91 research outputs found
Electrochemistry at nanoscale electrodes : individual single-walled carbon nanotubes (SWNTs) and SWNT-templated metal nanowires
Individual nanowires (NWs) and native single-walled carbon nanotubes (SWNTs) can be readily used as well-defined nanoscale electrodes (NSEs) for voltammetric analysis. Here, the simple photolithography-free fabrication of submillimeter long Au, Pt, and Pd NWs, with sub-100 nm heights, by templated electrodeposition onto ultralong flow-aligned SWNTs is demonstrated. Both individual Au NWs and SWNTs are employed as NSEs for electron-transfer (ET) kinetic quantification, using cyclic voltammetry (CV), in conjunction with a microcapillary-based electrochemical method. A small capillary with internal diameter in the range 30–70 μm, filled with solution containing a redox-active mediator (FcTMA+ ((trimethylammonium)methylferrocene), Fe(CN)64–, or hydrazine) is positioned above the NSE, so that the solution meniscus completes an electrochemical cell. A 3D finite-element model, faithfully reproducing the experimental geometry, is used to both analyze the experimental CVs and derive the rate of heterogeneous ET, using Butler–Volmer kinetics. For a 70 nm height Au NW, intrinsic rate constants, k0, up to ca. 1 cm s–1 can be resolved. Using the same experimental configuration the electrochemistry of individual SWNTs can also be accessed. For FcTMA+/2+ electrolysis the simulated ET kinetic parameters yield very fast ET kinetics (k0 > 2 ± 1 cm s–1). Some deviation between the experimental voltammetry and the idealized model is noted, suggesting that double-layer effects may influence ET at the nanoscale
Observation and control of maximal Chern numbers in a chiral topological semimetal
Topological semimetals feature protected nodal band degeneracies
characterized by a topological invariant known as the Chern number (C). Nodal
band crossings with linear dispersion are expected to have at most |C|=4, which
sets an upper limit to the magnitude of many topological phenomena in these
materials. Here we show that the chiral crystal PdGa displays multifold band
crossings, which are connected by exactly four surface Fermi-arcs, thus proving
that they carry the maximal Chern number magnitude of 4. By comparing two
enantiomers, we observe a reversal of their Fermi-arc velocities, which
demonstrates that the handedness of chiral crystals can be used to control the
sign of their Chern numbers.Comment: Accepted manuscrip
Structural correlations in heterogeneous electron transfer at monolayer and multilayer graphene electrodes
As a new form of carbon, graphene is attracting intense interest as an electrode material with widespread applications. In the present study, the heterogeneous electron transfer (ET) activity of graphene is investigated using scanning electrochemical cell microscopy (SECCM), which allows electrochemical currents to be mapped at high spatial resolution across a surface for correlation with the corresponding structure and properties of the graphene surface. We establish that the rate of heterogeneous ET at graphene increases systematically with the number of graphene layers, and show that the stacking in multilayers also has a subtle influence on ET kinetics. © 2012 American Chemical Society
Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction
The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing ‘pseudo-single-crystal’ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale
Colossal band renormalization and stoner ferromagnetism induced by electron-antiferromagnetic-magnon coupling
The interactions between electrons and antiferromagnetic magnons (AFMMs) are
important for a large class of correlated materials. For example, they are the
most plausible pairing glues in high-temperature superconductors, such as
cuprates and iron pnictides. However, unlike electron-phonon interactions
(EPIs), clear-cut observations regarding how electron-AFMM interactions (EAIs)
affect the band structure are still lacking. Consequently, critical information
on the EAIs, such as its strength and doping dependence, remains elusive. Here
we directly observe that EAIs induces a kink structure in the band dispersion
in BaKMnAs, and subsequently unveil several key
characteristics of EAIs. We found that the coupling constant of EAIs can be as
large as 6, and it shows huge doping dependence and temperature dependence, all
in stark contrast to the behaviors of EPI and beyond our current understanding
of EAIs. Such a colossal renormalization of electronic bands by EAIs drives the
system to the Stoner criteria, giving the intriguing ferromagnetic state in
BaKMnAs. Our results expand the current knowledge of EAIs,
which may facilitate the further understanding of many correlated materials
