DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3(1 0 0), SrTiO3(1 1 1) and SrTiO3(1 1 0)

Density Functional Theory has been used to determine the energetically preferred structures of submonolayer, monolayer, and multilayer Pt films on both ideal terminations of SrTiO₃(1 0 0), SrTiO₃(1 1 1), and SrTiO₃(1 1 0). The strength of the resulting metal/metal oxide interfaces was characterized by the adsorption energy of the film and the film’s work of separation. The two polar surfaces, SrTiO₃(1 1 1) and SrTiO₃(1 1 0), form significantly stronger interfaces than the non-polar SrTiO₃(1 0 0) surface. Approximate criteria were applied to predict the growth mode of Pt on each surface.     

Epitaxial growth of Co(0 0 0 1)hcp/Fe(1 1 0)bcc magnetic bi-layer films on SrTiO3(1 1 1) substrates

Co(0 0 0 1)_hcp/Fe(1 1 0)_bcc epitaxial magnetic bi-layer films were successfully prepared on SrTiO₃(1 1 1) substrates. The crystallographic properties of Co/Fe epitaxial magnetic bi-layer films were investigated. Fe(1 1 0)_bcc soft magnetic layer grew epitaxially on SrTiO₃(1 1 1) substrate with two type variants, Nishiyama–Wasserman and Kurdjumov–Sachs relationships. An hcp-Co single-crystal layer is obtained on Ru(0 0 0 1)_hcp interlayer, while hcp-Co layer formed on Au(1 1 1)_fcc or Ag(1 1 1)_fcc interlayer is strained and may involve fcc-Co phase. It has been shown possible to prepare Co/Fe epitaxial magnetic bi-layer films which can be usable for patterned media application.     

Electronic structure and topography of annealed SrTiO3(1 1 1) surfaces studied with MIES and STM

Perovskites of ABO₃ type like strontium titanate (SrTiO₃) are of great practical concern as materials for oxygen sensors operating at high temperatures. It is well known that the surface layer shows different properties compared to the bulk. Numerous studies exist for the SrTiO₃(1 0 0) and (1 1 0) surfaces which have investigated the changes in the electronic structure and topography as a function of the preparation conditions. They have indicated a rather complex behaviour of the surface and the near surface region of SrTiO₃ at elevated temperatures. Up to now, the behaviour of the SrTiO₃(1 1 1) surfaces under thermal treatment is not sufficiently known. This contribution is intended to work out the relation between alteration of the surface topography with respect to the preparation conditions and the simultaneous changes of the electronic structure. We applied scanning tunneling microscopy (STM) to investigate the surface topography and, additionally, metastable impact electron spectroscopy (MIES) to study the surface electronic structure of reconstructed SrTiO₃(1 1 1) surfaces. The crystals were heated up to 1000 °C under reducing and oxidizing conditions. Both preparation conditions cause strong changes of the surface topography and electronic structure. A microfaceting of the topmost layers is found.     

Scanning tunnelling microscopy studies of growth morphology in highly epitaxial c-axis oriented Pt thin film on (0 0 1) SrTiO3

A noble metal Pt thin film was successfully grown on (0 0 1) SrTiO₃ substrate by using a DC-sputtering technique. The surface morphology and growth features of the as-grown Pt films were investigated by scanning tunnelling microscopy. Growth conditions, such as pre-sputtering, deposition ambience, and oxygen ratio are found to greatly affect the orientation, the crystallinity, and the epitaxial behavior of Pt films on (0 0 1) SrTiO₃. Single-crystalline Pt films have been achieved by introducing a few percentage oxygen into the sputtering ambient. The in-plane-relationship of the c-axis oriented Pt thin films on (0 0 1) SrTiO₃ was determined to be (0 0 1)_Pt∥(0 0 1)_SrTiO3 and [0 0 1]_Pt∥[0 0 1]_SrTiO3. Oxygen in the sputtering ambient was found to be a key factor to achieve the epitaxial Pt films.     

