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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Enhanced Charge Transport in 2D Perovskites via Fluorination of Organic Cation

Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky spacer cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluorophenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with better aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained a power conversion efficiency of >13% for (F-PEA) 2 MA 4 Pb 5 I 16 -based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.

14 SOLAR ENERGY↗
DOE OSTI2013

Structure, Kinetics, and Thermodynamics of the Aqueous Uranyl(VI) Cation

Molecular simulation techniques are employed to gain insights into the structural, kinetic, and thermodynamic properties of the uranyl(VI) cation (UO22+) in aqueous solution. The simulations make use of an atomistic potential model (force field) derived in this work and based on the model of Guilbaud and Wipff (Guilbaud, P.; Wipff, G. J. Mol. Struct. (THEOCHEM) 1996, 366, 55-63). Reactive flux and thermodynamic integration calculations show that the derived potential model yields predictions for the water exchange rate and free energy of hydration, respectively, that are in agreement with experimental data. The water binding energies, hydration shell structure, and self-diffusion coefficient are also calculated and discussed. Finally, a combination of metadynamics and transition path sampling simulations is employed to probe the mechanisms of water exchange reactions in the first hydration shell of the uranyl ion. These atomistic simulations indicate, based on two-dimensional free energy surfaces, that water exchanges follow an associative interchange mechanism. The nature and structure of the water exchange transition states are also determined. The improved potential model is expected to lead to more accurate predictions of uranyl adsorption energies at mineral surfaces using potential-based molecular dynamics simulations.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Discovery and Mechanistic Studies of Facile N-Terminal Cα–C Bond Cleavages in the Dissociation of Tyrosine-Containing Peptide Radical Cations

Gas phase fragmentations of protein and peptide (M) ions in a mass spectrometer—induced by, for example, electron-capture dissociation1-2 and electron-transfer dissociation3-422 —form the foundation for top-down amino acid sequencing approaches for the rapid identification of protein components in complex biological samples. During these processes, protonated protein and peptide radicals ([M + nH]•(n – 1)+)5–8 are generated; their fragmentations are governed largely by the properties of the unpaired electron. Because of their importance in modern bioanalytical chemistry, considerable attention has been drawn recently toward understanding the radical cation chemistry behind the fragmentations of these odd-electron biomolecular ions in the gas phase.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Ionic liquids composed of phosphonium cations and organophosphate, carboxylate, and sulfonate as lubricant antiwear additives

Oil-soluble phosphonium-based ionic liquids (ILs) have recently been reported as potential ashless lubricant additives. This study is to expand the IL chemistry envelope and to achieve fundamental correlations between the ion structures and ILs’ physiochemical and tribological properties. Here we present eight ILs containing two different phosphonium cations and seven different anions from three groups: organophosphate, carboxylate, and sulfonate. The oil solubility of ILs seems largely governed by the IL molecule size and structure complexity. When used as oil additives, the ranking of effectiveness in wear protection for the anions are: organophosphate > carboxylate > sulfonate. All selected ILs outperformed a commercial ashless anti-wear additive. Surface characterization from the top and the cross-section revealed the nanostructures and compositions of the tribo-films formed by the ILs. Some fundamental insights were achieved: branched and long alkyls improve the IL’s oil solubility, anions of a phosphonium-phosphate IL contribute most phosphorus in the tribofilm, and carboxylate anions, though free of P, S, N, or halogen, can promote the formation of an anti-wear tribofilm.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Doping against the native propensity of MoS₂: Degenerate hole doping by cation substitution

