Stable aqueous dispersive colloidal Ag nanoparticles were prepared by reducing silver nitrate with sodium borohydride in the presence of 3-mercaptopropionic acid. The formation process of the Ag nanoparticles was investigated by UV-Visible spectroscopy and transmission electron microscopy. The results show that the spherical and rodlike particles and aggregates are formed in the initial stage of the reaction,then the rodlike particles and aggregates are gradually decomposed into small spherical particles,and the final obtained Ag nanoparticles with an average size of 8 nm are in uniform shapes and narrow size distribution,and the colloid remains stable for more than one month,which makes it convenient for use in practice. The presence of capping agent plays an extra role over nanoparticles stabilization and morphology. The presence of capping agent on the surface of Ag nanoparticle is confirmed by the X-ray photoelectron spectroscopy. It is found that Ag nanoparticles are negatively charged in alkaline condition,whereas they are positively charged in acid condition. Electrosteric effect is responsible for their long-term stability.
Isotropic negative permeability composite,composed of BST-MgO dielectric cubes with high permit-tivity dispersed in the Teflon substrate with low permittivity,was designed and fabricated based on Mie resonance and the effective medium theory.Measurements and simulations showed that the dielectric composite exhibited a strong sub-wavelength magnetic resonance at the first Mie resonance and possessed isotropic negative permeability,which resulted from the displacement current excited in the cubes.The dielectric particle was equivalent to a magnetic dipole at the magnetic resonance,which could be adjusted by the size and permittivity of the particles.It may provide a convenient method to design isotropic metamaterials and invisible cloak at infrared and visible frequencies.
