Lead‐free organic–inorganic hybrid scintillators for X‐ray detection

Scintillators, which can convert high‐energy particles (X‐rays) into detectable low‐energy ultraviolet–visible–near‐infrared photons, are essential components of X‐ray detectors and show extensive practical applications in nondestructive detection and medical imaging. Traditionally, inorganic scintillators represented by CsI:Tl have achieved definite progress. However, the harsh preparation conditions, high production cost, and poor mechanical properties impede their potential development in the high‐end X‐ray imaging field. Organic–inorganic hybrid metal complexes could be excellent alternatives, by virtue of their structural and spectral tunability, good solution processability, and excellent photophysical properties. This review mainly focuses on eco‐friendly lead‐free metal (Mn2+, Cu+, Sb3+, Sn2+, Ge2+, Ln3+, etc.) complex scintillators. The luminescence mechanisms are introduced and the scintillation performance, such as light yield, limit of detection, imaging resolution, etc., is highlighted. Moreover, the current challenges and perspectives in this emerging field are described. It is hoped to provide some theoretical guidance for the continuous development of the new scintillator systems.


INTRODUCTION
X-rays are a type of electromagnetic wave with wavelength ranging from 0.01 to 10 nm.12] At present, the commercialized inorganic scintillators (mainly including NaI:Tl, CsI:Tl, PbWO 4 , LYSO, etc.) are normally applied in the form of bulk crystals, suffering from complex production processes (high temperature and high pressure). [13]In addition, the mechanical rigidity makes inorganic scintillators unable to meet the increasing requirement for flexible electronic products.Thus, the above unfavorable factors greatly hamper their further application in the high-performance X-ray imaging field.Recently, metal-based organic-inorganic hybrid complexes (OIHCs) have attracted great interest, attributed to the following reasons: (1) easy production processes, they can be processed in solution; (2) adjustability, rich types of scintillators can be obtained by changing the metal centers and organic cations; (3) excellent photophysical properties, most of the metal-based OIHCs possess high photoluminescence quantum yield (PLQY) and large Stokes-shift; The general scintillation process of scintillators can be mainly divided into three stages: conversion, transport, and luminescence (Figure 1).At the first stage of conversion, the incident X-ray photons interact with the lattice heavy atoms of scintillators to produce a large number of hot electrons and deep holes, mainly through the photoelectric effect and Compton scattering effect.And then, the hot electrons generate massive energetic secondary electrons and auger threshold via coulomb scattering, resulting in the production of energetic charge carriers.This process will continue until the hot electrons have lost sufficient energy to further ionize the ions in the lattice.These resulting energetic charge carriers will undergo energy dissipation through interacting with phonons.After this process, a large number of low-energy electrons and holes gradually migrate and accumulate at the conduction band and valence band of the luminescence centers, that is, the transport stage.The last stage is luminescence that signifies efficient radiative recombination to emit plenty of low-energy (UV-vis-NIR) photons, which will then be accepted by a following photodetector to accomplish X-ray imaging and detection.Different metal (Cu + , Mn 2+ , Ln 3+ , Sb 3+ , Sn 2+ , etc.) complexes exhibit different luminescence mechanisms, such as band to band recombination of Cu +based complexes, 4f n-1 5d → 4f n transitions in Ln 3+ -based complexes, etc.

2.2
The key parameters in X-ray scintillators and indirect imaging Some important indexes are used to evaluate the scintillation performance, which mainly are X-ray absorption coefficient, LY, linear response and limit of detection (LOD), spatial resolution, radiation resistance, and so on.

X-ray absorption coefficient
The X-ray absorption coefficient (α) represents the efficiency of a scintillator to absorb the X-ray photons energy, which is dominated by the atomic number (Z) and density (ρ), as shown in Equation (1).
where A is the atomic mass, E is the radiation energy.It can be seen from the equation that the heavy atomic composition and large crystal density are beneficial to improving the X-ray absorption efficiency of the scintillator.

Light yield
LY indicates the X-ray conversion efficiency and can be defined as the number of emitted photons per each 1 MeV of absorbed X-ray energy, as shown in Equation (2).
where E is the energy of incident X-ray photons, β is a constant related to the host structure which typically ranges between 2 and 3, E g is the bandgap of the scintillator, S and Q represent the efficiency of the transport and luminescence stages, respectively.Some values (E, β, and S) in the above equation will not be easily changed, and the scintillators with small E g value and high Q show the trend of high LY.Ideally, the emitted photons can be completely collected by the back-end photodetector, but in fact, there will always be some losses due to the self-absorption effect, non-radiative processes, unexpected scattering, and internal reflection.Therefore, the actual LY is always lower than the theoretical LY value.The LY in the paper was estimated on radioluminescence (RL) intensity.

Linear response and LOD
The linear response of a scintillator demonstrates the variation of the proportionality between RL intensity and X-ray dose rate, indicating the dynamic range of X-ray detection.
LOD is an important figure of merit that ultimately determines the minimum dose rate used for imaging or detection, which usually refers to the dose rate when the signal-to-noise ratio (SNR) is 3. Fundamentally, LOD is determined by the detection ability of the whole measurement system including scintillators and photodetector.

Spatial resolution
The spatial resolution describes how detailed an object can be represented by the image, and is an important merit for evaluating the performance of an imaging system, which can be quantified by the modulation transfer function (MTF).MTF signifies the transfer ability of signal modulation of the spatial frequency relative to its output.As the spatial resolution improves, the number of pixels utilized in the construction of an image increases, and a clearer image will be collected.Generally, the MTF curve can be measured by the slanted-edge method.

