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Spectroscopic, Electronic, and Mechanistic Studies of Silicon Surfaces Chemically Modified with Short Alkyl Chains
Get PDFThe chemical, structural, and electronic properties of semiconductor surfaces are known to strongly influence the energetics at semiconductor interfaces. Inexpensive and scalable wet chemical modification of semiconductor surfaces provides a means to impart a desired functionality at semiconductor interfaces for the development of new devices based on precise and cost-effective chemistry. This thesis is composed of three studies that focused on identifying the spectroscopic, electronic, and mechanistic properties of reactions at Si surfaces. First, ethynyl- and propynyl-terminated Si(111) surfaces were prepared and characterized by vibrational and photoelectron spectroscopy as well as electrochemical scanning-tunneling microscopy. Ethynyl-terminated Si(111) exhibited ≡C−H, C≡C, Si−C stretching signals and a fractional monolayer (ML) coverage (Φ) of ΦSi−CCH = 0.63 ± 0.08 ML and ΦSi−OH = 0.35 ± 0.03 ML. Propynyl-terminated Si(111) showed (C−H)CH3 bending, Si−C stretching, and C≡C stretching with ΦSi−CCCH3 = 1.05 ± 0.06 ML. Deprotonation of ethynyl-terminated Si(111) surfaces formed a unique surface-bound lithium acetylide that acted as nucleophile. This work provides definitive spectroscopic and microscopic evidence for the covalent attachment of ethynyl and propynyl groups to the Si(111) surface.
Second, Si(111) surfaces were modified with 3,4,5-trifluorophenylacetylene (TFPA) groups to impart a positive dipole at the Si(111) surface. This negative surface dipole provides the necessary band-edge shift at the Si surface to maximize the interface between p-type Si and the proton reduction half reaction. Vibrational and photoelectron spectroscopy provided evidence for the attachment of TFPA groups to the Si(111) surface. Mixed methyl/TFPA monolayers were prepared and characterized using electrochemical and photoelectrochemical methods to show that the band-edge positions and open-circuit voltages were shifted positive with increasing fractional TFPA coverage on the surface. This work demonstrates that monolayer chemistry can be used to manipulate the band-edge positions of Si surfaces as a function of surface composition.
Finally, mechanistic studies of the reaction of liquid methanol with hydride-terminated Si(111) surfaces in the presence of an oxidant were carried out. Vibrational and photoelectron spectroscopy showed that acetylferrocenium, ferrocenium, and dimethyl-ferrocenium could serve as oxidants to promote an increased rate of methoxylation of the H–Si(111) surface in the dark. Illumination of intrinsic and n-type surfaces resulted in an increased rate of methoxylation, resulting from a positive shift in energy of the electron quasi-Fermi level in the presence of light. The results are described in the context of a kinetic charge transfer formalism that is consistent with the experimentally observed results. This work provides a general framework to describe the kinetics of charge transfer reactions that occur on semiconductor surfaces.</p
Nanoscale Thermal Transport with Photons and Phonons
Get PDFRecent progress in nanosciences challenges the conventional understanding of Fourier's law for heat conduction and Planck's law for thermal radiation, calling for theoretical and experimental advancement to improve our understanding at these length scales. Advances in both theoretical and experimental progress at these length scale have been made in the past two decades, but there are still many challenges and possibilities in further understanding how heat conducts or radiates at these length scales.
The first half of this thesis focuses on topics in nanoscale thermal radiation. First, we will discuss an effort to modify thermal emission using a hyperbolic metamaterial (HMM). Recent efforts in utilizing different metamaterial designs to modify thermal emission has led to greater control over the spectral and directional properties of thermal radiation, and the HMM is one such metamaterial. HMM is typically made up of sub-wavelength alternating layers of metal and dielectric that result in an anisotropic permittivity. Here we demonstrate that an annular, transparent HMM lens enables selective controlling of the plasmonic resonance such that a nanowire emitter, surrounded by an HMM, appears dark to incoming radiation from an adjacent nanowire emitter unless the second emitter is surrounded by an identical lens.
While many metamaterial schemes exist to modify thermal emission, these schemes are ultimately limited by the maximum possible emission of a blackbody. In an effort to further increase radiative thermal emission, we made another effort to explore the possibility of removing the enhanced but trapped thermal radiation energy density at sub-wavelength distances. Here, we propose and numerically demonstrate an active scheme that exploits the monochromatic nature of near-field thermal radiation to drive a transition in a laser gain medium, which, when coupled with external optical pumping, allows the resonant surface mode to be emitted into the far-field. We compare this proposed active radiative cooling (ARC) approach to the better-understood laser cooling of solids (LCS) technique, which achieves cooling by extracting phonons instead of thermal radiation. We show that LCS and ARC can be described with the same mathematical formalism and find that ARC can achieve higher efficiency and extracted power over a wide range of conditions.
