Two‐Dimensional Perovskite Single Crystals for High‐Performance X‐ray Imaging and Exploring MeV X‐ray Detection

Scintillation semiconductors play increasingly important medical diagnosis and industrial inspection roles. Recently, two‐dimensional (2D) perovskites have been shown to be promising materials for medical X‐ray imaging, but they are mostly used in low‐energy (≤130 keV) regions. Direct detection of MeV X‐rays, which ensure thorough penetration of the thick shell walls of containers, trucks, and aircraft, is also highly desired in practical industrial applications. Unfortunately, scintillation semiconductors for high‐energy X‐ray detection are currently scarce. Here, This paper reports a 2D (C4H9NH3)2PbBr4 single crystal with outstanding sensitivity and stability toward X‐ray radiation that provides an ultra‐wide detectable X‐ray range of between 8.20 nGyair s−1 (50 keV) and 15.24 mGyair s−1 (9 MeV). The (C4H9NH3)2PbBr4 single‐crystal detector with a vertical structure is used for high‐performance X‐ray imaging, delivering a good spatial resolution of 4.3 lp mm−1 in a plane‐scan imaging system. Low ionic migration in the 2D perovskite enables the vertical device to be operated with hundreds of keV to MeV X‐ray radiation at high bias voltages, leading to a sensitivity of 46.90 μC Gyair−1 cm−2 (−1.16 V μm−1) with 9 MeV X‐ray radiation, demonstrating that 2D perovskites have enormous potential for high‐energy industrial applications.

Scintillation semiconductors play increasingly important medical diagnosis and industrial inspection roles.Recently, two-dimensional (2D) perovskites have been shown to be promising materials for medical X-ray imaging, but they are mostly used in low-energy (≤130 keV) regions.Direct detection of MeV X-rays, which ensure thorough penetration of the thick shell walls of containers, trucks, and aircraft, is also highly desired in practical industrial applications.Unfortunately, scintillation semiconductors for high-energy X-ray detection are currently scarce.Here, This paper reports a 2D (C 4 H 9 NH 3 ) 2 PbBr 4 single crystal with outstanding sensitivity and stability toward X-ray radiation that provides an ultra-wide detectable X-ray range of between 8.20 nGy air s −1 (50 keV) and 15.24 mGy air s −1 (9 MeV).The (C 4 H 9 NH 3 ) 2 PbBr 4 single-crystal detector with a vertical structure is used for high-performance X-ray imaging, delivering a good spatial resolution of 4.3 lp mm −1 in a plane-scan imaging system.Low ionic migration in the 2D perovskite enables the vertical device to be operated with hundreds of keV to MeV X-ray radiation at high bias voltages, leading to a sensitivity of 46.90 μC Gy air −1 cm −2 (−1.16 V μm −1 ) with 9 MeV X-ray radiation, demonstrating that 2D perovskites have enormous potential for high-energy industrial applications.
increasing the applied bias, but the corresponding rise in dark current and dark current drift at large voltage are unfavorable for detection performance. [17,18]These adverse phenomena are attributable to polarization effect by ionic migration, which is well known to occur in the original three-dimensional (3D) lead halide perovskites (APbX 3 , where A = CH 3 NH 3 + and HC(NH 2 ) 2 + ; X = Cl − , Br − , and I − ). [19,20]ecently, two-dimensional (2D) OIHPs have emerged as viable solutions that address this concern; [18,21] their layered structures, which depend on large organic cations, are essential for inhibiting halide ion migration and preventing perovskite structural degradation. [22,23]oreover, 2D perovskites and perovskite-like materials are reportedly good candidates for X-ray imaging, exhibiting excellent operation stabilities in low-energy regions. [21,24]With this background in mind, we reasoned that 2D OIHPs with high ionic migration diffusion barriers should be sensitive and stable for medium−/high-energy X-ray detection at large bias voltages, with a significant potential for industrial applications.
