LEED studies of adsorption on clean (100) copper surfaces

Abstract

The interactions of nitrogen, oxygen and hydrogen with atomically clean (100) copper surfaces have been studied by low-energy electron diffraction (LEED) techniques. Cleaning methods included argon-ion bombardment, which produced (111) facets and decreased the work function to about 0.3 eV below that of the clean, annealed surface. The density of these facets was reduced by heat treatment. Retarding potential measurements indicated that the reflection coefficient of the clean, annealed surface attained a value of 0.70 in the very low energy region. Ion bombardment or contamination of the surface decreased the value of this parameter. Adsorption of N and N + on the clean surface produced a surface net described by Cu(100) c(2 × 2) — N, and Na + adsorbed to form an amorphous surface layer. Conversion of the molecular layer to the 2 × 2 structure did not occur. The ground state nitrogen molecule did not adsorb. A surface oxygen net, having oblique symmetry and providing a coverage of one-half monolayer, was formed by oxygen exposures of 1 × 10 −5 Torr-min at room temperature. Formation of this structure was accompanied by a slight increase in work function and a marked decrease in reflection coefficient. Oxygen exposures greater than 3 × 10 −5 Torr-min produced a three-dimensional structure providing a surface net described by Cu (100) 2 × 4–45° — 0. This structure formed preferentially at defect sites and had a work function about 0.4 eV lower than that of the clean surface. Oxygen exposures as high as 2 × 10 −1 Torr-min at room temperature caused the 2 × 4 structure to grow and did not produce the true oxide. Hydrogen did not adsorb.