UCLA

Semiconductor nanoplatelets (NPLs) are nanocrystals with quantum confinement only in the thickness dimension and precise monolayer thicknesses. They have excellent monodispersity giving them narrow linewidths, high absorption cross-sections, and high photoluminescence quantum yields. In addition to their exceptional photophysical properties, they can have lateral extents that exceed 1000 nm, making them analogous to two-dimensional semiconductors such as transition metal dichalcogenides with the advantage of colloidal preparation. In this dissertation I explore how interesting photophysical properties can arise within these materials as a result of their anisotropic structure. Chapter 1 provides the foundation for understanding the electronic and physical structures of NPLs, briefly summarizes the current understandings in the field, and presents open questions that this dissertation aims to address.

In Chapter 2, we develop a new type of heterostructure through cation exchange from CdTe to HgTe NPLs. HgTe NPLs are promising candidates for optoelectronic applications in the short-wave infrared (SWIR), but their absorption and photoluminescence has limited tunability. By taking advantage of their large, exposed surfaces following cation exchange, we grow HgTe quantum dots (QDs) on the lateral faces and establish a heterostructure capable of energy transfer from 2D NPL to 0D QD. We extend the tunability of photoluminescence through the energy transfer by controlling the confinement within the quantum dot, and observe high quantum yields across the SWIR.

Chapter 3 builds on our understanding of the HgTe NPL/QD heterostructure by exploring the mechanism and efficiency of energy transfer in greater depth. We compare the in-situ grown QD on NPL system to a system of mixed QDs and NPLs and demonstrate that the near unity energy transfer is only achieved using the in-situ method. We quantify this efficiency and find that the mixed case follows a F�rster Resonance Energy Transfer while the in-situ case is likely a near field energy transfer. Using our estimates, we also model the exciton diffusion within HgTe NPLs and find that the magnitude of the diffusion constant is comparable to other extended 2D semiconductors.

In Chapter 4 we turn our focus to synthetic procedures for CdTe NPLs to control lateral area. We develop a seeded growth mechanism that can be used to reproducibly extend NPLs from their small size after initial synthesis (100-500 nm) to mesoscale length scales (1-2 μm). We show that these methods can be applied to CdTe and CdSe, and cation exchange from these extended NPLs produces mesoscale HgTe and HgSe NPLs. Using in-situ monitoring, we examine how the seeded growth proceeds and show that the extension follows the understood mechanism of lateral ripening. Finally, we image the photoluminescence of the mesoscale NPLs using correlative microscopy and demonstrate that their large extents allow for spatial resolution of their photophysical properties.

Chapter 5 develops another synthetic procedure for controlling lateral extent, but without the need for seeded growth. By closely examining the parameters used during a slow injection procedure, we are able to understand how the precursor, temperature, and ligands play a role in NPL area and directly synthesize mesoscale NPLs in one step. We systematically show how each parameter affects the lateral size and monolayer thickness of the final NPL product. Using the largest mesoscale NPLs, we use transmission electron and atomic force microscopy to examine their propensity for stacking, as well as Raman spectroscopy to observe strain with spatial resolution.

Chapter 6 describes a handful of remaining experiments and projects that were not fully developed for publication. This also includes some techniques and lessons learned in sample handling which may be useful for future students.

Finally, Chapter 7 describes our efforts in chemical education research within the general chemistry laboratory course for life science majors at UCLA. In this chapter we examine how the style of pre-lab affects student performance when tasked with learning a new technique. A cohort of students critically evaluate a series of videos demonstrating exemplary technique and poor technique before attending their lab sections, and these students are compared to a cohort who simply watched the exemplary technique before starting the experiment. Our goal is to reduce cognitive load during the experiment by establishing the proper steps before attempting to practice the technique. We show that although the majority of the class is unaffected by the intervention, we are able to reduce the percentage of students performing as outliers. We also demonstrate that the intervention reduces the dependence of students on instructor feedback during the lab sessions which is advantageous for large classrooms.