The exciting properties of graphene have drawn considerable attention to 2D layered materials for electronic and opto-electronic applications. In this view, transition metal dichalcogenides (TMDs) take pride of place as they provide a wide range of electrical properties thank to the variety of their chemical composition. Most of these compounds are in form of layered structures that can be exfoliated down to mono-atomic two-dimensional sheets. Monolayers of TMDs exhibit properties that promote them to the forefront for future electronics.
Whereas the research community mainly focused on MoS2 and WS2, recent works, as the discovery of “titanic” magnetoresistance in WTe2, the prediction of magnetism in monolayer VS2, and the Weyl semi-metallic behaviour of 1T-MoTe2 underlined the importance of studying the entire family. To explore the whole range of chemical compositions, structural polytypes, and physical properties it is essential to grow large and high quality single crystals of all these TMDs.
Single crystals of TMDs are generally obtained by the chemical vapour transport method because of their high melting point, the strong volatility of components and the poorly known phase diagram. In this process, the choice of the transport agent is crucial, as it controls the reaction constant and kinetics, selects the polytype growth, and influences the defect formation. I will review the processing methods of all the TMDs (TM = Ti, Zr, Hf, V, Nb, Ta, Mo, W…), with particular focus on the beneficial role of metal chlorides as transport agent, and discuss the structural peculiarities and the suitability of these crystals for application in electronic devices.