Field emission microscopy of gold on (110), (100) and (211) tungsten surfaces

Abstract

Work function changes produced by gold adsorbed on the (100), (211) and (110) planes of tungsten were studied by field emission microscopy in an attempt to correlate behaviour observed in field emission with that deduced from low energy electron diffraction and reflection high energy electron diffraction studies. The initial increase in φ 100 agrees well with earlier findings and also with that observed in a low energy electron diffraction study and is therefore attributed to the formation of a P(2 × 1) array of gold adatoms. The transformation of this array into a compressed Au(100) layer, which takes place at a critical coverage on a low energy electron diffraction specimen, is accompanied by a sharp increase in work function. A similar sharp increase in φ 100 which has been observed on a field emitter contrasts with the more gradual increase in φ 100 which takes place on our field emitter substrates. Our inability to reproduce this sharp increase in φ leads us to the tentative conclusion that this transformation is nucleated at steps in the W(100) surface, and that on the (100) surface of an annealed field emitter the absence of such steps prevents transformation at a critical coverage. At spreading temperatures below 500 K gold atoms behave as point dipoles and are thought to occupy sites along the (211) furrows. Continued addition of gold then leads to the development of an Au(110)-like structure. Above 500 K, point dipole behaviour is no longer seen because of reorganization at the gold-tungsten interface. The gold layer which develops on this reconstructed surface undergoes a sharp increase in φ at a critical coverage, and this is tentatively attributed to a structural transformation from a loose surface structure into a normal Au(110) surface with a work function of 5.45 ± 0.03 eV. The binding energy of gold on W(110) is about 1 eV lower than that on other planes; consequently adsorbed gold at submonolayer coverage vacates the plane at temperatures above 330 K. Below 330 K gold atoms cannot escape from the plane and are believed to form two-dimensional clusters on the W(110) surface which grow and coalesce to form a strained Au(111) structure with φ = 5.2 eV; this is complete at about 1.3 θ . With increasing coverage, φ increases as islands with an Au(111) structure are formed. This gold layer is complete at θ ≈ 3.5 when φ = 5.32 ± 0.03 eV, and low energy electron diffraction indicates that the layer has Au(111)|W(110) with Au[121] rotated by 2.5° from W[110]. Three-dimensional gold crystallites which are observed by low energy electron diffraction to form at higher coverages do not form on the microscopic plane, probably owing to the absence of steps in the (110) surface at which such structures could be nucleated.