Tuning hexagonal NaYbF₄ nanocrystals down to sub-10 nm for enhanced photon upconversion

Area of Science:

• Materials Science
• Nanotechnology
• Photonics

Background:

• Upconversion emission in lanthanide-doped nanocrystals is crucial for applications.
• Increasing Ytterbium (Yb3+) concentration enhances emission but challenges small crystal synthesis.
• Existing methods struggle with high Yb3+ doping in small hexagonal NaLnF4 nanocrystals.

Purpose of the Study:

• To develop a controllable method for synthesizing small hexagonal NaYbF4 nanocrystals with high Yb3+ doping.
• To investigate the in situ transformation mechanism leading to ultrasmall nanocrystals.
• To enhance the upconversion emission of these nanocrystals for potential applications.

Main Methods:

• A repeatable and controllable synthesis method for hexagonal NaYbF4 nanocrystals.
• Monitoring reaction processes to understand nanocrystal formation.
• In situ transformation of cubic NaYbF4 to hexagonal NaYbF4.
• Surface passivation using a NaYF4 shell.

Main Results:

• Achieved tunable hexagonal NaYbF4 nanocrystal sizes down to ~7 nm without impurity doping.
• Observed enhanced upconversion emission in NaYbF4:Tm nanocrystals with higher Yb3+ doping.
• Demonstrated a ~100-fold enhancement in upconversion emission after NaYF4 shell coating.
• Achieved over 800 times stronger emission in ultraviolet and blue ranges.

Conclusions:

• A versatile method for synthesizing ultrasmall hexagonal NaYbF4 nanocrystals was established.
• High Yb3+ doping and NaYF4 shell coating significantly boost upconversion emission.
• The developed ultrasmall nanocrystals show promise for advanced applications, particularly in biology.