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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Probing Linewidths and Biexciton Quantum Yields of Single Cesium Lead Halide Nanocrystals in Solution

Cesium lead halide (CsPbX 3 , X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide composition and size. The spectroscopy of single PNCs enhances our understanding of the effect of confinement on excitations in PNCs in the absence of obfuscating ensemble averaging and can also inform synthetic efforts. However, single PNC studies have been hampered by poor PNC photostability under confocal excitation, precluding interrogation of all but the most stable PNCs, and leading to a lack of understanding of PNCs in the regime of high confinement. Here, we report the first comprehensive spectroscopic investigation of single PNC properties using solution-phase photon-correlation methods, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. With minimally perturbative solution-phase photon-correlation Fourier spectroscopy (s-PCFS), we establish that the ensemble emission linewidth of PNCs of all sizes and compositions is predominantly determined by the intrinsic single PNC linewidth (homogeneous broadening). The single PNC linewidth, in turn, dramatically increases with increasing confinement, consistent with what has been found for II–VI semiconductor nanocrystals. With solution-phase photon antibunching measurements, we survey the biexciton-to-exciton quantum yield ratio (BX/X QY) in the absence of user-selection bias or photodegradation. Remarkably, the BX/X QY ratio depends both on the PNC size and halide composition, with values between ~2% for highly confined bromide PNCs and ~50% for intermediately confined iodide PNCs. Our results suggest a wide range of underlying Auger rates, likely due to transitory charge carrier separation in PNCs with relaxed confinement.

single nanocrystal spectroscopy↗
DOE OSTI2017

Tunable UV-Emitters through Graphene Plasmonics

Control over the spontaneous emission of light through tailored optical environments remains a fundamental paradigm in nanophotonics. The use of highly confined plasmons in materials such as graphene provides a promising platform to enhance transition rates in the IR-THz by many orders of magnitude. However, such enhancements involve near-field plasmon modes or other kinds of near-field coupling like quenching, and it is challenging to use these highly confined modes to harness light in the far-field due to the difficulty of plasmonic outcoupling. Here in this paper, we propose that through the use of radiative cascade chains in multilevel emitters, IR plasmons can be used to enhance far field spectra in the visible and UV range, even at energies greater than 10 eV. Combining Purcell-enhancement engineering, graphene plasmonics, and radiative cascade can result in a new type of UV emitter whose properties can be tuned by electrically doping graphene. Varying the distance between the emitter and the graphene surface can change the strength of the far-field emission lines by 2 orders of magnitude. We also find that the dependence of the far-field emission on the Fermi energy is potentially extremely sharp at the onset of interband transitions, allowing the Fermi energy to effectively serve as a “switch” for turning on and off certain plasmonic and far-field emissions.

42 ENGINEERING↗
DOE OSTI2018

Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy

PEDOT:PSS, a transparent electrically conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased conductivity, a detailed understanding of the coupled electronic and morphological landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examination of electronic structure and morphology ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theoretical calculations and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. Here, we find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ultrafast Graphene Light Emitters

Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here in this paper, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Self-Templated Formation of P2-type K 0.6 CoO 2 Microspheres for High Reversible Potassium-Ion Batteries

We report that layered metal oxides have been widely used as the best cathode materials for commercial lithium-ion batteries and are being intensively explored for sodium-ion batteries. However, their application to potassium-ion batteries (PIBs) is hampered because of the poor cycling stability and low rate capability due to the larger ionic size of K + than of Li + or Na + . Herein, a facile self-templated strategy was used to synthesize unique P2-type K 0.6 CoO 2 microspheres that consist of aggregated primary nanoplates as PIB cathodes. The unique K 0.6 CoO 2 microspheres with aggregated structure significantly enhanced the kinetics of the K + intercalation/deintercation and also minimized the parasitic reactions between the electrolyte and K 0.6 CoO 2 . The P2-K 0.6 CoO 2 microspheres demonstrated a high reversible capacity of 82 mAh g –1 at 10 mA g –1 , high rate capability of 65 mAh g –1 at 100 mA g –1 , and long cycle life (87% capacity retention over 300 cycles). The high reversibility of the P2-K 0.6 CoO 2 full cell paired with a hard carbon anode further demonstrated the feasibility of PIBs. Lastly, this work not only successfully demonstrates exceptional performance of P2-type K 0.6 CoO 2 cathodes and microspheres K 0.6 CoO 2 ∥hard carbon full cells, but also provides new insights into the exploration of other layered metal oxides for PIBs.