In the second half of thesis, we switch our attention to nanoscale heat conduction where phonons are the dominant heat carriers. Phonons require a medium to travel, unlike thermal radiation, and thus experience much stronger interaction with the medium. Typical assumptions of many scattering events of phonons at the larger length scales break down at the nanoscale when phonon transport can no longer be accurately described by diffusion theory. Here, we present a numerical modeling effort using the Boltzmann Transport Equation to accurately model nanoscale phonon transport of a recent experiment. We show a calculated trend of pump beam size dependence on thermal conductivity similar to results from the time-domain thermal reflectance (TDTR) experiment. We also identify the radial suppression function that describes the suppression in heat flux, compared to Fourier's law, that occurs due to quasiballistic transport and demonstrate good agreement with experimental data.
While time-domain thermal reflectance (TDTR) experiment is widely used to characterize thermal transport, it is not ideal for in-plane thermal measurements compared to the transient grating (TG) techniques which utilize interference of two beams to create a in-plane grating pattern for thermal measurements. In the last part of my thesis, we highlight details of an experimental effort to develop the ultra-fast transient grating (TG) technique capable of measuring fast thermal decays. We will then highlight the results of thermal and acoustic measurements of molybdenum disulphide that can be obtained from this technique. Our results are in good agreement with other measurements and calculations.
With nanosciences paving way for the future of technology, understanding thermal management at the nanoscale is crucial for device performance and reducing energy waste. We believe that these results in thermal radiation and conduction will benefit thermal management at the nanoscale.</p
Theoretical and Experimental Investigation of Phonon Boundary Scattering in Thin Silicon Membranes
Get PDFThe thermal transport properties of thin semiconductor membranes play an important role in the performance of many technologies like micro-electronics and solid-state energy conversion. The dominant resistance to heat flow in thin membranes is offered by the scattering of thermal phonons at the membrane boundaries. In this dissertation, we examine the nature of microscopic phonon boundary scattering processes and their effect on the thermal conductivity of the thin membranes using a pump-probe experimental technique and computationally efficient solutions of the phonon Boltzmann transport equation (BTE).
First, we investigate the boundary scattering-limited thermal transport in nanostructures using an efficient variance-reduced Monte Carlo (MC) solution of the BTE to elucidate the impact of specular and diffuse phonon boundary scattering events on the thermal conductivity of the nanostructures. To directly measure the relative frequency of these two boundary scattering events, called the phonon specularity parameter, we design, implement and characterize a non-contact laser-based pump-probe experiment called the transient grating (TG) to perform phonon mode-dependent measurements of the specularity parameter in suspended free-standing thin silicon membranes. We describe the phenomenon of quasiballistic heat conduction, which enables the phonon mode-dependent measurements of the specularity parameter, and derive a transfer function based on the BTE with ab-initio phonon properties as inputs, to connect the specularity parameter with the experimentally measured thermal conductivity of the thin membranes.
