Ultrathin Y2O3(111) films on Pt(111) substrates

The growth of ultrathin films of Y₂O₃(111) on Pt(111) has been studied using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED). The films were grown by physical vapor deposition of yttrium in a 10^? 6 Torr oxygen atmosphere. Continuous Y₂O₃(111) films were obtained by post-growth annealing at 700 °C. LEED and STM indicate an ordered film with a bulk-truncated Y₂O₃(111)–1 × 1 structure exposed. Furthermore, despite the lattices of the substrate and the oxide film being incommensurate, the two lattices exhibit a strict in-plane orientation relationship with the [11?0] directions of the two cubic lattices aligning parallel to each other. XPS measurements suggest hydroxyls to be easily formed at the Y₂O₃ surface at room temperature even under ultra high vacuum conditions. The hydrogen desorbs from the yttria surface above ~ 200 °C.     

Oxidation of ultra-thin zinc films on Rh(100) surface

The oxidation of submonolayer zinc films on Rh(100) surface by O₂ gas has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). With a zinc coverage of 0.8 ML, an atomically flat ultra-thin zinc oxide film formed at an oxygen partial pressure of 2 × 10^? 8 mbar and a temperature of 150 °C. The zinc oxide film showed a c(16 × 2) LEED pattern. The high resolution STM image of the zinc oxide film showed single dotted spots and double dotted spots arranged linearly and periodically along the [01¯1] direction. We propose an atomic arrangement model of the film accounting for the LEED pattern, the STM image, and the atomic arrangement of the bulk ZnO(0001) surface.     

Surface structure of γ-Fe2O3(111)

The surface structure of γ-Fe₂O₃(111) has been investigated with a range of surface techniques. Two different surface structures were discovered depending upon surface preparation techniques. Sputtering followed by annealing in vacuum produced a reduced surface characterised by a (2 × 2) LEED pattern, whereas sputtering followed by annealing in 1 × 10^? 6 mbar oxygen produced a surface characterised by a (√3 × √3)-R30° LEED pattern. The latter appears to be a very low conductivity surface, whereas the former has the band gap expected for maghemite (~ 2.0 eV). We propose that the reduced surface is a magnetite-like layer, whereas the oxidised surface is an Fe₂O₃-like layer.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.     

Fabrication of Cu2O nanocrystalline thin films photosensor prepared by RF sputtering technique

Cuprous oxide (Cu₂O) nanocrystalline thin films were prepared on two types of substrates known as crystalline silicon and amorphous glass, by radio frequency reactive magnetron sputtering method. Scanning electron microscopy images confirmed that Cu₂O particles covered the entire surface of both substrates with smoothing distribution. The root mean square surface roughness for the prepared Cu₂O thin films on glass and Si (111) substrates is 4.16, and 3.36 nm, respectively. Meanwhile, X-ray diffraction results demonstrated that the two phases of Cu₂O and CuO were produced on Si (111) and glass substrates. The optical bandgap of Cu₂O thin films synthesised on glass substrate is 2.42 eV. Furthermore, the prepared Cu₂O nanocrystalline thin films have showed low reflectance value in the visible spectrum. Metal-Semiconductor-Metal photodetector based Cu₂O nanocrystalline thin films deposited onto Si (111) was fabricated using aluminium and platinum, with the current-voltage and photoresponse characteristic investigated under various applied bias voltages. The fabricated Metal-Semiconductor-Metal (M-S-M) photodetector had shown 126% sensitivity in the presence of 10 mW/cm² of 490 nm light with 1.0 V bias, displaying 90 and 100 ms response and recovery times, respectively. These findings have demonstrated the suitability of M-S-M Cu₂O photodetector as an affordable photosensor in the future.     

Dynamical LEED analyses of the clean and the NO-adsorbed Ir(111) surface

The adsorption structure of nitric oxide (NO) on Ir(111) was studied by thermal desorption spectroscopy (TDS) and dynamical analyses of low-energy electron diffraction (LEED). At the saturation coverage at about 100 K, a 2 × 2 pattern was observed by LEED and two peaks appeared at 365 and 415 K in TDS. No change in the LEED I–V curves was observed by annealing at 280 K, which means that the NO-saturated surface was retained at this temperature. On the contrary, partial desorption and changes of the LEED I–V curves were observed by annealing at 360 K. Combined with previous vibrational studies, it is suggested that one adsorption species is not affected, while another species is partially desorbed and the rest of them are dissociated by annealing at 360 K. Dynamical analyses of LEED were performed for the 280 K-annealed and the 360 K-annealed surfaces, which correspond to the NO-saturated and the NO-dissociated Ir(111) surfaces, respectively. These revealed that NO occupies the atop, fcc-hollow and hcp-hollow sites (atop-NO + fcc-NO + hcp-NO) for the NO-saturated Ir(111) surface with the saturation coverage of 0.75 ML. For the 360 K-annealed surface, the atop-NO is not affected but the fcc-NO and the hcp-NO are partially desorbed as NO and partially dissociated to N and O, both of which occupy the fcc-hollow site on the surface.     

