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- Title
- Mechanochemistry for Solid-State Syntheses and Catalysis.
- Creator
-
Restrepo, David, Blair, Richard, Kuebler, Stephen, Miles, Delbert, Clausen, Christian, Orlovskaya, Nina, University of Central Florida
- Abstract / Description
-
Traditional methods of synthesizing inorganic materials, such as hydrothermal, sol-gel, calcination and grinding steps, can typically require use of high temperatures, expensive precursors or use of solvents. Because of the energy-intensive nature or environmental impact these techniques, there is a push, especially from an industrial perspective, to move towards greener approaches. Mechanochemistry is a solvent-free alternative technique that can be used to synthesize a variety of materials...
Show moreTraditional methods of synthesizing inorganic materials, such as hydrothermal, sol-gel, calcination and grinding steps, can typically require use of high temperatures, expensive precursors or use of solvents. Because of the energy-intensive nature or environmental impact these techniques, there is a push, especially from an industrial perspective, to move towards greener approaches. Mechanochemistry is a solvent-free alternative technique that can be used to synthesize a variety of materials under ambient conditions. Due to this, there is an increase in attention towards the use of this approach in both solid-state inorganic and organic chemistry.This dissertation reports the mechanochemical synthesis of a few inorganic materials without the need of using high temperatures or solvents. Additionally, examples are presented in which mechanochemistry is used in conjunction with a secondary technique. This mechanical activation of the precursors lead to a decrease in calcination temperature and reactions times, as well as alteration of properties or unique reaction products. The synthesis of kaolinite, vanadia nanostructures, and spinels were carried out in this fashion. Mechanical activation of the precursors allowed for reduced hydrothermal treatment times in case of both kaolinite and vanadia nanostructures and the spinels are calcined at lower temperature for shorter periods of time. In addition, we report alternative template agents than previously reported for the formation of vanadia nanotubes, and report the formation of nanorods. Choosing the appropriate amine template can alter the structure and size of the material.Isomorphously substituted mixed oxides, kaolinite and spinels (MgAl2O4 and ZnAl2O4) were synthesized through a mechanically assisted process. Kaolinites are treated hydrothermally for 1 week at 250 (&)#186;C to produce an X-ray pure crystalline material. The spinels undergo calcination as low as 500 (&)#186;C to produce a nanocrystalline material. Rare-earth metals and transition metals were used as the substitutional atom. The substituted kaolinites exhibit strong order along the c axis, but less ordering along the a and b axes. Trivalent chromium and trivalent rare-earth metals, such as La, Ce, Pr, Nd, Eu, Gd, Ho, and Er, are used to replace aluminum in the structure. Likewise, divalent and trivalent transition, such as Mn, Ni, Cu and Cr, are used as the substitutional atoms in MgAl2O4 and ZnAl2O4. Cathodoluminescence studies on the substituted Spinel structure show that Mn2+ ions can occupy both the tetrahedral or octahedral holes to give a green and red emission, respectively. On the other hand, Cr3+ ions only occupy the octahedral holes to yield a red emission, similar to that in ruby. These isomorphously substituted materials may have potential applications in catalysis or glaze materials in ceramics.Oxidized graphite, an alternative to graphite oxide and graphene, can be synthesized rapidly by mechanochemical means. Grinding urea hydrogen peroxide adduct with graphite without the need of a solvent produces a product with an oxygen content of 5-15 wt%. The byproducts of this reaction are urea and water. This material is oxidized along the edges of the sheets, allowing it to be hydrophilic while retaining the conductivity. The material can suspend in water and processing allows for films of resistivities between 50 ? cm-2 and 10 k? cm-2. It was determined that the edges are fully oxidized to yield (-)COOH groups. This process offers a scalable, environmentally benign route to large quantities of oxidized graphite.An alternative method for the synthesis of nanostructured vanadia is reported. This process involves mechanical grinding of vanadium pentoxide, V2O5, with an amine template, such as diphenylamine, theophylline, rhodamine 6G and rhodamine, prior to hydrothermal treatment. This allows for the synthesis of VOx nanotubes and nanorods dependent on which template is used. Diphenylamine, theophylline, and rhodamine B produce nanorods. Use of rhodamine 6G produces asymmetric VOx nanorods.In addition to the mixed metals oxides mentioned above, sodium and calcium tantalates are synthesized mechanically. This route does not require the need of