Tracking Ultrafast Energy Flow in Plasmonic Nanocrystal Assemblies

Summary form only given. During the last decade, the photothermal properties of nanostructures have attracted an increasing interest for both fundamental and applied aspects, with potential impact on cancer therapy and solar devices [1]. In particular, metal nanoparticles have been widely exploited to absorb and concentrate light energy at the nanoscale, via plasmonic resonances. Most efforts in the field have dealt with the engineering of the individual nanoparticle configuration, from nanospheres to nanorods, nanoshells, nanocages, etc. (see e.g. Ref. 2 for an overview). Another route of research explored novel materials for plasmonics, including heavily doped semiconductors, and even all-dielectric materials with peculiar permittivity enabling quasi-static resonances without free carriers (see e.g. ref. 3 and references therein). However, the available degrees of freedom turned out to be relatively limited. A fresh new turn in the field is represented by plasmonic nanocrystal assemblies, i.e. so-called supraparticles or supracrystals of about 100 nm, consisting of few nm-sized metallic nanocrystals embedded in a matrix of organic ligands [4]. Thanks to their hybrid composition and their internal structure, these novel nanomaterials offer unprecedented possibilities for the engineering of both linear and nonlinear plasmonic phenomena. Here, we investigate the ultrafast optical response of water-soluble Au nanocrystal assemblies (Fig. 1.a) produced via nucleation from colloidal solution of -5.5-nm Au nanocrystals coated with 1-octadecane-thiol (C18113 7 SH) [4]. Broadband pump-probe experiments with -100 fs time resolution reveal a complex scenario in the transient optical response, with peculiar features having no counterpart in conventional plasmonic nanoparticles (Fig. 1.b). 2D fi nite element method electromagnetic simulations of the composite nanostructure and multi-temperature modeling of the energy flow dynamics (Fig. 1.c) [5] enable us to identify the mechanisms presiding over the light-heat conversion in this novel kind of nanomaterials. At fi rst, light is absorbed by the individual nanocrystals and subsequent excitation of out of equilibrium carriers takes place; in few ps electron phonon equilibration occurs within the individual nanocrystals; lastly, the heat is released from the metal lattice to the organic matrix embedding the nanocrystals. The latter process is revealed in the pump-probe map via negative thermooptic coefficient of the C18113 7 SH polymer, which modulates the plasmonic response of the whole assembly.