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Experimental and Theoretical Studies of Non-Equilibrium Systems: Motor-Microtubule Assemblies and the Human-Earth System
Get PDFSystems out of equilibrium are pervasive around us. In fact, being out of equilibrium is a key property of life, as described by Erwin Schrodinger in his series of essays "What is life?". Through the consumption of energy, i.e. food, living organisms achieve ordered states that would be very unlikely to occur at equilibrium, such as the mitotic spindle during cell division, swarms of bacteria, or flocks of starlings. The Earth system is another example of a non-equilibrium system. The state of the Earth has been evolving for billions of years, often under the influence of life. Today, humanity is a dominant influence forcing the Earth system to new states. Understanding these non-equilibrium systems has posed many challenges; in this thesis, we work towards quantitatively dissecting and gaining an intuition for the functioning of both a molecular scale and planetary scale non-equilibrium system.
Underlying many cellular functions such as cell division and transportation of organelles is the cytoskeleton composed of motor proteins and their constituent filaments. One of the key components are kinesin motors, which consume chemical energy to walk along and reorganize microtubules. Collections of these motors and microtubules are able to form organized structures. Understanding how these structures are formed has remained an open question. In Chapter 2, we develop a system of kinesin motors and microtubules wherein motor activity is controlled by light, thereby gaining spatiotemporal control over the formation of motor-microtubule assemblies. We demonstrate the creation of a variety of structures of different sizes and geometry, and measure how length and time scales of these assemblies depend on the activated region.
A remaining question was how the microscopic details of the interaction between motors and microtubule affect the dynamics and steady-state structure formed. With our scheme for light-control in hand, we extended the system to a variety of motor proteins that have different speeds, processivities (how many steps they take before unbinding from the microtubule), directionalities (which end of the microtubule they walk towards), and forces they are able to exert in Chapter 3. We found that the size of steady-state structures, distribution of motors within assemblies, and rate of contraction of networks depend on motor properties. Further, we demonstrate that various structures can be formed by combining different motors. This work begins to build a connection between the detailed microscopic interactions of cytoskeletal components to the larger scale structures they form.
Chapter 4 begins our work on understanding the state of the human-Earth system. A major hurdle to quantitatively understanding this system is the difficulty of finding and parsing the relevant data, which is often within long, complicated reports. In order to facilitate access to this data, we created the Human Impacts Database, which houses a collection of 300 carefully curated values related to human impacts on the Earth, introduced in Chapter 4. In this chapter, we describe the format of the database as well as demonstrate how it can be harnessed to gain a more holistic perspective on humanity's influence on the Earth.
Having this data is only a starting point towards deciphering the ways that humans are altering the state of the Earth, though. In Chapter 5, we combine these quantitative measurements with simple order-of-magnitude estimates to gain an intuition for the magnitude of several of the values. In this way, we show that many of the ways humanity is affecting the Earth can be tied back to how much land, water, and power we use. We further contextualize the magnitude of human influence by comparing human activities to natural analogs, finding that humans currently rival natural processes in influencing the state of the Earth system.</p
Spectroscopic Characterization of Electronic and Magnetic Relaxation Phenomena in Molecular Systems
Get PDFThe thesis herein describes the application of time-resolved spectroscopic techniques to the understanding of a variety of electronic and magnetic relaxation phenomena in molecular systems. Chapter I presents the techniques and theory behind transient absorption spectroscopy and electron paramagnetic resonance spectroscopy, which are two tools that are used throughout the thesis. Chapter II recounts the study of singlet fission in a series of bipentacene dipyridyl pyrrolides, including HDPP-Pent, Li₂(DPP-Pent)₂, and KDPP-Pent. Using transient absorption and kinetic modeling, we found that deprotonation and metal coordination induced a change in the rate of singlet fission (~7 fold increase going from HDPP-Pent to Li₂(DPP-Pent)₂) and ultimate triplet yield. Chapter III details the study of the temperature-dependent magnetic relaxation studies of S = ½ spin systems copper (II) phthalocyanine (CuPc) and vanadyl phthalocyanine (VOPc). Although the spin-lattice relaxation time (T₁) of CuPc is greater than that of VOPc at low temperatures (<30 K), the CuPc T₁’s decline more substantially with temperature than those of VOPc, which we attribute to the increased spin-orbit coupling constant of Cu over V. Ultimately, the phase memory times (T₂) are T₁-limited in CuPc by 150 K, whereas room temperature coherence is observed in VOPc. In Chapter IV, 2,9-dialkyl substituted 1,10-phenanthroline complexes of Cu(I) are studied computationally to assign entatic energies to the steric contributions attributed to the ligand that dictate the electrochemical and photophysical properties of the complexes. We performed experimental validation of reduction potential, low-temperature emission bandwidth and excited state relaxation energies, and ³MLCT lifetimes to support the computational work. In Chapter V, we present ongoing work toward the characterization of triplet and triplet pair states generated via singlet fission in HDPP-Pent, Li₂(DPP-Pent)₂, and KDPP-Pent by time-resolved electron paramagnetic resonance spectroscopy in collaboration with Drs. Jens Niklas and Oleg Poluektov. Finally, in Chapter VI, we present data collected toward the photophysical characterization of a series of Ni(II) 2,2’-bipyridine aryl halide complexes synthesized by David Cagan, which are relevant for photochemical transformations. We provide supporting materials for Chapters II, III, and V in Appendices A, B, and C, respectively
Photovoltaic Technologies Developed for Space-Based Solar Power
Get PDFIn this thesis, photovoltaic technologies were developed for space-based solar power. Two methods of realizing SBSP were introduced, namely concentrated photovoltaics (CPV) and radiation hard flat panel photovoltaics. Both techniques are instrumental to realizing SBSP as they are pathways to realizing high specific power and lower launch costs. Technologies developed to support these two forms of SBSP were then reported.
