Ultrafast electron-lattice thermalization in copper and other noble metal nanoparticles.

Using time-resolved ultrafast pump-probe spectroscopy we investigated the electron-lattice energy transfer in small copper nanospheres with diameters ranging from 3.2 to 23 nm, either embedded in a glass or dispersed in a solvent. Electron-lattice scattering rate is shown to increase with size reduction, in agreement with our previous results obtained on gold and silver nanoparticles in the low excitation regime. We attribute this effect to the reduction of the screening efficiency of electron-phonon interactions close to the nanoparticle surface. To understand the discrepancy between the results on the electron-lattice scattering in different metals reported in the literature (reduction, no dependence or increase with nanoparticle size), we discuss the experimental conditions required for the accurate determination of electron-lattice energy transfer time from time-resolved investigations in the weak and strong excitation regimes and present power-dependent experiments on gold nanospheres in solution. Our findings are derived from a theoretical analysis based on the two-temperature model predictions and on a complete modeling of the nanoparticle transient extinction cross-section through the resolution of Boltzmann equation in the presence of hot electrons.

Enhancing optofluidic actuation of micro-objects by tagging with plasmonic nanoparticles.

We report experimentally and theoretically on the significant exaltation of optical forces on microparticles when they are partially coated by metallic nanodots and shined with laser light within the surface plasmon resonance. Optical forces on both pure silica particles and silica-gold raspberries are characterized using an optical chromatography setup to measure the variations of the Stokes drag versus laser beam power. Results are compared to the Mie theory prediction for both pure dielectric particles and core-shell ones with a shell described as a continuous dielectric-metal composite of dielectric constant determined from the Maxwell-Garnett approach. The observed quantitative agreement demonstrates that radiation pressure forces are directly related to the metal concentration on the microparticle surface and that metallic nanodots increase the magnitude of optical forces compared to pure dielectric particles of the same overall size, even at very low metal concentration. Behaving as "micro-sized nanoparticles", the benefit of microparticles coated with metallic nanodots is thus twofold: it significantly enhances optofluidic manipulation and motion at the microscale, and brings nanometric optical, chemical or biological capabilities to the microscale.

Acoustic vibrations of Au nano-bipyramids and their modification under Ag deposition: a perspective for the development of nanobalances.

We investigated the acoustic vibrations of gold nanobipyramids and bimetallic gold-silver core-shell bipyramids, synthesized by wet chemistry techniques, using a high-sensitivity pump-probe femtosecond setup. Three modes were observed and characterized in the gold core particles for lengths varying from 49 to 170 nm and diameters varying from 20 to 40 nm. The two strongest modes have been associated with the fundamental extensional and its first harmonic, and a weak mode has been associated with the fundamental radial mode, in very good agreement with numerical simulations. We then derived linear laws linking the periods to the dimensions both experimentally and numerically. To go further, we investigated the evolution of these modes under silver deposition on gold core bipyramids. We studied the evolution of the periods of the extensional modes, which were found to be in good qualitative agreement with numerical simulations. Moreover, we observed a strong enhancement of the radial mode amplitude when silver is deposited: we are typically sensitive to the deposition of 40 attograms of silver per gold core particle. This opens up possible applications in the field of mass sensing, where metallic nanobalances have an important role to play, taking advantage of their robustness and versatility.

Surface plasmon resonance properties of single elongated nano-objects: gold nanobipyramids and nanorods.

The spectral characteristics (wavelength and line width) and the optical extinction cross-section of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanobipyramids have been quantitatively measured using the spatial modulation spectroscopy technique. The morphology of the same individual nanoparticles has been determined by transmission electron microscopy (TEM). The experimental results are thus interpreted with a numerical model using the TEM measured sizes of the particles as an input, and either including the substrate or assuming a mean homogeneous environment. Results are compared to those obtained for individual nanorods and also show the importance of the local environment of the particle on the detailed description of its spectral position and extinction amplitude.

3D morphology of Au and Au@Ag nanobipyramids.

The morphologies of Au and Au@Ag nanobipyramids were investigated using electron tomography. The 3D reconstruction reveals that the Au bipyramids have an irregular six-fold twinning structure with highly stepped dominant {151} facets. These short steps/edges stabilized via surface adsorbed CTAB favor the growth of silver on the lateral facets leading to strong blue shifts in longitudinal plasmon surface resonance.

Damping of acoustic vibrations in gold nanoparticles.

Studies of acoustic vibrations in nanometre-scale particles can provide fundamental insights into the mechanical properties of materials because it is possible to precisely characterize and control the crystallinity and geometry of such nanostructures. Metal nanoparticles are of particular interest because they allow the use of ultrafast laser pulses to generate and probe high-frequency acoustic vibrations, which have the potential to be used in a variety of sensing applications. So far, the decay of these vibrations has been dominated by dephasing due to variations in nanoparticle size. Such inhomogeneities can be eliminated by performing measurements on single nanoparticles deposited on a substrate, but unknown interactions between the nanoparticles and the substrate make it difficult to interpret the results of such experiments. Here, we show that the effects of inhomogeneous damping can be reduced by using bipyramidal gold nanoparticles with highly uniform sizes. The inferred homogeneous damping is due to the combination of damping intrinsic to the nanoparticles and the surrounding solvent; the latter is quantitatively described by a parameter-free model.