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- All Subjects: Materials Science
- Genre: Doctoral Dissertation
Description
Metal hydride materials have been intensively studied for hydrogen storage applications. In addition to potential hydrogen economy applications, metal hydrides offer a wide variety of other interesting properties. For example, hydrogen-dominant materials, which are hydrides with the highest hydrogen content for a particular metal/semimetal composition, are predicted to display high-temperature superconductivity. On the other side of the spectrum are hydrides with small amounts of hydrogen (0.1 - 1 at.%) that are investigated as viable magnetic, thermoelectric or semiconducting materials. Research of metal hydride materials is generally important to gain fundamental understanding of metal-hydrogen interactions in materials. Hydrogenation of Zintl phases, which are defined as compounds between an active metal (alkali, alkaline earth, rare earth) and a p-block metal/semimetal, were attempted by a hot sintering method utilizing an autoclave loaded with gaseous hydrogen (< 9 MPa). Hydride formation competes with oxidative decomposition of a Zintl phase. The oxidative decomposition, which leads to a mixture of binary active metal hydride and p-block element, was observed for investigated aluminum (Al) and gallium (Ga) containing Zintl phases. However, a new phase Li2Al was discovered when Zintl phase precursors were synthesized. Using the single crystal x-ray diffraction (SCXRD), the Li2Al was found to crystallize in an orthorhombic unit cell (Cmcm) with the lattice parameters a = 4.6404(8) Å, b = 9.719(2) Å, and c = 4.4764(8) Å. Increased demand for materials with improved properties necessitates the exploration of alternative synthesis methods. Conventional metal hydride synthesis methods, like ball-milling and autoclave technique, are not responding to the demands of finding new materials. A viable alternative synthesis method is the application of high pressure for the preparation of hydrogen-dominant materials. Extreme pressures in the gigapascal ranges can open access to new metal hydrides with novel structures and properties, because of the drastically increased chemical potential of hydrogen. Pressures up to 10 GPa can be easily achieved using the multi-anvil (MA) hydrogenations while maintaining sufficient sample volume for structure and property characterization. Gigapascal MA hydrogenations using ammonia borane (BH3NH3) as an internal hydrogen source were employed in the search for new hydrogen-dominant materials. Ammonia borane has high gravimetric volume of hydrogen, and additionally the thermally activated decomposition at high pressures lead to a complete hydrogen release at reasonably low temperature. These properties make ammonia borane a desired hydrogen source material. The missing member Li2PtH6 of the series of A2PtH6 compounds (A = Na to Cs) was accessed by employing MA technique. As the known heavier analogs, the Li2PtH6 also crystallizes in a cubic K2PtCl6-type structure with a cell edge length of 6.7681(3) Å. Further gigapascal hydrogenations afforded the compounds K2SiH6 and Rb2SiH6 which are isostructural to Li2PtH6. The cubic K2SiH6 and Rb2SiH6 are built from unique hypervalent SiH62- entities with the lattice parameters of 7.8425(9) and 8.1572(4) Å, respectively. Spectroscopic analysis of hexasilicides confirmed the presence of hypervalent bonding. The Si-H stretching frequencies at 1550 cm-1 appeared considerably decreased in comparison with a normal-valent (2e2c) Si-H stretching frequencies in SiH4 at around 2200 cm-1. However, the observed stretching modes in hypervalent hexasilicides were in a reasonable agreement with Ph3SiH2- (1520 cm-1) where the hydrogen has the axial (3e4c bonded) position in the trigoal bipyramidal environment.