In support of CPV, ultralight broadband mid-infrared coatings were developed for the concentrating mirrors used in our project. This was done to create radiative pathways for heat loss to ensure that the solar cells do not overheat. Using the rigorous coupled wave analysis technique, we optimized a backside single-layer coating using 2nm Cr/ 2μm CP1/ 500nm Ag that had an mIR emissivity of 0.6. Adding a second layer of this coating, we predicted that a 0.5nm Cr/ 1.9µm CP1/ 3nm Cr/ 2µm CP1/ 500nm Ag screen could achieve an emissivity of 0.8. We also optimized a 10nm ITO/ 2 μm CP1/ 500nm Ag frontside emitter which had a visible reflectivity of 0.896 and a mIR emissivity of 0.554. A backside emitter coating that was 0.927 emissive in the mIR with areal density 6.0 gm⁻² was successfully fabricated, as was a frontside mirror emitter coating with visible reflectivity of 0.896 and a mIR emissivity of 0.582 with areal density 4.1 gm⁻².
In support of radiation hard photovoltaics, organo-lead halide perovskites (OHLP) were investigated. Challenges facing their fabrication were explored, with special focus on the electron transport layer PCBM as well as OHLP formulation. It was found that doping PCBM with a surfactant CTAB was beneficial, but did not work with all surfaces. An ITO/NiOx/MAPbI3/CTAB+PCBM/Cu device with in-house champion efficiency of 12.41% was achieved, and an ITO/NiOx/FA0.85Cs0.15PbI3/PCBM/Cu device with in-house champion efficiency of 11.81% was achieved. Time-dependant drift diffusion modelling was employed to account for the S-kink arising from poor PCBM carrier concentration.
Finally, the proton degradation of OHLP devices and constituent transport layers were investigated to shed better light on how OHLP devices degrade under proton irradiation. Films of ITO, PEDOT, NiOx, PCBM, and PTCDi were found to degrade under 30keV and 75keV protons of up to 1.4 x 10¹⁴ p⁺cm⁻² fluence, but their electrical resistivity and optical transmissivity were not found to impact the cell as much as the OHLP absorber layer itself. Observing the light IV and EQE degradation of OHLP cells, it is evident that proton deposition in the OHLP layer itself causes the most damage, especially at 30keV and 75keV protons with fluences from 4.3 x 10¹³ p⁺cm⁻² to 1.7 x 10¹⁴ p⁺cm⁻². By considering the discrepancy in trends between Jsc and EQE, we concluded that the protons much accelerate intensity-based metastable photodegradation. Finally, by observing their anneal recovery, we concluded that it was temperature dependant and that maximum irrecoverable damage occurs at the OHLP/HTL interface.</p
The California Legacy Survey: a Three-Decade Census of Extrasolar Planets
Get PDFTaking an accurate census of planets orbiting other stars, otherwise known as exoplanets, is a crucial step toward understanding the nature of planet formation and placing Earth and the Solar System in a broader context. For my thesis, I present the culmination of a three-decade a high-precision radial velocity (RV) survey of 719 FGKM stars, known as the California Legacy Survey (CLS), and use this survey to perform a statistical study of exoplanets. I developed computational methods for planet search and RV sensitivity characterization, and detected 164 known exoplanets and 14 newly discovered or revised exoplanets and substellar companions in the CLS. I used this star and planet catalog to measure the occurrence rates of several exoplanet subtypes and probe formation pathways. I found that giant planet occurrence is greatly enhanced beyond 1 Earth-Sun distance (au), then decreases beyond 8 au. This implies that giant planet formation is much easier beyond the water-ice line of most stars, possibly due to greater ease of solid coagulation and pebble accretion, and eventually decreases with the density profile of of protoplanetary disks. It also means that