Nicolas Schaeffer

Fluorescent or not? Size-dependent fluorescence switching for polymer-stabilized gold clusters in the 1.1-1.7 nm size range

The synthesis of fluorescent water-soluble gold nanoparticles by the reduction of a gold salt in the presence of a designed polymer ligand is described, the size and fluorescence of the particles being controlled by the polymer to gold ratio; the most fluorescent nanomaterial has a 3% quantum yield, a 1.1 nm gold core and a 6.9 nm hydrodynamic radius.

Nanocrystals: Why Do Silver and Gold N-Heterocyclic Carbene Precursors Behave Differently?

Synthesizing stable Au and Ag nanocrystals of narrow size distribution from metal-N-heterocyclic carbene (NHC) complexes remains a challenge, particularly in the case of Ag and when NHC ligands with no surfactant-like properties are used. The formation of nanocrystals by one-phase reduction of metal-NHCs (metal = Au, Ag) bearing common NHC ligands, namely 1,3-diethylbenzimidazol-2-ylidene (L(1)), 1,3-bis(mesityl)imidazol-2-ylidene (L(2)), and 1,3-bis(2,6-(i)Pr2C6H3)imidazol-2-ylidene (L(3)), is presented herein. We show that both Au and Ag nanocrystals displaying narrow size distribution can be formed by reduction with amine-boranes. The efficiency of the process and the average size and size distribution of the nanocrystals markedly depend on the nature of the metal and NHC ligand, on the sequence in the reactant addition (i.e., presence or absence of thiol during the reduction step), and on the presence or absence of oxygen. Dodecanethiol was introduced to produce stable nanocrystals associated with narrow size distributions. A specific reaction is observed with Ag-NHCs in the presence of thiols whereas Au-NHCs remain unchanged. Therefore, different organometallic species are involved in the reduction step to produce the seeds. This can be correlated to the lack of effect of NHCs on Ag nanocrystal size. In contrast, alteration of Au nanocrystal average size can be achieved with a NHC ligand of great steric bulk (L(3)). This demonstrates that a well-defined route for a given metal cannot be extended to another metal.

Superior Oxygen Stability of N-Heterocyclic Carbene-Coated Au Nanocrystals: Comparison with Dodecanethiol.

The stability of Au nanocrystals (NCs) coated with different N-heterocyclic carbenes (NHCs) or dodecanethiol (DDT) to oxygen-based treatments was investigated. A dominant effect of the ligand type was observed with a significantly greater oxygen resistance of NHC-coated Au NCs compared to that of the thiol-based analogues. NHC-coated Au NCs are stable to 10 W oxygen plasma etching for up to 180 s whereas the integrity of DDT-coated Au NCs is strongly affected by the same treatment from 60-80 s. In the latter case, the average size of the NCs (from 2.6 to 6.3 nm) and the method of synthesis have no effect on the stability. NHC-coated Au NCs were found to generate of a smaller quantity of ligand-derived species under molecular oxygen treatment, which could account for the increased stability.

Hierarchy in Au nanocrystal ordering in supracrystals: 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 specimens are characterized by a low size distribution (below 7%). Those Au nanocrystals were used as building blocks to grow larger self-assembled crystalline structures or supracrystals. These crystalline growths were carried out by slow and controlled solvent evaporation at different temperatures and under non-null partial solvent vapor pressure (Pt). We show that the order within the supracrystals is temperature-dependent when they 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 diffraction (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.