Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N22: Artificially Structured Materials: Growth, Structure, and Related Phenomena |
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Sponsoring Units: DCMP Chair: Connie Li, Naval Research Laboratory Room: 324 |
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N22.00001: First-principle Investigation of the Stability and Vibrational Spectrum of MoSx Nanostructures Grown on Cu(111) Talat S. Rahman, Maral Aminpour, Duy Le, Marisol Alc\'antara Ortigoza Recent experiments have successfully synthetized MoS$_{\mathrm{X}}$ nanostructures in a controlled manner by evaporating Mo adatoms on the copper sulfide monolayer that forms on Cu(111) upon sulfur preloading [1,2]. STM observations and total-energy calculations based on density functional theory, including van-der-Waals interactions, have proposed several structures for MoS$_{\mathrm{X}}$/Cu(111). In this study, we investigate the plausibility of those structures and provide elements for further experimental substantiation or refutation. In particular, we perform density-functional-theory calculations (including ab intio van-der-Waals interactions) of vibrational spectrum of the proposed structures to (1) attest their dynamical stability; (2) compare their thermodynamic stability as obtained from the total free energy; and (3) provide the vibrational frequencies that uniquely fingerprint the proposed structures. \\[4pt] [1] Kim et al., Langmuir 27, 11650 (2011)\\[0pt] [2] Dezheng D Sun, Angew Chem. Int. Ed. \textbf{51}, 10284-8 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N22.00002: Vibrational and thermodynamic properties of transition-metal nanoclusters Valeri G. Grigoryan, Michael Springborg The knowledge of the vibrational spectrum of a cluster, which is the fingerprint of its structure, is necessary for the development of thermodynamics of clusters (melting, heat capacity, solid-solid structural transitions) and for the understanding of experimental vibrational spectra. In summary, the full vibrational spectrum of Ni$_N$ and Cu$_N$ nanoclusters with $N$ from $2$ to $150$ atoms has been determined using the analytical expression of the embedded-atom method (EAM) for the {\em force-constant tensor} for the first time. In the determination of the spectra we have employed the global-minimum structures obtained in our previous unbiased EAM studies (see e.g. Physical Review B, 2004; 2006). Furthermore, using those spectra and the superposition approximation, the thermodynamic properties of the clusters have been calculated quantum mechanically, including their heat capacity and solid-solid transition temperatures for several structural changes in the Ni and Cu clusters. Both the vibrational spectrum and the thermodynamic functions show strong cluster-size effects. We emphasized that our approach is general. It is based only on the (common) EAM form of the total energy and applicable to many other many-body potentials. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N22.00003: Material Improvements of ZnCdSe/ZnCdMgSe Heterostructures for Quantum Cascade Laser Applications with Incorporation of Growth Interruptions During MBE Growth Thor Garcia, Joel De Jesus, Arvind Ravikumar, Songwoung Hong, Claire Gmachl, Aidong Shen, Maria Tamargo We report on the growth of ZnCdSe/ZnCdMgSe/InP Quantum Cascade (QC) structures with improved electrical and material properties. Material quality has been previously addressed by limiting the lattice mismatch to within 0.2{\%} of InP. However, the yields of high quality material have remained low and lasing has not been observed. To address the low growth yields we have investigated possible mechanisms for degradation of the material. Growth interruptions during the MBE growth were added to the active core of the QC laser structure. High resolution XRD and PL were used to evaluate the material quality. Fabricated devices with growth interruptions show a dramatic improvement in the electroluminescence spectral properties. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N22.00004: New crystal structures in hexagonal CuInS$_{2}$ nanocrystals Xiao Shen, Emil A. Hern\'andez-Pagan, Wu Zhou, Yevgeniy S. Puzyrev, Juan C. Idrobo, Janet E. Macdonald, Stephen J. Pennycook, Sokrates T. Pantelides CuInS$_{2}$ is one of the best candidate materials for solar energy harvesting. Its nanocrystals with a hexagonal lattice structure that is different from the bulk chalcopyrite phase have been synthesized by many groups. The structure of these CuInS$_{2}$ nanocrystals has been previously identified as the wurtzite structure in which the copper and indium atoms randomly occupy the cation sites. Using first-principles total energy and electronic structure calculations based on density functional theory, UV-vis absorption spectroscopy, X-ray diffraction, and atomic resolution Z-contrast images