Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A40: Focus Session: Collective Diffusion and Self Organization |
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Sponsoring Units: DMP Chair: Michael S. Altman, Hong Kong University of Science and Technology Room: 349 |
Monday, March 18, 2013 8:00AM - 8:12AM |
A40.00001: Evidence for Collective Motion in LEEM Measurements of Metals on Semiconductors Shirley Chiang, Yu Sato, Jason Giacomo, Cory Mullet, Marshall van Zijll, Bret Stenger, Dylan Lovinger We review evidence for collective motion from LEEM measurements of three metal on semiconductor systems: Pb/Ge(111), Au/Ge(111), and Ag/Ge(110). Pb/Ge(111) shows a novel phase separation with fluctuating domains of $\sim$ 100nm size which spontaneously switch back and forth from the (1x1) to $\beta $ phase in the region of the phase diagram where the two phases coexist. This striking mechanism occurs because nm-scale domains can have thermally-induced density fluctuations comparable to the density difference between the two phases (PRL, \underline {99}, 096103 (2007)). Au/Ge(111) also shows evidence for fluctuating domains between the ($\surd $3x$\surd $3)R30 and (1x1) phases, both for small domains of 100nm diameter, and at the edges of large domains on a sample with low step density. LEEM movies also show ``hopping'' of large islands (tens to hundreds of nm in diameter) of Au on Ge(111). Self-assembly of large one dimensional (1D) islands (1-10 $\mu $m x 70-140 nm, $\sim$ 7nm high, for 7ML) along the [1,-1,0] direction occurs for Ag grown from 300-530 $^{\circ}$C on Ge(110). During the growth process, such 1D islands have been observed to collapse into other islands and defects in \textless\ 1sec. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A40.00002: Super-diffusive Motion of the Pb Wetting Layer on the Si(111) Surface M.S. Altman, K.L. Man, M.M.T. Loy, M.C. Tringides An unusual mass tansport behavior has been observed in the dense Pb wetting layer on the Si(111) surface. Mass transport is studied by observing non-equilibrium coverage profile evolution with low energy electron microscopy and micro-low energy electron diffraction (m-LEED). The strong sensitivity of diffraction features to Pb coverage in this system allows the Pb coverage profile to be determined precisely with high spatial resolution using m-LEED. Equilibration of an initial coverage step profile produced by laser induced thermal desorption proceeds by the exchange of mass between two steep coverage gradients that travel in opposite directions with invariant shapes. The coverage profile between these two moving edges unexpectedly exhibits a concave shape that apparently contradicts local mass conservation given by the continuity equation. The equilibration time is independent of Pb coverage above a critical coverage, 1.24 ML, but diverges sharply below. The observed spatio-temporal characteristics and lower cutoff for fast dynamics may signal a very unusual coverage dependence of diffusion or might suggest an exceptional collective super-diffusive mechanism by which diffusion is not driven by the local coverage gradient in the usual way. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A40.00003: Incorporation-limited growth of molecular film: Zn-phthalocyanine on Ag(100) Jerzy Sadowski, Abdullah Al-Mahboob Metal phthalocyanines draw considerable attention due to their potential for energy and environmental applications. However, the control of film fabrication, necessary for achieving optimized performance of organic devices, remains a challenge. Here, we present results of low-energy electron microscopy (LEEM) studies of incorporation-limited growth mechanism of zinc-phthalocyanine (ZnPc) thin films on Ag(100) substrate. We found that when ZnPc is deposited on a substrate kept at lower temperature (RT to 430K), the resulting film has a double domain epitaxial structure, resembling that of bulk ZnPc, with domain sizes in the sub-micrometer range. At above 440K, the film ordering changes into a 5x5 commensurate structure, having only a single crystalline orientation. In situ LEEM of the nucleation and growth of the film allowed us to find a crossover of equilibrium concentration of diffusing molecules versus temperature for both adsorption phases, and thus to tune the type of nucleation. We determined that at favorable growth conditions, the 5x5 phase undergoes a thermodynamic phase transition into the bulk-like structure and we were able to obtain ZnPc bulk-like films with unprecedented quality, with single crystalline domain sizes in the range of millimeters. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A40.00004: Modeling Patterning of Heteroepitaxial Overlayers from Nano to Micron Scales Invited Speaker: Tapio Ala-Nissila Thin heteroepitaxial overlayers have been proposed as templates to generate stable, self-organized nanostructures at large length scales, with a variety of important technological applications. However, modeling strain-driven self-organization is a formidable challenge due to a large span of length and time scales involved. In this talk, I will present a method for predicting the patterning of ultrathin films on micron length scales with atomic resolution [K.R. Elder {\it et al.}, Phys. Rev. Lett. {\bf 108}, 226102 (2012)]. It is based on the Phase-Field Crystal model, which allows one to reach diffusive time scales for relaxation of the system. We make quantitative predictions for the type of superstructures (stripes, honeycomb, triangular) and length scales of pattern formation of both compressively strained and tensile overlayers on metal-metal systems, including Cu on Ru(0001), Cu on Pd(111), and Ag on Cu(111). Our findings are in excellent agreement with previous experiments and call for future experimental investigations of such systems. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A40.00005: Dewetting of Epitaxial Silver Film on Silicon by Thermal Annealing Charlotte E. Sanders, Gary L. Kellogg, C.-K. Shih It has been shown that noble metals can grow epitaxially on semiconducting and insulating substrates, despite being a non-wetting system: low temperature deposition followed by room temperature annealing leads to atomically flat film morphology. However, the resulting metastable films are vulnerable to dewetting, which has limited their utility for applications under ambient conditions. The physics of this dewetting is of great interest but little explored. We report on an investigation of the dewetting of epitaxial Ag(111) films on Si(111) and (100). Low energy electron microscopy (LEEM) shows intriguing evolution in film morphology and crystallinity, even at temperatures below 100$^{\mathrm{o}}$C. On the basis of these findings, we can begin to draw compelling inferences about film-substrate interaction and the kinetics of dewetting. Financial support is from NSF, DGE-0549417 and DMR-0906025. This work was performed, in part, at the Center for Integrated Nanotechnologies, User Facility operated for the U.S. DOE Office of Science. Sandia National Lab is managed and operated by Sandia Corp., a subsidiary of Lockheed Martin Corp., for the U.S. DOE's National Nuclear Security Administration under DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A40.00006: Dewetting of nanometer-sized thin films on a solid substrate: A large-scale simulation study Trung Nguyen, Miguel Fuentes-Cabrera, Jason Fowlkes, Javier Diez, Alejandro Gonzalez, Lou Kondic, W. Michael Brown, Philip Rack Directing the assembly of nanoparticles into ordered arrays using interfacial instability has been of practical interest. Recent experimental and theoretical studies have revealed the role of the Raleigh-Plateau instability in determining the breakup process of fluidic thin films deposited on a solid substrate. Using all-atom models, we investigate the dynamic behavior of nanometer-sized thin rings as a function of initial geometry in the presence of Raleigh-Plateau-type instability and inward pressure due to initial azimuthal curvature. We consider systems at close-to-experiment scales consisting of hundreds of thousands to millions of atoms using LAMMPS, a massively parallel molecular dynamics package, with GPU acceleration. The simulation results are shown to be consistent with continuum modeling calculations in predicting the fastest growth mode and breakup times, both of which are important to the evolution of the thin films. Our study serves to stimulate future investigations connecting experimental and theoretical findings towards fabricating ordered arrays of nanoparticles. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A40.00007: Fe on graphene: interaction, growth morphology, and thermal stability Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Hai-Qing Lin, Kai-Ming Ho, Michael Tringides The nucleation and growth of Fe on graphene is highly unusual. Constantly increasing in island density with coverage is observed by experiment which indicates the presence of strong adatom predominantly repulsive interactions. We study Fe adatoms interactions on graphene by first-principles calculations and showed that the interactions between Fe adatoms consist of a short-range attraction and long-range repulsions. By investigating the adsorption energies and diffusion barriers for Fe adatoms on graphene, we also predict that Fe on graphene exhibit a three-dimensional growth mode. Fe nanostructures on graphene are also shown be stable against aggregation. The predictions from first-principles calculations are consistent with experimental observations. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A40.00008: Distribution of Steps with Finite-Range Interactions: Analytic Approximations and Numerical Results Diego Luis Gonz{\'a}lez, Diego Felipe Jaramillo, Gabriel T{\'e}llez, T.L. Einstein While most Monte Carlo simulations assume only nearest-neighbor steps interact elastically, most analytic frameworks (especially the generalized Wigner distribution) posit that each step elastically repels all others. In addition to the elastic repulsions, we allow for possible surface-state-mediated interactions. We investigate analytically and numerically how next-nearest neighbor (NNN) interactions and, more generally, interactions out to q'th nearest neighbor alter the form of the terrace-width distribution and of pair correlation functions (i.e.\ the sum over n'th neighbor distribution functions, which we investigated recently.\footnote{D.L. Gonz{\'a}lez, A. Pimpinelli, \& TLE, Phys. Rev. E 88, 011151 (2012)} For physically plausible interactions, we find modest changes when NNN interactions are included and generally negligible changes when more distant interactions are allowed. We discuss methods for extracting from simulated experimental data the characteristic scale-setting terms in assumed potential forms. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A40.00009: Developments in Characterizing Capture Zone Distributions in Island Growth T.L. Einstein, Alberto Pimpinelli, Diego Luis Gonz{\'a}lez, Rajesh Sathiyanarayanan The utility of using the distribution of capture zones (CZD) to characterize epitaxial growth continues to mount. For non-Poisson deposition (i.e.