Charge compensation via tetravalent doping for high-efficiency Mn²⁺-activated inorganic double perovskites toward high-resolution X-Ray imaging

Abstract

In recent years, lead-free halide double perovskite nanocrystals (NCs) have emerged as compelling scintillator candidates due to their high effective atomic number, ultrafast scintillation decay kinetics, eco-friendly composition, and bandgap tunability, all of which are essential for high-temporal-resolution radiation detection. Although nanostructured scintillators achieve exceptional optical uniformity, their nanocrystalline forms exhibit inherent limitations in terms of light emission intensity and overall scintillation efficiency. A critical limitation of X-ray flat-panel minidetectors is the absence of novel engineering approaches to amplify radioluminescence capabilities. Herein, we engineered a charge compensation strategy for Cs₂AgInCl₆ double perovskite NCs through co-doping tetravalent ions with Mn²⁺ emissive centers to stabilize the heterovalent substitution of In³⁺ sites with Mn²⁺. The co-doping of Zr⁴⁺ or Ce⁴⁺ enhanced Mn²⁺-activated emission and induced sequential improvements in photoluminescence lifetime and quantum yield for CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺. XPS analysis confirmed 1.46-fold (CAIC:Mn²⁺,Zr⁴⁺) and 1.59-fold (CAIC:Mn²⁺,Ce⁴⁺) increases in the effective Mn²⁺/Mn⁴⁺ doping concentration relative to CAIC:Mn²⁺. Ce⁴⁺ effectively suppressed Mn²⁺ oxidation to higher valence states, enhancing the Mn²⁺/Mn⁴⁺ ratio by 3.4 times. The CAIC:Mn²⁺,Ce⁴⁺@PDMS scintillator film demonstrated a superior light yield of 16 807 photons MeV^–1, spatial resolution of 7.7 lp mm⁻¹, and detection limit of 619.2 nGy_air s⁻¹.

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Introduction

Lead halide perovskite nanocrystals (NCs) have recently emerged as promising scintillator materials owing to their exceptional optoelectronic properties, including high photoluminescence quantum yields, superior X-ray absorption coefficients, rapid response characteristics, and tunable bandgaps via compositional engineering.^1,2 Liu et al. demonstrated CsPbBr₃ NCs with remarkable X-ray sensitivity, achieving a radioluminescence detection limit as low as 13 nGy_air s⁻¹, which is 423 times lower than the dose required for medical diagnosis.³ However, the intrinsic toxicity of lead and environmental degradation of Pb-based perovskites under operational conditions impede their commercial viability, driving the exploration of lead-free alternatives.⁴ Recent advances in lead substitution strategies focus on designing double perovskite with the general formula A₂B_IB_IIIX₆ (where A = monovalent cation, such as Cs⁺; B_I = monovalent metal ions, such as Na⁺ and Ag⁺; B_III = trivalent metal ions, such as In³⁺ and Bi³⁺ and X = halogen anion, such as Cl⁻ and Br⁻).^5,6 The dual-metal-site architecture further enables precise tuning of electronic and optical properties through compositional variation, offering a versatile platform to address the limitations of lead-based systems while retaining high-performance scintillation.^7–10 However, the absence of Pb in lead-free double perovskite still diminishes X-ray attenuation efficiency due to its lower effective atomic number than those of lead-containing counterparts, resulting in diminished radioluminescence intensity under equivalent irradiation doses. This inherent limitation significantly hinders their ability to achieve high-sensitivity, low-dose detection and sub-micron-scale spatial resolution. These performance indexes are crucial for meeting the requirements of next-generation medical diagnostics and non-destructive testing applications.^11–13

