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

Abstract

Laser pyrolysis, an aerosol nanosynthesis technique, uses a 10.6 μm CO 2 laser to rapidly heat precursors to induce decomposition and particle nucleation. A photosensitizing agent, usually SF 6 , absorbs the laser beam and transfers energy to the precursors. Here, we exploit the presence of SF 6 in the system and use it as a safe source of fluorine to synthesize NaYF 4 nanocrystals in a single-step process. NaYF 4 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 NaYF 4 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.

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“…NaYF 4 :Yb,Er@NaYF 4 consists of NaYF 4 :Yb,Er nucleus and NaYF 4 shell, as shown in figure 1(b) [38]. Under 980 nm excitation, the active nucleus of NaYF 4 :Yb, Er achieves thermal visible light emission through the energy transfer of Yb 3+ ions to Er 3+ ions, while the inert shell of NaYF 4 acts as a protective layer to prevent the NaYF 4 :Yb,Er nucleus from being affected by non-radiation attenuation, thus improving the upconversion emission of high SNR [39]. The physical structure of the sensor doped with UCNPs, as shown in figure 1(c).…”