elevated temperatures or expensive and hazardous materials. X-ray diffraction analysis of NaTaO3, Ca2Ta2O7, Ca4Ta2O9 and CaTa2O6 shows that these are the only phases detected after 4 h, 10 h, 27 h and 10 h of milling, respectively. During the synthesis of Ca2Ta2O7, an intermediate phase, Ca4Ta2O9, forms within 1 h, which reacts after 5 h to form the desired product. Reference Intensity Ratio analysis shows that the material synthesized mechanically is nanocrystalline Ca2Ta2O7.Nanocrystalline ZrSi2 can also be obtained through mechanochemical synthesis. This method allows for size control and results in crystallites ranging from 9 to 30 nm. Dilution with CaCl2 enables the size control process. A linear relationship exists between the concentration of CaCl2 and the crystallite size. Contrary to a typical self-propagating metathesis reaction, this process does not allow for self-propagation and requires continuous input of mechanical energy to continue. However, this method allows for non-passivated nanoparticles of ZrSi2, which can be incorporated into composites as a reinforcement material for several applications.Hard and ultra-compressible borides, such as ReB2 and OsB2, can be synthesized mechanically. The traditional synthesis of ReB2 requires excess boron due to treatment at high temperatures. This can lead to amorphous boron aggregating at the grain boundaries, which in turn, this would degrade the properties of the material. The mechanochemical approach requires mechanical treatment of Re and B powders in stoichiometric quantities for 80 h. Mechanical synthesis of OsB2 powders requires a 1:3 ratio of Os and B powders. After 12 h of milling time, h-OsB2 begins to form, and is the major phase present after 18 h. The lattice parameters corresponding to the hexagonal OsB2 were determined to be a = b = 2.9047 (&)#197;, c = 7.4500 (&)#197;, ? = ? = 90(&)#186;, ? = 120(&)#186;. Treatment of the OsB2 powder at 1050 (&)#186;C under vacuum for 6 days did not induce a phase change, suggesting the hexagonal phase is very stable.Mechanocatalysis of the depolymerization of cellulose and hydrogenation of olefins over BN are reported as well. Heterogeneous catalysis is difficult to apply to solids, such as cellulose. However, mechanical grinding of kaolin and cellulose allows for the catalysis to occur in the solid state. This process allows for a variety of different biomasses to be used as feedstock without inhibition. Kaolinite was found to be the best acid catalyst due to high surface acidity and its layered structure, allowing for up to 84% conversion of the cellulose to water-soluble compounds. This process allows for reduction of waste, insensitivity of feedstock, multiple product pathways and scalability.Hydrogenation reactions are carried out using transition-metals catalysts. These metals have desirable catalytic properties not seen in main group elements, but there is growing concern over their use. A metal-free heterogeneous hydrogenation catalyst based on frustrated Lewis pairs would significantly reduce the health, environmental, and economic concerns associated with these metal-based catalysts. We report the first metal-free heterogeneous hydrogenation catalyst. Hydrogenation of trans-cinnamic acid is carried out over defect-laden h-BN. The reactor we use is designed to maximize the defects produced in BN sheets. The introduction of defects in BN creates frustrated Lewis pairs. DFT calculations show that the carbon double bond is weakened over boron substitution for nitrogen sites, vacancies of both boron and nitrogen, and Stone-Wales defects.A new method for crystalline germanium deposition occurring at lower temperatures (210-260 (&)#186;C) is reported. This method involves mechanical treatment of the precursors to reduce the particle size. A ground mixture of Ge and CuI are heated under vacuum to synthesize GeI2. In situ disproportionation of this compound at 210 (&)#186;C allows for the deposition of polycrystalline Ge films onto a both glass and polymer substrates. The rate of deposition is found to be 25 ng min-1. The byproducts of this process are GeI2, GeI4 and Cu3Ge, which are valuable precursors for the synthesis of germanium nanostructures and organogermanium compounds.Mechanochemistry is also utilized for the synthesis of trisubstituted pnictides. Mechanochemical treatment of bromobenzene with either Na3Sb or Na3Bi allows for the formation of triphenylstibine or triphenylbismuthine, respectively. The synthesis of the alkali metals pnictide precursors is reported as well. The synthesis of triphenylstibine produces SbPh3 as the major product from the reaction. The synthesis of triphenylbismuthine produces more Wurtz-type coupling products, which are due to the BiPh3 acting as a catalyst. Tributyl and triphenyl analogues are reported as well. The trialkylated analogues for both Sb and Bi produce more Wurtz type coupling products. This would allow for a more cost effective and scalable, alternative methods than what is currently in use today.
