Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G53: Focus Session: Diffusion, Quantum Size Effects, and Growth of Metals, Islands, and Grain Boundaries on Surfaces |
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Sponsoring Units: DMP Chair: Harald Brune, Ecole Polytechnique Federale de Lausanne Room: Mile High Ballroom 2C |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G53.00001: Collective Mass Transport in Ag/Ge(110) 1D Nanoisland Growth Shirley Chiang, Cory Mullet, Michael Tringides, Marshall van Zijll, Bret Stenger, Emilie Huffman, Dylan Lovinger The growth of Ag deposited on Ge(110) was studied with low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). The LEEM studies showed the formation of long one-dimensional (1D) multi-height islands over the temperature range 430C-530C. During deposition, the length of the islands increases at a constant rate ($\sim$ 106 atoms/sec reaching $\sim$ 20 microns) and constant width (100-200nm) for 9ML total deposition. Stochastic diffusion cannot account for these very high island growth rates. Similarly when island decay is observed, it happens exceedingly fast and cannot be explained by uncorrelated detachment of Ag atoms. Both processes indicate a more collective mass transport, which must be related to the mobility of the wetting layer. STM images show the crystalline structure of the 1D Ag islands and also that the reconstructed regions between the islands consist of bare Ge; thus they confirm that the wetting layer provides the material for the islands to grow at these high rates. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G53.00002: Growth of fcc(111) Dy multi-height islands on 6H-SiC(0001) graphene Matthew Hershberger, Myron Hupalo, Patricia Thiel, Michael Tringides Graphene based spintronic devices require understanding the growth of magnetic metals. Rare earth metals have large bulk magnetic moments so they are good candidates for such applications, and it is important to identify their growth mode. Dysprosium was deposited on epitaxial graphene, prepared by thermally annealing 6H-SiC(0001). The majority of the grown islands have triangular instead of hexagonal shapes. This is observed both for single-layer islands nucleating at the top of incomplete islands and for fully completed multi-height islands. We analyze the island shape distribution and stacking sequence of successively grown islands to deduce that the Dy islands have fcc(111) structure, and that the triangular shapes result from asymmetric barriers to corner crossing. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G53.00003: QSE induced oscillatory electric field on stepped Pb(111) film and its influence on surface reactivity Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Hai-Qing Lin, Kai-Ming Ho, Michael Tringides When the thickness of ultrathin metal films is comparable to the Fermi wavelength , significant effects on the structure stability and the electronic properties emerge due to electron confinement. Using first-principles calculations, we showed that quantum size effects (QSE) can induce oscillatory electrostatic potential and thus alternating electric field on the surface of wedge-shaped Pb(111) films. The alternating electric field has significant influence adatom diffusion, leading to selective even or odd layer nucleation preference depending on the charge state of the adatom. This QSE induced alternatively modulated electric field is confirmed in growth experiments with the odd-layer preference of Mg adsorption on wedge-shaped Pb/Si(111) films. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G53.00004: 2014 Beller Lectureship: Quantum Size Effects: surface morphology and the stability of low dimensional structures Invited Speaker: Bene Poelsema Quantum Size Effects (QSE) play an important role in determining the surface morphology of certain epitaxial metal films on metal and semiconductor substrates. These give rise to distinct preferred film heights, imposed by a favourable relationship between the film's Fermi wave length and its interlayer spacing. QSE in thin films are usually observed on substrates with a surface projected band gap. We obtained evidence for QSE in Pb and Bi films deposited on Ni(111) [1]. For Pb on Ni(111) a classic QSE induced stabilization of specific Pb film thicknesses is obtained in a way very similar to Pb/Si(111). Slow heating of large QSE-stabilized Pb mesas leads to their ultrafast collapse and evidences collective motion of giant numbers of Pb atoms, resulting in mass transport rates much higher than expected form the activation energies derived from STM observations and DFT calculations for individual processes. For ultrathin Bi fims on Ni(111), the QSE lead to the evolution of distinctly different crystalline structures. No longer, a sequence of preferred heights with one unique crystal structure is observed, but rather the emergence of different crystalline structures with increasing thickness. This remarkable observation is attributed to the establishment of specific favourable relationships between the Fermi wave length and the interlayer spacing. The film's crystal structure is imposed by QSE, facilitated by the inclination of Bi towards allotropism. QSE not only lead to preferred thicknesses of thin layers (2D), as reported for various systems in the recent past, but can also lead to quantized nanowire lengths (1D), as we reported for Ir/Ge(001). The preferred Ir-nanowire lengths correspond to multiples of six unit cells and SPS measurements support the QSE induced nature of the length quantization. \\[4pt] [1] T. R. J. Bollmann, R. van Gastel, H. J. W. Zandvliet, and B. Poelsema, Phys. Rev. Let. 107, 136103 (2011); PRL 107, 176102 (2011). \\[0pt] [2] T. F. Mocking, P. Bampoulis, N. Oncel, B. Poelsema, Nature Commun. 