Local Electron Density Research Articles

Converting electrons into emergent fermions at a superconductor–Kitaev spin liquid interface

We study an interface between a Kitaev spin liquid (KSL) in the chiral phase and a non-chiral superconductor. When the coupling across the interface is sufficiently strong, the interface undergoes a transition into a phase characterized by a condensation of a bound state of a Bogoliubov quasiparticle in the superconductor and an emergent fermionic excitation in the spin liquid. In the condensed phase, electrons in the superconductor can coherently convert into emergent fermions in the spin liquid and vice versa. As a result, the chiral Majorana edge mode of the spin liquid becomes visible in the electronic local density of states at the interface, which can be measured in scanning tunneling spectroscopy experiments. We demonstrate the existence of this phase transition, and the non-local order parameter that characterizes it, using density matrix renormalization group simulations of a KSL strip coupled at its edge to a superconductor. An analogous phase transition can occur in a simpler system composed of a one-dimensional spin chain with a spin-flip $\mathbb{Z}_2$ symmetry coupled to a superconductor.

Shaped femtosecond laser-regulated deposition sites of galvanic replacement for simple preparation of large-area controllable noble metal nanoparticles

Controllable deposition sites in galvanic replacement (GR) are crucial for the application performance of noble metal nanoparticles (NMNPs). However, the simplicity and effectiveness of the existing research methods for controlling GR deposition sites are still needed to be improved. Herein, a novel method is proposed to simply and effectively control the deposition sites of NMNPs (Au and Pt) in the GR reaction by shaped femtosecond laser surface pre-modification. First, the localized free electron density of silicon is regulated by spatially shaped femtosecond laser to efficiently produce large-area uniform periodic amorphous transformation, which periodically changes the chemical reducibility of silicon. Second, this difference in chemical reducibility guides the subsequent GR reaction to produce large-area NMNPs with controllable deposition sites. By controlling the distribution and density of particles, these controllable NMNPs exhibit a superior SERS performance to the particles prepared by GR reaction alone: the maximum EF is 8.46 × 107 with high signal uniformity (RSD < 9.7%) and superior chemical stability (4.8% intensity deviation after 2 months in the air), which indicated the advantages of NMNPs with controllable deposition sites. This study provides a facile method to control the GR deposition sites and a new idea for Raman-enhanced substrate preparation.

Investigation of the role of defects on channel density profiles and their effect on the output characteristics of a nanowire FET

This work investigates the effect of defects on the electron density profiles of nanowire FETs with a rectangular cross-section. It also presents a framework for the discretization of the nanowire channels with defects. A self-consistent procedure using Schrodinger-Poisson solver with density matrix formalism calculates the local electron density profiles. The local electron density decreases due to defect-induced scattering potentials. The electron density profiles vary according to the nature of the intrinsic defects. The effect of defect-induced potentials on the output characteristics of the nanowire FET device is studied using the non-equilibrium Green’s function (NEGF) methodology. An increase in scattering potential in the nanowire channel causes a considerable decrease in the saturation voltage and current. This results in a faster saturation which changes the overall device performance. Hence, defect-controlled channels can be utilized to fabricate FETs with desired characteristics.

Machine learning to estimate the local quality of protein crystal structures

Low-resolution electron density maps can pose a major obstacle in the determination and use of protein structures. Herein, we describe a novel method, called quality assessment based on an electron density map (QAEmap), which evaluates local protein structures determined by X-ray crystallography and could be applied to correct structural errors using low-resolution maps. QAEmap uses a three-dimensional deep convolutional neural network with electron density maps and their corresponding coordinates as input and predicts the correlation between the local structure and putative high-resolution experimental electron density map. This correlation could be used as a metric to modify the structure. Further, we propose that this method may be applied to evaluate ligand binding, which can be difficult to determine at low resolution.

Structural and Componential Engineering of Co2P&CoP@N-C Nanoarrays for Energy-Efficient Hydrogen Production from Water Electrolysis.

