Optical and structural properties of doped ZnS nanoparticles produced by the sol-gel method

Optical and structural properties of Mn2+-doped ZnS nanoparticles in an org anic matrix are experimentally and theoretically studied. The nanoparticles , which were produced by the sol-gel method, are nearly monodisperse with a diameter of approximately 3 nm and show the characteristic orange-red lumi nescence of Mn2+ centers in a crystalline ZnS matrix. The absorption spectr um of the embedded ZnS nanoparticles is slightly blue shifted and broadened compared to the reference system containing ZnS microparticles. This blue shift is caused by quantum size effects, whereas the broadening is due to d efects such as lattice distortions, and vacancies, which are probably locat ed close to the surface in the case of small particles. With increasing tem perature the absorption spectra shift to the red and are broadened due to t hermal activated diffusion of ions close to the surface. In contrast, the s pectral feature of the emission spectra via the Mn2+ center is nearly uncha nged compared to the ZnS microparticles. Furthermore, the quantum efficienc y is increased and the decay time of the electron-hole pairs is shortened t o the nanosecond regime because of the enhanced probability of the electron -hole pairs to "see" the Mn2+ center. Therefore, the only effect of doping of ZnS nanoparticles with Mn2+ center is the suppression of the relaxation of electron-hole pairs via surface defects generating a highly efficient an d fast relaxation of the electron-hole pairs via the Mn2+ center.