[Show abstract][Hide abstract] ABSTRACT: The extraordinary character of Surface Plasmon modes of disordered metallic systems
has been predicted theoretically. We here demonstrate through Electron Energy Loss Spectroscopy
that percolating fractal metal films sustain numerous strongly confined Surface Plasmon modes.
[Show abstract][Hide abstract] ABSTRACT: Structural and optical modifications induced by low-energy (≤80 eV) bias-plasma annealing of silver nanoclusters (2–25 nm) grown by magnetron sputtering deposition are reported. By combining postmortem structural characterizations and real-time optical measurements, we show that etching effects associated with enhanced Ag mobility result in progressive and irreversible changes of both the morphology and organization of the nanoclusters (i.e., decrease of the cluster size and intercluster distance as well as increase of their out-of-plane aspect ratio). Surface plasmon resonance bands of the nanoclusters are also modified by plasma treatment, which causes a blue-shift together with an amplitude decrease and a narrowing of the band. In addition, the kinetics of plasma-induced modifications can be easily controlled by varying the applied bias voltage. Therefore, plasma annealing could emerge as an efficient alternative to more traditional thermal annealing treatments for tuning the plasmonic properties of noble metal nanoclusters with great flexibility.
Journal of Nanoparticle Research 02/2014; 16(3):1-13. · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Low-energy ion beam sputtering of alumina thin films followed by growth of metallic nanoparticles by glancing angle deposition is optimized in order to produce arrays of silver nanoparticle chains with a strong plasmonic dichroism. A systematic study is undertaken in order to establish the influence of the angle of silver deposition and the ordering of the pre-patterned rippled surface on the morphology and organization of the nanoparticles, and on their associated optical properties. High ion fluence for ripple formation and low glancing angle for metal deposition favor the formation of aligned and elongated particles with sub-nanometer gaps. Numerical simulations show that these nanoparticle arrays generate high electric field enhancements for an excitation parallel to the particle chains, and therefore can be used for surface enhanced spectroscopies.
[Show abstract][Hide abstract] ABSTRACT: We report on the identification and nanometer scale characterization over a large energy range of random, disorder-driven, surface plasmons in silver semicontinuous films embedded in silicon nitride. By performing spatially resolved electron energy loss spectroscopy experiments, we experimentally demonstrate that these plasmons eigenmodes arise when the films become fractal, leading to the emergence of strong electrical fields (“hot spots”) localized over few nanometers. We show that disorder-driven surface plasmons strongly depart from those usually found in nanoparticles, being strongly confined and randomly and densely distributed in space and energy. Beyond that, we show that they have no obvious relation with the local morphology of the films, in stark contrast with surface plasmon eigenmodes of nanoparticles.
Physical Review B 09/2013; 88(11). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Real-time surface differential reflectance spectroscopy in the visible range is used to study the optical response of silver nanoclusters, prepared by magnetron sputtering deposition, during cyclic treatments in different oxygen atmospheres and low-energy bias argon plasma. Changes in the reflectance show that the exposure to non-ionized (or partially ionized) oxygen causes a red-shift and damping (or complete vanishing) of the resonance, while bias plasma annealing induces the opposite effects, due to oxygen desorption and structural reshaping of the nanoclusters. These results open up new opportunities for developing plasmon-based devices with high tunability of the surface plasmon resonance (energy, width and amplitude) due to an interplay between morphological and chemical modifications of the nanoclusters.
[Show abstract][Hide abstract] ABSTRACT: Sub-wavelength arrays of silver nanoparticles embedded in a dielectric matrix are elaborated by a low-cost bottom-up technique, which combines substrate pre-patterning by defocused ion-erosion (resulting in the generation of one-dimensional nanoripples) with subsequent Volmer-Weber growth by physical vapor deposition at glancing incidence. The arrays are constituted by regular chains (∼20 nm period) of small particles (size less than 10 nm) with elongated shape in the direction of the ripples. This shape anisotropy and the resulting interparticle coupling inside the chains (gap between the particles less than 10 nm) are responsible for the plasmonic dichroism of the array leading to polarization-selective broad-band absorption in visible and near infrared. This dichroism can be tuned by adjusting the amount of deposited metal, the elaboration temperature, the nature of the dielectric host and the deposition geometry. The technique is at last employed to elaborate 3D nanostructures.