Radiation resistance
The radiation resistance expresses the capability of a scintillator to maintain its initial performance under continuous X-ray irradiation.High-energy X-rays can destroy scintillator structures, leading to a decline in scintillation efficiency.Hence, radiation resistance is an important criterion for the application of scintillators.
In addition, energy resolution, emission wavelength, response time, and chemical stability are also critical metrics for scintillators.Among them, response time in the paper is for photoluminescence (PL) decay.

MANGANESE(II)-BASED OIHCS
Manganese(II)-based OIHCs hold the features of low preparation cost and excellent photophysical properties, which are expected to become substitutes for lead(II)-based OIHC scintillators. [22,23]In general, manganese(II)-based OIHCs with different charge characteristics can be divided into ionic and neutral types (Figure 2). [24,25]

Luminescence mechanism of manganese(II)-based OIHCs
Manganese (Mn) has an outer electron configuration of 3d 5 4s 2 and can be oxidized into a variety of valence states, among which bivalence Mn 2+ is most easily generated.The electrons of Mn 2+ can transit from the ground state 6 A 1 to the high-energy quartet states 4 E, 4 A 1 (G), and 4 E(D).The absorption and emission transitions of Mn 2+ mainly include D-terms and G-terms transitions.Due to the strong splitting energy of the ligand field of D-term, the absorption of D-term is stronger than G-term.Hence, the first group of absorption peaks are usually derived from the transitions of 6 A 1 → 4 E(D) and 6 A 1 → 4 T 2 (D), while the second group of absorption peaks are generated by the transitions of 6 A 1 → 4 A 1 , 4 E(G), 6 A 1 → 4 T 2 (G) and 6 A 1 → 4 T 1 (G).The main emission of Mn 2+ originates from the transition of the lowest excited state energy level 4 T 1 to 6 A 1 (d-d).This transition is normally forbidden and leads to the small molar absorption coefficient of Mn 2+ .With the help of organic ligands, the forbidden can be broken.The working mechanism of manganese(II)based OIHCs relies on the structural reorganization of the excited state.Under irradiation, the generated excitons of organic ligands experience a quick intersystem crossing (ISC) from a single excited state (S 1 ) to a triplet excited state (T 1 ) and then transit to the lowest excited state ( 4 T 1 ) of Mn 2+ through energy transfer.Finally, luminescence will be produced by the radiative transition from the lowest excited state energy level 4 T 1 to 6 A 1 .In addition, different coordination environments of Mn 2+ endow the compounds with different luminescent colors.By changing the coordination numbers, the emission of manganese(II)-based OIHCs can cover multiple luminescent regions, including green, yellow, orange, and red.

