Suppression of Defect-Induced Quenching via Chemical Potential Tuning: A Theoretical Solution for Enhancing Lanthanide Luminescence

Nonradiative decay occurring at lattice defect sites may constitute an essential pathway for luminescence quenching in lanthanide-doped upconversion nanomaterials. Considerable efforts have been dedicated to alleviating such quenching effects through controlled single-crystal growth and stringent chemical processing. However, it is not feasible to remove all lattice defects in the crystals. The fabrication of highly luminescent upconversion materials is thus impeded by an incomplete understanding of defect-induced quenching behavior. Here, we propose a theoretical solution for enhancing the luminescence efficiency through the deactivation of deleterious defect centers. To address the exact nature of defect-induced energy dissipation, we systematically study the electronic structure and the stability of five different types of intrinsic defects in cubic NaYF4 crystal through ab initio calculations. Based on the calculated single-particle energy levels and absorption coefficients, we identify optically responsive defect centers which can effectively harvest 980 nm excitation energy due to their larger absorption coefficients. Such active defect centers are more capable of trapping excitation energy than the lanthanides, thus significantly mitigating the process of photon upconversion. By tuning the position of the Fermi level within the range of 1.75-6.94 eV, the initially active defect centers can be deactivated, resulting in the formation of inert defects and the suppression of energy dissipation. These findings not only provide new insight into the underlying mechanism of defect-induced luminescence quenching in lanthanide-doped crystals but also offer a theoretical toolbox that enables rapid identification of defect-tolerable hosts in search of high-efficiency upconversion phosphors.