Nanocrystal formation and luminescence properties of ZnS based doped nanophosphors

There is an ongoing interest on the photoluminescence (PL) and related properties of doped nano ZnS since there is a large change in their PL properties with particle size reduction. Such PL properties are theoretically predicted to be much enhanced with ZnS particle sizes below the Bohr radius, i.e., in ZnS for particle sizes less than 5 nm or so. We start this discussion by first suggesting some nanoparticle formation mechanisms. The type of capping mechanism used dictates the temporal PL stability under different ambient conditions and other post-processing device requirements. We then describe results on PL of our different doped nanocrystalline ZnS and compare our interpretation of results with those available in literature. We have used 3-D plots featuring the effect of different excitation wavelengths on the PL emission profile and its intensity variation on nanocrystalline ZnS doped with different ions. Reported results and our own data suggests that the nanoparticle formation leads to drastic change in PL emission peaks-such that they are often greatly different from those of similar bulk samples. These features are seen in cases where PL is due to electron recombination from dopant and codopant sites, e.g., during Cu and Ag doping. Such results are explained due to greatly modified band structure with nanoparticle formation and bandgap enhancement where dopant and codopant levels have also greatly changed. However, in nanocrystalline samples where the PL mechanism is due to intraion transitions of dopants (e.g., in Mn doping), the luminescence properties remain similar to those of the bulk--this is because as long as the host material and its related crystal field is the same, such electron transition levels remain similar.