We studied the electronic properties of phosphorus-doped silicon nanocrystals using the real-space first-principles pseudopotential method. We simulated nanocrystals with a diameter of up to 6 nm and made a direct comparison with experimental measurement for the first time for these systems. Our calculated size dependence of hyperfine splitting was in excellent agreement with experimental data. We also found a critical nanocrystal size below which we predicted that the dopant will be ejected to the surface.

Wave functions obtained from plane-wave density-functional theory ͑DFT͒ calculations using normconserving pseudopotential, ultrasoft pseudopotential, or projector augmented-wave method are efficiently and robustly transformed into a set of spatially localized nonorthogonal quasiatomic orbitals ͑QOs͒ with pseudoangular momentum quantum numbers. We demonstrate that these minimal-basis orbitals can exactly reproduce all the electronic structure information below an energy threshold represented in the form of environmentdependent tight-binding Hamiltonian and overlap matrices. Band structure, density of states, and the Fermi surface are calculated from this real-space tight-binding representation for various extended systems ͑Si, SiC, Fe, and Mo͒ and compared with plane-wave DFT results. The Mulliken charge and bond order analyses are performed under QO basis set, which satisfy sum rules. The present work validates the general applicability of Slater and Koster's scheme of linear combinations of atomic orbitals and points to future ab initio tight-binding parametrizations and linear-scaling DFT development.

An intriguing growth morphology of Pb islands on a Si(111) surface is observed in our STM experiments: the growth of a Pb layer on Pb islands with unstable heights starts from the periphery and moves towards the center, while the nucleation of the next layer on stable Pb islands starts away from the periphery. Using first-principles total energy calculations, we have studied the diffusion barriers of Pb adatoms on a freestanding Pb(111) film as a function of film thickness. The diffusion barriers are found to be very low (<60 meV), and a bi-layer oscillation due to the quantum size effect (QSE) is observed, with a lower barrier on the odd-layered, relatively unstable Pb films. The diffusion barrier difference between the odd- and even-layered film is as large as 40 meV. The observed unusual growth can be attributed to this big difference in the diffusion barriers due to QSE.