UC Berkeley

The investigation of the properties of matter under high pressure is becoming increasing important to many scientific fields, including chemistry, physics, and biology. From a fundamental standpoint, the application of high pressure allows one to investigate the effect of volume on the properties of a material. It also allows for the examination of the conversion of one phase of matter to another, whether through the crystallization of a material under high pressure, or the transformation of a solid from one crystal structure to another. This dissertation explores the properties of nanoscale materials under high pressure.

The size dependence for solid-solid phase transitions in II-VI nanocrystals under high pressure has been previously investigated. Nanoparticles represent an important size regime where the surface atoms make up a significant percentage of the total atoms in the crystal. This leads to the surface playing a larger role in the thermodynamics of the phase transition in these nanocrystals.

The first part of this dissertation examines the effect of small changes in the structure of the nanomaterial on its high pressure behavior. By changing the surface of cadmium selenide nanocrystals through the introduction of a zinc sulfide shell, it was determined that the phase transition could be tuned for shell thicknesses below the critical thickness. In addition to structural phase transitions in this core/shell system, the optical properties under high pressure were also investigated. The fluorescence of the core/shell particles splits into multiple peaks due to a breaking of the crystal symmetry and spin exchange in the excitons under pressure.

This theme was continued through the investigation of bismuth selenide nanoribbons with and without copper intercalated into the lattice. Bismuth selenide has been found to be a topological insulator, while copper bismuth selenide is a superconductor. The phase transition from the layered rhombohedral structure to the monoclinic phase in bismuth selenide nanomaterials could be pushed to higher pressures through the intercalation of copper in the van der Waals gap between the crystal layers.

The dissertation continues by exploring the mechanism of the wurtzite to rocksalt phase transition in cadmium sulfide nanocrystals on ultrafast timescales. A shock wave was initiated through the sample using a laser and the phase transition was probed using an ultrafast X-ray probe pulse. This allowed for the collection of the necessary structural data from the diffraction patterns at different time delays. A h-MgO type intermediate was found to be present under lower shock stresses, but it was not observed at higher shock stresses. This indicates that multiple phase transition pathways are simultaneously occurring in the sample.

Steps towards achieving the ability to image the fluorescence of single semiconductor nanoparticles under high pressure are discussed. At a single particle level new phenomena have been observed due to the inherent heterogeneity of the sample and a lack of averaging over multiple events. There are many hurdles that must be overcome before such experiments can be achieved.

The last part of the dissertation discusses the use of the high pressure behavior of these semiconductor nanocrystals to sense forces in a biological sample. The spectral shift of tetrapod nanocrystals was used to quantify the force exerted on a substrate by beating HL-1 cardiomyocytes. This was performed through the use of an acousto-optic tunable filter to provide the measurements with spectral, temporal, and spatial resolution.

Through the investigation of the fundamental properties of nanomaterials under high pressure, a better understanding of these materials, including their thermodynamics, was obtained. As a result, new applications of these materials to fields such as force sensing are discussed and implemented.