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International audience; The coupling effects affecting the vibrations of two close nanostructures (e.g., two metal nanoplates or nanospheres separated by a thin dielectric layer) may considerably alter their vibrational eigenfrequencies, as demonstrated by several recent experimental studies. In this work, we present theoretical investigations of these coupling processes based on a continuum mechanics approach, considering various systems composed by two identical nanostructures mechanically coupled by a spacer made of a different material and computing their eigenfrequencies as a function of the spacer thickness. We first discuss the vibrations of stacked slabs, a one-dimensional problem which can be treated analytically. The more complex configurations of dimers of rods or spheres coupled by a finite cylindrical spacer are then treated numerically. In all cases, the frequency shifts occurring for thin spacers can be simply interpreted as a modification of the boundary conditions of the problem as compared to the single nanostructure case, while those predicted near specific spacer thicknesses are ascribed to an avoided crossing effect, happening when the individual building blocks of the dimers (nanostructures and spacer) present common eigenfrequencies.

International audience; In most conventional optical experiments on substrate-supported nanoparticles, detectors are located in the far-field space whereas their intrinsic properties (scattering, absorption cross sections) are commonly computed in the near-field by using numerical approaches. In this respect, a connection between experiment and theory can be usefully made on the basis of the generalized optical theorem (GOT) that gives access, from the far-field expansion of the scattered wave, to the total extinction cross section as the sum of “forward” and “backward” contributions related to the power changes of the waves respectively transmitted and reflected by the substrate. The purpose of this work is to address this issue as clearly as possible by performing quantitative measurements on individual objects and on both type of waves, as far a comprehensive description of the global extinction process is intended. We show that the spatial modulation spectroscopy technique applied to a single nanoparticle is especially well suited in this context. This is illustrated by the model case of gold nanocubes supported either on a thin dielectric film or on a thick glass slide. Using the GOT in the particular case of a plane wave excitation at normal incidence, we were able to access the total extinction cross section of the supported scatterer. Moreover, we give evidence for the invariance of the extinction cross section relative to the transmitted wave, whether the nanoparticle is located in the front or behind the substrate with respect to the incoming beam. This effect can be seen as a manifestation of the optical reciprocity theorem with regard to this specific problem. We also discuss in which framework the experimental measurements can be pertinently reproduced by theoretical calculations, considering the setup geometry and the intrinsic optical response of the nanoparticle.

SSCI-VIDE+ECI2D+LPI; International audience; By combining a plasmonic metal with a reactive one, nanoalloys become valuable systemsfor optical purposes by using the Localized Surface Plasmon Resonance (LSPR) as theprobe for chemical processes. It is thus crucial to develop experimental devices allowing thein situ or in operando monitoring of these nanoparticles in order to identify the mechanisminvolved during restructuring processes. For this purpose, we developed a new setupallowing in situ optical measurements of a nanoparticle (NP) assembly, complementary tosingle NP environmental transmission electron microscopy (ETEM) observations [1]. The setup was tested on iron-silver NPs, which combine a plasmonic material with a ferromagnetic element [2].A detailed study of the Ag-Fe bimetallic system was thus conducted for two silver compositions (50% and 80%), by investigating the chemical properties of the NPs throughTEM and environmental optical spectroscopy, and interpreting the combined physicochemical observations with the help of parallel tempering Monte Carlo (MC) simulations of their structural stability at finite temperature. We show that these metals are initially segregated with a silver-enriched surface and that oxidation induces some iron migration towards the surface. In situ optical measurements carried out successively under oxidizing and reducing atmospheres indicate that iron oxidation and reduction are reversible processes that are directly mirrored in the LSPR changes (see Figure 1), in good agreementwith optical simulations. Based on these results we propose a scenario of the evolution of the chemical ordering under reducing and oxidizing atmospheres.REFERENCES1 Julien Ramade, Cyril Langlois, Michel Pellarin, Laurent Piccolo, Marie-Ange Lebeault, Thierry Epicier, Mimoun Aouine, and Emmanuel Cottancin, Tracking the restructuring of oxidized silver-indium nanoparticles under a reducing atmosphere by environmental HRTEM,Nanoscale, 2017, 9, 13563-135742 Julien Ramade, Emmanuel Cottancin, Marie-Ange Lebeault, Cyril Langlois, Laurent Piccolo, Michel Broyer, Matthias Hillenkamp, Jean Lermé, Florent Calvo and Michel Pellarin, Environmental Plasmonic Spectroscopy of Silver-Iron Nanoparticles: Chemical OrderingUnder Oxidizing and Reducing Conditions. Journal of Physical Chemistry C 2019, in press.

International audience; To be Raman-active (or, more generally, detectable using optical spectroscopy techniques), a vibrational mode of a nanosystem has to modulate its optical response. For small, isolated nanospheres, this is the case for only two categories of vibrational modes, namely, quadrupolar and radial ones. However, assembling nanospheres as dimers makes additional modes Raman-active, as previously demonstrated by the detection in the ultralow frequency range of a hybridized quasi-translation mode in previous measurements on single and ensembles of gold nanosphere dimers. In the present work, we use our recently developed single-particle Raman spectroscopy setup to compare inelastic light scattering by single isolated and dimerized gold nanospheres in an extended frequency range (0–40 GHz). The Raman spectra acquired on isolated nanoparticles present a single peak associated with their fundamental quadrupolar mode, consistently with previous ensemble measurements. In contrast, the spectra measured on dimers are richer and display a number of peaks increasing with decreasing interparticle distance, with all l = 2–8 Lamb modes detected in the quasi-contact case. These observations are rationalized using a recently developed classical model of inelastic light scattering by nanospheres. Importantly, our modeling approach takes into account the real electric field within the nanoparticles (computed using standard or generalized Mie theories) instead of relying on the frequently used Born and quasistatic approximations. This ingredient appears decisive for reaching a qualitative understanding of the measured spectra, explaining in particular the dominance of the l = 2 quadrupolar mode for isolated spheres and the increasing contribution of higher-order modes for increasing electromagnetic interactions in nanosphere dimers.

