Investigation of surface self-diffusion of Ni atoms on the Ni(100) plane by means of work function measurements and Monte Carlo calculations

Abstract

A novel method is presented to study migration of adsorbed metal atoms on a clean ( hkl) surface of a metal substrate. The system used was Ni on Ni(100). We make use of the assumption that each single step of random walk motion of an individual adatom on the metal substrate refers to an activation energy Δ E if (indices i and f giving the number of occupied nearest neighbour sites in the initial and final position respectively). Furthermore, we assume that the dipole moment p v of an adatom is determined by the number v of occupied nearest neighbour sites. Via their dipole moments the adsorbed atoms induce a change of work function ΔΦ of the substrate metal, ΔΦ being related to the average dipole moment per unit area by the Helmholtz equation. To measure ΔΦ we use a pendulum device. At fixed low coverage θ the variation of ΔΦ with temperature T has been calculated numerically within the framework of a Monte Carlo simulation (MCS) model, using various activation energies Δ E if and dipole moments p v as input parameters. By fitting these data to the experimental curves we could derive the following dipole moments and activation energies: p 0 = 0.45 ± 0.05 D, p 1 = 0.36 ± 0.05 D, p 2 = 0.27 ± 0.05 D, p 3 = 0.18 ± 0.05 D, ΔE 00 0.60 ± 0.02 eV, ΔE 10 = 0.70 ± 0.025 eV, ΔE 21 = 0.80 ± 0.05 eV, ΔE 20 = 0.90 ± 0.05 eV. We compare these results with those of other workers.