Ionic manganese(II)-based OIHC scintillators
At present, a lot of ionic manganese(II)-based OIHC scintillators have been reported and the organic ligands were mainly phosphonium and ammonium types. [26]Ma et al. prepared a 0D manganese(II)-based OIHC scintillator 1 for the first time. [27]The LY of the scintillator was up to 79,800 photon MeV −1 , which was three times that of commercial scintillator Ce: LuAG.To implement X-ray imaging, a flexible membrane (Figure 3A) was prepared by blending scintillator powder with polydimethylsiloxane (PDMS).However, the decay lifetime of manganese(II)-based OIHCs reached the level of milliseconds or even seconds, resulting in a severe ghosting effect in X-ray imaging, which was not suitable for dynamic detection and high-end medical imaging.To address this issue, Ma et al. reported a multicomponent scintillation system (Figure 3B) using a manganese(II)-based OIHC 2 to sensitize organic fluorescent dyes (PM 570). [28]This system displayed a short decay lifetime of 7.1 ns, which was much shorter than that of intrinsic manganese(II)-based OIHCs, demonstrating its potential in dynamic imaging.In addition, as the halogen atomic number gradually increases, the spin-orbit coupling effect can be strengthened, which will accelerate the radiation transition and thus reduce the decay lifetime.Kuang et al. prepared a single crystal scintillator 3 by a local heating solvent evaporation method. [29]The decay lifetime of the complex was 57.62 µs, which was lower than most of the reported manganese(II)-based OIHCs.
At present, the way of mixing scintillators with organic composites (PDMS, PVDF, TPU, etc.) is the most popular preparation technology for scintillator membranes.However, this traditional technology usually leads to light scattering and thus the spatial resolution of films will be reduced.In order to break through this bottleneck, some novel strategies have been proposed.For instance, combining scintillators with substrates in an in situ growth can avoid the powder aggregation and inhomogeneity, and the substrates can be divided into organic polymer and transparent ceramic.Yang et al. proposed an in situ growth method (Figure 3C) to prepare large-area flexible scintillation screen films with MnBr 2 , C 24 H 20 BrP precursors and thermoplastic polyurethane (TPU). [30]This flexible film could be repeatedly bent and stretched.The LY of the scintillation film 2-TPU was 6 times more than that of the widely studied CsPbBr 3 -PMMA film.Based on this flexible scintillation film, high-contrast X-ray imaging with a high spatial resolution of 14.5 lp mm −1 was successfully implemented.Liang et al. developed an in-situ fabrication method to prepare a nanocrystal film 4 with organic ligand BTPBr, MnBr 2 , and PVDF. [31]The light cross-talk of the scintillator film was greatly suppressed due to the dense distribution of nanocrystals.The manganese(II)-based OIHC scintillator displayed an impressively high spatial resolution of 23.8 lp mm −1 .Recently, transparent ceramic scintillators have attracted much attention due to their excellent transparency. [32,22]ia et al. fabricated a large-area wafer through an in situ growth method with the manganese(II)-based OIHC 2 (Figure 3D). [33]The wafer exhibited a high optical transparency of above 68% and a high spatial resolution of 15.7 lp mm −1 .Jin et al. reported a glass scintillator 5 based on manganese(II)-based OIHC through the melt-quenching method. [34]The glass scintillator displayed a high LY of 38,000 photon MeV −1 and a high spatial resolution of 17.28 lp mm −1 .
In addition, single-crystal manganese(II)-based OIHCs possess excellent optical transparency and sufficient X-ray attenuation ability and can be directly used as scintillators without any substrate, which showed enormous potential in X-ray imaging.Kuang et al. explored a single-crystal scintillator with the manganese(II)-based OIHC 3 through a localheating solvent evaporation method (Figure 3E). [29]The spatial resolution of the single-crystal scintillator was as high as 25 lp mm −1 , exceeding that of the majority of commercial scintillators (CsI:Tl, 10 lp mm −1 ).After that, they explored a series of single-crystal manganese(II)-based OIHC scintillators with different halogen atoms (X = Cl, Br, I) based on 2-dimethylaminopyridinium. [21] These single-crystal The X-ray photographs of scintillator 1. Reproduced with permission. [27]Copyright 2020, Springer Nature.(B) Schematic X-ray sensitization process with manganese(II)-based OIHC 2 and organic dye PM 570 (donor and acceptor).Reproduced with permission. [28]Copyright 2022, American Chemical Society.(C) Scheme depicting the fabrication of the scintillation film 2-TPU.Reproduced with permission. [30]Copyright 2022, Wiley.(D) The image of ceramic scintillator based on manganese(II)-based OIHC 2. (E) Schematic diagram of the X-ray imaging system.Reproduced with permission. [29]opyright 2022, American Chemical Society.(F) Conceptual illustration of the minimization of optical crosstalk by polarized RL. (G) The proof of concept of X-ray imaging using chiral crystals pairs and achiral crystals.Reproduced with permission. [35]Copyright 2022, Wiley.(H) Structure diagram of the cationic part of manganese(II)-based OIHC 9. Reproduced with permission. [39]Copyright 2021, Wiley.scintillators all showed high spatial resolution of above 10 lp mm −1 , among them, the spatial resolution of iodide complex 6 with a thickness of 0.31 mm was up to 25 lp mm −1 .
Furthermore, controlling the propagation direction of optical signals can also reduce the optical crosstalk effect in imaging.Under X-ray irradiation, the isotropic propagation of RL signals of achiral scintillators can cause the generation of optical crosstalk between adjacent scintillators and pixels (Figure 3F).Circularly polarized luminescence (CPL) refers to the luminescent system emitting different strengths of left and right circularly polarized light, which can reflect the excited state structure information.The luminescence dissymmetric factor (g lum ) is an important index to evaluate the intrinsic properties of CPL materials, which is the difference between the strength of I L and I R divided by their average total luminescence intensity.Generally, chiral scintillators with the property of CPL can adjust the propagation direction of light signals, partially reducing the optical crosstalk (Figure 3F). [35,36]As proof of concept, Wang et al. assembled a polarized scintillator pair with two chiral scintillator crystals. [35]As shown in Figure 3G, when the linearly polarized plate was rotated (0 or 90 degrees), the adjacent pixels of chiral scintillators would not be affected.In contrast, the achiral scintillators (LYSO and CsI:Tl) exhibited an unsolved image boundary because of the optical crosstalk from the isotropic RL.Zhao et al. designed and synthesized two pairs of chiral manganese(II)-based OIHC clusters (R/S-1 and R/S-2). [37]Among them, the photophysical property of R-2 (7) with high PLQY (86.3%) and |g lum | (7.1 × 10 −3 ) was the most superior and thus was chosen to test scintillation performance.Artem'ev et al. synthesized manganese(II)-based OIHC (R/S-[MBA-Me 3 ]MnBr 4 ) scintillators with high PLQY (89%) and |g lum | (4.5 × 10 −3 ), and demonstrated an excellent imaging effect. [38]The researches on OIHC scintillators with the feature of CPL property were few, but this strategy provided a possibility to improve X-ray imaging performance.

Neutral manganese(II)-based OIHC scintillators
Neutral manganese(II)-based OIHCs formed by Mn 2+ and organic ligands through the coordination bonds, have been extensively considered to show better stability than ionic manganese(II)-based OIHCs.Neutral manganese(II)-based OIHCs are usually synthesized with phosphine oxide or bis(phosphine oxide) ligands.Zhao et al. reported a neutral manganese(II)-based OIHC scintillator 8 by using organic ligand DDXPO ((9,9-dimethyl-9h-xanthene-4,5-diyl) bis(diphenylphosphine oxide)) and MnBr 2 . [39]Interestingly, unlike most current tetrahedral or octahedra manganese(II)based OIHCs, this complex exhibited a peculiar fivecoordinated mode in a distorted trigonal-bipyramidal geometry (Figure 3H).The manganese(II)-based OIHC exhibited a bright yellow emission centered at 583 nm with a high PLQY of 93% and an LY of 18,400 photon MeV −1 .As a scintillator, this complex demonstrated excellent radiation hardness, humidity resistance, and heat stability, indicating its application potential in harsh environments.Soon afterwards, Zhao et al. further prepared and investigated a series of binuclear neutral manganese(II)-based OIHC scintillators with excellent stability for X-ray imaging. [40]What is more, Guo et al. synthesized a 0D neutral manganese(II)-based OIHC scintillator 9 with excellent water resistance. [41]This scintillator could maintain an intense green emission when immersed in water.
By and large, manganese(II)-based OIHCs have displayed rich structures, tunable photophysical properties and easy-to-process characteristics.Regulating the structure of organic cations and the type of halogen atoms, manganese(II)-based OIHCs with different luminescence properties can be obtained (Table 1), which endowed them with extraordinary talents in the field of X-ray scintillation.However, due to the characteristics of multiple energy levels, manganese(II)-based OIHCs suffer from selfabsorption effect, restricting the efficient light out.Moreover, attributed to their inherent spin-orbit forbidden transitions (d-d), the decay time of manganese(II)-based OIHC scintillators is relatively long, reaching the level of hundreds of microseconds or even milliseconds, which cannot meet the requirements of fast X-ray scintillation response.Therefore, although manganese(II)-based OIHC scintillators displayed good photoelectric properties, there is also a large room for improvement.

