Oxygen Vacancies In SrTiO3 Research Articles

Tuning oxygen vacancies in complex oxides using 2D layered materials

Abstract The hybrid interface between 2D materials and complex oxides offers a rich platform to explore fascinating physical phenomena like helical edge states, broken-symmetry phases, and giant magnetoresistance. While current research primarily focuses on the influence of complex oxides on layered 2D materials, the reverse - how layered 2D materials affect complex oxides - remains largely unexplored. Here, we examined the impact of graphene layers on the formation of oxygen vacancies in SrTiO3 (STO) during high-temperature annealing. Our findings, supported by Raman spectroscopy and X-ray photoelectron spectroscopy, indicate that increasing the number of graphene layers progressively leads to a reduced oxygen vacancy content in STO, demonstrating the efficacy of graphene in modulating oxygen vacancy formation in bulk STO. Additionally, using photoluminescence, we showed that graphene layers can tune the in-gap states induced by oxygen vacancies in STO. Our first principal calculations further revealed that graphene layers increase the energy barrier for the outward diffusion of oxygen atoms, thereby inhibiting the formation of oxygen vacancies in STO. These results highlight a new route for tailoring the physical properties of complex oxides by engineering the interface with layered 2D materials.

Competing conduction mechanisms for two-dimensional electron gas at LaTiO3/SrTiO3 heterointerfaces

In pursuit of understanding the mechanism of two-dimensional electron gas (2DEG), artificial heterostructures based on SrTiO3 have sparked a wealth of exciting experimental and theoretical results. However, to date, the physical origin of this phenomenon still remains controversial. In our endeavor to unravel the mystery, we have synthesized a series of LaTiO3+δ/SrTiO3 films by controlling growth temperature and oxygen pressure, respectively. X-ray absorption results identify the transition of Ti ions from Ti3+ to Ti4+. It is proved that electrical transport properties in the LaTiO3/SrTiO3 film are dominated by oxygen vacancies in SrTiO3. In addition, the metallic behavior observed in the amorphous LaTiO3/SrTiO3 film suggests contributions from growth-induced donor defects to 2DEG. Our work presents direct evidence of competing conduction mechanisms for the observed 2DEG at the LaTiO3/SrTiO3 interface and highlights the important role of oxygen diffusion from the substrate SrTiO3. Meanwhile, we emphasize the significance of LaTiO3 as an oxygen scale meter to investigate interfacial properties of oxide heterostructures.

Laser-induced structural modulation and superconductivity in SrTiO3

Under normal conditions, stoichiometric SrTiO3 is an excellent dielectric. It shows a structural phase transition, from cubic to tetragonal, below 105 K. In this structure, well separated domains hosting tetragonal phases with different long axes exist giving rise to the so-called X, Y, and Z domains. At very low temperatures, it becomes a quantum paraelectric in which local ferroelectric domains are found at the X, Y, and Z domain boundaries. Creation of oxygen vacancy in SrTiO3 makes it conducting with low carrier density which also undergoes an unconventional superconducting transition at sub-kelvin temperatures. We have created structural phase separation with clear domain boundaries (as in the X, Y, and Z domains) at room temperature on single crystals of SrTiO3 by irradiating the surface with high power density excimer laser pulses. We find that the domain boundaries are dominantly conducting, and the irradiated crystals undergo a superconducting phase transition below 180 mK indicating that the superconducting phase appears at the domain boundaries. This concurrence of local ferroelectricity and superconductivity in lightly doped SrTiO3 supports a ferroelectric fluctuation mediated Cooper pairing in the system. The results also point out the possibility of controlling ferroelectricity and superconductivity in functional electronic devices through surface engineering.

Favoring Product Desorption by a Tailored Electronic Environment of Oxygen Vacancies in SrTiO3 via Cr Doping for Enhanced and Selective Electrocatalytic CO2 to CO Conversion.

The electrochemical CO2 reduction reaction (ECO2RR) into value-added products is crucial to address the herculean task of CO2 mitigation. Several efforts are being made to develop active ECO2RR catalysts, targeting enhanced CO2 adsorption and activation. A rational design of ECO2RR catalysts with a facile product desorption step is seldom reported. Herein, ensuing the Sabatier principle, we report a strategy for an enhanced ECO2RR with a faradaic efficiency of 85% for CO production by targeting the product desorption step. The energy barrier for product desorption was lowered via a tailored electronic environment of oxygen vacancies (Ovac) in Cr-doped SrTiO3. The substitutional doping of Cr3+ for Ti4+ into the SrTiO3 lattice favors the generation of more Ovac and modifies the local electronic environment. Density functional theory analysis evinces the spontaneous dissociation of COOH# intermediates over Ovac and lower CO intermediate binding on Ovac reducing the energy demand for CO release due to Cr doping.

