Optical response of single anisotropic nano-objects : from gold to transition metals

La réponse optique de nanoparticules (NPs) de métaux nobles est dominée par une résonance géante diterésonance de plasmon de surface (RPS), très sensible à la taille, la morphologie et l’environnement diélectriquedes NPs. Elle est étudiée sur des NPs individuelles grâce à un dispositif de Spectroscopie à Modulation Spatiale(SMS) permettant d’accéder à leur section efficace d’extinction absolue sur un large domaine spectral (300-900nm) en corrélation avec leur morphologie observée indépendamment par microscopie électronique àtransmission (MET) ou à balayage (MEB).Mon travail de thèse a d’abord consisté à développer un nouveau dispositif afin de mesurer l’extinction et de ladiffusion d’un même nano-objet unique, donnant ainsi accès à des mesures quantitatives de la section efficace dediffusion moyennant une connaissance a priori du diagramme angulaire de répartition de la lumière diffusée.La seconde partie concerne des études optiques (expérimentales et théoriques) et structurales (MET ou MEB) denano-objets exotiques. Tout d’abord, une étude systématique réalisée sur un grand nombre de bipyramides d’orélaborées par voie chimique a montré que leur RPS, située dans le rouge, est extrêmement sensible à leurmorphologie et à leur environnement, ce qui en fait des candidats de choix pour des capteurs biologiques. Parailleurs, l‘émergence d’une RPS induite par couplage plasmonique a été mis en évidence sur des nano-antennesnanolithographiées à base de métaux de transition (Pd, Pt, Cr). Ces résultats ouvrent des perspectivesd’applications nouvelles en élargissant la plasmonique à des métaux aux propriétés chimiques très variées(photo-catalyse, magnéto-optique).The optical response of noble metal nanoparticles (NPs) are known to be dominated by the Localized SurfacePlasmon Resonance (LSPR), which is highly sensitive to the size of the NPs, their shape and their environment.This optical response can be studied on single nanoparticles thanks to a highly sensitive setup based on theSpatial Modulation Spectroscopy (SMS) which gives access to their absolute extinction cross-section on a widespectral range (300–900 nm). Moreover, the morphology of the same objects studied in optics is characterized bya direct observation in Transmission or Scanning Electron Microscopy (TEM or SEM).In this work, a new setup allowing the measurement of both the extinction and the scattering of a single nanoobjecthas been developed. This technique allows a quantitative measurement of the scattering cross-sectionprovided the angular distribution of the scattered light by the NP is known.The second part is related to experimental and theoretical optical studies and morphological observationsthrough TEM and SEM of exotic nano-objects. First, a systematic study performed on a large number of goldbipyramids, chemically elaborated, has shown that the LSPR located in the red is highly sensitive to theirmorphology and to the environment. Thus, these objects can likely be used as biological sensors. In addition,emergence of a resonance induced by plasmon coupling has been evidenced on lithographed nano-antennasbased on transition metal (Pd, Pt, Cr) for which no LSPR is usually expected. This opens up prospects for novelapplications by extending the field of plasmonics to metals of various chemical properties (photocatalysis,magneto-optics)

Réponse optique de nano-objets uniques anisotropes : de l’or aux métaux de transition

