Spectrally Tunable Lead-Free Perovskite Rb2ZrCl6:Te for Information Encryption and X-ray Imaging

A series of lead-free Rb2ZrCl6:xTe4+ (x = 0%, 0.1%, 0.5%, 1.0%, 2.0%, 3.0%, 5.0%, 10.0%) perovskite materials were synthesized through a hydrothermal method in this work. The substitution of Te4+ for Zr in Rb2ZrCl6 was investigated to examine the effect of Te4+ doping on the spectral properties of Rb2ZrCl6 and its potential applications. The incorporation of Te4+ induced yellow emission of triplet self-trapped emission (STE). Different luminescence wavelengths were regulated by Te4+ concentration and excitation wavelength, and under a low concentration of Te4+ doping (x ≤ 0.1%), different types of host STE emission and Te4+ triplet state emission could be achieved through various excitation energies. These luminescent properties made it suitable for applications in information encryption. When Te4+ was doped at high concentrations (x ≥ 1%), yellow triplet state emission of Te4+ predominated, resulting in intense yellow emission, which stemmed from strong exciton binding energy and intense electron-phonon coupling. In addition, a Rb2ZrCl6:2%Te4+@RTV scintillating film was fabricated and a spatial resolution of 3.7 lp/mm was achieved, demonstrating the potential applications of Rb2ZrCl6:xTe4+ in nondestructive detection and bioimaging.

Due to its stability and distinctive luminescent properties, the all-inorganic Rb 2 ZrCl 6 attracted widespread attention.Zhang synthesized Rb 2 ZrCl 6 microcrystals via a hydrothermal method and utilized them as phosphors to prepare WLEDs [29].The Rb 2 ZrCl 6 microcrystals exhibited blue-white emission characteristics, with a high PLQY value of up to 60%.Notably, they demonstrated significant stability when exposed to heat, ultraviolet light, and environmental conditions involving water/oxygen.Zheng pioneered the environmentally friendly wet grinding method to synthesize a series of highly crystalline Rb 2 ZrCl 6 :xTe 4+ microcrystals.Furthermore, the research indicated that the effective emission of these perovskite materials could be attributed to their direct bandgap characteristics and zerodimensional structure [30].Shi introduced the tunable emission characteristics of Rb 2 ZrCl 6 : xSb 3+ phosphors that depend on the different excitation wavelengths [31].In 2023, Liu synthesized Te 4+ doped Rb 2 ZrCl 6 microcrystals, and conducted a preliminary study on their optical properties to manufacture white light emitting diodes [32].These studies on Rb 2 ZrCl 6 proved that it has excellent luminescence properties.
However, the emission of Rb 2 ZrCl 6 required excitation at high-energy wavelengths, posing a challenge for effective photon excitation when used in conjunction with commercial UV chips and electroluminescent devices [33].Furthermore, the single emission of Rb 2 ZrCl 6 could not meet the requirements of multiple anti-counterfeiting and information encryption applications.Therefore, the development of an effective wavelength tuning technique for Rb 2 ZrCl 6 was of significant importance in expanding its applications.Doping technology emerged as an effective means of tuning the luminescent properties of perovskite halides.Intelligent light-emitting materials achieved an intelligent response through photochromism [34].Photochromism entailed emitting different colors of light under various excitation wavelengths.Materials with photochromic properties were excellent candidates for anti-counterfeiting applications where light of different colors was emitted under different excitation wavelengths.It has been reported that Rb 2 ZrCl 6 :Te 4+ exhibited characteristics of emitting blue and yellow light at 255 nm and 388 nm, respectively, making it a promising material for anti-counterfeiting applications [32].In addition, Rb 2 ZrCl 6 was a scintillator material with potential.However, the anti-counterfeiting of photochromism applications and scintillation properties of Rb 2 ZrCl 6 powder samples had not been studied systematically.
In this study, the anti-counterfeiting application of Rb 2 ZrCl 6 :Te 4+ was explored and the luminescence mechanism of Rb 2 ZrCl 6 :Te 4+ was studied in detail, and its application in anti-counterfeiting and biological imaging was expanded and innovated.Rb 2 ZrCl 6 :xTe 4+ microcrystals were synthesized through a hydrothermal method, and the crystal structure and spectral characteristics of Rb 2 ZrCl 6 :xTe 4+ were analyzed.Remarkably, this modification resulted in the emergence of an additional yellow emission when subjected to low-energy excitation at 388 nm, facilitating the transition from the initial blue emission to a vibrant yellow emission.We designed an anti-counterfeiting information encryption and decryption experiment using Rb 2 ZrCl 6 :0.1%Te 4+ powder to emit different colors under different excitations.Simultaneously, X-ray imaging tests were conducted on scintillating films of Rb 2 ZrCl 6 @RTV and Rb 2 ZrCl 6 :2%Te 4+ @RTV, and both demonstrated imaging capabilities.This discovery demonstrated the potential applications of Rb 2 ZrCl 6 :xTe 4+ in information encryption, anti-counterfeiting, and bioimaging.

Synthesis of Rb 2 ZrCl 6 :xTe 4+ Microcrystals
The synthesis of Rb 2 ZrCl 6 :xTe 4+ involved a conventional hydrothermal method.Figure 1a schematically illustrates the Rb 2 ZrCl 6 :xTe 4+ growth method.Specifically, 2 mmol of RbCl, 1 − x mmol of ZrCl 4 and x mmol TeCl 4 were dissolved in 5 mL of HCl and placed in a 25 mL Teflon high-pressure reactor.After being heated at 180 • C for 12 h, the solution was gradually cooled for over 8 h to reach room temperature.Rb 2 ZrCl 6 : xTe 4+ microcrystals were observed to form once the temperature reduced to room temperature.Subsequently, Rb 2 ZrCl 6 :xTe 4+ microcrystals were subjected to three methanol washes and then dried in an oven at 80 • C for a total for 6 h.
