Liquid-based cationic ligand engineering in one-dimensional bismuth bromide perovskites: A-site influence on scintillation properties

Abstract

Low-dimensional bismuth-based hybrid organic–inorganic halide perovskites (Bi-HOIPs) are intriguing scintillating materials due to their anisotropic nature. However, a systematic investigation on Bi-HOIP frameworks for X-ray detection imaging remains lacking. In this work, we present the diverse X-ray detection responses in a series of Bi-HOIPs that are incorporated into a polydimethylsiloxane (PDMS) matrix utilizing ionic liquids (ILs) as organic ligands, namely, 1-butyl-1-methyl-pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole (API), and 1-butyl-3-methyl-imidazolium (BMI). The as-synthesized Bi-HOIPs in this series exhibited a modest light yield of ∼1000 photons per keV at room temperature. Interestingly, the incorporation of tin (Sn), instead of bismuth, resulted in radioluminescence intensity enhancement at low temperatures. An abrupt thermal quenching occurred in the case of APISnBiBr₅ and APISn₂Br₁₀, leading to an absence of light yield at RT. Interestingly, thermoluminescence (TL) measurements showed that the glow curve was absent in BMIBiBr₄ and APIBiBr₅, demonstrating the weak contribution of a defect within the crystal lattices. On the contrary, BMPBiBr₄, APISnBiBr₅ and APISn₂Br₁₀ displayed glow curves at temperatures above 100 K. We believe that controlling the number of defects promoted tunable modest distribution traps, manifesting their light yield profile. In terms of the decay profile, BMIBiBr₄ exhibited a fast decay component of 4 ns, while APIBiBr₅ and BMPBiBr₄ yielded a fast decay of 6 ns. A distinct constituent within the ILs could serve as a tuning factor, thereby generating different optical responses and scintillation features. We postulate that this can be attributed to the band gap and polaron signature in BMIBiBr₄, which manifested a faster quenching rate than BMPBiBr₄ and APIBiBr₅. This work sheds new insights on how the population of direct manipulation traps in IL-based bismuth halide perovskites can be driven by regulating the number of halogens at the X-sites of the targeted compounds.

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1. Introduction

Scintillating materials have been extensively designed and used for radiation detection applications in various fields, including X-ray protection, medical imaging, harmful sensor detection, and nuclear cameras, as they possess the ability to transform ionizing radiation into visible or ultraviolet photons.^1,2 In general, scintillators are constructed entirely from high-density heavy elements. Commercial scintillating materials with high light yields, including CsI:Tl (∼66 000 photons per MeV),³ LaBr₃:Ce (∼61 000 photons per MeV),⁴ and Gd₂O₂S:Tb (∼60 000 photons per MeV),^4,5 have recently been developed. Nonetheless, assembling scintillators in a high-temperature and vacuum environment is considered a time-consuming and expensive method. Thus, it is not a desirable approach for the environment.^6,7 Consequently, an ideal scintillator should be directed towards a solution-processable, easy to use, and inexpensive route.

Hybrid organic–inorganic halide perovskites (HOIPs) have been at the center stage as an alternative scintillating material owing to their good luminescent properties, high absorption coefficients for ionizing radiation, and solution-processability.^8–11 Birowosuto et al. reported various lead-based halide perovskites as scintillating materials, including methylammonium lead iodide (MAPbI₃), methylammonium lead bromide (MAPbBr₃), and 2,2′-(ethylenedioxy)bis(ethylammonium) lead chloride ((EBDE)PbCl₄).¹² At low temperatures, all of these perovskite single crystals displayed strong X-ray stimulated luminescence yields of more than 120 000 photons per MeV. Additionally, at room temperature, (EBDE)PbCl₄ generated a modest light yield of 9 000 photons per meV. The influence of organic chain length on lead-based HOIPs was also investigated by Maulida et al.¹³ Benzylammonium as the organic cation-based HOIPs exhibited the highest light yield value at RT. This was reflected in the alteration of the trap states, as the longer organic chain ligands in lead-based HOIPs were inclined to hinder the exciton recombination and yield a longer decay time. Sheikh et al. implemented a dual organic cation (phenethylammonium and benzylammonium) in a lead bromide perovskite structure, which resulted in >80% rapid component scintillation decay time and light yields of over 149 000 photons per eV at RT.¹⁴ The encapsulation of MAPbBr₃ nanocrystals (NCs) in a metal–organic framework has been designed to generate high exciton binding energy, bright and efficient luminescence, and improve stability.¹⁵ Consequently, MOF⊃MAPbBr₃ NC scintillator exhibits a linear response to X-ray with a detection limit determined to be 170 nGyair s⁻¹. It can be observed that lead-based HOIPs display good scintillation properties. However, owing to non-toxicity challenges and stability requirements, there are challenges that remain to be addressed in subsequent years.

