Oxygen adatoms at SrTiO3(0 0 1): A density-functional theory study

We present a density-functional theory study addressing the energetics and electronic structure properties of isolated oxygen adatoms at the SrTiO₃(0 0 1) surface. Together with a surface lattice oxygen atom, the adsorbate is found to form a peroxide-type molecular species. This gives rise to a non-trivial topology of the potential energy surface for lateral adatom motion, with the most stable adsorption site not corresponding to the one expected from a continuation of the perovskite lattice. With computed modest diffusion barriers below 1 eV, it is rather the overall too weak binding at both regular SrTiO₃(0 0 1) terminations that could be a critical factor for oxide film growth applications.     

Electronic charge transfer between Au nano-particles and TiO2-terminated SrTiO3(0 0 1) substrate

The interaction between Au nano-particles and oxide supports is recently discussed in terms of the catalytic activities. This paper reports the electronic charge transfer between Au nano-particles and TiO₂-terminated SrTiO₃(0 0 1) substrate, which is compared with that for stoichiometric(S)-, pseudo-stoichiometric(S¹)- and reduced(R)-TiO₂(1 1 0) supports. We observed the photoelectron spectra of Au 4f, O 2s, Ti 3p, and Sr 4p lines and also measured the work functions for Au/oxides supports using synchrotron-radiation light. As the results, all the O 2s, Ti 3p, and Sr 4p lines for Au/SrTiO₃(0 0 1) show lower binding energy shifts in a quite same manner and abrupt increase in the work function is seen in an initial stage. This clearly evidences an electronic charge transfer from the substrate to Au probably due to a much larger work function of Au than SrTiO₃(0 0 1), which leads to an upward band bending (0.3 eV) just like a Schottky contact. Electronic charge transfers also take place at Au/S- and Au/S¹-TiO₂(1 1 0) and Au/R-TiO₂(1 1 0) interfaces, where electrons are transferred from Au to S- and S¹-TiO₂ and from R-TiO₂ to Au, as predicted by ab initio calculations.     

Thickness dependence of the soft ferroelectric mode in SrTiO3 thin films deposited on MgO

We have measured the complex dielectric constants of SrTiO₃ thin films deposited on MgO substrate, by using the broadband terahertz time-domain spectroscopy. The dielectric dispersion of SrTiO₃ thin films with thickness of 1046, 460 and 55 nm has been observed in the frequency range from 0.1 to 8 THz. The dispersion mainly consists of the TO1 ferroelectric soft mode with a slight absorption of the TO2 phonon mode. From the analysis of the obtained dispersion, we found that the soft mode frequency hardens as the thickness of the film becomes thinner, and is extremely large compared with the bulk crystals. The damping of the soft mode is also larger than that of bulk SrTiO₃, which suggests the extrinsic nature of the broadening of the soft mode dispersion. In the thinnest film of 55 nm, even the shape of the dielectric dispersion changes, which may be related to integrated defects near the interface.     

Structural relaxation and metal-insulator transition at the interface between SrTiO3 and LaAlO3

The electronic structure of interfaces between LaAlO₃ and SrTiO₃ is studied using local spin density approximation (LSDA) with intra-atomic Coulomb repulsion (LSDA + U). We find that the nature of the interface metallic states is strongly affected by the type of the structure (sandwich or bilayer) and by the termination surface of LaAlO₃. In all structures the atomic relaxation plays a crucial role in the electronic properties of the system. While in sandwiches the structural relaxation produces a significant polarization in SrTiO₃ and a splitting of Ti 3d_xy orbitals, in AlO₂-terminated bilayers the relaxation occurs primarily in LaAlO₃ and results in an insulator-metal transition which has been observed experimentally with increasing thickness of the LaAlO₃ layer.     

Oxygen assisted deposition of Sr2FeMoO6 thin films on SrTiO3(1 0 0)

Epitaxial thin films of Sr₂FeMoO₆ (SFMO) were prepared by pulsed laser deposition on SrTiO₃(1 0 0) substrate. Thin films have been grown under different gas environments and they were structurally characterised by XRD. In contrast to previous reports, deposition carried out in the presence of a small amount of O₂ with Ar yields high-quality SFMO films with a saturation magnetic moment of 3.8 μ_B. These SFMO films were strained in such a way that they were elongated along the c-axis and compressed in the ab-axes directions. The large low-field magnetoresistance seen in these films has been attributed to the tunneling across the antisite boundaries.     

