Adsorption of nickel on tungsten: a probe-hole field emission microscopy study

In a probe-hole field emission microscopy experiment, submonolayer amounts of high-purity nickel were deposited on to a tungsten tip in clean, ultra high vacuum conditions and observations were made on W(1 1 0), (1 2 1) and (1 1 1) planes. The changes in the work function, Δφ, and corresponding pre-exponential factors, ln B, have been obtained by ten point Fowler–Nordheim plots. The results show that the change in average work function of nickel covered tungsten surface increases and attains a saturation value of +0.52 eV. This is accompanied by a decrease in the pre-exponential factor. Adsorption of nickel on the W(1 1 0) plane causes a decrease in Δφ in the lower coverage region and then exhibits an abrupt change to reach a saturation value of +0.5 eV as the coverage increases. This has been attributed to surface reconstruction. In the case of the W(1 2 1) plane Δφ decreases with the number of doses to a value of −0.45 eV. On the W(1 1 1) plane, the Δφ increases and saturates to a value of +0.50 eV after a brief initial decrease in the low-coverage region. The changes in the work function are explained in terms of atomic and electronic structure of nickel overlayers on various single-crystal planes of tungsten. A comparison of nickel with iron and cobalt adsorption on tungsten shows that the behaviour of nickel is similar to iron.