Understanding the interplay of surface structure and work function in oxides: A case study on SrTiO3

The work function is one of the most fundamental surface properties of a material, and understanding and controlling its value is of central importance for manipulating electron flow in applications ranging from high power vacuum electronics to oxide electronics and solar cells. Recent computational studies using Density Functional Theory (DFT) have demonstrated that DFT-calculated work function values for metals tend to agree well (within about 0.3 eV on average) with experimental values. However, a detailed validation of DFT-calculated work functions for oxide materials has not been conducted and is challenging due to the complex dipole structures that can occur on oxide surfaces. In this work, we have focused our investigation on the widely studied perovskite SrTiO3 as a case study example. We find that DFT can accurately predict the work function values of clean and reconstructed SrTiO3 surfaces vs experiment at about the same level of accuracy as metals when direct comparisons can be made. Furthermore, to aid in understanding the factors governing the work function of oxides, we have performed systematic studies on the influence of common surface features, including surface point defects, doping, adsorbates, reconstructions, and surface steps, on the work function. The relationships between the surface structure and work function for SrTiO3 identified here may be qualitatively applicable to other complex oxide materials.