COPPER(I)-BASED OIHCS
Copper(I) ions can easily combine with N, S, and P atoms to attain rich complexes, bringing in highly emissive nature and decent luminescence properties. [42,43]Generally, copper(I)based OIHCs (Figure 4) can be divided into three types, namely ionic, neutral and all-in-one (AIO).

Luminescence mechanism of copper(I)-based OIHCs
The PL mechanisms of copper(I)-based OIHCs mainly include metal-centered (MC) emission, triplet clustercentered ( 3 CC) emission, self-trapped exciton (STE) emission, and defect-bound exciton (DBE) emission.Generally, the MC transition is originated from d-d transitions with the d 10 configuration of Cu(I) (d 9 s 1 → d 10 ).The 3 CC emission is originated from the cluster centers of copper(I)-based OIHCs.STE, a local electronic state, is induced by the strong electron-phonon coupling in copper(I)-based OIHCs.The DBE emission originate from the excitons bound to intrinsic or extrinsic defects in copper(I)-based OIHCs.

Neutral copper(I)-based OIHC scintillators
For neutral copper(I)-based OIHC scintillators, amine-based derivatives are the most widely studied organic ligands.N atom has strong coordination with Cu(I) ion, which is beneficial to the stability of scintillators.Liu et al. reported a series of neutral copper(I)-based OIHC scintillators with pyridinebased molecules and Cu 4 I 4 . [44]Among them, the complex 10 with pyridine group exhibited the optimal optical properties, whose LY was more than 2.5 times that of BGO.In virtue of strong coordination between N atom in pyridine and Cu(I) atom, the scintillator exhibited extraordinary radiation resistance under continuous X-ray irradiation.When repeated on-off X-ray irradiation for 100 cycles, the RL intensity of the scintillator showed no obvious changes (Figure 5A).In addition, the complex with a hydrophobic nature also displayed intense moisture resistance and could remain stable after storing at a relative 60% humidity for 13 days (Figure 5B).A flexible and dense film based on the copper(I)-based OIHC scintillator combined with PVA delivered a high resolution of close to 20 lp mm −1 .
The MC emission of copper(I)-based OIHCs mainly relies on the structural reorganization of the excited state.The excitons first transit from the ground state (S 0 ) to S 1 , which is attributed to the metal/halide-to-ligand charge transfer ( 1 M/XLCT) transition.Later, the excitons transit to the 3 M/XLCT through the ISC process.Finally, luminescence F I G U R E 5 (A) Radiation resistance and (B) moisture resistance of the cluster scintillator 10.Reproduced with permission. [44]Copyright 2022, Wiley.(C) The HOMO and LUMO of the copper(I)-based OIHC scintillator 11 (77-273 K). (D) A standard resolution test pattern plate of the complex 11.Reproduced with permission. [45]Copyright 2022, Wiley.(E) Energy diagrams of copper(I)-based OIHC scintillator 12. (F) The MTF curve of X-ray images based on the complex 12. Reproduced with permission. [47]Copyright 2022, Wiley.
will be produced by the radiative transition from 3 M/XLCT to S 0 .Zhao et al. developed a copper(I)-based OIHC scintillator 11 with the MC emission. [45]From the density functional theory calculations, the highest occupied molecular orbital (HOMO) was mainly located on the Cu and I atoms and the lowest unoccupied molecular orbital (LUMO) was mainly distributed in the 3-methylpyridine, which were assigned to the mixed MLCT and XLCT state (Figure 5C).The LY of the scintillator was 28,385 ± 1335 photons MeV −1 and the flexible film showed a high spatial resolution of 9.8 lp mm −1 , displaying great potential in practical X-ray imaging (Figure 5D).
For 3 CC emission, the excitons transit from 1 M/XLCT to 3 M/XLCT through the internal conversion and then Reproduced with permission. [49]Copyright 2022, Wiley.(D) Schematic illustration of STE emission based on the copper(I)-based OIHC scintillator 16. (E) Structure of complex 16.Reproduced with permission. [54]Copyright 2021, American Chemical Society.(F) PL decay profiles of complex 20.Reproduced with permission. [58]Copyright 2022, American Chemical Society.transit to 3 CC through the ISC process.Finally, luminescence will be generated by the radiative transition from 3 CC to S 0 .The rate-constant of 3 CC radiative pathway is faster than that of MC emission. [46]The 3 CC emission is associated with the geometrical deformation of the clusters.Xia et al. conducted a novel copper(I)-based OIHC scintillator 12 with the 3 CC emission (Figure 5E) through the evaporation method. [47]The scintillator underwent an intense luminescence process exposed to irradiation and obtained a high LY of 20,000 ± 700 photons MeV −1 .The X-ray imaging test showed that a relatively high spatial resolution of 11.71 lp mm −1 was achieved based on the homogeneous scintillation film (Figure 5F).Afterwards, Mohammed et al. reported two neutral copper(I)-based OIHC scintillators with 3 CC emission. [48]Both of the cubane-like scintillators (13 and 14) displayed high LYs of exceeding 30,000 photons MeV −1 and outstanding spatial resolutions above 20 lp mm −1 .