Herein, we report a sensitive and stable direct-type X-ray detector fabricated using hundreds of micrometer thick (C 4 H 9 NH 3 ) 2 PbBr 4 (BA 2 PbBr 4 ) perovskite single crystals.BA 2 PbBr 4 is a typical 2D Ruddlesden-Popper-type perovskite that has a layered structure due to the long A-site chain of the BA cation.Less halide ion migration occurs in the layered structure of 2D OIHPs compared to a cubic-structured 3D perovskite, which is beneficial for maintaining structural stability as well as inhibiting dark current increase and drift at large applied biases.Moreover, the vertical structure is used in our single-crystal device to further reduce background signals in the dark.BA 2 PbBr 4 vertical devices exhibit outstanding low-energy X-ray detection performance and operational stability, and the ability to X-ray image was also verified using a plane-scan imaging system.Our BA 2 PbBr 4 single-crystal detector exhibits a wide detectable X-ray range and excellent sensitivity in medium−/high-energy energy regions, and is expected to motivate new strategies for industrial radiography applications.

Result and Discussions
A millimeter-sized BA 2 PbBr 4 single crystal is about 160 μm thick (Figure 1a), which is similar to that of α-Se used in commercial flatpanel detectors. [25]A relatively smooth crystal surface devoid of obvious defects is observed by scanning electron microscopy (SEM) (Figure S1, Supporting Information).The transparent thin-slice single crystal was synthesized from a saturated hydrobromic acid solution by a solvothermal method with slow cooling (1 °C h −1 ).A stable temperature field in a closed reactor with programmed cooling is necessary for the slow growth of high-quality crystals.It should be noted that BA 2 PbBr 4 grows horizontally due to the layered structure that consists of layers of inorganic [PbBr 6 ] 2− octahedra and intercalated bulky BA cations (Figure 1b), which is favorable for further fabricating large-area film devices.
The peaks in the powder X-ray diffraction (XRD) pattern of BA 2 PbBr 4 are well-matched with the calculated data, and the crystal XRD pattern shows obvious preferred (002), (004), (006), and (008) planar orientations (Figure 1c).Our sample crystals grew parallel to the (001) plane, which coincides with the layered-structure feature.The high-resolution X-ray rocking curve reveals that the four crystal planes exhibit narrow full-width at half-maximum (0.063-0.076°), suggestive of highly crystalline BA 2 PbBr 4 single crystals (Figure S2, Supporting Information).The element maps produced by energy dispersive X-ray spectroscopy (EDS) show that the target elements (C, N, Br, and Pb) are uniformly distributed in the crystal surface (Figure S3, Supporting Information).In addition, thermogravimetric analysis shows a weight loss of 45  1d).The optical bandgap was determined to be 2.94 eV from the Tauc plot (Figure 1d, inset); this value is higher than that of a classical 3D hybrid lead bromide perovskite (MAPbBr 3 : [26] 2.21 eV; FAPbBr 3 : [27] 2.13 eV).Less noise from visible light and thermal interference associated with the broad bandgap of BA 2 PbBr 4 is essential for the operational stability of an X-ray detector.The bandgap calculated using density-functional theory (DFT) is 0.46 eV lower than the experimental value (Figure S5a, Supporting Information), which is a reasonable broadly recognized difference. [26]To better understand the orbital contributions of the valence and conduction bands in BA 2 PbBr 4 , we calculated the projected density of states, which reveals that the −3.5 to 4 eV energy segment is mainly dominated by occupied Br-4p and Pb-6p states, with other parts composed of occupied and unoccupied C, H, N, Br, and Pb states (Figure S5b, Supporting Information).These results show that the 4p orbital of Br − mainly contributes to the valence band, with the conduction band mostly stemming from the 6p orbital of Pb 2+ .The charge densities corresponding to the valence band maximum and conduction band minimum are only distributed through the inorganic layer, which is composed of [PbBr 6 ] 2− octahedra (Figure S5c, Supporting Information).
We further examined the electronic properties of BA 2 PbBr 4 , with the relative dielectric constant in the high-frequency region determined to be 16.67, as calculated by the equation shown in Figure S7a, Supporting Information.The resistivity of BA 2 PbBr 4 was determined to be in the 10 11 -10 12 Ω cm range, with an average value of 8.51 × 10 11 Ω cm calculated for seven single crystals (Table S1, Supporting Information).This ultrahigh resistivity is attributable to the suppression of charge transport in the [001] orientation, and is at least one order of magnitude larger than values previously reported for perovskites, such as MAPbBr 3 [28] (10 7 -10 8 Ω cm) and Cs 2 AgBiBr 6 [29]   (10 9 -10 11 Ω cm).The high intrinsic resistivity of the BA 2 PbBr 4 single crystal results in a low dark current that favors optoelectronic detection applications.
