Size dependence of refractive index of gold nanoparticles

LUCIA B. SCAFFARDI; JORGE O. TOCHO

Margarita Island, Venezuela

Congreso; 5th Iberoamerican Meeting on Optics and 8th Latin American Meeting on Optics, Lasers and Their Applications; 2004

SPIE, OSA

In order to study the influence of particles in the nature or to control industrial processes it is essential to develop methods to measure the size. The optical methods are often preferred because they are not-destructive nor invasive, of low cost and the analysis can be very fast. The spectral method of extinction is one of the most used because it covers an extended interval of sizes and it requires a simple optical disposition, as it is the slight adaptation of a spectrophotometer. This work shows how to determine the size of gold particles with radius between 0.5 nm and 5 nm from the measurement of the optical extinction made with a commercial spectrophotometer. The experimental results are compared with calculations made from Mie´s theory for spherical particles, counting on the knowledge of the refractive index in terms of the wavelength, of particles and the solvent. In order to process the data, the optical constants of bulk must be modified to consider the limitation that introduces the size of nanoparticles. A form is discussed to consider the separated contribution of free electrons and bound electrons on the optical properties and their variation with the size. Key words: sizing, Mie´s scattering, spectral extinction, gold nanoparticles, dielectric function, free and bound electrons.