Self-Flux Synthesis of Transition Metal Dichalcogenides—Impact of Thermal Cycling and Refinement Procedures on Point Defect Density and Crystal Size

Two-dimensional transition metal dichalcogenides (TMDs) have generated great interest due to their unique optoelectronic properties and applications in moiré physics through complex heterostructures. Yet, many of these desired properties are limited by the presence of point defects in TMDs, highlighting the need for TMDs as close to crystallographic perfection as possible. To date, TMDs synthesized with a two-step self-flux method have been shown to contain point defect densities more than an order of magnitude lower than those made by any other synthesis method, leading to maximized Hall carrier mobilities and photoluminescent trion quantum yields [1-3]. However, a single thermal cycle of the two-step flux synthesis takes over 1 month due to long dwells at elevated temperatures and slow cooling rates, restricting high throughput synthesis of large TMD bulk crystals using the two-step flux method. Recent work has shown increase in size of flux MoSe₂ crystals of &gt;30% by using multiple thermal cycles, raising exfoliation and fabrication yields during electronic device and heterostructure fabrication.<br/><br/>In this work, we investigate the crystal growth mechanism and point defect density evolution of WSe₂ and MoSe₂ during a flux thermal cycle using optical imaging and conductive atomic force microscopy. Ampules containing WSe₂ and MoSe₂ are quenched at varying points during the thermal cycle to determine the effects of dwell length and cooling temperature on final crystal size and point defect density. We determine high-quality TMD crystals are formed with only a multi-day high-temperature dwell and rapid quench in water, decreasing total synthesis time by 75% and enabling the use of quick, additional thermal cycles. The fully-formed crystals are then refined by putting them back through a thermal cycle with fresh selenium precursor at an elevated chalcogen-to-TMD molar ratio, and the effects on point defect density and crystal size are investigated with each additional cycle. The refining process’ influence on electronic and optoelectronic properties are explored through transport and photoluminescent studies. Use of these growth optimizations greatly decrease the total time required for low point-defect flux TMDs with diameters approaching a centimeter, which increase exfoliation yield and monolayer size, enabling increased applications in transistor arrays or terahertz spectroscopy studies.<br/><br/>[1] S. Liu, <i>et al</i>. <i>ACS Nano</i> 17, 17 (2023), pp. 16587–16596, https://doi.org/10.1021/acsnano.3c02511<br/><br/>[2] Pack, J., <i>et al. </i>arXiv:2310.19782, (2023),<br/>https://doi.org/10.48550/arXiv.2310.19782<br/><br/>[3] B. Kim, <i>et al</i>. <i>ACS Nano</i> 16, 11 (2022), pp. 140-147, https://doi.org/10.1021/acsnano.1c04331