Electronic and Structural Transitions of LaAlO3 /SrTiO3 Heterostructure Driven by Polar Field-Assisted Oxygen Vacancy Formation at the Surface

Feld-induced modulation of two-dimensional electron gas at LaAlO3/SrTiO3 interface by polar distortion of LaAlO3

Since the discovery of two-dimensional electron gas at the LaAlO3/SrTiO3 interface, its intriguing physical properties have garnered significant interests for device applications. Yet, understanding its response to electrical stimuli remains incomplete. Our in-situ transmission electron microscopy analysis of a LaAlO3/SrTiO3 two-dimensional electron gas device under electrical bias reveals key insights. Inline electron holography visualized the field-induced modulation of two-dimensional electron gas at the interface, while electron energy loss spectroscopy showed negligible electromigration of oxygen vacancies. Instead, atom-resolved imaging indicated that electric fields trigger polar distortion in the LaAlO3 layer, affecting two-dimensional electron gas modulation. This study refutes the previously hypothesized role of oxygen vacancies, underscoring the lattice flexibility of LaAlO3 and its varied polar distortions under electric fields as central to two-dimensional electron gas dynamics. These findings open pathways for advanced oxide nanoelectronics, exploiting the interplay of polar and nonpolar distortions in LaAlO3.

Theoretical and Experimental Analysis of Hydroxyl and Epoxy Group Effects on Graphene Oxide Properties

In this study, we analyzed the impact of hydroxyl and epoxy groups on the properties of graphene oxide (GO) for the adsorption of methylene blue (MB) dye from water, addressing the urgent need for effective water purification methods due to industrial pollution. Employing a dual approach, we integrated experimental techniques with theoretical modeling via density functional theory (DFT) to examine the atomic structure of GO and its adsorption capabilities. The methodology encompasses a series of experiments to evaluate the performance of GO in MB dye adsorption under different conditions, including differences in pH, dye concentration, reaction temperature, and contact time, providing a comprehensive view of its effectiveness. Theoretical DFT calculations provide insights into how hydroxyl and epoxy modifications alter the electronic properties of GO, improving adsorption efficiency. The results demonstrate a significant improvement in the dye adsorption capacity of GO, attributed to the interaction between the functional groups and MB molecules. This study not only confirms the potential of GO as a superior adsorbent for water treatment, but also contributes to the optimization of GO-based materials for environmental remediation, highlighting the synergy between experimental observations and theoretical predictions in advances in materials science to improve sustainability.

Enhanced Generation of Reactive Oxygen Species via Piezoelectrics based on p-n Heterojunctions with Built-In Electric Field

Tuning the charge transfer processes through a built-in electric field is an effective way to accelerate the dynamics of electro- and photocatalytic reactions. However, the coupling of the built-in electric field of p-n heterojunctions and the microstrain-induced polarization on the impact of piezocatalysis has not been fully explored. Herein, we demonstrate the role of the built-in electric field of p-type BiOI/n-type BiVO4 heterojunctions in enhancing their piezocatalytic behaviors. The highly crystalline p-n heterojunction is synthesized by using a coprecipitation method under ambient aqueous conditions. Under ultrasonic irradiation in water exposed to air, the p-n heterojunctions exhibit significantly higher production rates of reactive species (·OH, ·O2-, and 1O2) as compared to isolated BiVO4 and BiOI. Also, the piezocatalytic rate of H2O2 production with the BiOI/BiVO4 heterojunction reaches 480 μmol g-1 h-1, which is 1.6- and 12-fold higher than those of BiVO4 and BiOI, respectively. Furthermore, the p-n heterojunction maintains a highly stable H2O2 production rate under ultrasonic irradiation for up to 5 h. The results from the experiments and equation-driven simulations of the strain and piezoelectric potential distributions indicate that the piezocatalytic reactivity of the p-n heterojunction resulted from the polarization intensity induced by periodic ultrasound, which is enhanced by the built-in electric field of the p-n heterojunctions. This study provides new insights into the design of piezocatalysts and opens up new prospects for applications in medicine, environmental remediation, and sonochemical sensors.

