Matching Items (29)

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Application and study of water oxidation catalysts and molecular dyes for solar-fuel production

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Developing a system capable of using solar energy to drive the conversion of an abundant and available precursor to fuel would profoundly impact humanity's energy use and thereby the condition

Developing a system capable of using solar energy to drive the conversion of an abundant and available precursor to fuel would profoundly impact humanity's energy use and thereby the condition of the global ecosystem. Such is the goal of artificial photosynthesis: to convert water to hydrogen using solar radiation as the sole energy input and ideally do so with the use of low cost, abundant materials. Constructing photoelectrochemical cells incorporating photoanodes structurally reminiscent of those used in dye sensitized photovoltaic solar cells presents one approach to establishing an artificial photosynthetic system. The work presented herein describes the production, integration, and study of water oxidation catalysts, molecular dyes, and metal oxide based photoelectrodes carried out in the pursuit of developing solar water splitting systems.

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Date Created
  • 2013

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Insulator based dielectrophoretic trapping of single mammalian cells

Description

This work demonstrated a novel microfluidic device based on direct current (DC) insulator based dielectrophoresis (iDEP) for trapping individual mammalian cells in a microfluidic device. The novel device is also

This work demonstrated a novel microfluidic device based on direct current (DC) insulator based dielectrophoresis (iDEP) for trapping individual mammalian cells in a microfluidic device. The novel device is also applicable for selective trapping of weakly metastatic mammalian breast cancer cells (MCF-7) from mixtures with mammalian Peripheral Blood Mononuclear Cells (PBMC) and highly metastatic mammalian breast cancer cells, MDA-MB-231. The advantage of this approach is the ease of integration of iDEP structures in microfliudic channels using soft lithography, the use of DC electric fields, the addressability of the single cell traps for downstream analysis and the straightforward multiplexing for single cell trapping. These microfluidic devices are targeted for capturing of single cells based on their DEP behavior. The numerical simulations point out the trapping regions in which single cell DEP trapping occurs. This work also demonstrates the cell conductivity values of different cell types, calculated using the single-shell model. Low conductivity buffers are used for trapping experiments. These low conductivity buffers help reduce the Joule heating. Viability of the cells in the buffer system was studied in detail with a population size of approximately 100 cells for each study. The work also demonstrates the development of the parallelized single cell trap device with optimized traps. This device is also capable of being coupled detection of target protein using MALDI-MS.

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Date Created
  • 2013

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An in situ Surface Stress Study of Electrochemical Phenomena: Electrodeposition and Molecular Adsorption

Description

Over the last decade copper electrodeposition has become the dominant process by which microelectronic interconnects are made. Replacing ultra-high vacuum evaporative film growth, the technology known as the Cu damascene

Over the last decade copper electrodeposition has become the dominant process by which microelectronic interconnects are made. Replacing ultra-high vacuum evaporative film growth, the technology known as the Cu damascene process has been widely implemented in the microelectronics industry since the early 2000s. The transition from vacuum film growth to electrodeposition was enabled by solution chemistries that provide "bottom-up" or superfilling capability of vias and trenches. While the process has been and is used widely, the actual mechanisms responsible for superfilling remain relatively unknown. This dissertation presents and discusses the background and results of experimental investigations that have been done using in situ electrochemical surface stress monitoring techniques to study the evolution of stress on Cu{111} thin film electrodes. Because of its extreme sensitivity to the structure on both the electrode and solution sides of the interface, surface stress monitoring as analytical technique is well suited for the study of electrodeposition. These ultra-high resolution stress measurements reveal the dynamic response of copper electrodes to a number of electrochemical and chemical experimental variables. In the case of constant current pulsed deposition and stripping, the surface stress evolution depends not only on the magnitude of the current pulse, but also shows a marked response to plating bath composition. The plating bath chemistries used in this work include (1) additive free, (2) deposition suppressing solutions that include polyethylene glycol (PEG) and sodium chloride (NaCl) as well as (3) full additive solution combinations which contain PEG, NaCl, and a one of two deposition accelerating species (bis-(sodiumsulfopropyl)disulfide (SPS) or mercaptopropane sulfonic acid (MPS)). The development of thin film stress is further investigated through a series of solution exchange experiments that correlate the magnitude of electrode exchange current density and the stress state of the film. Remarkably, stress changes as large as ~8.5 N/m are observed during solution exchanges at the open circuit potential. Overall, this research demonstrates that solution chemistry can have a large impact on thin film stress evolution, even for very small deposition thicknesses (e.g. <10 ML) or in the absence of net addition or removal of material from the electrode.

