Matching Items (99)
Description
Layer structured two dimensional (2D) semiconductors have gained much interest due to their intriguing optical and electronic properties induced by the unique van der Waals bonding between layers. The extraordinary success for graphene and transition metal dichalcogenides (TMDCs) has triggered a constant search for novel 2D semiconductors beyond them. Gallium chalcogenides, belonging to the group III-VI compounds, are a new class of 2D semiconductors that carry a variety of interesting properties including wide spectrum coverage of their bandgaps and thus are promising candidates for next generation electronic and optoelectronic devices. Pushing these materials toward applications requires more controllable synthesis methods and facile routes for engineering their properties on demand.
In this dissertation, vapor phase transport is used to synthesize layer structured gallium chalcogenide nanomaterials with highly controlled structure, morphology and properties, with particular emphasis on GaSe, GaTe and GaSeTe alloys. Multiple routes are used to manipulate the physical properties of these materials including strain engineering, defect engineering and phase engineering. First, 2D GaSe with controlled morphologies is synthesized on Si(111) substrates and the bandgap is significantly reduced from 2 eV to 1.7 eV due to lateral tensile strain. By applying vertical compressive strain using a diamond anvil cell, the band gap can be further reduced to 1.4 eV. Next, pseudo-1D GaTe nanomaterials with a monoclinic structure are synthesized on various substrates. The product exhibits highly anisotropic atomic structure and properties characterized by high-resolution transmission electron microscopy and angle resolved Raman and photoluminescence (PL) spectroscopy. Multiple sharp PL emissions below the bandgap are found due to defects localized at the edges and grain boundaries. Finally, layer structured GaSe1-xTex alloys across the full composition range are synthesized on GaAs(111) substrates. Results show that GaAs(111) substrate plays an essential role in stabilizing the metastable single-phase alloys within the miscibility gaps. A hexagonal to monoclinic phase crossover is observed as the Te content increases. The phase crossover features coexistence of both phases and isotropic to anisotropic structural transition.
Overall, this work provides insights into the controlled synthesis of gallium chalcogenides and opens up new opportunities towards optoelectronic applications that require tunable material properties.
In this dissertation, vapor phase transport is used to synthesize layer structured gallium chalcogenide nanomaterials with highly controlled structure, morphology and properties, with particular emphasis on GaSe, GaTe and GaSeTe alloys. Multiple routes are used to manipulate the physical properties of these materials including strain engineering, defect engineering and phase engineering. First, 2D GaSe with controlled morphologies is synthesized on Si(111) substrates and the bandgap is significantly reduced from 2 eV to 1.7 eV due to lateral tensile strain. By applying vertical compressive strain using a diamond anvil cell, the band gap can be further reduced to 1.4 eV. Next, pseudo-1D GaTe nanomaterials with a monoclinic structure are synthesized on various substrates. The product exhibits highly anisotropic atomic structure and properties characterized by high-resolution transmission electron microscopy and angle resolved Raman and photoluminescence (PL) spectroscopy. Multiple sharp PL emissions below the bandgap are found due to defects localized at the edges and grain boundaries. Finally, layer structured GaSe1-xTex alloys across the full composition range are synthesized on GaAs(111) substrates. Results show that GaAs(111) substrate plays an essential role in stabilizing the metastable single-phase alloys within the miscibility gaps. A hexagonal to monoclinic phase crossover is observed as the Te content increases. The phase crossover features coexistence of both phases and isotropic to anisotropic structural transition.
Overall, this work provides insights into the controlled synthesis of gallium chalcogenides and opens up new opportunities towards optoelectronic applications that require tunable material properties.
ContributorsCai, Hui, Ph.D (Author) / Tongay, Sefaattin (Thesis advisor) / Dwyer, Christian (Committee member) / Zhuang, Houlong (Committee member) / Arizona State University (Publisher)
Created2018
Description
Nanoporous materials, with pore sizes less than one nanometer, have been incorporated as filler materials into state-of-the-art polyamide-based thin-film composite membranes to create thin-film nanocomposite (TFN) membranes for reverse osmosis (RO) desalination. However, these TFN membranes have inconsistent changes in desalination performance as a result of filler incorporation. The nano-sized filler’s transport role for enhancing water permeability is unknown: specifically, there is debate around the individual transport contributions of the polymer, nanoporous particle, and polymer/particle interface. Limited studies exist on the pressure-driven water transport mechanism through nanoporous single-crystal nanoparticles. An understanding of the nanoporous particles water transport role in TFN membranes will provide a better physical insight on the improvement of desalination membranes.
