Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A11: Superlattices, Nanostructures, Materials: Synthesis, Growth and Characterization |
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Sponsoring Units: DCMP Chair: Joe Tischler, Naval Research Laboratory Room: 305 |
Monday, March 16, 2009 8:00AM - 8:12AM |
A11.00001: Coverage-Dependent Faceting of Au Chains on Si(557) F.J. Himpsel, I. Barke, F. Zheng, S. Bockenhauer, K. Sell, V. v. Oeynhausen, K.H. Meiwes-Broer The structural and electronic phase diagram of Au on Si(557) is established using scanning tunneling microscopy (STM) and angle-resolved photoemission (ARPES). Five phases consisting of altogether seven facets are observed in the sub-monolayer regime. Four of them consist of two coexisting structures. In order of increasing Au coverage the five phases are: Si(111)7$\times$7 + Si(112), Si(557)1$\times$2-Au, Si(111)5$\times$2-Au + Si(335)-Au, Si(111)$\sqrt{3}$$\times$$\sqrt{3}$-Au + Si(335)-Au, and Si(111)$\sqrt{3}$$\times$$\sqrt{3}$-Au + Si(5 5 11)-Au. The relative surface areas of the five phases and seven facets are determined accurately by depositing a Au wedge ranging from 0 to 0.8 monolayer and performing automatic pattern recognition on large-scale STM images. Angle-resolved photoemission spectra are decomposed into contributions from the five phases. The Fermi wave vectors of various facets are identified. Using Si(557)1$\times$2-Au as reference we find a coverage of 3 Au chains per unit cell for the frequently-studied Si(111)5$\times$2-Au surface (instead of the widely-used value of 2 Au chains). The impact of this finding on structural models is discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A11.00002: The Alignment of Gold Nanorods in Macroscopic Domains Jake Fontana, Peter Palffy-Muhoray, Ashish Agarwal, Nicholas Kotov The uniform alignment of nanoparticles in domains with macroscopic length scales is critical to the production of self-assembled composite metamaterials for optical applications. We describe methods of self assembly leading to films and suspensions with a high loading of orientationally ordered nanoparticles in macroscopic domains. The nanoparticles are short aspect ratio gold nanorods, with both plasmon peaks in the visible spectrum. Orientational order can be achieved via applied electric field, mechanical strain, as well as via interactions with anisotropic hosts and among nanoparticles. We have determined the orientational order in our samples from polarized absorbance measurements. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A11.00003: Role of the three Si suboxides at the surface of Si quantum dots and in Si/SiO$_2$ quantum wells on optical response Pierre Carrier The Si/SiO$_2$ interface structure has been extensively studied in the past, especially for MOSFET applications. Recent applications of Si/SiO$_2$ nanostructures in solar cells and LEDs are now investigated using Si quantum dots (QD) or Si/SiO$_2$ quantum wells (QW). The Si/SiO$_2$ interface contains three Si suboxides, each bonded to 1, 2, or 3 oxygen atoms, respectively referred to as Si$^{1+}$, Si$^{2+}$, and Si$^{3+}$. Models that contain all three suboxides are difficult to construct; results in the literature on oxygenated Si QD usually include Si$^{1+}$ and Si$^{2+}$ only. The models presented here contain the 3 suboxides and are based on a Si/SiO$_2$ surface model originally constructed by Pasquarello \textit{et al.}, Appl.\ Phys.\ Lett.\ \textbf{68}, 625 (1996). This model was used later by the author in the study of Si/SiO$_2$ QW [Phys.\ Rev.\ B \textbf{65}, 165339 (2002)] and is now extended to Si QD. It is shown that the band gap or optical response depends strongly on the Si suboxide atomic configuration at the surface of QD or at the interface of QW. Trends on the band gap variations as function of the three suboxides will be discussed. All models (QW and QD) are structurally relaxed using the program PARSEC [Phys.\ Rev.\ Lett.\ \textbf{72}, 1240 (1994)]. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A11.00004: Formation of Colloidal Nanoparticle Superlattices in a Two Solvent System Chenguang Lu, Austin Akey, Irving Herman A two solvent system consisting of a high boiling point solvent and a low boiling point solvent was found to greatly aid the self-assembly of nanoparticle superlattices. Nanoparticle mixtures were prepared under multiple suitable solvent evaporation conditions and the products were analyzed by SEM and TEM. The formation process of various binary nanoparticles superlattices was investigated to elucidate the optimal conditions for self-assembly. Superlattice formation in this two solvent system was further investigated with various spatial confinement conditions. Here, the capillary effect during the evaporation of solvents may be the driving factor in the self-assembly. Micrometer scale superlattices of CdSe nanoparticles were fabricated with this technique. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A11.00005: Modeling the Self-assembly of Nanorod Superlattices Alexey Titov, Petr Kral Colloidal semiconductor CdSe/CdS nanorods (NR) of diameters of 3-10 nm and lengths of 4-40 nm typically self-assemble into nematic and smectic phases, which are parallel to the substrate, or simple hexagonal (SH) superlattices, which are perpendicular to the substrate [1-2]. We model the formation of these structures by semi-classical means, starting from the forces between the nanorods, their coupling to the substrate and to the external electric fields. We determine the conditions under which superlattices with different number of particles, number of monolayers, aspect ratios of nanorods, etc. can be observed [3], and show that the obtained results agree well with the available experimental data. Our previous results of modeling superlattices of self-assembled monodisperse nanoparticles are also presented. \\[3pt] [1] L. S. Li and A. P. Alivisatos, Adv. Mater. 