Abstract View

Laser Pyrolysis Synthesis and Applications of Rare-Earth Fluorides for Optical Upconversion

Mohammad Malekzadeh, Khirabdhi Mohanty, Vishvajeet Mane, MARK SWIHART, University at Buffalo - SUNY

Abstract Number: 180
Working Group: Nanoparticles and Materials Synthesis

Abstract
Nanoparticulate rare-earth fluorides have been traditionally synthesized by solution-phase methods that require multiple steps to obtain the final products, and that employ organometallic precursors, costly ligands, and large volumes of organic solvents. Laser pyrolysis, a flexible aerosol nanosynthesis technique, provides a green single-step route to synthesize a wide variety of metal fluoride nanoparticles by a single method. Because most precursor molecules cannot absorb the 10.6 μm CO₂ laser that drives the laser pyrolysis, a photosensitizer is used to absorb light and transfer energy to the precursors. The most common photosensitizer is sulfur hexafluoride (SF₆) which is inert, non-toxic, and has exceptionally strong absorbance at the laser wavelength. Under focused illumination, the SF₆ can also decompose and serve as a fluorine source. This allows us to use spray delivery of low-cost fluorine-free metal salts as precursors for metal fluorides. Among the materials we have prepared by this approach are rare-earth based optical upconverting nanoparticles that absorb multiple infrared photons and emit at shorter wavelengths. We synthesized NaYF₄:Yb³⁺/Er³⁺ nanoparticles that upconverted near-infrared light into visible-light range with sharp green (550 nm) and red (650 nm) emission. Varying processing parameters led to emission of green, yellow, and red light under 980 nm diode laser illumination of dispersions of nanoparticles in various solvents ranging from highly polar to highly non-polar. In addition, we synthesized NaYF₄:Yb³⁺/Tm³⁺ nanoparticles with sharp near-infra-red emission at 800 nm and NaYF₄:Yb³⁺/Ho³⁺ nanoparticles with sharp green emission at 550 nm. Because we are not using any hydrophobic ligands during the synthesis process, our upconverting nanoparticles are directly dispersible in water. Accordingly, we used them in water-based nano-inks without any further coating process. The variety of upconverted emission wavelengths achievable by these nanoparticles, alone and in combination makes them useful in anti-counterfeiting and related applications, allowing one to embed images visible only under 980 nm illumination within other images. This presentation will discuss the synthesis, emission optimization, and printing of these nanoparticles.