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Double protected all-inorganic perovskite nanocrystals by crystalline matrix and silica for triple-modal anti-counterfeiting codes Leimeng Xu, Jiawei Chen, Jizhong Song, Jianhai Li, Jie Xue, Yuhui Dong, Bo Cai, Qingsong Shan, Boning Han, and Haibo Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06436 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017

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ACS Applied Materials & Interfaces

Double protected all-inorganic perovskite nanocrystals by crystalline

matrix

and

silica

for

triple-modal

anti-counterfeiting codes Leimeng Xu #, Jiawei Chen #, Jizhong Song*, Jianhai Li, Jie Xue, Yuhui Dong, Bo Cai, Qingsong Shan, Boning Han, Haibo Zeng*

MIIT Key Laboratory of Advanced Display Materials and Devices Institute of Optoelectronics and Nanomaterials Herbert Gleiter Institute of Nanoscience School of Materials Science and Engineering Nanjing University of Science and Technology, Nanjing 210094, China Email address: Haibo Zeng: [email protected] (corresponding author) Jizhong Song: [email protected] (corresponding author) Abstract: Novel fluorescence with highly covert and reliable features is quite desirable to combat the sophisticated counterfeiters. Herein, we report a simultaneously triple-modal fluorescent characteristic of CsPbBr3@Cs4PbBr6/SiO2 by the excitation of thermal, ultraviolet (UV) and infrared (IR) light for the first time, which can be applied for the multiple modal anti-counterfeiting codes. The diphasic structure CsPbBr3@Cs4PbBr6 nanocrystals (NCs) were synthesized via the typical re-precipitation method followed by uniformly encapsulation into silica microspheres. Cubic CsPbBr3 is responsible for the functions of anti-counterfeiting, while Cs4PbBr6 crystalline and SiO2 are mainly to protect unstable CsPbBr3 NCs from being destroyed by ambient conditions. The as-prepared CsPbBr3@Cs4PbBr6/SiO2 NCs possess improved stability and are capable of forming printable ink with organic binders for patterns. Interestingly, the fluorescence of diphasic CsPbBr3@Cs4PbBr6/SiO2 capsule patterns can be reversibly switched by the heating, UV and IR light irradiation, which has been applied as triple-modal fluorescent anti-counterfeiting codes. The results demonstrate that the perovskite@silica capsules are highly promising for myriad applications in areas such as fluorescent anti-counterfeiting, optoelectronic devices, medical diagnosis, and biological imaging. Keywords: inorganic perovskites, anti-counterfeiting, Cs4PbBr6, two photon photoluminescence, thermal responsive luminescence

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Introduction Counterfeiting is a global longstanding problem that is causing significant negative impacts on the social economy and present serious security threat to both individuals and communities.1-5 Thus, anti-counterfeiting is in high demand for protecting important and valuable items that should not be replicated, such as brands, luxury items, banknotes, tickets, and certificates. During the last decades, a wide variety of security and anti-counterfeiting technologies have been developed, including the recent magnetic response, plasmonic security labels, and luminescence printing6-8. Among these techniques, photoluminescence (PL) offers advantages in easy handling, high-throughput, and facile design, and is thus the most widely applied against counterfeiting. However, the traditional luminescent materials used in anti-counterfeiting

generally

exhibit

unicolor,

and

sometimes

dual-modal

luminescence upon excitation by ultraviolet (UV) or/and near-infrared (NIR) light (that is, the PL or/and up-conversion (UC) PL modes)9-11. Unfortunately, similar emission characteristics can be obtained by utilizing certain substitutes, thus exhibit poor anti-counterfeiting performance, which makes these techniques appear to be comparatively less effective in protecting documents, and easily cracked. Therefore, it is still highly desirable to develop novel PL materials with more covert and reliable features capable of combating sophisticated counterfeiters. PL materials possessing multi-modal emission properties are considered to be a powerful strategy to address this problem because it is more difficult for counterfeiters to replicate these behaviors.12 Consequently, exploring a fluorescent system with multiple mode features would provide a much higher level of security against counterfeiting. In the last two years, the halide perovskite materials, especially all-inorganic composition perovskite cesium lead halides (CsPbX3, CsPb2X5, Cs4PbX6, X = Cl, Br, and I) have been in the forefront of semiconductor fields on account of their amazing and unique luminescent and photoelectric properties13-19, some of which could be superexcellent option for advanced anti-counterfeiting. First, CsPbBr3 nanocrystals (NCs) under UV light excitation exhibit high PL quantum yield (QY), tunable light


