Christian Ricolleau

Size and shape effects on the order -> disorder phase transition in CoPt nanoparticles

Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (T(C)(NP)) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4-3-nm particles, T(C)(NP) is found to be 325-175 degrees C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that T(C)(NP) is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of T(C)(NP). This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties.

Cobalt-based anisotropic particles prepared by the polyol process

Cobalt and Co80Ni20 metal particles were prepared by reduction of acetate salts in a liquid polyol. The relative amount of the hcp phase and the probability of stacking faults in this phase vary with the experimental conditions of synthesis. The formation of either isotropic or anisotropic particles appears to be closely related to the stacking faults. Several kinds of anisotropic particles were detected by TEM: platelets, rods or diabolo-like particles. For isolated diabolos and rods, HRTEM and electron diffraction showed that the crystallographic c axis is parallel to the rotation axis of the particles. Magnetic property measurements showed high values of coercivity and remanence for the anisotropic particles. The anisotropic shapes differ markedly from those previously observed for cobalt-based particles prepared by the polyol process. A mechanism that involves a solid intermediate phase as either precursor or template is proposed.

Phase diagram for mesoporous CTAB–silica films prepared under dynamic conditions

Using θ–2θ and grazing incidence synchrotron X-ray diffraction experiments, the domain of existence of the different phases is established for spin-coated mesoporous silica films as a function of the surfactant/Si molar ratio and the condensation rate of the initial silica sol. These phases derive from three different micellar structures: the 3D-hexagonal structure (P63/mmc), formed from spherical micelles, the cubic structure (Pm3n), made up of ellipsoidal micelles, and the 2D-hexagonal structure (p6m), composed of cylindrical micelles. Electron microscopy investigations show that the organization of films is generally maintained after removal of the surfactant by calcination. However, structural distortions take place in mesoporous films as a consequence of the usual contraction during drying and calcination of films.

Structural study of 3D-hexagonal mesoporous spin-coated sol–gel films

Mesostructured films deposited on glass substrates by spin coating are studied by X-ray diffraction, small angle X-ray scattering using grazing incidence (GISAXS) and high resolution electron microscopy. The films generally present a gradient of ordering from the air/sol interface to the interior of the film, with a 3D-hexagonal mesophase near the surface and a disordered micellar structure at the interior. However, the 3D-hexagonal ordering and the texturing can be extended over all the film thickness by adjusting the composition of the deposited solution. In particular, critical values are clearly observed both for the size of silica units forming the walls in the mesophase and for the film thickness. All these results can be understood from the competition between structuration and gelation of the films during alcohol removal.

Growth and structural properties of CuAg and CoPt bimetallic nanoparticles

Core/shell CuAg and alloyed CoPt have been synthesized using two vapor phase deposition techniques. For CuAg prepared by Thermal Evaporation (TE), the size and the morphology of the Cu cores are the key parameters to promote the formation of the core/shell arrangement. For CoPt synthesized by Pulsed Laser Deposition (PLD), the growth kinetics of nanoparticles, depending on the deposition rate, the substrate nature and the temperature, controls the nanoparticle morphology. The competition between the growth and the ordering kinetics governs the nanoparticle structure. By reducing the growth kinetics, as-grown L1(0) ordered nanoparticles are obtained according to the bulk phase diagram.

Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy

The growth of colloidal nanoparticles is simultaneously driven by kinetic and thermodynamic effects that are difficult to distinguish. We have exploited in situ scanning transmission electron microscopy in liquid to study the growth of Au nanoplates by radiolysis and unravel the mechanisms influencing their formation and shape. The electron dose provides a straightforward control of the growth rate that allows quantifying the kinetic effects on the planar nanoparticles formation. Indeed, we demonstrate that the surface-reaction rate per unit area has the same dose-rate dependent behavior than the concentration of reducing agents in the liquid cell. Interestingly, we also determine a critical supply rate of gold monomers for nanoparticle faceting, corresponding to three layers per second, above which the formation of nanoplates is not possible because the growth is then dominated by kinetic effects. At lower electron dose, the growth is driven by thermodynamic and the formation and shape of nanoplates are directly related to the twin-planes formed during the growth.

A TEM in situ experiment as a guideline for the synthesis of as-grown ordered CoPt nanoparticles

CoxPt1−x nanoparticles on amorphous alumina are synthesized by a finely tuned pulsed laser deposition (PDL) technique. By means of an alternate deposition experiment using pure Co and Pt targets, we have shown that the composition of the nanoparticles can be controlled with a precision of 2%. This composition varies linearly with the lattice parameter of the nanoparticles following the well-known Vegard law. During the synthesis, the substrate temperature is a key parameter to control the structure and the morphology of the nanoparticles. We have performed an in situ heating experiment in an electron microscope using disordered Co55Pt45 nanoparticles to study their thermodynamic behaviour. We have observed both ordering within the nanoparticles and coalescence between the nanoparticles. The temperature regimes evidenced by the in situ experiment are used to determine the temperature of the substrate to synthesize as-grown L10 ordered nanoparticles with better control on their size and composition.

Performances of an 80–200 kV microscope employing a cold-FEG and an aberration-corrected objective lens

The performances of a newly developed 80-200 kV cold field emission gun (CFEG) transmission electron microscope (TEM) integrating a spherical aberration corrector for a TEM image-forming lens have been evaluated. To begin, we show that the stability of both emission and probe currents makes use of this new CFEG much friendlier. The energy spread of electrons emitted from the CFEG has been measured as a function of emission current and shows a very last 0.26 eV energy resolution at 200 kV and even 0.23 eV at 80 kV. The combination of the CFEG and the CEOS™ aberration corrector, associated with enhanced mechanical and electrical stabilities of this new microscope, allows reaching an information transfer below 75 pm at 200 and 80 pm at 80 kV. This unseen resolution at 200 kV has allowed us to study the structure of CoPt nanoparticles by observing direct images of their atomic arrangement along the high indexes zone axis. We have evidenced the presence of defects in these nanostructures that are not parallel to the electron beam. The precise stoichiometry of two iron oxides, FeO and Fe2O3, has been determined from an analysis of iron valence state that was obtained from a direct analysis of EELS fine structures spectrum of the two oxides.

Epitaxial growth of ZnS on CdS in CdS/ZnS nanostructures

CdS/ZnS core/shell-like nanostructures have been elaborated by using the chemistry of colloids. The structural properties of the CdS core and the CdS/ZnS core-shell have been studied by high resolution transmission electron microscopy (HRTEM). The morphology and the structures of CdS have been determined. In a second step, the epitaxy of ZnS on CdS and the CdS/ZnS interface have been investigated. The accommodation mode between both materials by partial dislocations has been evidenced. Finally, the ZnS layer grows with the same structure as the CdS core, which is either in the cubic blende-type structure or in the hexagonal wurtzite-type structure.

Silver nanoparticle growth in 3D-hexagonal mesoporous silica films

The 3D-hexagonal mesoporous films are used as templates to grow uniform silver nanoparticles. The grafting of hydrophobic groups at the pore surface, significantly slows down the silver ion diffusion, anchoring small silver clusters in micropores and leading to organized domains of silver particles in mesopores with a narrow size distribution.