Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B42: Focus Session: Kinetics of Self-Assembly at Surfaces |
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Sponsoring Units: DMP Chair: Ray Phaneuf, University of Maryland Room: Colorado Convention Center 505 |
Monday, March 5, 2007 11:15AM - 11:51AM |
B42.00001: Theory of kinetics of surface evolution: simulations, Langevin, and Fokker-Planck approach Invited Speaker: The study of the the time evolution of morphological features at crystal surfaces has become a topic of great importance, motivated in part by the need of achieving controlled fabrication of nanostructures, and in part by the fundamental statistical mechanics questions that it raises. For the evolution of nanostructure, the control of step dynamics is crucial, since the steps are the fundamental building blocks of crystalline surfaces. Kinetic Monte Carlo (KMC) simulations, coupled to Langevin-type analysis of the scaling properties of step dynamics is a powerful tool for assessing the universal features of surface fluctuations close and far from equilibrium. We discuss the results of KMC simulations of unstable growth of vicinal surfaces, that exhibit anomalous scaling exponents as well as multiscaling. We also discuss a tentative interpretation of the numerics within the Langevin approach, as well as the usefulness of the results in interpreting experimental data. Other theoretical tools, such as the spacing distribution of the eigenvalues of random matrices have been employed, e.g. for investigating the evolution of the terrace width distribution (TWD) on stepped surfaces. We discuss KMC simulations of the relaxation of non equilibrium surface structures towards the equilibrium state. Our results show that relaxation far from equilibrium may be driven by microscopic processes, such as detachment of three-bonded atoms, that differ from those that drive step fluctuations close to equilibrium. Applications of the formalism to out-of-equilibrium states, such as step flow growth, are discussed. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B42.00002: Strain induced metastability in the shape evolution of self-assembled nanoislands on Si(111): real-time electron microscopy observations and numerical simulations Nikhil Medhekar, Vivek Shenoy, James Hannon We present real-time Low-Energy Electron Microscopy (LEEM) observations of the growth and equilibrium shapes of (7x7) reconstructed domains on (1x1) reconstructed Si(111) surface which show several intriguing features in its shape evolution due to strain mediated interactions. We find that the shapes of large domains are fundamentally different from the compact shapes of smaller domains. In contrast, large islands show more ramified shapes resembling branched pine-tree when grown at faster rate and connected-triangles morphology when growth is near equilibrium. Using a phase-field model, we show that the key to understanding this behavior is the strain induced metastability of domain shapes that are trapped in the local minima of the complex energy landscape. The consideration of growth shapes that show spontaneous formation of side branches is necessary to establish the presence of unstable orientations and thus, our work shows that in estimating the thermodynamic and kinetic parameters, the conclusions solely drawn based on the analysis of equilibrium shapes can be erroneous. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B42.00003: Atom-Scale Mechanisms for Unstable Growth on Patterned GaAs(001) Tabassom Tadayyon-Eslami, Hung-Chih Kan, Lynn Calhoun, Ray Phaneuf Molecular beam epitaxy on patterned GaAs(001) under standard conditions of temperature ($\sim $600\r{ }C), rate ($\sim $ 0.3 nm/s) and flux ratio (As$_{2}$/Ga$\sim $10:1) leads to a transient instability toward perturbation of the flat surface [1]. Lowering the temperature through approximately 540$^{o}$C, roughly coincident with the preroughening temperature changes the mode of this instability [2]; however, as we show in this talk, observations of the As$_{2}$ flux dependence rule out both preroughening and a reconstructive phase transition as driving the growth mode change. Instead, we find evidence that the change in unstable growth mode can be explained by a competition between decreased adatom collection rate on small terraces and a small anisotropic multi-step Ehrlich-Schwoebel barrier. We relate these effects to the up-down symmetry breaking term which commonly appears in continuum equations for growth. [1] H.-C. Kan, S. Shah, T. Tadayyon-Eslami and R.J. Phaneuf, Phys. Rev. Lett., \textbf{92}, 146101 (2004). [2] T. Tadayyon-Eslami, H.-C. Kan, L. C. Calhoun and R. J. Phaneuf, Phys. Rev. Lett., \textbf{97}, 126101 (2006). [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B42.00004: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:27PM - 12:39PM |
