Scintillators have drawn tremendous attention for their ability to convert high-energy radiation into low-energy UV–vis light, showing their great potential in security inspection, biomedicine, and astrophysics [1–6]. At present, traditional inorganic scintillators (CsI:Tl, CaF₂:Eu, PbWO₄, Bi₄Ge₃O₁₂, etc.) have been successfully commercialized due to the advantages of excellent absorption coefficient and high light yield [7–12]. However, harsh preparation conditions, such as high processing temperatures and complicated preparation processes, increase the production cycle and costs, and seriously hinder their use in the development of novel flexible X-ray imaging [13–17]. Moreover, certain inorganic scintillators demonstrate significant susceptibility to moisture-induced degradation, posing additional challenges for long-term device storage [18–23]. The development of scintillator materials with excellent processability, cost-effectiveness, and high performance holds immense scientific and practical importance for advancing various fields.

Recently, zero-dimensional (0D) scintillator materials including the lead-halide perovskite-based nanocrystals and quantum dots have been applied in large-scale scintillation screens for X-ray imaging due to their outstanding solution processability [24–29]. However, the photon self-absorption effect that severely leads to aggregation-caused luminescence attenuation and inherently highly toxic metal Pb may both be crucial reasons to limit their commercial applications [30–34]. To overcome the agglomeration, the addition of surfactants (such as PVP) is an efficient synthetic strategy for the formation of homogeneous nano or micro-crystal scintillators [35–39]. Inspired by the unique optical behavior of crystallization-induced emission enhancement [40–42], we envision that this property can effectively prevent scintillators from aggregation-caused luminescence attenuation, in addition, the nano and micro-scale crystals with excellent optical properties can be prepared under mild conditions for the preparation of flexible scintillation screen. Currently, copper iodide clusters emerge as a new class of eco-friendly X-ray scintillators due to their rich structural diversity and excellent photophysical properties, such as exceptional X-ray absorption capability and high PLQY [43–46]. In addition, the unique cluster-centered emission makes Cu(Ⅰ)-Ⅰ clusters exhibit a shorter decay time in comparison to the MLCT/XMCT photoluminescence of common Cu(Ⅰ)-based complexes reported, further demonstrating the great potential for dynamic X-ray imaging applications [47,48].

In this work, we synthesized a uniformly sized 0D Cu₄I₆ microcrystalline scintillator by a simple solution protocol on a large scale at room temperature, which shows crystallization-induced emission enhancement property. The water oxygen stable Cu₄I₆ microcrystals display a narrow emission band at 503 nm with a PLQY up to 61.23%. The unique optical properties and heavy X-ray absorption ability endow Cu₄I₆ microcrystals high-quality radioluminescence (RL) performance, with an ultralow X-ray detection limit of 38.4 nGy/s, a high scintillator light yield of 21,700 photons/MeV, and a high resistance to radiation. More importantly, due to the highly dispersed microcrystalline morphology and low economic cost of Cu₄I₆, large-area (up to 100 cm²) and flexible X-ray scintillator screens were fabricated for high-quality, sensitive static and dynamic X-ray imaging applications of large-volume objects with a high resolution of 12.3 LP/mm. This work suggests that those CIEE-active copper iodide clusters hold great promise in high-energy detection and X-ray imaging applications.

The high-quality crystals of Cu₄I₆(DABCO—CH₃)₂ were obtained by the liquid phase diffusion method at room temperature. Briefly, the ethanol solution of DABCO—CH₃ was slowly added to a solution of CuI in acetonitrile and dichloromethane. Then, the resulting clear solution was allowed to evaporate in a dark environment at 4 ℃ to afford crystals of Cu₄I₆ for X-ray diffraction. In contrast to the conventional synthesis of microcrystals, the Cu₄I₆ microcrystals were synthesized by a simple, eco-friendly, and low-cost solution method, in absence of toxic surfactants such as polyvinyl pyrrolidone (PVP). CuI and DABCO—CH₃ powders with a molar ratio of 2:1 were dissolved in fresh acetonitrile and ethanol solution, respectively, to form a transparent solution at room temperature. The CuI in acetonitrile was poured into the ethanol solution of DABCO—CH₃ to immediately produce a large amount of Cu₄I₆ microcrystals with a high yield of ~95% (Fig. S1 in Supporting information). The detailed experimental procedure can be seen in Supporting information. This method of synthesizing Cu₄I₆ clusters at the gram-scale under mild conditions has offered substantial benefits for the fabrication of large-sized scintillator screens.

