Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W36: Nanoscale Complex Structures |
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Sponsoring Units: DCMP Chair: Wei Pan, Sandia National Laboratories Room: 408 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W36.00001: Conductance of a fully equilibrated quantum wire Tobias Micklitz, Jerome Rech, K. A. Matveev We study electronic transport properties of a long weakly interacting homogeneous quantum wire, connected to non-interacting leads. From Galilean invariance of the system we infer that in a state with a finite electric current, the electrons reach thermal equilibrium in a frame moving with their drift velocity. At non-zero temperature the resulting distribution function inside the wire is slightly different from the distribution supplied by the leads. This gives rise to a small correction to the quantized value of conductance $2e^2/h$, which can be found by performing a careful analysis of the conservation laws. The correction is of the order of $\left(T/E_F\right)^2$ and does not depend on the details of the electron-electron interaction. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W36.00002: Simulation of electron conduction in a prototypical three-terminal molecular transistor Haiying He, Ravindra Pandey, Shashi Karna In a single molecule, electronic charge can be modulated either by electrical field or by chemical effects, thereby opening up the possibility of their use as active elements in electronic devices. In this talk, we present the results of a theoretical study on the electronic conduction of a novel, three-terminal molecular architecture, analogous to a heterojunction bipolar transistor. In this architecture, two diode arms consisting of donor-acceptor molecular wires fuse through a ring, while a gate modulating wire is a $\pi $-conjugated wire. The calculated results show the enhancement or depletion mode of a transistor by applying a gate field along the positive or negative direction. A small gate field is required to switch on the current in the proposed architecture. The changes in the electronic conduction can be attributed to the intrinsic dipolar molecular architecture in terms of the evolution of molecular wavefunctions, specifically the one associated with the terphenyl group of the modulating wire in the presence of the gate field. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W36.00003: Synthesis and metal-to-semiconductor conversion of carbon nanotubes by light irradiation Lewis Gomez De Arco, Akshay Kumar, Yi Zhang, Koungmin Ryu, Alexander Badmaev, Chongwu Zhou We report on the synthesis of aligned nanotubes on Sapphire and quartz substrates, transfer, device fabrication and scalable metal-to-semiconductor conversion of carbon nanotubes on field-effect transistor (CNTFETs) channels by broadband light irradiation at environmental conditions. Inactivation of metallic nanotubes in the channels was achieved as a consequence of a light-assisted photochemical process that led to a controlled sp$^{2}$ to sp$^{3}$ transition in the nanotubes structure, and hence localization of $\pi $-electrons. Stronger gate bias dependence with improvements in the drain current On/Off ratio up to 10$^{5}$ was found in around 90 percent of the CNTFETs. The possibility of fabricating mostly semiconducting carbon nanotube transistors by simple light irradiation in air over entire wafers constitutes an important achievement in terms of assembly, integration and large scale fabrication of nanotube-based circuits. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W36.00004: Spectroscopic and structural studies of L-arginine doped Potassium Dihydrogen Phosphate crystals Jayesh Govani, Cristian Botez, William Durrer, Felicia Manciu We report in this study the spectroscopic and structural characterization of standard and L-arginine doped potassium dihydrogen phosphate crystals synthesized by a solution growth technique. The infrared absorption and Raman results demonstrate chemical functionalization between the amino (NH$_{3}^{+})$ groups of the organic material and the phosphate units of the inorganic crystals. This affirmation, which also implies the achievement of successful doping, is supported by the existence of extra vibrational lines in the IR and Raman spectra of L-arginine doped potassium dihydrogen phosphate crystals; these vibrational lines exhibit shifting towards lower frequencies as compared with the characteristic bands of L-arginine. Incorporation of the amino acid into the structure of the inorganic material is revealed by X-ray diffraction results also, where the shifting of diffraction lines and the appearance of a new one are observed. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W36.00005: Heterborane Analogs of Silicon clusters: Experimental and Theoretical Studies on Bi$_{2}$Si$_{n}$ Kiran Boggavarapu, Miley Jackson, Xiang Li, Andrej Grubisic, Di Wang, Kit Bowen, Anil Kandalam, Haopeng Wang Despite numerous studies, silicon clusters continue to fascinate. Part of the intrigue comes from the fact that, unlike metallic clusters which have strongly delocalized electrons and prefer to follow simple electron counting rules such as those originating from Jellium models, there are no simple rules of thumb that can be used to understand the diverse structures of silicon clusters. However, over the last couple of decades, there have been attempts to connect the structure and bonding of silicon clusters to a large class of well-studied three dimensional boron hydride compounds namely, \textit{closo-}boranes, B$_{n}$H$_{n}^{2-}$. By equating the $\sigma $-lone pair of divalent silicon to a B-H bond, it was shown that the frontier orbitals of both units are similar. Theoretical studies have concluded that the silicon clusters (Sin$^{2-})$ adopt similar structural patterns to those of boranes, when n = 5, 6, 7, 8, 10 and 13. The question then arises, whether neutral analogs of Si$_{n}^{2-}$ and neutral heteroboranes, X$_{2}$B$_{n}$H$_{n}$ (X = N, P, Sb, Bi), can be envisioned. Here, we present the scope and limitation such analogy based on our recent theoretical (DFT) and experimental (anion-photoelectron spectroscopy) studies on Bi$_{2}$Si$_{n}$ (n = 4-8). In particular, we show that that both Bi$_{2}$Si$_{5}$ and Bi$_{2}$B$_{5}$H$_{5}$ adopt similar pentagonal bipyrmidal (PBP) geometries and have analogous orbital energy patterns. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W36.00006: In situ characterization of crystal structure and physical properties of individual nanostructures in as-fabricated devices Marcel Lucas, Zhong Lin Wang, Elisa Riedo Nanostructures have potential applications as electronic components, catalysts, sensors, biomarkers, and energy harvesters. Control over their morphology and structure is essential, since their physical properties depend on their dimensions and crystallographic structure. Although in situ transmission electron microscopy can correlate the structure and physical properties of individual one-dimensional nanostructures, it usually damages the sample and is unable to recover the characterized nanostructure for next-step device fabrication and application. Here, we demonstrate a method combining atomic force microscopy and polarized Raman spectroscopy to characterize in situ the morphology, crystal structure and physical properties of individual nanostructures that can be as-fabricated devices without sample damage. Based on scanning probe microscopy, our method can be extended to study the electronic, mechanical, and tribological properties of inorganic/biological nanostructures. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W36.00007: Simulating self-assembly of porphyrin nanorods Gregory K. Guthe, Adam V. Subhas, Walter F. Smith, Joshua Schrier Diacid meso-tetra(4-sulfonatophenyl)porphine (TPPS$_{4}^{2-}$ ) monomers have been shown to self assemble into nanorods with well-defined cross-section$^{1}$ and intriguing photoelectronic properties$^{2}$. However, the structure and conduction mechanism of these nanorods is poorly understood, and questions remain about the aggregation process. Using density functional theory (DFT), we first obtain optimized geometries and atomic-charges for the monomers, which we then use for subsequent molecular dynamics (MD) simulations to observe the initial stages of the self-assembly process. This work uses the resources of the National Energy Research Scientific Computing Center. $^{1}$A.D. Schwab \textit{et al.