The optical response of noble metal nanoparticles (NPs) are known to be dominated by the Localized SurfacePlasmon Resonance (LSPR), which is highly sensitive to the size of the NPs, their shape and their environment.This optical response can be studied on single nanoparticles thanks to a highly sensitive setup based on theSpatial Modulation Spectroscopy (SMS) which gives access to their absolute extinction cross-section on a widespectral range (300–900 nm). Moreover, the morphology of the same objects studied in optics is characterized bya direct observation in Transmission or Scanning Electron Microscopy (TEM or SEM).In this work, a new setup allowing the measurement of both the extinction and the scattering of a single nanoobjecthas been developed. This technique allows a quantitative measurement of the scattering cross-sectionprovided the angular distribution of the scattered light by the NP is known.The second part is related to experimental and theoretical optical studies and morphological observationsthrough TEM and SEM of exotic nano-objects. First, a systematic study performed on a large number of goldbipyramids, chemically elaborated, has shown that the LSPR located in the red is highly sensitive to theirmorphology and to the environment. Thus, these objects can likely be used as biological sensors. In addition,emergence of a resonance induced by plasmon coupling has been evidenced on lithographed nano-antennasbased on transition metal (Pd, Pt, Cr) for which no LSPR is usually expected. This opens up prospects for novelapplications by extending the field of plasmonics to metals of various chemical properties (photocatalysis,magneto-optics).La réponse optique de nanoparticules (NPs) de métaux nobles est dominée par une résonance géante diterésonance de plasmon de surface (RPS), très sensible à la taille, la morphologie et l’environnement diélectriquedes NPs. Elle est étudiée sur des NPs individuelles grâce à un dispositif de Spectroscopie à Modulation Spatiale(SMS) permettant d’accéder à leur section efficace d’extinction absolue sur un large domaine spectral (300-900nm) en corrélation avec leur morphologie observée indépendamment par microscopie électronique àtransmission (MET) ou à balayage (MEB).Mon travail de thèse a d’abord consisté à développer un nouveau dispositif afin de mesurer l’extinction et de ladiffusion d’un même nano-objet unique, donnant ainsi accès à des mesures quantitatives de la section efficace dediffusion moyennant une connaissance a priori du diagramme angulaire de répartition de la lumière diffusée.La seconde partie concerne des études optiques (expérimentales et théoriques) et structurales (MET ou MEB) denano-objets exotiques. Tout d’abord, une étude systématique réalisée sur un grand nombre de bipyramides d’orélaborées par voie chimique a montré que leur RPS, située dans le rouge, est extrêmement sensible à leurmorphologie et à leur environnement, ce qui en fait des candidats de choix pour des capteurs biologiques. Parailleurs, l‘émergence d’une RPS induite par couplage plasmonique a été mis en évidence sur des nano-antennesnanolithographiées à base de métaux de transition (Pd, Pt, Cr). Ces résultats ouvrent des perspectivesd’applications nouvelles en élargissant la plasmonique à des métaux aux propriétés chimiques très variées(photo-catalyse, magnéto-optique)

Linking Ag nanoparticles by aliphatic α,ω-dithiols: A study of the aggregation and formation of interparticle hot spots

Linear α,ω-dithiols have been used as linkers to control the aggregation of silver nanoparticles. The characterization of the resulting nanoparticle clusters thus formed was carried out using plasmon resonance spectroscopy and transmission electron microscopy in both independent and correlated measurements. The obtained nanoparticle assemblies present morphologies that vary according to the dithiol length, displaying controllable-size interparticle gaps (plasmonic hot spots) with potential application in the detection of a large list of hydrophobic analytes. © 2013 American Chemical Society.ACKNOWLEDGMENTS This work has been supported by the Spanish Ministerio de Economía y Competitividad (grant FIS2010-15405), by the Comunidad de Madrid, through MICROSERES II network (grant S2009/TIC-1476). I.I.-L. also acknowledges CSIC and FSE 2007- 2013 for a JAE-CSIC predoctoral grant.Peer Reviewe

Plasmon Coupling in Silver Nanocube Dimers : Resonance Splitting Induced by Edge Rounding

International audienceAbsolute 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

Optical Properties of a Particle above a Dielectric Interface: Cross Sections, Benchmark Calculations, and Analysis of the Intrinsic Substrate Effects

