Structural and electronic impact of SrTiO3 substrate on TiO2 thin films

We demonstrate that the atomic structures, electronic states, and bonding nature of the interface between SrTiO₃ substrate and anatase TiO₂ thin films could be related and technologically manipulated at the atomic level. Applying advanced transmission electron microscopy, the grown anatase TiO₂ thin films are found to make a clean and direct contact to the SrTiO₃ substrates in an epitaxial, coherent, and atomically abrupt way. The atomic-resolution microscopic images reveal that the interface comprises SrO-terminated SrTiO₃ and Ti-terminated TiO₂ with the interfacial Ti of TiO₂ sitting above the hollow site, which is confirmed theoretically to be the most energetically favorable. Quantitatively, the first-principles calculations predict that the oxygen sublattice at the interface undergoes a notable reconstruction, i.e., the interfacial O atoms of TiO₂ are displaced largely toward the SrO plane of the SrTiO₃, flattening the originally zigzag TiO₂ atomic chains. Consequently, the interfacial layers suffer a remarkable modification in the charge accumulation and also a deviation in the density of states from their bulk counterparts, indicating that the substrate can have an impact on the deposited thin films electronically. Using several analytic methods, the SrTiO₃/TiO₂ interface is found to take on a metallic nature, and the interfacial bonding is determined to be of a mixed covalent and ionic character. This combined experimental and theoretical investigation gains insight into the complex atomic and electronic structures of the buried interface, which are fundamental for relating the atomic-scale structures to their properties on a quantum level.     

Structure–Property Relation of SrTiO3/LaAlO3 Interfaces

A large variety of transport properties have been observed at the interface between the insulating oxides SrTiO₃ and LaAlO₃ such as insulation, 2D interface metallicity, 3D bulk metallicity, magnetic scattering, and superconductivity. The relation between the structure and the properties of the SrTiO₃/LaAlO₃ interface can be explained in a meaningful way by taking into account the relative contribution of three structural aspects: oxygen vacancies, structural deformations (including cation disorder), and electronic interface reconstruction. The emerging phase diagram is much richer than for related bulk oxides due to the occurrence of interface electronic reconstruction. The observation of this interface phenomenon is a display of recent advances in thin film deposition and characterization techniques, and provides an extension to the range of exceptional electronic properties of complex oxides.     

Dielectric properties and relaxation behavior of Sm substituted SrTiO3 ceramics

Sr_1–1.5xSm_xTiO₃ (x = 0–0.025) ceramics were fabricated by the conventional solid-state reaction method. Experimental results show that all ceramics are pure cubic perovskite structure, and the lattice parameters and average grain sizes of the ceramics decrease with the Sm concentration increasing. Compared with the pure SrTiO₃ ceramics, the relative dielectric constant could be enhanced to 3,681 (at 1 kHz and room temperature)with dielectric loss less than 0.02 when x = 0.02. The good stability of ε _r under 0–20 kV/cm is beneficial to applying in high voltage conditions. Two sets of relaxation peaks of Sm doped SrTiO₃ ceramics are observed which are all related to the oxygen vacancies. The medium-temperature relaxation is ascribed to the coupling of oxygen vacancies with strontium vacancies and the origin of high-temperature relaxation is the motion of oxygen vacancies.     

Surface Pyroelectricity in Cubic SrTiO3

Symmetry‐imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO₃ is not pyroelectric, the (100) surface of TiO₂‐terminated SrTiO₃ is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO₃. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO₂ layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO₂ capping layer. The observation of pyroelectricity emerging at the SrTiO₃ surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo‐ and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.     

