First Principles Studies On Some Nano-Materials

With the rapid progress in density functional theory (DFT) and its numerical methods, DFT based first-principles calculation has become a routine method for condensed matter theory, quantum chemistry, and material science. Due to its novelty in optical, electronic, magnetic, thermal, or mechanic properties, nanomaterials have attracted an intense research interest. Although it is still at an immature stage, nano-technology is believed to play an important role in our modern life in the 21th Century. Theoretical studies are indispensable for understanding the properties of nano-materials and designing better nano-materials. In this dissertation, we study some novel materials with different dimensions, such as cluster, nanotube, and nanoribbon, based on first-principles calculations.In the first chapter, we introduce the basic concept of DFT and review its recent progress. In electron density based DFT, all ground state properties can be derived from the electron density. Finding good approximation for exchange-correlation functional is one of the main targets in DFT. With the development of modern functionals, DFT leads to more and more accurate results. All these progresses lead DFT applicable to a broad range of problems. At last, we introduce briefly the density functional packages used in the current work.In chapter 2, we begin to focus on graphene. First, we give simple introduction to such materials. Due to its novel properties, such as anomalous quantum hall effect and massless dirac fermions, graphene has been widely studied. Then, we study on electronic structures of the deformed graphene nanoribbons (GNRs) based on first-principles calculation. Our theoretical results show that the electronic properties of zigzag GNRs are not sensitive to uniaxial strain, while the energy gap modification of armchair GNRs (AGNRs) as a function of uniaxial strain displays a nonmonotonic relationship with a zigzag pattern. The subband spacings and spatial distributions of the AGNRs can be tuned by applying an external strain. Scanning tunneling microscopy dI/dV maps can be used to characterize the nature of the strains, compressive or tensile, of AGNRs. In addition, we find that the nearest-neighbor hopping integrals betweenπ-orbitals of carbon atoms are responsible for energy gap modification under uniaxial strain based on our tight-binding simulations.In chapter 3, we investigated the electronic and magnetic properties of SiC NRs with armchair- and zigzag-shaped edges by spin-polarized first-principles calculations. The armchair nanoribbons are nonmagnetic semiconductor, while the zigzag nanorib-bons are magnetic metal. The spin polarization in the zigzag SiC nanoribbons is originated from the unpaired electrons localized on the ribbon edges. Interestingly, the zigzag nanoribbons narrower than～4 nm present half-metallic behavior. Without the aid of external field or chemical modification, the metal-free half-metallicity predicted for narrow SiC zigzag nanoribbons opens a facile way for nanomaterials based spin-tronics applications.In chapter 4, we study a kind of novel nano-materials, tellurium nanotube. Recently Yu et al. synthesized ultralong and uniform single crystalline Te nanotubes. R-T, and I-V curves of individual Te nanotube, which was prepared by a surfactant assisted solvothermal approach, have been firstly measured in the temperature range of 5-300 K, indicating metallic character of the as-prepared Te nanotube with a T~2 dependence of the resistance, which was also consistent with the results by photoconductivity measurement. A p-type doping metallic behavior in our modeling results was shown for Na(N)-doped Te nanotubes, which was consistent with the experimental results.In chapter 5, we present DFT calculations on the Nano-Diamondoids. HOMO(the highest occupied molecular orbital)-LUMO(the lowest unoccupied molecular orbital) gaps decreases as the size of the nanoparticles increases. For nanoparticles larger than 1.5 nm, the HOMO-LUMO gap is smaller than the gap of bulk diamond. The LUMO remains delocalized on the surface. We also find out the bulk-LUMO-like unoccupied orbital and obtain the gap of bulk-like diamond, which is consistent with the DFT results.