Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F53: Focus Session: Growth of Metal Clusters and Islands on Surfaces |
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Sponsoring Units: DMP Chair: Bene Poelsema, University of Twente Room: Mile High Ballroom 2C |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F53.00001: Inhomogeneous strain of single-crystalline polyhedral gold nanocrystals revealed by coherent x-ray diffraction imaging Jong Woo Kim, Edwin Fohtung, Sohini Manna, Sebastian Dietze, Andrew Ulvestad, Ross Harder, Eric Fullerton, Oleg Shpyrko Coherent x-ray diffractive imaging was used to measure strain in gold nanocrystals grown by a single-step thermal chemical vapor deposition (CVD). Gold nanocrystals with well-defined facets such as triangular thin plates and octahedra were investigated. The inhomogeneous strain distributions were observed in both nanocrystals. This strain likely results from defects on the substrate in triangular plate nanocrystal. The resulting strain on the spherical surface of octahedral nanocrystal shows good congruence with theoretical prediction, but shows a discrepancy near the bottom surface on a silicon substrate. This inhomogeneous stain fields may be attributed to not only the dissimilar interface energies during growth, but also different thermal expansions between nanocrystals and the substrate after cooling down. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F53.00002: Electromigration-driven assembly of single-layer epitaxial islands on substrates: An approach to nanopatterning Dwaipayan Dasgupta, Dimitrios Maroudas We study an approach to surface nanopatterning based on the current-driven assembly of single-layer epitaxial islands on crystalline substrates. We develop a fully nonlinear model for the driven morphological evolution of single-layer epitaxial islands on crystalline elastic substrates with diffusional mass transport limited to the island edge. We validate our model by comparisons of simulation results for individual islands with experimental data for Ag island morphology and migration speed. We report oscillatory dynamics for islands on $\left\langle {110} \right\rangle $-oriented substrate surfaces and explore the dependence of the stable time-periodic state on the angle between the applied electric field and fast edge diffusion directions. Toward current-driven nanopatterning, we study the evolution of different-size island pairs driven to coalescence and its dependence on three key geometrical parameters: the sizes of the two islands of the pair and their center-to-center line misalignment with respect to the electric-field direction. We discover various patterns ranging from equal- and different-size stable steady island-pair configurations to many-island patterns that can be tailored by controlling the initial-pair geometrical parameters as well as the duration of application of the electric field. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F53.00003: Role of strain in the stability of hetero-epitaxial island on nanopillars Maxime Ignacio, Yukio Saito, Peter Smereka, Olivier Pierre-Louis Optoelectronics and microelectronics call for new techniques aiming at producing even smaller crystalline components of higher quality. Hetero-epitaxial growth on nanopatterned substrates such as nanopillar forests, is a promising direction to reduce mismatch strain and to obtain higher quality crystals. Indeed, 3D islands are grown on top of the pillars in a configuration which is similar to that of superhydrophobic liquid drops. However, as opposed to the case of liquids, elastic strain plays a major role in hetero-epitaxy. Using Kinetic Monte Carlo Simulations including elastic effects, we have studied in details the stability of a solid hetero-epitaxial island at the top of a nanopillar. We show that mismatch strain strain induces novel states for the island, including spontaneous symmetry-breaking and partial impalement of the islands in the nanopillars. Our results also suggest possible instabilities for solid-state catalytic particles governing nanowire growth.\\[4pt] [1] M. Ignacio, Y. Saito, P. Smereka, O. Pierre-Louis, preprint (2013).\\[0pt] [2] M. Ignacio, O. Pierre-Louis, Phys Rev. B 86 23410 (2012).