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 .

peer journals

Point-contact probe microscopy of nanoparticle supracrystals.

Highly ordered three-dimensional colloidal crystals (supracrystals) of gold nanoparticles have been imaged and analysed using a combination of scanning tunnelling microscopy and dynamic force microscopy. By exploring the evolution of both force and tunnel current with respect to tip-sample separation we make the surprising _nding that single nanoparticle resolution is readily obtained in tunnelling microscopy images acquired more than 1 nm into the repulsive (i.e. positive force) regime of the probe-nanoparticle interaction potential. This implies that the contrast mechanism involves a form of \point contact » imaging, rather than the through-vacuum tunneling, which underpins traditional tunneling microscopy and spectroscopy. Constant height force microscopy has been used to map tip-sample interactions in this point contact regime, revealing inhomogeneities that arise from the convolution of the tip structure with the ligand distribution at the nanoparticle surface.

Source : Point-contact probe microscopy of nanoparticle supracrystals. A. Sweetman, N. Goubet, I. Lekkas, M. P. Pileni, and Philis Moriarty J. Nanotechnology, 2014 , ID 6647414

peer journals

Water-Dispersed Hydrophobic Au Nanocrystal Assemblies with a Plasmon Fingerprint

Hydrophobic Au nanocrystal assemblies (both ordered and amorphous) were dispersed in aqueous solution via the assistance of lipid vesicles. The intertwine between vesicles and Au assemblies was made possible through a careful selection of the length of alkyl chains on Au nanocrystals. Extinction spectra of Au assemblies showed two peaks that were assigned to a scattering mode that red-shifted with increasing the assembly size and an absorption mode associated with localized surface plasmon, that was independent of their size. This plasmon fingerprint could be used as a probe for investigating the optical properties of such assemblies. Our water-soluble assemblies enable exploring a variety of potential applications including solar energy and biomedicine.

Source : Water-Dispersed Hydrophobic Au Nanocrystal Assemblies with a Plasmon Fingerprint. Yang, C.Deeb, J.L Pelouard, N. Felidj and M.P. Pileni ACS Nano, 2017,11, 7797−7806.