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emission wavelength (380 nm-700 nm), narrow emission peak with full width at half maximum (FWHM) of about 20 nm. Second, CsPbBr3 NCs with a nonlinear absorption coefficient of up to 0.085 cm/GW have been proven to have two-photon induced up-conversion PL (UC-PL) properties20-21. More interestingly, some literatures indicated that CsPbBr3 NCs presented extremely heat response abilities, which can emit bright light at room temperature, while almost no light when heated to a certain temperature21-22. Thus, CsPbBr3 reveals above triple-modal PL functions, that show a huge potential in the field of anti-counterfeiting and advanced security. However, CsPbBr3 exhibits relatively poorer stability and serious fluorescence quenching, which make it circumscribed for practical application. Hence developing new strategy for inorganic CsPbBr3 with high stability and outstanding PL properties simultaneously is significant for advanced anti-counterfeiting fields. CsPbBr3 possesses the potential for novel anti-counterfeiting application, while its stabilities can not meet the requirement. Relatively, its derivative phase Cs4PbBr6 shows better immunity against dampness and heating.23-25 Meanwhile, previous report indicated that Cs4PbBr6 lattice spacing could match well with the cubic CsPbBr3 lattice constants24, laying the foundation for effective recombination. Herein, we report on a novel strategy for synthesis of highly stable CsPbBr3@Cs4PbBr6/SiO2 composites with triple-modal (UV, IR, and thermal) fluorescent anti-counterfeiting characteristics for the first time. We synthesized the diphasic CsPbBr3@Cs4PbBr6 composite NCs via the modified re-precipitation and further wrapped them by silica matrix

to

enhance

the

stability.

The

CsPbBr3@Cs4PbBr6/SiO2 composites

simultaneously remain the super triple-modal fluorescence characteristics and high stability. Benefitting from silicon-oxygen bonds, the as-prepared CsPbBr3@/SiO2 composites can be readily dispersed in polydimethylsiloxane (PMDS) silica gel forming

highly

stable

ink

for

subsequent

pattern

film

processes.

The

CsPbBr3@Cs4PbBr6/silica composite films exhibit highly sensitive response to UV and IR light irradiation, and heating changes. Thus, exceedingly sensitive response to triple modes makes CsPbBr3@Cs4PbBr6/silica NCs provide a strengthened and more reliable anti-counterfeiting effect, which can render various platform highly



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challenging to be forged. Results and discussion The perovskite CsPbBr3@Cs4PbBr6/SiO2 composites were synthesized by a modified ligand-assisted re-precipitation method, as shown in Figure 1. The dimethylsulfoxide (DMSO) precursor solution of PbBr2 and CsBr was injected into toluene with oleic acid (OA), oleyl amine (OAm), and (3-aminopropyl) triethoxysilane (APTES) at room temperature. In this commixture system, cubic CsPbBr3 were formed immediately as the injection, and Cs4PbBr6 were grown surround the CsPbBr3 nanocrystals subsequently. For the sluggish hydrolysis reaction, the formation of silica matrix around nanocrystals happened later, which attributed to cross-linking

reactions

between

Si-O-C2H5

groups

attached

to

the

CsPbBr3@Cs4PbBr6 nanocrystals. The OA and OAm were used as surfactants to reduce the dimension and control the surface state of nanocrystals, thus maintain high PL QY (Fig. S1). Shown from the Fig. S1a, without the surfactant of OAm and OA, the particle would be very large, and its PL emission became weak. With an increase in the surfactant concentration, the PL properties was evidently enhanced, and the PL QY reached 84.5%. The SEM images (Fig. S1c) further exhibit that particle were evidently reduced after adding the surfactants. In this system, cubic CsPbBr3 grew inside rhombohedral Cs4PbBr6 by endotaxy, forming CsPbBr3@Cs4PbBr6 diphasic composite structure24. And the APTES as a kind of oxysilane, were transformed into the silica matrix capping the diphasic NCs surface with the reaction time increasing. Meanwhile, the hydrolysis properties of the Si-O-C2H5 group can prevent the polar molecules from destroying the perovskite nanocrystals, further enhancing the stability of the NCs26. The CsPbBr3@Cs4PbBr6/SiO2 composites form the “jujube cake structure” ultimately, as the diagram shown in Figure 1. Cs4PbBr6 crystalline matrix and silica wrapper provide dual protection for CsPbBr3 nanocrystals to maintain their high optical properties. The

phase

and

microstructure

transformation

process

of

CsPbBr3@Cs4PbBr6/SiO2 composites are shown explicitly from X-Ray Powder Diffraction and SEM analysis (Fig. 2). When the precursor was injected, the cubic


