Direct Evidence for Coupled Surface and Concentration Quenching Dynamics in Lanthanide-Doped Nanocrystals

Abstract

Luminescence quenching at high dopant concentrations generally limits the dopant concentration to less than 1-5 mol% in lanthanide-doped materials, and this remains a major obstacle in designing materials with enhanced efficiency/brightness. In this work, we provide direct evidence that the major quenching process at high dopant concentrations is the energy migration to the surface (i.e., surface quenching) as opposed to the common misconception of cross-relaxation between dopant ions. We show that after an inert epitaxial shell growth, erbium (Er) concentrations as high as 100 mol% in NaY(Er)F/NaLuF core/shell nanocrystals enhance the emission intensity of both upconversion and downshifted luminescence across different excitation wavelengths (980, 800, and 658 nm), with negligible concentration quenching effects. Our results highlight the strong coupling of concentration and surface quenching effects in colloidal lanthanide-doped nanocrystals, and that inert epitaxial shell growth can overcome concentration quenching. These fundamental insights into the photophysical processes in heavily doped nanocrystals will give rise to enhanced properties not previously thought possible with compositions optimized in bulk.

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

“…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. 10,11,19−21,30 In this work we systematically study the energy-loss pathways in UC NCs (Figure 1c, Figure S1 and Table S1) that lead to these lower efficiencies, as a function of the dopant concentration, for core-only and core–shell geometries and for NCs dispersed in a range of solvents.…”

Simultaneous Enhancement of Photoluminescence, MRI Relaxivity, and CT Contrast by Tuning the Interfacial Layer of Lanthanide Heteroepitaxial Nanoparticles

“…This increase is attributed to the improved shielding of Yb 3+ and Er 3+ luminescent centers from the surface by thicker NaLuF 4 layers. 57 Second, for the thicker NaLuF 4 layer the MRI relaxivity was enhanced by the β -Yb/Er@Lu CS structures acting as the supporting substrates for outer NaGdF 4 layers. Increasing the interfacial NaLuF 4 layer thickness slowed the NP tumbling, 52,64 thus enhancing the MRI relaxivity from 36.6 mM −1 s −1 to 51.7 mM −1 s −1 (Figure 5g and Figure S17).…”

“…56,57 Kinetically stable α -NaLuF 4 and α -NaGdF 4 NPs were synthesized and used as sacrificial NPs to grow shells on the β -NaYb/ErF 4 core NPs. The α -NaLuF 4 NPs had an average diameter of 8.27 nm (Figure S1), while α -NaGdF 4 NPs were 6.18 nm (Figure S2).…”

“…In stark contrast to the β -Yb/Er@Lu@Gd CSS with circularity close to 1, the circularity of β -Yb/Er@Gd CS decreased to 0.57 when approximately 9 mmol NaGdF 4 was deposited on the β -Yb/Er cores resulting in a nonuniform shell (Figure S20). Without the uniform and quasi-spherical CS architecture, the optically active Yb 3+ /Er 3+ luminescent centers were more susceptible to surface quenching because of the shortened distance between them and the quenchers around the NPs; 57 therefore, the PL emission intensity of β -Yb/Er@Gd CS was less than 10% of that of β -Yb/Er@Lu@Gd CSS-NPs with the same absorbance at 980 nm and excitation at the same power density (Figure 6d). While the thick NaGdF 4 shell deformed the CS structures and resulted in quenching of PL, it on the other hand reintroduced the limitation on MRI by preventing most of the Gd 3+ paramagnetic centers from accessing surrounding water protons.…”

Synthesis of Small Ce3+-Er3+-Yb3+ Tri-Doped BaLuF5 Active-Core-Active-Shell-Active-Shell Nanoparticles with Strong Down Conversion Luminescence at 1.5 μm

“…This was due to the Yb 3+ ions in the shell could transfer energy from the pump source to the core and make a contribution to the DC emissions [31]. When the Yb 3+ concentration in the shell continuously increased from 5% to 10%, the luminescence intensity of the BaLuF 5 :Yb 3+ ,Er 3+ ,Ce 3+ core-active-shell NPs gradually decreased since the concentration quenching effect occurred [31,38]. These results indicate that the optimum concentration of Yb 3+ in the shell was about 5% for BaLuF 5 :Yb 3+ ,Er 3+ ,Ce 3+ core-active-shell NPs, the intensity of the UC emissions of BaLuF 5 :Yb 3+ ,Er 3+ ,Ce 3+ @BaLuF 5 :Yb 3+ core-active-shell NPs was increased by 9.4 times compared that of the BaLuF 5 :Yb 3+ ,Er 3+ ,Ce 3+ core NPs, and was increased by 24.6 times compared to that of the BaLuF 5 :Yb 3+ ,Er 3+ core NPs without doping Ce 3+ ions.…”