Up-Converting Lanthanide Ions Doped Fluoride Nanophosphors: Advances from Synthesis to Applications

Abstract

The up-converting nanophosphors are the nano-crystalline materials that produce luminescence by converting low-energy radiation (e.g., infrared or near-infrared) into high energy radiation (ultraviolet and visible) via an anti-Stokes shift process. These can be prepared by incorporating up-converting luminescent centers such as lanthanide ions (Ln3+ ions, where Ln = Nd, Ho, Er, and Tm) and/or transition metal ions (e.g., Mn2+, Ti2+, Ni2+, Mo3+, Re4+, and Os4+) into a suitable nano-crystalline host material. The choice of dopant–host combination is decisive in determining the luminescence characteristics of a nanophosphor. Inorganic fluoride hosts such as ALnF4 and LnF3 (where A and Ln refer to alkali metal ions and lanthanide ions, respectively) and BF2 (B stands for Ca and Sr) are found suitable for up-converting Ln3+ ions as they exhibit; (i) low phonon energy, (ii) promising doping conditions, (iii) favorable electronic structure, and (iv) excellent chemical, thermal, and photo-stability. Ln3+ ions doped fluoride nanophosphors (where Ln = Ho3+, Er3+, and Tm3+) exhibit unique up-conversion luminescence characteristics such as strong emission in visible window, substantial anti-Stokes shifts (>600 nm), prolonged luminescence lifetimes (up to several milliseconds), and sensitization under infrared (IR) or near-infrared (NIR) irradiation. Due to these fascinating luminescence properties, they find potential applications in bio-imaging, drug delivery, tumor targeting, solid-state lighting, energy harvesting. Interestingly, their luminescence characteristics can be altered by varying the doping concentration, dopant–host combination, morphology, crystal structure, and the functional group present over the surface of the nanoparticles. This has given prime focus toward developing novel synthesis techniques that are promising in preparing the fluoride-based nanophosphors of desired morphological, compositional, structural, and optical characteristics. The solution-based synthesis methods or wet chemical methods are very promising in producing nanophosphors of controlled size–shape, phase, and chemical composition. Several solution-based methods were developed in past for the preparation of fluoride-based nanocrystals, with each of them having some merits and demerits. Therefore, a prior understanding of each method is essential before adapting it for material preparation. In this regard, this chapter provides a brief description about a few of solution-based synthesis methods such as hydrothermal, co-precipitation, and thermolysis, which are most versatile in the controlled preparation of a variety of Ln3+-doped fluoride nanophosphors (e.g., ALnF4, BF2, and LnF3, where A, B, and Ln stand for alkali metal, alkaline earth metal, and lanthanide ions, respectively).The critical role of reaction parameters such as pH of the reaction medium, precursor amount, ligands, reaction temperature and time on the controlled preparation of Ln3+-doped fluoride nanophosphors will be discussed in light of available literature. Further, the impact of morphological, structural, and compositional characteristics of the nanophosphors on their luminescence behavior will be discussed in regard to their biomedical, energy harvesting, and lighting applications.