Growth control of oxygen stoichiometry in homoepitaxial SrTiO3 films by pulsed laser epitaxy in high vacuum

Recent advancements in non-enzymatic electrochemical sensor development for the detection of organophosphorus pesticides in food and environment

Organophosphorus Pesticides (OPPs) are among the extensively used pesticides throughout the world to boost agricultural production. However, persistent residues of these toxic pesticides in various vegetables, fruits, and drinking water poses detrimental health effects. Consequently, the rapid monitoring of these harmful chemicals through simple and cost-effective methods has become crucial. In such an instance, electrochemical methods offer simple, rapid, sensitive, reproducible, and affordable detection pathways. To overcome the limitations associated with electrochemical enzymatic sensors, non-enzymatic sensors have emerged as promising and simpler alternatives. The non-enzymatic sensors have demonstrated superior activity, reaching detection limit up to femto (10-15) molar concentration in recent years, leveraging higher selectivity obtained through the molecularly imprinted polymers, synergistic effects between carbonaceous nanomaterials and metals, metal oxide alloys, and other alternative approaches. Herein, this review paper provides an overview of the recent advancements in the development of non-enzymatic electrochemical sensors for the detection of commonly used OPPs, such as Chlorpyrifos (CHL), Diazinon (DZN), Malathion (MTN), Methyl parathion (MP) and Fenthion (FEN). The design method of the electrodes, electrode functioning mechanism, and their analytical performance metrics, such as limit of detection, sensitivity, selectivity, and linearity range, were reviewed and compared. Furthermore, the existing challenges within this rapidly growing field were discussed along with their potential solutions which will facilitate the fabrication of advanced and sustainable non-enzymatic sensors in the future.

Ferroelectric Hf0.5Zr0.5O2 films with improved endurance obtained through low temperature epitaxial growth on seed layers

Crystallization temperature is a critical parameter in the stabilization of the metastable ferroelectric phase of HfO₂. The optimal crystallization temperature used for polycrystalline films is too low to grow epitaxial films. We have developed a new growth strategy, based on the use of an ultrathin seed layer, to obtain high-quality epitaxial films of orthorhombic Hf_0.5Zr_0.5O₂ at a lower temperature. The threshold temperature for epitaxy is reduced from about 750 °C to about 550 °C using a seed layer. Epitaxial films deposited at low temperatures exhibit highly enhanced endurance, and films grown at 550-600 °C show high polarization, no wake-up effect, and greatly reduced fatigue and improved endurance in comparison with the films deposited at high temperatures without a seed layer. We propose that the endurance enhancement is due to a positive effect of the defects, which limits the propagation of pinned ferroelectric domains.

Real-Time Diagnostics of 2D Crystal Transformations by Pulsed Laser Deposition: Controlled Synthesis of Janus WSSe Monolayers and Alloys

Energetic processing methods such as hyperthermal implantation hold special promise to achieve the precision synthesis of metastable two-dimensional (2D) materials such as Janus monolayers; however, they require precise control. Here, we report a feedback approach to reveal and control the transformation pathways in materials synthesis by pulsed laser deposition (PLD) and apply it to investigate the transformation kinetics of monolayer WS₂ crystals into Janus WSSe and WSe₂ by implantation of Se clusters with different maximum kinetic energies (<42 eV/Se-atom) generated by laser ablation of a Se target. Real-time Raman spectroscopy and photoluminescence are used to assess the structure, composition, and optoelectronic quality of the monolayer crystal as it is implanted with well-controlled fluxes of selenium for different kinetic energies that are regulated with in situ ICCD imaging, ion probe, and spectroscopy diagnostics. First-principles calculations, XPS, and atomic-resolution HAADF STEM imaging are used to understand the intermediate alloy compositions and their vibrational modes to identify transformation pathways. The real-time kinetics measurements reveal highly selective top-layer conversion as WS₂ transforms through WS_2(1-x)Se_2x alloys to WSe₂ and provide the means to adjust processing conditions to achieve fractional and complete Janus WSSe monolayers as metastable transition states. The general approach demonstrates a real-time feedback method to achieve Janus layers or other metastable alloys of the desired composition, and a general means to adjust the structure and quality of materials grown by PLD, addressing priority research directions for precision synthesis with real-time adaptive control.

