Two-Step Flux Synthesis of Ultrapure Transition-Metal Dichalcogenides

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted tremendous interest due to the unusual electronic and optoelectronic properties of isolated monolayers and the ability to assemble diverse monolayers into complex heterostructures. To understand the intrinsic properties of TMDs and fully realize their potential in applications and fundamental studies, high-purity materials are required. Here, we describe the synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than those in TMDs grown by a single-step flux technique. For WSe₂, we show that increasing the Se/W ratio during growth reduces point defect density, with crystals grown at 100:1 ratio achieving charged and isovalent defect densities below 10¹⁰ and 10¹¹ cm^-2, respectively. Initial temperature-dependent electrical transport measurements of monolayer WSe₂ yield room-temperature hole mobility above 840 cm²/(V s) and low-temperature disorder-limited mobility above 44,000 cm²/(V s). Electrical transport measurements of graphene-WSe₂ heterostructures fabricated from the two-step flux grown WSe₂ also show superior performance: higher graphene mobility, lower charged impurity density, and well-resolved integer quantum Hall states. Finally, we demonstrate that the two-step flux technique can be used to synthesize other TMDs with similar defect densities, including semiconducting 2H-MoSe₂ and 2H-MoTe₂ and semimetallic T_d-WTe₂ and 1T'-MoTe₂.