P. Blaudeck

Observation of (3 × 3)R30° diamond (111) on vapour-grown polycrystalline films

Abstract (111) facets of polycrystalline diamond films are investigated on the atomic scale by scanning tunnelling microscopy. Besides bulk-like structures, a ( 3 × 3 ) R30° superstructure is found. Molecular dynamics calculations are performed, which base on quantum mechanically derived interatomic forces. They exhibit stable molecular trimer-structures consistent with the observations. Comparison of the simulation to the experimentally obtained images gives evidence that the trimer structures are formed by reconstruction of a triple-dangling-bond layer and that they are centred at the hollow sites of the single-dangling-bond layer underneath.

A method and results for realistic molecular dynamic simulation of hydrogenated amorphous carbon structures using a scheme consisting of a linear combination of atomic orbitals with the local-density approximation

A method for realistic molecular dynamic (MD) simulations of the chemical bonding formation in extended hydrogenated amorphous carbon (a-C:H) structures of varying density and incorporated hydrogen content is presented. Applying the Born-Oppenheimer approximation, the forces moving the atoms via MD on the potential energy surface are calculated within an approximated MD-density functional theory which uses localized basis functions. The method is shown to describe correctly the ground state configurations of Cn microclusters, CnHm hydrocarbon molecules and radicals, as well as bulk crystalline carbon. Application to dynamical structure simulation of a-C and a-C:H results in realistic metastable configurations which are characterized electronically by a well defined gap in the electronic density of states around the Fermi energy. A reasonable structure statistics is obtained and compared with fully ab initio calculations and experiments.

Electrical transport and electronic properties of a amorphous carbon thin films

Abstract The present state of the experimental characterization of the electrical transport and electronic properties of amorphous carbon structures is briefly reviewed and our own reflections on this topic are classified. The theoretical interpretation of experimental data is based on a theory that derives the π electron energy spectra and the density of states. An estimate is given to test the applicability of the approximate electrical transport theories commonly used for different types of thin amorphous carbon films.

Structure of amorphous hydrogenated carbon: experiment and computer simulation

Abstract The microstructure of amorphous hydrogenated carbon films has been studied by electron diffraction measurements and comparison of the results with simulated diffraction data which have been modelled by molecular dynamics (MD) calculations. The films have been produced partly by a plasma-enhanced chemical vapour deposition process and partly by a plasma beam deposition method. The MD simulation is based on an annealing process cooling down a liquid phase ensemble of 64 carbon and a corresponding number of hydrogen atoms using a density functional approach to account for the interatomic forces.

Stability and reconstruction of diamond (100) and (111) surfaces

Abstract Results of scanning tunnelling microscopy (STM) and molecular dynamic (MD) annealing studies based on quantum mechanically derived interatomic forces using a semiempirical density functional approach are combined for analysing diamond surface structures. Experimentally obtained STM images of diamond (100) and (111) faces on polycrystalline films reveal (1 × 1), (3√ × 3√) R30° and possibly (2 × 1) structures. The (100) faces show stable (2 × 1) reconstruction with dimer formation. Surface structures with and without adsorbed hydrogen are determined and their stability is proved by MD simulated annealing techniques. The bulk-like (111) and (3√ × 3√) R30° structures, as they are observed on grown (111) facets, are attributed to the two different single-atomic (111) layers, which supports growth mechanisms in which the two alternating single-atomic layers grow by turns and not simultaneously.

Structure and chemical bonding in high density amorphous carbon

Abstract The microstructure of high density amorphous carbon materials prepared by direct and filtered cathodic are deposition was studied by electron diffraction measurements and molecular dynamic (MD) as well as Monte-Carlo (MC) modelling. The MD simulation, performed by quenching of a liquid, is based on a semiempirical density functional (DF) approach. The MC simulation uses a modified WWW algorithm and an empirical classical description of the atomic interactions. By comparison of the experimental results with theoretically simulated diffraction data, the atomic structure and chemical bonding in the a-C films are analysed and structure-property correlations are discussed.

Molecular dynamic investigations of amorphous carbon: π bonding vs. electronic defect generation

Abstract The local electronic bonding properties of amorphous carbon (a-C) structures with varying microscopic mass densities, ranging from 2.0 to 3.5 g cm−3, are analysed. Using a semiempirical density functional approach the model structures were generated by molecular dynamics performing a rapid quenching of 128-atom liquid carbon clusters within periodically arranged cubic supercells. The chemical bonding properties are evaluated within a local valence orbital description giving rise to a strong control of electronic properties and quality in a-C materials by the balance between π bonding formation and electronic defect generation.

Stability and structure of amorphous hydrogenated carbons: a molecular dynamic investigation

An approximate ab initio local-orbital quantum molecular dynamics is used to study the stability and structure of quenched amorphous hydrogenated carbons dependent on the mass density for different fixed hydrogen concentrations. Comparing the total structure energies for supercell clusters of equal composition and atom number we obtain the stable phase line for optimal chemical bonding corresponding to certain mass densities. We present a structural analysis of the most stable a-C:H modifications and discuss the cluster effects which in turn are mediated by the incorporated hydrogen.

Structure and chemical bonding in amorphous diamond

Abstract The atomic structures of relaxed amorphous carbon models with diamond density are investigated, and the related scattering, electronic, and vibrational properties are analysed. Purely tetrahedrally coordinated models were generated using a bond switch Monte Carlo algorithm similar to the Wooten-Winer-Weaire method for amorphous silicon. These defect-free models are compared with models produced by density-functional-based tight-binding molecular dynamics as well as by molecular dynamics using the classical Tersoff potential. The stability and properties of “amorphous diamond” are discussed, and the consequences for network strain and gap states of frequently appearing undercoordinated atoms are deduced.

Molecular dynamic structure investigations of the adsorption and bonding of CxHy-hydrocarbon molecules/radicals on a diamond (111) surface

Abstract Elementary reaction mechanisms for adsorption and chemical bonding of CxHy fragments (molecules, radicals, clusters) to a clean, unreconstructed and fully or partly hydrogenated diamond (111) surface are studied by molecular dynamic (MD) structure investigations. Applying the Born-Oppenheimer approximation the forces moving the hydrocarbon species via MD to the bonding formation with the crystalline substrate are calculated within an alternative MD-density functional (DF) approach, which uses localized atomic orbital basis functions. In our first results using the method we obtain the stable and metastable ground state structure configurations of CH, CH2, CH3, C2H, and C2H2 molecules and radical absorbed on a diamond (111) surface, modelled by a finite cluster. We comment on the dependence of the adsorbed cluster geometries on the surface dangling bond density and we discuss the energetically most favourable configuration in relation to low temperature, low pressure diamond growth conditions.