Because of their structural similarities, bismuth-based HOIPs have been proposed as replacements for lead-based HOIPs. Bismuth has a comparable mass to lead so that its compounds have the same radiation stopping power as lead compounds although it is much better for the environment.^16,17 Moreover, bismuth-based HOIPs create various configurations of [Bi-halide] connection,^18,19 which has potential to be as a game changer driven by their dimensional control, hence influencing the scintillation characteristics. However, many studies on bismuth perovskite compounds have focused on direct X-ray detectors. Zhuang et al. previously synthesized a 2D (NH₄)₂Bi₂I₉ layered perovskite that performed well for X-ray detection, with a limit detection of 55 nGyair s⁻¹.²⁰ Tao et al. employed top-seed solution growth to produce 1D (H₂MDAP)BiI₅, which can convert X-rays to electrical impulses with a sensitivity of roughly one μCGyair⁻¹ cm⁻².²¹ Yao et al. applied delayed solvent evaporation to synthesis 1D (DMEDA)BiI₅.²² The perovskite has an X-ray detection sensitivity of 72.5 μCGyair⁻¹ cm⁻² at 300 V. Furthermore, the employment of ionic liquid as cationic or anionic sites established a potential strategy for changing the electronic characteristics of bismuth-based HOIPs.^23,24 For scintillators, Jin et al. reported the hydrothermal synthesis of [Emim]BiCl₄(bp₂do), which displayed X-ray excited luminescence spectrum spanning from 425 to 750 nm, peaking at about 485 nm.²⁵ The use of ILs improves the compactness of supramolecular packing, resulting in longer lifetimes up to the second level time and larger quantum yields of 25.05%. It should be mentioned that the modification of organic cations impacts the resultant characteristics of HOIPs, which affect the scintillation outcomes. Thus, our motivation lies in the absence of a structure–property relationship in recent years to modulate the A-site in one-dimensional lead-free hybrid perovskites by rational design.²⁶

Previously, we investigated the structural, vibrational, and electronic properties of bismuth-based HOIPs with various ILs, including 1-butyl-1-methyl-pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole (API), and 1-butyl-3-methyl-imidazolium (BMI), resulting in BMPBiBr₄, APIBiBr₅, and BMIBiBr₄ perovskite.²⁶ However, the scintillation characteristics of such perovskites have not yet been studied. In this study, we focused on the following five compounds: BMPBiBr₄, BMIBiBr₄, APIBiBr₅, APISnBiBr₅, and APISn₂Br₁₀ perovskites. The monoclinic structure of APIBiBr₅ facilitated the observation of a fast decay of 6 ns (15%), with the domination of slow decay (709 ns, 85%). We find that the slow component is dominant in the case of APIBiBr₅ with a monoclinic structure, while BMIBiBr₄ and BMPBiBr₄ share a different number of nitrogen atoms within their skeleton backbone, yielding slow scintillation decay behaviour. The slow component of BMPBiBr₄ is slower than that of BMIBiBr₄ even though both perovskite crystal systems share a triclinic structure. This finding indicated that BMPBiBr₄ had a high number of self-trapped electrons and defects, as evidenced by the presence of a glow curve above 100 K in the thermoluminescence (TL) measurement. Furthermore, the deconvolution of the BMPBiBr₄ curvature indicated that three peaks are observed, highlighting shallow and depth trap variation. In particular, a sharp peak in APISn₂Br₁₀ suggests the occurrence of first order kinetics, while a broadened line shape exists for APISnBiBr₅ in terms of distribution traps at 312, 473, and 501 meV, which can be further improved by regulating the defect number in their lattice. This effort further extends the attempt to manipulate the trap number of the IL-based Bi-HOIPs by providing an excellent scintillating hybrid material.

2. Experimental

2.1. Synthetic procedures

The preparation of bismuth-based HOIPs was reported in our previous work.²⁶ Because pristine perovskite samples are very sensitive and too small for further optical characterization, the prepared bismuth-halide perovskites do not have transparent features in nature, leading to an undesired scattering process during scintillation measurement. We therefore grounded the as-prepared perovskites into fine powders and subsequently introduced them into the PDMS matrix. The samples were incorporated with polymer adopted from the previous studies by Maddalena et al.,²⁷ Haposan et al.,²⁸ and recent work by Makowski et al.²⁹ The perovskite samples were mixed with polydimethylsiloxane (PDMS) at a weight ratio of 20% and subsequently cured for 3 h on a hot plate in aluminum foil cuvette at 120 °C. The resulting perovskite microcrystal had a diameter and thickness of 1.0 cm and 0.5 cm, respectively.

