Near-infrared persistent luminescence of intrinsic Ti3+ in Na0·5Y0·5TiO3 and Na0·5Y0·5TiO3:Pr for dual-modal optical anti-counterfeiting

Persistent luminescence (PersL) materials have unique long-term separable excitation-emission properties. In this study, intrinsic near-infrared (NIR) PersL of Ti 3+ is found at 760 nm in undoped Na 0·5 Y 0·5 TiO 3 (NYT) after being illuminated to room light or red LED illumination. The low symmetric monoclinic perovskite structure of NYT leads to energy level splitting of Ti 3+ into 2 E 1/2 and 1 E 2/2 , which generates a new PersL excitation peak at ∼650 nm. Direct electron transfer from excited Ti 3+ to oxygen vacancy is verified during the PersL charging process by X-ray photoelectron spectroscopy which avoids cross-band transition. Further, double band visible/NIR PersL is realized by introducing Pr 3+ into NYT (Na 0·5 Y 0·5 TiO 3 :Pr, NYT-Pr) at 614 and 760 nm, respectively. By observing the anti-Stokes PersL of Pr in NYT-Pr at 614 nm that is rechargeable at 660 nm, a phonon-assisted charging mechanism is proposed, in which Ti 3+ absorbs excitation and phonon energy to realize electron transfer to oxygen vacancy ( V O ). Based on single band PersL NYT and double band PersL NYT-Pr, triple modal optical information encryption was realized by applying their visible fluorescence/visible PersL/invisible NIR PersL properties, showing different levels of optical information. NYT-Pr integrates traditional visible PersL by naked eyes and invisible NIR PersL imaged by CCD camera together, and will find promising applications in modern optics and bio-optics.