Atypical grain growth for (2 1 1) CdTe films deposited on surface reconstructed (1 0 0) SrTiO3 substrates

The (1 0 0) SrTiO₃ substrate has emerged as the oxide substrate of choice for the deposition of a wide variety of materials. The substrate's unavoidable miscut leads to a step-terrace morphology when heated to high temperatures. This morphological transition is accompanied by an atomic scale repositioning of the uppermost terrace atoms, the nature of which is strongly dependent on the substrate temperature and ambient atmosphere used. Here, we report the deposition of CdTe films on the as-received and reconstructed surfaces of (1 0 0) SrTiO₃. The as-received substrate gives rise to a [1 1 1] CdTe film with four equally distributed in-plane grain orientations. The surface reconstruction, on the other hand, gives rise to an unprecedented reorientation of the film's grain structure. For this case, a [2 1 1] CdTe film emerges having twelve unevenly distributed in-plane orientations. We attribute the film's grain structure to an atomic scale surface reconstruction, with the anisotropic distribution of grain-types arising from a preferential formation due to the step edges.     

SrTiO3(0 0 1) reconstructions: the (2 × 2) to c(4 × 4) transition

Scanning tunneling microscopy (STM) is used to investigate the (0 0 1) surface structure of Nb doped SrTiO₃ single crystals annealed in ultra high vacuum (UHV). Atomically resolved images of the (2 × 2) reconstructed surface are obtained after annealing a chemically etched sample. With further annealing dotted row domains appear, which coexist with the (2 × 2) reconstruction. The expansion of these domains with further annealing gives rise to the formation of a TiO₂ enriched c(4 × 4) reconstruction.     

Growth,structure and thermal reduction of MOCVD-deposited Fe films on Al2O3 (0 0 0 1) substrates

Fe films with strong preferred orientation were prepared on Al₂O₃ (0 0 0 1) substrates by a new two-step method using low-pressure metal-organic chemical vapor deposition (LP-MOCVD) method. X-ray diffraction (XRD) and a vibrating sample magnetometer were employed to characterize the structure and magnetic properties of the Fe films before and after thermal reduction, which was performed in hydrogen flow at 723–1023 K. XRD patterns indicate that the films changed into α-Fe (bcc) mono-phase from a mixture of α-Fe₂O₃ and/or Fe (bcc).     

Growth of highly-oriented CaCu3Ti4O12 thin films on SrTiO3 (1 0 0) substrates by a chemical solution route

Highly-oriented CaCu₃Ti₄O₁₂ (CCTO) thin films deposited directly on SrTiO₃ (1 0 0) substrates have been developed successfully using a chemical solution coating method. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were employed to characterize the structure and the morphology. It was observed that the CCTO thin films had the 1 μm × 1 μm domain-like microstructure that consists of compact grains of about 0.1 μm in size. The cross sectional SEM image shows that the CCTO grains grow regularly close to the clear interface between the CCTO film and the SrTiO₃ substrate. The result was discussed in terms of lattice mismatch between CCTO and SrTiO₃.     

Scanning tunneling microscopy and spectroscopy of Mo clusters grown on TiO2(1 1 0)

Molybdenum was deposited in two steps (3 eq. ML and 1 eq. ML) on the light blue rutile TiO₂(1 1 0) (1 × 1) surface at room temperature, each Mo deposition cycle being followed by an annealing up to 950-1000 K. This procedure was found to lead to formation of separated clusters having a size in very wide range (1-20 nm). Scanning tunneling microscopy showed a dependence of the cluster morphology as a function of the size. The scanning tunneling spectra of Mo clusters was studied as a function of cluster dimensions and discussed in comparison with photoelectron spectroscopy results previously obtained for homogeneous Mo films. The dI/dV curves do not display the valence band structure of deposited material, which could be explained by the Schottky barrier formation.     