The synthesis of Rb2ZrCl6:xTe 4+ involved a conventional hydrothermal method.ure 1a schematically illustrates the Rb2ZrCl6:xTe 4+ growth method.Specifically, 2 mm RbCl, 1 − x mmol of ZrCl4 and x mmol TeCl4 were dissolved in 5 mL of HCl and place a 25 mL Teflon high-pressure reactor.After being heated at 180 °C for 12 h, the solu was gradually cooled for over 8 h to reach room temperature.Rb2ZrCl6:xTe 4+ microcry were observed to form once the temperature reduced to room temperature.Subseque Rb2ZrCl6:xTe 4+ microcrystals were subjected to three methanol washes and then drie an oven at 80 °C for a total for 6 h.

Preparation of Rb2ZrCl6@RTV and Rb2ZrCl6:2%Te 4+ @RTV Scintillation Screens
The Rb2ZrCl6 powder was thoroughly mixed with RTV in a 2:1 ratio, and Rb2ZrCl6@RTV scintillation film was fabricated using the screen-printing method.production process of the Rb2ZrCl6:2%Te 4+ @RTV rigid scintillation screen follows the s method as that of the Rb2ZrCl6@RTV scintillation film.

Measurement and Characterization
The X-ray diffraction (XRD) diffractometer utilized in this study was the PANaly instrument (Almelo, The Netherlands) employed for polycrystalline powder charac zation.The testing conditions included a tube voltage set at 40 kV, with a Cu target m rial selected, emitting radiation at a wavelength of 1.540598 Å. Scanning was condu within an angular range of 10° to 90° at a rate of 6°/min, with a step size of 0.02°.Scan Electron Microscope (SEM) image measurements were performed using an SU3500 f Hitachi, Tokyo, Japan, and the samples were subjected to energy dispersive spectrom (EDS) elemental mapping using a scanning electron microscope (Zeiss Gemini SEM 2.3.Preparation of Rb 2 ZrCl 6 @RTV and Rb 2 ZrCl 6 :2%Te 4+ @RTV Scintillation Screens The Rb 2 ZrCl 6 powder was thoroughly mixed with RTV in a 2:1 ratio, and the Rb 2 ZrCl 6 @RTV scintillation film was fabricated using the screen-printing method.The production process of the Rb 2 ZrCl 6 :2%Te 4+ @RTV rigid scintillation screen follows the same method as that of the Rb 2 ZrCl 6 @RTV scintillation film.

Measurement and Characterization
The X-ray diffraction (XRD) diffractometer utilized in this study was the PANalytical instrument (Almelo, The Netherlands) employed for polycrystalline powder characterization.The testing conditions included a tube voltage set at 40 kV, with a Cu target material selected, emitting radiation at a wavelength of 1.540598 Å. Scanning was conducted within an angular range of 10 • to 90 • at a rate of 6 • /min, with a step size of 0.02 • .Scanning Electron Microscope (SEM) image measurements were performed using an SU3500 from Hitachi, Tokyo, Japan, and the samples were subjected to energy dispersive spectrometer (EDS) elemental mapping using a scanning electron microscope (Zeiss Gemini SEM 300, ZEISS, Jena, Germany).In this study, the experimental apparatus set to a voltage of 5 kV.PL, time-resolved photoluminescence (TRPL), and PLQY measurements were conducted using a PL spectrometer (Edinburgh, FLS 1000, Livingston, UK).Thermo-gravimetric (TG) data and Differential Scanning Calorimetry (DSC) data were collected with the aid of a Netzsch instrument (STA 449 F5 Jupiter, Burlington, MA, USA).The temperature range spanned from room temperature to 1273 K, with a heating rate of 10 K/min, under an Ar 2 atmosphere for testing.The UV-VIS spectrum was obtained using a UV-VIS spectropho-tometer (HITACHI, U-3900H).The imaging system used in this study was a custom-built platform consisting of an X-ray source, a scintillator screen, a CCD detector, as well as image processing and display equipment.The experimental setup involved a voltage of 100 kV, a current of 1000 µA, and a peak energy of 165 kVp.

Results and Discussion
Rb 2 ZrCl 6 :xTe 4+ microcrystals were synthesized via a hydrothermal method, and the structural properties of Rb 2 ZrCl 6 :xTe 4+ were characterized.Phase analysis of Rb 2 ZrCl 6 :xTe 4+ was conducted, followed by an examination of the structural changes in the crystal before and after doping.Figure 1 presents the test results of structural performance characterization.
Figure 1b demonstrates the XRD patterns of Rb 2 ZrCl 6 :xTe 4+ samples.Both the hydrothermally synthesized Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :xTe 4+ samples exhibited a common hybrid peak at a diffraction angle of 27.072 • [36].When compared with the PDF card, this hybrid peak precisely matches the diffraction peak of RbCl (200) [31], which is not luminous.Due to the ionic radius of Te 4+ being larger than Zr 4+ , the substitution of Te 4+ for Zr 4+ results in lattice expansion.Consequently, the diffraction peak position at (220) shifts notably towards smaller angles with increasing Te 4+ content.In order to ascertain the successful incorporation of Zr into the lattice of Rb 2 ZrCl 6 , calculations of the lattice constants of the material were performed using the WinCSD software (Version 4).The computed results indicated that with the introduction of Zr, the lattice parameter of the material increased from 7.14081 Å at x = 0 to 7.17377 Å at x = 10%.As illustrated in Figure 1c, the Rb 2 ZrCl 6 perovskite crystal structure is a typical cubic phase.Unlike the conventional 3D perovskite ABX 3 , Rb 2 ZrCl 6 features a vacancy-ordered structure composed of [ZrCl 6 ] 2− octahedra and Rb + ions.Te 4+ replaces the Zr 4+ sites.The Zr 4+ in Rb 2 ZrCl 6 substitutes for B 2+ sites, leading to a 50% periodic vacancy in [ZrCl 6 ] 2− .This causes the octahedral clusters to be fully separated by the A-site cation Rb + , resulting in a distinct 0D perovskite structure.To gain a comprehensive understanding of the sample's morphology and elemental distribution, perovskite microcrystalline powder was meticulously examined through SEM and EDS.As depicted in Figure 1d, the SEM image reveals that the perovskite exhibits a microcrystalline tetrahedral structure, and the average size of Rb 2 ZrCl 6 :2%Te 4+ microcrystals was 2 µm.Moreover, in Figure 1e, the mapping of Cs, Zr, and Sb elements in a single crystal revealed that the Te 4+ dopants were homogeneously introduced into the Rb 2 ZrCl 6 host lattice.
