Electronic Reconstruction Research Articles

Electronic reconstruction at the polar (111)-oriented oxide interface

Atomically flat (111) interfaces between insulating perovskite oxides provide a landscape for new electronic phenomena. For example, the graphene-like coordination between interfacial metallic ion layer pairs can lead to topologically protected states [Xiao et al., Nat. Commun. 2, 596 (2011) and A. Rüegg and G. A. Fiete, Phys. Rev. B 84, 201103 (2011)]. The metallic ion/metal oxide bilayers that comprise the unit cell of the perovskite (111) heterostructures require the interface to be polar, generating an intrinsic polar discontinuity [Chakhalian et al., Nat. Mater. 11, 92 (2012)]. Here, we investigate epitaxial heterostructures of (111)-oriented LaAlO3/SrTiO3 (LAO/STO). We find that during heterostructure growth, the LAO overlayer eliminates the structural reconstruction of the STO (111) surface with an electronic reconstruction, which determines the properties of the resulting two-dimensional conducting gas. This is confirmed by transport measurements, direct determination of the structure and atomic charge from coherent Bragg rod analysis, and theoretical calculations of electronic and structural characteristics. Interfacial behaviors of the kind discussed here may lead to new growth control parameters useful for electronic devices.

Insight into the Sb(III) and Sb(V) removal mechanisms on porous Fe-Ti-chitosan composite: Experiment and DFT calculations

Currently, increasing attention has been concentrated on antimony (Sb) pollution and its ecological damage in ecosystems, where efficient and green treatment technologies, particularly for aquatic Sb(III) and Sb(V), are urgently necessary. Herein, a novel Fe-Ti-chitosan (FTC) composite was prepared via a facile coprecipitation method for aquatic Sb(III) and Sb(V) elimination. A 2/1 mol ratio of Fe/Ti (FTC-2/1) was evaluated as the suitable proportion with the highest adsorption performance for Sb(III) and Sb(V). Detailed morphological analysis indicated that chitosan could not only act as the porous framework for Fe and Ti loading but also effectively inhibit the particle aggregation, in which the BET surface area of FTC-2/1 reached 235.23 m2·g−1. The unique three-dimension porous framework endowed the material with abundant surface hydroxyl groups and further promoted the diffusion kinetics of Sb(III) and Sb(V) in the porous structure. According to the density functional theory (DFT) calculations, the highest adsorption efficiency of FTC-2/1 was rationally interpreted by the preferential adsorption of the Fe-Ti bidentate complex towards Sb(III) and Sb(V), which was attributed to the redistribution of the interfacial electron induced by the Fe-O-Ti linkage. The driving force of the highest adsorption energy was derived from the electronic structure reconstruction of adsorbed antimony molecules and Fe-Ti atoms, in which a stable pd orbital hybridization occurred between O 2p and Fe/Ti 3d. This work demonstrated that FTC-2/1 is promising and environmentally friendly adsorbent for antimony elimination, and may provide a potent perspective for the interfacial analysis of binary oxides.

Microscopic origin of multiferroic order in monolayer NiI2

The discovery of multiferroic behavior in monolayer NiI2 provides a new symmetry-broken state in van der Waals monolayers, featuring the simultaneous emergence of helimagnetic order and ferroelectric order at a critical temperature of T = 21 K. However, the microscopic origin of multiferroic order in NiI2 monolayer has not been established, and in particular, the role of non-collinear magnetism and spin–orbit coupling in this compound remains an open problem. Here we reveal the origin of the two-dimensional multiferroicity in NiI2 using first-principles electronic structure methods. We show that the helimagnetic state appears as a consequence of the long-range magnetic exchange interactions, featuring sizable magnetic moments in the iodine atoms. We demonstrate that the electronic density reconstruction accounting for the ferroelectric order emerges from the interplay of non-collinear magnetism and spin–orbit coupling. We demonstrate that the ferroelectric order is controlled by the iodine spin–orbit coupling, and leads to an associated electronically-driven distortion in the lattice. Our results establish the microscopic origin of the multiferroic behavior in monolayer NiI2, putting forward the coexistence of helical magnetic order and ligand spin–orbit coupling as driving forces for multiferroic behavior in two-dimensional materials.

