U. Stephan

Structure and electronic properties of amorphous carbon: from semimetallic to insulating behaviour

Abstract Correlations between the atomic-scale structure and electronic properties in amorphous carbon and its hydrogenated analogues are analyzed. The metastable amorphous modifications with varying density 2.0–3.5 g/cm3 and different amount of hydrogen have been generated by density-functional-based molecular dynamics applying different annealing regimes. The atomic-scale structure is characterized with special emphasis on comparing neutron scattering with simulated diffraction data. The global electronic band gap properties are related to the chemical bonding and π-cluster formation. While at low density the π−π ∗ gap closes owing to the large size of π-clusters and the residual strain on the π-system from the rigid bonding environment, the internal strain at high density of 3.0 g/cm3 is maximally reduced by the separation of smaller π-clusters. In the latter case, the π-bonds optimally relax consistent with the opening of large π−π ∗ gaps up to 3 eV. While the internal strain again increases with further increase in the density, incorporation of hydrogen at 3.0 g/cm3 additionally supports the removal of internal strain by enforcing two-phase separation tendencies between chemically differently bonded carbon atoms.

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.

Model studies of the structure and chemical bonding in amorphous carbon films prepared by magnetron sputtering

Abstract The microstructure and chemical bonding properties of amorphous carbon with 2.0 g/cm 3 mass density prepared by magnetron sputtering have been studied by electron diffraction measurements, molecular dynamics and Monte Carlo simulations. The model structures were generated by simulated annealing techniques applying both classical Tersoff and Keating potentials and a quantum mechanical density functional method. By comparison of the experimental results with theoretically simulated diffraction data, the atomic scale structure is analyzed. The chemical bonding properties of the models are evaluated within a local valence orbital description. In this description, the electronic band gap properties are related to the formation of π-bonded sp 2 clusters. Good agreement is found between experimental and simulated diffraction data for models showing a minimum in their electronic density of states at the Fermi energy. In these models, 60% of the atoms belong to one large π-bonded cluster and, hence, from the matrix of the structures. Isolated aromatic clusters of any size are not found. In the best fitting model, only 14% of all five-, six- and seven-membered rings are completely π-bonded.

Density-Functional Based MD Studies of Low-Energy Atom Collisions onto Diamond and Graphite

The near-surface implantation of hyperthermal neutral atoms with (15 - 75) eV onto diamond (111) and graphite substrates is studied by molecular-dynamics (MD) using a density-functional (DF) based non-orthogonal tight-binding (TB) scheme. Depending on the initial energy and the impact point the atoms penetrate beneath the surface forming regions of local disorder and stress. The energy partition during the collision is analyzed yielding results about penetration and displacement threshold. After a final relaxation of the structures the penetration depths of the colliding particles are determined. The structural topology and the electronic properties of the induced defects and surface modifications are discussed. The penetration thresholds for noble gas atoms, hydrogen and carbon and the atomic-size dependent bulge arising after the subplantation process into a graphite substrate have been determined.