Ultrafast and high-resolution X-ray imaging based on zero-dimensional organic silver halides

Abstract

Supersensitive and fast X-ray imaging is of great importance in medical diagnosis, industrial flaw detection, security and safety inspection, and frontier science exploration. As the core of detection devices, new generation scintillators require small self-absorption capacity, short fluorescence lifetime, simple film-making protocol, excellent stability and non-toxicity. Herein, a new type of lead-free organic silver halide TPPAgX₂ (TPP = C₂₄H₂₀P and X = I, Br, and Cl) is rationally developed with a large Stokes shift (176 nm) and ultralow photoluminescence decay (3.8 ns lifetime). It achieves an ultrafast fluorescent response that is the best among all the Pb-free perovskite scintillators. Temperature-dependent PL and DFT calculations together confirm that TPPAgCl₂ follows an emission mechanism in which a triplet exciton can be rapidly upconverted via thermal activation. A series of TPPAgX₂-based flexible scintillator films were fabricated and tested. A detection limit of 0.447 μGy_air s⁻¹ was obtained for TPPAgCl₂, which is one order of magnitude lower than the medical X-ray diagnosis requirement. In addition, it exhibits a superior X-ray imaging resolution of 11.87 lp mm⁻¹. The excellent performance and simple preparation methodology are expected to be potentially applicable for large-scale manufacturing.

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1. Introduction

X-rays are highly energetic electromagnetic waves characterized by extremely short wavelengths and potent penetrating capabilities. They find extensive applications in medical diagnostics, industrial flaw detection, security and safety inspections, and non-destructive testing.^1–7 In these applications, scintillators play an important role in converting high-energy X-rays into low-energy ultraviolet or visible light.^8,9 Short life, high scintillation yield, low cost, scalable manufacturing, stability and non-toxicity are the main considerations for scintillators. Among these factors, a short life span is crucial for X-ray imaging.^10–12 The response time of a scintillator to incident radiation particles directly determines the time resolution and response speed of X-ray imaging.¹³ For example, dynamic high-speed X-ray imaging requires a frame rate of 2 ns (GHz), with the scintillator's reaction time reaching the nanosecond level to achieve high-resolution images.¹⁴ In medicine-related applications, the impact of X-ray on the human body is mainly determined on the basis of irradiation dose and time. Only a short fluorescent life can reduce the damage caused by X-ray imaging.

Recently, Wang et al. successfully fabricated the zero-dimensional organocopper halide composite scintillator (18-crown-6)₂Na₂(H₂O)₃Cu₄I₆(CNCI) with an ultrahigh optical yield of 109 000 photons per MeV, but its fluorescence lifetime is as long as 1.98 μs.¹⁵ For MAPbI₃ and MAPbBr₃ hybrid lead halides, which exhibit a decay time of few nanoseconds,^16,17 the luminous yield is less than 10 000 photons per MeV at room temperature and is toxic. To reduce the lead content while preserving the perovskite structure, Pb²⁺ ions are often replaced by non-toxic and stable metal ions. In previous studies, Mn²⁺ ions were introduced into the lattice to replace the original Pb²⁺ ions, which reduced the content of toxic composition and improved the stability of the material.¹⁸ Inspired by this, manganese-based metal halides have been thoroughly synthesized and studied, with a focus on their optical properties.¹⁹ However, their long luminescence lifetime remains an unsolved problem.

According to Fermi's golden rule, excitonic radioluminescence can fundamentally provide fast and intense scintillation. For example, Shen et al. reported the successful synthesis of centimeter-scale BDAPbI₄ (BDA = NH₃C₄H₈NH₃) single crystals. Their charge transport under X-ray was examined, and the shortest fluorescence lifetime to date (2.09 ns) was obtained.²⁰ However, the presence of Pb²⁺ in this material greatly limits its practical application.

Therefore, it is particularly important to find scintillators with short decay lifetimes and no toxicity. Because the ability to decay X-rays is mainly determined by the effective atomic number of the material, lead-free silver halide with a higher atomic number and shorter decay time can be utilized as an excellent substitute for a short carrier decay time.²¹ In recent years, several polycrystalline silver halide powders and single crystals have been synthesized using expensive techniques,^22,23 but their phase purity is low. Therefore, the development of high-quality silver halide scintillators with excellent performance is urgent and attractive.

