  In atomically thin transition metal dichalcogenide semiconductors, there is a
crossover from indirect to direct bandgap as the thickness drops to one
monolayer, which comes with a fast increase of the photoluminescence signal.
Here, we show that for different alloy compositions of WS2(1-x)Se2x this trend
may be significantly affected by the alloy content and we demonstrate that the
sample with the highest Se ratio presents a strongly reduced effect. The
highest micro-PL intensity is found for bilayer WS2(1-x)Se2x (x = 0.8) with a
decrease of its maximum value by only a factor of 2 when passing from mono- to
bi-layer. To better understand this factor and explore the layer-dependent band
structure evolution of WS2(1-x)Se2x, we performed a nano-angle resolved
photoemission spectroscopy study coupled with first-principles calculations. We
find that the high micro-PL value for bilayer WS2(1-x)Se2x (x = 0.8) is due to
the overlay of direct and indirect optical transitions. This peculiar high PL
intensity in WS2(1-x)Se2x opens the way for spectrally tunable light-emitting
devices.