\\[0pt] [3] K. Takano, Y. Saito, O. Pierre-Louis, Phys Rev B 82 075410 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F53.00004: Size-Ladder in Ripening by Cluster Diffusion Starting from Single Atoms Invited Speaker: Harald Brune We present a novel approach to create metal islands on close-packed single crystal metal surfaces with well defined sizes in the range of a few atoms. For elements with large cohesive energies, we observe that small clusters such as dimers and trimers diffuse as a whole at much lower temperatures than needed for their dissociation. Since the diffusion barriers increase with increasing island size we observe a stepwise increase of the mean island size from 1 to 2.5, to 4.5, to 7.0. The fact to able to produce large number densities of islands with these sizes enables to investigate the evolution of the chemical and physical properties with size in an atom-by-atom way. We demonstrate for the case of Co/Pt(111) how the experimental transition temperatures between the respective size plateaus can be used to infer monomer, dimer, and trimer migration barriers. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F53.00005: Controlled Growth of Silver Nanoclusters: A Molecular Dynamic Study Mine Konuk, Sondan Durukanoglu We have investigated the growth processes of various Ag nanoclusters with different shape and morphology. In order to understand the shape evaluation of nanocluster at the atomistic level, the energy barriers and reaction rates of different pathways are determined using nudged elastic band method and molecular dynamic simulations based on the potentials extracted from embedded atom method. Growth processes are controlled using varying initial nucleation conditions: deposition angle and rate, temperature, cluster size and shape. Our results show that the reaction conditions control the formation of atoms into clusters and determine the shape of nanocrystals. We also discuss our simulation results with the experimental studies based on the shape-controlled synthesis. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F53.00006: Interaction of Nanometer-Sized Gold Nanocrystals with Rutile (110) Surface Steps Revealed at Atomic Resolution Wenpei Gao, Ann Se Choi, Jian-Min Zuo We show that progress in atomic resolution Z-contrast imaging now enables a detailed understanding of nanocrystal (NC) interactions with surface steps. The interaction is studied based on the shape, orientation, strain and interfacial energy of Au NCs supported on surface steps of TiO$_{2}$ (110) surfaces with a small miscut angle. Au NCs with the approximate Au(111)$_{[-110]}\parallel $TiO$_{2}$ (110)$_{[001]}$ epitaxial relationship observed as it is on flat surfaces are selected for study. The presence of surface steps induces a small rotation in the NC in an amount less than the surface miscut angle. From the recorded images, we measured the atomic displacement around the surface steps. It shows that there is significant strain near the surface step inside Au NC. Experimental measurements of NC geometry on low miscut surfaces, however, reveal an approximate similarity in NC shapes. From this, an analysis of the average NC shape is performed using the modified Wulff-Kaishew theorem. Compared to NCs on flat surfaces, the measurements show a large height/width ratio, lowered interfacial energy and increased triple line energy for NCs observed on vicinal rutile (TiO$_{2})$ surfaces. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F53.00007: Structure and stability of stepped Au(111)/TiO$_{2}$(110) interface Bora Lee, Dallas R. Trinkle Au nanoparticles supported on TiO$_{2}$ surfaces has been widely studied due to its interesting catalytic properties. However, the Au/TiO$_{2}$ interface possesses a complex structure, making property determination difficult. In this study, Au layers on TiO$_{2}$ support associated with complex step structure have been investigated using energy density method (EDM) based on density functional theory. EDM provides the energy for each of atoms. This allows analysis of structure stability from the changes in atomic energy and work of adhesion is evaluated without spurious error, which leads accurate energy of complex interface structure of Au/TiO$_{2}$ with step configuration. We examine the changes with a stepped TiO$_{2}$ and Au surfaces; in particular, steps on surfaces of (110) TiO$_{2}$, (1x1) reduced surface and (1x2) added-row reconstructed surface, and both (001) step microfacet and with (111) step microfacet of Au (111) surfaces are considered. EDM results indicate that the step structure energy is localized, showing a large energy variation within one