Probing blue long persistent Eu2+/Nd3+ activated (Ca$_{0.9}$Si$_{0.1}$)Al$_2$O$_6$ emerging phosphor based novel invisible ink for knocking out fake security papers and combat counterfeiting

In the current work, a novel class of (Ca0.9Si0.1)Al2O6:Eu2+,Nd3+ long persistent phosphor was synthesized using high-temperature (1000-1500oC) solid-state reaction method. Further, the phosphor material was formulated into a phosphorescent ink using epoxy based colloidal suspension. Interestingly the phosphorescent ink absorbs ambient energies such as white light from D65 and tungsten lamps, sunlight, artificial lights etc., stores up the energy and emit as efficient blue colored photoluminescence (PL) apparently visible for many hours for dark adapted human eye. However, the optimum PL emission centred at ~450 nm has been registered at ~365 nm excitation that is attributed to transitions from 4f65d1 to 4f7 energy levels of Eu2+ ions. The longer persistence of PL aroused due to trapping and de-trapping of holes at Nd3+ sites situated proximate to valance band. In the optimised stoichiometric composition of the phosphor, the persistence lasted for >4 hours in extremely dark conditions. Furthermore, the phenomenon of long persistence enables to create/design unique hidden markings on goods and certificates using quick response (QR) code patterns. Systematic studies have been performed on various templates kept under UV (~365 nm) excitation to identify fake currencies, barcodes and combat counterfeiting effectively.