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.