Synthesis and electron emission properties of aligned carbon nanotube arrays

Carbon nanotubes (CNTs) have become one of the most interesting allotropes of carbon due to their intriguing mechanical, electrical, thermal and optical properties. The synthesis and electron emission properties of CNT arrays have been investigated in this work. Vertically aligned CNTs of different densities were synthesized on copper substrate with catalyst dots patterned by nanosphere lithography. The CNTs synthesized with catalyst dots patterned by spheres of 500 nm diameter exhibited the best electron emission properties with the lowest turn-on/threshold electric fields and the highest field enhancement factor. Furthermore, CNTs were treated with NH3 plasma for various durations and the optimum enhancement was obtained for a plasma treatment of 1.0 min. CNT point emitters were also synthesized on a flat-tip or a sharp-tip to understand the effect of emitter geometry on the electron emission. The experimental results show that electron emission can be enhanced by decreasing the screening effect of the electric field by neighboring CNTs. In another part of the dissertation, vertically aligned CNTs were synthesized on stainless steel (SS) substrates with and without chemical etching or catalyst deposition. The density and length of CNTs were determined by synthesis time. For a prolonged growth time, the catalyst activity terminated and the plasma started etching CNTs destructively. CNTs with uniform diameter and length were synthesized on SS substrates subjected to chemical etching for a period of 40 minutes before the growth. The direct contact of CNTs with stainless steel allowed for the better field emission performance of CNTs synthesized on pristine SS as compared to the CNTs synthesized on Ni/Cr coated SS. Finally, fabrication of large arrays of free-standing vertically aligned CNT/SnO2 core-shell structures was explored by using a simple wet-chemical route. The structure of the SnO2 nanoparticles was studied by X-ray diffraction and electron microscopy. Transmission electron microscopy reveals that a uniform layer of SnO2 is conformally coated on every tapered CNT. The strong adhesion of CNTs with SS guaranteed the formation of the core-shell structures of CNTs with SnO2 or other metal oxides, which are expected to have applications in chemical sensors and lithium ion batteries.

Synthetic studies of azulenyl and pseudoazulenyl nitrones

Free radicals have been implicated in various pathological conditions such as, stroke, aging and ischemic heart disease (IHD), as well as neurodegenerative diseases like Alzheimer’s, Parkinson’s, and Huntington’s disease. The role of antioxidants in protection from the harmful effects of free radicals has long been recognized. Trapping extremely reactive free radicals and eliminating them from circulation has been shown to be effective in animal models. Nitrone-based free radical traps have been extensively explored in biological systems. Examples include nitrones such as PBN, NXY-059, MDL-101,002, DMPO and EMPO. However, these nitrones have extremely high oxidation potentials as compared to natural antioxidants such as Vitamin E (α-tocopherol), and glutathione. Becker et al. (1995) synthesized novel azulenyl nitrones, which were shown to have oxidation potentials much lower than that of any of the previously reported nitrone based spin traps. Another azulenyl nitrone derivative, stilbazulenyl nitrone (STAZN), was shown to have an even lower oxidation potential within the range of natural antioxidants. STAZN, a second generation free radical trap, was found to be markedly superior than the two most studied nitrones, PBN and NXY-059, in animal models of cerebral ischemia and in an in vitro assay of lipid peroxidation. In this study, a third generation azulenyl nitrone was synthesized with an electron donating group on the previously synthesized STAZN derivative with the aim to lower the oxidation potential even more. Pseudoazulenes, because of the presence of an annular heteroatom, have been reported to possess even lower oxidation potential than that of the azulenyl counterpart. Therefore, pseudoazulenyl nitrones were synthesized for the first time by extracting and elaborating valtrate from the roots of Centranthus ruber (Red valerian or Jupiter’s beard). Several pseudoazulenyl nitrones were synthesized by using a facile experimental protocol. The physical and biological properties of these pseudoazulenyl nitrones can be easily modified by simply changing the substituent on the heteroatom. Cyclic voltammetry experiments have shown that these pseudoazulenyl nitrones do indeed have low oxidation potentials. The oxidation potential of these nitrones was lowered even more by preparing derivatives bearing an electron donating group at the 3-position of the five membered ring of the pseudoazulenyl nitrone.