obtained in an aberration-corrected scanning transmission electron microscope, we show that CuInS$_{2}$ nanocrystals do not form random wurtzite structure. Instead, the CuInS$_{2}$ nanocrystals consist of several wurtzite- related crystal structures with ordered cation sublattices, some of which are reported for the first time here. This work is supported by the NSF TN-SCORE (JEM), by NSF (WZ), by ORNL's Shared Research Equipment User Program (JCI) sponsored by DOE BES, by DOE BES Materials Sciences and Engineering Division (SJP, STP), and used resources of the National Energy Research Scientific Computing Center, supported by the DOE Office of Science under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N22.00005: Measuring the Elastic Modulus of the Grain Boundary Component of Nanocrystalline Copper Guo-Jie Gao, Yunjiang Wang, Shigenobu Ogata In the past twenty years, it has been widely accepted that the Young's modulus of the grain boundary (GB) part of nanocrystalline metals is about $70\%$ of that of the crystalline core component. However, this belief is an assumption based on numerical studies of specific grain boundary like $\Sigma5$ twist boundary where atoms interact with one another via simplified Lennard-Jones potential at $0K$ or experimental studies assuming the GB behaves like amorphous alloys. A thorough investigation driven from completely realistic atomic simulation at finite temperature is still lacking. We reexamine this assumption by measuring the Young's modulus of pure copper (Cu) with grain size ranging from $3$ to $25~nm$ at $300K$ using molecular dynamics (MD) uniaxial tensile tests. We implement a novel Voronoi protocol to build nanocrystalline structures of fully dense pure Cu with well-controlled grain size distribution and Mishin embedded atom model (EAM) potential. We find the following key results concerning the stiffness for nanocrystalline metals at finite temperature: 1) The GB is more thermally sensitive and therefore elastically much softer than the crystalline interior. 2) The Young's modulus of the GB is about $20\%$ or less of that of the grain interior. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N22.00006: Structural and compositional characterization of ``covetics'' a new class of materials containing high C concentration R.A. Isaacs, A. Herzing, D.R. Forrest, A.N. Mansour, M.C. LeMieux, J. Shugart, L. Salamanca-Riba ``Covetics'' are a new class of materials formed by the incorporation of high concentrations ($>6 wt\%$) of nanoscale carbon in a metal matrix. The carbon incorporates into the crystal structure of the host metal and remains dispersed after subsequent melting and re-solidification. The carbon is highly stable in these materials despite the absence of a predicted solid solution at such concentrations in the binary phase diagrams. Covetics have been shown to exhibit enhanced electrical, mechanical and thermal properties when compared with non-covetic metals. We have performed energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), SEM, TEM, STEM/electron energy loss spectroscopy (EELS), AFM, and Raman spectroscopy to investigate the structure of Al, Cu, and Ag covetics. Both bulk samples and thin films are investigated. Carbon was detected in the form of nanoparticles 5 nm - 200 nm in diameter with an interconnecting carbon matrix. The carbon is detectable by EDS and XPS, but not by analytical methods such as LECO and GDMS. Raman indicates a similar signal to that of CNTs in covetics. A detailed investigation of the morphology of the nanocarbon and the structure of several covetics will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N22.00007: Computational Investigations of a Possible New Class of Materials: A Superatom Ionic Solid Karl Sohlberg, Violeta Nasto A ``superatom'' is a cluster of atoms that shows high stability. High stability can arise from the geometric arrangement of the atoms in the cluster. For example, when atoms are close packed, clusters containing an integer number of closed shells of atoms, (i.e. 13, 55, 137... atoms) exhibit enhanced stability and are termed ``magic clusters.'' High stability can also arise from the electronic structure. High symmetry metal clusters that have exactly 8, 20, 40..., valence electrons show enhanced stability. Superatoms can act chemically like a single atom of a different element. We have used electronic structure calculations to test the idea that a new class of materials may be formed based on the periodic arrangement of superatom ions, instead of the typical atomic or polyatomic ions of a conventional ionic solid. A solid is formed based on crystalline packing of anionic (Al@Cu$_{54}^-$) and cationic (Ce@C$_{60}^+$), nearly spherical superatom species that show exceptional stability. According to radius-ratio rules, these ions will favor a CsCl crystal structure with a body-centered (bcc) type of unit cell. Calculations on this material suggest that it is stable, semiconducting and less dense than common metal oxides, but that the metal anion clusters deform within the material. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N22.00008: Electronically Guided Self Assembly within Quantum Corrals Rongxing Cao, Bingfeng Miao, Zhangfeng Zhong, Liang Sun, Biao You, Wei Zhang, Di Wu, An Hu, Samuel Bader, Haifeng Ding A grand challenge of nanoscience is to master the control of structure and properties in order to go beyond present day functionality. The creation of nanostructures via atom manipulation by means of a scanning probe represents one of the great achievements of the nano era. Here we build on this achievement to self-assemble nanostructures within quantum corrals. We constructed circular and triangular Fe quantum corrals on Ag(111) substrate via STM manipulation and studied the quantum confinement of electronic states and the diffusion of Gd atoms inside the corrals. Statistical results reveal the motion of the Gd atoms forming several individual orbits that are closely related to the local density of states. We experimentally demonstrate that different self-organized Gd atomic structures are formed within 30-nm circular and triangular Fe quantum corrals with a step-by-step guiding process. The findings demonstrate that quantum confinement can be used to engineer atomic structures and atom diffusion. And 30-nm resolution can be reached by means of advanced lithography. Adding quantum engineering to augment it opens new possibilities for local functionality design down to the atomic scale. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N22.00009: Simultaneous hypersonic and optical mirrors in nanometric porous silicon multilayers Jesus Manzanares-Martinez, Paola Castro-Garay, Damian Moctezuma-Enriquez, Yohan Jasdid Rodriguez-Viveros We study by theoretical simulations the non-perpendicular propagation of electromagnetic and elastic waves in Porous Silicon Multilayers (PSM). Our work is inspired by recent experimental results where the angular variation of the optical and hypersonic stop bands has been explored in PSM. [L. C. Parsons and G. T. Andrews, J. Appl. Phys. 111, 123521 (2012)] We proceed in three steps. First, we found the conditions to obtain a simultaneous photonic-phononic mirror at normal incidence. Second, we determined the angular variation of the mirrors computing the projected band structure. Finally, we found the conditions to obtain an omnidirectional mirror for hypersonic waves. However, we have found that for the optical case the mirror is limited to an angular cone. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N22.00010: Dynamic Structural Disorder in Supported Nanoparticles F.D. Vila, J.J. Rehr, S.D. Kelly, S.R. Bare Supported Pt based nanoclusters are of wide interest in nano-scale physics and have many industrial applications, yet an understanding of their structure is far from complete. Experimental probes such as x-ray absorption spectroscopy (XAS) only yield globally averaged properties, e.g., mean bond distances and mean-square radial disorder (MSRD), which can give a misleading characterization of such complex systems. To obtain a more detailed picture we have carried out finite temperature DFT/MD simulations\footnote{F. Vila \textit{et al.},\textit{UW preprint} (2012).} of Pt and PtSn nanoclusters up to 600 K (\textit{operando} conditions). These show that the nano-scale structure and charge distribution are inhomogeneous and dynamically fluctuating over several time-scales, ranging from fast (200-400 fs) bond vibrations to slow fluxional bond breaking ($>$10 ps). In particular the anomalous behavior of the MSRD is not static, but rather due to ``dynamic structural disorder'' (DSD) driven by stochastic motion of the center of mass over 1-4 ps time-scales. In addition the nanoclusters exhibit a semi-melted, Sn-rich surface. These findings show that, and how an improved XAS interpretation of supported nano-scale structure must take into account DSD and other structural inhomogeneities. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N22.00011: The Orientation Control of Iodine Molecules inside nano-scale channels Dingdi Wang, Juanmei Hu, Wenhao Guo, Shengwang Du, Z.K. Tang We demonstrate a technique to control the spatial orientation of iodine molecules inside nano-scale channels of an AlPO$_{4}$-11 zeolite crystal. The orientation of iodine molecules can be precisely controlled by the water molecule density inside the channels due to the interaction between iodine and water molecules. Without the presence of water molecules, all the embedded iodine molecules are directed along the direction of nano channels. As increasing the number of water molecules, the iodine molecules gradually ``stand up'' insde nano channels. The experimental results obtained from polarized Raman spectroscopy agree well with the theoretical analysis using molecular dynamics simulation. This technique may be used for engineering molecular orientation in nano-structured devices. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N22.00012: Epitaxial growth of YSi$_{2}$ nanowires the on Si(110) surface Saban Hus, Hanno Weitering High-aspect-ratio YSi$_{2}$ nanowires have been grown epitaxially on the Si (110) surface. In contrast to epitaxial growth on the Si (100) surface, YSi$_{2}$ nanowires on Si (110) grow in a single orientation and show a clear preference of nucleating at terrace edges, thus providing a promising method for fabricating regular nanowire arrays with controlled wire separation. The thinnest YSi$_{2}$ nanowires have a cross section of $\sim$ 0.5 x 2.8 nm$^{2}$ with wire lengths of up to a few hundred nm, while thicker nanowires can grow up to several $\mu$m long. Scanning tunneling spectroscopy measurements on individual nanowires indicate that the nanowires have metallic properties while the surface between the nanowires has a band gap of $\sim$ 1eV. These nanowires thus represent an ideal platform for studies of quasi one-dimensional electrical transport. Such studies are currently underway in our laboratory. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N22.00013: Absence of Dirac Electrons in Silicene on Ag (111) Surfaces Zhi-Xin Guo, Shinnosuke Furuya, Jun-ichi Iwata, Atsushi Oshiyama We report first-principles calculations that clarify stability and electronic structures of silicene on Ag(111) surfaces. We find that several stable structures exist for silicene /Ag(111), exhibiting a variety of surface morphology. We also find that Dirac electrons are absent near Fermi energy in all the stable structures due to buckling of the Si monolayer and mixing between Si and Ag orbitals. We propose that either BN substrate or hydrogen processing of Si surface is a good candidate to preserve Dirac electrons in silicene. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N22.00014: Engineering of the Static Interface Dipole in Metal/Organic Nanohybrid Materials Axel Enders, Donna Kunkel, Justin Nitz, Peter Dowben, Lucie Routaboul, Bernard Doudin, Pierre Braunstein, Scott Simpson, Eva Zurek We studied small molecules with large intrinsic electrical dipole as model system for molecular films adsorption on surfaces for altering the interface dipole screening. More specifically, we investigated the self-assembly and electronic interface properties of zwitterionic molecules of type C6H2(\textellipsis NHR)2(\textellipsis O)2 (R $=$ H, ...), adsorbed on Cu(111), Ag(111), Au(111) surfaces with scanning tunneling microscopy in UHV. These molecules carry positive and negative charges on opposite parts of the molecule, resulting in a large electric dipole of typically 10 Debye. We find that the dipole of the surface-supported molecule is decreased with respect to free species and of order of 1 - 2 Debye, depending on the substrate material. The molecules self-assemble into 2D structures upon adsorption, where the substrate-dependent strength of the dipolar interactions between the adsorbed molecules dictates the network architecture. DFT calculations were performed to analyze adsorption geometry, charge transfer and dipole moment. We will show that zwitterionic molecules provide a unique opportunity to engineer the interface dipole in metal/organic hybrid structures, which ultimately controls the charge injection barrier in devices. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N22.00015: Self-Organized Growth of Single Crystalline Copper Nanobead Strings by Electrodeposition Cong Meng, Ruwen Peng, Mu Wang Here we report a self-organized growth of single-crystalline strings of nano sized copper beads electrodeposited from an ultrathin layer of CuSO4 electrolyte solution without adding any additives. Spontaneous oscillation of voltage/current has been observed when potentiostatic/galvanostatic mode is applied. Scanning electron microscopy indicates that the filaments developed from the cathode are made of smooth copper beads a few hundreds of nanometers in size connected by thin single-crystalline rods. The periodicity along the string may vary from 500nm to one micron, and the spatial periodicity is strict up to hundreds of microns. To pinpoint the growth mechanism, we intentionally terminate the growth at different stage of the spontaneous oscillation of the voltage across the electrodes, and established the relation of the microscopic deposit morphology and the voltage oscillations. A growth mechanism is proposed based on the experimental observations. Structural and luminescent properties of the copper strings have been investigated, and the possible applications of such a unique structure have been discussed. [Preview Abstract] |
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