Layered transition metal dichalcogenides (TMDs) draw much attention as the key semiconducting material for two-dimensional electrical, optoelectronic, and spintronic devices. For most of these applications, both n- and p-type materials are needed to form junctions and support bipolar carrier conduction. However, typically only one type of doping is stable for a particular TMD. For example, molybdenum disulfide (MoS₂) is natively an n-type presumably due to omnipresent electron-donating sulfur vacancies, and stable/controllable p-type doping has not been achieved. The lack of p-type doping hampers the development of charge-splitting p–n junctions of MoS₂, as well as limits carrier conduction to spin-degenerate conduction bands instead of the more interesting, spin-polarized valence bands. Traditionally, extrinsic p-type doping in TMDs has been approached with surface adsorption or intercalation of electron-accepting molecules. However, practically stable doping requires substitution of host atoms with dopants where the doping is secured by covalent bonding. In this work, we demonstrate stable p-type conduction in MoS₂ by substitutional niobium (Nb) doping, leading to a degenerate hole density of ~3 × 10¹⁹ cm⁻³. Structural and X-ray techniques reveal that the Nb atoms are indeed substitutionally incorporated into MoS₂ by replacing the Mo cations in the host lattice. van der Waals p–n homojunctions based on vertically stacked MoS₂ layers are fabricated, which enable gate-tunable current rectification. A wide range of microelectronic, optoelectronic, and spintronic devices can be envisioned from the demonstrated substitutional bipolar doping of MoS₂. From the miscibility of dopants with the host, it is also expected that the synthesis technique demonstrated here can be generally extended to other TMDs for doping against their native unipolar propensity.

molybdenum disulfide↗
DOE OSTI2017

Electrically driven cation exchange for in situ fabrication of individual nanostructures

Cation exchange (CE) has been recognized as a particularly powerful tool for the synthesis of heterogeneous nanocrystals. Presently, CE can be divided into two categories, namely ion solvation-driven CE reaction and thermally activated CE reaction. Here we report an electrically driven CE reaction to prepare individual nanostructures inside a transmission electron microscope. During the process, Cd is eliminated due to Ohmic heating, whereas Cu + migrates into the crystal driven by the electrical field force. Contrast experiments reveal that the feasibility of electrically driven CE is determined by the structural similarity of the sulfur sublattices between the initial and final phases, and the standard electrode potentials of the active electrodes. These experimental results demonstrate a strategy for the selective growth of individual nanocrystals and provide crucial insights into understanding of the microscopic pathways leading to the formation of heterogeneous structures.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Opposite correlations between cation disordering and amorphization resistance in spinels versus pyrochlores

Understanding and predicting radiation damage evolution in complex materials is crucial for developing next-generation nuclear energy sources. Here, using a combination of ion beam irradiation, transmission electron microscopy and X-ray diffraction, we show that, contrary to the behaviour observed in pyrochlores, the amorphization resistance of spinel compounds correlates directly with the energy to disorder the structure. Using a combination of atomistic simulation techniques, we ascribe this behaviour to structural defects on the cation sublattice that are present in spinel but not in pyrochlore. Specifically, because of these structural defects, there are kinetic pathways for the relaxation of disorder in spinel that are absent in pyrochlore. This leads to a direct correlation between amorphization resistance and disordering energetics in spinel, the opposite of that observed in pyrochlores. Furthermore, these results provide new insight into the origins of amorphization resistance in complex oxides beyond fluorite derivatives.

36 MATERIALS SCIENCE↗
DOE OSTI2018

99 TcO 4 - remediation by a cationic polymeric network

Direct removal of 99 TcO 4 - from the highly acidic solution of used nuclear fuel is highly beneficial for the recovery of uranium and plutonium and more importantly aids in the elimination of 99 Tc discharge into the environment. However, this task represents a huge challenge given the combined extreme conditions of super acidity, high ionic strength, and strong radiation field. Here in this paper we overcome this challenge using a cationic polymeric network with significant TcO 4 - uptake capabilities in four aspects: the fastest sorption kinetics, the highest sorption capacity, the most promising uptake performance from highly acidic solutions, and excellent radiation-resistance and hydrolytic stability among all anion sorbent materials reported. In addition, this material is fully recyclable for multiple sorption/desorption trials, making it extremely attractive for waste partitioning and emergency remediation. The excellent TcO 4 - uptake capability is elucidated by X-ray absorption spectroscopy, solidstate NMR measurement, and density functional theory analysis on anion coordination and bonding.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Anion-adaptive crystalline cationic material for 99 TcO 4 - trapping

