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

Impact of Nanocrystallinity Segregation on the Growth and Morphology of Nanocrystal Superlattices.

A colloidal solution of 5 nm Au tetradecanethiol-coated nanoparticles is synthesized. After fast evaporation of one drop, ordered monolayers both composed of single domain and polycrystalline nanocrystals are obtained. On increasing the amount of materials and the evaporation time, nanocrystal films with irregular outlines are produced together with close-packed 3D superlattices exhibiting a truncated-tetrahedral shape. Using low-frequency micro-Raman scattering spectroscopy and electron microscopy the building block nanocrystallinity is characterized. Spontaneous nanocrystallinity segregation is revealed: the truncated-tetrahedral supracrystals are shown to mainly contain single domain building blocks while the supracrystalline films are composed of a mixture of single domain and polycrystalline nanocrystals. This observation points out the correlation between the nanocrystallinity segregation involved in the growth of the nanocrystal superlattices and their morphology

Source : Impact of Nanocrystallinity Segregation on the Growth and Morphology of Nanocrystal Superlattices. Y. Wan, H. Portalès, N. Goubet, A. Mermet, and M.P. Pileni Nanoresearch,2013, 6, 611–618.

peer journals

Impact of the metallic crystalline structure on the properties of nanocrystals and their mesoscopic assemblies.

The spontaneous assembly of uniform-sized globular entities into ordered arrays is a universal phenomenon observed for objects with diameters spanning a broad range of length scales. These extend from the atomic scale (10−8  cm), through molecular and macromolecular scales with proteins, synthetic low polymers, and colloidal crystals (∼ 10−6  cm), to the wavelength of visible light (∼ 10−5  cm). The associated concepts of sphere packing have had an infl uence in diverse fi elds ranging from pure geometrical analysis to architectural models or ideals. Self-assembly of atoms, supramolecules, or nanocrystals into ordered functional superstructures is a universal process and prevalent topic in science. About fi ve billion years ago in the early solar system, highly uniform magnetite particles of a few  hundred nanometers in size were assembled in 3D arrays. Thirty million years ago, silicate particles with submicrometer size were self-organized in the form of opal. Opal is colorless when composed of disordered silicate microparticles whereas it shows specifi c refl ectivity when particles order in arrays. Nowadays, nanocrystals, characterized by a narrow size distribution and coated with alkyl chains to maintain their integrity, self-assemble to form crystallographic orders called supracrystals. Nanocrystals and supracrystals are arrangements of highly ordered atoms and nanocrystals, respectively. The morphologies of nanocrystals, supracrystals, and minerals are similar at various scales from nanometer to millimeter scale. Such suprastructures, which enable the design of novel materials, are expected to become one of the main driving forces in material research for the 21st century. Nanocrystals vibrate coherently in a supracrystal as atoms in a nanocrystal. Longitudinal acoustic phonons are detected in supracrystals as with atomic crystals, where longitudinal acoustic phonons propagate through coherent movements of atoms of the lattice out of their equilibrium positions. These vibrational properties show a full analogy with atomic crystals: In supracrystals, atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains), which act like mechanical springs holding together the nanocrystals. Electronic properties of very thick (more than a few micrometers) supracrystals reveal homogeneous conductance with the fi ngerprint of the isolated nanocrystal. Triangular single crystals formed by heat-induced (50 °C) coalescence of thin supracrystals deposited on a substrate as epitaxial growth of metal particles on a substrate with specifi c orientation produced by ultrahigh vacuum (UHV). Here we demonstrate here that marked changes can occur in the chemical and physical properties of nanocrystals diff ering by their nanocrystallinity, that is, their crystalline structure. Furthermore, the properties (mechanical, growth processes) of supracrystals also change with the nanocrystallinity of the nanoparticles used as building blocks.

Source : Impact of the metallic crystalline structure on the properties of nanocrystals and their mesoscopic assemblies. MPPileni. Acc. Chem. Res., 2017, 50, 1946–1955