STM and XPS characterisation of vacuum annealed nanocrystalline WO3 films

Scanning tunnelling microscopy and X-ray Photoelectron Spectroscopy were conducted on magnetron sputtered WO₃ thin films, following a sequence of ultra high vacuum anneals from 100 °C to 900 °C. Annealing from 100 °C to 400 °C induced an upward surface band bending of about 0.3 eV, attributed to the oxygen migration from the bulk to the surface, but no changes in the surface topography. Chemical changes occurred from 600 °C to 800 °C, associated with the formation of secondary oxide species. STM imaging showed that the film surface consists of amorphous particles 35 nm in size up to 600 °C, while higher temperatures resulted in an increase in particle size. Crystallisation of the nanoparticles started to occur after annealing at 600 °C. The implications in terms of gas sensing are discussed.     

Overpotential deposition of Ag monolayer and bilayer on Au(1 1 1) mediated by Pb adlayer underpotential deposition/stripping cycles

Ultra-thin Ag films on the Au(1 1 1) surface were prepared via overpotential deposition (OPD) in the presence of Pb²⁺ ions. By carrying out repetitive Pb adlayer underpotential deposition (UPD) and stripping cycles during Ag bulk deposition, the two-dimensional growth of Ag films was significantly enhanced in high OPD. The Ag monolayer sample was made by comparing the voltammetry curves, in which the signatures for Pb adlayer UPD on Au(1 1 1) changed to that on Ag(1 1 1). As demonstrated by the X-ray specular reflectivity measurements, nearly complete monolayer and bilayer films can be made with optimized deposition procedures. On subatomic scale, however, we found that these films have significant higher root-mean-square displacement amplitudes than those underpotentially deposited Ag monolayer and bilayer on either Au(1 1 1) or Pt(1 1 1).     

Surface alloys, overlayer and incommensurate structures of Bi on Cu(1 1 1)

Using surface X-ray diffraction, we have determined the structure of three different sub-monolayer phases of Bi on Cu(1 1 1). In contrast to an early report, we find that at a coverage of 1/3 monolayer a substitutional surface alloy is formed with a (√3 × √3)R30° unit cell. For increasing coverage, de-alloying occurs, leading to an overlayer structure at a coverage of 0.5 ML in which the Bi atoms form zigzag chains. The surface contains three domains of this phase. Finally, at a slightly higher coverage of 0.53 ML, the unit cell is compressed in one direction, leading to a uniaxial-incommensurate phase with three rotational domains.The structure determination includes relaxations in the topmost layers and therefore allows a detailed comparison of the most important bond distances. This shows that an increased charge density of the Cu(1 1 1) surface is the main driving force for the different phases.     

Three-dimensional structure of the calcite-water interface by surface X-ray scattering

The three-dimensional structure of the calcite (104)-water interface has been determined with surface X-ray scattering. Nine crystal truncation rods (including specular and non-specular rods) were measured providing both vertical and lateral sensitivity to the interfacial structure. The results reveal that calcite is nearly ideally terminated with a single surface hydration layer that includes two inequivalent water molecules having distinct heights of 2.3 ± 0.1 and 3.5 ± 0.2 Å, each with a well-defined lateral registry with respect to the calcite surface. No additional layering of water is observed beyond this surface hydration layer. Small displacements in the outer two calcium carbonate layers were also observed. These results are compared with previous experimental and computational results.     

Molecular organization of discotic mesogenic cis-dichlorobis(3,4,5-trialkoxyphenylisonitrile)platinum (II) complexes on Langmuir and Langmuir–Blodgett films

Langmuir and Langmuir–Blodgett films of a new class of square planar metallomesogens, the cis-dichlorobis(3,4,5-trialkoxyphenylisonitrile)platinum (II) complexes (PtC_n), have been prepared.

The structure and the stability of the floating monolayers of these discotic complexes have been investigated by means of compression isotherms, surface potential measurements and Brewster angle microscopy (BAM). A certain influence of the peripheral substituents on the properties of the Langmuir films are observed. It is shown that the hexoxy derivative (PtC₆), which shows mesogenic behavior at room temperature, has a more marked tendency to self-organization in ordered multilayers than the butoxy complex (PtC₄) which is not liquid crystalline at room temperature.

