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

Abstract

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.

1. Introduction

Perovskite had basic and practical significance in photoelectric research, such as solar power, information encryption, and scintillation applications [1]. There were many types of perovskite, which could be divided into Pb-based perovskites [2,3,4,5,6] and non-Pb perovskites [7,8,9,10] according to whether the compound contained lead (Pb). Lead halide perovskites (LHPs) garnered significant research interest in the optoelectronic field, including solar cells, LEDs, and anti-counterfeiting [11,12,13,14,15], due to their excellent light absorption properties, tunable bandgap width, and high photoluminescence quantum yield (PLQY). LHP nanocrystals were identified as high-performance scintillators due to their cost-effectiveness, tunable emission, and fast decay time [16,17]. However, unfortunately, the large-scale commercial application of this material was hindered by stability and toxicity issues [18]. Therefore, lead-free perovskite materials that displayed non-toxicity, stability, and exceptional luminescent properties became the focus of significant attention, emerging as a primary area of research. In recent years, researchers developed numerous lead-free perovskite materials by substituting lead with various low-toxicity and non-toxic elements, for example, tin-based [19,20], copper-based [21,22,23,24,25], manganese-based [26], and zirconium-based perovskites [27,28].

Due to its stability and distinctive luminescent properties, the all-inorganic Rb₂ZrCl₆ attracted widespread attention. Zhang synthesized Rb₂ZrCl₆ microcrystals via a hydrothermal method and utilized them as phosphors to prepare WLEDs [29]. The Rb₂ZrCl₆ 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 Rb₂ZrCl₆:xTe⁴⁺ microcrystals. Furthermore, the research indicated that the effective emission of these perovskite materials could be attributed to their direct bandgap characteristics and zero-dimensional structure [30]. Shi introduced the tunable emission characteristics of Rb₂ZrCl₆: xSb³⁺ phosphors that depend on the different excitation wavelengths [31]. In 2023, Liu synthesized Te⁴⁺ doped Rb₂ZrCl₆ microcrystals, and conducted a preliminary study on their optical properties to manufacture white light emitting diodes [32]. These studies on Rb₂ZrCl₆ proved that it has excellent luminescence properties.

However, the emission of Rb₂ZrCl₆ 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 Rb₂ZrCl₆ could not meet the requirements of multiple anti-counterfeiting and information encryption applications. Therefore, the development of an effective wavelength tuning technique for Rb₂ZrCl₆ 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 Rb₂ZrCl₆:Te⁴⁺ 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, Rb₂ZrCl₆ was a scintillator material with potential. However, the anti-counterfeiting of photochromism applications and scintillation properties of Rb₂ZrCl₆ powder samples had not been studied systematically.

In this study, the anti-counterfeiting application of Rb₂ZrCl₆:Te⁴⁺ was explored and the luminescence mechanism of Rb₂ZrCl₆:Te⁴⁺ was studied in detail, and its application in anti-counterfeiting and biological imaging was expanded and innovated. Rb₂ZrCl₆:xTe⁴⁺ microcrystals were synthesized through a hydrothermal method, and the crystal structure and spectral characteristics of Rb₂ZrCl₆:xTe⁴⁺ 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 Rb₂ZrCl₆:0.1%Te⁴⁺ powder to emit different colors under different excitations. Simultaneously, X-ray imaging tests were conducted on scintillating films of Rb₂ZrCl₆@RTV and Rb₂ZrCl₆:2%Te⁴⁺@RTV, and both demonstrated imaging capabilities. This discovery demonstrated the potential applications of Rb₂ZrCl₆:xTe⁴⁺ in information encryption, anti-counterfeiting, and bioimaging.

2. Experiment Section

2.1. Materials

Rubidium chloride (RbCl, 99.95%, CN), zirconium chloride (ZrCl₄, 99.9%, CN), and antimony chloride (TeCl₄, 99%, CN) were purchased from Aladdin Ltd. Hydrochloric acid (CP, 36.0~38.0%, CN) and methanol (GR, 99.7%, CN) was purchased from Sinopharm Chemical Reagent Co, Ltd. None of the materials required further purification.

2.2. Synthesis of Rb₂ZrCl₆:xTe⁴⁺ Microcrystals

The synthesis of Rb₂ZrCl₆:xTe⁴⁺ involved a conventional hydrothermal method. Figure 1a schematically illustrates the Rb₂ZrCl₆:xTe⁴⁺ growth method. Specifically, 2 mmol of RbCl, 1 − x mmol of ZrCl₄ and x mmol TeCl₄ 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. Rb₂ZrCl₆:xTe⁴⁺ microcrystals were observed to form once the temperature reduced to room temperature. Subsequently, Rb₂ZrCl₆:xTe⁴⁺ microcrystals were subjected to three methanol washes and then dried in an oven at 80 °C for a total for 6 h.

