  Transition-metal dichalcogenides (TMDs) are promising for two-dimensional
(2D) semiconducting devices and novel phenomena. For 2D applications, their
work function, ionization energy, and electron affinity are required as a
function of thickness, but research on this is yet to cover the full family of
compounds. Here, we present the work function, ionization energy, and electron
affinity of few-layer and bulk MX2 (M = Mo, W and X = S, Se, Te) in 2H phase
obtained accurately by the density functional theory and GW calculations. For
each compound, we consider one-, two-, three-, four-layer, and bulk geometry.
In GW calculations, accurate results are obtained by nonuniform q sampling for
two-dimensional geometry. From band energies including the GW self-energy
correction, we estimate the work function, band gap, ionization energy, and
electron affinity as functions of the number of layers. We compare our results
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 TMDs.
