Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W25: Nanowires IV: Nitrides and Others |
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Sponsoring Units: DCOMP DMP Chair: Andrea Trave, LLNL Room: LACC 501A |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W25.00001: Thermal conductivity of B-C-N and BN nanotubes Chih-Wei Chang, Wei-Qiang Han, Alex Zettl We have measured the temperature-dependent thermal conductivity \textit{$\kappa $(T)} of boron-carbon-nitride (B-C-N) and boron nitride (BN) nanotube mats between room temperature and 20K. For both materials, \textit{$\kappa $(T)} increases with increasing temperature, with no sign of saturation. We employ a new analysis method to estimate the intrinsic \textit{$\kappa $(T)} of BN nanotubes converted from B-C-N nanotubes, and find that at room temperature \textit{$\kappa $(T)} of a multi-walled BN nanotube is comparable to that of a multi-walled carbon nanotube. At low temperature, the functional form of \textit{$\kappa $(T)} reflects dimensional confinement. [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W25.00002: Band-gap and the Electronic structure of BN nanotubes David Okawa, Shaul Aloni, William Mickelson, Gavi Begtrup, Alex Zettl A verity of BN nanotubes were studied via high resolution electron energy loss spectroscopy. Careful examination of low energy loss spectra (0-50 eV) the band gap of the single tube can be extracted. Moreover the spectral features give insight into the BN nanotube valence band structure. [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W25.00003: First Success on Growing Boron Nitride Nanotubes on Substrates Yoke Khin Yap, Jiesheng Wang, Vijaya Kayastha We report on the first success of growing boron nitride nanotubes (BNNTs) on Si substrates at 600 \r{ }C. BNNTs are expected to have a band gap of 5.5 eV, which is independent of the tube diameter, number of walls, and chirality. Furthermore, it is possible to tune this band gap down to $\sim $1.3 eV by substitution of carbon. Previously, BNNTs have been synthesized by arc discharge, laser ablation, and chemical pyrolysis, typically at growth temperatures $>$1100 \r{ }C. These products are in a powder form with impurities including catalyst and BN particles. Now, we succeeded on the patterned growth of BNNTs at 600 \r{ }C. These BNNTs are grown by a RF-plasma assisted pulsed-laser deposition (PLD) technique. The growth of BNNTs is sensitive to the types of catalysts used, plasma density, growth temperatures, and laser pulsed energy. The growth sites of BNNTs are highly controllable by the catalyst. At optimum condition, a well-defined growth region of BNNTs has been identified and explained. According to high-resolution field-emission scanning electron microscopy (FESEM), these BNNTs are grown vertically aligned on the substrates in a base growth mode. This is confirmed by high-resolution transmission electron microscopy (HRTEM). These BNNTs are constructed of well-defined tubular structures with diameters $\sim $10 to 20 nm. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W25.00004: Effect of Screening on electronic properties of Boron Nitride Nanotube under an electric field in the proximity of a possible metal insulator transition Jay Sau, Marvin Cohen Previous theoretical calculations have shown that the band gap of boron nitride nanotubes can change significantly in a transverse electric field. For large nanotubes it is possible to close the LDA gap with moderately high field strengths. The conduction and valence states in this system are spatially separated creating the possibilty of long-lived excitons. LDA calculations indicate a metallic screening of the field after gap closure. However due to the innately weak screening in Boron Nitride and 1D geometry one can also expect excitonic and other correlation effects to be strong in this system. The screening properties are critical to understanding whether metallic ground state can be reached and whether the possibly long-lived excitons have large binding energies. We estimate the field induced modification of the screening using a tight-binding approach similar to the tight-binding GW/Bethe-Salpeter approach that has previously been used for nanocrystals. The implications of the screening on the possible electronic structures are also examined. This research is supported by National Science Foundation Grant No. DMR-39768, Office of Energy Research, the Office of Basic Energy Sciences and Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Computational Resources were provided by NPACI and by NERSC. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W25.00005: Ab initio study of the quasiparticle band structure and opticalproperties of bulk h-BN and BN nanotubes. Cheol Hwan Park, Catalin D. Spataru, Steven G. Louie In the crystalline phase, different stacking of the hexagonal basal layers affects the electronic properties of bulk h-BN significantly. We have studied this stacking effect by first performing calculations on the Kohn-Sham band structure within the local density approximation to the density functional theory. Next, we compute the quasiparticle band structure by including electron self-energy effects within the GW appromixation. Finally, we obtain the optical excitation spectrum including electron-hole interaction (excitonic) effects by solving the Bethe-Salpeter equation. For single- walled boron-nitride nanotubes, we have also carried out a similar series of calculations to obtain the optical response of these tubes as a function of diameter and chirality. This work was supported by the NSF under Grant No. DMR0087088, and the U.S. DOE under Contract No. DE-AC03-76SF00098. Computer time was provided by NERSC and NPACI. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W25.00006: Controlled growth of GaN nanowires by focused ion beam Pt-catalyst patterning Jinyong Kim, Chang-Yong Nam, Douglas Tham, John E. Fischer One-dimensional GaN nanostructures are promising materials for nano-device applications. Control of nucleation density and position is a critical issue to fabricate such devices using GaN nanowires (NWs). However, only a few studies of controlled GaN growth are reported so far.$^{2)}$ An attractive approach is to use the spatial selectivity and resolution of focused ion beam (FIB) to deposit platinum (Pt) catalyst seeds for GaN NW growth at desired positions. In the present work, we demonstrate the successful patterned growth of GaN NWs on SiO$_{2}$/Si substrates or SiN$_{x}$ TEM grids using FIB Pt-patterning. The thermal reaction of Ga$_{2}$O$_{3}$ and NH$_{3}$ on the substrates at $\sim$920$^{\circ}$C was used for GaN NW growth.$^{3)}$ Grown NWs were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We conclude that Pt acts as a catalyst for GaN NW growth, and that FIB provides the control over density and position of GaN NW growth, which ultimately can be utilized for nano-device fabrication. This work is supported by the NSF NIRT Program under Grant No. DMR-0304178 1) Y. Huang, X. Duan, Y. Cui, and C. M. Lieber, Nano Lett., \textbf{2(2)}, 2002, 101-104. 2) J. Li, C. Lu, B. Maynor, S. Huang, and J. Liu, Chem. Mater., \textbf{16}, 2004, 1633-1636. 3) C. Y. Nam, D. T. Tham and J. E. Fischer, Appl. Phys. Lett. (in press, December 6, 2004). [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W25.00007: Low Contact Resistance Ohmic Junctions in GaN Nanowire Devices by Rapid Thermal Annealing G. Chen, Qihua Xiong, H. R. Gutierrez, J.J. Zengel, J. Wu, S. Tadigadapa, P.C. Eklund, K. Byon, J. Fischer GaN nanowires grown by a thermal evaporation method using Au nanoparticles as catalysts on silicon or alumna substrates have been studied. The wires have typical diameter $\sim $ 10-40 nm and are 5-10 $\mu $m in length. The growth proceeds by the VLS mechanism. Electrical contacts (Ti/Au) to the wires on Si/SiO2 substrates were made by standard e-beam lithography, e-beam evaporation and lift-off procedures. The as-prepared devices usually exhibit I-V behavior consistent with Schottky barrier injection. However, using rapid thermal annealing (a few minutes) in vacuum at temperature in the range 450-600\r{ }C, we have been able to produce dramatically lower contact resistance and linear I-V ohmic connections to our n-type GaN nanowires. Field-effect transistor (FET) and 4-probe resistivity characteristics of the devices are presented over the temperature range 10-300K and the data are discussed in terms of the electronic structure of the GaN nanowires. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W25.00008: Deterministic Synthesis and assembly of Germanium Nanowires Dunwei Wang, Hongjie Dai During the past decade, Ge has gained renewed interest due to its high carrier mobilities and small band gap, which is ideal especially for device scaling down to sub-100nm range. On the other hand, chemically derived low dimensional materials such as nanotubes and nanowires are attractive because of their easiness in produce, structure perfection and unique electronic properties. However, relatively few studies have been carried out on controlled synthesis of Ge nanowires (GeNWs). We demonstrate that controlled synthesis of high quality GeNWs can be performed at low temperatures below 300\r{ }C via a chemical vapor deposition reaction. 100{\%} yield with regard to catalyst particles was obtained for the first time, and this growth technique can be extended to produce grams of GeNWs in a small reaction. Sizes of GeNWs are determined by catalyst particle sizes and length is determined by growth time. As-produced GeNWs were also shown to be excellent field effect transistor materials. In addition, various post-growth assembly methods were studied and aligned GeNWs with tunable density were obtained. This work provides insights in growth mechanisms of nanowires in general and it can be applied to produce large volume of single crystalline GeNWs. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W25.00009: Growth and optical properties of GaAs-GaInP core-shell nanowires N. Skold, M.-E. Pistol, J. Tragardh, L. Samuelson, M. Larsson Nanowires are potentially interesting as ideal systems for 1D-transport of charge and excitons. They may also serve as waveguides for light. Adding a large band gap shell to a nanowire can improve several properties. The shell reduces the effect of surface states, enhances emission efficiency and improves the cavity properties of the nanowire. If the shell is grown lattice-mismatched to the core, elastic strain induced in the core offers flexibility in band structure engineering. We have synthesized GaAs-Ga$_{x}$In$_{1-x}$P core-shell nanowires by metal-organic vapor phase epitaxy in order to study such phenomena. The nanowire core was grown in the vapor-liquid-solid growth mode, size controlled by, and seeded from, Au aerosol particles. The core was grown at a low temperature where almost no growth takes place on the side facets. The shell was grown at a higher temperature where the kinetic hindrance of the side facet growth can be overcome. Photoluminescence measurements on individual nanowires (at 5 K) show enhanced emission efficiency from the core compared to uncapped samples. Luminescence from the shell indicates alloy ordering. Strain induced in the core by lattice-mismatched shells was studied and confirmed by deformation potential calculations. Time-resolved measurements will also be presented. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W25.00010: Band-gap engineering in III-V nanowires Ann I. Persson, Mikael T. Bj\"ork, S\"oren Jeppesen, Magnus W. Larsson, L. Reine Wallenberg, Lars Samuelson Freestanding nanowires can be used for probing the physics of one-dimensional transport, as well as creating new devices based on quantum physics. Growth of epitaxially nucleated nanowires is usually described by the vapor-liquid-solid (VLS) growth mechanism, where metallic seed particles, are used to form a eutectic system with the growth material. We have strong indications that the seed particle, often described as being in liquid phase, is in solid phase and that we have a growth regime different from VLS, based on a solid-phase diffusion. Chemical Beam Epitaxy is used as growth technique, with the advantage of a fast response in the flow at the substrate surface and a slow growth rate. This makes it possible to realize size-controlled heterostructures with atomically abrupt changes in materials within a single nanowire. Here we present results of InAs$_{1-x}$P$_{x}$ heterostructures in InAs nanowires, showing the alloying mechanism of As and P as a function of the As/P ratio. The results have been characterized by electrical measurements and TEM giving us information necessary to carry out band-gap engineering in nanowires based on the InAs$_{1-x}$P$_{x}$ system. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W25.00011: Light scattering study of acoustic phonons in 300nm wide rectangular wires Anthony Link, Rudra Bandhu, R. Sooryakumar, Konrad Bussmann We have investigated the acoustic properties of 300 nm wide polymethyl methaacrylate (PMMA) rectangular wires supported on a 100 nm thick silicon nitride (Si$_{3}$N$_{4})$ free-standing membrane by Brillouin light scattering. The large elastic mismatch between PMMA and Si$_{3}$N$_{4 }$results in effective confinement of most low frequency acoustic modes ($<$ 20 GHz) to within the boundaries of the PMMA wire. For wave-vector transfer along the wire width, dispersionless excitations arising from mode quantization along the width and thickness of the PMMA wires are observed. In addition, dispersive signatures of the flexural mode of the Si$_{3}$N$_{4 }$membrane are evident. The mode amplitudes were analyzed using the \textit{xyz }algorithm. Our results reveal the quantized character of the modes and the presence of dilatational, torsional, shear and edge type modes. The results are compared to the behavior of acoustic modes propagating along the wire axis. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W25.00012: Molecular Electronics with Peptide Nanotubes: Calculation of Conventional and Complex Band Structure Ribeka Takahashi, Hao Wang, James Lewis We have studied the electronic structure of three peptide nanotubes which have been proposed as a potential nanowire. We choose nanotubes which have different sequences of amino-acid residue: (L-Gln, D-Ala)$_{4}$, (L-Gln, D-Leu)$_{4}$, and (L-Gln, D-Phe)$_{4}$. We calculated the conventional band structure and the complex band structure of the peptide nanotubes. Our study of the conventional band structure shows that the peptide nanotubes generally have large band gaps (4 eV); however, the band gap decreases due to the presence of aromatic rings in the side chain of peptide nanotubes. Our study of the complex band structure predicts that the molecular tunneling conductivity of peptide nanotubes is small. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W25.00013: Thinnest Insulating and Semiconducting Nanotubes of SiOx (x = 1 and 2) Abhishek Singh, Vijay Kumar, Yoshiyuki Kawazoe While carbon nanotubes have been extensively studied, nanotubes of inorganic materials are of much interest due to their novel properties. Among these silica nanotubes are of special interest because of their hydrophilic nature, easy colloidal suspension formation, and surface functionalization. Here we show from \textit{ab initio} total energy calculations using ultrasoft pseudopotential plane wave method that nanotubes of SiOx (x = 1 and 2) can be formed with different diameters and lengths. The optimizations have been performed using conjugate gradient technique within the generalized gradient approximation for the exchange-correlation energy. We have studied finite and infinite nanotubes of SiO$_{x}$ in triangular, square, pentagonal and hexagonal crossection. Further we show that for both stoichiometries, the pentagonal nanotubes are energetically most favorable. The bonding nature in these nanotubes is predominantly covalent. The band structure of infinite SiO nanotube shows it to be semiconducting. However, SiO$_{2}$ nanotubes are insulating. These are the thinnest possible nanotubes of SiO$_{x}$. These nanotubes along could be the ideal cicuit element for the nano-electronics. [Preview Abstract] |
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W25.00014: Epitaxial GaN Nanorod Array Formation on (111) Si by MBE H. W. Seo, Q. Y. Chen, X. M. Wang, L. W. Tu, C. L. Hsiao, M. Chen, M. N. Iliev, Wei-Kan Chu We have studied the nanorod and nanotrench formation of GaN by molecular beam epitaxy (MBE) on (111)-Si substrates. The chemical makeup of the substrate surface is crucial to the creation or diminishing of the nanostructure. We have used ion implantations to tailor the surface chemistry or surface structure on which the nanorods are formed. We will discuss the growth mechanism and its implications for large scale nano array fabrications. [Preview Abstract] |
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