25 ENERGY STORAGE↗
DOE OSTI2018

Morphological Engineering of Winged Au@MoS 2 Heterostructures for Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS 2 ) has been recognized as a promising cost-effective catalyst for water-splitting hydrogen production. However, the desired performance of MoS 2 is often limited by insufficient edge-terminated active sites, poor electrical conductivity, and inefficient contact to the supporting substrate. To address these limitations, we developed a unique nanoarchitecture (namely, winged Au@MoS 2 heterostructures enabled by our discovery of the “seeding effect” of Au nanoparticles for the chemical vapor deposition synthesis of vertically aligned few-layer MoS 2 wings). The winged Au@MoS 2 heterostructures provide an abundance of edge-terminated active sites and are found to exhibit dramatically improved electrocatalytic activity for the hydrogen evolution reaction. Theoretical simulations conducted for this unique heterostructure reveal that the hydrogen evolution is dominated by the proton adsorption step, which can be significantly promoted by introducing sufficient edge active sites. Our study introduces a new morphological engineering strategy to make the pristine MoS 2 layered structures highly competitive earth-abundant catalysts for efficient hydrogen production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Effect of Nucleophilicity on the Kinetics of CO 2 Insertion into Pincer-Supported Nickel Complexes

The insertion of CO 2 into metal-element σ bonds such as M-H, M-OH, and M-NH 2 is often proposed to be a key step in catalytic CO 2 conversion. Nevertheless, there are few experimental studies measuring the kinetics of CO 2 insertion or comparing insertion into L n M-E (E = H, OH, NH 2 ) complexes with the same ancillary ligands. Here, we use a rapid-mixing stopped-flow instrument to measure the rate of CO 2 insertion into ( tBu PCP)Ni(OH) ( tBu PCP = 2,6-C 6 H 3 (CH 2 P t Bu 2 ) 2 ) and show that it proceeds at a faster rate in comparison to insertion into the corresponding metal hydride. We also demonstrate that the rate of CO 2 insertion into ( tBu PCP)Ni(OH) can generally be increased by using solvents with higher acceptor numbers or by modifying the ligand scaffold to reduce steric bulk or increase electron donation to the metal center. We also attempt to measure the rate of CO 2 insertion into ( tBu PCP)Ni(NH 2 ); however, this reaction is too fast to measure with our methodology. This work enables the direct comparison of CO 2 insertion into analogous L n M-E (E = H, OH, NH 2 ) systems and provides quantitative experimental evidence that the rate of insertion into a metal-element σ bond correlates with the nucleophilicity of E.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Tuning Porosity and Surface Area in Mesoporous Silicon for Application in Li-Ion Battery Electrodes

This work aims to improve the poor cycle lifetime of silicon-based anodes for Li-ion batteries by tuning microstructural parameters such as pore size, pore volume, and specific surface area in chemically synthesized mesoporous silicon. In this paper, we have specifically produced two different mesoporous silicon samples from the magnesiothermic reduction of ordered mesoporous silica in either argon or forming gas. In situ X-ray diffraction studies indicate that samples made in Ar proceed through a Mg 2 Si intermediate, and this results in samples with larger pores (diameter ≈ 90 nm), modest total porosity (34%), and modest specific surface area (50 m 2 g –1 ). Reduction in forming gas, by contrast, results in direct conversion of silica to silicon, and this produces samples with smaller pores (diameter ≈ 40 nm), higher porosity (41%), and a larger specific surface area (70 m 2 g –1 ). The material with smaller pores outperforms the one with larger pores, delivering a capacity of 1121 mAh g –1 at 10 A g –1 and retains 1292 mAh g –1 at 5 A g –1 after 500 cycles. For comparison, the sample with larger pores delivers a capacity of 731 mAh g –1 at 10 A g –1 and retains 845 mAh g –1 at 5 A g –1 after 500 cycles. The dependence of capacity retention and charge storage kinetics on the nanoscale architecture clearly suggests that these microstructural parameters significantly impact the performance of mesoporous alloy type anodes. Our work is therefore expected to contribute to the design and synthesis of optimal mesoporous architectures for advanced Li-ion battery anodes.

25 ENERGY STORAGE↗
DOE OSTI2017

Graphene–Titanium Interfaces from Molecular Dynamics Simulations

Unraveling the physical and chemical properties of graphene–metal contacts is a key step toward the development of graphitic electronic nanodevices. Although many studies have revealed the way that various metals interact with graphene, few have described the structure and behavior of large pieces of graphene–metal nanostructures under different conditions. In this paper, we present the first classical molecular dynamics study of graphene–titanium (G–Ti) structures, with and without substrates. Physical and chemical properties of equilibrium structures of G–Ti interfaces with different amounts of titanium coverage are investigated. Adhesion of Ti films on graphene is shown to be enhanced by the vacancies in graphene or the electrostatic influence of substrates. The dynamics of pristine G–Ti structures at different temperatures on planar and nonplanar substrates are investigated, and the results show that G–Ti interfaces are thermally stable, that is, not prone to any reaction toward the formation of titanium carbide.

