DOI: 10.1002/adma.200800832 Vibration Dynamics of Supra-Crystals of Cobalt Nanocrystals Studied With Femtosecond Laser Pulses** By Isabelle Lisiecki, Vale´rie Halte´, Christophe Petit, Marie-Paule Pileni, * and Jean-Yves Bigot* Self-assembly of nanometer-sized species in 2D or 3D supra structures is a fundamental process in nature. A well-known example of such ordering are opals, where the crystalline arrangement of silica spheres gives rise to a wavelength- dependent diffraction, generating their iridescent colors. [1] We are interested in self-assembled metallic nanocrystals, which are known to form so-called supra-crystals. Their intrinsic physical properties (mechanical, optical, magnetic) display specific behaviors revealing a collective response to external perturbations. [2–4] Indeed, self-organized silver-nanocrystal supra-crystals display collective breathing modes. [5,6] Here, the real-time observation of coherent motion in cobalt nanocrystals in supra-crystals excited by ultrashort laser pulses is reported. This motion is observed in the time-dependent reflectivity measured with a time-resolved confocal microscope designed to provide 150 fs temporal resolution and 300 nm spatial accuracy simultaneously. [7] Time-resolved pump-probe spectroscopy of metallic nano- particles [8,9] using femtosecond laser pulses is a powerful technique, which allows the unravelling of physical processes occurring from a few tens of femtoseconds up to 1 ns, such as Coulomb interactions between electrons, [10–12] their coupling to vibrational degrees of freedom of atoms, [13–15] as well as heat diffusion to the environment. [16,17] As previously reported, [18,19] it is possible to produce both face-centered cubic (fcc) supra-crystals and disordered aggregates from the same batch of cobalt nanocrystals by controlling the evapora- tion process. This procedure was performed for cobalt nanocrystals of two different diameters, 6.3  0.4 and 7.3  0.4 nm. As the 2D and 3D mesostructures are similar for both sizes, we only show here the complete characterization of the 6.3-nm nanocrystal population. As shown in the transmission electron microscopy (TEM) image in the left inset of Figure 1A, the nanocrystals, once deposited on highly oriented pyrolitic graphite (HOPG), form a compact 2D hexagonal network spontaneously. The long-range ordering of the nanocrystals is due to the sufficiently low size distribution (less than 13%), [20] combined with a size segregation process. [5] When the colloidal solution containing these particles is quickly evaporated under vacuum, [21] the grazing-incidence small-angle X-ray scattering (GISAXS) pattern (inset of Fig. 1B) shows a very diffuse and low-intensity ring, typical of amorphous materials. In the case of slow evaporation, the diffraction pattern displays several reflections that are typical to fcc structures [19] (right inset of Fig. 1A). The two spots labeled 3 and 4 correspond to the (111) and (222) reflections of fcc structures, respectively. The first-order reflection is very narrow and nearly resolution limited (0.0045 nm 1 ), indicating an out-of-plane long-range ordering of the magnetic nano- crystals. This gives us the minimum value for the coherence length, 140 nm. From the position of the (111) reflection, we deduce a stacking periodicity of 7.6  0.1 and 8.5  0.1 nm for the 6.3 and 7.3 nm particles, respectively. [19] The interparticle gaps (D i–p ) and the center-to-center interparticle distance (D c–c ) are 3.0  0.5, 3.1  0.5 nm and 9.3  0.1, 10.4  0.1 nm, respectively. These gaps are the result of the alkyl-chain interdigitation of 6 C–C bond distances, i.e., 0.9 nm. The other spots, labeled 1, 2, and 5, are indexed as (1,1,1), (2,0,0), and (1,3,1) for fcc, and reveal a 3D in-plane long-range ordering in the sample. The 3D assemblies of cobalt nanocrystals coated with dodecanoic acid chains, ordered or not, form an inhomogeneous film, as shown in the scanning electron microscopy (SEM) images (Figure 1). The film thickness varies from a few micrometers at the border to tens of nanometers in the center. Samples exposed to air for several days do not oxidize as previously reported, [3] and no coalescence of Co nanocrystals is observed in the long term. The ultrafast reflectivity dynamics DR(t)/R of the amor- phous assemblies and fcc supra-crystals of Co nanoparticles described above have been investigated using the femtosecond pump-probe technique (see Experimental Section). The maximum excitation density of the pump was 40 mJ cm 2 . Dynamic measurements were performed on similar individual pavements of both ordered and amorphous samples. As it is well known, after the pump-pulse excitation the femtosecond electron dynamics of metals can be described by an initial athermal distribution, which thermalizes within a few hundred femtoseconds into a hot-electron distribution via electron– electron scattering. The electrons then relax to the lattice via electron–phonon interaction, with characteristic time t e–l . This COMMUNICATION [*] Prof. M. P. Pileni, Dr. I. Lisiecki, Prof. C. Petit Laboratoire des Mate ´riaux Me ´soscopiques et Nanome ´triques LM2N; UMR 7070 CNRS, Universite ´ Pierre et Marie Curie 4 place Jussieu, 75252 Paris Cedex 05, France E-mail: pileni@sri.jussieu.fr Dr. J.-Y. Bigot, Dr. V. Halte ´ Institut de Physique et Chimie des Mate ´riaux de Strasbourg UMR7504 CNRS, Universite ´ Louis Pasteur BP42, 23 rue du Loess, 67034 Strasbourg Cedex 02, France E-mail: jean-yves.bigot@ipcms.u-strasbg.fr [**] This work has been done within the framework of project DANMA ANR-05-NANO-007-01 and ANR-05-NANO-007-02, financed by the ‘‘Agence Nationale de la Recherche’’, France. 4176 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 4176–4179