SCXRD analysis revealed that Cu₄I₆ crystallizes in the cubic space group Pa\overline{3} and consists of four Cu atoms, six I atoms, and two DABCO—CH₃ ligands in each cluster (Fig. 1a). The overall structure shows an I₆ octahedron being halved crosswise by an Cu₄ parallelogram (Fig. 1b), and the DABCO—CH₃ ligands coordinated to the exposed Cu atoms. The packing diagrams shown in Fig. S2 (Supporting information) display that Cu₄I₆ self-assembles into a hexagonal closest packing (HCP) pattern with perfect face-to-face contact of neighboring Cu₄I₆ and interdigitation of intercluster surface ligands.

The crystalline phase purity of the gram-scale synthesized Cu₄I₆ powder was confirmed by powder X-ray diffraction (PXRD) (Fig. 1c). Meanwhile, the scanning electron microscopy (SEM) image of Cu₄I₆ exhibited a uniform size distribution of microcrystals with rhombic block morphology (Fig. 1d), which is beneficial to forming a dense and uniform scintillator screen. Furthermore, the Cu₄I₆ microcrystals can maintain stability until approximately 280 ℃, as revealed by the thermogravimetric analysis (Fig. S3 in Supporting information), manifesting a significantly good thermal stability.

The photophysical performance of the fresh Cu₄I₆ crystals was studied by UV–vis absorption and PL spectra. The UV–vis absorption spectra of Cu₄I₆ displayed optical bandgaps of 2.99 eV (Fig. S4 in Supporting information), and the low bandgap favors better RL [48]. Fig. 1e shows the PL and PL excitation (PLE) spectra of Cu₄I₆, which exhibit intense color-pure green emission centered at 503 nm with a full width at half-maximum (FWHM) of 80 nm under UV-light excitation. The large Stokes shift (140 nm) and corresponding microsecond scale lifetime (3.59 µs) indicate the phosphorescence nature (Fig. S5 in Supporting information). Moreover, a high PLQY of 61.23% was acquired by adopting an absolute PL measurement system (Fig. S6 in Supporting information). It is worth noting that the large Stokes shift and high PLQY are beneficial for obtaining a decent scintillation light yield. It is noted that the structure of metal clusters has a significant effect on optical properties. For instance, the Cu₄I₆ clusters with identical core but different organic cation ligands have been reported previously, but exhibiting several PLQY and emission wavelengths [35,38]. Meanwhile, the Xia’s group reported the influence of different halide anion on scintillator performance of two copper(Ⅰ)-based clusters Cu₃X₃ (X = Cl, Br). They have considered that Cu₃Br₃ cluster with low halogen electronegativity further weakened halogen-to-ligand charge transfer (XLCT) emission, enabling enhanced quantum efficiency and scintillation performance than that of the chloride one [47]. Furthermore, the crystal size of metal clusters directly affects their electronic structure, especially the quantization degree of the energy level spacing. When the cluster size is reduced to the nanometer level, the quantum size effect causes its electronic energy level to change from a continuous state to a discrete state, thereby affecting the light absorption and emission characteristics [50]. Density functional theory (DFT) computation was conducted to understand the PL mechanism of Cu₄I₆. The electron densities of HOMO and LUMO were mainly localized in the regions surrounding the Cu/I atoms and the planar tetranuclear copper structure, respectively, indicating a cluster centered (CC) electronic transitions (Fig. S7 in Supporting information) [47].

In nanomaterials, AIEE or CIEE is a photophysical phenomenon that receives wide attention, in which luminophores experience a rise in PL efficiency with their self-assembly and aggregation than in dilute solution. As the solution of Cu₄I₆ in MeCN is non-emissive, the intense solid-state emission may be attributed to the AIE and CIE effects. Therefore, the CIEE activities of Cu₄I₆ were systematically analyzed in a MeCN/Et₂O solvent system.

