Luminescent properties of ZnS:Mn²⁺ nanocrystals/SiO₂ hybrid phosphor synthesized by in situ surface modification co-precipitation

Colloidal solution of ZnS:Mn²⁺ nanocrystals (2-5 nm in diameter) coated with silica was prepared by in situ surface modification co-precipitation in the presence of a surface modifier of 3-mercaptopropyl trimethoxysilane (MPS) and a dispersion stabilizer of sodium citrate, followed by addition of sodium silicate. The change in the optical absorption due to the interband transition of ZnS was measured during irradiation by a Xe lamp. No appreciable change in the optical absorption by irradiation was observed for the colloidal solution prepared in the presence of MPS and sodium citrate, while the absorbance decreased for the nanocrystals modified by either MPS or sodium citrate. This indicates that the photo-dissolution reaction of ZnS:Mn²⁺ is suppressed in the former by perfect surface modification. Formation of SiO₂ layer around ZnS:Mn²⁺ nanocrystals induced quantum confinement effect and the passivation of surface defects to enhance the orange photoluminescence (PL) due to the d-d transition of Mn ²⁺ ions. In addition, the broad cathodoluminescence (CL) spectrum was observed around 800 nm for ZnS:Mn²⁺ nanocrystals with SiO ₂ modification by electron bombardment at 10 kV. The difference in the PL and CL spectra might be attributed to the emission of isolated Mn ²⁺ ions for PL and the emission of the locally concentrated Mn ²⁺ ions for CL, although the CL might be the emission due to defects or impurities in the SiO₂ layer.