Optical and vibrational properties of new "Nano-Designed" materials produced by pulsed laser deposition

These réalisée en cotutelle entre l'Université Paul Sabatier, le CEMES, le CSIC et la Universidad Autonoma de MadridWe have developed new concepts for the fabrication of thin layer of nanostructured materials based on the deposit by alternate laser ablation of metal (Ag) and dielectric (Al2O3). This technique allows us to reach an optimal control on the morphological parameters of the nanostructures, which determine the optical response of the thin layers. We produced nanospheres, nanolentils and nanocolumns, self-organized and oriented, embedded in an amorphous alumina matrix. Their optical response was studied according to their morphology and was compared with theoretical simulations. Simultaneously, the vibrational answer of nanostructures was analyzed using low frequency Raman spectrometry. Finally, nanostructures containing alternate layers of Co and Ag nanoparticles, respectively, and separated by few nanometers have also been produced. We have shown that the interaction between the surface plasmon and the spheroidal acoustic vibration modes of the nanoparticles, activates the Raman signal.Nous avons développé de nouveaux concepts de fabrication de matériaux nanostructurés en couche mince basés sur le dépôt par ablation laser alterné d'un métal (Ag) et d'un diélectrique (Al2O3). Cette technique permet d'atteindre un contrôle optimal sur les paramètres morphologiques des nanostructures qui déterminent la réponse optique des couches minces. Nous avons produit des nanosphères, des nanolentilles et des nanocolonnes, auto-organisées et orientées, encapsulées dans une matrice d'alumine amorphe. Leur réponse optique a été étudiée en fonction de leur morphologie et comparée à des simulations théoriques. En parallèle à cette étude, la réponse vibrationnelle des nanostructures a été analysée par spectrométrie Raman basse fréquence. Finalement, des nanostructures contenant des couches alternées de nanoparticules de Co et d'Ag séparées par une distance de quelques nanomètres ont aussi été fabriquées. Il a été montré que l'interaction entre le plasmon de surface et les modes sphéroïdaux de vibration des nanoparticules active le signal Raman

Spectroscopie Brillouin : introduction et exemples

International audienceÀ l’image de la spectroscopie Raman, la spectroscopie Brillouin est basée sur la diffusion inélastique de la lumière induite par des variations de polarisabilité, et en ce sens ces deux techniques sont très comparables. Dans ce chapitre, nous commencerons par exposer les différences entre spectroscopies Raman et Brillouin, puis nous décrirons rapidement les bases de la diffusion Brillouin, ainsi que les instrumentations disponibles à l’heure actuelle. Pour finir, nous présenterons succinctement plusieurs études impliquant la spectroscopie Brillouin et qui montrent la diversité des questions qu’elle permet d’aborder et plus particulièrement son ouverture récente vers les matériaux biologiques

Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy

International audienceAcoustic vibrations of assemblies of gold nanoparticles were investigated using ultralow frequency micro-Raman scattering and finite element simulations. When exciting the assemblies resonantly with the surface plasmon resonance of electromagnetically coupled nano-particles, Raman spectra present an ultralow frequency band whose frequency lies below the lowest Raman active Lamb mode of single nanoparticles that was observed. This feature was ascribed to a Raman vibration mode of gold nanoparticle " supermolecules " , that is, nanoparticles mechanically coupled by surrounding polymer molecules. Its measured frequency is inversely proportional to the nanoparticle diameter and sensitive to the elastic properties of the interstitial polymer. The latter dependence as well as finite element simulations suggest that this mode corresponds to the out-of-phase semirigid translation (l = 1 Lamb mode) of each nanoparticle of a dimer inside the matrix, activated by the mechanical coupling between the nanoparticles. These observations were permitted only thanks to the resonant excitation with the coupling plasmon excitation, leading to an enhancement up to 10^4 of the scattering by these vibrations. This enhanced ultralow frequency Raman scattering thus opens a new route to probe the local elastic properties of the surrounding medium

Contact laws between nanoparticles: the elasticity of a nanopowder

International audienceStudies of the mechanical contact between nanometer-scale particles provide fundamental insights into the mechanical properties of materials and the validity of contact laws at the nanoscale which are still under debate for contact surfaces approaching atomic dimensions. Using in situ Brillouin light scattering under high pressure, we show that effective medium theories successfully predict the macroscopic sound velocities in nanopowders if one takes into account the cementation of the contacts Our measurements suggest the relevance of the continuum approach and effective medium theories to describe the contact between nanoparticles of diameters as small as 4 nm, i.e. with radii of contact of a few angstroms. In particular, we demonstrate that the mechanical properties of nanopowders strongly depend on the surface state of the nanoparticles. The presence of molecular adsorbates modifies significantly the contact laws