The doping strategy of transition metal ions has emerged as an effective strategy for enhancing the scintillation performance of lead-free halide double perovskite NCs.^9,14,15 As an economical and eco-friendly transition metal dopant, Mn²⁺ generates efficient luminescent centers via the ⁴T₁ → ⁶A₁ transition in an octahedral coordinate, producing tunable broadband emission that spans the green-to-orange spectral range through crystal field modulation.^16,17 The extended carrier recombination lifetime of Mn²⁺ further enhances the photoluminescence quantum yield (PLQY) of the host matrix, enabling a high light yield under X-ray excitation.^16–19 Although Mn²⁺-doped double perovskite NCs have been demonstrated as promising scintillation materials, their practical implementation remains hindered by the intrinsically low Mn²⁺ doping efficiency.^20–23 This limitation stems primarily from the non-equivalent substitution of the In³⁺ site by Mn²⁺ and lattice matching, which suppresses radiative recombination efficiency and ultimately degrades the luminescence performance of Mn²⁺-doped double perovskite NCs.^24–26 This fundamental constraint underscores the imperative to develop innovative doping optimization strategies. Charge compensation has emerged as a promising approach to mitigate doping-induced defects, as evidenced by its successful implementation in conventional phosphors, where charge-balancing mechanisms effectively suppress substitution-related lattice distortions by maintaining valence equilibrium. Zhou et al. employed Mg²⁺/Zn²⁺/Ca²⁺ co-doping in SrAl3BO7:Mn⁴⁺ to neutralize Mn⁴⁺-induced charge imbalance, achieving 89% luminescence enhancement through defect suppression.²⁷ Zheng et al. developed a Be²⁺-mediated charge compensation strategy in CaAl12O19:Mn⁴⁺ that simultaneously addresses charge imbalance and suppresses Mn⁴⁺ → Mn²⁺ self-reduction, achieving prominent luminescence enhancement with an internal quantum efficiency of 74.9% and external quantum efficiency of 56.3%.²⁸ Despite its proven efficacy in conventional phosphors, the charge compensation strategy remains conspicuously underexplored in double perovskite NCs, particularly for synchronously addressing doping limitations and enhancing scintillator performance.

In this study, we report significant enhancements in both the photoluminescence and radioluminescence of Mn²⁺-doped Cs₂AgInCl₆ double perovskite NCs through co-doping with tetravalent ions (Ce⁴⁺ or Zr⁴⁺). The underlying mechanism of the luminescence enhancement was investigated. The charge imbalance caused by the substitution of trivalent In³⁺ with divalent Mn²⁺ was rectified via Mn⁴⁺–X⁴⁺ co-substitution for In³⁺–In³⁺. Notably, Ce⁴⁺ doping not only compensated for the charge imbalance but also suppressed the oxidation of Mn²⁺ to Mn⁴⁺. This synergistically increased the effective doping amount of Mn and enhanced the Mn²⁺ relative occupancy of Mn²⁺/Mn⁴⁺. A flexible thin-film scintillator was fabricated by mixing PDMS with double perovskite NCs, resulting in an outstanding light yield of 16,807 photons per MeV, spatial resolution of 7.7 lp mm^-1, and detection limit of 619.2 nGy_air s^-1. Our study provides new insights into optimizing the scintillation performance of inorganic double perovskite NCs.

Results and discussion

Cs₂AgInCl₆:Mn²⁺ (CAIC:Mn²⁺), Cs₂AgInCl₆:Mn²⁺,Zr⁴⁺ (CAIC:Mn²⁺,Zr⁴⁺), and Cs₂AgInCl₆:Mn²⁺,Ce⁴⁺ (CAIC:Mn²⁺,Ce⁴⁺) NCs were successfully synthesized through an optimized hot-injection protocol.²²Fig. 1a illustrates the schematic crystallographic configurations of pristine CAIC, CAIC:Mn²⁺, CAIC:Ce⁴⁺, and CAIC:Mn²⁺,Ce⁴⁺. As established in previous studies, under an octahedral coordination environment (CN = 6), the ionic radius of Mn²⁺ is 0.83 Å, and that of Ce⁴⁺ is 0.86 Å. These values closely match the ionic radius of In³⁺ (0.80 Å, CN = 6). This high degree of similarity in ionic size minimizes lattice strain and facilitates stable substitution at the In³⁺ site. This rationale is strongly supported by the results of previous theoretical and experimental studies.^22,29–32 The same site preference in analogous perovskite systems has been observed for other heterovalent cations with similar ionic radii, such as Te⁴⁺.³³ All the X-ray diffraction (XRD) patterns of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺, and CAIC:Mn²⁺,Ce⁴⁺ (Fig. 1b) clearly demonstrate characteristic diffraction peaks corresponding to the cubic phase structure of CAIC with space group Fm3̄m (ICSD No. 257115), indicating high crystallinity and the absence of impurity phases. Significantly, no obvious changes in the diffraction peaks were observed for the doped samples when compared with the undoped CAIC samples. This phenomenon can be reasonably explained by the low doping concentrations of Mn²⁺, Zr⁴⁺, and Ce⁴⁺ ions, which are less than 1% of the total ion content, consistent with previous reports.^21,29 The lattice parameters and unit cell dimensions of the prepared series of double perovskite NCs are summarized in Table S1. The lattice constants of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺, and CAIC:Mn²⁺,Ce⁴⁺ were determined to be 10.166 Å, 10.181 Å, and 10.192 Å, respectively. This progressive lattice dilatation correlates with the ionic radii hierarchy of the dopants (rCe⁴⁺ > rZr⁴⁺ > rMn²⁺), confirming successful substitution at the octahedral sites.