The development of electrocatalysts for efficient water splitting is a pivotal and challenging task. Transition-metal phosphides (TMPs) have been known as one of the most promising candidates for the efficient hydrogen evolution reaction (HER) due to their favorable intrinsic reactivity. However, structural engineering related to the gas bubbles evolution and tiny regulation of components concerned with the electronic structure remained as a significant challenge that requires further optimization. Herein, the nanoarrays (NAs) composed of ultrasmall Co2P and CoP nanoparticle-embedded N-doped carbon matrix (Co2P&CoP@N-C) are prepared and demonstrated an overpotential of 62.8 ± 4.7 mV at 10 mA cm-2 in 1.0 M KOH. The nanoarray-structured electrocatalyst revealed the superaerophobicity and facilitates the detachment of the in situ formed hydrogen gas bubbles, ensuring abundant catalytic sites and electrode-electrolyte interface for the mass transfer process. The amount of P doping modulated the local electron density around Co and P atoms, which attains a favorable compromise to afford sufficient electrons for the electrocatalysis and inhibit the negative influence of H2 desorption. Significantly, the lowered overpotential induced by the electrocatalyst surface architecture is much stronger than that of the component content and promotes the electrocatalytic activity.

Localized Electron Density Redistribution in Fluorophosphate Cathode: Dangling Anion Regulation and Enhanced Na‐Ion Diffusivity for Sodium‐Ion Batteries

AbstractPolyanionic transition metal polyphosphate (TMPO)‐type Na3V2(PO4)2O2F (NVPO2F) is promising as cathode for large‐scale sodium‐ion batteries (SIBs) on account of its considerable capacity and highly stable structure. However, the redox of transition metal and phase transitions along with the (de)intercalation of Na+ lead to its slow kinetics and inferior rate performance. Herein, chlorine (Cl) is applied as a heteropical dopant to obtain Cl‐doped NVPO2F (NVPO2−xClxF) cathode material for SIBs. Density functional theory investigation reveals that Cl doping tunes the localized electronic density and structure in NVPO2F lattice, causing the electron redistribution on vanadium center and dangling anions. Hence, the NVPO2−xClxF cathode exhibits a revised redox behavior of vanadium for Na+ extraction/insertion, increases Na+ diffusion rate, as well as lowers charge transfer resistance. A Na+ storage mechanism of reversible transformations between three phases and V4+/V5+ redox couple for NVPO2−xClxF cathode is verified. The NVPO2−xClxF cathode reveals a high rate capacity of ≈63 mAh g−1 at 30C and great cycle stability over 1000 cycles at 10C. More importantly, outstanding rate property (314 Wh kg−1 at 5850 W kg−1) and cycling capability are obtained for the NVPO2−xClxF//3DC@Se full cell. This study demonstrates a brand‐new strategy to prepare advanced cathode materials for superior SIBs.

Combined the Photocatalysis and Fenton-like Reaction to Efficiently Remove Sulfadiazine in Water Using g-C3N4/Ag/γ-FeOOH: Insights Into the Degradation Pathway From Density Functional Theory.

Sulfadiazine (SDZ) is a common antibiotic pollutant in wastewater. Given that it poses a risk as an environmental pollutant, finding effective ways to treat it is important. In this paper, the composite catalytic material g-C3N4/Ag/γ-FeOOH was prepared, and its degradation performance was studied. g-C3N4/Ag/γ-FeOOH had a superior degradation effect on SDZ than g-C3N4 and γ-FeOOH. Compared with different g-C3N4 loadings and different catalyst dosages (5, 10, 25, and 50 mg/L), 2 mg/L g-C3N4/Ag/γ-FeOOH with a g-C3N4 loading of 5.0 wt% has the highest degradation promotion rate for SDZ, reaching up to 258.75% at 600 min. In addition, the photocatalytic enhancement mechanism of the catalyst was studied. Density functional theory (DFT) calculations indicated that the enhancement of photocatalytic activity was related to the narrowing of the forbidden band and the local electron density of the valence band. The bandgap of the catalyst was gradually narrowed from 2.7 to 1.05 eV, which can increase the light absorption intensity and expand the absorption edge. The density of states diagram showed that the local resonance at the interface could effectively improve the separation efficiency of e−-h+ pairs. Four degradation paths of SDZ were speculated based on DFT calculations. The analysis confirmed that the degradation path of SDZ primarily included Smiles-type rearrangement, SO2 extrusion, and S-N bond cleavage processes.