}, J. Phys. Chem. B \textbf{107}, 11339 (2003). $^{2}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W36.00008: Electric field directed growth of cuprous oxide nanostructures for photon sensing Sangeeta Sahoo, Saroj Nayak, Pulickel Ajayan We demonstrate an electro-deposition technique to synthesize cuprous oxide nanomaterials in various types of nanostructural form at room temperature. We apply an electric field under de-ionized water between two electrodes one of which is made of Cu. Using this method, direct growth of nanostructures has been achieved on different types of substrates. We show that the structural evolution depends strongly on the electric potential between the electrodes and also on the type of substrates. We have studied the growth mechanism on flat Si substrate and on Transmission electron Microscopy grid. A variety of structures from simple one dimensional nanowires to different complex two and three dimensional structures are successfully grown directly on substrates with this method. Direct integration of these nanostructures on Si substrate brings us one step ahead towards the fabrication of electronic devices. Taken together, this novel technique of Cu$_{2}$O nanostructure production is highly reproducible, catalysts free, fast and a low cost simple process. In addition, the electrical characteristics indicate the usefulness of these structures for photo-sensing and optoelectronic applications. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W36.00009: Ca$_{1+\varepsilon }$Co$_{4}$B$_{4}$ and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$: New Borides with One-Dimensional Channel Structures Yukari Katsura, Hiraku Ogino, Yutaka Matsumura, Kazumasa Sugiyama, Toetsu Shishido, Shigeru Horii, Jun-ichi Shimoyama, Kohji Kishio We discovered two novel borides Ca$_{1+\varepsilon }$Co$_{4}$B$_{4}$ (\textit{$\varepsilon $} $\sim $0.10) and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$ (\textit{$\varepsilon $} $\sim $0.18) as the first members of Gd$_{1+\varepsilon }$Fe$_{4}$B$_{4}$- and Pr$_{1+\varepsilon }$Re$_{4}$B$_{4}$-type ternary borides with a divalent metal at the rare earth sites. In these compounds, tetrahedral chains of transition metals and boron form tetragonal channel structures, which contain single atomic chains of Ca. These are composite structures of Ca sublattice and CoB/RuB sublattice, with common $a$-axis lengths and independent $c$-axis lengths. The two structural types are distinguished by configurations of the tetrahedral chains. Resistivity and magnetization measurements showed that these compounds are paramagnetic metals down to 2 K. Preliminary first-principle calculations indicated the presence of covalent bonds between transition metals and boron, and electrical conductivity originating from the $d$-bands of the transition metals. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W36.00010: Photoluminescence Studies of Hydrogenated a-Silicon Carbide Somilkumar Rathi, Feng Zhu, Joshua Gallon, Brian Simonds, George Radziszewski, P. Craig Taylor A series of PECVD grown silicon carbide samples differing in their carbon content was investigated by a near-IR FT-Photoluminescence (PL) technique. The goal of this study was to establish a correlation between carbon concentration in a-SiC and the observed luminescence signal. Variations in the observed temperature dependences of the PL signals in a-SiC are attributed to differences in the carbon content. The samples, initially kept at 18 K on a closed-cycle helium cryostat, were excited with Ar-ion laser light at 514.5 nm (with power ranging 5-30 mW; and a fluence of 0.5 to 3 W/cm$^{2})$, and the luminescence was recorded with an InGaAs detector over the range of 1.5 eV to 0.67 eV. There was no change in PL intensity below 50 K; above 50 K the intensity decayed logarithmically up to room temperature. The increase of carbon content caused an increase in the PL on the higher energy side of the emission peak (approximately centered at 1.29 eV; FWHH = 0.3 eV) with concurrent intensity decrease on the lower energy side. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W36.00011: Electrical transport in YSi$_{2}$ nanowires V. Iancu, P.R.C. Kent, T.-H. Kim, A.-P. Li, L.D. Menard, J.M. Ramsey, H.H. Weitering When a small amount of yttrium is deposited onto a gently heated Si(100)2x1 surface in ultrahigh vacuum, the yttrium atoms self-assemble into highly uniform silicide nanowires with dimensions of the order of 0.4x1.1x1000 nm$^{3}$. These YSi$_{2}$ nanowires are among the thinnest silicide structures fabricated to date. Their electrical properties have been explored using a four-probe scanning tunneling microscope (STM). The wires exhibit ohmic conductance at room temperature but the conductance decreases at lower temperature. STS measurements [1] indicated a small gap opening at low temperature in the thinnest YSi$_{2}$ wires, which appears to be associated with the charge-order fluctuations seen in STM. The YSi$_{2}$ nanowires not only represent an interesting model system for exploring 1D quantum transport, but they can also be used as electrodes or interconnects in nanoscale electronic devices on a silicon platform. The research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, U.S. DOE. [1] C. Zeng \textit{et al. }Nat. Mat. 7, 539 (2008) [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W36.00012: Gate-tunable magnetic exchange and giant g-factor fluctuations in InAs nanowire quantum dots Szabolcs Csonka, Lukas Hofstetter, Frank Freitag, Stefan Oberholzer, Christian Schonenberger, Thomas Sand Jespersen, Martin Aagesen, Jesper Nygard We use the spin-1/2 Kondo effect, which is observed in every other charge ground state with odd elec-trons, to measure the field-induced splitting of the spin-doublet, and hence the $g$- factor. We do this in hybrid quantum dots using both normal (N), ferromagnetic (F) and superconducting (S) contacts. Unlike to previous studies, the $g$-factors of neighboring states can vary a lot: $g$ can scatter between $2$ and $18$ and can therefore be even larger than in the bulk ($g \sim 15$). We demonstrate further the electric gate tunability of the $g$- factor in a single charge state. When using F contacts, a zero- field split-ting is induced. This proximity induced exchange field has recently been measured for the first time by Hauptmann et al. (Nature Physics \textbf{4}, (2008)) in carbon nanotubes. Here, we show the same ef-fect in a semiconducting nanowire, demonstrating that this effect is universal. Employing a pair of S and F contacts, the proximity-induced exchange shows up as a minigap in superconducting spectroscopy. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W36.00013: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W36.00014: First principles study of very thin TiO nanowires Oguz Gulseren, Deniz Cakir We have systematically investigated structural, electronic and magnetic properties of very thin TiO$_{x}$ ($x$=1,2) nanowires as well as bulk-like (110) rutile nanowires by using the first principles plane wave pseudopotential calculations based on density functional theory. A large number of different possible structures have been searched via total energy calculations in order to find the ground state structures of these nanowires. Three dimensional structures are more energetic than planar ones for both of the stoichiometries (i.e. $x$=1,2). The stability of TiO$_x$ nanowires enhances with its increasing radius, thus reaching sufficient coordination number of Ti and O atoms. All stoichiometric TiO$_{2}$ nanowires studied exhibit semiconducting behavior and have nonmagnetic ground state. There is a correlation between binding energy ($E_b$) and energy band gap ($E_g$) of TiO$_{2}$ nanowires. In general, $E_b$ increases with increasing $E_g$. In TiO nanowires, both metallic and semiconductor nanowires are resulted. In this case, in addition to paramagnetic TiO nanowires, there are also ferromagnetic ones. We have also studied the structural and electronic properties of bulk-like rutile (110) nanowires. There is a crossover in terms of energetics and bulk-like nanowires are more stable than the thin nanowires for larger radius wires after a critical diameter. These (110) rutile nanowires are all semiconductors. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W36.00015: Giant Flexoelectric Effect In Two-Dimensional Boron-Nitride Layers Ivan Naumov, Alexander Bratkovsky, Vivek Ranjan The direct conversion of ambient motion into electrical energy is a challenging fundamental and technological problem that is currently a focus of research. Boron-Nitride non-centrosymmetric monolayers are piezoelectrics that can sustain much larger structural and produce very large (a few Volts) voltage drop across flexed nanostrips. We show, by way of first-principles calculations, the existence of giant nonlinear flexoelectric effect in BN 2D strips. The induced polarization is quadratic in amplitude of atomic displacements $A$, yet the dipole moment per unit cell is about four times larger compared to PbZrTiO3 [1]. The new effect may find a variety of practical applications and, in particular, as nanogenerators and tactile sensors powered by an ambient motion or agitation. BN material is inert and can be used in biological environment.\\[3pt] [1] I.Naumov, A.Bratkovsky, V.Ranjan, arXiv:0810.1775 (2008). [Preview Abstract] |
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