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.     

The interaction of H2O molecules with iron films studied with MIES,UPS and XPS

X-Ray Photoelectron Spectroscopy (XPS), Metastable Induced Electron Spectroscopy (MIES) and Ultraviolet Photoelectron Spectroscopy (UPS) were applied to study the interaction of H₂O molecules with iron films.During the interaction with H₂O molecules under ultrahigh vacuum conditions, an oxide film is formed on the iron surface. UPS and XPS still show metallic contributions, even for a surface which is exposed to about 10³ L. The oxide film thickness amounts to about 1.8 nm. No hydroxide formation is observed at all, neither in UPS nor in MIES. Further impinging H₂O molecules do not interact with the surface, because the oxide film inhibits the dissociation of impinging molecules.H₂O exposure beyond 10⁹ L does not lead to a significant increase of the oxide layer, which saturates at a thickness of 1.8 nm. In particular, no surface hydroxide is observed at this exposure. Neither XPS UPS nor MIES reveal any indication for this.     

Effect of H2O chemisorption on passivation of Ge(100) surface studied by scanning tunneling microscopy

The electronic passivation of a Ge(100) surface, via the chemisorption of H₂O at room temperature (RT), and the temperature dependence of H₂O coverage were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). With a saturation H₂O dose at RT, a highly-ordered structure, due to the dissociative chemisorption of H₂O, was observed on a Ge(100) surface with a coverage of 0.85 monolayers (ML). Annealing the room temperature H₂O-dosed Ge surface to 175 °C decreased the coverage of H₂O to 0.6 ML. Further annealing at 250 °C decreased the coverage of H₂O sites to 0.15 ML, and the surface reconstruction of Ge dimers was observed over much of the surface. Annealing above 300 °C induced Ge suboxide structures, similar to the oxygen-dosed Ge surface. STS measurements confirmed that the surface dangling bond states near Fermi energy are removed by the H₂O chemisorption because the dangling bonds of Ge atoms are terminated by ―OH and ―H. The H₂O pre-dose at room temperature provides a template for the ultrathin passivation of Ge(100) surface via atomic layer deposition (ALD) at RT, since near monolayer nucleation can be obtained with a 1/2 hydroxylated and 1/2 hydrogenated Ge surface.     

Interactions between glycine and amorphous solid water nanoscale films

The interactions of glycine (Gly) with amorphous solid water (ASW) nanolayers (≤ 100 ML), vapor-deposited on single crystalline AlO_x surfaces at 100 K, have been investigated by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K-edge, temperature-programmed thermal desorption (TPD), X-ray photoelectron spectroscopy (XPS), and temperature-dependent work function measurements. Gly-on-ASW, ASW-on-Gly, and Gly on top of ASW-on-Gly ultrathin films have been fabricated. In contrast to the uniform ASW films grown directly on the hydrophilic AlO_x, water molecules adsorb on the hydrophobic Gly films in the form of 3D ASW clusters. This leads to significant differences in the NEXAFS and work function data obtained from ASW-on-AlO_x and ASW-on-Gly films, respectively. Furthermore, these structural differences influence the chemical state of Gly molecules (neutral vs. zwitterionic) adsorbed on top of ASW films. N1s XPS measurements revealed an increased amount of neutral Gly molecules in the film top-deposited on the ASW-on-Gly structure in comparison to the neutral Gly in the films directly condensed on AlO_x or grown on the ASW substrate. H₂O TPD spectra demonstrate that the crystallization and desorption processes of ASW are affected in a different way by the Gly layers, top-deposited on to ASW-on-AlO_x and ASW-on-Gly films. At the same time, Gly adlayers sink into the ASW film during crystallization/desorption of the latter and land softly on the alumina surface in the form of zwitterionic clusters.     

Interaction of water molecules with bare and deuterated polycrystalline diamond surface studied by high resolution electron energy loss and X-ray photoelectron spectroscopies

In this work we study the interaction of water molecules with deuterated and bare polycrystalline diamond surfaces upon exposure to water vapor by X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HR-EELS). To distinguish the molecular origin of hydrogen bonds (i.e. C–H, O–H, C–O–H, etc.) formed on the diamond surface upon interaction with the water molecules, deuterated and hydrogenated gases were used in our experiments. Diamond films were deposited from a deuterated gas mixture to induce C(di)-D surface terminations. Water adsorption on bare diamond surface gives rise to the appearance of well defined and pronounced C–H and C–OH vibrational HR-EELS peaks and an intense O (1s) XPS peak. These chemically adsorbed water fragments survive 300 °C anneal temperature under ultra-high vacuum conditions. Annealing at 600 °C of the water exposed bare diamond surface results in disappearance of the C–OH vibrational modes alongside with a pronounced reduction of the C–H vibrational modes, whilst only upon annealing to ~ 800 °C the O (1s) XPS peak decreased substantially in intensity. We associate these effects with dissociative adsorption of the water molecules on the bare diamond surfaces.Water exposure onto a deuterated surface, on the other hand, does not result in the appearance of the C–OH vibrational peaks but only to an increase of the C–H vibrational HR-EELS mode along side with the appearance of a weaker XPS O (1) peak, as compared to the same experiment, performed on the bare surface. 300 °C anneal significantly diminishes surface oxygen concentration, as monitored by XPS. We associate these results with H₂O decomposition reactions and also with molecular adsorption on deuterated diamond surfaces. Annealing of the water exposed deuterated diamond surface, results in a pronounced decrease and disappearance of the O (1s) XPS peak at a temperature of ~ 800 °C.     