ContributorsPuhakainen, Kati (Author) / Häussermann, Ulrich (Thesis advisor) / Seo, Dong (Thesis advisor) / Kouvetakis, John (Committee member) / Wolf, George (Committee member) / Arizona State University (Publisher)
Created2013
Description
The thesis studies new methods to fabricate optoelectronic Ge1-ySny/Si(100) alloys and investigate their photoluminescence (PL) properties for possible applications in Si-based photonics including IR lasers. The work initially investigated the origin of the difference between the PL spectrum of bulk Ge, dominated by indirect gap emission, and the PL spectrum of Ge-on-Si films, dominated by direct gap emission. It was found that the difference is due to the supression of self-absorption effects in Ge films, combined with a deviation from quasi-equilibrium conditions in the conduction band of undoped films. The latter is confirmed by a model suggesting that the deviation is caused by the shorter recombination lifetime in the films relative to bulk Ge. The knowledge acquired from this work was then utilized to study the PL properties of n-type Ge1-ySny/Si (y=0.004-0.04) samples grown via chemical vapor deposition of Ge2H6/SnD4/P(GeH3)3. It was found that the emission intensity (I) of these samples is at least 10x stronger than observed in un-doped counterparts and that the Idir/Iind ratio of direct over indirect gap emission increases for high-Sn contents due to the reduced gamma-L valley separation, as expected. Next the PL investigation was expanded to samples with y=0.05-0.09 grown via a new method using the more reactive Ge3H8 in place of Ge2H6. Optical quality, 1-um thick Ge1-ySny/Si(100) layers were produced using Ge3H10/SnD4 and found to exhibit strong, tunable PL near the threshold of the direct-indirect bandgap crossover. A byproduct of this study was the development of an enhanced process to produce Ge3H8, Ge4H10, and Ge5H12 analogs for application in ultra-low temperature deposition of Group-IV semiconductors. The thesis also studies synthesis routes of an entirely new class of semiconductor compounds and alloys described by Si5-2y(III-V)y (III=Al, V= As, P) comprising of specifically designed diamond-like structures based on a Si parent lattice incorporating isolated III-V units. The common theme of the two thesis topics is the development of new mono-crystalline materials on ubiquitous silicon platforms with the objective of enhancing the optoelectronic performance of Si and Ge semiconductors, potentially leading to the design of next generation optical devices including lasers, detectors and solar cells.
ContributorsGrzybowski, Gordon (Author) / Kouvetakis, John (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2013
Description
Group IV alloy films exhibit the ability to tune both band structure and lattice parameters and have recently attracted attention for their potential applications in Si-photonics and photovoltaics. In this work, several new approaches to produce these alloys directly on Si(100) and Ge(100) wafers are developed. For photovoltaics, use of Ge-buffered Si(100) wafers as a low cost platform for epitaxy of In1-xGaxAs layers was explored. The results indicate that this approach has promise for transitioning from bulk Ge platforms to virtual substrates for a significant cost reduction. The electrical and optical properties of Ge and Ge1-ySny layers produced using several different techniques were explored via fabrication of high performance heterostructure photodiodes. First, a new CVD approach to Ge-like materials was developed in which germanium is alloyed with very small amounts of tin. These alloys exhibited no significant difference in their structural properties or band gap compared to pure Ge, however superior photo response and reduced dark currents were observed from fabricated devices relative to pure Ge on Si reference diodes. Additionally, pure Ge/Si(100) photodiodes were fabricated using layers grown via reactions of Ge4H10 on Si(100) and found to exhibit low dark current densities with high collection efficiencies. Ge1-x-ySixSny materials represent the newest member of group IV alloy family. The ability to decouple the lattice constant and the band gap in this system has led to strong interest both for strain/confinement layers in quantum well structures, and as the possible "missing" 1 eV junction in multijunction photovoltaics. Recent progress in this field has allowed for the first time growth, fabrication and measurement of novel photodiodes based on Ge1-x-ySixSny. This work presents the material, electrical and optical properties of Ge1-x-ySixSny layers and photodiodes grown directly on Ge and Si wafers using two different synthetic approaches. A series of photodiodes containing Sn concentrations from 1-5%, all lattice matched to Ge, was fabricated. The devices exhibited low dark current densities with high collection efficiencies as required for photovoltaics. By measuring the photoresponse, tunable band gaps ranging from 0.85 eV to 1.02 eV were observed.
