First-principles investigation of the electronic properties of niobium and molybdenum mononitride surfaces

Using first-principles calculations we investigated the electronic properties of niobium and molybdenum mononitride (NbN and MoN, respectively) surfaces and their dependence on the surface orientation and termination. Work functions calculated for polar surfaces strongly depend on the surface termination, with nitrogen-terminated surfaces yielding the highest value, up to 6.6 eV for the fully N-covered MoN(0 0 1) surface. The dependence of the work function on coverage for the polar surface is monotonic for nitrogen termination, but does not follow the same trend in the case of metal termination. The work function decreases by ∼2 eV for MoN from a 100% metal-terminated surface to at least 25% metal-terminated surface, and then increases rapidly between 25% and 0% metal-terminated surface to recover its nitrogen termination result. The same trend was obtained for NbN. We observed a significant increase in the charge of the surface metal atom, up to its bulk value, with decreasing metal surface coverage. Electron transfer from the metal surface atoms to the subsurface atoms can explain these submonolayer metal coverage results. Finally we found that for the non-polar surfaces, the mononitrides work functions are generally lower than the work functions of the corresponding simple metal surfaces.