Next, the optical properties of Rb 2 ZrCl 6 :xTe 4+ were investigated.The ultraviolet fluorescence spectrum of Rb 2 ZrCl 6 :xTe 4+ showed diverse luminescent characteristics under various excitation wavelengths-bright blue at 254 nm excitation and bright yellow at 365 nm excitation, which resulted in a luminescence wavelength range that could be adjusted within 455 nm to 575 nm, transitioning from blue to yellow, demonstrating excellent spectral tunability.Figure 2 displays the results of optical property testing.
As shown in Figure 2a, the precise control of the Te 4+ doping level allowed for effective spectral tuning can result in the transformation from blue to yellow light emission.
As shown in Figure 2b, the emission spectrum of Rb 2 ZrCl 6 at 255 nm partially overlaps with that excitation spectrum of Rb 2 ZrCl 6 :0.5%Te 4+ at 575 nm, indicating that the blue emission of Rb 2 ZrCl 6 can be absorbed by Te 4+ .The distinctive emission peak of Rb 2 ZrCl 6 is situated at 455 nm, featuring a considerably wide full-width at half-maximum (FWHM) of 141 nm.Calculations reveal a substantial Stokes shift of 200 nm.The presence of such a substantial Stokes shift, along with the broad spectral characteristics, strongly implies that the PL mechanism in Rb 2 ZrCl 6 may indeed originate from the emission of self-trapped excitons [37].As shown in Figure S1, The UV-VIS absorption spectrum of Rb 2 ZrCl 6 :xTe 4+ reveals a broad absorption range spanning from 250 to 475 nm.Compared to Rb 2 ZrCl 6 , the Te 4+ doped Rb 2 ZrCl 6 sample exhibits significantly enhanced absorption features in the wavelength range of 370-425 nm.
Figure 2c depicts the Gaussian peak-splitting curve for Rb 2 ZrCl 6 :1%Te 4+ .Under 255 nm excitation, three distinct peaks are readily apparent.Notably, the peak at 575 nm exhibits the highest intensity, corresponding to the 3 P 1 → 1 S 0 triplet energy transition of Te 4+ .The peaks at 380 nm and 455 nm originate from the 1 P 1 → 1 S 0 singlet transition of Te 4+ and the STE emission of Rb 2 ZrCl 6 , respectively [32].As shown in Figure 2a, the precise control of the Te 4+ doping level allowed for effective spectral tuning can result in the transformation from blue to yellow light emission.
As shown in Figure 2b, the emission spectrum of Rb2ZrCl6 at 255 nm partially overlaps with that excitation spectrum of Rb2ZrCl6:0.5%Te 4+ at 575 nm, indicating that the blue emission of Rb2ZrCl6 can be absorbed by Te 4+ .The distinctive emission peak of Rb2ZrCl6 is situated at 455 nm, featuring a considerably wide full-width at half-maximum (FWHM) of 141 nm.Calculations reveal a substantial Stokes shift of 200 nm.The presence of such a substantial Stokes shift, along with the broad spectral characteristics, strongly implies that the PL mechanism in Rb2ZrCl6 may indeed originate from the emission of self-trapped excitons [37].As shown in Figure S1, The UV-VIS absorption spectrum of Rb2ZrCl6:xTe 4+ reveals a broad absorption range spanning from 250 to 475 nm.Compared to Rb2ZrCl6, the Te 4+ doped Rb2ZrCl6 sample exhibits significantly enhanced absorption features in the wavelength range of 370-425 nm.
Figure 2c depicts the Gaussian peak-splitting curve for Rb2ZrCl6:1%Te 4+ .Under 255 nm excitation, three distinct peaks are readily apparent.Notably, the peak at 575 nm exhibits the highest intensity, corresponding to the 3 P1→ 1 S0 triplet energy transition of Te 4+ .The peaks at 380 nm and 455 nm originate from the 1 P1→ 1 S0 singlet transition of Te 4+ and the STE emission of Rb2ZrCl6, respectively [32].
The characteristic emission peak of Rb2ZrCl6:2%Te 4+ is centered at 575 nm, exhibiting a noteworthy full width at a half-peak of 132 nm, and a calculated Stokes shift of 187 nm (Figure 2d).
As depicted in Figure 3a, when excited under 255 nm UV light, the emission spectrum curves of Rb2ZrCl6 exhibit distinct variations based on different Te 4+ doping concentrations.Rb2ZrCl6 itself exhibits an emission peak at 455 nm, while the introduction of Te 4+ results in the emergence of a novel emission peak at 575 nm.As shown in Figure S2, as the concentration of Te 4+ doping is increased, the intensity of the 455 nm peak gradually diminishes until it eventually fades away, while the 575 nm peak continues to rise until it reaches a maximum, after which it gradually decreases with higher doping ion concentrations.Therefore, the optical performance test in this paper is mainly based on The characteristic emission peak of Rb 2 ZrCl 6 :2%Te 4+ is centered at 575 nm, exhibiting a noteworthy full width at a half-peak of 132 nm, and a calculated Stokes shift of 187 nm (Figure 2d).
As depicted in Figure 3a, when excited under 255 nm UV light, the emission spectrum curves of Rb 2 ZrCl 6 exhibit distinct variations based on different Te 4+ doping concentrations.Rb 2 ZrCl 6 itself exhibits an emission peak at 455 nm, while the introduction of Te 4+ results in the emergence of a novel emission peak at 575 nm.As shown in Figure S2, as the concentration of Te 4+ doping is increased, the intensity of the 455 nm peak gradually diminishes until it eventually fades away, while the 575 nm peak continues to rise until it reaches a maximum, after which it gradually decreases with higher doping ion concentrations.Therefore, the optical performance test in this paper is mainly based on Rb 2 ZrCl 6 :2%Te 4+ .It can be seen from Figure 3a that the two emission peaks change regularly with different doping concentrations, and the energy transfer between Rb 2 ZrCl 6 and Te 4+ is observed.
