Metallic Electronic States Research Articles

Atomic and electronic structures of the [formula omitted]-FeSi2/Si(001) interface

The atomistic interface structure between β-FeSi2 and Si substrate was investigated by cross-sectional scanning transmission electron microscopy and density functional theory calculations (DFT). A high quality single crystalline β-FeSi2 film was epitaxialy grown on Si(001) substrate. We demonstrate that the β-FeSi2(100) plane is bonded to the Si(001) plane with two Si dimers per unit cell. With such interface structure, DFT band calculations reveal that metallic electronic states are localized at the interface. This finding implies the presence of two dimensional electronic states at the interface of β-FeSi2/Si(001), which may have potential applications in silicon-based electronic devices.

Probing the metallic state, magnetic anisotropy, and large Curie temperature in CaCuFeReO[formula omitted]: Strain engineering

Rhenium-based double perovskite oxides (DPOs) are a particularly fascinating class of materials because of high carrier spin polarization, large magnetic anisotropy energy (MAE), and complex electrical behavior along with high Curie temperature (TC). Herein, we discussed the impact of biaxial ([110]) strain ranging from −5% to ＋ 5% along the ab-plane on the electronic and magnetic properties of CaCuFeReO6 (CCFRO) DPO using ab-inito calculations. The unstrained system exhibits a ferrimagnetic (FiM) spin ordering as an energetically stable magnetic ground state due to electron hopping between Fe+3/Re+5t2g and Cu+2eg orbitals along with a metallic electronic state. Moreover, [010] (b-axis) is the magnetic easy axis that produces the MAE of 5.36 meV, giving a giant MAE constant of 1.97×107 erg/cm3. The evaluated partial spin-magnetic moment on the Cu1/Cu2/Fe/Re ion is 0.14/0.44/4.04/−0.64μB, where “−” sign on the Re moment means that its spin is antiparallel to the other ions, verifying the FiM ordering in the system. Also, the eg, t2g/eg, and t2g orbital characteristics of Cu+2, Fe+3, and Re+5 ions are visible in the spin-magnetization density iso-surfaces plots, respectively. Using the classical Heisenberg model, the computed TC for the unstrained structure is 576 K, which is quite close to the experimentally reported value of 567 K. In addition, it is shown that the FiM metallic behavior in the motif is robust against biaxial ([110]) strain within the considered range. It has also been revealed that TC increased to 5.73%/3.30% for the considered range of −5% compressive/＋5% tensile strain due to improved structural distortions.

Deep Electron Redistributions Induced by Dual Junctions Facilitating Electroreduction of Dilute Nitrate to Ammonia.

The electronic states of metal catalysts can be redistributed by the rectifying contact between metal and semiconductor e.g., N-doped carbon (NC), while the interfacial regulation degree is very limited. Herein, a deep electronic state regulation is achieved by constructing a novel double-heterojunctional Co/Co3O4@NC catalyst containing Co/Co3O4 and Co3O4/NC heterojunctions. When used for dilute electrochemical NO3 - reduction reaction (NO3RR), the as-prepared Co/Co3O4@NC exhibits an outstanding Faradaic efficiency for NH3 formation (FENH3) of 97.9%, -0.4V versus RHE and significant NH3 yield of 303.5mmol h-1 gcat -1 at -0.6V at extremely low nitrate concentrations (100ppm NO3 --N). Experimental and theoretical results reveal that the dual junctions of Co/Co3O4 and Co3O4/NC drive a unidirectional electron transfer from Co to NC (Co→Co3O4→NC), resulting in electron-deficient Co atoms. The electron-deficient Co promotes NO3 - adsorption, the rate-determining step (RDS) for NO3RR, facilitating the dilute NO3RR to NH3. The design strategy provides a novel reference for unidirectional multistage regulation of metal electronic states boosting electrochemical dilute NO3RR, which opens up an avenue for deep electronic state regulation of electrocatalyst breaking the limitation of the electronic regulation degree by rectifying contact.

