Relative Reactivity of Biogenic and Chemogenic Uraninite and Biogenic Non-Crystalline U(IV)

Aqueous chemical extractions and X-ray absorption spectroscopy (XAS) analyses were conducted to investigate the reactivity of chemogenic uraninite, nanoparticulate biogenic uraninite, and biogenic monomeric U(IV) species. The analyses were conducted in systems containing a total U concentration that ranged from 1.48 to 2.10 mM. Less than 0.02% of the total U was released to solution in extractions that targeted water-soluble and ion exchangeable fractions. Less than 5% of the total U was solubilized via complexation with a 0.1 M solution of NaF. Greater than 90% of the total U was extracted from biogenic uraninite and monomeric U(IV) after 6 h of reaction in an oxidizing solution of 50 mM K2S2O8. Additional oxidation experiments with lower concentrations (2 mM and 10 mM) of K2S2O8 and 8.2 L-1 dissolved oxygen suggested that monomeric U(IV) species are more labile than biogenic uraninite; chemogenic uraninite was much less susceptible to oxidation than either form of biogenic U(IV). These results suggest that noncrystalline forms of U(IV) may be more labile than uraninite in subsurface environments. This work helps fill critical gaps in our understanding of the behavior of solid-associated U(IV) species in bioremediated sites and natural uranium ore deposits

Data-driven approach to understanding exciton-exciton interactions in CsPbBr₃ nanocrystals

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 105-109).Lead halide perovskites are a rapidly developing class of materials of interest for optoelectronic applications. They have a number of desirable properties such as long carrier diffusion lengths and defect tolerance that arise from the materials' unique dielectric properties. Although much of the initial interest in lead halide perovskites was geared towards producing highly efficient solar cells from the bulk material, cubic perovskite nanocrystals are a strong candidate system for light-emitting applications. Optical gain in semiconductor nanocrystals relies on emission from biexciton or doubly excited states. Knowledge of the spectral properties of biexciton states is critical for understanding optical gain development as well as many-body interactions between charge carriers more broadly. In this thesis, we develop and demonstrate a data-driven approach to characterizing the energetics and dynamics of biexciton states in CsPbBr₃ nanocrystals using TA spectroscopy.We then use the understanding developed using the TA data to guide experiments using other techniques and further examine the physical phenomena that influence these excited states. In Chapter 2, we describe our data-driven method in detail and demonstrate its effectiveness in extracting spectral information about CsPbBr₃ nanocrystals. The method combines the target analysis fit commonly employed in organic systems with Bayesian inference and a Markov chain Monte Carlo sampler to accurately characterize the model uncertainty and vet the model itself. In Chapter 3, we apply the analysis developed in Chapter 2 to a size-series of CsPbBr₃ nanocrystals to extract the biexciton and exciton component TA spectra as a function of nanocrystal size. We find that the exciton and biexciton spectra have distinctive shapes, in contrast with the common assumption about these spectra.The biexciton spectra a broader and slightly blue-shifted from the exciton spectrum, and the broadening and blue-shifting both increase as the nanocrystal size decreases. We verify this with our own time-resolved photoluminescence experiments. In Chapter 4, we propose and discuss in detail the development of an experiment to verify our hypothesis for why the exciton-exciton interaction is repulsive - the effect of polaron formation. We describe the development of a femtosecond stimulated Raman spectroscopy experiment to directly observe polaron formation and the challenges of performing this technique at high repetition rate. The central goal of this thesis is to describe a more careful approach to analyzing spectroscopic data.by Matthew N. Ashner.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Chemical Engineerin

High repetition-rate femtosecond stimulated Raman spectroscopy with fast acquisition

Time-resolved femtosecond stimulated Raman spectroscopy (FSRS) is a powerful tool for investigating ultrafast structural and vibrational dynamics in light absorbing systems. However, the technique generally requires exposing a sample to high laser pulse fluences and long acquisition times to achieve adequate signal-to-noise ratios. Here, we describe a time-resolved FSRS instrument built around a Yb ultrafast amplifier operating at 200 kHz, and address some of the unique challenges that arise at high repetition-rates. The setup includes detection with a 9 kHz CMOS camera and an improved dual-chopping scheme to reject scattering artifacts that occur in the 3-pulse configuration. The instrument demonstrates good signal-to-noise performance while simultaneously achieving a 3-6 fold reduction in pulse energy and a 5-10 fold reduction in acquisition time relative to comparable 1 kHz instruments. ©2018 Optical Society of America.NSF - Division of Chemistry and Division of Materials Research (Award no. 1452857