Biologically inspired materials for electro-responsive coatings and the photo-oxidation of water

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 193-202).Evolving out of research on biomineralization, a new field devoted to studying the interactions between inorganic materials and proteins is emerging. In natural systems, proteins are responsible for the assembly of complex hierarchical structures such as the nacre of abalone. Tools such as phage and yeast display libraries have enabled the combinatorial screening of peptides against a multitude of materials to which natural systems typically have no exposure. These techniques have yielded peptides that can bind and assemble technologically relevant materials such as gold and CdS. In this work, combinatorial phage and yeast display libraries are used to identify peptide sequences that bind to electrode materials and metal oxides. As in nature, it is observed that the context of a particular peptide dramatically influences its properties. While a peptide sequence may exhibit good adhesion to a particular surface when displayed on yeast, the same peptide may have little affinity towards that same surface when displayed on bacteriophage. To probe the interactions between peptides and materials in a context-free environment, rationally designed synthetic peptides were screened against a number of inorganic materials. A synthetic peptide, covalently linked to either microspheres, quantum dots, or a polymer, was able to mediate adhesion of those entities to electrode surfaces. In nature, proteins play important roles beyond biomineralization. For example, membrane proteins contain voltage-gated ion channels that open and close in response to a voltage bias. Inspired by the electro-responsive activity of ion channels, the interactions between peptides, surfaces and electric fields was investigated. The peptide sequences that exhibited significant adhesion to metal oxides were dominated by positively charged residues. A high voltage, pulsed electric field was used to overcome the inherent negative charge of the metal oxide electrode surface, thereby controlling peptide adhesion to an electrode surface. Drawing further inspiration from the way nature employs peptides, a synthetic photocatalytic system for water oxidation was developed using photosystem II (PSII) as a model. Proteins form the structural scaffold for PSII, assembling dye molecules as well as the metal-oxo catalytic center; furthermore, peptides play an active role in shuttling charge throughout PSIL. The D1 peptide in PSII is an electro-responsive peptide of sorts, releasing plastiquinone upon the two electron reduction of the molecule. The system developed in this work uses: iridium oxide as a metal-oxo catalyst assembled by a peptide expressed on the M13 bacteriophage; metalloporphyrin photosensitizers that are covalently assembled on the protein framework of the bacteriophage; and a synthetic Ce(IV) dipicolinate electron accepting molecule. The electron accepting molecule, developed to fill the role of plastiquinone in PSII, is believed to be the first non-sacrificial electron acceptor capable of driving the metalloporphyrin-sensitized photocatalytic oxidation of water.by Andrew P. Magyar.Ph.D

Fabrication of Thin, Luminescent, Single-crystal Diamond Membranes

The formation of single-crystal diamond membranes is an important prerequisite for the fabrication of high-quality optical cavities in this material. Diamond membranes fabricated using lift-off processes involving the creation of a damaged layer through ion implantation often suffer from residual ion damage, which severely limits their usefulness for photonic structures. The current work demonstrates that strategic etch removal of the most highly defective material yields thin, single-crystal diamond membranes with strong photoluminescence and a Raman signature approaching that of single-crystal bulk diamond. These optically-active membranes can form the starting point for fabrication of high-quality optical resonators.Comment: To appear in AP

Coupling of silicon-vacancy centers to a single crystal diamond cavity

Optical coupling of an ensemble of silicon-vacancy (SiV) centers to single-crystal diamond microdisk cavities is demonstrated. The cavities are fabricated from a single-crystal diamond membrane generated by ion implantation and, electrochemical liftoff followed by homo-epitaxial overgrowth. Whispering gallery modes which spectrally overlap with the zero-phonon line (ZPL) of the SiV centers and exhibit quality factors ~2200 are measured. Lifetime reduction from 1.8 ns to 1.48 ns is observed from SiV centers in the cavity compared to those in the membrane outside the cavity. These results are pivotal in developing diamond integrated photonics networks

Deterministic coupling of delta-doped NV centers to a nanobeam photonic crystal cavity

The negatively-charged nitrogen vacancy center (NV) in diamond has generated significant interest as a platform for quantum information processing and sensing in the solid state. For most applications, high quality optical cavities are required to enhance the NV zero-phonon line (ZPL) emission. An outstanding challenge in maximizing the degree of NV-cavity coupling is the deterministic placement of NVs within the cavity. Here, we report photonic crystal nanobeam cavities coupled to NVs incorporated by a delta-doping technique that allows nanometer-scale vertical positioning of the emitters. We demonstrate cavities with Q up to ~24,000 and mode volume V ~ $0.47({\lambda}/n)^{3}$ as well as resonant enhancement of the ZPL of an NV ensemble with Purcell factor of ~20. Our fabrication technique provides a first step towards deterministic NV-cavity coupling using spatial control of the emitters.Comment: 13 pages, 3 figure

Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism