The work function of a material is commonly used as an intrinsic reference for band alignment; however, it is notoriously susceptible to extrinsic conditions. Following the classification of Bardeen we calculate values for the bulk binding energy of electrons in rutile-structured ${\mathrm{SnO}}_2$ and the effect of the surface on the work function, thus highlighting the role of the surface in determining the energy levels of a material. Furthermore we demonstrate how, through the use of ultrathin heteroepitaxial oxide layers (${\mathrm{SiO}}_2$, ${\mathrm{TiO}}_2$, ${\mathrm{PbO}}_2$) at the surface, the work function can be tuned to achieve energy levels commensurate with important technological materials. This approach can be extended from transparent conducting oxides to other semiconducting materials.

• Received 26 November 2013
• Revised 13 March 2014

DOI:https://doi.org/10.1103/PhysRevB.89.115320