Energy structure and fluorescence ofEu2+in ZnS:Eu nanoparticles

${\mathrm{Eu}}^{2+}$-doped ZnS nanoparticles with an average size of around 3 nm were prepared, and an emission band around 530 nm was observed. By heating in air at 150 °C, this emission decreased, while the typical sharp line emission of ${\mathrm{Eu}}^{3+}$ increased. This suggests that the emission around 530 nm is from intraion transition of ${\mathrm{Eu}}^{2+}.$ In bulk ${\mathrm{Z}\mathrm{n}\mathrm{S}:\mathrm{E}\mathrm{u}}^{2+},$ no intraion transition of ${\mathrm{Eu}}^{2+}$ was observed because the excited states of ${\mathrm{Eu}}^{2+}$ are degenerate with the continuum of the ZnS conduction band. We show that the band gap in ${\mathrm{Z}\mathrm{n}\mathrm{S}:\mathrm{E}\mathrm{u}}^{2+}$ nanoparticles opens up due to quantum confinement, such that the conduction band of ZnS is higher than the first excited state of ${\mathrm{Eu}}^{2+},$ thus enabling the intraion transition of ${\mathrm{Eu}}^{2+}$ to occur.