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The Cr/α-Al2O3(0001) interface has been explored by X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS) and ab initio first-principles calculations of core level shifts including final state effects. After an initial oxidation via a reaction with residual surface OH but no reduction of the alumina substrate, Cr grows in a metallic form without any chemical effect on the initially oxidized Cr. However, Cr metal lacks crystallinity. Long-range (reflection high energy electron diffraction) and short-range (XAS) order are hardly observed. Thus photoemission combined with atomistic simulations becomes a unique tool to explore the chemistry and environment at the Cr/alumina interface. Cr 2p, O 1s and Al 2s shifted components are all explained by the formation of moieties involving Cr3+ and/or Cr4+ and of metallic Cr0, which supports the previously found Cr buffer mechanism for poorly adhesive metals. Beyond the situation under study, the present data demonstrate the ability of a combined experimental and theoretical approach of core-level shifts to exhaustively describe the general case of disordered metal/oxide interfaces. [ABSTRACT FROM AUTHOR]

The morphology and adhesion energy of nanosized metal particles supported on dielectrics are a puzzling issue since, due to the increasing contribution of surfaces and interfaces in their energetics, their equilibrium shape escapes the rules established for large objects. The evolution of wetting during Volmer–Weber growth of nanoparticles is herein studied by in situ ultraviolet/visible surface differential reflectivity spectroscopy (SDRS). The integrated s-polarized SDR signal is shown to be proportional to the oscillator strength of the optically excited plasmon resonances parallel to the surface. Dielectric modelings show that this quantity, which is marginally affected by the size and density of the objects, depends mainly on the aspect ratio of the particles from which adhesion energy can be derived. Applied to noble (Ag, Au) or transition metals (Cr, Ni) and Zn on weakly interacting dielectric (Al2O3, SiO2, KBr) and semiconducting (TiO2, ZnO) substrates, this plasmonic approach evidences a robust U-shaped variation of the aspect ratio with film thickness and therefore size. In line with the thorough study of the Ag/Al2O3(0001) growth and linear elasticity predictions of the equilibrium shape of strained epitaxial particles, the first branch of the "U" is assigned to a size-dependent equilibrium shape related to surface/interface stress effects. A significant decrease in adhesion energy parallels a rounding of the particles. The second branch partly stems from flattening due to incomplete coalescence. The common behavior of poorly wetting supported metal nanoparticles that is revealed herein, with strong changes in shape and adhesion as a function of particle size, had not been evidenced so far. Both the proposed optical methodology and the final findings about adhesion at the nanoscale are of interest in the wide field of application of supported metal nanoparticles that involves heterogeneous catalysis and thin film growth. [ABSTRACT FROM AUTHOR]

For self‐assembling of TiO2 nanoparticles in multiple layers by layer‐by‐layer deposition to be applied to TiO2 thin films with defined and homogeneous thickness for large‐scale applications, the proper functionalization of substrate surface is a prerequisite to guarantee sufficient adhesion. The substrates selected and tested in the present paper were conductive, fluorine‐doped tin oxide (FTO) glass, nonconductive silica glass, and titanium alloy. The current study focusses on the analytical control of the stepwise functionalization of the substrates with 3‐aminopropyltriethoxysilane and glutaraldehyde (GA) for both the FTO glass and silica glass and with 3‐aminepropyl phosphonic acid and GA for Ti alloy. The analyses have been conducted by means of surface sensitive methods, X‐ray photoelectron spectroscopy, Auger electron spectroscopy, and time‐of‐flight secondary ions mass spectrometry. Chemical composition of surface of functionalized substrates shows differences in the degree and type of modification in dependence on substrate. It could be demonstrated that the best functionalized substrates were the conductive FTO glasses. The analysis of the functionalized Ti substrates has revealed that the surface coverage with 3‐aminepropyl phosphonic acid and GA molecules is an inhomogeneous one, and further optimization of the two‐step functionalization on the Ti alloy substrate is necessary. [ABSTRACT FROM AUTHOR]

