Chloride-Driven Chemical Vapor Transport Method for Crystal Growth of Transition Metal Dichalcogenides Alberto Ubaldini, Jacim Jacimovic, Nicolas Ubrig, and Enrico Giannini* Dé partement de Physique de la Matie ̀ re Condense ́ e, University of Geneva, quai E.-Ansermet 24, 1211 Geneva 4, Switzerland 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. ■ INTRODUCTION Transition metal dichalcogenides MX 2 with the honeycomb- like structure (M = Ti, Zr, Hf, V, Nb, Ta, Mo, W and X = S, Se, Te) are an interesting class of materials with a wide spectrum of electronic properties, including semiconductors, semimetals, metals and superconductors, and can exhibit complex charge ordering. 1-3 Both the electronic and mechanical peculiarities of these materials make them suitable for a variety of potential applications, ranging from superlubricants 4 to ultralow thermal conductivity devices, 5 from high-performance field-effect transistors 6 to optoelectronic devices, 7 catalysts in redox- based reactions, 8 solar cell converters, 9 and others. Known for a long time and widely studied in the past, also thanks to their low cost, these materials are experiencing a resurgence of interest, because of the possibility of being mechanically exfoliated down to two-dimensional (even monolayer) single crystalline pieces of solid matter that remain stable, keep the structure of the bulk crystal and exhibit exciting electronic properties. 10 These quasi-2-dimensional materials have shown immense potential as promising candidates for a new generation of electronic devices, complementary or even competitive to graphene. 1,11 Indeed, contrary to graphene, the existence of a band gap in 2-dimensional MX 2 allows the fabrication of transistors that can be switched off. Most of the research effort is currently focused on studying the transport properties of MoS 2 in the field-effect transistor (FET) geometry, thanks to a band gap of the order of few meV. 7 The first demonstration of a switchable field effect transistor based on ultrathin MoS 2 was published in 2012, 1 and silicon- level performance was shown to be possible. Larentis et al. 12 reported similar results in the case of MoSe 2 . Single layers to be used in 2D electronic devices can be exfoliated from bulk crystals using an adhesive-backed-tape technique, similarly to what is done with graphite in order to prepare graphene, and the electronic band structure is strongly affected by the number of layers in the 2D flakes. 7,13-15 The peculiar properties of MX 2 arise from their crystalline structure that consists of vertically stacked layers, held together by weak van der Waals-like interactions. 16,17 Each layer is formed by sheets of cations, having a 6-fold environment, packed between two sheets of anions. The two most important structures are the so-called “1T” (CdI 2 structure type, space group P3̅m1), where each transition metal ion is octahedrally surrounded by six anions, and the so-called “2H” structure (NbS 2 structure type, space group P63mmc), where the coordination is trigonal prismatic. 16 MoS 2 and MoSe 2 crystallize mainly in the latter, MoTe 2 and tantalum chalcogenides can crystallize in both types of structures, with the exception of TaTe 2 that is, found to crystallize in a monoclinic distortion of the “1T”-type (space group C2/m). 18 Different stacking of dichalcogenides layers can make MoS 2 and MoSe 2 crystallize in the metastable “3R”-polytype (R3mh space group, 19,20 ), which is stabilized either by high pressure 21 or Re substitutions. 22 The growth of large and high quality single crystals of metal dichalcogenides is one of the critical aspects that can limit or even hinder their application. These crystals are generally Received: June 25, 2013 Revised: August 21, 2013 Article pubs.acs.org/crystal © XXXX American Chemical Society A dx.doi.org/10.1021/cg400953e | Cryst. Growth Des. XXXX, XXX, XXX-XXX