Browsing by Subject "Nanomaterials"
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Item Biodegradable NIR-active contrast agents with amplifying photoacoustic effect for cancer theranostics(2018-12) Changalvaie, Behzad; Johnston, Keith P., 1955-This study discusses two classes of contrast agents, which can potentially be used in photoacoustic imaging and improve the early detection rate of cancer. The developed agents exhibit high adsorption in the near-infrared window (wavelength range of 700-1000 nm), high photo-stability, and efficient conversion of heat energy to produce acoustic waves. The first developed agent is comprised of gold nanoclusters. The inter-particle interactions between ~5 nm gold nanoparticles (Au NPs) were manipulated to form biodegradable, NIR-active nanoclusters. The inter-particle spacing was tuned to <1 nm within the clusters so that the surface plasmon resonance shifted to the NIR window. This was achieved by employing optimized binary ligand systems on the surface of Au NPs. The clusters were able to dissociate back to individual ~5 nm Au NPs in physiological media. This dissociation allows for the renal clearance of individual particles. In addition to Au NPs with binary ligands, another subclass of Au NP clusters was developed using glutathione (GSH) to cover the surface of the nanoparticles. The inter-particle interactions were tuned by using pH of the solution as a control knob to create and dissociate nanoclusters reversibly. The second class of contrast agents consists of organic materials that are already approved by the US Food and Drug Administration (FDA). J-aggregates of Indocyanine green (ICG) show a sharp, intense peak at a wavelength of 890 nm; however, they are not stable in physiological environment. These J-aggregates were protected by being encapsulated in poly(ethylene glycol)-b-poly(lactic-co-glycolic acid) (PEG-PLGA) nanocapsules, which show excellent stability and optimal degradation kinetics in physiological environments. Additionally, a novel coating method was developed to protect ICG J-aggregates with polyethylenimine (PEI) so that the J-aggregates maintain their form during encapsulation. The following chapters describe novel methods to synthesize biocompatible contrast agents with control over their size, stability, and degradation kinetics. These agents have the potential to be utilized in cancer theranosticsItem Experimental investigations of energy carrier interactions with atomic disorders and artificial long-range orders(2022-10-06) Smith, Brandon Paul; Shi, Li, Ph. D.; Orbach, Raymond L; Akinwande, Deji; Bank, Seth R; Wang, YaguoThe field of nanoscale energy transport, conversion, and storage is at an exciting time with next-generation devices manipulating discrete energy carriers, e.g. phonons, photons, and electrons, in confined dimensions arriving closer to commercialization, such as solid-state flexible electronics and optoelectronics utilizing one dimension (1D) and two dimensional (2D) nanomaterials. The transport dynamics of quasiparticles and their coupling are modified, notably in low dimensional nanomaterials, with the inclusion of disorder and artificial long-range order. Through this lens, it is possible to probe interesting physics and draw out intrinsic properties of the nanomaterials. This is especially important for electronic systems and energy conversion & storage devices where heat generation and dissipation within nano- and microscale locations of nonequilibrium impedes continued advancement. This thesis examines outstanding questions concerning nanoscale thermal and thermoelectric transport in low-dimensional materials to further understanding of crystal disorder and artificial long-range order. Specifically, the material systems investigated are alloy disorder and surface roughness in semiconducting silicon germanium (SiGe) nanowires, microscale rippling in layered molybdenum disulfide (MoS2) flakes, intra- and interlayer interactions in bulk and monolayer MoS2, and artificially created, long-range domain walls in twisted bilayer graphene (TBG). The fundamental questions are addressed through electrothermal, optothermal, and scanning probe metrology techniques. First, eight-probe thermal conductivity measurements of SiGe nanowires show that alloying suppresses thermal transport, and the mean-free-paths of low-frequency phonons are suppressed by diffuse surface roughness scattering in nanowires. The diffuse surface scattering results in length-independent thermal conductivity for lengths over two micrometers. Similarly, four-probe thermal conductivity measurements reveal that microscale ripples have negligible effects on phonon transport in 2D layers as the ripple wavelengths and curvatures are much