AVS1996 Session NS+SS-WeM: Properties of Nanostructures I

Wednesday, October 16, 1996 8:20 AM in Room 202A

Wednesday Morning

Start	Invited?	Item
8:20 AM		NS+SS-WeM-1 Dilute Magnetic Alloy of Fe in Cr(100) Y. Choi, I. Jeong, J. Park, S. Kahng, J. Lee, Y. Kuk (Seoul National University, Korea) It was known in the bulk that Fe can dissolve up to ~ 5% at 400 \super o\C into a Cr single crystal. The dilute Fe alloy can be a good candidate to study effect of exchange interaction in a magnetic alloy. Since Fe(100) and Cr(100) surfaces reveal surface states at different energy, this energy dependence can be utilized to chemically identify individual atoms in Scanning Tunneling Microscopy. As reported earlier\super 1\, individual Fe atoms can be identified when imaged at above Fermi level with the Fe coverage of <1ML. After annealing the surface, two types of alloying mechanisms were observed : Bulk alloying and surface alloying. The slight shift of Fe surface states was observed in those two types. We will propose a growth model of this Fe dilute alloy based on the STM study. Exchange interaction in the presence of magnetic field will also be discussed. ref. 1) J.A.Strosio, D.T.Pierce, A.Davies, and R.J.Celotta, Phys.Rev.Lett. 75, 2960 (1995)
8:40 AM		NS+SS-WeM-2 Electron Diffraction and High Resolution TEM Studies in Fe-Mg Thin Films X. Meng-Burany, L. Withanawasam, G. Hadjipanayis (University of Delaware) In the past, granular solids have been used in the investigation of size effects in the physical properties of small particles and clusters. In the last five years magnetic granular films have been studied extensively because of their novel magnetic properties including giant magnetoresistance. Most of the studies so far have been focused on Fe(Co)-Ag(Cu) granular solids. In this study we have prepared granular Fe-Mg films and we studied their structural and magnetic properties. Transmission electron microscopy, especially electron diffraction and high resolution imaging are essential for the understanding of size effects in nanoparticles. Fe-Mg granular films with Fe content in the range of 0-100% were made using magnetron sputtering. In this paper results for two of the films with 32%Fe and 48%Fe are reported. It is well known that it is difficult to distinguish between amorphous and small particles with a size of a few tens of nanometers, as well as, between crystalline Fe and Mg nanoparticles because their major d-spacings are close. Electron diffraction patterns obtained using a larger camera length show clearly the nanocrystalline diffraction ring to be embedded in the amorphous halo ring. High resolution images reveal the Mg d--spacings and therefore the location of Mg particles in the films. More detailed studies will be reported and correlated with the magnetic properties.
9:00 AM		NS+SS-WeM-3 STM Study of Atomic Structures on the Faceted Pd/W(111) Surfaces C. Nien, T. Madey (Rutgers University) The Scanning Tunneling Microscope (STM) has been used to study pyramidal faceting of Pd/W(111) surfaces, where (211) facets with dimensions ranging from ~ 3 to 15 nm can be induced by ultrathin films ( \>=\ 1 monolayer ) of Pd upon annealing [1]. With newly achieved atomic-resolution STM-images of these surfaces, we obtain direct confirmation of the (211) structure on individual facets of the 3-sided pyramids. In addition, the atomic structure of the facet edges implies an important role of the edge energy in this reconstruction. The STM data also shed light on the initial stages of the faceting transformation, revealing how each 3-sided pyramid is built up. When the as-deposited coverage of Pd is greater than the critical value ( ~ 1 monolayer ) for inducing faceting, the extra Pd atoms are found to form 3-dimensional clusters ( some having crystalline structures ) after annealing. To investigate the temperature dependence of the facet sizes in detail, we have developed a new statistical-analysis method for determining the size distribution of pyramids from STM images. The results of this analysis are used to quantify the increase in facet size with annealing temperature, and to determine that certain facet sizes exhibit unusual stability. [1] J. Guan, R.A. Campbell and T.E. Madey, Surf. Sci. 341 (1995) 311
9:20 AM		NS+SS-WeM-4 Characterization of Metal Clusters (Pd, Au and Ni) Supported on Various Metal Oxide Surfaces (MgO, TiO\sub 2\ and Al\sub 2\O\sub 3\) C. Xu, W. Oh, G Liu (Texas A&M University); D. Kim (Hallym University, Korea); D. Goodman (Texas A&M University) The chemical, electronic and morphological properties of various metal clusters (Pd, Au and Ni) supported on oxide thin films (MgO, TiO\sub 2\ and Al\sub 2\O\sub 3\) have been investigated with an array of surface science techniques including XPS, AES, ISS, and STM. Supported Au clusters exhibit an enhanced ability to adsorb CO as demonstrated by the TPD and FTIR measurements and a relatively high activity for CO oxidation at low temperature. The utilization of thin oxide films as a support allows the use of STM to study the morphology and local electronic structure of the metal clusters. A gradual nonmetal-to-metal transition is observed with increasing cluster size for Au and Pd. The thermal and chemical stability of the metal clusters as well as their interactions with the oxide supports have been also studied and will be discussed.
