Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P16: Nanowires: Electronic and Optical Properties |
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Sponsoring Units: DCMP Chair: Jason Petta, Princeton University Room: B115 |
Wednesday, March 17, 2010 8:00AM - 8:12AM |
P16.00001: Aharonov-Bohm interference in topological insulator nanoribbons Desheng Kong, Hailin Peng, Keji Lai, Stefan Meister, Yulin Chen, Xiao-Liang Qi, Shou-Cheng Zhang, Zhi-Xun Shen, Yi Cui Topological insulators represent novel phases of quantum matter with an insulating bulk gap and gapless edges or surface states. Barrier exists in studying topological surface states by direct transport measurements, presumably due to the predominance of bulk carriers from crystal defects or thermal excitations. Topological insulator nanoribbons are promising candidate to manifest the surface effects due to larger surface-to-volume ratio than bulk materials. We provide unambiguous transport evidence of topological surface states from magnetoresistance measurements of topological insulator material Bi$_{2}$Se$_{3}$ nanoribbons. Pronounced Aharonov-Bohm magnetoresistance oscillations clearly demonstrate the coherent propagation of two-dimensional electrons around the perimeter of the nanoribbon surface, as expected from the topological nature of the surface states. [Preview Abstract] |
Wednesday, March 17, 2010 8:12AM - 8:24AM |
P16.00002: Shape-tunable electronic properties of H-passivated Si nanowires Abraham Hmiel, Yongqiang Xue In this work we explore the structure, energetics and electronic properties of hydrogen-passivated silicon nanowires (SiNWs) with different surface structures and growth directions using first principles density functional theory. We first demonstrated that [112]-oriented Si nanowires (SiNWs) show a indirect-to-direct band gap transition induced solely by varying the cross sectional aspect ratio for both monohydride and trihydride passivation, which is explained by the different confinement effects on the conduction band state at Gamma point by the two facets. By analyzing the free energy of formation, we found that direct band gap [112] SiNWs are thermodynamically possible to form at experimentally relevant conditions. We next study water monomer adsorption on [112] and [110] SiNWs. We present results on adsorption geometry/energy and adsorption-induced electronic effects and investigate their shape/diameter dependence. [Preview Abstract] |
Wednesday, March 17, 2010 8:24AM - 8:36AM |
P16.00003: Quantum confinement effect on Li-Segregation in Si nanowires Chuan-Ding Dong, Xin-Gao Gong, Ru-Qian Wu Silicon nanowires (SiNWs) have attracted extensive research attention due to their excellent properties that are promising for various applications. The incorporation of impurity atoms and change of size play a crucial role in the control of their electronic properties. Using density functional calculations, we studied the segregation behavior of Li atom in SiNWs with different sizes.~ It is striking that the doping and segregation energetics also strongly depend on the size of SiNWs. Doping Li into the nanowire with a diameter less than 1.3 nm is energetically unfavorable. In contrast, Li prefers to take the center sites in large SiNWs. The phenomenal size dependence can be understood through the quantum confinement effect toward the electronic states introduced by the Li atom. The reduced size also leads to strong quantum confinement for states at the conduction band minimum and increases their energies. We will discuss other properties of Li in SiNWs, which are important for applications such as using SiNWs as electrode materials in Li-batteries. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P16.00004: Energetics and electronic structure of double-walled boron nanotubes Hui Tang, Sohrab Ismail-Beigi Single-walled boron nanotubes have been studied extensively since their first successful fabrication in experiments. On the other hand, double-walled or multi-walled boron nanotubes have not yet been discussed in literature. Here, using density functional theory, we present a stable semiconducting two-dimensional double-layered boron sheet, which is 0.14 eV/atom more stable than the most stable single-layered $\alpha$-sheet [1]. This double-layered sheet is stabilized due to the formation of inter-layer bonds. We show that double-walled boron nanotubes made from this double-layered sheet are all semiconducting. These double-walled nanotubes are more stable than single-walled ones for large nanotubes, but become less energetically favorable when the tube radius is smaller than 20 {\AA} due to their large curvature energies. To reduce the large curvature energies, we construct double-walled nanotubes whose inner and outer walls have different number of atoms around their circumference. The resulting nanotubes are more stable than single-walled ones for all radii.