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

由于其半径小，纳米线的特点是具有极大的表面体积比，因此其表面的结构和性质强烈影响纳米线的机械、光学和电学行为。因此，为了对纳米线进行现实的理论描述，将这些效应包括进来是至关重要的。对于具有(i)非常小的半径（=2nm）的纳米线，可以直接应用平面波伪势形式化中的传统密度泛函理论（DFT）方法；（ii）非常大的半径（größer ~ 50nm），可以通过单个面对表面进行建模。然而，许多将在重点项目中进行的实验的规模正好在这两个边界之间，因此无法通过标准方法获得。因此，我们的目标是应用分层方法，我们将从建立良好的密度泛函方法开始，以确定具有小半径（kleiner als 2nm）的半导体纳米线的平衡几何形状，形成能量和电子结构。在握手与这些结果近似DFT方法（DFTB）将应用于这些研究扩展到实验相关的长度尺度（高达20nm）。该方法将应用于裸/钝化独立和功能化Si， Ge, SiGe， SiC和GaN纳米线。扫描隧道显微镜/光谱模拟将允许直接连接到实验。在确定平衡结构的基础上，将dftb方法与非平衡格林函数技术相结合，计算输运性质。在金属接触开放边界条件下的完全自一致处理中，这种方法将允许研究纳米线中的非平衡电子传输，并解决相干与非相干传输，电子-声子相互作用以及通过表面功能化改变电流通量的问题。

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.