4, 2387 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G53.00005: Long range repulsive interactions in Fe on epitaxial graphene Myron Hupalo, Xiaojie Liu, Steven Binz, Cai-Zhuang Wang, Wen-Cai Lu, Patricia Thiel, Kai-Ming Ho, Edward Conrad, Michael Tringides The understanding of metal nucleation on graphene is essential for promising future applications, especially of magnetic metals which can be used in spintronics. A common method to study the grown morphology is to measure the nucleated island density n as a function of growth parameters. Surprisingly the growth of Fe on graphene is found not to follow classical nucleation: n is unexpectedtly high, it increases continuously with the deposited amount $\theta $ and shows no temperature dependence. These unusual results indicate the presence of long range repulsive interactions. Kinetic Monte Carlo simulations and DFT calculations support this conclusion. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G53.00006: Tuning Fe nucleation morphology via charge doping of graphene substrate Wenmei Ming, Feng Liu Graphene with Fe deposition can be potentially used as magnetic storage device when Fe atoms assume island morphology, or as magnetic electric contact for spin injection in spintronics device when Fe atoms assume a uniform thin-film morphology. We propose that the Fe initial growth morphology on graphene can be tuned in a controllable manner via charge doping of graphene. On one hand, charge doping may either increase or decrease the bonding strength between Fe adatom and graphene, affecting Fe adsorption; on the other hand, it may modulate the Fe adatom--adatom interaction, affecting the Fe island nucleation. Using first principles calculations, we have investigated the following diverse effects of charge doping on Fe deposition on graphene as a function of doping concentration: (1) adatom adsorption energy; (2) local magnetic moment; (3) dipole moment; (4) elastic deformation energy; (5) adatom diffusion barrier; (6) adatom-adatom interaction. Furthermore, using kinetic Monte Carlo simulations augmented with first-principles parameters, we have studied the nucleation morphology of Fe deposition on the charge doped graphene. Our results shed new light on understanding and control of the growth morphology of metal atoms on graphene. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G53.00007: Diffusion of small Cu islands on the Ni(111) surface: Results of the self learning kinetic Monte Carlo(II) simulations Shree Ram Acharya, Syed Islamuddin Shah, Talat S. Rahman We have examined the diffusion of two dimensional Cu islands (up to 10 atoms) on the Ni(111) surface using Self-Learning Kinetic Monte Carlo (SLKMC-II)[1] method which allows occupancy of both fcc and hcp sites on the fcc(111) surface for the identification of local neighborhood of a diffusing atom.The SLKMC-II reveals various single-atom , multi-atoms and concerted processes on the fly and stores them in a database. Energy barriers for these processes are calculated using semi-empirical embedded-atom method potential. Here we discuss some of the novel processes and their energy barriers found during the simulations and compare them with those found for the diffusion of Cu islands on Cu(111). We also report temperature dependence of the diffusion constants and frequency of occurrence of single-atom, multi-atom and concerted processes for these islands. The size dependence of effective energy barriers derived from the Arrhenius plots is also discussed. [1].S.I.Shah, et al., J.Phys.: Condens. Matter 24(2012)354004 [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G53.00008: Further Developments in Characterizing Capture Zone Distributions (CZD) in Island Growth T.L. Einstein, Alberto Pimpinelli, Diego Luis Gonz\'alez As argued previously, analysis of the distribution of the areas of capture zones (i.e. proximity polygons [or Voronoi tesselations] with respect to island centers) is often the best way to extract the critical nucleus size in studies of epitaxial growth. For non-Poisson deposition (i.e. when island nucleation is not fully random) the areas of these Voronoi cells can be well described by the generalized Wigner distribution (GWD), particularly in the central region around the mean area where the distribution is largest. We discuss several recent applications to experimental systems, catelogued in a recent minireview,\footnote{TLE, AP, \& DLG, arXiv 1311.xxxx} showing how this perspective leads to insights about the critical nucleus size. In contrast, several (but not all) studies have shown that the GWD may not describe the numerical data from painstaking simulations in both tails.$^2$ We discuss some refinements that have been proposed, as well as scaling forms. Finally, we comment on applications to social phenomena. Emphasis is on very recent developments. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G53.00009: Inhibition of surface instabilities by combined action of electric fields and thermal gradients Lin Du, Dwaipayan Dasgupta, Georgios I. Sfyris, Dimitrios Maroudas Surface instabilities, such as the Asaro-Tiller-Grinfeld (ATG) and the Stranski-Krastanow (SK) instabilities, originating due to the competition between surface free energy and elastic strain energy, pose serious reliability problems for device fabrication. Elastic strain energy is stored in bulk-like crystalline solids due to externally applied or process-induced stress and in epitaxial thin films on substrates due to the lattice mismatch between the film and substrate materials. We demonstrate that proper application of sufficiently strong external field(s) can eliminate ATG and SK instabilities based on linear stability analysis according to a fully nonlinear three-dimensional model of driven surface morphological evolution. We find that the simultaneous action of an electric field and a thermal gradient, in conjunction with substrate engineering, is capable of reducing the critical external field strength requirement by several orders of magnitude. We also derive the conditions for the synergy or competition between the two external fields toward surface stabilization. We validate the linear stability theory by comparisons of its predictions with results of self-consistent dynamical simulations of electrically and thermally driven surface evolution. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G53.00010: Dislocation mechanisms in stressed crystals with surface effects Chi-Chin Wu, Joshua Crone, Lynn Munday Understanding dislocation properties in stressed crystals is the key for important processes in materials science, including the strengthening of metals and the stress relaxation during the growth of hetero-epitaxial structures. Despite existing experimental approaches and theories, many dislocation mechanisms with surface effects still remain elusive in experiments. Even though discrete dislocation dynamics (DDD) simulations are commonly employed to study dislocations, few demonstrate sufficient computational capabilities for massive dislocations with the combined effects of surfaces and stresses. Utilizing the Army's newly developed FED3 code, a DDD computation code coupled with finite elements, this work presents several dislocation mechanisms near different types of surfaces in finite domains. Our simulation models include dislocations in a bended metallic cantilever beam, near voids in stressed metals, as well as threading and misfit dislocations in as-grown semiconductor epitaxial layers and their quantitative inter-correlations to stress relaxation and surface instability. Our studies provide not only detailed physics of individual dislocation mechanisms, but also important collective dislocation properties such as dislocation densities and strain-stress profiles and their interactions with surfaces. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G53.00011: Unraveling micro-mechanisms of grain boundary migration using molecular dynamics simulation and reaction path techniques Shijing Lu, Donald Brenner Understanding grain boundary (GB) migration mechanisms plays a key role in understanding the deformation mechanics of nano-crystalline materials. Despite the many theories have been proposed, there still exists widespread disagreement in the research community. For instance, the normal direction diffusion model is often assumed in conventional grain growth models, but recent studies have suggested that shear-coupled grain boundary migration is dominate for high angle structures during stress driven dynamics. This study addresses the competition between the two mechanisms by using molecular dynamics simulations to characterize symmetric tilt grain boundary migration in response to an external driving force. The fundamental idea is to first determine an order parameters using principal coordinate analysis and then find the reaction pathways under different simulation conditions by minimum free energy path (MFEP) search techniques. Once the MFEP is found, the free energy profile for GB migration can be computed from thermodynamic integration. Our preliminary results show that migration behavior of a symmetric tilt grain boundary with various misorientation angles can be well represented by two order parameters, and surprisingly the MFEP for most misorientation GBs has a zigzag shape instead of the commonly observed a smoothed interface. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G53.00012: Investigation of Grain Boundary Grooving Using Phase Field Crystal Model Shang-Chun Lin, Kuo-An Wu Dynamics of Grain Boundary (GB) grooving plays an important role in microstructure evolution. Classical theory on GB grooving assumes the solid-solid interface as a homogeneous boundary where details of GB structures are ignored. This assumption clearly requires certain modifications for cases such as low angle GB. The advantage of phase field crystal (PFC) method is its capability to describe materials with atomic resolutions. We investigate how dislocations influence dihedral angle in low angle GB. Furthermore, we find interesting phenomena occur during GB grooving, such as grain rotation and dislocation translation, which provide an alternative way to control grain growth at the nanoscale. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G53.00013: Energetics of Cs in 3 grain boundary of 3C-SiC Pushpa Raghani Energetics of Cs defects at 3 grain boundaries of 3C-SiC has been studied using density functional theory to understand the role of the grain boundaries in Cs diffusion and its eventual release from the tristructural isotropic fuel particles (TRISO). Cs is shown to be much more stable at the 3 grain boundary than in bulk of SiC with a significant decrease (7 - 17 eV) in the formation energies at grain boundaries than in bulk. It is found to have even lower formation energies than those of Ag at the 3 grain boundaries, while this trend was opposite in the bulk SiC as demonstrated previously from similar density functional theory calculations. Based on these results, a possible route to control Cs release from SiC layer via grain-boundary-engineering is suggested. [Preview Abstract] |
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