Surface defects and their impact on the electronic structure of Mo-doped CaO films: an STM and DFT study

The functionality of doped oxides sensitively depends on the spatial distribution of the impurity ions and their interplay with compensating defects in the lattice. In our combined scanning tunneling microscopy (STM) and density functional theory (DFT) study, we analyze defects occurring in Mo-doped CaO(001) films at the atomic scale. By means of topographic imaging, we identify common point and line defect in the doped oxide, in particular Mo donors and compensating Ca vacancies. The influence of charged defects on the oxide electronic structure is analyzed by STM conductance spectroscopy. The experimentally observed defect features are connected to typical point defects in the CaO lattice by means of DFT calculations. Apart from the identification of individual defects, our study reveals a pronounced inhomogeneity of the oxide electronic structure, reflecting the uneven spatial distribution of dopants in the lattice. Our results provide the basis for a better understanding of adsorption and reaction patterns on doped oxides, as widely used in heterogeneous catalysis.