**2. Perovskites for anti-counterfeiting applications**

Taking advantage of the high PLQY of halide perovskites, security information in a luminescent tag can be easily and rapidly identified by the human eye or spectrum. The excitation-dependent emission of perovskites can also be tuned from the monochromatic to broadband white light [36–38], giving an added complexity to the optical readout of tags. Combined with versatile encryption and decryption strategies, the security level of an individual tag can be enhanced multidimensionally and output in a simplified digital form [39]. The anti-counterfeiting mechanism of security tags during the flow of goods is illustrated in **Figure 1**, where the authentication is implemented by the communication between preloaded database and third parties.

#### **2.1 Fundamental structure of perovskites and their luminescent properties**

Perovskite mineral (calcium titanium oxide, CaTiO3) was discovered in the Ural Mountains by German mineralogist Gustav Rose in 1839 [40]. The crystal structure of perovskite oxide was not determined by X-ray diffraction until nearly a century later [41] and was proved to comprise three fundamental phases, i.e. cubic,

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

#### **Figure 1.**

*Anti-counterfeiting mechanism of security tags during the flow of goods.*

tetragonal, and orthorhombic based on the rigid 3D lattice. Halide perovskites share the similar crystal structure to perovskite oxide, of which the compounds were first synthesized in the late nineteenth century by H. L. Wells [42]. Typically, 3D perovskites (defined by a chemical formula of ABX3, where A is a monovalent cation, B is a divalent cation, and X is a halide anion) have direct bandgaps that can be widely tuned by altering the composition of A- and B-site cations and halide anions [21, 24, 43, 44]. Besides, 3D perovskites normally feature low exciton binding energy (*E*b) of dozens of millielectronvolts. Relaxation of an exciting photon in 3D perovskites normally releases a photon with equal energy, making the band maximum of PL spectra representative of their bandgaps. These properties are important for accurate encryption of perovskite patterns for wide-color-gamut luminescence.

Two-dimensional (2D) perovskites feature corner-sharing metal-halide octahedra intercalated by the bulky cations. Emission spectra of 2D perovskites can be structurally correlated with the interlayer spacing, quantum well (QW) thickness, and its distribution [45, 46]. Strong electron-photon coupling that originated from the deformable lattice was previously demonstrated for some 2D perovskite single crystals, which introduces permanent trap states [47]. The self-trapped excitons (STEs) were later revealed to be a type of transient defect driven by the electron-photon coupling and will contribute to the broadband emission of 2D perovskites [48, 49]. Further lowering the dimensionality of 2D perovskites leads to one-dimensional (1D) and zero-dimensional (0D) perovskites whose octahedra are shared by edge or face. STEs can also be responsible for the broadband emission of these materials with large Stokes shift [50–53]. The white light or dual−/multiband emissions under different excitations are favorable for those luminescent tags that demand a high security level.

Double perovskites are defined by a chemical formula of A2BB'X6, where B is a monovalent cation and B′ is a trivalent cation and feature a rock salt arrangement of BX6 and B'X6 octahedra. In addition, A2B(IV)X6 compounds are also grouped as double perovskites because of their vacancy-ordered structure [54, 55]. The phasepure double perovskites usually have room-temperature (RT) indirect bandgaps and exhibit band-to-band or downshifting emissions that can be strongly influenced by the specific metal dopants [55–59]. The in-depth reason was ascribed to lattice distortion since metal dopants will basically affect the length and angle of B − X − B′ bonds and hence change the electronic wave function coupling of metal cations [60].