Enhance the stability of oxygen vacancies in SrTiO3 via metallic Ag modification for efficient and durable photocatalytic NO abatement

Oxygen vacancy engineering is an appealing strategy in the direction of photocatalytic pollutant purification. Unfortunately, the short lifetime of oxygen vacancies significantly limits photocatalytic efficiencies and their application. Herein, we report that such a scenario can be resolved via plasmonic silver metal modification SrTiO3 containing oxygen vacancies, which can achieve a high NO removal rate of 70.0% and long stability. This outstanding photocatalytic activity can be attributed to the increased optical response range and carrier separation by metallic Ag with the unique character of localized surface plasmonic resonance (LSPR) effect. Moreover, the intrinsic mechanism of how the plasmonic metal could enhance the stability of oxygen vacancies is proposed. The plasmon-driven hot carriers inject SrTiO3 support that promotes the regeneration of oxygen vacancies around the interface, meanwhile, the introduction of Ag nanoparticles prevents the oxygen vacancies from being filled by the reactant. This work elucidates the unique role of plasmonic metal in photocatalysis, providing an innovative idea for improving catalytic stability.

White electroluminescence of diamond/boron/diamond/SrTiO3 composite film

Diamond/boron/diamond/SrTiO3 composite film (BDDSCF) electroluminescent (EL) devices were deposited by microwave plasma chemical vapor deposition (MPCVD) technology and vacuum electron beam vapor deposition (EBVD) technology. The samples were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDXS). The current-voltage (I–V) characteristic of the sample was measured by an adjustable direct current power supply. The EL spectra of the samples obviously exhibit three emission peaks, which are centered at 440 nm (blue area), 592 nm (yellow area) and 736 nm (red area). In the indoor environment, it can be observed by the naked eye that the surface of the sample will emit uniform and high brightness white light. The blue peak is from the recombination of the conduction band electron in SrTiO3 film and the quasi-acceptor (trapped hole) associated with oxygen vacancy, the yellow peak is caused by the self-trapping exciton (STE) recombination caused by oxygen vacancies in SrTiO3, and the red peak is attributed to the silicon impurity centers or carbon neutral vacancy in diamond films.

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.

Analysis of flexochemical effect and its application in scanning probe microscopy

Electrochemical processes in solids are affected by the properties of various interfaces, where the flexoelectric effect manifests itself considerably due to the inevitable strong gradient fields. Thus, it is crucial to study the coupling between the electrochemical process and the flexoelectric effect. Based on the continuum theory, we conducted the finite element implementation for the flexochemical effect, being the coupling between flexoelectricity, Vegard effect and chemical reactions. Then, the developed method is employed to investigate the flexochemical effect arising in scanning probe microscopy (SPM), including evaluating the contributions from the flexoelectric effect and Vegard effect to the electromechanical response on material SrTiO3 (STO) in piezoresponse force microscopy (PFM) as well as to mechanical redistribution of oxygen vacancy in STO. It is found that at room temperature the nanoscale electromechanical response of the undoped STO in PFM imaging is mainly induced by the converse flexoelectricity while the contribution of direct Vegard effect is negligible. Furthermore, the contact force exerted by SPM tip in manipulating the redistribution of oxygen vacancies is multifunctional, including diminishing vacancies underneath the contact area and enriching the regions around the tip-surface contact edge and inside the sample below the tip, resulting from the synergy of the converse Vegard effect and the direct flexoelectricity. These analyses explain some experimental observations well. This paper provides a continuum framework for the analysis of electrochemomechanical systems with the flexoelectric effect.

Imaging Se diffusion across the FeSe/ SrTiO3 interface

Monolayer FeSe on SrTiO$_3$ superconducts with reported $T_\mathrm{c}$ as high as 100 K, but the dramatic interfacial $T_\mathrm{c}$ enhancement remains poorly understood. Oxygen vacancies in SrTiO$_3$ are known to enhance the interfacial electron doping, electron-phonon coupling, and superconducting gap, but the detailed mechanism is unclear. Here we apply scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to FeSe/SrTiO$_3$ to image the diffusion of selenium into SrTiO$_3$ to an unexpected depth of several unit cells, consistent with the simultaneously observed depth profile of oxygen vacancies. Our density functional theory (DFT) calculations support the crucial role of oxygen vacancies in facilitating the thermally driven Se diffusion. In contrast to excess Se in the FeSe monolayer or FeSe/SrTiO$_3$ interface that is typically removed during post-growth annealing, the diffused Se remains in the top few unit cells of the SrTiO$_3$ bulk after the extended post-growth annealing that is necessary to achieve superconductivity. Thus, the unexpected Se in SrTiO$_3$ may contribute to the interfacial electron doping and electron-phonon coupling that enhance $T_\mathrm{c}$, suggesting another important role for oxygen vacancies as facilitators of Se diffusion.

