Laser Pyrolysis Synthesis of Upconverting Lanthanide-Doped NaYF4 Nanocrystals for Anticounterfeiting Applications

Laser pyrolysis, an aerosol nanosynthesis technique, uses a 10.6 μm CO2 laser to rapidly heat precursors to induce decomposition and particle nucleation. A photosensitizing agent, usually SF6, absorbs the laser beam and transfers energy to the precursors. Here, we exploit the presence of SF6 in the system and use it as a safe source of fluorine to synthesize NaYF4 nanocrystals in a single-step process. NaYF4 is an excellent host for rare-earth ions to create upconverting nanocrystals (UCNCs) that absorb infrared photons and emit at shorter wavelength. We thus dope the NaYF4 nanocrystals during synthesis with ytterbium as a sensitizer and erbium, thulium, or holmium as an upconversion emitter. Each emitter produces upconversion emission at specific wavelengths in the visible spectral range after near-infrared light (∼980 nm peaked emission from laser diode) is absorbed by ytterbium followed by energy transfer to emitter. We then show that calcining these materials, in a simple second step, dramatically increases upconversion emission intensity. Finally, we optimize the laser pyrolysis process and postsynthesis calcination to produce the most intense emission at each emission color. We also show that these laser-synthesized UCNCs can be dispersed in a broad range of solvents and retain their visible upconversion emission. This dispersibility could be particularly valuable in creating inks for printing in anticounterfeiting applications. Proof-of-concept use of the synthesized UCNC for anticounterfeiting applications was demonstrated by drop-casting a dispersion of UCNC in stencils on paper, to create various luminescent patterns that were visible under irradiation by a 980 nm laser diode. Overall, this study demonstrates a promising vapor-phase route to rapid, single-step, continuous synthesis of lanthanide-doped fluoride nanomaterials for anticounterfeiting and related applications.