International audienceElectron-lattice energy exchanges are investigated in gold and silver nanoparticles with sizes ranging from 30 to 2.2 nm embedded in different environments. Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect

The optical extinction spectra of single silver nanoparticles coated with a silica shell were investigated in the size range 10-50 nm. Measurements were performed using the spatial modulation spectroscopy technique which permits independent determination of both the size of the metal nanoparticle under study and the width of its localized surface plasmon resonance (LSPR). These parameters can thus be directly correlated at a single particle level for the first time. The results show a linear increase of the width of the LSPR with the inverse diameter in the small size regime (less than 25 nm). For these nanoparticles of well-controlled environment, this can be ascribed to quantum confinement of electrons or, classically, to increase of the electron surface scattering processes. The impact of this effect was measured quantitatively and compared to the predictions by theoretical models.

The internal thermalization dynamics of the conduction electrons is investigated in silver nanoparticles with radius ranging from 13 to 1.6 nm using a femtosecond IR pump-UV probe absorption saturation technique. A sharp increase of the electron energy exchange rate is demonstrated for nanoparticles smaller than 5 nm. The results are consistent with electron-electron scattering acceleration due to surface induced reduction of the Coulomb interaction screening by the conduction and core electrons.

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International audienceThe conduction electron energy exchanges are investigated in gold and silver nanoparticles with average size ranging from 2 to 26 nm, embedded in different matrices. The experimental studies were performed by following the internal thermalization dynamics of photoexcited nonequilibrium electrons with a femtosecond pump-probe technique. The probe wavelength dependent measurements are in qualitative agreement with the results of a theoretical model based on bulk metal electron kinetics and band-structure modeling. In both metals, the measured electron thermalization times are close to the bulk ones for nanoparticles larger than 10 nm and sharply decrease for smaller ones. The results are independent of the nanoparticle environment and synthesis technique showing that the observed size behavior reflects an increase of the efficiency of the electron-electron energy exchanges in small nanoparticles. It is in agreement with a simple model based on a bulk metal approach of the electron kinetics modified to introduce surface effects. The observed increase of the electron-electron interaction with size reduction is ascribed to reduction of the screening of the Coulomb interaction by the conduction and core electrons close to the nanoparticle surface

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Absolute extinction cross sections of individual silver nanocube dimers are measured using spatial modulation spectroscopy in correlation with their transmission electron microscopy images. For very small interparticle distances and an incident light polarized along the dimer axis, we give evidence for a clear splitting of the main dipolar surface plasmon resonance which is found to be essentially induced by cube edge rounding effects. Supported by discrete dipole approximation and finite element method calculations, this phenomenon highlights the high sensitivity of the plasmonic coupling to the exact shape of the effective capacitor formed by the facing surfaces of both particles, especially in the regime of very close proximity.