Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J32: Structure and Morphology: Metal Surfaces |
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Sponsoring Units: DMP DCMP Chair: Theodore Einstein, University of Maryland Room: E142 |
Tuesday, March 16, 2010 11:15AM - 11:27AM |
J32.00001: Island Size Selectivity during 2D coarsening of Ag/Ag(111) Surface Giridhar Nandipati, Abdelkader Kara, Syed Islamuddin Shah, Talat Rahman We have carried out realistic kinetic Monte Carlo [1] simulations to study initial stages of submonolayer Ag island coarsening on the Ag(111) surface. We find that during initial stages, coarsening proceeds through sequence of magic island sizes and this selectivity occurs via adatom detachment/attachment processes. We will present detailed analysis of island creation/annihilation (selection) and its dependence on island shape and also its dependence on temperature and initial Island size distributions. These simulations were carried out using a very large database of activation barriers for processes identified by their local environment. This database has been generated from previous self-learning kinetic Monte Carlo [2] simulations of small and medium sized clusters and the energy barriers were calculated using embedded atom method.\\[4pt] [1] J. Phys.: Cond. Mat. {\bf 21}, 084214 (2009).\\[0pt] [2] Phys. Rev. B {\bf 72}, 115401 (2005). [Preview Abstract] |
Tuesday, March 16, 2010 11:27AM - 11:39AM |
J32.00002: Strain Dependence of Microscopic Parameters for Growth of Ag on Ag(100) Christian Ratsch Many technologically relevant system have strain (due a lattice mismatch between different materials), and such strain is often the driving force behind the formation and self-organization of quantum dots and other nano-scale structures. It is therefore of paramount importance to understand and to be able to model growth of strained system. Some previous studies have discussed the strain dependence of adatom diffusion for a number of systems. In this talk, we will present density-functional theory calculations that examine the effect of strain on a number of microscopic growth parameters, such as diffusion, dissociation of small islands, detachment of adatoms from islands, and diffusion of adatoms along island edges. We will use growth of Ag on Ag(100) as our model system. We will then illustrate in growth simulations that employ the level-set technique how such strain dependence of microscopic parameters affects the ordering during growth. [Preview Abstract] |
Tuesday, March 16, 2010 11:39AM - 11:51AM |
J32.00003: Low-temperature Ag/Ag(100) growth revisited Jacques G. Amar, Yunsic Shim Recent experiments on Ag/Ag(100) growth indicate that the surface roughness exhibits a non-monotonic temperature-dependence - with peaks at approximately T = 90 K and 200 K. While the high-temperature peak has been previously explained the low-temperature peak has not. Here we present the results of hybrid molecular-dynamics/kinetic Monte Carlo simulations - with activation barriers obtained from recent parallel-temperature accelerated dynamics simulations - which were carried out in order to understand the growth behavior over the temperature range $T = 55 - 180$ K. Our simulations indicate that even at high temperature the surface roughness depends sensitively on a competition between a variety of low-barrier processes including downward funneling of depositing atoms, island relaxation via edge-zipping and edge-diffusion, atom-attraction, and concerted interlayer diffusion at kinks, while the short-range attraction of depositing atoms to microprotrusions also plays a crucial role. By taking these processes into account in our simulations, good agreement with experiment is obtained over the entire temperature range. A comparison between DFT calculations and EAM predictions for certain key barriers will also be presented. [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:03PM |
J32.00004: Controlling Self-Assembly in Al(110) Homoepitaxy Yogesh Tiwary, Kristen Fichthorn Homoepitaxial growth on Al(110) exhibits nanoscale self-assembly into huts with well-defined (100) and (111) facets [1]. Although some of the diffusion mechanisms underlying this kinetic self-assembly were identified and incorporated into a two-dimensional model [2], we used density-functional theory (DFT) to identify many other mechanisms that are needed to describe the three-dimensional assembly seen experimentally [3]. We developed a three-dimensional kinetic Monte Carlo (KMC) model of Al(110) homoepitaxy. The inputs to the model were obtained from DFT [3,4]. Our model is in agreement with experimentally observed trends for this system. We used KMC to predict self-assembly under various growth conditions. To achieve precise placement of Al nanohuts, we simulated thermal-field-directed assembly [5]. Our results indicate that this technique can be used to create uniform arrays of nanostructures. [1] F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Valbusa, E. Wang, and Z. Zhang, Phys. Rev. Lett. 91, 016102 (2003). [2] W. Zhu, F. Buatier de Mongeot, U. Valbusa, E. G. Wang, and Z. Y. Zhang, Phys. Rev. Lett. 92, 106102 (2004). [3] Y. Tiwary and K. A. Fichthorn, submitted to Phys. Rev. B. [4] Y. Tiwary and K. A. Fichthorn, Phys. Rev. B 78, 205418 (2008). [5] C. Zhang and R. Kalyanaraman, Appl. Phys. Lett. 83, 4827 (2003). [Preview Abstract] |
