The adsorption of copper on tungsten

Abstract

A detailed examination has been made by field emission microscopy, of the behaviour of small amounts of copper evaporated onto a clean tungsten point in ultra high vacuum. With increasing thickness, the behaviour of the copper deposit and its effect on work function, change in a way which leads to classification of three types of copper layer. Small amounts of copper form a layer on the surface which raises the work function from 4.52 to 4.83 eV, and can be desorbed therm ally with activation energy 95±6 kcal/mole. It is termed type I, and is believed to form the first monolayer of copper in which all adatoms are bonded directly to tungsten and are therefore polarized negative outward, giving rise to the observed increase in work function. Copper adsorbed upon the type I layer forms type II, which lowers the work function to 4.2 eV, diffuses over the tip surface with activation energy 20 kcal/mole, and is desorbed therm ally with activation energy 70 ± 7 kcal/mole. It is believed to comprise a layer 2 atoms thick which forms a copper lattice strained to conform to the substrate structure. Copper absorbed upon the type II layer is of type III. It is less strongly bound than type II, being desorbed with activation energy 64±3 kcal/mole and diffusing over the tip surface with activation energy 12kcal/mole. The increase in measured work function from 4.2 to 4.3 eV produced by type III is thought of about 3 atom layers, and addition of further copper produces nuclei which are thought to be crystallites having a true copper lattice structure. The observations reported are believed to be consistent with a description of epitaxial overgrowth given by van der Merwe (1963), and thus contrast with much other work on epitaxial overgrowth of pairs of metals with differing lattice dimensions.