The growth and characterization of rhenium-doped WS2single crystals

Single crystals of rhenium-doped WS2 have been grown by the chemical vapour transport method using bromine as a transporting agent. From x-ray diffraction pattern analysis, the doped crystals are found to crystallize in mixed polytypes of the three-layer rhombohedral (3R) and two-layer hexagonal (2H) structures while the undoped one is in the 2H form. The x-ray photoelectron spectra (XPS) in the vicinity of W 5p core-level regime exhibit strongly asymmetric line-shape character and the weak shoulder located at higher energy side is attributed to the presence of a small amount of rhenium in WS2. Hall coefficient measurements at room temperature indicate that the samples are n-type in nature. The doping effects of the materials are characterized by temperature-dependent conductivity, room temperature optical absorption and piezoreflectance (PzR) measurements. The drastic decrease in resistivity with increasing doping concentration suggests that more charge carriers are available for conduction in the doped compounds. The indirect energy gap for the doped samples shows red shifts on increasing the doping concentration. The direct band edge excitonic transition energies show a slight red shift due to the presence of a small amount of Re and the broadening parameter of the excitonic transition features increases due to impurity scattering.