Electron power absorption in micro atmospheric pressure plasma jets driven by tailored voltage waveforms in He/N2

In atmospheric pressure capacitively-coupled microplasma jets, voltage waveform tailoring (VWT) was demonstrated to provide ultimate control of the electron energy distribution function (EEDF), which allows us to enhance and adjust the generation of selected neutral species by controlling the electron power absorption dynamics. However, at the fundamental level, the physical origin of these effects of VWT remained unclear. Therefore, in this work, the electron power absorption dynamics is investigated in a He/N2 jet with a nitrogen concentration of 0.05% driven by a valleys voltage waveform at a base frequency of 13.56 MHz for different numbers of harmonics, using a self-consistent particle-in-cell simulation coupled with a spatio-temporally resolved analysis of the electron power absorption based on moments of the Boltzmann equation. Due to the local nature of the transport at atmospheric pressure, ohmic power absorption is dominant. Increasing the number of harmonics, due to the peculiar shape of the excitation waveform, the sheath collapse at the grounded electrode is shortened relative to the one at the powered electrode. As a consequence, and in order to ensure flux compensation of electrons and positive ions at this electrode, a high current is driven through the discharge at the time of this short sheath collapse. This current is primarily driven by a high ohmic electric field. Close to the grounded electrode, where the electron density is low and the electric field is therefore high, electrons are accelerated to high energies and strong ionization, as well as the formation of a local electron density maximum, are observed. This electron density maximum leads to a local ambipolar electric field that acts as an electric field reversal and accelerates electrons to even higher energies. These effects are understood in detail to fundamentally explain the unique potential of VWT for EEDF control in such plasmas.

Diagnosing asymmetric bipolar HiPIMS discharges using laser Thomson scattering

The temporal evolution of the electron temperature T e and density n e has been measured at two positions on the centre-line of an asymmetrically pulsed bi-polar HiPIMS plasma using incoherent laser Thomson scattering (LTS). The magnetron was operated with a tungsten target in argon atmospheres. The results show that in the plasma afterglow when positive voltage pulses are applied (above a threshold of at least 200 V) significant heating of the electrons can occur in which T e can rise to values comparable to the those measured in HiPIMS on-time. The on-set of the rises in T e are significantly delayed relative to the start of the positive pulse, with the delay time decreasing with the magnitude of the positive voltage. The delay is only weakly dependent on the operating pressure. The presence of large positive pulses can also affect the local electron density with n e seen to decay significantly more quickly in the afterglow than for the corresponding unipolar pulsing case, in which no positive pulse is applied. The LTS measurements were complemented by a time-resolved study of the plasma optical emission (neutral argon and tungsten lines). With increasing positive potentials applied in the afterglow the Ar(I) line intensities grow consistent with increasing T e. Interestingly, W(I) line intensities are detected in the afterglow with positive voltages >200 V despite the termination of all target sputtering, suggesting that tungsten is being re-sputtered from the vessel walls. With the aid of emissive probe measurements of the spatial and temporal evolution of plasma potential profile along the centre-line we discuss the phenomena of plasma electron heating and wall sputtering in the positive pulse. This is done in terms of the existence of a non-sustained reverse discharge, in which the vessel walls become an effective cathode.