In this paper, a series of nontoxic silver-based halide single crystals, TPPAgX₂ (X = I, Br, Cl), with homogeneous size and morphology are systematically prepared using a simple antisolvent method. As-synthesized TPPAgI₂ shows a Stokes shift of 84 nm and a short decay time of 105.8 ns, which can be further engineered and improved via the C-site regulation. Fluorescence lifetimes of 45.5 ns and 3.8 ns were therefore achieved on the TPPAgBr₂ and TPPAgCl₂, respectively. They also displayed Stokes shifts of 176 nm and 44 nm, respectively, without any self-absorption. Thermodynamically stable TPPAgCl₂ exhibits thermally activated undelayed fluorescence emission, which is mainly derived from the triplet state of STEs under X-rays, ensuring a large Stokes shift and excellent X-ray imaging capability. In addition, homogeneous films with a lateral size of 16 cm² and sub-millimeter in thickness were prepared by the mixture of TPPAgX₂ particles, polydimethylsiloxane (PDMS) and coagulant. High-resolution static X-ray imaging resolution (11.87 lp mm⁻¹) and detection limit (0.412 μGy_air s⁻¹) were consequently achieved on TPPAgCl₂-based film. The newly developed X-ray imaging materials with extremely short fluorescence lifetime, non-toxicity, and large Stokes shift are expected to be promising alternatives for future commercial applications.

2. Experimental section

2.1 Chemical

Tetraphenylphosphonium iodide (C₂₄H₂₀PI), tetraphenylphosphonium bromide (C₂₄H₂₀PBr), tetraphenylphosphonium chloride (C₂₄H₂₀PCl), AgI, AgBr, and AgCl were all purchased from Aladdin Industries. Dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), tri-N-octylphosphine, polydimethylsiloxane (PDMS), and ether were all used as received without further purification.

2.2 Synthesis of TPPAgX₂ (X = I, Br, Cl) single crystals

The primary coordination model for Ag halides is the tetrahedral model. In the tetrahedral model, the halide ions are located around the silver ions, forming an orthotetrahedral structure. For charge balance in the compound system, the material ratio of AgI/Br/Cl to TPPI/Br/Cl should be controlled between 1 : 1 and 1 : 2 because TPPI/Br/Cl, which is protonated, is only positively charged. To satisfy the coordination determination of metal Ag as much as possible, we finalized the ratio of AgI/Br/Cl to TPPI/Br/Cl to be 1 : 1. Specifically, the antisolvent method was used to synthesize TPPAgX₂ single crystals. 1 mmol TPPI (466.29 mg) and 1 mmol AgI (234.77 mg) were added in a clean bottle, with 2 ml of DMF and 0.5 ml of DMSO. The sealed bottle was set on a heating table at 60 °C, stirring until the chemicals were completely dissolved and a transparent liquid was obtained (about 30 minutes). After filtering by applying a syringe, the solution was put into another clean bottle and set inside a closed jar with an ether counter-solvent. Single crystals were obtained while standing for at least 48 h.

2.3 Synthesis of TPPAgX₂ (X = I, Br, Cl) – PDMS films

As-synthesized TPPAgX₂ (X = I, Br, Cl) crystals were ground by agate mortar to reduce their grain size and obtain a uniform size distribution. PDMS was prepared by mixing the prepolymer and curing agent in a volume ratio of 10 : 1. To synthesize TPPAgX₂ (X = I, Br, Cl) – PDMS thin films, approximately 454 mg of TPPAgX₂ (X = I, Br, Cl) single crystal powder was slowly mixed with 4 ml of PDMS (1.058 g ml⁻¹) under ultrasound and then stirred for 5 minutes to obtain a uniformly mixed solution. The mixed solution was centrifuged at a speed of 800/min for 3 minutes. TPPAgX₂ (X = I, Br, Cl) – PDMS thin films were prepared by spin coating and cured at 60–80 °C for 1 hour. The speed of spin coating can control the thickness of the film in the range of 50–500 μm.

2.4 Physical characterization

Powder X-ray diffraction (PXRD) measurements were performed using a PANalytical X’Pert Powder diffractometer. During the measurement, Bragg's diffraction angle (2°) range was set to 10°–80° and the scanning time was 20 minutes. Scanning electron microscopy (SEM) images were taken using Quattro S. Cold FE-SEM, Hitachi High-Technologies with an acceleration voltage of 10 KV, and EDX were used to characterize the elements of the doping samples. Pure TPPAgX₂ (X = I, Br, Cl) was scanned by applying a ZEISS GeminiSEM 300 field emission scanning electron microscope, and all elements were characterized by energy dispersive spectroscopy (EDS). The UV-Vis transmission spectra and absorption spectra were recorded using a UV-3600 spectrophotometer (Shimadzu, Japan) in the wavelength range of 200–800 nm. The decay curves were recorded by applying a time-resolved fluorescence spectrophotometer (FLS1000, Edinburgh Instrument Ltd., Edinburgh, UK). The photoluminescence (PL) spectra were measured using a 450 W Xe lamp for ozone removal as the excitation source and an FLS1000 spectrophotometer (Edinburgh Instrument Ltd., Edinburgh, UK). The thermogravimetric analysis of crystals was performed by TGA/DSC1/1600LF (Mettler Toledo, Switzerland) at a heating rate of 20 °C min⁻¹ for up to 800 °C. The DFT calculation was performed using the Cambridge sequential total energy package (CASTEP) module.²⁴ The generalized gradient approximation with the Burke-Ernzerhof potential was used for periodic solids. The cutoff energy was set as 400 eV, and the Brillouin zone used a 3 × 3 × 3 Monkhorst–Pack grid. Spin-polarized calculations within the DFT+U framework have been applied for Ag ions to correct the well-known DFT self-interaction errors.^25,26 For the structural optimization, the maximum force limit, maximum energy variation tolerance and maximum displacement were set as 0.03 eV Å⁻¹, 10⁻⁵ eV per atom and 0.002 Å, respectively. The self-consistent field (SCF) tolerance was set as 2 × 10⁻⁶ eV per atom. Excitation energy diagrams were performed using the B3LYP hybrid function implemented in the Gaussian-16 suite of program.²⁷ The time-dependent density functional theory (TDDFT) calculations were performed with PBE0-D3(BJ)/def2-SVP level of theory on the geometries obtained from XRD experiments.²⁸ An effective core potential associated with the def2-SVP basis set was used on the Ag atoms. This functional was shown to perform well for excited states²⁹ and spin–orbit coupling (SOC) calculations.³⁰ The above calculations were carried out using Gaussian 16 programs. The SOC matrix elements were calculated by applying ORCA 5.0.3 software based on the Douglas–Kroll–Hess 2nd order (DKH2) scalar relativistic method. The PBE0 functional and SARC/J auxiliary basis were used. Relativistically recontracted basis set DKH-def2-SVP was used for C, H and P atoms.³¹ For metal atoms, segmented all-electron relativistically contracted (SARC) DKH-TZVP was used for Ag.³² The radioluminescence (RL) spectrum was characterized using a Maya2000 Pro spectrometer. X-ray radiation for detection and imaging was produced by applying an X-ray source (HAMAMATSU, L10321). The type of charge-coupled device (CCD) chip used for detection and imaging was KAF-16803.

3. Results and discussion

The TPPAgX₂ (X = I, Br, Cl) single crystals (Fig. 1a) were synthesized by applying an anti-solvent method.^33–35 All the structures were analyzed and confirmed by powder X-ray diffraction (PXRD) measurements (Fig. 1b) and single crystal X-ray diffraction (Fig. 1c–e). It is clear that the results are in perfect agreement, confirming the successful synthesis of TPPAgX₂ single crystals. Detailed information is summarized in Tables S2–S4 (ESI†).

Fig. 1 (a) Schematic diagram of using the anti-solvent method for preparing TPPAgX₂ (X = I, Br, and Cl) crystals. (b) A comparison between the simulation results of TPPAgX₂ (X = I, Br, and Cl) single-crystal analysis and the XRD results tested. (c)–(e) Crystal structure of TPPAgX₂ (X = I, Br, and Cl) created with Mercury software based on crystallographic information obtained via X-ray single-crystal structural analyses. (f)–(h) Outline of TPPAgX₂ (X = I, Br, and Cl) crystal obtained through scanning electron microscopy and the growth of the TPPAgX₂ (X = I, Br, and Cl) crystal.

Scanning electron microscope (SEM) graphs show that TPPAgI₂ is a blocky-like solid, while TPPAgBr₂/Cl₂ is in the form of a small rod-like crystal (Fig. 1f–h and Fig. S1, ESI†). The dimensions of TPPAgX₂ (X = I, Br, Cl) are confirmed as 1.25 mm, 0.75 mm, and 0.05 mm, respectively. Energy dispersive spectroscopy (EDS) mapping (Fig. S1 and Table S1, ESI†) shows that all the elements C, P, Ag, and I/Br/Cl are uniformly distributed. The detected elemental contents of C, P, Ag, and I in TPPAgX₂ approached the stoichiometric ratios well. X-ray photoelectron spectroscopy (XPS) was used to further determine the composition and valence. By analyzing the integral area of the high-resolution spectra of C 1s, P 2p, Ag 3d, I 3d, Br 3d, and Cl 2p, the compositions were verified as TPP : Ag : X = 1 : 1 : 2 (Fig. S2a–S4a, ESI†). Two distinct peaks at 368 and 374 eV were found on the Ag 3d spectra, proving the presence of Ag⁺ (Fig. S2bc–S4bc, ESI†).

