Search

When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO2 layers. Computationally, we find that many-body interactions in Pd and CoO2 layers are highly different. Holes in the CoO2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials. PdCoO2 belongs to a class of materials where both weakly and strongly correlated electrons co-exist side-by-side in different layers of the crystal structure. Here, the authors investigate PdCoO2 using standing wave photoemission spectroscopy and many-body calculations reporting layer-specific details about the electronic structure. [ABSTRACT FROM AUTHOR]

Distinct properties of multiple phases of vanadium oxide (VOx) render this material family attractive for advanced electronic devices, catalysis, and energy storage. In this work, phase boundaries of VOx are crossed and distinct electronic properties are obtained by electrochemically tuning the oxygen content of VOx thin films under a wide range of temperatures. Reversible phase transitions between two adjacent VOx phases, VO2 and V2O5, are obtained. Cathodic biases trigger the phase transition from V2O5 to VO2, accompanied by disappearance of the wide band gap. The transformed phase is stable upon removal of the bias while reversible upon reversal of the electrochemical bias. The kinetics of the phase transition is monitored by tracking the time-dependent response of the X-ray absorption peaks upon the application of a sinusoidal electrical bias. The electrochemically controllable phase transition between VO2 and V2O5 demonstrates the ability to induce major changes in the electronic properties of VOx by spanning multiple structural phases. This concept is transferable to other multiphase oxides for electronic, magnetic, or electrochemical applications. [ABSTRACT FROM AUTHOR]

The discovery of polar metal opens the door to incorporating electric polarization into electronics with the potential to invigorate next‐generation multifunctional electronic devices. Especially, electric polarization can be induced by geometric design in non‐polar perovskite oxides. Here, the epitaxial strain exerted on the deposited single‐crystalline NdNiO3 thin films is systematically varied in both sign and amplitude by choosing substrates with different lattice mismatch. The pseudocubic NdNiO3(111) film, which is non‐polar in its bulk state, is induced to be polar under both compressive and tensile strain. The fine‐tuning of epitaxial strain is realized by continuously varying the film thickness using the "thickness‐wedge" growth technique, and from the elucidated thickness dependence, the electric polarization and metallicity can be further optimized. Moreover, transitioning from isotropic to anisotropic epitaxial strain gives rise to an ideal polar metal state in the pseudocubic NdNiO3(102) film on an orthorhombic substrate, achieving a remarkably low resistivity of 173 µΩ cm at room temperature. The metal–insulator transition in NdNiO3 is completely suppressed and the polar metal state becomes the ground state at all temperatures. These results demonstrate alluring possibilities of induction and manipulation of both electric polarization and electric transport properties in functional perovskite oxides by epitaxial strain engineering. [ABSTRACT FROM AUTHOR]

Oxygen defects are essential building blocks for designing functional oxides with remarkable properties, ranging from electrical and ionic conductivity to magnetism and ferroelectricity. Oxygen defects, despite being spatially localized, can profoundly alter global properties such as the crystal symmetry and electronic structure, thereby enabling emergent phenomena. In this work, we achieved tunable metal–insulator transitions (MIT) in oxide heterostructures by inducing interfacial oxygen vacancy migration. We chose the non-stoichiometric VO2-δ as a model system due to its near room temperature MIT temperature. We found that depositing a TiO2 capping layer on an epitaxial VO2 thin film can effectively reduce the resistance of the insulating phase in VO2, yielding a significantly reduced ROFF/RON ratio. We systematically studied the TiO2/VO2 heterostructures by structural and transport measurements, X-ray photoelectron spectroscopy, and ab initio calculations and found that oxygen vacancy migration from TiO2 to VO2 is responsible for the suppression of the MIT. Our findings underscore the importance of the interfacial oxygen vacancy migration and redistribution in controlling the electronic structure and emergent functionality of the heterostructure, thereby providing a new approach to designing oxide heterostructures for novel ionotronics and neuromorphic-computing devices. [ABSTRACT FROM AUTHOR]

