Non classé

Difference between the magnetic properties of the magnetotactic bacteria and those of the extracted magnetosomes: Influence of the distance between the chains of magnetosomes.

We report structural characterization and magnetic properties of various assemblies of chains of magnetosomes. The same magnetic properties are observed for the magnetotactic bacteria and for the extracted chains of magnetosomes isolated in a polymer. When the extracted chains of magnetosomes form a denser structure than that observed in the bacteria, the magnetic properties change markedly. A decrease in the coercivity and reduced remanence is observed. This behavior is attributed to an enhancement of the dipolar interactions between the chains of magnetosomes in the limit of a weakly interacting system; that is, the magnetostatic energy is lower than the anisotropy energy. The effect of the dipolar interactions is more pronounced at 250K than at 10 K. This behavior is attributed to the existence of a family of small magnetosomes, which undergo a transition from a ferromagnetic to a superparamagnetic state.

Source : Difference between the magnetic properties of the magnetotactic bacteria and those of the extracted magnetosomes: Influence of the distance between the chains of magnetosomes. 
E. Alphandery, A.T. Ngo, C. Lefevre, I. Lisiecki, L.F. Wu and M.P. Pileni  J. Phys. Chem. C., 2008, 112, 12304-12309.

peer journals

Self-organization of magnetic nanoparticles: a Monte Carlo study.

To understand the self-organization of magnetic nanocrystals in an applied field, we perform Monte Carlo simulations of Stockmayer fluids confined between two parallel walls. The system is examined in the gasliquid coexistence region of its phase diagram and the field is applied perpendicular to the walls. Gibbs ensemble simulations are carried out to determine the phase coexistence curves of the confined Stockmayer fluid. In canonical simulations, different types of organizations appear dependent on particle density: columns, walls, and elongated and spherical holes. The morphology and size of structures are in good agreement with results obtained by free energy minimization and experiments. The influence of a distribution of particle sizes on the particle organization is investigated.

Source : Self-organization of magnetic nanoparticles: a Monte Carlo study. J. Richardi, M.P. Pileni and J.J. Weis, Phys. Rev. E , 2008, 77, 061510-1-9.

peer journals

Quantitative absorption spectroscopy of a single gold nanorod.

The spectrally- and polarization-resolved absorption cross-sections of a single gold nanorod have been investigated using the spatial modulation spectroscopy technique. The ensemble of its optical features, that is, longitudinal and transverse surface plasmon resonances and interband absorption, has been quantitatively characterized. The results are compared with numerical simulations using the discrete dipole approximation and the finite element method, yielding information on the investigated nanorod size and shape.

Source : Quantitative absorption spectroscopy of a single gold nanorod. O.L. Muskens, G. Bachelier, N. Del Fatti, F. Vallée, A. Brioude, X.C. Jiang and M.P. Pileni, J. Phys. Chem. C, 2008, 112, 8917-8921.

Non classé

Vibration Dynamics of Supra-Crystals of cobalt nanocrystals studied with femtosecond laser pulses.

We report the first real-time observation of the collective motion of nanocrystals self-assembled in a 3D supracrystal. The long-range ordering in the fcc cobalt supra-crystals studied here is ideal to launch coherent vibrations of the nanocrystals when suddenly heated by femtosecond laser pulses. The interdigitated aliphatic chains linking the particles act as mechanical nanosprings, which can be resonantly excited by ultrashort laser perturbations. This macroscopic supramolecular motion finds its origin at the microscopic level where, similarly, the cobalt atoms in each nanocrystal have their own dynamics, including their electron and lattice relaxation energies.

Source : Vibration Dynamics of Supra-Crystals of cobalt nanocrystals studied with femtosecond laser pulses. I.Lisiecki, V. Halté, C. Petit, M-P Pileni and J-Y Bigot Adv. Mat., 2008, 20, 1-4.

peer journals

Tuning of solid phase in supracrystals made of silver nanocrystals.

Decanethiol-passivated silver nanocrystals are shown, by small-angle X-ray diffraction, to organize into hexagonal close packed or face centered cubic (fcc) structures depending on the substrate temperature. When the nanocrystals are passivated by dodecanethiols, fcc and body centered cubic lattices as well as disordered arrangements are observed. The different crystalline phases correspond to thermodynamic equilibrium states. The passivant chain length is shown to control the interactions between the nanocrystals and consequently the superlattice structure.

Source : Tuning of solid phase in supracrystals made of silver nanocrystals. A.I. Henry, A. Courty, M.P. Pileni, P.A. Albouy and J. Israelachvili Nano Lett. 8, 2008, 2000-2005

Non classé

Do directional primary and secondary cracks patterns in thin films of maghemite nanocrystals follow a universal feature ?

Cracks due to a shrinking film restricted by adhesion to a surface are observed in nature at various length scales ranging from tiny crack segments in nanoparticle films to enormous domains observed in the earth’scrust. Here, we study the formation of cracks in magnetic films made of maghemite (g-Fe2O3) nanocrystals. The cracks are oriented by an external magnetic field applied during the drying process which presents a new method to produce directional crack patterns. It is shown that directional and isotropic crack patterns follow the same universal scaling law with the film height varying from micrometer to centimeter scales. Former experimental studies of scaling laws were limited to small variations in height (1 order of magnitude). The large variation in height in our expriments becomes possible due to the combined use of nanocrystals and electron microscopy. A simple two-dimensional computer model for elastic fracture leads to structural and scaling behaviors, which match those observed in the experiments.

Source : Do directional primary and secondary cracks patterns in thin films of maghemite nanocrystals follow a universal feature ? A.T.Ngo, J.Richardi and M.P.Pileni J.Phys.Chem.B., 2008, 112, 14409-14414.

Non classé

High resolution patterned transfer using needle-shaped ferrite nanocrystals.

Graphite and silicon nanoneedles are fabricated by different etching processes on highly oriented pyrolitic graphite (HOPG) and SiO2/Si. The masks used to engrave such substrates are needle-shaped ferrite nanocrystals. We are able to control the height of these nanoneedles by manipulating the plasma etching parameters such as gas selectivity and etching time. The morphology of the engraved structures is characterized by scanning electron microscopy and atomic force microscopy. We show that the limiting dimension in the transfer using nanocrystals as masks is not that of the mask objects themselves but that of the distance separating them. We demonstrate that nanocrystals achieve the role of colloidal masks leading to resolutions down to 10 nm in distance between objects when the etched substrate is SiOx and less than 3 nm when the substrate is HOPG. This resolution is better than that reported in the literature. The difference in lateral resolution is discussed.

Source : High resolution patterned transfer using needle-shaped ferrite nanocrystals. D.Ingert and M.P.Pileni, J.Phys.Chem.C., 2008,112, 19329-19335.