The electronic properties of amorphous carbon structures with varying microscopic mass densities, ranging from 2.0 to 3.52 g/cm, are analyzed. Using a semiempirical density-functional approach the model structures were generated by molecular dynamics performing a simulated cooling of liquid carbon clusters, containing 128 atoms, within periodically arranged cubic supercells. By investigation of properties related to chemical bonding we find a strong control of the band gap by the balance between x bonding and electronic defect generation. As important sources that in8uence the x -vr' band gap the distance distribution between non-o-bonded hybrid orbitals as well as the topological properties of clustering sp (or sp) units are discussed. A number of local densities of states are calculated at representative atoms to validate the classification of x clusters and electronic defect states used.

Results of scanning-tunneling-microscopy (STM) and molecular-dynamics (MD) annealing studies based on quantum-mechanically derived interatomic forces using a semiempirical density-functional approach are combined for analyzing diamond surface structures. Experimentally obtained STM images of diamond (100) and (111)faces on polycrystalline films reveal (1 X 1),(&3 X &3) R 30', and possible (2 X 1) structures. The (100) faces show stable (2 X 1) reconstruction with dimer formation. Surface structures with and without adsorbed hydrogen are determined and their stability is obtained by MD simulated annealing techniques. The bulklike and (&3X&3) 830 structures, as they are observed on grown (111) facets, are attributed to the two diff'erent single atomic (111)layers, which support growth mechanisms, in which the two alternating single atomic layers grow in turn and not simultaneously. The equilibrium surface modifications which have been realized are electronically characterized by investigating the local electronic density of states at selected surface atoms. This information is compared and related to the features seen in the STM images.

A semiempirical molecular-dynamic density-functional approach is used to perform a systematic investigation of the stability, structure, and properties of quenched pure and hydrogenated amorphous carbon dependent on the mass density and the hydrogen concentration. By comparing the total structure energies for supercell clusters of equal composition and atom number, we obtain the most stable a-C:H configurations characterized by optimal chemical bonding corresponding to certain mass densities and hydrogen concentrations which control the short-and medium-range order in the material. We present a detailed structural analysis of stable and metastable a-C:H modifications comparing theoretically simulated di8'raction data with electron-scattering results. In addition, we discuss correlations between the atomic structure and the electronic properties which in turn are mediated by the chemical bonding and the clustering of different hybridized atoms.

Collisions of Cso with hydrogen-terminated diamond-(111) surfaces were studied by moleculardynamics simulations based on a semiempirical density-functional approach. The dominating factor determining the energy partition in the Cso-diamond-(111)-surface collision at constant impact energy is the orientation of the molecules relative to the initial impact points on the surface. In agreement with the experimental results using velocity-selective time-of-Sight mass spectroscopy the center-of-mass kinetic energies after the collisions are distributed around a mean value which is only slightly affected by the initial impact energy. The final energy content of the C60 s increases with the incident kinetic energy, depends strongly on the impact scattering topology, and yields a narrow distribution with increasing mean values in correlation with lower center-of-mass kinetic energies.

We report a simulation study on competition between cracking and peeling, in a layer of clay on desiccation and how this is affected by the rate of drying, as well as the roughness of the substrate. The system is based on a simple 2-dimensional spring model. A vertical section through the layer with finite thickness is represented by a rectangular array of nodes connected by linear springs on a square lattice. The effect of reduction of the natural length of the springs, which mimics the drying is studied. Varying the strength of adhesion between sample and substrate and the rate of penetration of the drying front produces an interesting phase diagram, showing cross-over from peeling to cracking behavior. Changes in the number and width of cracks on varying the layer thickness is observed to reproduce experimental reports.