  Alloying enables engineering of the electronic structure of semiconductors
for optoelectronic applications. Due to their similar lattice parameters, the
two-dimensional semiconducting transition metal dichalcogenides of the MoWSeS
group (MX2 where M= Mo or W and X=S or Se) can be grown as high-quality
materials with low defect concentrations. Here we investigate the atomic and
electronic structure of Mo(1-x)WxS2 alloys using a combination of
high-resolution experimental techniques and simulations. Analysis of the Mo and
W atomic positions in these alloys, grown by chemical vapour transport, shows
that they are randomly distributed, consistent with Monte Carlo simulations
that use interaction energies determined from first-principles calculations.
Electronic structure parameters are directly determined from angle resolved
photoemission spectroscopy measurements. These show that the spin-orbit
splitting at the valence band edge increases linearly with W content from MoS2
to WS2, in agreement with linear-scaling density functional theory (LS-DFT)
predictions. The spin-orbit splitting at the conduction band edge is predicted
to reduce to zero at intermediate compositions. Despite this,
polarisation-resolved photoluminescence spectra on monolayer Mo0.5W0.5S2 show
significant circular dichroism, indicating that spin-valley locking is
retained. These results demonstrate that alloying is an important tool for
controlling the electronic structure of MX2 for spintronic and valleytronic
applications.