larger than the phonon mean free paths and wavelengths. The peak thermal conductivity is found to increase with decreasing Raman scattering intensity in the frequency range with vanishing phonon density of states in MoS2 indicating an important role of point defect scattering. In addition, this dissertation presents an experimental effort to employ micro-Raman spectroscopy to investigate local nonequilibrium among different phonon polarizations in MoS2 inside the focused laser spot. It also describes an exploration of ultra-high vacuum scanning probe microscopy for probing the local thermoelectric property of twisted bilayer graphene moiré superstructures.Item Experimental investigations of thermal transport in carbon nanotubes, graphene and nanoscale point contacts(2011-05) Pettes, Michael Thompson, 1978-; Shi, Li, Ph. D.As silicon-based transistor technology continues to scale ever downward, anticipation of the fundamental limitations of ultimately-scaled devices has driven research into alternative device technologies as well as new materials for interconnects and packaging. Additionally, as power dissipation becomes an increasingly important challenge in highly miniaturized devices, both the implementation and verification of high mobility, high thermal conductivity materials, such as low dimensional carbon nanomaterials, and the experimental investigation of heat transfer in the nanoscale regime are requisite to continued progress. This work furthers the current understanding of structure-property relationships in low dimensional carbon nanomaterials, specifically carbon nanotubes (CNTs) and graphene, through use of combined thermal conductance and transmission electron microscopy (TEM) measurements on the same individual nanomaterials suspended between two micro-resistance thermometers. Through the development of a method to measure thermal contact resistance, the intrinsic thermal conductivity, [kappa], of multi-walled (MW) CNTs is found to correlate with TEM observed defect density, linking phonon-defect scattering to the low [kappa] in these chemical vapor deposition (CVD) synthesized nanomaterials. For single- (S) and double- (D) walled (W) CNTs, the [kappa] is found to be limited by thermal contact resistance for the as-grown samples but still four times higher than that for bulk Si. Additionally, through the use of a combined thermal transport-TEM study, the [kappa] of bi-layer graphene is correlated with both crystal structure and surface conditions. Theoretical modeling of the [kappa] temperature dependence allows for the determination that phonon scattering mechanisms in suspended bi-layer graphene with a thin polymeric coating are similar to those for the case of graphene supported on SiO₂. Furthermore, a method is developed to investigate heat transfer through a nanoscale point contact formed between a sharp silicon tip and a silicon substrate in an ultra high vacuum (UHV) atomic force microscope (AFM). A contact mechanics model of the interface, combined with a heat transport model considering solid-solid conduction and near-field thermal radiation leads to the conclusion that the thermal resistance of the nanoscale point contact is dominated by solid-solid conduction.Item First principles-based molecular modeling of thermal transport in silicon-based nanomaterials(2014-08-15) Lee, Yongjin; Hwang, Gyeong S.; Ekerdt, John; Shi, Li; Mullins, Charles; Sanchez, IsaacIn today’s nanotechnology, a critical issue is to gain the ability to control the structure and function of matter with a deeper understanding of the quantitative and qualitative relationship among their synthesis conditions, structures, and properties. Experiments may provide information regarding the behavior of nanomaterials, but their interpretations are often controversial due largely to the difficulty of direct measurement. Hereupon, with the amazing advance in computer technology since the late 20th century, computational modeling in science and engineering is increasingly important particularly in the fields of nanoscience and nanotechnology while it can provide researchers with significant insights into atomic-level interactions in various materials systems and underlying fundamental theories. The ability of engineering thermal conductivity of materials on the nanoscale has become extremely important in various applications including electronics and energy storage/conversion technologies. Due to technical difficulties in experimentally measuring the thermal conductivity of disordered and complex nanostructures, there has been much interest in use of theoretical and computational methods to investigate thermal transport properties nanostructured materials. One computational method that can perform an