Fig. 1 (a) Schematic diagram of the crystal structure. (b) XRD patterns of CAIC:8%Mn²⁺, CAIC:9%Mn²⁺,0.6%Zr⁴⁺ and CAIC:12%Mn²⁺,0.4%Ce⁴⁺ NCs. (c–e) TEM and HRTEM images of (c) CAIC:Mn²⁺, (d) CAIC:Mn²⁺,Zr⁴⁺ and (e) CAIC:Mn²⁺,Ce⁴⁺ NCs. (f–h) Size distribution estimated from TEM images of (f) CAIC:Mn²⁺, (g) CAIC:Mn²⁺,Zr⁴⁺ and (h) CAIC:Mn²⁺,Ce⁴⁺ NCs.

The morphological and structural characteristics of the CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺, and CAIC:Mn²⁺,Ce⁴⁺ NCs were further investigated by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The low-magnification TEM images (Fig. 1c–e) reveal uniformly dispersed double perovskite NCs with cubic morphology. The average sizes of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺, and CAIC:Mn²⁺,Ce⁴⁺ NCs are approximately 18.7 nm, 19.4 nm, and 20.1 nm, respectively, indicating a systematic size increase that correlates with dopant ionic radii (Fig. 1f–h). The HRTEM images (insets in Fig. 1c–e) demonstrate that all the samples possess similar phase structures, confirming their high crystallinity. The measured interplanar spacing of the (220) crystal plane increases sequentially from 0.36 nm (CAIC:Mn²⁺) to 0.37 nm (CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺). This observation is consistent with the lattice parameter variations deduced from the XRD analysis. Ion doping has no significant influence on the morphology and crystallinity of the double perovskite NCs.

X-ray photoelectron spectroscopy (XPS) was systematically employed to probe the elemental doping states and compositional evolution in the CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺, and CAIC:Mn²⁺,Ce⁴⁺ NCs. As illustrated in Fig. 2, the XPS spectra of all three samples confirm the presence of Cs, Ag, In, Cl, and Mn elements. Specifically, the high-resolution XPS spectra of Zr 4d and Ce 3d for the CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ NCs (Fig. S1) validate the successful incorporation of Zr and Ce dopants, respectively. Additionally, the quantitative analysis of the elemental contents, which was derived from the XPS results, is summarized in Table S2. The XPS results revealed a 46% and 60% enhancement in Mn incorporation efficiency for Zr⁴⁺- and Ce⁴⁺-co-doped systems, respectively, compared to singly Mn²⁺-doped CAIC. This synergistic doping effect possibly originates from charge compensation mechanisms, wherein higher-valent Zr⁴⁺ and Ce⁴⁺ substitutions at In³⁺ sites stabilize Mn²⁺ occupancy through electrostatic balancing.³⁴

Fig. 2 (a–e) High-resolution XPS spectra of (a) Cl 2p, (b) In 3d, (c) Ag 3d, (d) Cs 3d, and (e) Mn 2p for CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ NCs.

