Dielectric properties of silver nanoparticles coated with silica shells of different thicknesses
Core/shell nanoparticles having metallic silver nanoparticle cores of similar to 38 nm in diameter and silica shells of different thicknesses ranging from similar to 3.6-20 nm were prepared. For the silica coating, a slightly modified Stober method was used which allowed preparing grams of core/shell nanoparticles for the first time. The particles were characterized by UV-vis spectroscopy, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray scattering. Their dielectric properties were measured as pellets in parallel-plate capacitors. It was found that the permittivity is much influenced by the silica shell thickness with an increase in permittivity for thinner shells. A shell thickness of 20 +/- 2 nm allowed fabrication of capacitors which have similar characteristics to those of silica, thus, there is no influence of the metal core on the dielectric properties anymore. However, by decreasing the silica shell to 17 +/- 2, 8 +/- 1.5, and 6.6 +/- 1.5 nm the permittivity at high frequencies is increasing from 10, 34, to 41, respectively. The insulator to metal transition was observed for a silica shell thickness of 3.6 +/- 1 nm. Functionalization of the silica surface with a hydrophobic coating removes surface adsorbed water as observed by the flat dielectric permittivity over a large frequency domain.