Optical study of Langmuir-Schaeffer films of gold colloid nanoparticles

Thin films of 5 nm gold colloid nanoparticles were deposited onto solid substrates (silicon, glass, gold coated glass) by the Langmuir-Schaeffer (LS) method at a surface pressure of 10 mN/m. The films produced were characterised with optical techniques of UV-visible absorption spectroscopy, surface plasmon resonance (SPR), and spectroscopic ellipsometry. Film thickness was found to increase linearly with the increase in the number of layers, yielding the values of 2.5 nm and 5.2 nm per deposition stroke for LB and LS films, respectively. Gold colloid films produce SPR curves broadened and shifted to the larger angles in respect to SPR of evaporated gold, which may be related to the effect of quantum confinement of electrons in gold colloid particles. Fitting of both SPR and ellipsometry data yields the value of 7 nm for the Au particles size, which is close to the values obtained by AFM. We also report on the reduction in the effective thickness of evaporated gold films after depositing gold colloid on top.