Ruoting Yang,Dongwen Yang,Meng Wang,Fei Zhang,Xinzhen Ji,Mengyao Zhang,Mochen Jia,Xu Chen,Di Wu,Xin Jian Li,Yu Zhang,Zhifeng Shi,Chongxin Shan
High‐Efficiency and Stable Long‐Persistent Luminescence from Undoped Cesium Cadmium Chlorine Crystals Induced by Intrinsic Point Defects
cesium cadmium chloride,de‐trapping,information storage,long‐persistent luminescence,stability
Abstract Application of long‐persistent luminescence (LPL) materials in many technological fields is in the spotlight. However, the exploration of undoped persistent luminescent materials with high emission efficiency, robust stability, and long persistent duration remains challenging. Here, inorganic cesium cadmium chlorine (CsCdCl3) is developed, featuring remarkable LPL characteristics at room temperature, which is synthesized by a facile hydrothermal method. Excited by ultraviolet light, the CsCdCl3 crystals exhibit an intense yellow emission with a large photoluminescence quantum yield of ≈90%. Different from the reported systems with lanthanides or transition metals doping, the CsCdCl3 crystals without dopants perform yellow LPL with a long duration of 6000 s. Joint experiment‐theory characterizations reveal the intrinsic point defects of CsCdCl3 act as the trap centers of excited electrons and the carrier de‐trapping process from such trap sites to localized emission centers contributes to the LPL. Encouraged by the attractive fluorescence and persistent luminescence as well as good stability of CsCdCl3 against environment oxygen/moisture (75%), heat (100 °C for 10 h), and ultraviolet light irradiation, an effective dual‐mode information storage‐reading application is demonstrated. The results open up a new frontier for exploring LPL materials without dopants and provide an opportunity for advanced information storage compatible for practical applications.
High‐Efficiency and Stable Long‐Persistent Luminescence from Undoped Cesium Cadmium Chlorine Crystals Induced by Intrinsic Point Defects

Application of long‐persistent luminescence (LPL) materials in many technological fields is in the spotlight. However, the exploration of undoped persistent luminescent materials with high emission efficiency, robust stability, and long persistent duration remains challenging. Here, inorganic cesium cadmium chlorine (CsCdCl3) is developed, featuring remarkable LPL characteristics at room temperature, which is synthesized by a facile hydrothermal method. Excited by ultraviolet light, the CsCdCl3 crystals exhibit an intense yellow emission with a large photoluminescence quantum yield of ≈90%. Different from the reported systems with lanthanides or transition metals doping, the CsCdCl3 crystals without dopants perform yellow LPL with a long duration of 6000 s. Joint experiment‐theory characterizations reveal the intrinsic point defects of CsCdCl3 act as the trap centers of excited electrons and the carrier de‐trapping process from such trap sites to localized emission centers contributes to the LPL. Encouraged by the attractive fluorescence and persistent luminescence as well as good stability of CsCdCl3 against environment oxygen/moisture (75%), heat (100 °C for 10 h), and ultraviolet light irradiation, an effective dual‐mode information storage‐reading application is demonstrated. The results open up a new frontier for exploring LPL materials without dopants and provide an opportunity for advanced information storage compatible for practical applications.

