Rare-Earth Doped Nanoparticles in Security Printing Applications
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Rare-Earth Doped Nanoparticles in Security Printing Applications William Cross1, Tyler Blumenthal1, Jon Kellar1, P. Stanley May2, Jeevan Meruga1 and QuocAnh Luu2 1
Department of Materials and Metallurgical Engineering, South Dakota School of Mines and Technology, 501 E. St. Joseph St., Rapid City, SD 57701, U.S.A. 2 Department of Chemistry, Churchill-Haines Laboratories, Room 115, University of South Dakota, 414 E. Clark St., Vermillion SD 57069, U.S.A. ABSTRACT In this work, lanthanide-doped, sodium yttrium fluoride nanocrystals were prepared and dispersed in a solvent consisting of 90 vol% toluene and 10 vol% methyl benzoate. Poly (methyl methacrylate) polymer was dissolved in the solvent, in addition to the nanocrystals. Inks were printed using direct-write techniques. Substrates used included Kapton®, bond paper, metal and glass. Stencil patterns and QR codes were printed with these inks. An overview of direct write printing for security applications is given. On many substrates, these printed traces are difficult to detect in ambient lighting, but can be easily read using near-infrared (NIR) illumination, making them very useful for covert and semi-covert security printing applications. INTRODUCTION Upconversion phosphors that can efficiently convert near-infrared (NIR) into visible luminescence have numerous potential applications in science and technology [1-15]. One new area is in security devices. Counterfeiting costs government and private industries billions of dollars. Nanoparticle-based security films have emerged onto the research scene in recent years to help address this issue [13-16]. Current security films often utilize UV-to-visible downconversion of light [14]. These downconversion inks are widely available, allowing for counterfeit production with inkjet printers (e.g., IDs, money, and significant documents). The development of NIR-to-visible upconversion printed films demonstrates that a new level of semicover/covert and forensic security is possible. This research involves using lanthanide-doped β-NaYF4 nanoparticles for security printing applications. Inks comprised of Yb3+/Er3+ and Yb3+/Tm3+ doped β-NaYF4 nano-particles with oleic acid as the capping agent in toluene and methyl benzoate with PMMA (poly(methyl methacrylate)) as the binding agent were used to print a variety of covert security products. EXPERIMENTAL DETAILS Upconversion nanoparticles were synthesized using the method of Lin et al. [17, 18]. 1-2 wt% of upconversion nanocrystals were dispersed in toluene (99.5%, Sigma-Aldrich) by stirring vigorously for 2 h, followed by sonication for 10 min. PMMA (poly(methyl methacrylate), ~15,000 molecular weight, Scientific Polymer Products) beads were added at 1 wt% to the nanocrystal dispersion along with enough methyl benzoate (99% Acros Organics) to bring the final composition of the solution to 90:10 v/v toluene/methyl benzoate. The resulting mixture was then stirred for 3 h to achieve complete dissolution of the polymer. Samples were printed at
room temperature (21 °C) with an avera
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