Thin NaCl films on silver (001): island growth and work function

The surface work function (WF) and substrate temperature dependence of the NaCl thin-film growth on Ag(001) have been studied by noncontact atomic force microscopy and Kelvin probe force microscopy. In the sub-monolayer range, the NaCl film is composed of large crystalline islands, which decrease in density and increase in size with increasing temperature during deposition. Each island is composed of a large base island 2 monolayers (ML) thick (for T > 343 K), which collects impinging NaCl molecules that form ad-islands on top. Kelvin probe force microscopy (KPFM) measurements show a reduction of the silver WF by $\Delta \varphi =\varphi _{\mathrm {Ag}}-\varphi _{\mathrm {NaCl/Ag}}=0.69\pm 0.03\, \mathrm {eV}$ with no dependence on the film thickness (1–6 ML), in agreement with recent theoretical calculations. The previously observed nanometer-sized moiré pattern on islands that are 45° rotated with respect to the silver lattice could be observed in the scanning tunneling microscopy mode. However, no contrast could be obtained in KPFM images

Defect mediated manipulation of nanoclusters on an insulator

With modern scanning probe microscopes, it is possible to manipulate surface structures even at the atomic level. However, manipulation of nanoscale objects such as clusters is often more relevant and also more challenging due to the complicated interactions between the surface, cluster and apparatus. We demonstrate the manipulation of nanometer scale gold clusters on the NaCl(001) surface with a non-contact atomic force microscope, and show that the movement of clusters is in certain cases constrained to specific crystallographic directions. First principles calculations explain this kinetic anisotropy as the result of the cluster attaching to surface defects: cation vacancies allow the clusters to bond in such a way that they only move in one direction. Constraining the movement of clusters could be exploited in the construction of nanostructures or nanomechanical devices, and the manipulation signatures may also be used for identifying cluster-defect complexes.Peer reviewe

Quantitative structure of an acetate dye molecule analogue at the TiO2- acetic acid interface

The positions of atoms in and around acetate molecules at the rutile TiO2(110) interface with 0.1 M acetic acid have been determined with a precision of ±0.05 Å. Acetate is used as a surrogate for the carboxylate groups typically employed to anchor monocarboxylate dye molecules to TiO2 in dye-sensitised solar cells (DSSC). Structural analysis reveals small domains of ordered (2 x 1) acetate molecules, with substrate atoms closer to their bulk terminated positions compared to the clean UHV surface. Acetate is found in a bidentate bridge position, binding through both oxygen atoms to two five-fold titanium atoms such that the molecular plane is along the [001] azimuth. Density functional theory calculations provide adsorption geometries in excellent agreement with experiment. The availability of these structural data will improve the accuracy of charge transport models for DSSC

Synthesis of TiO2(110) ultra-thin films on W(100) and their reactions with H2O

International audienceWe present a study of the growth and reactivity of ultra-thin films of TiO2 grown on W(100). Three approaches to film growth are investigated, each resulting in films that show order in low-energy diffraction (LEED) and a low level of non-stoichiometry in X-ray photoelectron spectroscopy (XPS). H2O is used as a probe of the reactivity of the films, with changes in the Ti 2p and O 1s core levels being monitored by XPS. Evidence for the dissociation of H2O on the TiO2(110) ultra-thin film surface is adduced. These results are discussed with reference to related studies on native TiO2(110

Quantitative Structure of an Acetate Dye Molecule Analogue at the TiO2−Acetic Acid Interface

The positions of atoms in and around acetate molecules at the rutile TiO2(110) interface with 0.1 M acetic acid have been determined with a precision of ±0.05 Å. Acetate is used as a surrogate for the carboxylate groups typically employed to anchor monocarboxylate dye molecules to TiO2 in dye-sensitized solar cells (DSSC). Structural analysis reveals small domains of ordered (2 × 1) acetate molecules, with substrate atoms closer to their bulk terminated positions compared to the clean UHV surface. Acetate is found in a bidentate bridge position, binding through both oxygen atoms to two 5-fold titanium atoms such that the molecular plane is along the [001] azimuth. Density functional theory calculations provide adsorption geometries in excellent agreement with experiment. The availability of these structural data will improve the accuracy of charge transport models for DSSC.The skillful technical assistance of Helena Isern and Lionel Andréat the ID32 beamline is gratefully acknowledged. This work was funded by ERC Advanced Grant ENERGYSURF (GT), EPSRC (UK) (EP/C541898/1), EU COST action CM1104, and M.E.C. (Spain) through project MAT2012- 38213-C02-02. This work made use of ARCHER, the U.K.’s national high-performance computing services provided through the U.K.’s HPC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202).Peer reviewe

Orientation-dependent chemistry and band-bending of Ti on polar ZnO surfaces

International audienceOrientation-dependent reactivity and band-bending are evidenced upon Ti deposition (1-10 Å) on the polar ZnO(0001)-Zn and ZnO(000 1)-O surfaces. At the onset of the Ti deposition, a downward band-bending was observed on ZnO(000 1)-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 + 2 ZnO → TiO2 + 2 Zn on ZnO(0001)-Zn and Ti + y ZnO → TiZnxOy + (y-x) Zn on ZnO(000 1)-O. Consistently, upon annealing thicker Ti adlayers, the metallic zinc is removed to leave ZnO(0001)-Zn surfaces covered with TiO2-like phase and ZnO(000 1)-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

An In Situ and Real Time Plasmonic Approach of Seed/Adhesion Layers: Chromium Buer Eect at the Zinc/Alumina Interface

International audienceThe effect of additives on metal/oxide interfaces is explored in situ and in real time on evaporated films by a combination of surface science techniques, among which a very flexible optical method shows a unique ability to scrutinize the growth and wetting properties of supported clusters that involve several elements. The study focuses on Cr at the Zn/α-Al2O3(0001) interface at 300 K. A particular interest of the present interface is that Zn does not stick at all on bare alumina. The sticking and morphology of both Cr and Zn films during their growth are analyzed from sub-monolayer to multilayer thicknesses. After an initial oxidation reaction with residual OH groups, shown to be detrimental to Zn adhesion, Cr growth proceeds through the formation of high aspect ratio particles that percolate around an average thickness of 10 Å. With regard to Zn growth on a Cr deposit, two very distinct stages can be distinguished. In the sub-monolayer thickness range, Cr forms a seed layer that drastically increases the Zn sticking coefficient from 0 to nearly 1 due to a diffusion length of physisorbed Zn adatoms before desorption larger than Cr island separation; Zn clusters are anchored on the Cr seeds that they encapsulate, but their wetting behavior is dictated by the interaction with alumina. In a second stage, as soon as the Cr film percolates, it forms an adhesion layer on which Zn grows in a nearly two-dimensional mode. In all cases, Cr films are stable upon annealing. On Cr-covered alumina, the Zn desorption energy is enhanced as compared to bare surfaces, which, in line with atomistic simulations, is assigned to the formation of more favorable Cr–Al2O3 and Cr–Zn than Zn–Al2O3 bonds. Generally speaking, the ability demonstrated herein of small amounts of additives to dramatically increase the adhesion of films is of great practical interest. It shows that noncontinuous and partially oxidized films of additives, closer to realistic cases of application, can strongly enhance the sticking of films. Also, anchoring a functional film by discrete predeposited seeds can keep its properties intact

Geometric structure of Ti O 2 ( 110 ) ( 1 × 1 ) : Achieving experimental consensus

International audienc

Surface and Epitaxial Stresses on Supported Metal Clusters

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