A study by high field microscopy of the effect of substrate surface structure on the work function of layers of group 1b metals adsorbed on tungsten

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Abstract The extent to which the microstructure of a tungsten substrate surface can affect the work function of an overlying layer of condensed metal has been investigated by field emission and field ion microscopy. The value of work function φmax which is produced by approximately 1 monolayer of gold is found to depend upon the smoothness of the substrate surface, ranging between 5.3 eV for gold on a field evaporated end form and 5.1 eV for a monolayer spread at low temperature (∼300°K) on a thermally annealed end form. Intermediate values of φmax which are obtained at higher spreading temperatures result from the known ability of gold to smooth the tungsten surface. This behaviour is consistent with the belief that it is the variation in degree of alignment of gold-tungsten dipoles formed by the first monolayer, which causes variations in φmax with surface roughness. From data obtained with a field evaporation end form, the dipole moment μ0 at zero coverage is 6.8 ± 1.7 × 10−30 C m with polarizability α=12.7±4.8 A 3 . The range over which φmax varies is found to be smaller for copper than for gold, and φmax for silver is found to be independent of surface roughness. The thermally induced transition in work function from 4.8 eV to 5.3 eV for θ > 1.7, which is the most striking feature of the gold-tungsten system, is shown not to depend upon the condition of the tungsten surface. It precedes the a and β rearrangements reported previously and takes place entirely within the gold layer. Observations of field evaporation of gold and the dependence of the transition on coverage and temperature, leads to the conclusion that the transition results from a structural reorganisation of gold. Reconstruction of each gold monolayer commences at (110) and, after heating to induce reorganisation at a given temperature, the extent of reconstruction within each monolayer is characteristic of the temperature used, and increases with distance of the monolayer from the substrate surface. This effect is ascribed to the progressive relaxation of mismatch between “normal” gold and its un-reconstructed form, which presumably lies close to that of tungsten. Some evidence is presented for the existence of a comparable reconstruction of copper layers, but no evidence has been found for reconstruction within silver layers. From a comparison of the behaviour of copper, silver and gold, it is concluded that the stronger the adsorbate-substrate bond, the greater is the effect of the substrate on the work function, and presumably the structure, of the overlayer. The overlayer attains its saturated “bulk” work function (and probably structure) more readily the stronger the interatomic binding within the layer.