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Solid and liquid states

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<p> Quantum dots (QDs) are the foundation of many optoelectronic devices because their optical and electronic properties are synthetically tunable. The inherent connection between synthetically controllable physical parameters, such as size, shape, and surface chemistry, and QD electronic properties provides flexibility in manipulating excited states. The properties of the ligands that passivate the QD surface and provide such synthetic control, however, are quite different from those that are beneficial for use in optoelectronic devices. In these applications, ligands that promote charge transfer are desired. To this end, significant research efforts have focused on post-synthetic ligand exchange to shorter, more conductive ligand species. Surface ligand identity, however, is a physical parameter intimately tied to QD excited state behavior in addition to charge transfer. A particularly interesting group of ligands, due to the extraordinarily thin ligand shell they create around the QD, are the chalcogenides S2-, Se2-, and Te2-. While promising, little is known about how these chalcogenide ligands affect QD photoexcited states. This dissertation focuses on the impact of chalcogenide ligands on the excited state dynamics of cadmium chalcogenide QDs and associated implications for charge transfer. This is accomplished through a combination of theoretical (Chapters 2, 3, and 6) and experimental (Chapters 2, 4, 5 and 6) methods. We establish a theoretical foundation for describing chalcogenide capped QD photoexcited states and measure the dynamics of these excited states using transient absorption spectroscopy. The presented results highlight the drastic effects surface modification can have on QD photoexcited state dynamics and provide insights for more informed design of optoelectronic systems.</p>

Advances in spectroscopic measurement techniques enabling highly accurate measurements of trace gases in the atmosphere are critical for furthering our understanding of the chemical processes that impact both climate and human health. This dissertation presents the development and deployment of laser-based instruments for measuring parts per trillion (pptv) concentrations of iodine monoxide and glyoxal. Iodine, which is primarily released from oceanic sources, is highly reactive in the atmosphere. Despite its trace concentrations, iodine plays a potentially important role in ozone destruction, the catalysis of mercury deposition, and the formation of marine clouds. An in situ instrument to detect iodine monoxide (IO) using laser-induced fluorescence was developed and then validated during a deployment to the Shoals Marine Laboratory (Appledore Island, ME) in August and September 2011. Mixing ratios up to 10 pptv of IO were observed with a strong tidal dependence. The instrumental detection limit \((3\sigma)\) of 0.36 pptv in 1 minute is indicative of unprecedented sensitivity. Glyoxal, the smallest alpha-dicarbonyl, serves as an atmospheric tracer of both the oxidation of biogenic volatile organic compounds in forest environments as well as secondary organic aerosol. Modeling studies indicate that production of glyoxal on a global scale is driven primarily by biogenic emissions, specifically emissions of isoprene. However, measurements of glyoxal in environments where isoprene dominates its production are limited. An instrument to detect glyoxal in situ by laser-induced phosphorescence was developed. The 3σ limit of detection of this instrument was 3.9 pptv in 1 minute. During July and August 2009, gas-phase measurements of glyoxal were made during the Community Atmosphere-Biosphere Interactions Experiment at the PROPHET tower in an isoprene-dominated forest site in northern Michigan. Additional measurements made throughout the campaign have been used to constrain a box model using the Master Chemical Mechanism. The model over-predicts glyoxal relative to the observed mixing ratios. Theoretically predicted reaction pathways implemented in many isoprene oxidation schemes exacerbate this disagreement. / Chemistry and Chemical Biology

Much progress has been made in the study of chemical kinetics on the basis that many reaction mechanisms involve the participation of 'free radicals'. In such mechanisms the overall reaction is assumed to consist of a recurring series, or 'chain', of elementary reactions which can be initiated and propagated by stable but very reactive fragments of molecules, or radicals.

Note: / It has long been assumed that when a metal is placed into a solution of its ions an equilibrium is rapidly established in which a reversible interchange of atoms and ions take place. If one of the components of the system contains a radioactive isotope of the metal, it is possible to follow this exchange by observing the distribution of the active isotope among the atoms and ions persent.

Whenever an evacuated solid is exposed to a gas some of the gas is taken up by the solid. If the gas molecules adhere to the surface of the solid they are generally considered to be adsorbed. If the gas molecules enter the solid the term absorption is generally applied, although the less popular term, occlusion has occasionally been used. [...]

Hydrogen, more than any other gaseous substance,has been investigated in electric discharges and a greatdeal of evidence has been accumulated for its existence inthe atomic state.

Active nitrogen, as it is now commonly known, was first discovered by Lewis (1) in 1900. He used relatively pure nitrogen, through which a condensed discharge was passed. He noted that the discharge tube was filled with a "beautiful chamois-yellow mist," which persisted for several seconds after the current was interrupted. The spectrum of this afterglow was banded and could be identified with part of the nitrogen spectrum. The use of sodium to remove the last traces of oxygen and water eliminated the afterglow. [...]