Control Over Cadmium Chalcogenide Nanocrystal Heterostructures via Precursor Conversion Kinetics (2018)

Semiconductor nanocrystals have immense potential to make an impact in consumer products due to their narrow, tunable emission linewidths. One factor limiting their use is the ease and reproducibility of core/shell nanocrystal syntheses. This thesis aims to address this issue by providing chemical control over the formation of core/shell nanostructures by replacing engineering controls with kinetic controls. Chapter 1 contextualizes our study on nanoparticle synthesis with a brief discussion on the physics of quantum confinement and the importance of narrow size dispersities, core/shell band alignments, and low lattice mismatches and strain at core/shell nanocrystal interfaces. Next, the evolution of cadmium chalcogenide nanocrystal reagents is described, ranging from the original organometallic reagents used in the 1980s to modern approaches involving cadmium phosphonates and carboxylates. This is followed by a description of chalcogen precursors, highlighting the recent introduction of molecules whose well-controlled and tunable reaction rates allow for the size tuning of nanocrystals at 100% yield, and accompanying theories on nanocrystal nucleation. Chapter 2 covers work to expand the library of available sulfur precursors to a wider range of molecules relevant for the synthesis of cadmium sulfide nanocrystals. Using thioureas alone, only very fast or very slow precursor conversion rates can be accessed. This limits the accessible sizes of cadmium sulfide nanocrystals using a single hot injection of precursor at standardized reaction conditions. We observe that thiocarbonate and thiocarbamate precursors with varying electronic substituents allow access to intermediate precursor conversion rates and cadmium sulfide nanocrystal sizes. Interestingly, we note that these new precursor classes nucleate particles with higher monodispersity than ones synthesized from thioureas. These results indicate that in addition to precursor structure controlling precursor conversion rate, precursor structure additionally impacts nanocrystal monodispersity. Chapter 3 expands the library of sulfur and selenium precursors to include cyclic thiones and selenones which extends chemical control of precursor conversion kinetics to cover five orders of magnitude. This unprecedented breadth of rate control allows for the simultaneous hot injection of multiple precursors to generate core/shell or alloyed nanoparticles using precursor reactivity. Using this new synthetic strategy, we observe that kinetic control runs into several issues which we partially attribute to differences in cadmium sulfide and cadmium selenide critical concentrations and growth rates. Nevertheless, combined with a syringe pump shelling method, we are able to access core/shell and alloyed nanocrystals with photoluminescence quantum yields of 67-81%. Chapter 4 applies the concept of nanostructure control via precursor conversion kinetics to a better model system: two-dimensional nanoplatelets. Cadmium chalcogenide nanoplatelets are highly desirable materials due to their exceptionally narrow emission full width half max (FWHM) values which make them pure emitters relative to quantum dots or organic dyes. We synthesize 3 monolayer thick nanoplatelets whose lateral dimensions vary from 10 nm x 10 nm to 186 x 100 nm and demonstrate compositional control on the smallest platelet sizes with STEM EELS.