Finally, we present the methodology adopted to invert the BTE transfer function to extract the phonon specularity parameter from the thermal conductivity measurements in the TG experiment, while rigorously accounting for the experimental uncertainties. We find that the observed magnitudes and trends of the thermal conductivity of the thin membranes cannot be explained by the 50-year old Ziman's model for the phonon specularity parameter and the Fuchs-Sondheimer theory of phonon boundary scattering. We also find that the partially specular boundary scattering picture of phonon boundary interactions works well for one of the membranes, enabling a direct measurement of the mode-dependent phonon specularity parameter for the first time in an experiment. We discuss the possibility of phonon mode conversion at the boundaries of a few membranes for which the partially specular phonon boundary scattering picture fails to explain the observed thermal conductivity trends. Considering the importance of understanding phonon boundary scattering to engineer and improve nanoscale device performance, we expect that the new experimental and computational tools developed in this work will advance a variety of nanoscale energy applications and further our understanding of nanoscale heat transport.</p
Prototyping Diverse Synthetic Biological Circuits in a Cell-Free Transcription-Translation System
Get PDFSynthetic biological circuits are the foundation for the ultimate goals of controlling cells and building artificial cells from the ground up. To get closer to these goals in a more efficient way, we utilize a cell-free transcription-translation system to help perfect biological circuits for the simplicity, freedom, and convenience that the system offers. In this thesis, we demonstrate three distinct aspects of biological circuits in a cell-free transcription-translation system: circuit dynamics, phosphorylation, and membrane proteins. We start with a simple feedforward circuit, which shows dynamic responses to the input. We first prototype the feedforward circuit in the cell-free system with the aid of mathematical modeling. Then, based on the knowledge learned from prototyping, we successfully implement the circuit in cells. Not only do we show that a circuit with dynamics can be prototyped in the cell- free system, but we also test a more complicated circuit involving a phosphorylation cycle. The phosphorylation-based insulator circuit is prototyped and then a model created for the circuit is shown to be identifiable in the cell-free system. To further expand the capability of the cell-free system, we demonstrate that biologically active membrane proteins can be generated in the cell-free system with engineering, suggesting that even biological circuits requiring membrane proteins can be prototyped in the system. These results help advance our knowledge of both biological circuits and the cell-free transcription-translation system, and bring us one step closer to our ultimate goals of implementing control theory in synthetic biology
The Implications of Privacy-Aware Choice
Get PDFPrivacy concerns are becoming a major obstacle to using data in the way that we want. It's often unclear how current regulations should translate into technology, and the changing legal landscape surrounding privacy can cause valuable data to go unused. In addition, when people know that their current choices may have future consequences, they might modify their behavior to ensure that their data reveal less---or perhaps, more favorable---information about themselves. Given these concerns, how can we continue to make use of potentially sensitive data, while providing satisfactory privacy guarantees to the people whose data we are using? Answering this question requires an understanding of how people reason about their privacy and how privacy concerns affect behavior.
In this thesis, we study how strategic and human aspects of privacy interact with existing tools for data collection and analysis. We begin by adapting the standard model of consumer choice theory to a setting where consumers are aware of, and have preferences over, the information revealed by their choices. In this model of privacy-aware choice, we show that little can be inferred about a consumer's preferences once we introduce the possibility that she has concerns about privacy, even when her preferences are assumed to satisfy relatively strong structural properties. Next, we analyze how privacy technologies affect behavior in a simple economic model of data-driven decision making. Intuition suggests that strengthening privacy protections will both increase utility for the individuals providing data and decrease usefulness of the computation. However, we demonstrate that this intuition can fail when strategic concerns affect consumer behavior. Finally, we study the problem an analyst faces when purchasing and aggregating data from strategic individuals with complex incentives and privacy concerns. For this problem, we provide both mechanisms for eliciting data that satisfy the necessary desiderata, and impossibility results showing the limitations of privacy-preserving data collection.</p
Defining Gravity: Effective Field Theory, Entanglement, and Cosmology
Get PDFMany of the most exciting open problems in high-energy physics are related to the behavior and ultimate nature of gravity and spacetime. In this dissertation, several categories of new results in quantum and classical gravity are presented, with applications to our understanding of both quantum field theory and cosmology.
A fundamental open question in quantum field theory is related to ultraviolet completion: Which low-energy effective field theories can be consistently combined with quantum gravity? A celebrated example of the swampland program---the investigation of this question---is the weak gravity conjecture, which mandates, for a U(1) gauge field coupled consistently to gravity, the existence of a state with charge-to-mass ratio greater than unity. In this thesis, we demonstrate the tension between the weak gravity conjecture and the naturalness principle in quantum field theory, generalize the weak gravity conjecture to multiple gauge fields, and exhibit a model in which the weak gravity conjecture solves the standard model hierarchy problem. Next, we demonstrate that gravitational effective field theories can be constrained by infrared physics principles alone, namely, analyticity, unitarity, and causality. In particular, we derive bounds related to the weak gravity conjecture by placing such infrared constraints on higher-dimension operators in a photon-graviton effective theory. We furthermore place bounds on higher-curvature corrections to the Einstein equations, first using analyticity of graviton scattering amplitudes and later using unitarity of an arbitrary tree-level completion, as well as constrain the couplings in models of massive gravity. Completing our treatment of perturbative quantum gravity, outside of the swampland program, we also reformulate graviton perturbation theory itself, finding a field redefinition and gauge-fixing of the Einstein-Hilbert action that drastically simplifies the Feynman diagram expansion. Furthermore, our reformulation also exhibits a hidden symmetry of general relativity that corresponds to the double copy relations equating gravity amplitudes to sums of squares of gluon amplitudes in Yang-Mills theory, a surprising correspondence that yields insights into the structure of quantum field theories.
