SrTiO3 Surface Research Articles

In‐Situ Annealing of the (110) and (001) Surfaces of SrTiO3 Nanocuboids by High‐Resolution Transmission Electron Microscopy

Thermal annealing of SrTiO3 substrates is used in a wide range of applications, including films and nanoparticle growth, electronic devices, catalysis, and photocatalysis. To predict and control reaction kinetics during high temperature treatments, it is essential to understand dynamics in real time. For this purpose, in‐situ electron microscopy techniques are increasingly used, as structural changes can be monitored at the nanoscale. Herein, the authors report on the surface evolution of the (110) and (001) surfaces of SrTiO3 nanocuboids observed in‐situ during thermal treatment at 800 °C, using aberration corrected high‐resolution transmission electron microscopy. By establishing a threshold for the cumulative electron dose, below which only temperature induced growth occurs, the authors observe the formation of epitaxial, faceted TiO islands, in agreement with previously reported results on the thermal treatment of (001) surfaces of SrTiO3 in reducing environments. When surface evolution is a consequence of the combined effects of electron beam damage and temperature‐induced growth, structural modification of the cuboids is highly dynamic. In this case formation of faceted islands occurs rapidly in conjunction with sublimation of the cuboids at considerably larger speeds than in the case of thermal treatment.

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.

Observation of Out-of-Plane Spin Texture in a SrTiO_{3}(111) Two-Dimensional Electron Gas.

We explore the second order bilinear magnetoelectric resistance (BMER) effect in the d-electron-based two-dimensional electron gas (2DEG) at the SrTiO_{3}(111) surface. We find evidence of a spin-split band structure with the archetypal spin-momentum locking of the Rashba effect for the in-plane component. Under an out-of-plane magnetic field, we find a BMER signal that breaks the sixfold symmetry of the electronic dispersion, which is a fingerprint for the presence of a momentum-dependent out-of-plane spin component. Relativistic electronic structure calculations reproduce this spin texture and indicate that the out-of-plane component is a ubiquitous property of oxide 2DEGs arising from strong crystal field effects. We further show that the BMER response of the SrTiO_{3}(111) 2DEG is tunable and unexpectedly large.

Low‐angle twist grain boundary in SrTiO3 fabricated by spark plasma sintering techniques

AbstractA SrTiO3 bicrystal with a low‐angle twist grain boundary was fabricated using the spark plasma sintering (SPS) instrument. The atomic and electronic structure of the grain‐boundary core was characterized using scanning transmission electron microscopy techniques. It was determined that the boundary is comprised of 2 types of defects with distinct electronic structures: screw dislocations and dislocations with an [001] edge component. The dislocations with an [001] edge component dissociated into 2 partial dislocations, separated by a stacking fault consisting of 2 Ti–O layers. The screw dislocations are attributed to the twist component of the grain boundary, while dislocations with an [001] edge component are attributed to surface steps on the original (100) SrTiO3 surfaces prior to diffusion bonding. The observed repeat distances between the dislocations with edge components along the grain‐boundary plane are smaller than those discovered during traditional diffusion bonding experiments. The higher planar defect density observed in this study results partly from higher heating rates, lower processing temperatures, and shorter holding times during SPS processing.

Guideline to atomically flat TiO2-terminated SrTiO3(001) surfaces

The use of SrTiO3(001) substrates with single TiO2- or SrO-terminated surface is often required in order to grow high quality epitaxial thin films. Many surface preparation recipes can be found in the literature, based on different etching solutions (acid, neutral and basic pH) and combined with different annealing treatments. In this work, we compare three different methods usually used to obtain atomically flat TiO2-terminated SrTiO3 surfaces in which water, aqua regia (HCl-HNO3) and BHF (NH4F-HF) solutions are used as reactive agent. Atomic force microscopy measurements (recorded in topography, phase detection and Kelvin-probe force modes) show that using the BHF solution leads to a reliable selective dissolution of SrO on the substrate surface, thus allowing to efficiently obtain atomically flat single TiO2-terminated surfaces. The aging of the resulted surfaces as well as the possibility of layer-by-layer deposition atop is also discussed.