International audience; The intrinsic optical response of a scattering object is known to be modified when it is placed on a substrate. Moreover, the total extinction through ohmic losses and Rayleigh scattering is basically dependent on the reversal of the direction of the excitation wave. Considering, as stated by the generalized optical theorem, that the total extinction of the incident wave is shared among the extinction of the waves transmitted and reflected by the non-absorbing substrate, we show that only the contribution of the transmitted wave is insensitive to the direction of illumination, by analogy with the textbook case of transmission through a planar stratified medium. This property was recently confirmed experimentally [J. Phys. Chem. C 123, 15217 (2019)] and is established here on theoretical grounds in the frameworks of the Green's function integral equation method and the Lorentz reciprocity theorem. Some of its limitations with regard to the incidence and the polarization of the excitation wave are also discussed in detail.

The physical properties of metals change when their dimensions are reduced to the nano-scale and new phenomena such as the localized surface-plasmon resonance (LSPR) appear. This collective electronic excitation can be tuned over a large spectral range by adapting the material, size and shape. The existing literature is as rich as it is controversial—for example, size-dependent spectral shifts of the LSPR in small metal nanoparticles, induced by quantum effects, are reported to the red, to the blue or entirely absent. Here we report how complementary experiments on size-selected small silver nanoparticles embedded in silica can yield inconsistent results on the same system: whereas optical absorption shows no size effect in the range between only a few atoms and ~10 nm, a clear spectral shift is observed in single-particle electron spectroscopy. Our quantitative interpretation, based on a mixed classical/quantum model, resolves the apparent contradictions, not only within our experimental data, but also in the literature. Our comprehensive model describes how the local environment is the crucial parameter controlling the manifestation or absence of quantum size effects. The origin of size-dependent shifts of surface plasmon resonances in metal nanoparticles has been controversial for decades. A combined experimental and theoretical study on silver samples and their environments now provides a quantitative picture.

International audience; Plasmonic sensors owe their sensitivity to the susceptibility of collective electron modes to tiny changes of the local environment. Simple sensors are based on metal nanoparticles where spectral shifts of the resonance are frequently treated in view of an “effective” dielectric medium surrounding each particle. Using single-object spectroscopy probing a large number of silver particles on Si(111)-(7 × 7) in a photoemission electron microscope (PEEM), we show that this picture breaks down because of the formation of image dipoles in the substrate. Even at particle sizes as small as 10 nm, that is, small compared to the excitation wavelength, the dipole approximation is no longer valid because the system inherently creates higher order multipole modes. To validate this scenario, we compare plasmon energies, lifetimes, and their variability to the case of a surface with lower permittivity. Model calculations based on generalized Mie theory reveal that the formation of multipole modes is extremely sensitive to the local geometry on the scale of one ångström, in particular to the intradimer separation. Hence, such systems may enable novel ultrasensitive plasmonic detection schemes such as high-precision plasmonic rulers and probes for molecular interfaces or spacer layers.

The acoustic vibrations of single monomers and dimers of gold nanoparticles were investigated by measuring for the first time their ultralow-frequency micro-Raman scattering. This experiment provides access not only to the frequency of the detected vibrational modes but also to their damping rate, which is obscured by inhomogeneous effects in measurements on ensembles of nano-objects. This allows a detailed analysis of the mechanical coupling occurring between two close nanoparticles (mediated by the polymer surrounding them) in the dimer case. Such coupling induces the hybridization of the vibrational modes of each nanoparticle, leading to the appearance in the Raman spectra of two ultralow-frequency modes corresponding to the out-of-phase longitudinal and transverse (with respect to the dimer axis) quasi-translations of the nanoparticles. Additionally, it is also shown to shift the frequency of the quadrupolar modes of the nanoparticles. Experimental results are interpreted using finite-element simulations, which enable the unambiguous identification of the detected modes and despite the simplifications made lead to a reasonable reproduction of their measured frequencies and quality factors. The demonstrated feasibility of low-frequency Raman scattering experiments on single nano-objects opens up new possibilities to improve the understanding of nanoscale vibrations with this technique being complementary with single nano-object time-resolved spectroscopy as it gives access to different vibrational modes.

Sensitive, robust and stable sensors are required to bring biosensing techniques from the forefront of research to clinical and commercial settings. To this end, we report on the development of new robust plasmonic sensors consisting of gold nano-bipyramids (BPs) grafted to a glass substrate via silanization, associated with a novel setup based on Spatial Modulation Spectroscopy allowing the measurement of the optical response of individual nano-objects in a liquid environment. We thereby show that changes in the refractive index of the medium around individual silanized BPs can be detected by measuring their plasmonic shift with sensitivities comparable to values reported elsewhere and in good agreement with theoretical calculations. The optical response is furthermore shown to be stable and robust allowing for repeated measurements in different media and storage over many months. This work opens up new perspectives in the field of plasmonic bio-sensing as our setup is readily adaptable to dynamic liquid measurements and a wide range of applications such as the detection of clinically important analytes or pollutants in water.