unit cell. The relaxed geometry of stepped interface Au/TiO$_{2}$ is consistent with the experimentally observed result with transmission electron microscopy. The detailed analysis including the charge density and electronic structures will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F53.00008: Composition dependent reduction of size-selected CoPt bimetallic clusters on Al2O3 thin film Bing Yang, Eric Tyo, Soenke Seifert, Ghassan Khadra, Juliette Tuaillon, Veronique Dupuis, Stefan Vajda Atomic ratio in CoPt bimetallic nanoparticles has a great impact on tailoring the oxidation state and catalytic performance of supported CoPt catalysts. Here, we produced size-selected CoPt bimetallic clusters with atomic precision in both size and composition, soft-landed on alumina thin films. Upon landing, an immediate oxidation of Co is observed and aging in air leads to further oxidation of both Co and Pt as characterized by XPS. In-situ grazing incidence X-ray Absorption Spectroscopy and Small Angle Scattering was performed to monitor the oxidation state, and the size and shape of the catalyst under reducing conditions, respectively. A strong composition dependent behavior is observed in the reduction of the two metallic components. Co reduction in CoPt cluster occurs at 65oC, while the reduction of other clusters (Co3Pt, Co, CoPt3) shifts to higher temperature range (105$^{\circ}$C-225$^{\circ}$C). Pt in all Pt containing clusters (Pt, CoPt3, CoPt and Co3Pt) compositions was reduced already at 25$^{\circ}$C. Our results open up the possibility to tune the physical/chemical properties of nanoscale matter by precise control of their atomic ratio. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F53.00009: The Investigation of Epitaxy and Morphology of Au on MgO (001), (110), and (111) Timothy Pulliam, Siddharth Gopal, Michael Pierce, Vladimir Komanicky, Hoydoo You, Andi Barbour, Chenhui Zhu Au nano-crystals serve a central role in catalysis and surface chemistry, with the catalytic properties of the crystals highly dependent on physical characteristics. Characteristics such as surface area to volume ratio, crystal symmetry, and surface energy define the catalytic properties. We present our analysis of the morphology of deposited Au on substrates and how they vary with macroscopic parameters. Au was evaporated onto single crystals of each of the MgO (001), (110), and (111) principal facets to study the epitaxy, morphology, and overall crystalline nature of the nano-particles on the substrates. The depositions were performed in vacuum at 700$^{\circ}$C using an e-beam evaporator. The samples were then analysed using x-ray diffraction (XRD) and atomic force microscopy (AFM) techniques revealing epitaxy, and morphology respectively. The samples were then annealed at progressively higher temperatures and the measurements repeated. Au nano-crystals deposited on TiO$_2$ (110) were also concurrently studied. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F53.00010: Microscopic insights into the pathways of mass transport in oxygen-induced reversible morphologic transformation of faceted rhenium surfaces Hao Wang, Wenhua Chen, Robert Bartynski The shape (morphology) of supported metal nanoparticles often varies under reaction conditions, which in turn can induce changes in their catalytic activity. Faceted metal surfaces, free of any support materials, can be used as model catalysts or templates for synthesizing new catalysts due to their well-defined facet structures and controlled facet sizes on the nanometer scale. Here we present reversible morphology changes on a faceted Re($11\bar{{2}}1)$ single crystal surface under ultra-high vacuum (UHV) conditions, which are controlled by tuning adsorbed oxygen coverage, using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find microscopic structural connections between the various morphologies on the faceted Re($11\bar{{2}}1)$ surface, which provide a natural explanation for the mass transport pathways in the structural evolution. Our findings motivate a more detailed future exploration of oxygen-induced morphology transitions on catalytically active metal single crystal surfaces, which is of importance for development of new catalysts operating under oxygen rich conditions. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F53.00011: Direct observation of frozen gallium gas on wurtzite gallium nitride (000\underline {1}) using low-temperature scanning tunneling microscopy Khan Alam, Andrew Foley, Wenzhi Lin, Joseph Corbett, YingQiao Ma, Jeongihm Pak, Arthur Smith Gallium nitride layers are ordinarily grown under gallium-rich growth conditions by molecular beam epitaxy (MBE) to obtain the highest material quality. In 1997, Smith \textit{et al.} reported the family of reconstructions existing on the growth surface at room temperature, the highest-order being the c(6x12).[1] Additional gallium deposition does not lead to new reconstructions. Instead, excess gallium atoms are presumed to exist in a 2-dimensional gas state. Using a custom-built MBE/low-temperature (4.2 K) STM system, we have imaged this gallium gas for the first time by freezing out the motion. The frozen-out gallium atoms are visualized as asymmetric `L-shaped' features, with left-handed and right-handed L's scattered randomly across the surface. Interestingly, on any given atomic terrace we observe a 4x greater probability of left-handed versus right-handed L's (or vice versa), which inverts across bilayer-height steps. The cause of this asymmetry is explored by zooming in with atomic resolution, revealing two inequivalent adsorption sites. [1] A. R. Smith \textit{et al.} Phys. Rev. Lett., \textbf{79}, 3934 (1997). [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F53.00012: Heteroepitaxial growth and surface structure of L1$_{0}$-MnGa(111) ultra-thin films on GaN(0001) Andrada-Oana Mandru, Reyes Garcia Diaz, Kangkang Wang, Kevin Cooper, Muhammad Haider, David Ingram, Noboru Takeuchi, Arthur R. Smith Ferromagnetic MnGa(111) deposited on semiconducting GaN(0001) is a promising system due to the interest in developing nitride spintronic systems and the observed ideal lattice matching and sharp growth interface. [1] The experiments are carried out in a molecular beam epitaxy (MBE) system interconnected to an ultra-high vacuum (UHV) analysis chamber containing room-temperature scanning tunneling microscopy (STM). Ultra-thin MnGa films (23 nm) are grown heteroepitaxially on GaN(0001) substrates, while maintaining the Mn:Ga flux ratio at about 1.09. After growth, the sample is transferred in-situ to the analysis chamber for STM and Auger electron spectroscopy (AES) studies. STM imaging reveals the presence of smooth terraces and angular step edges and also the existence of both 1$\times$2 and 2$\times$2 surface structures. Additional Rutherford backscattering spectroscopy (RBS) measurements help clarify the important relationship between surface and bulk. Theoretical work has also been carried out and the resulting structural models and simulated STM images for both surface structures are compared to the STM images. [1] E. Lu, D. C. Ingram, A. R. Smith, J. W. Knepper and F. Y. Yang, Phys. Rev. Lett. 97, 146101 (2006) [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F53.00013: Structural, electronic, and magnetic properties of the Mn3N2(001) surfaces J. Guerrero-Sanchez, Kangkang Wang, Noboru Takeuchi, Arthur R. Smith Structural, electronic, and magnetic properties of theMn3N2(001) surfaces have been investigated experimentally in recent years. The molecular beam epitaxy technique has been used for the sample preparation. Scanning tunneling microscopy measurements show two different surface terminations: a Mn3N2(001)-(1x1)structure with MnN-layer termination and a Mn3N2-c(4x2) structure with the formation of Mn tetramers [1]. Spin-polarized STM studies have revealed a spin anisotropy in antiferromagnetic Mn3N2(001) nano-pyramids. To explain these results it has been proposed an induced anisotropy as a result of the atomic strain generated by the absence of N atoms that drives the structure to a non-collinear magnetic configuration or by the formation of molecule-like Mn tetramers on the surface [2]. In this work we perform first principles total energy calculations to investigate the Mn3N2(001)-(1x1) and Mn3N2(001)-(2x2) surfaces. We have determined the structural, electronic and magnetic properties of bulk Mn3N2 and compare results with those reported previously. The most favorable configuration for both surfaces has been obtained by calculating the surface formation energy. We have also studied the electronic and magnetic properties of the most stable surface structures. References [1] Rong Yang.; et al. Appl. Phys. Lett. 88, 173101 (2006). [2] Kangkang Wang and Arthur R. Smith, Nano Lett. 12, 5443 (2012). [Preview Abstract] |
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