The initial growth structure of Ni1-xFex (x=0.6–0.8) films dc-biased plasma-sputter-deposited on Ni/MgO(0 0 1), and on Fe/MgO(0 0 1)

Cross sectional and plane-view transmission electron microscopy (X- and PV-TEM) were used to investigate the initial growth phase of 5, 10, 20 and 40 nm thick Ni_1-xFe_x (x=0.6–0.8) films, prepared on MgO(0 0 1) covered with a buffer layer of Fe or Ni as well as on naked MgO(0 0 1). The 100 nm thick buffer layers of Fe and Ni were pre-grown on MgO(0 0 1). All of Ni_0.20Fe_0.80, Ni_0.40Fe_0.60, Fe and Ni films could be epitaxially grown at 250°C by dc-biased plasma sputtering at 2.9 kV in pure Ar gas.The films of Ni_0.20Fe_0.80 and Ni_0.40Fe_0.60 were grown in their own stable phase, bcc and fcc on MgO(0 0 1), respectively. However, Ni_0.20Fe_0.80 film could be grown in fcc phase pseudomorphic with Ni(0 0 1) up to 20 nm thick on Ni/MgO(0 0 1), while Ni_0.40Fe_0.60 film in bcc phase pseudomorphic with Fe(0 0 1) up to 10 nm thick on Fe/MgO(0 0 1). With increasing thickness, their growth phases transformed into their own stable phases. Whether or not the pseudomorphic phase may be induced and what its critical thickness may be should depend primarily on the lattice misfit between the crystal planes in contact. The growth mode of Ni_0.40Fe_0.60 film was investigated more in details to be compared with the simulations of the average strain energy versus thickness and with those of the critical thickness of the pseudomorphic films versus the lattice misfit between the contacted crystal planes.     

A high (0 0 1)-oriented CoPt/Ag film deposited on glass substrate

The magnetic properties and structures of CoPt_15 nm/Ag_0−100 nm films deposited by DC magnetron sputtering on glass substrates have been studied. The (0 0 1) texturing was improved by introducing an Ag underlayer. As the Ag underlayer thickness was 100 nm, a nearly perfect (0 0 1)-textured CoPt film was obtained.     

Common fingerprint of hydroxylated non-polar steps on MgO smoke and MgO films

The infrared spectra of MgO smoke exposed to water are compared to high-resolution electron energy loss spectra (HREELS) of hydroxylated ultrathin MgO(1 0 0)/Ag(1 0 0) films. Very similar bands are observed at 3458–3480 cm^?1 and 3710–3714 cm^?1. On the basis of first principle calculations, these bands are interpreted as the stretching modes of the two distinct OH groups that are formed at monatomic steps parallel to 〈1 0 0〉 crystallographic directions. The lower frequency band is due to H adsorbed on O at the step edge, while the higher frequency one originates from OH groups that are twofold coordinated with Mg. Consistently, scanning tunnelling microscopy images of MgO films, prepared in similar conditions as during the HREELS experiments, show that the MgO/Ag(1 0 0) island edges are mainly non-polar, i.e. oriented along the 〈1 0 0〉 direction. In spite of that, a minor contribution to the OH-stretch intensity from hydroxylated polar 〈1 1 0〉 steps cannot be excluded; for such geometry density functional theory predicts indeed a single OH species with a vibrational frequency very close to the high-frequency band of OH adsorbed at 〈1 0 0〉 steps.     

Novel PdAgCu ternary alloy as promising materials for hydrogen separation membranes: Synthesis and characterization

The use of the sequential electroless plating method allowed us to obtain the PdAgCu ternary alloy on top of dense stainless steel (SS) 316 L disks. The XRD analysis indicated that initially the nucleation of the two phases of the alloy (FCC and BCC) takes place, but the FCC/BCC ratio increases with the annealing time at 500 °C in H₂ stream. After 162 h, the film contained only the FCC phase, which presents promising properties to be applied in the synthesis of hydrogen selective membranes. SEM cross-section results showed that a dense, continuous, defect-free film was deposited on top of the SS support, and the EDS data indicated that no significant gradient was present on the thickness of the film. XPS and LEIS allowed us to determine that Cu and Ag surface segregation takes place after annealing up to 500 °C/5 days. In the top-most surface layer, Ag enrichment takes place as determined by ARXPS experiments which can be the result of the lower surface tension of Ag compared to that of Cu and Pd. Increasing the annealing temperature results in an increase of the Ag surface segregation while the Cu concentration in the top-most surface layer decreases.     