Electrochemical potential controlling preparation of oxygen vacancies modified SrTiO3 with Ti3+ and Ti2+ self-doping in molten salt

Self-doping of Ti3+ and Ti2+ in oxygen vacancies modified SrTiO3 crystal could significantly improve its photocatalytic properties. In this paper, a series of dark gray oxygen vacancies modified SrTiO3 with self-doping of Ti3+ and Ti2+ were synthesized by electrochemical reduction of SrTiO3 in molten SrCl2–NaCl. The samples were characterized by XRD, SEM, and UV–vis diffuse reflectance spectroscopy, respectively. The valence state of Ti ions on the surface of sample and its oxygen vacancies types were analyzed by XPS spectrum. XRD patterns showed that the diffraction angle (2θ) of samples shifted towards higher 2 theta as the increased cell voltage and the volume of the crystal decreased. XPS spectra indicated that the concentration of Ti3+ ions and oxygen vacancies in SrTiO3 increased as the increased cell voltage. The concentration of Ti3+ and oxygen vacancies in SrTiO3 obtained at cell voltage of 2.9 ​V were 66% and 24%, respectively. The absorption characteristic peak of the self-doping of Ti2+ in SrTiO3 was showed when the cell voltage exceeded 2.7 ​V. The optical absorption of the SrTiO3 extended to the visible light and its bandgap energy narrowed.

Oxygen vacancy and photoelectron enhanced flexoelectricity in perovskite SrTiO3 crystal

Photo-enhanced flexoelectricity or flexoelectricity-enhanced photovoltaic effect, named photo-flexoelectric, is an interesting topic and has application potential in photo-electro-mechanical devices. However, this effect is far from being well understood. In this work, we demonstrate the photoflexoelectric effect in perovskite-structured SrTiO3 (STO) single crystal and reveal the coupling mechanism between its photovoltaic and flexoelectric effect. Driven by the flexoelectric field, light-induced electrons can tunnel through the Schottky junction at the Au/STO interface, giving rise to enhanced flexoelectricity, i.e., photoflexoelectric effect. Thermal annealing in vacuum induces oxygen vacancies in STO and results in stronger light absorption and enlarged photoflexoelectric effect.

Unveiling Temperature-Induced Structural Domains and Movement of Oxygen Vacancies in SrTiO3 with Graphene.

Heterointerfaces coupling complex oxides exhibit coexisting functional properties such as magnetism, superconductivity, and ferroelectricity, often absent in their individual constituent. SrTiO3 (STO), a canonical band insulator, is an active constituent of such heterointerfaces. Temperature-, strain-, or mechanical stress-induced ferroelastic transition leads to the formation of narrow domains and domain walls in STO. Such ferroelastic domain walls have been studied using imaging or transport techniques and, often, the findings are influenced by the choice and interaction of the electrodes with STO. In this work, we use graphene as a unique platform to unveil the movement of oxygen vacancies and ferroelastic domain walls near the STO surface by studying the temperature and gate bias dependence of charge transport in graphene. By sweeping the back gate voltage, we observe antihysteresis in graphene typically observed in conventional ferroelectric oxides. Interestingly, we find features in antihysteresis that are related to the movement of domain walls and of oxygen vacancies in STO. We ascertain this by analyzing the time dependence of the graphene square resistance at different temperatures and gate bias. Density functional calculations estimate the surface polarization and formation energies of layer-dependent oxygen vacancies in STO. This corroborates quantitatively with the activation energies determined from the temperature dependence of the graphene square resistance. Introduction of a hexagonal boron nitride (hBN) layer, of varying thicknesses, between graphene and STO leads to a gradual disappearance of the observed features, implying the influence of the domain walls onto the potential landscape in graphene.

Tensile strain for band engineering of SrTiO3 for increasing photocatalytic activity to water splitting

SrTiO3 is a well-known highly active photocatalyst with high energy conversion efficiency. In this study, we investigated the formation of oxygen vacancy by using the chemo-mechanical effect that was introduced by the dispersion of metal particles into grain and photocatalytic activity to water splitting. Au dispersion on SrTiO3 followed by sintering treatment was studied for introduction of chemo-mechanical strain because of a different thermal expansion coefficient; the introduced chemo-mechanical strain generated oxygen vacancy in SrTiO3. Thus, induced chemo-mechanical strain shows change in electronic band structure resulting in increasing lowest unoccupied molecular orbital (LUMO) level with increasing Au content. Since photoluminescence was significantly decreased by sintering after Au dispersion, the introduced strain effects may work for increasing a charge separation efficiency and adsorption site in water splitting. Therefore, the photocatalytic activity was much increased by sintering treatment after Au dispersion on SrTiO3.

