Scanning tunneling microscopy studies of oxygen adsorption on Cu(111)

Abstract

The adsorption of O2 on Cu(1 1 1) at room temperature has been investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Adsorption of oxygen leads to formation of a surface oxide by incorporation of Cu atoms from step edges and terraces. This process is most rapid along the close packed direction of the surface and leads to mesoscopic changes in surface morphology. Three characteristic features were observed during the initial stage of adsorption; dark fringes along the Cu(1 1 1) step edges, dark domains within the Cu(1 1 1) terrace, and rather mobile light patches on top of the Cu terraces. Within these regions atomic scale features could be imaged and the structure related to that of Cu2O. The dark fringes and dark domains grew slowly in oxygen, whereas the bright patches only became visible when gas-phase O2 was evacuated. STM observation at elevated temperatures indicates mobilization and rearrangement of surface features formed during room temperature adsorption. O/Cu(1 1 1) surfaces annealed at 473–623 K indicated a well ordered (73R5.8°×21R−10.9°) lattice structure (‘44’-structure) on the terraces which was also confirmed by LEED. Annealing up to 723 K exhibited slight disordering of this lattice structure presumably due to inter-diffusion of O and Cu atoms with the bulk. The mesoscopic process of oxygen adsorption onto the clean Cu(1 1 1) surface was also investigated at elevated temperature by STM. The reaction of the surface maintained at temperatures between 373 and 773 K was investigated for oxygen pressures in the 10−5–10−3 Pa range. The oxidized surface formed at 373 K was largely disordered with only small areas of the ‘44’-structure (√73R5.8°×√21R−10.9°). Well-ordered adlattices of the ‘44’-structure were formed upon O2 exposure at 473 K. Such surfaces could be converted to the ‘29’-structure (√13R46.1°×7R21.8°) by annealing at 673 K in vacuum. On the basis of the STM images we propose a new model for the ‘29’ structure. Annealing at higher temperatures (∼773 K) retrieved the ‘44’-structure in small domains on a disordered background. A disordered oxide surface which was produced at room temperature and annealed at 723 K can be ordered into the ‘44’-structure by O2 exposure and heating at 623 K.