Quantum transport in nanodevices

The focus of this thesis is the simulation of electron transport properties in nanoscale devices. Work related to this dissertation has resulted in four peer re-viewed journal articles and ten proceedings. The outline is as follows:;Chapter II introduces the necessary formalism. Here the foundations of density functional theory, basis sets, quantum scattering theory and complex absorbing po- tentials are described. The extension of the complex absorbing potential transport framework to the general case of N electrodes is a key development presented in this chapter.;Chapter III discusses computational aspects in detail. The parallelization strategy for a density functional theory code with an atomic orbital basis set is presented. Also discussed are the implementation of a caching algorithm for memory efficient calculations, solution of self consistent problems and efficient application of finite difference operators. Finally, the convergence of transport properties is discussed with a CO molecule adsorbed onto a mono-atomic gold wire as an example.;Chapter IV presents calculations related to nanowires. Transmission spectra for mono-atomic gold chains, silicon nanowires and graphene nanoribbons with kinks and related defects are shown. It is found that kink defects generally cause large drops in conductance due to quantum interference effects. The conductance properties of elongated gold nanowires are also studied, with geometries produced by accurate molecular dynamics simulations. Polytetrahedral structures formed during elongation are found to cause non-integer values in conductance traces.;Chapter V presents transport calculations for multi-terminal devices. A tight- binding model is solved analytically to show the accuracy of the formalism developed in Chapter II. Simulations of model systems with 8 terminals highlight some of the quantum interference effects present in multi-terminal systems. Finally, more realistic examples of a four terminal graphene cross junction and six terminal carbon nanotube junction are simulated with a density functional theory Hamiltonian.;Chapter VI describes electronic devices which incorporate graphene. Graphene is widely touted as an ideal electrode material, and some groups have considered all-carbon devices by combining graphene and carbon nanotubes. In this chapter the Schottky barrier in a graphene-carbon nanotube junction is calculated and the potential of creating functional devices is discussed. Silicon is an important material in modern electronics, yet the interaction between graphene and silicon surfaces is not well studied. In this chapter the interaction between graphene nanoribbons and silicon with vacancy defects is examined. Since graphene does not possess a band gap other two-dimensional materials have been proposed to combine with graphene to form devices. Molybdenum disulphide is a two-dimensional material with a band gap. The interaction of graphene and MoS2 is explored in this chapter.