Long‐persistent luminescence (LPL) materials that still emit light after ceasing the excitation source have acquired enduring research interest owing to their fundamental scientific importance and versatile applications.[ , , , ] Generally, traps and emitters as active centers are related to the occurrence of LPL. The former determines the afterglow performances including the persistent emission intensity and decay time, while the latter determines the emission wavelength. In host matrices, the defects‐related traps store excitation energy and then release it to the emitters under external stimulation. Since its first discovery in the early 20th century, a large number of persistent luminescence materials have been explored by changing host matrices with different crystal structures and dopants with different combinations of transition metals or lanthanides.[ , ] Among them, materials with activators doped in different oxide hosts (aluminates, silicates, stannate, etc.)[ , , ] exhibit excellent LPL performance, and the corresponding spectral range covers ultraviolet (UV), visible, and even near‐infrared light.[ , , ] But the high lattice‐formation energy (≈3000 kJ mol−1) of such materials results in the requirement of high synthesis temperature for phase generation,[ ] leading to an increase in manufacturing costs and safety risks.[ , , ] Moreover, a higher synthesis temperature would induce agglomerated products with irregular morphology with strong scattering effect, limiting the signal collection.[ , , ] Besides, organic LPL materials are also constantly being explored,[ ] which can be easily synthesized without the need for high temperatures.[ ] While traditional organic LPL materials always have shorter lifetimes at room temperature (RT) compared with the inorganic counterparts.[ ] To prolong the charge transfer transition of organic LPL materials, many effective strategies have been developed, such as engineering crystallization,[ , ] host‐guest interactions,[ , , ] heavy metal doping,[ ] and hydrogen bond interactions.[ , ] Although rapid progresses in this field have been witnessed in recent years, some intrinsic disadvantages of organic LPL materials cast a gloomy shadow for their further practical applications.[ , , , ] For instance, the organic molecules are very susceptible to air environment; oxygen and moisture would cause quenching of triplet excitons. In the past several years, metal‐halide perovskites have gained increasing attention by virtue of their great potential in optoelectronic applications.[ , , , , , , , ] Such materials of both low lattice‐formation energy and versatile doping sites could enable a low‐temperature synthesis of LPL crystals.[ ] In the past two years, much interesting progress on LPL characteristics of perovskites has been reported successively. For instance, Xu et al. prepared the CsPbBr3:Ln3+ nanocrystals with high color‐purity LPL via a melt‐quenching process, which creates the carrier capture centers by replacing Pb2+ with Ln3+.[ ] Zhang et al. prepared the Cs2NaxAg1‐xInCl6:Mn2+ and Cs2Na0.2Ag0.8InCl6:Yb3+ single crystals (SCs) via a simple hydrothermal process, exhibiting excellent orange and near‐infrared LPL, respectively.[ , ] Further, they employed a similar method to grow Tb3+‐doped Cs2NaScCl6 SCs at 180 °C, which exhibit a bright green LPL after ceasing the X‐ray excitation.[ ] The above results suggest the LPL performances achieved from metal‐halide perovskites are promising. However, it is an indisputable fact that the reported LPL from metal‐halide perovskites is all realized by creating carrier capture centers in matrix with a strategy of rare‐earth ions doping.[ ] From an application perspective, the realization of considerable LPL in pristine perovskite systems without dopants is valuable and irresistible, considering the following three aspects mainly. First, the distribution of traps in hosts is sensitive to the concentration of ion doping, and the controllability of trap engineering is controversial. Second, ion doping requires higher activation energy to be entered into the matrix lattice, increasing the requirement for material synthesis. Finally, as mature activators for persistent luminescence, rare‐earth ions are rare strategic resources, and their inefficient doping process is not conducive to the rational allocation of resources. In this work, a high‐performance LPL material was developed based on undoped inorganic metal‐halide CsCdCl3, which was synthesized by a facile hydrothermal approach. Under UV light (254 nm) excitation, the synthesized CsCdCl3 crystals exhibit a broadband yellow emission related with the self‐trapped excitons formed in Cd—Cl octahedron, and the photoluminescence quantum yield (PLQY) is as high as ≈90%. Excitingly, a persistent luminescence was observed at RT with a long duration time of 6000 s. A combination of experimental characterizations together with theoretical calculations identifies the carrier de‐trapping mechanisms from shallow and deep trap sites induced by the intrinsic point defects of CsCdCl3. Importantly, CsCdCl3 SCs exhibit excellent environmental, thermal, and light stability. Enlightened by the present results, we further demonstrate the applications of such LPL material in dual‐mode information storage‐reading. This study offers an example of the development of LPL materials without dopants and opens up avenues for advanced applications in information storage.