KTaO3 -The New Kid on the Spintronics Block

Long after the heady days of high-temperature superconductivity, the oxides came back into the limelight in 2004 with the discovery of the 2D electron gas (2DEG) in SrTiO₃ (STO) and several heterostructures based on it. Not only do these materials exhibit interesting physics, but they have also opened up new vistas in oxide electronics and spintronics. However, much of the attention has recently shifted to KTaO₃ (KTO), a material with all the "good" properties of STO (simple cubic structure, high mobility, etc.) but with the additional advantage of a much larger spin-orbit coupling. In this state-of-the-art review of the fascinating world of KTO, it is attempted to cover the remarkable progress made, particularly in the last five years. Certain unsolved issues are also indicated, while suggesting future research directions as well as potential applications. The range of physical phenomena associated with the 2DEG trapped at the interfaces of KTO-based heterostructures include spin polarization, superconductivity, quantum oscillations in the magnetoresistance, spin-polarized electron transport, persistent photocurrent, Rashba effect, topological Hall effect, and inverse Edelstein Effect. It is aimed to discuss, on a single platform, the various fabrication techniques, the exciting physical properties and future application possibilities of this family of materials.

Unconventional interlayer exchange coupling via chiral phonons in synthetic magnetic oxide heterostructures

Chiral symmetry breaking of phonons plays an essential role in emergent quantum phenomena owing to its strong coupling to spin degree of freedom. However, direct experimental evidence of the chiral phonon-spin coupling is lacking. In this study, we report a chiral phonon-mediated interlayer exchange interaction in atomically controlled ferromagnetic metal (SrRuO₃)-nonmagnetic insulator (SrTiO₃) heterostructures. Owing to the unconventional interlayer exchange interaction, we have observed rotation of spins as a function of nonmagnetic insulating spacer thickness, resulting in a spin spiral state. The chiral phonon-spin coupling is further confirmed by phonon Zeeman effect. The existence of the chiral phonons and their interplay with spins along with our atomic-scale heterostructure approach unveil the crucial roles of chiral phonons in magnetic materials.

Evaluation of Optical and Acoustical Properties of Ba1-xSrxTiO3 Thin Film Material Library via Conjugation of Picosecond Laser Ultrasonics with X-ray Diffraction, Energy Dispersive Spectroscopy, Electron Probe Micro Analysis, Scanning Electron and Atomic Force Microscopies

Wide-range continuous spatial variation of the film composition in lateral compositionally graded epitaxial films requires the development of high throughput measurement techniques for their local and non-destructive characterization with the highest possible spatial resolution. Here we report on the first application of the picosecond laser ultrasonics (PLU) technique for the evaluation of acoustical and optical parameters of lateral compositionally graded film, the Ba_1−xSr_xTiO₃ (0 ≤ x ≤ 1) material library. The film was not dedicatedly prepared for its opto-acousto-optic evaluation by PLU, exhibiting significant lateral variations in thickness and surface roughness. Therefore, the achieved measurements of the sound velocity and of the optical refractive index, and characterization of the surface roughness confirm the robustness of the PLU technique for thin film evaluation. We hope that the first measurements of the acoustical and optical properties of epitaxial grown Ba_1−xSr_xTiO₃ (0 ≤ x ≤ 1) by PLU technique accomplished here provide the parameters required for more extended predictive design of the phononic, photonic and phoxonic mirrors and cavities with superior properties/functionalities for novel multifunctional nanodevices.

Correlated oxide Dirac semimetal in the extreme quantum limit

Quantum materials (QMs) with strong correlation and nontrivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Here, we report that strain-induced symmetry modification in correlated oxide SrNbO₃ thin films creates an emerging topological band structure. Dirac electrons in strained SrNbO₃ films reveal ultrahigh mobility (μ_max ≈ 100,000 cm²/Vs), exceptionally small effective mass (m* ~ 0.04m_e), and nonzero Berry phase. Strained SrNbO₃ films reach the extreme quantum limit, exhibiting a sign of fractional occupation of Landau levels and giant mass enhancement. Our results suggest that symmetry-modified SrNbO₃ is a rare example of correlated oxide Dirac semimetals, in which strong correlation of Dirac electrons leads to the realization of a novel correlated topological QM.