ContributorsBeeler, Richard (Author) / Kouvetakis, John (Thesis advisor) / Menéndez, Jose (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2012
Description
This thesis describes the fabrication of several new classes of Ge1-x-ySixSny materials with the required compositions and crystal quality to engineer the band gaps above and below that of elemental Ge (0.8 eV) in the near IR. The work initially focused on Ge1-x-ySixSny (1-5% Sn, 4-20% Si) materials grown on Ge(100) via gas-source epitaxy of Ge4H10, Si4H10 and SnD4. Both intrinsic and doped layers were produced with defect-free microstructure and viable thickness, allowing the fabrication of high-performance photodetectors. These exhibited low ideality factors, state-of-the-art dark current densities and adjustable absorption edges between 0.87 and 1.03 eV, indicating that the band gaps span a significant range above that of Ge. Next Sn-rich Ge1-x-ySixSny alloys (2-4% Si and 4-10% Sn) were fabricated directly on Si and were found to show significant optical emission using photoluminescence measurements, indicating that the alloys have direct band gaps below that of pure Ge in the range of 0.7-0.55 eV. A series of Sn-rich Ge1-x-ySixSny analogues (y>x) with fixed 3-4% Si content and progressively increasing Sn content in the 4-10% range were then grown on Ge buffered Si platforms for the purpose of improving the material's crystal quality. The films in this case exhibited lower defect densities than those grown on Si, allowing a meaningful study of both the direct and indirect gaps. The results show that the separation of the direct and indirect edges can be made smaller than in Ge even for non-negligible 3-4% Si content, confirming that with a suitable choice of Sn compositions the ternary Ge1-x-ySixSny reproduces all features of the electronic structure of binary Ge1-ySny, including the sought-after indirect-to-direct gap cross over. The above synthesis of optical quality Ge1-x-ySixSny on virtual Ge was made possible by the development of high quality Ge-on-Si buffers via chemical vapor deposition of Ge4H10. The resultant films exhibited structural and electrical properties significantly improved relative to state-of-the-art results obtained using conventional approaches. It was found that pure Ge4H10 facilitates the control of residual doping and enables p-i-n devices whose dark currents are not entirely determined by defects and whose zero-bias collection efficiencies are higher than those obtained from samples fabricated using alternative Ge-on-Si approaches.
ContributorsXu, Chi (Author) / Kouvetakis, John (Thesis advisor) / Menéndez, Jose (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Drucker, Jeffrey (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2013
Description
The utilization of solar energy requires an efficient means of its storage as fuel. In bio-inspired artificial photosynthesis, light energy can be used to drive water oxidation, but catalysts that produce molecular oxygen from water are required. This dissertation demonstrates a novel complex utilizing earth-abundant Ni in combination with glycine as an efficient catalyst with a modest overpotential of 0.475 ± 0.005 V for a current density of 1 mA/cm2 at pH 11. The production of molecular oxygen at a high potential was verified by measurement of the change in oxygen concentration, yielding a Faradaic efficiency of 60 ± 5%. This Ni species can achieve a current density of 4 mA/cm2 that persists for at least 10 hours. Based upon the observed pH dependence of the current amplitude and oxidation/reduction peaks, the catalysis is an electron-proton coupled process. In addition, to investigate the binding of divalent metals to proteins, four peptides were designed and synthesized with carboxylate and histidine ligands. The binding of the metals was characterized by monitoring the metal-induced changes in circular dichroism spectra. Cyclic voltammetry demonstrated that bound copper underwent a Cu(I)/Cu(II) oxidation/reduction change at a potential of approximately 0.32 V in a quasi-reversible process. The relative binding affinity of Mn(II), Fe(II), Co(II), Ni(II) and Cu(II) to the peptides is correlated with the stability constants of the Irving-Williams series for divalent metal ions. A potential application of these complexes of transition metals with amino acids or peptides is in the development of artificial photosynthetic cells.