The thermal stability of TPPAgX₂ crystals was investigated by TG-DSC measurements (Fig. S5, ESI†). These materials show fairly excellent stability from room temperature to 200 °C. The first heat absorption occurs at 200 °C without significant loss of weight until the temperature rises up to 450 °C. The sharp weight loss occurs at 450–550 °C, concurrently accompanied by significant heat absorption. Higher than this temperature, the crystal is considered to be decomposed.

According to previous reports, various kinds of organic molecules can emit light when excited by ultraviolet light.³⁶ Light emission largely originates from the electron transitions, after which an inherent loss channel is formed during the luminescence.^10,37 Specifically, excited high-energy electrons and holes at the Lowest Unoccupied Molecular Orbital (LUMO) orbitals are expected to be in a ratio of 1 : 3 for singlet and triplet states.^38–41 Triplet excitons with 75% proportion follow the rule of phosphorescence at room temperature. To verify whether our materials exhibit phosphorescence behavior, PL, PLE and fluorescence lifetimes were tested for TPPI, TPPBr, TPPCl feedstock and TPPAgX₂ crystals, respectively (Fig. 2a–d and Fig. S6, ESI†). The PL and PLE test results of TPPAgBr₂ and TPPAgCl₂ are similar.⁴²

Fig. 2 (a)–(c) Normalized excitation spectrum and emission spectrum of TPPAgX₂ (X = I, Br, and Cl). (d) Time-resolved PL curve of the TPPAgX₂ (X = I, Br, and Cl). The fluorescence lifetime of TPPAgI₂ is obtained upon an excitation of 378 nm and emission of 462 nm. The fluorescence lifetime of TPPAgBr₂ was obtained upon an excitation of 316 nm and emission of 466 nm. The fluorescence lifetime of TPPAgCl₂ was obtained upon an excitation of 312 nm and emission of 467 nm. (e) Absorption spectrum of TPPAgX₂ (X = I, Br, and Cl). The indirect bandgap calculated by applying the Tauc method is shown in (f). (g)–(i) Band structures for TPPAgX₂ (X = I, Br, and Cl).

From Fig. 2a and Fig. S6a (ESI†), the PL and PLE waveforms of pure organic TPPI/Br/Cl and TPPAgX₂ are very similar with certain differences. Compared to the original organic emission, the hybrid Ag-halogen units with TPP molecules cause enhanced luminescence intensity of TPPAgBr/Cl without any shift, indicating that the luminescence behavior changes from pure molecules to the cooperation of AgX₂ and molecules. As the size of the halogen atom increases, the luminescence center of TPPAgI₂ changes from 429 nm to 462 nm, suggesting that the luminescence manner depends on halogen element regulation.

The fluorescence lifetimes of TPPI/Br/Cl and TPPAgX₂ were compared in Fig. 2d (Fig. S6b, d, and f, ESI†). The fluorescence lifetime of TPPI reaches a microsecond level (0.62 μs) due to the emission inhibition of triplet excitons, which is far from being used for quick detection. The addition of Ag provides a new avenue for TPPI/Br/Cl to convert the triplet excitons efficiently. The triplet excitations can up-convert to singlet states through thermally activated reverse intersystem crossing (RISC) and ultimately take advantage of the fluorescence decay channel to rapidly emit photons.⁴³ Fig. S7 (ESI†) shows the PLQY that the light yield of TPPI/Br/Cl is slightly higher than that of TPPAgX₂. The addition of Ag improves the luminescence of organic molecules and greatly shortens the fluorescence lifetime.

From their PL and PLE, the Stokes shift can be calculated as 84 nm, 176 nm, and 44 nm for TPPAgI₂/Br₂/Cl₂ cases. Minimized self-absorption is generally associated with a large Stokes shift. TPPAgBr₂ thus possesses the largest Stokes shift with the corresponding smallest self-absorption. Because zero self-absorption is highly desirable for scintillators, especially for thick films, our materials exhibit considerable potential for scintillator device fabrication.

According to PL decay and the single exponential function line (Fig. 2d), the good fitting of the curves gives an accurate calculation of fluorescence time. The formulas for double exponential fitting and average life calculation are as follows:

where I(t) and I₀ represent the emission intensity at t and 0, respectively; A₁ and A₂ denote constants; and τ₁ and τ₂ are the decay times of the exponential component.

The fluorescence lifetime can be calculated as 105.8 ns for TPPAgI₂, 45.5 ns for TPPAgBr₂ and 3.8 ns for TPPAgCl₂. The prominent value of 3.8 ns outperforms those of well-known materials, such as Gd₂O₂S:Tb (600 000 ns),⁴⁴ BGO (300 ns),⁴⁵ CsI:Tl (1000 ns),⁴⁶ and CsPbBr₃QDs (45 ns).⁴⁷ To further compare the lifespan of our results with that of the literature, a series of data are presented in Table 1.