Tailoring heme -thiolate proteins into efficient biocatalysts with high specificity and selectivity

Cytochrome P450 monooxygenases, one of the most important classes of heme-thiolate proteins, have attracted considerable interest in the biochemical community because of its catalytic versatility, substrate diversity and great number in the superfamily. Although P450s are capable of catalyzing numerous difficult oxidation reactions, the relatively low stability, low turnover rates and the need of electron-donating cofactors have limited their practical biotechnological and pharmaceutical applications as isolated enzymes. The goal of this study is to tailor such heme-thiolate proteins into efficient biocatalysts with high specificity and selectivity by protein engineering and to better understand the structure-function relationship in cytochromes P450. In the effort to engineer P450cam, the prototype member of the P450 superfamily, into an efficient peroxygenase that utilizes hydrogen peroxide via the “peroxide-shunt” pathway, site-directed mutagenesis has been used to elucidate the critical roles of hydrophobic residues in the active site. Various biophysical, biochemical and spectroscopic techniques have been utilized to investigate the wild-type and mutant proteins. Three important P450cam variants were obtained showing distinct structural and functional features. In P450camV247H mutant, which exhibited almost identical spectral properties with the wild-type, it is demonstrated that a single amino acid switch turned the monooxygenase into an efficient preoxidase by increasing the peroxidase activity nearly one thousand folds. In order to tune the distal pocket of P450cam with polar residues, Leu 246 was replaced with a basic residue, lysine, resulting in a mutant with spectral features identical to P420, the inactive species of P450. But this inactive-species-like mutant showed catalytic activities without the facilitation of any cofactors. By substituting Gly 248 with a histidine, a novel Cys-Fe-His ligation set was obtained in P450cam which represented the very rare case of His ligation in heme-thiolate proteins. In addition to serving as a convenient model for hemoprotein structural studies, the G248H mutant also provided evidence about the nature of the axial ligand in cytochrome P420 and other engineered hemoproteins with thiolate ligations. Furthermore, attempts have been made to replace the proximal ligand in sperm whale myoglobin to construct a heme-thiolate protein model by mimicking the protein environment of cytochrome P450cam and chloroperoxidase.

Tailoring Heme-Thiolate Proteins into Efficient Biocatalysts with High Specificity and Selectivity

Cytochrome P450 monooxygenases, one of the most important classes of heme-thiolate proteins, have attracted considerable interest in the biochemical community because of its catalytic versatility, substrate diversity and great number in the superfamily. Although P450s are capable of catalyzing numerous difficult oxidation reactions, the relatively low stability, low turnover rates and the need of electron-donating cofactors have limited their practical biotechnological and pharmaceutical applications as isolated enzymes. The goal of this study is to tailor such heme-thiolate proteins into efficient biocatalysts with high specificity and selectivity by protein engineering and to better understand the structure-function relationship in cytochromes P450. In the effort to engineer P450cam, the prototype member of the P450 superfamily, into an efficient peroxygenase that utilizes hydrogen peroxide via the “peroxide-shunt” pathway, site-directed mutagenesis has been used to elucidate the critical roles of hydrophobic residues in the active site. Various biophysical, biochemical and spectroscopic techniques have been utilized to investigate the wild-type and mutant proteins. Three important P450cam variants were obtained showing distinct structural and functional features. In P450camV247H mutant, which exhibited almost identical spectral properties with the wild-type, it is demonstrated that a single amino acid switch turned the monooxygenase into an efficient preoxidase by increasing the peroxidase activity nearly one thousand folds. In order to tune the distal pocket of P450cam with polar residues, Leu 246 was replaced with a basic residue, lysine, resulting in a mutant with spectral features identical to P420, the inactive species of P450. But this inactive-species-like mutant showed catalytic activities without the facilitation of any cofactors. By substituting Gly 248 with a histidine, a novel Cys-Fe-His ligation set was obtained in P450cam which represented the very rare case of His ligation in heme-thiolate proteins. In addition to serving as a convenient model for hemoprotein structural studies, the G248H mutant also provided evidence about the nature of the axial ligand in cytochrome P420 and other engineered hemoproteins with thiolate ligations. Furthermore, attempts have been made to replace the proximal ligand in sperm whale myoglobin to construct a heme-thiolate protein model by mimicking the protein environment of cytochrome P450cam and chloroperoxidase.