Microscopic Exploration of Electronic States in Nickelate Superconductors

The multilayered nickelates, La3Ni2O7 and La4Ni3O10 , were investigated using nuclear magnetic resonance (NMR) at ambient pressure. Metallic electronic states under the density wave order were observed microscopically for both compounds.

Charge-loop current order and Z3 nematicity mediated by bond order fluctuations in kagome metals

Recent experiments on geometrically frustrated kagome metal AV3Sb5 (A = K, Rb, Cs) have revealed the emergence of the charge loop current (cLC) order near the bond order (BO) phase. However, the origin of the cLC and its interplay with other phases have been uncovered. Here, we propose a novel mechanism of the cLC state, by focusing on the BO phase common in kagome metals. The BO fluctuations in kagome metals, which emerges due to the Coulomb interaction and the electron-phonon coupling, mediate the odd-parity particle-hole condensation that gives rise to the topological current order. Furthermore, the predicted cLC+BO phase gives rise to the Z3-nematic state in addition to the giant anomalous Hall effect. The present theory predicts the close relationship between the cLC, the BO, and the nematicity, which is significant to understand the cascade of quantum electron states in kagome metals. The present scenario provides a natural understanding.

A pragmatic protocol for determining charge transfer states of molecules at metal surfaces by constrained density functional theory.

Electron transfer from a metal surface to a molecule is very important at the gas-surface interface, which can lead to electron-mediated energy transfer during molecular scattering from the surface, as evidenced by numerous state-to-state molecular beam experiments of NO and CO scattering from noble metal surfaces. However, it remains challenging to determine relevant charge-transfer states and their nonadiabatic couplings from first principles in such systems involving a continuum of metallic electronic states. In this work, we propose a pragmatic protocol for this purpose based on the constrained density functional theory (CDFT) approach. In particular, we discuss the influence of the charge partitioning algorithm used in CDFT to define the constraint property in molecule-metal systems. It is found that the widely used Bader charge analysis is adequate to define the physically sound CDFT diabatic states corresponding to a molecule with or without extra electron transferred from the metal. Numerical tests validate that the proposed CDFT scheme properly describes the electron transfer behaviors in several benchmark systems, namely, NO or CO interacting with Au(111) or Ag(111). The effects of the surface work function and the molecular electron affinity on electron transfer are discussed in detail by comparing the CDFT states of the four systems. This pragmatic CDFT protocol lays the foundation for constructing accurate global diabatic potential energy surfaces for these important systems and can be generalized to study other interfacial electron transfer related problems.

Effect of the number of graphene layers on the electron transfer kinetics at metal/graphene heterostructures

We theoretically investigated the kinetics of outer-sphere electron transfer (ET) at graphene with different number of layers in the presence and absence of metal substrates using density functional theory (DFT) calculations. It is shown that the experimental data on the electrocatalytic activity of the metal substrate for few-layer graphene can be explained under the assumption of the nonadiabatic electron transfer. However, the electron transfer at the metal-supported single-layer graphene is likely adiabatic. The enhanced electron transfer in the presence of the metal substrate is explained by the hybridization of the metal and graphene electronic states, which provides more efficient interaction of the metal substrate with the reactant near the graphene surface.

Hydrogen evolution reaction in acidic and basic medium on robust cobalt sulphide electrocatalyst

Huge research interest has been paid to produce the robust, efficient and large area electrodes based on non-noble electrocatalyst as an alternative of Pt. Herein, we report the facile synthesis of cobalt sulphide (CoS) based electrocatalyst with polycrystalline structure and non-uniform morphology. Also, flexible, robust, large area and bio-degradable electrodes on paper are prepared. Owing to different metal electronic states like Co2+ and Co3+, CoS electrocatalyst show rapid hydrogen evolution reaction. CoS based electrodes shows HER activity with overpotential of 284 mV in alkaline medium and 198 mV in acidic medium. In alkaline medium, electrodes follow sluggish Volmer reaction while in acidic, dynamic Heyrovsky-Volmer reaction is step determining reaction step. Paper based electrodes functionalised by CoS electrocatalyst advocate the futuristic development of non-noble electrodes for clean energy generation via electrocatalytic HER.

