Crystal structural effects on up/down-conversion luminescence properties of GdInO:Tm,Yb perovskite phosphors for effective dual-mode anti-counterfeit applications.

Developing advanced luminescent materials that are recognizable under specified conditions provides better opportunity for reliable optical anti-counterfeiting techniques. In this work, to the best of our knowledge, novel GdInO:Tm,Yb perovskite phosphors with ultrafine sizes and rounded morphologies were successfully synthesized by a facile chemical precipitation route. Two-type perovskites with orthorhombic and hexagonal structures could be obtained by calcining the precursor at 850 and 1100 °C, respectively. Under 980 nm excitation, the two phosphors exhibited cyan-bluish emission at ∼460-565 nm, red emission at 645-680 nm, and near-infrared emission at 770-825 nm arising from G+ D→H, F→H, and H→H transitions of Tm, respectively, where the hexagonal perovskite phosphor had relatively strong and sharp red emission as well as red-shifted cyan-bluish emission via successive cross relaxations. The Yb sensitizer enhanced the upconversion luminescence via effective Yb→Tm energy transfer and the optimal Yb concentrations were 10 at.% for orthorhombic perovskite and 5 at.% for hexagonal one. The upconversion mechanism mainly ascribed to two-photon processes while three-photon was also present. Upon excitation at 254 nm, their down-conversion spectra exhibited broad multibands in the wavelength range of 400-500 nm deriving from combined effects of the defect-induced emission of GdInO and the D→F + G→H emissions of Tm. The energy transfer from GdInO defect level to Tm excitation state was observed for the first time. The unclonable security codes prepared by screen printing from those dual-mode emitting perovskite phosphors were almost invisible under natural light, which had promising potential for anti-counterfeiting application.