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CsPbBr3 were produced immediately. As seen in Fig. 2a and 2e, the majority were cubic phase after 10 s, mingled with a little fraction of rhombohedral phase. After 30 min, rhombohedral Cs4PbBr6 crystals were totally formed as a crystalline matrix, which could be seen intuitively from Fig. 2b. The XRD patterns in Fig. 2f show the coexistence of both cubic and rhombohedral phase, confirming the CsPbBr3 embedded into Cs4PbBr6. With the advance of the reaction, laminar Si-O frame were gradually built up around rhombic CsPbBr3@Cs4PbBr6 nanocrystals due to the hydrolysis between APTES molecules (Fig. 2c). The broad band ranging from ~14 to 25 degree in Fig. 2g is attributed to amorphous structure of silica. With the increasing time, the silica wrapper is getting thicker and CsPbBr3@Cs4PbBr6/SiO2 nanocrystals grow larger (up to 200 nm for 12 h), as shown in Fig.2d. The spherical silica wrapper is owed to the strong steric hindrance of the APTES molecules26. The more obvious broad band and unchanged peaks of XRD pattern (Fig. 2h) indirectly indicates the encapsulation of perovskite composites in the silica matrix. The SEM images and XRD patterns exhibit the evolution process of nanocrystals from CsPbBr3 to CsPbBr3@Cs4PbBr6/SiO2, confirming the successful passivation of CsPbBr3 by Cs4PbBr6 crystalline matrix and silica wrapper. The microstructure of CsPbBr3@Cs4PbBr6/SiO2 was further analyzed as shown in Fig. 3. Transmission electron microscopy (TEM) image exhibits a typical CsPbBr3@Cs4PbBr6/SiO2 particle (Fig. 3a). After reaction for 12 h, the SiO2 matrix grew into spheres (∼200 nm) because of the strong steric hindrance derived from branched

APTES,

and

multiple

CsPbBr3@Cs4PbBr6

were

homogeneously

incorporated into the spheres with some nanocrystals simply attached to the surface of the spheres (Fig. S2). We further illustrated the distribution of CsPbBr3@Cs4PbBr6 NCs in SiO2 matrix through diagrammatic figure (Fig. 3b), and the illustration of CsPbBr3@Cs4PbBr6 is also presented (Fig. 3c). Cubic CsPbBr3 NCs can align simultaneously along all three dimensions of the Cs4PbBr6 compound lattice24, so CsPbBr3 NCs could exist steadily inside Cs4PbBr6. The homogeneous layered morphology of silica integument around the CsPbBr3@Cs4PbBr6 NCs demonstrates the amorphous form of SiO2 indirectly, which tallies with the results of XRD. SEM



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image (Fig. S2) shows that the morphology of CsPbBr3@Cs4PbBr6/SiO2 NCs are nearly all ball shape, consistent with reports of MAPbBr3/SiO2 in the literature27. CsPbBr3

NCs

grow

inside

Cs4PbBr6 along

certain

lattice

plane,

and

CsPbBr3@Cs4PbBr6 NCs embed uniformly in SiO2 matrix forming the “jujube cake structure”. The structure of CsPbBr3@Cs4PbBr6/SiO2 was further demonstrated by element

mapping

analysis.

Element

mapping

images

of

a

typical

CsPbBr3@Cs4PbBr6/SiO2 with a diameter of 200 nm are shown in Figure 3d-i. The Cs, Pb, Br, and Si, O elements were found homogeneous distributed in the entire range of the crystal, confirming that perovskite composite NCs were successfully and uniformly encapsulated into the silica matrix. The green emission of CsPbBr3@Cs4PbBr6/SiO2 are all attributed to CsPbBr3, because Cs4PbBr6 NCs are colorless25. Even a very small amount of CsPbBr3 can be enough to provide bright green flsorescence for the whole NC. The as-prepared CsPbBr3@Cs4PbBr6/SiO2 powder have a narrow green emission peak at 520 nm with a full-width at half-maximum (FWHM) of 22 nm (Fig. S3), and have a PL QY of 48%, which is comparable to the previous work28. The slight red shift of PL compared to CsPbBr3 was due to size effect. Although the CsPbBr3 NCs shows extraordinary luminescence, but they quenched sharply without the surface passivation. Relevant performance about stability will be discussed later. Importantly, CsPbBr3@Cs4PbBr6/SiO2 can be dispersed in various organic binders, especially in silicone gel and UV gel. PDMS was used as the typical binder here because of the hydrophobicity and facile usage, meanwhile Si-O-C2H5 group attached to the CsPbBr3@Cs4PbBr6/SiO2 NCs had a certain interaction with this kind of siloxane gel. Hence the CsPbBr3@Cs4PbBr6/SiO2 NCs can be mingled well in PDMS. The as-synthesized CsPbBr3@Cs4PbBr6/SiO2 powder was light yellow (inset in

Fig.

