No abstract

This paper describes a Fortran program, IsGISAXS, for the simulation and analysis of grazing‐incidence small‐angle X‐ray scattering (GISAXS) of islands supported on a substrate. As is usual in small‐angle scattering of particles, the scattering cross section is expressed in terms of an island form factor and interference function. However, the emphasis is placed on the specificity of the grazing‐incidence geometry, in particular in the evaluation of the island form factor in the distorted‐wave Born approximation. A library of simple geometrical shapes is available. A full account of size and possible shape distributions is given in the decoupling approximation, where sizes and positions are not correlated, and in the local monodisperse approximation. Two types of island repartitions on the substrate are considered: disordered systems characterized by their particle–particle pair correlation functions, and bidimensional crystalline or paracrystalline systems of particles.

One challenge in the production of nanometer-sized objects with given properties is to control their growth at a macroscopic scale in situ and in real time. A dedicated ultrahigh-vacuum grazing-incidence small-angle x-ray scattering setup has been developed, yielding high sensitivity and dynamics. Its capabilities to derive the average particle shape and size and the film growth mode and ordering and to probe both surfaces and buried interfaces are illustrated for two prototypical cases: the model catalyst Pd/MgO(100) and the self-organized Co/Au(111) system. A wide range of technologically important systems can potentially be investigated in various gaseous environments.

The atomic structure of icosahedral B4C boron carbide is determined by comparing existing infra-red absorption and Raman diffusion measurements with the predictions of accurate ab initio lattice-dynamical calculations performed for different structural models, a task presently beyond X-ray and neutron diffraction ability. By examining the inter-and intra-icosahedral contributions to the stiffness we show that, contrary to recent conjectures, intra-icosahedral bonds are harder.

The present paper focuses on the analysis of grazing incidence small-angle x-ray scattering ͑GISAXS͒ of islands on a substrate. Getting accurate morphological parameters relevant for the elaboration process, i.e., growth curves, island equilibrium shape, and interfacial energy, implies a quantitative data analysis. The emphasis is put on the island form factor, i.e., the Fourier transform of the island shape. It is shown that the island shape and size can be obtained through the island symmetry, the presence of island facets, the asymptotic behavior at high momentum transfer for large polydispersity, and the zeros or minima of the intensity for small polydispersity. The specificity brought by the grazing incidence scattering geometry is highlighted by a careful comparison between the Born approximation and the more accurate distorted wave Born approximation. The interplay between the form factor and the interference function is all the more important in the total scattering intensity when incoherent diffuse scattering comes into play at small momentum transfer for disordered systems. Getting rid of these interpretation difficulties requires accurate measurements of the scattered intensity far in the reciprocal space. This analysis methodology is illustrated through recently acquired GISAXS patterns during the in situ molecular beam epitaxy of Pd nanoislands on MgO͑001͒ single crystals for different thicknesses and temperatures. The morphological parameters obtained agree very well with subsequent transmission electron microscopy-results. Finally, GISAXS diffuse scattering has been shown, originating from the growth-coalescence process and from the size dependence of the island capture area.

The origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of gold nanoparticles supported on TiO2(110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O2, Ar, or a mixture of O2 + CO and of Ar + CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle’s diameter of 2.1 ± 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both “model” and “real” catalysts.

No abstract

Plasmonics of Ag, Au, and Zn nanoparticles supported on Al 2 O 3 (0001), TiO 2 (110), and ZnO(0001) substrates has been probed by surface differential reflectivity spectroscopy (SDRS) during vapor deposition growth. Parallel and perpendicular interfacial susceptibilities (ISs), or "optical thicknesses", which characterize only the dielectric response of the film, are derived from experimental spectra in p-and spolarization using an inversion procedure based on Kramers−Kronig transform. The consistency of the approach is checked against sum rules. Plasmonic contributions are unraveled by decomposing ISs into damped oscillators and identified with the help of dielectric simulations of truncated supported spheres or spheroids. Beyond the common Drude behavior of Ag, Au, and Zn, the comparison between the three metals demonstrates the paramount role of interband transitions in the ISs profiles. While gold and silver show free electron plasmon modes, zinc exhibits polarization modes of bound electrons. However, despite those differences, the resonant modes that are identified herein are universal for supported particles. Particle shape, equilibrium aspect ratio, image field, polydispersity, and interface-induced damping are discussed by analyzing changes in frequencies, oscillator strengths, and broadenings. Deposit-induced band gap absorption for semiconductor substrate and switches from growth to coalescence regimes are evidenced. Static and dynamic coalescence are characterized by power law exponents as a function of particle size. Therefore, the unique framework that is proposed opens strong prospects in the optical characterization of growth, metal/ semiconductor interfaces, and gas adsorption.