Efficient anion recognition is of great significance for radioactive 99 TcO 4 - decontamination, but it remains a challenge for traditional sorbents. Herein, we put forward a tactic using soft crystalline cationic material with anion-adaptive dynamics for 99 TcO 4 - sequestration. A cucurbit[8]uril-based supramolecular metal-organic material is produced through a multi-component assembly strategy and used as a sorbent for effective trapping of TcO 4 - . Excellent separation of TcO 4 - /ReO 4 - is demonstrated by fast removal kinetics, good sorption capacity and high distribution coefficient. Remarkably, the most superior selectivity among metal-organic materials reported so far, together with good hydrolytic stability, indicates potential for efficient TcO 4 - removal. The structure incorporating ReO 4 - reveals that the supramolecular framework undergoes adaptive reconstruction facilitating the effective accommodation of TcO 4 - /ReO 4 - . The results highlight opportunities for development of soft anion-adaptive sorbents for highly selective anion decontamination.

36 MATERIALS SCIENCE↗
DOE OSTI2020

99 TcO 4 - removal from legacy defense nuclear waste by an alkaline-stable 2D cationic metal organic framework

Removal of 99 TcO 4 - from legacy defense nuclear tank waste at Savannah River Site is highly desirable for the purpose of nuclear safety and environmental protection, but currently not achievable given the extreme conditions including high alkalinity, high ionic strength, and strong radiation field. Herein, we present a potential solution to this long-term issue by developing a two-dimensional cationic metal organic framework SCU-103, showing ultrahigh stability in alkaline aqueous media and great resistance to both β and γ radiation. More importantly, it is very effective for 99 TcO 4 - separation from aqueous media as demonstrated by fast exchange kinetics, high sorption capacity, and superior selectivity, leading to the successful removal of 99 TcO 4 - from actual Savannah River Site high level tank waste for the first time, to the best of our knowledge. In addition, the uptake mechanism is comprehensively elucidated by molecular dynamics simulation and density functional theory calculation, showing a unique chemical recognition of anions with low charge density.

98 NUCLEAR DISARMAMENT, SAFEGUARDS, AND PHYSICAL P↗
DOE OSTI2021

Mechanistic insight into copper cation exchange in cadmium selenide semiconductor nanocrystals using X-ray absorption spectroscopy

In terms of producing new advances in sustainable nanomaterials, cation exchange (CE) of post-processed colloidal nanocrystals (NCs) has opened new avenues towards producing non-toxic energy materials via simple chemical techniques. The main processes governing CE can be explained by considering hard/soft acid/base theory, but the detailed mechanism of CE, however, has been debated and has been attributed to both diffusion and vacancy processes. In this work, we have performed in situ x-ray absorption spectroscopy to further understand the mechanism of the CE of copper in solution phase CdSe NCs. The x-ray data indicates clear isosbestic points, suggestive of cooperative behavior as previously observed via optical spectroscopy. Examination of the extended x-ray absorption fine structure data points to the observation of interstitial impurities during the initial stages of CE, suggesting the diffusion process is the fundamental mechanism of CE in this system.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Structural basis for inhibition of the Cation-chloride cotransporter $\mathrm{NKCC1}$ by the diuretic drug bumetanide

Cation-chloride cotransporters (CCCs) NKCC1 and NKCC2 catalyze electroneutral symport of 1 Na + , 1K + , and 2 Cl - across cell membranes. NKCC1 mediates trans-epithelial Cl - secretion and regulates excitability of some neurons and NKCC2 is critical to renal salt reabsorption. Both transporters are inhibited by the so-called loop diuretics including bumetanide, and these drugs are a mainstay for treating edema and hypertension. Here, our single-particle electron cryo-microscopy structures supported by functional studies reveal an outward-facing conformation of NKCC1, showing bumetanide wedged into a pocket in the extracellular ion translocation pathway. Based on these and the previously published inward-facing structures, we define the translocation pathway and the conformational changes necessary for ion translocation. We also identify an NKCC1 dimer with separated transmembrane domains and extensive transmembrane and C-terminal domain interactions. We further define an N-terminal phosphoregulatory domain that interacts with the C-terminal domain, suggesting a mechanism whereby (de)phosphorylation regulates NKCC1 by tuning the strength of this domain association.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Atomic-scale probing of short-range order and its impact on electrochemical properties in cation-disordered oxide cathodes