Moving beyond perturbation theory into nonperturbative questions in quantum gravity, we consider the deep relation between spacetime geometry and properties of the quantum state. In the context of holography and the anti-de Sitter/conformal field theory correspondence, we test the proposed ER=EPR correspondence equating quantum entanglement with wormholes in spacetime. In particular, we demonstrate that the no-cloning theorem in quantum mechanics and the no-go theorem for topology change of spacetime are dual under the ER=EPR correspondence. Furthermore, we prove that the presence of a wormhole is not an observable in quantum gravity, rescuing ER=EPR from potential violation of linearity of quantum mechanics. Excitingly, we also prove a new area theorem within classical general relativity for arbitrary dynamics of two collections of wormholes and black holes; this area theorem is the ER=EPR analogue of entanglement conservation. We next turn our attention to the emergence of spacetime itself, placing consistency conditions on the proposed correspondence between anti-de Sitter space and the Multiscale Entanglement Renormalization Ansatz, a special tensor network that constitutes a computational tool for finding the ground state of certain quantum systems. Further examining the role of quantum entanglement entropy in the emergence of general relativity, we ask whether there is a consistent microscopic formulation of the entropy in theories of entropic gravity; we find that our results weaken equation-of-state proposals for entropic gravity while strengthening those more akin to holography, guiding future investigation of theories of emergent gravity.
Finally, we examine the consequences of the Hamiltonian constraint in classical gravity for the early universe. The Hamiltonian constraint allows for the Liouville measure on the phase space of cosmological parameters for homogeneous, isotropic universes to be converted into a probability distribution on trajectories, or equivalently, on initial conditions. However, this measure diverges on the set of spacetimes that are spatially flat, like the observable universe. In this thesis, we derive the unique, classical, Hamiltonian-conserved measure for the subset of flat universes. This result allows for distinction between different models of cosmic inflation with similar observable predictions; for example, we find that the measure favors models of large-scale inflation, as such potentials more naturally produce the number of e-folds necessary to match cosmological observations.</p
Impact of Climate and Tectonics on the Morphodynamics of Alluvial Piedmonts, Implications for Sediment Transfer and the Stratigraphic Record
Get PDFAlluvial piedmonts encircle most of the Earth mountain ranges. The erosion product of these mountains must cross the piedmont domain before reaching the basins where they can enter the sedimentary record. The flux of sediment transfers environmental signals, e.g. tectonics and climate, from source to sink, their preservation is critical for the sedimentary record. Alluvial piedmonts are very reactive to external and internal forcing often incise by tens to hundreds of vertical meters over a few thousand years only to subsequently aggrade by roughly the same amount. In my thesis, I set to study the morphodynamics of alluvial piedmonts in two areas, Death Valley, California, and the Eastern Tian Shan.
In Death Valley I show that cycles of aggradation and incision repeatedly bury active fault scarps and that leads to the accumulation of tectonic slip until the next incision episode when a tall waterfall is released in the catchment. This process links the release of tectonic offset to climatic periods and it also accounts for many unexplained 10 to 30 m waterfalls in the Death Valley area. In the Eastern Tian Shan, I establish that the northern alluvial piedmont, that is incised by 100 to 300 m in the Holocene, undergoes repeated cycles of aggradation and incision driven by fluctuations in Westerlies moisture and insolation at a 21 kyr period without the need for monsoonal moisture to drive the system. The varying extent of glacial overprint in the Eastern Tian Shan is responsible for the morphological contrast between the incised north and aggraded south piedmonts. I produce more terrace abandonment ages that show that all rivers of the northern piedmont incised since the last deglaciation but did so out of sync because of the geometry of the upstream glaciated valleys, and internal feedbacks with alluvial valley walls during river entrenchment. With new luminescence ages sampled in the stratigraphy I establish that sediments from the last 0.5 Myr are remobilized in the modern sediment flux by the alluvial incision. I develop a numerical tool that predicts locally the probability of sediment mixing based on the essential length- and timescales of the individual rivers of an alluvial piedmont.</p
Analysis of Packaging and Deployment of Ultralight Space Structures
Get PDFThis thesis presents a new approach to modeling in finite element analysis (FEA) creased thin-film sheets such as those used for drag sails, as well as modeling the packaging behavior of coilable deployable booms. This is highly advantageous because these deployable space structures are challenging to test on the ground due to their lightweight nature and the effects of gravity and air resistance. Such structures are utilized in the space industry due to their low mass and ability to be packaged into a small volume during their launch into space.