4a)

and

possessed

strong

green

emission.

Fig.

4a

show

the

CsPbBr3@Cs4PbBr6/SiO2-PDMS ink photographs under daylight and UV light respectively, which exhibit high uniformity and excellent PL properties. The peak green opaque sol is due to the high concentration of CsPbBr3@Cs4PbBr6/SiO2 powder in PDMS gel. On account of the favorable fluidity, CsPbBr3@Cs4PbBr6/SiO2-PDMS


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mixture can be used to cast films by all kinds of solution process, such as spin-coating, spraying and ink-jet printing. A CsPbBr3@Cs4PbBr6/SiO2-PDMS film spin-coated on the glass substrate is displayed in the Fig. 4b, the film is almost invisible under the white light while exerts strong green luminescence under UV light. This kind of highly identifiability is very suitable for practical anti-counterfeiting system. Patterning is always an effective manner for anti-counterfeiting application, so a pattern film with the “INK” logo (Fig. 4b) has been made, and the logo exhibits high PL properties with PL QY 45%, only a slight decrease compared to powder. The high PL QY can be preserved attributed to the protection of dual protection shell. Profiting from excellent light emission properties and the high stability, this new kind of composite materials CsPbBr3@Cs4PbBr6/SiO2 have a huge potential in sensors, displays, light switches, security ink, optoelectronic devices, and advanced anti-counterfeiting. According

to

the

aforementioned

content,

the

fluorescence

of

CsPbBr3@Cs4PbBr6/SiO2 can be used for triple-modal optical anti-counterfeiting codes for its simultaneous switching characteristic driven by UV, IR and thermal excitation. The first model was PL emission as shown in Fig. 5a. While excited by 325 nm UV light, the electrons on the valence band jump into the conduction band after absorbing photons, green emission is produced when electrons jump back to valence band. The schematic diagram is presented in Fig. 5a inset. The PL spectrum shows the actual fluorescence spectral changes of the composite films before and after UV light irradiation. It is clearly seen that the films initially exhibit a strong fluorescence emission at 525 nm under UV light. Whereas, the integrated fluorescence intensity of the sample under dark was significantly reduced to 0.6%, and the PL change is invisible. The visualized visual appearances of CsPbBr3@Cs4PbBr6/SiO2/PDMS films are presented in Fig. 4b. The repeated PL on/off switching in the films with alternate UV and visible-light irradiation was characterized as shown in Fig. S4a. After ten cycles of on/off switching, PL intensity shows no obvious change. Notably, the switching behavior is reproducible and reversible, which is prerequisite for practical application. The above-mentioned



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luminescent properties are the traditional fluorescent anti-counterfeiting features. Besides from the PL properties, the second model is the UC-PL, which is motivated by IR light. The UC-PL can be realized through excitation of an exciton (electron-hole pair) with two-photon absorption, which needs to simultaneously absorb two photons through a virtual state. The schematic diagram is shown in Fig.5b inset. It is needs to emphasize that the IR light used here is always the high-frequency laser. The right side of Fig. 5b presents the UC-PL spectrum of the CsPbBr3@Cs4PbBr6/SiO2/PDMS composite film under an NIR femtosecond pulse laser (800 nm). The PL emission excited at 800 nm is almost the same as the PL excited at 325 nm with a slightly redshift of 2 nm, which is unanimous to previous reports20-21. And the inset photographs in Fig. 5b reveal the strong green emission of CsPbBr3@Cs4PbBr6/SiO2/PDMS composite film under NIR laser while transparent in daylight. Identically, the UC-PL on/off switching behavior was tested (Fig. S4b), and it shows up the similar stability and repeatability as PL property. Simultaneously possessing above PL and UC-PL properties makes CsPbBr3@Cs4PbBr6/SiO2 composite materials own the super dual-modal fluorescent anti-counterfeiting function in contrast to traditional single modal fluorescent anti-counterfeiting materials. It will bring new choices for the current anti-counterfeiting industry bothered by mixed forged and fake commodity. Even more interestingly, besides above

