Introduction

Organic semiconductors are emerging as next-generation candidates for tissue-equivalent and biocompatible X-ray dosimeters/imagers. Their compatibility with emerging artificial intelligence (AI) technologies offers powerful support for real-time radiation safety control and mobile healthcare services, enabling multi-scenario applications such as home nursing for aging populations and rapid radiation monitoring in leakage-related emergencies1,2,3,4,5,6,7,8,9. As complementary alternatives to inorganic X-ray detectors (e.g., CdZnTe, CsPbBr3, and TlBr), which are often limited by their rigid, bulky form factors and toxic compositions10,11, organic detectors provide unique opportunities for flexible, lightweight, and safer devices.

Both indirect- and direct-type organic detectors have been extensively explored for X-ray detection and imaging applications12,13. Indirect-type detectors normally utilize an organic scintillator (such as thermally activated delayed fluorescence (TADF) organic small molecular materials) and a coupled photodetector, by which the incident X-ray beam is converted to visible light, then to charge signals14,15,16,17,18. An impressive imaging resolution of 50 lp mm−1 has been demonstrated using the organic scintillator with a necessary coupled-charged coupled device (CCD) camera, which induces a bulky imaging system and raises concerns for portable and wearable applications19. In contrast, the direct-type detector relies on an organic semiconductor that is ionized by the incident X-ray beam to produce charge signals, which is beneficial for high energy resolution, portability, as well as characteristics such as fast time response, a wide linear range, and compact size.

However, the practical deployment of direct-type organic X-ray detectors remains hindered by their inherently low detection sensitivity (S) and poor limit of detection (LoD). These shortcomings stem both from weak X-ray attenuation and limited charge carrier mobility20, which are intrinsic to their low-Z molecular composition and weak intermolecular interactions such as hydrogen bonding and van der Waals forces21,22,23,24,25,26. As a result, current organic detectors exhibit low responsivity, posing risks of inaccurate radiation dose monitoring and potential safety hazards during X-ray imaging.

Efforts over the past decade have focused on doping strategies that incorporate high-Z elements to enhance both X-ray absorption and S. For example, molecular engineering of TIPGe-pentacene has demonstrated a 22-fold increase in carrier mobility (from 0.018 to 0.4 cm2 V−1 s−1)27,28,29,30. High-Z nanoparticle doping strategies, such as Bi2O3 nanoparticle incorporation into P3HT:PCBM bulk-heterojunction films, have boosted the S up to 600 nC Gyair−1 cm−2 through enhanced X-ray scattering and absorption31,32,33,34,35. Furthermore, molecular tailoring approaches like replacing silicon with germanium in TIPS-pentacene36 have increased the S from 2.6 × 105 to 9.0 × 105 µC Gyair−1 cm−3. However, high-Z doping in organic semiconductors triggers three interlinked failure modes: interface defect accumulation, charge transport degradation, and temporal resolution collapse, collectively impairing detector reliability and temporal response. This multi-mechanism deterioration ultimately constrains radiation detection fidelity under dynamic operating, necessitating integrated organic detector designs that enhance S and spatial resolution while maintaining charge transport efficiency and medical regulatory compliance37.

Although organic X-ray detectors generally exhibit lower S compared to inorganic counterparts due to reduced absorption coefficients10,11,32,38,39,40,41,42, they allow performance tuning through charge transport engineering and dark current suppression. For instance, 4-hydroxycyanobenzene (4HCB, HO-C6H4-CN) has demonstrated low-dose detection down to 0.29 μGyair s−1 with a signal-to-noise ratio exceeding 3, attributed to its ultra-low dark current of 0.1 pA. However, its  S remains modest at ~25 μC Gyair−1 cm−2, limited by both low X-ray absorption and poor charge collection due to a hole mobility43 of only 3.4 cm2 V−1 s−1. This low mobility arises from 4HCB’s two-dimensional (2D) hydrogen-bonded crystal structure, where charge transport is inefficient and unstable. Additionally, the presence of the -CN group raises biocompatibility concerns23,24,43,44,45. These challenges highlight the potential of engineering stronger π-π interactions to replace weak hydrogen bonding, thereby enhancing charge mobility while maintaining the benefits of organic materials.