Table 1 Comparison of fluorescence lifetime of silver-based halide scintillators with different materials

Materials	PL decay	Ref.
TPPAgI 2	105.8 ns	This Work
TPPAgBr 2	45.5 ns	This Work
TPPAgCl 2	3.8 ns	This Work
Cs2Ag0.6Na0.4In0.85Bi0.15Cl6	16.0 μs	48
(DBA)4Cu4I4	20.3 μs	49
Cs2AgI3:Cu (5%)	192.8 ns	50
Cs2AgBiBr6	1.5 μs	51
Rb2AgBr3	5.3 ns	52
(ETP)2MnBr4	295.0 μs	53
Cs2AgBiCl6	4.4 ns	54
Cs2AgInCl6:Mn (0.5%)	0.6 ms	55
Cs2AgSbCl6:4%OA	7.9 ns	56
SC-Ag (Ag6S6L6)	15.5 μs	57
(DIET)3Cu3Br3	0.6 μs	58
Cu2AgBiI6	20.0 ns	59

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The UV-Vis absorption spectra were tested in the range of 250–800 nm (Fig. 2e), and TPPAgX₂ started to absorb light in the region approaching the ultraviolet light, with bands at 410 nm, 320 nm, and 330 nm. According to Formula S1 in the ESI,†n = 1/2 is used to estimate the direct band gap of TPPAgX₂. Fig. 2f shows that the band gap estimations of TPPAgX₂ from the Tauc plots are 2.85 eV, 3.36 eV, and 3.61 eV, respectively. To enhance our understanding of the photoluminescent mechanism of TPPAgX₂, the density of states (DOS), and band structures were calculated using periodic density functional theory (DFT).^60,61 DFT calculations indicate (Fig. 2g–i) that the band gaps of TPPAgX₂ (X = I, Br, Cl) are 2.457 eV, 3.017 eV, and 3.248 eV, respectively. Because the DFT generally underestimates the band gap of semiconductors, the calculated trends are basically consistent with the experimental results. The projected density of states (PDOS) of TPPAgX₂ was also calculated, as shown in Fig. S8 (ESI†). The valence band maximum (VBM) is mainly contributed from I/Br/Cl p orbitals, while the conduction band minimum (CBM) is mainly associated with the orbitals of carbon. This suggests that the excitation is from AgX₂ unit to TPP molecules, following a manner of inorganic–organic hybrid radioluminescence, which is in agreement with the PL results.

To better understand the relationship between excitation and emission of TPPAgX₂ crystals, temperature-dependent PL spectra were measured in the range of 80–290(380) K (with an interval of 30 K). As shown in Fig. 3a, the most prominent broad emission band does not split into multiple peaks along the temperature drop, implying that it follows a single radiation mechanism rather than a multiple radiation mechanism. The major PL emission shows a monotonically decreasing behavior associated with the temperature-dependent thermal expansion interaction and electron–phonon interaction (Fig. 3d). The intensity of PL emission gradually increases from 80 K to 170 K and then slowly decreases until the temperature reaches 320 K. Higher PL emission intensity at lower temperatures is opposite to the normal thermal burst PL behavior.⁶² Similar phenomena can also be found in the literature of (C₉NH₂₀)₂SnBr₄⁶³ and (H₂O)(C₆H₈N₃)₂Pb₂Br₁₀.⁶⁴ To investigate the luminescence mechanism, TPPAgI₂ was excited under the 305–445 nm irradiation (Fig. S9a, ESI†). Luminescence spectra centered at 462 nm come from the same excited state relaxation. Only one emission peak corresponded to the luminescence mechanism proposed in Fig. S10a (ESI†).

Fig. 3 (a)–(c) Contour map of TPPAgX₂ (X = I, Br, and Cl) PL spectra at low temperature (TPPAgI₂:80 = 320 K, TPPAgBr₂:80 = 380 K, TPPAgCl₂:80 = 380 K). (d)–(f) Emission intensity diagram and FWHM diagram at different temperatures (TPPAgI₂:80 = 320 K, TPPAgBr₂:80 = 380 K, TPPAgCl₂:80 = 380 K).

The temperature-dependent PL decay of 462 nm emission between 290 K and 80 K was monitored to study the radiation kinetics. Throughout the entire temperature range, the PL decay curve exhibits the same single exponential decay kinetics with almost no changes (Fig. S12a, ESI†).

