Upconversion Phenomena in Nanofluorides

Abstract Upconversion phenomena refer to mechanisms which produce population in an excited state whose energy exceeds that of the excitation photon. Emission at a lower wavelength than the excitation source is therefore possible and in the last century upconversion has been extensively studied in rare earth–doped fluoride-based materials. Then the development of nanoscience has increased the interest in the research of upconversion nanocrystals and especially in rare earth–doped nanofluorides. This chapter presents an overview of upconversion in nanofluoride-based crystals. Synthesis strategies developed to prepare rare earth–doped metal fluoride upconverting nanoparticles are described. Their upconversion properties and possible applications are presented in the case of simple nanoparticles as well as of more complex nanosystems with multishell structures and/or sophisticated surface functionalization.