Jupiter and Saturn are located in the orbital space of greatest giant planet occurrence, making the Solar System typical in giant placement. I then investigated the relationship between small close-in planets and cold outer giants, and found that not all giants host inner rocky companions. Rather, giants under about one-third of a Jupiter mass or beyond 3 au are more likely to have small companions than their warmer and more massive counterparts. Finally, I compared single giant and multi giant planetary systems. I performed novel characterizations of the orbital eccentricity distributions of these two populations, and discovered that multi-giant stellar hosts are on average significantly more metal-rich than single-giant hosts. This means that the Sun with its two giant planets is atypical among much more metal-rich giant hosts. I also found that lonely giants present a pile-up of `super-hot Jupiters' within 0.06 au not shared by neighborly giants, and that giants orbiting the same star tend to have similar masses. Taken together, these findings are a substantial leap forward in understanding the architectures and origins of planetary systems
Self-Heating of HEMT Low-Noise Amplifiers in Liquid Cryogenic Environments and the Limits of Microwave Noise Performance
Get PDFDetection and processing of microwave signals is of substantial scientific importance in fields ranging from radio astronomy to quantum computing. An essential component of the signal processing chain is the microwave amplifier, which adds gain to the signal so that it may be processed by subsequent microwave components. However, the amplifier itself adds its own internally generated noise into the measurement chain. As a result, amplifiers which add a minimal amount of noise are crucial to any high precision measurement scheme. A device which is commonly employed for this task is the high-electron-mobility transistor (HEMT) amplifier. Understanding the fundamental limits of the microwave noise performance of HEMT amplifiers is highly desirable. Noise temperatures in these devices as low as 3 times the quantum limit have been observed in the last decade, but the lack of understanding of the origin of the excess noise has hindered further improvements. Noise in HEMTs is attributed to a generator at the output, known as drain noise; and a generator at the input, which is attributed to thermal noise of the gate. At cryogenic temperatures of ~4 K, thermal noise is predicted to be negligible. However, a plateau in noise temperature has been observed at physical temperatures below ~20 K, with a negligible improvement in noise performance upon further cooling.
The primary noise mechanism responsible for this plateau is believed to be ohmic heating of the HEMT structure induced by current in the active device channel, a process known as self-heating. At room temperature the ambient thermal noise dominates the amplifier’s overall noise performance, but at the cryogenic temperatures required to achieve low-noise performance the self-heating effect produces thermal noise at the input of the HEMT gate which contributes significantly to the total noise. A potential mechanism to mitigate self-heating is to provide an additional thermal dissipation path for the Joule heating in the channel. However, given the sub-micron length scales and buried gate structure of HEMTs, thermal management is challenging. The primary heat conduction pathway, that of phonons travelling through the bulk HEMT substrate, decreases rapidly in magnitude at cryogenic temperatures. An alternative option is to submerge the HEMT in a cryogenic fluid, thereby presenting an alternate thermal conduction route through the HEMT surface into the fluid. This technique, while commonly employed in cryogenic thermal management of superconducting magnets, has not been investigated for HEMTs.
