Characterization of O−-Centers on Single Crystalline MgO(001)-Films

Defects play an important role for understanding the properties of oxide surfaces. However, a detailed atomistic characterization of the properties of defects in particular of point defects is still a challenging task. On polycrystalline material it is the large variety of different species in terms of local environment, electronic properties as well as the metastable nature of most of these species, which complicates matters. EPR spectroscopy has proven to be a versatile tool to characterize electronic properties as well as local environment of paramagnetic point defects on oxide surfaces. In this study we elucidate the properties of O--centers on MgO surfaces under ultrahigh vacuum (UHV) conditions using a single-crystalline MgO(001) surface as a well-defined model system. The O--centers were produced by reaction of N2O with previously prepared F+-centers, which were shown to be located at step edges of the MgO islands in a previous study. The experimental efforts are combined with ab-inito quantum chemistry calculations to gain a more detailed understanding of the electronic properties of the defects under consideration. The experimental and theoretical values of the g-tensor components are almost in quantitative agreement. In addition to the discussion of the properties of O--centers the paper will shed some light on the impact that doping of the surface in this case with Mo(V)-species present on the pristine surface has. In particular, we are able to provide evidence for the fact that there is redox chemistry between the O--centers and Mo-centers. The crosstalk between different redox active sites on the surface is an important phenomenon that is not limited to model systems as discussed here, but should also be taken into consideration if discussing the properties of high performance catalysts