Colloidal Synthesis of the 2H and 2M Phases of Tungsten Disulfide Nanocrystals

Book excerpt: Layered materials have recently gained a lot of attention, in particular group-VI transition metal dichalcogenides (TMDs). This class of materials exhibits unique phase-specific properties ranging in behavior from metallic to semiconducting to topologically insulating. Selecting for a single phase to utilize for their desired applications can be challenging, however, due to their similar ground state energies. Here we examine WS2 nanocrystals (NCs), which nucleate as the metastable 2M phase and gradually convert to the thermodynamically-favored 2H phase. We develop a phase-tunable colloidal synthesis by controlling the reactivity to either hinder or promote rapid phase conversion in order to access the 2M and 2H phases, respectively. We found that inclusion of oleic acid (OA) decreases the reactivity of the tungsten hexacarbonyl precursor, resulting in reduced phase-conversion upon reaction with sulfur. Reactivity of the W and S precursors can, however, be promoted by utilizing the one-pot method of sulfur addition, in which sulfur is present during the heating process. This increase in the direct W-S reactivity leads to rapid phase-conversion. Finally, we compare the reactivity of various S-precursors, including thiols and diorgano disulfides. The use of thiols results in more rapid phase-conversion, which we hypothesize is due to increased interlayer distances enabled by the incorporation of the alkyl group as a "crystal-bound" ligand. Collectively, these synthetic tools offer controllable access to either the 2M or 2H phase of WS2 NCs.