This dissertation investigates water permeation through single-crystal molecular sieve zeolite A particles in TFN membranes in four steps. First, the meta-analysis of nanoporous materials (e.g., zeolites, MOFs, and graphene-based materials) in TFN membranes demonstrated non-uniform water-salt permselectivity performance changes with nanoporous fillers. Second, a systematic study was performed investigating different sizes of non-porous (pore-closed) and nanoporous (pore-opened) zeolite particles incorporated into conventionally polymerized TFN membranes; however, the challenges of particle aggregation, non-uniform particle dispersion, and possible particle leaching from the membranes limit analysis. Third, to limit aggregation and improve dispersion on the membrane, a TFN-model membrane synthesis recipe was developed that immobilized the nanoparticles onto the support membranes surface before the polymerization reaction. Fourth, to quantify the possible water transport pathways in these membranes, two different resistance models were employed.
The experimental results show that both TFN and TFN-model membranes with pore-opened particles have higher water permeance compared to those with pore-closed particles. Further analysis using the resistance in parallel and hybrid models yields that water permeability through the zeolite pores is smaller than that of the particle/polymer interface and higher than the water permeability of the pure polymer. Thus, nanoporous particles increase water permeability in TFN membranes primarily through increased water transport at particle/polymer interface. Because solute rejection is not significantly altered in our TFN and TFN-model systems, the results reveal that local changes in the polymer region at the polymer/particle interface yield high water permeability.
This dissertation investigates water permeation through single-crystal molecular sieve zeolite A particles in TFN membranes in four steps. First, the meta-analysis of nanoporous materials (e.g., zeolites, MOFs, and graphene-based materials) in TFN membranes demonstrated non-uniform water-salt permselectivity performance changes with nanoporous fillers. Second, a systematic study was performed investigating different sizes of non-porous (pore-closed) and nanoporous (pore-opened) zeolite particles incorporated into conventionally polymerized TFN membranes; however, the challenges of particle aggregation, non-uniform particle dispersion, and possible particle leaching from the membranes limit analysis. Third, to limit aggregation and improve dispersion on the membrane, a TFN-model membrane synthesis recipe was developed that immobilized the nanoparticles onto the support membranes surface before the polymerization reaction. Fourth, to quantify the possible water transport pathways in these membranes, two different resistance models were employed.
The experimental results show that both TFN and TFN-model membranes with pore-opened particles have higher water permeance compared to those with pore-closed particles. Further analysis using the resistance in parallel and hybrid models yields that water permeability through the zeolite pores is smaller than that of the particle/polymer interface and higher than the water permeability of the pure polymer. Thus, nanoporous particles increase water permeability in TFN membranes primarily through increased water transport at particle/polymer interface. Because solute rejection is not significantly altered in our TFN and TFN-model systems, the results reveal that local changes in the polymer region at the polymer/particle interface yield high water permeability.
ContributorsCay Durgun, Pinar (Author) / Lind, Mary Laura (Thesis advisor) / Lin, Jerry Y. S. (Committee member) / Green, Matthew D. (Committee member) / Seo, Dong K. (Committee member) / Tongay, Sefaattin (Committee member) / Arizona State University (Publisher)
Created2018
Description
Carbon nanomaterials have caught tremendous attention in the last few decades due to their unique physical and chemical properties. Tremendous effort has been made to develop new synthesis techniques for carbon nanomaterials and investigate their properties for different applications. In this work, carbon nanospheres (CNSs), carbon foams (CF), and single-walled carbon nanotubes (SWNTs) were studied for various applications, including water treatment, energy storage, actuators, and sensors.