The structural properties of films, transferred at different pressures, have been studied by AFM and UV–VIS, FT-IR transmission and reflection–absorption infrared spectroscopy (RAIRS). The transfer of the monolayers onto solid substrates leads to Langmuir–Blodgett films in which face-on orientation is found for the molecule cores. The organization of the aliphatic chains depends on the pressure at which the monolayers are transferred.     

The ZnO non-polar (10 0) surface: an X-ray structural investigation

We performed a structural analysis of the non-polar ZnO (10 0) surface by means of grazing incidence X-ray diffraction. The analysis was conducted on ten rods, smooth surface domains, though of small coherent width, having been obtained after several Ar⁺ sputtering–annealing cycles. The surface derived from the bulk structure exposes one ZnO dimer per unit cell, parallel to the [001] axis. All the existing models, derived from ab initio calculations or low-energy electron diffraction (LEED) analyses, consist in a surface dimer whose O and Zn atoms are shifted inwards, with the O pointing outwards with respect to Zn. Whereas the LEED studies conclude on a dimer distance greater than in the bulk, the theoretical studies agree on a dimer contracted by amounts ranging from 5.5 to 7.5%. This contraction is interpreted as a result of the strong ionicity of ZnO, and is associated with a moderate dimer rotation. The latter, however, is found between 2.3 and 11.4°. Despite the discrepancies between the models, the Zn atom is always found shifted downwards by more than 0.25 Å. This is unambiguously rejected by our data, which show that the Zn atom keeps very close to its bulk position. It is displaced downwards by ΔZ_Zn=−0.06±0.02 Å, and it moves along [001] towards O by ΔX_Zn=0.05±0.02 Å. We denote a trend for the O atom to be displaced downwards too, with a concomitant displacement towards Zn. The faint X-ray scattering of O prevents us from assessing its position with accuracy. Depending on the choice of position for Zn in the error bar range, the buckling is evaluated as between −6.5 and 3°, or between −4 and 0.5°. The dimer distance is evaluated equal to 1.90 Å, with a deviation equal either to 0.06 or 0.11 Å.     

Analysis of roughness of Cs surfaces via evaluation of the autocorrelation function

At low temperature prepared quench-condensed Cs surfaces are analysed on a nanometer scale via scanning tunneling microscopy. The analysis of surface roughness is presented with the help of the evaluation of their autocorrelation function. In order to extract the correct autocorrelation function we present the requirement regarding the scan resolution of scanning probe microscopy (SPM) images in general. This is supported by a ‘numerical experiment’. Furthermore, we present some methods of deducing higher orders of autocorrelation lengths, which are needed to evaluate SPM images with non-random distribution of roughness amplitudes. These characteristic values of the autocorrelation function could play the key role in further statistical calculations, e.g., on how surface roughness alters the wetting behaviour of liquid helium adsorbed on the cesium surfaces.     

X-ray reflectivity study on gold films during sputter deposition

We performed in-situ X-ray reflectivity measurements of gold films during sputter deposition on polished silicon substrates. The measurements were performed at several substrate temperatures and under two argon pressures. The gold surfaces were also examined by scanning tunneling microscopy after deposition to obtain their real-space topographic images. These images were used to complement the X-ray reflectivity measurements in determining the effect of argon pressure on the gold surface and its height-height difference functions. An approximation for height-height difference functions was employed to analyze the X-ray reflectivity data. The measured interface width during growth followed a simple power law, consistent with recent theoretical results of dynamic scaling behavior. The scaling exponents, however, do not agree well with predictions based on some models in 2 + 1 dimensions.     

Structure analysis of stoichiometric LiNbO3(0 0 0 1) surfaces using low-energy neutral scattering spectroscopy

The atomic structure of LiNbO₃(0 0 0 1) surface was investigated by low-energy neutral scattering spectroscopy (LENS). Poled stoichiometric LiNbO₃ (SLN) samples were prepared for the measurements. The LENS was developed for surface structure and composition analysis particularly of highly insulating materials and was successfully applied to the structure analysis of the SLN(0 0 0 1) surface. The polar angle dependences of intensity of scattered He⁰ from the poled SLN surfaces indicate obvious differences between the negatively and the positively charged surfaces. It is suggested that O atoms cover the surfaces, and the first metal layers underneath the O layer consist of Li and Nb for negatively and positively charged surfaces, respectively, parallel to the applied electric field.     