Here, we propose a dimensionality engineering strategy to optimize charge transport in organic crystals, thereby improving X-ray detection and imaging performance without compromising response speed, tissue equivalence, and biocompatibility. Specifically, we replace the -CN group in 4HCB with a -COOCH3 group, yielding methyl 4-hydroxybenzoate (4MHB, HO-C6H4-COOCH3) crystals. This modification induces a three-dimensional (3D) π-stacked structure with dense intermolecular interactions, enabling faster charge transport compared to the hydrogen-bonded 2D 4HCB crystals. High-quality 4MHB single crystals are synthesized via a low-temperature solution method and systematically characterized for charge transport behaviors using 241Am alpha particle and time-of-flight (ToF) measurements. Their X-ray detection capabilities are further evaluated under 20 kVp tube voltage, demonstrating enhanced S and resolution.

Results and Discussion

Dimensionality engineering of organic crystals

Organic semiconductors, composed of elemental constituents similar to those in the human body (e.g., C, H, O, and N), offer intrinsic tissue equivalence for X-ray detection. This unique feature enables the measured X-ray dose to closely approximate the actual absorbed dose in biological tissues, eliminating the need for complex dose calibration procedures (Fig. 1a and Fig. S1). However, the performance of tissue-equivalent X-ray detectors (TEXDs) remains limited by poor charge transport, primarily due to weak intermolecular van der Waals interactions.

Fig. 1: Dimensionality engineering of 4HCB and 4MHB pure-organic single crystals.
Fig. 1: Dimensionality engineering of 4HCB and 4MHB pure-organic single crystals.The alternative text for this image may have been generated using AI.
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a Chemical composition of organic materials and human bodies for tissue-equivalent X-ray detectors. b Molecular structures of 4HCB and 4MHB with solution-grown single crystals. c Crystallographic structure of 4HCB and 4MHB. d and e XRD patterns of 4MHB and 4HCB single crystals along preferred facets. f–h Raman spectra, DSC curves, and UV transmittance spectra with insets of Tauc plot fitting of the optical band gap of both crystals, respectively.

In this work, we proposed a dimensionality engineering strategy to enhance the X-ray detection performance of TEXDs, specifically improving S, LoD, and operational stability, by optimizing charge carrier transport pathways. As a model system, we investigated 4HCB (HO-C6H4-CN) crystals, which adopted a 2D crystallographic structure. In this structure, charge transport along the out-of-plane direction (c-axis) was predominantly mediated by head-to-tail hydrogen bonds (-OH···NC-) between adjacent molecules (Fig. 1b, c and Fig. S2). The inherent weakness of these hydrogen bonds resulted in low charge mobility, electrical instability, and limited radiation tolerance46,47. To address these challenges, we modulated the structural dimensionality of 4HCB by substituting the planar -CN group with a sterically bulky -COOCH3 moiety (Fig. 1b). This substitution induced a 3D molecular packing (Fig. 1c) through two synergistic mechanisms: (1) the steric bulk of the methyl group disrupted the 2D planar stacking, promoting molecular extension along non-coplanar directions; and (2) the introduction of the -COOCH3 group facilitated the formation of a robust hydrogen-bonding network (O-H···O, C-H···O, and C-H···π contacts), which stabilized the 3D framework while reducing the interplanar π-π stacking distance between adjacent benzene rings.

