Oxygen-deficient SrTiO3 Research Articles

Improvement in electrochemical performance of SrTiO3 with rGO via composite (SrTiO3/rGO) strategy fabricated via solvothermal approach

Developing sustainable and highly effective electrocatalysts for the oxygen evolution reaction (OER) is a significant research target for precise custody of water splitting to fulfill worldwide energy demands. This study focuses on the fabrication of SrTiO3/rGO-based composite via a solvothermal technique, which works as an electrocatalyst for evaluating the efficiency of OER in alkaline media. The material displays outstanding OER efficacy in 1 M KOH solution with its cubic structure validated by the X-ray diffraction (XRD) study. The electrochemical study reveals that the SrTiO3/rGO composite has a minimal overpotential (222 mV) compared to SrTiO3 (289 mV) and the commercial catalyst RuO2 (367 mV). The SrTiO3/rGO composite also has a minimal Tafel plot gradient of 37 mV dec−1 whereas RuO2 has a Tafel value of 72 mV dec−1. The current material has superior properties compared to RuO2, a commercial catalyst for the OER. The SrTiO3/rGO composite shows exceptional stability for 40 h. The low conductivity of oxygen-deficient SrTiO3 perovskite can be improved by incorporating carbon-based rGO into it, as demonstrated by the minimal Rct (0.07 Ω) obtained from electrochemical impedance spectroscopy (EIS) analysis and an enlarged surface area of 250 cm2, as evidenced by electrochemical surface area (ECSA). Therefore, our research indicates that a particular morphology of metal oxide can improve the efficiency of electrocatalysis when combined with reduced graphene oxide (rGO), suggesting its potential for generating effective and environmentally friendly energy. Hence, these findings may improve the smooth passage of electrons and provide a novel perspective to function as an adequate substitute for RuO2, a commercial catalyst.

First-principles based Monte Carlo modeling of the magnetization of oxygen-deficient Fe-substituted SrTiO3.

Transition-metal (TM) substituted SrTiO3 has attracted much attention because its magnetism and/or ferroelectricity can be tuned via cation substitution, point defects, strain and/or oxygen deficiency. For example, Goto et al. [Phys. Rev. Applied, 7, 024006 (2017)] reported the magnetization of SrTi1-xFexO3-δ (STF) grown under different oxygen pressures and on various substrates. Here, we use hybrid density functional theory to calculate the effects of different oxygen vacancy (VO) states in STF on the magnetization for a variety of Fe cation arrangements. The magnetic states of the cations associated with the VO ground-states for x = {0.125, 0.25} are used within a Monte Carlo model for collinear magnetism to simulate the spontaneous magnetization. Our model captures several experimental features of STF, i.e., an increase in magnetization for small δ up to a maximum of ∼0.35μB per formula unit at an intermediate number of vacancies, with a slower decrease in magnetization with an increasing number of vacancies. Our approach gives insight into the relation between vacancy concentration and the oxygen pressure required to maximize the magnetization.

(Invited) Ferroelectric Effect in SrTiO3 Promotes Photoelectrochemical Water Oxidation

Ferroelectric materials, such as BaTiO3 or BiFeO3, retain an electric polarization after exposure to an external electric field. In semiconductor photoelectrodes, such a ferroelectric effect (FE) could be potentially used to enhance the carrier separation and photoelectrochemical activity under photoexcitation. However, to date, examples of FE-enhanced photoelectrochemistry are rare. Here, we report the discovery of a FE effect in oxygen vacancy rich SrTiO3 nanoparticles and single crystals, and their application for improved photoelectrochemical water oxidation. Single crystals terminated by [111] facets were subjected to hydrogen annealing at 1000 C to induce oxygen vacancies, as confirmed with optical absorption and X-ray photoelectron spectroscopy. In 0.5 M aqueous Na2SO4 solution 1.0 cm2 large crystals produce a 2.1 mA photocurrent at 0.6 V RHE and under 55 mW cm-2 UV illumination. Prior exposure to 10 kV cm-1 electric field increases the photocurrent to 4.5 mA or decreases it to 0.05 mA, depending on the orientation of the polarization. The effect of the electric polarization on the carrier separation direction is confirmed with surface photovoltage spectra. At room temperature, the FE in SrTiO3 is relatively short lived; samples stored in argon retain 85% of the FE effect after 24h, but samples stored in air loose 100% of the FE, likely due to oxygen uptake and disappearance of the vacancies. To obtain a microscopic interpretation of the observed FE effect we investigated ferroelectricity in oxygen-deficient SrTiO3 by density functional theory with effective many-body corrections. These calculations applied to SrTiO3 bulk and surfaces provide a rationale to interpret the experimental results. The finding of an FE in oxygen vacancy rich SrTiO3 is significant because it may be applicable to other metal oxides in the cubic perovskite structure type. Figure 1

