RESUMEN
Scanning-tunneling microscopy and density-functional theory have been employed to identify the spatial correlation between an oxygen vacancy and the associated Ce(3+) ion pair in a defective CeO(2)(111) film. The two Ce(3+) ions can occupy different cationic shells around the vacancy. The resulting variation in the chemical environment leads to a splitting of the filled Ce(3+) f levels, which is detected with STM spectroscopy. The position of the Ce(3+) ion pair is reflected in characteristic defect patterns observed in empty-state STM images, which arise from the bright appearance of Ce(4+) ions next to the defect while the Ce(3+) remain dark. Both findings demonstrate that at least one excess electron localizes in a Ce ion that is not adjacent to the O vacancy.
RESUMEN
Low-temperature scanning tunneling microscopy and spectroscopy have been employed to analyze the local electronic structure of the (111) surface of a ceria thin film grown on Ru(0001). On pristine, defect-free oxide terraces, the empty 4f states of Ce(4+) ions appear as the only spectral feature inside the 6 eV oxide band gap. In contrast, occupied states are detected between -1.0 and -1.5 eV below E(Fermi) in conductance spectra of different point and line defects, such as surface oxygen vacancies, grain boundaries and step edges. They are assigned to partially filled 4f states localized at the Ce(3+) ions. The presence of excess electrons indicates the oxygen-deficient nature of the direct oxide environment. The f state spectroscopy with the STM allows us to probe the spatial distribution of Ce(3+) ions in the ceria surface, providing unique insight into the local reduction state of this chemically important material system.
RESUMEN
The stabilization of single Fe atoms in the nanopores of an ultrathin silica film grown on Mo(112) is demonstrated with scanning tunneling microscopy (STM) and density functional theory (DFT). The Fe atoms are able to penetrate the openings in the oxide surface and adsorb in two different binding configurations at the metal-oxide interface. In the energetically preferred site, the Fe stays monomeric even at temperatures above 300 K. In the second configuration that is adopted in 10% of the cases, surface atoms can be attached to the subsurface species, resulting in the formation of Fe surface clusters. The interfacial Fe atoms preserve their magnetic moment, as shown by a distinct Kondo-like response in STM conductance spectra and DFT calculations.
RESUMEN
The possibility to modify the adsorption properties of a porous silica/Mo(112) film by controlling its work function has been studied by a combined STM and density-functional theory approach. While the original film is inert towards metal adsorption, Au atoms and clusters can be stabilized on the surface after Li doping. The Li atoms penetrate the topmost silica layer and bind as Li+ cations at the metal-oxide interface, thereby reducing the oxide work function. This induces a charge transfer into Au adatoms, which in turn enables strong Au-silica interaction mediated by a polaronic distortion of the oxide lattice.