peer journals

How nanocrystallinity and order define the magnetic properties of ε-Co supracrystals.

Single domain cubic ε -Co nanocrystals are synthesized via a high-temperature thermal decomposition of cobalt carbonyl in the presence of oleic acid and trioctylphosphane oxide (TOPO). The ε -Co nanocrystals are characterized by a low size distribution (σ  < 7%) and the average diameter is tuned from 7 nm to 9 nm by tailoring the molar ratio of the surfactants oleic acid and TOPO. Moreover, we have demonstrated the self-assembly of ε -Co nanocrystals in highly ordered three-dimensional (3D) face-centered cubic (fcc) structures called supracrystals. The layer-by-layer organization of these building blocks is achieved through solvent evaporation. Simultaneously, we produce. with the same ε -Co nanocrystals, disordered (amorphous) films. We demonstrate the presence of large interparticle magnetic interactions in the supracrystals by comparing their magnetic properties with the diluted samples. Then, by a detailed comparison of their collective magnetic properties with partially disordered films, the significant differences due to the change in anisotropy and distribution of dipolar interaction energies in the two systems are presented. This is attributed to the orientational and spatial ordering of single domain ε -Co nanocrystals markedly changing between ordered and disordered assemblies. The thermal evolution of the magnetization in ZFC/FC procedure presents three characteristic temperatures representing the blocking, the irreversibility and the maximum of Zeeman coupling temperatures. They are all affected by the presence of the order in supracrystals and they present different evolution trends as a function of nanoparticles size. While the variations of reduced remanent magnetizations in both condensed series are in good agreement with the previous theoretical calculations, the coercive fields present opposite evolutions.

Source : How nanocrystallinity and order define the magnetic properties of ε-Co supracrystals. J.Yang, K. Khazen, and M.P. Pileni, J Phys Condens Matter.,2014, 23-26 295303

peer journals

Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals.

8 nm Co nanoparticles with various crystalline structures called polymorphs were produced using different synthetic procedures, such as using reverse micelles, the thermal decomposition of organometallics approach or the hot injection process. These 8 nm Co nanoparticles differing by their crystalline structures are exposed to oxygen at elevated temperature. The fcc Co polycrystalline nanoparticles produce either Co–CoO yolk–shell or CoO hollow structures whereas amorphous Co nanoparticles produce Co–CoO core–shell nanoparticles. Furthermore, single domains with either hcp or e crystalline structure behave differently upon oxygen diffusion. Co–CoO nanoparticles were produced from the hcp phase while CoO hollow nanoparticles were the product for e-phase Co nanocrystals.

Source : Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals. Z .Yang, J.Yang, J.Bergström, K.Khazen, and M. P. Pileni Phys. Chem. Chem. Phys., 2014,16, 9791-9796.

peer journals

Control of the oxygen and cobalt atoms diffusion through Co nanoparticles differing by their crystalline structure and size.

The size-dependent Kirkendall effect is studied by using Co nanoparticles.The sizes of Co nanoparticles differing by their crystal structures callednanocrystallinity, namely amorphous, polycrystalline fcc, single crystalline hcp, and single crystalline ε  phase, are modulated from 4 to 10 nm. The nanoparticles self-assembled in 2D superlattices and differing by their nanocrystallinities are subjected to oxygen at 200 ° C for 10 min. With single-domain nanocrystals differing by their crystalline structure (ε  and hcp  phases), marked changes in the fi nal structures are observed: upon increasing the nanocrystal size, the ε  phase favors formation of a hollow structure whereas a transition from single-domain hollow to multidomain core/shell structures takes place with the hcp  phase. With polycrystalline fcc  Co nanocrystals, a transition from a hollow to a yolk/shell structure is observed, whereas with amorphous cobalt, solid CoO nanoparticles are produced at the smaller size and are converted to the core/shell structure at the larger one. These differences in size effect are attributed to the change in the control of the inward fl ow of oxygen atoms and the outward fl ow of Co atoms with the crystalline structure of cobalt nanoparticles. Such a diffusion process described here on the Kirkendall effect can be studied for other metal nanocrystals.

