Effects of structure and temperature on the nature of excitons in the Mo0.6W0.4S2 alloys

We have studied the nature of excitons in the transition metal dichalcogenide alloy Mo0.6W0.4 S2, compared to pure MoS2 and WS2 grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. Effects of temperature on the A and B exciton peak energies and linewidths in the optical transmission spectra were compared between the alloy and pure MoS2 and WS2. On increasing the temperature from 25 K to 293 K the energy of the A and B exciton peaks decreases, while their linewidth increases due to exciton-phonon interactions. The exciton-phonon interactions in the alloy are closer to those for MoS2 than WS2. This suggests that the exciton wave functions in the alloy have a larger amplitude on Mo atoms than on W atoms. The experimental absorption spectra could be reproduced by TDDFT calculations. Interestingly, for the alloy the Mo and W atoms had to be distributed over all layers. Conversely, we could not reproduce the experimental alloy spectrum by calculations on a structure with alternating layers, in which every other layer contains only Mo atoms and the layers in between also W atoms. For the latter atomic arrangement, the TDDFT calculations yielded an additional optical absorption peak that could be due to excitons with some charge transfer character. From these results we conclude that ALD yields an alloy in which Mo and W atoms are distributed uniformly among all layers.