Atom and cluster diffusion on Re(0001)

Diffusion of Re and W adatoms and Re clusters has been surveyed on the clos e-packed (0001) surface of rhenium, the first study of atom motion on an hc p metal. From the temperature dependence of the diffusivity, the activation energies to diffusion are found to be 11.11+/-0.43 and 11.04+/-0.34 kcal m ol(-1) for Re and W adatoms. The respective prefactors amount to 6.13(x 2.6 (+/-1)) x 10(-6) and 2.17(x 2.7(+/-1)) x 10(-3) cm(2) s(-1). The energy bar rier for single adatom motion over the step edge is estimated to be 15.7 +/ -0.4 and 19.3 +/-0.4 kcal mol(-1) for Re and W. Before descending the step edge, the adatoms become trapped at the edge, and have to overcome a barrie r of similar to 13 kcal mol(-1) to return to the plane's center. The behavi or of clusters containing from two to eight Re atoms has also been examined . Rotational Brownian motion is observed for Re-4 under the influence of a strong imaging held. The activation energies for diffusion of Re clusters o n Re(0001) are found to bear a striking resemblance to those of Ir clusters on the Ir(111) surface. Comparison of single atom diffusion on Re(0001) wi th that on other densely packed surfaces reveals that the diffusion barrier s do not scale quantitatively with the cohesive energy. (C) 1999 Elsevier S cience B.V. All rights reserved.