Synthesis of Large-Size 1T' ReS 2 x Se 2(1- x ) Alloy Monolayer with Tunable Bandgap and Carrier Type.

Chemical vapor deposition growth of 1T' ReS2 x Se2(1- x ) alloy monolayers is reported for the first time. The composition and the corresponding bandgap of the alloy can be continuously tuned from ReSe2 (1.32 eV) to ReS2 (1.62 eV) by precisely controlling the growth conditions. Atomic-resolution scanning transmission electron microscopy reveals an interesting local atomic distribution in ReS2 x Se2(1- x ) alloy, where S and Se atoms are selectively occupied at different X sites in each Re-X6 octahedral unit cell with perfect matching between their atomic radius and space size of each X site. This structure is much attractive as it can induce the generation of highly desired localized electronic states in the 2D surface. The carrier type, threshold voltage, and carrier mobility of the alloy-based field effect transistors can be systematically modulated by tuning the alloy composition. Especially, for the first time the fully tunable conductivity of ReS2 x Se2(1- x ) alloys from n-type to bipolar and p-type is realized. Owing to the 1T' structure of ReS2 x Se2(1- x ) alloys, they exhibit strong anisotropic optical, electrical, and photoelectric properties. The controllable growth of monolayer ReS2 x Se2(1- x ) alloy with tunable bandgaps and electrical properties as well as superior anisotropic feature provides the feasibility for designing multifunctional 2D optoelectronic devices.