Do 8 nm Co Nanocrystals in Long-Range-Ordered Face-Centered Cubic (fcc) Supracrystals Show Superspin Glass Behavior?

Here, we show evidence for superspin glass behavior in long-range-ordered face-centered cubic (fcc) supracrystals of 8 nm Co nanocrystals as has been well-demonstrated for disordered 3D assemblies. The dynamic behavior shows a critical slowing down, and the characteristic relaxation time is found to diverge to a finite static glass temperature. The collective nature of the glass state is supported by the existence of a memory effect. We conclude that, in the case of magnetic nanocrystal assemblies where the individual nanocrystal anisotropy is low, superspin glass behavior is observed whatever the mesoscopic order is

Unusual Effect of an Electron Beam on the Formation of Core/Shell (Co/CoO) Nanoparticles Differing by Their Crystalline Structures

In this study, an unusual effect of the electron beam in transmission electron microscopy (TEM) on the formation of Co/CoO core/shell structures is developed through careful in situ TEM/scanning TEM (STEM) analysis. By feature of the nanoscale precision of this approach, the electron beam-irradiated Co nanoparticles reveals remarkable resistance to oxidation compared to those without irradiation treatment. Moreover, the irradiated hcp single domain Co nanocrystals result in Co/CoO core/shell nanoparticles after oxidation, instead of the CoO hollow nanoparticles without irradiation treatment. This study highlights the electron beam can also play a role in nanoscale Kirkendall effect, in addition to the nanocrystallinity and 2D ordering effect that we have recently demonstrated. By careful in situ STEM-EELS (electron energy-loss spectroscopy) studies of the Co nanoparticles, it was found that the deliberately irradiated nanoparticles undergo an outward diffusion process of Co ions, forming an oxide layer with O species produced by the carboxylic group covalently bound to the Co atoms of the surface

Nanocrystallinity and the Ordering of Nanoparticles in Two-Dimensional Superlattices: Controlled Formation of Either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals

Here it is demonstrated that the diffusion process of oxygen in Co nanoparticles is controlled by their 2D ordering and crystallinity. The crystallinity of isolated Co nanoparticles deposited on a substrate does not play any role in the oxide formation. When they are self-assembled in 2D superlattices, the oxidation process is slowed and produces either core/shell (Co/CoO) nanoparticles or hollow CoO nanocrystals. This is attributed to the decrease in the oxygen diffusion rate when the nanoparticles are interdigitated. Initially, polycrystalline nanoparticles form core/shell (Co/CoO) structures, while for single-domain hexagonal close-packed Co nanocrystals, the outward diffusion of Co ions is favored over the inward diffusion of oxygen, producing hollow CoO single-domain nanocrystals

Coherent Longitudinal Acoustic Phonons in Three-Dimensional Supracrystals of Cobalt Nanocrystals

We use broadband picosecond acoustics to detect longitudinal acoustic phonons with few-gigahertz frequency in three-dimensional supracrystals (with face-centered cubic lattice) of 7 nm cobalt nanocrystal spheres. In full analogy with atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions, in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains) that act like mechanical springs holding together the nanocrystals. By repeating the measurements at different laser angles of incidence it was possible to accurately determine both the index of refraction of the supracrystal (<i>n</i> = 1.26 ± 0.03) and the room-temperature longitudinal speed of sound (<i>v</i>_s= 1235 ± 12 m/s), which is quite low due to the heavy weight of the spheres (with respect to atoms in a crystal) and the soft carbon chains (with respect to atomic bonds). Interestingly, the speed of sound inside the supracrystal was found to dramatically increase by decreasing the sample temperature due to a change in the stiffness of the dodecanoic acid chains which coat the Co nanocrystals