Ionic copper(I)-based OIHC scintillators
In addition to neutral copper(I)-based OIHCs, there are many ionic copper(I)-based OIHCs.Xia et al. synthesized a novel 0D ionic copper(I)-based OIHC scintillator 15 with strong and near-unity blue-emission at 450 nm. [49]The PLQY of the complex was as high as 99.7%, which is originated from the MC transition of copper(I)-based complexes.Under 305 nm irradiation, two emission peaks (450 and 650 nm) of the scintillator were observed at low temperature (80 K), demonstrating that the two emissions were derived from different excited states (Figure 6A,B).The low-energy emission at 650 nm disappeared as the temperature increased to 200 K and was attributed to 3 CC excited state (Figure 6C).The LY of the scintillator was 91,300 photon MeV −1 , which was superior to most copper(I)-based OIHC scintillators.
[52] The free excitons experienced ultrafast ISC from free excitons state to STE state (Figure 6D), generating a broadband emission. [53]hang et al. designed a 0D ionic copper(I)-based OIHC scintillator 16 with STE emission.The complex exhibited a sky-blue emission at 498 nm with a PLQY of 80.5% and a Stokes-shift of 209 nm. [54]The excellent photophysical properties of the scintillator mainly originated from the unique 0D electronic structures ((CuX 2 ) − quantum rods) featuring with strong quantum confinement effect and electron-phonon coupling (Figure 6E).The LY of the complex was 24,134 photons MeV −1 and the lifetime was 232.05 µs.Yue et al. reported a 0D ionic copper(I)-based OIHC scintillator 17 with a broadband emission. [55]This scintillator with STE emission exhibited a high LY of 27,706 photons MeV −1 due to the large Stokes-shift of 167 nm and negligible self-absorption.Moreover, Lei et al. synthesized a 0D ionic copper(I)-based OIHC scintillator 18 with an STE emission. [56]The scintillator with a large Stokes-shift of 191 nm displayed an outstanding LY of 57,974 photons MeV −1 .Kuang et al. have reported another copper(I)-based OIHC scintillator 19 with STE emission, highlighting a near unity PLQY and a high LY of 55,650 photons MeV −1 . [57]n addition to the above-mentioned emission nature (such as MC, 3 CC and STE) of copper(I)-based OIHC scintillators, the efficient luminescence could also come from the spin-allowed transition of DBE.This emission often shows fast scintillation decay.Wu et al. developed a copper(I)-based OIHC scintillator 20 through a facile solution method. [58]he complex had strong quantum confinement effects due to the 0D isolated electronic structure.Under 320 nm, the copper(I)-based OIHC showed an orange emission at 620 nm with a short decay lifetime of 56 ns (Figure 6F), which was shorter than most copper(I)-based OIHC scintillators.

AIO-based OIHC scintillators
Besides that, there is a class of AIO molecules comprising both ionic and coordinate bonds which can effectively balance the characteristics of neutral and ionic complexes. [59]ohammed et al. prepared a 0D AIO-based OIHC scintillator 21. [60] Under 375 nm irradiation, the scintillator emitted a green light at 527 nm with a high PLQY of 95.3%, whose LY was 32,600 photons MeV −1 .More importantly, the scintillator crystals were uniform nanoparticles with an average size of 69 nm, which could be prepared as water-dispersible inks.Then the authors adopted a 'vacuum-filtration' method to obtain a high-quality scintillation screen.Different from the traditional method, this environment-friendly approach could form a homogeneous film without powder grinding or physical mixing of different phases.Finally, the Cu(I) iodide ink-based scintillator screen exhibited extraordinary spatial resolution of >30 lp mm −1 , demonstrating its feasibility of applying in real-time dynamic X-ray imaging. [61]Based on this, a nude mouse without a ghosting effect was observed at an imaging rate of 6.45 fps in a real-time dynamic X-ray test.Moreover, the microcube scintillators with uniform size distribution are potentially applicable to the flexible X-ray imaging applications.In a word, most of copper(I)-based OIHC scintillators exhibited high LY, relatively short decay time, excellent linear response to X-ray dose rate, and so on, which was one of the candidates for lead-free OIHC scintillators (Table 2).However, their photophysical property stability was easily affected by the preparation conditions and the surrounding environment, such as ingredient proportion, reaction solvent, reaction temperature, etc.Besides that, the PL mechanisms of different copper(I)-based OIHC scintillators was originated from different excited states (MC, 3 CC, STE, and DBE), and it was not conducive to analyze the photophysical property.

LANTHANIDE(III)-BASED OIHCS
Lanthanide elements (Ln) possess abundant orbital energy levels and energy transitions and can achieve a wide range of luminescence from ultraviolet to near-infrared regions. [62,63]

Luminescence mechanism of lanthanide(III)-based OIHCs
The luminescence of lanthanide ions (Ln 3+ ) is normally weak because of the forbidden transition of f-f.[66] Generally, the organic ligands first absorb the energy of the incident source, and then the energy will be transferred to the excited state of Ln 3+ through the energy transfer process, mainly including Förster resonance energy transfer (FRET), Dexter excitation transfer (DET), etc. [67][68][69][70]