2.2. Radioluminescence (RL) and TL measurement

We utilized a single integrated setup to perform all these measurements. It is composed of an Inel XRG3500 X-ray generator Cu-anode tube (45 kV/10 mA), an Acton Research Corporation SpectraPro-500i monochromator, a Hamamatsu R928 photomultiplier tube (PMT), and an APD Cryogenic Inc. closed-cycle helium chiller. First, we evaluated RL at temperatures ranging from 10 to 350 K, beginning with the highest and progressing to the lowest, to prevent any interference through the thermal release of charge carriers to the emission yield. Following the RL measurements, we obtained low temperature afterglow and TL glow curves. Prior to TL evaluations, the crystals were irradiated to X-rays for 10 minutes at 10 K, with the monochromator adjusted to zeroth order. The glow curves were obtained at temperatures ranging from 10 to 350 K, with a heating rate of around 0.14 K s⁻¹. For imaging, the X-ray source was a PHYWE XR 4.0 expert unit (Mo source, 17.5 keV) operated at 35 kV voltage and 1 mA current, while the camera was a commercial colour camera with a 1-second exposure duration. The card with the chip was simply placed in front of the scintillator, while the back side was placed at the aperture of the X-ray tube, where the uncollimated X-ray emerged. They were put together as closely as possible to minimize light scattering.

2.3. Scintillation decay measurements

The scintillation time profile was performed using the delayed coincidence single photon counting method, which involves detecting correlated photon events to measure decay times accurately. In this method, a ¹³⁷Cs radioactive source was used for gamma radiation. The detection setup includes two Hamamatsu photomultiplier tubes, the R1104 for detecting the initial scintillation event (“start”) and the R928 for detecting subsequent events (“stop”). To operate the PMT, we applied a voltage of 1.19 kV. The timing of these events was precisely measured using a Canberra 2145 time-to-amplitude converter. These signals were then recorded and analysed by applying a TUKAN-8K-USB multichannel analyzer. Multi-exponential decay function fitting was then applied to the obtained spectra to obtain decay constants and their relative contributions, which are essential for calculating effective decay times.

3. Results and discussion

3.1. Characterization of ionic liquid bismuth hybrid perovskites

The traditional synthetic pathway of perovskites can be grown using slow solvent evaporation crystallization.²⁶ Because the chemical and mechanical stabilities of perovskites are very sensitive to humidity and thermal response, we therefore incorporated several targeted Bi-HOIPs into PDMS resin, as dictated in the photographs (Fig. S1, ESI†). PDMS was chosen because PDMS possesses the traits of the high elasticity, low temperature flexibility and full transparency and serves as a double protective matrix.^30,31 The incorporation of perovskites with PDMS can improve the stabilization of perovskites and produce samples of appropriate size for analysis.^32,33 Physically, the crystal has a sphere-like shape with diameter and thickness size of ±1.5 cm and 0.5 cm, respectively. In detail, SEM images of the crystal after grinding are depicted in Fig. S2 (ESI†), showing that the crystal is micrometer in size, which indicates that the incorporation of perovskite with PDMS yielded microcrystals of various sizes. The resulting perovskite crystals were subsequently used to analyse the scintillation features, which are discussed in the next section. The crystal structure analyses of the perovskites using Rietveld refinements are outlined in Fig. S3 (ESI†). The chemical states of APISn₂Br₁₀ were examined by applying an X-ray photoelectron spectrophotometer, as presented in Fig. S4 (ESI†), whereas other compounds can be found in ref. 26. The absorption, photoluminescence and time-resolved photoluminescence spectroscopies are outlined in Fig. S5 (ESI†). A detailed discussion is provided in the ESI.†

