Design of infrared-emitting rare earth doped nanoparticles and nanostructured composites

The unique atomic-like absorption and emission spectra of rare earth (RE) doped materials have shaped a unique niche of applications, especially those that benefit from a better utilization of the infrared (IR) electromagnetic range. These materials in the bulk form have mostly been utilized in their earliest development for illuminators, lasers and displays. In the past decade, new frontiers, especially in the field of biomedical imaging and theranostics, have emerged as a result of the advancements in chemical methods to synthesize nano-sized RE doped materials. The distinctive characteristics of RE doped nanoparticles (RENPs) to absorb IR photons and emit at any specific desired wavelength spanning from the ultraviolet (UV) to the IR region have unlocked an avenue to a myriad of applications. This review article covers briefly some aspects on the fundamental principles that govern the luminescence characteristics of these RE doped materials, such as the mechanisms for electronic transitions, dopants and host chemistries. Next, we will present an overview on the state-of-the-art methods to synthesize RENPs with controlled size, morphology, core-shell heterostructure and surface chemistry. The underlying relationships of the physical characteristics of RENPs, such as particle size and structure, with the optical emission properties will also be discussed so as to offer some material guidelines for designing brightly IR-emitting RENPs. The applications of RENPs are next broadly grouped according to the material form where they are required for some applications: (1) single particle systems, and (2) monolithic nanostructured optically active polymeric composites. This review will provide some guidelines for the engineering design of RENPs and their composites to meet the optical material performance requirements of various selected applications.