peer journals

Hydrophilic Gold Supracrystals Differing by the Nanoparticle Crystalline Structure

Very few studies concern water-soluble nanocrystals self-assembled in crystalline 3D superlattices called supracrystals. Furthermore, the control of the crystalline structure of nanocrystals known as nanocrystallinity has not been yet achieved with water-soluble nanocrystals. Here we produce, selectively, 5 nm Au single-domain (SD) and polycrystalline (POLY) water-soluble nanocrystals. These nanocrystals self-assembled in face-centered-cubic (fcc) supracrystals. The supracrystal stiff ness evolves with the nanocrystallinity, the nanocrystal surface charge, as well as the stericeff ect of the coating agent. The optical properties of SD and POLY nanoparticles and those of the related supracrystals are also presented. In addition, a nanocrystallinity segregation event was observed upon drying-assisted self-assembly of aqueous stoichiometric mixtures of SD and POLY NCs, as in the case of their hydrophobic counterparts.

Source : Hydrophilic Gold Supracrystals Differing by the Nanoparticle Crystalline Structure. S. Mourdikoudis, A. Çolak, I. Arfaoui, and M.P Pileni  J. Phys. Chem. C, 2017, 121, 10670−10680

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.

peer journals

Coating agent-induced mechanical behavior of 3D self-assembled nanocrystals 

The Young’s modulus of three-dimensional self-assembled Ag nanocrystals, as so-called supracrystals, is correlated with the type of coating agent as well as the nanocrystal morphology. The Young’s moduli of supracrystals of icosahedral Ag nanocrystals are measured in the range of tens to hundreds of megapascals revealing an extremely soft mechanical behavior. The alkylamine molecules used as coating agents weakly interact with the Ag nanocrystal surface favoring translational and orientational ordering of atomic lattice planes of nanocrystals. In such experimental conditions, both the average distance between nanocrystals and the increase of the nanocrystal diameter control the measured Young modulus: It increases with decreasing the interparticle distance and increasing the nanocrystal diameter. When dodecylamine (C12NH2) molecules is replaced by dodecanthiol (C12SH), the orientational ordering between nanocrystals, produced from the same batch as C12NH2, disappears by inducing a drop in the Young modulus. This is attributed to formation of a “skin” at the nanocrystal surface causing a transition from shaped to spherical nanocrystals. Finally, by comparing with various studies performed in our group with Co and Au nanocrystals, we explain the formation of such extremely soft materials with Ag nanocrystals by both the strength of the binding between nanocrystals and coating agent and the ligand-ligand interactions.

Source : Coating agent-induced mechanical behavior of 3D self-assembled nanocrystals. A. Çolak, J. Wei, I. Arfaoui,  M.P. Pileni  PCPC 2017, 19, 23887

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