Robust Preparation of Sub-20-nm-Thin Lamellae for Aberration-Corrected Electron Microscopy

Aberration-corrected scanning transmission electron microscopy (STEM) has been advancing resolution, sensitivity, and microanalysis due to the intense demands of atomic-level microstructural investigations. Recent STEM technologies require preparing a thin lamella whose thickness is ideally below 20 nm. Although focused-ion-beam/scanning-electron-microscopy (FIB/SEM) is an established method to prepare a high-quality lamella, nanometer-level controllability of lamella thickness remains a fundamental problem. Here, the robust preparation of a sub-20-nm-thin lamella is demonstrated by FIB/SEM with real-time feedback from thickness quantification. The lamella thickness is quantified by back-scattered-electron SEM imaging in a thickness range between 0 and 100 nm without any reference to numerical simulation. Using real-time feedback from the thickness quantification, the FIB/SEM terminates thinning a lamella at a targeted thickness. The real-time feedback system eventually provides 1-nm-level controllability of the lamella thickness. As a proof-of-concept, a near-10-nm-thin lamella is prepared from a SrTiO3 crystal by our methodology. Moreover, the lamella thickness is controllable at a target heterointerface. Thus, a sub-20-nm-thin lamella is prepared from a LaAlO3 /SrTiO3 heterointerface. The methodology offers a robust and operator-independent platform to prepare a sub-20-nm-thin lamella from various materials. This platform will broadly impact aberration-corrected STEM studies in materials science and the semiconductor industry.

Direct observation of strong surface reconstruction in partially reduced nickelate films

The polarity of a surface can affect the electronic and structural properties of oxide thin films through electrostatic effects. Understanding the mechanism behind these effects requires knowledge of the atomic structure and electrostatic characteristics at the surface. In this study, we use annular bright-field imaging to investigate the surface structure of a Pr0.8Sr0.2NiO2+x (0 < x < 1) film. We observe a polar distortion coupled with octahedral rotations in a fully oxidized Pr0.8Sr0.2NiO3 sample, and a stronger polar distortion in a partially reduced sample. Its spatial depth extent is about three unit cells from the surface. Additionally, we use four-dimensional scanning transmission electron microscopy (4D-STEM) to directly image the local atomic electric field surrounding Ni atoms near the surface and discover distinct valence variations of Ni atoms, which are confirmed by atomic-resolution electron energy-loss spectroscopy (EELS). Our results suggest that the strong surface reconstruction in the reduced sample is closely related to the formation of oxygen vacancies from topochemical reduction. These findings provide insights into the understanding and evolution of surface polarity at the atomic level.

Epitaxy and transport properties of alkali-earth palladate thin films

Topological insulators and semimetals are an interesting class of materials for new electronic and optical applications owing to their characteristic electromagnetic responses originating from the spin-orbit coupled band structures. However, topological electronic structures are rare in oxide materials despite their chemical stability being preferable for applications. In this study, given the theoretical prediction of Dirac bands in CaPd3O4, we investigate the fabrication and transport properties of SrPd3O4 and CaPd3O4 thin films as candidates of oxide Dirac semimetals. We have found that these materials are epitaxially grown on MgO (100) substrate under limited growth conditions by pulsed laser deposition. The transport properties show a weak temperature dependence, suggestive of narrow-gap properties, although unintentionally doped holes hinder us from revealing the presence of the Dirac band. Our study establishes the basic thermodynamics of thin-film fabrication of these materials and will lead to interesting properties characteristic of topological band structure by modulating the electronic structure by, for example, chemical substitutions or pressure.

Collective Control of Potential-Constrained Oxygen Vacancies in Oxide Heterostructures for Gradual Resistive Switching

Filamentary resistive switching in oxides is one of the key strategies for developing next-generation non-volatile memory devices. However, despite numerous advantages, their practical applications in neuromorphic computing are still limited due to non-uniform and indeterministic switching behavior. Given the inherent stochasticity of point defect migration, the pursuit of reliable switching likely demands an innovative approach. Herein, a collective control of oxygen vacancies is introduced in LaAlO3 /SrTiO3 (LAO/STO) heterostructures to achieve reliable and gradual resistive switching. By exploiting an electrostatic potential constraint in ultrathin LAO/STO heterostructures, the formation of conducting filaments is suppressed, but instead precisely control the concentration of oxygen vacancies. Since the conductance of the LAO/STO device is governed by the ensemble concentration of oxygen vacancies, not their individual probabilistic migrations, the resistive switching is more uniform and deterministic compared to conventional filamentary devices. It provides direct evidence for the collective control of oxygen vacancies by spectral noise analysis and modeling by Monte-Carlo simulation. As a proof of concept, the significantly-improved analog switching performance of the filament-free LAO/STO devices is demonstrated, revealing potential for neuromorphic applications. The results establish an approach to store information by point defect concentration, akin to biological ionic channels, for enhancing switching characteristics of oxide materials.