Tuesday, March 16, 2010 12:03PM - 12:15PM |
J32.00005: Role of Codeposited Impurities in Growth: Dependence of Morphology on Binding and Barrier Energies Rajesh Sathiyanarayanan, A. BH. Hamouda, A. Pimpinelli, T. L. Einstein The previous talk showed that codeposition of impurity atoms during epitaxial growth could be used for nanostructuring of surfaces. Based on their lateral nearest-neighbor binding energies ($E_{NN}$) to Cu and their diffusion barriers ($E_d$) on Cu(001), we classify the candidate impurity atoms into four sets. We find that codeposition of impurities from different sets produce qualitatively different surface morphologies both in the step-flow and the submonolayer ($\theta \le$ 0.7 ML) regimes. In the submonolayer regime, we characterize these differences through variations of the number of islands ($N_i$) and the average island size with coverage ($\theta$). Further, we compute the critical nucleus size ($i$) for all of these cases from the distribution of capture-zone areas using the generalized Wigner distribution.\footnote{A. Pimpinelli, T. L. Einstein, Phys. Rev. Lett. 99, 226102 (2007).} [Preview Abstract] |
Tuesday, March 16, 2010 12:15PM - 12:27PM |
J32.00006: Role of Codeposited Impurities in Growth: Explaining Cu(0 0 1) A. BH. Hamouda, Rajesh Sathiyanarayanan, A. Pimpinelli, T. L. Einstein A unified explanation of the physics underlying all the distinctive features of the growth instabilities observed on Cu vicinals\footnote{N. N$\acute{\mbox{e}}$el $\ldots$ H.-J. Ernst., J. Phys.: Condensed Matter 15, S3227 (2003).} has long eluded theorists. Recently, kinetic Monte Carlo studies showed that codeposition of impurities during growth could account for all the experimental observations.\footnote{A. Ben-Hammouda et al., Phys. Rev. B 77, 245430 (2008).} To identify the responsible impurity atom, we compute the nearest-neighbor binding energies ($E_{NN}$) and terrace diffusion barriers ($E_d$) for several candidate impurity atoms on Cu(1 0 0) using DFT-based VASP. Our calculations show that codeposition (with Cu) of mid-transition elements, such as Fe, Mn, and W, could cause the observed instabilities; when the experimental set-up is considered, W emerges to be the most likely candidate. We discuss the role of impurities in nanostructuring of surfaces. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J32.00007: Modeling Growth, Morphology and Optical Response of Chemically Deposited Silver Films Lawrence Davis, Elizabeth Kodpuak, Matthew Boggess, Miriam Deutsch A method for estimating the three dimensional (3D) morphology of metal films is presented. Rough silver films chemically grown on silica substrates are characterized via image analysis of scanning electron micrographs. While the latter technique provides only two dimensional structural projections, particle size distributions and time dependence of the surface coverage may be reliably determined. Three parameters governing film growth are extracted from the data and used as inputs for simulations. This allows for a simplified 3D film growth model, greatly reducing computing requirements while still providing direct access to the complete 3D structure of the films throughout the growth process. Two dimensional projections of the simulated films are compared to experimental data to determine the accuracy of the model. Information about the 3D structure of the films provided by this method is useful in predicting the optical response of the metal films. [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 12:51PM |
J32.00008: Terrace-Width Distributions (TWDs) of Touching Steps: Modification of the Fermion Analogy, with Implications for Measuring Step-Step Interactions on Vicinals T.L. Einstein, Rajesh Sathiyanarayanan, Ajmi BH. Hamouda, Kwangmoo Kim Using Monte Carlo simulations, we compute\footnote{RS, ABH, and TLE, Phys. Rev. B 80 (2009) 153415.} the TWDs of surfaces in which steps can touch each other, forming multiple-atomic height steps, but cannot cross (no overhangs), and so inconsistent with the standard mapping to spinless fermions. Our numerical results show that the generalized Wigner distribution, with minor modifications at small step separations, gives a very good fit for TWDs of touching steps. (We also generate analytic results by generalizing results for extended fermions.\footnote{Siew-Ann Cheong and C.L. Henley, arXiv:0907.4228v1.}) The interaction strength derived from the fit parameter $\varrho$ indicates an effective attraction between steps, weakening the overall repulsion. The strength of this effective attraction decreases for larger mean-step separations and decreasing step-touching energies; describable via finite-size scaling. Hence, accurate extraction of the true repulsion strength requires multiple vicinalities. [Preview Abstract] |
