Optical And Electronic Properties Of One-dimensional Inorganic Nanostructures

Nanoscience and nanotechnology have been one of important fields in basic and applied research. For their outstanding electronic, optical and magnetic properties, one-dimensional inorganic nanomaterials could be the most promising building blocks for the future nanoscale devices. To enable nanostructures to possess special electronic and optical properties which are required for excellent nanoscale devices is one of the important subjects for nanoscience. In this thesis, our research objects are some one-dimensional inorganic nanostructures. Electronic and optical properties of the nanostructures as well as the relation between the properties and structures are studied by the first principles. In addition, the properties are tuned by codoping and other methods. The thesis is organized as follows:In chapter 1, the background, structures and properties of one-dimensional inorganic nanomaterials are introduced.In chapter 2, stabilities, electronic and optical properties of zinc oxide and zinc selenide single-walled nanotubes are studied. It is found that the stain energies of zinc oxide and zinc selenide single-walled nanotubes are lower than those for single-walled carbon nanotube and boron nitride nanotubes, indicating that the zinc oxide and zinc selenide single-walled nanotubes can be existent under certain conditions. Zinc oxide and zinc selenide nanotubes show different properties from carbon and boron nitride nanotubes. Zinc oxide and zinc selenide nanotubes are direct band gap semiconductors and the gaps are independent of diameter and chirality. The optical properties of zinc oxide and zinc selenide nanotubes are independent of diameters and chirality also. The common feature of electronic and optical properties of zinc oxide and zinc selenide nanotubes could avoid the roadblock of selective synthesis of specific chirality nanotubes and may be advantageous for applications.In chapter 3, we investigate curvature effects and doping effects on the properties of one-dimensional carbon nanostructures. We demonstrate the curvature effects on magnetic properties of zigzag edges of finite length carbon nanotubes and rapheme nanoribbons. The magnetic moment at the non-passivated edge decrease with the increase of curvature: the magnetic moment is about 1.0μB for lower curvature (θ_Ï€-σ<110°), and it would be about 0.5μB for higher curvature (θ_Ï€-σ>110°). The magnetic coupling changes from ferromagnetic coupling to antiferromagnetic coupling as the curvature changes fromθ_Ï€-σ<110°toθ_Ï€-σ>110°.Next, we investigate the codoping effects on the properties of double-walled carbon nanotubes. For the pristine double-walled carbon nanotubes, the valence band maximum (VBM) is the state of shell tube and the conduction band minimum (CBM) is the state of core tube. For the codoped nanotubes where donor (potassium atom) is adsorbed outside shell and acceptor (NO₂ molecule) is adsorbed inside core, VBM is the state for core tube and CBM is the state for shell tube, which is the exact opposite of pristine double-walled carbon nanotubes. Optical calculations show that the transition between VBM and CBM for pristine nanotubes is forbidden while it is allowed for the codoped nanotubes. This transition leads to charge separation and photovoltaics. The electric field induced by charged dopants would inhibit electron-hole recombination and enhance the efficiency of photovoltaic devices.In chapter 4, we present the size effects and stain effects on the electronic and optical properties of zinc oxide one-dimensional nanostructures. Both zinc oxide nanowires and nanotubes which have same diameters are direct band gap semiconductor and have similar electronic structures. Optical properties show that the nanowires and nanotubes having same diameters have similar optical properties. The results show that properties of zinc oxide one-dimensional nanostructures are dependent on diameter while independent of surface shape. The band gap of zinc oxide nanowires decreases with the increase of diameter and approaches to the band gap of bulk of zinc oxide for very big nanowires. Furthermore, the strain effects on the properties of zinc oxide nanowires are obvious. The band gap of zinc oxide nanowires linearly decreases with the increase of strain except small nanowires at certain strain because different valence bands show different shift with increase of strain. In addition, we find that the optical properties of zinc oxide nanowires having different diameters show different dependence on strain. For parallel light polarization, the peaks of dielectric function for small nanowires show blue shift with the increase of strain while there is no shift for the main peaks of bigger nanowires. For perpendicular light polarization, the peaks show red shift with increase of strain except the first peak of small nanowire.In chapter 5, we calculated the electronic and optical properties of one-dimensional derivative nanostructure of rapheme, rapheme nanoribbons and graphane nanoribbons. According the edge shapes of nanoribbons, they could be classified into zigzag nanoribbons and armchair nanoribbons. In the electronic structures of zigzag rapheme nanoribbons, theÏ€band andÏ€^* band degenerate at the Fermi energy in certain range of Brillouin zone and this range increases with the ribbon width. Armchair rapheme nanoribbons are semiconductors and their band gaps present oscillations behavior and the oscillation amplitude decrease with the increase of ribbon width. For optical properties, the dielectric function peaks of zigzag rapheme nanoribbons show red shift with the increase of ribbon width for parallel light polarization, but for perpendicular light polarization there is no shift. For the armchair rapheme nanoribbons, the dielectric function peaks change with increase of ribbon width in three types for the parallel light polarization, but there is no changes for the perpendicular light polarization.In the graphane nanoribbons, the carbon atoms are bonded together via sp³ tetrahedral scheme like diamond. These nanoribbons show properties like diamond and independent on edge shapes. Graphane nanoribbons including zigzag nanoribbons and armchair nanoribbons are all direct band semiconductor and the gaps decrease with increase of the ribbon width and approach to diamond band gap for very wide ribbons. Optical properties of graphane nanoribbons are similar to the properties of graphane. For perpendicular light polarization, the dielectric functions have main peaks at about 7.0 eV and 13.0 eV. And they have the main peak at about 12.0 eV for the parallel light polarization. The electronic and optical properties of graphane nanoribbons are similar to those of graphane and independent of shapes of edge, different from the rapheme nanoribbons.Finally, we summarize the thesis and propose the future works in Chapter 6.