Th. Frauenheim

BORON-NITROGEN ANALOGUES OF THE FULLERENES : ELECTRONIC AND STRUCTURAL PROPERTIES

Abstract On the basis of a systematic density functional tight-binding study of boron-nitrogen polyhedra (BN) x composed entirely of four- and six-membered rings, it is predicted that octahedron-like structures B 12 N 12 , B 16 N 16 and B 28 N 28 are “magic” (i.e. anomalously stable) clusters. The infrared spectrum of B 12 N 12 is predicted. The similarities and differences between these “inorganic fullerenes” and the carbon-based equivalents are outlined. Ahigh stability of the (BN) x clusters is found to correlate with a large HOMO-LUMO gap.

Highly tetrahedral amorphous carbon films with low stress

We have deposited boronated highly tetrahedral amorphous carbon (ta‐C:B) films with low stress using a filtered cathodic vacuum arc. The sp3 fraction, hardness, and resistivity were measured as a function of the ion energy and were found to reach a maximum above 50 eV for B concentrations of 2% and 4%. The most significant result we found was that highly tetrahedral a‐C:B film (sp3≊80%) with low stress (1–3 GPa) with B concentrations up to 4% could be obtained. The B in the films was found to be predominantly (≊75%) sp2 bonded. Additionally, the stress in the films did not vary with the ion energy or sp3 fraction unlike in regular ta‐C films.

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.

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.

Boronated tetrahedral amorphous carbon (ta-C:B)

Abstract We have deposited boronated highly tetrahedral amorphous carbon (ta-C:B) films with low stress using a filtered cathodic vacuum arc (FCVA). The sp3 fraction, hardness and resistivity were measured as a function of the ion energy and were found to reach a maximum above 50 eV for B concentrations of 2 and 4%. The most significant result we found was that highly tetrahedral a-C:B film (sp3≈80%) with low stress (1–3 GPa) with B concentrations up to 4% could be obtained. The B in the films was found to be predominantly (∼75%) sp2 bonded The bond length and angle of ta-C:B found using the radial distribution function were similar to ta-C, confirming its tetrahedral nature. Additionally, the stress in the films did not vary with the ion energy or sp3 fraction unlike in undoped ta-C films. The ta-C:B films also exhibited higher resistivity than ta-C. This is believed to be related to the reduction of defect density measured by electron spin resonance, although the optical band gap was similar to ta-C (2.0–2.4 eV).

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.

Vibrational and electronic signatures of diamond surfaces

Stable clean and hydrogenated diamond (100) and (111) surface reconstructions found by density-functional molecular-dynamics (DF-MD) are characterized in their vibrational and electronic properties. For sufficiently large surface slab supercells we plot spatially resolved charge densities of the highest occupied surface states at constant height, which are compared with recently obtained images from scanning tunnelling microscopy. In addition, we have calculated the surface top-three-layer projected vibrational spectra and reproduce the main features obtained from experiments. The various reconstructions of the diamond surface yield rich spectra of surface modes involving excitations of dimers, trimers and chains. We classify most characteristic surface modes and discuss correlations to peaks observed in surface-sensitive experiments. Both calculated charge density distributions and vibrational spectra represent signatures of the considered surfaces that might be used to understand and to classify as grown diamond surfaces.

A theoretical study of boron and nitrogen doping in tetrahedral amorphous carbon

A density-functional based tight-binding (DF-TB) study of doping in tetrahedral amorphous carbon (ta-C) at a density of 3.0 g/cm3 using nitrogen (N) and boron (B) is presented in comparison with the corresponding results for diamond. We discuss the stability of single and pairs of impurity atoms at various sites in the amorphous matrix and investigate the effect on the electronic density of states. We discuss the implications for the doping of ta-C.

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.