Stability of oxygen vacancies at metal/oxide interfaces

We investigated the influence of work function and charge doping on the formation of oxygen vacancies in metal/oxide heterojunctions by first-principles calculations. SrTiO₃ is used as a typical oxide. Simple metals Pt, Au and Ag are used as electrodes. We show that electron doping could improve the formation energy of oxygen vacancies. In such a heterojunction, we found that the work function of the metal electrode affects the stability of oxygen vacancies in SrTiO₃. For an electrode with a smaller work function, more electrons are induced and accumulated in the oxides near the interface and improve the formation energy of oxygen vacancies. We also studied the effect of ferroelectric polarization in a heterojunction of metal/BaTiO₃ and found similar properties. We hope that our work could help in the design of complex-oxide-based electronic devices.

Characterization of simplified nonapeptides with broad-spectrum antimicrobial activities as potential food preservatives, and their antibacterial mechanism

Antimicrobial peptides (AMPs) have attracted attention in the field of food preservatives due to their favorable biosafety and potential antimicrobial activity. However, high synthetic cost, systemic toxicity, a narrow antimicrobial spectrum, and poor antimicrobial activity become the main bottlenecks for their practical applications. To address these questions, a set of derived nonapeptides were designed based on a previously discovered ultra-short peptide sequence template (RXRXRXRXL-NH₂) and screened to identify an optimal peptide-based food preservative with excellent antimicrobial properties. Among these nonapeptides, the designed peptides 3IW (RIRIRIRWL-NH₂) and W2IW (RWRIRIRWL-NH₂) presented a membrane-disruptive and reactive oxygen species (ROS) accumulation mechanism to execute potent and rapid broad-spectrum antimicrobial activity without observed cytotoxicity. Moreover, they exhibited favorable antimicrobial stability regardless of high ionic strength, heat, and excessive acid-base conditions, retaining potent antimicrobial effects for chicken meat preservation. Collectively, their ultra-short sequence length and potent broad-spectrum antimicrobial capacity may be beneficial for the further development of green and safe peptide-based food preservatives.

The Effect of La3+ on the Methylene Blue Dye Removal Capacity of the La/ZnTiO3 Photocatalyst, a DFT Study

Theoretically, lanthanum can bond with surface oxygens of ZnTiO₃ to form La-O-Ti bonds, resulting in the change of both the band structure and the electron state of the surface. To verify this statement, DFT calculations were performed using a model with a dispersed lanthanum atom on the surface (101) of ZnTiO₃. The negative heat segmentation values obtained suggest that the incorporation of La on the surface of ZnTiO₃ is thermodynamically stable. The bandgap energy value of La/ZnTiO₃ (2.92 eV) was lower than that of ZnTiO₃ (3.16 eV). TDOS showed that the conduction band (CB) and the valence band (VB) energy levels of La/ZnTiO₃ are denser than those of ZnTiO₃ due to the participation of hybrid levels composed mainly of O2p and La5d orbitals. From the PDOSs, Bader's charge analysis, and ELF function, it was established that the La-O bond is polar covalent. MB adsorption on La/ZnTiO₃ (-200 kJ/mol) was more favorable than on ZnTiO₃ (-85 kJ/mol). From the evidence of this study, it is proposed that the MB molecule first is adsorbed on the surface of La/ZnTiO₃, and then the electrons in the VB of La/ZnTiO₃ are photoexcited to hybrid levels, and finally, the MB molecule oxidizes into smaller molecules.

Oxide Two-Dimensional Electron Gas with High Mobility at Room-Temperature

The prospect of 2-dimensional electron gases (2DEGs) possessing high mobility at room temperature in wide-bandgap perovskite stannates is enticing for oxide electronics, particularly to realize transparent and high-electron mobility transistors. Nonetheless only a small number of studies to date report 2DEGs in BaSnO₃ -based heterostructures. Here, 2DEG formation at the LaScO₃ /BaSnO₃ (LSO/BSO) interface with a room-temperature mobility of 60 cm²  V^-1  s^-1 at a carrier concentration of 1.7 × 10¹³  cm^-2 is reported. This is an order of magnitude higher mobility at room temperature than achieved in SrTiO₃ -based 2DEGs. This is achieved by combining a thick BSO buffer layer with an ex situ high-temperature treatment, which not only reduces the dislocation density but also produces a SnO₂ -terminated atomically flat surface, followed by the growth of an overlying BSO/LSO interface. Using weak beam dark-field transmission electron microscopy imaging and in-line electron holography technique, a reduction of the threading dislocation density is revealed, and direct evidence for the spatial confinement of a 2DEG at the BSO/LSO interface is provided. This work opens a new pathway to explore the exciting physics of stannate-based 2DEGs at application-relevant temperatures for oxide nanoelectronics.

Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures

Polarity discontinuity across LaAlO₃/SrTiO₃ (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (t _c) and disappear simultaneously above t _c with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (V _O) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with V _O distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.