2.3. Preparation of Rb₂ZrCl₆@RTV and Rb₂ZrCl₆:2%Te⁴⁺@RTV Scintillation Screens

The Rb₂ZrCl₆ powder was thoroughly mixed with RTV in a 2:1 ratio, and the Rb₂ZrCl₆@RTV scintillation film was fabricated using the screen-printing method. The production process of the Rb₂ZrCl₆:2%Te⁴⁺@RTV rigid scintillation screen follows the same method as that of the Rb₂ZrCl₆@RTV scintillation film.

2.4. 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 Ar₂ atmosphere for testing. The UV–VIS spectrum was obtained using a UV–VIS spectrophotometer (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.

3. Results and Discussion

Rb₂ZrCl₆:xTe⁴⁺ microcrystals were synthesized via a hydrothermal method, and the structural properties of Rb₂ZrCl₆:xTe⁴⁺ were characterized. Phase analysis of Rb₂ZrCl₆:xTe⁴⁺ 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 Rb₂ZrCl₆:xTe⁴⁺ samples. Both the hydrothermally synthesized Rb₂ZrCl₆ and Rb₂ZrCl₆:xTe⁴⁺ 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 Te⁴⁺ being larger than Zr⁴⁺, the substitution of Te⁴⁺ for Zr⁴⁺ results in lattice expansion. Consequently, the diffraction peak position at (220) shifts notably towards smaller angles with increasing Te⁴⁺ content. In order to ascertain the successful incorporation of Zr into the lattice of Rb₂ZrCl₆, 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 Rb₂ZrCl₆ perovskite crystal structure is a typical cubic phase. Unlike the conventional 3D perovskite ABX₃, Rb₂ZrCl₆ features a vacancy-ordered structure composed of [ZrCl₆]²⁻ octahedra and Rb⁺ ions. Te⁴⁺ replaces the Zr⁴⁺ sites. The Zr⁴⁺ in Rb₂ZrCl₆ substitutes for B²⁺ sites, leading to a 50% periodic vacancy in [ZrCl₆]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 Rb₂ZrCl₆:2%Te⁴⁺ microcrystals was 2 μm. Moreover, in Figure 1e, the mapping of Cs, Zr, and Sb elements in a single crystal revealed that the Te⁴⁺ dopants were homogeneously introduced into the Rb₂ZrCl₆ host lattice.

Next, the optical properties of Rb₂ZrCl₆:xTe⁴⁺ were investigated. The ultraviolet fluorescence spectrum of Rb₂ZrCl₆:xTe⁴⁺ 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 Te⁴⁺ 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%Te⁴⁺ at 575 nm, indicating that the blue emission of Rb₂ZrCl₆ can be absorbed by Te⁴⁺. 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:xTe⁴⁺ reveals a broad absorption range spanning from 250 to 475 nm. Compared to Rb₂ZrCl₆, the Te⁴⁺ 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%Te⁴⁺. Under 255 nm excitation, three distinct peaks are readily apparent. Notably, the peak at 575 nm exhibits the highest intensity, corresponding to the ³P₁→¹S₀ triplet energy transition of Te⁴⁺. The peaks at 380 nm and 455 nm originate from the ¹P₁→¹S₀ singlet transition of Te⁴⁺ and the STE emission of Rb₂ZrCl₆, respectively [32].

The characteristic emission peak of Rb₂ZrCl₆:2%Te⁴⁺ 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 Te⁴⁺ doping concentrations. Rb₂ZrCl₆ itself exhibits an emission peak at 455 nm, while the introduction of Te⁴⁺ results in the emergence of a novel emission peak at 575 nm. As shown in Figure S2, as the concentration of Te⁴⁺ 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 Rb₂ZrCl₆:2%Te⁴⁺. 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 Te⁴⁺ is observed.

