Non classé

Influence of Cracks on the Optical Properties of Silver Nanocrystals Supracrystal Films

J.Wei, C.Deeb, J.L.Pelouard, M.PLPileni ACS Nano DOI: 10.1021/acsnano.8b07435

Physical properties of nanocrystals self-assembled into 3D superlattices called supracrystals are highly specific with unexpected behavior. The best example to support such claim was given, through STM/STS experiments at low temperature, of very thick supracrystals (around 1000 layers) where it was possible to image the surpracrystal surface and study their electronic properties. From previous studies, we know the optical properties of Ag nanocrystals self-assembled in hexagonal network (2D) or forming small 3D superlattices (from around 2 to 7 layers) are governed by dipolar interactions. Here, we challenge to study the optical properties of Ag supracrystals film characterized by large thicknesses (from around 27 to 180 Ag nanocrystals layers). In such experimental conditions, accordingtothe classical Beer-Lambert law, the absorption of Ag films is expected to be very large and the film transmission is closed to zero. Very surprisingly, we observe reduced transmission intensity with an increase of the notch linewidth, in the 300-800 nm wavelength range, as the supracrystal film thickness increased. By calculating the transmission through the supracrystal films, we deduced that the films were dominated by the presence of cracks with wetting layers existing at their bottoms. This result was also confirmed by optical micrographs. The cracks widths increased with increasing the film thickness leading to more complex wetting layers. We also demonstrated the formation of small Ag clusters at the nanocrystal surface. These results pave the way towards a class of plasmonic supracrystals.


Non classé

Intrinsic behavior of FCC supra crystals of nanocrystals self organized on mesoscopic scale

M.P. Pileni Chaos,2005,15, 47504-47510.

We describe intrinsic behavior due to the high ordering of nanocrystals at the mesoscopic scale. The first example shows well-defined columns in the formation of cobalt nanocrystals when an applied magnetic field is applied during the evaporation process. Collective breathing properties between nanocrystals are demonstrated. In both cases, these features are observed when the nanocrystals are highly ordered in fcc supra-crystals.

Non classé

From wormlike to spherical palladium nanocrystals: Digestive ripening.

Similar wormlike palladium nanocrystals are produced by various synthetic methods such as reverse micelles and phase-transfer reactions. Palladium nanocrystals, stabilized by dodecylamine as a coating agent, are produced by chemical reduction of PdCl2  by sodium borohydride, NaBH4. A soft digestive ripening process, without any reflux, in which wormlike palladium particles go to sphere is observed in the presence of a rather large amount of dodecanethiol. The wormlike particle diameter is similar to that of the spheres produced.



source.. From wormlike to spherical palladium nanocrystals: Digestive ripening.
K. Naoe, C. Petit and M.P. Pileni J. Phys. Chem. C,2007,111, 16249-16254.

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.

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.

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.