Reducing patient radiation dose while maintaining image quality in medical radiography requires a highly sensitive and stable direct-type X-ray detector with a low detection limit and fast response time.We note that an X-ray energy of <130 keV is required for medical operation, [30] which guarantees both patient safety and effective penetration.Here, the device performance of our BA 2 PbBr 4 single-crystal detector was evaluated using a low-energy 50 keV X-ray (peak energy at 50 kV tube voltage), in line with recently reported testing parameters for perovskite devices. [28,29,31]The vertical device structure of our perovskite detector consisted of upper and lower Ag electrodes and a BA 2 PbBr 4 single-crystal interlayer (Figure 2a, inset).The quasi-linear performance at a small applied bias is indicative of ohmic contact characteristics, with a small Schottky barrier of 0.2 V determined from the I-V plots with various X-ray dose rates (Figure S8a, Supporting Information).A negative voltage was used in subsequent experiments, due to the superior sensitivity exhibited at negative bias voltages (Figure S8b, Supporting Information).
The sensitivity of the BA 2 PbBr 4 single-crystal detector was determined at various biases, with a maximum value of 726.18 μC Gy air −1 cm −2 observed at a bias of −150 V (Figure 2a).It is worth mentioning that the relationship between current density and Xray dose rate shows a gradual reduction in slope in the 3.97-6.32mGy air s −1 range (Figure S9, Supporting Information), which indicates that sensitivity decreases with increasing dose rate at the same applied bias.Hence, we chose the 7.78-48.83[34] The maximum sensitivity at 0.92 V μm −1 is more than thirty times larger than that of a commercial α-Se X-ray detector [21] (20 μC Gy air −1 cm −2 ) in a 10 V μm −1 applied electrical field.Low detection limit is a hot topic in the X-ray detector field, as it is an important factor that limits the radiation dose experienced by the patient during medical imaging. [14,24,35]s shown in Figure 2b, the BA 2 PbBr 4 singlecrystal detector exhibited signal-to-noise ratios (SNRs) of 2.44 and 3.47 when exposed to dose rates of 3.97 and 13.28 nGy air s −1 , respectively.A detection limit of 8.20 nGy air s −1 was derived from the linear relationship between SNR and Xray dose rate according to an effective SNR of three, as shown in Figure S12, Supporting Information; this value is approximately three orders of magnitude lower than the dose rate (5.5 μGy air s −1 ) required for regular medical treatment, [36] and is lower than most reported perovskite single-crystal detectors (MAPbBr 3 : [35] 36 nGy air s −1 ; Cs 2 AgBiBr 6 : [29] 59.7 nGy air s −1 ; BDAPbI 4 : [21] 430 nGy air s −1 ).The ultralow detection limit of our vertical detector is ascribable to its large resistivity (10 11 -10 12 Ω cm) Energy Environ.Mater.2024, 7, e12487 that reduces background current, while the layered structure inhibits ionic migration. [24,29]The temporal response of our BA 2 PbBr 4 singlecrystal detector when exposed to a pulsed X-ray is shown in Figure 2c.
A rise time of 98 ms and a fall time of 307 ms were determined from the temporal response at an X-ray dose of 465.7 μGy air s −1 according to the time elapsed between 10% and 90% response-signal intensities; these values are moderate compared to those reported for vertical perovskite devices. [31,36,37]The fall time is significantly affected by the testing conditions, with lower fall times observed with increasing applied bias and dose rate (Figure S14, Supporting Information).Operation stability is a major issue for perovskite-based X-ray detectors, especially when operated with high-dose radiation and high relative humidity (RH).To determine stability, we subjected the BA 2 PbBr 4 single-crystal device to 50 cycles of repeated X-ray doses (50 keV, 564.7 μGy air s −1 ) in a 0.061 V μm −1 electric field at room temperature in ambient air, and the corresponding results are shown in Figure 2d (upper).Stable cyclic photocurrents were also observed over more cycles and under different conditions (applied bias, dose rate, and X-ray energy), as shown in Figures S15 and S16, Supporting Information.Meanwhile, this device was exposed to continuous X-ray radiation (133 min, 88.71 Gy air ), with no significant decline observed for the entire process (Figure 2d, bottom).The current stability of the detector was further examined at 86 AE 1.5% RH and a bias of −100 V in the absence of any encapsulation, which revealed stable photocurrent and dark current for at least 60 min (Figure S17, Supporting Information).Moreover, the difference in the mean photocurrents before and after 52 h of exposure under the above mentioned conditions is quite small (1.5% in Figure 2e).The corresponding XRD patterns show that the BA 2 PbBr 4 structure was not destroyed using high-dose radiation or in a high RH environment (Figure S19, Supporting Information), which is attributable to the hydrophobicity of the long BA cation chain and inhibited ionic migration that prevents crystal collapse.These results show that the BA 2 PbBr 4 single-crystal detector is outstandingly stable and meets most of the working requirements for actual applications.