Tuesday, March 16, 2010 12:51PM - 1:03PM |
J32.00009: Changing growth conditions during surface growth Yen-Liang Chou, Michel Pleimling, R. K. P. Zia Motivated by a series of experiments that revealed a temperature dependence of the dynamic scaling regime of growing surfaces, we investigate theoretically how a nonequilibrium growth process reacts to a sudden change of system parameters. We discuss quenches between correlated regimes through exact expressions derived from the stochastic Edwards-Wilkinson equation with a variable diffusion constant. Our study reveals that a sudden change of the diffusion constant leads to remarkable changes in the surface roughness. Different dynamic regimes, characterized by a power-law or by an exponential relaxation, are identified, and a dynamic phase diagram is constructed. We conclude that growth processes provide one of the rare instances where quenches between correlated regimes yield a power-law relaxation. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:15PM |
J32.00010: Far-from-equilibrium film growth on alloy surfaces: Ni and Al on NiAl(110) Jim Evans, Yong Han, Baris Unal, Dapeng Jing, Patricia Thiel We analyze the deposition of Ni and Al on NiAl(110) by STM and by KMC simulation of a multi-site lattice-gas model incorporating DFT energetics. The goal is to elucidate far-from- equilibrium growth of metal films on alloy surfaces, including self-growth. Deposition of Ni produces reversible formation of monolayer islands with some preference for diagonal steps at 300K and which are distorted-hexagons at 400K. Deposition of Al at 300K produces irreversible formation of irregular monolayer islands perhaps favoring [-110] steps. These features are recovered by the modeling, which captures distinct terrace diffusion pathways of Ni versus Al on NiAl(110), the details of island nucleation, and complex edge diffusion which controls growth shapes. Additional studies of sequential co-deposition reveal ``history-dependent'' structures far from perfect equilibrium alloy ordering. Depositing Al first then Ni creates monolayer islands with a core of Al surrounded by a ring of Ni. In contrast, depositing Ni first then Al creates monolayer Ni islands with significant second layer population by Al reflecting stronger binding of Al on top of the Ni islands. [Preview Abstract] |
Tuesday, March 16, 2010 1:15PM - 1:27PM |
J32.00011: Effect of misfit dislocation on surface diffusion Maral Aminpour, Oleg Trushin, Talat Rahman We apply molecular dynamics and molecular static methods to study the effect of misfit dislocations on adatom diffusion in close proximity to the dislocation core in heteroepitaxial systems, using many body interaction potentials. Our system consists of several layers (3-7) of Cu placed on top of a Ni(111) substrate. The misfit dislocation are created with the core located at the interface between the Cu film and Ni substrate, using a method described earlier[1]. Presence of the defect inside the structure leads to formation of strain fields on the surface, which affects adatom diffusion. To make quantitative estimates of the effect, we compare adatom diffusion on surfaces with misfit dislocation underneath, and on those without such defects. We find that presence of the defect under the surface strongly affects the adatom trajectory, creating anisotropy in atomic diffusion, even for the 7 layer Cu film. We also calculate the potential surface energy available to the adatom and compare the energy barriers for adatom diffusion in the core region and on the defect free sample. ref1. O Trushin \textit{et al}, J. Phys.: Condens. Matter 21 084211 2009 [Preview Abstract] |
Tuesday, March 16, 2010 1:27PM - 1:39PM |
