Enhanced Magnetic Anisotropy and Orbital Symmetry Breaking in Manganite Heterostructures

Manipulating magnetic anisotropy in complex oxide heterostructures has attracted much attention. Here, three interface‐engineering approaches are applied to address two general issues with controlling magnetic anisotropy in the La_2/3Sr_1/3MnO₃ heterostructure. One is the paradox arising from the competition between Mn_3d–O_2p orbital hybridization and MnO₆ crystal field. The other is the interfacial region where the nonuniform Mn? O bond length d and Mn? O? Mn bond angle θ disturb the structural modulation. When the interfacial region is suppressed in the interface‐engineered samples, the lateral magnetic anisotropy energy is increased eighteen times. The d‐mediated anisotropic crystal filed that overwhelms the orbital hybridization causes the lateral symmetry breaking of the Mn 3d_x²_?y² orbital, resulting in enhanced magnetic anisotropy. This is different from the classic Jahn–Teller effect where the lateral symmetry is always preserved. Moreover, the quantitative analysis on X‐ray linear dichroism data suggests a direct correlation between Mn 3d_x²_?y² orbital symmetry breaking and magnetic anisotropy energy. The findings not only advance the understanding of magnetic anisotropy in manganite heterostructures but also can be extended to other complex oxides and perovskite materials with correlated degrees of freedom.     

Broadband Enhanced Chirality with Tunable Response in Hybrid Plasmonic Helical Metamaterials

Designing broadband enhanced chirality is of strong interest to the emerging fields of chiral chemistry and sensing, or to control the spin orbital momentum of photons in recently introduced nanophotonic chiral quantum and classical optical applications. However, chiral light-matter interactions have an extremely weak nature, are difficult to control and enhance, and cannot be made tunable or broadband. In addition, planar ultrathin nanophotonic structures to achieve strong, broadband, and tunable chirality at the technologically important visible to ultraviolet spectrum still remain elusive. Here, these important problems are tackled by experimentally demonstrating and theoretically verifying spectrally tunable, extremely large, and broadband chiroptical response by nanohelical metamaterials. The reported new designs of all-dielectric and dielectric-metallic (hybrid) plasmonic metamaterials permit the largest and broadest ever measured chiral Kuhn's dissymmetry factor achieved by a large-scale nanophotonic structure. In addition, the strong circular dichroism of the presented bottom-up fabricated optical metamaterials can be tuned by varying their dimensions and proportions between their dielectric and plasmonic helical subsections. The currently demonstrated ultrathin optical metamaterials are expected to provide a substantial boost to the developing field of chiroptics leading to significantly enhanced and broadband chiral light-matter interactions at the nanoscale.     

Tailoring Ordered Mesoporous Titania Films via Introducing Germanium Nanocrystals for Enhanced Electron Transfer Photoanodes for Photovoltaic Applications

Based on a diblock-copolymer templated sol–gel synthesis, germanium nanocrystals (GeNCs) are introduced to tailor mesoporous titania (TiO₂) films for obtaining more efficient anodes for photovoltaic applications. After thermal annealing in air, the hybrid films with different GeNC content are investigated and compared with films undergoing an argon atmosphere annealing. The surface and inner morphologies of the TiO₂/GeO_x nanocomposite films are probed via scanning electron microscopy and grazing-incidence small-angle X-ray scattering. The crystal phase, chemical composition, and optical properties of the nanocomposite films are examined with transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible spectroscopy. Special focus is set on the air-annealed nanocomposite films since they hold greater promise for photovoltaics. Specifically, the charge–carrier dynamics of these air-annealed nanocomposite films are studied, and it is found that, compared with pristine TiO₂ photoanodes, the GeNC addition enhances the electron transfer, yielding an increase in the short-circuit photocurrent density of exemplary perovskite solar cells and thus, an enhanced device efficiency as well as a significantly reduced hysteresis.     

Oxygen Vacancy-Mediated Exciton Effect in Hierarchical BiOBr Enables Dichotomy of Energy Transfer and Electron Transfer in Photocatalysis

Using solar energy through green and simple artificial photosynthesis systems are considered as a promising way to solve the energy and environmental crisis. However, one of the important primary steps of photosynthesis, i.e., energy transfer, is long being ignored especially in inorganic semiconducting systems due to the small exciton binding energies. Herein, the simultaneous interrogation of the charge transfer and energy transfer steps in a photoexcitation process is proposed by utilizing few-layered nanosheet-assembled hierarchical BiOBr nanotubes with rich oxygen vacancies (OVs) as efficient multifunctional photocatalysts. Benefiting from the integrated 1D/2D structure and abundant OV defects, the excitonic effect strikes a delicate balance in the optimized BiOBr photocatalyst, showing not only improved charge carrier separation and transfer but also enhanced exciton generation. As a result, the hierarchical BiOBr nanotubes exhibit high efficiency toward photocatalytic CO₂ reduction with an impressive CO evolution rate of 135.6 µmol g⁻¹ h⁻¹ without cocatalyst or photosensitizer. The dominant reactive oxygen species of singlet oxygen (¹O₂) are discriminated for the first time, which originated from an energy transfer process, with electrophilic character, whereas the minor effect of superoxide anion radical (^•O₂⁻) with a nucleophilic rate-determining step in the photocatalytic aerobic oxidation of sulfides.     