\ when island nucleation is not fully random) the areas of these Voronoi cells (proximity polygons) can be well described by the generalized Wigner distribution (GWD), particularly in the central region around the mean area. We discuss several recent applications to experimental systems, showing how this perspective leads to insights about the critical nucleus size. In contrast, several studies have shown that the GWD may not describe the numerical data from painstaking simulations in both tails. We discuss some refinements that have been proposed. Finally, we comment on applications to social phenomena such as area distributions of secondary administrative units (like counties) and of Voronoi cells around Metro stops. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A40.00010: Magicity of Transition Metal Nanoclusters Based on Generalized Wulff Construction Shunfang Li, Xingju Zhao, X.S. Xu, Y.F. Gao, Zhenyu Zhang Nanoclusters with extra stability at certain cluster sizes are known as magic clusters which may exhibit exotic properties. Two dominant mechanisms have been invoked to define the magicity: electronic shell closure for simple and noble metal clusters, and atomic shell closure for rare-gas and other clusters. The latter mechanism is inherently rooted in the classic Wulff construction, which stipulates that the preferred structure of a cluster should minimize its total surface energy, resulting in close-shelled icosahedronal transition metal (TM) clusters with magic sizes of 13, 55, and 147. Here we use TM clusters around 55 as examples to demonstrate that the Wulff construction must be generalized to also include the contribution of edge atoms. Specifically, a majority of the TM$_{55}$ clusters are found to be fcc or hcp crystal fragments with much fewer edge atoms than the icosahedrons, and the magic number is shifted to its nearby even numbers. The generalized Wullf construction established here should be instrumental in fabricating nanoclusers with desirable functionalities. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A40.00011: Magicity of Ag nanoclusters on Si(111)-(7$\times$7) by atomic manipulation Fangfei Ming, Guohua Zhong, Kedong Wang, Zhenyu Zhang, Xudong Xiao Nanoclusters with extra stability at certain cluster sizes are known as magic clusters, whose magicity depends sensitively on the environments. Using scanning tunneling microscopy and first-principles calculations, we explore the dynamics and magicity of Agn (n$=$1-26) clusters constructed atom-by-atom on a Si(111)-(7$\times$7) surface. By measuring the thermal stability of clusters of increasing size, a set of magic clusters are distinctly established, which in return helps to reveal the preferred growth sequence towards geometrically close-shelled Ag10 and Ag25 clusters with extra inertness. We further use a probing atom to demonstrate that the adatom-cluster interaction is highly anisotropic, preserving the attractive nature of an Ag-Ag bond at short distances, but becoming repulsive at large distances mediated by the substrate. These innovative findings of fundamental importance are also expected to be significant in surface catalytic reactions and related technological areas. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A40.00012: Atomic Force Microscopy of Vertically Stacked Focused-Ion-Beam Induced Quantum Dots Marta Luengo-Kovac, Timothy Saucer, Andrew Martin, Joanna Millunchick, Vanessa Sih Control over the positioning of semiconductor quantum dots (QDs) could facilitate the coupling of QDs to photonic crystal cavities and has applications in the development of high-efficiency solar cells. QDs grown through self-assembly nucleate at random spatial locations. However, a focused ion beam (FIB) can be used to create preferential sites for QD nucleation, and this pattern can be transferred to subsequent layers of QDs, either due to strain or residual effects of the templating. Multilayer QD stacks can therefore maintain the lateral pattern of the initial layer while separating QDs from material damage induced by the patterning. Multilayer QD structures were grown on FIB-patterned GaAs(001) substrates with 10 nm thick GaAs spacers between the layers. The substrates were patterned with sixteen square arrays of holes with spacings of 0.25, 0.5, 1.0, and 2.0 $\mu$m each at FIB dwell times of 1.0, 3.0, 6.0, and 9.0 ms. We report on the effects of multilayer QD growth on the initial layers through atomic force microscope (AFM) imaging of single, two-, and three-layer FIB-templated QD samples. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A40.00013: Kinetic Monte-Carlo Simulation of Substrate Vacancy Diffusion in C$_{60}$ on Ag(111) Joseph Dulny III, Sangzi Liang, John Groh, Jorge Sofo, Renee Diehl Recently, clean Ag(111) surfaces with monolayer C$_{60}$ adsorbates have been studied with scanning tunneling microscopy and low energy electron diffraction. These studies revealed that the C$_{60}$ forms a commensurate $(2\sqrt{3}\times2\sqrt{3})R30^\circ$ phase on the Ag(111) substrate and when observed with STM, the C$_{60}$ molecules appear either ``bright'' or ``dim.'' LEED studies showed that these two species of C$_{60}$ are a result of the C$_{60}$ taking two different orientations on the Ag substrate, one of which only occurs when the C$_{60}$ is located over an Ag lattice vacancy. STM also shows the bright and dim C$_{60}$ molecules change location over time. This ``flipping'' behavior implies that vacancy diffusion in the Ag lattice is taking place. Here, using the kinetic Monte-Carlo algorithm, we model the diffusion of vacancies in the Ag lattice. Data collected from simulations is compared to experimental data on the flipping rate of the C$_{60}$ vs. temperature and the bright/dim C$_{60}$ ratio vs. temperature. Our model tells us that intralayer vacancy diffusion is taking place and that adsorption of C$_{60}$ on Ag(111) results in vacancy creation in the Ag(111) surface. Additional density functional theory calculations support the conclusions of the model. [Preview Abstract] |
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