Show less - Date Issued
- 2013
- Identifier
- CFE0004741, ucf:49785
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004741
- Title
- Selective Multivariate Applications in Forensic Science.
- Creator
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Rinke, Caitlin, Sigman, Michael, Campiglia, Andres, Yestrebsky, Cherie, Kuebler, Stephen, Richardson, Martin, University of Central Florida
- Abstract / Description
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A 2009 report published by the National Research Council addressed the need for improvements in the field of forensic science. In the report emphasis was placed on the need for more rigorous scientific analysis within many forensic science disciplines and for established limitations and determination of error rates from statistical analysis. This research focused on multivariate statistical techniques for the analysis of spectral data obtained for multiple forensic applications which include...
Show moreA 2009 report published by the National Research Council addressed the need for improvements in the field of forensic science. In the report emphasis was placed on the need for more rigorous scientific analysis within many forensic science disciplines and for established limitations and determination of error rates from statistical analysis. This research focused on multivariate statistical techniques for the analysis of spectral data obtained for multiple forensic applications which include samples from: automobile float glasses and paints, bones, metal transfers, ignitable liquids and fire debris, and organic compounds including explosives. The statistical techniques were used for two types of data analysis: classification and discrimination. Statistical methods including linear discriminant analysis and a novel soft classification method were used to provide classification of forensic samples based on a compiled library. The novel soft classification method combined three statistical steps: Principal Component Analysis (PCA), Target Factor Analysis (TFA), and Bayesian Decision Theory (BDT) to provide classification based on posterior probabilities of class membership. The posterior probabilities provide a statistical probability of classification which can aid a forensic analyst in reaching a conclusion. The second analytical approach applied nonparametric methods to provide the means for discrimination between samples. Nonparametric methods are performed as hypothesis test and do not assume normal distribution of the analytical figures of merit. The nonparametric permutation test was applied to forensic applications to determine the similarity between two samples and provide discrimination rates. Both the classification method and discrimination method were applied to data acquired from multiple instrumental methods. The instrumental methods included: Laser Induced-Breakdown Spectroscopy (LIBS), Fourier Transform Infrared Spectroscopy (FTIR), Raman spectroscopy, and Gas Chromatography-Mass Spectrometry (GC-MS). Some of these instrumental methods are currently applied to forensic applications, such as GC-MS for the analysis of ignitable liquid and fire debris samples; while others provide new instrumental methods to areas within forensic science which currently lack instrumental analysis techniques, such as LIBS for the analysis of metal transfers. The combination of the instrumental techniques and multivariate statistical techniques is investigated in new approaches to forensic applications in this research to assist in improving the field of forensic science.
Show less - Date Issued
- 2012
- Identifier
- CFE0004628, ucf:49942
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004628
- Title
- Cascaded plasmon resonances for enhanced nonlinear optical response.
- Creator
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Toroghi, Seyfollah, Kik, Pieter, Vanstryland, Eric, Kuebler, Stephen, Hagan, David, Belfield, Kevin, University of Central Florida
- Abstract / Description
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The continued development of integrated photonic devices requires low-power, small volume all-optical modulators. The weak nonlinear optical response of conventional optical materials requires the use of high intensities and large interaction volumes in order to achieve significant light modulation, hindering the miniaturization of all-optical switches and the development of lightweight transmission optics with nonlinear optical response. These challenges may be addressed using plasmonic...
