In atomically thin transition metal dichalcogenide semiconductors, there is a crossover from indirect to direct band gap 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 WS_2(1−x)Se_2x 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 WS_2(1−x)Se_2x (x = 0.8) with a decrease of its maximum value by only a factor of 2 when passing from mono-layer to bi-layer. To better understand this factor and explore the layer-dependent band structure evolution of WS_2(1−x)Se_2x, we performed a nano-angle-resolved photoemission spectroscopy study coupled with first-principles calculations. We find that the high micro-PL value for bilayer WS_2(1−x)Se_2x (x = 0.8) is due to the overlay of direct and indirect optical transitions. This peculiar high PL intensity in WS_2(1−x)Se_2x opens the way for spectrally tunable light-emitting devices.

在原子薄的过渡金属二卤化半导体中，随着厚度下降到一个单层，存在从间接带隙到直接带隙的交叉，这伴随着光致发光信号的快速增加。在这里，我们表明，对于WS _{2（1- x）} Se _{2 x的}不同合金成分，这种趋势可能会受到合金含量的显着影响，并且我们证明了具有最高Se比的样品表现出强烈的降低效果。发现双层WS _{2（1- x）} Se _{2 x}（x = 0.8），从单层到双层时，最大值仅减少2倍。为了更好地理解这一因素并探索WS _{2（1- x）} Se _{2 x}的层依赖性能带结构演化，我们进行了纳米角分辨光发射光谱研究，并结合了第一性原理计算。我们发现双层WS _{2（1- x）} Se _{2 x}（x  = 0.8）的高micro-PL值是由于直接和间接光学跃迁的叠加所致。WS _{2（1- x）} Se _{2 x中}这种特殊的高PL强度 为光谱可调的发光设备开辟了道路。