As depicted in Figure 3b, under UV excitation of 388 nm, Rb₂ZrCl₆ with different Te⁴⁺ doping concentrations presents a consistent emission peak at 575 nm. When the excitation and emission spectra of Rb₂ZrCl₆:xTe⁴⁺ 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 Te⁴⁺ [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 (λ_ex = 255 nm, λ_em = 455 nm), and the lifetime of Rb₂ZrCl₆:0.5%Te⁴⁺ is reduced to 8.03 μs (λ_ex = 255 nm, λ_em = 455 nm) compared to Rb₂ZrCl₆. This is due to the partial transfer of the exciton of Rb₂ZrCl₆ to the Te⁴⁺ 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%Te⁴⁺ (λ_ex = 255 nm, λ_em = 575 nm) and Rb₂ZrCl₆:0.5%Te⁴⁺ (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 Te⁴⁺. Under 255 nm excitation, the excitons acting on Te⁴⁺ are partly derived from Rb₂ZrCl₆, so the lifetime is longer.

Under 255 nm ultraviolet excitation, Rb₂ZrCl₆:1%Te⁴⁺ exhibits peaks at 455 nm and 575 nm, testing the temperature-dependent (low-temperature) PL curve of Rb₂ZrCl₆:1%Te⁴⁺, as illustrated in Figure 4c. The PL spectra obtained under 255 nm excitation exhibit a trend where the PL intensity of the 1%Te⁴⁺ 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%Te⁴⁺ 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%Te⁴⁺. 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 Te⁴⁺, 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 ¹S₀→³P₁ emission originated from the triplet emission of Te⁴⁺. Figure 3 displays the results.

As depicted in Figure 5a, as the optimal doping concentration was determined to be 2% Te⁴⁺, the temperature-dependent curve of FWHM of the emission peak at 575 nm for Rb₂ZrCl₆:2%Te⁴⁺ was fitted to the phonon broadening model. The equation is provided below:

$$FWHM\left(T\right)=2.36\sqrt{S}\mathrm{\hslash }{\omega }_{phonon}\sqrt{coth\left(\frac{\mathrm{\hslash }{\omega }_{phonon}}{2k_{B}T}\right)}$$

(1)

where S is the electron–phonon coupling, the ħɷ 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 ħɷ phonon is calculated to be 23.73 meV, and this relatively high S value indicates the presence of a robust electron–phonon coupling in Rb₂ZrCl₆:xTe⁴⁺. Figure 5b shows the temperature-dependent curve of integral strength of the PL curve. It is shown that the luminescent intensity of Rb₂ZrCl₆:2%Te⁴⁺ 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]:

$$I\left(T\right)=\frac{I_0}{1+A{e}^{\frac{-E_a}{k_{B}T}}}$$

(2)

In the Arrhenius formula, I₀ 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 Rb₂ZrCl₆ band results show the calculation results. The exchange-correlation 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%Te⁴⁺, 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 Te⁴⁺, 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 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 Rb₂ZrCl₆:xTe⁴⁺. 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 Te⁴⁺ concentration increases. This shift in color coordinates demonstrates the tunable nature of the emission spectrum.

Based on the discussion above, the mechanism of STEs for Rb₂ZrCl₆:Te under UV excitation was obtained, as shown in Figure 5h. When stimulated by light, Rb₂ZrCl₆:Te⁴⁺ will undergo lattice distortion, resulting in STE emission. Owing to the narrow bandgap between the S₁ and T₁ 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 S₁ state to the ground state, emitting photons and resulting in a blue emission at 455 nm under 255 nm excitation. The introduction of Te⁴⁺ as a dopant creates new energy levels. Additionally, the formation of [TeCl₆]²⁻ octahedra induces Jahn–Teller distortion, further enhancing the generation of self-trapped excitons. Electrons are excited from the valence band of Rb₂ZrCl₆ 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 Te⁴⁺ energy levels. In the case of Te⁴⁺, its ground state is categorized as the ¹S₀ energy level, and its excited state comprises three triplet energy levels, namely ³P₀, ³P₁, and ³P₂, in addition to a singlet energy level denoted as ¹P₁. Based on transition rules, transitions from ¹S₀ to ¹P₁ and ³P₁ are permitted. Transitions from ¹S₀ to ³P₀ and ¹S₀ to ³P₂ are entirely prohibited at the electric dipole transition level. Early studies indicate that the low-energy excitation region of Te⁴⁺ originates from the ¹S₀→³P₁ transition [42], with the absorption peaks in the 370–425 nm range attributed to the ¹S₀→³P₁ transition. Under 388 nm ultraviolet excitation, the 575 nm emission of Rb₂ZrCl₆:Te arises from the triplet transition of ¹S₀→³P₁. Under 255 nm, some electrons in the singlet state ¹P₁ 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 ¹P₁ to the triplet state ³P₁. Subsequently, under 388 nm, the triplet state relaxes back to the ground state, emitting a yellow light at 575 nm. The PLQY for Rb₂ZrCl₆ 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% Te⁴⁺ dopant.