Ultrafast Plasmonic Hot Electron Transfer in Au Nanoantenna/MoS2 Heterostructures

2D transition metal dichalcogenides are becoming attractive materials for novel photoelectric and photovoltaic applications due to their excellent optoelectric properties and accessible optical bandgap in the near‐infrared to visible range. Devices utilizing 2D materials integrated with metal nanostructures have recently emerged as efficient schemes for hot electron‐based photodetection. Metal‐semiconductor heterostructures with low cost, simple procedure, and fast response time are crucial for the practical applications of optoelectric devices. In this paper, template‐based sputtering method is used first to fabricate Au nanoantenna (NA)/MoS₂ heterostructures with low cost, simple preparation, broad spectral response, and fast response time. Through the measurement of femtosecond pump‐probe spectroscopy, it is demonstrated that plasmon‐induced hot electron transfer takes place in the Au NA/MoS₂ heterostructure on the order of 200 fs with an injected electron density of about 5.6 × 10¹² cm^?2. Moreover, the pump‐power‐dependent photoluminescence spectra confirm that the exciton energy of MoS₂ can be enhanced, coupled, and reradiated by the Au NA. Such ultrafast plasmon‐induced hot electron transfer in the metal‐semiconductor heterostructure can enable novel 2D devices for light harvesting and photoelectric conversion.     

Broadband Defects Emission and Enhanced Ligand Raman Scattering in 0D Cs3Bi2I9 Colloidal Nanocrystals

Excitonic 0D and 2D lead‐halide perovskites have been recently developed and investigated as new materials for light generation. Here broadband (>1 eV) emission from newly synthesized 0D lead‐free colloidal Cs₃Bi₂I₉ nanocrystals (NCs) is reported. The nature of their emissive states as well as the relative dynamics which are currently hotly debated are investigated. In particular, it is found that the broadband emission is made by the coexistence of emissive excitons and sub‐bandgap emissive trap‐states. Remarkably, evidence of enhanced Raman scattering from the ligands is observed when attached to the NCs surface, an effect that is preliminarily attributed to strong exciton‐ligands electronic coupling in these systems.     

Bandwidth Control and Symmetry Breaking in a Mott-Hubbard Correlated Metal

In Mott materials strong electron correlation yields a spectrum of complex electronic structures. Recent synthesis advancements open realistic opportunities for harnessing Mott physics to design transformative devices. However, a major bottleneck in realizing such devices remains the lack of control over the electron correlation strength. This stems from the complexity of the electronic structure, which often veils the basic mechanisms underlying the correlation strength. This study presents control of the correlation strength by tuning the degree of orbital overlap using picometer-scale lattice engineering. This study illustrates how bandwidth control and concurrent symmetry breaking can govern the electronic structure of a correlated SrVO₃ model system. This study shows how tensile and compressive biaxial strain oppositely affect the SrVO₃ in-plane and out-of-plane orbital occupancy, resulting in the partial alleviation of the orbital degeneracy. The spectral weight redistribution under strain is derived and explained, which illustrates how high tensile strain drives the system toward a Mott insulating state. Implementation of such concepts can push correlated electron phenomena closer toward new solid-state devices and circuits. These findings therefore pave the way for understanding and controlling electron correlation in a broad range of functional materials, driving this powerful resource for novel electronics closer toward practical realization.     

Evidence and Effect of Photogenerated Charge Transfer for Enhanced Photocatalysis in WO3/TiO2 Heterojunction Films: A Computational and Experimental Study

Semiconductor heterojunctions are used in a wide range of applications including catalysis, sensors, and solar‐to‐chemical energy conversion devices. These materials can spatially separate photogenerated charge across the heterojunction boundary, inhibiting recombination processes and synergistically enhancing their performance beyond the individual components. In this work, the WO₃/TiO₂ heterojunction grown by chemical vapor deposition is investigated. This consists of a highly nanostructured WO₃ layer of vertically aligned nanorods that is then coated with a conformal layer of TiO₂. This heterojunction shows an unusual electron transfer process, where photogenerated electrons move from the WO₃ layer into TiO₂. State‐of‐the‐art hybrid density functional theory and hard X‐ray photoelectron spectroscopy are used to elucidate the electronic interaction at the WO₃/TiO₂ interface. Transient absorption spectroscopy shows that recombination is substantially reduced, extending both the lifetime and population of photogenerated charges into timescales relevant to most photocatalytic processes. This increases the photocatalytic efficiency of the material, which is among the highest ever reported for a thin film. In allying computational and experimental methods, this is believed to be an ideal strategy for determining the band alignment in metal oxide heterojunction systems.     

