Physical Properties Of One-Dimensional Semicondutor Nanostructures And Device Design

Currently, low-dimensional nanoscale materials have attracted a great deal of research interest for their novel properties and potential application. Among them, one-dimensional nanostructure systems quickly become the focus of the present studies due to strong quantum confinement effects, size effects and surface effects. Especially semiconductor nano-device is now one of the hottest research topics because it is expected to be the only way for the future development of semiconductor device. However, a lot of physical phenomenon and laws about nanostructures are not well understood and found, such as size effects of electronic and mechanical properties, surface effects, electromechanical coupling effects. Furthermore how to use these nonomaterials to design practical nanodevices is one of the most significant but very complicated issues. In this thesis we study the structures, electronic properties, mechanical properties and their coupling effects of several important semiconductor nonomaterials by first-principle calculation method, molecular dynamical simulation method and classical and semi-classical model. We further proposed an all-carbon field-effect transistor design based on graphene and carbon nanotube hybrid structures and explore to how to control their performance. We conclude our main findings below:(1) Structural stability and electronic properties of bismuth nanowires and nanotube: We have studied the structural stability of bismuth nanowires oriented along the [012] direction. The shapes of cross section have an important effect on the stability of bismuth nanowires due to the different chemical bonding and surface reconstruction. Among the bismuth nanowires studied, the most stable ones are indirect gap semiconductors. The band gap decreases with increasing diameter, which explicitly demonstrates strong quantum size effects observed in previous experiments. For [012] bismuth nanotubes constructed from bismuth nanowires, our results show that the electronic properties of [012] bismuth nanotubes depend sensitively on the wall thicknesses rather than the diameters of nanotubes. Furthermore we studied layered bismuth nanotube constructed by rolling bismuth atom layers. We found that single-wall layered bismuth nanotube is more stable than [012] bismuth nanotubes with same wall thickness, while double-wall layered bismuth nanotube is nearly degenerate energetically with [012] bismuth nanotubes with same wall thickness. This indicates that with the increase of wall thickness both the layered and single-crystalline bismuth nanotubes are existent experimentally. Very recently double-wall bismuths have been synthesized experimentally, qualitatively and quantitatively confirming our theoretical results. Finally we study the structural and electronic properties of the bismuth/antimony superlattice nanowires. We found the confined states, resulting in the sharp the density of states peak near Fermi energy, which is a great advantage for improving the thermoelectric properties of materials. The confined electrons and holes distribute at different chemical zones, thus they belong to type-II superlattice. These findings might have important implications for understanding the structures and electronic properties of one-dimensional bismuth nanostructures and utilizing them as building blocks for nanoscale devices.(2) Electronic and mechanical properties of zinc oxide nanoblets and nanowires: The electronic and elastic properties of [0001] zinc oxide nanobelts with different lateral dimensions have been studied by employing first-principles approaches. We find that the different surface plays important effects on the energetic stability and the band gaps of the nanobelts, but minimal effects on the size dependence of the Young's modulus. Furthermore by applying continuum-based model we give a reasonable explanation for the size effect of Young's modulus of zinc oxide nanobelts and nanowires. We find that the continuum-based model proposed for the Young's modulus of nanostructures is applicable for the zinc oxide nanowires of about tens to hundreds nanometers in diameters, but not for those ultrathin nanowires and nanobelts due to the complicated relaxation in those ultrathin nanostuctures, highlighting the importance of surface effects and nonlinear effects.(3) Stability, transport properties and device design of graphene-based hybrid structures: Inspired by experimental results we propose a novel field-effect transistor device architecture, composed of carbon nanotube–graphene hybrids by molecular dynamic simulation. These all-carbon devices are proposed to be constructed from a bilayer graphene by contact probe cutting and high-temperature annealing. Then by ab initio transport calculations we demonstrate a large energy gap in the transmission spectrum of such junctions. The proposed device architecture has three important advantages: First, the way our devices are assembled ("self-folding"approaching) differs fundamentally from the traditional approaches of growing carbon nanotubes somewhere and then using them somewhere else. Second, it is made entirely out of carbon, all sp2 bonded carbon in fact, no dangling bonds, and high stable thermally and chemically. Third, the electronic band gap of self-folded nanotubes can be combined with the semi-metallicity of graphene electrodes to form a "metal-semiconductor-metal"junction. No metal electrodes are needed. The junction in between is smooth and coherent, which greatly improves the performance of the devices. .(4) Electromechanical coupling properties of boron nitride nanoribbons: By ab initio calculations we demonstrate that the band gap of zigzag boron nitride nanoribbon can be tuned significantly by applying uniaxial tensile strain and the wider boron nitride nanoribbons are, more easily the band gaps can be tuned within the same strain range, demonstrating distinctly different features from graphene nanoribbons. Definitely those properties are very important for the practical application because the wider the nanoribbons are, more easily they can be fabricated and controlled. This unexpected sensitivity of band gap to applied strain results from the electrostatic effect on band alignment. Further we find that zigzag boron nitride nanoribbons possess sizable dipole moment density due to spontaneous polarization and more importantly the magnitude and direction of dipole moment density can be tuned significantly by strain engineering in the elastic range. These features make them have great potential for the applications in the future nano-electro-mechanical devices.