As shown in Fig. 2a, the non-emissive transparent solution of Cu₄I₆ turned slightly cloudy when f_w (f_w = vol_Et2O/vol_MeCN+Et2O) was increased to 60% and exhibited a weak green emission under UV light. As f_w continued to increase from 60% to 70%, the PL intensity of the solution was sharply elevated (Figs. 2a and c). The PL intensity at f_w = 70% has risen >2200-fold (I_{70 %}/I₀ = 2223) in comparison to the pure MeCN solution of Cu₄I₆, with a high PLQY of 22.3%. Of note, the current microcrystal size is larger than that at f_w = 60% (Fig. 2a), which is similar to aggregate expansion in the AIE effect. Despite the microcrystals' quantity steadily increasing and size gradually aggregating as f_w increased from 70% to 90%, the PL intensity showed no significant change (Figs. 2c and d). In addition, the emission centered at 503 nm shows no obvious shift as the f_w increases (Fig. S8 in Supporting information), suggesting that the emission originates from the Cu₄I₆ microcrystals, which could be directly observed by transmission electron microscopy (TEM) images (Fig. 2b). Moreover, we found that there is a remarkable change in PLQY between the aggregated and crystal states of Cu₄I₆ (Fig. S9 in Supporting information), which can be attributed to defect-rich surfaces and low crystallinity induced by rapid crystallization in Et₂O solvents [51,52]. This phenomenon further proves the CIEE behavior of Cu₄I₆. Of note, this CIEE-active copper iodide clusters may effectively prevent clusters from aggregation-caused luminescence attenuation and further improving X-ray imaging ability.

Meanwhile, the dynamic light scattering (DLS) measurement reveals that the size of the aggregates increased with an increasing fraction of poor solvent (f_w: from 60% to 90%) (Fig. S10 in Supporting information). Finally, to verify the arrangement of Cu₄I₆ in aggregates, the aggregate particles (f_w = 70%) were collected for high-resolution TEM and selected area electron diffraction (SAED) experiments (Fig. S11 in Supporting information), and the diffuse rings in the SAED pattern indicate the crystalline state of Cu₄I₆.

The large-scale production of low-cost high-quality Cu₄I₆ microcrystals exhibiting excellent optical properties makes them promising candidates for X-ray imaging applications. Thus, the RL properties of Cu₄I₆, including the light yield, detection limit, and antiradiation stability under X-ray irradiation, were systematically examined. According to the XCOM web database, we calculated the attenuation coefficient of Cu₄I₆, which is lower than the commercial scintillator BGO but higher than the reported Cu₄I₄Py scintillator (Fig. 3a), because of the presence of two extra I atoms with high X-ray absorption cross sections [45]. The relationship between the X-ray attenuation efficiencies and the thickness of the Cu₄I₆ scintillators over the entire range of X-ray photon energies has been investigated, and a 0.5 mm-thick Cu₄I₆ scintillator could absorb ≈ 83.24% of the incident X-rays (Fig. S12 in Supporting information). Furthermore, the X-ray attenuation efficiencies as a function of photon energy at a thickness of 0.5 mm were studied (Fig. 3b). Taking a commercially available BGO (LY: 8000−10,000 photons/MeV) as the reference, the light yield of Cu₄I₆ scintillator was measured to be 16,814−21,700 photons/MeV by comparing the integrated area of the RL spectra (Fig. 3c). The minimum detection limit is closely related to the sensitivity of scintillators to X-rays, as an important parameter of the practical applications of scintillators. As shown in Fig. 3d and Fig. S13 (Supporting information), the RL intensity of the Cu₄I₆ scintillator showed a linear dependence on the X-ray dose rate. Of note, based on a three-time signal-to-noise ratio (SNR), the detection limit of the Cu₄I₆ scintillator is calculated to be 38.4 nGy/s, which is about 143 times lower than the dose of 5.5 µGy/s for the medical treatment requirement of X-ray diagnostics. Additionally, the radiation stability of the Cu₄I₆ scintillator was examined under cyclical X-ray irradiation, as shown in Fig. 3e, the RL intensity of the Cu₄I₆ scintillator barely changed under a high X-ray dose (278 µGy/s) for one hour. The result indicated that the Cu₄I₆ scintillator demonstrated excellent X-ray radiation stability, which provides a feasible opportunity for X-ray imaging. Fig. 3f and Table S1 (Supporting information) show the preliminary cost analysis of the Cu₄I₆ scintillator and a comparison with other commercial or reported scintillators, and our scintillator is highly cost-effective [53]. Besides, the light yield of Cu₄I₆ is comparable to that of those scintillators mentioned above (Fig. S14 in Supporting information). All these findings fully convince that Cu₄I₆ is a candidate material for X-ray imaging. The PL and RL spectra of the Cu₄I₆ crystals at room temperature are almost identical with a negligible difference in emission center, implying that the emission likely originated from the same excited states (Fig. S15 in Supporting information).

