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The counterfeiting of goods is a fast-growing issue worldwide, being a risk to human health, financial safety, and national security. Customized anti-counterfeiting patterning technologies enable unclonable tags on products, which ensure the reliable and convenient flow of goods such as daily foods, prescription medicines, and value-added components. In this chapter, we start with the introduction of recent advances of anti-counterfeiting technologies that generate unique physical tags on products for encryption and information storage. Various halide perovskite-based materials and their fabrication techniques for unreplicable luminescent patterns are then discussed, with a particular focus on the intelligent encoding principles that correlate with the chromism and other special optical readout of materials. The multilevel anti-counterfeiting functions that allow high-throughput authentication of products within a single tag are also exemplified, through which the increasing security demands can be fulfilled. We finally discuss the current issues encountered by perovskite anti-counterfeiting technologies and outline their future directions toward smarter and safer flow of goods.
Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, Sweden
Jin Xie
Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, China
Yu Hou*
Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, China
*Address all correspondence to: yhou@ecust.edu.cn
1. Introduction
The booming global businesses have largely facilitated the cross-border flow of goods, but meanwhile are threatened by the dramatically increased intellectual property (IP) crimes nowadays. According to the study by Organization for Economic Cooperation and Development (OECD), the value of counterfeit and pirated products is amounted to USD 464 billion in 2019, equal to 2.5% of world trade and more than half of the total value is carried by containerships between countries [1, 2]. The illicit trade hits company profits and nation tax revenue and endangers public health when pharmaceuticals and medical equipment are involved. For these reasons, advanced technologies that combat fake products demand prompt development to ensure reliable flow of goods while maintain its convenience.
Anti-counterfeiting idea was early raised by Philadelphia printer Benjamin Franklin in the 1700s [3], at that time colonies in North America were troubled by the circulation of counterfeit bills. Franklin deliberately misspelled Pennsylvania in the printed bills to baffle less-literate criminals. Meanwhile, he engraved the fine detail of copper on the leaf vein at the back of each bill, making these bills hard to be reproduced by counterfeiters. The unique copper engraving created by blocky lead printer has been regarded as a prototype for contemporary anti-counterfeiting patterning technologies. Since the 1950s, the development of holograms [4, 5, 6, 7], ink printing [8, 9, 10, 11], and exquisite laser engraving [12, 13, 14] have offered practical solutions to protect the market from malicious third parties.
Halide perovskites as an emerging family of semiconductor materials have achieved notable success in photovoltaics and other optoelectronics over the past decade [15, 16, 17, 18, 19, 20]. The intriguing photophysical property of perovskites, such as widely tunable bandgaps [21, 22, 23, 24, 25, 26], high photoluminescence quantum yield (PLQY) [27, 28, 29], and narrow emission width [30, 31, 32], are making them promising candidates for fabricating luminescent security tags. Meanwhile, the solution/ink processability of perovskites imparts them feasibility with a variety of printing technologies, enabling high-throughput generation of customized labels with enhanced encoding capacity and lowered processing cost [33, 34, 35].
Here, we give a retrospect to the recent advances of halide perovskite-based materials for anti-counterfeiting applications. Low-dimensional perovskites and double perovskites that are structural analogs to three-dimensional (3D) ones as well as other perovskite-like materials are included in the discussion. We summarize the patterning techniques that can lead to precise control of tag fabrication at high dim either flat surface or closed space. The luminescent security tags of perovskites are categorized by different encryption principles, with detailed phase transformation or compositional variation of materials being provided for each chromic case. Integration of luminescent properties that gives rise to multimodal anti-counterfeiting is discussed in respect of goods being strictly confidential. We then survey the special optical readout of security tags that is enabled by the exciton relaxation behavior and carrier dynamic of perovskites.
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, 37, 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.
Figure 1.
Anti-counterfeiting mechanism of security tags during the flow of goods.
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, 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 (Eb) 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, 51, 52, 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 phase-pure 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, 56, 57, 58, 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].
