Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H20: Focus Session: Assembly of Nanowires and Related Structures |
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Sponsoring Units: DMP Chair: Ken Shih, University of Texas at Austin Room: Morial Convention Center 212 |
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H20.00001: Formation and Properties of CdS-Ag$_2$S Nanorod Superlattices Denis Demchenko, Richard Robinson, Bryce Sadtler, Lin-Wang Wang, A. Paul Alivisatos, Can Erdonmez The mechanism of formation of recently fabricated$^1$ ordered CdS-Ag$_2$S nanorod superlattices is explained and their elastic and electronic properties are predicted theoretically. We show that diffusion-limited growth of Ag$_2$S islands in CdS nanorods partially contributes to the observed ordering, but cannot account for the full extent of the ordering alone. The valence force field (VFF) model results for the nanostructure show significant repulsion between Ag$_2$S segments due to strain created by the lattice mismatch between the two materials. This suggests that the interplay between the chemical interface energy and strain drives the spontaneous pattern formation. A first principles calculation of the energy levels in the superlattice shows a nested band alignment. The nanorod superlattice therefore corresponds electronically to a sequence of quantum wells of Ag$_2$S separated by barriers of CdS. The minibands formed in such superlattices make them desirable for applications in the solar cells. 1. R. D. Robinson, B. Sadtler, D. O. Demchenko, C. K. Erdonmez, L.-W. Wang, and A. P. Alivisatos, {\em Science} {\bf 317}, 355 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H20.00002: Transport studies on ultrathin silicide nanowires Violeta Iancu, Changgan Zeng, Stephen Jesse, Arthur Baddorf, Hanno Weitering Minute amounts of yttrium deposited on a silicon (100) surface assemble into ultrathin YSi$_{2}$ nanowires. Ultrathin nanowires receive a great deal of interest due to their possible uses as interconnects in nano-electronic devices or as nano-electrodes to measure e.g. the transverse current across DNA molecules in nanofluidic channels. Here we present electrical conductance measurements of a nanowire bridged by macroscopic electrodes and the characterization of the nanowire/electrode contact by scanning probe microscopy. The stability of the nanowires after exposure to air, water, and a KCl solution is also addressed for future use in nanofluidics. Research was conducted in part at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. DOE. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H20.00003: Copper Incorporation into ZnO Nanowires Susie Eustis, Douglas Meier, Babak Nikoobakht The applications of zinc oxide (ZnO) nanowires (NWs) in devices are promising due to the optical, mechanical and electrical properties of these one-dimensional structures, but current uses are limited by the ability to produce high quality nanowires at desired locations. Copper is an attractive catalyst for generating zinc oxide nanowires due to the long length and high density of ZnO NWs produced. However, defects due to impurities, oxygen deficiencies, and structural defects lead to decreased optical and electrical transport. Photoluminescence (PL) microscopy found that ZnO NWs grown by high temperature evaporation on a bulk copper substrate display the expected band gap emission at 380nm. A larger visible emission is also observed in the PL spectrum around 520nm due to defect states. High-resolution transmission electron microscopy (HR-TEM) shows the ZnO nanowires are single crystalline with hexagonal structure. The metallic tip shows a polycrystalline structure in HR-TEM images. Energy dispersive x-ray spectroscopy (EDS) mapping and auger electron spectroscopy (AES) clearly show copper throughout the length of the nanowire, which is most likely responsible for the strong deep trap emission from these nanowires. AES also finds large amounts of oxygen on the surface of these NWs, a further contributor to defect emission states. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H20.00004: Self-organization of atom wires on vicinal surfaces Invited Speaker: Self-organization is possibly the best way to produce nanostructures in large quantities. This also holds for the ultimate 1D system, atom wires; they can be self-assembled in large arrays on vicinal Si surfaces. Such atom wire systems often show intriguing electronic properties such as competing charge density waves and spin-orbit split one-dimensional bands. However, because of their low dimensionality, these wires also frequently show profound thermodynamic fluctuations that limit their \textit{structural} uniformity and have a large influence on their electronic properties. Therefore, in this talk I will focus on structural fluctuations in Ga atom wires self-organized on the Si(112) surface. In these atom wires, strain-relieving adatom vacancies self-organize into meandering vacancy lines (VLs) similar to the well-known nx2 superstructures for Ge on Si(100). The average spacing between these line defects can be experimentally controlled continuously by adjusting the chemical potential $\mu $ of the Ga adatoms. Significant VL correlations are discovered in STM experiments that cannot be captured within a mean field analysis. These structural flucuations are well described by a new lattice model that combines Density Functional Theory (DFT) calculations for perfectly ordered structures with the fluctuating disorder seen in experiment, and the experimental control parameter $\mu $. This hybrid approach of lattice modeling and DFT can be applied to other examples of line defects in hetero-epitaxy, especially in cases where correlation effects are significant and a mean field approach is not valid. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H20.00005: Growth and Properties of Self-Aligned MgO Nanowires Elena Cimpoiasu, Robert F. Klie, Ryan A. Munden, Mark A. Reed A simple VLS route was used to produce self-aligned MgO nanowires on both polished crystalline (c-axis sapphire) and ceramic (alumina) surfaces. Growth on alumina produces vertically-aligned, very thin nanowires, indicating enhanced growth at the liquid-solid interface. Growth on polished sapphire results in faceted MgO nanowires which are perpendicular to the $r$-plane of sapphire and show evidence of competing vapor-solid growth mechanism. The difference in the morphology and structure of the nanowires grown using the two different substrates clearly illustrates the affect of substrate on the growth process. \textit{This work was partially supported by DARPA, by the Department of Homeland Security, and by the National Science Foundation.} [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H20.00006: Novel Nanocomposites for Energy and Electromagnetic Applications via 3D self-assembly Amit Goyal, Sung-Hun Wee, Yanfei Gao, Junsoo Shin, Karren More, Yuri Zuev, Claudia Cantoni, Jianxin Zhong, Malcolm Stocks Nanocomposites comprising ordered 3D arrays of nanodots of one type of ceramic material coherently embedded in another ceramic matrix comprise are expected to exhibit novel physical properties tunable by adjusting the overall composition, concentration, feature size and spatial ordering of the nanodots. Wide-ranging applications such as photovoltaics, solid state lighting, ultra-high density storage and high temperature superconductivity are of interest. We report here on a joint experimental, theoretical and computational study on achieving 3D ordering via 3D self-assembly of nanodots of a complex ceramic material within another complex ceramic material, such as 3D self-assembly of BaZrO$_{3}$ nanodots in REBCO superconducting films. Examples will also be given for other ceramic and metal/ceramic systems. In all cases 3D self-assembly was obtained in epitaxial thick films grown via pulsed laser ablation on single-crystal or single-crystal-like substrates. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H20.00007: Terrace-width Distributions on Vicinal Surfaces: Effective Attraction Between Noninteracting Touching Steps Rajesh Sathiyanarayanan, Ajmi BHadj Hammouda, T.L. Einstein Terrace-width distributions (TWDs) characterize equilibrium as well as non-equilibrium morphology of vicinal surfaces. Using the terrace-step-kink (TSK) model, we apply Monte Carlo simulations (both Metropolis and kinetic) to study TWDs. Steps interact via inverse-square entropic and elastic interactions. Steps which cannot touch each other (except at corners) have monatomic height; their configurations, correspond to the worldlines of fermions in 1D. The associated TWDs are well described by the generalized Wigner surmise. The fit parameter ($\varrho$) is directly related to the dimensionless energetic interaction strength ($\tilde{A}$).\footnote{Hailu Gebremariam et al., Phys. Rev. B {\bf 69}(2004)125404} If steps are allowed to touch, one can find some double (or greater) height steps. For closely spaced steps, this can alter the TWD considerably. We simulated the TWD of steps with $\tilde{A}=0$ but touching allowed. Our results indicate an effective attraction between steps, as reflected by a value of $\varrho$ significantly less than 2 (the $\tilde{A}=0$ value with touching forbidden). As expected, this effective attraction becomes weaker as the terraces become wider; we discuss the crossover behavior. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H20.00008: Study of Fe atomic chains on Pt(997) vicinal surface Elijah Ayieta, Ya.B Losovyj, Cheng Ruihua The vicinal substrates form the promising templates for low cost effective bottom-up fabrications of nanostructures. A variety of one-dimensional atomic chains can be synthesized on the stepped surfaces. The electronic structure of a vicinal surface plays a significant role in determining the physical properties of atomic chains on stepped surface as well as the surface morphology. The Pt(997) surface is cut $6.5^o$ of Pt(111)surface forming terraces width of 2nm. The surface of the substrate is then characterized using scanning tunneling microscopy STM and angle resolved photoemission spectroscopy from synchrotron radiation source. The data shows that the surface has uniform steps with no reconstructions. Electron confinement is observed with wave vector perpendicular to the step direction. Fe atomic chains were then carefully deposited on the surface of Pt(997) and then finally characterized The exchange splitting of Fe 3$d$ bands is estimated according to the photoemission spectroscopy data. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H20.00009: Thermoelectric properties of Bi$_{2}$Te$_{3}$ films and nanowire arrays Cheng Lung Chen, Yang Yuan Chen, P. C. Lee, C. T. Chen, S. R. Harutyunyan, S. J. Lai, C. D. Chen, S. J. Lin The n-type Bi$_{2}$Te$_{3}$ nanowire arrays with diameter $\sim $120 nm and thin films with thickness $\sim $10 $\mu $m have been fabricated by electrochemical deposition from nitric acid bath, containing bismuth nitrate and tellurium dioxide. Extensive characterizations of the morphology, structure, and composition of the films and nanowires were performed by means of SEM, XRD, EDS, and TEM. The films have nanocrystalline structure whereas the nanowires are single crystallines. The influence of microstructure on thermoelectric properties was investigated by comparison charge carrier transport in two mutually perpendicular crystallographic directions. The measurements of Seebeck coefficient and electrical resistivity were carried out in temperature region of 180 to 300 K. The highest value of $\sigma $S$^{2}$ = 840 $\mu $W/m-K$^{2}$ was obtained at 285 K for film. The electrical resistivity of an individual Bi$_{2}$Te$_{3}$ nanowire is $\sim $4.3 $\mu \Omega $-m using e-beam lithography technique. Based on the Seebeck coefficient obtained from nanowire arrays, the calculated value of $\sigma $S$^{2}$ for single nanowire is $\sim $1000 $\mu $Wm$^{-1}$K$^{-2}$ which is higher than that of the film. These results may help in designing processes for thermoelectric microdevices. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H20.00010: Highly Aligned Epitaxial Nanorods with a Checkerboard Pattern in Oxide Films S. Park, Y. Horibe, T. Asada, N. Lee, S-W. Cheong, L.S. Wielunski, T. Gustafsson, P.L. Bonanno, S.M. O'Malley, A.A. Sirenko, A. Kazimirov, T. Tanimura One of the central challenges of nano-science is fabrication of nano-scale structures with well-controlled architectures using planar thin-film technology. Herein, we report our discovery of a periodic nanometer-scale self-assembly in spinel films, fabricated by manipulating spontaneous phase separation and substrate strain. The films consist of two types of chemically-distinct nanorods with mutually coherent interfaces, perfectly aligned along the film growth direction. This unique three dimensional epitaxy process contrasts with a typical behavior in conventional growth of highly lattice-mismatched films, and thus provides an important route for film fabrication of nano-structured arrays with periodically varied electronic and magnetic properties. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H20.00011: Photo-Induced Nanowire Formation on Polarity Patterned Ferroelectric Domains -- Wavelength Dependence Yang Sun, Robert Nemanich This research is focused on the bottom-up growth of nanostructures on periodically polled (congruent) lithium niobate. The formation of silver nanowires has been reported through a photo-induced reaction of UV exposed lithium niobate immersed in an aqueous silver nitrate solution. The metallic wires assemble predominantly at the domain walls of the periodically polled lithium niobate. In this study the process has been studied with Hg lamp excitation using a set of wavelength filters. Depending on the wavelength, the process can exhibit deposition over the whole surface or predominantly at the domain boundaries as previously described. The research has been extended to explore the photo-induced deposition of copper using an aqueous copper sulfate solution The process of copper wire formation on lithium niobate is occasionally observed, which suggests a dependence on surface preparation. When the thickness of the solution layer is reduced and the intensity of the UV light is increased, the lithium niobate surface is essentially uniformly covered with nanosized dots. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H20.00012: Low melting metal-induced synthesis of multi-component III-V semiconducting nanowires. Romaneh Jalilian, Zhiqiang chen, Gamini Sumanasekera Semiconductor multi-component alloys provide a natural means of tuning the band gap and other parameters to optimize and extend the application of semiconductor electronic/optoelectronic devices. In this study, multi-component nanowires have been synthesized in vapor phase by laser ablation of solid targets consisting of initial bulk materials. Growth of nano-crystals is believed to seed from low melting metallic droplets generated from laser bombardment and heating of the target. Curved tips have been observed at one end of the nanowires which contain same elemental components as the body of the nanowires. This technique is proven to be a successful approach to eliminate the need for external catalyst which can have detrimental consequences affecting the performance of an optoelectronic device. As-synthesized nanowires were characterized using TEM, SEM, XRD, EDS, photo luminescence, Vis-IR absorption and Raman spectroscopy. Results of transport properties of individual nanowires will also be presented. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H20.00013: Si nanowires on Ag(110): an experimental and electronic structure studies A. Kara, B. Ealet, C. Leandri, H. Oughaddou, G. LeLay, B. Aufray Si nanowires images on Ag(110) were obtained using Scanning Tunneling Microscopy. Wires with tailored width and very large lengths were observed with the wires growing along the open channels of Ag(110). For those wires 5 lattice constants wide, we observed 4 distinct bright spots across the wire with a periodicity of 2 times the Ag nearest neighbor distance along the wire. From the theoretical side, we propose several structures of Si on Ag(110) where Si atoms occupy either single or double layers, with the second layer may be formed by single or pair of atoms; and the first layer may be commensurate or incommensurate with the substrate. Comparison between calculated and observed STM images will be presented and a detailed analysis of the electronic structure of all the proposed structures will be discussed. [Preview Abstract] |
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