Oxides are most commonly found in divided forms with properties difficult to control since their crystallographic orientations usually escape analysis. To overcome this an appropriate model system can be provided by ZnO smoke which, obtained by combustion of Zn in air, exhibits nanoparticles with well-defined surface facets. The present work focuses on the interaction of water with ZnO smokes by combining density functional theory based simulations and infrared spectroscopy measurements with applied pressures from 10−7 to 1 mbar. We demonstrate that the use of ultra-high vacuum allows the analysis of the very first stages of the adsorption, and report on water structures on ZnO(112̅0) for the first time. We further show that ZnO powders behave as multi-facet single crystals involving (101̅0), (112̅0), (0001), and (0001̅) surfaces with the polar orientations corresponding to 25% of the total surface area. A great deal of cross-agreements between experimental results and simulation provides a simple approach for the examination of hydroxylated/hydrated ZnO smokes and can be widely applied on other ZnO-related powders. [ABSTRACT FROM AUTHOR]

Orientation-dependent reactivity and band-bending are evidenced upon Ti deposition (1–10 Å) on polar ZnO(0001)–Zn and ZnO(0001̅)–O surfaces. At the onset of the Ti deposition, a downward band-bending was observed on ZnO(0001̅)–O while no change occurred on ZnO(0001)–Zn. Combining this with the photoemission analysis of the Ti 2p core level and Zn L3(L2)M45M45 Auger transition, it is established that the Ti/ZnO reaction is of the form Ti + 2ZnO → TiO2 + 2Zn on ZnO(0001)–Zn and Ti + yZnO → TiZnxOy + (y−x)Zn on ZnO(0001̅)–O. Consistently, upon annealing thicker Ti adlayers, the metallic zinc is removed to leave ZnO(0001)–Zn surfaces covered with a TiO2-like phase and ZnO(0001̅)–O surfaces covered with a defined (Ti, Zn, O) compound. Finally, a difference in the activation temperature between the O-terminated (500 K) and Zn-terminated (700 K) surfaces is observed, which is tentatively explained by different electric fields in the space charge layer at ZnO surfaces. [ABSTRACT FROM AUTHOR]

With the advent of new steel grades, galvanic protection by zinc coating faces a new paradigm. Indeed, enrichment in strengthening elements prone to oxidation, such as Al, Mn, and Si, leads to the formation of oxide films that are poorly wet by zinc. We study herein routes for the improvement of adhesion at the model Zn/α-Al2O3 interface by the addition of metals. As a first step, with the help of ab initio results on the adsorption characteristics of transition metal adatoms at α-alumina surfaces, we establish and rationalize clear trends in both the behavior of metal-alumina interaction strength and the relative thermodynamic stability of configurations with weakly and strongly bound metal adatoms. The reasons for the enhanced binding strength of transition metals, such as Cr, maintained regardless of the precise alumina termination and the surface charge state are pointed out. On these grounds, possible improvements of adhesion under realistic conditions are discussed. It is predicted that enrichment in transition metals, such as Cr, may produce strongly adhesive interfaces that lead to cohesive cleavage. [ABSTRACT FROM AUTHOR]

A new spectral restoration algorithm of reflection electron energy loss spectra is proposed. It is based on the maximum likelihood principle as implemented in the iterative Lucy-Richardson approach. Resolution is enhanced and point spread function recovered in a semi-blind way by forcing cyclically the zero loss to converge towards a Dirac peak. Synthetic phonon spectra of TiO2 are used as a test bed to discuss resolution enhancement, convergence benefit, stability towards noise, and apparatus function recovery. Attention is focused on the interplay between spectral restoration and quasi-elastic broadening due to free carriers. A resolution enhancement by a factor up to 6 on the elastic peak width can be obtained on experimental spectra of TiO2(110) and helps revealing mixed phonon/plasmon excitations. [ABSTRACT FROM AUTHOR]