Electronic core levels of hydroxyls at the surface of chromia related to their XPS O 1s signature: A DFT + U study

The Cr 2p and O 1s binding energy (BE) levels have been calculated by first principles methods for different models of hydroxylated (0001)-Cr₂O₃ surfaces. Several surface terminations have been considered. The calculations allow us to reproduce the O 1s shifts between O in oxide and OH groups. It is found that two main effects account for the OH binding energy shifts. On the one hand, the increased covalency of the O–H bond with respect to the Cr–O bond, lowers the electronic O (1s and 2p) energy, and in consequence the BE of the core levels (O 1s) are higher. On the other hand, the lower the OH coordination number, the higher the valence and core levels energy, and the lower the BE. Consequently, mono-coordinated hydroxyls have a binding energy near that of O^2? in the oxide (ΔBE_OH–O = ? 0.2–0.0 eV). Two-fold coordinated hydroxyls have a slightly higher BE (ΔBE_OH–O = + 0.3 eV). Three-fold coordinated OH groups have a higher binding energy (ΔBE_OH–O = + 0.6?0.7 eV), corresponding to that experimentally measured for OH groups. Finally, water adsorbed above OH groups exhibits a still higher BE (ΔBE_HOH–O = + 0.9–1.0 eV). The ΔBE are slightly under-estimated under the initial state approximation, and overestimated under the final state (Z + 1) approximation.     

Silver sulphide growth on Ag(111): A medium energy ion scattering study

The interaction of S₂ with Ag(111) under ultra-high vacuum conditions has been investigated by medium energy ion scattering (MEIS). 100 keV He⁺ MEIS measurements provide a direct confirmation of a previous report, based on thermal desorption, that the growth of multilayer films of Ag₂S occurs through a continuous corrosion process. These films show a commensurate (√7 × √7)R19° unit mesh in low energy electron diffraction, consistent with the epitaxial growth of (111) layers of the high-temperature F-cubic phase of Ag₂S. The substantial range of co-existing film thicknesses found indicates that the growth must be in the form of variable-thickness islands. The use of 100 keV H⁺ incident ions leads to a very rapid decrease in the sulphide film thickness with increasing exposure that we attribute to an unusual chemical leaching, with implanted H atoms interacting with S atoms and desorption of H₂S from the surface.     

Diffusion and chemical composition of TiNxOy thin films studied by Rutherford Backscattering Spectroscopy

Thickness and chemical composition of the TiN_xO_y thin films deposited by reactive magnetron sputtering from Ti target at controllable oxygen flow rate were determined by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He⁺ ions. The films were deposited on carbon foils and amorphous silica (a-SiO₂) substrates at 25 °C and 250 °C. The estimated film thickness is of 75-100 nm. The O/Ti atomic ratio in the films increases up to 1.5 with increasing oxygen flow rate, while that of N/Ti decreases from about 1.1 for TiN to 0.4 at the highest oxygen flow rate. Substantial out-diffusion of carbon from the substrate is observed which is independent of the substrate temperature. Films grown onto a-SiO₂ substrates can be treated as homogeneous single layers without interdiffusion. It is more difficult to determine the nitrogen and oxygen content due to superposition of RBS signals arising from film and substrate. RBS analysis of the depth profile indicates that for the investigated films the carbon diffusion and oxidation not only at the topmost surface layers but over the bulk of the films were found. Comparison with XPS results indicates substantial oxygen adsorption at the surface of TiN_x thin films obtained at zero oxygen flow rate.     

Effects of growth rate and buffer-layer on Bi2Sr2CaCu2Oy thin film fabrication

The effect of the growth rate on the Bi₂Sr₂CaCu₂O_y (Bi2212) thin film quality on MgO substrate is investigated at several growth rates from 0.175 to 3 nm/min. The maximal step height on the film surface is improved from about 100 to 6 nm by the reduction of growth rate to 0.5 nm/min and simultaneously the superconducting critical temperature attaining to a zero resistance T_c(R=0), is also improved from 50 to 63 K. The surface morphologies of the upmost Bi-superconducting thin films with the intermediate layers on MgO substrate is also studied in contrast to that deposited directly on the MgO substrate.     

