### Session U35: Nanowires

 Thursday, March 16, 2006 8:00AM - 8:12AM U35.00001: Dielectrophoretic assembly of reversible and irreversible metal nanowire networks and vertically-aligned arrays Stergios J. Papadakis , Zhiyong Gu , David H. Gracias We demonstrate dielectrophoretic control of metallic nanowires in liquid suspensions. By varying parameters including the magnitude and frequency of the applied electric field, the liquid suspending the nanotubes, the nanowire metal, and the flow conditions, we can generate sparse or dense networks, multiply branching or predominately end-to-end networks, and vertically aligned nanowires standing on top of metal electrodes. The networks can be made reversibly or irreversibly. These results demonstrate the applicability of dielectrophoresis in aligning and positioning nanowires, either in the plane of the substrate or perpendicular to it, thereby suggesting a simple and versatile strategy for fabricating a range of integrated devices composed of nanowires. For example, sparse end-to-end networks are promising for individual electronic devices, dense branching networks take advantage of the large surface-to-volume ratio of nanowires for use as sensors, vertically-aligned arrays of nanowires may be used as vertical interconnects in damascene integration of microelectronic devices or in controlling the flow of fluid or light in microfluidic or nanophotonic devices, etc. Thursday, March 16, 2006 8:12AM - 8:24AM U35.00002: The Dielectrophoretically Guided Growth of Submicron, Near Single Crystal Indium Wires Bret Flanders , Ishan Talukdar , Birol Ozturk , Tetsuya Mishima Dielectrophoresis was used to direct the growth of crystalline indium wires between lithographic electrodes immersed in solutions of indium acetate. Determination of the conditions that suppress side branching on these structures has enabled the fabrication of arbitrarily long needle-shaped wires with diameters between 367nm and 556nm. Electron diffraction studies indicate that these wires are crystalline indium, that the unbranched wire segments are single-crystal domains, and that the predominant growth-direction is near $\langle$110$\rangle$. This outcome constitutes a critical step towards the use of simply prepared aqueous mixtures as a convenient means of controlling the composition of submicron, crystalline wires. Thursday, March 16, 2006 8:24AM - 8:36AM U35.00003: Growth Characteristics of Dielectrophoretically Fabricated Single Crystal Wires Ishan Talukdar , Birol Ozturk , Bret Flanders We investigate the mechanism underlying the dielectrophoretically guided growth of single-crystal metallic wires in aqueous solutions of metal-salts. The weak dependence of the mass deposition rate on the growth velocity suggests that the growth mechanism is consistent with microscopic solvability theory. We have also observed interesting dependencies of the growth velocities and the radii of these wires on the frequency of the dielectrophoretic voltage, and will report on our progress towards understanding these phenomena. The dependence of the wire's radius on the frequency is of potential technological interest in that it provides a sensitive means of controlling the submicron wire diameter. Thursday, March 16, 2006 8:36AM - 8:48AM U35.00004: Self-assembly of Eu$_{2}$O$_{3}$ nanoneedles and nanocrystals James Dickerson , Sameer Mahajan , Marcela Redigolo , Dmitry Koktysh Growth of anisotropic nanostructures starting from isotropic nanostructures is observed in Eu$_{2}$O$_{3}$ system. Anisotropic structures, like nanoneedles and nanospindles, are grown from the concentrated solution of Eu$_{2}$O$_{3}$ nanocrystals. This process involved the stepwise thermal growth of nanocrystals into ordered, high aspect ratio, one dimensional nanoneedles and the subsequent assembly of said nanoneedles into larger, oriented bundles (nanospindles). The Eu$_{2}$O$_{3}$ nanocrystals were synthesized following a room temperature, colloidal chemistry procedure, adapted from the synthesis of G. Wakefield \textit{et al}.$^{1}$ We present the results of this self-assembly phenomenon using 4-nm Eu$_{2}$O$_{3}$ nanocrystals. High resolution transmission electron microscopy was employed to characterize the approximate shape, size distribution, and crystallinity of the nanostructures. Absorption and photoluminescence measurements were performed to investigate what effect the size and shape of materials has on optical properties. 1. G. Wakefield, H. A. Keron, P. J. Dobson, and J. L. Hutchison, J. of Coll. Interf. Sci. \textbf{215}, 179, 1999. Thursday, March 16, 2006 8:48AM - 9:00AM U35.00005: Three-dimensional nanoscale composition mapping of semiconductor nanowires. Lincoln Lauhon , Daniel Perea , Jonathan Allen , Steven May , Bruce Wessels , David Seidman The composition of a single InAs nanowire was mapped in three dimensions with single-atom sensitivity and sub-nanometer resolution using local-electrode atom probe (LEAP) tomography. Arrays of epitaxial InAs nanowires were grown by chemical vapor deposition on GaAs substrates. Nanowires with diameters of 10-20 nm were analyzed over lengths of hundreds of nanometers. Three-dimensional reconstructions of the atoms in the nanowire showed hexagonal faceting, indicating that the LEAP analysis accurately reproduces the cross-section and shape of the nanowires. The Au catalyst particle sitting atop a nanowire was also analyzed; tomographic slices across the nanowire diameter, when displayed in 0.5 nm increments along the growth axis, revealed an extremely abrupt catalyst-nanowire interface that is also very flat. Despite the abruptness of the catalyst-nanowire interface, individual gold atoms were detected within the nanowire at a concentration of 100 ppm. These results indicate that LEAP microscopy can be used to (1) image buried nanowire interfaces in three dimensions and (2) analyze the concentration and distribution of dopant and impurity atoms in nanowires. Thursday, March 16, 2006 9:00AM - 9:12AM U35.00006: Ligand Functionality to Control Morphology, Solubility, and Assembly Behavior of CdSe Nanorods Wei Wang , Shengguo Jia , Sarbajit Banerjee , Michael Steigerwald , Irving Herman By varying the length of the carbon chain of the ligand, while keeping other reaction parameters the same, CdSe nanorods with different diameters and lengths, branched nanorods, and even nano-arrows have been synthesized. Since all the ligands used are phosphonic acids with the same binding group, the length of the carbon chain of the ligand can dramatically change the size and morphology of the nanocrystals. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been used to investigate the chemical composition of the nanorods. CdSe nanorod thin films have been made in hexane by electrophoretic deposition. The number and length of the ligands can dramatically change solubility, electrophoretic mobility of the nanorods, and quality of the nanorod film. We shall discuss mechanisms for ligand control of the nanocrystal structure. This work is supported by the MRSEC program of the National Science Foundation under Award No. DMR-0213574 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR). Thursday, March 16, 2006 9:12AM - 9:24AM U35.00007: Twinning Superlattices in Semiconducting Nanowires Qihua Xiong , J. Wang , Peter Eklund We report the first observation of quasi-periodic twinning superlattices (TSLs) in semiconducting nanowires. The periodicity of the superlattice appears to be controlled by $\Delta$T=T$_{m }$- T, i.e., the degree of undercooling of the liquid phase in contact with the solid phase during VLS growth, where T$_{m}$ is the melting point of the solid phase. We present results from two III-V systems (GaP, InP) in which the superlattice is generated by the periodic 180\r{ } flipping of the $<$112$>$ direction relative to the $<$111$>$ growth direction of the nanowire. We suspect that our observations mean that a TSL structure can be grown in many compound semiconducting nanowire systems. Control of the superlattice period should allow significant design possibilities for thermoelectric, electronic and electro-optic applications. This work was supported, in part, by NSF-NIRT DMR-0304178 Thursday, March 16, 2006 9:24AM - 9:36AM U35.00008: First Principles Studies of the Structural and Opto-Electronic Properties of Silicon Nanowires Trinh Vo , Andrew Williamson , Giulia Galli We report the results of first principles studies of the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires with [001], [011], and [111] growth directions and diameters ranging from 1 to 3 nm. We show that the growth direction, diameter, and surface structure all have a significant effect on the structural stability, electronic band gap, band structure, and band edge effective masses of the nanowires. The band gap is found to decrease with increasing diameter and to be further reduced by surface reconstruction. The band gap is direct for [001] and [011] NWs with diameters smaller than 3nm and [111] NWs with diameters smaller than 2nm. While the electron and hole effective masses are found to depend on NW size for [001] and [111] NWs, they are almost independent of size for [011] NWs. Finally, we use FEFF calculations to predict the EXAFS spectra produced by the relaxed atomic structure of the NWs and show that these spectra can serve as a tool for detection of surface reconstructions of the NWs. Thursday, March 16, 2006 9:36AM - 9:48AM U35.00009: Ab initio investigation into the stability and electronic properties of GaN-nanowires D.J. Carter , J.D. Gale , B. Delley , C. Stampfl Recent reports of successful fabrication of high quality gallium nanostructures such as quantum dots, nanocrystallites and nanowires, eg. [1], open the door to their possible role as important nanoscale building blocks for future optoelectronic, high-temperature/power and spintronic device applications. In the present work we perform \textit{ab initio} calculations, using the DMol$^{3}$ [2] and SIESTA [3] codes, for wurtzite GaN nanowires. We have examined nanowires in the [0001], $[10\bar {1}0]$, and $[11\bar {2}0]$ directions, and investigated the stability, electronic and atomic structures as a function of nanowire radius. We found that only nanowires in the [0001] direction are stable, and that wires in the other directions can be stabilised by saturating dangling bonds with hydrogen. We have also investigated the properties of key point defects and dopants. [1] J. C. Johnson, \textit{et al.} Nature Materials \textbf{1}, 106 (2002). [2] B. Delley, J. Chem. Phys. \textbf{92}, 508 (1990); \textit{ibid} \textbf{113}, 7756 (2000). [3] J.M. Soler, \textit{et al}. J. Phys.: Condens. Matter. \textbf{14}, 2745 (2002). Thursday, March 16, 2006 9:48AM - 10:00AM U35.00010: Au wetting of GaAs(111) studied on the atomic scale Emelie Hilner , Anders Mikkelsen , Jessica Eriksson , Edvin Lundgren , Hongsuk Yi , Peter Kratzer Because of the potential importance of semiconductor nanowires in future devices, a full understanding of their growth mechanism is desired. Au is commonly used to catalyze III-V nanowire growth, however the exact role of the Au in the growth process of the resulting wire is not known. This makes atomic scale studies of the Au/GaAs interface system well motivated. Here we report on the interaction of thin Au films and Au aerosol nanoparticles with the GaAs(111)B surface on the atomic scale using STM, LEED,LEEM, PEEM and ab-initio DFT calculations. We show that after deposition of Au either as a thin film or as aerosol nanoparticles and subsequent annealing to 450-600\r{ }C, a well ordered ($\surd$3x$\surd$3)R30$^{o}$ structure is formed. A structural model with a Au atom on every third threefold hollow hcp site of the Ga lattice is proposed based on theoretical calculations and experimental data.