Fe-doping induced electronic structure reconstruction in Ni-based metal-organic framework for improved energy-saving hydrogen production via urea degradation

Urea-assisted water electrolysis provides an energy-saving strategy toward hydrogen evolution and wastewater-relevant environmental crisis. Efficient bifunctional electrocatalyst for urea oxidization reaction (UOR) and hydrogen evolution reaction (HER) is the key to solve these challenges. Herein, we demonstrated a Fe-doped Ni-based MOF nanosheet arrays (FeNi-MOF NSs) through one-step hydrothermal synthesis for efficient energy-saving urea-assisted water electrolysis. When applied for UOR, such prepared FeNi-MOF NSs achieves a current density of 10 and 100 mA cm −2 at overpotentials of 131 and 155 mV, respectively. Based on both comprehensive experimental and theoretical investigations, the excellent UOR performance can be attributed to that the Fe-doping caused electronic structure reconstruction in MOF, which not only induced the formation of abundant high-valence Ni as active centers for UOR, but also created more electronic states with optimized adsorption energies of reactant and product molecules on the surface, benefiting the UOR kinetics. Based on the UOR and HER activities of FeNi-MOF NSs, a urea electrolyser has been assembled, which affords 10 mA cm −2 current density at a low cell voltage of 1.431 V with excellent stability during long-term electrolysis. • Fe-doped Ni-based MOF nanosheet arrays (FeNi-MOF NSs) have been synthesized. • Trace Fe-doping caused electronic structure reconstruction in Ni-MOF. • The formation of abundant high-valence Ni as active centers for UOR. • FeNi-MOF NSs require only 1.430 V to achieve 300 mA cm −2 for UOR. • Such urea electrolyser affords 10 mA cm −2 at a low cell voltage of 1.431 V.

Evolution of Electronic Structure in Pristine and Rb-Reconstructed Surfaces of Kagome Metal RbV3Sb5.

We report on in situ low-temperature (4 K) scanning tunneling microscope measurements of atomic and electronic structures of the cleaved surfaces of an alkali-based kagome metal RbV3Sb5 single crystals. We find that the dominant pristine surface exhibits Rb-1×1 structure, in which a unique unidirectional √3a0 charge order is discovered. As the sample temperature slightly rises, Rb-√3×1 and Rb-√3×√3 reconstructions form due to desorption of surface Rb atoms. Our conductance mapping results demonstrate that Rb desorption not only gives rise to hole doping but also reconstructs the electronic band structures. Surprisingly, we find a ubiquitous gap opening near the Fermi level in tunneling spectra on all the surfaces despite their large differences of hole-carrier concentration, indicating an orbital-selective band reconstruction in RbV3Sb5. The Rb desorption induced electronic reconstructions are further confirmed by our density functional theory calculations.

Correlation-driven electronic reconstruction in FeTe1\u2212xSex

Electronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstruction of the Fermi surface tuned independently by two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide family FeTe1−xSex. We demonstrate that this reconstruction is driven by the de-hybridization of a strongly renormalized dxy orbital with the remaining itinerant iron 3d orbitals in the emergence of an orbital-selective Mott phase. Our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase into an orbital-selective Mott phase in dxy as Se concentration is reduced.

Constructing High Efficiency CoZnx Mn2-x O4 Electrocatalyst by Regulating the Electronic Structure and Surface Reconstruction.

It is an effective strategy to develop novel electrocatalysts with controllable defects to enhance their electrocatalytic activity and stability. However, how to precisely design these catalysts on the atom scale remains very difficult. Herein, several vacancy-dependent CoZnx Mn2-x O4 catalysts are prepared through tailoring the concentration of Zn ions. The in situ activation of the obtained products accelerates the surface reconstruction. The superior electrocatalytic performance can be ascribed to the formations of MOOH (Mn, Co) active species and abundant oxygen vacancies, which are comparable to noble IrO2 and Pt/C catalysts. Zn-CoMn2 O4 -1.5 catalyst delivers a cell voltage of 1.63V and long durability. Density functional theory calculations demonstrate that the appropriate Zn ion doping can improve the density states of p electron on the surface of catalysts significantly and benefit the d-band center closing to Fermi level, suggesting their high charge carrier density and low adsorption energy.