ContributorsWang, Dong (Author) / Allen, James P. (Thesis advisor) / Ghirlanda, Giovanna (Committee member) / Redding, Kevin (Committee member) / Arizona State University (Publisher)
Created2014
Description
Natural hydrogenases catalyze the reduction of protons to molecular hydrogen reversibly under mild conditions; these enzymes have an unusual active site architecture, in which a diiron site is connected to a cubane type [4Fe-4S] cluster. Due to the relevance of this reaction to energy production, and in particular to sustainable fuel production, there have been substantial amount of research focused on developing biomimetic organometallic models. However, most of these organometallic complexes cannot revisit the structural and functional fine-tuning provided by the protein matrix as seen in the natural enzyme. The goal of this thesis is to build a protein based functional mimic of [Fe-Fe] hydrogenases. I used a 'retrosynthetic' approach that separates out two functional aspects of the natural enzyme. First, I built an artificial electron transfer domain by engineering two [4Fe-4S] cluster binding sites into an existing protein, DSD, which is a de novo designed domain swapped dimer. The resulting protein, DSD-bis[4Fe-4S], contains two clusters at a distance of 36 Å . I then varied distance between two clusters using vertical translation along the axis of the coiled coil; the resulting protein demonstrates efficient electron transfer to/from redox sites. Second, I built simple, functional artificial hydrogenases by using an artificial amino acid comprising a 1,3 dithiol moiety to anchor a biomimetic [Fe-Fe] active site within the protein scaffold Correct incorporation of the cluster into a model helical peptide was verified by UV-Vis, FTIR, ESI-MS and CD spectroscopy. This synthetic strategy is extended to the de novo design of more complex protein architectures, four-helix bundles that host the di-iron cluster within the hydrophobic core. In a separate approach, I developed a generalizable strategy to introduce organometallic catalytic sites into a protein scaffold. I introduced a biomimetic organometallic complex for proton reduction by covalent conjugation to biotin. The streptavidin-bound complex is significantly more efficient in photocatalytic hydrogen production than the catalyst alone. With these artificial proteins, it will be possible to explore the effect of second sphere interactions on the activity of the diiron center, and to include in the design properties such as compatibility with conductive materials and electrodes.
ContributorsRoy, Anindya (Author) / Ghirlanda, Giovanna (Thesis advisor) / Yan, Hao (Committee member) / Gust, Devens (Committee member) / Arizona State University (Publisher)
Created2014
Description
Atomic force microscopy (AFM) has become an important tool to characterize and image surfaces with nanoscale resolution. AFM imaging technique has been utilized to study a wide range of substances such as DNA, proteins, cells, silicon surfaces, nanowires etc. Hence AFM has become extremely important in the field of biochemistry, cell biology and material science. Functionalizing the AFM tip made it possible to detect molecules and their interaction using recognition imaging at single molecule level. Also the unbinding force of two molecules can be investigated based on AFM based single molecule force spectroscopy.
In the first study, a new chemical approach to functionalize the AFM tip in a simple and user-friendly way has been described. Copper-free click chemistry and a vinyl sulfone PEG linker have been utilized during the process. Using this technique, human thrombin and integrin were detected in separate experiments. Then a novel tri-arm linker with two recognition molecules on it was designed and two proteins (human thrombin and integrin) were detected simultaneously in the same experiment using recognition imaging. This technique can be applied to understand many multivalent interactions taking place in nature. Using the same tri-arm linker functionalized with two biotin molecules, the interaction of streptavidin with mono-biotin and bis-biotin ligands were investigated. The thermal stability of streptavidin-biotin complex was also studied using SDS-PAGE analysis.
In the final study, structure of native chromatin extracted from normal and cancer cell lines were analyzed using AFM imaging and agarose gel electrophoresis. Different salt fractions were used to extract chromatin region depending on their solubility. Mnase sensitivity of the chromatin sample was used to understand the open and closed structures of chromatin from different sources. The amount of chromatin in different salt fractions could act as an indicator of amount of open and condensed chromatin in normal and cancer cells. Eventually this ratio of closed and open structure of chromatin could be an indicator of tumorigenic nature of particular cell lines.
