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

Supra – and nanocrystallinities: a new scientific adventure

Nanomaterials exist in the interstellar medium, in biology, in art and also metallurgy. Assemblies of nanomaterials were observed in the early solar system as well as silicate particle opals. The latter exhibits unusual optical properties directly dependent on particle ordering in 3D superlattices. The optical properties of noble metal nanoparticles (Ag, Au and Cu) change with the ordering of atoms in the nanocrystals, called nanocrystallinity. The vibrational properties related to nanocrystallinity markedly differ with the vibrational modes studied. Hence, a drastic effect on nanocrystallinity is observed on the confined acoustic vibrational property of the fundamental quadrupolar modes whereas the breathing acoustic modes remain quasi-unchanged. The mechanical properties characterized by the Young’s modulus of multiply twinned particle (MTP) films are markedly lower than those of single nanocrystals. Two fcc supracrystal growth mechanisms, supported by simulation, of Au nanocrystals are proposed: heterogeneous and homogeneous growth processes. The final morphology of nanocrystal assemblies, with either films by layer-by-layer growth characterized by their plastic deformation or well-defined shapes grown in solution, depends on the solvent used to disperse the nanocrystals before the evaporation process. At thermodynamic equilibrium, two simultaneous supracrystal growth processes of Au nanocrystals take place in solution and at the air–liquid interface. These growth processes are rationalized by simulation. They involve, on the one hand, van der Waals interactions and, on the other hand, the attractive interaction between nanocrystals and the interface. Ag nanocrystals (5 nm) self-order in colloidal crystals with various arrangements called supracrystallinities. As in bulk materials, phase diagrams of supracrystals with structural transitions from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) and body-centered-cubic (bcc) structures are observed. They depend on the chain length of the coating agent and on the solvent used to disperse the nanocrystals before evaporation. The transition from fcc to hcp is attributed to specific stacking processes depending on evaporation kinetics whereas the formation of bcc supracrystals is attributed to van der Waals attractions. These results open up a new research area, which currently suffers from an extensive lack of knowledge.

Source : Supra – and nanocrystallinities: a new scientific adventure M.P.Pileni. J.Phys. Cond.Mat., 2011, 23, 503102.

peer journals

Supra and Nano crystallinity: Specific properties related to crystal growth mechanisms and nanocrystallinity.

The natural arrangement of atoms or nanocrystals either in well-defined assemblies or in a disordered fashion induces changes in their physical properties. For example, diamond and graphite show marked differences in their physical properties though both are composed of carbon atoms. Natural colloidal crystals have existed on earth for billions of years. Very interestingly, these colloidal crystals are made of a fixed number of polyhedral magnetite particles uniform in size. Hence, opals formed of assemblies of silicate particles in the micrometer size range exhibit interesting intrinsic optical properties. A colorless opal is composed of disordered particles, but changes in size segregation within the self-ordered silica particles can lead to distinct color changes and patterning. In this Account, we rationalize two simultaneous supracrystal growth processes that occur under saturated conditions, which form both well-defined 3D superlattices at the air!liquid interface and precipitated 3D assemblies with well-defined shapes. The growth processes of these colloidal crystals, called super- or supracrystals, markedly change the mechanical properties of these assemblies and induce the crystallinity segregation of nanocrystals. Therefore, single domain nanocrystals are the primary basis in the formation of these supracrystals, while multiply twinned particles (MTPs) and polycrystals remain dispersed within the colloidal suspension. Nanoindentation measurements show a drop in the Young’s moduli for interfacial supracrystals in comparison with the precipitated supracrystals. In addition, the value of the Young’s modulus changes markedly with the supracrystal growth mechanism. Using scanning tunneling microscopy/spectroscopy, we successfully imaged very thick supracrystals (from 200 nm up to a few micrometers) with remarkable conductance homogeneity and showed electronic fingerprints of isolated nanocrystals. This discovery of nanocrystal fingerprints within supracrystals could lead to promising applications in nanotechnology.

Source : Supra and Nano crystallinity : Specific properties related to crystal growth mechanisms and nanocrystallinity. M.P.Pileni, Account Chem Res., 2012,45, 1965-1972.

 

peer journals

Simultaneous Interfacial and Precipitated Supracrystals of Au Nanocrystals: Experiments and Simulations.

Under solvent saturation, a precipitation of fullgrownsupracrystals on the one hand and the formation of well defined supracrystalline fi lms at the air− liquid interface on the other hand were previously observed for the fi rst time (J. Am. Chem. Soc. 2012 , 134 , 3714− 3719). Here, these two simultaneous growth processes are studied by additional experiments and by Brownian dynamics simulations. The thickness of the supracrystalline films and the concentration of free nanocrystals within the solution are measured as a function of the nanocrystal size. The simulations show that the fi rst process of supracrystal growth is due to a homogeneous nucleation favored by solvent-mediated ligand interactions, while the second one is explained in terms of a diff usion process caused by a decrease in the surface energy when the particles penetrate the air− liquid interface. It is also verifi ed that the presence of thiol molecules at the air− solution interface does not hinder the formation of supracrystalline films.

Source : Simultaneous Interfacial and Precipitated Supracrystals of Au Nanocrystals: Experiments and Simulations. N Goubet, J. Richardi, P.A. Albouy and M. P. Pileni. J.Phys.Chem.B., 2013, 117, 4510!4516 .