Synthetic Studies of Azulenyl and Pseudoazulenyl Nitrones

Free radicals have been implicated in various pathological conditions such as, stroke, aging and ischemic heart disease (IHD), as well as neurodegenerative diseases like Alzheimer’s, Parkinson’s, and Huntington’s disease. The role of antioxidants in protection from the harmful effects of free radicals has long been recognized. Trapping extremely reactive free radicals and eliminating them from circulation has been shown to be effective in animal models. Nitrone-based free radical traps have been extensively explored in biological systems. Examples include nitrones such as PBN, NXY-059, MDL-101,002, DMPO and EMPO. However, these nitrones have extremely high oxidation potentials as compared to natural antioxidants such as Vitamin E (á-tocopherol), and glutathione. Becker et al. (1995) synthesized novel azulenyl nitrones, which were shown to have oxidation potentials much lower than that of any of the previously reported nitrone based spin traps. Another azulenyl nitrone derivative, stilbazulenyl nitrone (STAZN), was shown to have an even lower oxidation potential within the range of natural antioxidants. STAZN, a second generation free radical trap, was found to be markedly superior than the two most studied nitrones, PBN and NXY-059, in animal models of cerebral ischemia and in an in vitro assay of lipid peroxidation. In this study, a third generation azulenyl nitrone was synthesized with an electron donating group on the previously synthesized STAZN derivative with the aim to lower the oxidation potential even more. Pseudoazulenes, because of the presence of an annular heteroatom, have been reported to possess even lower oxidation potential than that of the azulenyl counterpart. Therefore, pseudoazulenyl nitrones were synthesized for the first time by extracting and elaborating valtrate from the roots of Centranthus ruber (Red valerian or Jupiter’s beard). Several pseudoazulenyl nitrones were synthesized by using a facile experimental protocol. The physical and biological properties of these pseudoazulenyl nitrones can be easily modified by simply changing the substituent on the heteroatom. Cyclic voltammetry experiments have shown that these pseudoazulenyl nitrones do indeed have low oxidation potentials. The oxidation potential of these nitrones was lowered even more by preparing derivatives bearing an electron donating group at the 3-position of the five membered ring of the pseudoazulenyl nitrone.

The correlation of structure and electronic properties near the surface of transition metal oxides

The strong couplings between different degrees of freedom are believed to be responsible for novel and complex phenomena discovered in transition metal oxides (TMOs). The physical complexity is directly responsible for their tunability. Creating surfaces/interfaces add an additional ' man-made' twist, approaching the quantum phenomena of correlated materials. ^ The dissertation focused on the structural and electronic properties in proximity of surface of three prototype TMO compounds by using three complementary techniques: scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and low energy electron diffraction, particularly emphasized the effects of broken symmetry and imperfections like defects on the coupling between charge and lattice degrees of freedom. ^ Ca1.5Sr0.5RuO4 is a layered ruthenate with square lattice and at the boundary of magnetic/orbital instability in Ca2-xSrxRuO4. That the substitution of Sr 2+ with Ca2+ causing RuO6 rotation narrows the dxy band width and changes the Fermi surface topology. Particularly, the γ(dxy) Fermi surface sheet exhibited hole-like in Ca1.5Sr0.5RuO4 in contrast to electron-like in Sr2RuO4, showing a strong charge-lattice coupling. ^ Na0.75CoO2 is a layered cobaltite with triangular lattice exhibiting extraordinary thermoelectric properties. The well-ordered CoO2-terminated surface with random Na distribution was observed. However, lattice constants of the surface are smaller than that in bulk. The surface density of states (DOS) showed strong temperature dependence. Especially, an unusual shift of the minimum DOS occurs below 230 K, clearly indicating a local charging effect on the surface. ^ Cd2Re2O7 is the first known pyrochlore oxide superconductor (Tc ∼ 1K). It exhibited an unusual second-order phase transition occurring at TS1 = 200 K and a controversial first-order transition at TS2 = 120 K. While bulk properties display large anomalies at TS1 but rather subtle and sample-dependent changes at TS2, the surface DOS near the EF show no change at T s1 but a substantial increase below TS2---a complete reversal as the signature for the transitions. We argued that crystal imperfections, mainly defects, which were considerably enhanced at the surface, resulted in the transition at TS2. ^