In this work we explore the use of liquid cryogenic cooling to directly mitigate the effect of HEMT self-heating. We test in particular the effectiveness of cooling using superfluid helium-4, which has the highest known thermal conductivity of any known substance. We report a systematic experimental investigation of the noise performance of a cryogenic packaged two-stage HEMT low-noise amplifier over a wide range of biases in a 4.0 - 5.5 GHz frequency band, with the device immersed in a variety of cryogenic baths including helium-4 vapor, liquid helium-4, superfluid helium-4, and vacuum. We present the details of the experimental apparatus which was constructed to perform microwave noise measurements of the low-noise amplifier when submerged in a liquid cryogen environment. We interpret our results using a small-signal model of the amplifier and compare our findings with the predictions of a phonon radiation model of heat dissipation. We find that liquid cryogenic cooling is unable to mitigate the thermal noise associated with self-heating. Considering this finding, we examine the implications for the lower bounds of cryogenic noise performance in HEMTs by incorporating the effects of self-heating into the existing noise modelling of HEMT amplifiers. Our analysis supports the general design principle for cryogenic HEMTs of maximizing gain at the lowest possible power.</p
Fragment Coupling Approach To C₁₉- AND C₂₀-Diterpenoid Alkaloids: Total Synthesis of (–)-Talatisamine, (–)-Liljestrandisine, and (–)-Liljestrandinine
Get PDFA unified, convergent fragment coupling approach to the C₁₉- and C₂₀-diterpenoid alkaloid natural products is presented. The highly-caged aconitine, denudatine, and napelline cores are disconnected through the central B-ring cyclohexane to an A/E/F-ring fragment common to the structures all three subfamilies. 1,2-addition of an appropriate organometallic C/D-bicycle to an A/F-ring hydrindane epoxy ketone fragment followed by a Lewis acid-catalyzed semipinacol reaction couples the two fragments together and sets a key all-carbon quaternary center at C11. This strategy is realized in the synthesis of the C₁₉-aconitine core by using a [3.2.1]-bicyclooctene C/D-fragment as the nucleophile in the 1,2-addition. This C/D-fragment is prepared using a meta-photocycloaddition; this represents an alternative approach to the commonly employed biomimetic Wagner-Meerwein rearrangement of a [2.2.2]-bicyclooctane.
To complete the aconitine core, a radical cyclization cascade to form the E-ring piperidine and B-ring cyclohexane in a single step is investigated. N-centered radicals were evaluated to initiate the cascade via a 6-exo-trig cyclization. A neutral aminyl radical gave rise to an unexpected Hoffman-Löffler-Freytag type product resulting from 1,5-hydrogen atom transfer. Employing Lewis-acidic single electron reducing metal catalyst led to formation of the E-ring cyclized product, however the second cyclization to close the B-ring did not occur.
As an alternative approach, the E-ring was closed via an intramolecular aziridination. Treatment of this aziridine with acetyl bromide results in an aziridine-opened alkyl bromide product. This alkyl bromide is used as a functional group handle to form the final ring of the aconitine core. From there, the total synteheses of the C₁₉-diterpenoid alkaloids (–)-talatisamine, (–)-liljestrandisine, and (–)-liljestrandinine were completed in short order. These synthetic efforts led to revision of the proposed structure of (–)-liljestrandisine.</p
Cascading Failures in Power Systems: Modeling, Characterization, and Mitigation
Get PDFReliability is a critical goal for power systems. Due to the connectivity of power grids, an initial failure may trigger a cascade of failures and eventually lead to a large-scale blackout, causing significant economic and social impacts. Cascading failure analysis thus draws wide attention from power system practitioners and researchers. A well-known observation is that cascading failures in power systems propagate non-locally because of the complex mechanism of power grids. Such non-local propagation makes it particularly challenging to model, analyze and control the failure process. In this thesis, we tackle these challenges by establishing a mathematical theory to model and characterize failure patterns, discover structural properties of failure propagation, and design novel techniques for failure mitigation.
First, we propose a failure propagation model considering both fast-timescale system frequency dynamics and the slow-timescale line tripping process. This model provides mathematical justifications to the widely used static DC model and can be generalized to capture a variety of failure propagation patterns induced by different control mechanisms of the power grid. More importantly, this model provides flexibility to design real-time control algorithms for failure mitigation and localization.
Second, we provide a complete characterization of line failures under the static DC model. Our results unveil a deep connection between the power redistribution patterns and the network block decomposition. More specifically, we show that a non-cut line failure in a block will only impact the branch power flows on the transmission lines within the block. In contrast, a cut set line failure will propagate globally depending on both the power balancing rules and the network topological structure. Further, we discuss three types of interface networks to connect the sub-grids, all achieving better failure localization performance.
Third, we study corrective control algorithms for failure mitigation. We integrate a distributed frequency control strategy with the network block decomposition to provide provable failure mitigation and localization guarantees on line failures. This strategy operates on the frequency control timescale and supplements existing corrective mechanisms, improving grid reliability and operational efficiency. We further explore the failure mitigation approach with direct post-contingency injection adjustments. Specifically, we propose an optimization-based control method with strong structural properties, which is highly desirable in large-scale power networks.</p
Swimming in Potential Flow
Get PDFActive bodies undergo self-propulsive motion in a fluid medium and span a broad range of length and time scales. This report focuses specifically on the motion at high Reynolds number, where inertial forces dominate the fluid dynamics. Many active systems spontaneously self-organize into visually striking structures: fish schooling, birds flocking, and bacterial colonies growing. Current models of emergent behavior in the inertial regime are mainly phenomenological and do not account for the fluid-mediated interactions between bodies. We seek to advance physical models of swimmers in high inertia environments. To this end, we explicitly model the hydrodynamics to discern what role the fluid medium plays in active group dynamics and whether it can reproduce the observed emergent phenomenon without the imposition of phenomenologically based interaction rules.