International audience; A key parameter for optimizing nanosized optical devices involving small metal particles is the spectral width of their localized surface plasmon resonances (LSPR). In the small size range the homogeneous LSPR line width is to a large extent ruled by the spatial confinement-induced broadening contribution which, within a classical description, underlies the popular phenomenological limited mean free path model. This unavoidable contribution to the LSPR line width is basically a quantum finite-size effect rooted in the finite extent of the electronic wave functions. This broadening reflects the surface-induced decay of the coherent collective plasmon excitations into particle–hole (p–h) excitations (Landau damping), the signature of which is a size-dependent fragmented LSPR band pattern which is clearly evidenced in absorption spectra computed within the time-dependent local density approximation (TDLDA). In this work we analyze the spatial confinement-induced LSPR damping contribution in the framework on an exact Hamiltonian formalism, assuming for convenience a jellium-type ionic density. In resorting to the harmonic potential theorem (HPT), a theorem stating that in the case of a harmonic external interaction the electronic center-of-mass coordinates separate strictly from the intrinsic motions of the individual electrons, we derive a simple approximate formula allowing to (i) quantify the size dependence of the LSPR damping in spherical nanoparticles (1/R law, where R is the sphere radius) and (ii) bring to the fore the main factors ruling the confinement-induced LSPR broadening. Then the modeling is straightforwardly generalized to the case of spheroidal (prolate or oblate) metal particles. Our investigations show that the LSPR damping is expected to depend strongly on both the aspect ratio of the spheroidal particles and the polarization of the irradiating electric field, that is, on the nature—transverse or longitudinal—of the collective excitation. It is found that the magnitude of the damping is tightly related to that of the LSPR frequency which rules the number of p–h excitations degenerate with the plasmon energy. Qualitative analysis suggests that the results are quite general and probably hold for other nonspherical particle shapes. In particular, in the case of elongated particles, as rods, the enlargement of the longitudinal LSPR band by the confinement effects is predicted to be much smaller than that of the transverse LSPR band.

International audience; In the presence of a nearby highly reflecting dielectric substrate the optical response of a metal nanoparticle exhibits a rich multipolar plasmonic structure. For interpreting and quantifying the spectral patterns, the quasistatic hydrodynamic plasmon hybridization (PH) method developed by the Nordlander group has been applied to the particle/interface system in the simple generic case of a jellium sphere (PI–PH model). In the quasistatic limit, the method of images allows the PI–PH model to be tightly related to the two-sphere dimer PH model (D–PH model). However, because the dynamics of the electron fluid in the mirror particle is enslaved to that of the particle above the interface, strong differences between both PH models result from the reduction by a factor two of the number of dynamical variables describing the electron fluids. In particular the PI–PH model involves only bonding bright modes, whereas bonding and antibonding, bright and dark modes are involved in the homodimer case. A very detailed description of both PH models is provided, with particular emphasis on the coupling with an external field and the transfer of the oscillator strength of the dipole excitations of the isolated particle(s) to high-order plasmon modes of the particle/interface or dimer systems. The predictions of both PH models are also compared, through absorption and extinction cross-section spectra, to exact results computed in the frame of the multipole expansion method, allowing the range of applicability and limits of both models to be determined. Some comments about heterodimers are also provided.

International audience; 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@AgxO 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.

International audience; Homodimers of noble metal nanocubes form model plasmonic systems where the localized plasmon resonances sustained by each particle not only hybridize but also coexist with excitations of a different nature: surface plasmon polaritons confined within the Fabry−Perot cavity delimited by facing cube surfaces (i.e., gap plasmons). Destructive interference in the strong coupling between one of these highly localized modes and the highly radiating longitudinal dipolar plasmon of the dimer is responsible for the formation of a Fano resonance profile and the opening of a spectral window of anomalous transparency for the exciting light. We report on the clear experimental evidence of this effect in the case of 50 nm silver and 160 nm gold nanocube dimers studied by spatial modulation spectroscopy at the single particle level. A numerical study based on a plasmon mode analysis leads us to unambiguously identify the main cavity mode involved in this process and especially the major role played by its symmetry. The Fano depletion dip is red-shifted when the gap size is decreasing. It is also blue-shifted and all the more pronounced that the cube edge rounding is large. Combining nanopatch antenna and plasmon hybridization descriptions, we quantify the key role of the face-to-face distance and the cube edge morphology on the spectral profile of the transparency dip. T he optical excitation of localized surface plasmon resonances (LSPR) in single metallic nanoparticles or multicomponent nanostructures is responsible for efficient resonant far-field scattering and near-field concentration of light. 1,2 Such nanoantennas offer the opportunity to manipulate light at scales much shorter than the wavelength by making the best use of the LSPR sensitivity to variations in particle shape, size, chemical composition, and dielectric environment. 3,4 The electrostatic coupling between several plasmonic subunits is an additional tool for tailoring the optical response over a wide spectral range. 5,6 In this respect, much attention is paid to analogues of Fano resonances in classical electrostatics and especially those arising from the coupling between a " dark " plasmonic mode (weak dipole moment and narrow line width) and a degenerate mode that is highly radiative over a much broader spectral range (" bright " mode, large dipole moment). On either side of the resonance, the rapid phase shift of the " dark " mode polarizability relative to that of the bright mode and the exciting field may induce constructive or destructive interference in the net far field scattering process. They result in the formation of a dip in the broad spectral band of the far field scattered light with a typical dissymmetric profile and a narrow width related to the " dark " resonance lifetime. 7−11 Fano resonances open sharp and transparent windows in the plasmonic response of metallic nanostructures. 12−14 Their high sensitivity to relative changes in the nanostructure dielectric environment can be efficiently exploited for chemical and biological sensing. 15,16 Most of the studies in this field deal with noble metal nanoantennas built from the assembly and coupling of subunits of various size, shape, and orientation: oligomers of colloidal particles (DNA assembled, 17,18 heterodimers 19) and predominantly objects engraved by electron-beam lithography. 20−22 This method offers considerable flexibility for designing the spectral response of intricate structures with an advantageous symmetry breaking. 13,23 In the far field, they act as wide band

