Robert Cortès

Epitaxial Growth of Gold on HSi(111): The Determining Role of Hydrogen Evolution

The potential dependence of gold electrodeposition on H-terminated Si(111) is studied in acidic electrolyte by means of atomic force microscopy and X-ray diffraction. The Au films (≤66 monolayers (ML)≈16 nm) are found to be (111)-oriented and in strong epitaxy with the Si(111) surface lattice, with two in-plane orientations separated by 180°. The deposit morphology is controlled by the deposition potential and can be islandlike or atomically flat. The flat morphology is accompanied by a preferential growth of 180°-rotated Au planes with respect to the Si bulk lattice which takes place at potentials where the hydrogen evolution reaction occurs. Obtaining ultraflat Au layers on Si(111) contrasts with the commonly observed islandlike morphology of electrodeposited films on semiconductors. This behavior is discussed in terms of a nucleation coupled with hydrogen evolution reaction (HER) and an enhanced Au adatom mobility induced by this reaction.

Intense photoluminescence of thin films of porous hydrogenated microcrystalline silicon

We report the preparation of highly photoluminescent porous silicon in the form of thin films with thicknesses of around 1 μm. These films are obtained by stain etching thin films of microcrystalline silicon deposited by plasma-enhanced chemical vapor deposition with a flow of silane gas (SiH4) that is highly diluted in hydrogen. Stain etching is performed with aqueous solutions of hydrofluoric acid and Fe(III) in the form of ferric chloride (FeCl3). The porous silicon obtained shows a strong photoluminescence with a quantum efficiency estimated in the 1%–10% range. It could be used for the design of inexpensive large-surface electroluminescent devices.We report the preparation of highly photoluminescent porous silicon in the form of thin films with thicknesses of around 1 μm. These films are obtained by stain etching thin films of microcrystalline silicon deposited by plasma-enhanced chemical vapor deposition with a flow of silane gas (SiH4) that is highly diluted in hydrogen. Stain etching is performed with aqueous solutions of hydrofluoric acid and Fe(III) in the form of ferric chloride (FeCl3). The porous silicon obtained shows a strong photoluminescence with a quantum efficiency estimated in the 1%–10% range. It could be used for the design of inexpensive large-surface electroluminescent devices.

Strain engineering of photo-induced phase transformations in Prussian blue analogue heterostructures

Heterostructures based on Prussian blue analogues (PBA) combining photo- and magneto-striction have shown a large potential for the development of light-induced magnetization switching. However, studies of the microscopic parameters that control the transfer of the mechanical stresses across the interface and their propagation in the magnetic material are still too scarce to efficiently improve the elastic coupling. Here, this coupling strength is tentatively controlled by strain engineering in heteroepitaxial PBA core-shell heterostructures involving the same Rb0.5Co[Fe(CN)6]0.8·zH2O photostrictive core and isostructural shells of similar thickness and variable mismatch with the core lattice. The shell deformation and the optical electron transfer at the origin of photostriction are monitored by combined in situ and real time synchrotron X-ray powder diffraction and X-ray absorption spectroscopy under visible light irradiation. These experiments show that rather large strains, up to +0.9%, are developed within the shell in response to the tensile stresses associated with the expansion of the core lattice upon illumination. The shell behavior is, however, complex, with contributions in dilatation, in compression or unchanged. We show that a tailored photo-response in terms of strain amplitude and kinetics with potential applications for a magnetic manipulation using light requires a trade-off between the quality of the interface (which needs a small lattice mismatch i.e. a small a-cubic parameter for the shell) and the shell rigidity (decreased for a large a-parameter). A shell with a high compressibility that is further increased by the presence of misfit dislocations will show a decrease in its mechanical retroaction on the photo-switching properties of the core particles.