Concentration Quenching in Upconversion Nanocrystals

Abstract

Despite considerable effort to improve upconversion (UC) in lanthanide-doped nanocrystals (NCs), the maximum reported efficiencies remain below 10%. Recently, we reported on low Er3+- and Yb3+-doped NaYF4 NCs giving insight into fundamental processes involved in quenching for isolated ions. In practice, high dopant concentrations are required and there is a trend toward bright UC in highly doped NCs. Here, additional quenching processes due to energy transfer and migration add to a reduction in UC efficiency. However, a fundamental understanding on how concentration quenching affects the quantum efficiency is lacking. Here, we report a systematic investigation on concentration-dependent decay dynamics for Er3+ or Yb3+ doped at various concentrations (1–100%) in core and core–shell NaYF4 NCs. The qualitative and quantitative analyses of luminescence decay curves and emission spectra show strong concentration quenching for the green-emitting Er3+ 4S3/2 and NIR-emitting 4I11/2 levels, whereas concentration quenching for the red-emitting 4F9/2 level and the IR-emitting 4I13/2 level is limited. The NIR emission of Yb3+ remains efficient even at concentration as high as 60% Yb3+, especially in core–shell NCs. Finally, the role of solvent quenching was investigated and reveals a much stronger quenching in aqueous media that can be explained by the high-energy O–H vibrations. The present study uncovers a more complete picture of quenching processes in highly doped UC NCs and serves to identify methods to further optimize the efficiency by careful tuning of lanthanide concentrations and core–shell design.

On the decay time of upconversion luminescence

“…27 The sensitizer's excited-state lifetime (τ 5 ) was varied over a large range (0.1τ 3 -10τ 3 ) to cover as many values reported in the literature. 29,30 As shown in Fig. 2a, when the sensitizer's excited-state lifetime (τ 5 ) is significantly smaller than τ 3 , the decay behavior of UCL can be well represented by its natural decay characterized by τ 3 (Fig.…”

Upconversion luminescence and temperature sensing properties of NaGd(WO4)2:Yb3+/Er3+@SiO2 core–shell nanoparticles

“…35,40 In those cases, the nonradiative decay from Er 3+ 4 I 11/2 to 4 I 13/2 state is enhanced due to the O-H quenching as the energy gap between 4 I 11/2 and 4 I 13/2 states matches the O-H vibration energy (3200-2600 cm À1 ). 41,42 Then, there is a high probability that the second IR photon excites Er 3+ from the 4 I 13/2 state to the red emitting 4 F 9/2 state. In our case, the core-shell UCNPs with thermal treatment are limited inuenced by the O-H quenching.…”

Optical and Mechanical Properties of Highly Translucent Dental Zirconia

“…However, SMS2 with 1.0339 wt.% of Er did not exhibit fluorescence. The peak fluorescent intensity would vary depending on the amount of the pigment due to the concentration Quenching [58].…”