Formation of the incommensurate Si(111)-∼5.4 × ∼5.4-In surface

We find that the 5 × 5 reconstruction can form only by indium deposition on the ‘rect’ structure of the Si(111)-7×3-In surface in a step-flow-like growth manner, which was previously known to form by deposition of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on the 7×3-rect surface. The 5 × 5 reconstruction is found to be an incommensurate structure that should be expressed as ∼5.4 × ∼5.4, instead of the 5 × 5. The ∼5.4 × ∼5.4 surface consists of a triple indium layer with about 3.1 ML coverage, and has a strong insulating character, in contrast to the metallic electronic states on the 7×3-rect surface. The ∼5.4 × ∼5.4 surface is considered to be rather unstable at RT.

Capturing Atom-Specific Electronic Structural Dynamics of Transition-Metal Complexes with Ultrafast Soft X-Ray Spectroscopy.

The atomic specificity of X-ray spectroscopies provides a distinct perspective on molecular electronic structure. For 3d metal coordination and organometallic complexes, the combination of metal- and ligand-specific X-ray spectroscopies directly interrogates metal-ligand covalency-the hybridization of metal and ligand electronic states. Resonant inelastic X-ray scattering (RIXS), the X-ray analog of resonance Raman scattering, provides access to all classes of valence excited states in transition-metal complexes, making it a particularly powerful means of characterizing the valence electronic structure of 3d metal complexes. Recent advances in X-ray free-electron laser sources have enabled RIXS to be extended to the ultrafast time domain. We review RIXS studies of two archetypical photochemical processes: charge-transfer excitation in ferricyanide and ligand photodissociation in iron pentacarbonyl. These studies demonstratefemtosecond-resolution RIXS can directly characterize the time-evolving electronic structure, including the evolution of the metal-ligand covalency.

Activation of anionic redox in d0 transition metal chalcogenides by anion doping

Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning. Pursuing with chalcogenides, but deviating from cationic substitution, we here present another twist to our band positioning strategy that relies on mixed ligands with the synthesis of the Li2TiS3-xSex solid solution series. Through the series the electrochemical activity displays a bell shape variation that peaks at 260 mAh/g for the composition x = 0.6 with barely no capacity for the x = 0 and x = 3 end members. We show that this capacity results from cumulated anionic (Se2−/Sen−) and (S2−/Sn−) and cationic Ti3+/Ti4+ redox processes and provide evidence for a metal-ligand charge transfer by temperature-driven electron localization. Moreover, DFT calculations reveal that an anionic redox process cannot take place without the dynamic involvement of the transition metal electronic states. These insights can guide the rational synthesis of other Li-rich chalcogenides that are of interest for the development of solid-state batteries.

Synthesis of N‐Heterocycles via Oxidant‐Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts

AbstractN‐Heterocycles, such as pyrroles, pyrimidines, quinazolines, and quinoxalines, are important building blocks for organic chemistry and the fine‐chemical industry. For their synthesis, catalytic borrowing hydrogen and acceptorless dehydrogenative coupling reactions of alcohols as sustainable reagents have received significant attention in recent years. To overcome the problems of product separation and catalyst reusability, several metal‐based heterogeneous catalysts have been reported to achieve these transformations with good yields and selectivity. In this Minireview, we summarize recent developments using both noble and non‐noble metal‐based heterogeneous catalysts to synthesize N‐heterocycles from alcohols and N‐nucleophiles via acceptorless dehydrogenation or borrowing hydrogen methodologies. Furthermore, this Minireview introduces strategies for the preparation and functionalization of the corresponding heterogeneous catalysts, discusses the reaction mechanisms and the roles of metal electronic states, and the influence of support Lewis acid–base properties on these reactions.

Synthesis of N-Heterocycles via Oxidant-Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts.

