N. I. Papanicolaou

Diffraction of helium from a stepped surface: Cu(117) — An experimental study

Abstract The scattering of an helium nozzle beam (incident energy 21 and 63 meV) from a copper (117) stepped surface has been studied for crystal temperature ranging from 70 to 773 K. The diffraction pattern from the step array is readily seen. The influence of residual impurity on the surface is carefully discussed. Special attention has been paid to the influence of experimental factors upon peak shapes. True peak amplitudes are deduced. The peak intensities versus temperature and versus incidence angle has been measured. Extrapolation of the intensities to 0 K allows a direct comparison of the data with the prediction of a hard corrugated wall model. A corrugation profile is given which fits quite well the data. Clear evidence for resonances with bound states has been found. Three energy levels of the potential well are thus determined (5.9,4.0, 2.1 meV) which agrees with a Morse or a 3–9 potential.

Molecular dynamics study of the vibrational and transport properties of copper adatoms on the (111) copper surface; comparison with the (001) face

Abstract The vibrational properties and the self-diffusion process of single adatoms on the Cu(111) surface have been studied and compared with the corresponding ones of the Cu(001) face, using molecular dynamics simulation based on a n-body potential. From the calculated phonon spectral densities it comes out that the adatoms modify the surface phonon spectrum and introduce new vibrational modes. The frequencies of these new suggest, in the in-plane directions, looser coupling of the adatoms with the surface atoms of the (111) than those of the (001) face and stronger coupling in the perpendicular direction. This is in agreement with the temperature dependence of the relaxed positions of both surface atoms and adatoms. The effect is more pronounced for the (001) face, indicating that this surface presents anharmonic behavior at high temperatures. In addition, the mean-square-displacements of the surface atoms in the normal direction, are greater in the case of the (001) face. The vibrational amplitudes of the adatoms on the (111) surface are smaller than those of the surface atoms, for a wide temperature range. The adatom self-diffusion on the (111) surface is a thermally activated process, exhibiting Arrhenius behavior at two distinct temperature regions. The detailed analysis of the diffusion process revealed a new hopping mechanism.

Adatom self-diffusion processes on (001) copper surface by molecular dynamics

Abstract Using molecular dynamics and an n -body potential adapted to copper we have studied the self-diffusion of Cu adatoms on Cu(001) surface. The simulations covered the temperature range between 700 and 1100 K. Besides the simple hopping and the exchange mechanism, the detailed trajectory analysis revealed multiple hopping events and complicated multi-particle exchange processes, involving several atoms that do not necessarily belong to the same nearest-neighbor row. These processes exhibit Arrhenius behavior from which we derived the migration energies associated with each process. It is found that the hopping mechanism requires an energy of 0.43 eV, in very good agreement with available experimental data, while the energy associated with the exchange mechanism is 0.70 eV. These results are in qualitative agreement with recent ab-initio calculations. In addition, we found that all mechanisms, even the most complicated, require about the same migration energy with the simple exchange and that for temperatures above 900 K they contribute almost equally to the total diffusion. Furthermore, the activation barriers for the hopping and the exchange mechanism deduced from energy minimizations, at T = 0 K, compare well with the simulation values.

Dynamical properties of Au from tight-binding molecular-dynamics simulations

We studied the dynamical properties of Au using our previously developed tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K were determined by computing the dynamical-matrix using a supercell approach. In addition, we performed molecular-dynamics simulations at various temperatures to obtain the temperature dependence of the lattice constant and of the atomic mean-square-displacement, as well as the phonon density-of-states and phonon-dispersion curves at finite temperature. We further tested the transferability of the model to different atomic environments by simulating liquid gold. Whenever possible we compared these results to experimental values.