Tuning the local electronic structure of SrTiO3 catalysts to boost plasma-catalytic interfacial synergy

Boosting plasma-catalyst synergy to enhance volatile organic compounds (VOCs) decomposition remains a challenge. Herein, rich oxygen vacancies (VO) were engineered into the SrTiO3 catalysts through a facile nitrogen incorporation strategy for the plasma-catalytic decomposition of toluene and ethyl acetate. 100% toluene conversion with 81% CO2 selectivity at a competitive energy efficiency was achieved under ambient conditions. The characterization results and theoretical calculations evidenced that the partial substitution of oxygen by nitrogen triggered the electronic reconstruction and local disorder, thus modulating the electronic properties and coordination structures contributed to the formation of VO-Ti3+ pairs. Quasi in-situ EPR, operando OES, and operando DRIFTS originally demonstrated that the VO-Ti3+ pairs as active sites promoted the plasma-catalytic synergy instead of isolated VO. Importantly, the VO-Ti3+ pairs with favorable electron transfer characteristics energetically preferred to capture and utilize vibrationally excited oxygen species. And the lattice oxygen supplied by the VO-Ti3+ pairs were more vigorously activated by the plasma to participate in the surface/interface reaction. This work advances our understanding of the real active sites in plasma-catalytic interfacial synergy and thus paving the way for the rational design of efficiently heterogeneous catalysts.

Electronic reconstruction and charge transfer in strained Sr2CoIrO6 double perovskite

The electronic, magnetic and optical properties of the double perovskite Sr$_2$CoIrO$_6$ (SCIO) under biaxial strain are explored in the framework of density functional theory (DFT) including a Hubbard $U$ term and spin-orbit coupling (SOC) in combination with absorption spectroscopy measurements on epitaxial thin films. While the end member SrIrO$_3$ is a semimetal with a quenched spin and orbital moment and bulk SrCoO$_3$ is a ferromagnetic (FM) metal with spin and orbital moment of 2.50 and 0.13 $\mu_{B}$, respectively, the double perovskite SCIO emerges as an antiferromagnetic Mott insulator with antiparallel alignment of Co, Ir planes along the [110]-direction. Co exhibits a spin and enhanced orbital moment of $\sim 2.35-2.45$ and $0.31-$0.45 $\mu_{B}$, respectively. Most remarkably, Ir acquires a significant spin and orbital moment of 1.21-1.25 and 0.13 $\mu_{B}$, respectively. Analysis of the orbital occupation indicates an electronic reconstruction due to a substantial charge transfer from minority to majority spin states in Ir and from Ir to Co, signaling an Ir$^{4+\delta}$, Co$^{4-\delta}$ configuration. Biaxial strain, varied from -1.02% ($a_{\rm NdGaO_3}$) through 0% ($a_{\rm SrTiO_3}$) to 1.53% ($a_{\rm GdScO_3}$), influences in partcular the orbital polarization of the $t_{2g}$ states and leads to a nonmonotonic change of the band gap between 163 and 235 meV. The absorption coefficient reveals a two plateau fearure due to transitions from the valence to the lower lying narrow $t_{2g}$ and the higher lying broader $e_{g}$ bands. Inclusion of many body effects, in particular, excitonic effects by solving the Bethe-Salpeter equation (BSE), increases the band gap by $\sim0.2$ and improves the agreement with the measured spectrum concerning the position of the second peak at $\sim 2.6$ eV.

Quadruple node of triple junctions of grain boundaries in a Eu2+-doped solid solution of the ions K+, Rb+, Cl- and Br-: an epifluorescence microscopy study using the doping ion as a fluorochrome.

