peer journals

Long-range 2D and 3D self-organizations of Co nanocrystals: A new challenge for a new physics.

The ability to grow long-range 2D and 3D organizations of metallic nanocrystals allows the emergence of unexpected physical properties. Here magnetic, structural and mechanical intrinsic properties due to the ordering of cobalt nanocrystals at the mesoscopic scale are discovered. Analogy between nanocrystals ordered in the supra-crystal and atoms in the bulk phase or in a nanocrystal related to the ordering concept is proposed.

Source : Long-range 2D and 3D self-organizations of Co nanocrystals: A new challenge for a new physics. I. Lisiecki, M.P. Pileni C.R.Chimie, 2009,12, 235-246. In honor of P.G.de Gennes

peer journals

How to tune the Au inter-nanocrystal distance in 2D self-ordered superlattices?

Hydrated and anhydrous Au nanocrystals with the same size dispersion and coating agent are synthesized from two procedures. Intriguing, the interparticle distance in two-dimensional (2D) self-ordered Au nanocrystals can be tuned by the amount of water molecules added to anhydrous Au nanocrystal solution. This behavior could be attributed to adsorption of water molecules either to the Au nanocrystal surface or on the dodecanethiol alkyl chains inducing their shrinkage. Taking into account various syntheses performed inhydrated media and the influence of water molecules on the appearance of attractive interactions between nanocrystals (measured by others), we claim that the presence of water molecules induces a competition between their adsorption at the metallic hydrophilic surface of Au nanocrystals and the chemisorption of the dodecanethiol. These adsorbed water molecules lead to additional attractive adhesion forces, which decrease the internanocrystal distance, whereas the strong thiol-Au bonding maintains the 2D long-range ordering of the nanocrystals.

Source : How to tune theAu inter-nanocrystal distance in 2D self-ordered superlattices? Salzemann, W. Zhai, N , Goubet and M.P. Pileni J. Phys. Chem. Lett., 2010, 1, 149-154.

peer journals

Do 8nm Co nanocrystals in long range ordered face-centered cubic (fcc) supracrystals show super-spin glass behavior ?

Here, we show evidence for superspin glass behavior in long-rangeordered face-centered cubic (fcc) supracrystals of 8 nm Co nanocrystals as has been well-demonstrated for disordered 3D assemblies. The dynamic behavior shows a critical slowing down, and the characteristic relaxation time is found to diverge to a finite static glass temperature. The collective nature of the glass state is supported by the existence of a memory effect. We conclude that, in the case of magnetic nanocrystal assemblies where the individual nanocrystal anisotropy is low, superspin glass behavior is observed whatever themesoscopic order is.

Source : Do 8nm Co nanocrystals in long range ordered face-centered cubic (fcc) supracrystals show super-spin glass behavior ? Parker, I. Lisiecki, M.P. Pileni J. Phys. Chem. Lett., 2010, 1, 1139–1142.

peer journals

Ability to discern the splitting between longitudinal and transverse plasmon resonances in Au compared to Ag nanoparticles in close-packed planar arrays.

The discrete dipole approximation method is used to simulate the optical response of ultrafine Au nanoparticles « NPs » with a diameter of 5 nm assembled in a planar hexagonal array. Similar calculations performed for Ag NPs arrays are also presented for comparison. For both cases, the absorption spectra are calculated for various incidence angles and interparticle distances to reveal the optical anisotropy related to the geometry of the system through the splitting of the surface-plasmon resonance « SPR » into the transverse and longitudinal modes. This effect usually results in the emergence of two SPR bands in the absorption spectrum as observed for Ag NPs arrays when the border-to-border interparticle distance becomes smaller than around one particle radius. Conversely, such a splitting is shown to be undistinguishable for Au NPs arrays whatever the interparticle distance and the incidence angle are. The different behaviors pointed out between Au and Ag NPs arrays are ascribed to the intrinsic dielectric properties of these two metals.

