Thin-film solar cells can address the cost barrier of solar power by virtue of less material usage and more efficient manufacturing processes like roll-to-roll printing. The thin-film material Cu/Zn/Sn/Se (CZTS) is of particular interest because it is composed of earth-abundant elements; however, the current record for CZTS device efficiency is 12.6%, which is far from the 20% efficiency that is needed to be commercially competitive. Low CZTS device efficiencies have been linked to low open-circuit voltages (Voc), which are likely due to high charge-carrier recombination. Recombination can be increased by irregularities in crystal structure and composition known as defects. In this project, different microscopies will be used to study the defects present at the interfaces of a working CZTS photovoltaic device in relation to the device's V_oc. The interfaces of concern include those between different material layers and those between CZTS crystal grains, i.e. grain boundaries. A variety of scanning probe microscopies (SPM) will be used to map local electronic properties at these interfaces, relating position on a device cross-section to current and voltage characteristics. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) will be used to correlate these electronic properties with CZTS device structure and composition. In making these correlations between SEM, EDS, and SPM data we hope to better understand the origins of Voc loss in CZTS devices and, in turn, inform more effective CZTS cell fabrication. Thus far, we have acquired SPM, SEM, and EDS images that display the distinct layers of the device cross-section. Further optimization of sample preparation is required to achieve both resolution of grain boundaries and a sample surface that is smooth enough for SPM to obtain reliable electronic data.

Solar technology is a potential way to help meet the growing demand for clean, renewable energy. Hybrid composites of inorganic quantum dots, with organic semiconducting polymers, offer a potential means of producing low-cost, solution-processable photovoltaics. Photovoltaics can convert sunlight directly into electricity to capture solar energy. The synthesis of quantum dots typically involves the use of large surfactant molecules to facilitate particle growth and solubility. These native ligands act as electrical insulators that impede charge transport in photovoltaic devices. Therefore, it is necessary to exchange these large ligands with small molecules to achieve efficient charge carrier photogeneration and transport. In this research, we examine bulk heterojunction blends of low band gap PbS quantum dots with the conjugated polymer poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b')dithiophene-2,6-diyl)(2-((dodecyloxy)carbonyl)thieno(3,4-b)thiophenediyl)) (PTB1) treated with different ligands including halide ions and organic crosslinkers. By using photoinduced absorption and transient photovoltage techniques, we study the long-lived charge generation dynamics and recombination kinetics of our devices. We find that the ligand treatments exhibiting higher device performance correlate to longer carrier recombination lifetimes in both spectroscopic and photovoltaic device measurements.

Composites of conjugated polymers with semiconducting nanocrystals (NCs) are of interest for low-cost, solution- processable photovoltaics. However, compared to typical polymer/fullerene composites, relatively little is known about factors influencing charge generation. Without understanding these factors, design principles for improving solar cell applications remain elusive. Previous studies of blends of the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) with PbS nanocrystal quantum dots showed the excited NC undergoing charge transfer by hole transfer from the NCs to the polymer. To further investigate this hole transfer mechanism, quasi-steady-state PhotoInduced Absorption (PIA) was performed on blends of P3HT with PbS nanocrystal quantum dots as a function of excitation energy. As the energy exciting the PbS quantum dots was increased, the yield, per photon absorbed, of positive charge carriers on the organic host polymer increased. This occurred even at excitation wavelengths that only the NCs could absorb at. Our result provide direct evidence that holes generated on the NC retain some of the additional energy from excitation, e.g. hot holes. Further understanding of these results could provide even more insight to the mechanisms of organic/inorganic photovoltaics.

Physicochemical DNA research contributes to cutting edge medical applications such as single nucleotide polymorphism (SNP) detection and drug delivery. DNA can be modified with azobenzene moieties to form azobenzene DNA. I used UV light to induce a trans to cis isomerization reaction of the azobenzene moiety which causes double stranded azobenzene DNA (dsDNA) to split into a single stranded DNA (ssDNA). We study the effects of UV light on the azobenzene photoisomerization of perfectly matched and mismatched DNA. For the experiments, a glass substrate is functionalized with amino-silane molecules and single stranded azobenzene DNA. I hybridized the azobenzene DNA with either a perfectly matched or a mismatched strand to form double stranded DNA. I destabilize the DNA duplex by adding UV light to the hybridized DNA sample. Since the mismatched strand has a single base mismatch, it is less intrinsically stable than a perfectly matched strand due to less contributing intermolecular forces. Consequently, the melting temperature is lower for the mismatched strand. The mismatched DNA strand returns a less intense fluorescent signal in my detection. Experimental parameters such as hybridization time, UV exposure time and temperature applied to the experimental set-up are varied in order to study changes in ssDNA vs. dsDNA fluorescence. Consistent reproduction of these results is vital for a reliable DNA chip assay that can detect single nucleotide polymorphisms (SNP) in fragments of DNA.

