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.

Non classé

Cracks in magnetic nanocrystal films: Do directional and isotropic crack patterns follow the same scaling law ?

In this letter, we show that the use of nanocrystals enables new insights into the scaling law of crack patterns. Directional and isotropic crack patterns made of γFe2O3 nanocrystals follow the same scaling law, with the film height varying by 3 orders of magnitude. A simple twodimensional computer model for elastic fracture also leads to the same scaling behavior for directional and isotropic cracks, in good agreement with the experiments.

Source : Cracks in magnetic nanocrystal films: Do directional and isotropic crack patterns follow the same scaling law ? A.T. Ngo, J. Richardi and M.P. Pileni Nano Lett., 2008, 8, 2485-2489.

peer journals

Probing atomic ordering and multiple twinning in metal nanocrystals through their vibrations.

Control of nanocrystal (NC) crystallinity currently raises great interest because of its potential benefits in both physics modeling and technological applications. Advances in methods for synthesizing perfect single-crystalline NCs are recent, so that the effect of crystallinity on NC properties has received only limited study and still needs to be properly investigated. Here, we report that crystallinity of gold NCs dramatically modifies their vibrations. Using low-frequency Raman scattering, we clearly demonstrate that single-domain NCs vibrate differently than their multiply twinned counterparts, through the splitting of the quadrupolar vibrations, which is only observed for the former. Using the resonant ultrasound approach, we calculate the vibrational frequencies of a gold sphere and show that elastic anisotropy induces a lift of degeneracy of the quadrupolar mode in good agreement with our experimental measurements. These findings open up challenging perspectives on using Raman spectroscopy to characterize nanocrystallinity.

Source : Probing atomic ordering and multiple twinning in metal nanocrystals through their vibrations. H. Portales, N. Goubet, L. Saviot, S. Adichtchev, D.B. Murray, A. Mermet, E. Duval and M.P. Pileni Proc. Natl. Acad. Sci. USA,  2008, 105, 14784-14789.

peer journals

Supracrystals of inorganic nanocrystals: An open challenge for new physical properties. 

When naturally occurring spherical objects self-organize, the physical properties of the material change. For example, a colorless opal is the result of a disordered aggregate of silica particles. When the silica particles are ordered, however, the opal takes on color, which is determined by the size of the self-assembled particles. In this Account, we describe how these 3D arrangements of nanomaterials can self-organize in 3D arrays called supracrystals; the 3D arrays can fall into the familiar categories of face-centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body-centered (bcc) crystals. The collective properties of these 2D and 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk. Comparison between the approach to saturation of the magnetic curve for supracrystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles, either highly crystallized or amorphous. We also demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self-ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supracrystal, analogous to a breathing mode vibration of atoms in a nanocrystal. Furthermore, we used 10 nm γ-Fe2O3 nanocrystals to gain new insight into the scaling law of crack patterns. We found that isotropic and directional crack patterns follow the same universal scaling law over a film height varying by 3 orders of magnitude. These data have led us to propose general analogies between supracrystals of nanocrystals, individual nanocrystals, and the molecules in the bulk phase for certain physical properties based on the ordering of the material. As we continue to study the physical properties of the ordered and disordered arrangements of nanomaterials, we will be able to go further in these analogies. And this exploration leads to new questions: first and foremost, is this behavior general ?

Source : Supracrystals of inorganic nanocrystals: An open challenge for new physical properties  M.P.Pileni Acc. Chem. Res., 2008,  41, 1799-1809.