Modified Self-Flux Synthesis of Centimeter-Scale Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDs) are a highly attractive class of materials due to their novel electronic and optoelectronic properties and the ability to use them as a platform for assembling heterostructures. Self-flux crystal growth of TMDs has been shown to grow bulk TMDs with the lowest defect densities of any synthesis technique, maximizing the ability to study their unique, intrinsic phenomena [1-3]. However, the current self-flux crystal growth method produces TMD crystals smaller than 1 mm² on average, making monolayer-exfoliation difficult and limiting applications to single devices. Additionally, these crystals are too small to use macroscopic exfoliation techniques such as gold-assisted exfoliation, which require crystals greater than 2 mm in diameter and can isolate entire monolayers from every layer of a bulk crystal. [4]<br/>In this work, we use a modified flux-synthesis thermal cycle containing an optimized dwell temperature and temperature cycling Ostwald ripening to increase the crystal area of ultralow defect density TMDs by more than ten-fold. The crystal growth kinetics as a function of temperature are determined, enabling increased cooling rate steps and shorter synthesis times. An additional post-processing self-flux recrystallization step is added and further increases the crystal size of TMDs and reduces the defect density. Lastly, gold-assisted exfoliation is used to isolate several centimeter-long monolayers from one modified self-flux synthesis MoSe₂ crystal, displaying monolayer yields per crystal more than five orders of magnitude greater than traditional scotch-tape exfoliation techniques. These self-flux synthesis modifications provide a route towards reproducibly exfoliating ultralow defect density TMD monolayers at the centimeter scale with high yields, greatly increasing the scalability of high-quality self-flux TMD bulk crystals for mass production.<br/><br/>[1] D. Rhodes, <i>et al.</i>, <i>Nat. Mater. </i>18 (2019), pp. 541-549, https://doi.org/10.1038/s41563-019-0366-8<br/><br/>[2] D. Edelberg, <i>et al.,</i> <i>Nano Lett. </i>19 (2019), pp. 4371-4379, https://doi.org/10.1021/acs.nanolett.9b00985<br/><br/>[3] S. Liu, <i>et al</i>.<i>,</i> <i>arXiv preprint arXiv:2303.16290 </i>(2023).<br/><br/>[4] F. Liu, <i>et al., Science </i>367 (2020), pp. 903-906, https://doi.org/10.1126/science.aba1416