S. Giorgio

Photoluminescence of silicon nanoclusters with reduced size dispersion produced by laser ablation

We report a photoluminescence study of silicon nanoclusters produced by laser ablation. It was found that by varying the preparation parameters it was possible to change the mean cluster size in the range 1–5 nm. Within this size variation, the photoluminescence band shifts in a wide spectral region from near ultraviolet to near infrared. This size-dependent photoluminescence of Si nanoclusters is consistent with a quantum confinement effect. The observed influence of cluster oxidation on the luminescence properties also supports the quantum confinement interpretation. We proposed a discrete size model which supposes that the spectral position of the luminescence band is essentially determined by the volume of clusters with a complete outer atomic layer. In the framework of this model, we were able to deconvolute the observed luminescence bands into a set of fixed Gaussian bands. The model is supported by the observation of a size selective doping of Si nanoclusters whose effect was well explained by Auge...

Growth and morphology of palladium particles epitaxially deposited on a MgO(100) surface

Abstract Palladium particles (1.5–12 nm) have been epitaxially grown on UHV-cleaved MgO(100) surfaces. Their crystalline structure and their morphology have been studied by various TEM techniques. At high deposition temperatures, homogeneous distributions of particles, displaying the single shape of an half octahedron truncated on the top by a (100) plane, have been obtained. At low deposition temperature particles were flatter with mainly (100) facets. At high temperature, on clean air-cleaved substrates, homogeneous distributions of very small particles (1.5–2 nm) with a high number density and sharp size distributions were obtained. These homogeneous distributions of well defined particles can be used in surface studies as model catalysts.

Shape variations of Pd particles under oxygen adsorption

Pd particles prepared under UHV on MgO single crystals were in situ annealed under various pressures of O2 at high temperature, then observed ex situ in HRTEM and WBDF imaging. Annealing induces extension of the (100) faces and particle flattening. These morphology variations are reversible after reduction in H2. The experimental shapes are close to the equilibrium forms. After oxygen adsorption, the evolution of the particle shapes is in good agreement with the calculated shapes from thermodynamic considerations.

Structure and morphology of small palladium particles (2–6 nm) supported on MgO micro-cubes

Abstract Small MgO cubes were prepared in a controlled atmosphere and protected against air contamination. Their clean surfaces were used as supports for Pd particles (2–6 nm) vacuum-deposited from a Knudsen cell in the same chamber. The Pd particles epitaxially oriented on the substrate were observed by HRTEM, microdiffraction and convergent-beam electron diffraction. An isotropic lattice expansion was determined by microdiffraction on isolated particles, increasing between 2% and 8% for particle sizes decreasing from 6 to 2 nm. From the HRTEM images of particles observed in two directions, parallel and perpendicular to the interface with the MgO substrate, the particles were characterized as single crystals with fcc structure and octahedral shape. The octahedra are truncated by (001) planes at the top and at the interface.

Dynamic observations of Au catalysts by environmental electron microscopy

Au clusters in the size range (1–8 nm), supported on MgO and TiO2, are studied at the atomic scale by High Resolution Transmission Electron Microscopy (HRTEM), in standard conditions and during cycles of gas treatments by environmental HRTEM. Their morphology, the structure of the interface with the substrate and their adhesion energy are deduced from top and profile views, according to the preparation techniques, by atomic deposition in ultra high vacuum or by deposition precipitation in wet conditions.Under hydrogen atmosphere the gold clusters exhibit a truncated octahedron shape while under oxygen they show a rounding of the shape. These evolution are reversible in hydrogen/oxygen cycles. In the case of the CO oxidation by Au clusters supported on TiO2, the formation of CO2 in the microscope sample holder is evidenced by a differentially pumped mass spectrometer connected to the environmental sample holder, at the same time as the rounding of the clusters. The implication of the shape evolution in CO oxidation is discussed.

High-resolution transmission electron microscopy study of gold particles (greater than 1 nm), epitaxially grown on clean MgO microcubes

Abstract Gold particles (greater than 1 nm) were vacuum deposited on clean surfaces of MgO microcubes in situ synthesized in a controlled atmosphere. The particles were observed simultaneously with the MgO substrate in plan view and in cross-section by high-resolution transmission electron microscopy, microdiffraction and convergent-beam electron diffraction. The gold particles (less than 4 nm) had a f.c.c. structure and were perfectly accommodated with the substrate, leading to a lattice expansion of 2.9%. In this size range (1-4 nm), all the particles were single crystals and had an half-octahedron shape with (001) truncations on the edges. The interface between the particles and the MgO substrate was flat; no defects were imaged. In contrast larger (greater than 4 nm) particles have the lattice parameter of the bulk material. The high stability of the particles in the electron beam was certainly due to their strong interaction with the clean MgO surface.

Enhancement of exciton emission from ZnO nanocrystalline films by pulsed laser annealing

Pulsed ArF laser annealing in air and in hydrogen atmosphere improves the optical properties of ZnO nanostructured films. Independently on the ambient atmosphere, laser annealing produces two major effects on the photoluminescence (PL) spectra: first, the efficiency of the exciton PL increases due to the decrease of the number of non-radiative recombination centers; second, the intensity of the defect-related orange band decreases because of the removing of excessive oxygen trapped into the films during deposition. However, annealing in the ambient air also increases the intensity of the green band related to oxygen vacancies. We show that the combination of laser annealing and passivation of oxygen vacancies by hydrogen results in films free of defect-related emission and keeps intact their nanostructural character.

Effect of the interface structure on the high-temperature morphology of supported metal clusters

Abstract The morphology of Pd clusters prepared by ultrahigh-vacuum condensation or wet impregnation on MgO and the structure of the interfaces are studied by weak-beam dark-field and high-resolution electron microscopy, according to the particie sizes. The shapes are stable when the particles are annealed at increasing temperature, they are close to the eauilibrium forms. Their adhesion energy β with the substrate is experimentally deduced from the Wulff-Kaishew construction. It decreases when the particie size increases, and with the formation of defects at the interface.

Au particles supported on (110) anatase-TiO2

Abstract Au particles were prepared by evaporation in ultra high vacuum at high temperature, on the surfaces of TiO2 micro-spheres with the anatase structure. The morphology and the structural deformation in Au deposits were studied by high resolution transmission electron microscopy and image simulations by the multislice technique. The particles were polyhedral, limited by (100) and (111) faces. Patches with a hexagonal lattice were found around the particles, which was interpreted as thin Au islands on the surface. In these islands the Au lattice was deformed and perfectly accommodated to the (110) surface of TiO2.

High-resolution transmission electron microscopy studies of structural deformations at the interface between Pd particles and MgO surfaces

Abstract Pd clusters were condensed on the surfaces of MgO microcubes, prepared in situ and observed by high-resolution transmission electron microscopy (HRTEM) in order to see directly both the profiles of the particles and the Pd-MgO interfaces. The HRTEM images in cross-sectional view were studied by numerical analysis. Small (1–2 nm) particles, with homogeneously expanded lattice distances, are perfectly accommodated to the MgO substrate and the dilatations are isotropic. In large (5–6 nm) particles, some dilatations of the d 200 distances were measured only at the interface, the top and the edges. Theoretical images of these particles (5 nm) were simulated by the multislice method. They provide evidence that the large Pd particles keep the bulk parameter, except in the first three deposited layers, which are expanded and progressively accommodated to the substrate. The deformations observed at the top and the edges are only artefacts of microscopy.