A facile spray pyrolysis synthesis technique was developed to synthesize individual CNSs with specific surface area (SSA) up to 1106 m2/g. The hollow CNSs showed adsorption of up to 300 mg rhodamine B dye per gram carbon, which is more than 15 times higher than that observed for conventional carbon black. They were also evaluated as adsorbents for removal of arsenate and selenate from water and displayed good binding to both species, outperforming commercial activated carbons for arsenate removal in pH > 8. When evaluated as supercapacitor electrode materials, specific capacitances of up to 112 F/g at a current density of 0.1 A/g were observed. When used as Li-ion battery anode materials, the CNSs achieved a discharge capacity of 270 mAh/g at a current density of 372 mA/g (1C), which is 4-fold higher than that of commercial graphite anode.
Carbon foams were synthesized using direct pyrolysis and had SSA up to 2340 m2/g. When used as supercapacitor electrode materials, a specific capacitance up to 280 F/g was achieved at current density of 0.1 A/g and remained as high as 207 F/g, even at a high current density of 10 A/g.
A printed walking robot was made from common plastic films and coatings of SWNTs. The solid-state thermal bimorph actuators were multifunctional energy transducers powered by heat, light, or electricity. The actuators were also investigated for photo/thermal detection. Electrochemical actuators based on MnO2 were also studied for potential underwater applications.
SWNTs were also used to fabricate printable electrodes for trace Cr(VI) detection, which displayed sensitivity up to 500 nA/ppb for Cr(VI). The limit of detection was shown to be as low as 5 ppb. A flow detection system based on CNT/printed electrodes was also demonstrated.
A facile spray pyrolysis synthesis technique was developed to synthesize individual CNSs with specific surface area (SSA) up to 1106 m2/g. The hollow CNSs showed adsorption of up to 300 mg rhodamine B dye per gram carbon, which is more than 15 times higher than that observed for conventional carbon black. They were also evaluated as adsorbents for removal of arsenate and selenate from water and displayed good binding to both species, outperforming commercial activated carbons for arsenate removal in pH > 8. When evaluated as supercapacitor electrode materials, specific capacitances of up to 112 F/g at a current density of 0.1 A/g were observed. When used as Li-ion battery anode materials, the CNSs achieved a discharge capacity of 270 mAh/g at a current density of 372 mA/g (1C), which is 4-fold higher than that of commercial graphite anode.
Carbon foams were synthesized using direct pyrolysis and had SSA up to 2340 m2/g. When used as supercapacitor electrode materials, a specific capacitance up to 280 F/g was achieved at current density of 0.1 A/g and remained as high as 207 F/g, even at a high current density of 10 A/g.
A printed walking robot was made from common plastic films and coatings of SWNTs. The solid-state thermal bimorph actuators were multifunctional energy transducers powered by heat, light, or electricity. The actuators were also investigated for photo/thermal detection. Electrochemical actuators based on MnO2 were also studied for potential underwater applications.
SWNTs were also used to fabricate printable electrodes for trace Cr(VI) detection, which displayed sensitivity up to 500 nA/ppb for Cr(VI). The limit of detection was shown to be as low as 5 ppb. A flow detection system based on CNT/printed electrodes was also demonstrated.
ContributorsWang, Chengwei, Ph.D (Author) / Chan, Candace K. (Thesis advisor) / Tongay, Sefaattin (Committee member) / Wang, Qing Hua (Committee member) / Seo, Dong (Committee member) / Arizona State University (Publisher)
Created2015
Description
This thesis presents systematic studies on angle dependent Raman and Photoluminescence (PL) of a new class of layered materials, Transition Metal Trichalcogenides (TMTCs), which are made up of layers possessing anisotropic structure within the van-der-Waals plane. The crystal structure of individual layer of MX3 compounds consists of aligned nanowire like 1D chains running along the b-axis direction. The work focuses on the growth of two members of this family - ZrS3 and TiS3 - through Chemical Vapor Transport Method (CVT), with consequent angle dependent Raman and PL studies which highlight their in-plane optically anisotropic properties. Results highlight that the optical properties of few-layer flakes are highly anisotropic as evidenced by large PL intensity variation with polarization direction (in ZrS3) and an intense variation in Raman intensity with variation in polarization direction (in both ZrS3 and TiS3).
