Adsorption of lead on low-index planes of tungsten

Abstract

Using probe-hole field emission microscopy the effect of adsorbed lead on the work function of the 100 and 110 planes of tungsten hasbeen studied and compared with the findings of Bauer et al. who studied the same system using LEED/Auger techniques. The effect of lead on the average work function \\ ̄ gf and that of (211) is also reported. Sub-monolayer lead increases φ(211) and this is ascribed to formation of a lead-tungsten dipole, the lead being negatively charged, with dipole moment 0.035 × 10 −30 C-m and polarizability 2.0 Å 3. On (110) lead reduces φ and behaves as a dipole with positively charged lead of moment 0.15 × 10 −30 C-m and polarizability 2.5 Å 3. φ(100) is also observed to decrease at low coverages equilibrated at low temperatures. This contrasts with Bauer's findings but is considered to result from failure of the Fowler—Nordheim model. With increasing lead coverage on all planes φ( hkl) tends to a constant value φ sat. By comparison with Bauer et al. we can identify φ sat on (110) as a compressed monolayer of lead. Likewise φ sat produced by low temperature (∼450 K) spreading on (100) is also associated with a compressed (1 × 1) structure. The lower value of φ(100) produced at higher temperatures (∼850 K) is identified with the microfacetted surface observed by Bauer et al. Lead is observed to be absent from (110) when mean adatom densities as high as 8 × 10 14 atoms cm −2 are thermally equilibrated, and this is shown to result from the relatively low binding energy of lead on (110). The general agreement between the present findings and those of Bauer lends confidence to the belief that both techniques can detect the same behaviour despite the very large (10 10) difference in the size of the area examined.