*Halide Perovskites as Emerging Anti-Counterfeiting Materials Contribute to Smart Flow of Goods DOI: http://dx.doi.org/10.5772/intechopen.105530*

behavior of layered FAn + 2PbnBr3n + 2 (FA = formamidinium) was recently studied with respect to its structural transformation under light irradiation [92]. The authors of this study demonstrated the UV damage to perovskite that can convert wide-bandgap 2D phase to narrow-bandgap 3D phase. Accordingly, perovskite film showed emission color changed from blue to green as response to the elongated irradiation time. The metastable 2D phase can meanwhile be transformed back by dark storage, showing reversible photochromism that is applicable for anti-counterfeiting patterns.

### *2.3.5 Multimodal luminescence*

Unlike unidirectional authentication methods, multimodal luminescence of perovskites allows the encryption and decryption to be conducted through multiple excited sources. Xu et al. [74] first demonstrated the triple-modal anti-counterfeiting of CsPbBr3@Cs4PbBr6/SiO2 composites in 2017, since the as-patterned codes showed reversible and switchable luminescence to heating, UV, and NIR irradiation. In addition, the dual-color emission of green and red of MAPbBr3@Eu-MOF composites was reported under 365-nm and 254-nm UV lamp [93], respectively, where the red emission under 254-nm excitation primarily comes from the photon upconversion (UC) of Eu-MOF species (**Figure 7a** and **b**). Solvatochromism was also observed for the composites, and the written pattern on paper showed reversible green emission via

#### **Figure 7.**

*(a) The dependence of PL spectra of MAPbBr3@Eu-MOF composites on the UV excitation wavelength. (b) Hydrochromism of "USTB" characters based on MAPbBr3@Eu-MOF composites and the MABr-induced recovery under 254-nm and 365-nm UV light. (c) Photographs of Cs2Ag0.6Na0.4InCl6:Yb3+/Er3+/Bi3+ (RE-1) under different excitations. (d) XEL, DS-PL, and UC-PL spectra of RE-1. (e) Photographs of RE-1 pattern under visible and 365 nm UV light. Reprinted with permission from refs. [58, 93]. Copyright 2018 American Chemical Society; copyright 2020 Wiley-VCH GmbH.*

water and MABr treatment. Notably, the UC luminescent component of perovskites can be further tuned by rational doping of lanthanides [94].

Overcoming the limited response range of conventional perovskite materials, the excitation source of Yb3+/Er3+/Bi3+ co-doped Cs2Ag0.6Na0.4InCl6 double perovskite was reported to be extended to X-ray, as a complementary to UV and NIR [58]. Bi3+ ions were demonstrated to reduce the structural disorder, promote the exciton localization, and lead to strong Jahn-Teller effect that would benefit both UC and X-ray excited luminescence (XEL) (**Figure 7c** and **d**). The as-synthesized double-perovskite single crystals were ground and dispersed in organic solvent for ink printing, and the patterns showed exceptional luminescent stability in thermal heating (up to 400°C), moisture, and high-dosage radiation conditions (**Figure 7e**). The combination of X-ray excited luminescence (XEL), downshifting (DS), UC luminescence, and other routine encryption methods enhance the confidential level of tags considerably, which offers a reliable solution for customized authentication of high-value products.

### *2.3.6 Other optical readout*

Some special optical readout of perovskites can be transformed into security information for anti-counterfeiting applications. Here, we exemplify the encryption principles of patterns based on afterglow phenomenon and carrier lifetime gating. The RT afterglow of perovskites was first reported for 2D PEA2PbCl4 (PEA = phenylethylammonium) perovskite doped with 1,8-naphthalimide (NI) spacers [95]. The as-printed pattern on paper showed UV-excited white emission in nitrogen atmosphere that comprises blue fluorescence from perovskite and yellow phosphorescence from NI organic cations. After UV light off, however, the blue fluorescence (PLQY: 25.6%) quenched quickly, while the yellow phosphorescence (PLQY: 56.1%) can maintain for a few seconds. This property caused the yellow afterglow of pattern that can be identified by both spectrum and human eye. Wei et al. [96] recently found the RT greenish afterglow of 0D BAPPIn1.996Sb0.004Cl10 (BAPP = C10H28N4) perovskite-like material after UV

#### **Figure 8.**

*(a) Molecular configuration of BAPP4+ cation and crystal structure of BAPPIn2Cl10. (b–d) Photographs of BAPPIn1.996Sb0.004Cl10 pattern under visible light, 365-nm UV light, and 365-nm UV light off (afterglow), respectively. (e) FLIM image and (f) time-correlated single-photon counting fluorescence lifetime imaging (TCSPC-FLI) image of tag patterned by CsPbBr3 and {en}FAPbBr3 NCs inks. (g) Fast-lifetime histograms of as-patterned inks and (h) binarization of lifetime for QR code generation. Reprinted with permission from refs. [96, 97]. Copyright 2021 American Association for the Advancement of Science; copyright 2021 Springer Nature.*

*Halide Perovskites as Emerging Anti-Counterfeiting Materials Contribute to Smart Flow of Goods DOI: http://dx.doi.org/10.5772/intechopen.105530*

light off, where the relaxation of excitons from BAPP organic cations were demonstrated to be responsible for the afterglow (**Figure 8a**–**d**). For CsPbBr3 NCs doped by lanthanide ions (Ln3+), the persistent time of afterglow is even up to 1800 s [98]. In addition, X-ray-induced afterglow was also reported for 0D Cs4EuX6 (X = Br, I) perovskite single crystals, despite the case did not involve anti-counterfeiting applications [99].

The carrier lifetime of perovskites is influenced by a variety of factors, among which the composition of perovskite can be the deterministic one. The EHD-printed security tags were reported to be encrypted based on the different carrier lifetime of CsPbBr3 and hollowed {en}FAPbBr3 NCs, which can then be decrypted by either fluorescence-lifetime imaging microscopy (FLIM) or time-of-flight fluorescence-lifetime imaging (ToF-FLI) (**Figure 8e**–**h**) [97]. These two imaging techniques enabled machine-readable lifetime of QR code that cannot be readily decoded by routine methods. Moreover, the system is highly reconfigurable due to the compositional versatility of perovskite NCs. The enhancement and Purcell factors of CsPbClxBr3 − x QDs that coupled to plasmonic silver cavity were also extracted for the encryption of QR code, where the factors are defined by the relationship among excitation efficiency, light extraction efficiency, quantum efficiency, and radiative rate [100].