2.2 Patterning techniques for perovskite security tags
Halide perovskites possess a high compatibility with printing techniques, since both the precursor solution and synthesized colloidal nanocrystals (NCs) can serve as inks. Using CsPbX3:Mn2+ (X = Cl, Br, I) NCs inks, Wang et al. [34] previously reported the fabrication of various patterns by screen, inkjet, and roll-to-roll printing techniques on flexible substrate (e.g. paper, polyethylene terephthalate, and banknotes). The patterns showed fluorescence as response to 254-nm and 365-nm ultraviolet (UV) light, and the CsPbBr3:Mn2+-based on maintained bright fluorescence after continuous UV irradiation for 60 days. Shi et al. [61] demonstrated an in situ growth of MAPbX3 (MA = methylammonium, X = Cl, Br, I) quantum dots (QDs) in polymer scaffold by directly inkjet printing precursor solution on polymer layer. The microdisk arrays of perovskite QDs showed high PLQY up to 80% and can be integrated for a variety of luminescent patterns. Specifically, the 2D code pattern fabricated on polyvinylidene chloride showed excellent water endurance and was still luminescent after being dipping in water for 100 days.
Nanoscale 3D printing technique was recently reported to fabricate perovskite nanopixels with programmed vertical height, location, and emission characteristics [35], which overcomes the low-resolution problem of conventional printing techniques. The authors of this study used femtoliter meniscus to guide the out-of-plane growth of MAPbX3 (X = Cl, Br, I) crystals from precursor solution, enabling ultrahigh integration density of red, green, and blue (RGB) nanopixel arrays with spacing of ~5 μm while maintaining its lateral resolution (Figure 2a). Numbers can be encoded for each discrete height of nanopixels and thus adds an additional level for encryption. Electrohydrodynamic (EHD) printing as another advanced printing technique was also reported to fabricate high-resolution CsPbX3 (X = Cl, Br, I) dot arrays with full-color display (Figure 2b) [62]. The size of a single dot was precisely controlled by the frequency and peak values of pulse voltage for precursor solution, and a minimum size of 5 μm can be achieved.
Figure 2.
(a) Schematic illustration of 3D printing of perovskite nanopixels. (b) Schematic illustration of EHD printing technique for perovskite patterning. (c) Representative laser processing system for perovskite patterning. Reprinted with permission from ref. [35, 62, 63]. Copyright 2021 American Chemical Society; copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; copyright 2020 Royal Society of Chemistry.
Laser beam was previously used to trigger the ultrafast crystallization of perovskite for both patterning and photovoltaic applications [64]. Figure 2c shows a typical laser processing system for perovskite patterning. Without any heat treatment, Zhang et al. [63] demonstrated the fabrication of CsPbBr3/CsPb2Br5-polymer nanocomposites fluorescent pattern by 532-nm femtosecond laser irradiation. Localized crystallization of perovskite was observed in the irradiated pathway, which was accompanied by the laser-induced polymerization of γ-butyrolactone solvent. The width of perovskite line was lowered down to 1.2 μm, and both the crystal quality and luminescent intensity can be fine-tuned by the power and moving speed of laser beam. In addition, laser engraving was introduced to directly create patterns on CsPbBr3 microplates [65]. The hidden security information provides a guidance for encryption on a miniaturized pattern.
Most recently, Sun et al. [66] reported the use of 3D lithography technique to fabricated separated CsPbX3 (X = Cl, Br, I) NCs in glass matrix. The strong thermal accumulation at the laser-irradiated region of borophosphate glass leads to local pressure and temperature above the liquidus of materials, which induces liquid nanophase separation of glass and perovskite. By tailoring the parameters of pulse duration, repetition rate, pulse energy, and irradiation time, the emission color of pattern was tuned from blue to red under 405-nm excitation. Perovskite NCs in glass matrix exhibited notable phase stability against long-term UV irradiation, organic solution, and high temperature. The patterns were used for both 3D multicolor and dynamic holographic displays, showing huge potential for stereoscopic optical storage and authentication. Accordingly, we provide an overall assessment of existing printing and laser processing techniques for perovskite security tags in Table 1.