dual-modal

performance,

this

material

has

the

third

virtue,

temperature-dependent PL intensity. The CsPbBr3@Cs4PbBr6 show up progressive PL emission intensity changes depended on the temperature change, which offers the third anti-counterfeiting modal. Perovskites are very sensitive to the thermal changes as described in the introduction. The temperature-dependent PL spectra of the CsPbBr3@Cs4PbBr6/SiO2/PDMS composite film measured in the temperature ranging from 25 to 150 °C were presented in Fig. 6a. The films can emit strong fluorescence at room temperature. As the temperature increased, the PL emission intensity decreased gradually. The fluorescence of films can be quenched close to the background at 150 °C. When the


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composite film was cooled to the initial temperature, the PL emission recovered its original green color completely. Interestingly, the fluorescence quenching and recovery of Cs4PbBr6 can be repetitive by the alternate heating and cooling, respectively. Subsequently, we tested the fluorescence quenching and recovering of composite film in the solid-state for real application. This PL measurement was executed in the condition of consecutive heating-cooling cycle over the temperature 30 ↔ 150 °C and 60 ↔ 150 °C respectively, and the result was shown in Figure 6b. The tests were carried in atmospheric environment. It can be noted that, there are no recession of the intensity in the process of circulation, which shows a good reversibility over 10 times. And the PL spectrums at RT before and after 10 cycles of cooling and heating were attested. There was no obvious deterioration of the intensity, position, and shape of the PL spectra for the samples tested, providing a fully reversible emission spectral change (Figure S5). More heating-cooling cycles of 50 times were tried (Fig.S6) and the attenuation is only 6%. This thermal-dependent stimulus-responsive property of Cs4PbBr6/SiO2/PDMS composite films is almost invisible by naked eyes at 150 °C, which is unique and therefore holds great promise for applications in anti-counterfeiting. The temperature-dependent PL intensity was analyzed by the Arrhenius formula (1)29, where I(T) is the (energy scale) integrated PL intensity at temperature T, I0 is the 0 K integrated intensity, Ea is the activation energy of thermal quenching, A is a constant, and kB is the Boltzmann constant. Figure S7 (Supporting Information) shows the Arrhenius plots for PL integrated intensities for composite films. The extracted activation energies, Ea, is 38 meV. The relative lower activation energies than those of classical semiconductor NCs30-33suggest the Cs4PbBr6 NCs are more sensitive to temperature.

I (T ) =

I0

(1)

E 1 + A exp(− a ) kBT



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This progressive decrease of PL intensity with temperature might be due to the thermally activated nonradiative deactivation of exciton in the NCs29, 34-35. As the temperature rise, nonradiative decay of NCs bring about the recession of PL intensity. To obtain information about the excited states of these NCs, we measured the fluorescence lifetimes at the temperature range from 30 to 150 °C. The PL lifetime data is presented in Fig. S8. As shown in the results, the fluorescence decay at 150 °C (1.1 ns) is much faster compared with that at 20 °C (9.8 ns), following the wll-documented trend for the temperature-dependent lifetimes of II–VI semiconductor QDs36-38. The observed shortening of the PL lifetime with increase in temperature signifies that the temperature-dependent nonradiative decay of CsPbBr3@Cs4PbBr6 NCs is responsible for the thermal quenching of PL intensity. Furthermore, these decreases in PL lifetimes at elevated temperature fully recover upon heating-cooling process (Fig. S9), supporting that this thermal-dependent quenching process is not intrinsic radiation and has reversible nature. This temperature-dependent PL quenching can be presented more intuitively by patterning films. Fig. 7b-f exhibits the pattern of films with a “flower” at different temperature after heating-cooling for 10 cycles, the visible photograph is consistent with the PL intensity shown in Fig. 7a. The “flower” can emit strong green light at 30 °C, and the emission gradually fades with the increasing temperature. When the temperature increases to 120 °C, the green color becomes very weak and completely disappear at 150 °C. But the strong green emission can be recovered when the temperature is cooled down to 30 °C. Similarly, a micro “NUST” logo was made to demonstrate the thermal-quenching of UC-PL, as seen in Fig. 7h-l. Under excited 800 nm fs laser, The UC-PL mapping of the “NUST” films heated from 30 °C to 150 °C exhibit the same tendency of that under UV light. Both fluorescence patterns show analogical thermal sensibility, which derive out the third modal security ability, thermal anti-counterfeiting. It was found that the highly fluorescent CsPbBr3@Cs4PbBr6-based system showed on/off modulation through a not only UV and IR light-driven but also thermally