A key parameter for optimizing nanosized optical devices involving small metal particles is the spectral width of their localized surface plasmon resonances (LSPR), which is intrinsically limited by the confinement-induced broadening (quantum finite size effects). I have investigated the size evolution of the LSPR width induced by quantum confinement in silver nanoparticles isolated in vacuum or embedded in transparent matrixes. Calculations have been performed within the time-dependent local density approximation in an extended size range, up to 40000 conduction electrons (diameter D ≈ 11 nm). The slope characterizing the 1/D linear evolution predicted by the simple classical “limited mean free path” model is found to depend noticeably on the surrounding matrix. The confinement-induced damping of the collective LSPR excitation is shown to be intimately related to the departure of the electron-background interaction from a pure harmonic law. In jellium-type models the damping is governed by the electronic...

International audience; The impact of quantum confinement on the width of the surface plasmon resonance of a metal nanoparticle is theoretically investigated in a model system formed by a silver nanosphere in different environments. Calculations are performed using the time-dependent local density approximation (TDLDA) for nanoparticle diameters up to about 11 nm, permitting precise quantification of the surface plasmon broadening due to size reduction. As expected, this is found to be inversely proportional to the particle diameter, but with an amplitude strongly depending on the environment (increasing by a factor of 4 when changing from vacuum to alumina). This is ascribed to the fact that damping is governed by the electronic surface spill-out (inherent in any quantum model) and thus strongly depends on the surface profile of the confining potential, that is, on the particle surface conditions.

We have studied the spectral response of individual pairs of gold nanoparticles chemically synthesized and stabilized on a transparent thin film. Very thin interspacing distances are explored by taking advantage of a residual layer of surfactant molecules at the metal surface. Absolute extinction cross section measurements by the spatial modulation spectroscopy technique are correlated to the corresponding high magnification images independently obtained from transmission electron microscopy. The signature of a transition from isolated to conductively coupled particles is evidenced in the optical spectra and is discussed in relation with their morphology. The strength of the electromagnetic coupling is controlled not only by their spacing but also by the shape of their opposite surfaces in the gap region.

The optical absorption of bimetallic nanoparticles M–Ag involving silver as an active plasmonic compound has been the subject of very extensive experimental studies, both for a large range of sizes and a large variety of associated metals. Considering the most commonly encountered core–shell segregated configuration M@Ag involving a transition metal M, the spectral response is found to be weakly discriminating with regard to the chemical order and composition and is characterized by a large unstructured plasmon resonance in the 2 eV to 4 eV range. The plasmon band is essentially shaped by the scars made in the absorption continuum of metal M by Fano-like induced resonances and is surprisingly little sensitive to the exact nature of this metal, giving birth to a “quasi universal” optical signature for M@Ag systems. In this paper, we show how this generic behaviour arises from the specific plasmonic response of silver and stress the role of interband transitions of both metals through their coupling with the free electron oscillation modes. This theoretical discussion will be illustrated through selected experimental results.

The extinction spectrum of single silver nanoparticles with size ranging from 20 to 80 nm is investigated with the spatial modulation spectroscopy technique using either a tunable laser or a white lamp as the broadband source. Results are in good agreement with the prediction of the Mie theory, permitting to extract the nanoparticle size from the measured absolute value of the optical extinction cross-section. In contrast, the deduced refractive index of the nanoparticle environment and the reduction of the electron mean free path show a large dependence on the precise value of the bulk silver dielectric function.

International audience; The optical properties of mixed (Ni0.5Ag0.5)n and Pt/Ag clusters are investigated in the size range 2-5 nm. Low Energy Ion Spectroscopy (LEIS) experiments show that the cluster surface is entirely covered by silver atoms for the two systems. The optical spectra of Ni/Ag clusters exhibit a large Surface Plasmon Resonance (SPR), damped and widened when the cluster size decreases, in agreement with a classical model assuming a core-shell geometry and including the reduction of the conduction electron mean-free path in the silver shell. For Pt/Ag clusters, no SPR emerges in the size range 2-5 nm, although it is predicted within a classical model, a pronounced SPR appearing only for clusters larger than 10 nm in diameter.

cited By 11; International audience; The optical properties of small Cu-Ag bimetallic clusters have been experimentally and theoretically investigated in relation to their chemical structure analyzed by high resolution transmission electron microscopy (HRTEM). Cu (1-x)Agx clusters of about 5 nm in diameter are produced in a laser vaporization source with a well-defined stoichiometry (x = 0, 25, 50, 75, and 100%) and dispersed in an alumina matrix. Absorption spectra are dominated by a broad and strong surface plasmon resonance whose shape and location are dependent on both cluster composition and sample aging. Detailed modeling and systematic calculations of the optical response of pure and oxidized mixed clusters of various chemical structures have been carried out in the framework of classical and semiquantal formalisms. Optical and HRTEM measurements combined with theoretical predictions lead to the conclusion that these bimetallic clusters are not alloyed at the atomic scale but rather present a segregation of chemical phases. Most likely, they adopt a Cu@Ag core-shell configuration. Moreover, the nanoparticle oxidation process is consistent with the formation of a copper oxide layer by dragging out inner copper atoms to the cluster surface. © 2015 American Chemical Society.

