Diffusion of vacancies in metal surfaces: theory and experiment

Surface vacancies were shown to be responsible for the motion of embedded In and Pd atoms in the Cu(0 0 1) surface. The density of surface vacancies at room temperature is extremely low, but they diffuse through the surface at an extremely high rate leading to significant diffusion rates of Cu(0 0 1) terrace atoms. In the STM measurements the rapid diffusion of these vacancies leads to long jumps of embedded tracer atoms. Measurements of the jump length distribution show a shape of the distribution that is consistent with the model that we discussed in Section 3. In turn, this shows that the vacancy-mediated diffusion process can be accurately described with the model that is presented in Section 3, provided that the interaction between the tracer atom and the surface vacancy is properly taken into account. The role of steps as sources and sinks for surface vacancies has been confirmed by measuring the position dependent jump length of embedded indium atoms. Near a step the jump rate of an indium atom is increased, but at the same time, given the shorter lifetime of vacancies near a step, the jump length is decreased. The effect of vacancy-tracer interaction was discussed for two different systems, In/Cu(0 0 1) (attractive interaction) and Pd/Cu(0 0 1) (repulsive interaction). The activation energies that are obtained for these two systems have to be interpreted differently. The measurements that we discussed comprehensively show that the motion of vacancies through a surface leads to significant motion of atoms in the terrace. Despite the fact that this motion is often impossible to visualize, it is an important mechanism for mass transport of atoms within a terrace. Moreover, as we showed in this chapter for the case of copper, in the more open (0 0 1) surface it is most likely the dominant mechanism. © 2003 Elsevier B.V. All rights reserved.