Lanthanide(III)-based OIHC scintillators
Lanthanide(III)-based OIHC scintillators usually exist in the form of metal-organic frameworks (MOFs).Wang et al. reported two isotypic lanthanide-based OIHC scintillators Eu(ox)(COO)(phen)(H 2 O) and Tb(ox)(COO)(phen)(H 2 O). [71]By adjusting the molar ratio of Eu 3+ and Tb 3+ , a linear multicolor visualization of X-ray irradiation from red to green was firstly achieved (Figure 7A).In addition to the rich luminescent properties, Eu 3+ and Tb 3+ -based organic frameworks also exhibited excellent stability.With the continuously increasing X-ray dose (0-53 Gy), Eu 3+ and Tb 3+ -based organic frameworks remained 85% and 87% of their initial X-ray excited luminescence (XEL) intensities, respectively.In contrast, the XEL intensity of commercial scintillator CsI:Tl was sharply decreased to 35% (Figure 7B).In order to facilitate efficient energy transfer, improving the energy level matching degree between the sensitizing centers and the resonance emission level plays a critical role.Zheng   [72] The 1,4-H 2 ndc and 2,6-H 2 ndc with a large number of π electrons were conducive to photons collection and energy transfer.The energy difference between Eu 3+ and T 1 was 2641 cm −1 , which was an appropriate value for energy transfer.Hereby, Eu 3+ was selected as the emission center to study the scintillation properties.Both of them  [73] Copyright 2022, Wiley.7C). [73]The strong density and structure rigidity of Ln-Cu 4 I 4 MOFs could reduce the nonradiative relaxation and increase the exciton utilization.The PLQY of Tb-Cu 4 I 4 MOF was 70.5%, while the PLQY of Tb-MOF without Cu 4 I 4 clusters was only 53.2%.The LY of Tb-Cu 4 I 4 MOF was 29,379 ± 3000 photons MeV −1 , which was 3 times higher than that of Tb-HNIC (11,513 ± 1000 photons MeV −1 ).Under irradiation, the Cu 4 I 4 clusters mainly absorbed the X-ray energy to populate triplet excitons via the X/MLCT state, and then the 3 X/MLCT excited state sensitized Tb 3+ for intense XEL (Figure 7D).
By and large, lanthanide(III)-based OIHC scintillators could achieve a wide range of luminescence from ultraviolet to near-infrared regions through adjusting the molar ratio of Ln 3+ , and show high X-ray radiation resistance due to the rigid structure.However, lanthanide(III)-based OIHC scintillators generally needed to be sensitized to luminesce, and the energy transfer between Ln 3+ and organic ligands would lead to energy dissipation.

Luminescence mechanism of antimony(III)-based OIHCs
Antimony ion (Sb 3+ ) has an outer electron configuration of 4d 10 5s 2 and possesses similar outer-shell electrons of ns 2 as Pb 2+ and the lone-pair electrons endow it with high stereo activity. [81,82]Under the excitation of high-energy photons, the excitons at the ground state first transit to the high-energy excited state, and then will be induced by the inorganic parts to instantaneously distort and form STE. At last, the excitons return to the ground state and complete the luminescence.The process tends to produce a broadband emission with a large Stokes-shift, which is beneficial to the light output of scintillators.

Antimony(III)-based OIHC scintillators
So far, there are few investigations on antimony(III)-based OIHC scintillators.Ma et al. prepared an antimony(III)based OIHC single crystal 22 with the configuration of the pyramid (Figure 9A). [83]The complex displayed a large Stokes-shift of 225 nm and a decay lifetime of 4.1 µs.The LY of the scintillator was derived as 49,000 photons MeV −1 (Figure 9B).Unfortunately, the X-ray imaging of the antimony(III)-based OIHC has not been deeply involved and explored.Afterwards, Zhao et al. obtained an antimony(III)based OIHC scintillator 23 and applied it to X-ray imaging (Figure 9C). [84]The decay lifetime of the scintillator was 2.67 µs and the Stokes-shift was 222 nm, which were consistent with the characteristics of STE.In the X-ray imaging test, the spatial resolution of the scintillator screen was 8.2 lp mm −1 .After that, some strategies have been presented to improve the spatial resolution of antimony(III)-based OIHC scintillators.Reducing the scattering of X-ray propagation between scintillator films and imaging items can effectively promote the spatial resolution.Yu et al. developed an antimony(III)-based OIHC scintillator 24 by an ionthermal method process. [85]The scintillator was used as the adhesive paint and attached to the target objects, displaying a high resolution of 12.5 lp mm −1 .Transparent scintillators with high PLQY and small self-absorption can achieve high spatial resolution.Liu et al. reported a transparent antimony(III)based OIHC scintillator 25 wafer through a melt-quenching method. [86]The wafer displayed a high optical transparency of 86%, a large Stokes-shift of 259 nm and a high PLQY of 99.3%.The transparent amorphous wafer showed a high spatial solution of 19 lp mm −1 and some items (shrimp, copper with a pore size of 95 µm and ballpoint pen) could be clearly imaged under X-ray irradiation.

OTHER METAL-BASED OIHCS
In addition to the above typical classifications, some other metal-based OIHCs used as scintillators (Figure 8) are also gradually being reported, such as tin, iridium, germanium, barium, uranium, and so on (Table 3).

Tin(II)-based OIHC scintillators
Tin(II)-based OIHCs are considered as emerging candidate materials to replace lead(II)-based OIHCs due to the unique ns 2 structure.Tin(II) with a lone pair is usually stereochemically active, and the biological toxicity is much lower than lead(II)-based OIHCs.Kovalenko et al. reported a novel 0D tin(II)-based OIHC scintillator 26. [88]At room temperature, both singlet and triplet exciton emissions of the scintillator were observed, which was attributed to the splitting of the energy levels originating from the Jahn-Teller (JT) perturbation of the ground state.The decay lifetimes of singlet emission and triplet emission in the complex were 10 ns ( 1 P 1 → 1 S 0 ) and 4 µs ( 3 P 1 → 1 S 0 ), respectively.Gu et al. fabricated a highly luminescent 2D layered tin(II)-based OIHC scintillator 27. [89]Under 350 nm, the complex obtained the emission peak at 596 nm with a high PLQY of 98% and the Stokesshift was as high as 246 nm.The poor stability of tin(II)-based scintillators inhibits its further development due to the easy oxidation of tin (Sn(II) → Sn(IV)).In order to suppress the oxidation of the sample and reduce the size of particles, probe-ultrasonication under an ice bath was chosen before the preparation of the membrane.The scintillator film with a thickness of 100 µm was achieved through a spin-coating approach and showed a high resolution of 200 µm.