Characterization of oxygen vacancies in SrTiO3 by means of anelastic and Raman spectroscopy

Oxygen vacancies in reduced SrTiO3 are investigated by Raman spectroscopy and dynamic elastic modulus measurements. The anelastic spectrum is used to evaluate the amount and state of the O vacancies, which result to be almost completely paired and otherwise aggregated at a concentration of 0.5 mol. %, with only 1/6 of them isolated at room temperature. Raman spectra recorded as a function of temperature in as-grown and reduced samples are examined regarding the different processes which can contribute to them. Chemometric analysis is used to highlight the specific influence of oxygen vacancies on the Raman spectra. It is shown that oxygen vacancies induce a relaxation of selection rules leading to the activation of first-order lines even in the cubic phase. Furthermore, regions around oxygen vacancies are demonstrated to be responsible for the asymmetric broadening of Raman lines.

Unravelling oxygen-vacancy-induced electron transfer at SrTiO3-based heterointerfaces by transport measurement during growth

Numerous studies have shown that oxygen vacancies play an important role on the formation of two-dimensional electron gas (2DEG) at SrTiO3-based heterointerfaces. Previously, it is widely believed that the main mechanism is that the oxygen vacancies in SrTiO3 directly contribute electrons to the 2DEG. Here, we performed transport measurements during the creation of 2DEG for depositing amorphous LaAlO3 on SrTiO3 substrates and related heterostructures. Our result suggests that, unlike the previous viewpoint, in this kind of 2DEG the determinant mechanism is the electron transfer from the oxygen vacancies in the film grown on SrTiO3, rather than the oxygen vacancies in SrTiO3 themselves. This effect is so striking that an amorphous film of less than 10% monolayer coverage on SrTiO3, or equivalently 0.04 nm, can already generate a highly conducting 2DEG. The present result may have a general implication and provide a possible way to understand the long-standing debate on the origin of 2DEG at SrTiO3-based heterointerfaces.

Ambient dependence of visible emissions in SrTiO3

We investigated the visible emission property of SrTiO3 (STO) single crystals with high temperature annealing in some ambient conditions. We found that the green emission in STO, which should be associated with intermediate states originating from functional ionic defects inside the samples, such as cation/oxygen vacancies, showed strong ambient dependence. While high temperature annealing in the O2 atmosphere suppressed the intensity of visible emission, annealing in an O2-free atmosphere, such as N2 or H2, increased it. The broad visible emissions were fitted with three sub-modes, whose intensities showed different evolutions with respect to the ambient condition. Our study demonstrated the systematic development of defect states with the amount of the oxygen vacancies in STO.

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

Abstract

Oxygen vacancy revived phonon-glass electron-crystal in SrTiO3

Controlling oxygen vacancy in undoped SrTiO3 was used to realize the phonon-glass electron-crystal (PGEC) concept. The undoped SrTiO3 samples were synthesized through solid-state-reaction method in air and then reduced through annealing under low oxygen partial pressure conditions. We show that decreasing the oxygen partial pressure (Po2) results in increase of the electrical conductivity, and the oxygen vacancies act as phonon scattering centers, which lead to decrease in the lattice thermal conductivity. Both the enhanced electrical conductivity (σ) and reduced thermal conductivity (κ) were achieved simultaneously through oxygen vacancy in undoped SrTiO3. This significantly increases the ratio of σ/κ that results in an enormously enhanced ZT value of about 13 times larger as compared with the samples reduced under relatively high Po2. Density-functional theory calculations also reveal that oxygen vacancies in SrTiO3 reduce the band-gap, and consequently increase the electrical conductivity.

Large positive linear magnetoresistance in the two-dimensional t2g electron gas at the EuO/SrTiO3 interface

The development of novel nano-oxide spintronic devices would benefit greatly from interfacing with emergent phenomena at oxide interfaces. In this paper, we integrate highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy results in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70 K of EuO, this spin-polarized two-dimensional t2g electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance (MR). Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the t2g nature of the carriers. First principles calculations strongly suggest that Zeeman splitting, caused by proximity magnetism and oxygen vacancies in SrTiO3, is responsible for the MR. This system offers an as-yet-unexplored route to pursue proximity-induced effects in the oxide two-dimensional t2g electron gas.

2D hole gas seen.

A p-type metallic sheet forms between two oxide insulators, LaAlO3 and SrTiO3. Suppression of oxygen vacancies in SrTiO3 plays a critical role in forming this sheet.