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driven. Such a unique response can be appropriately described by a triple logic gate with PL quenching as the optical output in response to three inputs of “UV light”, “IR light”, and “heat”. We believe that the as-prepared CsPbBr3@Cs4PbBr6-based films with the high-contrast reversible PL switching under triple operation have a huge potential in future anti-counterfeiting. To meet the requirements of practical application in advanced anti-counterfeiting system, the storage and thermal-stability of CsPbBr3@Cs4PbBr6/SiO2/PDMS films are tested by the attenuation of PL intensity as presented in Fig 8. As a contrast, CsPbBr3 and CsPbBr3@Cs4PbBr6 based films were also tested to verify the significantly enhanced stability of CsPbBr3@Cs4PbBr6/SiO2. The storage-stability test was

carried

in

ambient

conditions

with

RH

50%,

the

PL

properties

CsPbBr3@Cs4PbBr6/SiO2 of the green QD/silica powder has no obvious change over a period of two months with a slight decrease of 9%. While CsPbBr3@Cs4PbBr6 synthesized without SiO2 matrix have a remnant PL emission of 74% with a decrease of 26%. Distinctly, pure CsPbBr3 degenerates sharply to dark within 20 days. After adequate passivation, stability of CsPbBr3 in atmospheric environment are improved greatly, suitable for practical application. Thermal-based PL tests were operated from 30 to 150 oC in RH 50% ambient conditions. As mentioned before, PL intensity was gradually

decreased

with

the

increasing

temperature.

CsPbBr3

and

CsPbBr3@Cs4PbBr6 exerted steeper slump that is due to the fluorescence quenching of intrinsic radiation. Upon heating-cooling for ten cycles, CsPbBr3@Cs4PbBr6/SiO2 has no obvious degeneration (Fig. 6b), while CsPbBr3 and CsPbBr3@Cs4PbBr6 damp seriously, especially CsPbBr3 (Fig. S10). By the above results, the encapsulation of the CsPbBr3 NCs with Cs4PbBr6 crystalline and silica matrix is an effectively avenue to enhance the stability. In conclusion, the CsPbBr3@Cs4PbBr6/SiO2/PDMS films possess favorable storage and photo-stability, suitable for various practical applications, especially for fluorescent falsification-preventing. Inspired

by

the

above

exceptional

optical

properties

of

CsPbBr3@Cs4PbBr6/SiO2-based films, we design a small logo to simulate the



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conventional anti-counterfeiting codes. As shown in Figure 9, the pattern consisted of a Chinese character “中” and an English character “O” were handwritten on a label (containing a traditional anti-counterfeiting patterns, “100”) using the aforementioned Cs4PbBr6-based inks. The outline of the codes were marked with imaginary lines. The two characters on the bill was almost invisible in daylight, and no pattern of information could be obtained, reflecting the fact that CsPbBr3@Cs4PbBr6/SiO2-based films are almost invisible in normal light. Upon irradiation with a 365 nm UV lamp at room temperature, bright and high-contrast green characters “中” and “O” were seen (PL mode), which are similar to the existing anti-counterfeiting mark on the label itself (that is, the yellow number “100”). Upon excitation with a femtosecond pulse laser (800 nm), green colored characters “中” and “O” were observed (UC-PL mode), yet the yellow number “100” (anti-counterfeiting mark of the label itself) exhibited non-luminance. And the fluorescence images could be erased to the non-PL form without the irradiation of UV and IR light. Besides the excitation by UV light or IR excitation at room temperature, the temperature-dependent fluorescent characteristics in the two modals were also observed. When the temperature of the pattern increased to 150 °C, the fluorescence of NCs were gradually quenched to the background. After further cooling to room temperature, strong fluorescence of characters “O” and “中” can be recruited again at the note under UV light (III) or IR light (III’). While the commercial “100” label exhibit no evident change range from room temperature to 150 °C under UV light and IR light. Thus, the fluorescence of as-prepared CsPbBr3@Cs4PbBr6/SiO2-based composites could be motivated by UV and IR light, and this characteristic could respond sensitively to temperature in their surroundings. Compared to traditional fluorescent security materials, CsPbBr3@Cs4PbBr6/SiO2 composites present improved anti-counterfeiting performance, which could hardly be copied. Moreover, the printed characters retained their high stability after being observed for 2 months in an indoor environment (Fig. S11, inset film still exhibits dazzling green emission after storage of 2 months), which is beneficial for practical applications. These unique PL