Morphology and oxygen vacancy investigation of strontium titanate-based photo electrochemical cells

In this paper single band gap photo electrochemical cells (PECs) are presented, which consist of strontium titanate (SrTiO₃) photo anodes on nickel cathodes in potassium hydroxide electrolyte. SrTiO₃ powders are deposited on nickel substrates by electrophoresis before sintering with varying temperatures, times, cooling rates, gas types, and gas flow rates. The external quantum efficiency (EQE) of such PECs mainly depends on the morphology and the amount of oxygen vacancies in SrTiO₃ lattice. At first, the morphology is investigated, which can be adjusted by the particle size as well as the sinter temperature and time. Nanopowder-based PECs sintered above the starting sinter temperature indicate the best charge carrier transport and hence allow high EQEs. The sinter time influences the specific surface area, but not the EQE in this investigation. Secondly, the generation of oxygen vacancies is investigated, which depends on the oxygen partial pressure and the equilibration temperature. Low oxygen partial pressures and high equilibration temperatures increase the amount of oxygen vacancies, which can be set by the gas type and its flow rate or the cooling rate and an additional heating step, respectively. It can be shown that PECs have to possess a low amount of oxygen vacancies to reach high EQE values, but not too low to allow for sufficient conductivity. This point is shown through our finding that the samples with lower and higher concentrations exhibit very low photo activity. Oxygen vacancies can be considered as intrinsic donors and hence increase electrical conductivity which is necessary but also act as recombination centers. For SrTiO₃ nanopowder-based samples, which have been sintered at 1200 °C for 20 min with a cooling rate of 10 K/s in reducing gas (with 5 vol% H₂) and a low flow rate of 1.7 l/h, very high external quantum efficiencies of 64.2 % under 365 nm illumination can be achieved.     

The origin of varistor property of SrTiO3-based ceramics

The role of oxygen in the heat-treatment process of SrTiO₃ varistor ceramics has been investigated in this paper. The varistor voltage of SrTiO₃ ceramics has been found to be independent of the sample thickness and it increases with the heat-treatment temperature. It has been further revealed that the dielectric property is mainly governed by a highly resistive surface layer. The XPS results of Mn 2p and O 1s suggest that the surface layer is formed by oxygen diffusion and surface chemisorption at grain boundaries during the heat treatment in air. The chemisorption of oxygen in grain boundaries, which leads to the highly resistive surface layer, is the origin of the varistor property of SrTiO₃ ceramics.     

SrTiO3材料中氧空位的密度泛函理论

计算了SrTiO3-δ（δ=0，δ=0．125）体系电子结构，分析了氧空位对SrTiO3晶体的价键结构、能带、态密度、分波态密度、差分电荷密度的影响。所有计算都是基于密度泛函理论（DFT）框架下的第一性原理平面波超软赝势方法。计算结果表明：当氧空位浓度δ=0．125时，空位在母体化合物SrTiO3中引入了大量的传导电子，费米能级进入导带，体系显示金属型导电性。由于空位掺杂，导带底附近的态密度发生了畸变，刚性能带模型不再适合描述SrTiO2.875体系。同时，在导带底附近距离费米能级0．3eV处引入了空位能级，这和实验测得的SrTiO3材料内中性氧空位的电离能约为0．4eV相符。此外，Mulliken布局分析、分波态密度和差分电荷密度分析表明，该空位能级主要由与其最近邻的两个Ti原子的3d电子态贡献，并且由该空位引入的导电电子大部分都局域在空位最近邻的两个Ti原子周围。最后，计算了三种典型平衡条件下SrTiO3晶体内中性氧空位的形成能。     

SrTiO3 glass ceramics

This is the first in a series of two papers describing the crystallization and dielectric properties of glass-ceramics derived from a particular strontium titanium aluminosilicate glass composition. This first paper concerns the development of crystalline phases and microstructure of glass-ceramics prepared under various crystallization conditions. In the following paper, the dielectric properties of these glass-ceramics are described and correlated with the characterization results.Perovskite strontium titanate (SrTiO₃) was the primary crystalline phase in glass-ceramics crystallized over the temperature range of 800 to 1100° C. At crystallization temperatures below 950° C, SrTiO₃ formed with a spherulitic or dendritic growth habit. X-ray diffraction suggested that the SrTiO₃ crystallized in a perovskite-like precursor phase which transformed to perovskite SrTiO₃ with further crystallization time. However, electron diffraction indicated that this precursor phase was cubic perovskite SrTiO₃. At higher crystallization temperatures, perovskite SrTiO₃ was present as individual crystallites without evidence of the spherulitic habit. The crystallization of SrTiO₃ was followed by that of other phases, the hexacelsian and anorthite forms of SrAl₃Si₂O₈, and the rutile and anatase forms of TiO₂. The crystallization sequence and microstructure of the glass-ceramics were determined by the competition for strontium and titanium between the crystallizing phases, SrTiO₃ and SrAl₂Si₂O₈, and TiO₂.     