Source : Control of the oxygen and cobalt atoms diffusion through Co nanoparticles differing by their crystalline structure and size. Z. Yang, N. Yang,  J. Yang,  J. Bergström  and M.P. Pileni Adv.Funct.Mater.,2015, 25, 891-897.

Non classé, peer journals

Hierarchical mechanical behavior of cobalt supracrystals related to nanocrystallinity

We report on a hierarchical mechanical behavior of 500 nm-thick Co nanocrystal 3D superlattices (supracrystals) induced by the crystalline structure (nanocrystallinity) or by the length of the coating agent of Co nanocrystals. Increasing the nanocrystal shape anisotropy of Co nanocrystals through the control of their nanocrystallinity induces a higher level of ordering with both translational and orientational alignment of nanocrystals within the supracrystals. The hierarchy in ordering at various scales, i.e from the atomic lattice within the nanocrystals to the nanocrystal superlattice within supracrystal, is correlated with marked changes in the Young’s modulus of supracrystals: from 0.7 ± 0.4 GPa, to 1.7 ± 0.5 GPa and to 6.6 ± 1.5 GPa as the crystalline structure of Co nanocrystals changes from amorphous-Co, to -Co and to hcp-Co, respectively. Moreover, for supracrystals of 7 nm amorphous Co nanocrystals, the Young’s modulus decreases by one order of magnitude from 0.7 ± 0.4 GPa to 0.08 ± 0.03 GPa, by reducing the alkyl chain length of the ligands coated on Co nanocrystals from C18 (oleic acid) to C12 (lauric acid). The hierarchical mechanical behavior is rationalized using a dimensional model of stress-strain relationship in supracrystals.

Source : Hierarchical mechanical behavior of cobalt supracrystals related to nanocrystallinity.
M.Gauvin, N. Yang, Z. Yang, I.Arfaoui and M.P.Pileni Nanoresearch, 2015, 8, 3480-3487.

peer journals

Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure.

The Kirkendall eff ect is a classical phenomenon in materials science, and it is referred to as a nonreciprocal interdiff usion process through an interface of two metals with strikingly diff erent atomic diff usivities, leading to a formation of vacancies called Kirkendall voids. The nanoscale Kirkendall eff ect has been vastly applied in the fabrication of hollow nanostructures after the fi rst report on the synthesis of Co-based hollow nanocrystals. In this Feature Article, we briefl y start with an introduction on the Kirkendall effect concept, followed by the general synthetic strategy toward the production of hollow Kirkendall voids. The overall synthetic strategies are based on the design of diff usion couples at the nanoscale, and then, we discuss the factors that govern the formation of Kirkendall voids at the nanoscale, from the viewpoint of the nanoparticle size, nanoparticle crystallinity, and nanoparticle environment. We conclude with a summary and perspectives on the design of hollow nanostructures governed by the Kirkendall effect.

Source : Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure
ZYang, N. Yang and M.P.Pileni J.Phys.Chem.C., 2015,119, 22249-22260

peer journals

Dispersion of Hydrophobic Co Supracrystal in Aqueous Solution.

Assembly of nanoparticles into supracrystals provides a class of materials with interesting optical and magnetic properties. However supracrystals are mostly obtained from hydrophobic particles and therefore cannot be manipulated in aqueous systems limiting their range of applications. Here we show that hydrophobic shaped supracrystals self-assembled from 8.2 nm cobalt nanoparticles can be dispersed in water by coating the supracrystals with lipid vesicles. A careful characterization of these composites objects provides insights into their structure at different length scales. The novel composite, suspended in water, retains the crystalline structure and paramagnetic properties of the starting material, which can be moved with an applied magnetic field. It opens the routes to potential biological and biotechnological applications

Source : Dispersion of Hydrophobic Co Supracrystal in Aqueous Solution. N. Yang, Z. Yang, M. Held, P. Bonville, P.A. Albouy, R. Lévy, M.PPileni ACS Nano,2016 , 10, 2277–2286