Epitaxial MoOx nanostructures on TiO2(1 1 0) obtained using thermal decomposition of Mo(CO)6

Stoichiometric and highly-defective TiO₂(1 1 0) surfaces (called as yellow and blue, respectively) were exposed to Mo(CO)₆ vapours in UHV and in a reactive O₂ atmosphere. In the case of yellow-TiO₂, an O₂ reactive atmosphere was necessary to obtain the Mo(CO)₆ decomposition at 450 °C with deposition of MoO_x nanostructures where, according to core level photoemission data, the Mo⁺⁴ state is predominant. In the case of blue-TiO₂ it was possible to obtain Mo deposition both in UHV and in an O₂ atmosphere. A high dose of Mo(CO)₆ in UHV on blue-TiO₂ allowed the deposition of a thick metallic Mo layer. An air treatment of this sample at 580 °C led to the elimination of Mo as MoO₃ and to the formation of a transformed layer of stoichiometry of Ti_(1−x)Mo_xO₂ (where x is close to 0.1) which, according to photoelectron diffraction data, can be described as a substitutional near-surface alloy, where Mo⁺⁴ ions are embedded into the titania lattice. This embedding procedure results in a stabilization of the Mo⁺⁴ ions, which are capable to survive to air exposure for a rather long period of time. After exposure of the blue-TiO₂(1 1 0) substrate to Mo(CO)₆ vapours at 450 °C in an O₂ atmosphere it was possible to obtain a MoO₂ epitaxial ultrathin layer, whose photoelectron diffraction data demonstrate that is pseudomorphic to the substrate.     

Structures of Co and Pt nanoclusters on a thin film of Al2O3/NiAl(1 0 0) from reflection high-energy electron diffraction and scanning-tunnelling microscopy

With reflection high-energy electron diffraction (RHEED) and scanning-tunnelling microscopy (STM), we made measurements on Co and Pt nanoclusters grown by vapour deposition on a thin film of Al₂O₃/NiAl(1 0 0). The results show that the annealed Co nanoclusters (with mean diameters 2.5, 3.4, 5.8 nm and heights 0.7, 1.5, 1.5 nm, respectively) and Pt nanoclusters (with mean diameter 2.25 nm and height 0.4 nm) are highly crystalline and that their structures are significantly affected by the oxide substrate. Structural analysis based on the RHEED patterns indicates that both Co and Pt clusters have a fcc phase and grow with their (0 0 1) facets parallel to the θ-Al₂O₃(1 0 0) surfaces, and with their [1 1 0] and [−1 1 0] axes along the [0 1 0] and [0 0 1] directions of the oxide surface, respectively, so (Co(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0] and Pt(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0]). This growth is optimal as the Co and Pt fcc (0 0 1) facets match well with the oxygen mesh. To minimize the lattice mismatch, the lattice parameter of the Co clusters expands 4-5% relative to fcc Co bulk, whereas the lattice parameter of the Pt clusters remains near the bulk value, as the Pt fcc (0 0 1) plane has a close lattice match with the oxide surface.     

The CuGaSe2(0 0 1) surface: A (4 × 1) reconstruction

We report on the formation of a stable (4 × 1) reconstruction of the chalcopyrite CuGaSe₂(0 0 1) surface. Using Ar⁺ ion-bombardment and annealing of epitaxial CuGaSe₂ films grown on GaAs(0 0 1) substrates it was possible to obtain flat, well-ordered surfaces showing a clear (4 × 1) reconstruction. The cleanliness and structure were analyzed in situ by AES and LEED. AES data suggest a slight Se-enrichment and Cu-depletion upon surface preparation. Our results demonstrate that (0 0 1) surfaces of the Cu-III-VI₂(0 0 1) material can show stable, unfacetted surfaces.     

Magnetoresistance of magnetite thin films grown by pulsed laser deposition on GaAs(1 0 0) and Al2O3(0 0 0 1)

Magnetotransport properties of magnetite thin films deposited on gallium arsenide and sapphire substrates at growth temperatures between 473 and 673 K are presented. The films were grown by UV pulsed laser ablation in reactive atmospheres of O₂ and Ar, at working pressure of 8 × 10⁻² Pa. Film stoichiometry was determined in the range from Fe_2.95O₄ to Fe_2.97O₄. Randomly oriented polycrystalline thin films were grown on GaAs(1 0 0) while for the Al₂O₃(0 0 0 1) substrates the films developed a (1 1 1) preferred orientation. Interfacial Fe³⁺ diffusion was found for both substrates affecting the magnetic behaviour. The temperature dependence of the resistance and magnetoresistance of the films were measured for fields up to 6 T. Negative magnetoresistance values of ∼5% at room temperature and ∼10% at 90 K were obtained for the as-deposited magnetite films either on GaAs(1 0 0) or Al₂O₃(0 0 0 1).     