Growth,structure, and thermal stability of epitaxial BaTiO3 films on Pt(111)

The growth of epitaxial ultrathin BaTiO₃ films upon rf magnetron sputter deposition on a Pt(111) substrate has been studied by scanning tunnelling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy. The BaTiO₃ films have been characterized from the initial stages of growth up to a film thickness of 4 unit cells. The deposited films develop a long-range order upon annealing at 1050 K in UHV. In the submonolayer regime a wetting layer is formed on Pt(111). Thicker films reveal a Stranski–Krastanov-like structure as observed with STM. By XPS a good agreement of the thin film stoichiometry with BaTiO₃ single crystal data is determined. Due to annealing at 1150 K BaTiO₃ forms large two-dimensional islands on the Pt(111) substrate. Different surface structures develop on the islands depending on the O₂ partial pressure during annealing.     

In-situ imaging of the nucleation and growth of epitaxial anatase TiO2(001) films on SrTiO3(001)

The growth of TiO₂ anatase films on Nb‐doped SrTiO₃(001) molecular beam epitaxy has been studied in-situ by scanning tunneling microscopy. We show that the initial growth follows the Stranski–Krastanov mode, where islands form on top of a wetting layer consisting of two monolayers (ML) of TiO₂. The epitaxial islands subsequently nucleate and coalesce into large commonly oriented crystallites. The reconstruction observed by reflection high-energy electron diffraction (RHEED) is shown to result from the coexistence of individual (4 × 1) and (1 × 4) reconstructions present on different crystallite surfaces. The anatase grows in units of bilayers, resulting in a step height of 2 ML (~ 0.5 nm). This result explains the fact that the measured period of the RHEED specular-beam intensity oscillations corresponds to the time required for deposition of 2 ML. Ar ion sputtering and UHV annealing results in a transformation to coexisting (4 × 1) and (1 × 4) reconstructed terraces on individual crystallites, as commonly observed by ex-situ STM studies.     

Energy storage properties of antiferroelectric 0.92NaNbO3-0.08SrZrO3 film on (001)SrTiO3 substrate

The present study demonstrates the fabrication of an antiferroelectric 0.92NaNbO₃-0.08SrZrO₃ film deposited on a SrRuO₃ coated (001)SrTiO₃ single crystal substrate by pulsed laser deposition. In the 0.92NaNbO₃-0.08SrZrO₃ film, the domain with its c-axis aligned with the out-of-plane direction contributed to the stabilization of an antiferroelectric phase under the high electric field. The film had an energy storage density of 2.9 J cm⁻³ and storage efficiency of 67% at room temperature, which kept at 2.5 J cm⁻³ and 50% at high temperature of 150 °C.     

The effect of dispersed SrTiO3 particles on the ionic conduction of (La,Li)Ti2O6

Ionic conductivity of La_4/3 ? y Li_3yTi₂O₆ (LLTO), where y value is 0.21, dispersed by various amount of strontium titanate (SrTiO₃) was measured by complex impedance method, and powder X-ray diffraction (XRD) measurements and scanning electron microscope (SEM) observations were carried out. With increasing SrTiO₃ concentration, the conductivity decreased. At 34 mol%, the Bragg peaks at 11, 26 and 35° in LLTO disappeared in XRD patterns and small non-angulated crystals were shown in SEM. The behavior of ionic conductivity was discussed by using the effective medium approximation (EMA) theory.     

Room temperature ferroelectricity of tetragonally strained SrTiO3 thin films on single crystal Rh substrates

Epitaxial SrTiO₃ thin films were deposited on single crystalline Rh substrates by pulsed laser deposition. The tetragonally stained structure of the SrTiO₃ thin films with a c/a ratio of 1.04 was confirmed by x-ray diffraction experiments. The SrTiO₃ thin films exhibited good ferroelectric properties with a high remanent polarization (2P_r) of 8 μC/cm² and a canonical ferroelectric piezoresponse hysteresis loop at room temperature. We estimated a high activation electric field of about 6.4 MV/cm for domain wall creeping. This activation electric field is higher than that of typical ferroelectric materials such as PbTiO₃.     

Identification of iron oxide phases in thin films grown on Al2O3(0 0 0 1) by Raman spectroscopy and X-ray diffraction

We report on the identification of Fe₃O₄ (magnetite) and α-Fe₂O₃ (hematite) in iron oxide thin films grown on α-Al₂O₃(0 0 0 1) by evaporation of Fe in an O₂-atmosphere with a thickness of a few unit cells. The phases were observed by Raman spectroscopy and confirmed by X-ray diffraction (XRD). Magnetite appeared independently from the substrate temperature and could not be completely removed by post-annealing in an oxygen atmosphere as observed by X-ray diffraction. In the temperature range between 400 °C and 500 °C the X-ray diffraction shows that predominantly hematite is formed, the Raman spectrum shows a mixture of magnetite and hematite. At both lower and higher substrate temperatures (300 °C and 600 °C) only magnetite was observed. After post-annealing in an O₂-atmosphere of 5 × 10^?5 mbar only hematite was detectable in the Raman spectrum.     

