The effects of size, shape and organization on the surface plasmon resonances of Ag nanoclusters sandwiched between Si(3)N(4) layers are studied by transmission electron microscopy and anisotropic spectroscopic ellipsometry. We present an easy-to-handle model that quantitatively links the nanostructure and optical response of the films, which are considered as dielectric/metal:dielectric/dielectric trilayers, with the central nanocomposite layer being an effective medium whose optical properties are described by an anisotropic dielectric tensor. The components of this tensor are calculated using a generalization of the Yamaguchi theory taking into account the real organization, size and shape distributions of ellipsoidal nanoclusters, whose electronic properties are assumed to reflect shape-dependent finite size effects. Using this model, it is shown that the optical response of the films in the visible range is dominated by the excitation of the surface plasmon resonance of the clusters along their in-plane long axis, while no surface plasmon resonance resulting from an excitation along their in-plane short axis can be observed due to damping effects. Moreover, the spectral position of this resonance appears to be mainly affected by the average shape of the clusters, and weakly by their size, their shape distribution and the electromagnetic interaction between them.
Nanocermet trilayered thin films consisting of silver nanoclusters sandwiched between two dielectric layers (the buffer and the cap) have been synthesized by ion-beam sputtering with an alternate deposition of the metal and the dielectric species. The influence of the amount of silver, the nature of the buffer and the cap (BN or Si3N4), and a time delay before the cap deposition on clusters morphology and repartition have been investigated by transmission electron microscopy. It has been observed that the clusters display truncated ellipsoidal shapes in which the height to diameter ratio H∕D decreases as the amount of deposited silver increases. For a given amount of silver, this ratio is lower in the case of a Si3N4 cap, whatever the nature of the buffer. Two explanations are proposed to account for this “cap effect” on clusters morphology: the first one is based on a calculation of the H∕D minimizing the surface free energy of the clusters embedded between the buffer and the cap; the second one holds on the shape relaxation of the coalesced nonequilibrium clusters towards their equilibrium shape with the buffer, this process occurring until clusters are fully covered with the cap. Because of the higher deposition rate of Si3N4 compared to BN, a Si3N4 cap would allow a less efficient reshaping and consequently lead to flatter clusters. This explanation is supported by the temporal evolution of clusters morphology and repartition observed during the time delay before deposition of the cap. The evolution of the spectral position of the surface-plasmon resonance (SPR) of the trilayers as a function of their structure has also been investigated by optical transmittance measurements. The influence of cluster morphology, as well as the nature of the buffer and the cap on the SPR spectral position are discussed.
The real and imaginary parts of the dielectric function of VO2 thin films, deposited on r-plane sapphire via pulsed laser deposition, are measured by means of visible-infrared ellipsometry for wavelengths ranging from 0.4 to 15 μm and temperatures within its phase transition. For both the insulator-to-metal (heating) and metal-to-insulator (cooling) transitions, it is shown that the two ellipsometric signals exhibit three temperature-driven behaviors, which are well described by appropriate combinations of the Tauc-Lorentz, Gaussian, and Drude oscillator models. By fitting Bruggeman's effective medium model for the dielectric function to the corresponding measured experimental values, using the volumetric fraction of the VO2 metallic domains as a fitting parameter for different temperatures within the VO2 phase transition, we have found that this model is suitable for describing the dielectric function in visible and near-infrared wavelengths (∼0.4 to ∼3.0 μm), but it generally fails for longer infrared ones. Furthermore, the hysteresis loop of the VO2 emissivity averaged over a relevant interval of wavelengths is determined and shown to vary from ∼0.49, in the insulator phase, to ∼0.16, in the metallic one. These values, based on the VO2 dielectric function, are consistent with previous measurements reported in the literature, and therefore, our measured data are expected to be useful for describing the behavior of VO2 films involved in optical and radiative applications.
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