36 MATERIALS SCIENCE↗
DOE OSTI2017

LiMn 2 O 4 Surface Chemistry Evolution during Cycling Revealed by in Situ Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy

This work utilizes in situ electrochemical and analytical characterization during cycling of LiMn 2 O 4 (LMO) equilibrated at different potentials in an ultrahigh vacuum (UHV) environment. The LMO reacts with organic molecules in the vacuum to form a high surface concentration of Li 2 CO 3 (≈50% C) during initial charging to 4.05 V. Charging to higher potentials reduces the overall Li 2 CO 3 concentration (≈15% C). Discharging to 3.0 V increases the Li 2 CO 3 concentration (≈30% C) and over discharging to 0.1 V again reduces its concentration (≈15% C). This behavior is reproducible over 5 cycles. The model geometry utilized suggests that oxygen from LMO can participate in redox of carbon, where LMO contributes oxygen to form the carbonate in the solid electrolyte interphase (SEI). Similar results were obtained from samples cycled ex situ , suggesting that the model in situ geometry provides reasonably representative information about surface chemistry evolution. Finally, carbon redox at LMO and the inherent voltage instability of the Li 2 CO 3 likely contributes significantly to its capacity fade.

25 ENERGY STORAGE↗
DOE OSTI2018

Simple Fabrication Method for Mixed Matrix Membranes with in Situ MOF Growth for Gas Separation

Metal organic framework (MOF)/polymer composite membranes are of interest for gas separations, as they often have performance that exceeds the neat polymer. However, traditional composite membranes, known as mixed matrix membranes (MMMs), can have complex and time-consuming preparation procedures. The MOF and polymer are traditionally prepared separately and require priming and mixing to ensure uniform distribution of particles and compatibility of the polymer–particle interface. In this study, we reduce the number of steps using an in situ MOF growth strategy. Herein, MMMs are prepared by growing MOF (UiO-66) in situ within a Matrimid polymer matrix while simultaneously curing the matrix. The gas separation performance for MMMs, prepared using this approach, was evaluated for the CO 2 /N 2 separation and compared with MMMs made using the traditional postsynthesis mixing. It was found that MMMs prepared using both the in situ MOF growth strategy and by traditional postsynthesis mixing are equivalent in performance. Finally, using the in situ MOF growth allows for a simpler, faster, and potentially more economical fabrication alternative for MMMs.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Adipic Acid Production via Metal-Free Selective Hydrogenolysis of Biomass-Derived Tetrahydrofuran-2,5-Dicarboxylic Acid

Biomass-derived furans offer sustainable routes to adipic acid (AA), a key chemical in Nylon-6,6 synthesis. In this work, we show that tetrahydrofuran-2,5-dicarboxylic acid (THFDCA) is a viable precursor for AA production, achieving up to 89% yield in a metal-free system containing HI and molecular H 2 in a propionic acid solvent at 160 °C. Reactivity studies demonstrate that the interplay between HI, H 2 , and the solvent is essential for effective THFDCA ring opening. By measuring the reaction orders of HI and molecular H 2 and calculating an acid–base equilibrium constant in a nonaqueous solvent, we show that HI plays a multifaceted role in the reaction by acting both as a proton source and an iodide source to selectively cleave C–O bonds without overhydrogenation of carboxylic acid groups. Using reactivity studies, kinetic measurements, and first-principles computational insights, we demonstrate that metal-free activation of molecular H 2 plays a key role in the reaction, following HI-mediated cleavage of the etheric C–O bond in THFDCA.

09 BIOMASS FUELS↗
DOE OSTI2018

Redox-Mediator-Assisted Electrocatalytic Hydrogen Evolution from Water by a Molybdenum Sulfide-Functionalized Metal–Organic Framework

The Zr 6 -based metal–organic framework NU-1000 was successfully functionalized with candidate catalysts—MoS x units—via SIM (solvothermal deposition in MOFs) of molybdenum(VI), followed by reaction with H2S gas. The structure of the material, named MoS x - SIM, was characterized spectroscopically and through a single-crystal X-ray diffraction measurement. These measurements and others established that the catalyst is monometallic, with mixed oxygen and sulfur coordination, and that it forms from a MOF-node-supported molybdenum-based cluster featuring only oxy ligands. Notably, the formal potential for the MOF-grafted complex, like that for the metal–sulfur active site of hydrogenase, is nearly coincident with the formal potential for the hydrogen couple. Its effective concentration within the mesoporous MOF is several hundred millimolar, and its porous-framework-based immobilization/heterogenization obviates the need for aqueous solubility as a condition for use as a well-defined catalyst. MoS x - SIM was evaluated as an electrocatalyst for evolution of molecular hydrogen from aqueous acid. Although the MoS x - functionalized framework exhibits catalytic behavior, the highly insulating nature of the support inhibits high electrocatalytic performance. Introduction of an archetypal redox mediator (RM), methyl viologen (MV 2+ ), resulted in more than 20-fold enhancement in its catalytic performance on a turnover frequency basis, implying efficient RM-assisted electron transfer to otherwise electrochemically non-addressable MoS x moieties. Electrochemical kinetic studies with additional viologens as mediators reveal an unexpected square-root dependence of overall reaction rate on mediator concentration, as well as sensitivity to the strength of RM •+ as a reductant. These observations, together with observations of potential-dependent H/D isotope effects and potential-dependent pH effects, provide useful insights into the catalysis mechanism and help to explain how the MOF-affixed monometallic catalyst can effectively catalyze a two-electron reduction reaction, i.e., hydrogen evolution from acidified water.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