Effect of low temperature structural phase transitions in BaTiO3on electrical transport through a metal-ferroelectric-metal multilayer of AuCr/BaTiO3/Nb:SrTiO3

In this paper we report an investigation of electronic transport through the metal-ferroelectric-metal (MFM) multilayer consisting of AuCr/BaTiO₃/Nb:SrTiO₃over a temperature range of 100 K-300 K where BaTiO₃(BTO) shows a series of structural phase transitions leading to change of magnitude as well as the orientation of the polarizationP→. We observed that the bias dependent barrier heights associated with the interfaces carry strong signature of the phase transitions in the BTO layer which lead to a strong temperature dependent asymmetric transport, when cooled down below room temperature. Specifically, it is observed that the temperature dependence is closely correlated to low temperature transitions in the BTO layer as revealed through the temperature dependent x-ray diffraction (XRD), capacitance as well as resistivity behavior of the BTO layer. There is substantial enhancement of the asymmetry in the device current that occurs at or close to temperaturesT₂∼ 190 K where BTO shows a crystallographic phase change to the low temperature rhombohedral phase. The temperature dependent changes occur due to barrier modulation at the interfaces of AuCr/BaTiO₃as well as BaTiO₃/Nb:SrTiO₃that softens on cooling due to inhomogenities present there. The change in barrier on change of the bias direction has been observed belowT₂which arises from alignment of the polarization in-plane or out-of-plane as determined by tensile or compressive character of the in-plane strain in the BTO film. We also discuss the effect of space charge determined by the oxygen vacancies in the interface region, regulated by the applied bias.

Sensitive RHEED signature of Ti-excess enabling enhanced cationic composition control during the molecular beam epitaxy of SrTiO3 based solid solutions

Monitoring the appearance of half-order streaks along the [210] RHEED azimuths instead of along the [100] azimuths during the MBE growth of SrTiO₃ thin layers provides an improved accuracy of ±6.7% on the control of the cationic composition.

Interfacial stabilization for epitaxial CuCrO2 delafossites

ABO₂ delafossites are fascinating materials that exhibit a wide range of physical properties, including giant Rashba spin splitting and anomalous Hall effects, because of their characteristic layered structures composed of noble metal A and strongly correlated BO₂ sublayers. However, thin film synthesis is known to be extremely challenging owing to their low symmetry rhombohedral structures, which limit the selection of substrates for thin film epitaxy. Hexagonal lattices, such as those provided by Al₂O₃(0001) and (111) oriented cubic perovskites, are promising candidates for epitaxy of delafossites. However, the formation of twin domains and impurity phases is hard to suppress, and the nucleation and growth mechanisms thereon have not been studied for the growth of epitaxial delafossites. In this study, we report the epitaxial stabilization of a new interfacial phase formed during pulsed-laser epitaxy of (0001)-oriented CuCrO₂ epitaxial thin films on Al₂O₃ substrates. Through a combined study using scanning transmission electron microscopy/electron-energy loss spectroscopy and density functional theory calculations, we report that the nucleation of a thermodynamically stable, atomically thick CuCr_1-xAl_xO₂ interfacial layer is the critical element for the epitaxy of CuCrO₂ delafossites on Al₂O₃ substrates. This finding provides key insights into the thermodynamic mechanism for the nucleation of intermixing-induced buffer layers that can be used for the growth of other noble-metal-based delafossites, which are known to be challenging due to the difficulty in initial nucleation.

Orientation and lattice matching of CoNi nanowires embedded in SrTiO3: unveiling novel strain relaxation mechanisms in vertically aligned nanocomposites

Unveiling complex structural rearrangements and novel strain relaxation mechanisms in vertically aligned nanocomposites.