To elucidate the structural differences between 4HCB and 4MHB crystals, we analyzed their X-ray diffraction (XRD) patterns and vibrational spectra. The XRD measurements indicated that 4HCB crystallized in the orthorhombic system (Pbcn, a = 0.92 nm, b = 1.07 nm, c = 2.55 nm, β = 90°), while 4MHB adopted a monoclinic lattice (Cc, a = 1.3 nm, b = 1.73 nm, c = 1.22 nm, β = 129°), matching their theoretical lattice structures. The dominant crystal facets for solution-grown 4HCB and 4MHB were identified as (001) and (111), respectively (Fig. 1d,e). Notably, this structural transformation exemplified how targeted molecular tailoring enhanced the material stability, which is an essential criterion for radiation detection in harsh environments. Replacing the -CN group in 4HCB with a -COOCH3 group to form 4MHB significantly altered vibrational characteristics and intermolecular interactions, thereby improving thermal stability. Raman spectra (Fig. 1f) corroborated this transformation, that the characteristic C ≡ N stretching mode of 4HCB disappeared in 4MHB, being replaced by an intense C = O vibration, while enhanced π-π stacking signals indicated stronger intermolecular coupling, a critical factor for structural stability. Infrared (IR) spectra (Fig. S3) further supported these findings, with the C ≡ N band at ~2225 cm−1 in 4HCB completely absent in 4MHB, and replaced by sharp C = O stretching at ~1750 cm−1 and C-O-C stretching modes in the 1200–1300 cm−1 range, which were characteristic of ester functionality. These results highlighted the critical role of molecular packing in dictating the thermal stability of organic crystals.

Differential scanning calorimetry (DSC) analysis (Fig. 1g) revealed that 4MHB exhibited a melting point of 129 °C and no detectable phase transitions between 25 °C and 129 °C, underscoring its superior thermal stability compared to 4HCB (melting point: 115 °C). This enhanced robustness was attributed to the ester group in 4MHB, which promoted dipole-dipole interactions and rigidifies the molecular framework, thereby improving crystalline order. These findings highlighted the critical role of molecular design in tailoring structural stability for high-temperature applications. Additionally, the UV-Vis transmittance spectra (Fig. 1h) of both crystals were recorded, with their optical bandgaps (Eg) extracted using Tauc plots48, yielding values of 4.08 eV (4HCB) and 4.12 eV (4MHB). These wide bandgaps suggested effective suppression of thermally excited charge carriers, a property that minimized dark current and ensured low-noise operation under ambient conditions.

Charge transport of 4HCB and 4MHB organic single crystals

Crystallographic dimensionality, governed by molecular packing interactions, plays a pivotal role in determining the charge transport properties of organic single crystals. To elucidate this relationship, we systematically compared the electrical performance of 4HCB and 4MHB single-crystal detectors. Both detectors employed a sandwiched architecture with Au electrodes, where the cathode was affixed to the S-side (signal) using conductive carbon paste, while the anode was wire-bonded via a Cu connection to the G-side (ground) of a printed circuit board (Fig. 2a). Dark current density (J) versus electric field (E) characteristics were first evaluated (Fig. 2b). The extracted resistivity (ρ) was 1.94 × 1013 Ω cm for 4MHB, which was nearly two orders of magnitude higher than that of 4HCB (2.41 × 1011 Ω cm). In our previous research, Zhao et al. demonstrated that mechanical polishing enhanced 4HCB resistivity from 1012 to 1014 Ω cm, but introduced surface damage causing severe charge recombination and low charge collection efficiency43. In contrast, the as-grown 4MHB crystals intrinsically exhibited high surface quality due to the formation of stronger π-π bonds (Fig. S4), which is another benefit of molecular tailoring resulting in simultaneous enhancement of resistivity and charge transport properties for high-performance X-ray detection and imaging.

Fig. 2: Electrical properties of 4HCB and 4MHB single crystals.
Fig. 2: Electrical properties of 4HCB and 4MHB single crystals.The alternative text for this image may have been generated using AI.
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a Schematic diagram of detector structure with sandwiched Au electrodes and alpha-particle ToF characterization. b J-E curves of 4HCB and 4MHB detectors. c and d 241Am alpha pulses in 4HCB and 4MHB single crystals at a series of bias voltages (vertical gray lines indicate the fall time (tf)). e Pulse fall time histogram of 4HCB single crystals biased at −100 V. f Pulse fall time histogram of 4MHB single crystals biased at −200 V. g and h Hole mobility value extrapolation from the drift velocity of 4HCB and 4MHB vs. electric field linear plot. i and j Pulse height spectra obtained with digital pulse shape analysis of 4HCB and 4MHB at various bias voltages. k and l Gaussian peak centroid position of 4HCB and 4MHB vs. applied voltage plot and Hecht equation fitting. Data in (b, g, h, k, l) are presented as mean ± 5% error.