accurate analysis for the thermal conductivity of new or complex systems is molecular dynamics (MD), due to its capability of predicting the behaviors of atoms in large systems. In this work, we have developed a comprehensive MD-based computational platform capable of predicting and explaining thermal transport in disordered and complex nanostructured materials. The unique features include construction of realistic nanostructures, determination of reliable force fields, and direct simulation of large systems, which are allowed by coupling various state-of-the-art computational methods including quantum mechanics, molecular mechanics, statistical theories, and massively parallel computing. The computational scheme was applied to describe thermal transport in various silicon and carbon-based disordered and nanostructures. First, the effects of defects including vacancy clusters, substitutional dopants, and dopant-defect complexes on the thermal conductivity of bulk crystalline silicon were investigated. Next, we analyzed the factors affecting heat transport in silicon-germanium and ternary silicon-germanium-tin alloys. Lastly, we performed the analysis of heat transport in silicon-based nanostructures such as nanowires and polycrystalline structures.Item Germanium and silicon nanowires for use in water purification(2022-05-11) Sullivan, William (M.S. in chemical engineering); Korgel, Brain Allan, 1969-Germanium and silicon nanowires present an exciting opportunity for broadening the scope of membrane fouling mitigation research. Germanium nanowires provide a highly effective model system for investigating how to incorporate silicon nanowires into polymeric membranes, while providing relative ease in synthesis and workability compared to silicon nanowires. Silicon nanowires present an exciting area of investigation for fouling mitigation for two main reasons: they can be surface passivated to achieve desired chemical properties and they are photoactive. This work explores how to effectively incorporate germanium nanowires into polymeric membranes as a model to be used for silicon nanowires. Then the integration of silicon nanowires is further explored to determine the most effective methods of silicon nanowire incorporation into polymeric membranes. Successful integration of silicon nanowires into polymeric membrane systems is demonstrated, providing the groundwork for further exploration of the use of nanowires in water purification, specifically for fouling mitigation.Item Nanomaterials in complex environments : interactions affected by matrix composition, pH, natural organic matter, and bio- or geo-colloids(2021-05-07) Sabaraya, Indu Venu; Saleh, Navid B.; Kirisits, Mary Jo; Lawler, Desmond F; Rylander, Marissa N.Nanomaterials display unique properties as compared to their bulk counterparts due to their small size and high surface-area-to-volume ratio. These functional materials have emerged as an important constituent of many consumer products over recent decades. Engineered nanomaterials (ENMs) are being integrated into construction materials, biomedical devices and products, and energy -production and -storage materials, among many other applications. For ENMs to be effective in suspension-based products and devices and for understanding their fate and environmental effects upon release (during usage or at the end of the usable life), an assessment of ENM interaction with other particles of biological and geological origin in complex solution is essential. Aggregation, or lack of colloidal stability, of ENMs influences their functional efficacy as well as key physicochemical processes that ENMs might undergo in the environment, including sedimentation and dissolution. The functional promise and environmental fate of ENMs are strongly influenced by their inherent properties as well as the characteristics of the receiving medium in which they are applied or received. The receiving medium’s characteristics also can influence the pathways and mode of transformation of ENMs after environmental release. Although the nanomaterial literature is rich with studies of ENM interactions in simple and controlled media, there is a paucity of systematic studies evaluating the role of medium complexity on ENM interaction with bio- and geo-colloids. As described in the following goals, the work herein addresses this data gap by evaluating the interaction of nanomaterials in complex matrices: • Explore the relationship of ENM dispersibility within a simplified organic solvent mixture for a civil engineering application (i.e., asphalt matrix) with selected performance metric of the ENM-enabled asphalt binder. • Investigate the colloidal stability of next-generation two-dimensional (2D) ENMs in a heterogeneous