Reconstruction at a metallic interface studied by field ion and field emission microscopy

Reconstruction of a tungsten surface by adsorbed layers of gold, silver and copper has been studied by field emission and field ion microscopy. Gold reconstructs the surface in three ways, termed the α, β and γ rearrangements. The α rearrangement, which results in a smoothing of the tungsten surface, takes place at around 400° K with gold coverages of 5 monolayers (5θ), and is thought to be an increase in structural perfection of the tungsten surface by gold-assisted surface diffusion of tungsten atoms, β-reconstruction takes place in the temperature range 480–950°K at coverages ? 1.7θ, producing a faceted surface which comprises {211} and {110} facets, and is thought to result from the need to minimise the free energy at the gold/tungsten interface. The γ structure, which appears above 1400°K, is believed to represent a change in the shape of the tip by transport of tungsten to the (110) locality. Adsorbed silver produces neither β nor γ structures, and the degree of α rearrangement is very small, being confined to the {230} regions of the substrate. Copper lies between silver and gold in its ability to rearrange the tungsten surface, some degree of α rearrangement is detectable, and the β structure is very poorly developed unlike the γ structure which is clearly formed. The binding strength of copper to tungsten is greater than that of silver, but less than that of gold; the capacity of an adsorbate, to reconstruct the tungsten substrate is therefore thought to be related to the strength of the adsorbate-substrate bond.