International audience; In the presence of a nearby highly reflecting dielectric substrate, the optical response of a metal nanoparticle exhibits a rich multipolar plasmonic structure. For quantifying the multimodal plasmonic response of such hybrid particle/interface systems, the hydrodynamic plasmon hybridization (PH) method has been applied in the simple case of a solid metal sphere (PI-PH model). In this work I extend the formalism described in my previous paper ( J. Phys. Chem. C 2014, 118, 28118−28133) to the more general case where underlying dielectric media are present in the whole space (polarizable ionic background and matrix). In the quasistatic limit the method of images allows the PI-PH model to be closely related to the two-sphere dimer PH model. However, because the dynamics of the electron fluid and of the polarization charges in the mirror particle are enslaved to those in the real metal particle, strong differences between both PH models result from the reduction by a factor of two of the number of dynamical variables describing the system. As compared to jellium spheres in vacuum, the PH formalism is much more complex because the induced surface polarization charges in the underlying media contribute to the energetics of the system. In this work a very detailed analytical description of both PH models, allowing a direct numerical implementation, is provided. In the frame of the original and physically transparent method developed in my previous paper, particular emphasis is given to modeling the coupling of the nanosystem with an external applied field. In the presence of underlying dielectric media, I show that the taking into account of the field created by the additional polarization charge distributions induced by the bare applied field is essential for defining the effective external field that is coupled to the nanosystem. The predictions of both PH models are also compared, through absorption and extinction cross-section spectra, to exact results computed in the frame of the multipole expansion method, allowing the range of applicability and limits of both models to be determined. Finally, the PH formalism is shown to be generalizable to deal with underlying dielectric media characterized by complex (that is, absorbing) frequency-dependent permittivities.

International audience; The low frequency Raman scattering from gold nanoparticle bimodal assemblies with controlled size distributions has been studied. Special care has been paid to determining the size dependence of the Raman intensity corresponding to the quadrupolar Lamb mode. Existing models based on a microscopic description of the scattering mechanism in small particles (bond polarizability, dipole induced dipole models) predict, for any Raman-active Lamb modes, an inelastic intensity scaling as the volume of the nanoparticle. Surprisingly experimental intensity ratios are found to be anomalously much greater than theoretical ones, calling into question this scaling law. To explain these discrepancies, a simple mechanism of Raman scattering, based on the density fluctuations in the nanoparticles induced by the Lamb modes, is introduced. This modeling, in which the nanoparticle is described as an elastic isotropic continuous medium-as in Lamb theory, successfully explains the major features exhibited by low frequency Raman modes. Moreover this model provides a unified picture for any material, suitable for handling both small and large size ranges, as well as non-resonant and resonant excitation conditions in the case of metallic species. Published by AIP Publishing.

International audience; Anisotropic gold nanoparticles and in particular with shapes exhibiting tips are known to present an extremely strong localized electromagnetic field. This field is mostly located at the top of the tips and can be used in various optical applications. Moreover, as a consequence of their anisotropy, they present two plasmon resonance bands corresponding to the transverse and longitudinal resonance modes. Tuning the aspect ratio it becomes possible to display SPR bands near the near infrared region. This was particularly investigated in the case of nanorods and also for bipyramids. In this paper we report a high yield synthesis approach that allows one to precisely control the aspect ratio of bipyramids and to elongate the structure until they adopt a javelin-like aspect. We were able to prepare nano-javelins with surface plasmon resonances up to 1850 nm, opening important perspectives in terms of optical applications in the NIR and IR regions. The synthetic methods are fully reported and the optical properties were correlated with the theoretical approach, taking into consideration not only the aspect ratio but also the truncation of the nano-objects.

In this article, we show for the first time, both theoretically and empirically, that plasmonic coupling can be used to generate Localized Surface Plasmon Resonances (LSPRs) in transition metal dimeric nano-antennas (NAs) over a broad spectral range (from the visible to the near infrared) and that the spectral position of the resonance can be controlled through morphological variation of the NAs (size, shape, interparticle distance). First, accurate calculations using the generalized Mie theory on spherical dimers demonstrate that we can take advantage of the plasmonic coupling to enhance LSPRs over a broad spectral range for many transition metals (Pt, Pd, Cr, Ni etc.). The LSPR remains broad for low interparticle distances and masks the various hybridized modes within the overall resonance. However, an analysis of the charge distribution on the surface of the nanoparticles reveals these modes and their respective contributions to the observed LSPR. In the case of spherical dimers, the transfer of the oscillator strengths from the “dipolar” mode to higher orders involves a maximum extinction cross-section for intermediate interparticle distances of a few nanometers. The emergence of the LSPR has been then experimentally illustrated with parallelepipedal NAs (monomers and dimers) made of various transition metals (Pt, Pd and Cr) and elaborated by nanolithography. Absolute extinction cross-sections have been measured with the spatial modulation spectroscopy technique over a broad spectral range (300–900 nm) for individual NAs, the morphology of which has been independently characterized by electron microscopy imaging. A clear enhancement of the LSPR has been revealed for a longitudinal excitation and plasmonic coupling has been clearly evidenced in dimers by an induced redshift and broadening of the LSPR compared to monomers. Furthermore, the LSPR has been shown to be highly sensitive to slight modifications of the interparticle distance. All the experimental results are well in agreement with finite element method (FEM) calculations in which the main geometrical parameters characterizing the NAs have been derived from electron microscopy imaging analysis. The main advantage of dimers as compared to monomers lies in the generation of a well-defined and highly enhanced electromagnetic field (the so-called “hot spots”) within the interparticle gap that can be exploited in photo-catalysis, magneto-plasmonics or nano-sensing.

International audience; The effects of surface-induced lattice contraction on the size evolution of the surface plasmon resonance and of the electronic thermalization time in small silver clusters have been investigated in the framework of a mixed classical/quantum model. The increase of the conduction-electron density results in a blue-shift trend for decreasing cluster size. However this effect is counterbalanced by the increase of the dielectric function associated to the ionic-core background. Agreement with the blue-shift trend observed in experiment is recovered by introducing an inner surface skin of vanishing ionic-core polarizability having a thickness practically unchanged as compared to previous estimations. The influence of the lattice contraction on the electron dynamics is also discussed. It is shown that this influence is negligible as compared to the surface effects arising from the spillout and the inner skin of reduced ionic-core polarizability which are both responsible for a decrease of the electron thermalization time as the particle size decreases.