Sulfur‐Dopant‐Promoted Electroreduction of CO2 over Coordinatively Unsaturated Ni‐N2 Moieties

AbstractAtomically dispersed nickel–nitrogen–carbon (Ni‐N‐C) moieties are promising for efficient electrochemical CO2‐to‐CO conversion. To improve the intrinsic electrocatalytic activity, it is essential but challenging to steer the coordination environment of Ni centers for promoting the CO formation kinetics. Here, we introduce alien sulfur atoms to tune the local electronic density of unsaturated NiN2 species. A coordinated structure evolution is detected and S vacancies are generated at high overpotentials, as confirmed by X‐ray absorption spectroscopy. The sulfur dopants enhance CO selectivity and activity over normal unsaturated NiN2 structure, reaching a high CO Faradaic efficiency of 97 % and a large CO current density of 40.3 mA cm−2 in a H‐cell at −0.8 V and −0.9 V (vs. RHE), respectively. DFT calculations reveal both doped S atoms and evolved S vacancies in the NiN2 coordination environment contribute to the reduced energy barriers for CO2 electroreduction to CO.

Sulfur-Dopant-Promoted Electroreduction of CO2 over Coordinatively Unsaturated Ni-N2 Moieties.

Atomically dispersed nickel-nitrogen-carbon (Ni-N-C) moieties are promising for efficient electrochemical CO2 -to-CO conversion. To improve the intrinsic electrocatalytic activity, it is essential but challenging to steer the coordination environment of Ni centers for promoting the CO formation kinetics. Here, we introduce alien sulfur atoms to tune the local electronic density of unsaturated NiN2 species. A coordinated structure evolution is detected and S vacancies are generated at high overpotentials, as confirmed by X-ray absorption spectroscopy. The sulfur dopants enhance CO selectivity and activity over normal unsaturated NiN2 structure, reaching a high CO Faradaic efficiency of 97 % and a large CO current density of 40.3 mA cm-2 in a H-cell at -0.8 V and -0.9 V (vs. RHE), respectively. DFT calculations reveal both doped S atoms and evolved S vacancies in the NiN2 coordination environment contribute to the reduced energy barriers for CO2 electroreduction to CO.

Synthesis and characterization of the layered insensitive pentazolate salts based on two triazolium isomers

The all-nitrogen cyclo-pentazolate (c-N5ˉ) anion has attracted interest because of its promising application in high-energy materials. Now, nonmetallic pentazolate salts were prepared by assembling cyclo-N5ˉ and cations but it was shown to be challenging to balance excellent detonation performance and good stability, which results in restrictions for practical applications. However, we find the layer-by-layer crystal stacking structure can enhance the sensitivity of pentazolate salts. Herein, we designed and synthesized two targeted cyclo-pentazolate salts with layered stacking mode in their crystal structures based on two isomeric cations: 3,4,5-triamino-1,2,4-triazolium and 3-hydrazino-4-amino-1,2,4-triazolium. The results demonstrate that the aromaticity of triazole and pentazole rings arouse the π−π interaction force, promoting the formation of a layer-by-layer crystal stacking structure. Two compounds show very low sensitivity characteristics (FS>40 and IS>360) and good detonation properties (detonation velocity: 3, Dv= 8796m⋅s−1; 4, Dv=8574m⋅s−1. detonation pressure: 3, P = 26.6 GPa; 4, P = 25.1 GPa). Computational analysis based on noncovalent interaction index, localized orbital locator-π and electron density difference provide a better understanding about the effects of layer-by-layer stacking structure on the stabilities. These findings contribute to preparation of both high-energy and low sensitivity pentazolate salts.

Hard X-Ray Nanotomography for 3D Analysis of Coking in Nickel-Based Catalysts.

Understanding catalyst deactivation by coking is crucial for knowledge‐based catalyst and process design in reactions with carbonaceous species. Post‐mortem analysis of catalyst coking is often performed by bulk characterization methods. Here, hard X‐ray ptychographic computed tomography (PXCT) was used to study Ni/Al2O3 catalysts for CO2 methanation and CH4 dry reforming after artificial coking treatment. PXCT generated quantitative 3D maps of local electron density at ca. 80 nm resolution, allowing to visualize and evaluate the severity of coking in entire catalyst particles of ca. 40 μm diameter. Coking was primarily revealed in the nanoporous solid, which was not detectable in resolved macropores. Coke formation was independently confirmed by operando Raman spectroscopy. PXCT is highlighted as an emerging characterization tool for nanoscale identification, co‐localization, and potentially quantification of deactivation phenomena in 3D space within entire catalyst particles.