3.2. Structural order of one-dimensional bismuth-based HOIPs

Prior to discussing the scintillation properties, we revisit the impact of ionic liquid variations on their perovskite crystal arrangements, as depicted in Fig. 1. In agreement with the previous work,²⁶ the overall inorganic networks of Bi-HOIPs share a one axis arrangement formed by the 1D corner-sharing of zigzag chains, displaying a similar trend apart from the IL-type. In addition, we found that the crystal structures of BMPBiBr₄ and BMIBiBr₄ exhibit a triclinic crystal structure. In contrast, APIBiBr₅, APIBiSnBr₅ and APISn₂Br₁₀ share a monoclinic system (see Table S1, ESI†). Each of the bismuth atoms is coordinated with six bromine anions to form BiBr₆ octahedrons. Here, four bromine anionic sites are situated in the terminals, and the other two moieties serve as bridges between two neighbouring BiBr₆ octahedrons. In terms of Bi–Br connectivity and lattice parameters, we found that the three crystallographic axes of APIBiBr₅ and APIBiSnBr₅ were moderately extended. For instance, the lattice along the a-axis is changed upon Sn-introduction (from 15.7941 Å to 19.8608 Å, see Table S1, ESI†). Interestingly, the incorporation of Sn into the APIBiBr₅ crystal lattice did not generate significant geometric distortion (see Table S2, ESI†). For instance, we elaborate a fair geometric analysis by extracting Sn–Br–Sn bond angles between APIBiSnBr₅ and APISn₂Br₁₀ (see Computational details, ESI†). Here, a noticeable change upon fully substituting Sn atoms into the perovskite was observed. The reduction bond length is 0.19 Å. The Sn–Br–Sn of APISnBiBr₅ is 3° narrower than that of APISn₂Br₁₀. The wider Sn–Br–Sn in APISnBiBr₅ is fair because of the existence of Bi–Br–Sn and Bi–Br–Bi connectivity, which affects the bond angle for the stabilization of octahedral tilting.

Fig. 1 Calculated structures of perovskites, which were extracted from CIF files, including (a) BMPBiBr₄, (b) BMIBiBr₄, (c) APIBiBr₅, (d) APISnBiBr₅, and (e) APISn₂Br₁₀.

3.3 Radioluminescence measurements of Bi-HOIPs and PDMS matrix

The RL spectra of the admixture Bi-HOIPs and PDMS matrix are presented in Fig. 2. We note that two spectral areas are found in BMPBiBr₄, APIBiBr₅, and BMIBiBr₄. The first region is covered in the range of 325–520 nm, while the second region appears in the 525–560 nm spectral range. According to the earlier study, we found a resemblance feature in the recorded PL spectra (Fig. S5, ESI†) of the three aforementioned Bi-HOIPs with a maximum centered at 540 nm.²⁶ This peak is shifted approximately 20 nm towards a higher wavelength compared to the PL results because the different excitation sources of RL and PL were used.¹⁴ We indirectly relate that the second region in the RL mapping originated from the light-mater interaction dominantly governed by the excitonic contribution. Such findings can be addressed in the first area as a direct consequence of the chemical integration of perovskite into PDMS. As previously noted by Kim et al.,³⁴ the inclusion of perovskite using a PDMS matrix inhibited the deprotonation of organic cations owing to the interaction of the oxygen element with the metal via coordination bonds. A contrasting RL spectral behavior is observed in the case of BMPPBiBr₄ (Fig. 2(a)) and BMIBiBr₄ (Fig. 2(b)) centering at 550 nm. In addition, the A-site substitution of Bi-HOIPs with the API cation (Fig. 2(c)) led to a slight broadening and became higher in terms of intensity close to 200 K.

Fig. 2 Temperature-dependent radioluminescence (RL) spectra of perovskite samples, including (a) BMPBiBr₄, (b) BMIBiBr₄, (c) APIBiBr₅, (d) APISnBiBr₅, and (e) APISn₂Br₁₀, at wavelengths between 350–600 nm and temperatures between 10–300 K. The left color bar correlates with the intensity of the RL spectra, revealing a varied intensity range for each of the perovskite samples.

To investigate the impact of synergetic substitution, we introduce Sn and simultaneously increase the number of Br atoms into APIBiBr₅ structure and then APISn₂Br₁₀. In contrast, here, we observed that by partially replacing Bi, the metallic signature (close to 550 nm) is largely suppressed in APISnBiBr₅ (Fig. 2(d)) and smeared out for APISn₂Br₁₀ (Fig. 2(e)). The collected RL spectra of APIBiSnBr₅ at 10 K in the second area are significantly reduced although the RL spectra in the first region are greater than those in other perovskites. This result reveals that the substitution of Sn in APIBiBr₅ creates a new recombination center that contributes to the improvement of RL intensity. The RL intensity of APISnBiBr₅ decreases with increasing temperature, indicating the presence of thermal quenching (TQ), which is explored further below. APISn₂Br₁₀ exhibits the highest RL intensity in the first region compared to the others. This result further demonstrates that the existence of a small fraction of Sn in APIBiBr₅ influences the enhancement of RL intensity owing to the formation of a recombination center. Similarly, the RL profile of APISn₂Br₁₀ changes with temperature, resulting in the creation of broad-band spectra. The broad-band spectra show several recombination pathways of luminescence that are generated using high energetic X-ray irradiation.