Show moreThe continued development of integrated photonic devices requires low-power, small volume all-optical modulators. The weak nonlinear optical response of conventional optical materials requires the use of high intensities and large interaction volumes in order to achieve significant light modulation, hindering the miniaturization of all-optical switches and the development of lightweight transmission optics with nonlinear optical response. These challenges may be addressed using plasmonic nanostructures due to their unique ability to confine and enhance electric fields in sub-wavelength volumes. The ultrafast nonlinear response of free electrons in such plasmonic structures and the fast thermal nonlinear optical response of metal nanoparticles, as well as the plasmon enhanced nonlinear Kerr-type response of the host material surrounding the nanostructures could allow ultrafast all-optical modulation with low modulation energy. In this thesis, we investigate the linear and nonlinear optical response of engineered effective media containing coupled metallic nanoparticles. The fundamental interactions in systems containing coupled nanoparticles with size, shape, and composition dissimilarity, are evaluated analytically and numerically, and it is demonstrated that under certain conditions the achieved field enhancement factors can exceed the single-particle result by orders of magnitude in a process called cascaded plasmon resonance. It is demonstrated that these conditions can be met in systems containing coupled nanospheres, and in systems containing non-spherical metal nanoparticles that are compatible with common top-down nanofabrication methods such as electron beam lithography and nano-imprint lithography. We show that metamaterials based on such cascaded plasmon resonance structures can produce enhanced nonlinear optical refraction and absorption compared to that of conventional plasmonic nanostructures. Finally, it is demonstrated that the thermal nonlinear optical response of metal nanoparticles can be enhanced in carefully engineered heterogeneous nanoparticle clusters, potentially enabling strong and fast thermal nonlinear optical response in system that can be produced in bulk through chemical synthesis.
Show less - Date Issued
- 2014
- Identifier
- CFE0005556, ucf:50272
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005556
- Title
- Structure-Property Relationship of the Two-Photon Circular Dichroism of Compounds with Axial and Helical Chirality.
- Creator
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Diaz, Carlos, Hernandez, Florencio, Uribe Romo, Fernando, Kuebler, Stephen, Masunov, Artem, Del Barco, Enrique, University of Central Florida
- Abstract / Description
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Back in 1894 Lord Kelvin coined the term (")chiral(") in order to refer to molecules whose mirror images were not superimposable with themselves. Over the years, research has demonstrated the important role that chiral molecules play in life, chemistry, and biology as well as their importance in the development of new drugs and technologies.The efforts to understand chiral systems have been mainly driven by spectroscopic methods that leverage on the opposite responses that enantiomers have to...
Show moreBack in 1894 Lord Kelvin coined the term (")chiral(") in order to refer to molecules whose mirror images were not superimposable with themselves. Over the years, research has demonstrated the important role that chiral molecules play in life, chemistry, and biology as well as their importance in the development of new drugs and technologies.The efforts to understand chiral systems have been mainly driven by spectroscopic methods that leverage on the opposite responses that enantiomers have to linear or circularly polarized light of both handedness. More specifically, Electronic Circular Dichroism (ECD) which measures the differences in linear absorption of left and right circularly polarized light has been the method par excellence for the spectroscopic characterization of chiral compounds. Unfortunately, the fact that ECD is based on linear absorption severely limits the use of this method in the near to far UV region. This is mainly due to the interferences generated by the strong linear absorption of common organic solvents and buffers in this portion of the light spectrum. Nevertheless, the fact remains that many chiral biomolecules of interest related to deceases like Alzheimer and Parkinson, exhibit most of their linear absorption in the near to far UV region where ECD cannot be employed for their study. Therefore, it has become an urgent necessity to develop spectroscopic methods to study chiral molecules that can circumvent the limitations of ECD at shorter wavelengths. In order to overcome the existent limitations in linear chiral spectroscopy, the nonlinear equivalent of ECD arises as a promising alternative, i.e. Two-Photon Circular Dichroism (TPCD). Although, this