Synthesis of Colloidal WSe2 Nanocrystals: Polymorphism Control by Precursor-Ligand Chemistry

Syntheses of transition-metal dichalcogenides (TMDs) using colloidal-chemistry approaches are gaining significant interest in recent years, as these methods enable the morphology and properties of the nanocrystals to be tuned for targeted applications. In this work, by only varying the ligand used during synthesis, we synthesized nanoflowers with oleic acid (OA) and 1T′ phase dominated WSe2 nanosheets with oleylamine (OLA). WSe2 nanocrystals show slower rate of formation for the metastable 1T′ phase. Surface chemistry analyses of the synthesized nanocrystals by solution NMR establish that neither of the ligands bind strongly to the surface of nanocrystals but are in a dynamic coordination with the WSe2 surface. A further examination of the coordination of tungsten hexacarbonyl (W(CO)6) with the respective ligands confirms that W(CO)6 decomposes in OA, losing its octahedral symmetry, which leads to fast reactivity in the flask. In contrast to this, W(CO)6 reacts with OLA to form a new complex, which leads to slower reactivity and crystallization of the synthesized nanocrystals in the octahedral 1T′ phase. These insights into the influence of precursor-ligand chemistry on reaction outcome and the peculiar surface chemistry of colloidal TMD nanocrystals will be instrumental in developing future colloidal TMD nanocrystals.