3.4. Thermal effects on the radioluminescence profile

We argue that thermal interaction contributes to the RL spectral variation to some extent, as shown in Fig. 3. Here, we successfully unravel the peculiar observation as we break the RL into four distinct temperature profiles in the first region (see Fig. 3). To the best of our knowledge, there is no specific argument concerning the unusual spectra reported in the literature. We propose that the unusual RL spectra possibly originated from the multiple exciton recombination, bulk and surface defect, re-absorption and self-absorption, and photon replica.³⁵ Additionally, the formation of unusual shapes in RL spectra could be related to the presence of a vibrational structure in the spectrum.³⁶ The weak interfacial interaction between PDMS and perovskite can also trigger the formation of four RL regimes.³⁷ A high energy excitation promotes unconventional emission.

Fig. 3 Representation of RL spectra of perovskites at temperatures of (a) 10 K, (b) 100 K, (c) 200 K, and (d) 300 K.

Upon further investigation, the broad RL peaks of BMPBiBr₄ (Fig. 3(a)) preserve their profile throughout temperature-dependent RL analysis. This discovery shows that the electronic structure of BMPBiBr₄/PDMS (Fig. 3(b)) does not change as a result of the X-ray temperature effect. For the sake of clarity, we labelled each of the prominent deconvoluted RL maxima (i–v) within the first region to monitor the thermal effect at different temperatures. Although the overall trend seems to be random, we found that a certain peak of APISnBiBr₅ (green) exhibited a gradually reduced intensity. In particular, peak (iii) across 200 K (Fig. 3(c)) and subsequent heating to 300 K (Fig. 3(d)) led to a nearly depleted signal. For comparison, we also present the photoluminescence (PL) profile and charge dynamic behaviour of these compounds using time-resolved photoluminescence (TRPL), as depicted in Fig. S5 (ESI†). Here, we attribute that octahedral tilting contributes significantly to tailor the defect states of the 1D ionic liquid perovskites.

To further quantify the finding, we exploited temperature-dependent RL analysis (see Fig. 4) by carefully monitoring the intensity variation from low to high temperatures. As previously outlined by Thirumal et al. and Li et al., such a TQ process is triggered by the suppression of phono-assisted nonradiative recombination.^38,39 Temperature-dependent intensities are further examined to evaluate and compare the intrinsic differences of hybrid halide perovskites. The graphs in Fig. 4(a)–(e) are fitted using the Arrhenius fitting equation (eqn (1)),⁴⁰ and the parameters are summarized in Table 1.

where Γ₀/Γ_v, ΔE_q and k_B are the ratio between the thermal quenching rate at T = ∞ (attempt rate) and the radiative transition rate, the thermal quenching activation energy and the Boltzmann constant, respectively. The value of Γ₀/Γ_v varies depending on the type of IL employed in bismuth-based HOIPs. BMPBiBr₄ achieves the lowest Γ₀/Γ_v value, whereas its BMI counterpart is marginally improved. BMIBiBr₄ (Fig. 4(b)) has much larger Γ₀/Γ_v values than BMPBiBr₄ (Fig. 4(a)) and APIBiBr₅ (Fig. 4(c)). Furthermore, the value of Γ₀/Γ_v is consistent with the estimated ΔE_q. This characteristic suggests that the fast-quenching rate dominates in BMIBiBr₄ compared to the other compounds. The low Γ₀/Γ_v and ΔE_q of BMPBiBr₄ and APIBiBr₅ are caused by the de-trapping process of a bound exciton, resulting in increased RL intensity at lower temperatures.⁴¹ At low temperatures, these bound excitons lack sufficient energy to be reconverted into free excitons.³⁸

Fig. 4 Arrhenius fitting of thermal quenching behaviour of perovskites, including (a) BMPBiBr₄, (b) BMIBiBr₄, (c) APIBiBr₅, (d) APISnBiBr₅, and (e) APISn₂Br₁₀.

Table 1 Thermal quenching parameters of perovskites

Parameters	Perovskites
	BMPBiBr4	BMIBiBr4	APIBiBr5	APISnBiBr5	APISn2Br10
Γ 0/Γv (s^−1)	4.9 ± 1.3	1418.5 ± 530.5	6.5 ± 0.4	9298.2 ± 252.8	17 755 ± 4159
ΔEq (meV)	28 ± 6	185 ± 97	39 ± 2	81 ± 5	185 ± 59