15, 408 (2003).\\[0pt] [2] D. V. Talapin et al., J. Am. Chem. Soc. 126, 12984 (2004).\\[0pt] [3] A. V. Titov and P. Kral, Nano Lett. 8, 3605 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A11.00006: Morphology of Cu$_2$S-CdS and Ag$_2$S-CdS Nanorod Heterostructures Denis Demchenko, Bryce Sadtler, Haimei Zheng, A. Paul Alivisatos, Lin-Wang Wang A partial cation exchange has been used to synthesize Cu$_2$S-CdS and Ag$_2$S-CdS nanocrystal heterostructures, with two very different morphologies. Cu$^+$ cation exchange takes place preferentially at the ends of CdS nanorods, Cu$_2$S segments grow into the nanorod from both ends. Ag$^+$ exchange is non-selective, Ag$_2$S islands nucleate and grow over the entire surface of the nanorod. This leads to very different patterns, striped Ag$_2$S-CdS superlattice with several equidistant Ag$_2$S segments in a CdS nanorod, and an asymmetric Cu$_2$S-Cds heterostructure with Cu$_2$S segments at the ends of the CdS nanorod. We use first-principles calculations to obtain formation energies of the different epitaxial interfaces between Cu(Ag)$_2$S and different facets of CdS nanorods. Comparison of chemical and elastic contributions to the interface formation energy for the Cu(Ag)$_2$S-CdS shows that the relative stability of the interfaces determines the nucleation of Cu(Ag)$_2$S and the resulting morphology. Furthermore, since two end facets of CdS nanorod are not crystallographically equivalent a controlled asymmetric nucleation of Cu$_2$S can occur. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A11.00007: Self-assembly of molecular wires Andreas Riemann Scanning Tunneling Microscopy (STM) has been used to study the self-assembly of the naturally occurring amino acid L-methionine on different surfaces. It has been found that methionine forms highly regular structures on an Ag(111) surface under UHV conditions as well as on a graphite surface under ambient conditions. Methionine arranges itself into an array of molecular wires of uniform width and separation. The spacing of these wires can be controlled by means of the deposition amount. Molecular mechanics calculations are used to suggest a model for the methionine configuration on the surfaces. The width of the wires is determined by two methionine molecules arranged with their carboxyl group facing each other. The regular separation of individual wires suggests a long range interaction between these wires. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A11.00008: An STM Study of Atomic Co Wires Nader Zaki, Denis Potapenko, Peter Johnson, Danda Acharya, Percy Zahl, Peter Sutter, Richard Osgood Due to stronger electron-electron interactions, 1-D systems are predicted and, in some cases, have been shown to exhibit unique and exotic electronic properties. One route to the formation of 1-D systems is by self-assembly using low-index vicinal crystal surfaces. In this regard, we have successfully formed 1-atom wide Co wires using Cu(775), a 7-atom wide stepped array with (111) terraces. Contrary to a recently reported DFT prediction, the Co wires are not laterally encapsulated but are positioned exactly at the step edge. We will present STM studies of this system performed at room temperature and STS measurements made at low temperature. While vicinal Cu(111) does exhibit ``frizz'' at the steps when scanning above cryogenic temperatures, the Co wires pin the edges, visually accentuating their presence under STM. Furthermore, we observe a lower local density of states for the Co wires as compared with the Cu steps, which also serves to differentiate the two metals. Cu(111) possess a surface projected bandgap which may electronically decouple the wire electrons that reside in this gap. However, we also see resonances at the Fermi level which suggests electronic coupling between the vicinal Cu surface and the Co electrons. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A11.00009: III-V nanowires grown in a simple, homebuilt system. M. D. Schroer, J. R. Petta Semiconductor nanowires are promising experimental platforms for studying quantum transport due to their built-in one-dimensional confinement of charge carriers. To enable the study of III-V semiconducting nanowires, we built a simple tube furnace based MOCVD reactor. Growth of InP and InAs nanowires using trimethylindium, di-tert-butylphosphine and triethylarsenic has been studied as a function of temperature, pressure, precursor concentration and growth substrate. At optimal growth conditions, wires of 20-100 nm in diameter and up to 10 $\mu$m in length are achievable on InAs substrates. Characterization was performed using SEM, EDS and TEM; both wurtzite and zincblende structures have been observed. We will also present transport measurements of nanowires grown using this system. \vspace{8pt} [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A11.00010: Fabrication of nanowire-nanotube hybrid arrays in porous aluminum oxide membranes Zuxin Ye, Haidong Liu, Isabel Schultz, Wenhao Wu, D. G. Naugle, I. Lyuksyutov Fabrication of ordered nanowire-nanotube hybrid arrays embedded in porous anodic aluminum oxide (AAO) membranes is demonstrated. Arrays of TiO$_{2}$ nanotubes were first deposited into the pores of AAO membranes