Enhancement of the anomalous Nernst effect in epitaxial Fe4N films grown on SrTiO3(001) substrates with oxygen deficient layers

Anomalous Nernst effect of epitaxial Fe4N films on MgO(001), MgAl2O4(MAO)(001), and SrTiO3(STO)(001) substrates grown by molecular beam epitaxy was investigated. Moderately large anomalous Nernst coefficients (SANE) of 1.4 and 1.7 μV/K were obtained in the Fe4N films on the MgO(001) and MAO(001) substrates, respectively, and large anomalous Hall angles (∼0.06) and transverse thermoelectric conductivities [∼1.3 A/(m K)] were derived from the experimental results. On the other hand, a large effective SANE of 2.8 μV/K was obtained in the Fe4N film on the STO(001) substrate. The origin of the enhanced effective SANE is the negatively large Seebeck coefficient (SSE) in an oxygen deficient STO layer near the surface of the STO substrate. This indicates that it is possible to enhance the effective SANE of ferromagnetic materials by utilizing adjacent materials with large |SSE| such as the oxygen deficient STO layer.

Probing Photoexcited Charge Carrier Trapping and Defect Formation in Synergistic Doping of SrTiO3

Strontium titanate (SrTiO3) is widely used as a promising photocatalyst due to its unique band edge alignment with respect to the oxidation and reduction potential corresponding to oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, further enhancement of the photocatalytic activity in this material could be envisaged through the effective control of oxygen vacancy states. This could substantially tune the photoexcited charge carrier trapping under the influence of elemental functionalization in SrTiO3, corresponding to the defect formation energy. The charge trapping states in SrTiO3 decrease through the substitutional doping in Ti sites with p-block elements like Aluminium (Al) with respect to the relative oxygen vacancies. With the help of electronic structure calculations based on density functional theory (DFT) formalism, we have explored the synergistic effect of doping with both Al and Iridium (Ir) in SrTiO3 from the perspective of defect formation energy, band edge alignment and the corresponding charge carrier recombination probability to probe the photoexcited charge carrier trapping that primarily governs the photocatalytic water splitting process. We have also systematically investigated the ratio-effect of Ir:Al functionalization on the position of acceptor levels lying between Fermi and conduction band in oxygen deficient SrTiO3, which governs the charge carrier recombination and therefore the corresponding photocatalytic efficiency.

Critical points in photoluminescence spectra and their relation with phase transition in Nb-doped SrTiO3

Luminescence spectra are extremely sensitive to variations in structural environment; thus, result of structural change, such as phase transition, can be observed via luminescence intensity. The temperature dynamic of photoluminescence in the Nb-doped SrTiO3 demonstrates two critical points at 115 and 160 K, which correspond to temperatures of structural phase transitions for the bulk and the surface of SrTiO3, respectively. The absence of a hysteresis effect in the photoluminescence emission points out the correspondence of the critical points to a second-order phase transition. Similar critical behaviours were also observed in oxygen-deficient SrTiO3, confirming a relationship between the PL and phase transition. The existence of peaks in the temperature coefficient of resistivity at the same temperatures also confirms the correlation between photoluminescence and phase transition in the Nb-doped SrTiO3, providing a simple non-contact method to detect phase transitions in luminescence materials.

Negatively Charged Muonium and Related Centers in Solids

Muonium (Mu) centers formed upon implantation of $\mu^+$ in a solid have long been investigated as an experimentally accessible model of isolated hydrogen impurities. Recent discoveries of hydridic centers formed at oxygen vacancies have stimulated a renewed interest in H$^-$ and corresponding Mu$^-$ centers in oxides. However, the two diamagnetic centers, Mu$^+$ and Mu$^-$, are difficult to separate spectroscopically. In this review article, we summarize established and developing methodologies for identifying Mu$^-$ centers in solids, and review recent Mu studies on said centers supposedly formed in mayenite, oxygen-deficient SrTiO$_{3-x}$, and BaTiO$_{3-x}$H$_x$ oxyhydride.

Optimizing thermal conduction in bulk polycrystalline SrTiO3−δ ceramics via oxygen non-stoichiometry

Abstract

Unconventional Multiband Superconductivity in Bulk SrTiO_{3} and LaAlO_{3}/SrTiO_{3} Interfaces.