Solid-state dewetting of Pt on (100) SrTiO3

Continuous thin films of Pt on (100) SrTiO₃ substrates were dewetted to form Pt particles at 1,150 °C, using an oxygen partial pressure of 10^?20 atm. After retraction of thick (50 or 100 nm) Pt films, SrTiO₃ anisotropic rods, slightly depleted in Ti, were found on the surface of the substrate. Rods did not form after dewetting of thinner (10 nm) Pt films. After dewetting, a ~10 nm thick interfacial phase was found between the Pt and the SrTiO₃. The interfacial phase, based on Sr and containing ~25 at% oxygen, is believed to be a transient state, formed due to Ti depletion from the substrate, resulting in a Pt(Ti) solution in the particles. The interfacial phase forms due to the low oxygen partial pressure used to equilibrate the system, and is expected to influence the electrical properties of devices based on Pt–SrTiO₃.     

Luminescent impurity ion probe and low temperature phase of SrTiO3 nanoparticles

Abstract

Optical absorption and photoluminescence of Cr³⁺ impurity ion probe were studied on nanocrystalline SrTiO₃/Cr(0·1%) powders with an average particle size between 13 and 100 nm prepared by the Pechini type polymeric sol–gel method. Only O_h¹ cubic perovskite phase was revealed in the powders at room temperature. The optical absorption edge and the position of the zero-phonon emission R-line of the octahedral Cr³⁺ centres shifted to higher energies with decreasing particle size. The temperature shift of the R-line position appeared for all powders nearly the same as the 'dielectric related' one in the bulk crystals, evidencing that SrTiO₃ retains quantum paraelectric behaviour down to a particle size of about 10 nm. However, behaviour of the R-line position was unstable at low temperatures in powders with a particle size of about 13 and 20 nm. This instability was speculated as a manifestation of a possible low temperature ferroelectric phase transition in small enough SrTiO₃ nanoparticles.     

Synthesis of SrO(SrTiO3)n (n = 1, 2, ∞) compounds and electronic structure analysis

SrO(SrTiO₃)_n compounds were prepared by a modified sol-gel self-propagating combustion, which is a low-temperature combustion synthesis procedure using microwave-assisted sol-gel as precursors. The thermal treating conditions were determined by DTA/TG analysis of the powders. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the sample characterization. The results confirm that the high temperature and long reaction time which occur in classical solid-state reaction method are avoided. The band structure, total density of states (DOS), and partial density of states (PDOS) of SrO(SrTiO₃)_n were calculated in order to study the electronic structures of SrO(SrTiO₃)_n.     

Sol-Gel derived SrTiO3 and SrZrO3 coatings on SiC and C-fibers

Sols in the systems Sr-Ti-O and Sr-Zr-O have been prepared by an alkoxide sol/gel-process using Strontium acetate, Titanium-isopropoxide and Zirconium-n-propoxide. SiC- and C-fibers were coated with the SrTiO₃ and SrZrO₃ sols. Annealing in N₂-atmosphere at temperatures of 900 and 1100°C resulted in the formation of crack free coatings of monophase SrTiO₃ and SrZrO₃, respectively. The thickness of the coatings ranged from 350 to 500 nm. The SrTiO₃ and SrZrO₃ reaction products were characterized by differential thermal analysis (DTA), thermal gravimetry (TG), X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

The study of optical properties of In2O3 and of mixed oxides In2O3−MoO3 system deposited by coevaporation