High-energy laser source required, sophisticated optical paths and machines
Table 1.
Technical assessment of patterning methods.
2.3 Encryption principles of perovskite security tags
With the assistance of advanced patterning techniques, the intriguing luminescent properties found on perovskites can be transformed into security information for encryption and decryption of tags. Normally, these tags are invisible under visible light but can emit light under UV, visible, or near-infrared (NIR) excitations. In this section, we provide an overview of encryption principle of perovskite security tags, including pattern, thermochromism, solvatochromism, photochromism, and multimodal luminescence. Other optical readout, such as long-lived emission (afterglow) phenomenon and carrier lifetime gating, are discussed as special encryption methods for delicate authentication of goods. Figure 3 shows the representative cases of encryption principles being reported over the past few years.
Figure 3.
Timeline of pioneering works with new encryption principles being reported for perovskite security tags.
2.3.1 Pattern
Shape design of a pattern is a fundamental approach to encode the security data relative to the complexity of contours. Printing or laser processing techniques have been developed to create customized pattern shapes whose resolution now reach a few micropixels or below. Lin et al. [33] raised the concept of clonable shape, while unclonable texture for anti-counterfeiting tags is based on CsPbBr3 patterns. A large amount of patterns that grown on laser-engraved lyophilic 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) layer are grouped by the number of edges for both polygon and complex contour design. Using portable microscope and ShaptexMatch authentication software, the authors of this study matched the shape and texture of patterns, respectively, from the establishing database of 61st type of graphics. The effective encoding capacity of patterns was estimated up to 2.1 × 10623, and the authentication time was only 12.17 s for 4000 samples.
The vertical height of a single perovskite pixel can be also encoded as specific numbers [35], which is regarded as a complementary encryption strategy to lateral shape design of a pattern (Figure 4a and b). 3D confocal PL imaging was applied to recognize the height variation of perovskite pixels with the height interval of 5 μm. The height values were further converted into binary information matrix for digitalized decryption. As we have mentioned in Section 2.2, the pattern design at three dimensionalities enabled by 3D lithography technique allows more complex encryption on a security tag (Figure 4c–e) [66]. Random 3D luminescent patterns can therefore be spatially and temporally identified, offering an innovative platform for smart authentication of goods.
Figure 4.
(a) Tilt-view SEM image of as-printed perovskite nanopixel arrays. (b) Multicolor display of perovskite nanopixel arrays with different halide components under UV light. (c) Multicolor pattern with CsPbClxBr3 − x nanophases in glass under UV light. (d) 3D microhelix arrays of CsPbClxBr3 − x under UV light. (e) Dynamic holographic display of as-patterned “ZJUUSST” characters under 532-nm light. Reprinted with permission from ref. [35, 66] copyright 2021 American Chemical Society; copyright 2022 American Association for the Advancement of Science.
2.3.2 Thermochromism
Halide perovskites, especially organic–inorganic hybrid ones, feature considerably large thermal expansion coefficients [68, 69]. The thermochromic property of perovskites was first observed in thin film due to the phase transition between transparent hydrated phase (MA4PbI6·2H2O) and dark perovskite phase (MAPbI3) [70]. This phenomenon can be reversible by exposing perovskite film to ambient moisture at RT or heating condition at 60°C repeatably and was explored as the switchable photovoltaic performance for perovskite solar cells. The discoloration mechanism was recently developed for smart window applications based on hydrated MAPbClxI3 − x [71]. Similarly, Lin et al. [72] demonstrated the reversible thermochromic property of CsPbBrxI3 − x film coupled with dynamic transition of RT non-perovskite phase and high-temperature perovskite phase, which is also switched by the moisture and thermal annealing.
