**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

## **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.*

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

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.

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**.

## **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.

#### *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,


#### **Table 1.**

*Technical assessment of patterning methods.*

#### **Figure 3.**

*Timeline of pioneering works with new encryption principles being reported for perovskite security tags.*

while unclonable texture for anti-counterfeiting tags is based on CsPbBr3 patterns. A large amount of patterns that grown on laser-engraved lyophilic 1*H*,1*H*,2*H*,2*H*perfluorooctyltriethoxysilane (POTS) layer are grouped by the number of edges for both polygon and complex contour design. Using portable microscope and

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

#### **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.*

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.

#### *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 (*E*a, 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 doubleperovskite 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 [81–83]. 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

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

#### **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.*

patterns can be tailored for dual-color emission toward various shapes and characters in moisture atmosphere [82].

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 asfabricated 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].

#### **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.*

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.

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 *E*a 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
