peer journals

Dependence of the localized surface plasmon resonance of noble metal quasispherical nanoparticles on their crystallinity-related morphologies.

The absorption spectra of 5 nm noble metal nanoparticles (Ag, Au, and Cu) with typical morphologies of multiply twinned particles (MTPs) and single crystals are calculated by using the discrete dipole approximation method. Among the considered morphologies, it is found that icosahedral, cuboctahedral and truncated octahedral particles behave like quasispherical particles whereas the optical response of the decahedral particles significantly differs from the others. This result, which originates from the shape anisotropy of the decahedron, points out the capacity to discriminate decahedral MTPs from a population of particles with mixed crystallinities and related quasispherical shapes.

Source : Dependence of the localized surface plasmon resonance of noble metal quasispherical nanoparticles on their crystallinity-related morphologies. P.Yang, H.Portales and M.P.Pileni. J.Chem.Phys., 2011, 134, 024507-1/6.

peer journals

How to Predict the Growth Mechanism of Supracrystals from Gold Nanocrystals ?

Here we report the influence of the nanocrystal size and the solvent on the growth of supracrystal made of gold nanocrystals. These parameters may determine the final morphology of nanocrystals assemblies with either a layer-by-layer growth or a process of nucleation in solution. Experiments supported by simulations demonstrate that supracrystal nucleation is mainly driven by solvent-mediated interactions and not solely by the van der Waals attraction between nanocrystal cores, as widely assumed in the literature.

Source : How to Predict the Growth Mechanism of Supracrystals from Gold Nanocrystals ? N.Goubet, J. Richardi, P.A. Albouy and M.P. Pileni. J.Phys.Chem.Lett., 2011, 2, 417-422.

peer journals

Which forces do control the supracrystal nucleation in organic media ?

Here, two mechanisms of fcc Au supracrystal (assembly of Au nanocrystals) growth are proposed. The sizes of the Au nanocrystals and the solvent in which they are dispersed are major parameters that determine the final morphology of nanocrystal assemblies; fi lms by layer-by-layer growth (heterogeneous growth), characterized by their plastic deformation, or well-defined shapes grown in solution (homogeneous growth). Experiments supported by simulations demonstrate that supracrystal nucleation is mainly driven by solvent-mediated interactions and not solely by the van der Waals attraction between nanocrystal cores, as widely assumed in the literature.

Source : Which forces do control the supracrystal nucleation in organic media ? N.Goubet, J.Richardi, P.A. Albouy and M.P.Pileni. Adv.Funct.Mater., 2011,21, 2693-2704.

peer journals

Low Sensitivity of Acoustic Breathing Mode Frequency in Co Nanocrystals upon Change in Nanocrystallinity.

Cobalt nanocrystals (NCs) with narrow size distribution and polycrystalline structure in their native form are synthesized in reverse micelles. After annealing at 350 !C, these NCs are transformed into single crystalline phase with hexagonal close-packed structure. The vibrational dynamics of NCs di!ering by their nanocrystallinity is studied by femtosecond pump!probe spectroscopy. By recording the di!erential re »ectivity signal in the native and annealed Co NCs, the frequency of their fundamental breathing acoustic mode can be measured in the time domain. A small decrease of the breathing mode frequency is observed in single crystalline Co NCs compared to that measured in polycrystals, indicating low sensitivity of their fundamental radial mode upon change in crystallinity. This result is in agreement with predictions from calculations using the resonant ultrasound approach.

Source : Low Sensitivity of Acoustic Breathing Mode Frequency in Co Nanocrystals upon Change in Nanocrystallinity. D.Polli I. Lisiecki, H. Portalès, G. Cerullo and M.P. Pileni. ACS Nano., 2011, 5, 5785-5791.

peer journals

How to control the crystalline structure of supracrystals of 5-nm Ag nanocrystals ?

Supracrystals of 5-nm silver nanocrystals are characterized by various structures, ranging from face-centered-cubic (fcc), to hexagonal-close-packed (hcp), to body-centered-cubic (bcc) structures. Here, it is shown that the transition from fcc to hcp is solvent-dependent and attributed to specifi c stacking processes, depending on the evaporation kinetics. Hence, at a fi xed substrate temperature, the most volatile solvents (such as hexane and toluene) favor the growth of fcc superlattices, whereas with solvents that have a higher boiling point (such as octane, decane, and dodecane), hcp supracrystals are produced. In contrast, the formation of bcc structures is shown to be solvent-independent and is attributed to van der Waals attractions. The chain length of the coating agent and the deposition temperature govern the transition from compact (fcc/hcp) to bcc supracrystals. The experimentally phase transitions are interpreted by theoretical approaches.

Source : How to control the crystalline structure of supracrystals of 5-nm Ag nanocrystals ?A. Courty, J. Richardi, P.A. Albouy and M.P. Pileni, Chem Mat., 2011, 23, 4186-4192.

peer journals

How Can the nanocrystallinity of 7 nm Spherical Co nanoparticles dispersed in solution be improved?

We report a solution-phase annealing of spherical Co nanocrystals synthesized in reverse micelles and coated with dodecanoic acid. The deposition of a drop of solution on a transmission electron microscope grid shows that a progressive increase in the temperature to 316 ! C results in the progressive crystallographic transition from a polycrystalline and probably face-centered cubic Co phase to the single-crystalline hexagonal close-packed (hcp) Co phase. These nanocrystals are highly stable against oxidation and coalescence. We stress that, to our knowledge, this constitutes the firrst example in the literature of pure hcp-Co spherical single crystals dispersed in solution. These nanocrystals can be freely manipulated and, due to their, low size dispersion, can self-organize on various substrates.

Source : How Can the nanocrystallinity of 7 nm Spherical Co nanoparticles dispersed in solution be improved? M. Cavalier, M. Wall, I. Lisiecki, and M.P. Pileni. Langmuir, 2011, 27, 5014–5020.

peer journals

2D superlattices and 3D supracrystals of metal nanocrystals : a new scientific adventure.

Nanocrystals are able to self-assemble in hexagonal networks (2D) and in supracrystals (3D). Here it is shown that the interparticle distance is tuned by the presence of water molecules adsorbed at the nanocrystal interface and on the alkyl chains used as coating agents. By using an intrinsic property due to the nanocrystal ordering, a new, but destructive, method is proposed to detect defects on a large monolayer scale. The supracrystal growth mechanism changes with the nanocrystal size from a heterogeneous (layer-by-layer) to a homogeneous (growth in solution) process. Co supracrystals are highly stable after annealing at 350 !C with an improvement in the nanocrystal ordering, i.e., in the supracrystallinity. With Ag supracrystals it was possible, from the same batch of 5 nm Ag nanocrystals, to control the supracrystallinity with phase transitions of hcp to fcc and amorphous solids to hcp and bcc. Finally a tentative analogy between atoms and nanocrystals is proposed in the crystal growth process. These data open a new research area with a large potential for discovering new chemical and physical properties.

Source : 2D superlattices and 3D supracrystals of metal nanocrystals : a new scientific adventure.  M.P.Pileni J. Mater. Chem., 2011, 21, 16748 – 16758