Synthesis and Characterization of ReS₂ and ReSe₂ Layered Chalcogenide Single Crystals

Abstract

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.