Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V25: Nanowires III: Silicon Based Nanowires |
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Sponsoring Units: DCOMP DMP Chair: Peter Ecklund, Penn State University Room: LACC 501A |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V25.00001: Theory of quantum nanodevices induced by charged metallic nanowires in a Si(1-x)Ge(x)/Si heterostructure. Zinovi Gribnikov, George Haddad Modern nanoelectronics has giving birth to new quantum devices: QDs, QWrs, DQDs, QPCs, etc. As a rule, such devices are realized on the basis of modulation doped Al$_{x}$Ga$_{1-x}$As/GaAs or Si$_{1-x}$Ge$_{x}$/Si heterostructures controlled by negatively charged \textit{depleting }gates. Such a method of controlling is not obligatory for Si-based structures where the more effective \textit{accumulating }gates can be used. In this case, the modulation doping is eliminated and the gates move up from the structure surface to one of the levels in the multi-layered covering barrier. All the active devices are induced by the positively charged gates-nanowires (NWrs) parallel to the surface and placed on several levels in the barrier. Electrons from the peripheral n$^{+}$-contacts are delivered across the intermediate 2DEGs induced by the flat gates. The NWrs (parallel to each other or crossing) induce in the Si-QW parallel or crossing QWrs. The negatively charged NWrs in the barriers separate the induced 2DEGs and QWrs into different parts by gate-controlled tunnel barriers. QDs are induced under the intersection points of two positively charged NWrs or can be separated from the induced QWr by negative transverse NWrs. We calculate quantum states in the closed induced quantum elements (QD, DQD, Q ring) and estimate conductivities of the induced current-conducting devices: QWrs, DQWrs, QPCs, etc. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V25.00002: First Principles Calculations of the Structural, Electronic and Optical Properties of Silicon Nanowires Trinh Vo, Andrew Williamson, Giulia Galli First principles electronic structure calculations are used to investigate the electronic, structural and optical properties~of pure and germanium doped silicon nanowires. The effects of varying the wire diameter, growth direction and dopant concentration are investigated.~ Wires with diameters ranging from one to three nanometers and [001], [011] and [111] growth directions are studied. All the surfaces of the wires are hydrogen passivated and we investigate structures with dihydride, canted dihydride and 2x1 reconstructed structures. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V25.00003: Digital Pulse Force Microscopy Study Of Suspended SiC Nanowires Abdullah Alkhateeb, Devananda Gangadean, Daqing Zhang, David McIlroy, D. Eric Aston Silicon carbide nanowires could be potentially useful in designing nanomechanical systems which lead to the interest in understanding their mechanical properties. In this report the technique of digital pulse force microscopy (DPFM) has been used to analyze the mechanical properties of suspended SiC nanowires. The SiC nanowires were suspended on a silicon grating with trenches of 1.5 micron width and 1 micron height. Deflection measurements and hence calibrated force-distance curves along the length of the nanowire were obtained. Moreover, a deflection model for beams is discussed to understand the behavior of the nanowire during the deflection measurement and then to extract the elastic modulus for the nanowire. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V25.00004: Testing mechanical response of nanovolumes of material Catalin Picu, Deli Liu, Toh-Ming Lu, Gwo-Ching Wang The measurement of the mechanical properties of nanoscale volumes of materials is an important topic in nanotechnology. However, the fabrication of suitable nanosize test structures and the measurement of their mechanical properties are challenging. In this work, we grow isolated helical Si nanosprings and slanted straight Cu nanorods with wire diameter of about 200 nm and total length larger than 4 micrometers, which we use as test specimens. The structures, which are fabricated by the oblique angle deposition technique with substrate rotation, are fixed to the substrate at one end. The other end of the nanostructure is loaded in a purely mechanical test, using a tip-cantilever assembly attached to a conventional AFM. Both mechanical and electromechanical loading is performed. Appropriate modeling is used to interpret the results. The constitutive behavior of the material is derived and discussed. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V25.00005: Selective Fractionation of Nanowire Diameter by Centrifugation T.E. Trammell, K.W. Adu, H.R. Guti\'errez, Q. Xiong, E.C. Dickey, P.C. Eklund Small diameter semiconductor nanowires are excellent candidates for a new generation of optoelectronic devices. Many batch processes can produce a large amount of nanowires, but with a very wide diameter distribution. In this work, we show how centrifugation can be used in producing distinct diameter fractions from the as-grown diameter distribution. This separation technique was applied to wires of four different semiconductor materials (Si, Ge, GaP, ZnS). We observed that the mean diameter distribution of a given fraction systematically shifts to smaller nanowire diameters with increasing centrifugation time. Based on the classic equations of motion of particles in fluids, we have calculated the influence of different parameters such as solvent viscosity, nanowire material density, centrifugation time and speed on the final diameter distribution. This simple model will be shown to be a useful instrument for the selection of optimal centrifugation conditions for separating many distinct diameter fractions from a grown material. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V25.00006: Relative Stabilities of Silicon Nanowires of Different Orientations Alexander Tchernatinsky, Chris Leahy, Dmitry Migas, Ming Yu, Chacram Jayanthi, Shi-Yu Wu This work presents a theoretical study of equilibrium structures of silicon nanowires oriented along $<$001$>$, $<$110$>$, and $<$111$>$ directions with a particular attention to relative stabilities of these wires with respect to their diameters and their shapes. The diameters of nanowires considered in this work ranges from a few nm to ~ 10 nm. Because of the size of the system, we employ a semi-empirical Hamiltonian that: (i) captures electron screening and many-body effects via self-consistent (SC) and environment-dependent (ED) terms, and (ii) is built on the framework of the linear combination of atomic orbitals (LCAO). Initial configurations of wires were chosen carefully to minimize the number of dangling bonds. The reliability of the SCED-LCAO method was first established by comparing the results obtained from this method to ab-initio method for small-diameter silicon nanowires. To allow the relaxation of large-diameter wires up to 10 nm, a linear scaling algorithm is implemented within the SCED-LCAO molecular dynamics. Energetic considerations show that silicon nanowires oriented along the <011> direction are most stable in the diameter range from 3 to 10 nm and this result is in agreement with a recent experiment (C. M. Lieber et al., Nano Lett, 4, 433, 2004 ). [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V25.00007: Modeling transport through Si nanowires using a nonequilibrium Green function approach Fabiano Oyafuso, Paul von Allmen, Seungwon Lee, Gerhard Klimeck We present our recent work in modeling electronic transport through Si nanowires using an nonequilibrium Green function approach. Our calculation, which extends the 1D nanoelectronic modeling software (NEMO) to 3D, uses a 20-orbital, orthogonal, semi-empirical tight-binding basis in which coupling between orbitals of nearest neighbor atoms are position dependent, so that lattice deformations and alloys can be accurately modelled. In this work, we study two effects on coherent transport through Si nanowires that are naturally included within this atomistic description. First, the sensitivity of the transmission coefficient due to surface perturbations is examined. A comparison is made between conductance through a hydrogen-passivated nanowire and one in which dangling bonds are introduced. Second, a study of transport through Si(x)Ge(1-x) barriers is presented. It is found that the transport behavior is markedly different from that in which a uniform material is assumed. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V25.00008: Lattice Vibration in Silicon Nanowires Li Yang, Xinyuan Zhao, M.Y. Chou The lattice vibrations for silicon nanowires along the [110] direction are investigated by first-principles calculations. The distribution of phonon modes at the $\Gamma$ point is obtained for nanowires of various diameters. Two different frequency shifts are found for the optical modes and the collective modes, respectively. When the size of nanowires decreases, the frequencies of optical modes are red-shifted, while the frequencies of collective modes are blue-shifted. We provide an explanation for these trends based on a different quantum confinement effect. In addition, the controversy over clamped and free boundary conditions is resolved in light of our first-principles calculations. Finally, the relative Raman scattering activity in a small nanowire is evaluated. Based on these Raman modes, a practical method to estimate the size of nanowires is presented. We also find that quantum confinement considerably changes the sound velocity, which has a significant effect in the studies of transport properties in nanostructures. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V25.00009: Effects of Shape-Dependence and Diameter-Dependence on Photoluminescene properties of Silicon Nanowires and Porous Silicon Dmitri Migas, C.S. Jayanthi, S.Y. Wu It is well known that photoluminescence (PL) in porous silicon arise from silicon nanostructures. In this work in order to shed light on PL from porous silicon, we have carried out the first-principles electronic structure calculations of silicon nanowires (SiNWs). Specifically, we considered (001)-oriented SiNWs of different diameters and cross-sectional shapes with their dangling bonds passified by hydrogen atoms. Our work demonstrates that the PL originates from the inner core of hydrogenated SINWs, and that enhanced dipole matrix elements are obtained when the surface of the SiNW is characterized by dimers and covered by SiH species. However, we find that a surface covered by SiH$_{2}$ species obtained by breaking the dimer bonds helps in stabilizing the direct nature of the gap. This work also presents the interplay between the nature of the energy gap, the diameter, and the shape of the wire on the PL from porous silicon.. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V25.00010: Measurement of Gate-Modulated Thermoelectric Power in Si Nanowires Joshua Small, Meninder Purewal, Philip Kim We have measured the thermoelectric power (TEP) of individual Boron-doped p-type Si nanowires (SiNW). The measured TEP shows strong gate modulation as a function of gate voltage. The p-type SiNW shows positive TEP values at the turn-on regime gate voltage and a peak at the gate voltage in the subthreshold regime. We compare the electric and thermoelectric transport properties of SiNW's to CNT's and discuss their viability as good thermoelectric materials. [Preview Abstract] |
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V25.00011: An Ab Initio Study on Silicon and Germanium Nanotubes Prachi Pradhan, Asok K. Ray First principles calculations using hybrid density functional theory have been performed to examine the electronic and geometric structure properties of single-walled silicon (SWSiNT) and germanium (SWGeNT) nanotubes. Finite clusters $X_m H_n$(X = Si or Ge) are used to model the nanotubes ($e.g.$ the smallest SWSiNT is modeled as Si$_{60}$H$_{12}$). Hydrogen termination is done to simulate the effect of longer tubes as well as to take care of end effects. A pseudopotential basis set has been used for the silicon atoms$^{1}$ and complete geometry optimizations of the structures has been carried out using the Gaussian 03 suite of programs.$^{2}$ Computer simulations predict that the existence and stability of the nanotubes are highly dependent on the ratio of the \textit{sp}$^{2}$ to \textit{sp}$^{3}$ hybridization. Results will be presented on cohesive energies, HOMO- LUMO gaps, and other electronic structure properties and their dependence on the tube diameter. We will discuss the density of states (DOS) to explain the possible metallic or semi-conducting character of the tubes. Detailed comparisons with published data in the literature will also be presented. * Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525). $^{1 }$P. J. Hay and W. R. Wadt, J. Chem. Phys. \textbf{82}, 270 (1985). $^{2 }$\textit{Gaussian03}, Revision A.1, M. J. Frisch \textit{et al}.$,$ Gaussian Inc., Pittsburgh, PA , 2003. [Preview Abstract] |
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