It is shown that removing the crease bending stiffness in creased sheets still allows the deployment behavior of a benchmark problem to be captured, including deployment forces and equilibrium configurations. In addition, folding creased sheets from a flat state into a packaged configuration can result in crease crumpling and excessive wrinkling. To avoid this the Momentless Crease Force Folding (MCFF) technique is developed.
Further presented is the behavior of tape springs and Tubular Rollable and Coilable (TRAC) booms when coiled to radii greater than their natural bend radius. Under these conditions the booms can form multiple localized folds which may jam during boom deployment. Understanding this behavior is important for extending the use of these booms to large scale space structures such as orbital solar power stations.
A useful analytical model is developed determining when the localized folds in a tape spring will bifurcate and is verified against simulation results. Additionally, a numerical model of the wrapping an isotropic tape spring around a hub with a radius greater than the localized fold radii is validated against physical experiments. This model is used to predict trends in the force required to fully wrap a tape spring around a given hub radii.
Finally, when examining the coiling and uncoiling behavior of TRAC booms it was found that the tension force required to keep a TRAC boom tightly coiled is significantly less than the force required to initially coil the boom.</p
High-Efficiency Solar Fuel Devices: Protection and Light Management Utilizing TiO2
Get PDFGlobal climate change coupled with increasing global energy consumption drives the need for renewable and carbon-neutral alternatives to fossil fuels. Photoelectrochemical devices store solar energy in chemical bonds, and have the potential to provide cost-effective fuel for grid-scale energy storage as well as to serve as a feedstock for the production of carbon-neutral transportation fuels. A widely recognized goal is the demonstration of a monolithically-integrated solar-fuels system that is simultaneously efficient, stable, intrinsically safe, and scalably manufacturable. This thesis presents the development of three separate high-efficiency solar fuel devices protected by thin films of amorphous TiO2, and develops light management strategies to increase the performance of these devices.
First, high-efficiency monolithic cells were designed to perform solar water-splitting and CO2 reduction. These designs are driven by high-quality single-crystalline III-V semiconductors that are unstable when placed in direct contact with aqueous electrolytes but can be protected against corrosion by hole-conducting amorphous films. Experimental fabrication and characterization of this tandem device was realized in the form of a fully-integrated water-splitting prototype with a solar-to-hydrogen efficiency of 10% showing stability for over 80 hours of operation. This was followed by the demonstration of water-splitting and CO2 reduction devices enabled by bipolar membranes, which increased stability and alleviated materials-compatibility constraints by creating a pH difference between the anolyte and catholyte, maintained at steady-state. Finally, universal light management strategies were developed using high-aspect-ratio TiO2 nanocones, resulting in an increase in catalyst loading with ultrahigh broadband transmission.</p
Insights on the Formation and Fate of Organic Nitrates in the Atmosphere from Field and Laboratory Observations
Get PDFAlkenes are oxidized rapidly in the atmosphere by addition of OH and subsequently O2, leading to the formation of peroxy radicals. These peroxy radicals react with NO to form organic nitrates through a minor radical-terminating branching pathway. Over large regions of the continental boundary layer, the formation of organic nitrates control tropospheric ozone and the lifetime of NOx. Laboratory investigations described herein show that the yield of nitrates through this pathway is larger than previously described for alkenes, and the yield increases with the number of heavy atoms. This result is used to interpret field observations taken over Houston in the summer of 2013. These measurements show that small alkenes still play a large role in ozone production more than a decade after they had been identified as a causal factor.
In further studies, measurements of isoprene hydroxy nitrates (ISOPN) and hydroperoxides, formed from the OH oxidation of isoprene, are used to diagnose the complexities of reversible O2 addition for allylic hydroxy isoprene radicals. It is shown that over most of the atmosphere, isoprene's peroxy radical isomers are in their equilibrium distribution. In this regime, hydroxy peroxy radical isomers comprise approximately 95% of the radical pool, a much higher fraction than in the nascent (kinetic) distribution. Intramolecular H-shift isomerization from the Z hydroxy peroxy radical isomers produced from OH addition to C4 is estimated to be 4s-1 at 297K. While the Z isomer is initially produced in low yield, it is continually reformed via decomposition of the hydroxy peroxy radicals. As a result, unimolecular chemistry from this isomer contributes as much as half of the atmospheric fate of the entire pool of peroxy radicals formed via addition of OH at C4. In contrast, unimolecular chemistry following OH addition at C1 is slower and less important.
Field observations of alkyl nitrates over the Southeastern United States during the summer over forested environments show that there are still gaps in our understanding of the organic nitrate budget. The formation of isoprene hydroxy nitrates (ISOPN) is shown to be a dominant NOx loss pathway during the day