[Show abstract][Hide abstract] ABSTRACT: It is reported on a reactive magnetron sputtering-based deposition method to synthesize, at room temperature, photochromic nanocomposite thin films consisting of Ag nanoparticles sandwiched between nanoporous TiO2 layers. The fabrication process is compatible with large-scale production and functional flexible substrates. It is shown that when TiO2 is deposited in the metallic mode, the formation of Ag metal nanoparticles induces localized surface plasmon resonances in the visible range and therefore the as-deposited samples are colored. In contrast, when TiO2 is deposited in the compound mode, the trilayer samples are colorless because silver oxidizes during TiO2 deposition. It is demonstrated that the colorless samples can be colored under ultraviolet (UV) laser exposure at 244 nm due to the reduction of oxidized silver and the formation of metallic Ag nanoparticles. Moreover, irradiation at 647 nm wavelength of colored samples (as-prepared or after UV exposure) gives rise to changes in the particle morphology that strongly modifies the film absorbance and results in a color transition from blue to orange. The choice of the irradiation wavelength allows controlling the color saturation of the sample up to the complete discoloration by using a visible laser at 488 nm. All these photochromic mechanisms are repeatable during cyclic processes.
[Show abstract][Hide abstract] ABSTRACT: The progress of organic electronics demands an increased participation of nanotechnology, and it has already been shown that the presence of metallic nanoparticles and/or nanostructured thin films can enhance the device performance. Nevertheless, to gain control over the device final performance, it is crucial to achieve a profound understanding of the nanostructure development and assembly. We investigate the growth kinetics of silver (Ag) on a tris(8-hydroxyquinolinato)aluminum (Alq3) thin film via sputter deposition. The increase of the average electron density of the Ag nanostructured film is observed to follow a sigmoidal shape development as a function of the deposited Ag thickness, as a consequence of dominant island-mediated growth. The nanoclustered film is percolated at around a thickness of 5.0 ± 0.1 nm. At this film thickness the effective film density is about 50%. Moreover, our simulation results indicate that the shape of the nanoclusters changes from truncated spheres to cylinders upon surpassing the percolation threshold.
The Journal of Physical Chemistry C 02/2015; 119(8):4406. DOI:10.1021/jp512675w · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ultradense macroscopic arrays of ferromagnetic alloy nanowires exhibit unique properties that make them attractive both for basic physics studies and for prospective nanodevice applications in various areas. We report here on the production of self-organized equiatomic FePt nanowires produced by glancing-angle ion-beam codeposition on alumina nanoripple patterns at room temperature and subsequent annealing at 600 [degree]C. This study demonstrates that periodically aligned FePt nanowires with tunable size ([similar]10-20 nm width and [similar]0.5-10 nm height) can be successfully grown as a consequence of shadowing effects and low mobility of Fe and Pt on the rippled alumina surface. Moreover, the structure and magnetic properties of the FePt nanowires, which undergo a phase transition from a disordered A1 (soft) structure to a partially ordered L10 (hard) structure, can be modified upon annealing. We show that this behavior can be further exploited to change the effective uniaxial anisotropy of the system, which is determined by a strong interplay between the shape and magnetocrystalline anisotropies of the nanowires.
[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. DOI:10.1007/s11051-014-2328-z · 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). DOI:10.1103/PhysRevB.88.115427 · 3.74 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.
[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: We calculate, in the quasistatic coupled dipole approximation, the analytical expressions of the effective dielectric tensor of a single layer of polydisperse ellipsoidal nanoparticles with two of their principal axes in the layer's plane and embedded in a homogeneous dielectric medium. The organization (isotropic or anisotropic) and orientation (without or with a preferential in-plane orientation) of the nanoparticles is taken into account, together with their (possibly correlated) in-plane size, in-plane projected shape, and height distributions. In particular, we propose to describe the response of a layer of nanoparticles presenting a height distribution by using a vertically graded effective medium model. The expressions are tested in the case of finely characterized dielectric/silver/dielectric granular trilayers grown by means of vapor deposition in which the silver coalesced nanoparticles present correlated in-plane size and in-plane projected shape/height distributions and a moderate surface coverage of about 25$%$. A satisfactory quantitative agreement is obtained between the simulated and measured surface plasmon extinction bands of the metal nanoparticles. This agreement is permitted by the capability of the effective medium model of taking into account the ellipsoidal shape of the nanoparticles. The significant role of the size and shape distributions is also demonstrated.
Physical Review B 08/2012; 86(4):045415. DOI:10.1103/PhysRevB.86.045415 · 3.74 Impact Factor
[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). DOI:10.1103/PhysRevB.85.235415 · 3.74 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. DOI:10.1117/1.JNP.6.061502 · 1.69 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: Les systèmes nanostructurés comportant des îlots hétéroépitaxiés synthétisés sur des surfaces d'oxyde présentent un intérêt, d'une part fondamental et d'autre part technologique en vue des applications potentielles dans les domaines de l'électronique, et de l'optoélectronique. Dans de tels systèmes, le contrôle de l'organisation, de la forme et de la taille des îlots est d'une importance primordiale puisque ces caractéristiques déterminent les propriétés physiques finales. L'utilisation des surfaces vicinales semble être une méthode intéressante pour pouvoir organiser, voire auto-organiser des nanoparticules à la surface d'un substrat . En effet ces surfaces obtenues après la découpe d'un monocristal selon un plan légèrement désorienté par rapport à un plan cristallographique dense, présentent naturellement des marches atomiques. De plus, elles ont tendance à diminuer leur énergie de surface sous traitement thermique en formant des paquets de marches (facettes). Nous allons montrer dans cette communication que des traitements thermiques contrôlés permettent de réaliser à partir de ce processus de regroupement de marches des surfaces constituées de marches périodiques selon une et même deux directions. I. Démarche et techniques expérimentales Dans ce travail, les mécanismes de mise en « paquet de marches » ont été étudiés en fonction des angles de taille (1, 5 et 10°) de monocristaux d'alumine α (001) et des conditions de traitement thermique. L'un des objectifs est d'identifier des situations où les surfaces structurées sont stables ou tout du moins métastables. L'étude de l'évolution de ces substrats sous atmosphère contrôlée a été menée par microscopie à force atomique (AFM) et diffusion centrale des rayons X sous incidence rasante (GISAXS). Ces dernières mesures ayant été réalisées à l'aide de la source européenne de rayonnement X synchrotron (ESRF – Grenoble).
2ème colloque francophone PLUridisciplinaire sur les Matériaux, l’Environnement et l’Electronique, Limoges; 06/2011