\\[4pt] [1] H. Tang, and S. Ismail-Beigi, PRL 99, 115501 (2007). [Preview Abstract] |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P16.00005: Spin-filtering by field dependent resonant tunneling Zoran Ristivojevic We consider theoretically transport in a spinfull one-channel interacting quantum wire placed in an external magnetic field. For the case of two point-like impurities embedded in the wire, under a small voltage bias the spin-polarized current occurs at special points in the parameter space, tunable by a single parameter. At sufficiently low temperatures complete spin-polarization may be achieved, provided repulsive interaction between electrons is not too strong. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P16.00006: The Electronic Structure of Heterostructured and Superlatticed Si/Ge Nanowires: A Maximally-Localised Wannier Function Approach Matthew Shelley, Arash Mostofi In recent years, Si/Ge nanowires have generated much interest within condensed matter and electrical engineering communities due to the variety of tunable properties that they exhibit. Heterostructured or superlatticed Si/Ge nanowires have been identified as potential candidates for such thermoelectric applications\footnote{Li \emph{et al} Appl. Phys. Lett. \textbf{83} 3186 (2003)} and a detailed knowledge and understanding of their electronic structure would help exploit or maximise this effect. Modelling such systems is a serious challenge for traditional electronic structure methods, such as density functional theory (DFT). The study of systems that are non-periodic, or have very large periodic repeat units, is prohibitive with the traditional plane-wave (PW) formalism of DFT. We have therefore developed a method which combines the accuracy of large-scale PW-DFT calculations with the transferability of a compact basis of maximally-localised Wannier functions (MLWFs). Moving to a MLWF basis allows the Hamiltonians of fragments of a system to be combined to form model Hamiltonians of large disordered systems. We present results on heterostructured and superlatticed Si/Ge nanowires in the ballistic regime with a view to discuss their thermoelectric merit. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P16.00007: First Principle Study on Wurzite Core-shell Nanowires Heterostructures ZnO/ZnS. Tuoc Vu We present a first-principle study on the structural and electronic structure of II-VI wurtzite core-shell core-multishell ZnO/ZnS hexagonal unsaturated and saturated nanowire and examine the dependence of interface stress and formation energy on nanowire lateral size with diameter range from 20{\AA} up to 36.4{\AA}. We also calculated Young's modulus along growth direction and tensile test have been applied for various wires to show the diameter dependence's of their mechanical properties. The electronics properties of heterostructure nanowire (e.g. band structure, Density of State, charge transfer via Mulliken population analysis) also exhibit wire's diameter dependence behaviors.. [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P16.00008: A hybrid density functional study of double walled armchair SiC nanotubes Kapil Adhikari, Asok Ray As a continuation of our studies of single walled SiC nanotubes,\footnote{K. Alam and A. K. Ray, Nanotechnology, \textbf{18}, 495706 (2007); Physical Review B, \textbf{77}, 035436 (2008).} we report here a systematic study of double walled armchair type1 SiC nanotubes using the finite cluster approximation. The geometries of smaller armchair SiC nanotubes inside bigger armchair nanotubes have been spin optimized using the hybrid functional B3LYP (Becke's three-parameter exchange functional and the Lee-Yang-Parr exchange-correlation functional) and the Los Alamos National Laboratory double-$\zeta $ basis set as implemented in the GAUSSIAN 03 suite of programs. The results have been compared with published experimental and theoretical results in the literature on single walled and multi walled SiC nanotubes. A detailed study of binding energies, Mulliken charges, density of states and HOMO-LUMO gaps has been performed for type 1 SiC nanotube from (n,n)@(n+3,n+3) to (n,n)@(n+5,n+5) (n=3-7). Results on radial buckling and optimized diameter will also be reported to study and compare inter wall interactions between the two coaxial SiC nanotubes. Possibilities of work on