The Division of Chemistry supports Joshua Vura-Weis of the University of California - Berkeley as an American Competitiveness in Chemistry Fellow. Dr. Vura-Weis will use ultra-short (femtosecond) pulses of x-rays to probe the charge and exciting dynamics in semiconductor nanostructures. The femtosecond x-ray reflectivity experiments will be conducted in collaboration with Prof. Steve Leone. Nanomaterials will be made using the expertise and facilities of the Molecular Foundry at the Lawrence Berkeley National Laboratory. In addition, he will make steady-state x-ray measurements at the Advanced Light Source. For his plan for broadening participation, the PI will work to adapt curriculum materials developed for the SEPUP Program at the Lawrence Hall of Science for use by summer school students participating with the Breakthrough Collaborative -- a national consortium of 27 summer school programs for underserved but highly motivated students.
Research like that of Dr. Vura-Weis is aimed at developing a better understanding of the ways in which a technologically important class of materials interacts with light. The hope is that this will lead to the development of superior materials for e.g. solar energy applications. The efforts at broadening participation being pursued by Dr. Vura-Weis are aimed at enabling the best and brightest young people to pursue careers in science.
This project developed a new method of tabletop extreme ultraviolet (XUV) transient absorption spectroscopy on first-row transition metal oxide semiconductors such as α-Fe2O3 (hematite). The XUV probe was created with a tabletop ultrafast laser system using a process called high-harmonic generation, providing sub-100 femtosecond time resolution. Previous work on XUV transient absorption spectroscopy focused on gas-phase samples, and extension of this technique to solid-state samples required the design and construction of a new instrument with an order of magnitude higher flux and stability. The substrate itself absorbed 90% of the XUV photons, so the flux measured at the CCD detector was 10 times lower than in a gas-phase experiment with the same sample absorbance. It is also possible to excite the majority of the sample in gas-phase experiments because each laser pulse excites a new population of gas molecules. To avoid sample damage in the α-Fe2O3 film, only 5% of the Fe atoms could be excited with each laser pulse, giving correspondingly lower signal-to-noise ratio. High-harmonic generation was therefore performed in a semi-infinite gas cell to maximize photon flux, stability, and ease of alignment. In order to improve the spectral coverage and minimize the low-flux regions between odd harmonics, a two-color laser field (800 nm + 400 nm) was used to generate both even and odd harmonics. A schematic of this instrument is shown in Figure 1. Using this instrument, we collected the XUV absorption spectra of a series of transition metal oxides such as α-Fe2O3, Co3O4, and TiO2. The spectrum of α-Fe2O3is shown in Figure 2, along with a simulated spectrum using a semiempirical ligand field multiplet calculation. We showed that simulation techniques originally developed for higher-energy x-rays can be successfully used to simulate both ground and excited state spectra in the XUV spectral region. We then probed the ultrafast excited-state dynamics of hematite, which is an earth-abundant photocatalyst for hydrogen production from water. Transient absorption spectroscopy was performed in order to resolve competing theoretical assignments of the major visible-light absorption peak at 400 nm. By comparing the excited-state XUV absorption to semiempirical charge transfer multiplet calculations, we definitively assigned the initial photoexcited state as a ligand-to-metal charge transfer (LMCT) state and not a d-d excitation (Figure 3). Intellectual Merit: This research developed new instrumentation for studying the photophysics of transition metal oxide semiconductors. We extended theoretical treatments developed for higher-energy x-ray absorption to the extreme ultraviolet spectral region, building the intellectual foundation for studies of transition metals in this energy range. Finally, we used these techniques to resolve a long-standing controversy about the electronic structure of α-Fe2O3, an earth-abundant material that is the subject of intense study due to its potential use as a photocatalyst for water splitting. Broader Impacts: Societal Goals: A fundamental understanding of electron dynamics in solid-state materials is needed to develop the next-generation photocatalysts and photovoltaics that will be required to reduce the effects of global warming at acceptable financial costs. By extending a spectroscopic technique that has previously been used primarily for gas-phase dynamics into the solid state, we have enhanced research infrastructure and showed that a tabletop instrument can reveal oxidation state changes previously only accessible using synchrotron sources. Outreach: An NSF-funded, inquiry-based chemistry curriculum ("SEPUP") was implemented throughout a national consortium of summer schools for underserved middle school children ("The Breakthrough Collaborative"), of whom 92% are of color and 85% qualify for free or reduced-price lunches. Throughout the six-week summer school programs, students learned solution chemistry techniques, data analysis, and scientific presentation skills. The problem-based curriculum focused on a hypothetical industrial site which had to balance the costs and benefits of various pollution control strategies, and students learned to evaluate both financial and environmental costs of different remediation techniques. Project funds were used to purchase the SEPUP modules for 10 of the 35 Breakthrough programs, and a veteran middle school science teacher was recruited to record training videos and provide email support to teachers throughout the summer. The Breakthrough Collaborative employs rising high school seniors and undergraduates as teachers, so this project provided valuable training in modern teaching methods to a new generation of science teachers.