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Date Created
  • 2011

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Charge transport in single molecules

Description

Studying charge transport through single molecules is of great importance for unravelling charge transport mechanisms, investigating fundamentals of chemistry, and developing functional building blocks in molecular electronics.

First, a study of

Studying charge transport through single molecules is of great importance for unravelling charge transport mechanisms, investigating fundamentals of chemistry, and developing functional building blocks in molecular electronics.

First, a study of the thermoelectric effect in single DNA molecules is reported. By varying the molecular length and sequence, the charge transport in DNA was tuned to either a hopping- or tunneling-dominated regimes. In the hopping regime, the thermoelectric effect is small and insensitive to the molecular length. Meanwhile, in the tunneling regime, the thermoelectric effect is large and sensitive to the length. These findings indicate that by varying its sequence and length, the thermoelectric effect in DNA can be controlled. The experimental results are then described in terms of hopping and tunneling charge transport models.

Then, I showed that the electron transfer reaction of a single ferrocene molecule can be controlled with a mechanical force. I monitor the redox state of the molecule from its characteristic conductance, detect the switching events of the molecule from reduced to oxidized states with the force, and determine a negative shift of ~34 mV in the redox potential under force. The theoretical modeling is in good agreement with the observations, and reveals the role of the coupling between the electronic states and structure of the molecule.

Finally, conclusions and perspectives were discussed to point out the implications of the above works and future studies that can be performed based on the findings.

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  • 2017

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Foundational investigation of electrophoretic exclusion

Description

Electrophoretic exclusion is a counter-flow gradient focusing method that simultaneously separates and concentrates electrokinetic material at a channel entrance utilizing electric and fluid velocity fields. However, its effectiveness is heavily

Electrophoretic exclusion is a counter-flow gradient focusing method that simultaneously separates and concentrates electrokinetic material at a channel entrance utilizing electric and fluid velocity fields. However, its effectiveness is heavily dependent on the non-uniform field gradients about the entrance. This work assesses the capability of electrophoretic exclusion to capture and enrich small molecules and examines the channel entrance region both quantitatively and qualitatively to better understand the separation dynamics for future design.

A flow injection technique is used to experimentally evaluate electrophoretic exclusion of small molecules. Methyl violet, a cationic dye, and visible spectroscopy are used to monitor flow and electrophoretic dynamics at the entrance region resulting in successful capture and simultaneous enrichment of methyl violet at the channel interface. Investigation of the entrance region is performed using both experiment data and finite element analysis modeling to assess regional flow, electric fields, diffusion, convection, and electrophoretic migration. Longitudinal fluid velocity and electric field gradient magnitudes near the channel entrance are quantified using Particle Tracking Velocimetry (PTV) and charged fluorescent microspheres. Lateral studies using rhodamine 123 concentration monitoring agree qualitatively with simulation results indicating decreased gradient uniformity for both electric and fluid velocity fields closer to the channel wall resulting in a localized concentration enhancement at lower applied voltages than previously observed or predicted. Resolution interrogation from both a theoretical assessment and simulation construct demonstrate resolution improvement with decreased channel width and placement of an electrode directly at the interface. Simulation resolution predictions are in general agreement with early experimental assessments, both suggesting species with electrophoretic mobilities as similar as 10-9 m2/(Vs) can be separated with the current design. These studies have helped evolve the understanding of the interface region and set the foundation for further interface developments.