Figure 1a displays the schematic crystal structure of CsCdCl3, showing unique octahedral bonding, in which [Cd2Cl9]5− subunits formed with two face‐sharing octahedra share corners with independent [CdCl6]4− octahedron, thus unfold in three dimensions.[ , ] Specifically, each [CdCl6]4− octahedron shares corners with six [Cd2Cl9]5− dimers, and each [Cd2Cl9]5− dimer shares corners with six [CdCl6]4− octahedra (Figure 1b,c). Figure 1d shows the typical X‐ray diffraction (XRD) patterns of CsCdCl3 samples (SCs and the corresponding ground powders). One can see that the detected diffraction peaks for CsCdCl3 SCs and ground powders are all consistent with the standard card of hexagonal CsCdCl3 (PDF# 01‐070‐1615), and no additional diffraction peaks emerge, which suggests that there are no impurities inside the prepared CsCdCl3 crystals. The detailed crystal structure data and atomic positions of hexagonal CsCdCl3 were summarized in Tables S1 and S2, Supporting Information, respectively. Figure S1, Supporting Information, displays the Raman spectra of CsCdCl3 SCs and ground powder. The measured spectra are characterized by similar features with four peaks. The mode of 69 cm−1 is related to the E 2g, and the peak at 115, 129, and 248 cm−1 are attributed to A 1g modes of CsCdCl3.[ , ] This result further excludes the possibility that small domain of impurities is embedded in the crystal. X‐ray photoelectron spectroscopy (XPS) tests were conducted to examine the chemical valence of CsCdCl3 crystals. Figure S2, Supporting Information, presents a survey XPS spectrum, and the signals attributed to Cs, Cd, and Cl were obtained. Specifically, the Cd 3d spectrum has well‐separated spin‐orbit components, and the bands peaked at 410.9 and 404.2 eV are related to the Cd 3d3/2 and Cd 3d5/2, respectively. Two separate peaks at 736.5 and 722.5 eV are assigned to the Cs 3d3/2 and Cs 3d5/2, and two distinct peaks observed at 198.2 and 196.8 eV are associated with the 2p1/2 and 2p3/2 of Cl−, well consistent with the previous studies.[ , ] Figure 1e presents the dependence of diagonal length of CsCdCl3 SCs on the cooling rate, and a monotonically decreasing trend was revealed. Within a cooling rate window of 2.5–30 °C h−1, the diagonal length of CsCdCl3 SCs can be adjusted from 0.15 to 0.6 mm, while the colorless and transparent hexagonal shape was maintained well. Such size evolution can be ascribed to the reduced supersaturation at low cooling rate, which is conducive to promoting the nucleation and growth of the crystals. Figure 1f displays the typical morphology of the prepared CsCdCl3 SC, showing a smooth and flat surface with regular hexagonal shape. In combination with the XRD results of CsCdCl3 and the corresponding reciprocal lattice, we confirm that the hexagonal exposed facet in the scanning electron microscope (SEM) image is (006) crystal plane. From the corresponding energy dispersive spectroscopy (EDS) elemental mapping, one can observe that Cs, Cd, and Cl elements are homogeneously distributed throughout the crystal. Based on the results of EDS, the quantitative atomic ratio (%) of Cs:Cd:Cl is determined to be 0.86:0.77:2.68, which is consistent with the stoichiometry of CsCdCl3 (Table S3, Supporting Information). Clear lattice fringes can be observed in the high‐resolution transmission electron microscopy (TEM) image of CsCdCl3 SC in Figure 1g, corresponding to the (004) and (102) crystal planes of hexagonal CsCdCl3. Figure 1h displays the fast Fourier transformation (FFT) image, which obviously demonstrates the excellent crystallization of the CsCdCl3 crystals. Further, we investigated the electronic band structure and projected density of states (PDOS) of CsCdCl3 on the basis of the first‐principles calculations. Figure 2a exhibits the calculated band structure, which has a direct bandgap of 4.7 eV. The PDOS suggests that the conduction band of CsCdCl3 is mainly composed of Cl 3p and Cd 5s orbitals, and the valence band mostly originates from the Cl 3p and Cd 5d states, and Cs cation displays negligible influence to the frontier orbitals (Figure 2b). Correspondingly, the charge density concentrates in Cd—Cl polyhedrons rather than Cs atoms analyzed by the wave functions |Ψ|2 of conduction band minimum (CBM) and valence band maximum (VBM), which are clearly depicted in Figure 2c. That is, the carriers prefer to be located in the Cd—Cl polyhedron in the lattices of CsCdCl3. We further studied the optical properties of CsCdCl3 SCs experimentally. The UV–visible absorption spectrum of CsCdCl3 SCs exhibits an absorption peak located at 255 nm, and the corresponding Tauc plot exports a direct bandgap of 4.74 eV, close to the calculated data described above (Figure S3, Supporting Information). As shown in Figure 2d, under UV light excitation, the CsCdCl3 SCs display a broad yellow emission that peaked at 580 nm, and the full‐width at half‐maximum (FWHM) of the PL spectrum is ≈81 nm. The pale‐blue line shows a PL excitation (PLE) spectrum of CsCdCl3 SCs peaked at 258 nm. The broadband emission and the large Stokes shift (322 nm) suggest that the PL mechanisms are likely to result from the self‐trapped excitons which are always reported in low‐dimensional perovskites rather than the direct band‐edge transition.[ , , , ] Figure 2e presents the pseudocolor map of wavelength‐dependent PL and PLE spectra. The peak position and band shape of the PL spectra are almost independent of different excitation and emission wavelengths monitored, implying that the broadband yellow emission is derived from an excited state rather than the superposition of many defects‐related carrier recombination centers. Figure S4, Supporting Information, shows the excitation power‐dependent PL spectra of CsCdCl3, and the PL intensity is linearly dependent on the excitation power, excluding the emission that originates from the defect centers. Besides, the measured absolute PLQY of the CsCdCl3 crystals is ≈90% (Figure S5, Supporting Information), manifesting an outstanding emission property of products. The relationship of integrated PL intensity and reciprocal temperature is plotted to calculate the exciton binding energy of CsCdCl3 crystals, and the exciton binding energy of CsCdCl3 was calculated to ≈603.33 meV (Figure S6, Supporting Information). Such a large value is similar to that of the Cu‐based halide SCs with self‐trapped excitons nature,[ , ] which can guarantee the generation of excitons at RT and promote their radiative recombination.[ ] Figure 2f presents the PL decay curve monitored at 580 nm excited by 254 nm light, which is fitted with biexponential decay function (R 2 = 0.99), and the average PL lifetime is estimated as 0.6 s. Such a long PL lifetime is attributed to the LPL character of the CsCdCl3 sample, which will be explained in more detail later.[ ] Interestingly, the CsCdCl3 SC exhibits a long‐term yellow LPL after ceasing the UV irradiation (Figure 3a), which can be observed by naked eyes for one minute. Figure S7, Supporting Information, presents the time‐resolved LPL spectra recorded at different time periods after turning off the excitation light, showing an ever‐reducing peak intensity with a fixed peak position at 580 nm. The normalized LPL and PL spectra show that there is almost no difference in spectral shape or peak position (Figure S8, Supporting Information), implying the persistent luminescence originates from the same emission center to fluorescence in CsCdCl3. Further, the LPL decay curve of CsCdCl3 was recorded at RT to recognize the persistent luminescence character. As seen in Figure 3b, the LPL intensity drops abruptly in the first 150 s, followed by an ultra‐slow decay. The duration of LPL signal is up to 6000 s, remaining a signal level of two times larger than the noise level. It is worth mentioning that this duration time is longer than that of many non‐doping LPL systems, such as SrAl2O4 reported by Huang et al., Mg2SnO4 prepared by Hu et al., and Cs3In2Cl9 reported by Chen et al.[ , , ] Moreover, the duration of LPL in the present case is much higher than many doped metal halide systems,[ , ] demonstrating the application prospect of pristine CsCdCl3 as a promising LPL candidate. Although the research on LPL has been going on for years, the mechanisms of the LPL phenomenon are still controversial. In general, the LPL phenomenon is closely related to the intrinsic or extrinsic defects that act as the trapping centers in the matrix.[ ] To examine the distribution of active afterglow traps, the thermoluminescence (TL) spectroscopy was therefore measured.[ ] As shown in Figure 3c, the measured TL curve is a broad spectrum with two TL peaks obtained by Gaussian fitting at 385 and 451 K, respectively, suggesting that there are two kinds of trap sites (shallow traps and deep traps) that continuously distributed in the CsCdCl3 SC.[ , ] In addition, the light intensity‐time curve (I −1–t) converted from the LPL decay curve cannot be fitted to a straight line (Figure S9, Supporting Information), implying that the trapped carriers are returned to the conduction band by the thermal de‐trapping process.[ , ] Evidently, the shallow traps play a major role in LPL at RT, while the contribution of deep traps is negligible because a high activation energy is needed to overcome the energy barrier to enable the de‐trapping process of electrons. To prove this, a thermal stimulation treatment was therefore conducted to activate the deep trap sites, facilitating the de‐trapping process of electrons. As shown in Figure 3d, after the first LPL decay for 270 s (stage I), the sample is rapidly heated to 180 °C within 45 s (stage II), during which the LPL intensity increases abruptly, which could be assigned to the de‐trapping process of electrons in the deep traps that gain enough energy at high temperatures, implying the excellent energy storage capability of CsCdCl3 SC. During the subsequent temperature‐keeping stage (stage III) at 180 °C, the second LPL decay occurs. Obviously, two LPL decay behaviors at RT and 180 °C come from different electron de‐trapping processes, one from shallow trapping centers and another from deep trapping centers.[ ] To further demonstrate that thermal stimulation can activate deep trap sites, PL spectra at different temperatures were characterized (Figure S10, Supporting Information). At high temperatures, the electrons in the deep traps enable to escape and transfer to Cd—Cl octahedra to participate in emission, which overcomes the influence of thermal‐related non‐radiative recombination and results in a slight increase in PL intensity. To further explore the formation of trapping centers in CsCdCl3 SC, the first‐principles calculations are conducted to study energetics and transition energy levels of intrinsic defects, in which three vacancy defects (V Cs, V Cd, and V Cl), three interstitial defects (Cs i, Cd i, and Cl i), and two cation substitution defects (Cs Cd and Cd Cs) are considered. Two chemical potentials, Cd‐poor and Cd‐rich, are chosen to perform the calculations, due to the formation energy of a point defect is greatly affected by the chemical potential of component elements. As shown in Figure 3e, eight defects in CsCdCl3 are considered to calculate their formation energies at different Fermi level, in which the formation energies of V Cl and Cs i at Cd‐rich conditions are lower than other defects, implying that both types of defects are easily formed in hexagonal CsCdCl3. As the condition transits to Cd‐poor, the formation energy of V Cd is the lowest. Further, we calculated the energy level positions of the intrinsic defects in CsCdCl3. As illustrated in Figure 3f, the V Cl and Cs i defects could create relatively shallow energy levels within the bandgap, so they could act as the charge trap states in CsCdCl3. While, the energy level of V Cd defect does not locate within the bandgap of CsCdCl3, thus it could not be involved in the persistent emission even if its formation energy is rather low. Therefore, we consider that the intrinsic defects of V Cl and Cs i in CsCdCl3 SC form electron trapping centers near the conduction band, contributing to the LPL. Based on the above discussions, we illustrated the proposed LPL mechanisms of CsCdCl3 in Figure 3g. The entire LPL process is divided into two parts based on the UV lamp turning on or off. First, in the valence band, the electrons promoted by UV excitation are transformed from ground state to the excited state. After a period of relaxation, part of free electrons in the conduction band fall into the lower energy luminescent center and then recombine with holes in the valence band, emitting yellow fluorescence; another part of the electrons is captured and stored by the trapping centers close to the conduction band. Second, after the light excitation is stopped, the electrons stored in the traps are continuously released back to the conduction band under external stimuli (thermal de‐trapping), and subsequently captured by the luminescent center to emit yellow LPL. The stability of LPL materials is a crucial parameter to evaluate the reliability of their practical applications.[ ] Herein, combining the theoretical calculations and experimental characterizations, we evaluate the stability of the as‐prepared CsCdCl3 SCs. Figure S11, Supporting Information, shows the calculated phonon band structures of CsCdCl3, and no virtual frequencies were observed, verifying the dynamic stability of CsCdCl3. Figure 4a presents the variation diagram of the thermodynamic stability of CsCdCl3 with time at 300 K, and a small potential energy fluctuation manifests it have a fine thermodynamic stability. Furthermore, the calculations are extended to the decomposition enthalpy. As summarized in Figure 4b, the calculated decomposition enthalpy of CsCdCl3 is much larger than other all‐inorganic metal halides counterparts (CsPbBr3, CsCu2I3, Cs3Cu2I5, and Cs3Bi2Br9), indicating the better stability of CsCdCl3 and opening up huge opportunities for future practical applications. The high structural stability of CsCdCl3 may be due to the face‐sharing [CdCl6]4− octahedra that reduced the total energy of the structure. In addition, because of the large electronegativity of chlorine, the perovskite chloride generally has higher structural stability than those of bromide and iodide. Experimentally, we studied the influence of external environment (humidity, temperature, UV light) on the optical and structural properties of CsCdCl3. Figure 4c shows that the as‐prepared CsCdCl3 SCs are still pure phase and the structural integrity is maintained rather than destroyed, after storing a long period of time for 50 days in air (RT, 75% humidity). Moreover, the initial emission performance of sample can be efficiently maintained after 50‐day storage, only showing a small decay of about 11% (Figure 4d), indicating that the luminescence characteristic of CsCdCl3 SCs is stable even affected by moisture and