by an electroless sol-gel technique. For subsequent electrochemical deposition of Co nanowires into the TiO$_{2}$ nanotubes, a thick Au layer was first evaporated on one surface of the membrane to serve as the cathode. Co nanowires were then electrochemically deposited into the TiO$_{2}$ nanotubes through the other surface to form the hybrid nanowire-nanotube arrays. SEM and TEM measurements showed a high Co nanowire filling factor and a clean interface between the Co nanowires and the TiO$_{2}$ nanotubes. The TiO$_{2}$ nanotubes were found to be composed of nanometer sized TiO$_{2}$ crystals, while the Co nanowires were polycrystalline with Co crystal size comparable to the nanowire diameter. This technique can be extended to the fabrication of hybrid arrays of various materials. This work was supported by DOE No. DE-FG02-07ER46450, NSF No. DMR-0606529, and the Robert A. Welch Foundation A-0514. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A11.00011: First Observation of Quantum Size Effects in Metal Films on Insulator Hawoong Hong, Aaron Gray, Ruqing Xu, T.-C. Chiang Quantum size effects of metal films have been drawing lot of attention among surface science community. These quantum size effects have been reported for metal films on semiconductors and metals. Here the first observation of the quantum size effects in metal films on insulators will be reported. Pb thin films, which exhibit the most dramatic quantum-size-effects were chosen again for this effort. Sapphire (001) substrates were used after they were annealed at 1500 $^{o}$C in air and cleaned in a UHV chamber by heating. X-ray diffraction was measured during and after the deposition-annealing processes. Most of the structural aspects observed with Pb on Si(111) also appeared in Pb films on sapphire. The preferred island heights (or magic heights) appeared as 7, 10, 12, 14 layers from the preliminary analysis. This contrasts the magic heights on the Si substrates (5, 7, 9,{\ldots}). This difference is coming from the phase shifts of confined electrons through the different interfaces. This magic selection of island-heights stays quite strong even at elevated temperatures up to 200$\sim $300 $^{o}$C. Time resolved 3-D reciprocal space mapping also showed very strong ordering between islands during deposition and annealing of the films. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A11.00012: Surface Dislocation of Al Films on Ag(111) Bo Xu, Erkuang Zhu, Chao Lu, Yongjun Tian Ordered dislocation structures of metal surfaces are of particularly interests because they can provide templates for building nanostructures with novel electronic, magnetic, and catalytic properties. Here we report two dislocation structures formed for Al on Ag(111). Depending on substrate temperate, Al films demonstrate distinct surface structures. At room temperature, Al nanocrystals with the (111) orientation are formed. At 500 K, a herringbone reconstruction, similar with the well known Au(111) reconstruction surface, is formed, while at 600K, a trigonal reconstruction surface is formed. Molecular self assembly processes on these surfaces are investigated. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A11.00013: Theoretical Analysis of Equilibrium Surface Segregation in Ternary III-V and II-VI Semiconductor Nanostructures Sumeet Pandey, Tejinder Singh, Dimitrios Maroudas We present an atomic-scale analysis of equilibrium surface segregation in ternary compound (III-V and II-VI) semiconductor nanostructures. The analysis is based on a computational scheme for compositional and structural relaxation that combines Monte Carlo with conjugate-gradient methods according to properly modified/extended parameterizations of the valence-force-field (VFF) description; the VFF parameterizations employed in the analysis are validated by comparison with first-principles density functional theory calculations. We report equilibrium concentration distributions in slabs of In$_{x}$Ga$_{1-x}$As and ZnSe$_{1-x}$S$_{x}$ as a function of composition, x, slab thickness, and slab surface crystallographic orientation, as well as in In$_{x}$Ga$_{1-x}$As and ZnSe$_{1-x}$S$_{x}$ nanocrystals with well-defined surface facets as a function of x and nanocrystal size. The results are discussed in the context of synthesis of core/shell structures of ternary compound semiconductor nanocrystals for increased quantum-dot photoluminescence efficiency. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A11.00014: Growth and optical properties of highly oriented ZnSSe alloy nanowires Sui Kong Hark, Yao Liang ZnS, ZnSe and their alloys are important semiconductors for optical applications in the UV-blue spectral region. Nanowires, nanobelts and nanotubes of ZnS and ZnSe, but rarely their alloys, had been synthesized, typically as a random, inhomogeneous assembly. For future basic studies and applications, it is necessary to control the orientation and composition of the nanowires. We have grown ZnSSe alloy nanowires epitaxially on GaAs substrates by metal-organic chemical vapor deposition. Their orientation was adjusted by changing the crystallographic orientation of the substrate. Through controlled alloying, we have also achieved band gap engineering. The nanowires were characterized by SEM, HRTEM and XRD. Their optical properties were studied by Raman, cathodoluminescence and photoluminescence spectroscopy. In addition to the nanowires, the growth conditions and optical properties of ZnSSe alloy nano-tetrapods were studied. [Preview Abstract] |
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