peer journals

Enhanced stability against oxidation due to 2D self organization of hcp Co nanocrystals: A New intrinsic property.

Here we clearly show the remarkable resistance to oxidation of highly crystallized hcp Co nanocrystals close packed in 2D compared to the same nanocrystals either disordered or isolated on the substrate. From a magnetic point of view, these core–shell structures with a high crystalized hcp core are expected to lead to a significant change in the exchange compared to the fcc systems. This intriguing behavior of Co nanocrystals that is related to the permeability of the coating chains to O2  constitutes the first intrinsic property due to their 2D organization.

Source : Enhanced stability against oxidation due to 2D self organization of hcp Co nanocrystals: A New intrinsic property. I.Lisiecki, S.Turner, S.Balls, M.P.Pileni and G.Van Tendeloo. Chem. Mat., 2009, 21, 2335-2338.

peer journals

Immunity of coated self-organized nanocrystals afainst strong oxidation: A new intrinsic property due to nanocrystal ordering.

New materials made of two- and three-dimensional superlattices of nanoparticles exhibit unique collective properties arising from the ordering of the nanoparticles. Here, dodecanethiol-coated silver nanocrystals self-assembled in 2D were subjected to oxygen plasma using the reactive ion etching process. The careful investigation by transmission electronic microscopy (TEM) of the same areas before and after exposure allows one to distinguish two behaviors under plasma treatment, which are determined by the level of ordering of the 2D organizations. Higher ordered self-assemblies remain unchanged, while less ordered organizations coalesce into larger nanocrystals with spheroidal shapes that could be single crystals. Electron energy-loss spectroscopy (EELS) measurements show that there is no oxidation of the silver either on the large coalesced nanocrystals or on the stable self-assemblies. A new intrinsic property, immunity of highly self-ordered nanocrystals provided by the ordering, is reported, and a mechanism of the coalescence is proposed.

Source : Immunity of coated self-organized nanocrystals afainst strong oxidation: A new intrinsic property due to nanocrystal ordering. E.Klecha, D.Ingert and M.P.Pileni Langmuir, 2009, 25, 2824-2830.

peer journals

Optical response of ultrafine spherical silver particles arranged in hexagonal planar arrays studied by DDA method.

Absorption spectra of nanosized spherical silver particles hexagonally arranged in planar arrays have been calculated using the discrete dipole approximation (DDA). The silver dielectric function used in the calculations is size-corrected in order to account for the ultrafine particle size of 5-nm in diameter. The optical anisotropy arising from the dimensionality of the planar array of close-packed nanoparticles is clearly revealed in the absorption spectra by the splitting of the surface plasmon resonance (SPR) in two bands corresponding to the longitudinal and transverse modes. Under p-polarized light, the amplitude of the splitting is observed to sensitively increase for decreasing interparticle spacing. This behavior makes it possible to well distinguish the two SPR bands under the requirement that the interparticle spacing should be smaller than around one particle radius. Indeed, for larger interparticle spacing, the two plasmon modes tend to superimpose, so that the profile of the resulting single band looks nearly the same as the one of an isolated nanoparticle.

Source : Optical response of ultrafine spherical silver particles arranged in hexagonal planar arrays studied by DDA method. H.Portales, N.Pinna and M.P..Pileni J.Phys.Chem.A., 2009,113, 4094-4099.

peer journals

Identification of multipolar surfcae plasmon resonances in silver triangular nanoprisms with very high aspect ratio using the DDA method. 

The extinction spectra of 5-nm thick, triangular silver nanoprisms are calculated using the discrete dipole approximation (DDA) method. The calculations are proved to accurately converged by satisfying the usual criteria related to the applicability of the DDA. The ultrathin thickness of the nanoprisms considered here has the advantage of making it possible to largely tune their aspect ratio (AR) from 5 to 40, and simultaneously limit all their dimensions below the wavelength of the incident light. For nanoprisms with AR g  15, several intense bands are observed. These bands correspond to the well-known, in-plane, dipolar surface plasmon resonance (SPR) and several multipolar modes emerging at higher energies. Because of the high AR of the nanoprisms considered in this work, the multipolar SPR are particularly well observed, thus making it possible to examine them in detail. The calculated extinction spectrum shows a clear dependence on the edge length, the thickness, the aspect ratio and the volume of the nanoprism. The evolution of the extinction spectrum when simulating the presence of a substrate is also investigated as well as that induced by changing the size of the truncation in snipped nanoprisms. The qualitative agreement of the presented simulations with previous experimental observations made by other groups confirms the ability of the DDA method to predict the optical properties of such ultrathin triangular nanoprisms.

Source : Identification of multipolar surfcae plasmon resonances in silver triangular nanoprisms with very high aspect ratio using the DDA method. P.Yang, H.Portales and M.P.Pileni J.Phys.Chem. C., 2009, 113, 1157967- 11604.

peer journals

Self organization of cofined dipolar particles in parallel field.

Monte Carlo simulations of a Stockmayer fluid confined between two parallel walls are performed to investigate self-organization of magnetic nanocrystals in a field parallel to the walls as a function of density, field strength, and wall separation. In order to study the formation of mesoscopic structures, a large number of up to 12 000 particles have to be used. The particles organize into periodically spaced cylindrical-like columns whose width typically varies between 5 and 9 particle diameters at low density. At small heights the columns are quenched due to the parallel walls, while larger wall separations can accommodate several layers of columns in good agreement with experiments. An increase in density entails a clear increase in column thickness, whereas an increase in field strength seems to have the opposite effect.

Source : Self organization of cofined dipolar particles in parallel field. J.Richardi, M.P.Pileni and J.J.Weis. J.Chem.Phys., 2009, 130, 124515-124519.

peer journals

Mesoscopic void structures in cobalt nanocrystal film.

Here we report the formation of void (hole) structures when concentrated colloidal solutions ofmagnetic nanocrystals are subjected to a magnetic field during slow evaporation. This presents a new type of solid mesostructure obtained by self-assembly of nanocrystals. The voids are characterized by a cylindrical shape with either circular or elliptical base. We show that the morphology of these patterns is essentially controlled by the fraction of the volume occupied by the magnetic phase to the total volume of the film. Monte Carlo simulations carried out using a Stockmayer fluid model agree remarkably well with the experiments for the formation of void structures in the range of considered volume fractions.

Source : Mesoscopic void structures in cobalt nanocrystal film. C.Salzeman, J.Richardi, I.Lisiecki, J.J.Weis and M.P.Pileni 
Phys.. Lett., 2009,102, 144502-144506.

peer journals

Assemblies of aligned magnetotatic bacteria and extracted magnetosomes: What is the main factor responsible for the magnetic anisotropy ?

The origin of the magnetic anisotropy is explained in an assembly of aligned magnetic nanoparticles. For that, nanoparticles synthesized biologically by Magnetospirillum magneticum AMB-1 magnetotactic bacteria are used. For the first time, it is possible to differentiate between the two contributions arising from the alignment of the magnetosome easy axes and the strength of the magnetosome dipolar interactions. The magnetic anisotropy is shown to arise mainly from the dipolar interactions between the magnetosomes.

Source : Assemblies of aligned magnetotatic bacteria and extracted magnetosomes: What is the main factor responsible for the magnetic  anisotropy? E.Alphandry, Y.Ding, A.T.Ngo, Z.LWang, F.F.Wu and M.P.Pileni. ACS Nano, 2009, 3, 1539-1547.