Next, Rb₂ZrCl₆:2%Te⁴⁺ 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 Rb₂ZrCl₆:2%Te⁴⁺ by designing anti-counterfeiting and information encryption experiments.

Using Rb₂ZrCl₆:0.1%Te⁴⁺ 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 Rb₂ZrCl₆:0.1%Te⁴⁺ 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 Rb₂ZrCl₆:0.1%Te⁴⁺ and the emission band luminescent material. As shown in Figure 6c, the “520” numbers were Rb₂ZrCl₆:0.1%Te⁴⁺ powder, and the rest were Rb₂ZrCl₆ powder. Since both Rb₂ZrCl₆:0.1%Te⁴⁺ and Rb₂ZrCl₆ 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 Rb₂ZrCl₆:0.1%Te⁴⁺ powder emitted yellow light, while other digits filled with Rb₂ZrCl₆ 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 Rb₂ZrCl₆:0.1%Te⁴⁺ in fields such as anti-counterfeiting and confidential information encryption and decryption were broadened.

We tested the stability of Rb₂ZrCl₆:xTe⁴⁺. 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%Te⁴⁺ 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%Te⁴⁺ 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 Te⁴⁺, as evidenced in Figure S7. Neither Rb₂ZrCl₆ nor Rb₂ZrCl₆:2%Te⁴⁺ underwent a phase transition, maintaining their structural integrity at temperatures up to 884 K, both before and after the doping process.

Additionally, a circular Rb₂ZrCl₆:2%Te⁴⁺@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 Rb₂ZrCl₆:2%Te⁴⁺, 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 Rb₂ZrCl₆, Rb₂ZrCl₆:2%Te⁴⁺, and commercial BGO in terms of XEL, it is observed that Rb₂ZrCl₆ achieves an optical yield of 89,000 photons/MeV, while Rb₂ZrCl₆:2%Te⁴⁺ 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%Te⁴⁺ 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 Te⁴⁺ can better match the current mainstream CCD. Figure 7a compares the X-ray absorption capacity of Rb₂ZrCl₆:2%Te⁴⁺, Cs₃Mn₂Br₅, CsPbBr₃, and Cs₃Cu₂I₅, Rb₂ZrCl₆:2%Te⁴⁺ showed similar X-ray absorption capacity to CsPbBr₃. 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%Te⁴⁺@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%Te⁴⁺@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%Te⁴⁺@RTV films is 70 μm, as shown in Figure S9. As shown in Figure 7d, Rb₂ZrCl₆:2%Te⁴⁺@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%Te⁴⁺@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.

4. Conclusions

In summary, vacancy-ordered double perovskite Rb₂ZrCl₆ and Rb₂ZrCl₆:xTe⁴⁺ microcrystals were successfully prepared via hydrothermal method. The spectral properties of Rb₂ZrCl₆:xTe⁴⁺ were studied, and the application prospects of Rb₂ZrCl₆:xTe⁴⁺ in the fields of information encryption and X-ray detection are investigated. Rb₂ZrCl₆ 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, Rb₂ZrCl₆:xTe⁴⁺ presents different luminescence emissions at different excitation wavelengths. With the increase of Te⁴⁺ concentration, ³P₁ to ¹S₀ emission of Rb₂ZrCl₆:xTe⁴⁺ 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. Rb₂ZrCl₆:xTe⁴⁺ microcrystals have broad spectrum emission, longer PL lifetime, and a significant Stokes shift. The strongest doping concentration, Rb₂ZrCl₆:2%Te⁴⁺, exhibits a strong yellow emission, and the decay time and Stokes shift of Rb₂ZrCl₆:2%Te⁴⁺ are 3.51 μs and 187 nm, respectively, attributed to its triplet self-trapping exciton emission. Therefore, Rb₂ZrCl₆:2%Te⁴⁺ can be applied to anti-counterfeiting and information encryption design, and can achieve specific color anti-counterfeiting and information encryption design by adjusting Te⁴⁺ concentration. For example, Rb₂ZrCl₆:0.1%Te⁴⁺ 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 Rb₂ZrCl₆ and Rb₂ZrCl₆:2%Te⁴⁺ 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. Rb₂ZrCl₆:xTe⁴⁺ shows excellent application prospects in the field of information encryption and imaging.