Abstract Chemical short-range-order has been widely noticed to dictate the electrochemical properties of Li-excess cation-disordered rocksalt oxides, a class of cathode based on earth abundant elements for next-generation high-energy-density batteries. Existence of short-range-order is normally evidenced by a diffused intensity pattern in reciprocal space, however, derivation of local atomic arrangements of short-range-order in real space is hardly possible. Here, by a combination of aberration-corrected scanning transmission electron microscopy, electron diffraction, and cluster-expansion Monte Carlo simulations, we reveal the short-range-order is a convolution of three basic types: tetrahedron, octahedron, and cube. We discover that short-range-order directly correlates with Li percolation channels, which correspondingly affects Li transport behavior. We further demonstrate that short-range-order can be effectively manipulated by anion doping or post-synthesis thermal treatment, creating new avenues for tailoring the electrochemical properties. Our results provide fundamental insights for decoding the complex relationship between local chemical ordering and properties of crystalline compounds.

25 ENERGY STORAGE↗
DOE OSTI2023

Triarylmethyl cation redox mediators enhance Li–O 2 battery discharge capacities

A major impediment to Li–O 2 battery commercialization is the low discharge capacities resulting from electronically insulating Li 2 O 2 film growth on carbon electrodes. Redox mediation offers an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li 2 O 2 film growth and extending discharge lifetimes. As such, the exploration of diverse redox mediator classes can aid the development of molecular design criteria. Here, in this work, we report a class of triarylmethyl cations that are effective at enhancing discharge capacities up to 35-fold. Surprisingly, we observe that redox mediators with more positive reduction potentials lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structure–property relationships for future improvements in redox-mediated O 2 /Li 2 O 2 discharge capacities. Furthermore, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and the concentrations needed to achieve efficient redox mediation at a given current density. We expect this analysis to guide future redox mediator exploration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Carrier control in Sn–Pb perovskites via 2D cation engineering for all-perovskite tandem solar cells with improved efficiency and stability

All-perovskite tandem solar cells are promising for achieving photovoltaics with power conversion efficiencies above the detailed balance limit of single-junction cells, while retaining the low cost, light weight and other advantages associated with metal halide perovskite photovoltaics. However, the efficiency and stability of all-perovskite tandem cells are limited by the Sn-Pb-based narrow-bandgap perovskite cells. Here we show that the formation of quasi-two-dimensional (quasi-2D) structure (PEA) 2 GAPb 2 I 7 from additives based on mixed bulky organic cations phenethylammonium (PEA+) and guanidinium (GA+) provides critical defect control to substantially improve the structural and optoelectronic properties of the narrow-bandgap (1.25 eV) Sn-Pb perovskite thin films. Additionally, this 2D additive engineering results in Sn-Pb-based absorbers with low dark carrier density (~1.3 x 10 14 cm -3 ), long bulk carrier lifetime (~9.2 us) and low surface recombination velocity (~1.4 cm s -1 ), leading to 22.1%-efficient single-junction Sn-Pb perovskite cells and 25.5%-efficient all-perovskite two-terminal tandems with high photovoltage and long operational stability.

