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In atomically thin transition metal dichalcogenide semiconductors, there is acrossover from indirect to direct bandgap as the thickness drops to onemonolayer, which comes with a fast increase of the photoluminescence signal.Here, we show that for different alloy compositions of WS2(1-x)Se2x this trendmay be significantly affected by the alloy content and we demonstrate that thesample with the highest Se ratio presents a strongly reduced effect. Thehighest micro-PL intensity is found for bilayer WS2(1-x)Se2x (x = 0.8) with adecrease of its maximum value by only a factor of 2 when passing from mono- tobi-layer. To better understand this factor and explore the layer-dependent bandstructure evolution of WS2(1-x)Se2x, we performed a nano-angle resolvedphotoemission spectroscopy study coupled with first-principles calculations. Wefind that the high micro-PL value for bilayer WS2(1-x)Se2x (x = 0.8) is due tothe overlay of direct and indirect optical transitions. This peculiar high PLintensity in WS2(1-x)Se2x opens the way for spectrally tunable light-emittingdevices.

Indirect to direct band gap crossover in two-dimensional WS2(1-x)Se2x alloys