peer journals

Mechanical properties of Au supracrystals tuned by flexible ligand interaction. 

Here mechanical properties of face cubic centered colloidal crystals obtained out of equilibrium by solvent evaporation of coated Au nanocrystals suspension, called supracrystals, are reported as a function ligand chain length (n ) and interparticle edge-to-edge distance within the supracrystals (δpp ) for two nanocrystal sizes (d ). Young’ s modulus (E* ) and hardness (H ) are independent of δpp  and of the supracrystal morphology. Both E*  and H  are in the range of few tenths of a MPa to a few GPa. Tuning of δpp  by 50% is achieved by controlling the solvent vapor pressure (Pt ) during the evaporation process. For any nanocrystal size, at Pt  = 0, E*and H  values markedly increase with increasing n  from 12 to14. At Pt = 39% and 75%, such dependency disappears. This trend diff ers from classical nanocomposite materials and is attributed to a change in the conformation of fl exible ligands with n  and to free thiol-containing molecules trapped in the supracrystallattices.

Source : Mechanical properties of Au supracrystals tuned by flexible ligand interaction.  M.Gauvin, Y. Wan, I. Arfaoui and M.P. Pileni J.Phys.Chem.C.,2014, 118, 5005−5012

peer journals

How nanocrystallinity and order define the magnetic properties of ε-Co supracrystals.

Single domain cubic ε -Co nanocrystals are synthesized via a high-temperature thermal decomposition of cobalt carbonyl in the presence of oleic acid and trioctylphosphane oxide (TOPO). The ε -Co nanocrystals are characterized by a low size distribution (σ  < 7%) and the average diameter is tuned from 7 nm to 9 nm by tailoring the molar ratio of the surfactants oleic acid and TOPO. Moreover, we have demonstrated the self-assembly of ε -Co nanocrystals in highly ordered three-dimensional (3D) face-centered cubic (fcc) structures called supracrystals. The layer-by-layer organization of these building blocks is achieved through solvent evaporation. Simultaneously, we produce. with the same ε -Co nanocrystals, disordered (amorphous) films. We demonstrate the presence of large interparticle magnetic interactions in the supracrystals by comparing their magnetic properties with the diluted samples. Then, by a detailed comparison of their collective magnetic properties with partially disordered films, the significant differences due to the change in anisotropy and distribution of dipolar interaction energies in the two systems are presented. This is attributed to the orientational and spatial ordering of single domain ε -Co nanocrystals markedly changing between ordered and disordered assemblies. The thermal evolution of the magnetization in ZFC/FC procedure presents three characteristic temperatures representing the blocking, the irreversibility and the maximum of Zeeman coupling temperatures. They are all affected by the presence of the order in supracrystals and they present different evolution trends as a function of nanoparticles size. While the variations of reduced remanent magnetizations in both condensed series are in good agreement with the previous theoretical calculations, the coercive fields present opposite evolutions.

Source : How nanocrystallinity and order define the magnetic properties of ε-Co supracrystals. J.Yang, K. Khazen, and M.P. Pileni, J Phys Condens Matter.,2014, 23-26 295303

peer journals

Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals.

8 nm Co nanoparticles with various crystalline structures called polymorphs were produced using different synthetic procedures, such as using reverse micelles, the thermal decomposition of organometallics approach or the hot injection process. These 8 nm Co nanoparticles differing by their crystalline structures are exposed to oxygen at elevated temperature. The fcc Co polycrystalline nanoparticles produce either Co–CoO yolk–shell or CoO hollow structures whereas amorphous Co nanoparticles produce Co–CoO core–shell nanoparticles. Furthermore, single domains with either hcp or e crystalline structure behave differently upon oxygen diffusion. Co–CoO nanoparticles were produced from the hcp phase while CoO hollow nanoparticles were the product for e-phase Co nanocrystals.

Source : Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals. Z .Yang, J.Yang, J.Bergström, K.Khazen, and M. P. Pileni Phys. Chem. Chem. Phys., 2014,16, 9791-9796.

peer journals

Hierarchy in Au Nanocrystal Ordering in Supracrystal: III. Competition between Van der Waals and Dynamic Processes.

Au nanocrystals coated with thiol derivatives of varying chain sizes ranging from C12  to C16  were produced; two different size nanocrystals have been synthesized (5 and 7 nm in diameter) for each coating agent. All of those specimensare characterized by a low size distribution (below 7%). ThoseAu nanocrystals were used as building blocks to grow larger self-assembled crystalline structures or supracrystals. Thesecrystalline growths were carried out by slow and controlledsolvent evaporation at different temperatures and under nonnull partial solvent vapor pressure (Pt). We show that theorder within the supracrystals is temperature-dependent whenthey are made of hexadecanethiol-coated gold nanocrystals, regardless of the size of the nanocrystals. The interparticle distances within the various supracrystals that were produced were determined by small-angle X-ray diff raction (SAXRD). We demonstrate that the interparticle distance is controlled not only by the presence of physisorbed thiol residues, as previously reported, but also, at higher temperatures, by the dynamics of the organic chains and the van der Waals forces involved between the metallic cores of the nanocrystals forming the structure.

Source : Hierarchy in Au Nanocrystal Ordering in Supracrystal: III. Competition between Van der Waals and Dynamic Processes. N. Schaeffer, Y.Wan, M.P. Pileni. Langmuir ,2014, 30, 7177 (2014).

peer journals

Ag Nanorystals: 1. Effet of Ligands on Plasmonic Properties.

Silver nanocrystals (NCs) stabilized using amine-terminated coating agents (oleylamine or dodecylamine), their size ranging between 2 and 12 nm in diameter, are synthesized by hot injection methods. Their dispersion in size is relatively low (typically below 10%) without the needfor a postsynthesis size segregation process. The amineterminated coating agents are replaced by thiol-terminated molecules (dodecanethiol or hexadecanethiol) by ligand exchange, allowing the formation of alkanethiol coated Ag colloids. All NCs with various surface coatings are dispersed intoluene. Regardless of the nature of the coating agent, the surface plasmon resonance (SPR) is red-shifted with decreasing the NC size. For a given size, the SPR peak of thiol-stabilized NCs is shifted to lower energies compared to that of amine-stabilized. Furthermore, with thiol-stabilized Ag NCs, the position of the SPR peak was found to be sensitive to the length of the alkylchains of the coating agent, whereas minor diff erences are detected for Ag NCs coated with amines terminated with diff ering alkyl chain lengths.

Source : Ag Nanorystals: 1. Effet of Ligands on Plasmonic Properties. J.Wei, N.Schaeffer and M.P. Pileni J.Phys. Chem.B., 2014, 118, 14070−14075

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

Spontaneous formation of high-index planes in Au single domain nanocrystal superlattices.

Crystals of nanocrystals, also called supracrystalsand nanocrystal superlattices, are expected to exhibit  specifi c properties that diff er from both the corresponding bulk material and nanosized elementary units. In particular, their surfaces have a great potential as nanoscale interaction plateforms. However, control of the symmetry, compacity, and roughness of their surfaces remains an open question. Here, we describe the spontaneous formation of upper vicinal surfaces for supracrystals of Au nanocrystals grown on a sublayer of ordered Co nanocrystals. Stepped or kinked surfaces vicinal to the {100}, {110}, and {111} planes are observed to be extended on the micrometer range. The formation of such high-index planes is explained by a heteroepitaxial relationship between both Co and Au nanocrystal superlattice.

Source : Spontaneous formation of high-index planes in Au single domain nanocrystal superlattices. N. Goubet, J.Yang, P.A.Albouy and M.P.Pileni Nano. Lett.,2014, 14, 6632-6638