DS0305 - Apport des nanosciences et nanotechnologies aux matériaux fonctionnels et biotechnologies

From Individual Transitions to Surface-Plasmon Resonances in Nano-Gold and -Silver – FIT SPRINGS

Submission summary

Owing to their outstanding optical properties, noble-metal clusters are used for a wide range of applications, ranging from surface-enhanced Raman spectroscopy (SERS), biomolecule sensing, labeling of biomolecules, cancer therapy, the plasmonic absorption enhancement in solar cells to nanophotonics. Moreover, the optical properties can be exploited in the characterization of clusters that are not employed for their optical properties but for, e.g., catalysis.
--- Physically, the transition range of sizes between a few atoms and about 3 nm is the most interesting because here the transition between molecular-like excitations and the collective excitation of truly metallic nanoparticles occurs, opening unparalleled opportunities of nonmaterial engineering. The physics in this size range presents a terra incognita because only recently experiment has started to achieve the measurement of the individual spectral features and theory has started to carry out atomistic quantum calculations in this size range.
--- Before, only very small clusters or very "large" nanoparticles have been precisely treated. Clusters of a few atoms are well accessible in cluster beam experiments and amenable to the treatment with high-level quantum-chemistry methods. "Large" nanostructures, in particular those popular in the field of plasmonics due to their marked localized surface-plasmon resonances (LSPR), can be well described by classical electrodynamics and fabricated by a variety of approaches. Experimentally, the inhomogeneous line broadening due to the size distributions in ensemble measurements has been the main obstacle to the determination of individual spectral features influenced by quantum-confinement effects, or even of the precise determination of plasmon line widths. Only recently, single-particle experiments have become possible, based on either optical or electron energy-loss measurements. Alternatively, ultra-homogeneous samples of certain ligand-protected clusters have been produced and characterized, showing a rich spectral structure for clusters as large as 144 Au atoms.
--- However, the agreement between those experiments and available theory remains fair, at best. Reliable benchmark results are almost entirely missing. For that reason, a number of questions remain open in the theoretical description mostly done using time-dependent density-functional theory (TDDFT): What are the approximations of the exchange and correlation effects applicable to this size range, and why? In which situations is spin-orbit coupling important? What are the precise parameters that determine the width of molecular-like absorption peaks and of the LSPR? How exactly are the spectra influenced by the matrix or substrate and by oxidation?
--- In the present project, we will combine single-particle Electron Energy-Loss Spectroscopy (EELS) and ensemble optical measurements confronted with state-of-the-art atomistic TDDFT calculations. The latter will be compared with more simplified theories to cover the whole range of sizes. Our study will provide a better physical understanding of the transition from molecular transitions to the collective regime. For Au, Ag, and their alloys, we will carry out TDDFT calculations, constantly confronted with the measurements, which will enable optimization of both. This procedure will lead to a better understanding of how the optical properties of the clusters depend on the different parameters. Besides the improved understanding of the intricacies of the theoretical description, the project will thus create highly relevant practical knowledge on how to tune the properties of the widely applied nanoparticle-based materials. Due to this increased power of nanomaterial engineering, our challenging project has a strong bearing and a high technological relevance to the economically and technically important fields subsumed under the axis "Nanomaterials for biology and health," but also for energy applications and chemistry.

Project coordination

Hans-Christian Weissker (Centre National de Recherche Scientifique Délégation Provence et Corse Centre Interdisciplinaire de Nanoscience de Marseille)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CNRS DR12 CINAM Centre National de Recherche Scientifique Délégation Provence et Corse Centre Interdisciplinaire de Nanoscience de Marseille
ILM - CNRS Institut Lumière Matière
CNRS DR04 LPS Centre National de Recherche Scientifique Délégation Ile-de-France Ouest et Nord Laboratoire de Physique des Solides

Help of the ANR 369,774 euros
Beginning and duration of the scientific project: October 2014 - 48 Months

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