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.

International audienc

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.

In order to determine the possibilities to control the chemical configuration of bimetallic nanoparticles, we have considered CuAg nanoparticles synthesized by a physical route as a model in this study. The synthesis was made by pulsed laser deposition under ultra-high vacuum conditions, via a sequential deposition procedure. We show that the temperature of the substrate and the absolute quantity of Ag in a particle are the main parameters that drive the chemical configuration. To explain the transition from a core-shell configuration to a Janus configuration as a function of Ag quantity, we have conducted density-functional theory calculations and atomistic molecular dynamics simulations to investigate the stability of this system. The results are presented together with the experimental observations.

accessory. Samples were prepared by evaporating a drop of solution on a carbon (Agar) grid.Nonlinear Optical Measurements: Harmonic light scattering measurements were conducted with a Q-switched Nd:YAG (yttrium aluminum garnet) laser emitting pulses of about 40 ns at 1.91 lm. Metal nanoparticles with various shapes and organizations are desired in order to understand nanometer-scale properties, and they are attractive for several applications in the fields of optics, electronics, and magnetism. As far as magnetic storage is concerned, very high storage density requires high magnetic anisotropy to overcome thermal effects and to prevent superparamagnetic behavior, which appears as the size of the magnetic single-domain particles is reduced. Several kinds of magnetic anisotropy can be considered for this application, including: i) magnetocrystalline anisotropy (for example, of CoPt and FePt alloys with tetragonal structures); [1±3] ii) exchange anisotropy of ferromagnetic/antiferromagnetic core±shell particles; [4] and iii) shape anisotropy of elongated magnetic particles such as rods and wires.[5]The chemical synthesis of nanoparticles presents the advantage of simplicity and low cost compared with physical approaches. Moreover, it is now well-established that the structure of fine cobalt particles prepared by physical means under high vacuum is size-dependent: the face-centered cubic (fcc) phase is stabilized for mean diameters below 20 nm. [6,7] In contrast, several examples of hexagonal close-packed (hcp) cobalt nanoparticles prepared by wet-chemical processes have been reported. [8,9] The synthesis of metal nanoparticles with anisotropic shapes by liquid-phase processes is an interesting challenge, because in most cases the isotropic shapes minimize their surface energy in solution.In this context, several solid hosts have been used as templates for the anisotropic growth of metal particles, for example, mesoporous silica [10] and carbon nanotubes.[11] The most COMMUNICATIONS 338

No abstract

Fluorescence labeling is the prevailing imaging technique in cell biology research. When they involve statistical investigations on a large number of cells, experimental studies require both low magnification to get a reliable statistical population and high contrast to achieve accurate diagnosis on the nature of the cells' perturbation. Because microscope objectives of low magnification generally yield low collection efficiency, such studies are limited by the fluorescence signal weakness. To overcome this technological bottleneck, we proposed a new method based on metal-coated substrates that enhance the fluorescence process and improve collection efficiency in epifluorescence observation and that can be directly used with a common microscope setup. We developed a model based on the dipole approximation with the aim of simulating the optical behavior of a fluorophore on such a substrate and revealing the different mechanisms responsible for fluorescence enhancement. The presence of a reflective surface modifies both excitation and emission processes and additionally reshapes fluorescence emission lobes. From both theoretical and experimental results, we found the fluorescence signal emitted by a molecular cyanine 3 dye layer to be amplified by a factor approximately 30 when fluorophores are separated by a proper distance from the substrate. We then adapted our model to the case of homogeneously stained micrometer-sized objects and demonstrated mean signal amplification by a factor approximately 4. Finally, we applied our method to fluorescence imaging of dog kidney cells and verified experimentally the simulated results.

Ostwald ripening has been broadly studied because it plays a determinant role in the evolution of cluster size during both chemical and physical synthesis of nanoparticles. This thermoactivated process causes large particles to grow, drawing material from the smaller particles, which shrink. However, this phenomenon becomes more complex when considering the coarsening of metallic alloy clusters. The present experimental and theoretical investigations show that the relative composition of CoPt nanoparticles can be strongly modified during high temperature annealing and displays a size-dependent behavior. This compositional change originates from the higher evaporation rate of Co atoms from the nanoparticles. More importantly, this effect is expected in all alloy clusters containing species with different mobilities.