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• 61. [Article] Electron Transport Dynamics in Semiconductor Heterostructure Devices

  Modern semiconductor fabrication techniques allow for the fabrication of semiconductor heterostructures which host electron transport with a minimum of scattering sites. In such devices, electrons populate ...

  Citation × Citation Citation Abstract Copy Title: Electron Transport Dynamics in Semiconductor Heterostructure Devices Author: Pilgrim, Ian Year: 2014 Modern semiconductor fabrication techniques allow for the fabrication of semiconductor heterostructures which host electron transport with a minimum of scattering sites. In such devices, electrons populate a two-dimensional electron gas (2DEG) in which electrons propagate in exactly two dimensions, and may be further confined by potential barriers to form electron billiards. At sub-Kelvin temperatures, electron trajectories are determined largely by reflections from the billiard walls, while net conduction through the device depends on quantum mechanical wave interference. Measurements of magnetoconductance fluctuations (MCF) serve as a probe of dynamics within the electron billiard. Many prior studies have utilized heterostructures employing the modulation doping architecture, in which the 2DEG is spatially removed from the donor atoms to minimize electron scattering. Theoretical studies have claimed that MCF will be fractal when the confinement potential defining the billiard is soft-walled, regardless of the presence of smooth potentials within the billiard such as those introduced by remote ionized donors. The small-angle scattering sites resulting from these potentials are often disregarded as negligible; we use MCF measurements to investigate such claims. To probe the effect of remote ionized donor scattering on the phase space in electron billiards, we compare MCF measured on billiards in a modulation-doped heterostructure to those measured on billiards in an undoped heterostructure, in which this potential landscape is believed to be absent. Fractal studies are performed on these MCF traces, and we find that MCF measured on the undoped billiards do not exhibit measurably different fractal characteristics than those measured on the modulation-doped billiards. Having confirmed that the potential landscapes in modulation-doped heterostructures do not affect the electron phase space, we then investigate the effect of these impurities on the distribution of electron trajectories through the billiards. By employing thermal cycling experiments, we demonstrate that this distribution is highly sensitive to the precise potential landscape within the billiard, suggesting that modulation-doped heterostructures do not support fully ballistic electron transport. We compare our MCF correlation data with the dynamics of charge transfer within heterostructure systems to make qualitative conclusions regarding these dynamics. Title: Electron Transport Dynamics in Semiconductor Heterostructure Devices Author: Pilgrim, Ian Year: 2014 Modern semiconductor fabrication techniques allow for the fabrication of semiconductor heterostructures which host electron transport with a minimum of scattering sites. In such devices, electrons populate a two-dimensional electron gas (2DEG) in which electrons propagate in exactly two dimensions, and may be further confined by potential barriers to form electron billiards. At sub-Kelvin temperatures, electron trajectories are determined largely by reflections from the billiard walls, while net conduction through the device depends on quantum mechanical wave interference. Measurements of magnetoconductance fluctuations (MCF) serve as a probe of dynamics within the electron billiard. Many prior studies have utilized heterostructures employing the modulation doping architecture, in which the 2DEG is spatially removed from the donor atoms to minimize electron scattering. Theoretical studies have claimed that MCF will be fractal when the confinement potential defining the billiard is soft-walled, regardless of the presence of smooth potentials within the billiard such as those introduced by remote ionized donors. The small-angle scattering sites resulting from these potentials are often disregarded as negligible; we use MCF measurements to investigate such claims. To probe the effect of remote ionized donor scattering on the phase space in electron billiards, we compare MCF measured on billiards in a modulation-doped heterostructure to those measured on billiards in an undoped heterostructure, in which this potential landscape is believed to be absent. Fractal studies are performed on these MCF traces, and we find that MCF measured on the undoped billiards do not exhibit measurably different fractal characteristics than those measured on the modulation-doped billiards. Having confirmed that the potential landscapes in modulation-doped heterostructures do not affect the electron phase space, we then investigate the effect of these impurities on the distribution of electron trajectories through the billiards. By employing thermal cycling experiments, we demonstrate that this distribution is highly sensitive to the precise potential landscape within the billiard, suggesting that modulation-doped heterostructures do not support fully ballistic electron transport. We compare our MCF correlation data with the dynamics of charge transfer within heterostructure systems to make qualitative conclusions regarding these dynamics. Close

• 62. [Article] NANOARCHITECTURE-PROPERTY RELATIONSHIPS IN TISE2 BASED NANOLAMINATES FOR DEVELOPMENT OF NOVEL DESIGN STRATEGIES IN COMPOSITE THERMOELECTRIC MATERIALS

  This dissertation is centered on investigation of metastable thermoelectric thin film materials and is split into 3 primary sections. Section 1 focuses on formation mechanisms of FeSbx compounds from layered ...

