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Two-dimensional molybdenum disulfide (MoS) has recently drawn major attention due to its promising applications in electronics and optoelectronics. Chemical vapor deposition (CVD) is a scalable method to produce large-area MoS monolayers, yet it is challenging to achieve shape-uniform, high-quality monolayered MoS grains as random, diverse crystallographic orientations and various shapes are produced in the same CVD process. Here, we report the growth of high-quality MoS monolayers with uniform triangular shapes dominating (up to 89%) over other shapes on both SiO/Si and sapphire substrates. The new confined-space CVD process prevents contamination and helps regulate the Mo/S ratio during the deposition. The as-grown triangular MoS monolayers exhibit grain sizes up to 150 μm and possess better crystalline properties and lighter n-type doping concentration than those of the monolayers grown by common CVD methods. The corresponding field effect transistor devices show high electron mobilities of 50-60 cm V s and positive threshold voltages of 21-35 V. This mild n-type behavior makes it possible to regulate the formation of excitons by back-gate voltage due to the interaction of excitons with free charge carriers in the MoS channel. As a result, gate-tunable photoluminescence (PL) effect, which is rarely achievable for MoS samples prepared by common CVD or mechanical exfoliation, is demonstrated. This study provides a simple versatile approach to fabricating monolayered crystals of MoS and other high-quality transition metal dichalcogenides and could lead to new optoelectronic devices based on gate-tunable PL effect.

Two-dimensional MA2Z4, as another system of a two-dimensional material family, can obtain different materials and considerable properties by replacing the elements M, A, and Z. At present, the physical properties and optical response of MA2Z4 materials have been studied, but there is still a lack of research on the application of MA2Z4 as a transistor channel material to investigate its transistor performance. Here, we employ WGe2N4 as a representative to systematically study the bounce-to-transport properties and gate control capability of ML WGe2N4 field effect transistors below 10 nm via ab initio quantum transport calculations. Until the channel length is down to 3.0 nm, the optimized n/p-type doped WGe2N4 metal–oxide–semiconductor field-effect transistors with proper concentrations and underlap structures can satisfy the high-performance requirements of International Technology Roadmap for Semiconductors of 2013 version, by considering the on-current, subthreshold swing, intrinsic delay time, and dynamic power indicator. Therefore, we can estimate that the monolayer WGe2N4 is a competitive alternative for transistor channel materials in the post-silicon era.