Photoluminescence spectroscopy was utilized to systematically investigate the effects of charge compensation on the optical properties of Mn²⁺-doped double perovskite NCs. CAIC:Mn²⁺ exhibits a distinct photoluminescence emission peak centered at 620 nm under 310 nm excitation, corresponding to the spin-forbidden ⁴T₁(g) → ⁶A₁(S) d–d transition of Mn²⁺ ions in octahedral coordination (Fig. S2a).^21–23 The excitation spectrum of CAIC:Mn²⁺ exhibits a bimodal profile in the ultraviolet regime, with principal excitation maxima at 254 and 320 nm, corresponding to spin–orbit coupling-induced ¹S₀ → ³P₂ and ¹S₀ → ³P₁ transitions of Mn²⁺, respectively.²³ The concentration-dependent luminescent behavior of CAIC:Mn²⁺ was investigated. The photoluminescence emission intensity of CAIC:Mn²⁺ exhibits an initial increase as the doping concentration of Mn²⁺ increases from 10% to 20% and then decreases when further increasing to 30% (Fig. S2b), which is possibly due to the concentration quenching effect mediated by Mn–Cl–Mn exchange interaction and the proliferation of non-radiative recombination centers.²¹ A systematic comparison of photophysical properties, including emission spectral profiles, excited-state lifetimes and PLQY, was conducted among CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺. Fig. 3a and b show the concentration-dependent emission spectra of CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺, where (a) corresponds to the Zr⁴⁺ concentration series (0–5 mol%) and (b) depicts the Ce⁴⁺ concentration gradient (0–5 mol%), maintaining a constant Mn²⁺ doping level at 20 mol%. It is observed that as the doping levels of Zr⁴⁺ and Ce⁴⁺ increase, the emission intensity of Mn²⁺ first increases and then decreases, yet remains higher than that of single-doped CAIC:Mn²⁺, proving that the incorporation of Zr⁴⁺ and Ce⁴⁺ evidently enhances Mn²⁺ emission intensity. The intensities reached a maximum at the critical concentrations of 3 mol% Zr⁴⁺ and 2 mol% Ce⁴⁺. This intensity decrease may result from the accelerated formation of non-radiative recombination centers and defects.^23,29,35,36 CAIC:Mn²⁺,Ce⁴⁺ exhibits a more pronounced optimization effect than CAIC:Mn²⁺,Zr⁴⁺ (Fig. 3c). The excitation spectra of CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ show a subtle blue-shift relative to CAIC:Mn²⁺ (Fig. S3), which may result from the expanded double-perovskite band gap upon the incorporation of Zr⁴⁺ or Ce⁴⁺.^35,36

Fig. 3 (a) Photoluminescence spectra (λ_ex = 310 nm) of CAIC:20% Mn²⁺,x% Zr⁴⁺ (x = 0, 1, 2, 3, 4, 5) NCs. (b) Photoluminescence spectra (λ_ex = 310 nm) of CAIC:20% Mn²⁺,y% Ce⁴⁺ (y = 0, 1, 2, 3, 4, 5) NCs. (c) Photoluminescence spectra (λ_ex = 310 nm) of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺. (d) Photoluminescence lifetime decay curves of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ (λ_ex = 310 nm, λ_ex = 620 nm).

The photoluminescence lifetime decay curves of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ are shown in Fig. 3d. The average lifetime τ_ave can be calculated using the following formula:^21,23,29

where A₁ and A₂ represent the amplitudes of the two signal components, and τ₁ and τ₂ are the corresponding time constants, respectively. The measured lifetimes of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺, and CAIC:Mn²⁺,Ce⁴⁺ are 0.74, 1.04, and 1.34 ms, respectively (Table S4), which fall within the millisecond range and are comparable to those reported for other Mn²⁺-doped scintillations.^37–39 The PLQYs of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ are 6.72%, 13.22% and 18.46%, respectively (Fig. S4). The significant increase in both the lifetime and PLQY indicates that the incorporation of Zr⁴⁺ and Ce⁴⁺ reduces the non-radiative transitions of Mn²⁺ and improves the luminescence properties of CAIC:Mn²⁺.

The aliovalent substitution mechanisms in Mn²⁺-monodoped, and Mn²⁺, X⁴⁺ (X = Zr, Ce) co-doped CAIC double perovskite NCs were analyzed. When Mn²⁺ substitutes for In³⁺ sites, charge non-equivalence is introduced due to the valence difference (+3 for In³⁺vs. +2 for Mn²⁺), generating In cation vacancies. To maintain the charge balance in the CAIC host lattice, the substitution of trivalent In³⁺ by divalent Mn²⁺ follows a stoichiometric charge compensation ratio of 3 : 2. During the doping process, each Mn²⁺ replacing an In³⁺ generates a negatively charged defect and a positively charged vacancy , as described by process (1). The positively charged vacancy is energetically unstable and tends to capture electrons to reach equilibrium, potentially oxidizing Mn²⁺ to a higher valence state (e.g., Mn⁴⁺), as shown in processes (2) and (3).^40–42