Results suggest that light is efficiently absorbed when E-field of the polarized incident excitation laser is polarized along the chain (b-axis). It is greatly attenuated and absorption is reduced when field is polarized perpendicular to the length of 1D-like chains, as wavelength of the exciting light is much longer than the width of each 1D chain. Observed PL variation with respect to the azimuthal flake angle is similar to what has been previously observed in 1D materials like nanowires. However, in TMTCs, since the 1D chains interact with each other, it gives rise to a unique linear dichroism response that falls between 2D and 1D like behavior. These results not only mark the very first demonstration of high PL polarization anisotropy in 2D systems, but also provide a novel insight into how interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of Quasi-1D materials. The presented results are anticipated to have impact in technologies involving polarized detection, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.
Results suggest that light is efficiently absorbed when E-field of the polarized incident excitation laser is polarized along the chain (b-axis). It is greatly attenuated and absorption is reduced when field is polarized perpendicular to the length of 1D-like chains, as wavelength of the exciting light is much longer than the width of each 1D chain. Observed PL variation with respect to the azimuthal flake angle is similar to what has been previously observed in 1D materials like nanowires. However, in TMTCs, since the 1D chains interact with each other, it gives rise to a unique linear dichroism response that falls between 2D and 1D like behavior. These results not only mark the very first demonstration of high PL polarization anisotropy in 2D systems, but also provide a novel insight into how interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of Quasi-1D materials. The presented results are anticipated to have impact in technologies involving polarized detection, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.
ContributorsPant, Anupum (Author) / Tongay, Sefaattin (Thesis advisor) / Alford, Terry Lynn (Committee member) / He, Ximin (Committee member) / Arizona State University (Publisher)
Created2016
Description
The proposed research mainly focuses on employing tunable materials to achieve dynamic control of radiative heat transfer in both far and near fields for thermal management. Vanadium dioxide (VO2), which undergoes a phase transition from insulator to metal at the temperature of 341 K, is one tunable material being applied. The other one is graphene, whose optical properties can be tuned by chemical potential through external bias or chemical doping.
In the far field, a VO2-based metamaterial thermal emitter with switchable emittance in the mid-infrared has been theoretically studied. When VO2 is in the insulating phase, high emittance is observed at the resonance frequency of magnetic polaritons (MPs), while the structure becomes highly reflective when VO2 turns metallic. A VO2-based thermal emitter with tunable emittance is also demonstrated due to the excitation of MP at different resonance frequencies when VO2 changes phase. Moreover, an infrared thermal emitter made of graphene-covered SiC grating could achieve frequency-tunable emittance peak via the change of the graphene chemical potential.
In the near field, a radiation-based thermal rectifier is constructed by investigating radiative transfer between VO2 and SiO2 separated by nanometer vacuum gap distances. Compared to the case where VO2 is set as the emitter at 400 K as a metal, when VO2 is considered as the receiver at 300 K as an insulator, the energy transfer is greatly enhanced due to the strong surface phonon polariton (SPhP) coupling between insulating VO2 and SiO2. A radiation-based thermal switch is also explored by setting VO2 as both the emitter and the receiver. When both VO2 emitter and receiver are at the insulating phase, the switch is at the “on” mode with a much enhanced heat flux due to strong SPhP coupling, while the near-field radiative transfer is greatly suppressed when the emitting VO2 becomes metallic at temperatures higher than 341K during the “off” mode. In addition, an electrically-gated thermal modulator made of graphene covered SiC plates is theoretically studied with modulated radiative transport by varying graphene chemical potential. Moreover, the MP effect on near-field radiative transport has been investigated by spectrally enhancing radiative heat transfer between two metal gratings.
In the far field, a VO2-based metamaterial thermal emitter with switchable emittance in the mid-infrared has been theoretically studied. When VO2 is in the insulating phase, high emittance is observed at the resonance frequency of magnetic polaritons (MPs), while the structure becomes highly reflective when VO2 turns metallic. A VO2-based thermal emitter with tunable emittance is also demonstrated due to the excitation of MP at different resonance frequencies when VO2 changes phase. Moreover, an infrared thermal emitter made of graphene-covered SiC grating could achieve frequency-tunable emittance peak via the change of the graphene chemical potential.
