Transition Metal Oxide Heterostructures Research Articles

Bimetallic NiO/NiFe2O4 heterostructures with interfacial effects for boosting electrochemical water splitting applications

The present work reports the simple preparation of NiO/ NiFe2O4 heterostructures with interfacial effects as bifunctional electrocatalysts for overall water splitting. At increasing Ni/Fe precursor ratios, the evolution of NiO phases is observed in NFO heterostructures, which incorporate differential strain effects at the interface. The modulated heterojunction of NiO/NiFe2O4 heterostructure incorporates compressive strain at the interfaces enhancing its electrocatalytic performances. Notably, NiO/NiFe2O4 heterostructure prepared at equimolar Ni/Fe ratios (NFO-1) exhibits significant electrocatalytic performances due to modulated interfacial effects. As such, NFO-1 exhibits lower OER and HER overpotentials of about 242 mV and 106 mV for 10 mA cm−2 current density and exhibits improved water splitting activity, requiring 1.56 V to drive 10 mA cm−2 current density. The present work emphasizes the importance of modulating the heterostructures of transition metal oxides to enhance the interfacial effects as bifunctional electrocatalysts for water splitting applications.

Synergistic Active Phases of Transition Metal Oxide Heterostructures for Highly Efficient Ammonia Electrosynthesis

AbstractElectrochemically converting waste nitrate (NO3−) into ammonia (NH3) is a green route for both wastewater treatment and high‐value‐added ammonia generation. However, the NO3−‐to‐NH3 reaction involves multistep electron transfer and complex intermediates, making it a grand challenge to drive efficient NO3− electroreduction with high NH3 selectivity. Herein, an in‐operando electrochemically synthesized Cu2O/NiO heterostructure electrocatalyst is proven for efficient NH3 electrosynthesis. In situ Raman spectroscopy reveals that the obtained Cu2O/NiO, induced by the electrochemistry‐driven phase conversion, is the real active phase. This electronically coupled phase can modulate the interfacial charge distribution, dramatically lower the overpotential in the rate‐determining step and thus requiring lower energy input to proceed with the NH3 electrosynthesis. The orbital hybridization calculations further identify that Cu2O is beneficial for NO3− adsorption, and NiO could promote the desorption of NH3, forming an excellent tandem electrocatalyst. Such a tandem system leads to NH3 Faradaic efficiency of 95.6%, a super‐high NH3 selectivity of 88.5% at −0.2 V versus RHE, surpassing most of the NH3 electrosynthesis catalysts at an ultralow reaction voltage.

Low-dimensional electronic state at the surface of a transparent conductive oxide

Materials that blend physical properties that are usually mutually exclusive could facilitate devices with novel functionalities. For example, the doped perovskite alkaline earth stannates BaSnO3 and SrSnO3 show the intriguing combination of high light transparency and high electrical conductivity. Understanding such emergent physics requires deep insight into the materials’ electronic structures. Moreover, the band structure at the surfaces of those materials can deviate significantly from their bulk counterparts, thereby unlocking novel physical phenomena. Employing angle-resolved photoemission spectroscopy and ab initio calculations, we reveal the existence of a 2-dimensional metallic state at the SnO2-terminated surface of 1% La-doped BaSnO3 thin films. The observed surface state is characterized by a distinct carrier density and a lower effective mass compared to the bulk conduction band, of about 0.12me. These particular surface state properties place BaSnO3 among the materials suitable for engineering highly conductive transition metal oxide heterostructures.

Polaronic Trions Induced by Strong Interfacial Coupling in Monolayer WSe2

AbstractThe weak dielectric screening in 2D semiconducting transition metal dichalcogenides give rise to strongly bound quasiparticles, which provides a platform to investigate the diverse excitonic phenomena and correlated physics. However, how to effectively control these quasiparticles is still a challenge for their applications in optoelectronic and valleytronic devices. Herein, by means of fabricating monolayer WSe2 and transition metal oxide (TMO) heterostructures, polaronic trion, that is a trion dressed with soft rotational optical (RO) phonons, is realized due to the strong interfacial coupling. This Fröhlich bound state of trion dramatically increases the trion binding energy (BE) from room temperature to 65 meV at 80 K in WSe2/LaAlO3 (LAO). However, the increase of the trion BE for WSe2/SrTiO3 (STO) occurs below the phase transition temperature. This work expands the possibilities of the TMDs/TMOs heterostructures and promotes the development of 2D van der Waals materials for quasiparticle‐based devices.