First principle study of SrTiO3 (0 0 1) surface and adsorption of NO on SrTiO3 (0 0 1)

The results of first-principles calculations about the two possible terminations of (0 0 1) surface of SrTiO₃ perovskite and adsorption of NO on SrTiO₃ (0 0 1) surface were presented. Both surface parameters (atomic structures and electronic configurations) and adsorption parameters (bond, energy and charge) of NO on SrTiO₃ (0 0 1) surface, which have never been investigated before as far as we know were investigated using density functional theory calculations with the local-density approximation (DFT-LDA). It was found that the two possible terminations of SrTiO₃ (0 0 1) surface have large surface relaxation, which leads to surface polarization and exhibits different reactivity toward the dissociative adsorption of NO. The electronic states of TiO₂-terminated surface have larger difference than that of bulk, so it is more favorable for adsorption of NO than SrO-terminated surface.     

Study of c(2 × 2)-MnAu(0 0 1) layers on Mn(0 0 1) by means of scanning tunneling microscopy/spectroscopy

Intermixing, growth, geometric and electronic structures of gold films grown on antiferromagnetic stacking body-centered-tetragonal manganese (0 0 1) films were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2 × 2)-MnAu(0 0 1) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer (5 × 23)-like Au reconstruction appears instead of the c(2 × 2) structure and local density of states peaks obtained on the c(2 × 2)-MnAu surface disappear, pure Au layers likely grow from the fourth layer.     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Effects of N adsorption on the structural and electronic properties of SrTiO3(0 0 1) surface

The atomic configurations, bonding characteristics, and electronic structures of the N-adsorbed (directly and substitutionally) SrTiO₃(0 0 1) surface are studied by using first-principles method based on the density functional theory. From the analysis of the energetics and density of states, it is found that the stability of the directly adsorbed N depends on the relative position of N atom to the surface. To better understand the effects of the substitutionally adsorbed N on the surface, as an example, the behavior of Au atoms adsorbed on the N-substituted surface is discussed in detail. There is clearly a synergy effect between the substitution of N for O_s(I) and the adsorption of Au atoms on the SrTiO₃(0 0 1) surface.     

First-principles study of the (0 0 1) surface of cubic SrZrO3

Density functional calculations have been used to investigate the (0 0 1) surface of cubic SrZrO₃ with both SrO and ZrO₂ termination. Surface structure and electronic structure have been obtained. The SrO surface is found to be similar to its counterpart in SrTiO₃, while there are marked differences between the ZrO₂ and TiO₂ terminations in SrZrO₃ and SrTiO₃, respectively, concerning surface relaxation and rumpling. For the ZrO₂-terminated surface of SrZrO₃, the covalency of the interaction between the outmost Zr and the O beneath is enhanced as a result of their bond contraction. The band gap reduction and the presence of the surface states are also discussed in relation with the behavior of the electrostatic potential.     

Surface morphology of epitaxial lattice-matched Ba0.7Sr0.3O on Si(0 0 1) and vicinal Si(0 0 1)-4°[1 1 0] substrates

Exploring ways for self-organized structuring of insulating thin films, we investigated the possibility to produce replicas of step trains, given by a vicinal Si(0 0 1)-4°[1 1 0] surface, in layers of crystalline and perfectly lattice-matched Ba_0.7Sr_0.3O. For this purpose, we carried out high-resolution spot profile analyses in low-energy electron diffraction (SPA-LEED) both on flat Si(0 0 1) and on Si(0 0 1)-4°[1 1 0]. Oxide layers were generated by evaporating the metals in oxygen ambient pressure with the sample at room temperature. Our G(S) analysis of these mixed oxide layers reveals a strong influence of local compositional fluctuations of Sr and Ba ions and their respective scattering phases, which appears as an unphysically large variation of layer distances. Nevertheless, we are able to show that quite smooth and closed oxide films are obtained with an rms roughness of about 1 ML. These Ba_0.7Sr_0.3O films directly follow the step train of Sr-modified vicinal Si surfaces that form (1 1 3) oriented facets after adsorption of a monolayer of Sr. This proves that self-organized structuring of insulating films can indeed be an effective method.