Thickness dependence of work function, ionization energy, and electron affinity of bulk and few-layer transition metal dichalcogenides

We studied the work function, ionization energy, and electron affinity of bulk and few-layer transition-metal dichalcogenides (TMDs) in 2H phase using the density functional theory (DFT) and the GW approximation. We obtained DFT band energies of few-layer TMDs with respect to the vacuum level. For the vacuum level of bulk TMDs, we considered a sufficiently thick slab system which has a similar band gap with bulk. We introduced the GW approximation to obtain the quasiparticle energy shift of valence band maximum and conduction band minimum and estimated the work function, band gap, ionization energy, and electron affinity as functions of the number of layers. We compare our quasiparticle band energies of TMDs with available theoretical and experimental reports, and we discuss types of band alignments in in-plane and out-of-plane junctions of these few-layer and bulk transition-metal dichalcogenides.