Emergent ferromagnetism and insulator-metal transition in δ-doped ultrathin ruthenates

Heterostructures of complex transition metal oxides are known to induce extraordinary emergent quantum states that arise from broken symmetry and other discontinuities at interfaces. Here we report the emergence of unusual, thickness-dependent properties in ultrathin CaRuO3 films by insertion of a single isovalent SrO layer (referred to as δ-doping). While bulk CaRuO3 is metallic and nonmagnetic, films thinner than or equal to ~15-unit cells (u.c.) are insulating though still nonmagnetic. However, δ-doping to middle of such CaRuO3 films induces an insulator-to-metal transition and unusual ferromagnetism with strong magnetoresistive behavior. Atomically resolved imaging and density-functional-theory calculations reveal that the whole δ-doped film preserves the bulk-CaRuO3 orthorhombic structure, while appreciable structural and electronic changes are highly localized near the SrO layer. The results highlight delicate nature of magnetic instability in CaRuO3 and subtle effects that can alter it, especially the role of A-site cation in electronic and magnetic structure additional to lattice distortion in ruthenates. It also provides a practical approach to engineer material systems via highly localized modifications in their structure and composition that may offer new routes to the design of oxide electronics.

Defect-induced magnetism in homoepitaxial SrTiO3

Along with recent advancements in thin-film technologies, the engineering of complex transition metal oxide heterostructures offers the possibility of creating novel and tunable multifunctionalities. A representative complex oxide is the perovskite strontium titanate (STO), whose bulk form is nominally a centrosymmetric paraelectric band insulator. By tuning the electron doping, chemical stoichiometry, strain, and charge defects of STO, it is possible to control the electrical, magnetic, and thermal properties of such structures. Here, we demonstrate tunable magnetism in atomically engineered STO thin films grown on STO (001) substrates by controlling the atomic charge defects of titanium (VTi) and oxygen (VO) vacancies. Our results show that the magnetism can be tuned by altering the growth conditions. We provide deep insights into its association to the following defect types: (i) VTi, resulting in a charge rearrangement and local spin polarization, (ii) VO, leading to weak magnetization, and (iii) VTi–VO pairs, which lead to the appearance of a sizable magnetic signal. Our results suggest that controlling charged defects is critical for inducing a net magnetization in STO films. This work provides a crucial step for designing magnetic STO films via defect engineering for magnetic and spin-based electronic applications.

Manipulating the metal-to-insulator transition and magnetic properties in manganite thin films via epitaxial strain

Strain engineering of epitaxial transition metal oxide heterostructures offers an intriguing opportunity to control electronic structures by modifying the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, we demonstrate that the electronic structure, magnetic and transport properties of ${\mathrm{La}}_{0.9}{\mathrm{Ba}}_{0.1}{\mathrm{MnO}}_{3}$ thin films can be effectively controlled by epitaxial strain. Spectroscopic studies and first-principles calculations reveal that the orbital occupancy in Mn ${e}_{g}$ orbitals can be switched from the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbital to the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital by varying the strain from compressive to tensile. The change of orbital occupancy associated with Mn $3d$-O $2p$ hybridization leads to dramatic modulation of the magnetic and electronic properties of strained ${\mathrm{La}}_{0.9}{\mathrm{Ba}}_{0.1}{\mathrm{MnO}}_{3}$ thin films. Under moderate tensile strain, an emergent ferromagnetic insulating state with an enhanced ferromagnetic Curie temperature of 215 K is achieved. These findings not only deepen our understanding of electronic structures, magnetic and transport properties in the ${\mathrm{La}}_{0.9}{\mathrm{Ba}}_{0.1}{\mathrm{MnO}}_{3}$ system, but also demonstrate the use of epitaxial strain as an effective knob to tune the electronic structures and related physical properties for potential spintronic device applications.

