Intra-layer doping effects on the high-energy magnetic correlations in NaFeAs

We have used Resonant Inelastic X-ray Scattering (RIXS) and dynamical susceptibility calculations to study the magnetic excitations in NaFe$_{1-x}$Co$_x$As (x = 0, 0.03, and 0.08). Despite a relatively low ordered magnetic moment, collective magnetic modes are observed in parent compounds (x = 0) and persist in optimally (x = 0.03) and overdoped (x = 0.08) samples. Their magnetic bandwidths are unaffected by doping within the range investigated. High energy magnetic excitations in iron pnictides are robust against doping, and present irrespectively of the ordered magnetic moment. Nevertheless, Co doping slightly reduces the overall magnetic spectral weight, differently from previous studies on hole-doped BaFe$_{2}$As$_{2}$, where it was observed constant. Finally, we demonstrate that the doping evolution of magnetic modes is different for the dopants being inside or outside the Fe-As layer.Comment: 19 pages, 7 figure

Correlation of the superconducting critical temperature with spin and orbital excitations in (CaxLa1− x)(Ba1. 75− xLa 0. 25) Cu3Oy as measured by resonant inelastic x-ray scattering

Electronic spin and orbital (dd) excitation spectra of (CaxLa1−x )(Ba1.75−xLa0.25+x)Cu3Oy samples are measured by resonant inelastic x-ray scattering (RIXS). In this compound, Tc of samples with identical hole dopings is strongly affected by the Ca/Ba substitution x due to subtle variations in the lattice constants, while crystal symmetry and disorder as measured by linewidths are x independent. We examine two extreme values of x and two extreme values of hole-doping content y corresponding to antiferromagnetic and superconducting states. The x dependence of the spin-mode energies is approximately the same for both the antiferromagnetic and superconducting samples. This clearly demonstrates that RIXS is sensitive to the superexchange J even in doped samples. A positive correlation between J and the maximum of Tc at optimal doping (T max c) is observed. We also measured the x dependence of the dxy→dx2−y2 and dxz/yz→dx2−y2 orbital splittings. We infer that the effect of the unresolved d3z2−r2→dx2→y2 excitation on T max c is much smaller than the effect of J. There appears to be dispersion in the dxy→dx2−y2 peak of up to 0.05 eV. Our fitting furthermore indicates an asymmetric dispersion for the dxz/yz→dx2−y2 excitation. A peak at ∼0.8 eV is also observed and attributed to a dd excitation in the chain layer

Resolving the effect of oxygen vacancies on Co nanostructures using soft XAS/X-PEEM

Improving both the extent of metallic Co nanoparticle (Co NP) formation and their stability is necessary to ensure good catalytic performance, particularly for Fischer–Tropsch synthesis (FTS). Here, we observe how the presence of surface oxygen vacancies (Ovac) on TiO2 can readily reduce individual Co3O4 NPs directly into CoO/Co0 in the freshly prepared sample by using a combination of X-ray photoemission electron microscopy (X-PEEM) coupled with soft X-ray absorption spectroscopy. The Ovac are particularly good at reducing the edge of the NPs as opposed to their center, leading to smaller particles being more reduced than larger ones. We then show how further reduction (and Ovac consumption) is achieved during heating in H2/syngas (H2 + CO) and reveal that Ovac also prevents total reoxidation of Co NPs in syngas, particularly the smallest (∼8 nm) particles, thus maintaining the presence of metallic Co, potentially improving catalyst performance

Multipletes atómicos, hibridación y correlación electrónica en los difluoruros MF2 (M=Cr-Zn) /

 tesis que para obtener el grado de Doctor en Ciencias Químicas, presenta Paul Olalde Velasco ; asesor José Jiménez Mier y Terán. 194 páginas : ilustraciones. Doctorado en Ciencias Químicas UNAM, Instituto de Ciencias Nucleares, 201

Effect of excess lithium in LiMn 2

We performed a comparative study of the soft x-ray absorption spectroscopy of the LiMn2O4 and Li1.15Mn1.85O4 electrode materials with a quantitative analysis of Mn oxidation states. The revealed redox evolution of Mn upon electrochemical cycling clarifies the effect of excess Li in the materials, which naturally explains the different electrochemical performance. The spectral analysis perfectly agrees with different initial cycling capacities of the two materials. The results show unambiguously that Mn3+ starts to dominate the electrode surface after only one cycle. More importantly, the data show that, while LiMn2O4 electrodes follow the nominal Mn redox evolution, the formation of Mn3+ on the electrode surface is largely retarded for Li1.15Mn1.85O4 during most of the electrochemical processes. Such a different surface Mn redox behavior leads to differences in the detrimental effects of Mn2+ formation on the surface, which is observed directly after only two cycles. Our results provide strong evidence that a key effect of the (bulk) excess Li doping is actually due to processes on the electrode surfaces. Published by AIP Publishing.National Materials Genome Project [2016YFB0700600]; Guangdong Innovation Team Project [2013N080]; Shenzhen Science and Technology Research Grant (peacock plan) [KYPT20141016105435850]; Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy [DE-AC02-05CH11231]; Assistant Secretary for Energy Efficiency, Vehicle Technologies Office of the U.S. Department of Energy (U.S. DOE) under the Advanced Battery Materials Research (BMR)SCI(E)ARTICLE911

Polymers with tailored electronic structure for high capacity lithium battery electrodes.

A conductive polymer is developed for solving the long-standing volume change issue in lithium battery electrodes. A combination of synthesis, spectroscopy and simulation techniques tailors the electronic structure of the polymer to enable in situ lithium doping. Composite anodes based on this polymer and commercial Si particles exhibit 2100 mAh g -1 in Si after 650 cycles without any conductive additive. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Reducing Exciton Binding Energy by Increasing Thin Film Permittivity: An Effective Approach To Enhance Exciton Separation Efficiency in Organic Solar Cells

Photocurrent generation in organic solar cells requires that excitons, which are formed upon light absorption, dissociate into free carriers at the interface of electron acceptor and donor materials. The high exciton binding energy, arising from the low permittivity of organic semiconductor films, generally causes low exciton separation efficiency and subsequently low power conversion efficiency. We demonstrate here, for the first time, that the exciton binding energy in B,O-chelated azadipyrromethene (BO-ADPM) donor films is reduced by increasing the film permittivity by blending the BO-ADPM donor with a high dielectric constant small molecule, camphoric anhydride (CA). Various spectroscopic techniques, including impedance spectroscopy, photon absorption and emission spectroscopies, as well as X-ray spectroscopies, are applied to characterize the thin film electronic and photophysical properties. Planar heterojunction solar cells are fabricated with a BO-ADPM:CA film as the electron donor and C₆₀ as the acceptor. With an increase in the dielectric constant of the donor film from ∼4.5 to ∼11, the exciton binding energy is reduced and the internal quantum efficiency of the photovoltaic cells improves across the entire spectrum, with an ∼30% improvement in the BO-ADPM photoactive region