A minimal swimmer model consisting of three linked spheres is constructed, and we find self-propulsion without external forces or momentum transfer via vortex shedding. The inertial swimmer is also compared to an identical swimmer in the Stokes regime---where fluid inertia is neglected. The Stokes hydrodynamics are longer-ranged at leading order, and we demonstrate that the stronger hydrodynamic interactions lead to a greater center of mass translation after a period of articulation.</p
Progress in Low-Cost Gallium Arsenide Solar Cells
Get PDFIn order to prevent disastrous global warming the manufacturing capacity of renewable energy power sources must grow rapidly. Solar photovoltaics will likely be one of humanity's main sources of energy in the future due to the enormous available resource but increasing the manufacturing capacity of solar panels is hamstrung by both the limited profit margins of the highly-competitive renewable energy market and the enormous capital cost of building the factories that convert sand into semiconductor-grade silicon. Gallium arsenide is a material that can potentially help with the capital bottleneck because it absorbs light much more strongly than silicon and so the capital cost per unit weight of making the semiconductor can be spread over a larger number of devices and therefore effectively reduced. We present a number of results aimed at enabling low capital-cost GaAs solar cell manufacturing. First is a technique for open-tube, vapor phase zinc diffusion in GaAs. This method is dramatically simpler than its historical counterparts. Second, we use this technique to fabricate solar cells with Voc's greater than 960 mV and uncertified efficiencies over 23%, large improvements over the state of the art. We further demonstrate a base-metal, air-tolerant ohmic contact to n-type GaAs which is an improvement over traditional contacts that require noble metals and inert atmospheres. We also found the existence of melt-grown n-type GaAs with minority carrier diffusion length comparable to vapor grown material which helps with the economic viability of these devices. We also performed a technoeconomic analysis on our proposed devices and find that they satisfy the desired properties of both the capital and electricity being cheaper than silicon solar cells. We also demonstrate the first n-on-p diffused junction GaAs solar cells.
As a parallel path to low capital intensity GaAs solar cells we also investigated non-epitaxial heterojunction devices. In the course of this work we both developed and characterized passivation chemistries for GaAs. Results in include the first use of a carbene and dithiothreitol for GaAs passivation and achieving surface recombination velocities comparable to GaInP passivation. With passivated organic heterojunction solar cells we were able to achieve a Voc of 840 mV which is a record for this class of devices, but its unclear how to improve the result to make them competitive with diffused junctions.
We also explored nanowire solar cells as an alternative strategy to reducing material usage by exploiting their strong light-absorption. We developed a computational model for a non-epitaxial GaAs heterojunction nanowire solar cell and predict an optimized efficiency over 30%. Towards fabrication we used metal-assisted-chemical-etching to make nanowire arrays and found we were able to cleanly cleave the nanowires embedded in a polymer from a 110 oriented wafer.
We also share some preliminary work on using total internal reflection in a solar cell encapsulant to mitigate shading loss due to the contacts on the front of a solar cell. We developed a computational model arguing that these structures could increase energy yield by 8% and demonstrated proof-of-principle experiments.
Finally, we share work on designing solar cells for operation on Venus. We developed models for the optical properties and recombination that correctly model the temperature dependence of a reference solar cell and using that model predict that a GaInP single-junction solar cell is a good solar-cell design for general usage in the atmosphere.</p
A Fluvial Perspective on the Role of Sulfide Oxidation in the Global Carbon Cycle
No full textThis thesis contains an introduction and five studies on the role of sulfide oxidation in regulating the relationship between chemical weathering and climate. Chapter I provides an overview of the key question — what stabilizes the global carbon cycle? — and motivates the specific questions addressed in each subsequent chapter. Chapter II describes a new numerical tool for inverting observations of dissolved river chemistry for information about chemical weathering reactions. Chapters III, IV, and V describe observations from field sites in Iceland, Nepal, and Alaska that address the fidelity of fluvial sulfur isotope ratios, the role of sulfide oxidation in Cenozoic cooling, and sulfur dynamics within permafrost landscapes, respectively. Using the tools and lessons of the preceding chapters, Chapter VI quantifies the impact of global chemical weathering on the concentration of atmospheric carbon dioxide