Nanoscience and Nanotechnology Letters 01/2013; 5:19. · 1.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Surface plasmons (SP) are collective oscillations of the conduction electrons of noble metals at metal-dielectric boundaries and are commonly described as classical dispersive electromagnetic waves. Confined geometries of metallic nanoparticles induce a resonant behavior and gives rise to SP modes that dictate their optical properties. Among all SP related phenomena, the special case of optical excitations in disordered films has drawn during the last few years much theoretical  as well as experimental  attention. The peculiar broadband absorption properties of such systems are now commonly attributed to a SP mediated strong localization of light at the nanoscale within deep sub-wavelength areas called "hot spots" (HS). However, despite the great deal of work in the past 15 years, no comprehensive study of such films has been carried out – especially, full properties of the HS have never been studied experimentally. Thus many fundamental questions on the physics of SP and in particular HS in disordered films remain frustratingly open . Now, most of these questions might find answers when accessing the full spectral information at the (predicted) relevant scale of the HS, namely less than 20 nm. In this contribution, we will show how the extension of spatially resolved electron energy loss spectroscopy (EELS) for SP characterization from the now commonly performed nanoparticle SP analysis  to that of disordered films has helped us to unveil the HS spectral properties of disordered films. As an example, Figure 1 shows a STEM High Angle Angular Dark Field (HAADF) image of a silver (in grey) disordered film embedded in a (dark looking) Si 3 N 4 matrix, evidencing the random character of its morphology. Another HAADF image of a smaller substrate area, on which hyperspectral imaging measurements are performed, is displayed. EEL spectra extracted from three different positions of the electron probe are presented. Resonances associated to SP modes are underlined by the presence of peaks in the spectra. The change in the peak energies and widths for such close probing sites suggests a complex optical character, which is confirmed by strong fluctuations in the energy filtered EEL probability maps displayed in figure 2. However, these maps show that the electromagnetic energy is confined in sub-wavelength areas whose spatial location varies with the energy loss. This is equivalent to the Electromagnetic Local Density of states (EMLDOS) maps obtained with SNOM , but with far better spatial resolution and full spectral information . Fitting each spectrum by a sum of Gaussian functions allows discriminating all the SP modes that can be blurred by picturing the EMLDOS. We show in figure 3 the results of such data processing for the lowest energy peak. The hot spots are still visible in the amplitude maps, and associated to spatial regions where the peak energy and width are well defined, but changing from one to another. Therefore, not only EELS hyperspectral imaging allows to spatially probe the EMLDOS fluctuations in a disordered system  with nanometer resolution, but also provides a direct identification of the SP modes through physical observables resonant energy and width. Finally, we will try to show in which amount the observed HS behavior is very specific to random films, as opposite to a random collection of nanoparticles.   We thank Z. Mahfoud for help in data treatment, and R. Carminati and J.J. Greffet for enjoyable discussions.This work is partially supported by the Centre National de la Recherche Scientifique and the Délégation Générale de l'Armement. Figure 1. Left: Overall view STEM HAADF image of a silver disordered film embedded in a Si 3 N 4 matrix. Middle: Magnified view of the area indicated in the white dashed square. Right: EEL Spectra acquired at three probe positions indicated by the colored squares.
15th European Microscopy Congress EMC 2012; 09/2012
[Show abstract][Hide abstract] ABSTRACT: 3D reciprocal space mapping in the grazing incidence small-angle x-ray scattering geometry was used to obtain accurate morphological characteristics of nanoripple patterns prepared by broad beam-ion sputtering of Al2O3 and Si3N4 amorphous thin films as well as 2D arrays of Ag nanoparticles obtained by glancing angle deposition on Al2O3 nanorippled buffer layers. Experiments and theoretical simulations based on the distorted-wave Born approximation make it possible to determine the average 3D shape of the ripples and nanoparticles together with crucial information on their in-plane organization. In the case of nanoparticle arrays, the approach was also used to quantify the growth conformity of an additional capping layer, which proceeds by replication of the buried ripple pattern.
