Controllable synthesis of WS2(1-x)Se2x monolayers with fast photoresponse by a facile chemical vapor deposition strategy

Abstract

Two-dimensional transition metal dichalcogenides (TMDs) alloys such as WS2(1-x)Se2x have attracted much interest in the fundamental science and device fabrication for their composition-tunable chemical and physical properties. However, the traditional chemical vapor deposition (CVD) methods often rely on precursors with high melting point such as WO3, WS2 and WSe2. Here, we report a facile CVD strategy for the formation of high quality WS2(1-x)Se2x monolayers with composition-tunable optical and electronic properties by using highly evaporated sodium tungstate dihydrate (Na2WO4·2H2O) precursor as W source. The as-grown samples exhibit uniform allocation of W, S, and Se elements across the domain and the Se (or S) composition can be continually modulated from 0 to 2. The band gap evolves continuously from 1.97 eV for WS2 to intermediate value for WS2(1-x)Se2x and then to 1.63 eV for WSe2. Electrical transport studies further reveal the transition from n-type for WS2 to bipolar (or slightly n-type) for WS2(1-x)Se2x and then to p-type for WSe2. Furthermore, the as-grown WS2(1-x)Se2x-photodetectors exhibit good photo response to visible light (532 nm) with a fast response time of about 20 ms. Our facile and precise synthesis strategy can be used as a general route to fabricating such composition modulated 2D TMD alloys for optoelectronic applications.