Thickness Dependent Properties in Oxide Heterostructures Driven by Structurally Induced Metal–Oxygen Hybridization Variations

Thickness‐driven electronic phase transitions are broadly observed in different types of functional perovskite heterostructures. However, uncertainty remains whether these effects are solely due to spatial confinement, broken symmetry, or rather to a change of structure with varying film thickness. Here, this study presents direct evidence for the relaxation of oxygen‐2p and Mn‐3d orbital (p–d) hybridization coupled to the layer‐dependent octahedral tilts within a La_2/3Sr_1/3MnO₃ film driven by interfacial octahedral coupling. An enhanced Curie temperature is achieved by reducing the octahedral tilting via interface structure engineering. Atomically resolved lattice, electronic, and magnetic structures together with X‐ray absorption spectroscopy demonstrate the central role of thickness‐dependent p–d hybridization in the widely observed dimensionality effects present in correlated oxide heterostructures.     

Single‐Crystal Hybrid Perovskite Platelets on Graphene: A Mixed‐Dimensional Van Der Waals Heterostructure with Strong Interface Coupling

Van der Waals (vdW) heterostructures open up excellent prospects in electronic and optoelectronic applications. In this work, mixed‐dimensional metal‐halide perovskite/graphene heterostructures are prepared through selective growth of CH₃NH₃PbBr₃ platelets on patterned single‐layer graphene using chemical vapor deposition. Preferred growth of single‐crystal CH₃NH₃PbBr₃ platelets on graphene surfaces is achieved, which is accompanied by significant photoluminescence quenching. Raman spectra reveal that perovskite platelets cause p‐type doping in the graphene layer. A significant Fermi level decrease of 272 meV in graphene is estimated, which corresponds to a high doping density of 7.5 × 10¹² cm^?2. Surface potentials measured by Kelvin probe force microscopy indicate a negatively charged perovskite surface under illumination, which is consistent with the upward band bending deduced from conducting atomic force microscopy measurements. Moreover, a field‐effect phototransistor is fabricated using the perovskite/graphene heterostructure channel, and the increased Dirac voltage under illumination confirms an enhanced p‐type character in graphene. These findings enrich the understanding of strong interface coupling in such mixed‐dimensional vdW heterostructures and pave the way toward novel perovskite‐based optoelectronic devices.     

Emergent Noncentrosymmetry and Piezoelectricity Driven by Oxygen Octahedral Rotations in n = 2 Dion–Jacobson Phase Layer Perovskites

The loss of centrosymmetry via oxygen octahedral rotations is demonstrated in the n = 2 Dion–Jacobson family of layered oxide perovskites, A′LaB₂O₇ (A′ = Rb, Cs; B = Nb, Ta). Ab initio density functional theory calculations predict that all four materials should adopt polar space groups, in contrast to the results of previous experimental studies that have assigned these materials to the centrosymmetric P4/mmm space group. Optical second harmonic generation experiments confirm the presence of a noncentrosymmetric phase at ambient temperature. Piezoresponse force microscopy experiments also show that this phase is piezoelectric. To elucidate the symmetry‐breaking and assign the appropriate space groups, the crystal structure of CsLaNb₂O₇ is refined as a function of temperature from synchrotron X‐ray diffraction data. Above 550 K, CsLaNb₂O₇ adopts the previously determined centrosymmetric P4/mmm space group. Between 550 and 350 K, the symmetry is lowered to the noncentrosymmetric space group Amm2. Below 350 K, additional symmetry lowering is observed as peak splitting, but the space group cannot be unambiguously identified.     

Control of Structural Distortions in Transition‐Metal Oxide Films through Oxygen Displacement at the Heterointerface

Structural distortions in the oxygen octahedral network in transition‐metal oxides play crucial roles in yielding a broad spectrum of functional properties, and precise control of such distortions is a key for developing future oxide‐based electronics. Here, it is shown that the displacement of apical oxygen atom shared between the octahedra at the heterointerface is a determining parameter for these distortions and consequently for control of structural and electronic phases of a strained oxide film. The present analysis by complementary annular dark‐ and bright‐field imaging in aberration‐corrected scanning transmission electron microscopy reveals that structural phase differences in strained monoclinic and tetragonal SrRuO₃ films grown on GdScO₃ substrates result from relaxation of the octahedral tilt, associated with changes in the in‐plane displacement of the apical oxygen atom at the heterointerface. It is further demonstrated that octahedral distortions and magnetrotransport properties of the SrRuO₃ films can be controlled by interface engineering of the oxygen displacement. This provides a further degree of freedom for manipulating structural and electronic properties in strained films, allowing the design of novel oxide‐based heterostructures.     

Interface Carriers and Enhanced Electron-Phonon Coupling Effect in Al2O3/TiO2 Heterostructure Revealed by Resonant Inelastic Soft X-Ray Scattering

The electronic structure and the electron-phonon couplings in a novel mass-production-compatible Al₂O₃/TiO₂ 2D electron system (2DES) are investigated using resonant inelastic soft X-ray scattering. The experimental data from the samples of various TiO₂ thicknesses unequivocally show that the Ti³⁺ state indeed exists at the deep interface to serve as an n-type dopant for the 2DES. The electronic structure of Ti³⁺ species is scrutinized as entirely separated from that of the Ti⁴⁺ host lattice. Furthermore, features of sub-eV energy loss phonon modes are clearly observed, indicating substantial electron-phonon coupling effects. Such low energy loss features are enhanced in thinner TiO₂ samples, implying that polaronic local lattice deformation is enhanced due to the presence of Ti³⁺. These findings suggest that the 2DES properties can be controlled via well-established TiO₂ engineering, thereby enthroning the binary oxide heterostructure as a promising candidate for 2DES device applications.     