Germanium(II)-based OIHC scintillators
Germanium ions (Ge 2+ ) also have analogous ns 2 ion structure to Pb 2+ and the lone pair gives them high stereo activity.Kovalenko et al. introduced a novel germanium(II)-based OIHC scintillator 28 with a disphenoidal structure. [88]The F I G U R E 9 (A) Structure of the antimony(III)-based OIHC single crystal 22. (B) The RL spectra of 22 and CsI Tl.Reproduced with permission. [83]opyright 2022, American Chemical Society.(C) The diagrams of the antimony(III)-based OIHC 23.Reproduced with permission. [84]Copyright 2023, Wiley.(D) The efficient energy transfer from the donor to the acceptor.(E) Normalized emission spectra of the composite film.(F) X-ray imaging of the composite film.Reproduced with permission. [87]Copyright 2022, Elsevier.

Iridium(III)-based OIHC scintillators
Iridium(III)-based OIHC scintillators exhibit weak response under X-ray irradiation, which need to be sensitized by energy donors.Mohammed et al. synthesized a donoracceptor-based transparent composite film 29 for X-ray imaging. [87]The highly efficient triplet-triplet energy transferred from the donor to the acceptor can realize the near-unity exciton utilization (Figure 9D).From the DFT calculations, both the LUMO and HOMO of the donor were lower than those of the acceptor, which was the prerequisite for the electron exchange energy transfer mechanism (Figure 9E).The LY of the composite film was 12,000 photon MeV −1 .What is more important, the X-ray imaging resolution of the film was 19.8 lp mm −1 , demonstrating that the composition can afford multicomponent scintillation systems with high resolution (Figure 9F).

Uranium(VI)-based OIHC scintillators
It is well known that the X-ray absorption coefficient is closely related to the absorption cross-section of scintillators and is proportional to the fourth power of the effective atomic number (Z eff ).Uranium is the heaviest naturally occurring element in the world and usually exists in the form of an oxidation state (U VI ).The high atomic number and oxidation state of uranium-based complexes were endowed with great potential for X-ray detection.10A). [90]The dense structure of SCU-9 scintillator (C) Comparison of X-ray attenuation lengths for SCU-9 and CsI:Tl in the X-ray energy region ranging from 30 eV to 30 keV.Reproduced with permission. [90]opyright 2018, Wiley.(D) The coordination conditions of compounds.(E) The relative scintillation intensity of different complexes.Reproduced with permission. [91]Copyright 2022, Royal Society of Chemistry.(F) Total atomic structure of Cu 2 Au 2 (R-BTT) 4 .(G) Cross-sectional SEM images of a particledeposited film at different scales.(H) Corresponding MTF curves of particle-deposited films with different thicknesses.Reproduced with permission. [92]opyright 2023, American Chemical Society.
could effectively restrict non-radiative relaxation and make it have better radiation resistance hardness than CsI:Tl under high dose rate radiation (Figure 10B).In the energy region below 20 keV, the X-ray attenuation efficiency of SCU-9 was comparable to commercially available CsI:Tl, and it was suggested that SCU-9 had great stopping power to X-rays (Figure 10C).The radioactive nature of uranium isotopes limits their practical application, but there is still a lot of research value for X-ray detection because of their unique characteristics.

Barium(II)-based OIHC scintillators
Unlike uranium ions and lead ions, the heavy element barium with a large X-ray absorption cross-section is nonradioactive and non-toxic.10D), respectively. [91]The raw reactants (organic ligands, BaCl 2 ⋅2H 2 O and BaSO 4 ) all exhibited weak X-raystimulated luminescence.Once assembled into Ba-SMOF, the scintillator exhibited remarkable X-ray response signals (Figure 10E).The barium ions and naphthalene-sulfonic moieties worked synergistically.The barium ions were excited by X-rays generating many electrons, and then these electrons could transfer energy to the naphthalene-sulfonic moieties emitters, which was the antenna effect.Under different X-ray irradiation, the energy resolution of MOF-based scintillators remained unchanged, indicating that they have latent applications in practical high-energy detection.

Coinage-metal-based OIHC scintillators
The Cu-Au alloy cores with "soft" sulfur-based ligands are also called coinage-metal nanoclusters (CMNs).CMNs with heavy multinuclear metallic cores exhibit large X-ray absorption ability. [93]Zang et al. constructed a scintillator with TADF property based on Cu-Au alloy cores. [94]The small ΔE ST and large significant spin-orbit coupling (SOC) could accelerate the electron transfer between singlet and triplet excited states, which was in favor of reverse intersystem crossing (RISC).From DFT, the ΔE ST was 0.08 eV and the SOC was 147.62 cm −1 , so that the high exciton utilization could be realized through the RISC process with increasing temperature.To take advantage of this effect, an LY of 26,000 photons MeV −1 was obtained, which was comparable to that of traditional inorganic commercial scintillators.Bakr et al. also synthesized a coinage-metal-based scintillator (Cu 2 Au 2 (R-BTT) 4 ) with a LY of 13,600-17,000 photons MeV −1 (Figure 10F). [89]Normally, the spatial resolution of scintillator films was inversely proportional to the thickness of the film.The conventional method of preparing thick films (>10 um) tended to result in low X-ray absorption and RL output.In this work, a unique film preparation method was explored.The scintillation superparticles were deposited in the flexible polyethylene terephthalate to develop a particle-deposited scintillation film (Figure 10G).Surprisingly, the particle-deposited scintillator film with a thickness of 24.5 µm showed a spatial resolution of 16.6 lp mm −1 for X-ray imaging (Figure 10H).Furthermore, the LY of particle-deposited scintillation film could maintain 14,080-17,600 photons MeV −1 , which also indirectly proved the practicability of this film-preparation method.

