Generalized Effective Medium Theory to Extract the Optical Properties of Two-Dimensional Nonspherical Metallic Nanoparticle Layers

A new effective medium theory is introduced to describe the optical properties of a two-dimensional array of metallic nanoislands. This model which takes into account both the nanoisland orientation and their shape distribution is successfully used to interpret the ellipsometric measurements performed on gold nanoislands sputtered on a silicon substrate. By coupling ellipsometry to atomic force microscopy measurements, we show that the growth mechanism involves a Volmer–Weber growth mode. The optical anisotropy of uniaxial films was attributed to in-plane preferential self-orientation of gold nanoislands. Finally, we demonstrate that the optical birefringence and dichroïsm of nanoisland layers can be tuned during the film growth and are due to the splitting of the plasmon resonance into two modes: the transversal and the longitudinal modes of gold nanoislands

Ellipsometry of Colloidal Solutions: New Experimental Setup and Application to Metallic Colloids

An ellipsometric cell is developed to simultaneously determine the shape distribution, the volume fraction, and the complex refractive index of gold and silver colloids. Simulation reveals that this cell drastically improves the detection limit of ellipsometry. Indeed, Ag and Au nanoparticles (NPs) are detected at the ppmv level. We demonstrate that the NPs shape distribution can be estimated from ellipsometric measurements by analyzing them with a shape distributed effective medium theory (SDEMT). The obtained distributions from ellipsometry are in agreement with those deduced from transmission electron microcopy (TEM). Contrary to TEM, ellipsometry probes a large number of NPs estimated at about 10¹¹ NPs. Finally, we show that the complex refractive index of colloids as determined from ellipsometry is sensitive to the optical properties of the solvent and the plasmonic properties of NPs

Local Structure-Driven Localized Surface Plasmon Absorption and Enhanced Photoluminescence in ZnO-Au Thin Films

Nanocomposite films consisting of gold nanoparticles embedded in zinc oxide (ZnO-Au) have been synthesized with different gold loadings by reactive magnetron sputtering at near-room temperature followed by ex situ annealing in air up to 300 °C. Using X-ray diffraction and high resolution transmission microscopy it is shown that during deposition gold substitutes zinc in ZnO as isolated atoms and in nanoparticles still exhibiting the structure of ZnO. Both situations degrade the crystalline quality of the ZnO matrix, but thermal annealing cures it from isolated gold atoms and triggers the formation of gold nanoparticles of size higher than 3 nm, sufficient to observe a strong activation of localized surface plasmon resonance (LSPR). The amplitude of LSPR absorption observed after annealing increases with the gold loading and annealing temperature. Moreover, UV and visible photoluminescence from the ZnO matrix is strongly enhanced upon activation of LSPR showing strong coupling with the gold nanoparticles. Finally, modeling of spectroscopic ellipsometry measurements unambiguously reveals how curing the defects increases the optical bandgap of the ZnO matrix and modifies the optical dielectric functions of the nanocomposite and ZnO matrix

Gold Nanoparticle Chains: Synthesis, Characterization, and Modeling Using Spectroscopic Ellipsometry

In this paper, we explore the ability of ellipsometry to characterize colloidal suspensions composed of gold nanoparticle (NP) chains. The complex effective index of these suspensions is deduced from ellipsometric measurement by using a wavelength-by-wavelength inversion without any dispersion law. We show that the effective refractive index of these colloids is defined by the nature of the solvent, whereas their effective extinction coefficient is mainly sensitive to the plasmonic properties of NP chains. The influence of the NP radius distribution and arrangement on the effective extinction coefficient of NP chain are investigated through simulations based on the coupled point dipole method (CDM). We clearly show that this coefficient is mainly sensitive to the interparticle distance and the number of NPs in the longest segment of chains. We demonstrate that the distribution of the number of NPs in the longest segment of chains and their volume fractions can be directly deduced from the ellipsometry by using the CDM

Chiral Perovskite Nanocrystal Growth inside Helical Hollow Silica Nanoribbons

Helical perovskite nanocrystals (H-PNCs) were prepared using nanometric silica helical ribbons as platforms for the in situ growth of the crystals using the supersaturated recrystallization method. The H-PNCs grow inside nanometric helical porous silica, and their handedness is determined by the handedness of porous silica templates. They show both strong induced circular dichroism (CD) and strong induced circularly polarized luminescence (CPL) signals, with high dissymmetry g-factors. Right-handed and left-handed PNCs show respectively positive and negative CD and CPL signals, with a dissymmetry g-factor (abs and lum) of ∼±2 × 10–2. Simulations based on the boundary element method demonstrate that the circular dichroism originates from the chiral shape of H-PNCs