As stated above, X-ray detection performance and operation stability are greatly affected by ionic migration, leading to a notable dark current increase and drift at large bias and irreversible material degradation. [28,38]Accordingly, we determined the experimental activation energy and the theoretical ionic diffusion barrier to assess ionic migration in BA 2 PbBr 4 , the results of which are shown in Figure 2f,g.The activation energy (E a ) for ionic migration along the [001] axis was determined by measuring temperature-dependent conductivities (σ) in a 0.011 V μm −1 electric field.Two linear regions were obtained when the ln(σT) − 1000/T plot was fitted using the Nernst-Einstein relation under dark conditions. [24,32]The first linear region (247-370 K) has an E a of 0.04 eV, and the second (388-426 K) exhibits a larger E a 0 of 0.70 eV.The larger E a 0 at high temperature is assigned to ionic migration, with the smaller E a at low temperature considered to be associated with electronic conduction, as previously reported. [39]It is noteworthy that the conduction contribution transitions at a relatively high temperature (380 K), which provides strong evidence for room temperature operational stability.The Br − (vacancy) diffusion barrier (V Br À ) was subsequently simulated using DFT calculations, because halide ionic migration has previously been demonstrated to dominate conduction. [33]Different ionic-migration directions were also considered: equatorial-to-equatorial (Path 1) and axial-to-equatorial (Path 2), [19] as shown in Figure 2h.Path 1 plays a major role in halide ionic migration due to the higher barrier associated with Path 2; hence, the V Br À of BA 2 PbBr 4 was determined to be 0.76 eV.This calculated value is close to the experimental activation energy for ionic migration (0.70 eV), and is higher than the corresponding value for the 3D MAPbBr 3 perovskite (0.22 eV). [19]This result highlights that a 2D layered structure is better at suppressing ionic migration than the conventional 3D structure of an OIHP, thereby improving the device stability of an X-ray single-crystal detector.
To demonstrate the X-ray imaging capability of a BA 2 PbBr 4 single-crystal vertical device, we constructed a plane-scan single-pixel imaging system, as shown in Figure 3a.It is worth noting that the target object remains motionless and the X-ray detector is regularly moved by a 2D mobile platform to continuously detect signals in this imaging system, which is close to that of an actual clinical situation.The vertical device structure was still used in our X-ray Energy Environ.Mater.2024, 7, e12487 detector to provide an electric field along the [001] axis and to achieve low background noise and operational stability, as mentioned above.This device structure has also been adopted for many flat-panel detectors with array readout electronics, [11,14,40] which is a favorable trend in miniaturized radiography applications.Signal uniformity during the 2D scanning process is fundamental to high-quality X-ray imaging in our plane-scan imaging system.The uniformity was tested by exposing the detectors in a 15 × 30 mm background area to 350 μGy air s −1 Xray radiation.The detectors moved at a rate of 0.5 mm s −1 , and the same speed for detector movement was used in the following imaging experiments.No obtrusive dots were observed in the testing image (Figure 3b, top), and the photocurrents measured at each movement step fluctuated very little (1.8%) (Figure 3b, bottom), confirming that the moving detector responds in a stable manner.
The ability to resolve materials with different densities is also of great concern for an X-ray imaging system.As shown in Figure 3c, five metal plates (Pb, Cu, Sn, Ti, and Al) and polypropylene (PP) plate were used as 0.2-mm-thick X-ray attenuators, and a blank group (without plate) was set with initial X-ray radiation.The X-ray images exhibit clearly different colors (from black to white), with step changes in imaging photocurrent also observed.As shown in Figure S20, Supporting Information, the photocurrent intensity increases with decreasing material density, which suggests that our detectors have a superior ability to distinguish X-rays with different energies and dose rates, which are attenuated by those plates.Unexceptionally, this X-ray detector also shows a stepwise downward trend in response current for Al plates of different thickness, with excellent contrast observed in the corresponding images at different dose rates, as shown in Figure S21, Supporting Information (the minimum difference in dose rate between 0.5 and 0.6 mm Al plates: 8.3 μGy air s −1 ).Due to the significant difference between metal and air, narrow (1 mm) hollowed-out stainless steel letters "Fjirsm" were clearly observed in the X-ray image at a dose rate of 350 μGy air s −1 (Figure S22a, Supporting Information).Subsequently, multiple types of imaging object, crab (biological tissue), data cable (metallic and plastic product), and chicken feet contained a needle (biological tissue and metal), were clearly imaged under similar conditions (dose rate, bias, and distance ratio), as shown in Figure 3d,e and Figure S22b, Supporting Information.These X-ray images provide convincing evidence that the BA 2 PbBr 4 single-crystal detector can feasibly be used for X-ray imaging.