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properties of the CsPbBr3@Cs4PbBr6-based composites are derived from their intrinsic attributes, which is very rare and highly difficult to replicate, make it an optimal application in advanced anti-counterfeiting. Conclusion In summary, we have constructed a high-performance CsPbBr3@Cs4PbBr6/SiO2 composites via the modified re-precipitation method. This new kind of composites consist of cubic CsPbBr3 NCs wrapped by crystalline Cs4PbBr6 and silica matrix, which exhibit particular triple-mode emission simultaneously possessing PL, UC-PL and thermal-related PL. The composite structure was confirmed by XRD, SEM and element mapping. The outstanding anti-counterfeiting features mainly come from CsPbBr3 NCs, while Cs4PbBr6 and SiO2 matrix provide double protection to enhance the stabilities. The PL property, two-photon induced UC-PL and thermal-triggered PL emission of the composite NCs were represented by PL spectrums and visual photographs. Meanwhile, the storage stability and thermal stability were also tested through the attenuation of PL intensity. This kind of new photoelectric materials based on CsPbBr3@Cs4PbBr6/SiO2 inaugurate a new triple-mode prevention route for the future anti-counterfeiting technologies that could be commercially implemented for real world forensic authentications. Meanwhile, in situ formation of silica matrix around perovskite NCs provided a valid optimization for NCs. With higher stability and remarkable luminescence specialities, the CsPbBr3@Cs4PbBr6/SiO2 composites can be further applied in myriad areas such as optoelectronic devices, medical diagnosis, and biological imaging. Experiment Section Synthesis of CsPbBr3@Cs4PbBr6/SiO2 composites The synthesis procedure of CsPbBr3@Cs4PbBr6/SiO2 with OA and OAm ligands followed the modified re-precipitation method. Typically, the DMSO precursor solution was prepared by dissolving 0.16 mmol CsBr, 0.2 mmol PbBr2 in 5 mL DMSO solvent and sonicated until a transparent solution was formed. 1 mL of DMSO



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precursor solution was injected into 20 mL toluene with 200 µL oleyl amine (OAm), 250 µL oleic acid (OA), and 200 µL aminopropyltriethoxysilane (APTES) at room temperature. In order to form the silica matrix, the reaction solution was further stirred for 12 hours. After stirring 12 h, the precipitates were collected through centrifugation at 5000 rpm for 10 min, and the CsPbBr3@Cs4PbBr6/SiO2 powders were obtained by vacuum drying. Preparation of CsPbBr3@Cs4PbBr6/SiO2-based anti-counterfeiting ink With the mechanical stirring process, 60 mg CsPbBr3@Cs4PbBr6/SiO2 powders were dispersed homogeneously in 4 g Sylgard 184 including “base” and “curing agent” with a weight ratio of 10:1. The resultant mixture was dripped or spin-coated on a clear glass to form the CsPbBr3@Cs4PbBr6/SiO2/PDMS film. For comparison purpose, the pure CsPbBr3@Cs4PbBr6/PDMS films was also prepared following the same method mentioned above. Characterization Powder X-ray diffraction (XRD) patterns were performed on a Bruker D8 Advance X-ray Diffractometer at 40 kV and 40 mA using Cu Kα radiation (λ=1.5406 Å). The morphologies were investigated by the JEOL JSM-7800F field emission scanning electron microscope (FESEM), and the JEM-2100F and JEM-ARM200F transmission electron microscope (TEM) instruments. PL spectra were measured on an FLS920P fluorescence spectrometer (Edinburgh Instruments) equipped with a photomultiplier in a thermoelectrically cooled housing (R928P, Hamamatsu) with a 450 W xenon arc lamp as the excitation for steady-state spectra and a picosecond pulsed diode laser (EPD-405 nm, pulse width: 49 ps, Edinburgh Instruments) for PL lifetime (time-correlated single-photon counting) measurements. The absolute PL quantum yields were detected using a fluorescence spectrometer with an integrated sphere (Hamamatsu Photonics). The photo-stability measurements were analyzed in a temperature and humidity chamber (25 °C, RH 60%) using a 454 nm LED light (21 mW/cm2) provided by Ocean Optics LS-450. Temperature-dependent PL and PL