Passivation of GaAs(001) surface by the growth of high quality c-GaN ultra-thin film using low power glow discharge nitrogen plasma source

The benefits of using a low power glow discharge nitrogen plasma source to create high quality GaN layers on GaAs(001) surface are first highlighted. This uncommon type of plasma source has the particularity of working at a low power (3–10 W) and a low pressure (10^? 1 Pa) which induce creation of a small quantity of active nitrogen species. We put in evidence that this distinctiveness allows the growth of a stoichiometric and As-free GaN ultra-thin film on a GaAs(001) substrate by the mean of the inter-diffusion of As and N atoms. XPS, EELS, AFM are used to monitor surface composition and structure changes and to estimate the GaN thickness. A near saturation of the nitride layer thickness versus plasma exposure time is found. Furthermore, the possibility to crystallize the amorphous GaN layer by an annealing at 620 °C in a cubic structure with a lattice parameter close to that of c-GaN is put in evidence by means of TEM and LEED measurements. These measurements also show the homogeneity of the GaN thickness. In addition, the passivating effect of the GaN ultra-thin film to protect the GaAs surface is proved with the monitoring by XPS of the surface oxidation during several days of air exposure.     

Atomic and nanostructures of monolayer c(2 × 2)NiN on Cu(0 0 1)

Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.     

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.     

Adsorption of O2 and CO2 on the Si(111)-7 × 7 surfaces

The interaction of O₂ and CO₂ with the Si(111)-7 × 7 surface has been studied with X-ray photoelectron spectroscopy (XPS). It was found that both O₂ and CO₂ molecules can readily oxidize the Si(111)-7 × 7 surface to form thin oxide films. Two oxygen species were identified in the oxide film: oxygen atoms binding to on-top sites of adatom/rest atoms with an O 1s binding energy of ~ 533 eV as well as to bridge sites of adatom/rest atom backbonds at ~ 532 eV. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film formed by CO₂ has a lower O/Si ratio than that of O₂.     

Synthesis of epitaxial graphene on rhodium from 3-pentanone

The synthesis of high quality single layer graphene on rhodium, g/Rh(111), is reported. The graphene layers are grown at 1060 K by low pressure chemical vapor deposition (CVD) using 3-pentanone as a precursor molecule. The presented growth technique shows an easy high quality production method for epitaxial graphene monolayers. The chemical composition and structural properties of such self-assembled monolayers were characterized by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Scanning Tunneling Microscopy (STM) confirms the formation of a 3 nm super cell and a unique surface morphology which establishes the potential of g/Rh(111) as a template for molecules.     

The growth of Ag films on a TiO2(110)-(1×1) surface

We report here the growth of Ag film and its thermal stability on the TiO₂(1 1 0)-(1×1) surface using combination techniques of low-energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At a surface temperature as low as 125 K, a 2D growth of Ag films seems to occur for submonolayer coverages up to ∼0.8 ML. Annealing of low temperature grown Ag films to 500 K for coverage of 1–2.4 ML would result in the formation of metastable Ag layers with rest of Ag forming 3D needle-like islands on top of this Ag film.     

Preparation and characterization of iron–molybdate thin films

Mixed Fe–Mo oxides are used in industrial catalytic processes of selective oxidation of methanol to formaldehyde. For better understanding of the structure-reactivity relationships of these catalysts we aim to prepare well-ordered iron–molybdate thin films as model catalysts. Here we have studied Mo deposition onto Fe₃O₄ (111) thin films produced on Pt(111) as a function of Mo coverage and annealing temperature using LEED, AES, STM and IRAS. At low temperatures, the iron oxide film is covered by Mo = O terminated molybdena nanoparticles. Upon oxidation at elevated temperatures (T > 900 K), Mo species migrate into the film and form new bonds with oxygen in the film. The resulting films maintain the crystal structure of Fe₃O₄, and the surface undergoes a (√3 × √3)R30° reconstruction. The structure is rationalized in terms of Fe substitution by Mo in the surface layers.     