In the first study, a new chemical approach to functionalize the AFM tip in a simple and user-friendly way has been described. Copper-free click chemistry and a vinyl sulfone PEG linker have been utilized during the process. Using this technique, human thrombin and integrin were detected in separate experiments. Then a novel tri-arm linker with two recognition molecules on it was designed and two proteins (human thrombin and integrin) were detected simultaneously in the same experiment using recognition imaging. This technique can be applied to understand many multivalent interactions taking place in nature. Using the same tri-arm linker functionalized with two biotin molecules, the interaction of streptavidin with mono-biotin and bis-biotin ligands were investigated. The thermal stability of streptavidin-biotin complex was also studied using SDS-PAGE analysis.
In the final study, structure of native chromatin extracted from normal and cancer cell lines were analyzed using AFM imaging and agarose gel electrophoresis. Different salt fractions were used to extract chromatin region depending on their solubility. Mnase sensitivity of the chromatin sample was used to understand the open and closed structures of chromatin from different sources. The amount of chromatin in different salt fractions could act as an indicator of amount of open and condensed chromatin in normal and cancer cells. Eventually this ratio of closed and open structure of chromatin could be an indicator of tumorigenic nature of particular cell lines.
ContributorsSenapati, Subhadip (Author) / Lindsay, Stuart (Thesis advisor) / Zhang, Peiming (Thesis advisor) / Ghirlanda, Giovanna (Committee member) / Gould, Ian (Committee member) / Arizona State University (Publisher)
Created2015
Description
A vast amount of energy emanates from the sun, and at the distance of Earth, approximately 172,500 TW reaches the atmosphere. Of that, 80,600 TW reaches the surface with 15,600 TW falling on land. Photosynthesis converts 156 TW in the form of biomass, which represents all food/fuel for the biosphere with about 20 TW of the total product used by humans. Additionally, our society uses approximately 20 more TW of energy from ancient photosynthetic products i.e. fossil fuels. In order to mitigate climate problems, the carbon dioxide must be removed from the human energy usage by replacement or recycling as an energy carrier. Proposals have been made to process biomass into biofuels; this work demonstrates that current efficiencies of natural photosynthesis are inadequate for this purpose, the effects of fossil fuel replacement with biofuels is ecologically irresponsible, and new technologies are required to operate at sufficient efficiencies to utilize artificial solar-to-fuels systems. Herein a hybrid bioderived self-assembling hydrogen-evolving nanoparticle consisting of photosystem I (PSI) and platinum nanoclusters is demonstrated to operate with an overall efficiency of 6%, which exceeds that of land plants by more than an order of magnitude. The system was limited by the rate of electron donation to photooxidized PSI. Further work investigated the interactions of natural donor acceptor pairs of cytochrome c6 and PSI for the thermophilic cyanobacteria Thermosynechococcus elogantus BP1 and the red alga Galderia sulphuraria. The cyanobacterial system is typified by collisional control while the algal system demonstrates a population of prebound PSI-cytochrome c6 complexes with faster electron transfer rates. Combining the stability of cyanobacterial PSI and kinetics of the algal PSI:cytochrome would result in more efficient solar-to-fuel conversion. A second priority is the replacement of platinum with chemically abundant catalysts. In this work, protein scaffolds are employed using host-guest strategies to increase the stability of proton reduction catalysts and enhance the turnover number without the oxygen sensitivity of hydrogenases. Finally, design of unnatural electron transfer proteins are explored and may introduce a bioorthogonal method of introducing alternative electron transfer pathways in vitro or in vivo in the case of engineered photosynthetic organisms.