In the near field, a radiation-based thermal rectifier is constructed by investigating radiative transfer between VO2 and SiO2 separated by nanometer vacuum gap distances. Compared to the case where VO2 is set as the emitter at 400 K as a metal, when VO2 is considered as the receiver at 300 K as an insulator, the energy transfer is greatly enhanced due to the strong surface phonon polariton (SPhP) coupling between insulating VO2 and SiO2. A radiation-based thermal switch is also explored by setting VO2 as both the emitter and the receiver. When both VO2 emitter and receiver are at the insulating phase, the switch is at the “on” mode with a much enhanced heat flux due to strong SPhP coupling, while the near-field radiative transfer is greatly suppressed when the emitting VO2 becomes metallic at temperatures higher than 341K during the “off” mode. In addition, an electrically-gated thermal modulator made of graphene covered SiC plates is theoretically studied with modulated radiative transport by varying graphene chemical potential. Moreover, the MP effect on near-field radiative transport has been investigated by spectrally enhancing radiative heat transfer between two metal gratings.
ContributorsYang, Yue (Author) / Wang, Liping (Thesis advisor) / Phelan, Patrick (Committee member) / Wang, Robert (Committee member) / Tongay, Sefaattin (Committee member) / Rykaczewski, Konrad (Committee member) / Arizona State University (Publisher)
Created2016
Description
During the 1960s, the long-standing idea that traits or behaviors could be
explained by natural selection acting on traits that persisted "for the good of the group" prompted a series of debates about group-level selection and the effectiveness with which natural selection could act at or across multiple levels of biological organization. For some this topic remains contentious, while others consider the debate settled, even while disagreeing about when and how resolution occurred, raising the question: "Why have these debates continued?"
Here I explore the biology, history, and philosophy of the possibility of natural selection operating at levels of biological organization other than the organism by focusing on debates about group-level selection that have occurred since the 1960s. In particular, I use experimental, historical, and synthetic methods to review how the debates have changed, and whether different uses of the same words and concepts can lead to different interpretations of the same experimental data.
I begin with the results of a group-selection experiment I conducted using the parasitoid wasp Nasonia, and discuss how the interpretation depends on how one conceives of and defines a "group." Then I review the history of the group selection controversy and argue that this history is best interpreted as multiple, interrelated debates rather than a single continuous debate. Furthermore, I show how the aspects of these debates that have changed the most are related to theoretical content and empirical data, while disputes related to methods remain largely unchanged. Synthesizing this material, I distinguish four different "approaches" to the study of multilevel selection based on the questions and methods used by researchers, and I use the results of the Nasonia experiment to discuss how each approach can lead to different interpretations of the same experimental data. I argue that this realization can help to explain why debates about group and multilevel selection have persisted for nearly sixty years. Finally, the conclusions of this dissertation apply beyond evolutionary biology by providing an illustration of how key concepts can change over time, and how failing to appreciate this fact can lead to ongoing controversy within a scientific field.
explained by natural selection acting on traits that persisted "for the good of the group" prompted a series of debates about group-level selection and the effectiveness with which natural selection could act at or across multiple levels of biological organization. For some this topic remains contentious, while others consider the debate settled, even while disagreeing about when and how resolution occurred, raising the question: "Why have these debates continued?"
Here I explore the biology, history, and philosophy of the possibility of natural selection operating at levels of biological organization other than the organism by focusing on debates about group-level selection that have occurred since the 1960s. In particular, I use experimental, historical, and synthetic methods to review how the debates have changed, and whether different uses of the same words and concepts can lead to different interpretations of the same experimental data.
I begin with the results of a group-selection experiment I conducted using the parasitoid wasp Nasonia, and discuss how the interpretation depends on how one conceives of and defines a "group." Then I review the history of the group selection controversy and argue that this history is best interpreted as multiple, interrelated debates rather than a single continuous debate. Furthermore, I show how the aspects of these debates that have changed the most are related to theoretical content and empirical data, while disputes related to methods remain largely unchanged. Synthesizing this material, I distinguish four different "approaches" to the study of multilevel selection based on the questions and methods used by researchers, and I use the results of the Nasonia experiment to discuss how each approach can lead to different interpretations of the same experimental data. I argue that this realization can help to explain why debates about group and multilevel selection have persisted for nearly sixty years. Finally, the conclusions of this dissertation apply beyond evolutionary biology by providing an illustration of how key concepts can change over time, and how failing to appreciate this fact can lead to ongoing controversy within a scientific field.