particle system and in the presence of natural geo-colloids and organic matter to evaluate environmental fate of these ENMs. • Examine transformation pathways of 2D nanomaterials and their potential release from ENM-enabled devices in a simulated landfill leachate environment.Item Nanostructuring silicon and germanium for high capacity anodes in lithium ion batteries(2012-12) Harris, Justin Thomas; Korgel, Brian Allan, 1969-; Ekerdt, John G.; Hwang, Gyeong S.; Mullins, Charles B.; Stevenson, Keith A.Colloidally synthesized silicon (Si) and germanium (Ge) were explored as high capacity anode materials in lithium ion batteries. a-Si:H particles were synthesized through the thermal decomposition of trisilane in supercritical n-hexane. Precise control over particle size and hydrogen content was demonstrated. Particles ranged in size from 240-1500 nm with hydrogen contents from 10-60 atomic%. Particles with low hydrogen content had some degree of local ordering and were easily crystallized during Raman spectroscopy. The as-synthesized particles did not perform well as an anode material due to low conductivity. Increasing surface conductivity led to enhanced lithiation potential. Cu nanoparticles were deposited on the surface of the a-Si:H particles through a hydrogen facilitated reduction of Cu salts. The resulting Cu coated particles had a lithiation capacity seven times that of pristine a-Si:H particles. Monophenylsilane (MPS) grown Si nanowire paper was annealed under forming gas to reduce a polyphenylsilane shell into conductive carbon. The resulting paper required no binder or carbon additive and achieved capacities of 804 mA h/g vs 8 mA h/g for unannealed wires. Si and Ge heterostructures were explored to take advantage of the higher inherent conductivity of Ge. Ge nanowires were successfully coated with a-Si by thermal decomposition of trisilane on their surface, forming Ge@a-Si core shell structures. The capacity increased with increasing Si loading. The peak lithiation capacity was 1850 mA h/g after 20 cycles – higher than the theoretical capacity of pure Ge. MPS additives created a thin amorphous shell on the wire surfaces. By incubating the wires after MPS addition the shell was partially reduced, conductivity increased, and a 75% increase in lithiation capacity was observed for the nanowire paper. The syntheses of Bi and Au nanoparticles were also explored. Highly monodisperse Bi nanocrystals were produced with size control from 6-18 nm. The Bi was utilized as seeds for the SLS synthesis of Ge nanorods and copper indium diselenide (CuInSe2) nanowires. Sub 2 nm Au nanocrystals were synthesized. A SQUID magnetometer probed their magnetic behavior. Though bulk Au is diamagnetic, the Au particles were paramagnetic. Magnetic susceptibility increased with decreasing particle diameter.Item Novel synthesis of nanostructured electrode materials for lithium-ion batteries(2010-08) Theivanayagam, Murali Ganth; Manthiram, ArumugamLithium-ion batteries have revolutionized the portable electronics market, and they are currently pursued intensively for vehicle applications and storage of renewable energies (solar and wind energy). Cost, safety, cycle life, and energy and power densities are the critical parameters for these applications. With this perspective, there has been immense interest to develop new cathode and anode materials as well as to develop novel synthesis and processing approaches. This dissertation explores the use of novel synthesis approaches to obtain high-performance, nanostructured phosphate and silicate cathodes and iron oxide nanowire anodes and investigates their structure-property relationships. First, a novel microwave-solvothermal (MW-ST) approach has been developed to synthesize phase-pure, highly crystalline LiFePO₄ nanorods within 5-15 minutes at low temperatures of < 300 °C, without requiring reducing gas atmospheres. The LiFePO₄ nanorods, after forming a nanocomposite with conducting polymer or multi-walled carbon nanotubes or coating with conductive carbon, offer excellent cycle life and rate performance when implemented as cathodes in lithium-ion cells. In addition, other LiMPO₄ (M = Mn, Co, and Ni) olivine nanorods have also been synthesized by the MW-ST approach and characterized. The MW-ST process has then been extended to prepare a new class of carbon-coated, nanostructured silicates of the formula Li₂MSiO₄ (M = Fe and Mn). These materials have two times higher theoretical capacities (~ 330 mAh/g) than olivine phosphates (~ 170 mAh/g). Li₂FeSiO₄ exhibits practical discharge capacities of 148 mAh/g at room temperature and 203 mAh/g at 55 °C, with good rate capability and stable cycle life. Li₂MnSiO₄, on the other hand, shows higher discharge capacities of 