Maximising the resolving power of the scanning tunneling microscope

The usual way to present images from a scanning tunneling microscope (STM) is to take multiple images of the same area, to then manually select the one that appears to be of the highest quality, and then to discard the other almost identical images. This is in contrast to most other disciplines where the signal to noise ratio (SNR) of a data set is improved by taking repeated measurements and averaging them. Data averaging can be routinely performed for 1D spectra, where their alignment is straightforward. However, for serial-acquired 2D STM images the nature and variety of image distortions can severely complicate accurate registration. Here, we demonstrate how a significant improvement in the resolving power of the STM can be achieved through automated distortion correction and multi-frame averaging (MFA) and we demonstrate the broad utility of this approach with three examples. First, we show a sixfold enhancement of the SNR of the Si(111)-(7 × 7) reconstruction. Next, we demonstrate that images with sub-picometre height precision can be routinely obtained and show this for a monolayer of Ti2O3 on Au(111). Last, we demonstrate the automated classification of the two chiral variants of the surface unit cells of the (4 × 4) reconstructed SrTiO3(111) surface. Our new approach to STM imaging will allow a wealth of structural and electronic information from surfaces to be extracted that was previously buried in noise.

Origin of H2 Formation on Perfect SrTiO3 (001) Surface: A First-principles Study

We systematically investigate saturated adsorption of H atoms on the SrTiO3 (001) surface to reveal the origin of H2 molecule formation using density functional theory methods. We find that H atoms prefer to adsorb on O sites at low coverage, while adsorbing on Ti sites at higher coverage. Interestingly, H on O sites and H on neighboring Ti sites (HTi) dimerize to form H2 molecules provided that enough electrons are doped in the conduction bands of SrTiO3. Bader charges and electronic structures show that negatively charged HTi plays a decisive role in the formation of H2. Both calculated hydrogen coverage and electron density at saturated adsorption are in good agreement with experimental values. Our results provide an illuminating instance of H2 formation on perfect oxide surface, which shall pave the way to further understanding the detailed mechanism of H2 evolution on more complex oxide surfaces.

Chemical intermixing at oxide heterointerfaces with polar discontinuity

The stability of the (4 × 4) reconstruction is studied on SrTiO3(111) and LaTiO3(111) surfaces by scanning tunneling microscopy. While the reconstruction may effectively compensate the surface polarity, the associated large lattice distortion on SrTiO3 with relatively strong polarity has to be relaxed by extra adsorbates. Alternatively, substituting Sr by La can reduce the polarity and stabilize the clean reconstructed surface. This provides the driving force for La segregating towards the surface during the heteroepitaxy of SrTiO3 on LaTiO3-covered SrTiO3(111), and thus, the interface sharpness will be deteriorated. To improve the quality of heterointerfaces where the surface polarity of substrates tends to be increased by the epitaxial films, the growth kinetics must be optimized to play the major role.

Oxygen Vacancy Defect Migration in Titanate Perovskite Surfaces: Effect of the A-Site Cations

Oxygen vacancy formation energies and migration barriers in (001) surfaces of CaTiO3, SrTiO3, and BaTiO3 have been investigated using first principles density functional theory. The degree of distortion within the TiO2 sublattice in the presence of defects and consequently the defect formation energies in these titanate surfaces are determined by the size of the A-site cation (Ca2+ < Sr2+ < Ba2+). This is notably the case for CaTiO3, in which the presence of a vacancy defect leads to a heavily distorted local orthorhombic structure within the (001) slab depending on the defect position, despite the overall cubic symmetry of the material modelled. This effectively leads to the TiO2 sublattice acting as a thermodynamic trap for oxygen vacancy defects in CaTiO3. By contrast, calculated vacancy diffusion pathways in SrTiO3 and BaTiO3 indicate that vacancy diffusion with these larger A-site cations is kinetically and not thermodynamically controlled.

Single-layer TiOx reconstructions on SrTiO3 (111): (√7 × √7)R19.1°, (√13 × √13)R13.9°, and related structures

The atomic structures of two reconstructions, (√7 × √7)R19.1° and (√13 × √13)R13.9°, on the SrTiO3 (111) surface were determined using a combination of density functional theory and scanning tunneling microscopy data and APW + lo density functional theory minimizations and simulations. These reconstructions belong to the same structural family made up of an interconnected, single layer of edge-sharing TiO6 and TiO5[] octahedra. This family of reconstructions between 0.5 and 1.5 excess TiO2, representing the lowest-reported TiO2 coverages for reconstructions on this surface. This family is found to include the previously-solved (2 × 2)a reconstruction. They all follow a simple rule for surface composition, which serves as a tool for better understanding and predicting the structure of other reconstructions of arbitrary surface unit cell size on SrTiO3 (111). This reconstruction family and the calculations of surface energies for different hypothesis structures also shed light on the structure of Schottky defects observed on these reconstructed SrTO3 (111) surfaces.

New Insights into Sensitization Mechanism of the Doped Ce (IV) into Strontium Titanate.