Under UV excitation, photoinduced carriers relax simultaneously to self-trapped exciton (STE) via intersystem crossing (ISC) and are located in shallow trap states. At higher temperatures, thermal activation induces the carriers in the shallow trap state to return to the self-trapped state, which increases the carrier concentration and improves the emission efficiency. However, at lower temperatures, as the thermal energy is not sufficient to trap the carriers in the self-captured state, the carrier concentration of STE decreases. Consequently, we observe a decrease in the emission intensity. The capture process between shallow capture and excited states only affects the carrier concentration on the STE states rather than changing (Fig. S6, ESI†).⁶⁵

E _a gained by the Arrhenius formula (Formula S2, ESI†) is 462 MeV (Fig. S11a, ESI†), which is much higher than the room-temperature thermal ionization energy (∼26 MeV),⁶⁶ (DADPA)In_0.83Sb_0.17Cl₆H₂O (44.42 MeV),⁶⁷ indicating that the structural phase transition of TPPAgI₂ crystal MCs is difficult to occur. To evaluate the deformability of the 0D structure and electron–phonon coupling strength, we calculate the Huang–Rhys factor (S) based on Formula S3 (ESI†). The calculated ħ_ωphonon is 49.39 MeV. The extracted S factor of 11.34 is larger than those of traditional semiconducting materials, such as ZnSe (0.3)⁶⁸ and CdSe (1.0).⁶⁹

From Fig. 3b and c, the main emission signal of TPPAgBr₂ and TPPAgCl₂ did not split into multiple peaks, but the band width was sharply shrunk compared to TPPAgI₂. When the temperature decreases from 380 K to 230 K (Fig. 3e and f), the PL emission intensity gradually increases. Excitation light with 270–370 nm for TPPAgBr₂, and 265–375 nm for TPPAgCl₂ were used to obtain the emission spectra of (290–700) nm and (285–700) nm, respectively. Fig. S9b and c (ESI†) shows the emission spectra centralized in a single peak and maintained the same shape. This indicates that the emission at 492 nm (TPPAgBr₂) and 356/495 nm (TPPAgCl₂) comes from excited state relaxation. The emission peaks around 466 nm and 492 nm (Fig. 2b) were observed for TPPAgBr₂ and 356/467/495 nm (Fig. 2c) for TPPAgCl₂ at 80–380 K, which correspondingly yielded the luminescence pathway (Fig. S10b and c). The temperature-dependent PL decay at 470 nm for TPPAgBr₂ and TPPAgCl₂ was further monitored between 290 K and 80 K to observe the radiation kinetics (Fig. S12b and c, ESI†). The curves exhibit the same single-exponential decay kinetics throughout the entire temperature range. Using formula (4–5), the activation energies of TPPAgBr₂ and TPPAgCl₂ can be determined as 244 MeV and 401 MeV, respectively (Fig. S11b and c, ESI†). S factors for TPPAgBr₂ and TPPAgCl₂ are 10.38 and 8.9, respectively. The phonons obtained through the phonon broadening model for TPPAgBr₂ and TPPAgCl₂ are 85.73 MeV and 64.65 MeV, respectively (Fig. 3e and f, ESI†). All these data are summarized in Table 2.

Table 2 Parameters obtained from formulas (4) and (5) of TPPAgX₂

Materials	E a (MeV)	ℏωphonon (MeV)	S
TPPAgI2	462	61.86	11.34
TPPAgBr2	244	61.86	10.38
TPPAgCl2	401	61.86	8.90

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In addition, abnormal temperature-dependent PL luminescence was found on TPPAgCl₂. The luminous intensity shows a positive relation with the temperature, while a slight blue-shift occurs along the temperature decreases (Fig. 4a). When the temperature is higher than 230 K, the PL intensity quickly decreases due to the influence of hot quenching. To understand this emission behavior in-depth, the relationship between lifetime and temperature was drawn (Fig. 4b). The lifetime of TPPAgCl₂ exhibits a reverse relation with temperature variation. This is consistent with the delayed fluorescence mechanism proposed by Yersin.^70,71 This phenomenon is mainly due to the reverse intersystem crossing (RISC) caused by the thermal activation transition from the long-lived lowest excited triplet (T₁) to the short-lived lowest singlet (S₁), as demonstrated in Fig. 4c. Because TPPAgCl₂ freezes primarily in a triplet at low temperatures, for example, 40 K, the emission state can be assigned as a pure triplet. As the temperature increases, the singlet is filled with a continuous RISC process from the triplet, which makes the emission of the high-energy singlet possible, resulting in an overall reduction in the PL decay time. In addition, because the energy of the S₁ state is higher than the T₁ state, the emission spectrum shows a blue shift at 515–491 nm. The above results show that the emission of TPP comes from thermal activation delayed fluorescence (TADF), and the emission model can well fit the functional relationship between PL lifetime and temperature:^72,73