The charge transport dynamics were also measured by alpha (α)-particle-excited time-of-flight (ToF) method, which enables separate evaluation of electron-only and hole-only carrier drift dynamics, charge mobility (μ), and mobility-lifetime (μt) products49. Under negative bias voltage, α-particles penetrated from the cathode, generating electron-hole pairs within a depth of 19.1 μm (Fig. S5). The transient charge signals induced by hole drift between the electrodes were recorded to analyze hole transport behavior. For each α-particle event, the hole drift time under a given bias voltage was extracted from the falling edges of the transient charge pulses, and average drift times were determined using Gaussian statistical analysis of 30 α-particle events (Fig. S6). Hole drift times were systematically examined over a bias range from −100 to −600 V (Fig. 2c–f). The resulting hole mobilities exhibited a substantial enhancement in 4MHB crystals, reaching 19.91 cm2 V−1 s−1, compared to 8.93 cm2 V−1 s−1 for 4HCB (Fig. 2g, h). This improvement was attributed to the 3D π-π stacking in 4MHB, which facilitated enhanced intermolecular orbital overlap along the [110] crystallographic direction, in contrast to the hydrogen-bond-driven charge transport along the [001] direction in the 2D 4HCB structure.

The α-particle response pulse height spectra revealed superior energy resolution in 4MHB detectors compared to 4HCB (Fig. 2i, j). The μτ products, calculated using the Hecht equation49, further highlighted the advantage of 4MHB, with its hole mobility-lifetime product (1.7 × 10−4 cm2 V−1) exceeding that of 4HCB (8.1 × 10−5 cm2 V−1) by 2.1-fold (Fig. 2k, l). The molecular tailoring strategy resulted in the stronger π-π bonds between the neighboring 4MHB molecules, which induced not only better charge transport properties, but also better surface quality to reduce the leakage current and charge recombination. Therefore, both high bulk resistivity and excellent μτ products could be achieved simultaneously in as-grown 4MHB crystals, enabling superior X-ray detection and imaging.

X-ray detection and imaging performances

The 4MHB detector exhibited ultralow dark current density and enhanced charge collection efficiency, both of which benefited X-ray detection and imaging performance, especially for low-dose irradiation. The X-ray photocurrent vs. time (I-t) curves were measured under 20 kVp X-ray exposure (dose rate: 131.5 to 199.5 μGyair s−1). As shown in Fig. 3a, the 4MHB detector demonstrated a higher photocurrent response and substantially lower dark current compared to the 4HCB detector. This performance enhancement stemmed from molecularly engineered 3D π-orbital interactions in 4MHB, which simultaneously optimized charge collection efficiency and suppressed dark current through improved charge transport properties. Specifically, the 4MHB detector achieved an on-off ratio approximately 100 times higher than 4HCB detectors under high electric fields, a standard operating condition for radiation detection systems.

Fig. 3: X-ray detection characterizations of 4HCB and 4MHB detectors.
Fig. 3: X-ray detection characterizations of 4HCB and 4MHB detectors.The alternative text for this image may have been generated using AI.
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a I-t curves of 4HCB and 4MHB with the dose rate of 131.5–199.5 μGyair s−1 and switching X-ray on and off under 10 V mm−1. b Normalized I-t curves for 4HCB and 4MHB response time comparison at 58.76 μGyair s−1 under 10 V mm−1. c The On-off ratio of 4MHB and 4HCB detectors varying the electric field. d Photocurrent density (J) changes with irradiated X-ray dose rate at bias voltage from 10−200 V. e S for 20 kVp X-ray at a series of bias voltage. f I-t curve of 4MHB detector at the bias of 30 V and varying X-ray dose rates from 33.2 to 96.9 nGyair s−1, with the insertion showing the enlarged transient I-t curve at dose rate of 33.2 nGyair s−1. g Relationship between SNR and D. h The hole mobility-resistivity comparison of 4MHB in this work and other materials. i The S-LoD comparison of 4MHB in this work and other X-ray detector materials. Data in (d,g) are presented as mean ± 5% error.

