Colloidal Continuous Injection Synthesis of Fluorescent MoX2 (X = S, Se) Nanosheets as a First Step Toward Photonic Applications

Transition-metal dichalcogenide (TMD) nano-sheets have become an intensively investigated topic in the field of 2D nanomaterials, especially due to the direct semiconductor nature, and the broken inversion symmetry in the odd-layer number, of some of their family members. These properties make TMDs attractive for different technological applications such as photovoltaics, optoelectronics, valleytronics, and hydrogen evolu-tion reactions. Among them, MoX2 (X = S and Se) are turned to direct gap when their thickness is thinned down to monolayer, and thus, efforts toward obtaining large-scale monolayer TMDs are crucial for technological applications. Colloidal synthesis of TMDs has been developed in recent years, as it provides a cost-efficient and scalable way to produce few-layer TMDs having homogeneous size and thickness, yet obtaining a monolayer has proven challenging. Here, we present a method for the colloidal synthesis of mono-and few-layer MoX2 (X = S and Se) using elemental chalcogenide and metal chloride as precursors. Using a synthesis with slow injection of the MoCl5 precursor under a nitrogen atmosphere, and optimizing the synthesis parameters with a design of experiments approach, we obtained a MoX2 sample with the semiconducting (1H) phase and optical band gaps of 1.96 eV for H-1-MoS2 and 1.67 eV for 1H-MoSe2, respectively, consistent with a large monolayer yield in the ensemble. Both display photoluminescence at cryogenic and room temperature, paving the way for optical spectroscopy studies and photonic applications of colloidal TMD nanosheets.