Papers

AVS1997 Session SS1-TuM: Diffusion and Nucleation of Molecules and Clusters

Tuesday, October 21, 1997 8:20 AM in Room A3/4

Tuesday Morning

Start	Invited?	Item
8:20 AM		SS1-TuM-1 Effects of Vacancies and On-Top Adatoms on Mobilities of Two-Dimensional Pt Clusters on Pt(111) V. Chirita, E.P. Munger (Linkoping University, Sweden); J.E. Greene (University of Illinois, Urbana-Champaign); J.-E. Sundgren (Linkoping University, Sweden) We use molecular dynamics simulations to follow the dynamics of small two-dimensional (2-D) Pt clusters on Pt(111) at 1000 K, and the effects that adatoms and vacancies have on their mobilities. The structures investigated were those of close-packed hexagonal Pt₇ and Pt₁₉ clusters. As expected, the close-packed clusters were found to be extremely stable, characterized by high dissociation energies and small diffusion rates. However, the addition of single cluster vacancies and/or on-top adatoms dramatically changes the migration rates of Pt₇ clusters. It is found that both vacancies and adatoms induce rapid intracluster bond breaking, resulting in cluster reconfigurations, rotations, the introduction of stacking faults, and greatly enhanced cluster diffusion rates. Mapping center-of-mass motion for total simulation times > 145 ns revealed increases in average cluster velocities by much more than an order of magnitude. Cluster migration occurs primarily by concerted motion of all cluster atoms and a novel diffusion mechanism involving double shearing of dimers/trimers. Contrary to previous reports, edge-atom diffusion plays only a minor role in the overall migration of small clusters. The interaction of adatoms and vacancies with the larger Pt₁₉ structures is also interesting but does not have a significant effect on cluster mobilities. The ring-like Pt₁₈ clusters, obtained by removing the central atom in Pt₁₉ clusters, are also stable structures, maintain their shape and exhibit small diffusion rates over total simulation times of > 50 ns. The addition of on-top adatoms does not alter the diffusion rates of Pt₁₈ clusters. Adatoms are found to descend onto the terrace, primarily through push-out/exchange mechanisms. However, our simulations reveal, for the first time, that this mechanism can occur at the inner edge of the ring, i.e. adatoms can fill in cluster monovacancies.
8:40 AM		SS1-TuM-2 Cluster-Step and Cluster-Cluster Coalescence: Post-Deposition Dynamics in Ag/Ag(100) Adlayers C.R. Stoldt, J.-M. Wen, C.J. Jenks, P.A. Thiel, A.M. Cadilhe, J.W. Evans (Iowa State University) Post-deposition evolution of Ag/Ag(100) adlayers, for coverages below about 0.3ML, was recently shown to be dominated by the diffusion of large near-square 2D adatom clusters, rather than by their Ostwald ripening.¹ Here we report that this feature leads to dramatic morphological changes in the adlayer associated with: (i) collision and subsequent merging of clusters with monoatomic steps; (ii) both side-on and corner-to-corner collision of pairs of clusters, and their subsequent restructuring to form single larger near-square clusters. Scanning Tunneling Microscopy (STM) studies of the relaxation of such far-from-equilibrium step-edge nanostructures can provide more detailed information about the underlying mass transport mechanisms than can be obtained, e.g., from analyses of near-equilibrium step-edge fluctuations.² We describe extensive STM observations of such nanostructure evolution at 295K, indicating the dependence of characteristic times on step orientation and other local features of the morphology. Comparison with appropriate atomistic models for step-edge evolution facilitates extraction of rates or activation barriers for the atomic hopping or diffusion processes at the step edge which control this evolution. ¹J.-M. Wen, et al., Phys. Rev. Lett. 73 (1994) 2591; 76 (1996) 652. ²E.D. Williams, Surf. Sci. 299/300 (1994) 502.
