Brewster-Angle Variable Polarization Spectroscopy of Colloidal Au-Nanospheres and -Nanorods at the Silicon Surface

Colloidal Au-nanospheres and -nanorods were chemically synthesized from chloroauric acid containing solutions and adsorbed to hydrogen-terminated p-Si(111) surfaces. A comparative analysis of the respective plasmon resonance modes, in solution and at the silicon surface, was carried out by in situ optical transmission and surface reflection techniques. In solution, the absorbance was determined by VIS transmission spectroscopy and compared to Mie theory as well as finite difference time domain calculations. At the p-Si(111) surface, the reflectance was analyzed for the first time by Brewster-angle variable polarization spectroscopy (BA-VPS) and discussed in terms of anisotropic uniaxial thin-film properties. With BA-VPS, the strengths of parallel and orthogonal electric field components of the incident wave can be varied relative to the surface plane. Consequently, opposite changes of transverse and longitudinal resonance strengths are detected upon gradually incremented light polarization angles. Model considerations confirm that spherical and elongated particles can thereby be distinguished. The influence of particle–surface interaction and the dielectric environment is finally discussed.