Impact of high sensitizer doping on the transient multiband upconversion luminescence in β-NaYF4:Yb/Ho microcrystals

Abstract

Under intense near-infrared light excitation, upconversion luminescence (UCL) nanomaterials exhibit multiband emission and high sensitizer quenching concentration, indicating the potential to enhance upconversion (UC) quantum efficiency. In comparison to other common activator ions (e.g. Er3+, Tm3+, Ho3+), Ho3+ possesses abundant energy levels. However, previous studies mostly focused on the red (Ho3+:5F5 → 5I8) and green (Ho3+:5F4,5S2 → 5I8) emissions. In this paper, we have examined the transient UCL characteristics of β-NaYF4:Yb/Ho microcrystals (MCs) excited by a high-power 976 nm ns pulsed laser (∼0.32 GW/cm2). The energy transfer (ET) pathways with varying Yb3+ doping (20–98 mol%) are determined by the time evolution spectra. In high Yb3+ doping system (98 mol%), the electron population of Ho3+ accelerates, and the multiband UC rise is shortened by 6 times (from ∼3 μs to ∼500 ns). We identified a new pathway for electron populating with higher Yb3+ doping ( ≥ 40 mol%), which is Yb3+:2F5/2 + Ho3+:3L9 → Yb3+:2F7/2 + Ho3+:3M9. As Yb3+ concentration increases, the varying lifetime and intensity distribution of UCL leads to variations in the luminescence color evolution. Our result shows that altering the Yb3+ doping concentration provides a way to control the energy flow between energy levels of Ho3+, which can modulate the rise time and color evolution of Yb–Ho co-doped UC MCs. This study is expected to promote the advancement of fast-respond optoelectronic devices, UC displays, and micro-nano lasers. Kewordsmultiband upconversion luminescence; high-power excitation; upconversion luminescence dynamics; high sensitizer concentration; upconversion pathways.