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SUPRANANO: Nanocrystal supra-crystal, crystalline, nanometer, micrometer - Marie-Paule Pileni

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Marie Pinhas-Diena, in charge of scientific communications l tel: +33 (0)1 44 27 22 89 l email: marie.pinhas@upmc.fr

SUPRANANO: Nanocrystal supra-crystal, crystalline, nanometer, micrometer - Marie-Paule Pileni

From metal nanocrystal to supracrystal: crystallinity at nanometer and micrometer scales

The SUPRANO project is based on the control of crystal growth at different scales. At the nanoscale, the selective control of the atom order in a nanocrystal population is a real challenge. Thus, the project’s goal is to produce a collection of monocrystalline or polycrystalline nanocrystals. The physical and chemical properties of nanocrystals should depend on their nanocrystallinity. These nanocrystals can be assembled at the mesoscopic scale in a crystal lattice called "supra-crystal". Nanocrystallinity of the nanoparticles used in the assembly in supra-crystals should modify the chemical and physical properties of supra-crystals.

 

Marie-Paule Pileni. © Instant Harcourt, Paris

 

Some important results

The crystal structure of a nanocrystal plays a major role on the chemical and physical properties not only of the nanocrystal itself but also its assembly into a supra-crystal.

 

The diffusion of oxygen in cobalt nanocrystals depends on their nanocrystallinity. For an epsilon phase of cobalt nanocrystals, empty shells of cobalt oxide are formed, regardless of the nanocrystals size (5 nm-9-nm), while for hcp- (or fcc-) type nanocrystals, the diffusion depends on the size of the nanocrystals with the formation for small sizes of empty shells (4-6nm) and large sizes (7-8nm) of core/shell with a hcp- (or fcc-) type cobalt core and a cobalt oxide shell. In the case of amorphous nanocrystals, only cobalt oxide can be obtained.

 

The influence of nanocrystallinity on induced vibration modes by laser excitation depends on the mode studied. So for the quadrupole mode (l = 2) observed by low-frequency Raman, a lifting of degeneracy is observed for monocrystalline nanocrystals with a splitting of quadrupole peaks whereas in polycrystalline nanoparticles, only one peak is obtained. Regarding the breathing mode (l = 0) observed by femtosecond laser excitation, the oscillation frequencies remain similar for mono-domains or nanocrystals polycristallisés always with a slightly higher incidence (2%) for single crystals than for polycrystals.

 

a. High-resolution microscopy scanning shows a {221} type nanocrystal vicinal plan to the {111}. b. A {35 9 1} type nanocrystal vicinal plan {100}. c. A{430} nanocrystal vicinal plan to the {110}. Rights reserved.

 

The assembly of nanocrystals presents a single crystal orientational order which is not observed for polycrystals. This leads to very significant changes in the mechanical properties of supra-crystals. Monocrystal supra-crystals have a much higher Young's modulus (at least one order of magnitude) than those compounds of polycrystalline nanoparticles.

 

For a mixture of nanocrystals differing from their nanocrystallinity, from their assembly in supra-crystals, a segregation process occurs with nanoparticles of the same size with the formation of triangular supra-crystal compounds composed of mono-domain nanocrystals and supra-crystals in a film made of polycrystalline nanocrystals.

 

Cobalt nanocrystals that are mono-domain and ferromagnetic at room temperature induce the formation of "quasi" supra-crystals when in the presence of silver nanocrystals. The mixture cobalt nanocrystals that are of mono-domain and gold enables supra-crystal formation characterized by usually unstable vicinal surfaces in the case of atomic surfaces.

 

The transport properties studied by microscopy at a low temperature tunneling could be observed with supra-crystal thickness of a few micrometers while under such conditions it would be expected to be in an insulating medium. In addition, the response of the isolated nanocrystal in the assembly is observed. This indicates that the nanocrystal keeps its own properties when it is trapped in a supra-crystal.

 

The Monaris site « De la Molécule aux Nano-objets : Réactivité, Interactions et Spectroscopies » (CNRS/UPMC)Nouvelle fenêtre (in French)

 

The SUPRANANO site (in English)Nouvelle fenêtre



10/04/15