Above cases show the thermochromic phenomena of perovskites in the presence of moisture but may not be applicable to anti-counterfeiting tags that are fully encapsulated. Taking advantage of the inverse temperature crystallization (ITC) of hybrid perovskites, Bastiani et al. [73] reported the chromatic inks with wide color variation that depend on the halide constituent of perovskite precipitate. The RT yellow inks turned to orange, red, and black when temperature reached 60°C, 90°C, and 120°C, corresponding to the extrapolated absorption edges of MAPbBr2.7I0.3 at 597 nm, MAPbBr2.4I0.6 at 615 nm, and MAPbBr1.8I1.2 at 651 nm, respectively. The thermochromic behavior of perovskite inks showed consecutive cycling between RT and 60°C for several times.
The reversible thermochromic phenomena was also observed in diphasic perovskite material (CsPbBr3/Cs4PbBr6) wrapped by silica nanosphere [74]. The strong RT PL emission (at 525 nm) of composited patterns gradually decreased when temperature was elevated and almost disappeared at 150°C. Temperature-dependent PL spectra revealed the relatively low activation energy (Ea, 38 meV) of thermal quenching of composites, being ascribed to the thermal-sensitive nature of Cs4PbBr6 and should be responsible for the thermal-switchable PL emission. In addition, thermochromism can be found in both lead-based and lead-free 2D perovskites and double perovskites. Octahedral distortion and interlayer distance of 2D perovskites are strongly related to temperature and hence will result in phase transition during thermal heating or cooling [75, 76]. The structural change is accompanied by the bandgap variation of materials, leading to thermochromic behavior that can be identified by CIE coordinates. Ning et al. [77] reported the thermochromism of double-perovskite single crystal and thin film from RT to 250°C. The synergistic effect of anharmonic fluctuation and electron–phonon coupling as well as the spin-orbit coupling effect were unveiled to explain the thermochromic behavior of Cs2AgBiBr6 upon the bond length change of Ag-Br and Bi-Br. Security tags based on thermochromic perovskites enable the decryption through the information of discoloration or photoexcited emission relative to temperature.
2.3.3 Solvatochromism
Solvatochromism refers to chromic behavior of materials as response to water or other organic solvents. As we mentioned in Section 2.3.2, hybrid perovskites feature hydrochromism due to the formation of hydrated or non-perovskite phases in moisture atmosphere [70, 72]. Reversibly decomposition-induced hydrochromism was recently reported for CsPbBr3 NCs confined in mesoporous silica nanospheres (MSNs) [78]. Orthorhombic CsPbBr3 will decompose into nonluminescent tetragonal CsPb2Br5 and CsBr in the presence of water, and the dissolved CsBr component can be confined in MSNs. As a result, the green emission pattern turned to dark in moisture condition and recovered when water was removed (Figure 5a). Similar hydrochromic mechanism was also reported for CsPbBr3/Cs4PbBr6 nanocomposites, which maintained about half of its initial PL intensity after 10 wetting-drying cycles [80]. Cs3Cu2I5 as lead-free perovskite-like material was recently exploited for hydrochromism-based encryption and decryption of security tags [81, 82, 83]. Water functions as a switch of phase transition between blue emission Cs3Cu2I5 and yellow emission CsCu2I3 under UV excitation. Combined with water-resistant polymethyl methacrylate (PMMA) coating layer, moreover, the microarray patterns can be tailored for dual-color emission toward various shapes and characters in moisture atmosphere [82].
Figure 5.
(a) Reversible hydrochromism of CsPbBr3 pattern under 365-nm UV light and the corresponding phase transformation. (b) Reversible DMF-induced solvatochromism of InCl6(C4H10SN)4·Cl:Sb3+ pattern under 365-nm UV light and the corresponding phase transformation. Reprinted with permission from refs. [78, 79]. Copyright 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim; copyright 2021 American Chemical Society.