where K_S1 and K_T1 are the reciprocal of the fitting decay time of S₁ and T₁ states, respectively, and ΔE(S₁ − T₁) is their energy gap. As shown in Fig. 4b, the fitted decay lives of the S₁ and T₁ states are 0.11 ns and 4.4 ns, respectively. The fitted ΔE(S₁ − T₁) value of 0.0563 eV (22 nm) is consistent with the spectral shift (25 nm) in the temperature-dependent PL spectrum. This small energy splitting supports the generation of TADF by an efficient RISC process at ambient temperatures. As a TADF material, the smaller ΔE(S₁ − T₁) can effectively promote the emission collection of radiation-induced triplet excitons, which is fairly favorable for scintillator applications.⁴³

Fig. 4 (a) Temperature-dependent normalized PL spectra of TPPAgCl₂ measured from 80 to 380 K. (b) Temperature-dependent PL lifetime and the fitting curve of TPPAgCl₂. (c) Schematic diagram of the luminescence mechanism of TPPAgCl₂ (left) and energy level diagram of TPPAgCl₂ singlet and triplet excited molecular crystals (right). (d) Detailed energy level diagram of TPPAgCl₂.

To verify the heat exciton trapping mechanism, the geometric and electronic structures of TPPAgCl₂ were calculated (Fig. 4d). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) cover almost the entire molecular skeleton. The calculated HOMO frontier orbitals of TPPAgCl₂ are mainly distributed over the AgCl₂ core, while the LUMO orbitals mainly originated from the TPP component, which is perfectly consistent with the DOS calculation (Fig. 4c and Fig. S8c, ESI†). The energy of the singlet and triplet states is calculated by applying the TD-DFT method based on the tam-dancoff approximation. The singlet states are listed as S₁ (2.799 eV), S₂ (2.821 eV), S₃ (2.978 eV), S₄ (3.002 eV) and S₅ (3.098 eV), respectively. The triplet levels include T₁ (2.076 eV), T₂ (2.253 eV), T₃ (2.360 eV), T₄ (2.457 eV), T₅ (2.516 eV), T₆ (2.625 eV), T₇ (2.808 eV) and T₈ (2.991 eV).

The energy gap between T₈ and T₁ is as large as 0.915 eV, while it is much smaller between S5 and (T₈–T₅) because the lowest one is merely 0.107 eV. Therefore, TPPAgCl₂ has a large T_n–T₁ and a small T_n–S_m energy gap, which is necessary to inhibit the internal transformation of T_n to T₁ and accelerate the RISC of T_n to S_m.^74–76 It is worth noting that the experimental ΔE_ST fitting value (0.056 eV) (Fig. 4b) of TPPAgCl₂ is significantly different compared with the theoretical calculated value (0.722 eV) (Fig. 4d), which may be related to the large spectral blue shift caused by the contraction of the distance between molecules with increasing temperature.⁵⁷ The fitting curve (Fig. 4b) of TPPAgCl₂ research to a platform below 170 K temperature indicates that excitons are mainly frozen in the T₁ state. As temperature increases, the upper S₁ populations participate in emission through the RISC pathway with a shorter life span. The nature of the excited state of luminescent materials under high-energy X-ray radiation is fundamentally different from those under low-energy photoexcitation, as shown in Fig. S13 (ESI†).³⁶ The LUMO and LUMO of TPPCl were calculated for comparison. Fig. S14 (ESI†) shows that the difference between the HOMO and LUMO of TPPAgCl₂ is smaller than that of TPPCl. It is generally known that the exchange interaction between HOMO and LUMO is directly proportional to the ΔE_ST.⁴³ Smaller ΔE_ST means much easier electron excitation from HOMO to LUMO, indicating that the introduction of Ag guides all the thermal excitons towards the fast singlet state.

The ability to convert X-rays into visible light is key to scintillator materials in X-ray imaging.^77–79 High-quality images with high repetition rates and no hysteresis are essentially important.⁸⁰ Using the method described in the experimental section, a flexible film of TPPAgX₂ and PDMS was prepared, with a thickness of 0.55 mm and an area of 16 cm². The flexibility and fluorescence were tested under UV irradiation (Fig. S15 and S16, ESI†). Three TPPAgX₂ films further examined the imaging capability under 70 KV/500 μA X-ray irradiation. The radioluminescence spectra exhibit almost the same profile as the PL spectra, indicating the same radiative complexation manner (Fig. 5b). To evaluate the light yield, commercial scintillator BGO with a light yield of 10 000 photons per MeV was used as a standard reference. According to the relative method, the light yield can be determined by the following equation:⁸¹

where n_A(λ) and n_BGO(λ) are the numbers of photons with a wavelength of λ emitted by the TPPAgX₂ and BGO, respectively. η(λ) is the detection efficiency of the PMT detector, and 10 000 photons per MeV is the light yield of BGO (A represents TPPAgX₂). The light yields are estimated to be 25 600 photons per MeV for TPPAgI₂, 17 420 photons per MeV for TPPAgBr₂, and 21 700 photons per MeV for TPPAgCl₂.