J32.00012: Inhibition of Surface Mobility of Cu Adatoms on the Cu(111) Surface via Sn Alloying Zhengzheng Chen, Nicholas Kioussis, Nasr Ghoniem, King-Ning Tu, Jenn-Ming Yang We have investigated the effect of substitutional Sn on the diffusion of Cu-adatom on the Cu(111) surface by first principle methods. We have determined the energy landscape by accurately calculating the diffusion and adsorption energies of a Cu adatom as a function of distance from Sn. The results reveal two important electronic mechanisms: (1) each substitutional surface Sn atom introduces a forbidden adsorption region in its vicinity, within which Cu adatoms are not stable and spontaneously diffuse away from Sn; and (2) the binding of Cu adatoms close to Sn is weaker. Using the first principle calculated energy landscape, we have carried out Kinetic Monte Carlo simulations and found that Sn effectively impedes the diffusion of the Cu adatom Especially, intermetallic compound Cu$_3$Sn almost totally suppresses the diffusion of adatom at low and medium temperatures. Analysis of the change of surface state induced by Sn demonstrates Friedel oscillation, where the radius of the forbidden adsorption region corresponds to the first Friedel valley. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 1:51PM |
J32.00013: Strain-Induced Modification on Diffusion Barriers: An Ab-initio Study Handan Yildirim, Talat S. Rahman While it is known that adatom diffusion barriers via hopping mechanism increase with increasing tensile and decrease with increasing compressive strain, we show that the extent of variation in the barriers is system specific. In particular, our DFT based calculations for the self-diffusion of Cu, Ag and Pd adatoms on strained (100) terraces and near step edges show that the variation for Pd on Pd(100) is much smaller than that for Ag on Ag(100) and Cu on Cu(100). Electronic structure analysis, particularly the shift in the d-band center, shows that the electronic structure of Pd is the most affected. Although this perturbation is expected to induce the highest deviation in the barriers for Pd, our analysis shows the opposite. This contradiction is traced to the dissimilarity in their elastic responses. We find that the elastic response to strain is the least for Pd. We compare the relative importance of strain induced changes in surface electronic structure and atomic relaxations in determining the changes in diffusion barriers. [Preview Abstract] |
Tuesday, March 16, 2010 1:51PM - 2:03PM |
J32.00014: Patterns in strained epitaxial layers: Beyond the Frenkel-Kontorova model Kristen Fichthorn, Joshua Howe Although the structures of strained overlayers have been understood in terms of the classical Frenkel-Kontorova model [1], our recent work indicates that these structures can have a quantum mechanical origin. Experimental STM studies show that the second layer of Ag on Pt(111) forms a striped pattern [2]. We simulated the striped phase with a combination of DFT, to quantify Ag pair interactions up to the 53$^{rd}$ neighbor [3], and Monte Carlo (MC). Our MC simulations yield a striped phase with the experimental characteristics: stripes along the $[112] $ direction that alternate between fcc and hcp sites. Although our DFT pair potential can be fit in an average way to a theoretical expression with radial symmetry [4], this fit cannot produce the striped phase, indicating that angular variations in the potential are important in producing this phase. [1] J. Frenkel and T. Kontorova, Journal of Physics-USSR 1, 137 (1939). [2] H. Brune, H. Roder, C. Boragno, and K. Kern, Phys. Rev. B 49, 2997 (1994). [3] W. Luo and K. A. Fichthorn, Phys. Rev. B 72, 115433 (2005). [4] P. Hyldgaard and M. Persson, J. Phys: Cond. Matter 12, 2981 (2000). [Preview Abstract] |
Tuesday, March 16, 2010 2:03PM - 2:15PM |
J32.00015: Large-scale surface reconstructions from first principles: Au(100) and Pt(100) by all-electron DFT Paula Havu, Volker Blum, Ville Havu, Patrick Rinke, Matthias Scheffler We show that the large-scale, quasihexagonal surface reconstructions of Au(100) and Pt(100) are captured by all-electron density functional theory (DFT) in the local-density approximation and PBE generalized gradient approximation in excellent agreement with experiment. While the superstructure is often approximated as (5$\times$1) in first-principles calculations, larger, more realistic approximants turn out to be important for some aspects. For example, the reconstruction energy for Au(100) approximately doubles compared to (5$\times$1) when considering the much larger, more realistic series (5$\times N$) ($N$=10,15,20,25,30). For Pt(100), where an experimental reconstruction energy estimate exists [1], DFT based on these approximants is in close agreement. In addition to the energetics, our calculations reveal the full local surface geometry, and corroborrate relativistically enhanced $d$-$d$ hybridization as the electronic reconstruction driving force. Our calculations are based on 5-layer asymmetric slab geometries, two layers of which are fully relaxed, i.e. up to 786 atoms, with 336 relaxed, and are performed using the efficient, accurate all-electron electronic structure code FHI-aims [2]. [1] W.A. Brown, R. Kose, D.A. King, Chem. Rev. \textbf{98}, 797 (1998) [2] V. Blum \emph{et al}, Comp. Phys. Comm. \textbf{180}, 2175 (2009). [Preview Abstract] |
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