NIR-to-visible up-conversion nanomaterials have been investigated in many promising applications including nextgeneration displays, solar cells, and biological labels. When doped with different trivalent lanthanide ions, NaYF4 nanoparticles can produce up-converted emission from visible to infra-red wavelengths. However, the quantum yield of this class of materials is low. Noble metals in the vicinity of the phosphor can increase the phosphorescence by local field enhancement due to plasmonic resonances, and by modification of the radiative rate of the phosphor. Most previous studies have investigated the phenomenon by placing nanophosphors onto a metal substrate, or by fabrication of nano structures with spacers such as polymers, dielectric materials (silica). By contrast, we have studied the interaction between the luminescence and the surface plasmon using a core-shell type nanostructure where a uniform shell of silver is shown to grown on doped-NaYF4 nanophosphors by Ostwald ripening. We further demonstrate the proximity effect of metal-enhanced luminescence by exciting an undoped NaYF4 shell. The result shows a significant synergistic enhancement of up-conversion luminescence due to the active shell as spacer layer. In addition, we have shown this novel nanostructure may be useful in surface-enhanced Raman spectroscopy (SERS).
Some lanthanide-doped nanoparticles can absorb X-ray radiation and emit in the visible to near infrared spectrum via a down-conversion mechanism, making them a potentially valuable agent for in vivo imaging studies. We have studied a series of Gd<sup>3+</sup> and Eu<sup>3+</sup>compositions in lanthanide fluorides to optimize the emission from Eu<sup>3+</sup> upon X-ray excitation. The optimum concentration of Eu<sup>3+</sup> that produced the most intense emission in NaGdF<sub>4</sub> was found to be 15% molar concentration. The impact of the crystallographic phases (i.e. cubic or hexagonal) on the optical emission was investigated. Furthermore, an attempt to include a sensitizer (i.e Ce<sup>3+</sup>) in NaGdF<sub>4</sub>:Eu resulted in a reduction in the emission following X-ray excitation. A surface coating of NaGdF<sub>4</sub>:Eu nanoparticles with a gold shell showed a similar decrease in luminescence intensity by a factor of two although the gold shell offers other advantages in biomedical applications.