SrTiO3 and Ce4+ doped SrTiO3 were synthesized by a modified sol–gel process. The optimization synthesis parameters were obtained by a series of single factor experiments. Interesting phenomena are observable in Ce4+ doped SrTiO3 systems. Sr2+ in SrTiO3 system was replaced by Ce4+, which reduced the surface segregation of Ti4+, ameliorated agglomeration, increased specific surface area more than four times compared with pure SrTiO3, and enhanced quantum efficiency for SrTiO3. Results showed that Ce4+ doping increased the physical adsorption of H2O and adsorbed oxygen on the surface of SrTiO3, which produced additional catalytic active centers. Electrons on the 4f energy level for Ce4+ produced new energy states in the band gap of SrTiO3, which not only realized the use of visible light but also led to an easier separation between the photogenerated electrons and holes. Ce4+ repeatedly captured photoelectrons to produce Ce3+, which inhibited the recombination between photogenerated electrons and holes as well as prolonged their lifetime; it also enhanced quantum efficiency for SrTiO3. The methylene blue (MB) degradation efficiency reached 98.7% using 3 mol % Ce4+ doped SrTiO3 as a photocatalyst, indicating highly photocatalytic activity.

Out-of-plane interface dipoles and anti-hysteresis in graphene-strontium titanate hybrid transistor

The out-of-plane electric polarization at the surface of SrTiO3 (STO), an archetypal perovskite oxide, may stabilize new electronic states and/or host novel device functionality. This is particularly significant in proximity to atomically thin membranes, such as graphene, although a quantitative understanding of the polarization across graphene–STO interface remains experimentally elusive. Here, we report direct observation and measurement of a large intrinsic out-of-plane polarization at the interface of single-layer graphene and TiO2-terminated STO (100) crystal. Using a unique temperature dependence of anti-hysteretic gate-transfer characteristics in dual-gated graphene-on-STO field-effect transistors, we estimate the polarization to be as large as ≈12 μC cm−2, which is also supported by the density functional theory calculations and low-frequency noise measurements. The anti-hysteretic transfer characteristics is quantitatively shown to arise from an interplay of band bending at the STO surface and electrostatic potential due to interface polarization, which may be a generic feature in hybrid electronic devices from two-dimensional materials and perovskite oxides.

Electronic properties and surface reactivity of SrO-terminated SrTiO3 and SrO-terminated iron-doped SrTiO3

Surface reactivity and near-surface electronic properties of SrO-terminated SrTiO3 and iron doped SrTiO3 were studied with first principle methods. We have investigated the density of states (DOS) of bulk SrTiO3 and compared it to DOS of iron-doped SrTiO3 with different oxidation states of iron corresponding to varying oxygen vacancy content within the bulk material. The obtained bulk DOS was compared to near-surface DOS, i.e. surface states, for both SrO-terminated surface of SrTiO3 and iron-doped SrTiO3. Electron density plots and electron density distribution through the entire slab models were investigated in order to understand the origin of surface electrons that can participate in oxygen reduction reaction. Furthermore, we have compared oxygen reduction reactions at elevated temperatures for SrO surfaces with and without oxygen vacancies. Our calculations demonstrate that the conduction band, which is formed mainly by the d-states of Ti, and Fe-induced states within the band gap of SrTiO3, are accessible only on TiO2 terminated SrTiO3 surface while the SrO-terminated surface introduces a tunneling barrier for the electrons populating the conductance band. First principle molecular dynamics demonstrated that at elevated temperatures the surface oxygen vacancies are essential for the oxygen reduction reaction.

Chemical Synthesis of Multilayered Nanostructured Perovskite Thin Films with Dielectric Features for Electric Capacitors

We report a simple sol–gel method for the preparation of multilayered hybrid thin films composed of nanostructured SrTiO3 (STO) and BaTiO3 (BTO) layers. A nanoporous thin film of STO is prepared by the spin-coating method on Si/SiO2/Ti/Pt substrate. When the BTO is processed, the BTO is crystallized between STO crystal pores. A significant strain field is induced on the interface between the STO and BTO surfaces. The concave shape of the nanopores could enable the realization of a highly strained interfacial heterostructure composed of STO/BTO. Compared to nanostructured STO, BTO, and bilayer STO/BTO films, multilayered STO/BTO films show the highest dielectric constant and a high Curie temperature (Tc) at around 280 °C, which is suitable for electric capacitor applications.

