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Browsing by Author "Seifert, Soenke"
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Item Colloidal Synthesis of Single-Layer Quasi-Ruddlesden–Popper Phase Bismuth-Based Two-Dimensional Perovskite Nanosheets with Controllable Optoelectronic Properties(ACS, 2021-07) Lee, Jacob T.; Seifert, Soenke; Sardar, Rajesh; Chemistry, School of ScienceSingle- and few-layered two-dimensional (2D) nanomaterials have attracted intense research interest in the last two decades due to their unique electronic and optoelectronic properties leading to various potential applications. Herein, we report the colloidal synthesis of Bi-based 2D perovskite nanosheets (PEG6-NH3+)nCs3–nBi2X9, where X = Cl, Br, and I, through careful design of reaction conditions and selection of poly(ethylene glycol) (PEG6) surface passivating ligands. The 2D nanosheets are ∼5 nm in thickness with micron-sized lateral dimensions and display composition-dependent band gap and work function modulation. Small-angle X-ray scattering analysis substantiates that the individual inorganic crystal layer, Cs3–nBi2X9, is separated by the spacer, PEG6 ligand. Additionally, we determined that PEG6-NH2 is an essential passivating ligand and spacer for the formation of Bi-based 2D nanosheets. Most importantly, controlled crystallization of the colloidal dispersion of nanosheets results in the formation of superlattice microstructures of the quasi-Ruddlesden–Popper phase. These microstructures can be exfoliated to ultrathin nanosheets by overcoming the van der Waals interaction between the organic passivating layers. The controlled synthesis of lead-free 2D perovskite nanosheets presented here can expand their utility to photocatalytic and optoelectronic applications with reduced toxicity.Item Phase Coexistence in Single-Lipid Membranes Induced by Buffering Agents(American Chemical Society, 2014-08-26) Johnson, Merrell A.; Seifert, Soenke; Petrache, Horia I.; Kimble-Hill, Ann C.; Department of Biochemistry & Molecular Biology, IU School of MedicineRecent literature has shown that buffers affect the interaction between lipid bilayers through a mechanism that involves van der Waals forces, electrostatics, hydration forces and membrane bending rigidity. This letter shows an additional peculiar effect of buffers on the mixed chain 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers, namely phase coexistence similar to what was reported by Rappolt et al. for alkali chlorides. The data presented suggest that one phase appears to dehydrate below the value in pure water, while the other phase swells as the concentration of buffer is increased. However, since the two phases must be in osmotic equilibrium with one another, this behavior challenges theoretical models of lipid interactions.Item Programmable Colloidal Approach to Hierarchical Structures of Methylammonium Lead Bromide Perovskite Nanocrystals with Bright Photoluminescent Properties(ACS, 2017-04) Teunis, Meghan B.; Johnson, Merrell A.; Muhoberac, Barry B.; Seifert, Soenke; Sardar, Rajesh; Chemistry and Chemical Biology, School of ScienceSystematic tailoring of nanocrystal architecture could provide unprecedented control over their electronic, photophysical, and charge transport properties for a variety of applications. However, at present, manipulation of the shape of perovskite nanocrystals is done mostly by trial-and-error-based experimental approaches. Here, we report systematic colloidal synthetic strategies to prepare methylammonium lead bromide quantum platelets and quantum cubes. In order to control the nucleation and growth processes of these nanocrystals, we appropriately manipulate the solvent system, surface ligand chemistry, and reaction temperature causing syntheses into anisotropic shapes. We demonstrate that both the presence of chlorinated solvent and a long chain aliphatic amine in the reaction mixture are crucial for the formation of ultrathin quantum platelets (∼2.5 nm in thickness), which is driven by mesoscale-assisted growth of spherical seed nanocrystals (∼1.6 nm in diameter) through attachment of monomers onto selective crystal facets. A combined surface and structural characterization, along with small-angle X-ray scattering analysis, confirm that the long hydrocarbon of the aliphatic amine is responsible for the well ordered hierarchical stacking of the quantum platelets of 3.5 nm separation. In contrast, the formation of ∼12 nm edge-length quantum cubes is a kinetically driven process in which a high flux of monomers is achieved by supplying thermal energy. The photoluminescence quantum yield of our quantum platelets (∼52%) is nearly 2-fold higher than quantum cubes. Moreover, the quantum platelets display a lower nonradiative rate constant than that found with quantum cubes, which suggests less surface trap states. Together, our research has the potential both to improve the design of synthetic methods for programmable control of shape and assembly and to provide insight into optoelectronic properties of these materials for solid-state device fabrication, e.g., light-emitting diodes, solar cells, and lasing materials.Item Reorganization of Ternary Lipid Mixtures of Non-Phosphorylated Phosphatidylinositol Interacting with Angiomotin(ACS, 2018) Kimble-Hill, Ann C.; Petrache, Horia I.; Seifert, Soenke; Firestone, Millicent A.; Biochemistry and Molecular Biology, School of MedicinePhosphatidylinositol (PI) lipids are necessary for many cellular signaling pathways of membrane associated proteins, such as Angiomotin (Amot). The Amot family regulates cellular polarity, growth, and migration. Given the low concentration of PI lipids in these membranes, it is likely that such protein-membrane interactions are stabilized by lipid domains or small lipid clusters. By small-angle x-ray scattering, we show that non-phosphorylated PI lipids induce lipid de-mixing in ternary mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE), likely due to preferential interactions between the head groups of PE and PI. These results were obtained in the presence of buffer containing concentrations of Tris, HEPES, NaCl, EDTA, DTT, and Benzamidine at pH 8.0 that in previous work showed an ability to cause PC to phase separate but are necessary to stabilize Amot for in vitro experimentation. Collectively, this provided a framework for determining the effect of Amot on lipid organization. Using fluorescence spectroscopy, we were able to show that the association of Amot with this lipid platform causes significant reorganization of the lipid into a more homogenous organization. This reorganization mechanism could be the basis for Amot membrane association and fusigenic activity previously described in the literature and should be taken into consideration in future protein-membrane interaction studies.Item Ultrathin Plasmonic Tungsten Oxide Quantum Wells with Controllable Free Carrier Densities(ACS, 2020-03) Prusty, Gyanaranjan; Lee, Jacob T.; Seifert, Soenke; Muhoberac, Barry B.; Sardar, Rajesh; Chemistry and Chemical Biology, School of ScienceLocalized surface plasmon resonances (LSPR) of nanostructures can be tuned by controlling their morphology, local dielectric environment, and free carrier concentration. We report the colloidal synthesis of an ∼3 tungsten–oxygen (W-O) layer thick (∼1 nm), two-dimensional (2D) WO3-x nanoplatelets (NPLs) (x ≈ 0.55–1.03), which display tunable near-infrared LSPR properties and additionally high free electron density (Ne) that arises predominantly from the large shape factor of 2D NPLs. Importantly, the W to O composition ratios inferred from their LSPR measurements show much higher percentage of oxygen vacancies than those determined by X-ray diffraction analysis, suggesting that the aspect ratio of ultrathin WO3-x NPLs is the key to producing an unprecedentedly large Ne, although synthesis temperature is also an independent factor. We find that NPL formation is kinetically controlled, whereas thermodynamic parameter manipulation leads to Ne values as high as 4.13 × 1022 cm–3, which is close to that of plasmonic noble metals, and thus our oxide-based nanostructures can be considered as quasi-metallic. The unique structural properties of 2D nanomaterials along with the high Ne of WO3-x NPLs provide an attractive alternative to plasmonic noble metal nanostructures for various plasmon-driven energy conversions and design of photochromic nanodevices.