We show that the optical properties of a particle above a plane dielectric interface differ dramatically from those of the same particle embedded in a homogeneous matrix. Calculations for gold and silver spheres have been carried out in using the exact multipole expansion method, providing thus benchmark results for testing the accuracy of the available numerical methods. For silver spheres, the dependence of the extinction cross-section has been studied in detail as a function of the parameters characterizing the particle/interface system, namely, the radius of the sphere, the particle-surface distance, and the dielectric index of the substrate, as well as those characterizing the light excitation, that is, the angle of incidence and the polarization. Throughout this study we have separated the effects arising from the inhomogeneity of the applied field (interference between the incoming and reflected plane waves) from the intrinsic substrate effects resulting from the interaction with the induced surface charges on the surface. These last effects are, in the present formalism, encoded in the reflected scattered field impinging on the particle. For particles close to the interface, a rich multipolar plasmonic structure is observed, which can be described in the frame of a hybridization scheme similar to that developed for dealing with layered particles or dimers. Comparison with approximate models is also provided

Synthesis, electron tomography and single-particle optical response of twisted gold nano-bipyramids

International audienceA great number of works focus their interest on the study of gold nanoparticle plasmonic properties. Among those, sharp nanostructures appear to exhibit the more interesting features for further developments. In this paper, a complete study on bipyramidal-like gold nanostructures is presented. The nano-objects are prepared in high yield using an original method. This chemical process enables a precise control of the shape and the size of the particles. The specific photophysical properties of gold bipyramids in suspension are ripened by recording the plasmonic response of single and isolated objects. Resulting extinction spectra are precisely correlated to their geometrical structure by mean of electron tomography at the single-particle level. The interplay between the geometrical structure and the optical properties of twisted gold bipyramids is further discussed on the basis of numerical calculations. The influence of several parameters is explored such as the structural aspect ratio or the tip truncation. In the case of an incident excitation polarized along the particle long axis, this study shows how the plasmon resonance position can be sensitive to these parameters and how it can then be efficiently tuned on a large wavelength range

Plasmonic coupling with most of the transition metals: a new family of broad band and near infrared nanoantennas

International audienceno abstrac

Photo-Oxidation of Individual Silver Nanoparticles: A Real-Time Tracking of Optical and Morphological Changes

Absolute extinction measurements on individual silver nanoparticles under illumination show a steady evolution of their localized surface plasmon resonance. Their progressive transformation during light exposure and the influence of various parameters such as the nature of stabilizers, the local environment (oxygen rate), the spectral range of the incident light, and the shape of the nanoparticle (spheres or nanocubes) have been carefully investigated in correlation with transmission electron microscopy imaging. A combination of optics and electron microscopy gives evidence that photoaging mainly consists of the progressive formation of an oxide shell around a metallic silver core during light illumination. Moreover, in the case of nanocubes, the metallic core not only decreases in volume but also changes morphologically since edges and corners are rounded off during the photo-oxidation process. The generalized Mie theory and finite element method, used to calculate the optical extinction cross-section of core/shell Ag@Ag_<i>x</i>O nanoparticles, well account for the observed time evolutions of the absolute extinction spectra of the silver nanospheres and nanocubes. Furthermore, the calculated electromagnetic field at the nanocube surface, enhanced on edges and corners, can explain the higher efficiency of the photo-oxidation on edges and corners and the rounding increase under illumination

Synthesis of PEGylated gold nanostars and bipyramids for intracellular uptake

International audienceA great number of works have focused their research on the synthesis, design and optical properties of gold nanoparticles for potential biological applications (bioimaging, biosensing). For this kind of application, sharp gold nanostructures appear to exhibit the more interesting features since their surface plasmon bands are very sensitive to the surrounding medium. In this paper, a complete study of PEGylated gold nanostars and PEGylated bipyramidal-like nanostructures is presented. The nanoparticles are prepared in high yield and their surfaces are covered with a biocompatible polymer. The photophysical properties of gold bipyramids and nanostars, in suspension, are correlated with the optical response of single and isolated objects. The resulting spectra of isolated gold nanoparticles are subsequently correlated to their geometrical structure by transmission electron microscopy. Finally, the PEGylated gold nanoparticles were incubated with melanoma B16-F10 cells. Dark-field microscopy showed that the biocompatible gold nanoparticles were easily internalized and most of them localized within the cells