Fig. 3: Bandgap of bulk TMDC and dependence on the number of layers.
From: Bandgap engineering of two-dimensional semiconductor materials
a Band structure of bulk (top) and monolayer (bottom) WS2. High symmetry points are shown on the right side. Band structures of bulk MX2 are qualitatively similar to this, where the lowest energy gap is indirect. This allows for photoluminescence originating from indirect gap transitions, labeled as I, as well as direct gap transitions, labeled as A and B. b Photoluminescence (PL) spectra of monolayer (1ML) to 8-layer (8ML) WS2, measured at temperature T = 5 K, along with optical images of the samples (insets)56. Labels A and I refer to the direct and indirect transitions sketched in (a), respectively. The B transition has higher energy, outside the range of energies in the figure, and is therefore not shown. c Dependence of the energy of the most prominent PL peak on the number of layers for different TMDC58. Panel b used with permission from The Royal Society of Chemistry/Molas et al.56; panel c reprinted by permission from Springer Nature Nature Communications58, Copyright (2014), advance online publication, 06th February 2014 (https://doi.org/10.1038/ncomms4252 Nat. Commun.).
