G. Jungnickel

Autocatalysis during fullerene growth

Total energy calculations with a local spin density functional have been applied to the Stone-Wales transformation in fullerene (C60). In the formation of the almost exclusively observed Ih isomer of C60 with isolated pentagons, the final transformation must be from a C2v isomer with two pentagon pairs. It was found that the energy barrier for this rearrangement was substantially reduced in the presence of an extra carbon atom. Such atoms were found to bind loosely, preferentially to regions in which there were paired pentagons. Pentagon rearrangements, which are necessary steps in the growth of fullerenes, may therefore result from autocatalysis by carbon.

On the electronic structure of WS2 nanotubes

Abstract Electronic properties of WS 2 nanotubes have been studied using the Density Functional Based Tight Binding method (DFTB). We show that WS 2 nanotubes are all semiconducting possessing a non zero moderate gap. Only for zig-zag ( n , 0) nanotubes the band gap tends to vanish for very small tube diameters ( n , n ) tubes exhibit an indirect gap of similar size as the direct gap in ( n , 0) nanotubes of comparable diameter.

A molecular dynamics study of N-incorporation into carbon systems: Doping, diamond growth and nitride formation

Abstract An ab-initio-based tight-binding molecular-dynamics method is used to describe the behaviour of a variety of carbon-nitrogen systems, ranging from doping in CVD diamond and the effect of N on CVD diamond growth to that of the possible fabrication of super-hard C3N4 crystals. We describe why N does not dope in CVD diamond though it is incorporated predominantly on-site. This also has consequences for growth processes, in particular the (100) Harris growth mechanism. Towards higher N densities, we observe clear trends counteracting the formation of a low-compressibility crystalline phase: (1) N-incorporation strongly catalyses C-under-coordination, which, in turn; (2) causes the nitrogens to develop in a paracyanogen-like manner (CN-double and -triple) bonding; and (3) the most favourable densities for a-CN appear to be much lower than the desired hard crystalline ones. Finally, we investigate the possibility that an admixture of Si catalyses sp3 carbon formation, thus favouring increased network connectivity and crystallinity.

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.

Theory of Ga, N and H terminated GaN (0 0 0 1)(0 0 0 1) surfaces

Abstract We present a theoretical study of atomic structures, electrical properties and formation energies for a variety of possible reconstructions with 1 × 1 and 2 × 2 periodicity of the GaN (0 0 0 1) and (0 0 0 1 ) surfaces. We find that during MBE growth in the (0 0 0 1) direction 2 × 2 structures become stable under N rich growth conditions while Ga rich environment should yield structures with 1 × 1 periodicity. Considering MBE growth on (0 0 0 1 ) surfaces reconstructions with 1 × 1 periodicity have low energies. During MOCVD growth where H terminated surfaces may occur 1 × 1 periodicities are found to be stable for both growth directions.

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.

Structure and energetics of SinNm clusters: Growth pathways in a heterogenous cluster system

We present a detailed study of the structures and energetics of SinNm clusters with n+m⩽6. We have determined the lowest-energy isomers of these clusters as a function of total cluster size and cluster stoichiometry. The properties of the low-energy isomers were calculated using an accurate, all-electron full-potential density-functional method at both the local density approximation (LDA) and the generalized gradient approximation (GGA) levels of theory. We found the most stable clusters by conducting an extensive phase space exploration for all the clusters containing up to 6 atoms, checking all bonding topologies and all possible atom type decorations. The search was done using a fast, but accurate, density-functional based tight-binding method. The calculations reveal several trends in the silicon–nitrogen binary cluster system. For N-rich clusters, linear or quasi-linear structures predominate, with strong multiple-bond character. Si-rich clusters favor planar or three-dimensional structures. Near th...

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.

Cage-forming tendencies in SinNm clusters

Abstract We present the results of a computational study of the structure and properties of Si n N m clusters, where n + m =20. We use complementary tight-binding and first-principles density functional calculations to determine the optimal arrangement of N atoms on the clusters for m =2,4,6 and 8. The calculations indicate a transition in the basic geometrical structure of the substituted clusters from close-packed structures typical of pure Si clusters for m m ≥4. We discuss the predicted thermodynamic and chemical stabilities of these novel Si-based cages.

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.