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Date Created
  • 2015

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Mixed Polyanion and Clathrate Materials as Novel Materials for Lithium-ion and Sodium-ion Batteries

Description

This work describes the investigation of novel cathode and anode materials. Specifically, several mixed polyanion compounds were evaluated as cathodes for Li and Na-ion batteries. Clathrate compounds composed of silicon

This work describes the investigation of novel cathode and anode materials. Specifically, several mixed polyanion compounds were evaluated as cathodes for Li and Na-ion batteries. Clathrate compounds composed of silicon or germanium arranged in cage-like structures were studied as anodes for Li-ion batteries.

Nanostructured Cu4(OH)6SO4 (brochantite) platelets were synthesized using polymer-assisted titration and microwave-assisted hydrothermal methods. These nanostructures exhibited a capacity of 474 mAh/g corresponding to the full utilization of the copper redox in an conversion reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies were preformed to understand the mechanism and structural changes.

A microwave hydrothermal synthesis was developed to prepare a series compounds based on jarosite, AM3(SO4)2(OH)6 (A = K, Na; M = Fe, V). Both the morphology and electrochemical properties showed a compositional dependence. At potentials >1.5 V vs. Li/Li+, an insertion-type reaction was observed in Na,Fe-jarosite but not in K,Fe-jarosite. Reversible insertion-type reactions were observed in both vanadium jarosites between 1 – 4 V with capacities around 40 - 60 mAh/g. Below 1 V vs. Li/Li+, all four jarosite compounds underwent conversion reactions with capacities ~500 mAh/g for the Fe-jarosites.

The electrochemical properties of hydrogen titanium phosphate sulfate, H0.4Ti2(PO4)2.4(SO4)0.6 (HTPS), a new mixed polyanion material with NASICON structure was reported. A capacity of 148 mAh/g corresponding to2 Li+ insertion per formula unit was observed. XRD and XPS were used to characterize the HTPS before and after cycling and to identify the lithium sites. Evaluation of the HTPS in Na-ion cell was also performed, and a discharge capacity of 93 mAh/g was observed.

A systematic investigation of the role of the processing steps, such as ball-milling and acid/base etching, on the electrochemical properties of a silicon clathrate compound with nominal composition of Ba8Al16Si30 was performed. According to the transmission electron microscope (TEM), XPS, and electrochemical analysis, very few Li atoms can be electrochemically inserted, but the introduction of disorder through ball-milling resulted in higher capacity, while the oxidation layer made by the acid/base treatment prevented the reation. The electrochemical property of germanium clathrate was also investigated, unlike the silicon clathrate, the germanium one underwent a conversion reaction.

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  • 2017

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Microfluidic tools for protein crystallography

Description

X-ray crystallography is the most widely used method to determine the structure of proteins, providing an understanding of their functions in all aspects of life to advance applications in fields

X-ray crystallography is the most widely used method to determine the structure of proteins, providing an understanding of their functions in all aspects of life to advance applications in fields such as drug development and renewable energy. New techniques, namely serial femtosecond crystallography (SFX), have unlocked the ability to unravel the structures of complex proteins with vital biological functions. A key step and major bottleneck of structure determination is protein crystallization, which is very arduous due to the complexity of proteins and their natural environments. Furthermore, crystal characteristics govern data quality, thus need to be optimized to attain the most accurate reconstruction of the protein structure. Crystal size is one such characteristic in which narrowed distributions with a small modal size can significantly reduce the amount of protein needed for SFX. A novel microfluidic sorting platform was developed to isolate viable ~200 nm – ~600 nm photosystem I (PSI) membrane protein crystals from ~200 nm – ~20 μm crystal samples using dielectrophoresis, as confirmed by fluorescence microscopy, second-order nonlinear imaging of chiral crystals (SONICC), and dynamic light scattering. The platform was scaled-up to rapidly provide 100s of microliters of sorted crystals necessary for SFX, in which similar crystal size distributions were attained. Transmission electron microscopy was used to view the PSI crystal lattice, which remained well-ordered postsorting, and SFX diffraction data was obtained, confirming a high-quality, viable crystal sample. Simulations indicated sorted samples provided accurate, complete SFX datasets with 3500-fold less protein than unsorted samples. Microfluidic devices were also developed for versatile, rapid protein crystallization screening using nanovolumes of sample. Concentration gradients of protein and precipitant were generated to crystallize PSI, phycocyanin, and lysozyme using modified counterdiffusion. Additionally, a passive mixer was created to generate unique solution concentrations within isolated nanowells to crystallize phycocyanin and lysozyme. Crystal imaging with brightfield microscopy, UV fluorescence, and SONICC coupled with numerical modeling allowed quantification of crystal growth conditions for efficient phase diagram development. The developed microfluidic tools demonstrated the capability of improving samples for protein crystallography, offering a foundation for continued development of platforms to aid protein structure determination.

