Abstract

Upconversion nanoparticles (UCNPs)/nanomaterials have the unique property to absorb near-infrared (NIR) light which leads to an anti-Stokes emission from the ultraviolet (UV) over the visible to the NIR range (Zhang 2014). The NIR light is absorbed by sensitizer ions (e.g., Yb³⁺ and Nd³⁺), and then transferred to so-called activator ions (e.g., Tm³⁺, Er³⁺, and Ho³⁺) via a nonradiative, resonant energy transfer process. The lanthanide ions are preferably embedded in host lattices with low phonon energy and similar ionic radii to reduce nonradiative deactivation processes (Haase and Schäfer 2011; Suyver et al. 2006; Wang and Liu 2009). Fluorides combine low phonon energies (~300–400 cm⁻¹) with high chemical stability compared to other halide ions or oxide materials. The doping ratio of the sensitizer and activator ions is very critical, in order to generate highly fluorescent nanomaterials and to avoid self-quenching of the excited states. A NaYF₄ hexagonal host lattice doped with Yb³⁺ and Er³⁺ ions and a simplified overview of the energy transfers inside the crystal is depicted in Figure 4.1. Figure 4.1