Source : Ability to discern the splitting between longitudinal and transverse plasmon resonances in Au compared to Ag nanoparticles in close-packed planar arrays. Yang, H. Portalès and M.P. Pileni Phys. Rev.B, 2010, 81, 205405.

Non classé

How the level of ordering of 2D nanocrystals superlattices is controlled by their deposition mode?

Here, we show that droplet deposition of 5 nm Ag nanocrystals induces the formation of two bidimensionnal arrays differing by their level of long range ordering. This is valid for the alkanethiol/solvent combinations using decanethiol or dodecanethiol as the coating agent and hexane, octane, decane, or dodecane as the solvent to disperse the nanocrystals. The spacing between the nanocrystals does not diverge from one network to the other for a given combination but depends conversely on this combination. This is demonstrated by employing an intrinsic property of highly ordered Ag nanocrystals subjected to oxygen plasma and also classical methods usually used for this purpose.

Source : How the level of ordering of 2D nanocrystals superlattices is controlled by their deposition mode? E. Klecha, D. Ingert and M. P. Pileni,  J. Phys. Chem. Lett., 2010, 1, 1616–1622.

peer journals

Crystallinity Dependence of the Plasmon Resonant Raman Scattering by Anisotropic Gold Nanocrystals.

Au nanocrystals (NCs) with different crystalline structures and related morphologies are unselectively synthesized using an organometallic route. The acoustic vibrations of these NCs are studied by plasmon mediated low-frequency Raman scattering (LFRS). A splitting of the quadrupolar vibration mode is pointed out in the LFRS spectrum. Comparison of the measured frequencies with calculations and careful examination of the NCs morphologies by transmission electron microscopy ascertain this splitting as being an effect of crystallinity. The excitation dependence of the LFRS spectra is interpreted by the shape-selection of the NCs via plasmon!vibration coupling. These results give new insights into the crystallinity influence on both the vibrations of the NCs and their coupling with plasmons and demonstrate the relevance of elastic anisotropy in monodomain NCs.

Source : Crystallinity Dependence of the Plasmon Resonant Raman Scattering by Anisotropic Gold Nanocrystals. H. Portales, N. Goubet, l. Saviot; P.Yang, S. Sirotkin, E. Duval, A. Mermet, M.P. Pileni. ACS Nano, 2010, 4,  3490-3497.

peer journals

Simulations of Cracks Supported by Experiments: The Influence of the Film Height and Isotropy on the Geometry of Crack Patterns.

Here, we show that cracks give valuable information on the height and the isotropy of nanocrystal films. Two-dimensional crack patterns are systematically studied by simulations and experiments varying the height and the anisotropy of the applied stress. The simulations are carried out using a bundle-spring network model, which allows studying the influence of height on crack patterns. For the experiments, a model system made of magnetic ! -Fe2 O3  nanocrystals is used, which enables a change in the anisotropy of the stress by applying a magnetic field during the drying process. The crack pattern morphology is investigated by simulations using square, hexagonal, and isotropic spring arrangements and applying an isotropic or unilateral stress. The average crack distance as a function of the film height studied by simulations follows a universal scaling law, confirming the experimental data. We show that this implies that the morphology of the crack pattern does not change with the height. The frequency of crack fragments as a function of the number of their sides or neighbors is analyzed for isotropic crack patterns obtained from the simulations using various spring arrangements. Finally, the theoretical results are compared to the experiments. The observed linear scaling of crack distances with height is in good agreement with the theory. The frequency of crack fragments as a function of the number of sides and neighbors does not significantly change with height, except for very thin layers. The simulations show that the experimental frequencies indicate the isotropy of the nanocrystal films.

Source : Simulations of Cracks Supported by Experiments: The Influence of the Film Height and Isotropy on the Geometry of Crack Patterns. Richardi, A.T. Ngo, M.P. Pileni. J.Phys.Chem.,C , 2010, 114, 17324–17332.