The I-t curves of both detectors were obtained using a chopper-modulated X-ray beam (Fig. 3b), revealing that 4MHB exhibited a faster rise time of 83 ms compared to 130 ms for 4HCB, which was attributed to its superior hole mobility. Furthermore, the 4MHB detectors demonstrated a 100-fold higher X-ray on-off ratio than 4HCB across applied electric fields of 0–100 V mm−1 (Fig. 3c). The X-ray S of 4MHB and 4HCB was quantified by linearly fitting the photocurrent density versus incident dose rate curves under bias voltages ranging from 10 to 200 V (Fig. 3d and Fig. S7), with S values plotted in Fig. 3e. The 4MHB detector showed bias-dependent S enhancement below 100 V, followed by response saturation due to field-enhanced suppression of charge recombination, finally reaching 226.1 μC Gyair−1 cm−2 at 200 V. Furthermore, the 4MHB detector demonstrated an X-ray detection dose rate of 33.2 nGyair s−1 with a signal-to-noise ratio (SNR) of 29.8. Additionally, we deduced the LoD by extrapolating the D and the corresponding SNR curve, obtaining a LoD of 4.22 nGyair s−1 for the 4MHB detector (with SNR of 3, according to the IUPAC definition)50 as shown in Fig. 3f, g. This value is the lowest reported LoD among organic semiconductor X-ray detectors, thanks to the exceptional low dark current of 0.014 pA and the high hole mobility of 19.91 cm2 V−1 s−1. The outstanding X-ray S of the pure organic 4MHB detector arose from high charge collection efficiency, which was attributed to its high resistivity and excellent hole mobility. This mobility surpassed not only other organic semiconductors in the Table 1 but also lead-based perovskite detectors, resulting in the low charge recombination rate (Fig. 3h). Furthermore, 4MHB demonstrated an ultralow LoD of 4.22 nGyair s−1. As shown in Table 1, this LoD is significantly lower than other organic materials, being approximately 4.7 times better than 4HPA (20 nGyair s−1)37, and 1175 times better than DABCO-NH4Br3 (4960 nGyair s−1)51 (Fig. 3i). Coupled with its tissue-equivalent composition and biocompatibility, 4MHB demonstrated great potential for low-dose medical imaging, offering a high-performance, non-toxic alternative.

Table 1 X-ray detection performances of organic, perovskite, and Bismuth-based semiconductors
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Due to the 3D crystallographic structure and the enhanced π-π bonds, 4MHB detectors demonstrated great operational stability, with an ultralow dark current drift of 1.14 × 10−10 nA cm−1 s−1 V−1 at 100 V mm−1 (Fig. 4a, Fig. S8 and Table S1). This value even surpassed those of high-performance lead-based perovskite detectors, such as (BDA)PbI4 (6.06 × 10−9 nA cm−1 s−1 V−1)52, and bismuth-based perovskite detectors such as (R/S-PPA)2BiI5 (1.0 × 10−4 nA cm−1 s−1 V−1)53 by several orders of magnitude (Fig. 4b, Table 1). This breakthrough extended across a broad operational voltage window (10–150 V mm−1), where synchronized photocurrent enhancement and sub-picoampere dark current suppression validated its voltage-adaptive stability (Fig. 4c). Environmental durability was evidenced through rigorous cyclic testing (60 cycles over 3500 s) showing <5% signal variation under atmospheric conditions, complemented by long-term air-exposure stability with <5% photocurrent degradation after 42 days (Fig. 4d, e). Continuous X-ray endurance tests revealed robust performance under sustained irradiation at 33.21, 45.61, and 57.98 μGyair s−1 (15000 s drift <1 × 10−7 nA cm−1 s−1 V−1) and repetitive 8-h exposure cycles at 10.02 mGyair s−1, maintaining consistent photocurrent response through multiple irradiation intervals (Fig. 4f, g). The unified stability profile originated from 4MHB’s 3D π-π stacked architecture, which simultaneously restricted ionic defect migration and facilitated efficient charge transport through reinforced intermolecular interactions.

