Small Alkaline-Earth-based Core/Shell Nanoparticles for Efficient Upconversion.

Area of Science:

• Materials Science
• Nanotechnology
• Photonics

Background:

• Lanthanide-based upconversion efficiency is highly dependent on the host crystalline lattice and its crystal field.
• Alkaline-earth rare-earth fluorides (MLnF) offer potential as novel host materials for upconversion nanoparticles (UCNPs).

Purpose of the Study:

• To investigate the upconversion performance of six distinct alkaline-earth rare-earth fluoride host materials doped with Yb3+ and Er3+.
• To synthesize and characterize core/shell UCNPs with varying shell thicknesses and evaluate their upconversion quantum yield (UCQY).

Main Methods:

• Synthesis of ~5 nm UCNP cores using thermal decomposition.
• Growth of optically inert shell layers via a hot-injection method, creating core/shell structures with varying thicknesses.
• Measurement of UCQY for 36 unique UCNPs and comparison with β-NaGdF4.

Main Results:

• Core nanoparticles exhibited UCQY below the detection limit (<10-5%).
• UCQY increased by 4-5 orders of magnitude with shell thickness up to 4-6 nm.
• SrLuF nanoparticles achieved the highest UCQY (0.53% at 80 W/cm2), outperforming β-NaGdF4 by 5 times.

Conclusions:

• Ultrasmall alkaline-earth-based MLnF nanoparticles exhibit efficient and bright upconversion.
• Optimized core/shell structures significantly enhance UCQY.
• These UCNPs show promise for applications in biological imaging and optical sensing.