Influence of Sensitizer Concentration on Local Structural Disorder in NaYF₄:Yb³⁺/Er³⁺ Nanocrystals

Abstract

Lanthanide-doped upconverting materials have emerged as a highly promising class of materials in recent years, owing to their exceptional optical characteristics such as large anti-Stokes shifts, sharp line-type emissions, long emission lifetimes, outstanding photostability and efficient near-infrared excitation. These unique features enable efficient background-free photon upconversion and endow the materials with remarkable versatility across a broad spectrum of applications, including bioimaging, medical diagnostics, colour displays, optical thermometry and advanced photonic devices. NaYF₄ co-doped with Yb³⁺/Er³⁺ at an optimized composition of 79.5/20/0.5 is widely regarded as a benchmark upconversion system, as this ratio maximizes sensitizer-to-activator energy transfer while suppressing concentration quenching. Recent studies however highlight the critical role of defects and local disorder in enhancing upconversion efficiency by breaking local symmetry around rare-earth ions and partially relaxing parity-forbidden 4f–4f transitions. Alkali-metal-ion doping further amplifies these effects through lattice strain and crystal-field modulation. Since conventional diffraction techniques primarily probe long-range structural order, establishing a direct correlation between local structural disorder and optical performance becomes critically important. Here, we revisit NaYF₄:Yb³⁺/Er³⁺ nanocrystals by tuning sensitizer concentration to elucidate its impact on local structure and upconversion efficiency.