Experimental and Theoretical Studies of the Electronic Band Structure of Bulk and Atomically Thin Mo 1- x W x Se 2 Alloys

We present studies focused on the evolution of the electronic band structure of the Mo W Se alloy with the tungsten content, which was conducted by combining experimental and theoretical methods. Employed spectroscopic techniques, namely, photoreflectance, photoacoustic spectroscopy, and photoluminescence, allowed observing indirect and direct transitions at high and beyond high-symmetry points of the Brillouin zone (BZ). Two excitons (A and B) associated with the K point of the BZ were observed together with other optical transitions (C and D) related to band nesting. Moreover, we have also identified the indirect transition for the studied crystals. Obtained energies for all transitions were tracked with a tungsten content and compared with results of calculations performed within density functional theory. Furthermore, based on the mentioned comparison, optical transitions were assigned to specific regions of the BZ. Finally, we have obtained bowing parameters for experimentally observed features, for, i.e., thin-film samples: (A) = 0.13 ± 0.03 eV, (B) = 0.14 ± 0.03 eV, (C) = 0.044 ± 0.008 eV, and (D) = 0.010 ± 0.003 eV.