phenomenon was theoretically predicted in 1975, it was not until 2008, with the introduction of the double-L scan, that a reliable and versatile method for the measurement of TPCD was introduced. The high sensitivity of this method is based on the use of (")twin(") pulses that allow accounting for fluctuations in the excitation source that prevented the experimental realization of the measurement. The first measurement of a full TPCD spectrum was performed on BINOL enantiomers and the results were supported and discussed with the help of theoretical calculations. After that seminal work, we embarked in expanding the understanding of the structure-property relationship of TPCD by performing, systematically, a series of theoretical-experimental studies in chiral biaryl derivatives and compounds with helical chirality.In Chapter 2 we present the theoretical-experimental study of the effect of the ?-electron delocalization curvature on the TPCD of molecules with axial chirality. The targeted molecules for this part of our investigation were S-BINOL, S-VANOL, and S-VAPOL. Our findings revealed that an increase in the TPCD signal, within this series of compounds, was related to the curvature of the ?(-)electron delocalization. The contributions of the different transition moments to the two-photon rotatory strength support our outcomes. Then, in Chapter 3 we introduce the development of the Fragment-Recombination Approach (FRA) for the calculation of the TPCD spectra of large molecules. This simple but powerful method is based on the additivity of the TPCD signal, and is subject to a strict conditional fragmentation approach. FRA-TPCD is demonstrated, theoretically, in two hypothetical molecular systems from the biaryl derivatives family. Afterward, in Chapter 4 we show the first experimental demonstration of FRA-TPCD through the conformational analysis of an axially-chiral Salen ligand in solution (AXF-155). The FRA-TPCD spectra calculated for the different isomers of AXF-155 allowed narrowing the number of possible isomers of this complex molecule in THF solution to only two. This represents a significant improvement from previously reported results using ECD. Subsequently, in Chapter 5 we present the study of the effect of intramolecular charge transfer (ICT) in S-BINAP, an axially dissymmetric diphosphine ligand with strong ICT. The evaluation of the performance of two different exchange-correlation functional (XCF) confirmed that in order to properly predict the theoretical TPCD spectrum of a molecule exhibiting strong ICT, it is required to use an XCF such as CAM-B3LYP. In addition, our findings revealed the importance of considering an adequate number of excited states in order to be able to fully reproduce the experimental TPCD spectrum, thus avoiding wrong assignments of theoretical transitions to experimental spectral features. Finally, and expanding on our previous study, in Chapter 6 we investigated the effect of the nature of ICT on two hexahelicene derivatives. Our investigation demonstrated that the TPCD signal of chiral molecules with strong ICT does not only depend on the strength of this effect but on its nature, i.e. extension of the ?(-)electronic delocalization increasing beyond (EXO-ICT) or within (ENDO-ICT) the helicene core. In summary, with the results presented in this thesis we closed a first loop in the understanding of the structure-property relationship of TPCD. In the future, we expect to deepen in our knowledge of the structure-property relationship of this phenomenon by studying further helicene derivatives with donor-acceptor motif, and through the application of FRA-TPCD to the conformational analysis of amino acids in peptides. We foresee numerous applications of TPCD for the study of optically active molecules with implications in biology, medicine, and the drug and food industry, and applications in nanotechnology, asymmetric catalysis and photonics.
Show less - Date Issued
- 2015
- Identifier
- CFE0005787, ucf:50067
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005787
- Title
- QUANTITATIVE SCANNING TRANSMISSION ELECTRON MICROSCOPY OF THICK SAMPLES AND OF GOLD AND SILVER NANOPARTICLES ON POLYMERIC SURFACES.
- Creator
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Dutta, Aniruddha, Heinrich, Helge, Del Barco, Enrique, Chow, Lee, Chen, Bo, Kuebler, Stephen, University of Central Florida
- Abstract / Description
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Transmission Electron Microscopy (TEM) is a reliable tool for chemical and structural studies of nanostructured systems. The shape, size and volumes of nanoparticles on surfaces play an important role in surface chemistry. As nanostructured surfaces become increasingly important for catalysis, protective coatings, optical properties, detection of specific molecules, and many other applications, different techniques of TEM can be used to characterize the properties of nanoparticles on surfaces...