oxygen. Further, the thermal stability test of CsCdCl3 SCs was conducted to assess its temperature tolerance. The thermogravimetric analysis (TGA) displayed in Figure S12, Supporting Information, evidence that CsCdCl3 is thermally stable up to a high temperature of 550 °C. Besides, time‐dependent PL spectra of CsCdCl3 SC under continuous heating at 100 °C were performed. Figure 4e presents a pseudocolor map of the time‐dependent PL spectra for 10 h, over which the CsCdCl3 SC was continuously excited by UV light. Clearly, the emission properties of sample are very robust over the entire test with the spectral shape, peak intensity, and peak position almost unchanged. Subsequently, the sample was employed for successive five‐cycled LPL decay test. As displayed in Figure 4f, the persistent luminescence decay trend of CsCdCl3 SC can be well maintained over five cycles of re‐activation, evidencing the outstanding LPL stability of CsCdCl3. Overall, the results obtained indicate that CsCdCl3 has a good stability under the influence of moisture, oxygen, heat, and light, proving the reliable capability of this LPL material for practical applications in harsh conditions. Considering its attractive fluorescence and persistent luminescence properties as well as outstanding stability, the as‐prepared CsCdCl3 SCs are applied for information storage. Figure 5a shows a dual‐mode information storage‐reading method we designed. First, we use a 254 nm lamp to excite the prepared pattern for 5 min, and the photo‐generated electrons are trapped by trapping centers in CsCdCl3 to store energy as described in Figure 3g, which can be deemed as the charging process. Second, the pattern information is identified due to the LPL of CsCdCl3 at RT when the lamp is removed, because the carriers in shallow traps are released to the luminescent centers. This process is called fast reading (stage I in Figure 3d). Third, after removing the excitation light for 1 min, the electrons in the shallow traps are almost emptied, thus the emission intensity diminishes substantially and the information is gradually hidden, resulting in the un‐recognition of pattern information. Finally, by placing the pattern at a high temperature (180 °C) for 4 min, the information reappears, which is due to that the charge carriers in the deep traps obtain enough thermal energy to enable the de‐trapping process and reach the emission centers (stage III in Figure 3d). Thus, information re‐reading is achieved. Experimentally, the large‐sized CsCdCl3 SCs were ground into powder and then filled into different molds to prepare patterns with different information. Figure S13, Supporting Information, shows the photos of the as‐prepared flat patterns of quick response (QR) code and leaf in natural light with the pattern size of 4 cm × 4 cm. Figure 5b exhibits the information storage‐reading process for QR code and leaf patterns. The QR code pattern emits yellow fluorescence under UV light excitation. After stopping UV excitation to end the charging process, the QR code exhibits yellow LPL emission. The website homepage can be conveniently identified by scanning the LPL QR code with a handphone, to realize the information reading.[ ] Gradually, the LPL is too weak for the QR code information to be recognized. Extraordinarily, the information of QR code is re‐read by heating the pattern. Similarly, the leaf pattern exhibits a yellow LPL after ceasing UV light excitation, and the pattern information can be re‐read when the invalid pattern is heated to 180 °C. Therefore, dual‐mode information storage‐reading is achieved thanks to the LPL characteristics of CsCdCl3 SC, implying the application prospect of CsCdCl3 in the field of information storage.

In conclusion, we demonstrated a high‐performance LPL material based on an undoped inorganic metal halide CsCdCl3 synthesized by the facile hydrothermal method. Under UV light excitation, the CsCdCl3 SCs exhibit a broadband yellow emission with a PLQY as high as ≈90%, and the RT LPL emission lasts up to 6000 s. Joint experiment‐theory characterizations suggest that the carrier de‐trapping from shallow and deep trap sites induced by the intrinsic point defects in CsCdCl3 to localized emission centers contributes to the LPL. Moreover, the CsCdCl3 SCs demonstrate good optical and structural stability under the influence of moisture, oxygen, heat, and light. Even after 50 days of storage in 75% humidity air or 10 h heating at 100 °C, the CsCdCl3 SCs can still maintain LPL luminescence characteristics, manifesting a reliable LPL material compatible for practical application. Encouraged by the attractive fluorescence and persistent luminescence properties as well as outstanding stability of CsCdCl3 SCs, we successfully implemented a dual‐mode information storage‐reading application. The results obtained represent a groundbreaking advance in the fields of undoped metal halides LPL materials, highlighting the great potential in information storage applications.