From localized to delocalized plasmonic modes, first observation of superradiant scattering in disordered semi-continuous metal films

International audienceOur study proposes a new way to observe and explain the presence of extended plasmonic modes in disordered semi-continuous metal films before the percolation threshold. Attenuated total reflection spectroscopy allows us to follow the transition of plasmon modes from localized to delocalized resonances, but also reveals unobserved collective plasmon modes. These bright modes with out-of-plane polarization are transverse collective plasmonic resonances. By increasing the density of metallic nanoparticles in a wavelength scale, we observe an angular squeezing and spectral broadening of these modes. This behavior can be explained considering that transverse localized surface plasmon resonances of each nanoparticle, all resonant, interact in a collective and coherent way via a common confined light mode: the evanescent wave. These many-body resonances, which have never been clearly identified in such disordered semi-continuous metal films, can be described by analogy with atomic physics as superradiant modes. Our first simulations, using dyadic Green's formalism, demonstrate the existence of this mode for a dense array of plasmonic systems. In this regime, the radiation rate of the superradiant mode increases with the number of tied dipoles. This explains the spectral broadening observed in our work and constitutes the first manifestation of superradiance mode in plasmonic random structure

Inelastic Light Scattering by Multiple Vibrational Modes in Individual Gold Nanodimers

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Inelastic Light Scattering Contribution to the Study of the Onset of Sintering of a Nanopowder

International audienceThe onset of the sintering of 5 nm ZrO2 and TiO2 nanoparticles is investigated by various methods including inelastic light scattering. A special attention is paid to low-frequency Raman spectra where scattering from confined acoustic vibrations and quasielastic scattering manifest. Significant changes are observed between samples sintered at different temperatures or applied forces. A detailed analysis of the spectra enables to follow the variation of the size of the nanoparticles, the surface area, and the formation of internanoparticles necks in the sintered materials. Finally, low-frequency scattering is shown to be more sensitive to the onset of sintering than most other experimental methods

From silver nanolentils to nanocolumns: surface plasmon-polaritons and confined acoustic vibrations

International audienceOptical and vibrational properties of silver nanolentils, nanospheres, and nanocolumns are studied experimentally using optical transmission spectroscopy and low-frequency Raman scattering. The split of the surface plasmon–polariton resonance into transverse and longitudinal components due to the squeezing or to the elongation of the nanospheres is clearly observed. The vibration band frequencies are demonstrated to be strongly related to the average in-plane diameter, while the quadrupolar vibration modes observed depend on the shape of the nanoparticles

Inelastic light scattering by long narrow gold nanocrystals: when size, shape, crystallinity and assembly matter

International audienceWe report the synthesis of long narrow gold nanocrystals and the study of their vibrational dynamics using inelastic light scattering measurements. Rich experimental spectra are obtained for monodomain gold nanorods and pentagonal twinned bipyramids. Their assignment involves diameter-dependent non-totally symmetric vibrations which are modeled in the framework of continuum elasticity by taking into account simultaneously the size, shape and crystallinity of the nanocrystals. Light scattering by vibrations with angular momenta larger than 2 is reported. It is shown to increase with the ratio of the nanocrystals diameter to the interparticle separation. It originates from the plasmonic coupling due to the self-assembly of the nanocrystals after deposition

Ligand-dependent nano-mechanical properties of CdSe nanoplatelets: calibrating nanobalances for ligands affinity monitoring

International audienceThe influence of ligands on the low frequency vibration of different thicknesses cadmium selenide colloidal nanoplatelets is investigated using resonant low frequency Raman scattering. The strong vibration frequency shifts induced by ligand modifications as well as the sharp spectral linewidths make low frequency Raman scattering a tool of choice to follow ligand exchange as well as the nano-mechanical properties of the NPLs, as evidenced by a carboxylate to thiolate exchange study. Apart from their molecular weight, the nature of the ligands, such as the sulfur to metal bond of thiols, induces a modification of the NPLs as a whole, increasing the thickness by one monolayer. Moreover, as the weight of the ligands increases, the discrepancy between the massload model and the experimental measurements increase. These effects are all the more important when the number of layers is small and can only be explained by a modification of the longitudinal sound velocity. This modification takes its origin in a change of lattice structure of the NPLs, that reflects on its elastic properties. These nanobalances are finally used to characterize ligands affinity with the surface using binary thiols mixtures, illustrating the potential of low frequency Raman scattering to finely characterize nanocrystals surfaces