Optical characterizations of multi-shape nanoparticles in polymer thin film layers

M. Carlberg, F. Pourcin, O. Margeat, J. Le Rouzo, G. Berginc, R.-M. Sauvage, J. Ackermann, L. Escoubas
Aix Marseille University,

Keywords: plasmonics, metallic nanoparticles, PVP, perfect absorber, spectroscopic ellipsometry, FDTD


Metallic nanoparticles are currently a hot topic thanks to their tuneable optical properties. Indeed, their light absorption or scattering is material, size and shape, but also environment dependent. We aim to take advantage of these optical properties in order to realize a perfect absorber in the visible wavelength range for military applications as stealth technologies and photodetectors, and for solar energy harvesting as well in photovoltaic cells as in thermal solar cells. We synthetize our nanoparticles by chemical wet synthesis and deposit them in a transparent polymer host matrix. Optical characterizations are coupled with numerical calculations to increase the understanding of the optical phenomena occurring. In order to obtain absorbing nanoparticles in the visible wavelength range, the nanoparticle material chosen is silver. Silver also yields high electric field enhancement, allowing us to work with lower nanoparticle densities. To cover the whole visible spectrum, our hypothesis is to mix differently sized and shaped nanoparticles. Our main concern is to maximize the absorption, therefore the size of the nanoparticles is kept smaller than 100 nm. The nanoparticles are produced by chemical wet synthesis in mild conditions, making the process easily up scalable. Nanospheres are produced in a simple silver salt reduction. Nanoprisms and nanocubes are obtained in a two-step seed based process. The colloidal solutions absorb from 350 nm up to 1200 nm. The different nanoparticles are deposited in a transparent and non-absorbing polymer thin film layer in different configurations. In a first step thin film layers containing a single kind of nanoparticles are studied. Then the colloidal solutions of nanoparticles are mixed together and finally multi-layers are analyzed. The optical characterizations done are spectrophotometer measurements, i.e. total and diffuse reflectance and transmittance, of the colloidal solutions and the thin film layers, and spectroscopic ellipsometry, in order to obtain the complex optical indices n and k of the heterogeneous thin film layers. The mathematical models we apply to fit the spectroscopic ellipsometry measurements consist of a Cauchy law for non-absorbing polymer layers and one or more Lorentz laws adding the absorbing property of the different nanoparticles. Depending on the nanoparticle shape and size, the Lorentz laws are centered at different energies. The optical characterizations are completed by FDTD simulations of single particles, particles in lattice and in random arrangements. For example, experimental measured broadening of absorption peaks can be explained by calculating the optical properties of a nanosphere-nanoprism aggregate.