Multistaged discharge constructing heterostructure with enhanced solid-solution behavior for long-life lithium-oxygen batteries

Inferior charge transport in insulating and bulk discharge products is one of the main factors resulting in poor cycling stability of lithium-oxygen batteries with high overpotential and large capacity decay. Here we report a two-step oxygen reduction approach by pre-depositing a potassium carbonate layer on the cathode surface in a potassium-oxygen battery to direct the growth of defective film-like discharge products in the successive cycling of lithium-oxygen batteries. The formation of defective film with improved charge transport and large contact area with a catalyst plays a critical role in the facile decomposition of discharge products and the sustained stability of the battery. Multistaged discharge constructing lithium peroxide-based heterostructure with band discontinuities and a relatively low lithium diffusion barrier may be responsible for the growth of defective film-like discharge products. This strategy offers a promising route for future development of cathode catalysts that can be used to extend the cycling life of lithium-oxygen batteries.

Tuning the Electrical and Thermoelectric Properties of N Ion Implanted SrTiO3 Thin Films and Their Conduction Mechanisms

The SrTiO₃ thin films were fabricated by pulsed laser deposition. Subsequently ion implantation with 60 keV N ions at two different fluences 1 × 10¹⁶ and 5 × 10¹⁶ ions/cm² and followed by annealing was carried out. Thin films were then characterized for electronic structure, morphology and transport properties. X-ray absorption spectroscopy reveals the local distortion of TiO₆ octahedra and introduction of oxygen vacancies due to N implantation. The electrical and thermoelectric properties of these films were measured as a function of temperature to understand the conduction and scattering mechanisms. It is observed that the electrical conductivity and Seebeck coefficient (S) of these films are significantly enhanced for higher N ion fluence. The temperature dependent electrical resistivity has been analysed in the temperature range of 80–400 K, using various conduction mechanisms and fitted with band conduction, near neighbour hopping (NNH) and variable range hopping (VRH) models. It is revealed that the band conduction mechanism dominates at high temperature regime and in low temperature regime, there is a crossover between NNH and VRH. The S has been analysed using the relaxation time approximation model and dispersive transport mechanism in the temperature range of 300–400 K. Due to improvement in electrical conductivity and thermopower, the power factor is enhanced to 15 µWm⁻¹ K⁻² at 400 K at the higher ion fluence which is in the order of ten times higher as compared to the pristine films. This study suggests that ion beam can be used as an effective technique to selectively alter the electrical transport properties of oxide thermoelectric materials.

Enhanced magnetism in lightly doped manganite heterostructures: strain or stoichiometry?

Lattice mismatch induced epitaxial strain has been widely used to tune functional properties in complex oxide heterostructures. Apart from the epitaxial strain, a large lattice mismatch also produces other effects including modulations in microstructure and stoichiometry. However, it is challenging to distinguish the impact of these effects from the strain contribution to thin film properties. Here, we use La0.9Sr0.1MnO3 (LSMO), a lightly doped manganite close to the vertical phase boundary, as a model system to demonstrate that both epitaxial strain and cation stoichiometry induced by strain relaxation contribute to functionality tuning. The thinner LSMO films are metallic with a greatly enhanced TC which is 97 K higher than the bulk value. Such anomalies in TC and transport cannot be fully explained by the epitaxial strain alone. Detailed microstructure analysis indicates La deficiency in thinner films and twin domain formation in thicker films. Our results have revealed that both epitaxial strain and strain relaxation induced stoichiometry/microstructure modulations contribute to the modified functional properties in lightly doped manganite perovskite thin films.

Enhanced flexoelectricity at reduced dimensions revealed by mechanically tunable quantum tunnelling

Flexoelectricity is a universal electromechanical coupling effect whereby all dielectric materials polarise in response to strain gradients. In particular, nanoscale flexoelectricity promises exotic phenomena and functions, but reliable characterisation methods are required to unlock its potential. Here, we report anomalous mechanical control of quantum tunnelling that allows for characterising nanoscale flexoelectricity. By applying strain gradients with an atomic force microscope tip, we systematically polarise an ultrathin film of otherwise nonpolar SrTiO3, and simultaneously measure tunnel current across it. The measured tunnel current exhibits critical behaviour as a function of strain gradients, which manifests large modification of tunnel barrier profiles via flexoelectricity. Further analysis of this critical behaviour reveals significantly enhanced flexocoupling strength in ultrathin SrTiO3, compared to that in bulk, rendering flexoelectricity more potent at the nanoscale. Our study not only suggests possible applications exploiting dynamic mechanical control of quantum effect, but also paves the way to characterise nanoscale flexoelectricity.