Although discovered many decades ago, superconductivity in doped SrTiO_{3} remains a topic of intense research. Recent experiments revealed that, upon increasing the carrier concentration, multiple bands cross the Fermi level, signaling the onset of Lifshitz transitions. Interestingly, T_{c} was observed to be suppressed across the Lifshitz transition of oxygen-deficient SrTiO_{3}; a similar behavior was also observed in gated LaAlO_{3}/SrTiO_{3} interfaces. Such a behavior is difficult to explain in the clean theory of two-band superconductivity, as the additional electronic states provided by the second band should enhance T_{c}. Here, we show that this unexpected behavior can be explained by the strong pair-breaking effect promoted by disorder, which takes place if the interband pairing interaction is subleading and repulsive. A consequence of this scenario is that, upon moving away from the Lifshitz transition, the two-band superconducting state changes from opposite-sign gaps to same-sign gaps.

Mechanical dissipation from charge and spin transitions in oxygen-deficient SrTiO3\xa0surfaces

Bodies in relative motion separated by a gap of a few nanometers can experience a tiny friction force. This non-contact dissipation can have various origins and can be successfully measured by a sensitive pendulum atomic force microscope tip oscillating laterally above the surface. Here, we report on the observation of dissipation peaks at selected voltage-dependent tip-surface distances for oxygen-deficient strontium titanate (SrTiO3) surface at low temperatures (T = 5 K). The observed dissipation peaks are attributed to tip-induced charge and spin state transitions in quantum-dot-like entities formed by single oxygen vacancies (and clusters thereof, possibly through a collective mechanism) at the SrTiO3 surface, which in view of technological and fundamental research relevance of the material opens important avenues for further studies and applications.

Optical investigation of oxygen defect states in SrTiO3 epitaxial thin films

We investigated oxygen defect states in SrTiO3 (STO) thin films by using photoluminescence spectroscopy and spectroscopic ellipsometry. The oxygen-deficient STO homoepitaxial films were fabricated by using different oxygen partial pressures and flow rates for controlling the amount of oxygen vacancies systematically. The visible emission and absorption increased simultaneously with increasing oxygen vacancy concentration in the STO thin films. Given the absolute values of the optical constants, we discussed the origins of the visible absorption features in the STO thin films, in relation to the defect formation and related electronic reconstruction.

Effect of Ag and Ti electrodes on capacitance tunability of oxygen-deficient SrTiO3 based MIM varactors prepared by plasma sputtering

The role of oxygen deficiencies on the electrical performance of SrTiO3 films is investigated by depositing on the stack of TiN/Ti/SiO2/Si films using radio-frequency (RF) and direct current (DC) magnetron sputtering systems operating in oxygen-deficient plasma environment. Electrodes of silver (Ag) and titanium (Ti) are used to observe the effects of their sizes on the electrical properties of the SrTiO3 films. It is found that the nonlinearity in capacitance–voltage graphs (capacitance tunability) of the varactor can be controlled by varying the electrode area. In addition, the material of the electrode plays a vital role in controlling the non-linearity in the capacitance of the oxygen-deficient SrTiO3 dielectric films with varying voltage. It is found that the silver top electrode instead of titanium, delivers better performance with high capacitance tunability, low leakage current and good quality factor. Post deposition annealing at 550 °C in nitrogen ambient, creates more oxygen deficiencies in SrTiO3 film thereby increasing the linearity of the capacitance–voltage graph and decreasing the capacitance tunability.

Scavenging of oxygen from SrTiO3 during oxide thin film deposition and the formation of interfacial 2DEGs

SrTiO3 is a widely used substrate for the growth of other functional oxide thin films. The reactivity of the substrate with respect to the film during deposition, particularly with regard to redox reactions, has typically been glossed over. We demonstrate by depositing a variety of metals (Ti, Al, Nb, Pt, Eu, and Sr) and measuring the in situ core level spectra of both the metal and SrTiO3 that, depending on the oxide formation energy and work function of the metal, three distinct types of behavior occur in thin metal films on SrTiO3 (100). In many cases, there will be an interfacial layer of oxygen-deficient SrTiO3 formed at the interface with the overlying film. We discuss how this may affect the interpretation of the well-known two-dimensional electron gas present at the interface between SrTiO3 and various oxides.

Microstructure and dynamics of vacancy-induced nanofilamentary switching network in donor doped SrTiO3−x memristors

Donor doping of perovskite oxides has emerged as an attractive technique to create high performance and low energy non-volatile analog memories. Here, we examine the origins of improved switching performance and stable multi-state resistive switching in Nb-doped oxygen-deficient amorphous SrTiO3 (Nb:a-STOx) metal–insulator–metal (MIM) devices. We probe the impact of substitutional dopants (i.e., Nb) in modulating the electronic structure and subsequent switching performance. Temperature stability and bias/time dependence of the switching behavior are used to ascertain the role of substitutional dopants and highlight their utility to modulate volatile and non-volatile behavior in a-STOx devices for adaptive and neuromorphic applications. We utilized a combination of transmission electron microscopy, photoluminescence emission properties, interfacial compositional evaluation, and activation energy measurements to investigate the microstructure of the nanofilamentary network responsible for switching. These results provide important insights into understanding mechanisms that govern the performance of donor-doped perovskite oxide-based memristive devices.