Cadmium chloride (CdCl2, >99.99%), cesium chloride (CsCl, >99.9%), hydrochloric acid (HCl, 36 wt.% in water), and ethyl alcohol were purchased from Aladdin Chemistry Co., Ltd. (China).

The CsCdCl3 crystals with a millimeter size were prepared by hydrothermal technique. Specifically, 2 mmol of CsCl, 2 mmol of CdCl2, and 5 mL concentrated HCl were added to a 25 mL Teflon liner. The solution was then heated to 180 °C in the stainless steel Parrautoclave, and then maintained at this temperature for 12 h to fully dissolve the raw materials. Following that, the temperature of drying chamber was slowly cooled to RT at the rate of 2.5 °C h−1. Then, the unwanted hydrochloric acid solution was removed with a rubber tipped dropper, and the precipitated crystals were washed three times with ethylalcohol to remove the impurity. Finally, the prepared CsCdCl3 crystals were dried in a drying chamber (12 h at 80 °C).

Different patterns were prepared through two steps: first, the CsCdCl3 SCs were ground into small‐sized powders with a grinding bowl; second, the obtained powders were filled into the customized pattern templates; finally the patterns were scraped and pressed by a glass slide to get flat patterns in the shapes of leaves or QR code.

The structural characterizations of CsCdCl3 crystals were examined by XRD (TD‐3500, China). The morphology and chemical compositions were observed by SEM (JEOL, JSM‐7500F) and EDS. The microstructure was further characterized by TEM (JEOL, JEM‐3010). Absorption spectrum was performed by an UV–vis spectrophotometer (UV–vis, Hitachi UH4150). PL/PLE spectra were monitored by the fluorescence spectrophotometer (Horiba; Fluorolog‐3). The PLQY of the CsCdCl3 crystals was performed in FLS‐1000 spectrometer with a calibrated integrating sphere. The transient PL spectrum of CsCdCl3 crystals was measured by the time‐resolved spectrometer (Delta‐flex, Horiba Scientific). TGA was obtained successfully by TGA Q5000IR under the N2 atmosphere. The LPL decay curve was monitored at 580 nm by thermoluminescence spectrometer (TOSL‐3DS). When the remained signal level was two times to that of background, the corresponding time was considered as the LPL decay time. The samples were charged with a 254 nm lamp for 5 min before transferring into a heating chamber. TOSL‐3DS thermoluminescence spectrometer was used to obtain the thermoluminescence spectrum at a heating rate of 1 K s−1 (Guangzhou Rongfan Science and Technology Co., Ltd).

According to density functional theory (DFT), the first‐principles calculations were conducted. The Vienna Ab initio Simulation Package (VASP) was used to implement the plane‐wave pseudopotential method. Projector augmented wave (PAW) approach was chosen and treated 5s25p26s for Cs, 4d105s2 for Cd, and 3s23p5 for Cl as valence electrons. The DOS and electronic band structures were calculated with Perdew–Burke–Ernzerhof (PBE) exchange‐correlation functional. The canonical (NVT) ensemble with the Nosé thermostat was used to conduct the molecular dynamics simulations at 300 K, and the last time was 6 ps while the time step was 1 fs. The formation energy ∆H of point defects was calculated according to the following equation: in which, E D is the total amount of energy of supercells that contain impurities. E H is total amount of energy of the impurity‐free supercells. ni is a difference number of atoms for the ith atomic species between the defect‐containing and defect‐free supercells. µi is the relative chemical potential for the ith atomic species referenced to its bulk μibulk, ε VBMis the energy of the VBM of the host material, and ε f is the Fermi energy relative to the VBM. The 2×2×2 supercell including 48 formula units of CsCdCl3 and the 2×2×1 grid for k‐point sampling of the Brillouin zone were adopted to perform defect formation energy calculations. A finite‐size correction was used for the supercell calculation to conduct the correction of potential alignment and image charge. The image charge correction used a calculated static dielectric constant of 6.5.

The authors declare no conflict of interest.

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