Incorporating hydrogen into complex oxides holds promise for discovering exotic phenomena and novel functionalities by promoting couplings between ion and lattice/charge/spin/orbital degrees of freedom. Here, electrolyte gating‐driven hydrogenation is investigated in epitaxial brownmillerite SrFeO2.5 thin films in which the hydrogenation‐induced lattice distortion is qualitatively different from its counterpart SrCoO2.5. The achievable lattice expansion in hydrogenated SrFeO2.5 is weaker than in SrCoO2.5 and primarily occurs along the normal of the stacked octahedral FeO6 and tetrahedral FeO4 layers. Upon the substitution of Fe with Co, the lattice expansion monotonically increases with increase of Co/Fe ratio, implying an intrinsic difference in accommodating hydrogen between Fe‐ and Co‐based brownmillerites. Moreover, a net magnetic moment in hydrogenated SrFeO2.5 films is observed above room temperature, which gradually weakens with the increase of Co substitution, suggesting a stronger canted magnetism in Fe‐based hydrogenated brownmillerites. This work clarifies the electrolyte gating‐driven hydrogenation mechanisms in brownmillerite SrFeO2.5 epitaxial thin films and those with Co substitution, particularly the deterministic role of Co/Fe ratio in the evolution of structure and properties upon hydrogenation. [ABSTRACT FROM AUTHOR]

Mass production of green hydrogen via water electrolysis requires advancements in the performance of electrocatalysts, especially for the oxygen evolution reaction. In this feature article, we highlight how epitaxial nickelates act as model systems to identify atomic-level composition–structure–property–activity relationships, capture dynamic changes under operating conditions, and reveal reaction and failure mechanisms. These insights guide advanced electrocatalyst design with tailored functionality and superior performance. We conclude with an outlook for future developments via operando characterization and multilayer electrocatalyst design. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mn-Ni/rGO composite materials synthesized by a simple one-step hydrothermal method. • Ni(OH) 2 -decorated Mn 3 O 4 nanorods are well-dispersed on reduced-graphene sheets. • Strong synergism between graphene and metal composites on surfaces/interfaces. • Hybrid materials exhibit good catalytic activity in air–cathode for Al/Zn–air batteries. Manganese oxide, such as Mn 3 O 4 , is one of promising material as electrocatalyst in air–cathode for metal–air battery due to low cost, high catalytic activity and good stability. Although pure Mn 3 O 4 material exhibits excellent catalytic activity to ORR, it is severely limited by its intrinsically low conductivity and high over-potential toward OER. In this work, the hybrid materials of nickel-hydroxide-decorated nanorods of trimanganese-tetroxide supported on reduced-graphene have been prepared by one-step hydrothermal method. The composite materials are used as bifunctional cathode catalysts to ORR/OER, and investigated by the technologies of rotating disk electrode and metal–air batteries. The results indicate that the hybrid material of Mn-Ni/rGO use as catalyst in air–cathode for Al/Zn–air battery has good catalytic activity. Based on analysis of scanning electron microscope and X-ray photoelectron spectroscopy, the composite material shows good performance owing to the improvement of conductivity supported on graphene, more Mn4+ on the surface of hybrid material, the coexistence of Mn3+/Mn4+ redox couple, and the synergism between nickel-hydroxide and graphene. [ABSTRACT FROM AUTHOR]

We have developed an experimental system to simultaneously measure surface structure, morphology, composition, chemical state, and chemical activity for samples in gas phase environments. This is accomplished by simultaneously measuring x-ray photoelectron spectroscopy (XPS) and grazing incidence x-ray scattering in gas pressures as high as the multi-Torr regime while also recording mass spectrometry. Scattering patterns reflect near-surface sample structures from the nano-scale to the meso-scale, and the grazing incidence geometry provides tunable depth sensitivity of structural measurements. Scattered x rays are detected across a broad range of angles using a newly designed pivoting-UHV-manipulator for detector positioning. At the same time, XPS and mass spectrometry can be measured, all from the same sample spot and under ambient conditions. To demonstrate the capabilities of this system, we measured the chemical state, composition, and structure of Ag-behenate on a Si(001) wafer in vacuum and in O2 atmosphere at various temperatures. These simultaneous structural, chemical, and gas phase product probes enable detailed insights into the interplay between the structure and chemical state for samples in gas phase environments. The compact size of our pivoting-UHV-manipulator makes it possible to retrofit this technique into existing spectroscopic instruments installed at synchrotron beamlines. Because many synchrotron facilities are planning or undergoing upgrades to diffraction limited storage rings with transversely coherent beams, a newly emerging set of coherent x-ray scattering experiments can greatly benefit from the concepts we present here. [ABSTRACT FROM AUTHOR]