ContributorsVaughn, Michael David (Author) / Moore, Thomas (Thesis advisor) / Fromme, Petra (Thesis advisor) / Ghirlanda, Giovanna (Committee member) / Redding, Kevin (Committee member) / Arizona State University (Publisher)
Created2014
Description
The AAA+ ATPase Rubisco activase (Rca) regulates the activity of Rubisco, the photosynthetic enzyme responsible for catalyzing biological carbon fixation. However, the detailed mechanism by which Rca self-association controls Rubisco reactivation activity remains poorly understood. In this work, we are using fluorescence correlation spectroscopy (FCS) to better characterize the thermodynamics of the assembly process of cotton Rca. We present FCS data for Rca in the presence of Mg*ATPgS and Mg*ADP and for the D173N Walker B motif mutant in the presence of Mg*ATP. Our data are consistent with promotion and stabilization of hexamers by Mg*ATPgS and Mg*ATP, whereas Mg*ADP facilitates continuous assembly. We find that in the presence of Mg·ADP, Rca self-associates in a step-wise fashion to form oligomeric and higher order forms, with a strong size dependence on subunit concentration. The monomer is the dominant species below 0.5 micromolar, whereas the hexamer appears to be most populated in the 10-30 micromolar range. Large assemblies containing on the order of 24 subunits become dominant above 40 micromolar, with continued assembly at even higher concentrations. Our data are consistent with a highly dynamic exchange of subunits among oligomeric species of diverse sizes. The most likely ADP-mediated assembly mechanism seems to involve the formation of spiral supra-molecular structures that grow along the helical axis by the step-wise addition of dimeric units. To examine the effect of Mg·ATP on oligomerization, we have generated the D173N mutant of Rca, which binds but does not hydrolyze ATP. In range of 8 and 70 micromolar, 60-80% of Rca is predicted to form hexamers in the presence of Mg*ATP compared to just 30-40% with Mg*ADP. We see a clear trend at which hexamerization occurs at high ATP:ADP ratios and in addition, at increasing concentrations of free magnesium ions to 5 milimolar that results in formation of six subunits. We present an assembly model where Mg*ATP promotes and stabilizes hexamerization at low micromolar Rca concentrations relative to Mg*ADP, and suggest that this results from closed ring hexamer formation in Mg*ATP and open hexameric spiral formation in Mg*ADP .
ContributorsKuriata, Agnieszka (Author) / Wachter, Rebekka (Thesis advisor) / Redding, Kevin (Thesis advisor) / Ghirlanda, Giovanna (Committee member) / Ros, Alexandra (Committee member) / Arizona State University (Publisher)
Created2014
Description
Ge1-ySny alloys represent a new class of photonic materials for integrated optoelectronics on Si. In this work, the electrical and optical properties of Ge1-ySny alloy films grown on Si, with concentrations in the range 0 ≤ y ≤ 0.04, are studied via a variety of methods. The first microelectronic devices from GeSn films were fabricated using newly developed CMOS-compatible protocols, and the devices were characterized with respect to their electrical properties and optical response. The detectors were found to have a detection range that extends into the near-IR, and the detection edge is found to shift to longer wavelengths with increasing Sn content, mainly due to the compositional dependence of the direct band gap E0. With only 2 % Sn, all of the telecommunication bands are covered by a single detector. Room temperature photoluminescence was observed from GeSn films with Sn content up to 4 %. The peak wavelength of the emission was found to shift to lower energies with increasing Sn content, corresponding to the decrease in the direct band gap E0 of the material. An additional peak in the spectrum was assigned to the indirect band gap. The separation between the direct and indirect peaks was found to decrease with increasing Sn concentration, as expected. Electroluminescence was also observed from Ge/Si and Ge0.98Sn0.02 photodiodes under forward bias, and the luminescence spectra were found to match well with the observed photoluminescence spectra. A theoretical expression was developed for the luminescence due to the direct band gap and fit to the data.
ContributorsMathews, Jay (Author) / Menéndez, Jose (Thesis advisor) / Kouvetakis, John (Thesis advisor) / Drucker, Jeffery (Committee member) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2011