other types of SiC nanotubes currently underway in our group will also be discussed. [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P16.00009: First-principles Investigation of CdSe Nanowires using Wannier functions:Effects of Surface and Confinement on dielectric properties Yosuke Kanai, Jeffrey Grossman, Giancarlo Cicero Understanding how the electronic properties of a material change at the nanoscale is important for a wide range of technological applications as well as for basic science. One-dimensional nano-structures such as nanowires hold great promise for their potential application in opto-electronic devices. In this work, we investigate the electronic polarity behavior and transverse polarizability of hexagonal Cadmium Selenide (CdSe) nanowires of up to 3 nm in diameter, using a Wannier function description from density functional theory calculations. We address effects of quantum confinement and surfactant molecules on these nanowire properties via a local property analysis using the Wannier functions, revealing a few interesting insights in terms of local polarity changes. Interestingly, the transverse polarizability is enhanced and deviates significantly from the classical model for a dielectric cylinder at this scale. We will discuss our observation in terms of surface and confinement effects. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P16.00010: Temperature Dependence of doped ZnO Nanowire Photoconductance Dongdong Li, Liang Zhao, Paichun Chang, Ruqian Wu, Jia Grace Lu ZnO nanowires doped with impurity atoms have been fabricated by pulsed laser-assisted chemical vapor deposition method. The as-synthesized nanowires are constructed as a four-probe field-effect transistor with a global back gate. The temperature dependant transport properties based on nanowire field effect transistor are investigated. The ionized impurity scattering is predominant at low temperatures, which has a temperature dependence of $T^{1.5}$. While at high temperatures, electron-acoustic phonon scattering dominates, yielding a temperature dependence of$T^{-1.5}$ The conductance is dramatically affected by light illuminations - HeNe Laser (632 nm) and UV (254 nm). A smooth transition can be observed under laser illumination, $i.e.$ the resistance reaches a minimum at certain temperature, then rises steadily with continued increase of temperature. This stems from the drastic reduction of electron mobility due to the enhanced electron-phonon interaction. In contrast, two reproducible jumps under UV irradiation are observed for all doped nanowires. This originates from the existence of deep defect levels, which locates more than 2 eV below the conduction band edge. They serve as charge traps for electron excitation from the valence band. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P16.00011: Anisotropic surface plasmon excitation in Au/silica nanowire by STEM-EELS C. T. Wu, C. W. Chen, K. H. Chen, L. C. Chen, M. -W. Chu, C. H. Chen Recently it was reported that Au nanoparticles encapsulated in silica or Ga$_{2}$O$_{3}$ nanowires (Au nano-peapods) exhibited remarkable enhancement of photo-induced conductivities when irradiated with a light source with wavelength close to the surface plasmon resonance (SPR) of Au at $\sim $2.4 eV. No photo-response of the Au nano-peapods was observed when excited with light sources with wavelengths far away from the SPR. These observations strongly suggested that excitation of SPR might be responsible for the enhanced photoconductivity. Here we report the SP excitations of gold nanoparticles embedded nicely in silica nanowires by electron energy-loss spectroscopy (EELS) in conjunction with a scanning transmission electron microscope (STEM). In this study, we'll show that the STEM-EELS and energy-filtered TEM are important tools with very high spatial resolution for the mapping of optical excitation of surface plasmon for novel-metal nano-particles. Furthermore, anisotropic SP excitation intensities were observed along the axial and radial directions of the Au-silica nanowire after corrections for the thickness effect. This anisotropy of SP excitations suggests that the photo-induced electronic transport may be concentrated in a region near the surface of the Au-silica nanowire. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P16.00012: Optical Properties of PbSe Nanowires E. Clifton, J.G. Tischler, E.E. Foos, T.J. Zega, R.M. Stroud, J.E. Boercker, C.D. Cress, Al L. Efros, S.C. Erwin Colloidal PbSe nanocrystals show potential as a material for high efficiency photovoltaics, for two reasons: (1) tunability of the band gap through a wide range in the near infrared, and (2) efficient multiexciton generation. The charge mobility of PbSe NCs is relatively high compared to other nanocrystalline materials, but is still far from optimal. One possible way to increase the mobility is to use nanowires instead of nanocrystals. To this end we have synthesized high-aspect-ratio ($>$100) PbSe nanowires in solution. Here we investigate the optical properties of these nanowires using photoluminescence and transmission. We observe clear quantum confinement, and demonstrate that the bandgap can be tuned over the range required, $\sim$0.4eV, for photovoltaic applications [1]. Finally, we investigate the evolution of quantum confinement when going from 0D to 1D by comparing the optical properties of nanocrystals and nanowires. [1] Schaller et al., Nanoletters 6, 424(2006) [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P16.00013: The Optical Properties of Free Standing Titania Nanotube Arrays Fabricated by Electrochemical Anodization Mohamed AbdElmoula, Latika Menon Titania nanotube arrays fabricated by anodization has become the main interest of many research groups, mainly due to its promising applications. Solar energy harvesting is one of the most anticipated applications, in which the light conversion to electron-hole pairs can be carried out on the surface of these nanotubes, taking advantage of its high aspect ratio and low recombination probability. In our work we are studying the optical properties of free sanding titania nanotubes arrays, which allows us to measure the optical properties of the nanotubes more precisely. We are investigating the transmission of light through the nanotubes for different tube lengths. This will allow us to find the most effective tube length for light maximum absorption. In order to carry out a systematic study we will investigate the propagation of light incident from one side of the film (nanotubes open side). We will also investigate the propagation of light incident from the other side of the film (nanotubes closed side), and the reflection of light from the titanium layer underneath the nanotube arrays. Also an investigation of the effect of annealing conditions on the optical properties will be carried out. As a result, we will report the most effective nanotube parameters that maximize the solar spectrum absorption. [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P16.00014: Doped ZnO nanowires and their optical properties Athavan Nadarajah, Rolf Konenkamp We report on the optical properties of doped ZnO nanowires grown at 80C. The incorporation of impurities, the annealing behavior as well as electro- and photoluminescence results are presented. The comparison of spectra obtained from temperature-dependent photoluminescence (PL) measurements before and after thermal annealing indicates that the optical activity of impurities changes noticeably upon annealing. The internal quantum efficiency for PL was measured to be as high as 16 percent for Al-doped samples annealed at 380C. The PL measurements also show that the excitonic luminescence is preferentially guided while the defect emission is more isotropically emitted. Visible electroluminescence (EL) from a hybrid p-n junction arrangement consisting of a hole-conducting polymer and n-type ZnO nanowires was achieved. The observed EL spectra show an ultra-violet excitonic emission peak and a broad defect-related emission band in the visible range. After annealing at 380C the defect related EL peak exhibits a characteristic shift to higher wavelengths. The magnitude of the shift is dependent on the dopant type. [Preview Abstract] |
Wednesday, March 17, 2010 10:48AM - 11:00AM |
P16.00015: Photochemical Properties of Titania Nanotube Arrays Ragen McAdoo, Mohamed Abd Elmoula, Latika Menon Titania nanotubes have been fabricated by means of electrochemical anodization of titanium foils in an electrolyte. The nanotube dimensions (diameter, wall thickness and length) can be controlled in our fabrication approach by varying the electrolyte used and by adjusting the anodization voltage. Gold nanoparticles have been attached on the nanotube surface by means of a modified deposition-precipitation method. By adjusting the time of deposition and concentration of the solution, a high deposition density of the gold particles with good control over the size of the gold nanoparticles has been obtained. Results on the photochemical properties of such nanotube arrays, both blank and the Au-deposited titania nanotubes under simulated solar radiation will be reported. [Preview Abstract] |
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