Show moreTransmission Electron Microscopy (TEM) is a reliable tool for chemical and structural studies of nanostructured systems. The shape, size and volumes of nanoparticles on surfaces play an important role in surface chemistry. As nanostructured surfaces become increasingly important for catalysis, protective coatings, optical properties, detection of specific molecules, and many other applications, different techniques of TEM can be used to characterize the properties of nanoparticles on surfaces to provide a path for predictability and control of these systems.This dissertation aims to provide fundamental understanding of the surface chemistry of Electroless Metallization onto Polymeric Surfaces (EMPS) through characterization with TEM. The research focuses on a single EMPS system: deposition of Ag onto the cross-linked epoxide (")SU8("), where Au nanoparticles act as nucleation sites for the growth of Ag nanoparticles on the polymer surface. TEM cross sections were analyzed to investigate the morphology of the Au nanoparticles and to determine the thicknesses of the Ag nanoparticles and of the Ag layers. A method for the direct measurement of the volume and thickness of nanomaterials has been developed in the project using High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM). The morphology of Au and Ag NPs has been studied to provide reliable statistics for 3-D characterization. Deposition rates have been obtained as a function of metallization conditions by measuring the composition and thickness of the metal for EMPS. In the present work a calibration method was used to quantify the sensitivity of the HAADF detector. For thin samples a linear relationship of the HAADF signal with the thickness of a material is found. Cross-sections of multilayered samples provided by Triquint Semiconductors, FL, were analyzed as calibration standards with known composition in a TECNAI F30 transmission electron microscope to study the dependence of the HAADF detector signal on sample thickness and temperature.Dynamical diffraction processes play an important role in electron scattering for larger sample thicknesses. The HAADF detector intensity is not linearly dependent on sample thicknesses for thick samples. This phenomenon involves several excitation processes including Thermal Diffuse Scattering (TDS) which depends on temperature-dependent absorption coefficients. Multislice simulations have been carried out by Python programming using the scattering parameters available in the literature. These simulations were compared with experimental results. Wedge-shaped Focused Ion Beam (FIB) samples were prepared for quantitative HAADF-STEM intensity measurements for several samples and compared with these simulations. The discrepancies between the simulated and experimental results were explained and new sets of absorptive parameters were calculated which correctly account for the HAADF-STEM contrasts. A database of several pure elements is compiled to illustrate the absorption coefficients and fractions of scattered electrons per nanometer of the sample.In addition, the wedge-shaped FIB samples were used for studying the HAADF-STEM contrasts at an interface of a high- and a low-density material. The use of thick samples reveals an increased signal at the interfaces of high- and low-density materials. This effect can be explained by the transfer of scattered electrons from the high density material across the interface into the less-absorbing low-density material. A ballistic scattering model is proposed here for the HAADF-STEM contrasts at interfaces of thick materials using Python. The simulated HAADF-STEM signal is compared with experimental data to showcase the above phenomenon. A detailed understanding of the atomic number contrast in thick samples is developed based on the combination of experimental quantitative HAADF-STEM and simulated scattering. This approach is used to describe the observed features for Ag deposition on SU-8 polymers.
Show less - Date Issued
- 2014
- Identifier
- CFE0005485, ucf:50333
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005485
- Title
- Photophysical and photochemical factors affecting multi-photon direct laser writing using the cross-linkable epoxide SU-8.
- Creator
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Williams Jr., Henry, Kuebler, Stephen, Elsheimer, Seth, Zhai, Lei, Liao, Yi, Heinrich, Helge, University of Central Florida
- Abstract / Description
-
For the past decade, the epoxy based photoresist SU-8 has been used commercially and in the lab for fabricating micro- and nano-structures. Investigators have studied how processing parameters such as pre- and post-exposure bake temperatures affect the resolution and quality of SU-8 structures patterned using ultraviolet or x-ray lithography. Despite the advances in understanding the phenomena, not all of them have been explored, especially those that are specific to multi-photon direct laser...