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delays measurements were performed using a variable temperature liquid nitrogen optical cryostat Optistat DN-V2 controlled via a cryogenic programmable temperature controller MercuryiTC (Oxford Instruments) with a temperature stability of ±0.1 K (measured over a 10 min period). The temperature-dependent PL mapping were conducted by coupling the scanning galvanometer and WhiteLase™ SC400-4 fs laser source. The sample with the pattern “NUST” was prepared by the Focused ion beam scanning electron microscopy (FIB-SEM) (Zeiss Auriga) from Nanjing University of Science and Technology. Supporting information The effect of surfactant concentration; large scale SEM; contrast of PL spectra between original and improved sample; heat stability tests in two modals of luminescence; PL spectra before and after 10 cycles; more cycles of heat stability tests; Arrhenius plots for PL integrated intensities; PL decays curve at different temperature; PL decays curve before and after 10 cycles; contrast of heat stability tests; PL spectra of composite films and luminous photograph. Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2016YFB0401701), The National Basic Research Program of China (2014CB931702), NSFC (61604074, 51572128, 51672132), NSFC-RGC (5151101197), the Natural Science Foundation of Jiangsu Province (BK20160827), China Postdoctoral Science Foundation (2016M590455), the Fundamental Research Funds for the Central Universities (No. 30915012205, 30916015106), and PAPD of Jiangsu Higher Education Institutions. References 1.

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Figure caption Fig. 1 Schematic illustration of the formation of CsPbBr3@Cs4PbBr6/SiO2 composites. Fig. 2 SEM images and corresponding XRD patterns of productions for different reaction time: (a, e) CsPbBr3 NCs for 10s, (b, f) CsPbBr3@Cs4PbBr6 for 30min, (c, g) CsPbBr3@Cs4PbBr6 wrapped with few silica for 1h, (d, h) CsPbBr3@Cs4PbBr6/SiO2 composites for 12h. Fig. 3 Microstructure of CsPbBr3@Cs4PbBr6/SiO2 composites. (a) TEM image of a typical CsPbBr3@Cs4PbBr6/SiO2 composite. (b) Schematic diagram of a typical CsPbBr3@Cs4PbBr6/SiO2

composite.

(c)

Schematic

diagram

of

a

typical

CsPbBr3@Cs4PbBr6 embedded in silica matrix. (d–i) Element mapping analysis showing the elemental distribution of silica, oxygen, cesium, lead, and bromide, respectively. Fig. 4 (a) Photograph of the CsPbBr3@Cs4PbBr6/SiO2 composites dispersed in PDMS gel under ambient and UV light. Inset: CsPbBr3@Cs4PbBr6/SiO2 composites powder.


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(b) Photograph of spin coated films and handwritten “INK” pattern by CsPbBr3@Cs4PbBr6/SiO2 based PDMS gel under ambient condition and irradiated by 454 nm LED light sources. Fig. 5 (a) PL spectrum of the CsPbBr3@Cs4PbBr6/SiO2-based composite films with 325 nm excitations. Inset: schematic diagrams of one-photon absorption/PL process. (b) PL spectrum of the CsPbBr3@/SiO2-based composite films with 800 nm (TPA) excitations. Inset (left): schematic diagrams of two-photon absorption/UC-PL process. Inset (right): CsPbBr3@Cs4PbBr6/SiO2-based composite films under ambient and IR light. Fig. 6 (a) PL spectra of CsPbBr3@Cs4PbBr6/SiO2 composite films at various temperatures range from 20 oC to 150 oC. (b) Reversible fluorescent response of 10 consecutive cycles at 30-150 oC and 60-150 oC, respectively. Fig. 7 (a-f) PL photographs and (g-l) UC-PL images of the films at various temperatures range from 30 oC to 150 oC. Fig. 8 (a) Storage-stability tests of CsPbBr3@Cs4PbBr6/SiO2, CsPbBr3@Cs4PbBr6 and CsPbBr3 in ambient conditions with RH 50% for two months. (b) Thermal-stability tests of CsPbBr3@Cs4PbBr6/SiO2, CsPbBr3@Cs4PbBr6 and CsPbBr3 heated from RT to 150 oC in ambient conditions. Fig. 9 Demonstration of the triple-mode anti-counterfeiting CsPbBr3@Cs4PbBr6/SiO2 -based films. (I) excitation with a 365 nm UV lamp (turned ON) and dark (turned OFF); (II) excitation with a 800 nm femtosecond pulse laser (images are spots irradiated by the laser) (turned ON) and dark (turned OFF); (III, III’) excitation with a 365 nm UV lamp and a 800 nm femtosecond pulse laser at 150 °C (turned OFF) and room temperature (turned ON).



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Fig. 1


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Fig. 2



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Fig. 3


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Fig. 4



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Fig. 5


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Fig. 6



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Fig. 7


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Fig. 8



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Fig. 9


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