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Date Created
  • 2016

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Interfacial and electrode modifications in P₃HT:PC₆₁BM based organic solar cells: devices, processing and characterization

Description

The inexorable upsurge in world’s energy demand has steered the search for newer renewable energy sources and photovoltaics seemed to be one of the best alternatives for energy production. Among

The inexorable upsurge in world’s energy demand has steered the search for newer renewable energy sources and photovoltaics seemed to be one of the best alternatives for energy production. Among the various photovoltaic technologies that emerged, organic/polymer photovoltaics based on solution processed bulk-heterojunctions (BHJ) of semiconducting polymers has gained serious attention owing to the use of inexpensive light-weight materials, exhibiting high mechanical flexibility and compatibility with low temperature roll-to-roll manufacturing techniques on flexible substrates. The most widely studied material to date is the blend of regioregular P3HT and PC61BM used as donor and acceptor materials. The object of this study was to investigate and improve the performance/stability of the organic solar cells by use of inexpensive materials. In an attempt to enhance the efficiency of organic solar cells, we have demonstrated the use of hexamethyldisilazane (HMDS) modified indium tin oxide (ITO) electrode in bulk heterojunction solar cell structure The device studies showed a significant enhancement in the short-circuit current as well as in the shunt resistance on use of the hexamethyldisilazane (HMDS) layer. In another approach a p-type CuI hole-transport layer was utilized that could possibly replace the acidic PEDOT:PSS layer in the fabrication of high-efficiency solar cells. The device optimization was done by varying the concentration of CuI in the precursor solution which played an important role in the efficiency of the solar cell devices. Recently a substantial amount of research has been focused on identifying suitable interfacial layers in organic solar cells which has efficient charge transport properties. It was illustrated that a thin layer of silver oxide interfacial layer showed a 28% increase in power conversion efficiency in comparison to that of the control cell. The optoelectronic properties and morphological features of indium-free ZnO/Ag/MoOx electrodes was also studied. Organic solar cells on these composite electrodes revealed good optical and electrical properties, making them a promising alternative indium free and PEDOT:PSS-free organic solar cells. Lastly, inverted solar cells utilizing zinc oxide and yttrium doped zinc oxide electron transport was also created and their device properties revealed that optimum annealing conditions and yttrium doping was essential to obtain high efficiency solar cells.

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Date Created
  • 2015

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Photophysics of bio-inspired solar energy conversion

Description

Increased global demand for energy has led to prolific use of fossil fuels, which produce and release greenhouse gases, such as carbon dioxide. This increase in atmospheric carbon dioxide affects