When tetravalent ions X⁴⁺ (X = Zr, Ce) are incorporated into CAIC:Mn²⁺, each X⁴⁺ substitution for In³⁺ generates a positively charged defect and a negatively charged vacancy , as described by process (4). The negatively charged vacancy tends to release electrons to achieve charge equilibrium, counteracting the electron capture process (2). Specifically, the negatively charged and positively charged can neutralize each other to reach equilibrium, as shown in process (5).^41,42 Additionally, the positively charged and negatively charged defects can balance charges through process (6). This interaction effectively hinders the progression of process (3), thereby suppressing the oxidation of Mn²⁺ to a higher valence state.^34,41,42

More intriguingly, compared to Zr⁴⁺, Ce⁴⁺ facilitates charge compensation and introduces a dynamic redox equilibrium through the Ce⁴⁺/Ce³⁺ ion pair, which further suppresses the oxidation of Mn²⁺ to higher valence states. Consequently, Ce⁴⁺ exhibits superior optimization of Mn²⁺ emission.⁴¹ The Ce 3d XPS spectra can be deconvoluted into two spin–orbit components, 3d_5/2 and 3d_3/2, corresponding to the Ce³⁺ and Ce⁴⁺ oxidation states of Ce, respectively.^43,44 As evidenced by XPS analysis in Fig. S5a, both Ce³⁺ and Ce⁴⁺ coexist in CAIC:Ce⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺. Notably, the Ce⁴⁺/Ce³⁺ ratio increases significantly from 0.69 (CAIC:Ce⁴⁺) to 1.27 in the CAIC:Mn²⁺,Ce⁴⁺ (Fig. S5b). This observation stems from the substitution of trivalent In³⁺ by tetravalent Ce⁴⁺, which generates positively charged defects . These defects exhibit a high affinity for electron capture, driving their reduction to Ce³⁺ and establishing a Ce⁴⁺/Ce³⁺ redox equilibrium within the lattice.^30,41,42,45 Consequently, to maintain charge neutrality, a higher proportion of Ce³⁺ compared to Ce⁴⁺ was observed in the CAIC:Ce⁴⁺ sample. For CAIC:Mn²⁺,Ce⁴⁺, the charge compensation mechanism between Ce⁴⁺ and Mn²⁺ (process (6)), and redox interactions between the Ce⁴⁺–Ce³⁺ and Mn²⁺–Mn⁴⁺ lead to a decrease in the trivalent Ce³⁺ proportion and an increase in the tetravalent Ce⁴⁺. To further illustrate the impact of charge compensation on the Mn²⁺–Mn⁴⁺ conversion, the high-resolution Mn 2p XPS spectra were obtained for CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ (Fig. S6a). The Mn 2p_3/2 XPS spectra can be deconvoluted into distinct peaks at 640 eV (Mn²⁺) and 642 eV (Mn⁴⁺).²⁸ Remarkably, the effective doping content of Mn²⁺ at 640 eV is significantly higher than that of Mn⁴⁺ at 642 eV after co-doping with Zr⁴⁺ or Ce⁴⁺. To more intuitively visualize the changes in the proportions of Mn ions in different valence states, further integration of XPS fitting results was performed. The proportions of Mn²⁺ in both CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ increase significantly, with CAIC:Mn²⁺,Ce⁴⁺ exhibiting a more pronounced optimization effect (Fig. S6b). The Mn²⁺/Mn⁴⁺ ratio of CAIC:Mn²⁺,Ce⁴⁺ (4.58) is 3.4-fold higher than that of CAIC:Mn²⁺ (1.33) and 1.4-fold higher than that of CAIC:Mn²⁺,Zr⁴⁺ (3.22), while CAIC:Mn²⁺,Zr⁴⁺ exhibits a 2.4-fold increase in the Mn²⁺/Mn⁴⁺ ratio compared to CAIC:Mn²⁺. These results confirm that the charge compensation effect stabilizes Mn²⁺ within the lattice, optimizing its luminescent properties. Furthermore, upon self-reduction of Ce⁴⁺ to Ce³⁺, energy transfer to Mn²⁺ occurs, further intensifying its emission.⁴⁶

