UC Berkeley

Semiconducting nanocrystals have shown great use in a variety of light emitting and absorb- ing applications, such as solar cells, light emitting diodes, displays, and even stress sensors. These materials exhibit size-tunable physical properties, such as phase transition pressures and band gaps. Although a lot of work has gone into understanding the structure-property relationships in these materials, a fundamental understanding of how these materials in- teract under various perturbations is necessary to understand the material’s applicability in applications. This dissertation outlines the characterization of various nanocrystals and nanocrystal composites under two different perturbations: self-absorption of luminesced light and elevated pressure.

Chapter 1 outlines the general properties of semiconducting nanocrystals and gives a background on previously studied perturbations. Optoelectronic and structural studies of cadmium chalcogenide nanocrystals are highlighted and the complex phase transitions ob- served in CsPbBr3 are discussed in detail.

In Chapter 2 we investigate how the reabsorption of photoluminesced light, an effect known as the inner filter effect (IFE), can affect the nanocomposite’s properties under illu- mination. The IFE has been well studied in solutions, but has garnered less attention in regards to solid-state nanocomposites. We demonstrate that the IFE can result in a large spectral red-shift of over a third of the linewidth of the photoluminescence of the nanocom- posites over a distance of 100 μm. We then utilize this red-shift to develop a displacement sensor with sub-micrometer resolution that has high-temporal and spatial resolution.

We investigate the effects of pressure on NCs in Chapters 3-4. Chapter 3 outlines the use of a diamond anvil cell for generating gigapascal pressures. This technique is then utilized to understand how CsPbBr3 nanocrystals respond to pressure in Chapter 4. The crystal structure of CsPbBr3 is composed of corner sharing lead bromide octahedra with Cs+ sitting in the cavities. By investigate the optical shifts and structural changes in the material with applied pressure, we find that the material transitions to a high-pressure phase around 1.4 GPa. A crystal structure of this high pressure phase has not been previously reported and we find that it has P21/m symmetry. We further find that CsPbBr3 exhibits a size-dependent compressibility where we find that nanocrystals are at least 40% more compressible than the bulk material.

We finally present an outlook for future studies in Chapter 5. We present ideas for determining additional fundamental properties of the CsPbBr3 high-pressure phase transition and investigating the effect of the inorganic-inorganic interface in nanocrystal core/shell heterostructures on crystallographic phase stability.