B42.00005: Multiscale simulations of self-assembly of CdTe nanoparticles into sheets Zhenli Zhang, Zhiyong Tang, Nicholas Kotov, Sharon Glotzer By controlling the organic stabilizers on the surfaces of CdTe nanodots, these particles are found in experiments to self-assemble into one-dimensional wires and two-dimensional sheets[1,2]. To explore the underlying mechanisms for the two processes we perform simulations on multiple scales ranging from quantum mechanics to mesoscale stochastic simulations[2,3]. The simulations and corresponding energy analysis demonstrate that a delicate balance of anisotropic forces between nanoparticles is responsible for the different nanostructures they form. In particular, we show how nanoparticle shape, directional hydrophobic attraction, and electrostatic interactions determine the anisotropy of the interaction and final self-assembled structures. \medskip [1] Tang ZY, Kotov NA, Giersig M, Science, 297, 237-240, 2002. [2] Tang ZY, Zhang ZL, Wang Y, Glotzer SC and Kotov NA, Science, 314, 274-278, 2006. [3] Zhang ZL, Tang ZY, Kotov NA and Glotzer SC, preprint. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B42.00006: Charging in CdSe nanocrystals and mechanistic elucidation of the electrophoretic deposition of nanocrystal films Shengguo Jia, Sarbajit Banerjee, Irving Herman The charge on nanocrystals is not only used to stabilize the colloidal systems but also to assemble these materials into novel films and superlattices. Here, we propose a model for charging in nanocrystals involving the dissociation of ligand molecules from specific surface sites. We also develop a mechanistic model to explain the electrophoretic deposition of nanocrystal films based on electrophoretic mobility measurements, photoluminescence from nanocrystal solutions and films, and observations from deposition experiments. Even though equally thick nanocrystal films are obtained on both negative and positive electrodes, the numbers of positive and negative nanocrystals are not equal in solution. After appropriate reprecipitation cycles, the nanocrystals are ``sticky'' enough to be deposited on the electrodes and nanocrystal films can be formed by electrophoresis. The limiting factor for the maximum thickness to which the films can be grown is the concentration of the minority charged crystals (negatively charged nanocrystals in this case). The charge on the nanocrystal surfaces can be adjusted by the addition of ligands. This work was supported primarily by the MRSEC Program of the NSF under Award No. DMR-0213574 and by NYSTAR. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B42.00007: Phase Field Crystal Modeling of Island Formation and Dislocation Nucleation During Strained Film Growth Zhi-Feng Huang, Ken Elder We study the process of nanostructure self assembly during epitaxial growth of strained solid films through the use of the phase field crystal model. The model is derived from density functional theory and incorporates anisotropy, elasticity and plastic deformations on atomic length and diffusive time scales. We particularly address the formation and evolution of islands/mounds in strained thin films following an initial morphological instability and the nucleation and climb of misfit dislocations. The relation between film structural properties and materials and growth conditions are also discussed. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B42.00008: An Investigation into InAs/GaAs Thin Film Growth Maria Mignogna, Kristen Fichthorn Quantum dots self-assemble due to Stranski-Krastinov growth in heteroepitaxial systems with a lattice mismatch above 2\%, for example in the deposition of InAs on GaAs (001). However, there are many questions left unanswered about quantum dot growth, such as the role of strain in the wetting layer. Simulation techniques such as molecular dynamics (MD) can provide insight at the atomic scale. An empirical potential to study this system has recently been developed [1]. Using NPT MD, we studied the thermal properties and melting of bulk GaAs, as well as the stability of the GaAs(001)$\beta$2(2x4) reconstruction against melting. To probe diffusion and the preferred Ga-atom