Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P10: Focus Session: Growth, Structure, Dynamics, and Function of Nanostructured Surfaces and Interfaces -- Metals |
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Sponsoring Units: DMP Chair: Jacques Amar, University of Toledo Room: D221 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P10.00001: Interplay between Quantum Size Effect and Strain Effect on Growth of Nanoscale Metal Thin Film Miao Liu, Yong Han, Feng Liu Quantum Size Effect (QSE) has been shown to be a dominant factor in the growth of metal nanofilms on semiconductor substrates in the so-called electronic growth regime. On the other hand, the strain effect is ubiquitous in heteroepitaxial growth of semiconductor and metal thin films. Most time, however, these two important effects have been studied separately focusing on one while neglecting the other. Here, we develop a theoretical framework to investigate the interplay between QSE and strain effect on the stability of metal nanofilms. The QSE and strain effect are shown to be coupled through the concept of ``quantum stress''. First-principles calculations reveal large quantum oscillations in the surface stress of metal nanofilms as a function of film thickness, which adds extrinsically additional strain-coupled quantum oscillations to surface energy of strained metal nanofilms. Our theory enables a quantitative estimation of the amount of strain in experimental samples from the measured stability patterns, explaining a possible origin for some outstanding discrepancies between the existing theories and experiments. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P10.00002: Contrasting growth modes of Ru thin film nano-structures on Si and Pd Xiangshi Yin, Ao Teng, Mustafa \"Ozer, Hanno Weitering, Paul Snijders We have studied Ruthenium thin film growth on both Si (111) and Pd (111) surfaces. The films were deposited at low (LN2) temperature and at room temperature, and subsequently annealed at elevated temperatures, up to 600C. The surface structure, morphology, and chemical composition were investigated by LEED, STM, AES and XPS. Upon deposition at low temperature, nanoclusters are formed on both Si and Pd. Remarkably, the nanoclusters are approximately 3 nm in diameter and exhibit narrow size distributions on both substrates. In the case of Ru on Si, XPS spectra indicate silicide formation at the interface above 300C, but the nanocluster surface morphology survives up to 600C. On the other hand, nanoclusters on Pd smoothen into atomically flat films above 200C. The striking difference in adatom mobilities on these substrates is surprising in light of the very high melting temperature of Ru (2400C). [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P10.00003: Temperature-dependence of Ni+Al co-deposition on NiAl(110): Atomistic-level modeling of deviations from perfect alloy ordering Yong Han, Dapeng Jing, Baris Unal, P.A. Thiel, J.W. Evans Stoichiometric co-deposition of Ni and Al on NiAl(110) for high enough temperatures (below the order-disorder transition for NiAl) must produce near-perfect alloy islands and overlayers. However, at 300K, island structure is far from perfectly ordered and depends strongly on the deposition protocol (e.g., simultaneous vs. sequential). Realistic atomistic-level modeling of this non-equilibrium behavior must provide an accurate description of not just alloy thermodynamics (through adatom adsorption and interaction energies), but also of diffusion kinetics (for adatom attachment-detachment at and transport along island edges). This is achieved by multi-site lattice gas modeling with DFT input for adsorption and interaction energies for adatoms both at adsorption sites and at transition states for hopping [T. Duguet, Y. Han et al., Proc. Nat. Acad. Sci. 107 (2010) Special Issue on Surface Chemistry; Y. Han et al., submitted (2010)]. Model analysis by KMC simulation shows a transition from poor alloy order at 300K to almost perfect order at 600K. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P10.00004: Electronic origin of kinetic and dynamic processes at atomic steps on vicinal metal substrates Invited Speaker: Single-atomic-layer steps play an important role in the kinetics and dynamics of morphological evolution and structural formation at surfaces. In this talk, we will attempt to elucidate the importance of the electronic nature in determining the bonding of adatoms and the activation barriers for adatom descent at step edges. The insights gained through first-principles case studies have timely and important impacts in understanding the evolution of many nanostructured surfaces and prevention of electrical breakdown in nanodevices. In the first case [1], we establish a clear correlation between the preferred diffusion mechanism and step-edge barrier and the relative degree of electronic shell filling of the adatom and the substrate. We also find an approximate linear relation between the adatom step-edge hopping barrier and the adatom-surface bonding strength with a slope roughly proportional to the number of the nearest neighbors of the adatom in the initial state. These results can serve as simple guiding rules for predicting precise atomic surface morphologies and designing desirable surface nanostructures, such as atom wires [2]. In the second case [3], we discover an optimal surface electromigration inhibitor on the technologically important Cu(111) surface, characterized by energetically favoring and binding strongly at the kink of step edges. Finally, we will briefly discuss how the electronic bonding strengths influence the nucleation and growth behavior of carbon atoms at the step edges of various transition-metal surfaces, a crucial insight in designing optimal kinetic pathways for mass