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David Kreher, enseignant-chercheur

Nanostructured surfaces for nanophotonics, supra (macro) molecular assemblies (nanotubes, gels, core-shell particles)

In this topic, recently we demonstrated highly ordered 3D nanostructures on highly-oriented pyrolitic graphite (HOPG) at the liquid-solid interface at room temperature. Our strategy followed classical architectural paradigms based on the realization of a well-organized on-plane monolayer paving the HOPG, and the emergence, perpendicular to the substrate, of an array of standing organic nanopillars of tunable height in view of obtaining large atomically-precise alignments both in-plane and vertically controlled. To validate this concept, we chose the multilayered [2.2]paracyclophanyl (PCP) moiety as nanopillars with variable height. More precisely, we designed and synthesized a series of molecules bearing two functional 'clips' end-capping a central either benzene ring, or the lower deck of a two- and three-layered PCP unit.

 

We were then able to modify the chemistry of the upper-deck towards self-assemblies with photonic functions, by developing a strategy aimed at the decoupling of conjugated molecules from the surface, by lifting photoactive entities a few Å above the surface while maintaining the lateral organization of the array. To this aim, we introduced the concept of Janus-like 3D molecular tectons which are dual-functionalized building blocks exposing two opposite faces (A and B) linked by a rigid insulating spacer : the pedestal A manages the 2D selfassembly on the substrate (currently HOPG) via alkyl chain interdigitation whereas B provides the photonic functionality (see an example on the left).

 

Based on this strategy, we develop actually several projects, one being the obtaining of a nano-porous 3D network and to control the opening / closing of the 3D cavities formed. Our strategy is based of course on the realization of (i) a well-organized « first-level » paving the HOPG, (ii) the emergence, perpendicular to the substrate, of an array of pillars, and (iii) in this case the attachment on the top of a photoswitch (scheme below).

Another possibility we are looking for is to extend the concept of Janus tecton to molecules bearing magnetic center. Indeed, our recent strategy was aimed at the decoupling of conjugated molecules from the surface, by lifting photoactive entities a few Å above the surface while maintaining the lateral organization of the array. That's the reason why we propose to synthesize and adsorb on HOPG (and grapheme later) such building blocks where specific ligands for example will be attached to the upper deck of the pillar, in order to interact with nanocrystals and « trap » them in a very well ordered mannere.

 

We also showed that all of these multistory molecules are suitable for self-assembly, and few months ago modified our strategy to move towards gold, still for building up additional p-conjugated architectures perpendicular to the gold substrate, in order to target nanophotonics applications. To do so, in a first stage, we very recently designed a new binding unit based on alkyl chain interdigitation specifically designed for steering self-assembly on Gold.[Bleger 2012] Based on newly established molecular design rules, bare pedestal led to a structure with optimized non-covalent intermolecular bonding and much reduced conjugation of p electrons.

 

 

The self-organization of this first-floor alone on Au(111), presents a highly regular structure. In the Janus tecton approach framework, we expect this moiety to act as a pedestal, and associated with rigid insulating spacer, it should offer improved constitutional flexibility. In a close future, we would like consequently to both transfer our methodology of Janus tectons towards gold substrate but to a novel and important substrate also due to its properties, the graphene.

 

Self-assemblies : principal project

Self-Assembled Engineered Sub-Wavelength Emitters for STM-Excited Single Molecule Electroluminescence.
    So far, molecular self-assembly constitutes a promising route for the bottom-up manufacturing of nanostructures
    on atomically-flat surfaces...

>>> Read more...

 

Significant personal publications