Epifluorescence microscopy was used to unveil the geometry and relative spatial orientation of an arrangement of crystal defects, consisting of a quadruple node of triple junctions of grain boundaries, in a Eu2+-doped solid solution of K+, Rb+, Cl- and Br-. The doping ion was utilized as a fluorochrome. Microscopy images of different optical cross-sections of the arrangement of crystal defects under study were recorded and used to build an electronic three-dimensional reconstruction of this arrangement. The geometry is that of an irregular tristetrahedron, centred at the quadruple node, so that the tristetrahedron legs, deviating from the lattice <111>-zone axis directions, lie along the triple junctions, whereas the tristetrahedron faces collapse onto the grain boundaries. The deviation angles as well as the angles defined by different triple junction pairs and different grain boundary pairs were measured. The orientational deviation is larger than that in a KI single crystal but lower than that in a solid solution of K+, Cl- and Br-, meaning that the structural equilibrium depends on the matrix lattice structural character. Uncompensated local nanostrains, due to the ion substitution, are associated with the observed decrement in structural stability, in relation to the KI case, whereas compensated local nanostrains, provoked by the simultaneous substitution of cations and anions, are argued to be responsible for the observed increment in structural stability, in relation with the K(Cl, Br) case. The optical characterization of the fluorochrome, the recording and processing of the microscopy images, the building of the electronic 3D reconstruction, the angular measure methodology and the geometrical modelling are all carefully described.

Enhanced Tunneling Magnetoresistance Effect via Ferroelectric Control of Interface Electronic/Magnetic Reconstructions.

Magnetic tunnel junctions (MTJs) with tunable tunneling magnetoresistances (TMR) have already been proven to have great potential for spintronics. Especially, when ferroelectric materials are used as insulating barriers, more novel functions of MTJs can be realized due to interface magnetoelectric coupling. Here, we demonstrate a very large ferroelectric modulation of TMR (as high as 570% in low-resistance state) in the ferroelectric/magnetic La0.5Sr0.5MnO3/BaTiO3 (LSMO/BTO) junctions and find robust interfacial electronic and magnetic reconstructions via ferroelectric polarization switching. Through electrical, magnetic, and optical measurements combined with X-ray absorption and magnetic circular dichroism, we reveal that the interfacial electronic and magnetic (ferromagnetic/antiferromagnetic phase transition) reconstructions originate from strong electromagnetic coupling between BTO and LSMO at the interface and are driven by the modulation of hole/electron doping at the interface of LSMO/BTO through ferroelectric polarization switching. As a result, the ferroelectrically controlled interface barrier height and width and spin filter effect enable a giant electrical modulation of TMR. Our results shed new light on the intrinsic mechanisms governing magnetoelectric coupling and offering a new route to enhance magnetoelectric coupling for spin control in spintronic devices.

Visualizing band alignment across 2D/3D perovskite heterointerfaces of solar cells with light-modulated scanning tunneling microscopy

Graded 2D perovskite capping shells with continuously upshifting valence bands, produced by tailored dimensional engineering, can effectively extract holes from 3D perovskite cores. Real-space observation of electronic structures will fully reveal the operating mechanisms of 2D/3D hybrid perovskite solar cells (PSCs). Here, for the first time, light-modulated scanning tunneling microscopy visualizes the cross-sectional band alignment across 2D (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) n-1 Pb n I 3n+1 /3D CH 3 NH 3 PbI 3 stacked perovskites. By systematically analyzing their electronic configuration, the mixed-dimensional perovskite band structure along the vertical 3D-to-2D direction can be spatially resolved. Remarkably, the electric field in the 2D perovskite is larger under light illumination than under dark conditions, resulting in an increase in the concentration of holes and electrons distributed in the 2D and 3D perovskites, respectively. Benefiting from this electronic reconstruction, charge recombination is suppressed, thereby significantly promoting the 2D/3D PSC performance. Moreover, our method opens an avenue for direct, local mapping of optoelectronic device energy levels. • Real spacing energy band distribution of photo-activated 2D/3D perovskites is first mapped. • Visualization of photocarrier dynamics is demonstrated, promote efficiencies of 2D/3D perovskite solar cells. • The electric field in the 2D perovskite is enhanced under light illumination. • This LM-XSTM technology will be used for other semiconductor devices.