CONCLUSIONS
In summary, lead-free OIHCs are a novel and promising category of scintillators for X-ray detection and imaging owing to decent photophysical performance.In this review, we introduced the recent advancements in lead-free OIHCs as scintillators for X-ray detection.Specifically, the photophysical properties, luminescence mechanisms and scintillation performance (i.e., LY, LOD, spatial resolution) of various lead-free OIHCs were comprehensively summarized.Undoubtedly, lead-free OIHCs offer exciting opportunities for achieving high-end flexible X-ray imaging.However, their development as scintillators still needs in-depth study and expansion, and there are still some issues to be addressed.For instance, manganese(II)-based OIHC scintillators usually possess long decay lifetime (hundreds of microseconds or even milliseconds) because of the spin-orbit forbidden transitions (d-d), which limits their application in the field of dynamic X-ray imaging.Lanthanide(III)-based OIHC scintillators need to be sensitized to luminesce, leading to energy dissipation.Tin(II)-based OIHC scintillators can be oxidized easily, so encapsulation strategies are desired.The current photophysical properties of germanium(II)-based OIHC scintillators are still relatively poor, and cannot meet the requirements of high-efficiency scintillators.
Virtually, in order to obtain ideal X-ray scintillators with great absorption capacity, high LY, short decay time, low cost, high spatial resolution and strong radiation resistance, and some strategies about scintillators have been proposed.Firstly, the scintillators should contain high-Z metals, which can effectively improve the absorption capacity of X-rays and radiation resistance.Secondly, introducing the heavy halogen atoms (Br and I) to scintillators can reduce decay lifetimes.Thirdly, scintillators should possess small E g and high PLQY, and these features can improve LY.Therefore, more efforts should be made to design and develop new scintillators.Furthermore, the integration and size-controlled scintillation screens are all expected to be realized in the future.
Overall, it is foreseeable that the study of lead-free OIHC scintillators will promote the development of the X-ray detection field.In future work, more efforts should be made to explore novel scintillators with excellent scintillation performance and the integration of flexible scintillation screens.

A C K N O W L E D G E M E N T S
This work was supported by the National Key R&D Program of China (2023YFE0202500) and National Natural Science Foundation of China (62375142 and 62005241).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.

F I G U R E 2
Chemical structures of several representative manganese(II)-based OIHCs.1-7 are ionic types, while 8 and 9 are neutral ones.

F I G U R E 4
Chemical structures of several representative copper(I)-based OIHCs.10-14 are neutral types, 15-20 are ionic types, 21 is AIO type.

F I G U R E 6
Schematic illustration of PL recombination mechanism in copper(I)-based OIHC scintillator 15 at (A) 80 K and (B) RT. (C) Temperaturedependent (80-300 K) PL spectra of scintillator 15 upon 305 nm excitation along with temperature range from 80 to 300 K and temperature interval of 20 K.

F
I G U R E 7 (A) The 1D chain structures and linear CIE chromaticity diagram of lanthanide-based OIHC scintillators.(B) The relative XEL intensity of lanthanide-based OIHC scintillators and commercial CsI:Tl.Reproduced with permission. [71]Copyright 2020, Royal Society of Chemistry.(C) A schematic representation of Tb-Cu 4 I 4 MOFs.(D) Schematic diagram of the PL mechanism of Tb-Cu 4 I 4 MOFs.Reproduced with permission.
presented a good linear response to the X-ray dose rate.The spatial resolution of [Eu 2 (1,4-ndc) 3 (DMF) 4 ] n ⋅nH 2 O and [Eu 4 (2,6-ndc) 6 (µ 2 -H 2 O) 3 (H 2 O) 4 ] n ⋅2nH 2 O were 5.5 lp mm −1 and 3.0 lp mm −1 , respectively.The Cu 4 I 4 clusters with high-Z elements possess large Xray absorption cross-sections.Therefore, introducing Cu 4 I 4 clusters into lanthanide-based MOF scintillators can enhance the X-ray absorption, reduce the nonradiative relaxation and increase the exciton utilization.On this basis, Zhao et al. firstly designed a lanthanide-based MOF (Tb-Cu 4 I 4 MOF) scintillator with Cu 4 I 4 clusters and Tb(III) ions to be absorption centers and luminescent chromophores, respectively (Figure

F I G U R E 1 0
(A) Scheme of two different uranyl chains.(B) Under the same X-ray irradiation, the comparison of XEL intensities of SCU-9 and CsI:Tl.
This paper does not involve human investigation and animal experiments and thus does not need ethics committee approvals.O R C I D Qiang Zhao https://orcid.org/0000-0002-3788-4757R E F E R E N C E S 94.Q. Peng, Y. Si, Z. Wang, S. Dai, Q. Chen, K. Li, S. Zang, ACS Cent.Sci.2023, 9, 1419.How to cite this article: H. Cui, W. Zhu, Y. Deng, T. Jiang, A. Yu, H. Chen, S. Liu, Q. Zhao, Aggregate 2024, 5, e454.https://doi.org/10.1002/agt2.454A U T H O R B I O G R A P H I E S Haixia Cui graduated from Yanshan University in 2022.She is working as a Ph.D. candidate under the supervision of Prof. Qiang Zhao in the State Key Laboratory of Organic Electronics and Information Displays and Institute of Advanced Materials, Nanjing University of Posts and Telecommunications.Her current research interests mainly focus on the design, synthesis, and applications of scintillators.Shujuan Liu received her Ph.D. degree from Fudan University in 2006.She then joined Nanjing University of Posts and Telecommunications.Since 2013, she has been a full professor.Her research focuses on new optoelectronic materials and devices.Qiang Zhao received his Ph.D. degree in 2007 from Fudan University.He then became a postdoctoral fellow at Nagoya University of Japan.He joined Nanjing University of Posts and Telecommunications in 2008.He was promoted as a full professor in 2010.His research area is organic and flexible electronics.
TA B L E 1 Summary of copper(I)-based OIHC scintillators.
Summary of other metal-based OIHC scintillators.