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C60 molecules. We have studied films obtained by deposition of (C60)nSim clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C60)nSim were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C60)nSi n + , (C60)nSi n - 1 + and (C60)nSi n - 2 + species. This observation is in favor of the arrangement of these complexes as polymers where the C60 cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film (≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp3 lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C60 molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments.

Optical properties of mixed clusters (AuxAg 1 - x ) n and (NixAg 1 - x ) n , produced by laser vaporization and embedded in an alumina matrix, are reported. The size effects are investigated for different concentrations (x = 0.25, 0.5 and 0.75) in the diameter range 2-4 nm. For alloyed clusters (AuxAg 1 - x ) n of a given size an almost linear evolution of the surface plasmon frequency ω s with the concentration is observed (between those of pure gold and pure silver clusters). Moreover the blue-shift and the damping of the resonance with decreasing size is all the more important as the gold concentration in the particles increases. Such results are in agreement with theoretical calculations carried out in the frame of the time-dependent local-density-approximation (TDLDA) including an inner skin of ineffective screening and the porosity of the matrix. The optical response of (NixAg 1 - x ) n clusters exhibits a surface plasmon resonance in the same spectral range as the one observed for pure silver clusters, but considerably damped and broadened. For a given mean cluster size 3.0 nm, a blue-shift of the resonance is observed when increasing the nickel concentration (between x = 0.25 and x = 0.75). The results are in good qualitative agreement with classical predictions in the dipolar approximation, assuming a core-shell geometry.

International audience; The optical properties of nanocomposite thin films of gold, silver and bimetallic silver-gold clusters embedded in a porous alumina matrix have been investigated in the size range 2–6.7 nm. The metallic particles are produced by laser vaporization of either an Au0.5Ag0.5 alloy or a pure metal target whereas the dielectric matrix is evaporated by an electron gun. Samples involving a low metal concentration have been characterized by several complementary techniques in order to determine their composition, morphology and cluster size distribution. The mixed particles have the same stoichiometry as the target rod. Optical absorption spectra exhibit a surface plasmon resonance whose position is shifting with cluster mean size, giving evidence of finite size effects. Theoretical calculations in the framework of Time-Dependent-Local-Density-Approximation (TDLDA), taking into account an inner skin of ineffective screening and the porosity of the matrix, are consistent with observed size evolutions of the Mie frequency in each type of sample.

The optical properties of bimetallic silver-gold clusters are investigated in the size range 1.9--2.8 nm. Clusters, produced by laser vaporization of an alloy target Au-Ag (50%-50% atomic composition) are embedded with a low concentration in an alumina matrix. The mixed particles have the same stoichiometry as the rod. Absorption spectra exhibit a plasmon resonance band between those of pure Au and Ag embedded clusters, shifting towards higher energies with decreasing cluster size. Theoretical calculations in the frame of the time-dependent-local-density-approximation, including an inner skin of ineffective screening and the porosity of the matrix, are in good agreement with experimental results.

(C60)nSim+ cationic clusters are produced in a laser vaporization source by quenching the vapors from two independent C60 and silicon targets. They are analyzed in the gas phase by abundance and photofragmentation time-of-flight mass spectroscopy. For complexes containing only one C60 molecule, silicon is unlikely to wet the fullerene surface. Mass spectroscopic studies are rather in favor of a three-dimensional growth of silicon clusters weakly bound to C60. For larger systems, one can distinguish two classes of silicon atoms: most of them group in the form of compact islands (or clusters) and some others are directly involved in the linkage of C60 molecules. Particular geometric structures for the stable polymers (C60Si)n−2(C60)2+, (C60Si)n−1C60+, and (C60Si)n+ are postulated.

A theoretical mixed classical/quantal model for describing the optical properties of free and matrix-embedded metal clusters, based on the time-dependent local-density approximation, is presented. As compared to the previous models involving an inner or/and an outer dielectric medium, all the dielectric effects---screening and absorbing properties---are self consistently calculated. Especially, the interband transitions, which dominate the absorption spectra in the UV spectral range, and the mutual interplay between the conduction and core electron excitations, are correctly taken into account.

Absorption and low-frequency Raman-scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (\ensuremath{\approx}420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, the maximum of which depends on the cluster diameter D at the maximum of the cluster-size distribution in the sample according to the approximate law ${\ensuremath{\omega}}_{\mathrm{vib}}\ensuremath{\propto}{D}^{\ensuremath{-}1}.$ The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters, via the plasmon-phonon interaction. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is shown to reflect the ellipsoidal shape distribution of part of the embedded clusters.

We have investigated SiC films, obtained by the low-energy cluster beam deposition (LECBD) technique, and using AFM Raman and XPS spectroscopies. We produce these films at room temperature and in an ultrahigh vacuum environment to protect them from the pollution. The inner morphology of the clusters is close to an amorphous-like structure. However, most of the theoretical models predict an sp 3 hybridization, in disagreement with our experimental results. We find that the films are composed mainly by free components formed by rich silicon, rich carbon, and rich SiC regions, respectively, In our case, the mean hybridization of the rich carbon region is mainly sp n -like with 2 < n < 3.