Besides water, methanol (MeOH) was previously demonstrated capable to trigger the solvatochromism of MAPbBr3 NCs that are converted from lead-based metal–organic framework (MOF) [84]. The authors of this study found that MeOH impregnation can remove the organic perovskite species while leave lead ions in MOF matrix. The green emission of pattern under UV excitation therefore quickly quenched after impregnation but can be recovered by loading MABr solution (10 mg mL−1 in n-butanol) on top. Using n-butylamine (n-BA) and acetic acid (AcOH) as encryption and decryption reagents, respectively, Sun et al. [85] reported the solvatochromism of patterns based on CsPbX3 (X = Cl, Br, I) QDs. The RGB emission colors disappeared after n-BA treatment and then recovered by AcOH treatment. Specifically, the as-fabricated multicolor quick response (QR) code still showed clear optical readout after 100 times crumpling. Isopropanol (IPA)-induced solvatochromism was observed in 1D CsMnBr3 NCs which underwent phase decomposition to 0D Cs3MnBr5 and MnBr2 [86]. The pattern showed emission color changing from red to green under 365-nm UV light.
Solvatochromism can also be induced by new phase formation where solvent molecules are incorporated into perovskite lattice [79]. The 0D InCl6(C4H10SN)4·Cl:Sb3+ showed red-shifted emission peak from 550 nm to 580 nm and 600 nm when being exposed to ethanol (EtOH) and N,N-dimethylformamide (DMF) vapor, corresponding to the new phases of InCl6(C4H10SN)3·EtOH:Sb3+ and InCl5(C4H10SN)2·DMF:Sb3+, respectively. Pattern based on this environmentally friendly material displayed reversible emission colors between yellow and orange under 365-nm excitation, which was enabled by the alternate incorporation and release of DMF species (Figure 5b). Considering the peculiar chemical reaction between perovskites and solvents, solvatochromism of perovskite security tags provides an additional route for authentication of goods.
2.3.4 Photochromism
Photochromic property has been found in a variety of organics and organic–metal complexes in the case of light-mediated configuration change of molecules [87]. By anchoring the diarylethene (DAE) derivative onto CsPbBr3 QDs surface, Mokhtar et al. [88] observed the reversible photoswitchable luminescence of QDs-DAE hybrids. The open-ring isomer of DAE underwent cyclization under UV light and quickly turned off the green emission of printed pattern, while the green emission can be switched on again by exposing the pattern to visible light for DAE cycloreversion (Figure 6a). Similar photochromic behavior was reported for DAE derivative whose triethoxysilane (TEOS) moiety is altered by alkyl amine [90]. Following this strategy, a majority of photochromic molecules may be introduced as the surfactant to achieve the photochromism of perovskite QDs/NCs.
Figure 6.
(a) Photoswitchable cyclization and cycloreversion of DAE surfactant and the resultant photochromism of pattern based on CsPbBr3-DAE hybrids. (b) UV irradiation-induced reversible halide exchange at CsPbCl1.5Br1.5/MYE interface and the photochromism of QR code patterned by CsPbCl1.5Br1.5/MYE composites. Reprinted with permission from refs. [88, 89]. Copyright 2020 American Chemical Society; copyright 2021 American Chemical Society.
Photochromism also occurs under the circumstance of photoinduced compositional variation of perovskites. The emission color of CsPbCl1.5Br1.5 NCs that confined in macroporous Y2O3:Eu3+ (MYE) changed from red to green under continuous UV irradiation, which was explained by the halide migration between perovskite NCs and MYE matrix [89]. The small Ea of halide vacancy defects allows bromine ions to segregate to NCs domain, while the chlorine ions are fixed on MYE surface due to the stable Y-Cl bond. Therefore, the red emission is contributed by MYE matrix at the first stage, and the subsequent strong green emission that originated from Br-rich perovskite NCs will dominate the composites (Figure 6b). The QR code fabricated by stereolithography printing technique maintained 61% of the maximum green emission intensity after 25 encryption/decryption cycles. Yang et al. [91] reported the irreversible photochromism of CsPbCl1.5Br1.5@Ca0.9Eu0.1MoO4 (CEMO) composites enabled by interfacial redox reaction of Eu3+ + Pb0 → Eu2+ + Pb2+. The emission peak at 615 nm dominated the composites at the initial stage for a few seconds but was quickly overwhelmed by the emission peak at 519 nm during continuous UV irradiation, and patterns based on such composites ultimately displayed a mixed color of cyan.
