The growth, structure, and thermal stability of vapor deposited ultra-thin metal films: Rh on Ag(100), Au on Pd(110), and Pt on Pd(110)

摘要

The growth, structure, and thermal stability of ultra-thin metal films is investigated using surface sensitive techniques. The three systems studied present a variety of differing characteristics which can contribute to the growth mode, two-dimensional structure, and thermal stability of the films. The main factors contributing to the differing properties of the three systems presented here are: (1) the different substrate morphologies; (2) the differences in surface free energies between the overlayer and the substrate; (3) the degree of lattice mismatch for a particular system; and (4) the extent of miscibility of the two metals. These characteristics allow a means of comparison for the three systems studied;The Rh films are found to grow in a pseudo layer-by-layer manner on Ag(100) at 300 K. At higher temperatures Ag migrates to the surface of the Rh films forming a Ag-Rh-Ag sandwich structure. The differences in surface free energies, the high mobility of the Ag atoms, and the immiscibility of the two metals is thought to govern the equilibrium structure of this system;The Au films on Pd(110) grow layer-by-layer for the first two layers, followed by a trend toward three-dimensional growth for thicker films. The Au films reconstruct to a (1 x 2) superstructure at two monolayers, and to a (1 x 3) structure at higher coverages. This transition between the (1 x 2) and (1 x 3) is postulated to originate from a break down in the two-dimensional order as three-dimensional growth begins, which favors the formation of the (1 x 3) structure;The Pt films on Pd(110) follow a pseudo layer-by-layer growth at low temperatures, with three dimensional growth occurring at higher temperatures. The (1 x 2) and (1 x 3) superstructures are also observed for this system, the transition between the two being temperature dependent. Presumably the Pt is kinetically trapped into layer-by-layer growth at low temperatures. However, at higher temperatures three dimensional growth begins resulting in a similar transition between the two phases as is observed for Au on Pd(110);Fundamental research on metal overlayers, such as that described here, provides meaningful information on phenomena which govern interface formation and film growth processes.