Probing quantum confinement effects on the excitonic property and electronic band structures of MoS2

Abstract

We systemically investigate the effects of quantum confinement on the excitonic property and electronic band structures of centimeter-scale MoS2 films (number of layers: N = 1, 2, 4, and 18) through spectroscopic ellipsometry. The optical properties including optical conductivity, exciton binding energy and critical points (CPs) are traced as a function of layer number. We find that A and B excitons have the largest binding energies in monolayer MoS2 compared with the values in multilayer MoS2 because of the absence of interlayer coupling in former. Nevertheless, the peak positions of A and B excitons are almost the same, even when the layer number changes. This is because of the canceling out of differences between quasi-particle band gap and excitonic binding energy. Furthermore, the threshold energies of the eight CPs are calculated; using these values, the valence and conduction band alignments of the CPs are determined. It is noteworthy that the electronic band structures of MoS2 become stable when the layer number is more than four, which can be attributed to the largely weakened quantum confinement effect in these cases. The results of our study help us better understand the effect of quantum confinement on photoelectronic and photovoltaic applications using MoS2 films.