Surface and subsurface oxide formation on Ni(1 0 0) and Ni(1 1 1)

The oxidation of Ni(1 0 0) and Ni(1 1 1) at elevated temperatures and large oxygen exposures, typical of the methods used in the preparation of NiO(1 0 0) films for surface studies, has been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ incident ions. Oxide film growth proceeds significantly faster on Ni(1 1 1) than on Ni(1 0 0), but on both surfaces oxide penetration occurs to depths significantly greater than 100 Å with total exposures of 1200 and 6000 L respectively. The metal/oxide interface is extremely rough, with metallic Ni extending to the surface, even for much thicker oxide films on Ni(1 1 1). On Ni(1 1 1), NiO growth occurs with the (1 0 0) face parallel to the Ni(1 1 1) surface and the close-packed 〈1 1 0〉 directions parallel. On Ni(1 0 0) the MEIS blocking curves cannot be reconciled with a single orientation of NiO(1 0 0) (with the 〈1 1 0〉 directions parallel) on the surface, but is consistent with the substantial orientational disorder (including tilt) previously identified by spot-profile analysis LEED.     

Surface X-ray scattering studies of the growth of Pd thin films on the Pt(0 0 1) electrode surface and the effects of the adsorption of CO

The morphology of electrochemically deposited Pd films on the Pt(0 0 1) electrode surface has been examined through the combination of cyclic voltammetry (CV) and in situ surface X-ray scattering (SXS). Analysis of SXS measurements has indicated that the Pd grows via pseudomorphic island formation, with the partial occupation of successive layers occurring at a first layer occupation of 0.8 ML. Further Pd deposition sees the formation of larger islands built onto the now complete monolayer, characteristic of pseudomorphic Stranski–Krastanov (SK) growth. In the H_UPD potential region the effect of CO on the surface expansion of the multilayer Pd film is negligible. In the hydrogen evolution region, however, the effect of the adsorption of CO has been shown to produce surface normal expansion and in-plane disorder of the Pd film. It is suggested that hydrogen permeation into the Pd film is enhanced on the CO-poisoned surface.     

Surface structure analysis based on the exclusive use of the specular LEED spot – a theoretical study

The exclusive use of the specularly reflected beam (the (0,0) spot) may be a more practical way of collecting data for a LEED I–V structure analysis under certain experimental conditions. In this paper we discuss the special properties of the (0,0) spot intensity and test its sensitivity towards structural changes for the model system CO/Ni(1 1 1) within the framework of a R factor analysis. It is found that the (0,0) spot can, indeed, be used for a reliable structure determination if the energy range is increased by collecting data at different polar and azimuthal angles of incidence. The R factor contrast is, however, reduced with respect to a conventional LEED I–V analysis.     

What information can be obtained by RHEED applied on polycrystalline films?

In this paper, an analysis of reflection high-energy electron diffraction (RHEED) performed on polycrystalline films is presented. It is shown that it is possible to obtain qualitative and quantitative information about the crystallography of the deposit. Two series of test samples were used: (i) glass substrates recovered by Fe grains with a (200) fiber-like texture normal or inclined with respect to the surface normal, and (ii) the same films but with a (110) texture. These samples exhibit characteristic RHEED patterns. Using a simple model, it is possible to determine: (i) the texture of the film, (ii) the average angle of the texture axis with respect to the surface normal, and (iii) the angular width of the distribution of this angle about its average value.     

Darwin''s theory for the grazing incidence geometry

Darwin's dynamical theory of X-ray diffraction was studied in the case of grazing incidence geometry. It was shown that in such geometry the Darwin theory is not correct. In the two-beam case, the reflectivity of the specular reflection calculated by the original Darwin's theory was slightly different from that calculated by the well-known Fresnel formula. Furthermore, in the three-beam case, there were large discrepancies between the calculations by the Darwin theory and those by the extended dynamical theory proposed by Kishino and Kohra. These discrepancies were resolved when multiple scattering in an atomic plane was taken into account.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

