LUMINESCENT NANOPARTICLES: COLLOIDAL SYNTHESIS AND EMISSION PROPERTIES

摘要

This paper has described the different strategies that can be used for the colloidal synthesis of luminescent nanoparticles. The first studies concerned the case of Ⅱ-Ⅵ semiconductors, in which the luminescence is drastically affected by the surface trapping of excited carriers. Two strategies can be used to avoid surface trapping. The first consists of the elaboration of core/shell nanostructures by epitaxial growth of another semiconductor on the surface of the particles. This allows a limitation of the extension of the carrier's wavefunction at the surface of the particles and leads to a tremendous improvement of the emission. Another possibility consists of doping the nanoparticles with a luminescent ion (CdS:Mn). Rapid transfer of the energy of the excited carriers to the localized electrons of the luminescent ion avoids non radiative relaxation at the surface of the particles. High luminescent yields are also obtained, with an emission spectrum characteristic of the doping ion. The same strategy of using localized excited states is in fact commonly used for the elaboration of highly luminescent phosphors such as rare earth doped oxides. Recent investigations performed on YVO_4:Eu and LaPO_4:Ce,Tb show that nanoparticles exhibit high emission yields, which are nevertheless affected to some extent by the surface quenching species.