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We notice that this type of IL significantly affects the TQ outcome. Let us revisit the XRD data from the prior research, which shows that BMIBiBr₄ and BMPBiBr₄ compounds exhibit triclinic crystallography, but APIBiBr₅ has a monoclinic crystal structure. Such structural variation caused by the various ILs in the A-site could influence the quenching behaviour of bismuth-based HOIPs. To shed more light, we propose that the observation is related to the band gap energy implication of halide perovskite upon interchanging the number of nitrogen atoms and the skeletal backbone. Based on our previous investigation,²⁶ geometric consideration exhibited small displacement in terms of Bi–Br bond distance and angles of specific type ILs, hosting the minute band gap variation. In a similar triclinic structure, the apical Bi–Br bond of BMPBiBr₄ is longer than that of BMIBiBr₄, resulting in a narrow band gap of BMPBiBr₄ (3.04 eV) compared with BMIBiBr₄ (3.16 eV). Consequently, a faster quenching rate (1418.5 ± 530.5 s⁻¹, see Table 1) is obtained for BMIBiBr₄. However, we note that the band gap of APIBiBr₅ (3.01 eV) has a fast quenching rate (6.5 s⁻¹). We believe that the occurrence of double bonds in the API internal chemical structure governs the increasing number of small polarons. Because the photoexcited free electrons are feasible to capture by a small polaron, such a condition suppresses the radiative recombination, leading to a fast-quenching rate. In a similar approach, a BMI structure containing imidazole rings causes a significant outcome of a faster quenching rate compared to BMPBiBr₄ and APIBiBr₅. For instance, Sn substitution in the APIBiBr₅ structure elevates the Γ₀/Γ_v value by approximately 1500 times compared to APIBiBr₅. These data suggest that replacing partial bismuth with tin atoms promotes a rapid TQ succession rate. These findings show that the RL intensity of APISnBiBr₅ is reduced by 50% and approaches zero at 200 K. The findings are related to the mismatch of atomic size, leading to structural distortion and creating high numbers of defect states.

The occurrence of TQ behaviour in APISn_BiBr₅ (Fig. 4(d)) and APISn₂Br₁₀ (Fig. 4(e)) is consistent with a previous work by Xie et al.,⁴⁰ showing the presence of TQ in lead-based HOIPs. A comparable TQ process was also observed in the case of (C₆H₅CH₂NH₃)₂SnBr₄.⁴² APISn₂Br₁₀ has a comparable ΔE_q to (C₆H₅CH₂NH₃)₂SnBr₄ but has a greater Γ₀/Γ_v value than the latter. This finding shows that the TQ process in APISn₂Br₁₀ is faster than the TQ process in (C₆H₅CH₂NH₃)₂SnBr₄, which occurs at lower temperatures. The presence of TQ in bismuth-based HOIPs alters the light yield of the perovskite. Based on the RL measurements, three candidates such as BMPBiBr₄, APIBiBr₅, and BMPBiBr₄ had a low light yield at room temperature of approximately 1000 ph PER keV, indicating that they might be used as scintillating materials, particularly at low temperatures, owing to their great sensitivity at low temperatures.

3.5. High-energetic γ-ray scintillation decay time of Bi-HOIPs

The scintillation decays of BMPBiBr₄, BMIBiBr₄, and APIBiBr₄ were analysed by exposing gamma (γ) irradiation to the perovskite samples. The γ-ray scintillation decay curves of BMPBiBr₄, BMIBiBr₄, and APIBiBr₄ are presented in Fig. 5. All samples exhibit bi-exponential features, showing the fast and slow decay components. The Bi-HOIPs, which contain BMP (Fig. 5(a)) and API (Fig. 5(c)) as the ILs, exhibit a considerably fast decay component of 6 ns, while BMI exhibits a fast component of 4 ns. We propose that such a feature might be correlated to the type of IL structure used as the molecular backbone in the bismuth-based HOIP configuration. Let us consider that the different positions of nitrogen cations affect the resulting final geometry structure of bismuth-halide networks, which influences the charge carrier decay pathway. We note that the entire compound shows a dominant slow component rather than a fast counterpart. This result indicates that a high number of self-trapped electrons and defects heavily contribute to the secondary decay event, which is in line with the existence of the TQ process in the RL measurements. The undesired slow component of Bi-HOIPs can be mitigated under two approaches by suppressing the number of defect states within the crystals. First, the chemical composition optimalization during the synthesis of targeted bismuth-based halide perovskite promoted a high crystallinity order, thus lowering the chance of defect formation. Second, microstructure engineering is suggested as an applicable route to retaining the integrity of high-quality bismuth-based hybrid perovskites.^43,44 We revisit the fundamental mechanism of scintillation decay to illustrate the formation of fast and slow components. Initially, a free electron is created owing to the material exposure to gamma radiation, followed by a thermalization process into the conduction band. Thus, the electron is recombined at an ultrafast time.⁴⁵ Alternatively, electrons could also be transferred into the deep and shallow traps, subsequently emitting and generating a slow emission phenomenon.⁴⁶ This finding is further explained in the next section to elucidate the occurrence of trap states in the bismuth-based halide perovskites.

