T. Girardeau

Quantitative modelling of the surface plasmon resonances of metal nanoclusters sandwiched between dielectric layers: the influence of nanocluster size, shape and organization.

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.

Morphology and surface-plasmon resonance of silver nanoparticles sandwiched between Si3N4 and BN layers

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...

Optical and structural properties of Ag-Si 3 N 4 nanocermets prepared by means of ion-beam sputtering in alternate and codeposition modes

Ion-beam sputtering deposition has been used in two ways, as granular multilayers and as cosputtered film, to elaborate Ag-Si3N4 nanocermets. Multilayer deposition creates slightly oblate clusters, and cosputtering produces two cluster families: elongated clusters within the Si3N4 matrix and larger ones at the film surface. The transmittance spectra of these nanocermets are characterized by a surface plasmon resonance. In the reported research the position of this resonance is related to the morphological properties of silver nanoclusters, which are studied by transmission-electron microscopy, grazing-incidence small-angle x-ray scattering, and atomic-force microscopy.

Using ion beams to tune the nanostructure and optical response of co-deposited Ag : BN thin films

The present study is devoted to co-deposited Ag : BN nanocermet thin films and is focused on the influence of ion irradiation conditions on their structural and linear optical properties. Ion irradiation was performed in situ during the growth of the nanocermets using a 50 eV assistance beam (nitrogen/argon or nitrogen-ion assistance) and ex situ on as-grown films using a 120 keV argon-ion beam (post-irradiation). Grazing incidence small-angle x-ray scattering measurements show that (i) as-grown N-assisted films contain prolate spheroidal clusters (height-to-diameter ratio H/ D ≈ 1.8), (ii) N/Ar-ion assistance leads to the formation of more elongated clusters ( H/ D ≈ 2.1) and (iii) post-irradiation leads to a decrease of H/ Dto a value close to 1. These results are discussed on the basis of atomic diffusion processes involved during the growth of the nanocermets and during the post-irradiation. The optical transmittance spectra of these films measured at normal incidence display one absorption band, due to the excitation of the (1,1) plasmon mode of the clusters. In the case of the as-grown films, an additional band appears at oblique incidence for P-polarized light, as a consequence of the excitation of the (1,0) plasmon mode of the clusters. Our results show that the spectral position of the absorption bands (which can be tuned in the 400–600 nm range) depends on the H/ Dratio of the clusters, in good agreement with calculations of optical transmittance considering the nanocomposite layer as a uniaxial anisotropic medium whose dielectric tensor is described by an anisotropic Maxwell–Garnett model.

Linear and third-order nonlinear optical responses of multilayered Ag:Si3N4 nanocomposites

The linear and third-order nonlinear responses of tailored Si3N4/Ag/Si3N4 trilayers and (Si3N4/Ag)n/Si3N4 multilayers grown by alternating ion-beam sputtering have been studied by combining complementary characterization techniques such as transmission electron microscopy, spectroscopic ellipsometry and degenerate four-wave mixing. The linear optical response dominated by the surface plasmon resonance of Ag nanoparticles has been measured over the whole visible range while the third-order nonlinear susceptibility has been probed at the surface plasmon resonance wavelength. Due to the weak in-plane interaction between Ag nanoparticles, the linear and nonlinear optical responses of the Si3N4/Ag/Si3N4 trilayers are mainly influenced by the size and shape of the nanoparticles. A maximum value of 1.1 x 10(-7) esu has been found at 635 nm for the effective third-order nonlinear susceptibility of the trilayer with the highest amount of silver. The linear optical response of the (Si3N4/Ag)n/Si3N4 multilayers is shown to be dominated by the surface plasmon resonance of isolated layers of weakly interacting nanoparticles at wavelengths shorter than 600 nm whereas a contribution due to vertical interactions has been shown for higher wavelengths. Below the vertical percolation threshold, their nonlinear optical response at the surface plasmon resonance wavelength is similar to the one of an isolated assembly of nanoparticles, and the effective third-order nonlinear susceptibility is slightly increased by decreasing the thickness of the Si3N4 spacer.

Grazing incidence small-angle X-ray scattering of BN-Fe and BN-Ag nanocomposite films obtained by cosputtering and multilayer deposition techniques

We have elaborated BN-Fe and BN-Ag nanocomposite films by ion-beam sputtering codeposition and multilayer deposition. Grazing incidence small-angle X-ray scattering experiments have been performed to study the nanostructure of the films as a function of the deposition conditions and of the metallic species. The results show that the multilayer method generates a self-organized growth from plane to plane and a small polydispersity. The cosputtering technique displays a more random arrangement of nanoparticles but has the advantage of leading to a columnar growth when assisting the deposition with a nitrogen ion beam. Furthermore, the presence of two families of nanoparticles (small elongated clusters within the matrix and large particles on the surface) has been evidenced in the case of codeposition of BN and Ag under assistance.