Michael Sternberg

Atomistic simulations of complex materials: ground-state and excited-state properties

The present status of development of the density-functional-based tightbinding (DFTB) method is reviewed. As a two-centre approach to densityfunctional theory (DFT), it combines computational efficiency with reliability and transferability. Utilizing a minimal-basis representation of Kohn–Sham eigenstates and a superposition of optimized neutral-atom potentials and related charge densities for constructing the effective many-atom potential, all integrals are calculated within DFT. Self-consistency is included at the level of Mulliken charges rather than by self-consistently iterating electronic spin densities and effective potentials. Excited-state properties are accessible within the linear response approach to time-dependent (TD) DFT. The coupling of electronic and ionic degrees of freedom further allows us to follow the non-adiabatic structure evolution via coupled electron–ion molecular dynamics in energetic particle collisions and in the presence of ultrashort intense laser pulses. We either briefly outline or give references describing examples of applications to ground-state and excited-state properties. Addressing the scaling problems in size and time generally and for biomolecular systems in particular, we describe the implementation of the parallel ‘divide-and-conquer’ order-N method with DFTB and the coupling of the DFTB approach as a quantum method with molecular mechanics force fields.

Theoretical tools for transport in molecular nanostructures

We have developed a quantum simulation tool to investigate transport in molecular structures. The method is based on the joint use of a density functional tight binding (DFTB) and of a Greens function technique which allows us the calculation of current flow through the investigated structures. Typical calculations are shown for carbon-nanotube-based field effect transistors and for DNA fragments. Transport; molecular structures

Growth of (110) diamond using pure dicarbon

We use a density-functional based tight-binding method to study diamond growth steps by depositing dicarbon species onto a hydrogen-free diamond (110) surface. Subsequent C_2 molecules are deposited on an initially clean surface, in the vicinity of a growing adsorbate cluster, and finally, near vacancies just before completion of a full new monolayer. The preferred growth stages arise from C_2n clusters in near ideal lattice positions forming zigzag chains running along the [-110] direction parallel to the surface. The adsorption energies are consistently exothermic by 8--10 eV per C_2, depending on the size of the cluster. The deposition barriers for these processes are in the range of 0.0--0.6 eV. For deposition sites above C_2n clusters the adsorption energies are smaller by 3 eV, but diffusion to more stable positions is feasible. We also perform simulations of the diffusion of C_2 molecules on the surface in the vicinity of existing adsorbate clusters using an augmented Lagrangian penalty method. We find migration barriers in excess of 3 eV on the clean surface, and 0.6--1.0 eV on top of graphene-like adsorbates. The barrier heights and pathways indicate that the growth from gaseous dicarbons proceeds either by direct adsorption onto clean sites or after migration on top of the existing C_2n chains.

Molecular Devices Simulations Based on Density Functional Tight-Binding

We have developed a quantum simulation tool to investigate transport in molecular structures. The method is based on the joint use of a Density functional tight-binding (DFTB) and of a Greens function technique which allows us the calculation of current flow through the investigated structures. Typical calculations are shown for carbon-nanotube-based field effect transistors, sensors and for DNA fragments.

NOON — A non-orthogonal localised orbital order-N method

Abstract We present the implementation of an orbital-based linear scaling method for total energy calculations within a Tight-Binding approach. Our scheme explicitly incorporates charge self-consistency for both single- and multiple-species systems and uses non-orthogonal basis sets to construct non-orthogonal orbitals. The energy functional of Kim et al. [Phys. Rev. B 52 (1995) 1640] is minimised within a hierarchical iteration scheme for wave functions and the Lagrangian parameter. A number of safeguard mechanisms are applied to stabilise the algorithms on both levels. Compared to exact diagonalisations using extended states we reproduce the total energy typically within 100 meV when localizing wave functions within second neighbours and within 30 meV when including third neighbours. The charge transfer effects and related energies for self-consistent-charge corrections are reproduced accurately.

Density Functional Based Tight Binding Study of C2 and CN Deposition on (100) Diamond Surface

Abstract : A density-functional based tight binding method was used to study elementary steps in the growth of ultrananocrystalline (UNCD) diamond. It was shown previously that C(2) dimers are the dominant growth species in hydrogen-poor argon plasmas. Recent experimental evidence shows that nitrogen addition to the plasma profoundly changes the morphology of the UNCD film. CN species are believed to play a major role. Reactions of C(2) and CN molecules with reconstructed diamond (100) surfaces were studied. A single CN prefers an end-on attachment to a surface atom on the unhydrided (100) surface with its C end down. It is shown how further C(2) addition to the surface leads to CN-mediated diamond growth and how the CN species remain on top of the growing diamond layer.

Molecular dynamics simulation of impurities in nanocrystalline diamond grain boundaries

Nanocrystalline diamond films grown on Si substrates at 800 C from hydrogen-poor plasmas have a number of highly desirable mechanical and electronic properties. Impurities were found by SIMS measurements to be uniformly distributed throughout the thickness of the films at a level of 10{sup 17}--10{sup 18} cm{sup {minus}3}. It is likely that the impurities are located at the grain boundaries, which play a crucial role in controlling important characteristics of the films, such as electrical conductivity and electron emission. Density-functional based tight-binding (DFTB) molecular dynamics simulations were performed for diamond light-energy high-angle (100) twist grain boundaries with impurities such as N, Si and H.