9:00 AM		SS1-TuM-3 A New Understanding of Island Decay on Metal Surfaces M. Giesen (Forschungszentrum Jülich, Germany); J.B. Hannon (Sandia National Laboratories); C. Klünker, G. Schulze Icking-Konert (Forschungszentrum Jülich, Germany); N.C. Bartelt (Sandia National Laboratories); H. Ibach (Forschungszentrum Jülich, Germany); J.C. Hamilton (Sandia National Laboratories) The theory of the decay of two dimensional islands considers two cases. In the first case, the decay of the island area is limited by the terrace diffusion. Then, the island area decays approximately according to a t ^2/3 - law. In the second case, the island decay is limited by the attachment/detachment of atoms at the island edges. Then, the island area decays linearly in time. The attachment/detachment limited kinetics occurs when the sticking coefficient of the mass carrying species to the island edges is much smaller than 1. So far, in all experimental investigations on low indexed metal surfaces the measured time exponent was near 2/3, consistent with the understanding that there should be no activation barrier for the attachment of an atom on the terrace to the lower edge of the step. In this work we report that the island decay can be attachment/detachment limited also on metal surfaces. Using scanning tunneling microscopy, we have found for Cu (111) that the island decay is approximately according to a t ^2/3 - law. For Cu (100), however, the time dependence of the island decay deviates significantly from the t ^2/3 - law. For small islands, we find that the islands decay linearly in time. In order to exclude that the differences in the time laws measured on Cu (100) and (111) could be due to the influence of the different island environments on the time dependence, we have also performed computer simulations and compared them to the experiments. We conclude that for Cu (111) the island decay is diffusion limited whereas for Cu (100) the decay is attachment/detachment limited. We attribute the different kinetics to a different mass transport mechanism for the Cu (100) and the (111) surface. For Cu (100) we propose that vacancies rather than adatoms are the mass carrying species during the ripening process.
9:20 AM		SS1-TuM-4 Island Diffusion and Coarsening on Metal (100) Surfaces W.W. Pai, A.K. Swan, Z. Zhang, J.F. Wendelken (Oak Ridge National Laboratory) The diffusion and coarsening of two-dimensional homoepitaxial islands on Cu(100) and Ag(100) surfaces at room temperature have been studied using time-sequenced scanning tunneling microscopy. The results indicate that island diffusion occurs according to a new model of periphery diffusion which is mediated by the edge kink distribution. By assembling the STM images into movies, it becomes apparent that smaller islands move much faster than larger islands, and that coarsening occurs primarily through island collisions. To quantify the above observation, computer-based image analysis was used to precisely measure both the size dependence of the island diffusivity and the dynamics of island coarsening. We found the diffusion coefficients, D, as a function of island side length, L, can be fit to a power law, D~L^-alpha, where α exhibits a non-integer value of ~2.5 in the case of Cu and ~2.2 in the case of Ag. Corroborating these values of α, the island distribution in both cases was found to coarsen with time as ~t^0.23. In sharp contrast to the theoretical models which predict integer exponents of 1, 2, or 3 for α depending on the diffusion mechanism, our results strongly suggest that these models are oversimplified. A recent theoretical study by Khare et al. indicates that even when mass transport is strictly confined to island edges, the effective value of α can range continuously from 1 to 3¹. Attachment and detachment of atoms from kink sites as the "rate limiting" step can explain such continuous variation of α when the kink-kink separation distribution is accounted for. Our results support this model when reasonable kink densities are considered.² ¹S. V. Khare and T. L. Einstein, PRB 54, 11752 (1996). ²Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corp. for the U.S. DOE under Contract No. DE-AC05-96OR22464
9:40 AM		SS1-TuM-5 The Diffusion of Large, Adsorbed Molecules on Metal Surfaces K.A. Fichthorn (Pennsylvania State University) Although the diffusion dynamics of adsorbed atoms and small molecules have been the subject of many investigations, relatively few studies have focused on the diffusion of large, adsorbed molecules. To probe the unique features of molecular diffusion, we have conducted molecular-dynamics-simulation studies of diffusion in a series of physically adsorbed n-alkanes (C2-C50) on a model Pt(111) surface. In addition, we have used transition-state theory to estimate theoretical diffusion coefficients for these molecules and to determine the characteristic mechanisms by which they diffuse. These combined studies have yielded much insight into the unique mechanisms of molecular diffusion and their ramifications for molecular diffusion coefficients. Our studies also highlight differences between relatively small molecules, which diffuse in a concerted manner and have unique (but related) diffusion barriers, and large molecules, which share a common diffusion barrier that is independent of chain length. We discuss the similarities between the diffusion of large adsorbed molecules and that of atomic clusters.
