Chemical trends in the work function of modified Si(111) surfaces: A DFT study

Using first-principles, we provide predictions for chemical trends in the work function of silicon (111) surfaces modified with various terminations. For nonpolar terminating atomic species such as F, Cl, Br, and I, the change in the work function is directly proportional to the amount of charge transferred from the surface, thus relating to the difference in electronegativity of the adsorbate and silicon atoms. The change is a monotonic function of coverage in this case, and the work function increases with increasing electronegativity. Polar species such as -TeH, -SeH, -SH, -OH, -NH₂, -CH₃, and -BH₂ do not follow this trend due to the interaction of their dipole with the induced electric field at the surface. In this case, the magnitude and sign of the surface dipole moment need to be considered in addition to the bond dipole to generally describe the change in work function. Compared to hydrogenated surfaces, there is slight increase in the work function of H:Si(111)-XH, where X = Te, Se, and S, whereas reduction is observed for surfaces covered with -OH, -CH₃, and -NH₂. Our analysis contributes to a microscopic understanding of induced changes in experimentally studied silicon surfaces.