9:40 AM	Invited	NS+SS-WeM-5 Electrochemical Nanostructuring of Metal Surfaces D. Kolb, R. Ullmann, T. Will, J. Ziegler (University of Ulm, Germany) Metal clusters of a few Angstroms in height can be brought onto flat surfaces at predetermined positions via an STM tip, onto which the metal was deposited. The size of these tip-induced clusters can be varied in height between one and several atomic layers by the tunneling parameters. The mechanism of the cluster transfer from tip to surface, which involves a nondestructive tip approach initiated by either a sign change in tunnel voltage or a voltage pulse for the z-direction applied directly to the tube scanner, has been investigated. With the latter technique large cluster arrays for electrocatalytic studies and nanowires are readily manufactured. In the case of Cu it is shown that the shape of the clusters can depend markedly on the nature of the substrate onto which they are placed. While on gold surfaces three-dimensional clusters are formed, two-dimensional islands are created on silver substrates. The selective dissolution of individual Cu clusters by very positive tip potentials is demonstrated. Preliminary studies of the electrochemical stability of Cu clusters as a function of their size have been performed, and the effect of Ostwald ripening on the stability of Cu islands on Ag(111) is demonstrated.
10:20 AM		NS+SS-WeM-7 Electrochemical Deposition of Metal Nanocrystallites on the Atomically Smooth Graphite Basal Plane: Mechanism of Growth and Particle Size Control R. Stiger, J. Zoval, R. Penner (University of California, Irvine) Although electrochemical metal deposition has been investigated for over a century, only within the last year has it become possible to electrocrystallize extremely small particles having radii of less than 100 \Ao\. We have shown that the pulsed potentiostatic deposition of metals (Ag, Pt, and Cu) onto atomically smooth basal plane graphite electrode surfaces at coverages between 10\super -4\ and 10\super -2\ monolayers can yield dispersions of nanocrystallites which are narrowly distributed in size. These particle dispersions can be employed for fundamental investigationsof metal particle behavior, for probing metal particle size effects in electrocatalysis, and as precursors for semiconductor synthesis. In this paper, we focus on the mechanism of growth which gives rise to these dispersions with particular emphasis on understanding the origin of metal particle size polydispersity. In the simplest case - exemplified by silver deposition - deposition proceeds by an instantaneous nucleation and diffusion limited growth. Monte Carlo modeling of the growth process for this case allows for unambiguous identification of the principle contributors to polydispersity. With this information, a clear strategy for minimizing polydispersity can be proposed involving pulsed potentiostatic deposition. With other metals, such as Cu and Pt, the mechanism becomes complicated by kinetic effects and adsorption of the metal ion (or metal ion complex) at the electrode surface. Nevertheless, high overpotential deposition of these metals yields nanocrystallite dispersions with characteristics qualitatively similar to those of Ag.
10:40 AM		NS+SS-WeM-8 Electrochemical Synthesis of Semiconductor Nanocrystals M. Anderson, R. Nyffenegger, G. Hsiao, R. Penner (University of California, Irvine) In this presentation we show an electrochemical pathway towards the synthesis of metal- and subsequently semiconductor structures in the nanometer size regime. Semiconductor nanocrystallites have been formed directly on a highly oriented pyrolitic graphite (HOPG) surface by plating in metal ion containing electrolytes. A size dependence of the optical properties of these semiconductor nanocrystallites has been observed and is compared to size distributions found with scanning probe microscopy. Potential steps with time lengths of 10 to 100 ms were employed to deposit metal nanostructures onto the surface. These metal structures were then converted by chemical processes to semiconductor material by the introduction of the appropriate anion, e.g. I\super -\ to transform Cu into CuI. Alternately, certain semiconductor materials can be formed by direct electrochemical reduction, e.g. ZnO. In order to correlate the topography revealed by non-contact AFM imaging and the fluorescence signal, it was attempted to 1. reduce the spot size in the optical experiment to dimensions usually covered by scanning probe microscopy, and, 2. to investigate the very same nanocrystals by re-locating the imaged area later in the spectrograph.
11:00 AM		NS+SS-WeM-9 Two Dimensional Electrocrystallization of Metal Films on Thiol Covered Au(111) Electrodes S. Gilbert, O. Cavalleri, K. Kern (Ecole Polytechnique Federale de Lausanne, Switzerland) The electrochemical deposition of Cu on Au(111) surfaces covered by alkanethiol self-assembled monolayers has been followed in-situ by electrochemical STM. Monolayers formed by thiols of different chain lengths have been used to create organic spacer layers of systematically controlled thickness onto which electrocrystallization of Cu has been achieved. Cyclic voltammetry shows the absence of underpotential deposition peaks, while in-situ STM has revealed the formation of a uniform distribution monolayer-high Cu islands in the UPD potential region. These Cu islands range in diameter from 3-5 nm regardless of thiol chain length. Island formation in the UPD region is the initial phase of Cu layer growth. As potentials are brought into the bulk deposition (overpotential) region, 2D growth of Cu layers is observed on the thiol films composed of molecules whose chain lengths are 12 carbons or less. For longer chain thiols, no further deposition is observed beyond the island formation stage, regardless of overpotential. Results from in-situ IR and quartz crystal microbalance studies will be presented as well, to shed greater insight to the film growth process.