Nanophosphors: From rare earth activated multicolor-tuning to new efficient white light sources

The huge and impossible to list number of publications dealing with luminescent materials demonstrates widespread interest at both fundamental and technological level in their spectroscopic properties and applications. Moreover consequences, opportunities and challenges of spatial confinement have prompted worldwide interest in nanoscale-sized luminescent materials, termed nanophosphors (n-PSPs) as a valid alternative to fluorophores and semiconductor quantum dots. In this contribution the main classes of luminescent materials as well as spectroscopy of rare earth (RE)-activated phosphors (PSP) are first overviewed with focus on REs’ transitions, luminescence mechanisms, electron-lattice dynamics, thermal line broadening and line shift. Then, the corresponding scenario for n-PSPs is presented and discussed, by providing many examples, with focus on the effects of spatial confinement and surface-to-volume ratio on luminescence efficiency, lifetime of the excited states, electron-phonon dynamics, luminescence quenching, confinement of dopants and tuning of multicolor emission. In particular, RE-activated multicolor emission is intensively investigated to produce white light for indoor and outdoor lighting. In this respect, recent reports are mentioned about a new class of efficient broadband white light emitting n-PSPs consisting of nominally un-doped RE-free oxide nanopowders and TM-doped hosts with temperature-independent emission. The novelty of this approach lies in presenting a route alternative to the presently investigated strategies to obtain WL sources that mainly include RE dopants and single-phase compounds containing REs as stoichiometric components.