peer journals

Ordering at Various Scales: Magnetic Nanocrystals.

Here, it is shown that the internal crystallinity called nanocrystallinity of rather uniform Co nanoparticles can be improved by annealing. This induces marked changes in the magnetic properties such as an increase in the blocking temperature that can reach a value close to room temperature. It is shown that the acoustic breathing modes remain quite unchanged by changing the nanocrystallinity. Co nanocrystals with low size distribution are able to self-assemble either in fcc colloidal crystals called supracrystals or in #lms with voids. Collective intrinsic properties (chemical and physical) due to magnetic nanocrystal ordering in 2D and 3D superlattices are presented. Furthermore, when the nanocrystals are aligned, the magnetic properties of the assemblies are improved. By using magnetostatic bacteria, it is demonstrated that the magnetic anisotropy is mainly due to induced dipolar interactions with a low contribution of the in!uence of the orientation of the nanocrystal easy axes.

Source : Ordering at Various Scales: Magnetic Nanocrystals. I. Lisiecki and M. P. Pileni, J. Phys. Chem. C, 2012, 116, 3–14

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

Engineering the magnetic dipolar interactions in three-dimensional binary supracrystals via mesoscale alloying.

Inspired by metallic alloys in atomic solids, we used two distinct metallic nanoparticles, considered as “artificial metal atoms”, to engineer ordered binary nanoparticle alloys at the mesoscale, called binary supracrystals. Here, ferromagnetic 7.2-nm Co nanoparticles are used as large “A” site particles, while either ferromagnetic 4.6-nm Co or non-magnetic 4.0-nm Ag nanoparticles are used as small ‘B’ site particles to fabricate long-range ordered binary supracrystals with a stoichiometry of AB2 and AB13. The interparticle distances between 7.2-nm Co nanoparticles within the Co/Ag binary supracrystals can be tuned by a control of crystal structure from AB2 (CoAg2) to AB13 (CoAg13). A decrease of magnetic coupling between Co nanoparticles was observed as the Co-Co interparticle distance increases. Furthermore, by alloying 7.2-nm and 4.6-nm Co nanoparticles to form AB2 (CoCo2) binary supracrystals, a collective magnetic behavior of these two particle types, due to the dipolar interaction, was evidenced by observing a single peak in the Zero Field Cooling (ZFC) magnetization curve. Compared with the CoAg2 binary supracrystals, a spin orientation effect in sublattice that reduces the dipolar interactions in the supracrystals was uncovered in CoCo2 binary supracrystals.  

Source : Engineering the magnetic dipolar interactions in three-dimensional binary supracrystals via mesoscale alloying.
Z. Yang, J. Wei, P. Bonville, M.P. Pileni Adv. Funct.Mater.,2015, 25, 4908.

peer journals

Surface Plasmon Resonance Properties of Silver Nanocrystals Differing in Size and Coating Agent Ordered in 3D Supracrystals

Silver nanocrystals differing by their coating agents and sizes are self-assembled in thin supracrystalline films. The surface plasmon resonance (SPR) properties of such assemblies are presented. Nanocrystal size, interparticle distance and coating agent play key roles in the plasmonic coupling of Ag nanocrystals within supracrystals. Here, we demonstrate experimentally the predictions for 2D self-assemblies remains valid for thin 3D superlattices. The absorption spectra in the visible range are markedly dependent on the incidence of the light source and confirm the apparition of a splitting of the dipolar surface band into two components upon increasing the incidence angle. The major parameter inducing the splitting of the SPR band is the relative ratio between the average distance of nanocrystals and their diameters. The nature of the coating agent is also of particular importance; it is hereby shown that theoretical predictions and experimental data are in agreement for alkylamine coating agents, whereas they differ for thiol-coated nanocrystals.

Source : Surface Plasmon Resonance Properties of Silver Nanocrystals Differing in Size and Coating Agent Ordered in 3D Supracrystals
J.Wei, N. Schaeffer, P.A. Albouy, and M.P.Pileni, Chem.Mat., 2015,27, 5614−5621.

peer journals

Beyond Entropy: Magnetic Forces Induce Formation of Quasicrystalline Structure in Binary Nanocrystal Superlattices

It is shown that binary superlattices of Co/Ag nanocrystals with the same size, surface coating, differing by their type of crystallinity are governed by Co− Co magnetic interactions.By using 9 nm amorphous-phase Co nanocrystals and 4 nm polycrystalline Ag nanocrystals at 25 ° C, triangle-shaped NaCl-type binary nanocrystal superlattices are produced driven by the entropic force, maximizing the packing density. By contrast, usingferromagnetic 9 nm single domain (hcp) Co nanocrystals instead of amorphous-phase Co, dodecagonal quasicrystalline order is obtained, together with less-packed phases such as the CoAg13 (NaZn13 -type), CoAg (AuCu-type), and CoAg3  (AuCu3 -type) structures. On increasing temperature to 65 ° C, 9 nm hcp  Co nanocrystals become superparamagnetic, and the system yields the CoAg3  (AuCu3 -type) and CoAg2  (AlB2 -type) structures, as observed with 9 nm amorphous Co nanocrystals. Furthermore, by decreasing the Co nanocrystal size from 9 to 7 nm, stable AlB2 -type binary nanocrystal superlattices are produced, which remain independent of the crystallinity of Co nanocrystals with the superparamagnetic state.

Source : Beyond Entropy: Magnetic Forces Induce Formation of Quasicrystalline Structure in Binary Nanocrystal Superlattices
Z.Yang, J. Wei, P. Bonville and M. P. Pileni J.Am.Chem.Soc, 2015, 137, 4487-449

peer journals

Nano Supracrystallinity.

It is shown that the chemical and physical properties of a collection of nanocrystals either isolated or self assembled in 3D superlattices called supracrystals markedly depend on the crystalline structure of the nanocrystal.

Source : Nano Supracrystallinity M.P.Pileni EPL, 2015, 109 58001.

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