Synthesis and Characterization of ReS₂ and\nReSe₂ Layered Chalcogenide Single Crystals

Abstract

We report the synthesis of high-quality\nsingle crystals of ReS₂ and ReSe₂ transition\nmetal dichalcogenides using\na modified Bridgman method that avoids the use of a halogen transport\nagent. Comprehensive structural characterization using X-ray diffraction\nand electron microscopy confirm a distorted triclinic 1<i>T</i>′ structure for both crystals and reveal a lack of Bernal\nstacking in ReS₂. Photoluminescence (PL) measurements on\nReS₂ show a layer-independent bandgap of 1.51 eV, with\nincreased PL intensity from thicker flakes, confirming interlayer\ncoupling to be negligible in this material. For ReSe₂,\nthe bandgap is weakly layer-dependent and decreases from 1.31 eV for\nthin layers to 1.29 eV in thick flakes. Both chalcogenides show feature-rich\nRaman spectra whose excitation energy dependence was studied. The\nlower background doping inherent to our crystal growth process results\nin high field-effect mobility values of 79 and 0.8 cm²/(V\ns) for ReS₂ and ReSe₂, respectively, as extracted\nfrom FET structures fabricated from exfoliated flakes. Our work shows\nReX₂ chalcogenides to be promising 2D materials candidates,\nespecially for optoelectronic devices, without the requirement of\nhaving monolayer thin flakes to achieve a direct bandgap.