Lattice‐Matching Formed Mesoporous Transition Metal Oxide Heterostructures Advance Water Splitting by Active Fe–O–Cu Bridges

AbstractDeveloping efficient bifunctional electrocatalysts toward oxygen/hydrogen evolution reactions is crucial for electrochemical water splitting toward hydrogen production. The high‐performance electrocatalysts depend on the catalytically active and highly accessible reaction sites and their structural robustness, while the rational design of such electrocatalysts with desired features avoiding tedious manufacture is still challenging. Here, a facile method is reported to synthesize mesoporous and heterostructured transition metal oxides strongly anchored on a nickel skeleton (MH‐TMO) containing identified Fe–Cu oxide interfaces with high intrinsic activity, easy accessibility for reaction intermediates, and long‐term stability for alkaline oxygen/hydrogen evolution reactions. The MH‐TMO with the electrocatalytically active Fe–O–Cu bridge has an optimal oxygen binding energy to facilitate adsorption/desorption of oxygen intermediates for oxygen molecules. Associated with the high mass transport through the nanoporous structure, MH‐TMO exhibits impressive oxygen evolution reaction catalysis, with an extremely low overpotential of around 0.22 V at 10 mA cm−2 and low Tafel slope (44.5 mV dec−1) in 1.0 M KOH, realizing a current density of 100 mA cm−2 with an overpotential as low as 0.26 V. As a result, the alkaline electrolyzer assembled by the bifunctional MH‐TMO catalysts operates with an outstanding overall water‐splitting output (1.49 V@10 mA cm−2), outperforming one assembled with noble‐metal‐based catalysts.

Self-healing Growth of LaNiO3 on a Mixed-Terminated Perovskite Surface.

Developing atomic-scale synthesis control is a prerequisite for understanding and engineering the exotic physics inherent to transition-metal oxide heterostructures. Thus, far, however, the number of materials systems explored has been extremely limited, particularly with regard to the crystalline substrate, which is routinely SrTiO3. Here, we investigate the growth of a rare-earth nickelate─LaNiO3─on (LaAlO3)(Sr2AlTaO6) (LSAT) (001) by oxide molecular beam epitaxy (MBE). Whereas the LSAT substrates are smooth, they do not exhibit the single surface termination usually assumed necessary for control over the interface structure. Performing both nonresonant and resonant anomalous in situ synchrotron surface X-ray scattering during MBE growth, we show that reproducible heterostructures can be achieved regardless of both the mixed surface termination and the layer-by-layer deposition sequence. The rearrangement of the layers occurs dynamically during growth, resulting in the fabrication of high-quality LaNiO3/LSAT heterostructures with a sharp and consistent interfacial structure. This is due to the thermodynamics of the deposition window as well as the nature of the chemical species at interfaces─here, the flexible charge state of nickel at the oxide surface. This has important implications regarding the use of a wider variety of substrates for fundamental studies on complex oxide synthesis.

Complementary Insights from Neutron and Resonant X‐Ray Reflectometry for the Study of Perovskite Transition Metal Oxide Heterostructures

The nondestructive investigation of electronic and magnetic modifications at buried interfaces contributes significantly to the understanding of the underlying interactions. This knowledge is essential for the design of electronic devices based on complex quantum materials. Herein, the application of resonant X‐ray and neutron reflectometry for the study of perovskite transition metal oxide heterostructures is reviewed. Both methods are well suited for the nondestructive study of interfacial reconstructions and interactions of spin, charge, and orbitals at the nanoscale. Herein, cases are highlighted in which both methods are complementary and thus provide unique insights into the physics of oxide heterostructures.

Large phonon drag thermopower boosted by massive electrons and phonon leaking in LaAlO3/LaNiO3/LaAlO3 heterostructure.

An unusually large thermopower (S) enhancement is induced by heterostructuring thin films of the strongly correlated electron oxide LaNiO3. The phonon-drag effect, which is not observed in bulk LaNiO3, enhances S for thin films compressively strained by LaAlO3 substrates. By a reduction in the layer thickness down to three unit cells and subsequent LaAlO3 surface termination, a 10 times S enhancement over the bulk value is observed due to large phonon drag S (Sg), and the Sg contribution to the total S occurs over a much wider temperature range up to 220 K. The Sg enhancement originates from the coupling of lattice vibration to the d electrons with large effective mass in the compressively strained ultrathin LaNiO3, and the electron–phonon interaction is largely enhanced by the phonon leakage from the LaAlO3 substrate and the capping layer. The transition-metal oxide heterostructures emerge as a new playground to manipulate electronic and phononic properties in the quest for high-performance thermoelectrics.