The bandgap of perovskites is structurally dependent on the QW thickness; in this view, photochromism can be achieved in dimensionality-mixed perovskites whose QW thickness and distribution are self-adapted to light stimulus. The emission 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 water and MABr treatment. Notably, the UC luminescent component of perovskites can be further tuned by rational doping of lanthanides [94].
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.
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 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].
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.
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].
3. Challenges of anti-counterfeiting technology based on halide perovskites
We hereby briefly discuss the current challenges encountered by perovskite fluorescent tags prior to their real-world applications, including the potential overuse of toxic lead, the poor durability, and many clonable functions that can be easily reproduced by counterfeiters. Possible solutions are also provided with respect to each challenging case.
3.1 Lead contamination
Lead’s toxicity has been widely recognized due to its damage to the nervous system of biological individuals. Therefore, lead-based wastes are now under strict control in many developed countries. Despite perovskite security tags made by lead compounds feature many intriguing fluorescent properties, they can be highly risky when adhere to daily goods and cause potential lead leakage. Alternatively, more environmental-friendly perovskites (e.g. tin-, antimony-, bismuth-, and copper-based) can be developed to replace lead-based ones while maintaining the bright luminescence and high processability of tags [55, 57, 58, 79, 82, 83, 96].
3.2 Poor durability
The phase stability of halide perovskites, especially 3D ones, can be susceptible to environmental perturbations and hence fail to work during long term or repeated authentication. Lowering down the dimensionality of perovskites as well as composite strategies enable more robust perovskite phase, yet the stability of fluorescent tags can hardly rival the simple-patterned tags (e.g. QR code). Advanced sealing techniques alleviate this problem by isolating perovskites from environment; however, they are limit for those tags that need direct exposure to atmosphere, chemicals, or solvents. Inert matrix has been demonstrated to enhance the durability of both common and special perovskite fluorescent tags. Beside glass, silica, and polymers [66, 82, 101], other durable matrix materials remain to be exploited.
3.3 Clonable functions
Single-mode perovskite fluorescent tags work as response to certain stimulus, making their functions clonable by commercial phosphors or other functionalized luminescent materials. A safer communication between users and server database requires physically unclonable functions (PUFs) that generated by irregular encryption and decryption methods. In this view, multimodal anti-counterfeiting that combines two or more encoding and decoding pathways (see Section 2.3) is prompt to be developed for highly confidential security tags. In addition, authentication based on the digital readout of sophisticated machines can also fulfill the demands of PUFs [97, 100].
Increasingly rich encryption principles have been exploited for halide perovskite-based security tags owning to their intriguing luminescent properties as response to a wide range of stimuli. Apart from the existing cases, the mechanochromism upon mechanical stress as well as the magnetochromism under altered magnetic field can be studied for perovskites with the aim of further enriching the diversity of authentication methods [102, 103]. Perovskite memristors as a new rising technology was also demonstrated to deliver switching electronic signals relative to the charged defects and halide motions inside the materials, providing an additional solution toward the design of PUF system [104]. All these unique optical and digital readout may overcome the limit of conventional clonable tags such as QR codes, watermarks, and raised print.
Future development of perovskite security tags is supposed to follow the taxonomy of predominant PUFs, including high encoding capacity, tunable security level, logically/physically reconfigurable functions, and switchable access between private and public. Based on the rational screening strategy of perovskite materials, micro- and nanoscale patterning techniques allow these functions to be multidimensionally integrated in a minimized tag, making security information more robust against third parties. Halide perovskites are bound to play a more important role in anti-counterfeiting arena and contribute to future smart flow of goods in a more fair and orderly global market.
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Written By
Ziren Zhou, Jin Xie and Yu Hou
Submitted: 12 February 2022Reviewed: 24 May 2022Published: 29 July 2022
Open access peer-reviewed chapter