The X-ray scintillation mechanism was further investigated. For the 0D Cu₄I₆ cluster-base scintillator, X-ray-excited luminescence mainly goes through the following processes (Fig. 3g). Under the excitation of high-energy X-ray, the heavy atoms in the Cu₄I₆ scintillator interact with the X-ray photons through the photoelectric effect and Compton scattering, where the high-energy electrons escape from the inner shells of the atoms. Such high-energy electrons subsequently induce plentiful secondary electrons by colliding with atoms. Then these secondary electrons and holes quickly thermally relax to the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the Cu₄I₆ cluster, forming the recombination of electron-hole pairs, and the singlet (S_n) and triplet (T_n) excitons are generated with a 1:3 ratio due to the exciton spin statistics. Finally, the triplet excitons (T₁) decay to the ground state (S₀) through a radiative transition process, generating intense phosphorescence emission. Besides, the calculations of the excited states could offer a more profound understanding of the phosphorescence transition mechanism, and the calculated spin-orbit coupling (SOC) values between S₁ and T₁ (gap ≈ 0.19 eV) is high enough for intersystem crossing (ISC) to produce efficient triplet excitons for phosphorescence (Fig. S16 in Supporting information).

Compared with high-dimensional scintillators, zero-dimensional (0D) scintillation materials have attracted tremendous attention due to good processibility in flexible scintillation screens, and the homogeneous and small-size microcrystal is beneficial to improve the scintillation screen capability. To unfold the capability of X-ray imaging of Cu₄I₆, the large-area flexible scintillator screen (10 × 10 cm²) can be fabricated with soft and deformable PDMS (for details, see Supporting information). As shown in Figs. 4a and b, the processed scintillator screen is characterized by favorable homogeneity and flexibility, and it exhibits uniform green light emission across the entire area under both X-ray and 365 nm UV light excitation and also shows decent transverse tensile property (Fig. S17 in Supporting information). Additionally, we evaluated the X-ray imaging performance of the Cu₄I₆ scintillator screen. As shown in Fig. 4c, the modulation transfer functions (MTF) of X-ray images based on Cu₄I₆ scintillator was obtained. The spatial resolutions is acquired as 12.3 LP/mm where the spatial frequency (LP/mm) at MTF = 0.2, which is superior to some reported perovskite and copper-halide cluster flexible scintillator screens [47,49]. Next, the ballpoint pen and earphones were placed between the cluster scintillation screen and the X-ray source to test the X-ray imaging performance of the actual objects. As shown in Figs. 4d and e, under X-ray irradiation, the metal aperture in the millimeter scale of headset and the spring inside the ballpoint pen can be both clearly observed from X-ray imaging photographs. More importantly, to further evaluate the feasibility of the Cu₄I₆ scintillator screen for dynamic X-ray imaging, the dynamic imaging patterns of a piano metronome were measured. As shown in Fig. 4f, the X-ray imaging patterns of the metronome at different angles of swing can be obviously recorded. In addition, Video S1 (Supporting information) clearly captured the rapid oscillating behavior of the metronome displayed on the large-area scintillator screen (The video only has noise suppression processing, and the red flash is a small X-ray dose alarm.), under X-ray irradiation. All the above radiation experiments suggest the potential of the Cu₄I₆ scintillator for low-dose healthcare diagnostics and high-quality imaging of static and dynamic objects in practical applications.

In conclusion, an ecofriendly 0D microcrystalline scintillator Cu₄I₆ based on CIEE-active copper iodide cluster was successfully synthesized on a large scale via a facile solution method. The Cu₄I₆ cluster shows a high PLQY of 61.23%, narrowband green emission, large Stokes shifts, and short decay times (as short as 3.59 µs) at room temperature. Compared with the traditional commercial scintillator BGO, uniformly sized Cu₄I₆ microcrystals exhibit high-quality scintillation properties with a low detection limit of 38.4 nGy/s, and high relative light yields of 21,700 photons/MeV. In addition, due to monodispersity of Cu₄I₆ microcrystals, the large-area and flexible X-ray scintillator screen was readily fabricated, which was successfully used for high-resolution imaging on static and dynamic objects, with a spatial resolution of 12.3 lp/mm. We expect that this work can provide a novel tack for preparation of microcrystalline scintillation materials with excellent performance in high-energy detection and X-ray imaging applications.