As depicted in Figure 3b, under UV excitation of 388 nm, Rb 2 ZrCl 6 with different Te 4+ doping concentrations presents a consistent emission peak at 575 nm.When the excitation and emission spectra of Rb 2 ZrCl 6 :xTe 4+ are characterized, it is found that the excitation peak of 388 nm is detected at the emission wavelength of 575 nm, and this excitation peak is from the triplet emission of Te 4+ [32].
The decay times of Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :0.5%Te 4+ PL at room temperature are studied, as shown in Figure 4a.The lifetime of Rb 2 ZrCl 6 is about 14.47 µs (λ ex = 255 nm, λ em = 455 nm), and the lifetime of Rb 2 ZrCl 6 :0.5%Te 4+ is reduced to 8.03 µs (λ ex = 255 nm, λ em = 455 nm) compared to Rb 2 ZrCl 6 .This is due to the partial transfer of the exciton of Rb 2 ZrCl 6 to the Te 4+ level, which would have led to the reduction of the luminous exciton of Rb 2 ZrCl 6 , resulting in faster decay.This also corresponds to the results of excitation and emission spectra as shown in Figure 3a.As shown in Figure 4b, the lifetimes of Rb 2 ZrCl 6 :0.5%Te 4+ (λ ex = 255 nm, λ em = 575 nm) and Rb 2 ZrCl 6 :0.5%Te 4+ (λ ex = 388 nm, λ em = 575 nm) are 5.11 µs and 3.75 µs, respectively.This is because under the 388 nm excitation, luminous excitons act directly on Te 4+ .Under 255 nm excitation, the excitons acting on Te 4+ are partly derived from Rb 2 ZrCl 6 , so the lifetime is longer.
Materials 2024, 17, x FOR PEER REVIEW 6 of 14 Rb2ZrCl6:2%Te 4+ .It can be seen from Figure 3a that the two emission peaks change regularly with different doping concentrations, and the energy transfer between Rb2ZrCl6 and Te 4+ is observed.As depicted in Figure 3b, under UV excitation of 388 nm, Rb2ZrCl6 with different Te 4+ doping concentrations presents a consistent emission peak at 575 nm.When the excitation and emission spectra of Rb2ZrCl6:xTe 4+ are characterized, it is found that the excitation peak of 388 nm is detected at the emission wavelength of 575nm, and this excitation peak is from the triplet emission of Te 4+ [32].
The decay times of Rb2ZrCl6 and Rb2ZrCl6:0.5%Te4+ PL at room temperature are studied, as shown in Figure 4a.The lifetime of Rb2ZrCl6 is about 14.47 μs (λex = 255 nm, λem = 455 nm), and the lifetime of Rb2ZrCl6:0.5%Te 4+ is reduced to 8.03 μs (λex = 255 nm, λem = 455 nm) compared to Rb2ZrCl6.This is due to the partial transfer of the exciton of Rb2ZrCl6 to the Te 4+ level, which would have led to the reduction of the luminous exciton of Rb2ZrCl6, resulting in faster decay.This also corresponds to the results of excitation and emission spectra as shown in Figure 3a.As shown in Figure 4b, the lifetimes of Rb2ZrCl6:0.5%Te 4+ (λex = 255 nm, λem = 575 nm) and Rb2ZrCl6:0.5%Te4+ (λex = 388 nm, λem = 575 nm) are 5.11 μs and 3.75 μs, respectively.This is because under the 388 nm excitation, luminous excitons act directly on Te 4+ .Under 255 nm excitation, the excitons acting on Te 4+ are partly derived from Rb2ZrCl6, so the lifetime is longer.
Under 255 nm ultraviolet excitation, Rb2ZrCl6:1%Te 4+ exhibits peaks at 455 nm and 575 nm, testing the temperature-dependent (low-temperature) PL curve of Rb2ZrCl6:1%Te 4+ , as illustrated in Figure 4c.The PL spectra obtained under 255 nm excitation exhibit a trend where the PL intensity of the 1%Te 4+ doped Rb2ZrCl6 increases as the temperature decreases.A similar change in trend was observed for samples excited at 388 nm (Figure S3).This behavior can be attributed to the reduction in electron-phonon coupling at lower temperatures, which is more conducive to the inter-system crossing (ISC) process, consequently leading to an enhancement in the strength of the triplet STE [38]. Figure 4d presents a false-color representation of the changes in PL intensity and wavelength in response to temperature variations for Rb2ZrCl6:1%Te 4+ when excited with a 388nm light source.For a similar investigation under 255 nm excitation, please refer to Figure S4, which illustrates the pseudo-color diagram depicting PL intensity and wavelength changes as a function of temperature for Rb2ZrCl6:1%Te 4+ .As the temperature rises, there is a gradual reduction in the PL intensity attributed to heat-induced non-radiative recombination.Simultaneously, an increase in temperature is associated with a widening of the FWHM of the PL spectrum.This phenomenon indicates that at elevated temperatures, greater exciton-phonon coupling is at play, thereby promoting non-radiative recombination processes [39].Next, in this study, the temperature-dependent relationship of FWHM and PL integral strength was analyzed.Band structure calculations and DOS were performed on Rb2ZrCl6:Te, Rb2ZrCl6 had a large band gap of 3.69 eV.After doping Te 4+ , a new impurity level was introduced, and the band gap was 3.47 eV.We analyzed the luminescence mechanism of Rb2ZrCl6:Te, in which yellow 1 S0→ 3 P1 emission originated from the triplet emission of Te 4+ .Figure 3 displays the results.
