Photoluminescence enhancement of ZnS:Mn2+ nanocrystal phosphors:: Comparison of organic and inorganic surface modifications

ZnS:Mn2+ nanocrystals of 3 nm in diameter were coated with silica to form 20 +/- 4 nm core/shell particles, as observed by transmission electron spectroscopy. For ZnS:Mn2+ nanocrystals modified with silica, the intensity of photoluminescence (PL) due to the d-d transition of Mn2+ increases with increasing tetraethyl orthosilicate (TEOS) amount and hence with increasing the thickness of silica shell. This is attributed to the passivation of surface defects and the quantum confinement effect. On the contrary, the PL intensity of the ZnS:Mn2+ nanocrystals modified by both lauryl phosphate (HLP) and silica decreases with increasing TEOS amount. (31)p CP/MAS NMR spectroscopy reveals that the chemical interaction between phosphate groups and ZnS:Mn2+ is interrupted by the silica shell. These results suggest that the surface modification of HLP around the ZnS:Mn2+ nanocrystals is more effective than the formation of the silica shell from the viewpoint of PL enhancement.