210 mAh/g at room temperature and 250 mAh/g at 55 °C, but it exhibits poor rate performance and rapid capacity fade during cycling. In addition, carbon-coated olivine solid solution nano-particles of the formula LiM[subscript 1-y]M[subscript y]PO₄ (M = Fe, Mn, Co, and Mg), synthesized by a facile, high-energy mechanical milling process (HMME), have also been investigated. The electrochemical data reveal a systematic shift in the redox potential (open-circuit voltage) of the M²⁺/³⁺ couples in the LiM[subscript 1-y]M[subscript y]PO₄ solid solutions compared to those of the pristine LiMPO₄. The shifts in the redox potentials have been explained by the changes in the M-O covalence (inductive effect), which are caused by changes in the electronegativity of M or the M-O bond length or M-O-M interactions. Finally, a two-step microwave-hydrothermal process has been developed to synthesize carbon-decorated, single-crystalline Fe₃O₄ nanowires. The resulting iron oxide nanowires exhibit capacity values > 800 mAh/g with stable cycle life and high rate performance as an anode in lithium-ion cells.Item Optoelectronic, structural, and topological properties of van der Waals layered materials under extreme conditions(2018-08) Kim, Joonseok; Akinwande, Deji; Lin, Jung-Fu; Banerjee, Sanjay K; Dodabalapur, Ananth; Wang, YaguoThe concept of Internet of Things (IoT) has been discussed extensively in the recent years, where billions of smart devices and sensors communicate with each other and provide ubiquitous service. Two-dimensional (2D) materials for such application could be exposed to extreme conditions that IoT devices may experience, such as mechanically stressing, chemically reactive, high-temperature, and/or radiative environment. Therefore, it is crucial to understand the materials' properties under extreme conditions, and further engineer the properties from the acquired knowledge. In this dissertation, we focus on the effects of oxygen/moisture condition on air-sensitive 2D materials, and effects of hydrostatic pressure on 2D and other layered materials. In Chapter 2, we report detailed study on air-degradation of few-layer phosphorene films and field effect transistors, as well as an effective encapsulation method that enhances the stability of devices up to several months. In the later parts we explore the effects of hydrostatic pressure on layered materials, where the anisotropic van der Waals structure exhibit remarkably large pressure-modulation of material properties. In Chapter 3, pressure effects on Raman modes in bulk Mo₀.₅W₀.₅S₂ alloy are examined to discover strengthening of inter-layer interactions under pressure. In Chapter 4, pressure-induced structural transition of bulk WTe₂ is discussed, where layer sliding introduces inversion symmetry, similar to the case in monolayer WTe₂. In Chapter 5, evolution of optical band gaps of monolayer WS₂ and Mo₀.₅W₀.₅S₂ are studied, where we show different pressure-behaviors of band edges according to the composition. In Chapter 6, structural, vibrational, and topological electronic properties of Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂ topological insulator alloy is explored, to show that the topological states could be modulated by pressure, without transitions in the crystal structure.Item Solution grown silicon and germanium nanostructures : characterization and application as lithium ion battery anode materials(2012-05) Chockla, Aaron Michael; Korgel, Brian Allan, 1969-Solution-grown silicon and germanium nanowires were produced using various solvents and nanocrystalline seed materials. Silicon nanowires grown using monophenylsilane as the silicon source and gold catalyst seeds were made into a freestanding, lightweight, mechanically robust fabric and tested as a negative electrode material in lithium ion batteries. Annealing the fabric under reducing atmosphere converts the intrinsic poly(phenylsilane) shell into a highly conductive carbonaceous coating, improving Li storage behavior. Reduced graphite oxide (graphene) was studied as a freestanding support for gold-seeded germanium and silicon nanowires, the latter grown using trisilane. Graphene improves capacity retention for germanium nanowires but shows little improvement for silicon. Slurry-cast films of nanowires were also tested as negative electrodes in lithium ion batteries using a variety of electrolyte solvent / binder combinations. Gold is detrimental to performance of silicon nanowires grown using trisilane. Removing gold through a simple wet etching procedure dramatically improves capacity retention. Silicon nanowires were also synthesized using in-situ formed tin seeds. Tin-seeded nanowires are easier to produce and outperform gold-seeded wires in lithium ion