Refractive index of transparent nanoparticle films measured by surface plasmon microscopy

Abstract

Nanometer thin films of latex spheres or ferritin macromolecules are deposited on silver substrate and their structure is studied by means of the surface plasmon resonance method. A homogeneous particle layer is spread in a circular paraffin cell tightly attached to a silver film on glass. Appropriate drying of the suspension in the presence of surfactant creates a monolayer and multilayer of ordered nanoparticles. The layers modulate the surface plasmon and, hence, the deep and narrow minimum in film reflectivity. Specially designed experimental setup views the illuminated film area by an optical microscope and measures the layer reflectivity as a function of the incident angle. The brightness of image, obtained at the angle of minimum reflectivity, depends on the thickness of particle layer. The reflectivity data are fitted regarding plane parallel layer of a complex refractive index and using theoretical equations that separate the real and imaginary parts. The calculated layer thickness and the real part of refractive index are in a reasonable agreement with those known for similar systems. The imaginary part of the refractive index depends on the structural defects of the nanoparticle layers.