Atomistic aspects of epitaxial growth

Preface. Adatom, Vacancy, and Dimer Diffusion. Experimental Study of Surface Diffusion in Metal Overlayers on Anisotropic Metal Surfaces A.T. Loburets, et al. Ab Initio Modeling of Free Energy Profiles in Thermally-Activated Processes I. Stich, et al. Surface Diffusion with a Realistic Damping Coefficient O.M. Braun. Vibrational and Structural Properties of the Nb(001) Surface with and without a Nb Adatom by Tight-Binding Molecular Dynamics Ch.E. Lekka, et al. Adatoms and Vacancies on the A3B (001)Surfaces Ch. E. Lekka, G.A. Evangelakis. Long-Time-Scale Simulations of Al(100) Crystal Growth G. Henkelman, H. Jonsson. Diffusion of Dimers on Silicon and Germanium (001) Surfaces H.J.W. Zandvliet, et al. Island Nucleation and Multilayer Growth. Island Nucleation in Metal Thin-Film Growth K.A. Fichthorn, et al. Capture-Numbers and Island Size-Distributions in Irreversible Homoepitaxial Growth: A Rate Equation Approach M.N. Popescu, et al. Island Statistics Reflecting Growth Processes P.A. Mulheran. Growth of an Anisotropic Surface: The Case of Ag/Ag(110) C. Mottet, et al. Vibrational Properties of 2D Copper Islands on the Cu(111) Surface by MD Simulations E. Vamvakopoulos, G.A. Evangelakis. Irreversible Nucleation in Multilayer Growth P. Politi, C. Castellano. Second Layer Nucleation and the Shape of Wedding Cakes J. Krug, P. Kuhn. Steering Epitaxial Growth B. Poelsema, S. Van Dijken. Coarsening Mechanisms in Surface Morphological Evolution T. Michely, et al. Realistic Atomistic Modeling of Mound Formation during Multilayer Growth: Metal(100) Homoepitaxy K.J. Caspersen, J.W. Evans. Vicinal and Patterned Substrates. Patterning Surfaces by Self-Organized Growth K. Kern. Growth and Ion Erosion: Two Methods for Patterning Surfaces F. Buatier de Mongeot, et al. Oscillatory Driving of Crystal Surfaces: A Route to Controlled Pattern Formation O.-Pierre-Louis, M. Haftel. Reconstruction-Determined Growth of Silver on Silicon (111)-(7×7) P. Sobotik, et al. Electromigration of Si Adatoms on Si Surfaces: A Key to Understanding Step Bunching Instabilities during Sublimation and MBE Growth S. Stoyanov, et al. Atomic Steps on a Single-Crystal Surface Studied with in situ UHV Reflection-Electron Microscopy A.V. Latyshev, et al. Heteroepitaxy, Strain Relaxation, and Quantum Dots. Mechanisms and Anomalies in the Formation of InAs-GaAs(001) Quantum Dot Structures B.A. Joyce, D.D. Vvedensky. Ab initio Study of Stability of Surfaces and Nanonstructures J. Kollar, et al. Atomistic and Continuum Elastic Effects in Heteroepitaxial Systems A.C. Schindler, et al. An Initio Thermodynamics and Statistical Mechanics of Diffusion, Growth, and Self-Assembly of Quantum Dots M. Scheffler, P. Kratzer. Atomistic Aspects of SiGe Nanostructure Formation by Molecular-Beam Epitaxy O.P. Pchelyakov, et al. Stress-Induced Surface Modulation C. Misbah, et al. Entropy Effects in the Self-Organized Formation of Nanostructures V.A. Shchukin, et al. Dislocation-Free 3D Islands in Highly Mismatched Epitaxy: An Equilibrium Study with Anharmonic Interactions I. Markov, J.E. Prieto. Self-Assembly of Few-Atom Clusters in a Model of a Strained Submonolayer V.I. Tokar, H. Dreysse. Ab initio Study of the Influence of Epitaxial Strain on Magnetoelastic Properties M. Komelj, M. Fahnle. Strain Characterization of Epitaxially-Grown Superlattices by R

Second-moment interatomic potential for Cu-Au alloys based on total-energy calculations and its application to molecular-dynamics simulations

We have evaluated interatomic potentials of Cu, Au and Cu-Au ordered alloys in the framework of the second-moment approximation to the tight-binding theory by fitting to the volume dependence of the total energy of these materials computed by first-principles augmented-plane-wave calculations. We have applied this scheme to calculate the bulk modulus and elastic constants of the pure elements and alloys and we have obtained a good agreement with experiment. We also have performed molecular-dynamics simulations at various temperatures, deducing the temperature dependence of the lattice constants and the atomic mean square displacements, as well as the phonon density of states and the phonon-dispersion curves of the ordered alloys. A satisfactory accuracy was obtained, comparable to previous works based on the same approximation, but resulting from fitting to various experimental quantities.

Modification of phonon spectral densities of the (001) copper surface due to copper adatoms by molecular dynamics simulation

Phonon spectral densities of the (001) copper surface containing copper adatoms have been calculated at various temperatures using molecular dynamics simulation. The results reveal that the presence of adatoms is manifested mainly by the appearance of new dispersionless peaks which are broadened and shifted to lower energies as the temperature increases. The existent experimental results, as well as lattice dynamics, ab initio calculations and other simulations in the case of a clean surface, are in good agreement with our predictions.

Second-moment interatomic potential for Al, Ni and Ni–Al alloys, and molecular dynamics application

Abstract We present an interatomic potential for Al, Ni and Ni–Al ordered alloys within the second-moment approximation of the tight-binding theory. The potential was obtained by fitting to the total energy of these materials computed by first-principles augmented-plane-wave calculations as a function of the volume. The scheme was validated by calculating the bulk modulus and the elastic constants of the pure metals and alloys that were found to be in fair agreement with the experimental measurements. In addition, we performed molecular-dynamics simulations and we obtained the thermal expansion coefficient, the temperature dependence of the atomic mean-square displacements and the phonon density of states of the compounds. Despite the simplicity of the model, we found satisfactory agreement with the available experimental data.

Second-moment interatomic potential for gold and its application to molecular-dynamics simulations

We have obtained a new interatomic potential for Au in the framework of the second-moment approximation to the tight-binding model by fitting the total energy of the metal as a function of the volume computed by first-principles calculations. The scheme was validated by calculating the bulk modulus, elastic constants, vacancy formation energy and relaxed surface energies of Au, which were found to be in fair agreement with experiment. We also have performed molecular-dynamics simulations at various temperatures and we have determined the temperature dependence of the lattice constant, mean-square displacements, as well as the phonon density of states and the phonon dispersion curves of the metal. The agreement with the available experimental data is much better than previous works based on the same approximation.

Molecular dynamics study of gold adatom diffusion on low-index copper surfaces

Abstract The transport properties of single Au adatoms on low-index Cu surfaces have been studied using molecular dynamics. We found that on the Cu(001) face, the adatom substitutes a surface Cu atom via the exchange diffusion mechanism, while this process is absent on the Cu(111) surface. On the Cu(110) face, hopping and exchange diffusion processes coexist with almost equal probability. The migration energies associated with the various diffusion processes have been deduced. It turns out that the diffusion of Au adatoms differs significantly from Cu self-diffusion on the low-index Cu surfaces. In addition, the calculated adatom binding energies, as well as the temperature dependence of the mean-square-displacements and the relaxed positions in the normal to the surfaces direction are compatible with the diffusion results.