Fig. 4: Stability of organic detector.
Fig. 4: Stability of organic detector.The alternative text for this image may have been generated using AI.
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a Dark current drift curves of 4MHB and 4HCB with continuously 7300 s working. b Dark current drift of 4MHB with comparison to other materials. c I-t curves of 4MHB detector with 58.76 μGyair s−1 X-ray irradiation with varying bias voltages. d I-t curves of long-term work stability at bias voltage of 30 V (right panel is the extracted one on-off cycle from left figure to show the detailed rise and fall stages corresponding to X-ray on and off). e Comparison of photocurrent-dose rate curves before and after storage of 42 days, with inset showing I-t curves after 42-day storage. f Photocurrent stability of 4MHB under continuous X-ray irradiation at different dose rates (57.98, 45.61 and 33.21 μGyair s−1), with inset showing current drift rate at 1000 s. g Photocurrent cycling stability of 4MHB under continuous X-ray exposure (dose rate: 10.02 mGyair s−1) with 1-h irradiation intervals.

In addition, the 4MHB detector demonstrated exceptional stability at 58.76 μGyair s−1, validating its capability for high-fidelity imaging in low-dose applications such as pediatric imaging and longitudinal tumor monitoring. Here, we utilized a custom-built imaging system comprising an X-ray source, a 3D motion platform with a mounted imaging object, and a 4MHB imager sensor for grayscale resolution assessment by current-mode imaging process (Fig. 5a). Imaging resolution was quantified by calculating the modulation transfer function (MTF) vs. spatial resolution (lp mm−1) using the edge spread function (ESF) and line spread function (LSF) measured with a sharp-edge surface15,51,54,55. Spatial resolution characterization utilized a lead (Pb) resolution test pattern (Fig. 5b). The image quality was influenced by system parameters, including the scanning step size of the moving stage and the applied bias voltage across the detector, both of which were carefully optimized through experimental evaluation (Fig. 5c, Fig. S9 and Table 2). The 4MHB detector outperformed 4HCB in both spatial resolution (1.6 lp mm−1 vs. 1.0 lp mm−1) and MTF (2.2 vs. 1.3 lp mm−1) (Fig. 5d,e), validated through LSF Fourier analysis (Fig. S10). As shown in Fig. 5f, g, we demonstrated the low-dose X-ray imaging of a metal nut by the 4MHB detector under the exposure dose rate down to 4.72 μGyair s−1. Furthermore, we achieved X-ray imaging of a complex object (plastic capsule with an inserted metal wire) at 1.13 μGyair s−1, in which the components with different densities could be clearly recognized by 4MHB detectors, indicating the high imaging contrast (Fig. 5h). This value is around five times lower than the medical safety standards (5.5 μGyair s−1)56,57, exhibiting great potential for low-dose and human-friendly medical diagnosis. The excellent low-dose and high-contrast imaging capabilities of 4MHB detectors were attributed to the ultra-low dark current drift and efficient charge collection.

Fig. 5: X-ray imaging by organic imager sensors.
Fig. 5: X-ray imaging by organic imager sensors.The alternative text for this image may have been generated using AI.
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a Schematic diagram of the home-made imaging system. b MTF vs. spatial resolution curves of 4MHB and 4HCB sensors. c Effects of imaging parameters on image resolution. d, e X-ray imaging of Pb-resolution pattern card for determining the spatial resolution of 4MHB and 4HCB imager sensors. f X-ray imaging of a nut at 11.74 μGyair s−1. g X-ray imaging of a nut at 4.72 μGyair s−1. h X-ray imaging of plastic capsule with an inserted metal wire at 1.13 μGyair s−1.

Table 2 Effects of imaging parameters on image resolution
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In conclusion, this study demonstrated that modulating the dimensionality of pure organic crystals can significantly enhance their detection and imaging capabilities for low-dose X-ray beams by simultaneously suppressing dark current and improving charge transport properties. The superior electrical properties were attributed to the formation of 3D π-π intermolecular packing in 4MHB crystals, achieved by introducing a 3D-extended -COOCH3 group to replace the planar -CN group in the 2D structure of 4HCB. As a result, the 4MHB single-crystal detector exhibited outstanding hole mobility of up to 19.91 cm2 V−1 s−1, a high mobility-lifetime product of 1.7 × 10−4 cm2 V−1, and an ultralow dark current drift of 1.14 × 10−10 nA cm−1 s−1 V−1 at 100 V mm−1. These properties surpass those of 4HCB and other organic semiconductors, enabling stable and reliable device operation over extended periods. The 4MHB detector demonstrated a rapid X-ray rise time of 83 ms and a record LoD of 4.22 nGyair s−1 among all organic detectors, enabling a high spatial resolution of 1.6 lp mm−1 at the low X-ray exposure dose rate of 58.76 μGyair s−1. Notably, the enhancement in X-ray detection and imaging capability was achieved without introducing any heavy or toxic elements, thereby preserving the tissue equivalence and biocompatibility essential for organic X-ray detectors.