Regulating Local Electron Density of Iron Single Sites by Introducing Nitrogen Vacancies for Efficient Photo-Fenton Process.

The activity of heterogeneous photocatalytic H2 O2 activation in Fenton-like processes is closely related to the local electron density of reaction centre atoms. However, the recombination of electron-hole pairs arising from random charge transfer greatly restricts the oriented electron delivery to active center. Here we show a defect engineered iron single atom photocatalyst (Fe1 -Nv /CN, single Fe atoms dispersed on carbon nitride with abundant nitrogen vacancies) for the activation of H2 O2 under visible light irradiation. Based on DFT calculations and transient absorption spectroscopy results, the engineered nitrogen vacancies serve as the electron trap sites, which can directionally drive the electrons to concentrate on Fe atoms. The formation of highly concentrated electrons density at Fe sites significantly improves the H2 O2 conversion efficiency. Therefore, the optimized single atom catalyst exhibiting a higher ciprofloxacin degradation activity, which was up to 18 times that of pristine CN.

Regulating Local Electron Density of Iron Single Sites by Introducing Nitrogen Vacancies for Efficient Photo‐Fenton Process

AbstractThe activity of heterogeneous photocatalytic H2O2activation in Fenton‐like processes is closely related to the local electron density of reaction centre atoms. However, the recombination of electron‐hole pairs arising from random charge transfer greatly restricts the oriented electron delivery to active center. Here we show a defect engineered iron single atom photocatalyst (Fe1‐Nv/CN, single Fe atoms dispersed on carbon nitride with abundant nitrogen vacancies) for the activation of H2O2under visible light irradiation. Based on DFT calculations and transient absorption spectroscopy results, the engineered nitrogen vacancies serve as the electron trap sites, which can directionally drive the electrons to concentrate on Fe atoms. The formation of highly concentrated electrons density at Fe sites significantly improves the H2O2conversion efficiency. Therefore, the optimized single atom catalyst exhibiting a higher ciprofloxacin degradation activity, which was up to 18 times that of pristine CN.

Machine learning band gaps from the electron density

A remarkable consequence of the Hohenberg-Kohn theorem of density functional theory is the existence of an injective map between the electronic density and any observable of the many electron problem in an external potential. In this work, we study the problem of predicting a particular observable, the band gap of semiconductors and band insulators, from the knowledge of the local electronic density. Using state-of-the-art machine learning techniques, we predict the experimental band gaps from computationally inexpensive density functional theory calculations. We propose a modified Behler-Parrinello (BP) architecture that greatly improves the model capacity while maintaining the symmetry properties of the BP architecture. Using this scheme, we obtain band gaps at a level of accuracy comparable to those obtained with state of the art and computationally intensive hybrid functionals, thus significantly reducing the computational cost of the task.

Local computational methods to improve the interpretability and analysis of cryo-EM maps

Abstract Cryo-electron microscopy (cryo-EM) maps usually show heterogeneous distributions of B-factors and electron density occupancies and are typically B-factor sharpened to improve their contrast and interpretability at high-resolutions. However, ‘over-sharpening’ due to the application of a single global B-factor can distort processed maps causing connected densities to appear broken and disconnected. This issue limits the interpretability of cryo-EM maps, i.e. ab initio modelling. In this work, we propose 1) approaches to enhance high-resolution features of cryo-EM maps, while preventing map distortions and 2) methods to obtain local B-factors and electron density occupancy maps. These algorithms have as common link the use of the spiral phase transformation and are called LocSpiral, LocBSharpen, LocBFactor and LocOccupancy. Our results, which include improved maps of recent SARS-CoV-2 structures, show that our methods can improve the interpretability and analysis of obtained reconstructions.

