UC Berkeley

The motion of electrons and nuclei, or of charge carriers and lattice, respectively, in a solid system, are the most fundamental dynamics in nature. Understanding the ultrafast behavior of electrons, holes and phonons is critical to understanding and controlling the light-induced response of materials, most crucially for photovoltaic and photoelectrochemical application. In these materials, it is not understood why nanoparticles and other nanoscale systems exhibit such different macroscopic responses to solar irradiation, such as increased photocurrent or improved charge transfer efficiency. Unfortunately, few techniques exist with which to measure photoexcited phenomena on the femtosecond and picosecond timescales that electronic and nuclear motion occurs, and even fewer are sensitive to both charge carriers and lattice changes. Here, the technique of extreme ultraviolet (XUV) transient absorption spectroscopy is applied to a variety of nanoparticle and nanoscale solar energy relevant material systems to uncover the coupled carrier-lattice relaxation processes following absorption of a solar photon. XUV light initiates a core-to-valence transition, so it is uniquely able to simultaneously measure electrons, holes, and lattice dynamics with few-femtosecond time resolution. XUV is also sensitive to oxidation state and spin state and is element specific, making it ideal for answering the lingering questions in the solar energy field.

In this dissertation, we begin in Chapter 1 with a description of semiconductors and nanoparticles, focusing on the ultrafast dynamics of charge carriers and the lattice. Then, core-level spectroscopy is introduced, and the process of high-order harmonic generation to create the XUV core level spectrum is explored. In Chapter 2, the ultrafast laser and high vacuum setup is presented, followed by an analysis of nanoparticle thin film sample preparation. Chapter 3 delves into the first nanoparticle transient XUV absorption experiment, which utilizes iron oxide nanoparticles (goethite α-FeO(OH)) and bulk iron oxide films (hematite α-Fe2O3) to demonstrate a link between morphology, crystal structure and photoexcited small polaron formation. A nanoscale metal-oxide-semiconductor junction of Ni-TiO2-Si is explored in Chapter 4, and the ultrafast hole dynamics across all three layers of the junction are revealed. In Chapter 5, a hot phonon bottleneck is demonstrated to slow hot carrier cooling in silicon nanoparticles following above-gap photoexcitation when compared to a single-crystal silicon bulk. Finally, Chapter 6 outlines the current progress studying hot hole and lattice relaxation in the ferroelectric semiconducting halide perovskite CsGeI3 and proposes future directions in nanoparticles and thin films with extreme ultraviolet transient absorption spectroscopy.