Fig. 5 Scintillation decay of (a) BMPBiBr₄, (b) BMIBiBr₄, and (c) APIBiBr₄ collected at RT with biexponential features of the respective fast and slow decay component.

3.6. Afterglow, thermoluminescence, and radiation hardness

For further scintillation analysis of the samples, afterglow and TL experiments were used to analyse the energy trap profile, as represented in Fig. 6. Upon termination of X-ray excitation at 10 K, the emission yields up to thousands of seconds. There is no afterglow during the temperature increase in BMIBiBr₄ (Fig. 6(b)) and APIBiBr₅ (Fig. 6(c)), revealing the absence of energy traps. This result is similar to previous work by Xie et al.⁴⁰ and Diguna et al.,⁴² as reported in lead-based and tin-based HOIPs, respectively. Meanwhile, BMPBiBr₄ (Fig. 6(a)) shows the existence of afterglow (see Fig. S6, ESI†), indicating the presence of the energy trap. Similarly, the substitution of Sn in APIBiBr₅ and the replacement of Bi with Sn indicate the presence of afterglow. The glow curve of APISnBiBr₅ (Fig. 6(d)) is extremely broad, implying the existence of a trap distribution in APISnBiBr₅. We revisit previous work that highlights that Sn substitution increases the length of the axial Bi–Br bonds from 3.41 Å to 3.8 Å and consequently introduces a geometric distortion within the system, introducing defect sites in the crystal lattice of APISnBiBr₅. This in turn influences the scintillation performance of APISnBiBr₅.

Fig. 6 Afterglow measurement results of perovskites: (a) BMPBiBr₄, (b) BMIBiBr₄, (c) APIBiBr₅, (d) APISnBiBr₅, and (e) APISn₂Br₁₀. The red dashed lines denote the time when the samples were heated, while the black dashed lines denote the exponential decay fits.

In addition, we believe that the distortion can trigger the existence of self-trapped excitons owing to the enhanced electron–phonon coupling,⁴⁷ which is mutually related to the current finding. To determine the trap, we analyse the experiments after heating the samples (at 3580 seconds, red dashed lines in Fig. 6 and heat profile in ESI,† Fig. S5), and we present the TL spectra, as shown in Fig. 7.

Fig. 7 Classic Randall–Wilkins fitting of TL glow curve. (a), (b), and (c) are BMPBiBr₄, APISn₂Br₁₀, and APISnBiBr₅, respectively.

The classic Randall–Wilkins approach (eqn (2)) was used to calculate the parameter of trap arising in TL spectra of BMIBiBr₄, APISn₂Br₁₀, and APISnBiBr₅, as shown in Fig. 7(a)–(c), including concentration, depth and frequency factors (presented in Table 2).

where I, n₀, s, E, T, k_B and β are the TL intensity, initial trap concentration, factor of frequency, trap depth, temperature, the Boltzmann constant and heating rate, respectively. As shown in Fig. 7, the TL peak of BMPBiBr₄ is pronounced at >100 K, which is higher compared to our previous study.⁴¹ This finding can be associated with the different organic ligand chains and sizes between BMP and tetramethylammonium, which generate the different trap positions in bismuth-based HOIPs.

Table 2 Tabulation of trap parameters according to Randall–Wilkins fitting results

Parameter	Perovskites
	BMPBiBr5			APISnBiBr2	APISn2Br10
Peak	Peak 1	Peak 2	Peak 3	Peak 1	Peak 1
n 0 (cm^−3)	3.6 × 10^3	8.8 × 10^3	6.9 × 10^3	3.7 × 10^5	8.1 × 10^3
E (meV)	349	692	910	672	1022
s (s^−1)	3.8 × 10^5	4.4 × 10^11	5.3 × 10^9	7.1 × 10^10	2.3 × 10^23

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Distribution	Distribution 1	Distribution 2	Distribution 1
A (cm^−3)	3.5 × 10^3	7.2 × 10^3	2.2 × 10^2
E (meV)	312	473	501
σ (meV)	101	104	102

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According to the fitting, three deconvoluted peaks associated with the shallow and depth traps are extracted. Here, we consider the lowest energy (peak 1) corresponding to the shallow traps, while peak 2 and peak 3 are associated with the depth traps. The initial trap concentration is comparable with several hybrid perovskite systems. In APISn₂Br₁₀, the glow curve corresponds to the first order kinetic. The sharp first order peak is correlated with the depth traps; meanwhile, the broad deconvoluted peak is related to the distribution traps. Similarly, the substitution of Sn in APIBiBr₅ shows a sharp first order peak, which is attributed to the depth traps. Two broad deconvoluted peaks are yielded, indicating the occurrence of distribution traps in APISnBiBr₅. This results in evidence that the substitution of Sn induces the formation of several defect sites, which act as the distribution traps in the lattice.

