Quasi-One-Dimensional Nanotubes: Geometry And Electronic Structure Tuning From First-Principles Calculation

Quasi-one-dimensional nanotubes have been extensively studied both theoretically and experimentally. The unique geometries and quantum-confinement-effects endow the nanotubes distinctive nanoelectronic, photonic, mechanic, and magnetic properties, such as the upgrade of the binding energy of excitons, the blue-shift of absorption peak, the promotion of chemical reactivity, the promotion of mechanical property, the change of magnetical correlation, and so forth. The hollow space in nanotubes makes it an ideal candidate material in wide-range fields, such as gas storage and delivery, confined chemical reaction, and light and gas sensor. Ever since the discovery of carbon nanotubes (CNTs) in 1991, abundant theoretical and experimental researches have been paid on revealing its novel mechanical and electronic properties. Additionally, other inorganic nanotubes such as WS2 and BN nanotubes have been synthesized successfully, which have applications in lubrication, composing the scanning probe tips, lithium cells, field-emission devices, etc. The relationship between the structures of nanomaterials and their functions is therefore quite crucial and becomes the goal of research. Searching for nanotubular materials with novel geometries and fascinating properties is highly desirable, which is the aim of present thesis.The first-principles caculatiosn on the basis of density-functional-theory (DFT) combined with molecular dynamics simulations (MDSs) have been proved to be a useful theoretical method in revealing the structures and properties of nanomaterials. The geometric configutaions, electronic structures, optoelectronic absorption and excitation, magnetism, mechanic properties, dynamics of chemical reaction, the procedure of atomic and molecular collisions, etc., can be predicted using this theoretical scheme. The first-principles calculations have also been successfully employed to deal with the problems of defects devising and restoring, doping and functionalization, self-assembly, gas sensor and storage, and nanoelectric circuit device, which increases the efficiency and shortens the development period.In this thesis, we performed first-principles calculations in conjunction with molecular dynamics simulations to reveal the structures and electronic properties of four typical nanotubes, SiC, CoSi2, SiN and TiO2. These nanotubes possess different structural features and tuning mechanisms of electronic properties, which are quire crucial for their applications in nanoscaled devices. The roles of defects in modulating the electronic structures are predicted. The main conclusions are summarized as follows.·SiC nanotubes possess high surface-to-bulk, which makes them sensitive to foreign decoration. Using first-principles method, we found that N and NHx (x=1,2) groups can be chemically incorporate into the network of SiC nanotubes in different ways, accompanied with the formation of N-C and N-Si bonds. The adsorbing energy of N and NHx (x=1,2) groups on (5,5) and (8,0) SiC nanotubes ranges from -1.82 to -7.19 eV. The electronic structures of SiC nanotubes can be effectively modified by these groups and display diverse characters ranging from semiconducting to semimetallic, depending on the chirality of SiC nanotubes as well as the way of the incorporation of these functional groups. The N-adsorbed (5,5) SiC nanotube which has tetrahedral adsorbing configuration, and silicon-site NH2-adsorbed (5,5) and (8,0) SiC nanotubes, are p-type semiconductors. The carbon-site NH2-adsorbed (5,5) and (8,0) SiC nanotubes, and N-substituted (5,5) SiC nanotubes, are n-type semiconductors. The N-substituted (8,0) SiC nanotube is semimetallic. ·Spin-polarized DFT calculations showed that the incorporation of Co atoms into Si nanotubes can not only stabilize these tubes but also tune their electronic properties. The stable configurations have the adsorbed Co atoms locating on an intra-layer between the outer layer and the inner layer of the tube wall. There is no energy barrier for a Co atom to enter the Si nanotubes through the center of the Si hexagon. The formation energies of (5,5) and (8,0) CoSi2 nanotubes are much lower than those of corresponding prinstine Si nanotubes by about 67% and 66%, respectively. The stabilities of Si nanotubes with Co and other TM atoms (TM= Cr, V and Mo) adsorbed were examined by either CG optimization or NVT dynamic simulations at 300 and 1000 K with a Nose thermostat for 2 ps, and only the CoSi2 nanotubes were found to be stable at these temperatures. The potential energy profiles indicate that with the increase of Co concentration, Co atoms favor the formation of a Co intra-layer in the walls of the Si nanotubes. Isolated CoSi2 nanotubes favor gathering together and forming bundles with lower energy. It was found that electrons transfer from 7r-orbital of Si nanotubes and atomic Co to the interspace between the Si hexagon and Co. The electronic structures of these CoSi2 nanotubes exhibit the characters of metals with high electron density states at Fermi level.·The hydrogen-stabilized silicon nitride nanosheets and nanotubes with the stoichiometry of HSiN were studied by first-principles calculations. The stable H-SiN nanosheet has a two-dimensional hexagonal grid of Si and N atoms with the Si dangling bonds being passivated by H atoms, which are placed alternatively on the two sides of the sheet, whereas H-SiN nanotubes can be built from rolling up the nanosheet. The hydrogen arrangement strongly affects the energetic favorability of the H-SiN nanotubes and makes some distinctively favorable configurations possible. The stable H-SiN nanosheet and zigzag tubes have a direct band gap at theΓpoint, which are crucial for building nanoscale optical and photonic devices.·We propose stable layered structures and ultrathin tubular configurations of titanium oxide (TiO2) nanomaterials on the basis of first-principles calculations within density functional theory. The layered structures are stable for the TiO2 nanosheets containing less than four (001) rutile bilayers. When TiO2 nanosheet is composed of a single (001) bilayer, the energetically most favorable configuration has a two-dimensional triangular structure where each atom is fully coordinated. Ultrathin TiO2 nanotubes can be modeled by rolling up this triangular sheet with different rollup vectors. The strain energies of these nanotubes decrease with the increase of tube diameters. Armchair tubes are indirect-band gap semiconductors, while zigzag tubes are direct-band gap semiconductors. The band gap values of the tubes increase with increasing tube diameter. NH doping reduces the band gap of the (7,7) tube by about 0.66 eV and that of the (12,0) tube by only 0.16 eV. The incorporation of C atoms into (7,7) ultrathin TiO2 nanotube induces metallization of the tube.·First-principles calculations indicate that TiO2 nanoribbons can be formed from the already-synthesized ultra-thin TiO2 nanosheet. Zigzag TiO2 nanoribbons terminated by oxygen atoms are energetically more preferable than the armchair ones with the same ribbon width. Both zigzag and armchair TiO2 nanoribbons are semiconductors with band gap larger than 2.93 eV in our L(S)DA+U procedure. The band gaps as a function of ribbon width are separated into two different categories with a hierarchy of gap size given by Eg,2m＞Eg,2m+1.For the TiO2 nanoribbons containing even number of Ti lines (NZ or NA= 2m), the band gap (Eg,2m) decrease with the increase of width, whereas for the TiO2 nanoribbons with NZ (or NA)= 2m+1, the band gap (Eg,2m+1) increase with increasing width. The spin-polarization of edge states can be achieved in the nonstoichiometric TiO2-x nanoribbons containing threefold-coordinated Ti atoms at the edge which may be formed under poor oxygen conditions. When Vo defects are formed at one edge of TiO2-x nanoribbons, the spin-up branch has a band gap of 0.06 eV, while the band gap of spin-down branch is 2.31 eV. When Vo defects are formed at both edges, the TiO2-x nanoribbon becomes a half-metal. The hydrogenation of Vo defects quenches the magnetic moments. The tunable band gaps, half-metallicity, and the sensitivity to H atoms of ultra-thin TiO2 nanoribbons containing Vo defects imply their potential applications in solar cells, spintronics-based devices, and sensors.