Physical Review B 06/2012; 85(23). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Surface differential reflectance spectroscopy (SDRS) is sensitive enough to observe the minute changes in the surface plasmon resonance (SPR) of noble metal nanoparticles (NPs), which is extremely dependent on the morphology and organization of the NPs as well as on the chemical atmosphere surrounding them. Taking this SPR as a signature, we have studied the reactivity of Ag NPs using a dedicated in situ SDRS setup adapted on a magnetron sputtering deposition machine. This configuration allowed us to analyze the SPR modifications in real-time, not only during the growth of Ag NPs, but also during their exposure to molecular O2 and during their capping by a dielectric (Si3N4) matrix. Real-time SDRS analysis reveals that, upon exposure of the Ag NPs to O2, their SPR characteristics (position, amplitude, and width of the absorption band) alter immediately, indicating the instantaneous reactive interactions between Ag NPs and adsorbed O2 molecules. In addition, during the deposition of the Si3N4 matrix, real-time SDRS reveals possible breaking of Ag–O2 interactions. Moreover, with increasing Ag NP size, SPR modifications are seen to be reduced in O2 atmosphere, suggesting the diminution of Ag–O2 reactive interactions in the bigger NPs compared to the smaller ones.
Journal of Nanophotonics 04/2012; 6(1):061502. · 1.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Surface differential reflectance spectroscopy (SDRS), an optical characterization technique, is sensitive enough to observe the minute changes in the surface plasmon resonance (SPR) of noble metal nanoparticles (NPs). This SPR, which causes a sharp absorption of light in the visible range, is extremely sensitive not only to the morphology and organization of the NPs, but also to the chemical atmosphere surrounding them. Hence, taking SPR as a signature phenomenon, we have studied the reactivity of Ag NPs using a dedicated in situ SDRS set-up mounted on a magnetron sputtering machine. Real-time optical characterizations were possible not only during the deposition of Ag NPs, but also during their exposure to gases such as O2, N2, Ar, either non-ionized or partially ionized. This optical study reveals that Ag NPs are reactive to non-ionized O2 exposure, which induces modifications in the SPR characteristics (width, amplitude and position of the absorption band) in contrast to N2 and Ar. Moreover, this study also evidences a complete disappearance of the SPR when Ag NPs are exposed to partially ionized oxygen species O2(+) as well as a significant reactivity of the NPs exposed to N2(+), while Ar remains non-reactive in both non-ionized and partially ionized forms.
Nanostructured Thin Films IV - SPIE Proceedings; 08/2011
[Show abstract][Hide abstract] ABSTRACT: Multilayered films of Au/ZnO were prepared by physical vapor deposition. Varying the Au thickness, tAu, and the number of bilayers, n, allowed us to investigate the role of these parameters on the sample structural and electronic properties. X-ray diffraction, X-ray absorption spectroscopy, grazing incidence small angle X-ray scattering and transmission electron microscopy experiments, have been combined to UV-visible and infrared spectroscopy to characterize the multilayers in the as-prepared state and after annealing. In the as-prepared state, the strong Au and ZnO lattice interaction leads to ZnO epitaxy on Au. Gold appears either as continuous layers or in form of nanoparticles. ZnO experiences a structural transformation from wurztite to rock salt monitored by the Au morphology. Annealing at 500 °C destroys the lattice matching. The electronic and optical properties of the systems are understood in line with the Au morphology and ZnO structural state.
Journal of Applied Physics 05/2011; 109(9):094308-094308-10. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The self-alignment and optical dichroism of Au nanoparticle chains grown by glancing incidence deposition on rippled Al2O3 thin films is investigated. Although the nucleation of the nanoparticles is almost isotropic, their growth is strongly anisotropic resulting in a sharp dependence of their optical transmittance on the orientation of the polarization of the incident light. We show that both the frequency and the spectral width of the transverse and longitudinal surface plasmon resonances can be easily tuned by varying the amount of deposited metal. Such nanostructured materials open perspectives for the development of plasmonic devices endowed with tunable optical dichroism both in the visible and the near infrared regimes.
[Show abstract][Hide abstract] ABSTRACT: Morphological and crystallographic structures of multilayered W/Cu nanocomposite thin films elaborated by physical vapor deposition were studied by varying copper and tungsten thicknesses. Sample examinations were performed by x-ray diffraction (XRD), grazing incidence small-angle x-ray scattering and transmission electron microscopy (TEM). Samples were found to be composed of copper nanoparticles, homogeneously dispersed in planes parallel to the film-substrate interface and periodically separated by tungsten layers along the growth direction. Our observations revealed an original texture development of the tungsten matrix from a mixture of unexpected α-W〈111〉 and α-W〈110〉 components to unique α-W〈110〉 component as the copper coverage passes a thickness threshold of 0.6 nm. Local TEM texture stereology investigations revealed simultaneous columnar growth of both preferential orientations posterior to polycrystalline development while XRD reveals strong compressive residual stresses in both texture components. Physical origins of the preferential crystallographic orientation evolution are discussed. Copper mono layers adsorption on W surfaces promotes surface energy anisotropy and diminution which is effective over the threshold. Below, the presence of a W(Cu) solid solution which does not affect substantially the texture is revealed by the stress-free lattice parameter value extracted from XRD.
