Structure Determination of Disordered Metallic Sub-Monolayers by Helium Scattering: A Theoretical and Experimental Study

An approach based on He scattering is used to develop an atomic-level structural model for an epitaxially grown disordered sub-monolayer of Ag on Pt(111) at 38K. Quantum scattering calculations are used to fit structural models to the measured angular intensity distribution of He atoms scattered from this system. The structure obtained corresponds to narrowly size-dispersed compact clusters with modest translational disorder, and not to fractals which might be expected due to the low surface temperature. The clusters have up to two layers in height, the lower one having few defects only. The relations between specific features of the angular scattering distribution, and properties such as the cluster sizes and shapes, the inter-cluster distance distribution etc., are discussed. The results demonstrate the usefulness of He scattering as a tool for unraveling new complex surface phases.Comment: 5 pages, 3 figures, to appear in Surf. Sci. Lett. Related papers available at http://neon.cchem.berkeley.edu/~dani/He-papers.htm

He Scattering from Random Adsorbates, Disordered Compact Islands and Fractal Submonolayers: Intensity Manifestations of Surface Disorder

A theoretical study is made on He scattering from three fundamental classes of disordered ad-layers: (a) Translationally random adsorbates, (b) disordered compact islands and (c) fractal submonolayers. The implications of the results to experimental studies of He scattering from disordered surfaces are discussed, and a combined experimental-theoretical study is made for Ag submonolayers on Pt(111). Some of the main theoretical findings are: (1) Structural aspects of the calculated intensities from translationally random clusters were found to be strongly correlated with those of individual clusters. (2) Low intensity Bragg interference peaks appear even for scattering from very small ad-islands, and contain information on the ad-island local electron structure. (3) For fractal islands, just as for islands with a different structure, the off-specular intensity depends on the parameters of the He/Ag interaction, and does not follow a universal power law as previously proposed in the literature. In the experimental-theoretical study of Ag on Pt(111), we use first experimental He scattering data from low-coverage (single adsorbate) systems to determine an empirical He/Ag-Pt potential of good quality. Then, we carry out He scattering calculations for high coverage and compare with experiments. The conclusions are that the actual experimental phase corresponds to small compact Ag clusters of narrow size distribution, translationally disordered on the surface.Comment: 36 double-spaced pages, 10 figures; accepted by J. Chem. Phys., scheduled to appear March 8. More info available at http://www.fh.huji.ac.il/~dani

Stability of Disk and Stripe Patterns of Nanostructures at Surfaces

The ordering of two-dimensional Ag islands embedded in the Pt(111) surface layer has been investigated using He-atom scattering and scanning tunneling microscopy. At lower Ag coverage the embedded islands consist of compact clusters (disks) arranged into a short-range ordered two-dimensional array. At higher coverage the Ag islands have an elongated shape leading to a ''labyrinthine'' pattern of regularly spaced meandering stripes. As the temperature is increased, both the disk and stripe arrays of embedded Ag clusters transform reversibly into a disordered 2D Ag-Pt mixture. The observed behavior is explained in terms of strain-induced long-range interactions

Untersuchungen zum heteroepitaktischen Wachstum im Submonolagenbereich mittels Streuung thermischer Heliumatome

Heteroepitaxial growth of thin metal films on metal surfaces has been analyzed in the submonolayer regime by thermal helium atom scattering (TEAS). Experiments have been performed on two different substrates: the non-reconstructed Pt(111) surface and the Au(111) surface which exhibits a long-range (~ 22 x $\sqrt{3}$)-reconstruction. Ag was deposited on the Pt substrate and Co on the Au substrate. Based on the analysis of the growth of Ag on Pt(111) it is found that below T = 45K the monomer is the stable nucleus. At these low temperatures small islands are formed with mean size and separation depending on the deposition rate and temperature. It is shown that these structures are not thermally stable and that their arrangement is governed by kinetic processes. Upon increasing the deposition temperature the size of the critical nucleus increases and the growth mode changes from a three dimensional growth to a layer-by-layer growth based on a "step flow" mechanism. At all temperatures the growth of the first Ag layer is pseudomorphic (i. e. with the same lattice parameter and orientation as the Pt(111) substrate). Surprisingly, at a deposition temperature of about T = 600 K, however, submonolayer deposition of Ag again leads to the formation of small islands with an initial diameter of about 10 $\mathring{A}$. Furthermore, these islands are arranged in a rather regular pattern. The size as well as the separation of the islands depends neither on the deposition rate nor on the deposition temperature. To generate this structure the Pt(111) surface covered with submonolayer amounts of Ag only has to be heated once above a critical temperature of about T = 600 K. It is demonstrated that these quasi-regularly arranged islands represent the thermal equilibrium configuration. For this phase two distinct structural phase transitions are observed: (i) Upon increasing the Ag coverage the structure of the Ag islands changes from a two-dimensional array of compact islands (disk phase) with an average separation of about 30 $\mathring{A}$ at $\Theta$ = 0.2 ML to a one-dimensional "striped" pattern at coverages around $\Theta$ = 0.5 ML with approximately the same average distance of $\approx$ 30 $\mathring{A}$ but no preferential orientation of the stripes. (ii) An order-/ disorder-transition with a critical temperature of about T$_{c}$ = 620 K. At low coverage (disk phase) the regular arrangement of the islands is destroyed whereas at higher coverage (stripe phase) the one-dimensional structure startsto meander and to disintegrate into smaller clusters. The existence of disk and stripe phases as well as the observed transitions can be understood within a simple model based on elastic interactions (originating from the lattice mismatch of the Ag adlayer and the Pt(111) substrate). A main result of this model is that the ratio between the initial size of an island and the minimum separation between islands (i. e. the minimum periodicity of the pattern) is 1/3 and that this ratio does not depend on the symmetry of the pattern (stripes vs. disks). This result is corroborated by the present experiments. A comparison of the structure with a regular arrangement of Ag islands obtained after [...

Morphology of fcc Co(110) films on Cu(110)

The surface morphology of fee Co(110) films deposited on Cu(110) using oxygen as a surfactant is characterized by He atom scattering. Interference measurements reveal that the thicker Co films are quite flat and that this flatness is preserved after removal of the oxygen with atomic hydrogen. During growth of the first few layers a rough film morphology is observed which is related to the deconstruction of the Cu(110)-(2 x 1)O interface and the formation of a (1 x 2) reconstructed Co-Cu-O phase after deposition of the first Co monolayer. With increasing film thickness the measured step height changes from ( 1.28 +/- 0.02)Angstrom characteristic of the Cu(110) surface to (1.21 +/- 0.02)Angstrom attributed to the interlayer spacing of pseudomorphic fee Co(110). (C) 2000 Elsevier Science B.V. All rights reserved