Adsorption and Direct Electron Transfer from Hemoglobin into a Three‐Dimensionally Ordered Macroporous Gold Film

Application of protein‐based, direct electron communication in bioelectronic devices, biosensors, or biofuel cells usually requires high stability and function density of the immobilized proteins or enzymes. Traditional methods have been used to increase the function density using multilayer immobilization techniques at the expense of losing stability and electron‐communication rate, that is, generally only protein molecules near the electrode surface are electroactive. In order to overcome the above problems, a three‐dimensional, ordered, macroporous gold film electrode is synthesized electrochemically by an inverted colloidal crystal template technique. The uniform, three‐dimensional macroporous gold provides superior conductivity, high stability, and large surface area. Its interconnected macroporous structure, containing gold nanoparticles, significantly enhances the amount of adsorbed hemoglobin (Hb) molecules at the monolayer level and also provides a good microenvironment for retaining the biological activity of the adsorbed protein, as confirmed by electrochemical and attenuated total reflection Fourier‐transform infrared spectroscopy. Therefore, direct electron transfer between the adsorbed Hb and the electrode is achieved. Adsorption of Hb on the macroporous gold film electrode is monitored using electrochemical impedance spectroscopy. The saturated adsorption amount, Γ, of the Hb is determined to be 6.55×10^–10 mol cm^–2 with a surface coverage of 88.1 %. The electrochemical behavior and the adsorption mechanism of Hb on the macroporous gold film electrode are discussed on the basis of the experimental results.     

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO3

Strain gradients induced by nanoindentations in bulk undoped strontium titanate (SrTiO₃) are investigated. After indenting a 850 nm deep pattern on the SrTiO₃ surface with a 8 GPa hardness and an 83 GPa indentation modulus by a Berkovich tip, the resulting deformation patterns are imaged by Raman spectroscopy and by second harmonic generation microscopy. Cross‐shaped compressively stressed regions along the cubic SrTiO₃ primary slip planes are observed. These zones relax toward the unstrained bulk material through localized strain gradients on a length scale of several microns. Second harmonic tomography reveals the reduction of the crystal's centrosymmetry in the vicinity of the crosslike features due to a strain gradient. These results indicate the appearance of flexoelectricity following the nanoindentation process due to the creation of an inhomogeneous strain.     

Symmetry Modulation and Enhanced Multiferroic Characteristics in Bi1‐xNdxFeO3 Ceramics

BiFeO₃ is recognized as the most important room temperature single phase multiferroic material. However, the weak magnetoelectric (ME) coupling remains as a key issue, which obstructs its applications. Since the magnetoelectric coupling in BiFeO₃ is essentially hindered by the cycloidal spin structure, here efforts to improve the magnetoelectric coupling by destroying the cycloidal state and switching to the weak ferromagnetic state through symmetry modulation are reported. The structure is tuned from polar R3c to polar Pna2₁, and finally to nonpolar Pbnm by forming Bi_1‐xNd_xFeO₃ solid solutions, where two morphotropic phase boundaries (MPBs) are detected. Greatly enhanced ferroelectric polarization is obtained together with the desired weak ferromagnetic characteristics in Bi_1‐xNd_xFeO₃ ceramics at the compositions near MPBs. The change of magnetic state from antiferromagnetic (cycloidal state) to ferromagnetic (canted antiferromagnetic) is confirmed by the observation of magnetic domains using magnetic force microscopy. More interestingly, combining experiments and first‐principles‐based simulations, an electric field‐induced structural and magnetic transition from Pna2₁ back to R3c is demonstrated, providing a great opportunity for electric field‐controlled magnetism, and this transition is shown to be reversible with additional thermal treatment.     

Broadband Electromagnetic Response and Enhanced Microwave Absorption in Carbon Black and Magnetic Fe3O4 Nanoparticles Reinforced Polyvinylidenefluoride Composites

Journal of Electronic Materials - High-efficiency Electromagnetic Interference (EMI) shielding materials are essential in the harsh environment created by unwanted electromagnetic (EM) signals. In...     