We next further investigated the spatial resolution of our BA 2 PbBr 4 device with a standard line-pair card (0.1 mm lead-equivalent) in the X-ray imaging system, which revealed a maximal discernible line pair of 4.3 lp mm −1 in the X-ray image, as observed by the human eye (Figure 3f, top).The modulation transfer function (MTF) corresponding to 4.3 lp mm −1 calculated from the line-pair profile was 21.7%, which is higher than the standard value of 20% (Figure 3f, bottom).The slanted-edge method was also used to assess the detector, which revealed a value smaller than that obtained using the line-pair card (Figure S24, Supporting Information).BA 2 PbBr 4 detectors reportedly exhibit relatively high spatial resolutions (3.1-4.9 lp mm −1 ). [14,15,31,32]he resolution is still limited according to pixel size (0.4 mm 2 ) in our single-pixel imaging system; hence, the BA 2 PbBr 4 single-crystal detector is expected to exhibit a higher spatial resolution with precision machining.
[43] Notably, these two applications have significantly different X-ray energy requirements, as shown in Figure 4a.As mentioned above, low-energy X-rays are used in medical imaging applications; such X-rays are sufficient to penetrate the majority of the body.On the other hand, the steel walls of industrial target objects are generally more than 2 mm thick; consequently, higher energy X-rays are required to ensure penetration depth.However, many conventional scintillators are not suitable for MeV X-ray detection in industrial applications, and research on novel scintillation semiconductor for use in high-energy region is currently scarce.Here, the detection performance of our 2D BA 2 PbBr 4 single-crystal detector was examined using medium-energy (200-400 keV) and highenergy (9 MeV) X-rays that have the typical maximum steel penetrations of 35-350 mm. [41]o explore the potential of our BA 2 PbBr 4 vertical device for use with medium-/high-energy X-rays, we need to consider detectionperformance criteria that differ from those required for medical imaging, since higher X-ray energies are used and non-human objects are being imaged.The maximum detectable energy is an essential figure of merit that determines the applicable scope of the X-ray detector.Scintillators gradually absorb fewer photons with increasing X-ray energy (Figure S25, Supporting Information), eventually leading to no response signal changes.A positive correlation was observed between response photocurrent and X-ray source tube current with 200-400 keV X-rays (Figure 4b), which indicates that the single-crystal detector still responds normally when exposed to 400 keV radiation; hence, the maximum detectable energy is at least 400 keV in this study.Moreover, the detectable dose-rate range is also an important Energy Environ.Mater.2024, 7, e12487 X-ray detector parameter.It should be noted that while lower limits are frequently discussed for many reported detectors used in medical applications, upper limits are rarely mentioned.A higher dose rate is used for industrial X-ray imaging application to ensure superior imaging quality with a short work time, while the photocurrent saturation threshold is more easily reached at high dose rates in the high-energy region.Therefore, the upper detection limit of the detector needs to be seriously considered in the same manner as the lower limit.Notably, the photocurrent intensity was still found to be enhanced by 6.7% when the dose rate was increased to 1.64 mGy air s −1 (the highest value available using the maximum power of 4500 W in our X-ray tube), as shown in Figure 4c.The detectable dose rate range was determined to be 0.29-1.64mGy air s −1 at 400 keV for our detector; more data acquired under medium-energy conditions are shown in Figure S27, Supporting Information.