As depicted in Figure 5a, as the optimal doping concentration was determined to be 2% Te 4+ , the temperature-dependent curve of FWHM of the emission peak at 575 nm for Rb2ZrCl6:2%Te 4+ was fitted to the phonon broadening model.The equation is provided below: Under 255 nm ultraviolet excitation, Rb 2 ZrCl 6 :1%Te 4+ exhibits peaks at 455 nm and 575 nm, testing the temperature-dependent (low-temperature) PL curve of Rb 2 ZrCl 6 :1%Te 4+ , as illustrated in Figure 4c.The PL spectra obtained under 255 nm excitation exhibit a trend where the PL intensity of the 1%Te 4+ doped Rb 2 ZrCl 6 increases as the temperature decreases.A similar change in trend was observed for samples excited at 388 nm (Figure S3).This behavior can be attributed to the reduction in electron-phonon coupling at lower temperatures, which is more conducive to the inter-system crossing (ISC) process, consequently leading to an enhancement in the strength of the triplet STE [38]. Figure 4d presents a false-color representation of the changes in PL intensity and wavelength in response to temperature variations for Rb 2 ZrCl 6 :1%Te 4+ when excited with a 388nm light source.For a similar investigation under 255 nm excitation, please refer to Figure S4, which illustrates the pseudo-color diagram depicting PL intensity and wavelength changes as a function of temperature for Rb 2 ZrCl 6 :1%Te 4+ .As the temperature rises, there is a gradual reduction in the PL intensity attributed to heat-induced non-radiative recombination.Simultaneously, an increase in temperature is associated with a widening of the FWHM of the PL spectrum.This phenomenon indicates that at elevated temperatures, greater exciton-phonon coupling is at play, thereby promoting non-radiative recombination processes [39].
Next, in this study, the temperature-dependent relationship of FWHM and PL integral strength was analyzed.Band structure calculations and DOS were performed on Rb 2 ZrCl 6 :Te, Rb 2 ZrCl 6 had a large band gap of 3.69 eV.After doping Te 4+ , a new impurity level was introduced, and the band gap was 3.47 eV.We analyzed the luminescence mechanism of Rb 2 ZrCl 6 :Te, in which yellow 1 S 0 → 3 P 1 emission originated from the triplet emission of Te 4+ .Figure 3 displays the results.
As depicted in Figure 5a, as the optimal doping concentration was determined to be 2% Te 4+ , the temperature-dependent curve of FWHM of the emission peak at 575 nm for Rb 2 ZrCl 6 :2%Te 4+ was fitted to the phonon broadening model.The equation is provided below: where S is the electron-phonon coupling, the hω phonon is the phonon frequency, and k B is the Boltzmann constant.The fitting results are shown in Figure 5a.The Huang-Rhys factor (S) is found to be 23, the hω phonon is calculated to be 23.73 meV, and this relatively high S value indicates the presence of a robust electron-phonon coupling in Rb 2 ZrCl 6 :xTe 4+ .Figure 5b shows the temperature-dependent curve of integral strength of the PL curve.It is shown that the luminescent intensity of Rb 2 ZrCl 6 :2%Te 4+ microcrystals demonstrates a decrease as the temperature rises, indicating the occurrence of thermal quenching of luminescence (Figure 5b).The temperature dependency of PL intensity can be aptly described using the Arrhenius formula fitting [40]: In the Arrhenius formula, I 0 represents the PL intensity at 0 K, E a stands for the activation energy, and k B denotes the Boltzmann constant.The E a , determined through Arrhenius formula fitting, is calculated to be 398 meV.Notably, this E a value significantly exceeds the thermal energy at room temperature, suggesting the formation of a stable STE with highly localized characteristics [41].
In Figure 5c, the Rb 2 ZrCl 6 band results show the calculation results.The exchangecorrelation functional employed the generalized gradient approximation (GGA), with a cutoff energy set at 550 eV.Brillouin zone sampling was achieved using a 4 × 4 × 4 Monkhorst-Pack κ-point grid.The convergence criteria for structural optimization were met when the energy per atom was below 10 −8 eV and the forces were less than 10 −3 eV/Å, which shows that Rb 2 ZrCl 6 has a direct band gap of 3.69 eV, after doping 2%Te 4+ , it becomes 3.47 eV, as shown in Figure 5d.According to Figure 5e, the CBM is mainly composed of Zr 4d orbitals, supplemented by a small contraction of the Cl 3p orbital, while the VBM is mainly Cl 3p orbitals.As shown in Figure 5f, after doping with Te 4+ , an impurity level is introduced with the top 3.47 eV of the valence band, including the Te 5s and Cl 3p orbitals.The CBM is occupied by the Zr 4d, Cl 3p, and Te 5p orbitals.
By DOS analysis, Rb does not play a role in bandgap formation.This implies that electron bands predominantly arise from zirconium and halogen contributions within isolated [ZrCl 6 ] 2− octahedrons.The movement of electrons and holes is confined in three spatial dimensions, leading to the emergence of a 0D electronic structure.This unique structure fosters efficient emission of STEs [30]. Figure 5g depicts the CIE color coordinates of Rb 2 ZrCl 6 :xTe 4+ .Notably, these coordinates exhibit a systematic shift, transitioning from the cold white light region (0.20, 0.23) to the warm white light region (0.45, 0.50) as the Te 4+ concentration increases.This shift in color coordinates demonstrates the tunable nature of the emission spectrum.In the Arrhenius formula, I0 represents the PL intensity at 0 K, Ea stands for the activation energy, and kB denotes the Boltzmann constant.The Ea, determined through Arrhenius formula fitting, is calculated to be 398 meV.Notably, this Ea value significantly exceeds the thermal energy at room temperature, suggesting the formation of a stable STE with highly localized characteristics [41].
