| Density functional theory (DFT) and time-dependent DFT (TDDFT) are applied to study the electronic structure of two categories of technologically important systems, silicon nanostructures and carbon nanotubes. Effect of different chemical passivation schemes on the electronic and optical properties of 1--2 nm diameter core/shell silicon nanocrystals was investigated in detail. Our calculation result explains the experimental observation that hydrogen-passivated Si nanocrystals luminesce in the blue, while oxide-passivated Si nanocrystals luminesce in the yellow-red. Nanocrystals containing P or B impurity atoms, either on the surface or in the interior, are explored to understand electrical doping in strongly quantum-confined nanostructures. We find the behavior of donor and acceptor in nanostructures is quite different from their properties in bulk. We also use finite clusters to model silicon nanowires and Si (001) channel/gate oxide interface of field effect transistor (FET). Our theoretical results provide useful information for guiding the fabrication of improved nanometer scale devices, such as silicon CMOS FET.; We also study the geometrical, electronic and optical properties of finite-length metallic (5,5) armchair single wall carbon nanotube. The C-C bond length alternation, band gap, relative stability, optical transitions, Fermi energy and charging energy as a function of length are explored. We use Coulson model to explain the peculiar band gap oscillatory variation pattern as length of finite metallic carbon nanotubes. Through our calculation, we predict that an intensive transition in visible region of the optical spectrum, which energy depends on tube length, is an identifying signature during the synthesis and identification of this class of short nanotubes in the future. |