Journal of Applied Physics 01/2011; 109(1):014305-014305-12. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For the first time, grazing incidence small-angle X-ray scattering (GISAXS) analysis is used to characterize the morphology of TiO(2) thin films grown by glancing angle physical vapor deposition (GLAD). According to cross-section scanning electron microscopy (SEM) images, the films consist of near isotilted TiO(2) columns of different length and width depending on film thickness. The obtained GISAXS patterns show a characteristic asymmetry with respect to the incidence plane, which is associated with the tilted geometry of the TiO(2) columns. The patterns also show the existence of two populations of columns in these GLAD-TiO(2) films. The population of the thinnest columns appears related to the first grown layer and is common for all the films investigated, while the second population of columns grows with the thickness of the films and has been related to wider columns formed by shadowing at the expense of the initially formed columns.
[Show abstract][Hide abstract] ABSTRACT: The Maxwell-Garnett's and Toudert's methods detailed in Appl. Opt.48, 778 (2009)APOPAI0003-693510.1364/AO.48.000778, based on the Maxwell-Garnett and Yamaguchi effective medium models, respectively, have been used for calculating the absorbance alpha of a (BaF(2)/Ag)(5)/BaF(2) nanocomposite thin film identical to the one presented in Fig. 1 of Appl. Opt.48, 778 (2009)APOPAI0003-693510.1364/AO.48.000778]. We propose that the discrepancies observed in this reference between the alpha spectra calculated by the two methods are due to a non rigorous use of both effective medium models by the author and show that adequate calculations lead to superposed spectra.
[Show abstract][Hide abstract] ABSTRACT: Au / Si <sub>3</sub> N <sub>4</sub> films have been prepared by gold and silicon nitride sequential sputtering with different Au thicknesses. The goal is to investigate the Au growth mode and to identify the regime where Au nanoparticles (NPs) can be prepared. The number of Au atoms, the bilayer thickness, the Au environment, the size, and the organization of the particles have been determined. Gold appears in metallic form. The thickest Au layer (8 nm) is continuous whereas the thinnest ones (with average values of 1–3 nm) are formed by nearly spherical Au NPs with diameters in the 5–7 nm range. Au NPs spatial self-organization is observed for average thickness of t <sub> Au </sub>=2.9 nm . These growth modes are discussed and compared with those observed for other metal/ Si <sub>3</sub> N <sub>4</sub> systems prepared in similar conditions.
Journal of Applied Physics 06/2010; · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In situ and real time surface differential reflectance (SDR) spectroscopy is employed to study the growth of metallic Ag and/or dielectric Si3N4 films during deposition by magnetron sputtering. The measurements during Si3N4 sputtering allow determining both the refractive index and the deposition rate. During Ag sputtering, the SDR presents a maximum in the visible range, typical of a surface plasmon resonance (SPR) indicating the 3D growth of silver nanoclusters. After a certain deposition thickness, the SDR change corresponds to a continuous layer growth and allows determining the Ag deposition rate. During Ag/Si3N4 alternate deposition, the SDR spectroscopy enables to follow the SPR modifications (position, amplitude and width) not only during the formation of the Ag nanoclusters but also during their capping by a Si3N4 matrix and during intermediate steps (holding time after the silver sputtering, Si3N4 target ignition and pre-sputtering before the Si3N4 deposition) where significant changes are detected. It suggests possible nanocluster reshaping or physicochemical processes occurring at the nanocluster interface during the different steps.
Thin Solid Films 03/2010; 518(10):2637–2643. · 1.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The possibility to investigate complex magnetic profiles throughout an ultrathin magnetic film or an interface by soft x-ray resonant magnetic reflectivity is presented. The determination of in- and out-of-plane magnetic profile is shown to be possible with a subnanometer resolution by measuring the reflectivity over a wide angular range. The technique is applied to a granular magnetic multilayer and to a perpendicular exchange bias coupled system.
Journal of Physics Conference Series 02/2010; 211(1):012015.