Construction of GQDs‐Decorated Ultrathin Bi2WO6 Nanosheets Hydrogel: a Recyclable‐Flexible Platform with Excellent Piezo‐Photocatalytic Activity for High‐Performance Water Decontamination and its Theoretical Interpretation

AbstractIt is demonstrated that, by using the electronic reconstruction of ultrathin Bi2WO6 (BWO) nanosheets with graphene quantum dots (GQDs) upon a unique process of laser irradiation in liquid (LAL), a kind of GQDs modified BWO nanosheets under metastable state were synthesized, which performed excellent visible‐light piezo‐photocatalytic organics degradation. Furthermore, using commercial cpolyacrylamids (PAM) as the additional materials, a kind of electronic reconstruction GQDs‐BWO nanosheets hydrogel was achieved by dispersed the functionalized powders into a super absorbent polymers (SAPs) system upon in situ polymerization method, which showed that it is a bracing idea to use SAPs for water decontamination reactors to enable full use of vibration energy synergistic with light of nature to driven pollution degradation. First‐principles calculation was also simultanously performed in this study. This study may provide a new enlightenment on fabricating novel 2D nanomaterials hydrogel for efficient effluent purification and potentially other environmental or energy applications.

Tunable Rashba spin-orbit coupling and its interplay with multiorbital effect and magnetic ordering at oxide interfaces

The complex oxide heterostructures such as LaAlO3/SrTiO3 (LAO/STO) interface are paradigmatic platforms to explore emerging multi-degrees of freedom coupling and the associated exotic phenomena. In this study, we reveal the effects of multiorbital and magnetic ordering on Rashba spin-orbit coupling (SOC) at the LAO/STO (001) interface. Based on first-principles calculations, we show that the Rashba spin splitting near the conduction band edge can be tuned substantially by the interfacial insulator-metal transition due to the multiorbital effect of the lowest t_2g bands. We further unravel a competition between Rashba SOC and intrinsic magnetism, in which the Rashba SOC induced spin polarization is suppressed by the interfacial magnetic ordering. These results deepen our understanding of intricate electronic and magnetic reconstruction at the perovskite oxide interfaces and shed light on the engineering of oxide heterostructures for all-oxides-based spintronic devices.

Electronic surface reconstruction of TiO2 nanocrystals revealed by resonant inelastic x-ray scattering

The identification of lattice multiphases in TiO2 nanocrystals is studied by high resolution transmission electron microscope and electron diffraction patterns. Based on the spectroscopic analysis using soft x-ray absorption and resonant inelastic soft x-ray scattering, it is believed that the oxygen vacancies at the interface exhibit structural distortion of the TiO68− cluster around the defect site as for the multiphase lattice. We elucidate that the extra 3d electrons nearby induce the inelastic scattering features with the excitation energy dependence owing to different energy relaxation processes, a characteristic of the electron-phonon coupling or the nature of the electron-hole pair at the intermediate state. The manifold dd excitations driven by the strong interaction between Ti-3d and O-2p electrons are noticeably rich, coexisting on both Ti and O sites. This sophisticated experiment can advance the perspective of nanocomposite TiO2 for various interactions of surface Ti3+ in applications of future devices.

Orbital engineering in YVO3−LaAlO3 superlattices

Oxide heterostructures provide unique opportunities to modify the properties of quantum materials through a targeted manipulation of spin, charge, and orbital states. Here, we use resonant x-ray reflectometry to probe the electronic structure of thin slabs of ${\mathrm{YVO}}_{3}$ embedded in a superlattice with $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$. We extend the previously established methods of reflectometry analysis to a general form applicable to ${t}_{2g}$ electron systems and extract quantitative depth-dependent x-ray linear dichroism profiles. Our data reveal an artificial, layered orbital polarization, where the average occupation of $xz$ and $yz$ orbitals in the interface planes next to $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ is inverted compared to the central part of the ${\mathrm{YVO}}_{3}$ slab. This phase is stable down to 30 K and the bulklike orbital ordering transitions are absent. We identify the key mechanism for the electronic reconstruction to be a combination of epitaxial strain and spatial confinement by the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ layers, in good agreement with predictions from ab initio theory.