Silicon–carbon binary clusters with various mean compositions are generated in a laser vaporization source from targets processed as mixtures of graphite and silicon powders. Their size distribution is first analyzed by time-of-flight mass spectroscopy, which shows the stability of carbon fullerenes doped with silicon atoms in substitutional sites. Further investigations on the level of silicon doping are carried out by means of the laser-induced fragmentation of selected sizes. The photoproduct size distributions give evidence for at least nine silicon atoms substituted into still stable fullerene networks. The synthesis of heterofullerenes is mainly assisted by the nucleation mechanisms from Si-C mixed atomic vapors. Just as in the case of externally doped fullerene precursors, the laser-induced annealing of stoichiometric silicon-carbide clusters appears as an alternative route to produce heterofullerenes in the gas phase.

Silicon carbon binary clusters are generated in a laser vaporization source from SixC1−x mixed targets (x=0 to 50%). We have first analyzed stoichiometric (SiC)n (n⩽40) clusters grown from a silicon carbide target (x=50%). Both high fluence photoionization of (SiC)n neutral clusters and photofragmentation of size-selected (SiC)n+ natural positive ions show that silicon-doped fullerenes emerge as stable photoproducts through the laser induced annealing of these clusters. They are detected as stable species as soon as a sufficient amount of silicon is eliminated through unimolecular processes involving the sequential losses of Si2C and Si3C neutral molecules in the earliest evaporation steps. This result is in favor of an efficient substitution of silicon atoms (about 12) into stable “cagelike” carbon networks. We will also show that an efficient doping of carbon fullerenes with silicon atoms can be obtained in carbon-rich mixed clusters directly grown as positive ions from nonstoichiometric targets (x

High-resolution ion mobility measurements have been used to examine In n − and In n + clusters with n =2–30. The cluster–He scattering cross-sections of the anions are systematically larger than those of the cations. The difference (∼25% for In 2 to ∼2% for In 30 ) is attributed to the electron density extending further from the surface of the cluster in the anions than in the cations. Geometric cross-sections estimated from electron densities determined from a charged jellium model are in good agreement with the experimental data: the change in cross-section with charge state is reproduced over the entire range.

The optical properties of nanocomposite thin films of gold and silver clusters embedded in an alumina matrix have been investigated. The samples have been characterized by several techniques in order to determine their composition, morphology and cluster size distribution. Ellipsometry and photoabsorption measurements have been performed. The differences in the optical properties of the various samples reflect finite size effects. Time Dependent Local-Density-Approximation (TDLDA) calculations, taking into account the d-electrons and matrix screening effects, are consistent with observed size evolutions of the Mie frequency. Low frequency Roman scattering spectra exhibit a strong size-dependent band related to quadrupolar vibration modes of the clusters.

The optical response of free and matrix-embedded gold metal clusters AuN is investigated in the framework of the time-dependent local-density-approximation (TDLDA). The characteristics of the surface plasmon resonance are carefully analyzed as a function of the model parameters and the particle radius. The strong influence of the frequency-dependence of the 5d core-electron dielectric function in the vicinity of the interband threshold is emphasized. The size evolution of the Mie-frequency in free gold clusters exhibits a noticeable blue-shift trend as the particle size decreases, much stronger than in silver clusters. The width and shape of the resonance, essentially ruled by the decay via the interband transitions, are found closely correlated to the imaginary component of the core-electron dielectric function. In presence of a surrounding matrix the blue-shift trend is largely rubbed out. Agreement with recent experimental results on size-selected gold clusters embedded in an alumina matrix may be achieved by taking into account the porosity effects at the metal/matrix interface. The comparison with the predictions of classical models is also provided.

The surface plasmon energy of silver clusters is investigated in the framework of the time-dependent local density approximation. The results are in good agreement with experimental data on free ${\mathrm{Ag}}_{N}^{+}$ clusters and on ${\mathrm{Ag}}_{N}$ clusters embedded in solid argon and alumina. The size effects are weak and very sensitive to the matrix, the porosity at the interface, and the cluster charge. It is shown that the core-electron contribution to the metal dielectric function is mainly responsible for the quenching of the size effects in the optical response.

International audience; A great number of studies focus their interest on the photophysical properties of fluorescent hybrid gold nanoparticles for potential applications in biotechnologies such as imaging and/or treatment. Spherical gold nanoparticles are known to quench a chromophore fluorescent signal, when moieties are located in their close vicinity. The use of a polymer spacer on such a system allowed only partial recovery of the dye emission by controlling the surface to dye distance. Gold-based anisotropic sharp nanostructures appear to exhibit more interesting features due to the strong electric field generated at their edges and tips. In this paper, a complete study of hybrid fluorescent bipyramidal-like gold nanostructures is presented. We describe the chemical synthesis of gold bipyramids functionalized with fluorescent water-soluble polymers and their photophysics both in solution and on a single object. We show that the use of a bipyramidal shape instead of a spherical one leads to total recovery of the fluorescence and even to an enhancement of the emission of the dyes by a factor of 1.4.

cited By 9; International audience; Owing to the very different electrovalences of indium and silver, nanoparticles made of these elements are among the simplest examples of hybrid plasmonic systems retaining a full metallic character. The optical properties of small indium-silver clusters are investigated here for the first time in relation to their structural characterization. They are produced in the gas phase by a laser vaporization source and co-deposited in a silica matrix. The optical absorption of fresh samples is dominated by a strong surface plasmon resonance (SPR) in the near UV, in an intermediate position between those of pure elements. A combination of SPR analysis and electron microscopy imaging provides evidence for the favourable surface segregation of indium. After a prolonged exposure to ambient air and because of the silica matrix porosity, changes in the SPR reflect the spontaneous formation of a dielectric indium oxide shell around a metallic silver core. The metallic character of indium can nevertheless be recovered by annealing under a reducing atmosphere. The reversibility of these processes is directly mirrored in optical measurements through SPR shifts and broadenings as supported by multi-shell Mie theory calculations. By controlling their oxidation level, In-Ag clusters can be considered as new candidates to extend SPR spectroscopy in the UV range and model plasmonic systems consisting of a silver particle of potentially very small size, fully protected by a dielectric oxide shell. © 2014 the Owner Societies.