  Citation × Citation Citation Abstract Copy Title: NANOARCHITECTURE-PROPERTY RELATIONSHIPS IN TISE2 BASED NANOLAMINATES FOR DEVELOPMENT OF NOVEL DESIGN STRATEGIES IN COMPOSITE THERMOELECTRIC MATERIALS Author: Bauers, Sage Year: 2017 This dissertation is centered on investigation of metastable thermoelectric thin film materials and is split into 3 primary sections. Section 1 focuses on formation mechanisms of FeSbx compounds from layered precursors. It was found that a compositionally favorable and homogeneous nucleation environment allowed for the nucleation of a metastable phase, which surprisingly resembles the local coordination environment of the precursors, even in cases where they are compositionally unfavorable. Over the course of this work, the technique of normal-incidence thin film pair distribution function analysis is introduced, which allows for rapid acquisition and analysis of local structure data from intact thin films. Section 2 investigates changes in the stacking sequences of ([PbSe]1+δ)m(TiSe2)n nanolaminate materials, which consist of interleaved layers of each compound in the chemical formula, and how these changes effect the thermoelectric power factor. Homologous series of systematically varying m and n values are investigated and measured properties are correlated back to the designed nanoarchitecture of the laminate materials. It is found that the compounds are stabilized by electron exchange between constituents at the interfaces, and that ‘doping’ of the laminate structure by changing the relative amounts of each constituent is an effective means of optimizing their transport properties. It is also shown that interface density between constituents can be utilized to optimize performance. Section 3 moves from the case of PbSe layers, which maintain their structure, to SnSe layers that significantly distort as the layer size is changed. The distortions in SnSe are observed to occur from templating off TiSe2 layers. As the size of the SnSe layers increases, relatively fewer templated interfacial atoms exist and stabilization of interior atoms must also be considered. The coarse behaviors developed in ([PbSe]1+δ)m(TiSe2)n hold, but the structural distortions in SnSe likely change the band structure of this constituent and hence the composite material, complicating the analysis. In some cases, these changes allow for radically different behavior, best exemplified with high TiSe2 ratios in ([SnSe]1+δ)1(TiSe2)n displaying significant enhancement of the Seebeck coefficient at cryogenic temperatures over the low-n and PbSe-containing analogues. This dissertation includes previously published and unpublished coauthored material. Title: NANOARCHITECTURE-PROPERTY RELATIONSHIPS IN TISE2 BASED NANOLAMINATES FOR DEVELOPMENT OF NOVEL DESIGN STRATEGIES IN COMPOSITE THERMOELECTRIC MATERIALS Author: Bauers, Sage Year: 2017 This dissertation is centered on investigation of metastable thermoelectric thin film materials and is split into 3 primary sections. Section 1 focuses on formation mechanisms of FeSbx compounds from layered precursors. It was found that a compositionally favorable and homogeneous nucleation environment allowed for the nucleation of a metastable phase, which surprisingly resembles the local coordination environment of the precursors, even in cases where they are compositionally unfavorable. Over the course of this work, the technique of normal-incidence thin film pair distribution function analysis is introduced, which allows for rapid acquisition and analysis of local structure data from intact thin films. Section 2 investigates changes in the stacking sequences of ([PbSe]1+δ)m(TiSe2)n nanolaminate materials, which consist of interleaved layers of each compound in the chemical formula, and how these changes effect the thermoelectric power factor. Homologous series of systematically varying m and n values are investigated and measured properties are correlated back to the designed nanoarchitecture of the laminate materials. It is found that the compounds are stabilized by electron exchange between constituents at the interfaces, and that ‘doping’ of the laminate structure by changing the relative amounts of each constituent is an effective means of optimizing their transport properties. It is also shown that interface density between constituents can be utilized to optimize performance. Section 3 moves from the case of PbSe layers, which maintain their structure, to SnSe layers that significantly distort as the layer size is changed. The distortions in SnSe are observed to occur from templating off TiSe2 layers. As the size of the SnSe layers increases, relatively fewer templated interfacial atoms exist and stabilization of interior atoms must also be considered. The coarse behaviors developed in ([PbSe]1+δ)m(TiSe2)n hold, but the structural distortions in SnSe likely change the band structure of this constituent and hence the composite material, complicating the analysis. In some cases, these changes allow for radically different behavior, best exemplified with high TiSe2 ratios in ([SnSe]1+δ)1(TiSe2)n displaying significant enhancement of the Seebeck coefficient at cryogenic temperatures over the low-n and PbSe-containing analogues. This dissertation includes previously published and unpublished coauthored material. Close

• 63. [Article] Circuit and Behavioral Analysis of Klinotaxis in Caenorhabditis elegans

  The nervous system is a complex organ that functions in most metazoans to sense and respond to a constantly changing world. How the nervous system does this is a major focus of systems-level neuroscience. ...

