Imad Arfaoui

Simultaneous Growths of Gold Colloidal Crystals

Natural systems give the route to design periodic arrangements with mesoscopic architecture using individual nanocrystals as building blocks forming colloidal crystals or supracrystals. The collective properties of such supracrystals are one of the main driving forces in materials research for the 21st century with potential applications in electronics or biomedical environments. Here we describe two simultaneous supracrystal growth processes from gold nanocrystal suspension, taking place in solution and at the air-liquid interface. Furthermore, the growth processes involve the crystallinity selection of nanocrystals and induce marked changes in the supracrystal mechanical properties.

Modulating physical properties of isolated and self-assembled nanocrystals through change in nanocrystallinity.

For self-assembled nanocrystals in three-dimensional (3D) superlattices, called supracrystals, the crystalline structure of the metal nanocrystals (either single domain or polycrystalline) or nanocrystallinity is likely to induce significant changes in the physical properties. Previous studies demonstrated that spontaneous nanocrystallinity segregation takes place in colloidal solution upon self-assembling of 5 nm dodecanethiol-passivated Au nanocrystals. This segregation allows the exclusive selection of single domain and polycrystalline nanoparticles and consequently producing supracrystals with these building blocks. Here, we investigate the influence of nanocrystallinity on different properties of nanocrystals with either single domain or polycrystalline structure. In particular, the influence of nanocrystallinity on the localized surface plasmon resonance of individual nanocrystals dispersed in the same dielectric media is reported. Moreover, the frequencies of the radial breathing mode of single domain and polycrystalline nanoparticles are measured. Finally, the orientational ordering of single domain nanocrystals markedly changes the supracrystal elastic moduli compared to supracrystals of polycrystalline nanocrystals.

Periodic Positioning of Multilayered [2.2]Paracyclophane-Based Nanopillars†

The development of conjugated organic molecules as building blocks capable of ordering on well-defined surfaces is an active research area in view of future applications in molecular electronics, photonics, and nanomechanical devices. 4] Bottom-up approaches based on supramolecular chemistry are of particular interest for formation of preprogrammed organic self-assembled monolayers. Extensive efforts have been concentrated, independently, in two directions. The first, based on upright-oriented chemisorbed molecules such as thiols, consists of stacking functional moieties in the off-plane direction, which is usually detrimental for lateral order. The second, the objective of which is the mastering of increasingly complex lateral organizations, is based on two-dimensional molecules forming essentially planar structures. Combining complex in-plane self-organizations and off-plane protrusions in self-assembled molecular architectures is an important current challenge. This is a key requirement, for example, for the realization of biological molecular receptors or for decoupling electronically active molecules from a conducting substrate. The latter is currently achieved either through insulating layers or by introduction of molecular spacer units. However, such modifications strongly perturb the lateral organization. Moreover, complex combinations of in-plane and off-plane structures are precluded. A first step towards such combinations is the nanoscale positioning of rigid upstanding elements. We recently proposed a strategy to form “on-demand” noncovalent self-assemblies with predetermined 2D topologies on highly oriented pyrolitic graphite (HOPG) at the liquid/solid interface at room temperature. This method is based on a novel functional unit which acts as a cliplike noncovalent bond on HOPG. Here, we exploit this approach for the realization of 2D patterns of well-defined 3D nanostructures. Our strategy follows classical architectural paradigms based on the realization of a well-organized in-plane monolayer on HOPG and the emergence, perpendicular to the substrate, of a periodic array of standing organic nanopillars of tunable height. To validate this concept, we chose the multilayered [2.2]paracyclophane (PCP) moiety as a nanopillar of variable height. More precisely, we designed and synthesized a series of compounds (Scheme 1) bearing two

Charge Transport in a Single Superconducting Tin Nanowire Encapsulated in a Multiwalled Carbon Nanotube

The charge transport properties of single superconducting tin nanowires encapsulated by multiwalled carbon nanotubes have been investigated by multiprobe measurements. The multiwalled carbon nanotube protects the tin nanowire from oxidation and shape fragmentation and therefore allows us to investigate the electronic properties of stable wires with diameters as small as 25 nm. The transparency of the contact between the Ti/Au electrode and nanowire can be tuned by argon ion etching the multiwalled nanotube. Application of a large electrical current results in local heating at the contact which in turn suppresses superconductivity.

Surface enhanced SHG from macrocycle, catenane and rotaxane thin films: experiments and theory.

Surface enhanced second harmonic generation experiments on supramolecules: macrocycles, catenanes and rotaxanes, monolayers and multilayers deposited by vacuum evaporation on silver layers are reported and described. The measurements show that the molecules are ordered in thin films. The highest order is observed in the case of macrocycles and the lowest in thin films of fumaramide [2] rotaxanes. Also a better ordering is observed in the case of monolayers. The observed second harmonic generation activity is interpreted in terms of electric field induced second harmonic generation. The electric field contributing to SHG signal is created by silver atoms on the surface of silver layers. The measured second order NLO susceptibilities for a fumaramide [2] rotaxane is compared with that obtained by considering only EFISH contribution to SHG intensities. The electric filed on the surface of silver layer is calculated using TINKER molecular mechanics/dynamics software and the Embedded Atom model. An excellent agreement is observed between the calculated and the measured SHG susceptibilities.

Directed Organization of Platinum Nanocrystals through Organic Supramolecular Nanoporous Templates

We propose a novel approach to trap 2 nm Pt nanocrystals using nanoporous two-dimensional supramolecular networks for cavity-confined host-guest recognition process. This will be achieved by taking advantage of two features of supramolecular self-assembly at surfaces: First, its capability to allow the formation of complex 2D architectures, more particularly, nanoporous networks, through noncovalent interactions between organic molecular building-blocks; second, the ability of the nanopores to selectively host and immobilize a large variety of guest species. In this paper, for the first time, we will use isotropic honeycomb networks and anisotropic linear porous supramolecular networks to host 2 nm Pt nanocrystals.