Structure and Disorder in Squaraine–C60 Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor–Acceptor Interface

Organic solar cells based on the combination of squaraine dyes (as electron donors) and fullerenes (as electron acceptors) have recently garnered much attention. Here, molecular dynamics simulations are carried out to investigate the evolution of a squaraine–C₆₀ bilayer interface as a function of the orientation and order of the underlying squaraine layer. Electronic couplings between the main electronic states involved in exciton dissociation and charge (polaron pair) recombination are derived for donor–acceptor complexes extracted from the simulations. The results of the combined molecular‐dynamics?quantum‐mechanics approach provide insight into how the degree of molecular order and the dynamics at the interface impact the key processes involved in the photovoltaic effect.     

High‐Performance,Transparent Thin Film Hydrogen Gas Sensor Using 2D Electron Gas at Interface of Oxide Thin Film Heterostructure Grown by Atomic Layer Deposition

A high‐performance, transparent, and extremely thin (<15 nm) hydrogen (H₂) gas sensor is developed using 2D electron gas (2DEG) at the interface of an Al₂O₃/TiO₂ thin film heterostructure grown by atomic layer deposition (ALD), without using an epitaxial layer or a single crystalline substrate. Palladium nanoparticles (≈2 nm in thickness) are used on the surface of the Al₂O₃/TiO₂ thin film heterostructure to detect H₂. This extremely thin gas sensor can be fabricated on general substrates such as a quartz, enabling its practical application. Interestingly, the electron density of the Al₂O₃/TiO₂ thin film heterostructure can be tailored using ALD process temperature in contrast to 2DEG at the epitaxial interfaces of the oxide heterostructures such as LaAlO₃/SrTiO₃. This tunability provides the optimal electron density for H₂ detection. The Pd/Al₂O₃/TiO₂ sensor detects H₂ gas quickly with a short response time of <30 s at 300 K which outperforms conventional H₂ gas sensors, indicating that heating modules are not required for the rapid detection of H₂. A wide bandgap (>3.2 eV) with the extremely thin film thickness allows for a transparent sensor (transmittance of 83% in the visible spectrum) and this fabrication scheme enables the development of flexible gas sensors.     

Atomic‐Scale Control of Electronic Structure and Ferromagnetic Insulating State in Perovskite Oxide Superlattices by Long‐Range Tuning of BO6 Octahedra

Control of BO₆ octahedral rotations at the heterointerfaces of dissimilar ABO₃ perovskites has emerged as a powerful route for engineering novel physical properties. However, its impact length scale is constrained at 2–6 unit cells close to the interface and the octahedral rotations relax quickly into bulk tilt angles away from interface. Here, a long‐range (up to 12 unit cells) suppression of MnO₆ octahedral rotations in La_0.9Ba_0.1MnO₃ through the formation of superlattices with SrTiO₃ can be achieved. The suppressed MnO₆ octahedral rotations strongly modify the magnetic and electronic properties of La_0.9Ba_0.1MnO₃ and hence create a new ferromagnetic insulating state with enhanced Curie temperature of 235 K. The emergent properties in La_0.9Ba_0.1MnO₃ arise from a preferential occupation of the out‐of‐plane Mn d_3z²_?r² orbital and a reduced Mn e_g bandwidth, induced by the suppressed octahedral rotations. The realization of long‐range tuning of BO₆ octahedra via superlattices can be applicable to other strongly correlated perovskites for exploring new emergent quantum phenomena.     

Synergistically Tuning Electronic Structure of Porous β‐Mo2C Spheres by Co Doping and Mo‐Vacancies Defect Engineering for Optimizing Hydrogen Evolution Reaction Activity

The development of novel non‐noble electrocatalysts with controlled structure and surface composition is critical for efficient electrochemical hydrogen evolution reaction (HER). Herein, the rational design of porous molybdenum carbide (β‐Mo₂C) spheres with different surface engineered structures (Co doping, Mo vacancies generation, and coexistence of Co doping and Mo vacancies) is performed to enhance the HER performance over the β‐Mo₂C‐based catalyst surface. Density functional theory calculations and experimental results reveal that the synergistic effect of Co doping with Mo vacancies increases the electron density around the Fermi‐level and modulates the d band center of β‐Mo₂C so that the strength of the Mo? H bond is reasonably optimized, thus leading to an enhanced HER kinetics. As expected, the optimized Co₅₀‐Mo₂C‐12 with porous structure displays a low overpotential (η₁₀ = 125 mV), low‐onset overpotential (η_onset = 27 mV), and high exchange current density (j₀ = 0.178 mA cm^?2). Furthermore, this strategy is also successfully extended to develop other effective metal (e.g., Fe and Ni) doped β‐Mo₂C electrocatalyst, indicating that it is a universal strategy for the rational design of highly efficient metal carbide‐based HER catalysts and beyond.