14 SOLAR ENERGY↗
DOE OSTI2017

Mechanisms and time-resolved dynamics for trihydrogen cation (H 3 +) formation from organic molecules in strong laser fields

Strong-field laser-matter interactions often lead to exotic chemical reactions. Trihydrogen cation formation from organic molecules is one such case that requires multiple bonds to break and form. Here, we present evidence for the existence of two different reaction pathways for H 3 + formation from organic molecules irradiated by a strong-field laser. Assignment of the two pathways was accomplished through analysis of femtosecond time-resolved strong-field ionization and photoion-photoion coincidence measurements carried out on methanol isotopomers, ethylene glycol, and acetone. Ab initio molecular dynamics simulations suggest the formation occurs via two steps: the initial formation of a neutral hydrogen molecule, followed by the abstraction of a proton from the remaining CHOH 2+ fragment by the roaming H 2 molecule. This reaction has similarities to the H 2 +H 2 + mechanism leading to formation of H 3 + in the universe. These exotic chemical reaction mechanisms, involving roaming H 2 molecules, are found to occur in the ~100 fs timescale. Roaming molecule reactions may help to explain unlikely chemical processes, involving dissociation and formation of multiple chemical bonds, occurring under strong laser fields.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

Paclitaxel loading in cationic liposome vectors is enhanced by replacement of oleoyl with linoleoyl tails with distinct lipid shapes

Abstract Lipid carriers of hydrophobic paclitaxel (PTX) are used in clinical trials for cancer chemotherapy. Improving their loading capacity requires enhanced PTX solubilization. We compared the time-dependence of PTX membrane solubility as a function of PTX content in cationic liposomes (CLs) with lipid tails containing one (oleoyl; DOPC/DOTAP) or two (linoleoyl; DLinPC/newly synthesized DLinTAP) cis double bonds by using microscopy to generate kinetic phase diagrams. The DLin lipids displayed significantly increased PTX membrane solubility over DO lipids. Remarkably, 8 mol% PTX in DLinTAP/DLinPC CLs remained soluble for approximately as long as 3 mol% PTX (the solubility limit, which has been the focus of most previous studies and clinical trials) in DOTAP/DOPC CLs. The increase in solubility is likely caused by enhanced molecular affinity between lipid tails and PTX, rather than by the transition in membrane structure from bilayers to inverse cylindrical micelles observed with small-angle X-ray scattering. Importantly, the efficacy of PTX-loaded CLs against prostate cancer cells (their IC50 of PTX cytotoxicity) was unaffected by changing the lipid tails, and toxicity of the CL carrier was negligible. Moreover, efficacy was approximately doubled against melanoma cells for PTX-loaded DLinTAP/DLinPC over DOTAP/DOPC CLs. Our findings demonstrate the potential of chemical modifications of the lipid tails to increase the PTX membrane loading while maintaining (and in some cases even increasing) the efficacy of CLs. The increased PTX solubility will aid the development of liposomal PTX carriers that require significantly less lipid to deliver a given amount of PTX, reducing side effects and costs.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Lysosomal enzyme binding to the cation-independent mannose 6-phosphate receptor is regulated allosterically by insulin-like growth factor 2

The cation-independent mannose 6-phosphate receptor (CI-MPR) is clinically significant in the treatment of patients with lysosomal storage diseases because it functions in the biogenesis of lysosomes by transporting mannose 6-phosphate (M6P)-containing lysosomal enzymes to endosomal compartments. CI-MPR is multifunctional and modulates embryonic growth and fetal size by downregulating circulating levels of the peptide hormone insulin-like growth factor 2 (IGF2). The extracellular region of CI-MPR comprises 15 homologous domains with binding sites for M6P-containing ligands located in domains 3, 5, 9, and 15, whereas IGF2 interacts with residues in domain 11. How a particular ligand affects the receptor’s conformation or its ability to bind other ligands remains poorly understood. To address these questions, we purified a soluble form of the receptor from newborn calf serum, carried out glycoproteomics to define the N-glycans at its 19 potential glycosylation sites, probed its ability to bind lysosomal enzymes in the presence and absence of IGF2 using surface plasmon resonance, and assessed its conformation in the presence and absence of IGF2 by negative-staining electron microscopy and hydroxyl radical protein footprinting studies. Together, our findings support the hypothesis that IGF2 acts as an allosteric inhibitor of lysosomal enzyme binding by inducing global conformational changes of CI-MPR.

59 BASIC BIOLOGICAL SCIENCES↗