Magnetic anisotropy of epitaxial Fe layers grown on Si(0 0 1)

The magnetic properties of epitaxial iron films up to 80 monolayers (ML) thickness grown on Si(0 0 1) by using a template technique were investigated by means of superconducting quantum interference device and magneto-optic Kerr effect techniques. The thinnest films investigated (∼3 ML) exhibit a composition close to Fe₃Si with a Curie temperature below room temperature (RT) and strong out-of-plane remanent magnetization that reflects the presence of a dominant second order surface anisotropy term. Thicker films (⩾4 ML) are ferromagnetic at RT with remanent magnetization in film-plane and a composition closer to pure Fe with typically 8–10% silicon content. When deposited at normal incidence such films show simple in-plane fourfold anisotropy without uniaxial contribution. The relevant fourth-order effective anisotropy constant K₄^eff was measured versus film thickness and found to change its sign near 18 ML. The origin of this remarkable behavior is investigated by means of a Néel model and mainly traced back to fourth-order surface anisotropy and magneto-elastic effects related to the large biaxial in-plane compressive strain up to 3.5% in the thinnest (⩽25 ML) films.     

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.     

Effect of film thickness on the transport properties of MgB2 synthesized by spray pyrolysis

Polycrystalline MgB₂ films of different thickness have been prepared by employing spray pyrolysis technique on MgO (1 0 0) substrate. The MgB₂ and other phases have been confirmed using X-ray diffraction technique and no trace of impurities phases have been found. The resistivity behavior shows that the superconducting transition temperature lies in the range of 37–39 K with narrow transition width. The transport critical current density vary with films thickness and achieved highest value ～1.2 × 10⁶ A/cm² at 20 K for 2.0 μm thick film and its values increase as thickness increases.     

Adsorption and dissociation of water on LaB6(100) investigated by surface vibrational spectroscopy

The chemisorption of water (H₂O and D₂O) on a LaB₆(100) surface was studied with reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS). The clean surface was exposed to H₂O and D₂O at temperatures from 90 K to room temperature, and spectra were acquired after heating to temperatures as high as 1200 K. It was found that water molecularly adsorbs on the surface at 90 K as a monomer at low coverages and as amorphous solid water at higher coverages. Water adsorbs dissociatively at room temperature to produce surface hydroxyl species as indicated by OH/OD stretch peaks at 3676/2701 cm^?1. Room temperature adsorption also reveals low frequency loss features in HREEL spectra near 300 cm^?1 that are quite similar to results obtained following the dissociative adsorption of O₂. In the latter case, the loss features were attributed to the LaO stretch of O atoms bridge-bonded between two La atoms. In the case of dissociative adsorption of H₂O, the low frequency loss features could be due to either the LaO vibrations of adsorbed O or of adsorbed OH.     

Proton transfer in water–hydroxyl mixed overlayers on Pt(1 1 1): Combined approach of laser desorption and spatially-resolved X-ray photoelectron spectroscopy

Proton transfer in water–hydroxyl mixed overlayers on a Pt(1 1 1) surface was studied by a combination of laser induced thermal desorption (LITD) method and spatially-resolved X-ray photoelectron spectroscopy (micro-XPS). The modulated pattern OH + H₂O/H₂O/OH + H₂O was initially prepared by the LITD method; vacant area with a 400 μm width was first formed in the mixed OH + H₂O overlayer by irradiation of focused laser pulses, and followed by refilling the vacant area with pure H₂O. Spatial distribution changes of OH and H₂O were measured as a function of time with the micro-XPS technique, which indicated that H₂O molecules in the central region flow into the OH + H₂O region. From quantitative analyses using a diffusion equation, we found that the proton transfer in the mixed overlayer consists of at least two pathways: direct proton transfer from H₂O to OH in the nearest site and the proton transfer to the next-nearest site via H₃O⁺ formation. The time scale of first and second path was estimated to be 5.2 ± 0.9 ns and 48 ± 12 ns at 140 K, respectively. In the presence of water capping layer, however, the rate of proton transfer is reduced by an order of magnitude, which would be explained by peripatetic behavior of proton into H₂O capping layer.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

Interface reaction of Ta/NiO and its effect on exchange coupling

Ta/NiO/NiFe/Ta multilayers, utilizing Ta as the buffer layer, were prepared by RF reactive and DC magnetron sputtering. The exchange coupling field between NiO and NiFe reached a maximum value of 120 Oe at a NiO film thickness of 50 nm. The composition and chemical state at the interface region of Ta/NiO/Ta were studied using the X-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that there is an `intermixing layer’ at the Ta/NiO (and NiO/Ta) interface due to a thermodynamically favorable reaction: 2Ta+5NiO=5Ni+Ta<sub>2</sub>O<sub>5</sub>. This interface reaction has an effect on the exchange coupling. The thickness of the `intermixing layer’ as estimated by XPS depth-profiles was about 8–10 nm.