Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF₄:Yb³⁺,Er³⁺

Abstract

Here, we show that the long-accepted mechanism for the production of red and blue emission through upconversion (UC) of 1 μm excitation in Yb(3+)/Er(3+)-doped materials does not apply in the popular β-NaYF4 host. We propose a new mechanism involving Yb(3+)-to-Er(3+) energy-transfer UC out of the green-emitting (2)H11/2,(4)S3/2 states that quantitatively accounts for all of the observed optical behavior. Rate constants for the relevant radiative and nonradiative processes are reported along with a prediction of the power dependence of the pulsed and continuous-wave UC quantum efficiency.

Quenching Pathways in NaYF4:Er3+,Yb3+ Upconversion Nanocrystals

“…Both the green and the red UC luminescence can arise after two energy-transfer steps followed by partial nonradiative relaxation (indicated in Figure 1b), although additional pathways involving three energy-transfer steps have been identified leading to red luminescence. 35−37 The energy-transfer processes relevant to UC compete with undesired processes such as (multi)phonon relaxation and cross-relaxation that reduce the UC emission. The spectral characteristics of nanocrystalline β-NaYF 4 phosphors with diameters down to sub-10 nm (refs (20 and 50)) are similar to those of bulk β-NaYF 4 , but the emission efficiencies are significantly lower.…”

“…For example, some studies have proposed complex population pathways for the red-emitting level, involving multiple multiphonon and/or energy-transfer processes. 35−37 Moreover, at high excitation powers the state populations can saturate, and levels higher in energy than the green- or red-emitting ones become involved in the energy-transfer and cross-relaxation pathways. 15,50 It is also important to consider that cross-relaxation and nonradiative recombination steps may not necessarily lead to irreversible energy loss, but the energy may be used in a next energy-transfer UC step.…”

Lanthanide-Doped KLu2F7 Nanoparticles with High Upconversion Luminescence Performance: A Comparative Study by Judd-Ofelt Analysis and Energy Transfer Mechanistic Investigation

“…Recently, an EBT process involving Er 3+ 2 H 11/2 / 4 S 3/2 and 4 I 13/2 manifolds was proposed and proved to account for the greatly enhanced red emission262837. A new UC mechanism involving the population of 4 F 9/2 manifold through an EBT process from high-lying level 4 G 11/2 was proposed3839. From our previous study40, the population of Er 3+ red-emitting manifold should not be tailored mainly by cross-relaxation or multiphonon processes in Yb 3+ -Er 3+ system.…”

Enhanced 1.53μm band fluorescence in Er3+/Ce3+ codoped tellurite glasses containing Ag NPs

“…However, the relatively low phonon energy ($750 cm À1 ) for tellurite glass hosts makes the multiphonon relaxation process from the pumping level 4 I 11/2 to the fluorescence level 4 I 13/2 of Er 3+ too slow, and as a result strong visible up-conversion emission originated from the excited state absorption (ESA) of Er 3+ at the 4 I 11/2 level can be observed in the Er 3+ -doped tellurite glasses [11][12][13]. Therefore, in order to increase the 980 nm pumping efficiency and in turn enhance the 1.53 lm fluorescence emission, it is critical to increase the relaxation rate of 4 I 11/2 ?…”