N‐Heterocycles, such as pyrroles, pyrimidines, quinazolines, and quinoxalines, are important building blocks for organic chemistry and the fine‐chemical industry. For their synthesis, catalytic borrowing hydrogen and acceptorless dehydrogenative coupling reactions of alcohols as sustainable reagents have received significant attention in recent years. To overcome the problems of product separation and catalyst reusability, several metal‐based heterogeneous catalysts have been reported to achieve these transformations with good yields and selectivity. In this Minireview, we summarize recent developments using both noble and non‐noble metal‐based heterogeneous catalysts to synthesize N‐heterocycles from alcohols and N‐nucleophiles via acceptorless dehydrogenation or borrowing hydrogen methodologies. Furthermore, this Minireview introduces strategies for the preparation and functionalization of the corresponding heterogeneous catalysts, discusses the reaction mechanisms and the roles of metal electronic states, and the influence of support Lewis acid–base properties on these reactions.

Mirror twin boundaries in MoSe2 monolayers as one dimensional nanotemplates for selective water adsorption.

Water adsorption on transition metal dichalcogenides and other 2D materials is generally governed by weak van der Waals interactions. This results in a hydrophobic character of the basal planes, and defects may play a significant role in water adsorption and water cluster nucleation. However, there is a lack of detailed experimental investigations on water adsorption on defective 2D materials. Here, by combining low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we study in that context the well-defined mirror twin boundary (MTB) networks separating mirror-grains in 2D MoSe2. These MTBs are dangling bond-free extended crystal modifications with metallic electronic states embedded in the 2D semiconducting matrix of MoSe2. Our DFT calculations indicate that molecular water also interacts similarly weak with these MTBs as with the defect-free basal plane of MoSe2. However, in low temperature STM experiments, nanoscopic water structures are observed that selectively decorate the MTB network. This localized adsorption of water is facilitated by functionalization of the MTBs by hydroxyls formed by dissociated water. Hydroxyls may form by dissociating of water at undercoordinated defects or adsorbing of radicals from the gas phase in the UHV chamber. Our DFT analysis indicates that the metallic MTBs adsorb these radicals much stronger than on the basal plane due to charge transfer from the metallic states into the molecular orbitals of the OH groups. Once the MTBs are functionalized with hydroxyls, molecular water can attach to them, forming water channels along the MTBs. This study demonstrates the role metallic defect states play in the adsorption of water even in the absence of unsaturated bonds that have been so far considered to be crucial for adsorption of hydroxyls or water.

Interfacial Engineering on Electronic States and Gas Sensing Performances of Metal Oxide/Graphene Composites

Heterojunctions of two-dimensional materials have been attracting considerable attention because of their outstanding tunable optical and electrical properties. Taking the composites of CeO2/graphene and SnO2/graphene as examples, the interfacial effects on the electronic states and gas sensing properties are studied. For the CeO2/graphene composites, the electron is transferred from graphene onto CeO2 {111} facets showing the Schottky contact, but from CeO2 {100} facets onto graphene resulting in Ohmic contact. Furthermore, since CeO2{100} surface is the polar surface, and NO2 is a polar molecule, the interactions between NO2 and CeO2 with {100} polar facets should be stronger thus promoting adsorption of NO2. The internal electric field near the polar surface promotes charge separation and accelerates charge exchange between NO2 and the composites. As a result, the CeO2 {100}/graphene composites deliver substantially enhanced gas sensing performance to NO2, as compared to CeO2 {111}/graphene composites. It was also found that oxygen vacancies (Ov) have significant influences on the gas sensing performance. According to the first-principle calculations, the tunable electronic states and enhanced gas sensing performances owing to interfacial effects can be well understood.

Antiferromagnetic metallic state as proved by magnetotransport in epitaxially stabilized perovskite PbRuO3

Perovskite ${\mathrm{PbRuO}}_{3}$, which has been synthesized only in bulk polycrystalline form under high pressure, is an orthorhombic Pbnm at room temperature, exhibiting structural phase transition to Imma at around 90 K. This structural transition is accompanied with an orbital ordering and antiferromagnetic spin fluctuation. Here, we report the fabrication of single crystalline perovskite ${\mathrm{PbRuO}}_{3}$ thin films on various substrates. Magnetotransport measurements reveal that the films have metallic electronic ground state without any clear electronic transitions reported in previous literature. Evidenced by a shift of transition temperature, suppression of magnetic spin fluctuation due to epitaxial strain is implicated. Nonlinear Hall effect is also observed with indiscernible hysteresis, plausibly originating from antiferromagnetic spins, yet multiband effect cannot be completely ruled out.

