Interaction and diffusion of potassium onCr2O3(0001)/Cr(110)

Abstract

The interaction of potassium atoms on top of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)/\mathrm{C}\mathrm{r}(110)$ has been studied using work-function (\ensuremath{\Delta}\ensuremath{\Phi}), temperature programmed desorption (TPD), and optical second-harmonic generation (SHG) measurements. Potassium grows via the completion of a first layer, followed by a second layer in the form of two-dimensional (2D) islands, and at higher coverage 3D clusters are formed. This growth model is supported by and consistent with the results obtained from all three methods. Work-function data suggest that annealing at temperatures above 350 K results in the formation of a surface potassium oxide compound, provided the potassium coverage is higher than 0.5 monolayers (ML). Diffusion of alkali-metal atoms on an oxide surface is reported here over distances of several micrometers. This was measured using optical SH diffraction from coverage gratings that were generated by laser-induced thermal desorption. The activation energy for surface diffusion of potassium on ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)/\mathrm{C}\mathrm{r}(110)$ has been determined to be $11\ifmmode\pm\else\textpm\fi{}0.5\mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}$ with a preexponential factor ${D}_{0}{=10}^{5}{\mathrm{cm}}^{2}/\mathrm{s}\mathrm{e}\mathrm{c}$ in the coverage range of 1.5--2.5 ML, dropping to 9 kcal/mol and ${D}_{0}=3\ifmmode\times\else\texttimes\fi{}{10}^{3}{\mathrm{cm}}^{2}/\mathrm{s}\mathrm{e}\mathrm{c}$ at a coverage of 3.0 ML. These results are consistent with the diffusion of atoms in the third layer, on top of two-dimensional potassium islands in the second layer, the activation energy represent the barrier for descending from the 2D islands.