The unimolecular decomposition of size selected cluster cations of trivalent metals (Aln+, Gan+, and Inn+), induced by high fluence laser ionization, has been investigated in the n=7 to n=85, 55, and 75 size ranges, respectively. This method is applied for the first time to photoexcited trivalent clusters generated in an evaporative ensemble and the experimental data cover a size range that was not explored in previous pioneering experiments on their dynamics. Small clusters dissociate through the loss of a neutral or a charged atom whereas clusters larger than a well defined critical size merely dissociate through the first channel. In the framework of the RRK statistical theory, the measured evaporation rates provide some information about the size evolution of the cluster dissociation energies and their ionization potentials in the low size range. The competition between the ion and the atom evaporation is found to be consistent with the size evolution of the ionization potentials independently measure...

A new reflectron time-of-flight mass spectrometer with a cylindrical electrostatic mirror instead of the usual plane reflector is described. When it is mounted at right angles to the direction of an incoming molecular beam, this system automatically compensates for the transverse drift of the ions in the spectrometer due to their initial velocity parallel to the molecular beam direction. As a consequence, ions of any mass can be collected onto the detector, whatever their initial transverse velocity is. The mass range accessible in a single scan with our cylindrical reflectron is thus strongly increased as compared to the case of a usual plane reflectron. This article presents the general design of our cylindrical reflectron. A detailed description of the apparatus is given and its performances are illustrated on selected examples. Cluster mass spectra extending over a very wide size range are presented. A mass resolution of about 4000 is achieved in mass spectra of laser photoionized aluminum clusters. Moreover, the cylindrical geometry of the reflector gives rise to focusing properties onto the detector, which could be interesting in view of collection efficiency improvement.

Cluster assembled thin films of covalent materials are studied by Raman spectroscopy. The main properties are discussed in terms of frustration through several examples (carbon, silicon, and boron nitride). Some applications such as luminescence are discussed.

Using the pseudo-Hamiltonian technique we investigate, through complete self-consistent ground-state calculations, nonlocal ion-pseudopotential effects on the dipole strength distribution and the electronic shell structure of alkali clusters. The results obtained for sodium clusters are compared to the predictions of the standard jellium model (JM) and to experiments. Some wrong JM-predictions — shift of some low magic sizes, insufficient redshift of the surface plasmon frequency relative to the Mie frequency — are corrected, but a noticeable discrepancy remains with regard to the supershell structure. In the second part of this paper we restore the reliability of the pseudo-Hamiltonian approximation, on which some doubts have been recently cast, because of incorrect predictions concerning the bulk properties of lithium. From a careful analysis of the nonlocal effects in a smoothed ionic background, we show that a specific pseudo-Hamiltonian parametrization has to be systematically selected for ensuring maximum accuracy. We successfully test the suitability of this parametrization in the case of lithium clusters, through a comparison with recently published results obtained using accurate norm-conserving nonlocal pseudopotentials.

Unimolecular dissociation of aluminum, gallium and indium clusters is investigated. Small sizes dissociate into two channels: either the evaporation of a neutral or a charged monomer. Above a given size nc, only dissociation of a neutral atom subsists. The evaporation of a charged monomer is characteristic of trivalent metal clusters and is consistent with the size evolution of the ionization potential towards the atomic value. The experiments are interpreted in the framework of the statistical R.R.K. model. For smaller sizes (n < nc), as two evaporation processes are in competition, we have evaluated cluster relative dissociation energies and ionization potentials. The competition between the two evaporation channels is well mirrored by the evolution of the ionization potentials independently measured by near-threshold photoionization experiments. For gallium, our measurements have revealed that the covalent to metal transition occurs for larger sizes (n = 30–50 atoms) than for aluminum clusters.

The low-energy cluster beam deposition technique (LECBD) is applied to produce cluster assembled films with hitherto unknown nanostructured morphologies and properties. Neutral clusters having the very low energy gained in the supersonic expansion at the exit of the inert gas condensation-type source are deposited without fragmentation upon impact on the substrate. Depending on the deposition conditions (nature, size and flux of incident clusters, nature and temperature of the substrate, vacuum conditions), granular nanostructures resulting from the diffusion and coalescence of supported clusters are obtained with materials of any type (covalent or metallic). A critical size for coalescence limits the supported grain size and, finally, highly porous thick films growing by random stacking of nanoparticles are obtained. A recent model developed by combining several dynamical processes simultaneously occurring on the substrate (deposition - diffusion - aggregation, DDA) is used to simulate the cluster assembled film morphology in good agreement with the experimental observations. Examples of novel materials obtained by LECBD are presented to illustrate the interesting potentialities of the technique. In the case of covalent materials such as carbon and silicon, 'amorphon'-type disordered structures, different from the conventional amorphous structures (a-C and a-Si), are obtained with some unique properties. With transition metal (Fe, Co and Ni) cluster assembled films, a specific magnetic behaviour, resulting from the competition between the intrinsic properties of the grains (magnetocrystalline anisotropy) and the interactions between grains, is observed. Also, films of clusters embedded in various co-deposited matrices are produced in order to control the interactions between grains via the matrix materials (insulating, conducting ...). Interesting optical properties (from metallic clusters in ) or giant magnetoresistance effects (from Co clusters in silver) are reported for such systems, emphasizing the future role of LECBD in various fields of applications such as optical and optoelectronic nanostructures, magnetic and magneto-optic nanostructures and quantum devices.

International audience; 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.