where EAIs play a critical role, such as high-temperature superconductors
A new view of electrochemistry at highly oriented pyrolytic graphite
Major new insights on electrochemical processes at graphite electrodes are reported, following extensive investigations of two of the most studied redox couples, Fe(CN)64–/3– and Ru(NH3)63+/2+. Experiments have been carried out on five different grades of highly oriented pyrolytic graphite (HOPG) that vary in step-edge height and surface coverage. Significantly, the same electrochemical characteristic is observed on all surfaces, independent of surface quality: initial cyclic voltammetry (CV) is close to reversible on freshly cleaved surfaces (>400 measurements for Fe(CN)64–/3– and >100 for Ru(NH3)63+/2+), in marked contrast to previous studies that have found very slow electron transfer (ET) kinetics, with an interpretation that ET only occurs at step edges. Significantly, high spatial resolution electrochemical imaging with scanning electrochemical cell microscopy, on the highest quality mechanically cleaved HOPG, demonstrates definitively that the pristine basal surface supports fast ET, and that ET is not confined to step edges. However, the history of the HOPG surface strongly influences the electrochemical behavior. Thus, Fe(CN)64–/3– shows markedly diminished ET kinetics with either extended exposure of the HOPG surface to the ambient environment or repeated CV measurements. In situ atomic force microscopy (AFM) reveals that the deterioration in apparent ET kinetics is coupled with the deposition of material on the HOPG electrode, while conducting-AFM highlights that, after cleaving, the local surface conductivity of HOPG deteriorates significantly with time. These observations and new insights are not only important for graphite, but have significant implications for electrochemistry at related carbon materials such as graphene and carbon nanotubes
Influence of COVID-19 on anemia in patients on program hemodialysis
The aim of the study - to study changes in the level of hemoglobin, iron and the volume of erythropoietin therapy in patients on program hemodialysis during the COVID-19 pandemic.Цель исследования - изучить изменения уровня гемоглобина, железа и объема терапии эритропоэтином у пациентов, находящихся на программном гемодиализе, в период пандемии COVID-1
Enhancement of Rydberg-mediated single-photon nonlinearities by electrically tuned Förster resonances
We demonstrate experimentally that Stark-tuned Förster resonances can be used to substantially increase the interaction between individual photons mediated by Rydberg interaction inside an optical medium. This technique is employed to boost the gain of a Rydberg-mediated single-photon transistor and to enhance the non-destructive detection of single Rydberg atoms. Furthermore, our all-optical detection scheme enables high-resolution spectroscopy of two-state Förster resonances, revealing the fine structure splitting of high-n Rydberg states and the non-degeneracy of Rydberg Zeeman substates in finite fields. We show that the ∣50S1/2,48S1/2⟩↔∣49P1/2,48P1/2⟩ pair state resonance in 87Rb enables simultaneously a transistor gain G>100 and all-optical detection fidelity of single Rydberg atoms F>0.8. We demonstrate for the first time the coherent operation of the Rydberg transistor with G>2 by reading out the gate photon after scattering source photons. Comparison of the observed readout efficiency to a theoretical model for the projection of the stored spin wave yields excellent agreement and thus successfully identifies the main decoherence mechanism of the Rydberg transistor
Direct observation of orbital hybridisation in a cuprate superconductor
The minimal ingredients to explain the essential physics of layered
copper-oxide (cuprates= materials remains heavily debated. Effective low energy
single-band models of the copper-oxygen orbitals are widely used because there
exists no strong experimental evidence supporting multiband structures. Here we
report angle-resolved photoelectron spectroscopy experiments on La-based
cuprates that provide direct observation of a two-band structure. This
electronic structure, qualitatively consistent with density functional theory,
is parametrised by a two-orbital ( and ) tight-binding
model. We quantify the orbital hybridisation which provides an explanation for
the Fermi surface topology and the proximity of the van-Hove singularity to the
Fermi level. Our analysis leads to a unification of electronic hopping
parameters for single-layer cuprates and we conclude that hybridisation,
restraining d-wave pairing, is an important optimisation element for
superconductivity.Comment: supplementary material available on reques
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