ContributorsDimond, Christopher C (Author) / Collins, James P. (Thesis advisor) / Gadau, Juergen (Committee member) / Laubichler, Manfred (Committee member) / Armendt, Brad (Committee member) / Lynch, John (Committee member) / Arizona State University (Publisher)
Created2014
Description
For interspecific mutualisms, the behavior of one partner can influence the fitness of the other, especially in the case of symbiotic mutualisms where partners live in close physical association for much of their lives. Behavioral effects on fitness may be particularly important if either species in these long-term relationships displays personality. Animal personality is defined as repeatable individual differences in behavior, and how correlations among these consistent traits are structured is termed behavioral syndromes. Animal personality has been broadly documented across the animal kingdom but is poorly understood in the context of mutualisms. My dissertation focuses on the structure, causes, and consequences of collective personality in Azteca constructor colonies that live in Cecropia trees, one of the most successful and prominent mutualisms of the neotropics. These pioneer plants provide hollow internodes for nesting and nutrient-rich food bodies; in return, the ants provide protection from herbivores and encroaching vines. I first explored the structure of the behavioral syndrome by testing the consistency and correlation of colony-level behavioral traits under natural conditions in the field. Traits were both consistent within colonies and correlated among colonies revealing a behavioral syndrome along a docile-aggressive axis. Host plants of more active, aggressive colonies had less leaf damage, suggesting a link between a colony personality and host plant health. I then studied how aspects of colony sociometry are intertwined with their host plants by assessing the relationship among plant growth, colony growth, colony structure, ant morphology, and colony personality. Colony personality was independent of host plant measures like tree size, age, volume. Finally, I tested how colony personality influenced by soil nutrients by assessing personality in the field and transferring colonies to plants the greenhouse under different soil nutrient treatments. Personality was correlated with soil nutrients in the field but was not influenced by soil nutrient treatment in the greenhouse. This suggests that soil nutrients interact with other factors in the environment to structure personality. This dissertation demonstrates that colony personality is an ecologically relevant phenomenon and an important consideration for mutualism dynamics.
ContributorsMarting, Peter (Author) / Pratt, Stephen C (Thesis advisor) / Wcislo, William T (Committee member) / Hoelldobler, Bert (Committee member) / Fewell, Jennifer H (Committee member) / Gadau, Juergen (Committee member) / Arizona State University (Publisher)
Created2018
Description
The Energiewende aims to drastically reduce Germany’s greenhouse gas emissions, without relying on nuclear power, while maintaining a secure and affordable energy supply. Since 2000 the country’s renewable-energy share has increased exponentially, accounting in 2017 for over a third of Germany's gross electricity consumption. This unprecedented achievement is the result of policies, tools, and institutional arrangements intended to steer society to a low-carbon economy. Despite its resounding success in renewable-energy deployment, the Energiewende is not on track to meet its decarbonization goals. Energiewende rules and regulations have generated numerous undesired consequences, and have cost much more than anticipated, a burden borne primarily by energy consumers. Why has the Energiewende not only made energy more expensive, but also failed to bring Germany closer to its decarbonization goals? I analyzed the Energiewende as a complex socio-technical system, examining its legal framework and analyzing the consequences of successive regulations; identifying major political and energy players and the factors that motivated them to pursue socio-technical change; and documenting the political trends and events in which the Energiewende is rooted and which continue to shape it. I analyzed the dynamics and the loopholes that created barriers to transition, pushed the utility sector to the brink of dissolution, and led to such undesirable outcomes as negative wholesale prices and forced exports of electricity to Germany’s European neighbors. Thirty high-level energy experts and stakeholders were interviewed to find out how the best-informed members of German society perceive the Energiewende. Surprisingly, although they were highly critical of the way the transition has unfolded, most were convinced that the transition would eventually succeed. But their definitions of success did not always depend on achieving carbon-mitigation targets. Indeed, Germany jeopardizes the achievement of these targets by changing too many policy and institutional variables at too fast a pace. Good intentions and commitment are not enough to create economies based on intermittent energy sources: they will also require intensive grid expansion and breakthroughs in storage technology. The Energiewende demonstrates starkly that collective action driven by robust political consensus is not sufficient for steering complex socio-technical systems in desired directions.