batteries. Germanium nanowires perform exceptionally well under high current loads when cycled using electrolyte solutions that contain fluoroethylene carbonate and show promise for high- power applications. Controlled synthesis of solution-grown germanium nanorods is demonstrated using nanocrystalline bismuth seeds. The addition of poly(vinylpyrrolidinone) / hexadecene copolymer leads to branched nanorods. Absorbance spectra were calculated using the discrete dipole approximation to compare against spectra obtained experimentally. The absorbance spectra and electric field internal to the nanorods depend highly on nanorod orientation. The presence of bismuth or gold at the tip of the nanorods also significantly alters the spectra and electric fields. Ligand and surface chemistry of solution grown indium phosphide nanowires is also examined. Octylphosphonic acid and hexadecylamine are both essential for the growth of single crystalline indium phosphide nanowires. Potential solution synthesis routes to indium (III) oxide nanowires and indium phosphide nanowires with twinning superlattice structure are presented. Various phosphoric acid derivatives were tested in place of octylphosphonic acid and the efficacy of each is discussed.Item Structural engineering and electronic tuning of non-noble transition metal-based electrocatalysts(2021-04-21) Fang, Zhiwei, Ph. D.; Yu, Guihua (Assistant professor); Manthiram, Arumugam; Johnston, Keith P.; Liu, YuanyueCatalysis, a process that can accelerate chemical reactions, has become a pivotal role in producing renewable energy (e.g. fuel cells, solar energy, biofuels, etc.) s by environmentally friendly routes. Heterogeneous electrocatalysis has prompted intensive efforts, owing to their low thermodynamic requirements, cost-effective energy, high coulombic efficiency, and reduced carbon footprint. However, the unfavorable kinetics of most electrochemical reactions severely limits the large-scale applications of energy conversion devices. To reduce the reaction barrier, efficient electrocatalysts, with high active-site accessibility, abundant surface areas, good electrical conductivity, desirable electrical conductivity and long-term stability, are necessarily required. This dissertation offers a dual-tuning strategy combining structural design and electronic tuning of non-noble-metal-based electrocatalysts. To push the mass/charge transfer of non-noble-meal-based catalysts for practical applications, strategies including structural engineering and optimized electronic modification are applied to achieve efficient and stable electrocatalysts. Porosity engineering is firstly introduced in 2D transition metal-based electrocatalysts to alleviate the restacking issue of the 2D nanomaterials, offering large active surface areas and fast ion transfer (Chapter 3). Besides, to overcome the inferior electron transfer during the electrochemical process, electronic modification, such as anionic substitution, is employed to boost the electron transfer. By applying structural engineering and electronic modification in 2D electrocatalyst, both mass transport and charge transfer are improved. The density of state and the local electronic/atomic structure optimizations of electrocatalysts are further studied by modeling computation. To extend the structural design and electronic modification to a broad range of electrocatalysts, gel-based electrocatalysts with enhanced mass/charge transfer are further introduced. Unlike conventional electrocatalysts prepared from bulky powders suffering from severe issues on mass transport and electron transfer, gel-based electrocatalysts offer larger numbers of active sites, due to unique hierarchical structures, compositional tunability, ease of functionalization, and high wettability for electrolyte penetration (Chapter 4). By introducing functional dopants or alloying with transition metals, not only the electron transfer of gel-derived alloys can be improved, but also the N adsorption energy can be regulated (Chapter 5). Finally, key strategies combining structural design and electronic tuning of non-noble-metal-based electrocatalysts are summarized and possible future directions are provided (Chapter 6).Item Surface chemistry and material integration of metal oxide nanocrystals(2022-09-12) Lakhanpal, Vikram Shri; Milliron, Delia (Delia Jane); Ganesan, Venkat; Lynd, Nathaniel; Yu, GuihuaMetal oxide nanocrystals have a variety of chemical, electronic, and optical properties unique not only to their material composition but also to their size and shape. Control and tunability over these physical parameters can be achieved through colloidal synthesis. In this process, small nuclei form in a heated mixture and long organic molecules known as