Browsing by Subject "Colloidal synthesis"
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Item Synthesis and characterization of germanium-based nanocrystals(2021-04-06) Kim, Hyun Gyung; Korgel, Brian Allan, 1969-; Mullins, Charles B.; Delia, Milliron; Roberts, Sean T.Approaches to colloidal synthesis have rapidly developed to control the size, shape, and composition of various semiconductors, offering cost reductions, controllability, and scalability. Of semiconductor materials, germanium nanomaterials are known to be the most difficult to synthesize in solution-based methods because of their high crystallization temperature. Zero-dimensional germanium nanocrystals were synthesized by the heat-up method, without any strong reducing agent. Subsequently, finely controlled size-selective precipitation narrowed size distributions, and size-selected nanocrystals successfully created a monolayer germanium nanocrystals superlattice. One-dimensional germanium nanorods were synthesized by the solution–liquid–solid method using tin nanoparticles as seeds. By forming a liquid alloy with the tin seed at the eutectic temperature, which is much lower than the crystallization temperature, germanium nanorods were grown from the tin seed. A monophenylsilane enhanced the yield of germanium nanorods by promoting the phenyl redistribution of diphenylgermane, a germanium precursor. Using a mixture of HCl and HF, tin seeds were completely removed from the tips of the germanium nanorods, leaving germanium crystalline nanorods. Nonvolatile memories, a key component in various electronics and portable systems, include phase-change memory, a leading technology that has seen exponential growth in demand over the last decade. One important class of phase change materials are compounds on the GeTe–Sb2Te3 tie line. Despite interesting properties of the nanomaterials, colloidal synthesis of phase change material nanocrystals has only been rarely reported. In the present study, three representative phase change material nanocrystals, GeTe, Sb2Te3, and Ge2Sb2Te5, were successfully synthesized using the hot-injection method. A poly(vinylpyrrolidinone)–hexadecane (PVP–HDE) polymer was essential for the nanocrystal dispersion and making ternary Ge2Sb2Te5 nanocrystals. Two solvents, oleylamine and trioctylphosphine, were studied for synthesizing all three nanocrystals and reveal the conversion chemistry of phase change material precursors.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.