Metal oxides as electrocatalysts for water splitting: On plasmon‐driven enhanced activity

AbstractMany technological approaches have been searched in order to overcome the main challenges concerning the world energy crisis and global environmental issues. Among them, plasmon‐driven photoelectrochemical reactions towards water electrolysis attract great attention due to their capacity to efficiently harvest solar energy. Synergism between tunable optical features and catalysts active sites of plasmonic nanomaterials gives rise to a singular perspective for photochemical processes. Through resonant photonic excitation, hot carriers’ motion facilitates the charge transfer process on the catalyst surface for chemical reactions. In this minireview, recent experimental research with emphasis on water splitting reactions have been summarized with the purpose of understanding the mechanistic hot electrons generation and transfer on the plasmonic noble metal nanoparticles (MNPs) and transition metal oxides (MOs) heterostructures. Examples of plasmonic nanomaterials are highlighted and compared for both water electrolysis semi reactions. Finally, this work concludes by describing the remaining challenges and gives some perspectives regarding the promising future of plasmon‐driven reactions investigations.

Spin texture driven spintronic enhancement at the LaAlO3/SrTiO3 interface

Recent experiments have shown that transition metal oxide heterostructures such as SrTiO$_3$-based interfaces, exhibit large, gate tunable, spintronic responses. Our theoretical study showcases key factors controlling the magnitude of the conversion, measured by the inverse Edelstein and Spin Hall effects, and their evolution with respect to an electrostatic doping. The origin of the response can be linked to spin-orbital textures. These stem from the broken inversion symmetry at the interface which produces an unusual form of the interfacial spin-orbit coupling, provided a bulk atomic spin-orbit contribution is present. The amplitudes and variations of these observables are direct consequences of the multi-orbital subband structure of these materials, featuring avoided and topological crossings. Interband contributions to the coefficients lead to enhanced responses and non-monotonic evolution with doping. We highlight these effects using analytical approaches and low energy modeling.

Atomic Layer and Interfacial Oxygen Defect Tailored Magnetic Anisotropy and Dzyaloshinskii–Moriya Interaction in Perovskite SrRuO3/SrTiO3 Heterostructures

It is significant to modulate the magnetic anisotropy energy (MAE) and Dzyaloshinskii-Moriya interaction (DMI) in perovskite-type transition metal oxides heterostructures toward the multifunctional...

Berry phase engineering at oxide interfaces

Geometric phases in condensed matter play a central role in topological transport phenomena such as the quantum, spin and anomalous Hall effect (AHE). In contrast to the quantum Hall effect - which is characterized by a topological invariant and robust against perturbations - the AHE depends on the Berry curvature of occupied bands at the Fermi level and is therefore highly sensitive to subtle changes in the band structure. A unique platform for its manipulation is provided by transition metal oxide heterostructures, where engineering of emergent electrodynamics becomes possible at atomically sharp interfaces. We demonstrate that the Berry curvature and its corresponding vector potential can be manipulated by interface engineering of the correlated itinerant ferromagnet SrRuO$_3$ (SRO). Measurements of the AHE reveal the presence of two interface-tunable spin-polarized conduction channels. Using theoretical calculations, we show that the tunability of the AHE at SRO interfaces arises from the competition between two topologically non-trivial bands. Our results demonstrate how reconstructions at oxide interfaces can be used to control emergent electrodynamics on a nanometer-scale, opening new routes towards spintronics and topological electronics.

Controlling antiferromagnetic domains in patterned La0.7Sr0.3FeO3 thin films

Transition metal oxide thin films and heterostructures are promising platforms to achieve full control of the antiferromagnetic (AFM) domain structure in patterned features as needed for AFM spintronic devices. In this work, soft x-ray photoemission electron microscopy was utilized to image AFM domains in micromagnets patterned into La0.7Sr0.3FeO3 (LSFO) thin films and La0.7Sr0.3MnO3 (LSMO)/LSFO superlattices. A delicate balance exists between magnetocrystalline anisotropy, shape anisotropy, and exchange interactions such that the AFM domain structure can be controlled using parameters such as LSFO and LSMO layer thickness, micromagnet shape, and temperature. In LSFO thin films, shape anisotropy gains importance only in micromagnets where at least one extended edge is aligned parallel to an AFM easy axis. In contrast, in the limit of ultrathin LSFO layers in the LSMO/LSFO superlattice, shape anisotropy effects dominate such that the AFM spin axes at micromagnet edges can be aligned along any in-plane crystallographic direction.

