Wafer-Scale Uniform Synthesis of 2D Transition Metal Dichalcogenides Single Crystals via Chemical Vapor Deposition

Abstract

ConspectusTwo-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs), most with a formula of MX2 (M = Mo, W; X = S, Se, etc.), have emerged as ultrathin channel materials in next-generation electronics, due to their atomic thickness, tunable bandgap, and relatively high carrier mobility, etc. To propel their practical applications in integrated circuits, large-scale preparation of large-domain MX2 single crystals and single-crystal films is significantly important. Among all the synthetic methods, chemical vapor deposition (CVD) has shown great promise for the controllable syntheses of large-area uniform 2D materials with electronic-grade quality and reasonable cost. So far, intensive efforts have been devoted to the controllable growth of MX2 single crystals with large sizes, controlled thicknesses, fast growth rates, and unidirectional orientations. However, due to the complexity of the precursor species (mostly like solid powders) and their gradient feeding distributions along the gas flow direction, it is still a huge challenge to synthesize large-area MX2 samples with centimeter-scale uniformity, which inevitably hinders their practical applications in industry. More significantly, due to the noncentrosymmetric structures of MX2, antiparallel domains and twin grain boundaries usually evolved on most substrates (e.g., mica, C-sapphire). These defective grain boundaries usually exhibit metallic characteristics and serve as conducting channels, seriously impairing the electrical and optical properties of related devices.This review thereby focuses on the recent progresses on the CVD syntheses of large-scale uniform, large-domain MX2 flakes and single-crystal films, primarily on the single-crystal insulating substrates, amorphous insulating substrates and single-crystal metal substrates. Some promising growth strategies toward improving the thickness uniformity, domain size and crystal quality of MX2 are highlighted, through modulating the key parameters in the CVD processes, such as precursor type, substrate category/crystallinity or growth promoter. The key mechanisms that directing the uniform orientations of individual domains and their merging toward single-crystal films are also highlighted. Besides, the applications of the 2D MX2 in high-performance electronic devices are also discussed. Finally, the current challenges for the wafer-scale syntheses of MX2 single crystals and their applications are prospected. The potential opportunities in this field are also proposed in terms of the wafer-scale production and thickness control of 2D MX2, epitaxial growth of wafer-scale MX2 single-crystals, damage-free, and easy-processing transfer of wafer-scale MX2, as well as the high-performance electronics/optoelectronics based on wafer-scale MX2. We believe this review can provide a basic guidance for the controllable growth of wafer-scale MX2 single crystals, and inspire more researchers to design innovative synthesis routes to promote their practical applications in new-generation electronic devices.