Compound I is the intermediate responsible for substrate oxidation in the catalytic cycles of a variety of heme-containing metalloenzymes, such as peroxidases, oxygenases, and importantly, cytochrome P450s. Cytochrome P450s, through compound I, oxidize approximately 75% of foreign substances that enter the body, such as pharmaceuticals and chemicals from the environment, so there is an interest in the mechanism by which this reaction occurs. The goal of this project is to probe the potential reversibility of Compound I formation, a step traditionally thought to be irreversible. Due to its stability and availability, horseradish peroxidase (HRP) was used as the model enzyme. In the case of HRP, hydrogen peroxide is reduced to water at the heme cofactor, while oxidizing a variety of substrates. In the presence of an oxidant and the absence of a substrate to oxidize, hydrogen peroxide coordinates to the iron of the heme, the O-O bond is cleaved, which forms water and compound I. A key experiment reacts HRP with unlabeled H₂O₂ in ¹⁸O-labeled water, and determines whether mixed-labled H₂O₂ is formed. The presence of mixed-labeled H₂O₂ indicates that there is reversibility of the O-O bond cleavage that forms compound I. The existence of mixed-labeled H₂O₂ is ascertained by first removing HRP by filtration, leaving the H₂O₂ in the filtrate. This is followed by the addition of a water soluble phosphine. In this case, the phosphine cannot be oxidized by Compound I, but can readily react with free hydrogen peroxide in solution to give the corresponding phosphine oxide. Mass spectroscopy is used to determine whether the phosphine oxide has incorporated the ¹⁸O label. Results show that when the experiment is performed in the presence of ¹⁸OH₂, there was no observable increase in the intensity of a peak corresponding to ¹⁸O-phosphine oxide, so reversibility of Compound I formation was not confirmed.

Ion mobility mass spectrometry is a powerful technique for analyzing proteins and protein complexes in the gas phase. Ions are generated using nanoelectrospray ionization from aqueous, buffered solutions containing the protein or protein complex of interest. The resulting ions are native-like; their structures have low, narrow charge-state distributions and resemble those in the condensed-phase. There is an interest in probing different charge states of protein and protein complexes. High charge-state distributions of proteins and protein complexes, for example, can be seen after addition of ‘supercharging’ reagents, e.g., sulfolane or m-nitrobenzyl alcohol, to the electrospray buffer. Several mechanisms have been proposed to explain the mechanism of supercharging. Here, we used ion mobility mass spectrometry to probe the charge-state distributions and collision cross sections of a large set of native-like protein and protein complex ions, with masses ranging from 6–468 kDa, generated from solutions containing 1% m-nitrobenzyl alcohol. We determined the extent of supercharging by calculating the percent increase in highest charge state observed between supercharged and non-supercharged cations. We observed a large range of percent increases in highest charge state but in general, there is evidence that small proteins (<100 kDa) exhibited a greater increase than larger proteins in these experiments. Upon comparison of these results to those for ions generated from sulfolane-containing solutions, we found that the maximum extent of supercharging observed with m-nitrobenzyl alcohol was less than that observed when sulfolane was used. The most supercharged cations generated with m-nitrobenzyl alcohol had up to a 96% increase in collision cross section compared to ions generated without supercharging agents, but the differences in collision cross sections with and without supercharging were less than 2% for large protein complexes (> 100 kDa). These results suggest that ions supercharged using m-nitrobenzyl alcohol may retain their structures better than those supercharged using sulfolane.

We investigate which structural features of sterols most strongly influence miscibility of lipids in membranes. Lipid membranes serve as simple models of cell membranes. Within cell membranes, discrete domains in which lipids, sterols, and proteins are non-uniformly distributed are thought to be essential to cellular function. Sterols including cholesterol in mammals and ergosterol in yeast are often discussed as being evolutionarily optimized to interact with membranes. Here we examine the miscibility phase behavior of membranes that contain either a plant sterol or a yeast sterol with the eventual goal of comparing the interactions between lipids and plant sterols, or lipids and yeast sterols, with the interactions between cholesterol, and lipids. To do so, we map miscibility phase diagrams of vesicle membranes containing a sterol (stigmasterol, beta-sitosterol, or ergosterol), a lipid with a low melting temperature (DOPC), a lipid with a high melting temperature (DPPC), and a small amount of a fluorescently labeled lipid that preferentially partitions into one of the two membrane phases. We compare our resulting phase diagram to a well-studied system, specifically membranes composed of DOPC/DPPC/cholesterol. The method that we use to generate giant unilamellar vesicles is electroformation, and we observe phase separation via fluorescence microscopy. Our results suggest that minor structural changes in the sterol have large effects on membrane miscibility.

We describe an exo-selective cyclization of enantioenriched allenols to form cyclic ethers containing an α-tetrasubstituted stereocenter. Cyclic ethers bearing α-tetrasubstituted stereocenter are present in acetogenins, macrodiolides, ionophores, and lignans. These natural products have been found to have anti-fugal, anti-tumor, and/or antibiotic properties. However, an asymmetric catalytic synthesis of α-tertrasubstituted cyclic ethers remains a major synthetic challenge. Our method works with as little as 0.02 mol % of a gold catalyst for the exo-selective cyclization of enantioenriched allenols, and proceeds with excellent chirality transfer, and high functional group compatibility.