  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.