Show moreFor the past decade, the epoxy based photoresist SU-8 has been used commercially and in the lab for fabricating micro- and nano-structures. Investigators have studied how processing parameters such as pre- and post-exposure bake temperatures affect the resolution and quality of SU-8 structures patterned using ultraviolet or x-ray lithography. Despite the advances in understanding the phenomena, not all of them have been explored, especially those that are specific to multi-photon direct laser writing (mpDLW). Unlike conventional exposure techniques, mpDLW is an inherently three-dimensional (3D) process that is activated by nonlinear absorption of light.This dissertation reports how several key processing parameters affect mpDLW using SU-8 including pre-exposure bake duration, focal depth, incident laser power, focal-point scan speed, and excitation wavelength. An examination of solvent content of films at various stages in the mpDLW by 1H-NMR shows that even moderate solvent content (over 1 wt-%) affects film viscosity and photoacid diffusion lengths, and can greatly affect the overall fidelity of small features. A study of micro-fabricated feature size versus writing depth in the material shows that even slight refractive index mismatch between SU-8 and the medium between it and the focusing objective introduces spherical aberration that distorts the focus, causing feature size to decrease or even increase in size with writing depth, depending on the average exposure power used. Proper adjustment of the average exposure power was demonstrated as a means to fabricate more uniform features with writing depth. Third, when varying the power and scan speed, it was observed that the feature-size scales with these two parameters in a manner that is consistent with a three-photon absorption mechanism at an excitation wavelength of 800 nm. When aniiiexcitation wavelength of 725 nm is used, the feature-size scaling becomes consistent with that of two photon absorption. This shows that the photoinitiators in the SU-8 can be activated by either two- or three-photon absorption over this wavelength range. Using an irradiance of ~2 TW cm-2 and elongated femtosecond pulses resulted in an observed fourth order power dependence. This observation is in agreement with the literature and suggests that the effective absorptive nonlinearity is also sensitive to pulse duration. These findings will be useful for creating accurate models of the process of mpDLW in SU-8. These models could be used to optimize the processing parameters and develop new processing methods and materials for high-resolution fabrication of robust 3D microstructures. Some of the findings were used to develop a method for fabricating functional microlenses on the tip of optical fibers. This approach opens a new route to functional integrated photonic devices.
Show less - Date Issued
- 2013
- Identifier
- CFE0005441, ucf:50403
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005441
- Title
- Biochemical Characterization of the NifB Enzyme and NifB-cofactor.
- Creator
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Gevorkyan, Jirair, Igarashi, Robert, Belfield, Kevin, Hernandez, Florencio, Kuebler, Stephen, Vonkalm, Laurence, University of Central Florida
- Abstract / Description
-
The Mo-nitrogenase complex is composed of two components, Fe-protein and MoFe-protein. This complex is able to catalyze the reduction of N2 through the MgATP dependent transfer of electrons from the Fe-protein Fe4S4 cluster to the MoFe-protein P-cluster and, subsequently, to the iron-molybdenum cofactor (FeMo-co). FeMo-co is a Fe7S9MoC-(R)-homocitrate cluster and has two biosynthetic precursors, NifB-co and L-cluster, of unknown structure and composition. The biosynthesis of FeMo-co is an...
Show moreThe Mo-nitrogenase complex is composed of two components, Fe-protein and MoFe-protein. This complex is able to catalyze the reduction of N2 through the MgATP dependent transfer of electrons from the Fe-protein Fe4S4 cluster to the MoFe-protein P-cluster and, subsequently, to the iron-molybdenum cofactor (FeMo-co). FeMo-co is a Fe7S9MoC-(R)-homocitrate cluster and has two biosynthetic precursors, NifB-co and L-cluster, of unknown structure and composition. The biosynthesis of FeMo-co is an enigmatic process that minimally requires NifB, NifEN, Fe-protein, MoO42-, (R)-homocitrate and S-adenolsylmethionine.A means to isolate the NifB enzyme for characterization has been developed through use of a GST-fusion tag. Double recombination of A. vinelandii strains with a constructed vector has yielded strains capable of nif promoter regulated expression of GST-NifB. Extracts of strains containing GST-NifB were shown to activate the Mo-nitrogenase complex in biochemical complementation assays. Mass spectroscopy was then used to verify successful isolation of GST-NifB by GSH-Sepharose affinity purification.The number of NifB-co ligand binding sites and ligand types were examined by EXAFS analysis of samples containing selenol and thiol ligands. A Fe6S9C model for NifB-co was optimized to best fit the EXAFS data, where a 2-fold discrepancy in binding sites implied by thiol or selenol only ligand samples suggests Fe-(?2S)-Fe binding in the absence of Se. Samples containing heterogeneous ligand types indicated that NifX bound NifB-co ligates to four cysteine residues and one molecule of DTT.
Show less - Date Issued
- 2013
- Identifier
- CFE0004682, ucf:49865
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004682
