Abstract
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.
Highlights
Following the discovery of graphene,[1] a vibrant research area on two-dimensional (2D) transition-metal dichalcogenides (TMDs) has emerged in recent years to investigate the exciting and diverse properties of these materials.[2−4] TMDs are layered materials characterized by strong covalent bonding within single layers and weak interlayer van der Waals forces
Keeping all synthesis parameters except the ligand type fixed, we found that reactions in the presence of oleic acid (OA) yielded a flowerlike morphology in the 2H crystal structure along with some contribution from the 1T′ phase, whereas the use of oleylamine (OLA) led to nanosheets with a 1T′ crystal structure
We focused on the formation of WSe2 nanocrystals by reacting tungsten hexacarbonyl (W(CO)6) and elemental Se at high temperature (∼330 °C) in the presence of oleic acid or oleylamine as the coordinating solvent
Summary
Following the discovery of graphene,[1] a vibrant research area on two-dimensional (2D) transition-metal dichalcogenides (TMDs) has emerged in recent years to investigate the exciting and diverse properties of these materials.[2−4] TMDs are layered materials characterized by strong covalent bonding within single layers and weak interlayer van der Waals forces. Mahler et al synthesized WS2 nanocrystals in the 2H or 1T crystal phase using a mixture of ligands with or without a catalyst,[22] whereas Geisenhoff et al showed the phase tuning of WSe2 nanocrystals from the 2H to 1T′ crystal phase by increasing the concentration of oleic acid in the reaction flask.[36] Altavilla et al used oleylamine as a ligand and reducing agent to decompose single-source precursors to produce WS2 and MoS2 nanosheets.[37] Despite these multiple examples, few studies have addressed in detail the relation between the addition of ligands to the reaction mixture and the precursor reactivity or the crystal structure and surface characteristics of the resulting TMD nanocrystals. A detailed description of the results can be found in Section S10 in the Supporting Information
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