Photoinduced Electron Transfer in Heterosupramolecular Assemblies of TiO2 Nanoparticles and Terthiophene Carboxylic Acid in Apolar Solvents

Monodisperse stearic acid coated titanium dioxide nanoparticles (< 3 nm) have been prepared and characterized. The nanoparticles are soluble in common apolar organic solvents and bind to carboxylic acid functionalized π‐conjugated oligomers to form heterosupramolecular assemblies. When [2,2′;5′,2″]terthiophene‐5‐carboxylic acid was coupled to the nanoparticles, the fluorescence of the terthiophene moiety of the heterosupramolecular system was completely quenched due to a photoinduced electron transfer from terthiophene to titanium dioxide. Time‐resolved fluorescence and Stern–Volmer analysis revealed that the quenching is predominantly static rather than dynamic and occurs in apolar solvents.     

Molybdenum Dioxide Nanoparticles Anchored on Nitrogen-Doped Carbon Nanotubes as Oxidative Desulfurization Catalysts: Role of Electron Transfer in Activity and Reusability

Electron transfer between metal-oxides and supports considerably affects the oxidative desulfurization (ODS) performance of catalysts, while this is far from being well understood. Herein, molybdenum dioxide with oxygen vacancies (V_O-MoO₂) catalysts derived from Mo-based metal-organic frameworks are anchored on electron-rich nitrogen-doped carbon nanotubes (NC) to obtain excellent ODS activity and reusability. Results show that either dibenzothiophene (DBT) or 4,6-dimethyldibenzothiophene (4,6-DMDBT) is removed 100% on the composite catalyst (V_O-MoO₂@NC) within 40 min of reaction when cumene hydroperoxide is chosen as an oxidant. After five cycles of reaction, DBT and 4,6-DMDBT removal still exceeded 99.5 and 95.0%, respectively. Results from density functional theory calculations and characterizations confirm that the strong electron-donating effect of NC on V_O-MoO₂ can promote the dispersion of V_O-MoO₂ and reduce the bond energy of the Mo O bond, leading to exposure of active sites and enrichment of oxygen vacancies (V_O). Furthermore, the strong interfacial electrostatic interaction caused by the electron transfer from NC to V_O-MoO₂ can reduce the leaching of active sites of the catalyst. This study provides a versatile strategy of constructing strong electronic interaction between metal-oxide and support via anchoring on NC for the design of high-performance ODS catalysts.     

Harvesting Lost Photons: Plasmon and Upconversion Enhanced Broadband Photocatalytic Activity in Core@Shell Microspheres Based on Lanthanide‐Doped NaYF4, TiO2, and Au

Efficiently harvesting solar energy for photocatalysis remains very challenging. Rational design of architectures by combining nanocomponents of radically different properties, for example, plasmonic, upconversion, and photocatalytic properties, offers a promising route to improve solar energy utilization. Herein, the synthesis of novel, plasmonic Au nanoparticle decorated NaYF₄:Yb³⁺, Er³⁺, Tm³⁺‐core@porous‐TiO₂‐shell microspheres is reported. They exhibit high surface area, good stability, broadband absorption from ultraviolet to near infrared, and excellent photocatalytic activity, significantly better than the benchmark P25 TiO₂. The enhanced activity is attributed to synergistic effects from nanocomponents arranged into the nanostructured architecture in such a way that favors the efficient charge/energy transfer among nanocomponents and largely reduced charge recombination. Optical and energy‐transfer properties are modeled theoretically to support our interpretations of catalytic mechanisms. In addition to yielding novel materials and interesting properties, the current work provides physical insights that can contribute to the future development of plasmon‐enhanced broadband catalysts.     

Simultaneously Realizing Rapid Electron Transfer and Mass Transport in Jellyfish‐Like Mott–Schottky Nanoreactors for Oxygen Reduction Reaction

Fundamental understanding of constructing elevated catalysts to realize fast electron transfer and rapid mass transport in oxygen reduction reaction (ORR) chemistry by interface regulation and structure design is important but still ambiguous. Herein, a novel jellyfish‐like Mott–Schottky‐type electrocatalyst is developed to realize fast electron transfer and decipher the structure–mass transport connection during ORR process. Both spectroscopy techniques and density functional theory calculation demonstrate electrons spontaneously transfer from Fe to N‐doped graphited carbon at the heterojunction interface, thus accelerating electron transfer from electrode to reactant. Dynamic analysis indicates unique structure can significantly improve mass transport of oxygen‐species due to two factors: one is electrolyte streaming effect caused by tentacle‐like carbon nanotubes; the other is effective collision probability in the semi‐closed cavity. Therefore, this Mott–Schottky‐type catalyst delievers superior ORR performance with high onset potential, positive half wave potential, and large current density. It also exhibits low overpotential when serving as an air cathode in Zn–air batteries. This work deepens understanding of the two key factors—electron transfer and mass transport—on determining the kinetic reaction of ORR process and offers a new avenue in constructing efficient Mott–Schottky electrocatalysts.