ligands, which are typically amines or carboxylates, regulate their growth. These ligands also provide long term stability to the nanocrystals when dispersed in solution, as their bulky chains prevent the particles from aggregating. However, many of the properties that make nanocrystals so intriguing involve interaction with their surfaces, which necessitates the removal of the ligands. A variety of methods for ligand stripping have been explored, all with the goal of obtaining ligand-free nanocrystals that retain their properties in dispersions that are stable long enough to combine with other materials, such as polymers, or to process into a functional device, such as a film or coating. In this dissertation, I relate my work during my PhD in various methods of ligand stripping and nanocrystal processing. In particular, I focus on nanocrystalline cerium oxide, a rare-earth metal oxide with a highly reactive surface, and cerium-doped indium oxide, an optically transparent conductor. Cerium oxide nanocrystals are ligand stripped with an organic salt and transferred into dimethylformamide, a polar organic solvent, after which they are mixed with poly(ethylene oxide) to make composite thin films. Protons are generated from water vapor by the cerium oxide surface, turning the films into a proton conducting electrolyte. Cerium-doped indium oxide nanocrystals are stripped with potassium hydroxide in order to transfer them into water, where it is mixed with the conductive polymer PEDOT:PSS in order to create conductive films with enhanced transparency over more opaque films containing just the polymer. Following the mixed results of these projects, I developed and adapted the potassium hydroxide ligand stripping process to a broader range of metal oxide nanocrystals, providing a general method for ligand stripping and transfer into aqueous media.Item Synthesis and electrochromic properties of niobium oxide nanocrystals(2022-04-26) Lu, Hsin-Che; Milliron, Delia (Delia Jane); Korgel, Brian A.; Mullins, Charles B.; Yu, GuihuaNiobium oxide (Nb₂O [subscript 5-x]) nanocrystals hold promise for improving the performance of conventional electrochromic smart windows due to their tunable electrochromic properties within various polymorphs and ideal electrochemical and optical stabilities. By tuning the nanocrystal structure, this study aimed at providing experimental tools to control the electrochromic spectral range and switching kinetics of Nb₂O [subscript 5-x] nanocrystals for electrochromic applications. Alongside the experimental exploration, theoretical background that elucidates the change of electrochromic spectral range and switching kinetics brought by Nb₂O [subscript 5-x] nanocrystals was also investigated. Experimentally, the colloidal synthesis of Nb₂O [subscript 5-x]nanocrystals that produces monoclinic Nb₁₂O₂₉ nanoplatelets was achieved by precisely arranging the structure of niobium precursors. Upon progressively reducing the nanoplatelets, increasing absorbance in the near-infrared region is attributed to a surface-dominated mechanism, whereas the secondary absorbance mode in the visible region is brought by Li⁺ intercalation, establishing the dual-mode electrochromism of the monoclinic Nb₁₂O₂₉ nanoplatelets. The colloidal synthesis was further modified to produce both nanorods and nanoplatelets of monoclinic Nb₁₂O₂₉. This synthetic endeavor allows the investigation on the influence of shape anisotropy on the electrochromic spectral range. Both experimental analysis and calculations based on density functional theory were utilized to show that, in nanoplatelets, the presence of both square planar and crystallographic shear sites enables a higher degree of charge localization during Li⁺ intercalation, leading to absorbance increase in both visible and near-infrared regions, while in nanorods, the Li⁺ only intercalates into the square planar sites with lower degree of charge localization and the absorbance is limited within the near-infrared region. Lastly, nanocrystals of orthorhombic Nb₂O₅, monoclinic Nb₁₂O₂₉, and Sn-doped In₂O₃ were utilized to demonstrate the influence of various charge storage mechanisms on the switching kinetics of electrochromic nanocrystals. The absorbance change over time was collected experimentally and modeled by an exponential-growth equation to quantitatively elucidate the key parameters that control the switching kinetics. We concluded that, for the surface-dominated mechanisms, dual-stage switching kinetics were observed regardless of the materials, suggesting that the switching kinetics are efficient at early stage but becoming slower over time. As for the intercalation mechanism, single-stage switching kinetics controlled by the Li⁺ diffusion was observed.