  Citation × Citation Citation Abstract Copy Title: Circuit and Behavioral Analysis of Klinotaxis in Caenorhabditis elegans Author: McCormick, Kathryn Year: 2013, 2014 The nervous system is a complex organ that functions in most metazoans to sense and respond to a constantly changing world. How the nervous system does this is a major focus of systems-level neuroscience. This dissertation investigates the neural basis of the sensorimotor transformation underlying a spatial orientation strategy in the nematode Caenorhabditis elegans. Motile organisms rely on spatial orientation strategies to navigate to environments that are conducive to organismal fitness and comfort, e.g. environments with the correct temperature, light level, or access to food and mates. As such, spatial orientation strategies as a class represent a key behavior common to most forms of life on earth. To explore the behavioral mechanism used by C. elegans for spatial orientation, we designed and manufactured a microfluidic device that breaks the feedback loop between self-motion and environmental change by partially restraining the animal. The device takes advantage of laminar flow at small scale to provide distinct environments across the dorsoventral undulation that constitutes locomotion in this animal without using a physical barrier. This device allowed us to conclude that worms use the change in chemical concentration sensed between lateral extremes of the locomotion cycle to direct forward locomotion toward a favorable stimulus, an orientation strategy termed klinotaxis. We then investigated the neuronal basis of this behavior using laser ablation, calcium imaging, and optogenetic stimulation. We found a minimal neuronal network for klinotaxis to sodium chloride including the ASE, AIY, AIZ, and SMB neuron classes that displays left/right asymmetry across the sensory neuron, interneuron, and motor neuron levels. We extended these results by ablating other neurons that have been implicated in klinotaxis in other studies. Finally, we imaged the ASE neurons during klinotaxis in microfluidic device and found that these neurons are active on the timescale of individual head swings. Additionally, we found anecdotal evidence that photostimulation of ASE neurons expressing the light sensitive ion channel Channel Rhodopsin (CHR2) is sufficient to stimulate klinotaxis behavior. This dissertation includes previously published co-authored material. Title: Circuit and Behavioral Analysis of Klinotaxis in Caenorhabditis elegans Author: McCormick, Kathryn Year: 2013, 2014 The nervous system is a complex organ that functions in most metazoans to sense and respond to a constantly changing world. How the nervous system does this is a major focus of systems-level neuroscience. This dissertation investigates the neural basis of the sensorimotor transformation underlying a spatial orientation strategy in the nematode Caenorhabditis elegans. Motile organisms rely on spatial orientation strategies to navigate to environments that are conducive to organismal fitness and comfort, e.g. environments with the correct temperature, light level, or access to food and mates. As such, spatial orientation strategies as a class represent a key behavior common to most forms of life on earth. To explore the behavioral mechanism used by C. elegans for spatial orientation, we designed and manufactured a microfluidic device that breaks the feedback loop between self-motion and environmental change by partially restraining the animal. The device takes advantage of laminar flow at small scale to provide distinct environments across the dorsoventral undulation that constitutes locomotion in this animal without using a physical barrier. This device allowed us to conclude that worms use the change in chemical concentration sensed between lateral extremes of the locomotion cycle to direct forward locomotion toward a favorable stimulus, an orientation strategy termed klinotaxis. We then investigated the neuronal basis of this behavior using laser ablation, calcium imaging, and optogenetic stimulation. We found a minimal neuronal network for klinotaxis to sodium chloride including the ASE, AIY, AIZ, and SMB neuron classes that displays left/right asymmetry across the sensory neuron, interneuron, and motor neuron levels. We extended these results by ablating other neurons that have been implicated in klinotaxis in other studies. Finally, we imaged the ASE neurons during klinotaxis in microfluidic device and found that these neurons are active on the timescale of individual head swings. Additionally, we found anecdotal evidence that photostimulation of ASE neurons expressing the light sensitive ion channel Channel Rhodopsin (CHR2) is sufficient to stimulate klinotaxis behavior. This dissertation includes previously published co-authored material. Close