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

Area of Science:

• Materials Science
• Nanotechnology
• Physical Chemistry

Background:

• Lanthanide-doped upconversion (UC) phosphors convert infrared light to visible light.
• Nanocrystals (NCs) exhibit lower UC efficiencies compared to bulk materials despite extensive research.
• Understanding UC loss pathways is crucial for developing efficient UC NCs.

Purpose of the Study:

• Investigate the excited-state dynamics of β-NaYF₄ co-doped with Yb³⁺ and Er³⁺.
• Identify and quantify loss pathways limiting UC efficiency in NCs.
• Develop a model to predict and mitigate UC quenching.

Main Methods:

• Studied excited-state dynamics of Yb³⁺/Er³⁺-doped β-NaYF₄ NCs.
• Measured and modeled energy transfer, spontaneous emission, and decay processes.
• Investigated solvent/ligand interactions and defect-induced quenching.

Main Results:

• Identified energy transfer to solvent/ligand vibrations as a significant quenching pathway.
• Developed a quantitative model incorporating cross-relaxation and Förster resonance energy transfer (FRET) to molecular vibrations.
• Found quenching of near-infrared energy levels (Er³⁺: ⁴I₁₁/₂ and Yb³⁺: ²F₅/₂) is dominant, likely due to OH⁻ defects.

Conclusions:

• Energy transfer to surrounding molecules and defects significantly reduces UC NC efficiency.
• The developed model provides insights into required inert-shell thickness to prevent solvent quenching.
• Minimizing vibrational coupling to defects like OH⁻ is critical for enhancing UC NC performance.