Structure and Chemical State of Cesium on Well-Defined Cu(111) and Cu2O/Cu(111) Surfaces

The deposition of cesium (Cs) onto the metallic and oxidized surfaces of Cu(111) was investigated using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory calculations (DFT) to elucidate the properties of alkali metals when supported on metal and oxide surfaces. At low coverages, cesium adopts partially cationic (Csδ+), highly mobile, and likely atomic structures on the metal surface of Cu(111). Such structures are not observable at room temperature using STM due to rapid surface mobility but can be quantified using XPS. This is further verified by DFT calculations which show that Cs adsorption on Cu(111) is site insensitive, where atop, bridge, and hollow sites yield identical adsorption energy. Reaction with O2 (1 × 10–7 Torr) at room temperature of the Cs/Cu(111) surface results in both CsOx and CuxO formation, initially from the Cs sites and Cu step edges and then subsequently encompassing the terraces. The presence of Cs promotes the oxidation process by O2, and we have identified the oxidized Csδ+ and Cu1+ using XPS. In contrast to the metallic substrate, the deposition of Cs onto the preoxidized surface of Cu2O/Cu(111) allows for the anchoring of the oxidized Csδ+ nanostructures (few nm) which appear indiscriminately on the oxide surface with high dispersion and low mobility. While clearly distinguishable using STM, we have revealed a rich geometric heterogeneity of the nanostructures of Cs on Cu2O/Cu(111), likely templated through a strong interaction between the Cs and the Cu2O substrate. In addition, it is also evident that Cs imparts a destabilizing effect on the ordered oxide substrate, as observed through the increase of surface defects. Finally, the thermal stability of the Cs structures was studied, using sequential annealing steps revealing that Cs remained stable up to 550 K with some loss of both cesium and oxygen at higher temperatures of 650 K. DFT calculations show that unlike Cu(111), the adsorption energy of Cs on CuxO/Cu(111) is highly dependent on adsorption site, and electronic effects enabled through the interaction between Cs, O, and Cu.