Atomic Structure, electronic, optical and electrical properties of freestanding, passivated, and functionalized semiconductor nanowires

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Due to their small radius nanowires are characterized by an extreme large surface to bulk ratio - structure and properties of their surfaces thus strongly influence the mechanical, optical, and electrical behaviour of nanowires. For a realistic theoretical description of nanonwires it is therefore crucial to include such effects. This is straightforward for nanowires with (i) very small radii (=2nm) where conventional density functional theory (DFT) methods in the plane wave pseudopotential formalism can be directly applied and (ii) very large radii (größer als 50nm) where the surface can be modelled by individual facets. However, many of the experiments which will be performed within the focused project have a size just in between these two boundaries and which are thus not accessible by the standard approaches. We therefore aim to apply a hierarchical approach where we will start from well established density-functional methods to identify the equilibrium geometry, formation energies, and electronic structure of semiconductor nanowires with small radii (kleiner als 2nm). In handshaking with these results approximate DFT methods (DFTB) will be applied to extend these studies to the experimentally relevant length scale (up to 20nm). The approach will be applied on naked/passivated freestanding and functionalised Si, Ge, SiGe, SiC and GaN nanowires. Scanning tunnelling microscopy/spectroscopy simula-tions will allow a direct connection to experiment. Based on the identified equilibrium structure transport properties will be calculated by interfacing the DFTB-method with nonequilibrium Green-function techniques. In a fully self-consistent treatment with open boundary conditions for metal contacts this approach will allow to study nonequilibrium electron transport in the nanowires and to address questions on coherent versus incoherent transport, electron-phonon interactions and the modification of current flux by surface functionalisation.