S. Altieri

Structure and stability of nickel/nickel oxide core–shell nanoparticles

The results of a combined x-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HR-TEM) study of Ni nanoparticles (NP), before and after oxidation, are presented. An experimental set-up was realized for the preparation and study of pre-formed NP films, concentrating the attention on Ni NP in the diameter range between 4 and 8 nm. The XPS data were taken in situ from NPs after different stages of oxidation, including controlled dosing of O(2) gas in the experimental system and exposure to the atmosphere. The Ni 2p structure is a combination of spectra from metallic Ni in the NP core and from the oxide shell. The signal from the NP core was observed even for samples after exposure to air. From the comparison of HR-TEM experimental images with theoretical simulations, it was found that the Ni NP core has a regular multitwinned icosahedral structure, composed of single-crystal tetrahedra with (111) faces. The NiO phase is clearly observed forming islands on the NP surface.

Structural study of thin MgO layers on Ag(001) prepared by either MBE or sputter deposition

Abstract Growth, morphology and structure of thin MgO films prepared on Ag(001) substrates by different preparation procedures have been investigated by AES, LEED and modulated electron emission. Oxide layers were prepared by Mg MBE in an oxygen atmosphere or sputter deposition from bulk MgO target. In both cases stoichiometric MgO forms, which initially grows in a rocksalt tetragonally distorted structure. For 3-ML films, 3.6% expansion of the interlayer spacing along the growth axis has been measured. The misfit strain progressively reduces as the film thickness increases, and the equilibrium spacing is completely recovered at 8–10 ML thickness. In spite of the similarity in structure and strain, different deposition procedures lead to different film morphology. Sputter deposited films only partially cover the substrate. Fractional coverage (50%) has been evaluated for the 3-ML thick film, and full coverage only occurs at approximately 10 ML thickness. The MBE procedure results in almost complete layers. Fourfold broadening of spot profiles has been observed in LEED patterns of both MBE and sputter deposited films, indicating the occurrence of a similar large-scale atomic arrangement and surface morphology.

Thickness-dependent strain in epitaxial MgO layers on Ag(0 0 1)

Abstract Thin MgO films (1–20 ML) were synthesised by evaporating the metallic component in oxygen atmosphere on Ag(0xa00xa01). The reduced lattice misfit (3.1%) between the (0xa00xa01) surface unit meshes of the Ag fcc and MgO rock salt structure leads to the formation of epitaxial MgO layers with the (0xa00xa01)MgO∥(0xa00xa01)Ag and [1xa00xa00]MgO∥[1xa00xa00]Ag orientation relationships. The oxide layers were structurally characterized by LEED and modulated electron emission, while their surface morphology was studied by STM. In spite of the weak interaction between the oxide overlayer and the substrate, the occurrence of a significant tetragonal distortion of the MgO structure has been detected. We investigated in detail the strain in the oxide layer as a function of the thickness. Thin (

Structure and morphology of ultrathin NiO layers on Ag(001)

Abstract The structure and morphology of thin NiO films prepared on Ag(001) by reactive growth at 460 K has been investigated as a function of the film thickness in the 3–20 monolayers range. Emphasis was on the study of the oxide layer misfit strain. Primary beam diffraction modulated electron emission and low energy electron diffraction experiments allowed the determination of the in-plane and out-of-plane strain in the oxide layer, while scanning tunneling microscopy, X-ray photoelectron spectroscopy and secondary electron imaging have been used to monitor the film morphology, stoichiometry and structure, respectively. The film strain begins to be removed at a critical thickness of 10 ML, while at 20 ML the film is fully relaxed. Strain analysis indicates that the Poisson ratio of the oxide layer is nearly equal to that of the bulk material.