Strain-Dependent Chemical Reaction on Inhomogeneous Surfaces

Gd islands grow on an annealed Gd(0001)/W(110) layer in the Stransky Krastanov mode. When hydrogen is adsorbed on strained Gd islands, we find that the GdH₂ nuclei are formed on the islands and not on the wetting layer between them and preferably close to the edges of the islands. This is in contrast to the usual case where the reaction is on the edge. The Gd islands on the W surface are under tensile stress, which decays due to relaxation upon approaching the edge. The lattice constant of GdH₂ is smaller than that of strained Gd, but is larger (by 4%) than that of unstrained Gd. Since the surface energy of GdH₂ is dependent on the strain, the rate of nucleation depends on it, too, and the nuclei are formed favorably in the areas of no strain between the strained Gd and GdH₂. The distribution of the hydride nuclei around these areas is a quantitative indication to the surface energy and its dependence on strain (surface stress and elastic energy). Thus, the hydride nuclei distribution can serve as a tool for measurement of the local values of surface energy, surface stress and elastic energies. These considerations apply to every gas-solid interaction, starting by nucleation on a strained surface.