Synthesis and Characterization of ReS₂ and ReSe₂ Layered Chalcogenide Single Crystals

We report the synthesis of high-quality single crystals of ReS₂ and ReSe₂ transition metal dichalcogenides using a modified Bridgman method that avoids the use of a halogen transport agent. Comprehensive structural characterization using X-ray diffraction and electron microscopy confirm a distorted triclinic 1T′ structure for both crystals and reveal a lack of Bernal stacking in ReS₂. Photoluminescence (PL) measurements on ReS₂ show a layer-independent bandgap of 1.51 eV, with increased PL intensity from thicker flakes, confirming interlayer coupling to be negligible in this material. For ReSe₂, the bandgap is weakly layer-dependent and decreases from 1.31 eV for thin layers to 1.29 eV in thick flakes. Both chalcogenides show feature-rich Raman spectra whose excitation energy dependence was studied. The lower background doping inherent to our crystal growth process results in high field-effect mobility values of 79 and 0.8 cm²/(V s) for ReS₂ and ReSe₂, respectively, as extracted from FET structures fabricated from exfoliated flakes. Our work shows ReX₂ chalcogenides to be promising 2D materials candidates, especially for optoelectronic devices, without the requirement of having monolayer thin flakes to achieve a direct bandgap.