Quantum Yields, Surface Quenching, and Passivation Efficiency for Ultrasmall Core/Shell Upconverting Nanoparticles.

Area of Science:

• Materials Science
• Nanotechnology
• Photonics

Background:

• Ultrasmall hexagonal-phase NaGdF4: Yb3+, Er3+ upconversion nanoparticles (UCNPs) are promising for bioimaging.
• Controlling shell thickness is crucial for optimizing luminescence properties and preventing core-shell intermixing.

Purpose of the Study:

• To synthesize and characterize ultrasmall UCNPs with varying NaYF4 shell thicknesses.
• To investigate the influence of shell thickness on upconversion and downshifting photoluminescence for combined visible and short-wave infrared (SWIR) imaging.
• To identify optimal UCNP structures for enhanced imaging performance.

Main Methods:

• Synthesis of core UCNPs (3.7 ± 0.5 nm) and subsequent shell growth (monolayer to 10 nm).
• Photoluminescence characterization including excitation power-dependent emission spectra, slope factors, quantum yields, and decay kinetics.
• Analysis of core-shell intermixing using spectroscopic tools.

Main Results:

• Upconversion quantum yield enhanced by >10,000x at low excitation power.
• Excitation power density-independent SWIR quantum yield (0.1-14%) achieved.
• Optimized 5 nm shell thickness for combined visible and SWIR imaging.
• Continuous tuning of lifetimes and quantum yields with shell thickness observed.

Conclusions:

• Shell thickness is a critical parameter for tuning UCNP photoluminescence for dual-mode imaging.
• A 5 nm NaYF4 shell provides optimal performance for combined visible and SWIR applications.
• Observed lack of saturation in UQY and decay kinetics indicates significant core-shell intermixing, detectable via spectroscopy.