Strongly correlated and topological states in [111] grown transition metal oxide thin films and heterostructures

We highlight recent advances in the theory, materials fabrication, and experimental characterization of strongly correlated and topological states in [111] oriented transition metal oxide thin films and heterostructures, which are notoriously difficult to realize compared to their [001] oriented counterparts. We focus on two classes of complex oxides, with the chemical formulas ABO3 and A2B2O7, where the B sites are occupied by an open-shell transition metal ion with a local moment and the A sites are typically a rare earth element. The [111] oriented quasi-two-dimensional lattices derived from these parent compound lattices can exhibit peculiar geometries and symmetries, namely, a buckled honeycomb lattice, as well as kagome and triangular lattices. These lattice motifs form the basis for emergent strongly correlated and topological states expressed in exotic magnetism, various forms of orbital ordering, topological insulators, topological semimetals, quantum anomalous Hall insulators, and quantum spin liquids. For transition metal ions with high atomic number, spin–orbit coupling plays a significant role and may give rise to additional topological features in the electronic band structure and in the spectrum of magnetic excitations. We conclude this perspective by articulating open challenges and opportunities in this actively developing field.

Artificial quantum confinement in LaAlO3/SrTiO3 heterostructures

Heterostructures of transition metal oxides (TMO) perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial 2D electron system (2DES) discovered at the LAO3/STO3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the dependence of the electronic structure on the width of the confining potential using soft X-ray ARPES combined with ab-initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3d t2g bands and, interestingly, redistributes the charge between the dxy and dxz/dyz bands.

Atomically-smooth single-crystalline VO2 (101) thin films with sharp metal-insulator transition

Atomically-abrupt interfaces in transition metal oxide (TMO) heterostructures could host a variety of exotic condensed matter phases that may not be found in the bulk materials at equilibrium. A critical step in the development of such atomically-sharp interfaces is the deposition of atomically-smooth TMO thin films. Optimized deposition conditions exist for the growth of perovskite oxides. However, the deposition of rutile oxides, such as VO2, with atomic-layer precision has been challenging. In this work, we used pulsed laser deposition to grow atomically-smooth VO2 thin films on rutile TiO2 (101) substrates. We show that an optimal substrate preparation procedure followed by the deposition of VO2 films at a temperature conducive for step-flow growth mode is essential for achieving atomically-smooth VO2 films. The films deposited at optimal substrate temperatures show a step and terrace structure of the underlying TiO2 substrate. At lower deposition temperatures, there is a transition to a mixed growth mode comprised of island growth and layer-by-layer growth modes. VO2 films deposited at optimal substrate temperatures undergo a sharp metal to insulator transition, similar to that observed in bulk VO2, but at a transition temperature of ∼325K with ∼103 times increase in resistance.

Polarity and Spin-Orbit Coupling Induced Strong Interfacial Exchange Coupling: An Asymmetric Charge Transfer in Iridate-Manganite Heterostructure.

Charge transfer is of particular importance in manipulating the interface physics in transition-metal oxide heterostructures. In this work, we have fabricated epitaxial bilayers composed of polar 3d LaMnO3 and nonpolar 5d SrIrO3. Systematic magnetic measurements reveal an unexpectedly large exchange bias effect in the bilayer, together with a dramatic enhancement of the coercivity of LaMnO3. Based on first-principle calculations and X-ray absorption spectroscopy measurements, such a strong interfacial magnetic coupling is found closely associated with the polar nature of LaMnO3 and the strong spin-orbit interaction in SrIrO3, which collectively drive an asymmetric interfacial charge transfer and lead to the emergence of an interfacial reentrant spin/superspin glass state. Our study provides a new insight into the charge transfer in transition-metal oxide heterostructures and offers a novel means to tune the interfacial exchange coupling for a variety of device applications.