Shape-Uniform, High-Quality Monolayered MoS2 Crystals for Gate-Tunable Photoluminescence.

Two-dimensional molybdenum disulfide (MoS2) 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 MoS2 monolayers, yet it is challenging to achieve shape-uniform, high-quality monolayered MoS2 grains as random, diverse crystallographic orientations and various shapes are produced in the same CVD process. Here, we report the growth of high-quality MoS2 monolayers with uniform triangular shapes dominating (up to 89%) over other shapes on both SiO2/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 MoS2 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 cm2 V-1 s-1 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 MoS2 channel. As a result, gate-tunable photoluminescence (PL) effect, which is rarely achievable for MoS2 samples prepared by common CVD or mechanical exfoliation, is demonstrated. This study provides a simple versatile approach to fabricating monolayered crystals of MoS2 and other high-quality transition metal dichalcogenides and could lead to new optoelectronic devices based on gate-tunable PL effect.