Control Over Precursor Conversion for the Synthesis of Tailored Group-VI Transition Metal Dichalcogenide Nanocrystals

Book excerpt: Transition metal dichalcogenides are a class of materials that are built from two-dimensional layers held together via Van der Waals interactions. These materials are structurally diverse and can adopt multiple phases. In particular, the group VI TMDs (MoS2, MoSe2, WS2, WSe2) exist in either a metastable 2M phase or the thermodynamically favored 2H phase. Material properties can be tuned by controlling the phase or the number of layers in the crystal, making controllable syntheses of these materials highly desired. Colloidal synthesis offers a solution-phase route to solid-state materials. Conducting the synthesis in the solution phase allows a synthetic chemist to access a diverse parameter space that is usually not afforded with more traditional solid-state syntheses. This includes being able to tune reaction temperature, ligand environment, and precursor reactivity, which can easily lead to kinetically controlled reaction regimes and direct synthesis of metastable phases. This dissertation demonstrates that colloidal synthesis is a viable tool for phase tunable syntheses of TMDs, as well as investigating aspects that govern phase conversion.The coordination and reactivity of the tungsten precursor used can be influenced via the ligands used. Reactions performed in solutions of either the strongly coordinating oleic acid, or the weakly coordinating trioctylphosphine oxide are detailed in chapter 2. The size and phase of the nanocrystals are tuned, where increased amounts of oleic acid decrease the reactivity leading to large nanocrystals of the metastable 2M phase. Building upon the work outlined in chapter 2, chapter 3 leverages the reduced reactivity of the metal precursor in great amounts of oleic acid to synthesize metal selenide/tungsten diselenide heterostructures via a one-pot method. The delayed reactivity in oleic acid allows for the nucleation of other metal selenides prior to the secondary growth of WSe2. How the coordination environment of tungsten influences reactivity and nanocrystal formation is more rigorously evaluated in chapter 4. Heating W(CO)6 in the presence of either trioctylphosphine oxide or trioctylphosphine forms W(CO)6-x(L)x intermediates. Phosphine oxides promote CO labilization while phosphines hinder this process. Thus, performing reactions in TOPO lead to low-temperature syntheses of the metastable 2M phase, while those with TOP require additional heating. Insights into how morphology impacts phase conversion from the metastable 2M phase to the thermodynamically favored 2H phase are investigated in chapter 5. Reactions performed in trioctylphosphine oxide with differing concentration results in WSe2 nanocrystals with differing layer numbers. Here, high precursor concentrations lead to nanocrystals with increased number of layers without changing the lateral size of the nanocrystals. This change in nanocrystal morphology is accompanied by a reduction in the phase conversion rate from the 2M phase to the 2H phase. Phase conversion is likely slowed with increased interlayer binding energies. Finally, a synthetic route to monolayer TMD nanocrystals is outlined in chapter 6. Controlled heat-ups in the presence of excess trioctylphosphine result in ligands tightly bound to the basal plane of the TMDs. These ligands permanently separate the layers from one another, producing monolayers, and influence the electronic properties of the nanocrystals. Additionally, phase conversion is rapid in these monolayer-like systems.