The growth of vapour-deposited silver nanoparticles on &agr;-Al2O3 was studied in situ from 190 to 675 K by surface differential reflectivity spectroscopy in the UV-visible range. Changes in size, shape and density were derived from the plasmonic response modelled in the framework of interface susceptibilities by assuming that supported clusters were in the form of truncated spheres. The sticking coefficient of silver on alumina is close to one up to T ' 575 K before entering a regime of incomplete condensation. The Arrhenius dependence of the saturation density indicates a nucleation on defects at low temperature (T ≤ 300 K) and detrapping above. The particle size D evolution follows temporal power laws, independent of temperature and flux, which characterize the growth (D ∼ t0:31) and coalescence (D ∼ t0:55) of the film. These are indicative of the growth of isolated particles at constant density and dynamic coalescence, respectively. The wetting angle of the silver clusters is shown to increase during the growth regime, which is assigned to a combination of surface stress and mismatch-induced strain, and to decrease upon coalescence, which is attributed to plastic relaxation. For particles larger than 10 nm in size, the values of contact angle and adhesion energy level off with asymptotic limits (&thgr;c D =127:5° ± 1° and 0:48 ±0:02 J m-2) that nicely agree with tabulated data. This work highlights the ability of nanoplasmics to monitor in situ the growth kinetics of thin supported films. [ABSTRACT FROM AUTHOR]

The growth of a titanium thin film on the α-Al2O3(0 0 0 1) surface occurs in two steps. In the first stage, titanium is oxidised via a chemical reaction with the surface oxygen species which are characterised by a shift of 1.5 eV of the O 1s level. These species involve removable OH groups ((5±2)×1014/cm2 at saturation) and—likely—residual OH groups ((3±1.5)×1014/cm2, minimum value). At higher titanium coverage, the titanium film becomes metallic. [Copyright &y& Elsevier]

Determines the local structure centered on sodium after diffusion in silica. Use of extended X-ray absorption fine structure; Representation of soda-silica glass in local structure; Presence of ionic conductivity in oxide glasses.

This comment article introduces a series of four review papers about oxide surfaces which are published in this journal.[sup 1 - 4] It is emphasized that oxides concern a very broad area of applications and that the examination of the extreme surface of these materials is crucial to the understanding of an increasing number of phenomena. [ABSTRACT FROM AUTHOR]

Periodic arrays of anisotropic silver nanoparticles having peculiar optical properties are fabricated at a macroscopic scale. The proposed scalable method is based on temperature‐assisted solid‐state dewetting of a continuous thin layer deposited on a silica substrate patterned by the nanoimprint technique. The resulting nanoparticles are shaped like diamonds and are half‐embedded into the patterned silica. A period‐dependent optimum in film thickness for the quality of spatial organization is found and discussed in terms of thermodynamics and, for the first time, in terms of the role of grains in the dewetting process. The optical properties of the arrays are driven by not only simply the particle shape but also the lattice period and the degree of order. A surface lattice resonance that disperses with the underlying period is evidenced experimentally and confirmed by optical simulations. The opportunity to fabricate and tune such an assembly of plasmonic particles on transparent substrate opens interesting perspectives for not only fundamental photonics but also potential optical applications. [ABSTRACT FROM AUTHOR]

Thanks to a dedicated modelling of intensities, the depth sensitivity of X-ray photoemission is used to probe the segregation profile of aluminium at the (110), (100) and (111) low index surfaces of the body-centred Fe 0.85 Al 0.15 random alloy. Sputtered surface composition is close to the nominal bulk one, thus excluding preferential sputtering. Surface enrichment in aluminium upon annealing starts at around 700 K before reaching a stationary state above 1000 K. The average surface composition is close to Fe 0.5 Al 0.5 , corresponding to the B 2 CsCl structure of the phase diagram. The impacted depth, that is in the range of 2.5–3.5 nm, is quite significant. Although not evidenced previously in surface science conditions at FeAl single crystal surfaces, it is qualitatively in agreement with the segregation at grain boundaries and shear planes of Al-alloyed steels. This segregation tendency is rationalized through ab initio calculations. Unlabelled Image • Severe aluminium segregation above 700 K at the surfaces of bcc Fe 0.85 Al 0.15 • Photoemission determination of the concentration profile in a random alloy • Nanometer deep impacted layer with a Fe 0.5 Al 0.5 composition • Ab initio modelling of segregation trends [ABSTRACT FROM AUTHOR]

