  The unique optical and electronic properties of two-dimensional transition
metal dichalcogenides (2D TMDs) make them promising materials for applications
in (opto-)electronics, catalysis and more. Specifically, alloys of 2D TMDs have
broad potential applications owing to their composition-controlled properties.
Several important challenges remain regarding controllable and scalable
fabrication of these alloys, such as achieving control over their atomic
ordering (i.e. clustering or random mixing of the transition metal atoms within
the 2D layers). In this work, atomic layer deposition (ALD) is used to
synthesize the TMD alloy Mo$_{1-x}$W$_{x}$S$_{2}$ with excellent composition
control along the complete composition range 0 $\leq$ x $\leq$ 1. Importantly,
this composition control allows us to control the atomic ordering of the alloy
from well-mixed to clustered while keeping the alloy composition fixed, as is
confirmed directly through atomic-resolution HAADF-STEM imaging. The control
over atomic ordering leads to tuning of the bandgap, as is demonstrated using
optical transmission spectroscopy. The relation between this tuning of the
electronic structure and the atomic ordering of the alloy was further confirmed
through ab-initio calculations. Furthermore, as the atomic ordering modulates
from clustered to well-mixed, the typical MoS$_{2}$ and WS$_{2}$ A$_{1g}$
vibrational modes converge. Our results demonstrate that atomic ordering is an
important parameter that can be tuned experimentally to finely tune the
fundamental properties of 2D TMD alloys for specific applications.