Increased global demand for energy has led to prolific use of fossil fuels, which produce and release greenhouse gases, such as carbon dioxide. This increase in atmospheric carbon dioxide affects the global weather system and has been cited as a cause for global warming. For humans to continue to meet demands for energy while reducing greenhouse emission, a sustainable, carbon-neutral energy source must be developed. The sun provides energy for the majority of life on earth, as well as the energy stored in the chemical bonds of fossil fuels. This dissertation investigates systems inspired by the biological mechanism of solar energy capture and storage. In natural photosynthesis, organisms use chlorophyll as a chromophore to absorb the sun's energy. Bio-inspired systems use close analogues like porphyrins and phthalocyanines. In this dissertation, a soluble, semiconducting porphyrin is reported. The polymer was synthesized via a Buchwald-Hartwig style coupling of porphyrin monomers which produced a polyaniline-like chain with porphyrins incorporated into the backbone. Spectroscopic and electrochemical studies were performed, which show evidence of excited state charge transfer and a first oxidation state of 0.58 V (vs SCE). These properties suggest that the polymer could be involved in excited state electron donation to fullerenes and other electron acceptors, which could be beneficial in organic photovoltaics, sensors, and other applications. Molecular dyads and triads capable of charge separation have been studied for decades, and the spectroscopic properties of two novel systems are reported in this dissertation. A peripherally-connected zinc-phthalocyanine-C60 dyad was studied, and showed excited state electron transfer from the phthalocyanine excited state to the C60, with a long-lived charge separated state. An axially-linked carotene-Si-pthalocyanine-C60 triad was studied, showing excited state electron transfer from the phthalocyanine to the C60, but fast recombination before hole transfer can occur to the carotene. Analogues of the electron transport mechanisms used in many biological systems use iron-sulfur clusters to shuttle electrons from donors to acceptors. In this dissertation, the spectroscopic properties of a de novo protein were studied. Nanosecond transient absorption was used to characterize the electron and energy transfer of an excited water-soluble porphyrin to the oxidized [FeS] clusters incorporated in the de novo protein. The triplet state of the porphyrin was strongly quenched with the holo-protein without a rise in porphyrin plus signal, suggesting that only Dexter-type energy transfer occurs between the sensitized porphyrin and the [FeS] clusters.

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Date Created
  • 2014

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Synthesis and characterization of an ionomer for zinc-air battery cathodes

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The work presented in this thesis covers the synthesis and characterization of an ionomer that is applicable to zinc-air batteries. Polysulfone polymer is first chloromethylated and then quaternized to create

The work presented in this thesis covers the synthesis and characterization of an ionomer that is applicable to zinc-air batteries. Polysulfone polymer is first chloromethylated and then quaternized to create an ion-conducting polymer. Nuclear magnetic resonance (NMR) spectra indicates that the degree of chloromethylation was 114%. The chemical and physical properties that were investigated include: the ionic conductivity, ion exchange capacity, water retention capacity, diameter and thickness swelling ratios, porosity, glass transition temperature, ionic conductivity enhanced by free salt addition, and the concentration and diffusivity of oxygen within the ionomer. It was found that the fully hydrated hydroxide form of the ionomer had a room temperature ionic conductivity of 39.92mS/cm while the chloride form had a room temperature ionic conductivity of 11.80mS/cm. The ion exchange capacity of the ionomer was found to be 1.022mmol/g. The water retention capacity (WRC) of the hydroxide form was found to be 172.6% while the chloride form had a WRC of 67.9%. The hydroxide form of the ionomer had a diameter swelling ratio of 34% and a thickness swelling ratio of 55%. The chloride form had a diameter swelling ratio of 32% and a thickness swelling ratio of 28%. The largest pore size in the ionomer was found to be 32.6nm in diameter. The glass transition temperature of the ionomer is speculated to be 344°C. A definite measurement could not be made. The room temperature ionic conductivity at 50% relative humidity was improved to 12.90mS/cm with the addition of 80% free salt. The concentration and diffusivity of oxygen in the ionomer was found to be 1.3 ±0.2mMol and (0.49 ±0.15)x10-5 cm2/s respectively. The ionomer synthesized in this research had material properties and performance that is comparable to other ionomers reported in the literature. This is an indication that this ionomer is suitable for further study and integration into a zinc-air battery. This thesis is concluded with suggestions for future research that is focused on improving the performance of the ionomer as well as improving the methodology.

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Date Created
  • 2012