• Similar aluminium segregation deep profile at the surfaces of bcc Fe 0.85 Al 0.15. • Orientation (1 1 0)/(1 0 0)/(1 1 1) dependent surface structures. • Al-rich long-range nanometer size surface reconstruction at (1 1 0) surface. • Pronounced nano-faceting at (1 1 1) surface. Based on previous photoemission findings, the influence of newly observed aluminum surface segregation at the low (1 1 0), (1 0 0) and (1 1 1) index surfaces of the A 2 body-centered Fe 0.85 Al 0.15 random alloy is explored. Scanning Tunneling Microscopy, Low-Energy Electron Diffraction and Grazing-Incidence X-ray Diffraction are combined to explore surface structures and topographies. The formation of a surface composition close to the B 2 Fe 0.5 Al 0.5 occurs with the appearance of (i) a long-range pseudo-hexagonal incommensurate superstructure with a periodicity of ~ 18 Å on the (1 1 0) surface, (ii) a (1 × 1) termination on the (1 0 0) surface and (iii) a intense vicinal faceting of the (1 1 1) surface in the form of triangular pits. [ABSTRACT FROM AUTHOR]

The spectroscopic fingerprints of the point defects of titanium dioxide remain highly controversial. Seemingly indisputable experiments lead to conflicting conclusions in which oxygen vacancies and titanium interstitials are alternately referred to as the primary origin of the Ti 3d band-gap states. We report on experiments performed by electron energy loss spectroscopy whose key is the direct annealing of only the very surface of rutile TiO2 (110) crystals and the simultaneous measurement of its temperature via the Bose-Einstein loss/gain ratio. By surface preparations involving reactions with oxygen and water vapor, in particular, under electron irradiation, vacancy- and interstitial-related band-gap states are singled out. Off-specular measurements reveal that both types of defects contribute to a unique charge distribution that peaks in subsurface layers with a common dispersive behavior. [ABSTRACT FROM AUTHOR]

This paper reports on the long-standing puzzle of the atomic structure of the Ag/α-Al 2 O 3 (0001) interface by combining X-ray absorption spectroscopy, to determine Ag local environment [ i.e. average Ag-Ag ( d Ag-Ag ) and Ag-O ( d Ag-O ) interatomic distances and Ag coordination numbers (CN)], and numerical simulations on nanometric-sized particles. The experimental key was the capability of a structural study of clusters involving only a few atoms. The concomitant decrease of d Ag-Ag and CN with decreasing cluster size provides unambiguous fingerprints for the dimensionality of the Ag clusters in the subnanometric regime leading to a series of unexpected results regarding the size-dependent interface structures. At low coverage, Ag atoms sit on surface Al sites to form buckled monolayer-thick islands associated with a Ag-Ag distance (2.75 Å) which fits the alumina lattice. Upon increasing Ag coverage, as 3D clusters appear, the Ag interface atoms tend to leave Al sites to sit atop O atoms as d Ag-Ag increases. The then highlighted size-dependent evolution, is built on structural models which seemed so far contradictory in a static vision of the interface. Theory generalizes the case as it predicts the existence of alumina-supported 2D clusters of Pd and Pt at small coverage and a similar 2D-3D transition upon increasing the size. The structural transformation from 2D Ag clusters to macroscopic 3D islands is accompanied by a noticeable reduction of adhesion energy at the Ag/α-Al 2 O 3 (0001) interface.

The Cr/α-Al 2 O 3 (0001) interface has been explored by X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS) and ab initio first-principles calculations of core level shifts including final state effects. After an initial oxidation via a reaction with residual surface OH but no reduction of the alumina substrate, Cr grows in a metallic form without any chemical effect on the initially oxidized Cr. However, Cr metal lacks crystallinity. Long-range (reflection high energy electron diffraction) and short-range (XAS) order are hardly observed. Thus photoemission combined with atomistic simulations becomes a unique tool to explore the chemistry and environment at the Cr/alumina interface. Cr 2p, O 1s and Al 2s shifted components are all explained by the formation of moieties involving Cr 3+ and/or Cr 4+ and of metallic Cr 0 , which supports the previously found Cr buffer mechanism for poorly adhesive metals. Beyond the situation under study, the present data demonstrate the ability of a combined experimental and theoretical approach of core-level shifts to exhaustively describe the general case of disordered metal/oxide interfaces.