Fig. 5 (a) Schematic diagram of the scintillator image testing system. (b) Radioluminescence spectra of TPPAgX₂ (X = I, Br, Cl) under the irradiation at 70 KV, 500 μA with an average X-ray photon energy of about 21 keV. (c) Detection limit (DL) of TPPAgX₂ (X = I, Br, Cl). (d) Modulation transfer function (MTF) curve of fabricated scintillation screen. (e) Comparison of spatial resolution of TPPAgCl₂ scintillation screen and other halide scintillators. (f) Digital photographs of the target objects (USB data cable head) under natural light (upper row) and X-ray irradiation (lower row). Among them, the left is the X-ray image of TPPAgI₂, the middle is the X-ray image of TPPAgBr₂, and the right is the X-ray image of TPPAgCl₂.

As a proof of concept, a dedicated system consisting of an X-ray tube, imaging object, reflector, and a commercial digital camera was set up for the X-ray imaging study (Fig. 5a). The USB header and 74LS161 counter chip were selected as objects to examine the imaging ability of the TPPAgX₂ scintillator. Under X-ray irradiation, four wires inside the USB header and the wiring configuration inside the chip could be clearly identified (Fig. 5f). Under natural light, it is impossible to distinguish an internal structure. Due to the different X-ray transmittances of metal and plastic, X-rays can finely reveal internal structures. When the signal-to-noise ratio is 3, the detection limit can be derived from the slope of the fitted line (Fig. 5c). We calculated the detection limits of 0.447 μGy_air s⁻¹ for TPPAgI₂, 0.491 μGy_air s⁻¹ for TPPAgBr₂, and 0.412 μGy_air s⁻¹ for TPPAgCl₂, which are much lower than the dose rate required for standard medical X-ray diagnosis (5.500 μGy_air s⁻¹).⁸² To evaluate the spatial resolution of X-ray imaging of the best-performed TPPAgCl₂, the resolution was evaluated using a standard card (Fig. S17, ESI†). It ended up with a resolution of 11.87 lp mm⁻¹ (Fig. 5d). It is superior to other metal halide scintillators such as CsCu₂I₃ (7.5 lp mm⁻¹), BA₂PbBr₄ : 10%Mn (10.7 lp mm⁻¹), Cs₂Ag_0.6Na_0.4In_0.85Bi_0.15Cl₆ (4.3 lp mm⁻¹), and (C₈H₂₀N)₂MnBr₄ (4.9 lp mm⁻¹) (Fig. 5e).^{48,81,83–85} In summary, these solid experiment results support the hypothesis and potential of TPPAgX₂ (X = I, Br, Cl) for X-ray imaging application.

4. Conclusion

In summary, non-toxicity TPPAgX₂ (X = I, Br, Cl) scintillators with extremely low fluorescence lifetime, zero self-absorption, excellent optical yield and thermal stability are rationally designed and fabricated. The fluorescence lifetime of TPPAgCl₂ is only 3.8 ns, which is the best among all the Pb-free perovskite scintillators. Together with temperature-related PL and DFT calculations, it was confirmed that TPPAgCl₂ follows the emission mechanism of ultrafast up-conversion of triplet exciton upon thermal activation. Flexible TPPAgX₂-PDMS films were carefully prepared and tested, and the best TPPAgCl₂ exhibited considerably high resolution (11.87 lp mm⁻¹), low detection limit (0.412 μGy_air s⁻¹) and good radiation stability to the X-ray imaging. Using the low-cost and rapid synthesis method, these flexible films can be extended to the large-scale mass production of scintillators. This work is expected to provide alternative materials for next-generation X-ray detection and imaging.

Data availability

Research data are not shared.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (62375032), Natural Science Foundation of Chongqing (No. CSTB2023TIAD-KPX0017, CSTB2022NSCQ-MSX0360), China Postdoctoral Science Foundation (Grant No. BX20230355). Sponsored by Natural Science Foundation of Chongqing, China, cstc2019jcyj-msxmX0737; National Natural Science Foundation of China under Grant 52302059.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2321717–2321719. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qm00362d

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