Facile Synthesis of MoS$_{2}$ and Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ Triangular Monolayers

Single- and few-layered transition metal dichalcogenides (TMDs) such as MoS$_{2}$ and WS$_{2}$ are emerging two dimensional materials exhibiting numerous and unusual physico-chemical properties that could be advantageous in the fabrication of unprecedented optoelectronic devices. Here we report a novel and alternative route to synthesize triangular monocrystals of MoS$_{2}$ and Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ by annealing MoS$_{2}$ and MoS$_{2}$/WO$_{3}$ precursors, respectively, in the presence of sulfur vapor. In particular, the Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ triangular monolayers show gradual concentration profiles of W and Mo whereby Mo concentrates in the islands' center and W is more abundant on the outskirts of the triangular monocrystals. These observations were confirmed by atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM), as well as Raman and photoluminescence (PL) spectroscopy. The presence of tunable PL signals depending on the Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ stoichiometries in 2D monocrystals opens up a wide range of applications in electronics and optoelectronics.