ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
Nanoparticles' morphology is a key parameter in the understanding of their thermodynamical, optical, magnetic and catalytic properties. In general, nanoparticles, observed in transmission electron microscopy (TEM), are viewed in projection so that the determination of their thickness (along the projection direction) with respect to their projected lateral size is highly questionable. To date, the widely used methods to measure nanoparticles thickness in a transmission electron microscope are to use cross-section images or focal series in high-resolution transmission electron microscopy imaging (HRTEM "slicing"). In this paper, we compare the focal series method with the electron tomography method to show that both techniques yield similar particle thickness in a range of size from 1 to 5 nm, but the electron tomography method provides better statistics since more particles can be analyzed at one time. For this purpose, we have compared, on the same samples, the nanoparticles thickness measurements obtained from focal series with the ones determined from cross-section profiles of tomograms (tomogram slicing) perpendicular to the plane of the substrate supporting the nanoparticles. The methodology is finally applied to the comparison of CoPt nanoparticles annealed ex situ at two different temperatures to illustrate the accuracy of the techniques in detecting small particle thickness changes.
ABSTRACT
We present the first derivation of the phase field equations using a coarse-graining procedure on a microscopic master equation. The procedure leads to a mesoscopic nonlinear Fokker-Planck equation equivalent to a Cahn-Hilliard equation supplemented with a noise, but with specific prescriptions for the mobilities and the noise term. All the ingredients (chemical potentials, mobilities, stiffness coefficient) depend on the coarse-graining size. Finally, we show the ability of the phase field equations to describe a precipitation kinetics involving a nucleation and growth mechanism.
ABSTRACT
Studying the structure of nanoparticles as a function of their size requires a correlation between the image and the diffraction pattern of single nanoparticles. Nanobeam diffraction technique is generally used but requires long and tedious TEM investigations, particularly when nanoparticles are randomly oriented on an amorphous substrate. We bring a new development to this structural study by controlling the nanoprobe of the Bright and Dark Field STEM (BF/DF STEM) modes of the TEM. The particularity of our experiment is to make the STEM nanoprobe parallel (probe size 1 nm and convergence angle <1 mrad) using a fine tuning of the focal lengths of the microscope illumination lenses. The accurate control of the beam position offered by this technique allowed us to obtain diffraction patterns of many single nanoparticles selected in the digital STEM image. By means of this technique, we demonstrate size effects on the order-disorder transition temperature in CoPt nanoparticles when their size is smaller than 3 nm.
