than the dissociation to CH3 + H, suggesting that methanol is a likely product of methane activation on this surface. A novel aspect of this work is the focus on stable surface structures where each Ag dopant substituting Ce forms a complex with a charge-compensating surface oxygen vacancy. These complexes are found to play a critical role and accounting for their presence is essential for a proper description of the surface reactivity
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