In Figure 5c, the Rb2ZrCl6 band results show the calculation results.The exchangecorrelation functional employed the generalized gradient approximation (GGA), with a cutoff energy set at 550 eV.Brillouin zone sampling was achieved using a 4 × 4 × 4 Monkhorst-Pack κ-point grid.The convergence criteria for structural optimization were met when the energy per atom was below 10 −8 eV and the forces were less than 10 −3 eV/Å, which shows that Rb2ZrCl6 has a direct band gap of 3.69 eV, ,after doping 2%Te 4+ , it becomes 3.47eV, as shown in Figure 5d.According to Figure 5e, the CBM is mainly composed of Zr 4d orbitals, supplemented by a small contraction of the Cl 3p orbital, while the VBM is mainly Cl 3p orbitals.As shown in Figure 5f, after doping with Te 4+ , an impurity level is introduced with the top 3.47 eV of the valence band, including the Te 5s and Cl 3p orbitals.The CBM is occupied by the Zr 4d, Cl 3p, and Te 5p orbitals.
By DOS analysis, Rb does not play a role in bandgap formation.This implies that electron bands predominantly arise from zirconium and halogen contributions within isolated [ZrCl6] 2− octahedrons.The movement of electrons and holes is confined in three spa- Based on the discussion above, the mechanism of STEs for Rb 2 ZrCl 6 :Te under UV excitation was obtained, as shown in Figure 5h.When stimulated by light, Rb 2 ZrCl 6 :Te 4+ will undergo lattice distortion, resulting in STE emission.Owing to the narrow bandgap between the S 1 and T 1 states generated by STEs, the triplet exciton undergoes thermal activation to the vibrational level of the singlet state through RISC.Subsequently, it undergoes radiative transition from the S 1 state to the ground state, emitting photons and resulting in a blue emission at 455 nm under 255 nm excitation.The introduction of Te 4+ as a dopant creates new energy levels.Additionally, the formation of [TeCl 6 ] 2− octahedra induces Jahn-Teller distortion, further enhancing the generation of self-trapped excitons.Electrons are excited from the valence band of Rb 2 ZrCl 6 to the conduction band, followed by a nonradiative relaxation process.This relaxation results in a dual outcome: one portion leads to bulk STE emission, specifically in the form of blue light emission, while the other fraction transitions to the Te 4+ energy levels.In the case of Te 4+ , its ground state is categorized as the 1 S 0 energy level, and its excited state comprises three triplet energy levels, namely 3 P 0 , 3 P 1 , and 3 P 2 , in addition to a singlet energy level denoted as 1 P 1 .Based on transition rules, transitions from 1 S 0 to 1 P 1 and 3 P 1 are permitted.Transitions from 1 S 0 to 3 P 0 and 1 S 0 to 3 P 2 are entirely prohibited at the electric dipole transition level.Early studies indicate that the low-energy excitation region of Te 4+ originates from the 1 S 0 → 3 P 1 transition [42], with the absorption peaks in the 370-425 nm range attributed to the 1 S 0 → 3 P 1 transition.Under 388 nm ultraviolet excitation, the 575 nm emission of Rb 2 ZrCl 6 :Te arises from the triplet transition of 1 S 0 → 3 P 1 .Under 255 nm, some electrons in the singlet state 1 P 1 transition back to the ground state, resulting in PL at approximately 380 nm.Meanwhile, the remaining electrons undergo a process known as ISC, transitioning from the singlet state 1 P 1 to the triplet state 3 P 1 .Subsequently, under 388 nm, the triplet state relaxes back to the ground state, emitting a yellow light at 575 nm.The PLQY for Rb 2 ZrCl 6 at room temperature is notably high, reaching 62.64%.Figure S5 illustrates that the PLQY remains at 44.74% at room temperature even with a 2% Te 4+ dopant.
Next, Rb 2 ZrCl 6 :2%Te 4+ was tested for stability, and exhibited good air stability over a period of four months.Moreover, the dual-state emission characteristic provides theoretical support for research and experimental design in information encryption applications.Figure 6 showed the wide application prospect of Rb 2 ZrCl 6 :2%Te 4+ by designing anticounterfeiting and information encryption experiments.and emission process, the application prospects of Rb2ZrCl6:0.1%Te 4+ in fields such as anticounterfeiting and confidential information encryption and decryption were broadened.We tested the stability of Rb2ZrCl6:xTe 4+ .The PL spectra of Rb2ZrCl6 powder, following a four-month exposure to atmospheric conditions (at 293 K and humidity levels of 20-30%), exhibited a 7% reduction in intensity when compared to freshly prepared powder.In contrast, the PL strength of Rb2ZrCl6:2%Te 4+ decreased by only 6% after being stored for the same duration under identical environmental conditions (Figure S6).The exceptional stability of both Rb2ZrCl6 and Rb2ZrCl6:2%Te 4+ could be attributed to their structural characteristics.Rb2ZrCl6 was a vacancy-ordered double perovskite, resulting from the combination of quaternary elements and vacancies; this unique structure contributes to its robust stability.In addition, Rb2ZrCl6 exhibited robust thermal stability both before and after the introduction of Te 4+ , as evidenced in Figure S7.Neither Rb2ZrCl6 nor Using Rb 2 ZrCl 6 :0.1%Te 4+ spectral adjustable properties, different colors were displayed through different wavelengths to achieve the function of anti-counterfeiting and information encryption.Figure 6a is a schematic diagram of the concept of information encryption and decryption.Figure 6b shows an example of anti-counterfeiting.The Rb 2 ZrCl 6 :0.1%Te 4+ powder pattern appeared white under ambient light; at 254 nm UV light, it emitted blue light, and at 365 nm UV light, it emitted yellow light, which could be clearly distinguished by the naked eye.In order to further enhance the security level of optical information encryption, an optimization scheme was designed based on the dual excitation source of Rb 2 ZrCl 6 :0.1%Te 4+ and the emission band luminescent material.As shown in Figure 6c, the "520" numbers were Rb 2 ZrCl 6 :0.1%Te 4+ powder, and the rest were Rb 2 ZrCl 6 powder.Since both Rb 2 ZrCl 6 :0.1%Te 4+ and Rb 2 ZrCl 6 powders appeared white under natural light, the nine digital patterns appeared white as a whole, representing the encryption process.When excited by 254 nm UV light, all nine numbers produced blue emission.Under excitation at 365 nm UV light, only the "520" filled with Rb 2 ZrCl 6 :0.1%Te 4+ powder emitted yellow light, while other digits filled with Rb 2 ZrCl 6 powder did not emit any light, this constituted the decryption process.The additional excitation and emission of luminescent information enhanced the overall security level, providing a more secure approach for anti-counterfeiting and confidential information encryption and decryption.Through this enhanced luminescent information excitation and emission process, the application prospects of Rb 2 ZrCl 6 :0.1%Te 4+ in fields such as anti-counterfeiting and confidential information encryption and decryption were broadened.