AFM, SEM and GIXRD studies of thin films of red polycarbazolyldiacetylenes

In this paper we report the results of a morphological and structural investigation on film properties of a soluble polydiacetylene, the poly[1,6-bis(3,6-dihexadecyl-N-carbazolyl)-2,4-hexadiyne] (polyDCHD-HS). The red films of this polymer, prepared by standard spin-coating techniques, revealed absence of linear dichroism and birefringence in contrast with the ordered mesophases detected by powder X-ray studies. In order to interpret the optical behavior of this polymer, we performed AFM and SEM studies of polyDCHD-HS films spun on hydrophylic and hydrophobic glass substrates. We found the presence of surfaces organized in rod-like particles, more regularly oriented on the hydrophylic substrate. GIXRD studies, carried out on films sufficiently thick to allow the observation of the diffraction pattern, reveled the presence of a lamellar structure with a spacing of 3.22 nm. The low intensity of the diffraction peaks and the isotropic linear optical properties of the films show that the lamellar mesophases are not extended over large areas. These findings were compared with the data obtained from AFM and SEM studies on films of two other polydiacetylenes, the poly[1-(3,6-dihexadexyl-N-carbazolyl)-6-(N-carbazolyl)-2,4-hexadyine] (polya-DCHD) and the poly[1,6-bis(3,6-dipalmitoyl-N-carbazolyl)-2,4-hexadyine] (polyDPCHD), spun on hydrophylic glass substrate. The results confirmed the presence of nodular morphologies which seem to be a general characteristic of this class of materials. The particles organization appears instead related to the chemical nature of the substituents on the carbazolyl rings.     

Effect of surface roughness on the electron-stimulated desorption of D from microporous D2O ice

The electron-stimulated desorption (ESD) of D⁺ from microporous D₂O ice films condensed on Pt(111) has been investigated. The total D⁺ yield as a function of temperature from 90–180 K depends sensitively on the film roughness, surface temperature and ice phase. In particular, we observe an irreversible increase in the cation yield as the microporous thin film is heated from 90–120 K, which we associate with a decrease in surface roughness as the micropores collapse. We present evidence which suggests that the number of surface sites available for emission, the surface roughness, and reneutralization or reactive scattering of the D⁺ desorbate play major roles in determining the ion yield. A simple model which qualitatively addresses the role of surface roughening on ESD ion yields shows good agreement with the data.     

Self-assembled germanium nano-clusters on silver(1 1 0)

The adsorption of germanium on Ag(1 1 0) has been investigated by scanning tunnelling microscopy (STM), as well as surface X-ray diffraction (SXRD). At 0.5 germanium monolayer (ML) coverage, Low Energy Electron Diffraction (LEED) patterns reveals a sharp c(4 × 2) superstructure. Based on STM images and SXRD measurements, we present an atomic model of the surface structure with Ge atoms forming tetramer nano-clusters perfectly assembled in a two-dimensional array over the silver top layer. The adsorption of the germanium atoms induces a weak perturbation of the Ag surface. Upon comparison with results obtained on the (1 1 1) and (1 0 0) faces, we stress the role played by the relative interactions between silver and germanium on the observed surface structures.     

STM observation of initial growth of Sn atoms on Ge(0 0 1) surface

We have studied initial growth of Sn atoms on Ge(0 0 1) surfaces at room temperature and 80 K by scanning tunneling microscopy. For Sn deposition onto the Ge(0 0 1) substrate at room temperature, the Sn atoms form two kinds of one-dimensional structures composed of ad-dimers with different alignment, in the 〈3 1 0〉 and the 〈1 1 0〉 directions, and epitaxial structures. For Sn deposition onto the substrate at 80 K, the population of the dimer chains aligning in the 〈3 1 0〉 direction increases. The diffusion barrier of the Sn adatom on the substrate kinetically determines the population of the dimer chain. We propose that the diffusion barrier height depends on surface strain induced by the adatom. The two kinds of dimer chains appearing on the Ge(0 0 1) and Si(0 0 1) surfaces with adatoms of the group-IV elements are systematically interpreted in terms of the surface strain.     

Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2O3-mullite multi-coatings for carbon/carbon composites

Oxidation protective SiC-Al₂O₃-mullite multi-coatings for carbon/carbon (C/C) composites were prepared with a two-step pack cementation process. The influence of preparation temperature and SiO₂/Al₂O₃ ratio of the pack powder on the phase, microstructure and oxidation resistance of the multi-coatings were investigated. It showed that the multi-coatings that contained mullite could be produced at 1700-1800 °C. A denser coating surface was acquired with the decrease of SiO₂/Al₂O₃ ratio in the pack chemistries while a little damnification to the interface of the coating and C/C substrate. The as-prepared coating could effectively protect C/C composites from oxidation at 1600 °C for 81 h.