  Transition metal dichalcogenide (TMDC) monolayers are newly discovered
semiconductors for a wide range of applications in electronics and
optoelectronics. Most studies have focused on binary monolayers that share
common properties: direct optical bandgap, spin-orbit (SO) splittings of
hundreds of meV, light-matter interaction dominated by robust excitons and
coupled spin-valley states of electrons. Studies on alloy-based monolayers are
more recent, yet they may not only extend the possibilities for TMDC
applications through specific engineering but also help understanding the
differences between each binary material. Here, we synthesized highly
crystalline Mo$_{(1-x)}$W$_{x}$Se$_2$ to show engineering of the direct optical
bandgap and the SO coupling in ternary alloy monolayers. We investigate the
impact of the tuning of the SO spin splitting on the optical and polarization
properties. We show a non-linear increase of the optically generated valley
polarization as a function of tungsten concentration, where 40% tungsten
incorporation is sufficient to achieve valley polarization as high as in binary
WSe2. We also probe the impact of the tuning of the conduction band SO spin
splitting on the bright versus dark state population i.e. PL emission
intensity. We show that the MoSe2 PL intensity decreases as a function of
temperature by an order of magnitude, whereas for WSe2 we measure surprisingly
an order of magnitude increase over the same temperature range (T=4-300K). The
ternary material shows a trend between these two extreme behaviors. These
results show the strong potential of SO engineering in ternary TMDC alloys for
optoelectronics and applications based on electron spin- and valley-control.