Sensitivity usually declines with increasing X-ray energy, which is of particular concern for medium-/high-energy X-ray detection applications. [15]To assess the sensitivity degradation in the medium-energy region, the detector was exposed to 200-400 keV X-rays at the same dose rate as the control group, where the detector was exposed to 50 keV X-rays (Figure 4d).Sensitivity and current density in the medium-energy region were determined to be 10-40% lower than the values obtained at 50 keV (Figure 4d, inset), with a sensitivity of 61.51 μC Gy air −1 cm −2 (−50 V bias) achieved at 400 keV.X-ray detector signal stability is also critical for practical industrial imaging.
Our device was exposed to continuous X-rays (400 keV, 2.35 Gy air ), with response currents at different dose rates (0.29, 0.88, and 1.46 mGy air s −1 ) consistent before and after X-ray irradiation (Figure 4e and Figure S29, Supporting Information).This superior detection performance confirms that our BA 2 PbBr 4 single-crystal detector can competently detect X-rays with energies of hundreds of keV.
To investigate X-ray detection performance in the high-energy region, the BA 2 PbBr 4 single-crystal detector was exposed to 9 MeV X-rays, which approaches the beam energy limit set by the World Health Organization for actual industrial inspection applications. [44]s shown in Figure 5a, the response photocurrent of the singlecrystal device positively correlates with bias and dose rate, which indicates that the photocurrent saturation threshold had not been reached.Quasi-linear photocurrent responses at different biases are shown in the inset of Figure 5a, verifying that the BA 2 PbBr 4 singlecrystal detector responds effectively when exposed to 9 MeV X-rays; hence the maximum detectable energy of our detector is updated to 9 MeV in this work.A wide detectable dose rate range (1.02-15.24mGy air s −1 ) was determined for the BA 2 PbBr 4 single-crystal detector at a distance of 371 mm from the accelerator, with the following beam parameters: 9 MeV energy, 100 mA peak beam current, and 10-150 Hz pulse repetition frequency (Figure S31, Supporting Information).Remarkably, a dose rate of 209.67 mGy air s −1 at a distance of 100 mm (where the imaging object is located) and 150 Hz is of the same order of magnitude as the simulated and measured values for cargo scanning; [45] hence, the detectable dose rate range of our detector meets actual application requirements.
The sensitivity of our vertical device at various biases is presented in Figure 5b, which reveals that a higher bias leads to better sensitivity, with an optimal value of 46.90 μC Gy air −1 cm −2 (−100 V bias), which is much higher than that of CsPbBr 3 [46] (2.20 μC Gy air −1cm −2 at 1 V) at 6 MeV.Compared with the control group at 50 keV, our device exhibited consistent sensitivity degradation (65%) at 9 MeV at various biases (Figure S32, Supporting Information).To further evaluate the sensitivity of the BA 2 PbBr 4 single-crystal detector when exposed to high-energy X-rays, steel plates with various thicknesses (0.25 to 2.5 cm) were used as attenuators in front of our detector (Figure 5c, inset).As shown in Figure 5c, the response photocurrent decreased with increasing steel thickness, demonstrating that our device can effectively distinguish steel with a minimum thickness of 0.25 cm at 9 MeV.The increasingly lower signal also shows that detector response is mostly driven by the 9 MeV X-ray (instead of relatively low-energy scattered X-rays from other directions).This radiation-resistant capability is essential for high-energy X-ray detection, as shown in Figure 5d, in which the photocurrent was observed to fluctuate less than 1% using 9 MeV 9.0 Gy air radiation, with no obvious decline in the stepwise photocurrent observed at different dose rates before and after continuous irradiation.This stable performance when exposed to high-energy radiation at high bias is attributable to the high ionic diffusion barrier associated with the layered structure.Meanwhile, these results highlight an opportunity for exploring novel and high-performance industrial imaging devices using our 2D OIHPs single crystals with wide band-gap and less ionic migration.

Conclusion
We prepared a 2D BA 2 PbBr 4 single crystal with outstanding performance for low-energy (50 keV) X-ray detection and imaging.We also explored the use of the 2D OIHPs device to detect medium-/high-energy (200 keV to 9 MeV) X-rays for the first time.
The BA 2 PbBr 4 detector exhibited a high sensitivity of 726.18 μC Gy air −1 cm −2 , an ultralow detection limit of 8.20 nGy air s −1 , and outstanding operational stability when irradiated (88.71Gy air ) with 50 keV X-rays and at 86 AE 1.5% RH.At the same time, the good spatial resolution (4.3 lp mm −1 ) observed for our plane-scan imaging system highlights the feasibility of our single-crystal vertical device for highenergy X-ray imaging applications.The BA 2 PbBr 4 single-crystal device shows a wide detectable dose rate range with X-rays of up to 9 MeV in energy, and is sensitive below 9 MeV (46.90 μC Gy air −1 cm −2 at −1.16 V μm −1 ).Such sensitive and stable 2D OIHPs offer novel perspectives for exploring scintillation materials for industrial high-energy CT applications.