Electron Density Modulation of MoO2/Ni to Produce Superior Hydrogen Evolution and Oxidation Activities.

Hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) have aroused great interest, but the high price of platinum group metals (PGMs) limits their development. The electronic reconstruction at the interface of a heterostructure is a promising strategy to enhance their catalytic performance. Here, MoO2/Ni heterostructure was synthesized to provide effective HER in an alkaline electrolyte and exhibit excellent HOR performance. Theoretical and experimental analyses prove that the electron density around the Ni atom is reduced. The electron density modulation optimizes the hydrogen adsorption and hydroxide adsorption free energy, which can effectively improve the activity of both HER and HOR. Accordingly, the prepared MoO2/Ni@NF catalyst reveals robust HER activity (η10 = 50.48 mV) and HOR activity (j0 = ∼1.21 mA cm-2). This work demonstrates an effective method to design heterostructure interfaces and tailor the surface electronic structure to improve HER/HOR performance.

Electronic, Orbital, and Spin Reconstructions at the Interface between Metallic La0.7Sr0.3MnO3 and Charge-Disproportionated Insulator SrCoO2.5

Electronic, Orbital, and Spin Reconstructions at the Interface between Metallic La_0.7Sr_0.3MnO₃ and Charge-Disproportionated Insulator SrCoO_2.5

Self-driven Ru-modified NiFe MOF nanosheet as multifunctional electrocatalyst for boosting water and urea electrolysis

Urea electro-oxidation reaction (UOR) has been a promising strategy to replace oxygen evolution reaction (OER) by urea-mediated water splitting for hydrogen production. Naturally, rational design of high-efficiency and multifunctional electrocatalyst towards UOR and hydrogen evolution reaction (HER) is of vital significance, but still a grand challenge. Herein, an innovative 3D Ru-modified NiFe metal-organic framework (MOF) nanoflake array on Ni foam (Ru-NiFe-x/NF) was elaborately designed via spontaneous galvanic replacement reaction (GRR). Notably, the adsorption capability of intermediate species (H*) of catalyst is significantly optimized by Ru modification. Meanwhile, rich high-valence Ni active species can be acquired by self-driven electronic reconstruction in the interface, then dramatically accelerating the electrolysis of water and urea. Remarkably, the optimized Ru-NiFe-③/NF (1.6 at% of Ru) only requires the overpotential of 90 and 310 mV to attain 100 mA cm−2 toward HER and OER in alkaline electrolyte, respectively. Impressively, an ultralow voltage of 1.47 V is required for Ru-NiFe-③/NF to deliver a current density of 100 mA cm−2 in urea-assisted electrolysis cell with superior stability, which is 190 mV lower than that of Pt/C-NF||RuO2/NF couple. This work is desired to explore a facile way to exploit environmentally-friendly energy by coupling hydrogen evolution with urea-rich sewage disposal.

Spin-polarized electronic states and atomic reconstructions at antiperovskite Sr3SnO(001) polar surfaces

We report a first-principles investigation of the atomic and electronic properties at the perfect and defective (001) surfaces of the antiperovskite ${\mathrm{Sr}}_{3}\mathrm{Sn}\mathrm{O}$. We first performed a thermodynamical study of the atomic structure terminations and demonstrated that SrSn-terminated surfaces should be the most stable one, either with a perfect $(1\ifmmode\times\else\texttimes\fi{}1$) structure or with a $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction induced by the formation of Sn vacancies. We detailed the surface gap states obtained for these surfaces, which we compare with those of other surface terminations, also having relatively low energies. These gap states, located near the Fermi level, could have a major contribution to the transport properties. Due to the lack of inversion symmetry associated with the surface, we predict that they also experience spin splittings, an important property for spin-orbitronic applications. Finally, we found that ${\mathrm{Sr}}_{2}\mathrm{O}$-terminated surfaces could display a ferromagnetic ordering resulting from the population of $4d$ orbitals of Sr atoms at the surface and that this could lead to the formation of a spin-polarized two-dimensional electron gas.