Probing vacancy behavior across complex oxide heterointerfaces

Oxygen vacancies ( V O • • ) play a critical role as defects in complex oxides in establishing functionality in systems including memristors, all-oxide electronics, and electrochemical cells that comprise metal-insulator-metal or complex oxide heterostructure configurations. Improving oxide-oxide interfaces necessitates a direct, spatial understanding of vacancy distributions that define electrochemically active regions. We show vacancies deplete over micrometer-level distances in Nb-doped SrTiO₃ (Nb:SrTiO₃) substrates due to deposition and post-annealing processes. We convert the surface potential across a strontium titanate/yttria-stabilized zirconia (STO/YSZ) heterostructured film to spatial (<100 nm) vacancy profiles within STO using (T = 500°C) in situ scanning probes and semiconductor analysis. Oxygen scavenging occurring during pulsed laser deposition reduces Nb:STO substantially, which partially reoxidizes in an oxygen-rich environment upon cooling. These results (i) introduce the means to spatially resolve quantitative vacancy distributions across oxide films and (ii) indicate the mechanisms by which oxide thin films enhance and then deplete vacancies within the underlying substrate.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Control of Polar Orientation and Lattice Strain in Epitaxial BaTiO3 Films on Silicon

Conventional strain engineering of epitaxial ferroelectric oxide thin films is based on the selection of substrates with a suitable lattice parameter. Here, we show that the variation of oxygen pressure during pulsed laser deposition is a flexible strain engineering method for epitaxial ferroelectric BaTiO₃ films either on perovskite substrates or on Si(001) wafers. This unconventional growth strategy permits continuous tuning of strain up to high levels (ε > 0.8%) in films greater than one hundred nanometers thick, as well as selecting the polar axis orientation to be either parallel or perpendicular to the substrate surface plane.

UV radiation enhanced oxygen vacancy formation caused by the PLD plasma plume

Pulsed Laser Deposition is a commonly used non-equilibrium physical deposition technique for the growth of complex oxide thin films. A wide range of parameters is known to influence the properties of the used samples and thin films, especially the oxygen-vacancy concentration. One parameter has up to this point been neglected due to the challenges of separating its influence from the influence of the impinging species during growth: the UV-radiation of the plasma plume. We here present experiments enabled by a specially designed holder to allow a separation of these two influences. The influence of the UV-irradiation during pulsed laser deposition on the formation of oxygen-vacancies is investigated for the perovskite model material SrTiO3. The carrier concentration of UV-irradiated samples is nearly constant with depth and time. By contrast samples not exposed to the radiation of the plume show a depth dependence and a decrease in concentration over time. We reveal an increase in Ti-vacancy-oxygen-vacancy-complexes for UV irradiated samples, consistent with the different carrier concentrations. We find a UV enhanced oxygen-vacancy incorporation rate as responsible mechanism. We provide a complete picture of another influence parameter to be considered during pulsed laser depositions and unravel the mechanism behind persistent-photo-conductivity in SrTiO3.

Strain control of oxygen kinetics in the Ruddlesden-Popper oxide La1.85Sr0.15CuO4

Oxygen defect control has long been considered an important route to functionalizing complex oxide films. However, the nature of oxygen defects in thin films is often not investigated beyond basic redox chemistry. One of the model examples for oxygen-defect studies is the layered Ruddlesden-Popper phase La2-xSr x CuO4-δ (LSCO), in which the superconducting transition temperature is highly sensitive to epitaxial strain. However, previous observations of strain-superconductivity coupling in LSCO thin films were mainly understood in terms of elastic contributions to mechanical buckling, with minimal consideration of kinetic or thermodynamic factors. Here, we report that the oxygen nonstoichiometry commonly reported for strained cuprates is mediated by the strain-modified surface exchange kinetics, rather than reduced thermodynamic oxygen formation energies. Remarkably, tensile-strained LSCO shows nearly an order of magnitude faster oxygen exchange rate than a compressively strained film, providing a strategy for developing high-performance energy materials.