XPS quantitative analysis further demonstrated that Zr⁴⁺ or Ce⁴⁺ co-doping significantly enhances Mn²⁺/Mn⁴⁺ incorporation efficiency, with effective doping concentration increasing to 1.46 times (Zr⁴⁺) and 1.59 times (Ce⁴⁺) relative to the CAIC:Mn²⁺ sample, as quantified by peak-area integration of Mn 2p_3/2 orbitals (Table S2). ICP-OES elemental analysis was also performed to monitor cationic stoichiometric evolution before and after tetravalent doping. Zr⁴⁺ or Ce⁴⁺ co-doping increased the effective Mn²⁺/Mn⁴⁺ doping amounts by 1.12-fold and 1.5-fold, respectively, with the most pronounced augmentation observed in CAIC:Mn²⁺,Ce⁴⁺ (Table S3). This aligns with XPS elemental analysis, further validating that the charge compensation effect enhances the effective incorporation of Mn²⁺/Mn⁴⁺ into the lattice. The co-doping of tetravalent cations (Zr⁴⁺, Ce⁴⁺) into CAIC:Mn²⁺ substantially reduces the defect density induced by the non-isovalent Mn²⁺ substitution at the In³⁺ sites in double perovskite NCs, simultaneously increasing the effective Mn²⁺/Mn⁴⁺ doping concentration and optimizing its luminescent performance.

Previous studies have confirmed that charge compensation engineering effectively enhances the photoluminescence efficiency of CAIC:Mn²⁺; however, its influence on radiation-induced luminescence remains unexplored. Given the critical dependence of scintillator performance on defect density, carrier mobility, and exciton self-trapping dynamics, the scintillation characteristics of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ have been systematically investigated. First, we evaluated the X-ray absorption coefficients of CAIC:Mn²⁺,Ce⁴⁺ against commercial scintillators (CsPbBr₃, Si, CsI:Tl, and YAG:Ce) based on the photon cross-section database. As illustrated in Fig. 4a, CAIC:Mn²⁺,Zr⁴⁺ presents superior absorption efficiency across the photon energy range from 1 keV to 100 MeV, significantly outperforming YAG:Ce and Si. Additionally, its X-ray absorption rates are comparable to commercial scintillators CsPbBr₃ and CsI:Tl. The X-ray absorption efficiency of a material exhibits a linear dependence on its thickness. The thickness-dependent X-ray attenuation efficiency of CAIC:Mn²⁺,Ce⁴⁺, CsI:Tl, and YAG:Ce was quantified at a fixed X-ray photon energy of 22 keV (Fig. 4b). The synthesized CAIC:Mn²⁺,Ce⁴⁺ demonstrates excellent X-ray attenuation efficiency compared to CsI:Tl and YAG:Ce within the thickness range of 0–1.5 mm. CAIC:Mn²⁺,Ce⁴⁺ scintillator achieves complete absorption of X-ray energy at 0.75 mm, highlighting its potential as a high-performance X-ray-absorbing scintillation material for practical applications.

Fig. 4 (a) X-ray absorption coefficients of CsPbBr₃, Si, CsI-Tl, YAG:Ce and CAIC:Mn²⁺,Ce⁴⁺ scintillators as a function of photon energy. (b) Attenuation efficiency of Si, CsI-Tl, YAG:Ce and CAIC:Mn²⁺,Ce⁴⁺ scintillators as a function of thickness under 22 keV X-ray photon radiation. (c) Radioluminescence spectra of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺. (d) Proposed radioluminescence mechanism diagram. (e) Radioluminescence intensities of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺, CAIC:Mn²⁺,Ce⁴⁺ and YAG:Ce scintillators as a function of X-ray dose rate. (f) SNR values of CAIC:Mn²⁺,Ce⁴⁺ as a function of an X-ray dose rate. (g) Radioluminescence spectra of CAIC:Mn²⁺,Ce⁴⁺ and YAG:Ce scintillators. (h) Radioluminescence intensity of CAIC:Mn²⁺,Ce⁴⁺ over 30 consecutive X-ray irradiation on/off cycles (beam current: 100 μA, tube voltage: 50 kV).