production of quality epitaxial graphene [4]. \\[4pt] [1] Y. Mo, W. G. Zhu, E. Kaxiras, and Z. Y. Zhang, Phys. Rev. Lett. 101, 216101 (2008). \\[0pt] [2] Y. Mo, K. Varga, E. Kaxiras, and Z. Y. Zhang, Phys. Rev. Lett. 94, 155503 (2005). \\[0pt] [3] K. H. Bevan, W. G. Zhu, H. Guo, and Z. Y. Zhang, Phys. Rev. Lett. (submitted). \\[0pt] [4] H. Chen, W. G. Zhu, and Z. Y. Zhang, Phys. Rev. Lett. 104, 186101 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P10.00005: Bilayer islands in heteroepitaxy of transition metals: insights from first principles Talat S. Rahman, Marisol Alcantara Ortigoza, Sergey Stolbov Although not in equilibrium configuration, bilayer islands have been observed in the heteroepitaxy of some transition metals for four decades. Its physical origin, however, was investigated experimentally recently for Ru on Pt(111) [1]. By introducing an energy-gain criterion (upon adlayer formation) and by analyzing the density of electronic states of 1 to 3 Ru adlayers on Pt(111), we show that, even though no bonding stronger than that of atoms in bulk Ru is involved, the energy gain for the formation of the second layer is the largest. We find that the effect of the lattice mismatch is not trivial to elucidate from experiment since the electronic structure of a clean substrate changes in the presence of strain and/or chemical bonding with other species. The lattice mismatch, however, is the key factor for the instability in the formation of a third-layer. We extend the model to explain the well-known case of Co/Cu(111) and to predict other possible bilayer systems. \\[4pt] [1] A. Bergbreiter et al., Vacuum \textbf{84} 13 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P10.00006: Collective Super-Diffusive Motion of the Pb Wetting Layer on Si(111) Michael Altman, K.L. Man, M.M.T. Loy, M.C. Tringides An unusual mass transport behavior has been discovered in the Pb/Si(111) wetting layer. Mass transport is studied by observing non-equilibrium coverage profile evolution with low energy electron microscopy (LEEM). Equilibration of an initial coverage step profile does not exhibit the profile broadening and gradual, x$\sim $t$^{1/2}$, time-dependent evolution that is expected from classical considerations. Instead, the profile edge is displaced linearly in time, x$\sim $t, much faster than expected for thermally activated hopping and without dispersal. LEEM also reveals a wave-like disturbance in the wetting layer that propagates in the direction opposite the step profile motion. The Pb coverage that is left in the wake of this disturbance can be determined accurately and with high lateral resolution using selected-area LEED due to the Devil's Staircase (DS) phases in this system. The expanding wave converts an initial homogeneous DS phase with coverage $\theta >\theta _{c}$ to a final phase with $\theta _{c}$ = 1.25 ML, thereby conserving mass across the initial step profile position. This identifies a collective super-diffusive motion of the Pb layer that can extend rapidly over macroscopic distances. Such motion may facilitate the remarkably efficient self-organization of uniform height, quantum size effect-induced Pb islands on Si(111). [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P10.00007: Crossover from concerted motion to periphery diffusion for Cu clusters on Cu(111): Application of Fine Grid On-Lattice SLKMC Syed Islamuddin Shah, Giridhar Nandipati, Altaf Karim, Abdelkader Kara, Talat S. Rahman The ``fine grid on-Lattice'' Self-Learning Kinetic Monte Carlo (SLKMC) technique combines the ideas embedded in the SLKMC [1] method with a new pattern recognition scheme which incorporates both fcc and hcp sites to characterize and store configurations. Application of methods for saddle point searches have revealed several new mechanisms involving multiple atoms which contribute to cluster migration. We present results for the diffusion of 2D Cu islands on Cu(111), using semi-empirical interatomic potentials [2], at three temperatures (300K, 500K and 700K). Long time simulations show a trend in crossover from concerted motion to periphery diffusion for clusters containing more than 14 atoms. The calculated trends in effective energy barriers and diffusion constants are compared with those obtained earlier from the SLKMC Method [1] which allowed only surface fcc site occupancy.\\[4pt] [1] A. Karim et al. Phys. Rev. B 73, 165411 (2006)\\[0pt] [2] S. M. Foiles et al. Phys. Rev. B 33, 7983 (1986) [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P10.00008: Shape transitions in strained islands: kinetics versus energetics Yunsic Shim, Yevgen Kryukov, Jacques Amar Recently, it has been argued that the shape transition from compact to ramified islands observed experimentally in submonolayer Cu/Ni(100) growth is not due to kinetics but can be understood in terms of energetic arguments. In order to determine the responsible mechanisms we have carried out temperature-accelerated dynamics (TAD) simulations as well as energetics calculations. Surprisingly, our results indicate that the strain-energy contribution to the dependence of island-energy on shape is relatively weak. In contrast, our TAD simulations indicate that unexpected concerted motions occurring at step edges may be responsible. The energy barriers for these concerted motions are significantly lower than for Cu/Cu(100) and Ni/Ni(100), decrease with increasing island size, and appear to saturate for islands larger than 300 - 400 atoms. These results suggest that the shape transition is of kinetic origin but is strongly mediated by strain. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P10.00009: Temperature-induced crossovers in the static roughness of a one-dimensional interface Elisabeth Agoritsas, Vivien Lecomte, Thierry Giamarchi At finite temperature and in presence of disorder, a one-dimensional elastic interface displays different scaling regimes at small and large lengthscales. Using a replica approach and a Gaussian variational method (GVM), we explore the consequences of a finite interface width $\xi$ on its small-lengthscale geometrical fluctuations. We compute analytically the static roughness $B(r)$ of the interface as a function of the distance $r$ between two points on the interface, in the specific case of short-range elasticity and random-bond disorder. We find that for a finite $\xi$ two temperature regimes exist, and we determine the corresponding different roughness regimes and their crossover lengthscales. In addition, using a directed polymer description, we study via a second GVM procedure and generic scaling arguments, a modified toy model that provides further insight on those results, which apply to experimental interfaces such as e.g. ferromagnetic domain walls in thin films, subjected to a quenched uncorrelated disorder. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P10.00010: Mean field approach to fluctuations of surface line defects Dionisios Margetis Below the roughening transition temperature, the dynamics of crystal surfaces are driven by the motion of line defects (steps) of atomic size. According to the celebrated Burton Cabrera-Frank (BCF) model, the steps move by mass conservation, as adsorbed atoms (adatoms) diffuse on terraces and attach/detach at step edges. The resulting deterministic equations of motion incorporate nonlinear couplings due to entropic and elastic-dipole step-step interactions. In this talk, I will discuss a formal theory for stochastic aspects of step motion by adding noise to the BCF model in 1+1 dimensions. I will define systematically a ``mean field'' that enables the conversion of the coupled, nonlinear stochastic equations for the distance between neighboring steps (terrace widths) to a single Langevin-type equation for an effective terrace width. In the course of my study, I invoke the Bogoliubov-Born-Green Kirkwood-Yvon (BBGKY) hierarchy for joint terrace-width probability densities and a decorrelation ansatz for terrace widths. By using an example drawn from epitaxial growth (with material deposition from above), I will compare the mean field approach to an exact result from a linearized growth model. [D. Margetis, J. Phys A: Math. Theor. 43, 065003 (2010).] [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P10.00011: Spacing distribution functions for 1D point island model with irreversible attachment Diego Gonzalez, Theodore Einstein, Alberto Pimpinelli We study the configurational structure of the point island model for epitaxial growth in one dimension. In particular, we calculate the island gap and capture zone distributions. Our model is based on an approximate description of nucleation inside the gaps. Nucleation is described by the joint probability density p$^{xy}_{n}$ (x,y), which represents the probability density to have nucleation at position x within a gap of size y. Our proposed functional form for p$^{xy}_{n}$ (x,y) describes excellently the statistical behavior of the system. We compare our analytical model with extensive numerical simulations. Our model retains the most relevant physical properties of the system. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P10.00012: Capture zone area distributions for homogeneous nucleation and growth of islands during deposition Jim Evans, Yong Han, Maozhi Li The size distribution of islands formed by homogeneous nucleation and growth during deposition is known to encode information about the nucleation mechanism. The same was recently proposed for the distribution, g(A), of areas, A, of capture zones (CZ) surrounding islands [1], where most atoms landing within a CZ aggregate with the associated island. We have developed a precise theory for g(A) whose evolution is driven by the nucleation of new islands [2]. g(A) has a complicated form controlled by details of the spatial aspects of nucleation. However, it is reasonably approximated by a Generalized Gamma distribution, g(A) $\sim $ A$^{\beta }$ exp[-cA$^{n}$]. For compact 2D islands, one has n $\sim $ 1.5, and $\beta \quad \sim $ 3(i+2)/2 for critical size i. Here, $\beta $ follows from analysis of the creation of new small capture zones between nearby pairs of islands, and n from analysis of the likelihood that a new CZ overlaps an existing large CZ. \\[4pt] [1] Pimpinelli {\&} Einstein, PRL 99 (2007) 226102; 104 (2010) 149602;\\[0pt] [2] Li, Han {\&} Evans, PRL 104 (2010) 149601. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P10.00013: Modeling Island-Growth Capture Zone Distributions (CZD) with the Generalized Wigner Distribution (GWD): New Developments in Theory and Experiment Alberto Pimpinelli, T.L. Einstein, Diego Luis Gonz\'alez, Rajesh Sathiyanarayanan, Ajmi BH. Hamouda Earlier we showed [PRL 99, 226102 (2007)] that the CZD in growth could be well described by $P(s)=a s^\beta \exp(-bs^2)$, where $s$ is the CZ area divided by its average value. Painstaking simulations by Amar's [PRE 79, 011602 (2009)] and Evans's [PRL 104, 149601 (2010)] groups showed inadequacies in our mean field Fokker-Planck argument relating $\beta$ to the critical nucleus size. We refine our derivation to retrieve their $\beta \approx i + 2$ [PRL 104, 149602 (2010)]. We discuss applications of this formula and methodology to experiments on Ge/Si(001) and on various organics on SiO$_2$, as well as to kinetic Monte Carlo studies homoepitaxial growth on Cu(100) with codeposited impurities of different sorts. In contrast to theory, there can be significant changes to $\beta$ with coverage. Some experiments also show temperature dependence. [Preview Abstract] |
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