Strain coupling of oxygen non-stoichiometry in perovskite thin films

The effects of strain and oxygen vacancies on perovskite thin films have been studied in great detail over the past decades and have been treated separately from each other. While epitaxial strain has been realized as a tuning knob to tailor the functional properties of correlated oxides, oxygen vacancies are usually regarded as undesirable and detrimental. In transition metal oxides, oxygen defects strongly modify the properties and functionalities via changes in oxidation states of the transition metals. However, such coupling is not well understood in epitaxial films, but rather deemed as cumbersome or experimental artifact. Only recently it has been recognized that lattice strain and oxygen non-stoichiometry are strongly correlated in a vast number of perovskite systems and that this coupling can be beneficial for information and energy technologies. Recent experimental and theoretical studies have focused on understanding the correlated phenomena between strain and oxygen vacancies for a wide range of perovskite systems. These correlations not only include the direct relationship between elastic strain and the formation energy of oxygen vacancies, but also comprise highly complex interactions such as strain-induced phase transitions due to oxygen vacancy ordering. Therefore, we aim in this review to give a comprehensive overview on the coupling between strain and oxygen vacancies in perovskite oxides and point out the potential applications of the emergent functionalities strongly coupled to oxygen vacancies.

Tuning electromagnetic properties of SrRuO3 epitaxial thin films via atomic control of cation vacancies

Elemental defect in transition metal oxides is an important and intriguing subject that result in modifications in variety of physical properties including atomic and electronic structure, optical and magnetic properties. Understanding the formation of elemental vacancies and their influence on different physical properties is essential in studying the complex oxide thin films. In this study, we investigated the physical properties of epitaxial SrRuO₃ thin films by systematically manipulating cation and/or oxygen vacancies, via changing the oxygen partial pressure (P(O₂)) during the pulsed laser epitaxy (PLE) growth. Ru vacancies in the low-P(O₂)-grown SrRuO₃ thin films induce lattice expansion with the suppression of the ferromagnetic T_C down to ~120 K. Sr vacancies also disturb the ferromagnetic ordering, even though Sr is not a magnetic element. Our results indicate that both A and B cation vacancies in an ABO₃ perovskite can be systematically engineered via PLE, and the structural, electrical, and magnetic properties can be tailored accordingly.

Oxygen Partial Pressure during Pulsed Laser Deposition: Deterministic Role on Thermodynamic Stability of Atomic Termination Sequence at SrRuO3/BaTiO3 Interface

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO₃/BaTiO₃/SrRuO₃ (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO₂ or TiO₂-SrO) with oxygen partial pressure (P_O2) during PLD. We found that a uniform SrO-TiO₂ termination sequence at the SRO/BTO interface can be achieved by lowering the P_O2 to 5 mTorr, regardless of the total background gas pressure (P_total), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures.

Kinetically Controlled Fabrication of Single-Crystalline TiO2 Nanobrush Architectures with High Energy {001} Facets

This study demonstrates that precise control of nonequilibrium growth conditions during pulsed laser deposition (PLD) can be exploited to produce single-crystalline anatase TiO2 nanobrush architectures with large surface areas terminated with high energy {001} facets. The data indicate that the key to nanobrush formation is controlling the atomic surface transport processes to balance defect aggregation and surface-smoothing processes. High-resolution scanning transmission electron microscopy data reveal that defect-mediated aggregation is the key to TiO2 nanobrush formation. The large concentration of defects present at the intersection of domain boundaries promotes aggregation of PLD growth species, resulting in the growth of the single-crystalline nanobrush architecture. This study proposes a model for the relationship between defect creation and growth mode in nonequilibrium environments, which enables application of this growth method to novel nanostructure design in a broad range of materials.