10:20 AM		SS1-TuM-7 Mechanisms for Vacancy Formation in the Rh/Ag(001) System* L.D. Roelofs, C.J. Rockwell (Haverford College); R.J. Behm (Universität Ulm, Germany) Room temperature deposition of Rh on Ag(001) results in a complicated morphology¹. At low Rh coverage (θ ≤ 0.05 ML) one sees via STM the incorporation of some of the Rh and the growth of small islands of mixed Ag and Rh constitution. At higher coverages (0.5 ≤ θ ≤ 1 ML) one sees apparent fjords etched into surface steps, multiple layer islands and multiple layer extended vacancies (pits) of appreciable size. Such etching phenomena are not generally seen in bimetallic systems. To elucidate the underlying mechanisms we have developed a simulation of the kinetic behavior of this system. The activation energies which parametrize our simulation are obtained from total energy calculations and from experimental input. The simulation reproduces the main features observed in the experiment and reveals two mechanisms by which the pits can form. We display simulation results reproducing the observed morphology in general, present the kinetics parameters that are the basis for the simulation, and describe two mechanisms by which the vacancies form. Both mechanisms probably play a role in the actual physical system. *Supported by the Alexander von Humboldt Foundation and the NSF under DMR 95 10623. ¹S.-L. Chang, J.-M. Wen, P. A. Thiel, S. Günther, J. A. Meyer and R. J. Behm, Phys. Rev. B53, 13747 (1996).
10:40 AM		SS1-TuM-8 Kinetics of Ultrathin Film Growth and Island Thickening of Au on TiO₂(110) S.C. Parker, A.W. Grant, C.T. Campbell (University of Washington) The kinetics and energetics of the various basic migrational steps of metal adatoms on oxide surfaces and over metal particles on these oxides are fundamental in the understanding of film morphology in epitaxial growth, dispersion of oxide-supported metal catalysts, and strength of metal / ceramic contacts. Vapor deposition of late transition metals on single crystal oxide surfaces has been used to explore these elementary processes. Generally, 2-D islands form until a certain "critical coverage" is reached, after which the islands grow thicker and their lateral growth is significantly suppressed. Measurable characteristics of the metal islands such as their separation, thickness, area, density, and coverage depend upon variables such as time, temperature, and flux as well as the kinetics and energetics of metal atom migration steps. By examining Au deposition on TiO₂(110) with XPS, LEIS, SPA-LEED, and STM, information about these kinetics and energetics has been obtained. When heated, these metal islands irreversibly thicken, exposing more of the oxide surface. These thickening kinetics further elucidate details of these atomic steps. Chemical modification of the surface also affects these parameters.
11:00 AM		SS1-TuM-9 Growth and Epitaxial Studies of Nanometer Au islands on TiO₂ (110) L. Zhang, F. Cosandey, T.E. Madey (Rutgers, The State University of New Jersey) We have studied the growth and epitaxy of individual three-dimensional (3-D) Au islands on the TiO₂ (110) surface by using High Resolution Scanning Electron Microscopy (HRSEM) and Electron Backscattered Diffraction (EBSD) in the HRSEM. The Au was evaporated under UHV conditions onto clean and stoichiometric TiO₂ (110) with substrate temperatures ranging from 300 to 775 K. In this temperature range, Au grows as 3-D islands on TiO₂ (110). At 300K, the island morphology evolves with increasing Au thickness, from hemisperical islands initially, to coalesced worm-like island structures, to percolation at ~8.5 nm average Au thickness. At high temperature and for high Au thickness (>5nm), Au islands exhibit crystalline shapes. Epitaxial studies of individual islands have been performed both on high temperature annealed surfaces, and for samples onto which Au is deposited at high temperature. For deposition at 300K and annealing at 775K, Au islands are highly faceted with hexagonal shapes. EBSD studies of individual Au clusters (average diameter ~80nm) show that the majority are oriented along the fcc Au [111] direction, which is parallel to the TiO₂ [110]. In plane [11-2] Au is parallel to [1-10] TiO₂. Domains corresponding to two epitaxial variants that are rotated by 180 have been determined. In contrast, for deposition and growth at 775K, the Au clusters are oriented quite differently; the [11-2] direction is parallel to the [110] TiO₂ surface normal, and in plane [111] Au is parallel to [1-10] TiO₂.