Water adsorption on SrTiO3(001): I. Experimental and simulated STM

Density functional theory-based simulations of scanning tunneling micrographs were used for comparison to published experimental images of reconstructed SrTiO₃(001) surfaces. It was found that the addition of dissociatively adsorbed H₂O to the presently accepted structural solution of the 2 × 1 reconstruction is more consistent with the experimental data. A proposed model for the c(4 × 4) reconstruction, based on the hydrated 2 × 1 structure, agrees well with experiment and is consistent with a formation process consisting of the simple dehydration of a wet 2 × 1 structure.     

Water adsorption on SrTiO3(001): II. Water,water, everywhere

The role of water adsorption on Ti-rich SrTiO₃(001) surface reconstructions is studied. Density functional calculations with hybrid functionals of numerous adsorption configurations indicate that the relative stability of the different reconstructions is strongly altered by the addition of water, with all the reconstructions having comparable energy for half-monolayer coverage, most with a fair degree of hydrogen bonding. This strongly suggests that which reconstruction is observed depends upon a competition between the kinetics of ordering and dehydration. X-ray photoelectron spectra are consistent with the theoretical predictions for the dehydration of the 2 × 1 and c(4 × 2) reconstructions.     

Colossal magnetoresistance effect in epitaxially grown La2/3Ca1/3MnO3 perovskite-like manganite thin films

We report in this work, study on colossal magnetoresistance (CMR) effect in epitaxial La_2/3Ca_1/3 MnO₃ thin films grown on SrTiO₃ (0 0 1) substrates by pulsed laser deposition (PLD) technique. The films were grown on as-received SrTiO₃ substrates and on SrTiO₃ substrates prepared by HF etching (Koster et al., Appl. Phys. Lett. 73 (1998) 2920; V. Leca et al., Wet etching methods for perovskite substrates, University of Twente, MESA+ Research Institute, Low Temperature Division). Two of the samples were annealed in different conditions to investigate the films heat treatment effect on electric and magnetic properties. Electrical resistance was done using the four-probe method at temperatures in the range of 2–375 K without a magnetic field and in an external field of 5 T applied in the film plane. Resistance-magnetic field (R vs. H) at 77 K for the two annealed samples was done in a 5 T sweep magnetic field. The surface morphology and structural information of the films were obtained using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. Secondary ion mass spectroscopy (SIMS) analysis was performed on the annealed samples to investigate any possible chemical reaction between La_2/3Ca_1/3MnO₃ thin films and SrTiO₃ substrate.     

Chemical solution processing and properties of Sr2FeMoO6 thin films

Crack-free and oriented Sr₂FeMoO₆ (SFMO) thin film with double perovskite structure has been fabricated by the chemical solution deposition (CSD) method. A homogeneous and stable SFMO precursor solution was successfully prepared by controlling the reaction of starting metal-organic compounds in a mixture solvent of 1-propanol and 2-methoxyethanol. SFMO thin films with c-axis preferred orientation could successfully be synthesized on MgO (0 0 1) and SrTiO₃ (0 0 1) substrates by optimizing the several processing conditions. SFMO thin film prepared on SrTiO₃ (0 0 1) showed a magnetoresistance effect at a low magnetic field.     

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Preparation and characterization of Fe3O4(1 1 1) nanoparticles and thin films on Au(1 1 1)

Fe₃O₄ nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe₃O₄ was formed by annealing α-Fe₂O₃(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe₂O₃(0 0 0 1) structures into Fe₃O₄ nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe₃O₄ initially appears as nanoparticles at low coverages, and forms thin films at ～2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe₃O₄ particles with periodicities of ～50 and ～42 Å, respectively. As the Fe₃O₄ particles form more continuous films, the ～50 Å feature was the predominant superstructure observed. The Fe₃O₄ structures at all coverages show a hexagonal unit cell with a ～3 Å periodicity in the atomically resolved STM images.     

In situ RHEED monitor of the growth of epitaxial anatase TiO2 thin films

Epitaxial anatase TiO₂ thin films were successfully grown on (0 0 1) SrTiO₃ substrates by the laser molecular-beam epitaxy (laser-MBE) method. The whole growth process is monitored by in situ reflection high-energy electron diffraction (RHEED). RHEED monitoring shows a transition from a streaky pattern to a spot pattern during deposition, indicating different growth modes of TiO₂ film. The RHEED patterns are in consistent with the RHEED intensity oscillation results. The atomic force microscopy (AFM) and X-ray diffraction (XRD) investigation show that the thin films have single crystalline orientation with roughness less than three unit cells.