Chemical Vapor Deposition of Monolayer Mo_1-xW_xS₂ Crystals with Tunable Band Gaps

Band gap engineering of monolayer transition metal dichalcogenides, such as MoS₂ and WS₂, is essential for the applications of the two-dimensional (2D) crystals in electronic and optoelectronic devices. Although it is known that chemical mixture can evidently change the band gaps of alloyed Mo_1-xW_xS₂ crystals, the successful growth of Mo_1-xW_xS₂ monolayers with tunable Mo/W ratios has not been realized by conventional chemical vapor deposition. Herein, we developed a low-pressure chemical vapor deposition (LP-CVD) method to grow monolayer Mo_1-xW_xS₂ (x = 0-1) 2D crystals with a wide range of Mo/W ratios. Raman spectroscopy and high-resolution transmission electron microscopy demonstrate the homogeneous mixture of Mo and W in the 2D alloys. Photoluminescence measurements show that the optical band gaps of the monolayer Mo_1-xW_xS₂ crystals strongly depend on the Mo/W ratios and continuously tunable band gap can be achieved by controlling the W or Mo portion by the LP-CVD.