Robert Cortes’s research while affiliated with École Polytechnique and other places

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, the studies of the microscopic parameters which 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 a same Rb0.5Co[Fe(CN)6]0.8. zH2O photostrictive core and isostructural shells with a similar thickness and variable mismatch with the core lattice. The shell deformation and the quantification of the optical electron transfer at the origin of photostriction are monitored by combined in situ and in 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 small lattice mismatch i.e., small a-cubic parameter for the shell) and the shell rigidity (decreased for large a-parameter). A shell with a high compressibility that is further increased by the presence of misfit dislocations will decrease its mechanical retroaction on the photo-switching properties of the core particles.

Iron epitaxial electrodeposition on Au(111) substrates is investigated using in situ scanning tunneling microscopy (STM) and ex situ X–ray diffraction (XRD). STM observations show that Fe grows quasi layer–by–layer at sufficiently negative potentials. XRD results indicate that Fe layers thicker than 3 ML are bcc and present the epitaxial relationship Fe(110) <1‒10> ║ Au(111) <11‒2>. They also show that the Fe lattice is uniaxially in–plane strained along Fe<1-10> and that the strain is progressively relieved with increasing layer thickness. The growth of Fe on a Ni layer deposited on Au(111) leads to strain free Fe layers with the same epitaxial relationship. The specific shape of Fe islands suggest that the first Fe monolayer deposited on Au(111) presents a centered rectangular lattice similar to that of bcc Fe(110) but is stretched along Fe<1-10> by more than 8%.

The growth and atomic structure of thin Co films electrodeposited onto Au(111) have been studied using in situ surface X-ray diffraction (SXRD) and scanning tunneling microscopy (STM). SXRD is used to monitor the growth in real-time and, following Co deposition, the strain of the Co film is characterized as a function of the Co thickness. In-plane SXRD measurements show that Co deposition leads to the formation of a reconstructed Au layer at the Au-Co interface which is composed of an unfaulted, single Au monolayer uniformly compressed with respect to the underlying bulk Au lattice. The surface normal structure of the Au-Co interface and Co films are examined by measurement of the extended X-ray reflectivity (or specular crystal truncation rod (CTR)) for a nominally two monolayer thick (2 ML) Co deposit. In situ STM has also been used to obtain the morphology of the Co films and the Au(111) reconstruction pattern both before and after Co dissolution. The STM data give complementary information to the SXRD results. The influence on the Co structure of the adsorption of carbon monoxide is also investigated in detail for a 2-3 ML thick Co film. It is shown that CO adsorption induces small vertical relaxations in the Co layers. In contrast, varying the electrochemical potential has negligible influence on the Co structure and affects only the electrolyte layering in the electrochemical double layer. The results are discussed in relation to the magnetic properties, in particular the change in the spin reorientation transition caused by CO adsorption.

High-quality core–shell particles, which associate a photostrictive core (Rb0.5Co[Fe(CN)6]0.8·zH2O, RbCoFe) and a ferromagnetic shell (Rb0.2Ni[Cr(CN)6]0.7·z′H2O, RbNiCr), were successfully grown by a multistep protocol based on coprecipitation in water. High-resolution transmission electron microscopy shows that well-defined heterostructures are formed and that the core–shell interface is abrupt with the epitaxial relationship [001](001)RbCoFe//[001](001)RbNiCr, confirmed by simulations of the X-ray diffraction line widths. The core particles are monocrystalline, with 50 nm sides, and the shell consists of large platelet-like crystallites, with a height that corresponds to the shell thickness and lateral dimensions comparable to the size of the core particles. Analysis of the diffracted intensities as a function of shell thickness (9–26 nm) shows that the epitaxial shell growth does not lead to a thick pseudomorphic layer at the interface. In contrast, Williamson–Hall plots suggest that a structural relaxation takes place to adapt the mismatched lattices, with the formation of misfit dislocations distributed over the entire shell thickness. This later finding is indicative of an effective mechanical coupling within the heterostructures. However, a magnetization increase by only a few percent was observed under light irradiation for these RbCoFe@RbNiCr particles. We showed from in situ synchrotron X-ray diffraction measurements that these small changes most likely reflect confinement effects as photoswitching of the core phase is partly or completely blocked depending on the shell thickness.

The influence of cobalt doping on the magnetic anisotropy of γ-Fe2O3 nanoparticles has been investigated using two different approaches: (i) simultaneous precipitation of Fe2+, Fe3+, and Co2+ precursors in water and (ii) adsorption of Co2+ ions onto the surface of preformed iron oxide particles followed by diffusion in the solid phase upon heat treatment. The incorporation of small amounts of Co dopants, less than 1 at %, was monitored by magnetization measurements combined with X-ray absorption spectroscopy experiments at the Co K-edge. These latter measurements were carried out in fluorescence mode using a crystal analyzer spectrometer for an enhanced sensitivity. Analyses of the X-ray absorption fine structures allowed for unraveling the differences in local atomic structure and valence state of Co in the two series of samples. A thermally activated diffusion in the spinel lattice was observed in the 250–300 °C range, leading to a substantial increase in magnetocrystalline anisotropy. At higher annealing temperature, magnetic anisotropy was still found to increase due to an enhanced surface contribution associated with dehydroxylation of terminal Fe atoms. This study not only provides direct correlations between magnetic anisotropy and dopant localization in Co doped γ-Fe2O3 but also demonstrates for the first time that simultaneous coprecipitation of Fe2+, Fe3+, and Co2+ may actually lead to heterogeneous doping, with a significant part of the Co dopants adsorbed at the particle surface.

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.

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The magnetism in ultrathin films and multilayers is an active field of research in strong relationship with applications. Advances in the understanding of such systems came from an improved control of growth modes and detailed structural studies coupled with magnetic measurements, especially in the UHV environment. Thanks to the development of in situ methods, similar studies can now be conducted in the electrolytic environment and investigating the magnetism of nanostructures on the nanometre scale has become a reality. After a brief review of our work about the epitaxial growth and magnetism of ultrathin magnetic layers of Ni, Co and Fe on Au(1 1 1), with an emphasis on the early stages of their electrochemical growth, this article introduces ongoing works which draw attention about the specificity and the opportunities offered by the electrochemical interface.

Films of mesoporous silica are formed on silicon single crystal by high-potential anodisation in neutral fluoride electrolyte. The films formed at 7–15 V potential exhibit a bright coloured aspect, characteristic of thin films of submicrometer thickness with flat parallel surfaces. SEM examination of cleaved profiles and X-ray reflectivity curves point to a stratified structure of these films on the nanometric scale. The stacking period, on the order of 10 nm, is an increasing function of the preparation potential. This structure is clearly related to the well-known oscillatory electrochemical behaviour observed above 3 V potential in the silicon/acidic-fluoride-electrolyte system. These observations confirm the idea that this oscillatory behaviour is accompanied by significant changes in the oxide morphology, namely a modulation of the porosity. The surviving of this oscillatory behaviour even though the interface is buried below a stack of several tens of oxide layers is a challenging observation for the theoretical modelling of the oscillation mechanism. We propose that periodic cracking of the inner oxide film may originate from a tensile stress due to electrochemically-induced desiccation near the silicon/oxide interface. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)