binding sites, we calculated the minimum potential-energy surface for a gallium atom on the GaAs (001) $\beta$2(2x4) reconstruction. We also evaluated various diffusion pathways and energy barriers using the nudged elastic-band method. The potential captures the location and energy of the deepest binding minimum as compared to DFT values and also achieves good agreement for the diffusion barriers. We used accelerated MD simulations to obtain diffusion coefficients as a function of temperature and these compare favorably to previous results from experiment and DFT. [1] T. Hammerschmidt, PhD Thesis (2006) [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B42.00009: Nanocrystal Formation in Ion-Beam Synthesized GaAs:N and InAs:N A. Wood, W. Ye, X. Weng, P.T. Wang, R.S. Goldman, Y.Q. Wang Ion-implantation followed by thermal annealing offers a unique approach to custom tailoring of semiconductor nanocomposites. For N ion-implanted GaAs (GaAs:N), an amorphous layer with crystalline GaAs remnants is often observed. Subsequent furnace or rapid-thermal annealing (RTA) leads to the formation of zincblende (ZB) GaN nanocrystals [1], which transform to wurtzite (WZ) following extended furnace annealing [2]. For N ion-implanted InAs (InAs:N), nanocrystal formation and evolution has not been previously reported. We are studying the formation and evolution of GaAs:N and InAs:N nanocomposites, synthesized using 100keV ion-implantation with a dose of 5x10$^{17}$cm$^{-2}$, at 300C and 77K. In all cases, the as-implanted structures are primarily amorphous. For GaAs:N, RTA up to 625C leads to an amorphous layer with crystalline GaAs remnants, while RTA in the range 675-700C results in both ZB and WZ nanocrystallites. For InAs:N, 500C RTA leads to the formation of ZB InN-rich and InAs-rich nanocrystals, with amorphous matrices and domains. We will discuss the role of crystalline remnants in the nucleation and growth of ZB nanocrystals, and the mechanisms of the ZB-WZ transformation. [1] X. Weng, et al, \textit{J. Appl. Phys}., \textbf{92} 4012 (2002) [2] X. W. Lin, et al, \textit{Appl. Phys. Lett}. \textbf{67}, 2699 (1995) [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B42.00010: Dual-surfactant effect on enhancing Zn-Doping of GaP Junyi Zhu, Gerald Stringfellow, Feng Liu We report first-principles calculations demonstrating a dual-surfactant effect of Sb and H on enhancing Zn-doping in vapor phase epitaxially grown GaP thin films. The combined effects of Sb and H lower significantly the doping energy of Zn in GaP, while neither Sb nor H can work alone as effectively. The role of H is to provide the extra electron accommodating the p-type dopant incorporation to satisfy the electron counting rule. Our finding has an important general implication that p-type doping in III-V thin films can be achieved by chemical deposition with H, but difficult by physical deposition without H. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B42.00011: Design Principles Incorporating Surface Dynamics for Creating Ordered Organic Nanostructures on Si and SiC Dimerized Surfaces via Car-Parrinello Molecular Dynamics Robin Hayes, Mark Tuckerman Self-assembled organic nanostructures on Si-type semiconducting surfaces promise to impact nanoelectronics, sensors, and nanolithography. Experimentalists have long exploited cycloaddition reactions between small conjugated molecules and Si surface dimers, but with limited success in creating well-ordered structures. Cycloaddition of 1,3-cyclohexadiene (CHD) to the Si(100)-2x1 surface provides a rich test case to explore the role surface dynamics plays in adduct formation because of the variety of bonding motifs and discrepancy of the product distribution with thermodynamic predictions. Car-Parrinello molecular dynamic simulations reveal that the local surface environment, including dimer tilt angle and dimer flipping, matters. CHD often travels over several dimers before forming an adduct by a two step process. First the C=C reacts with a ``down'' Si. The intermediate can persist for over 4 ps and can cause nearby dimers to flip, allowing CHD to complete the reaction with any of the adjacent Si. Thereby, accounting for most of the experimental product distribution. Previously formed adducts protect Si within a 5.5 $\AA$ radius and direct the surface exploration of unbound CHD. These principles are tested on reactions between 1,3-CHD and the closely related 3C-SiC(001)-3x2 surface. [Preview Abstract] |
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