Resolving Dirac electrons with broadband high-resolution NMR

Detecting the metallic Dirac electronic states on the surface of Topological Insulators (TIs) is critical for the study of important surface quantum properties (SQPs), such as Majorana zero modes, where simultaneous probing of the bulk and edge electron states is required. However, there is a particular shortage of experimental methods, showing at atomic resolution how Dirac electrons extend and interact with the bulk interior of nanoscaled TI systems. Herein, by applying advanced broadband solid-state 125Te nuclear magnetic resonance (NMR) methods on Bi2Te3 nanoplatelets, we succeeded in uncovering the hitherto invisible NMR signals with magnetic shielding that is influenced by the Dirac electrons, and we subsequently showed how the Dirac electrons spread inside the nanoplatelets. In this way, the spin and orbital magnetic susceptibilities induced by the bulk and edge electron states were simultaneously measured at atomic scale resolution, providing a pertinent experimental approach in the study of SQPs.

Local atomic structure and lattice defect analysis in heavily Co-doped ZnS thin films using X-ray absorption fine structure spectroscopy

X-ray absorption fine structure (XAFS) spectroscopy was used to address both the local atomic structure and the local electronic structure around Zn, Co, and S in Co-doped ZnS thin films. X-ray absorption near-edge spectroscopy (XANES) revealed changes in the strong pre-edge feature of the S K-edge spectra that depend on the Co concentration and are related to atomic bound-state transitions through hybridization between 3d transition metal and sp host semiconductor electronic states. These changes reveal intrinsic effects, such as S vacancies and dopant effects. From the Co K-edge XANES spectra, substitution of Co2+ into ZnS is observed. The local atomic structure around the dopant, obtained by XAFS spectroscopy, indicates a reduction of the Co–S bond length. Optical analysis shows deep absorption centers or intermediate states in the bandgap, ascribed to d-d transitions in the Co atoms that modify the electronic band structure and cause a decrease in the bandgap as a function of Co concentration. This work provides direct evidence of dopant effects in heavily Co-doped ZnS, including bond distances and Debye-Waller factors, providing key elements to generate a realistic atomic structural model, which is crucial to understand the observed electronic properties of this material.

Non-trivial surface states of samarium hexaboride at the (111) surface

The peculiar metallic electronic states observed in the Kondo insulator, samarium hexaboride (SmB6), has stimulated considerable attention among those studying non-trivial electronic phenomena. However, experimental studies of these states have led to controversial conclusions mainly due to the difficulty and inhomogeneity of the SmB6 crystal surface. Here, we show the detailed electronic structure of SmB6 with angle-resolved photoelectron spectroscopy measurements of the three-fold (111) surface where only two inequivalent time-reversal-invariant momenta (TRIM) exist. We observe the metallic two-dimensional state was dispersed across the bulk Kondo gap. Its helical in-plane spin polarisation around the surface TRIM indicates that SmB6 is topologically non-trivial, according to the topological classification theory for weakly correlated systems. Based on these results, we propose a simple picture of the controversial topological classification of SmB6.

Electron-transfer-induced metallic electronic states in a H/Fe3O4(001) film subsurface

We investigate the local electronic properties of Fe atoms in a H/Fe3O4(001) film subsurface using scanning tunneling microscopy/spectroscopy (STS). Local barrier height measurements show a H-adsorption-induced reduction of the local work function, indicating electron transfer from H to surface atoms. The STS results and high-resolution spatial mapping of the occupied density of states near the Fermi level reveal that subsurface Fe atoms located beneath a OH group exhibit a metallic electronic state, different from the semiconducting state for intrinsic subsurface Fe atoms. These results can be explained by considering the extension of electron transfer to the subsurface layer.