Role of doubleTiO2layers at the interface of FeSe/SrTiO3superconductors

We determine the surface reconstruction of SrTiO3 used to achieve superconducting FeSe films in experiments, which is different from the 1x1 TiO2 terminated SrTiO3 assumed by most previous theoretical studies. In particular, we identify the existence of a double TiO2 layer at the SrTiO3-FeSe interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. As a better electron donor than previously proposed interfacial structures, the double layer helps to remove the hole pocket in the FeSe at the {\Gamma} point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this novel superconductor.

Research Update: Plentiful magnetic moments in oxygen deficient SrTiO3

Correlated band theory is employed to investigate the magnetic and electronic properties of different arrangements of oxygen di- and tri-vacancy clusters in SrTiO3. Hole and electron doping of oxygen deficient SrTiO3 yields various degrees of magnetization as a result of the interaction between localized magnetic moments at the defect sites. Different kinds of Ti atomic orbital hybridization are described as a function of the doping level and defect geometry. We find that magnetism in SrTiO3−δ is sensitive to the arrangement of neighbouring vacancy sites, charge carrier density, and vacancy-vacancy interaction. Permanent magnetic moments in the absence of vacancy doping electrons are observed. Our description of the charged clusters of oxygen vacancies widens the previous descriptions of mono- and multi-vacancies and points out the importance of the controlled formation at the atomic level of defects for the realization of transition metal oxide based devices with a desirable magnetic performance.

Magnetism and metal-insulator transition in oxygen-deficientSrTiO3

First-principles calculations to study the electronic and magnetic properties of bulk, oxygen-deficient SrTiO$_3$ (STO) under different doping conditions and densities have been conducted. The appearance of magnetism in oxygen-deficient STO is not determined solely by the presence of a single oxygen vacancy but by the density of free carriers and the relative proximity of the vacant sites. We find that while an isolated vacancy behaves as a non-magnetic double donor, manipulation of the doping conditions allows the stability of a single donor state, with emergent local moments coupled ferromagnetically by carriers in the conduction band. Strong local lattice distortions enhance the binding of this state. The energy of the in-gap local moment can be further tuned by orthorhombic strain. Consequently we find that the free-carrier density and strain are fundamental components to obtaining trapped spin-polarized electrons in oxygen-deficient STO, which may have important implications in the design of optical devices.

Rectifying effect through the interface of SrTiO3−δ/GaAs heterojunctions

Oxides-semiconductor junctions have attracted great attention and exhibited promising potential in integrated devices in which the passing current is controlled by applying voltage. It is found that an oxygen-deficient SrTiO3−δ/p-GaAs junction displays an obvious rectifying effect. The SrTiO3−δ thin film is in the anomalous in-plain compressive strain, as confirmed by structural characterization. Investigations on the current–voltage curve show that the current decreases with the increasing temperature at the reverse bias, which suggests that the rectifying behavior may be attributed to the strain-assisted tunneling mechanism. The effect of strain or film thickness on the transport property is also discussed.

Revealing optically induced magnetization in SrTiO3 using optically coupled SQUID magnetometry and magnetic circular dichroism

In this work, we study the time- and temperature-dependence of optically induced magnetization in bulk crystals of slightly oxygen-deficient SrTiO$_{3-\delta}$ using an optically coupled SQUID magnetometer. We find that a weak ($\sim$5$\times$10$^{-7}$~emu) but extremely long-lived (hours) magnetic moment can be induced in SrTiO$_{3-\delta}$ at zero magnetic field by circular-polarized sub-bandgap light. We utilize this effect to demonstrate that SrTiO$_{3-\delta}$ crystals can be used as an optically addressable magnetic memory by writing and subsequently reading magnetic patterns with light. The induced magnetization is consistent with that of a polarized ensemble of independent oxygen-vacancy-related complexes, rather than from collective or long-range magnetic order.

Photocarrier injection and photo-resistance in SrTiO3−δ/GaAs p-n junctions

Heterojunctions composed of n-type oxygen-deficient SrTiO3−δ and p-type GaAs were fabricated using pulsed laser deposition method. The good crystallinity of SrTiO3−δ was confirmed by X-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy. These heterojunctions exhibited excellent rectifying behavior from 40 K to room temperature. The photocarrier injection effect and a large photo-resistance were observed in a wide temperature range. The photo-resistance is nearly 100% at low temperatures and ∼40% at room temperature under −0.5 V bias. Strong dependences on both temperature and bias voltage were found as well, which might be understood by considering the band structure of the formed p-n junction.