Experimental Section
Crystal growth and detector fabrication: Butylammonium bromide (BABr; >99.5%, Xi'an p-OLED corp.) and lead bromide (PbBr 2 ; >99.9%, Xi'an p-OLED corp.) were dissolved in 4 mL hydrobromic acid (HBr; 48 wt% aqueous solution; Aladdin Reagent Ltd.) at a 0.2:1 mass ratio and added to a Teflon-lined stainless steel autoclave.The mixture was heated at a constant temperature of 120 °C for 4 h, and then cooled to 20 °C at a programmed rate of 1 °C h −1 in a heating oven.Silver electrodes (1.0 Troy oz.; Structure Probe.Inc.) were sequentially fabricated on the top and bottom surfaces of a BA 2 PbBr 4 single crystal, and gold bonding wire (JC204C; JoBo) was attached to the silver electrodes as conductive testing wire.
Material characterization: Power XRD patterns were acquired using Cu Kα radiation (40 kV, 15 mA) with 0.02°steps on a Rigaku Miniflex600 diffractometer.Rocking curves were collected on a Rigaku SmartLab instrument with Cu Kα Xrays (40 kV, 30 mA).UV-Visible absorption spectra were obtained on a Hitachi U4100UV spectrometer in the 390-800 nm region.Thermogravimetric analyses were conducted on a Setaram Setsys16 thermal gravimetric analyzer under continuous nitrogen flow.Scanning electron microscopy images and EDS elemental maps were obtained on a Hitachi S4800 field-emission scanning electron microscope.Capacitances were tested using a Solartron SI-1260 impedance/gain-phase analyzer at a bias of 0.2 V.The temperature-conductivity curve was acquired on a Novocontrol Technologies broadband dielectric/impedance spectrometer at a bias of 1 V. Electrode area and crystal thickness were measured using a JTVMS-3020 video measuring system (Dongguan Jaten Precision Instrument Co., Ltd.).Optical images were acquired using a cell phone (HUAWEI Mate 20 pro) or a digital camera (Canon EOS 5D Mark IV).
Theoretical calculation: DFT calculations were performed using the Vienna ab initio simulation package (VASP) [47] and ionic potentials, including the effect of core electrons, were described using the projector augmented wave (PAW) method. [48]Perdew-Burke-Ernzerhf (PBE), generalized gradient approximation (GGA), and exchange-correlation (XC) functionals were used to relax the structural configurations and calculate the band structure. [49]The K-point pathway was defined as follows: G(0 0 0), X(0 0.5 0), S(−0.5 0.5 0), Y(−0.5 0 0).A planewave energy cutoff of 520 eV was used in all calculations.All structures were geometrically relaxed until the total force on each ion was below 0.01 eV Å−1 .The climbing image nudged elastic band (CI-NEB) method with the limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) optimizer. [50,51]was used to search for ion-diffusion pathways in BA 2 PbBr 4 .Initial and final configurations were obtained after full structural relaxation.The number of inserted images used in the CI-NEB calculations depended on the reaction coordinate between the initial and final configuration.
X-ray detection: To examine detector performance using low-energy X-ray radiation, we used a tungsten anode X-ray tube (Moxtek TUB00146-W06) to produce continuous 30-50 kV X-ray with a maximum output power of 12 W, and a home-built chopper fabricated from lead sheets (0.5 mm) was used to provide a pulsed X-ray beam.The low-energy X-ray dose rate was calibrated using a Radical AGDM+ dosimeter with a Radical 10X6-180 ion chamber, and a 2 mm Ruisen Optical JB400 attenuator was used to produce a low dose rate.To detect mediumenergy (200-400 keV) X-rays, we used a tungsten anode X-ray tube (COMET AG MXR-451) to provide continuous 200-400 keV X-rays with a maximum output power of 4500 W. To detect high-energy (9 MeV) X-rays, we used the 9 MeV linear accelerator at the Institute of High Energy Physics of the Chinese Academy of Sciences to provide continuous X-ray radiation.The medium−/high-energy X-ray dose rate was calibrated using a dosimeter (PTW UNIDOSE) with two ion chambers (PTW TW23361 and TW30013).A computer-controlled source meter (Keithley Model 2450) with a test fixture (Keithley Model 8101-PIV) was used to provide the bias voltage and collect current data.Low-energy X-ray experiments were performed in a shielded box constructed of 2-mm-thick lead plates to decrease environmental interference (such as light and magnetic fields), while experiments with medium-/high-energy X-ray were conducted in a dark shielded house with thick concrete walls to ensure no measurement disturbances and human safety.Temperature and humidity data were recorded using a TH22R-EX temperature and humidity logger (Shenzhen Inste Technology Co., Ltd.), and the 86 AE 1.5% RH environment was produced using saturated KCl solution.
