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

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

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.