A discussion of the optical properties of two systems of dielectric films i.e. In₂O₃ and of mixed oxides In₂O₃−MoO₃ system is presented. Film thickness, substrate temperature, annealing and composition (in molar%) have a profound effect on the structure and optical properties of these films. The decrease in optical band gap with the increase in film thickness of In₂O₃ is interpreted in terms of incorporation of oxygen vacancies in the In₂O₃ lattice. The decrease in optical band gap with the increase in substrate temperature and annealing of In₂O₃ thin films is ascribed to the release of trapped electrons by thermal energy or by the outward diffusion of the oxygen-ion vacancies, which are quite mobile even at low temperature. For the mixed oxides In₂O₃−MoO₃ system the results are found to be compatible with the reduction in the value of optical band gap of these materials as the molar fraction of MoO₃ increases in the In₂O₃ thin films and is attributed to the incorporation of Mo(VI) ions in an In₂O₃lattice that causes the indium orbital to become a little less tightly bound. The decrease in optical band gap of mixed oxides In₂O₃−MoO₃ system, with increasing film thickness is interpreted in terms of incorporation of oxygen vacancies in both In₂O₃ and MoO₃ lattice which are also believed to be the source of conduction electrons in In₂O₃–MoO₃ complex. The decrease in optical band gap with increasing substrate temperature and annealing of mixed oxides In₂O₃−MoO₃ system is due to the increasing concentration of oxygen vacancies, formation of indium and molybdenum species of lower oxidation state and indium interstitials. The blue colouration of mixed oxides In₂O₃–MoO₃ samples is due to the inter-electron transfer from oxygen 2p to molybdenum 4d level due to which Mo species of lower oxidation states are formed.     

Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film using Graphene

Quantum Hall conductance in monolayer graphene on an epitaxial SrTiO₃ (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate‐voltage sweep range is gradually increased in the device, systematic generation and annihilation of oxygen vacancies, evidenced from the hysteretic conductance behavior in the graphene, are observed. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and noninvasive probe for understanding the electronic and ionic phenomena in complex transition‐metal oxides without impairing the oxide layer underneath.     

Fourier transform-infrared and optical studies on sol-gel synthesized SrTiO3 precursor films

SrTiO₃ precursor thin films were deposited on KBr single crystals and fused silica substrates by the sol-gel technique. Fourier transform-infrared (FT-IR) spectra, ultraviolet-visible spectra and X-ray diffraction spectra were recorded after the samples were cured at various temperatures. FT-IR spectra showed a gradual decrease in -OH concentration indicating removal of hydroxyls and reduction in peaks belonging to organic groups indicating the removal of organics. After annealing at 500 °C, the peak due to SrTiO₃ crystallites (500 cm^–1) began to appear, indicating crystallization. The optical spectra showed an increase in refractive index due to densification as the curing temperature increased. With increase in curing temperature, the thickness of the film decreased, indicating densification due to pore collapse. The optical band gap also changed with annealing due to the structural transformation from amorphous to crystalline phase. X-ray diffraction spectroscopy confirmed the structural changes.     

An improved method for preparation of SrTiO3 nanoparticles

SrTiO₃ nanoparticles were synthesized for the first time via a modified polymeric precursor method. The samples were characterized by thermogravimetry, X-ray diffraction (XRD), BET surface area, micro-Raman spectroscopy, field emission scanning and transmission electron microscopy (FE-SEM and FE-STEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence measurements. It is found that calcination atmosphere (air, nitrogen and oxygen) plays an important role of both crystal size and photolumiscence behavior of the SrTiO₃ nanocrystallites. Results show that the powders obtained in nitrogen/oxygen atmosphere possess controllable particles size of approximately 11 nm presenting the highest photoluminescence emission.     