The radioluminescence spectra of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ are shown in Fig. 4c. A sequential radioluminescence intensity enhancement is observed: CAIC:Mn²⁺ < CAIC:Mn²⁺,Zr⁴⁺ < CAIC:Mn²⁺,Ce⁴⁺, aligning with the corresponding photoluminescence trends shown in Fig. 3c. Specifically, in the absence of charge compensation, lattice charge neutrality is maintained by generating cation vacancies (such as In vacancy ).^47–49 These vacancies act as non-radiative recombination centers, leading to the quenching of active luminescent centers. In contrast, the co-doping of Ce⁴⁺ and Zr⁴⁺ as charge compensators effectively eliminates the non-radiative recombination centers, thereby enhancing the emission intensity.²⁹ The proposed radioluminescence mechanism (Fig. 4d) highlights a critical distinction from the photoluminescence process. Radioluminescence involves an additional energy relaxation and thermalization process of high-energy electrons generated via X-ray interactions. X-ray photons first interact with the heavy atoms in the CAIC:Mn²⁺,Ce⁴⁺ scintillator. Through the photoelectric effect and Compton scattering, the atoms absorb radiation energy via electron excitation: the photoelectric effect ejects electrons directly, while Compton scattering generates high-energy electrons. These high-energy electrons undergo thermalization on an ultrafast timescale, which simultaneously loses energy through inelastic collisions with lattice constituents, thereby inducing secondary electron generation. The primary ejected electrons and secondary electrons generated by irradiation eventually become trapped by Mn²⁺ luminescent centers. Subsequently, these centers release energy via radiative transitions, producing intense red radioluminescence.^31,50–53 A detailed energy level diagram illustrating this process for Mn²⁺ is shown in Fig. S8.

The X-ray dose rate dependence of the radioluminescence intensities of CAIC:Mn²⁺, CAIC:Mn²⁺,Zr⁴⁺ and CAIC:Mn²⁺,Ce⁴⁺ scintillators was recorded (Fig. 4e). These scintillators consistently exhibit a broad linear response within the dose rate range of 0.15–1.65 mGy_air s⁻¹. The radioluminescence intensity of CAIC: Mn²⁺,Ce⁴⁺ is higher than that of YAG:Ce at an equivalent X-ray dose rate, highlighting its superior X-ray detection sensitivity. The detection limit, defined as the dose rate corresponding to a signal-to-noise ratio (SNR) of 3, was calculated as 619.2 nGy_air s⁻¹ for CAIC:Mn²⁺,Ce⁴⁺ (Fig. 4f). This value achieves a 8.6-fold reduction compared to the medical diagnostic standard of 5.5 μGy_air s⁻¹. The light yield, a crucial parameter for determining scintillator performance and high-resolution X-ray imaging capability, quantifies photon emission per unit of absorbed ionizing energy. The light yield of CAIC:Mn²⁺,Ce⁴⁺ scintillator was determined to be 16 807 photons per MeV using a commercial YAG:Ce single crystal (14 000 photons per MeV) as a calibration standard under identical X-ray tube operating conditions (Fig. 4g). The radioluminescence intensity of the CAIC:Mn²⁺,Ce⁴⁺ scintillator remained nearly unchanged over 30 consecutive on/off cycles when exposed to an X-ray source with a tube current of 100 μA and a tube voltage of 50 kV (Fig. 4h). The scintillator's excellent stability under X-ray irradiation demonstrates its practical application potential. Furthermore, no discernible changes were observed in the XRD diffraction pattern of CAIC:Mn²⁺,Ce⁴⁺ after prolonged X-ray exposure (Fig. S7), indicating its outstanding structural stability and resistance to radiation-induced degradation.

Finally, we evaluated the X-ray imaging performance of the scintillators using a custom-designed system (Fig. 5a). The CAIC:Mn²⁺,Ce⁴⁺@PDMS scintillator film exhibits bright red photoluminescence under ultraviolet irradiation. Critically, it demonstrates exceptional spatial uniformity across the entire film, in addition to excellent flexibility that allows for easy folding and twisting (Fig. 5b and c). The profile of a metal key was well resolved under X-ray irradiation (Fig. S6d and e). The metal spring enclosed within the ball-point pen was invisible to the naked eye under daylight conditions; however, the internal structural outline of the spring enclosed within the pen was distinctly revealed under X-ray illumination (Fig. 5f and g). To quantify spatial resolution, the modulation transfer function (MTF) was derived via a slanted-edge analysis of the acquired radiographs. The spatial resolution is 7.7 lp mm⁻¹ at an MTF value of 0.2 (Fig. 5h). This result indicates that X-ray images generated by CAIC:Mn²⁺,Ce⁴⁺@PDMS scintillator screen exhibit high resolution, validating their practical viability for non-destructive evaluation of complex components and highlighting their exceptional performance in high-resolution X-ray imaging.