X-ray imaging: In this study, imaging was preformed using a self-build planescan single-pixel X-ray imaging system.The system consisted of a 50-keV X-ray tube, a 2D mobile platform, an X-ray detector (a BA 2 PbBr 4 single crystal with a test fixture in a lead protective case), as well as a data acquisition system (a Keithley 2450 source meter controlled by computer).A 0.1-mm lead-equivalent linepair card (Type 81) and a 1-mm-thick tungsten edge (IEC 62220-1-3:2008) were used in this work.Other imaging objects (crab, data cable, chicken feet, and metallic plates) were purchased from the market and online shops (Taobao.com).All imaging was performed at room temperature in a dark environment, and remotely controlled by the researcher from outside the shielded house.

Figure 2 .
Figure 2. Low-energy X-ray detection performance of BA 2 PbBr 4 single-crystal devices.a) Sensitivity of the BA 2 PbBr 4 single-crystal detector at different biases.Inset: schematic showing the device structure.b) X-ray dose rate-dependent signal-to-noise ratio (SNR) at a bias of −10 V. c) Temporal X-ray response of the BA 2 PbBr 4 single-crystal detector.d) Operational stability of the BA 2 PbBr 4 singlecrystal detector over 50 repeated cycles of X-ray irradiation at 20 s time intervals (bottom) and continuous irradiation (top) at a bias of −10 V. e) Humidity stability of the BA 2 PbBr 4 single-crystal detector in an environment with 86 AE 1.5% relative humidity (RH).f) Temperature-dependent conductivity in the dark.g) Calculated energy profiles along two Br − migration paths in BA 2 PbBr 4 .h) Schematic showing Br − ionic migration paths.

Figure 3 .
Figure 3. Imaging performance of a BA 2 PbBr 4 single-crystal device in the plane-scan system.a) Schematic of the plane-scan single-pixel imaging system.b) X-ray image (top) and distribution curve (bottom) acquired during photocurrent stability testing over a 15 × 30 mm scan area.c) Optical images (top) and X-ray images with mean current curve (bottom) of materials with different densities.Optical (left) and X-ray (right) images of d) a crab irradiated at 470 μGy air s −1 (−100 V bias).e) a data cable irradiated at 350 μGy air s −1 (−80 V bias).Scale bars: 5 mm.f) X-ray image (top) and contour profile (bottom) of a line-pair card irradiated at 650 μGy air s −1 (−50 V bias).

Figure 4 .
Figure 4. Medium-energy X-ray detection performance of BA 2 PbBr 4 single-crystal devices.a) X-ray imaging applications classified according to photon energy.b) X-ray tube current-dependent response currents at various X-ray energies (−50 V bias).c) X-ray response current of the BA 2 PbBr 4 detector at five dose rates.d) Dose-rate-dependent X-ray current densities as functions of X-ray energy.Inset: ratio of sensitivity and current density at higher X-ray energies and 50 keV.e) Detection signal stability when continuously irradiated with 400 keV X-rays (−50 V bias).

Figure 5 .
Figure 5. High-energy X-ray detection performance of the BA 2 PbBr 4 single-crystal device.a) Voltagedependent response currents at various X-ray (9 MeV) dose rates.Inset: Dark current and photocurrent of the BA 2 PbBr 4 single-crystal detector.b) Dose-rate-dependent X-ray current densities at three biases.c) Response photocurrent curves acquired from steel plates with different thicknesses.Inset: optical image of six steel plates (0.25 to 2.5 cm).d) Signal stability of a BA 2 PbBr 4 single-crystal detector when exposed to 9 MeV X-rays (−50 V bias).