Macroscopic Spontaneous Polarization and Surface Oxygen Vacancies Collaboratively Boosting CO2 Photoreduction on BiOIO3 Single Crystals

Prompt recombination of photogenerated electrons and holes in bulk and on the surface of photocatalysts harshly impedes the photocatalytic efficiency. However, the simultaneous manipulation of photocharges in the two locations is challenging. Herein, the synchronous promotion of bulk and surface separation of photoinduced charges for prominent CO₂ photoreduction by coupling macroscopic spontaneous polarization and surface oxygen vacancies (OVs) of BiOIO₃ single crystals is reported. The oriented growth of BiOIO₃ single-crystal nanostrips along the [001] direction, ensuing substantial well-aligned IO₃ polar units, renders a large enhancement for the macroscopic polarization electric field, which is capable of driving the rapid separation and migration of charges from bulk to surface. Meanwhile the introduction of surface OVs establishes a local electric field for charge migration to catalytic sites on the surface of BiOIO₃ nanostrips. Highly polarized BiOIO₃ nanostrips with ample OVs demonstrate outstanding CO₂ reduction activity for CO production with a rate of 17.33 µmol g⁻¹ h⁻¹ (approximately ten times enhancement) without any sacrificial agents or cocatalysts, being one of the best CO₂ reduction photocatalysts in the gas–solid system reported so far. This work provides an integrated solution to governing charge movement behavior on the basis of collaborative polarization from bulk and surface.     

Dielectric properties of donor-doped polycrystalline SrTiO3

Various donor dopants such as Y³⁺ and Nb⁵⁺ were incorporated in SrTiO₃ in amounts 2.5 mol % during sintering in air at 1450° C for 15 h. Dense ceramic materials with grains of optimum uniformity and largest size were obtained when the cation stoichiometry was adjusted to allow for charge compensation of the donor ions by strontium vacancies. For donor levels 0.3 mol%, the dielectric constant measured at 25° C and 1 kHz was up to two orders of magnitude higher than that of undoped strontium titanate. The increase in permittivity, however, was dependent on grain size, was influenced by the method of electrode application, and was suppressed by the presence of 0.1 mol% Mn in the ceramic. These observations, together with data obtained from electrical measurements at other temperatures and frequencies, were consistent with interpretation of the anomalously high dielectric constant as a boundary-layer effect resulting from semiconducting grains and weakly insulating grain-surfaces. It is suggested that donor doping influences the electrical properties of SrTiO₃ mainly by increasing the volatility of oxygen from the grains during sintering, and by decreasing the rate of re-oxidation during cooling.     

Probing electronic defect states in manganite/SrTiO3 heterostructures by surface photovoltage spectroscopy

The energetic distribution of electronic defect states in oxide heterostructures made of ultrathin lanthanum manganite (La_0.7Ca_0.3MnO₃, La_0.7Ce_0.3MnO₃) films on SrTiO₃ substrates is investigated by surface photovoltage (SPV) spectroscopy. Within a comparative evaluation of the SPV spectra of both the film/substrate structures and pure substrates at different temperatures we were able to elaborate a map of defect states across the SrTiO₃-bandgap and we find that the defect state distribution is mainly affected by the substrate, i.e., no specific film-induced defect states were detected. Possible origins of the defect states are discussed within the framework of a semiconductor band scheme, taking into account complementary photoconductivity and SPV transient data.     

Core level and valence band investigation of WO3 thin films with synchrotron radiation

In this work, the electronic properties of the surface of WO₃ films with thickness of 150 nm, thermally evaporated in high vacuum onto Si(100) substrates and pre-treated in air by a 24-h-long annealing at 300 °C and 500 °C (obtaining polycrystalline monoclinic samples) have been studied by surface and bulk sensitive core level (W 4f) and angle integrated valence band photoemission using synchrotron radiation (ELETTRA Synchrotron). The photon energy ranged from 50 eV to 200 eV. The line shape analysis of W 4f core level spectra has shown that the surface presents a sub-stoichiometric WO₃ component assigned to oxygen vacancies ultimately responsible for the gas sensitivity of this material. Correspondingly, valence band spectra show well-defined metallic states W 5d in the gap and near the Fermi level. The variations of surface chemical composition caused by Ultra High Vacuum annealing, and prolonged exposure to UV beam has been monitored by changes in spectral line shape. A general consequence of annealing in vacuum is the segregation of oxygen from the bulk toward the surface as confirmed by independent scanning tunnelling spectroscopy measurements.