ContributorsSturm, Christine (Author) / Sarewitz, Daniel (Thesis advisor) / Miller, Clark (Committee member) / Anderies, John (Committee member) / Hirt, Paul (Committee member) / Arizona State University (Publisher)
Created2018
Description
Microplastics are emerging to be major problem when it comes to water pollution and they pose a great threat to marine life. These materials have the potential to affect a wide range of human population since humans are the major consumers of marine organisms. Microplastics are less than 5 mm in diameter, and can escape from traditional wastewater treatment plant (WWTP) processes and end up in our water sources. Due to their small size, they have a large surface area and can react with chlorine, which it encounters in the final stages of WWTP. After the microplastics accumulate in various bodies of water, they are exposed to sunlight, which contains oxidative ultraviolet (UV) light. Since the microplastics are exposed to oxidants during and after the treatment, there is a strong chance that they will undergo chemical and/or physical changes. The WWTP conditions were replicated in the lab by varying the concentrations of chlorine from 70 to 100 mg/L in increments of 10 mg/L and incubating the samples in chlorine baths for 1–9 days. The chlorinated samples were tested for any structural changes using Raman spectroscopy. High density polyethylene (HDPE), polystyrene (PS), and polypropylene (PP) were treated in chlorine baths and observed for Raman intensity variations, Raman peak shifts, and the formation of new peaks over different exposure times. HDPE responded with a lot of oxidation peaks and shifts of peaks after just one day. For the degradation of semi-crystalline polymers, there was a reduction in crystallinity, as verified by thermal analysis. There was a decrease in the enthalpy of melting as well as the melting temperature with an increase in the exposure time or chlorine concentration, which pointed at the degradation of plastics and bond cleavages. To test the plastic response to
ii
UV, the samples were exposed to sunlight for up to 210 days and analyzed under Raman spectroscopy. Overall the physical and chemical changes with the polymers are evident and makes a way for the wastewater treatment plant to take necessary steps to capture the microplastics to avoid the release of any kind of degraded microplastics that could affect marine life and the environment.
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UV, the samples were exposed to sunlight for up to 210 days and analyzed under Raman spectroscopy. Overall the physical and chemical changes with the polymers are evident and makes a way for the wastewater treatment plant to take necessary steps to capture the microplastics to avoid the release of any kind of degraded microplastics that could affect marine life and the environment.
ContributorsKelkar, Varun (Author) / Green, Matthew D (Thesis advisor) / Tongay, Sefaattin (Committee member) / Halden, Rolf U. (Committee member) / Arizona State University (Publisher)
Created2017
Description
A new class of layered materials called the transition metal trichalcogenides (TMTCs) exhibit strong anisotropic properties due to their quasi-1D nature. These 2D materials are composed of chain-like structures which are weakly bound to form planar sheets with highly directional properties. The vibrational properties of three materials from the TMTC family, specifically TiS3, ZrS3, and HfS3, are relatively unknown and studies performed in this work elucidates the origin of their Raman characteristics. The crystals were synthesized through chemical vapor transport prior to mechanical exfoliation onto Si/SiO¬2 substrates. XRD, AFM, and Raman spectroscopy were used to determine the crystallinity, thickness, and chemical signature of the exfoliated crystals. Vibrational modes and anisotropic polarization are investigated through density functional theory calculations and angle-resolved Raman spectroscopy. Particular Raman modes are explored in order to correlate select peaks to the b-axis crystalline direction. Mode III vibrations for TiS3, ZrS3, and HfS3 are shared between each material and serves as a unique identifier of the crystalline orientation in MX3 materials. Similar angle-resolved Raman studies were conducted on the novel Nb0.5Ti0.5S3 alloy material grown through chemical vapor transport. Results show that the anisotropy direction is more difficult to determine due to the randomization of quasi-1D chains caused by defects that are common in 2D alloys. This work provides a fundamental understanding of the vibrational properties of various TMTC materials which is needed to realize applications in direction dependent polarization and linear dichroism.
ContributorsKong, Wilson (Author) / Tongay, Sefaattin (Thesis advisor) / Wang, Liping (Committee member) / Green, Matthew (Committee member) / Arizona State University (Publisher)
Created2017