We tested the stability of Rb 2 ZrCl 6 :xTe 4+ .The PL spectra of Rb 2 ZrCl 6 powder, following a four-month exposure to atmospheric conditions (at 293 K and humidity levels of 20-30%), exhibited a 7% reduction in intensity when compared to freshly prepared powder.In contrast, the PL strength of Rb 2 ZrCl 6 :2%Te 4+ decreased by only 6% after being stored for the same duration under identical environmental conditions (Figure S6).The exceptional stability of both Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :2%Te 4+ could be attributed to their structural characteristics.Rb 2 ZrCl 6 was a vacancy-ordered double perovskite, resulting from the combination of quaternary elements and vacancies; this unique structure contributes to its robust stability.In addition, Rb 2 ZrCl 6 exhibited robust thermal stability both before and after the introduction of Te 4+ , as evidenced in Figure S7.Neither Rb 2 ZrCl 6 nor Rb 2 ZrCl 6 :2%Te 4+ underwent a phase transition, maintaining their structural integrity at temperatures up to 884 K, both before and after the doping process.
Additionally, a circular Rb 2 ZrCl 6 :2%Te 4+ @RTV thin film scintillation screen with a thickness of 70 µm and a diameter of 3 cm was also prepared, achieving a X-ray imaging spatial resolution of 3.7 lp/mm and a light output of 18,000 Photons/MeV for Rb 2 ZrCl 6 :2%Te 4+ , equivalent to 2.3 times that of BGO, demonstrating potential applications in X-ray imaging.Figure 7 showed the results of X-ray imaging testing.
Comparing Rb 2 ZrCl 6 , Rb 2 ZrCl 6 :2%Te 4+ , and commercial BGO in terms of XEL, it is observed that Rb 2 ZrCl 6 achieves an optical yield of 89,000 photons/MeV, while Rb 2 ZrCl 6 :2%Te 4+ reaches 18,000 photons/MeV, which is 11 times and 2.3 times of that of BGO, respectively, as shown in Figure S8.The future applications of Rb 2 ZrCl 6 :2%Te 4+ and Rb 2 ZrCl 6 in X-ray imaging were investigated.Considering that photodiodes generally peak in their responsiveness within the wavelength range of 500-600 nm [43].Therefore, the emission light of Rb 2 ZrCl 6 doped with Te 4+ can better match the current mainstream CCD. Figure 7a compares the X-ray absorption capacity of Rb 2 ZrCl 6 :2%Te 4+ , Cs 3 Mn 2 Br 5 , CsPbBr 3 , and Cs 3 Cu 2 I 5 , Rb 2 ZrCl 6 :2%Te 4+ showed similar X-ray absorption capacity to CsPbBr 3 .Figure 7b illustrates the X-ray imaging equipment utilized in this study.During the experimental procedures, the settings were configured with a voltage of 100 kV, a current of 1000 µA, and a peak energy of 165 kVp.The Rb 2 ZrCl 6 @RTV and Rb 2 ZrCl 6 :2%Te 4+ @RTV composite films with a diameter of 3 cm were prepared using a screen printing method.When observed under natural light, the Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :2%Te 4+ @RTV films appear white and light yellow respectively, and when excited by 254 nm UV light, they show blue and yellow respectively, as shown in Figure 7c.The thickness of Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :2%Te 4+ @RTV films is 70 µm, as shown in Figure S9.As shown in Figure 7d, Rb 2 ZrCl 6 :2%Te 4+ @RTV film as the scintillator screen is used to image the resolution plate, and a resolution of 3.7 lp/mm can be obtained.Its corresponding gray value is shown in Figure 7d.The resolution of Rb 2 ZrCl 6 @RTV film is 3.1 lp/mm, as shown in Figure S10. Figure 7e,f show the X-ray image of the chip and the ballpoint pen by using Rb 2 ZrCl 6 :2%Te 4+ @RTV film as the scintillator screen, showing clear internal structure.In the same way, the X-ray imaging test of Rb 2 ZrCl 6 @RTV film can also be used for the chip and the ballpoint pen imaging applications, as shown in Figure S11.Comparing Rb2ZrCl6, Rb2ZrCl6:2%Te 4+ , and commercial BGO in terms of XEL, it is observed that Rb2ZrCl6 achieves an optical yield of 89,000 photons/MeV, while Rb2ZrCl6:2%Te 4+ reaches 18,000 photons/MeV, which is 11 times and 2.3 times of that of BGO, respectively, as shown in Figure S8.The future applications of Rb2ZrCl6:2%Te 4+ and Rb2ZrCl6 in X-ray imaging were investigated.Considering that photodiodes generally peak in their responsiveness within the wavelength range of 500-600 nm [43].Therefore, the emission light of Rb2ZrCl6 doped with Te 4+ can better match the current mainstream CCD. Figure 7a compares the X-ray absorption capacity of Rb2ZrCl6:2%Te 4+ , Cs3Mn2Br5, CsPbBr3, and Cs3Cu2I5, Rb2ZrCl6:2%Te 4+ showed similar X-ray absorption capacity to CsP-bBr3.Figure 7b illustrates the X-ray imaging equipment utilized in this study.During the experimental procedures, the settings were configured with a voltage of 100 kV, a current of 1000 μA, and a peak energy of 165 kVp.The Rb2ZrCl6@RTV and Rb2ZrCl6:2%Te 4+ @RTV composite films with a diameter of 3 cm were prepared using a screen printing method.When observed under natural light, the Rb2ZrCl6 and Rb2ZrCl6:2%Te 4+ @RTV films appear white and light yellow respectively, and when excited by 254 nm UV light, they show blue and yellow respectively, as shown in Figure 7c.The thickness of Rb2ZrCl6 and Rb2ZrCl6:2%Te 4+ @RTV films is 70 μm, as shown in Figure S9.As shown in Figure 7d, Rb2ZrCl6:2%Te 4+ @RTV film as the scintillator screen is used to image the resolution plate, and a resolution of 3.7 lp/mm can be obtained.Its corresponding gray value is shown in Figure 7d.The resolution of Rb2ZrCl6@RTV film is 3.1 lp/mm, as shown in Figure S10. Figure 7e,f show the X-ray image of the chip and the ballpoint pen by using Rb2ZrCl6:2%Te 4+ @RTV film as the scintillator screen, showing clear internal structure.In the same way, the X-ray imaging test of Rb2ZrCl6@RTV film can also be used for the chip and the ballpoint pen imaging applications, as shown in Figure S11.

