Internal OH\u2212 induced cascade quenching of upconversion luminescence in NaYF4:Yb/Er nanocrystals

Yansong Feng,Zhi Li,Hong Zhang,Qiqing Li,Jun Yuan,Langping Tu,Lixin Ning

doi:10.1038/s41377-021-00550-5

Abstract

Internal hydroxyl impurity is known as one of the main detrimental factors affecting the upconversion (UC) efficiency of upconversion luminescence (UCL) nanomaterials. Different from surface/ligand-related emission quenching which can be effectively diminished by, e.g., core/shell structure, internal hydroxyl is easy to be introduced in synthesis but difficult to be quantified and controlled. Therefore, it becomes an obstacle to fully understand the relevant UC mechanism and improve UC efficiency of nanomaterials. Here we report a progress in quantifying and large-range adjustment of the internal hydroxyl impurity in NaYF4 nanocrystals. By combining the spectroscopy study and model simulation, we have quantitatively unraveled the microscopic interactions underlying UCL quenching between internal hydroxyl and the sensitizers and activators, respectively. Furthermore, the internal hydroxyl-involved UC dynamical process is interpreted with a vivid concept of “Survivor effect,” i.e., the shorter the migration path of an excited state, the larger the possibility of its surviving from hydroxyl-induced quenching. Apart from the consistent experimental results, this concept can be further evidenced by Monte Carlo simulation, which monitors the variation of energy migration step distribution before and after the hydroxyl introduction. The new quantitative insights shall promote the construction of highly efficient UC materials.

Highlights

The great application prospect in biology, medicine, optogenetics, photovoltaics, and sustainability has enabled lanthanide (Ln) ion-doped upconversion nanoparticles (UCNPs) to attract widespread attention, which derives mainly from their superior anti-Stokes spectroscopic property[1,2,3,4,5,6,7,8,9,10,11,12,13]
According to the simulation results, we proposed a so-called “survivor effect” to intuitively explain the internal OH− effects on UC, i.e., as shown in Scheme 1, the excited states involved in UC are the survivors of energy migration processes
As OH−-free UCNPs are the premise of the quantitative analysis of internal OH−, we have modified the reported protocol[40] and have developed a convenient procedure to synthesize the ultra-small OH−

Summary

Introduction

The great application prospect in biology, medicine, optogenetics, photovoltaics, and sustainability has enabled lanthanide (Ln) ion-doped upconversion nanoparticles (UCNPs) to attract widespread attention, which derives mainly from their superior anti-Stokes spectroscopic property[1,2,3,4,5,6,7,8,9,10,11,12,13]. Despite a significant progress in synthesis chemistry of UCNPs, the relatively low upconversion (UC) efficiency, especially under restricted excitation power density, e.g., that allowed in clinics, remains a major bottleneck on their way of actual applications[14,15,16,17,18]. Nanomaterials are much more vulnerable to charged impurities, among which OH− is the most critical, as it may be readily brought into the nanocrystals during synthesis and passivation may increase the UC efficiency by 10–1000 times[32,33]. The quenching mechanism of the surface OH− has been well documented either from the comparison of bare core

Methods

Results

Conclusion