11:20 AM		SS1-TuM-10 Intermixing and Growth Anisotropy for Ni/Au(111) W.G. Cullen, P.E. Quesenberry, P.N. First (Georgia Institute of Technology) Growth of Ni on reconstructed Au(111) has been studied in the coverage range 0.02 - 5 ML by Scanning Tunneling Microscopy (STM) and Auger Electron Spectroscopy (AES). It is well established that the elbows of the 22 x √ 3 herringbone reconstruction contain dislocations of the surface lattice which serve as nucleation sites for Ni, as well as for the transition metals Fe, Co and Rh. Submonolayer islands are two dimensional and hexagonally shaped with edges along close packed directions, as found in previous studies. Our results indicate that the islands have a tendency toward threefold symmetry, and an anisotropy exists in the growth direction of islands nucleated at alternate elbows, i.e. at "pinched in" or "pinched out" elbows. This is accompanied by an alternation of island orientation by 180 degrees on alternate elbows. The anisotropic features are readily observed at a coverage of 0.25 ML, and are seen to be a consequence of the crystallography of the reconstruction. Noticeable structure exists within the islands, with topographic variations of approximately 0.25 Å, suggesting intermixing. Growth morphology at higher coverage demonstrates a persistence of the two-dimensional layer-by-layer growth mode. Annealing results in dramatic changes in island morphology for the submonolayer growth, and is accompanied by an increase in the amount of Au in the surface layer throughout the coverage range, as determined by AES. Results will be presented in comparison with the systems Co/Au(111) and Fe/Au(111).¹ ¹Supported by ONR and NSF
11:40 AM		SS1-TuM-11 Effects of Low Energy N₂⁺ Irradiation on Surface Morphological Evolution during TiN(001) Epitaxy by Reactive Magnetron Sputtering J.E. Greene, B.W. Karr, I.G. Petrov, D.G. Cahill (University of Illinois, Urbana) The group IV-B transition metal nitride TiN is widely employed as a wear-resistant coating on mechanical components and as a diffusion barrier in microelectronic devices. We use the epitaxial growth of single crystal TiN as a model system for insight on the evolution of surface morphology and microstructure in more complex polycrystalline films. Atomically-flat MgO substrates, prepared by air annealing at 950°C for 12 hours, are verified by atomic force microscopy (AFM). Epitaxial TiN layers are grown on MgO(001) by reactive magnetron sputter deposition in pure N₂ at 650 < T_s < 750°C. Without energetic ion bombardment, our scanning tunneling microscopy (STM) results show that the development of surface morphology is dominated by growth mounds with an aspect-ratio of approximately 0.006; both the roughness amplitude and average separation between mounds follow an approximate power law dependence on film thickness, t^gamma, with γ = 0.25±0.07. The island edges exhibit dendritic geometries characteristic of limited step-edge mobility. Low-energy N₂⁺ ion irradiation during film growth at T_s = 650°C leads to both surface and step edge smoothing when V_s is increased from 3 to 43 V. These results provide the first direct evidence that low energy ion irradiation can reduce low-temperature kinetic surface roughening. However, increasing T_s to 750°C reduces the effects of N₂⁺ ion bombardment between 3 and 48 V, but increasing V_s > 48 V leads to enhanced surface roughening with decreased in-plane length scales resulting from bulk defect production