Size-Effect on Electrochemical Hydrogen Evolution Reaction by Single-Size Platinum Nanocluster Catalysts Immobilized on Strontium Titanate

Size-specific chemical properties of metal nanoclusters (NCs) have motivated their applications in catalysis, whose properties were found to be dramatically different from bulk materials. We have reported herein the size-dependent activity of platinum (Pt) NCs for the electrochemical hydrogen-evolution-reaction (HER) in neutral water. The single-size NC catalysts were prepared by the soft-landing of mass-selected Pt NCs on a strontium titanate (SrTiO3) (100) surface using an intensive, size-selective NC source based on high-power impulse magnetron sputtering. Successful fabrications of single-size Pt NC catalysts were confirmed with scanning tunneling microscopy. It was found from electrochemical measurements that catalytic HER activity per unit Pt shows a maximum at Pt30 in the range of n = 1–45. Taking into account the electron affinities of Pt NCs measured by gas-phase anion photoelectron spectroscopy, the reaction mechanism of HER is deduced: the size-specific HER activity originates from the matching of energy levels of Pt NCs with the band structure of SrTiO3 surfaces.

Fabrication of Excellent Heterojunction Assisting by Interfaced Oxygen Vacancy to Improve the Separation Capacity of Photogenerated Carriers

AbstractHeterojunction connecting by interface chemical bonds is a promising method as it can provide more efficient route for photogenerated charge separation. In this work, the oxygen vacancies are introduced into the surface of SrTiO3 by a one‐step hydrogenation method. SrTiO3 with oxygen vacancy (STO) is compounded with Cd0.5Zn0.5S (CZS) by hydrothermal method, resulting in the successful formation of a good heterojunction structure. By comparing with the pristine SrTiO3/CZS composites, it is found that the oxygen vacancies play an important role in the formation of excellent heterojunctions, in addition to the traditional reports as charge traps and adsorption sites or the ability to cause band changes. X‐ray photoelectron spectroscopy (XPS) and high‐resolution transmission electron microscope results disclose that S2− enters the oxygen vacancy on the (110) plane of STO and interacts with the adjacent Ti in the van der Waals force to form the Ti‐O‐S group, which results in the formation of excellent heterojunctions and makes the CZS nanoparticles growth evenly on the STO nanoplates. Furthermore, the formation of excellent heterojunction and the introduction of interfaced oxygen vacancy significantly improve the separation efficiency of photogenerated charge carriers, which dramatically increases the photoelectrochemical performance.

Interaction of SrO-terminated SrTiO3 surface with oxygen, carbon dioxide, and water

The interaction of SrO terminated SrTiO3 surface with molecular carbon dioxide and water has been investigated using first-principle theoretical methods and surface analysis techniques.

Optical Anisotropy of SrTiO3(110) for Different Surface Terminations

In this work we investigate the SrTiO3(110) surface by reflectance anisotropy spectroscopy (RAS) at varying preparation steps including ex situ wet etching, high temperature oxygen annealing, as well as in situ vacuum annealing. Different surface terminations show distinctly different RAS spectra which are correlated to an altered surface stoichiometry measured by X‐ray photoelectron spectroscopy (XPS) and an altered valence band structure measured by UV‐photoelectron spectroscopy (UPS) in cases of conductive samples. We link the changes in the observed RAS spectra to various surface reconstructions, with particular focus of the signature of (3 × 1) and (1 × 4) reconstructed surfaces, and a metallic surface state observed in vacuum annealed SrTiO3.

Catalytic Mechanisms of NO Reduction in a CO–NO Atmosphere at Co- and Cu-Doped SrTiO3(100) Surfaces

The NO reduction in a CO–NO atmosphere at the Co- and Cu-doped SrTiO3(100) surfaces is investigated by density functional theory calculations. The energy profiles for several mechanisms are computed and compared. On this basis, a rationale for the available experimental data is proposed. Oxygen vacancies formed as intermediate species in the CO oxidation process play a key role in the NO reduction. Their concentration is also relevant because, if low, formation of N2O prevails on that of N2, leading to an incomplete reduction. The formation of oxygen vacancies is favored by Co and Cu impurities, the latter being most effective. Overall, doping promotes the catalytic reduction of NO on SrTiO3.

Core-shell SrTiO3/graphene structure by chemical vapor deposition for enhanced photocatalytic performance

Direct growth of high quality graphene on the surface of SrTiO3 (STO) was realized through chemical vapor deposition (CVD), to construct few-layer ‘graphene shell’ on every STO nanoparticle. The STO/graphene composite shows significantly enhanced UV light photocatalytic activity compared with the STO/rGO reference. Mechanism analysis confirms the role of special core-shell structure and chemical bond (TiC) for rapid interfacial electron transfer and effective electron-hole separation.