To investigate the structure endurance upon high-energy irradiation, we present the radiative hardness experiments, as depicted in Fig. S7 (ESI†). The results exhibit a linear profile between the normalized integrated intensity and dose in the log-scale plot. We note the absence of strong saturation in all the under-study perovskites, which is comparable to the previous work in lead-based halide perovskites.⁴⁰ We noticed that BMPBiBr₄ and BMIBiBr₄ shared similar patterns, while APIBiBr₅ exhibited a ramp profile. This finding implies that the latter has a low radiation hardness compared to the BMPBiBr₄ and BMIBiBr₄. Based on this test, we believe that the Bi-HOIPs with monoclinic crystal structure are more stable towards radiation compared to the triclinic counterparts. The substitution of Bi with Sn atoms in the APIBiBr₅ system decreases the radiation hardness, suggesting the consequence of an atomic size mismatch between Bi³⁺ and Sn⁴⁺. The full conversion of Bi with Sn atoms in the B-site of the API system improves radiation hardness. This finding infers that a reduction in defect density within the crystal lattice of APISn₂Br₁₀ is achieved.

4. Conclusions

In summary, the utilization of ILs as organic cations in bismuth-based HOIPs exhibits modest scintillation features. All perovskites exhibit high RL spectra at low temperatures, and an opposite trend at elevated temperatures, indicating the existence of the TQ process. By rational design, we demonstrate that A-site tuning is used to tailor the optical response and scintillation properties of Bi-HOIPs, particularly their impact on the thermal quenching rate. We rationalize that the finding originates from the IL internal structure composition, which is regulated by the nitrogen atom and skeletal backbone, yielding a minute change within the band gap and polaron contribution. Consequently, BMPBiBr₄, BMIBiBr₄ and APIBiBr₅ show a light yield value of approximately 1000 ph per keV. A fast TQ process is observed at APISnBiBr₅ and APISn₂Br₁₀, leading to the absence of RL spectra at RT. In TL analysis, BMIBiBr₄ and APIBiBr₅ show no trap in their crystal lattice. Meanwhile, BMPBiBr₄ generates shallow and depth traps according to glow curve fitting. We found that the imidazole ring provided fewer defect sites than the pyrroline ring in BMP. The depth traps and distribution traps are observed in APISnBiBr₅ and APISn₂Br₁₀. The presence of distribution traps induced by X-rays reveals that the perovskites have a high defect level in their crystal lattice. Several Bi-HOIPs show modest scintillation results, which could be improved via subsequent treatments.

Author contributions

R. Subagyo: writing – original draft, visualization, resources, methodology, formal analysis. M. S. Eid: data curation, formal analysis, investigation, resources. N. Safitri: methodology, visualization. R. Hanifah: methodology, visualization. A. A. Afkauni: visualization, formal analysis, investigation. L. Zhang: data curation, formal analysis. M. Makowski: data curation, formal analysis. M. A. K. Sheikh: data curation, writing – review & editing. M. H. Mahyuddin: formal analysis, methodology, resources, software. D. Kowal: data curation, writing – review & editing. M. Witkowski: formal analysis, resources. W. Drozdowski: formal analysis, resources, validation. M. D. Birowosuto: conceptualization, data curation, supervision, methodology, validation, investigation, funding acquisition, writing – review & editing. A. Arramel: conceptualization, formal analysis, investigation, supervision, validation, writing – review & editing. Y. Kusumawati: conceptualization, supervision, resources, investigation, project administration, funding acquisition, validation. All authors have given their approval to the final manuscript version.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Authors from Institut Teknologi Sepuluh Nopember acknowledge funding of this research from Asian Development Bank (ADB) through the Partnership Research HETI (Penelitian Kemitraan HETI, Higher Education for Technology and Innovation) with grant number 0005/01.PKS/PPK-HETI/2023. Authors from Nano Center Indonesia express their gratitude to PT. Nanotech Indonesia Global Tbk for the start-up research grant. The funder was not involved in the study design, collection, analysis, data interpretation, article writing, or the decision to submit it for publication. M. D. B. and D. K. acknowledge funding from the National Science Center, Poland under grant OPUS-24 no. 2022/47/B/ST5/01966. M. H. M. acknowledges the research fund from Institut Teknologi Bandung under the “Penelitian, Pengabdian Masyarakat, dan Inovasi (PPMI) 2023” research scheme (grant no. 3AJ/IT1.07/SK-KP/2023).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2410659. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc04390a

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