Chloride-Driven Chemical Vapor Transport Method for Crystal Growth of Transition Metal Dichalcogenides

Abstract

Single crystals of Mo and Ta dichalcogenides, MX 2 (M = Mo, Ta and X = S, Se, Te), have been grown by the vapor transport method in closed atmosphere, using a novel transport reaction that involves a mixture of M, MCl 5 and X as a source. The most important parameters that must be kept under control for succeeding in crystal growth are the temperature, which depends mainly on X, and the initial M:MCl 5 ratio. The best growth temperature is found to be the highest in the case of tellurides, the lowest in the case of sulfides and intermediate for the selenides. The optimal M:MCl 5 molar ratio decreases with the atomic number of the chalcogen, ranging from 50 for sulfides to 15 for tellurides. Values out of this range made it not possible to control the nucleation process. The high quality of the crystals grown by using this method is confirmed by X-ray diffraction studies, Raman spectroscopy and electrical resistivity measurements, and proved to be higher than that of commercially available MoS 2 crystals. In the case of TaS 2 and TaSe 2, using a high starting chloride fraction and a low reaction temperature, the growth of thin (∼μm) and long (∼mm) whiskers is observed. An explanation of their growth mechanism is proposed.