Surface stoichiometry of La0.7Sr0.3MnO3 during in vacuo preparation; A synchrotron photoemission study

We present a study of the surface stoichiometry and contamination of La_0.7Sr_0.3MnO₃ thin films following exposure to air and subsequent in vacuo preparation. Samples were studied using both soft X-ray synchrotron photoemission (hν = 150 to 350 eV) and traditional Mg-Kα XPS (hν = 1253.6 eV) whilst annealing incrementally to ≈ 510°C in low pressures of O₂. In all cases, a Mn depleted and Sr rich surface oxide layer is observed, it is of reduced crystalline quality and is charge depleted. This surface layer is weakly affected by subsequent annealing, and is partially reversed by annealing in higher O₂ pressure. Surface carbon contamination is incrementally removed by annealing at increased temperatures, and at 270 °C, it is reduced to ≈ 0.4% of the topmost unit cell. The modification of the surface stoichiometry and electronic properties is consistent with the reported loss of magnetic properties in thin LSMO films.     

Effect of impurities on the surface morphology of Fe(111)

The surface composition and morphology of Fe(111) have been examined through a combined analysis that includes low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The preferential segregation of sulfur has been clearly identified by AES upon annealing. The STM images exhibit numerous triangular pits of various sizes, and the LEED patterns show diffused n × 1 spots. The triangular pits reveal a Sierpinski gasket fractal. For sulfur-free Fe(111), nitrogen segregates to the surface upon annealing, forming a 4√3 × 4√3 superstructure that is identified by LEED patterns and STM images. The STM images show nanoscale triangular clusters regularly aligned in a hexagonal 4√3 × 4√3 configuration. Ultra-thin chromium film deposited on a nitrogen-segregated Fe(111) surface with post-annealing induces further nitrogen segregation, resulting in the formation of triangular pyramid-shaped CrN nanoclusters.     

Ti/CeOx(111) interfaces studied by XPS and STM

Low coverage of Ti was deposited on the well-ordered CeO_x(111) (1.5 < x < 2) thin films grown on Ru(0001) by physical vapor deposition at room temperature. The structure and interaction of Ti/ceria interfaces were investigated with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques under ultrahigh vacuum conditions. XPS data indicate that the deposition of Ti on both oxidized and reduced ceria surfaces causes the partial reduction of Ce from + 4 to + 3 state. Ti is formally in the + 4 state. STM data show the formation of small atomic-like titania features at 300 K, which coalesce to form chain structures upon heating. It is demonstrated in the study that the deposition of Ti can form mixed metal oxides at the interface and modify both electronic and structural properties of the ceria support. The structural study of Ti/ceria interfaces can be a key for understanding the higher catalytic activity of the Ti–CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

The oxidation of Fe(111)

The oxidation of Fe(111) was studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS) and scanning tunnelling microscopy (STM). Oxidation of the crystal was found to be a very fast process, even at 200 K, and the Auger O signal saturation level is reached within ~ 50 × 10^? 6 mbar s. Annealing the oxidised surface at 773 K causes a significant decline in apparent surface oxygen concentration and produces a clear (6 × 6) LEED pattern, whereas after oxidation at ambient temperature no pattern was observed. STM results indicate that the oxygen signal was reduced due to the nucleation of large, but sparsely distributed oxide islands, leaving mainly the smooth (6 × 6) structure between the islands. The reactivity of the (6 × 6) layer towards methanol was investigated using temperature programmed desorption (TPD), which showed mainly decomposition to CO and CO₂, due to the production of formate intermediates on the surface. Interestingly, this removes the (6 × 6) structure by reduction, but it can be reformed from the sink of oxygen present in the large oxide islands simply by annealing at 773 K for a few minutes. The (6 × 6) appears to be a relatively stable, pseudo-oxide phase, that may be useful as a model oxide surface.     

Reactivity of the Cu2O(1 0 0) surface: Insights from first principles calculations

We present a summary of results of systematic first principles calculations of the electronic and geometric structures of the Cu₂O(1 0 0) surface and the process of CO oxidation on this surface (energetics and pathways of adsorption, diffusion and reactions of CO and O₂ on the surface). The (p, T) phase diagram of the Cu₂O(1 0 0) in equilibrium of with gas phase O₂ built using the ab initio thermodynamics approach suggests that the O-terminated surface is preferred over the Cu-terminated one within the entire ranges of pressures and temperatures in which the compound exists. Metastable Cu-terminated Cu₂O(1 0 0) is found to undergo a surface reconstruction in agreement with experiment. We find CO to oxidize spontaneously on the O-terminated Cu₂O(1 0 0) surface by consuming surface O atoms. Our calculations also show that the surface O-vacancies left in the course of the CO oxidation can be easily filled with dissociative adsorption of the gas phase O₂ molecules, which are usually present in reaction environment.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.