A study of superluminous stars with the Fermi-Large Area Telescope

ABSTRACT The γ-ray emission from stars is induced by the interaction of cosmic rays with stellar atmospheres and photon fields. This emission is expected to come in two components: a stellar disc emission, where γ-rays are mainly produced in atmospheric showers generated by hadronic cosmic rays, and an extended halo emission, where the high density of soft photons in the surroundings of stars create a suitable environment for γ-ray production via inverse Compton (IC) scattering by cosmic ray electrons. Besides the Sun, no other disc or halo from single stars has ever been detected in γ-rays. However, by assuming a cosmic ray spectrum similar to that observed on Earth, the predicted γ-ray emission of superluminous stars, e.g. Betelgeuse and Rigel, could be high enough to be detected by the Fermi Large Area Telescope (LAT) after its first decade of operations. In this work, we use 12 yr of Fermi-LAT observations along with IC models to study nine superluminous nearby stars, both individually and via stacking analysis. Our results show no significant γ-ray emission, but allow us to restrict the stellar γ-ray fluxes to be on average &amp;lt;3.3 × 10−11 ph cm−2 s−1 at a 3σ confidence level, which translates to an average local density of electrons in the surroundings of our targets to be less than twice of that observed for the Solar system.

Intrinsic Electron Localization of Metastable MoS2 Boosts Electrocatalytic Nitrogen Reduction to Ammonia.

The advancement of efficient electrocatalysts toward the nitrogen reduction reaction (NRR) is critical in sustainable ammonia synthesis under ambient pressure and temperature. Manipulating the electronic configuration of electrocatalysts is particularly vital to form metal-nitrogen (MN) bonds during the NRR through regulating the active electronic states of sites. Here, in sharp contrast to stable 2H MoS2 without metal chains, MoMo bonding in metastable polymorphs of MoS2 bulk (zigzag chain in the 1T' phase and diamond chain in the 1T″' phase) is discovered to significantly increase intrinsic electron localization around the metal chains. This can enhance the charge transfer from the adsorbed nitrogen molecule to the metal chains, allowing for boosted NRR kinetics. The electrochemical experiments show that the NH3 yield rate and the faradaic efficiency of the metastable 1T″' MoS2 rich with abundant Mo-Mo bonds are about 9 and 12 times above average than those of 2H MoS2 , correspondingly. Theoretical simulations reveal the high local electron density surrounding the MoMo chains and sites can promote π back-donation, which is beneficial for increasing nitrogen adsorption, strengthening the MN bonds, and reducing the cleavage barrier of the triple NN bond.

Experimental and theoretical electronic absorption spectra, optical, photoelectrical characterizations of 1, 2, 3-Thiazaphosphinine and 1, 2-Azaphospholes bearing a chromone ring: Solvatochromic effect and TD/DFT approach

Geometry, global energetic and dipole moment of the studied structures 1-4 in the ground state are calculated using the DFT/B3LYB/6-311++G (d,p) level of theory. It has been uncovered that compounds containing 1, 2, 3-thiazaphosphinine and 1, 2-azaphospholes bearing a Chromone ring structure displays noteworthy biological properties. The studied compounds 1-4 are non-planar, as indicated from the dihedral angles. Using frontier molecular orbital (FMO) analysis, various spectroscopic and quantum chemical parameters are evaluated. Besides, absorption energies, oscillator strength, and electronic transitions of 1, 2, 3-thiazaphosphinine and 1, 2-azaphospholes 1-4 molecules have been derived at TD-DFT/CAM-B3LYP/6-311++G (d, p) computations utilizing a PCM and measured in different solvents polar and non-polar experimentally in Uv-Vis spectra. The second-order perturbation interactions between donor and acceptor MOs of the ground state and the natural bond orbital (NBO) analysis show a localization and delocalization of electron density, intermolecular Charge Transfer CT character of n-π*, π-π* transitions. The calculated at the same level of theory which NLO, α, Δα and first order β were showed promising optical properties. For the understanding of reactivity points, the molecular electrostatic potential surfaces (MEPS) plots have been computed. All the calculations have been performed in the gas phase.