Fig. 5 (a) Schematic diagram of a custom-designed X-ray imaging system. (b and c) Photographs of the CAIC:Mn²⁺,Ce⁴⁺@PDMS film under an ultraviolet lamp (290 nm). (d and e) Images of a metal key under the illumination of (d) X-ray and (e) daylight. (f and g) Images of a ballpoint pen under the illumination of (f) X-ray and (g) daylight. (h) MTF curve of the CAIC:Mn²⁺,Ce⁴⁺@PDMS film measured using the slanted-edge method.

Conclusion

In conclusion, a series of Mn²⁺-doped CAIC double perovskite NCs were synthesized using an optimized hot-injection method. A rational charge compensation strategy via co-doping with tetravalent cations (Zr⁴⁺ or Ce⁴⁺) was implemented to enhance their photoluminescence properties. The introduction of Zr⁴⁺ and Ce⁴⁺ significantly amplified Mn²⁺-activated emission intensity, with lifetimes increasing sequentially from 0.74 ms (CAIC:Mn²⁺) to 1.04 ms (CAIC:Mn²⁺,Zr⁴⁺) and 1.34 ms (CAIC:Mn²⁺,Ce⁴⁺). The corresponding PLQY improved from 6.72% (CAIC:Mn²⁺) to 13.22% (CAIC:Mn²⁺,Zr⁴⁺) and 18.46% (CAIC:Mn²⁺,Ce⁴⁺). XPS quantitative analysis confirmed a 1.46-fold (CAIC:Mn²⁺,Zr⁴⁺) and 1.59-fold (CAIC:Mn²⁺,Ce⁴⁺) increase in the effective doping concentration of Mn²⁺/Mn⁴⁺ relative to CAIC:Mn²⁺. The redox-active Ce⁴⁺ effectively suppressed Mn²⁺ oxidation to higher valence states, yielding a 3.4-fold enhancement in Mn²⁺/Mn⁴⁺ ratio for CAIC:Mn²⁺,Ce⁴⁺ compared to CAIC:Mn²⁺, while CAIC:Mn²⁺,Zr⁴⁺ achieved a 2.4-fold improvement. The influence of the charge compensation effect on radiation-induced luminescence was also explored. The CAIC:Mn²⁺,Ce⁴⁺@PDMS scintillator film exhibited intense red radioluminescence under X-ray irradiation, featuring a higher light yield than the commercial scintillator YAG:Ce single crystal. The scintillator film demonstrated a spatial resolution of 7.7 lp mm⁻¹ and a low detection limit of 619.2 nGy_air s⁻¹. Furthermore, its excellent radiation stability and prolonged operational lifetime under continuous X-ray irradiation significantly facilitate the practical application of inorganic nanoscintillators in high-resolution X-ray microimaging.

Author contributions

Gaoyuan Xing: writing – original draft, investigation, data curation. Endian Cui and Xiangyang Yuan: data curation, visualization. Bing Wang: resources. Yanan Zhao, Jianfeng Tang: supervision, conceptualization, resources. Jiucun Chen: resources. Jing Liu: supervision, writing – review & editing.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information contains a detailed Experimental Section, and characterization data: photoluminescence excitation spectra, photoluminescence emission spectra, PLQYs, XPS analyses, XRD spectra, etc. See DOI: https://doi.org/10.1039/d5qi01582k.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJZD-K202400201, and KJQN202200221), Fundamental Research Funds for the Central Universities (SWU-KT25003), and the Natural Science Foundation of Chongqing (CSTB2024NSCQ-MSX0459). We acknowledge the critical and quantity of testing work supported by Beijing Zhongkebaice Technology Service Co., Ltd. We thank Prof. Yingshuai Liu for his guidance. We thank Analytical and Testing Center of Southwest University for their assistance with UV-Vis absorption spectra and photoluminescence spectra measurements. We thank Zolix Instruments Co., Ltd for assisting with X-ray imaging.

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