Role of Oxygen Deposition Pressure in the Formation of Ti Defect States in TiO2(001) Anatase Thin Films

We report the study of anatase TiO₂(001)-oriented thin films grown by pulsed laser deposition on LaAlO₃(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti³⁺-localized states and their relationship with the structural and electronic properties on the surface and the subsurface. Localized in-gap states are analyzed using resonant X-ray photoelectron spectroscopy and are related to the Ti³⁺ electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti³⁺ only in a well-defined range of deposition pressure; outside this range, Ti³⁺ and the strength of the in-gap states are reduced.

Unraveling the enhanced Oxygen Vacancy Formation in Complex Oxides during Annealing and Growth

The reduction of oxides during annealing and growth in low pressure processes is a widely known problem. We hence investigate the influence of mere annealing and of growth in vacuum systems to shed light on the reasons behind the reduction of perovskites. When comparing the existing literature regarding the reduction of the perovskite model material SrTiO₃ it is conspicuous that one finds different oxygen pressures required to achieve reduction for vacuum annealing and for chemically controlled reducing atmospheres. The unraveling of this discrepancy is of high interest for low pressure physical vapor depositions of thin films heterostructures to gain further understanding of the reduction of the SrTiO₃. For thermal annealing, our results prove the attached measurement devices (mass spectrometer/ cold cathode gauge) to be primarily responsible for the reduction of SrTiO₃ in the deposition chamber by shifting the thermodynamic equilibrium to a more reducing atmosphere. We investigated the impact of our findings on the pulsed laser deposition growth at low pressure for LaAlO₃/SrTiO₃. During deposition the reduction triggered by the presence of the laser plume dominates and the impact of the measurement devices plays a minor role. During post annealing a complete reoxidization of samples is inhibited by an insufficient supply of oxygen.

Photoelectrochemical water splitting with a SrTiO3:Nb/SrTiO3 n+–n homojunction structure

An n⁺–n homojunction, which was constructed by thermal diffusion of niobium in STO, was found to make a significant impact on the performance of the STO photoanodes by affecting their depletion width.

Direct Nanoscale Analysis of Temperature-Resolved Growth Behaviors of Ultrathin Perovskites on SrTiO3

Revealing growth mechanism of a thin film and properties of its film-substrate interface necessarily require microscopic investigations on the initial growth stages in temperature- and thickness-resolved manners. Here we applied in situ scanning tunneling microscopy and atomic force microscopy to investigate the growth dynamics in homo- (SrTiO3) and hetero- (SrRuO3) epitaxies on SrTiO3(001). A comparison of temperature-dependent surface structures of SrRuO3 and SrTiO3 films suggests that the peculiar growth mode switching from a "layer-by-layer" to "step-flow" type in a SrRuO3 films arises from a reduction of surface migration barrier, caused by the change in the chemical configuration of the interface between the topmost and underlying layers. Island densities in perovskite epitaxies exhibited a clear linear inverse-temperature dependence. A prototypical study on island nucleation stage of SrTiO3 homoepitaxy revealed that classical diffusion model is valid for the perovskite growths.

Investigations of metastable Ca2IrO4 epitaxial thin-films: systematic comparison with Sr2IrO4 and Ba2IrO4

We have synthesized thermodynamically metastable Ca2IrO4 thin-films on YAlO3 (110) substrates by pulsed laser deposition. The epitaxial Ca2IrO4 thin-films are of K2NiF4-type tetragonal structure. Transport and optical spectroscopy measurements indicate that the electronic structure of the Ca2IrO4 thin-films is similar to that of Jeff = 1/2 spin-orbit-coupled Mott insulator Sr2IrO4 and Ba2IrO4, with the exception of an increased gap energy. The gap increase is to be expected in Ca2IrO4 due to its increased octahedral rotation and tilting, which results in enhanced electron-correlation, U/W. Our results suggest that the epitaxial stabilization growth of metastable-phase thin-films can be used effectively for investigating layered iridates and various complex-oxide systems.