Conclusions
In summary, vacancy-ordered double perovskite Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :xTe 4+ microcrystals were successfully prepared via hydrothermal method.The spectral properties of Rb 2 ZrCl 6 :xTe 4+ were studied, and the application prospects of Rb 2 ZrCl 6 :xTe 4+ in the fields of information encryption and X-ray detection are investigated.Rb 2 ZrCl 6 has broadband STE emission, exhibiting a blue-white emission spectrum with fluorescence decay time and Stoker shift of 14.47 µs and 200 nm, respectively, without self-absorption.After doping, Rb 2 ZrCl 6 :xTe 4+ presents different luminescence emissions at different excitation wavelengths.With the increase of Te 4+ concentration, 3 P 1 to 1 S 0 emission of Rb 2 ZrCl 6 :xTe 4+ gradually dominates, and the emission wavelength can be adjusted in the range of 455 nm to 575 nm, showing a transition from blue to yellow light, which means excellent spectral tunability.Rb 2 ZrCl 6 :xTe 4+ microcrystals have broad spectrum emission, longer PL lifetime, and a significant Stokes shift.The strongest doping concentration, Rb 2 ZrCl 6 :2%Te 4+ , exhibits a strong yellow emission, and the decay time and Stokes shift of Rb 2 ZrCl 6 :2%Te 4+ are 3.51 µs and 187 nm, respectively, attributed to its triplet self-trapping exciton emission.Therefore, Rb 2 ZrCl 6 :2%Te 4+ can be applied to anti-counterfeiting and information encryption design, and can achieve specific color anti-counterfeiting and information encryption design by adjusting Te 4+ concentration.For example, Rb 2 ZrCl 6 :0.1%Te 4+ can produce different colors under different ultraviolet excitation energies, showing blue under 254 nm excitation and yellow under 365 nm excitation, which can realize the information encryption of blue and yellow color transformation.The application prospect in X-ray imaging was also investigated.The light yield of Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :2%Te 4+ was 89,000 photon/MeV and 18,000 photon/MeV, respectively, which were 11 and 2.3 times that of BGO.Scintillation films with uniform thickness (70 µm) were prepared by combining the material with RTV, and spatial resolutions of 3.1 lp/mm and 3.7 lp/mm were achieved, respectively.Rb 2 ZrCl 6 :xTe 4+ shows excellent application prospects in the field of information encryption and imaging.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma17112530/s1, Figure S1 (c,d) respectively represent the thickness of the glass sheet, the total thickness of the glass sheet and Rb 2 ZrCl 6 :2%Te 4+ @RTV film; Figure S10: X-ray image of the resolution plate by using Rb 2 ZrCl 6 @RTV film as the scintillator screen; Figure S11: X-ray image of the chip (left) and the ballpoint pen (right) by using Rb 2 ZrCl 6 @RTV film as the scintillator screen.

Figure 6 .
Figure 6.(a) A Schematic representation of the concept of information encryption and decryption.(b) Anti-counterfeiting of Rb2ZrCl6:0.1%Te 4+ pattern.(c) Information encryption and decryption test.

Figure 6 .
Figure 6.(a) A Schematic representation of the concept of information encryption and decryption.(b) Anti-counterfeiting of Rb 2 ZrCl 6 :0.1%Te 4+ pattern.(c) Information encryption and decryption test.

Materials 2024 , 14 Figure 7 .
Figure 7. (a) X-ray absorption coefficients of Rb2ZrCl6:2%Te 4+ , Cs3Mn2Br5, CsPbBr3, and Cs3Cu2I5.(b) X-ray imaging prototype equipment system diagram.(c) Photographs of Rb2ZrCl6 and Rb2ZrCl6:2%Te 4+ @RTV film under ambient light (left) and 254 nm UV light (right).(d) The graph of the gray value change in standard X-ray resolution test pattern plate.(e) X-ray image of the chip by using Rb2ZrCl6:2%Te 4+ @RTV film as the scintillator screen.(The upper right corner of the picture is the chip picture).(f) X-ray image of the ballpoint pen by using Rb2ZrCl6:2%Te 4+ @RTV film as the scintillator screen.(The upper right corner of the picture is the ballpoint pen).

Figure 7 .
Figure 7. (a) X-ray absorption coefficients of Rb 2 ZrCl 6 :2%Te 4+ , Cs 3 Mn 2 Br 5 , CsPbBr 3 , and Cs 3 Cu 2 I 5 .(b) X-ray imaging prototype equipment system diagram.(c) Photographs of Rb 2 ZrCl 6 and Rb 2 ZrCl 6 :2%Te 4+ @RTV film under ambient light (left) and 254 nm UV light (right).(d) The graph of the gray value change in standard X-ray resolution test pattern plate.(e) X-ray image of the chip by using Rb 2 ZrCl 6 :2%Te 4+ @RTV film as the scintillator screen.(The upper right corner of the picture is the chip picture).(f) X-ray image of the ballpoint pen by using Rb 2 ZrCl 6 :2%Te 4+ @RTV film as the scintillator screen.(The upper right corner of the picture is the ballpoint pen).