Methods

Materials and Crystal Growth

The 4HCB (4-hydroxycyanobenzene, 98%) and 4MHB (methyl 4-hydroxybenzoate, 99%) were purified via multi-step recrystallization to enhance material quality. For 4MHB, 0.5–2.0 g of pre-purified powder was dissolved in 10 mL of acetone under continuous stirring until completely dissolved, as depicted in Fig. S11. The resulting solution was filtered through a 0.42-μm disposable organic filter to remove impurities and then transferred into a clean, dry beaker. The beaker was covered with two layers of perforated plastic wrap and placed in a refrigerator at 0 °C for 36–48 h to allow slow evaporation of acetone. This process initially yielded seed crystals, which were subsequently used for further growth in a saturated solution. The seed crystals were introduced into a freshly prepared saturated solution, and conditions, such as temperature and evaporation rate, were carefully controlled to promote uniform growth, resulting in large, high-quality single crystals. For 4HCB, a similar crystallization method was employed, using diethyl ether as the primary solvent and 2 mL of petroleum ether as a cosolvent to enhance crystallization efficiency. The solution concentration was adjusted to 0.012 g mL−1, and crystal growth was conducted at 0 °C. This study successfully prepared 4HCB crystals with high structural integrity by leveraging precise control of the evaporation process and solution composition.

Structural and Thermal Property Characterization

Powder and single-crystal XRD were conducted to determine the crystallinity and structure of 4MHB and 4HCB. Measurements were carried out using Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA, with a scanning range of 5°–40° and a scanning speed of 5° min−1. Ultraviolet-visible (UV-Vis) spectroscopy was used to characterize the optical properties and calculate the band gaps of the crystals. Measurements were performed in the wavelength range of 200–800 nm using a Shimadzu UV-2600 spectrometer. Fourier-transform infrared (FTIR) spectroscopy was performed to identify functional groups and molecular vibrations. The spectra were recorded with a Shimadzu IRTracer-100 spectrometer over the range of 800–4000 cm−1 at a resolution of 4 cm−1. Raman spectroscopy was conducted to detect vibrational modes and validate the molecular structure of the crystals. Measurements were carried out using a Horiba LabRAM HR Evolution spectrometer with a 325 nm excitation source, scanning the range of 50–4000 cm−1. Thermal behavior was evaluated using DSC. The measurements were conducted under a nitrogen atmosphere, with heating and cooling rates of 5 °C min−1 over a temperature range of 20 °C–150 °C.

Electrode Fabrication and Electrical Measurements

Gold electrodes were deposited onto the crystal surfaces via vacuum evaporation method to form ohmic contacts. The crystal was carefully mounted on a printed circuit board (PCB) using carbon glue to establish the cathode connection. Fine copper wires were used to bond the gold electrode on the crystal surface to another pin on the PCB, serving as the anode. Current-voltage (I–V) characteristics were measured to determine the resistivity of 4MHB and 4HCB crystals. Charge carrier mobility was characterized using α-particle excitation from a 241Am source (5.49 MeV) under ambient conditions. The source and detector were positioned within a copper-shielded chamber at 1.5 cm separation to minimize environmental interference. To ensure the α-particle energy at this specific distance was sufficient for effective excitation, its energy attenuation was modeled utilizing the SRIM software package. ToF measurements were performed by recording drift times at applied voltages ranging from –100 to −600 V, enabling calculation of the mobility-lifetime (μτ) product via Hecht equation fitting.

The resistivity (ρ) can be calculated using the following formula:

$$\rho=\frac{{SV}}{{LI}}$$
(1)

where I is the current, V is the applied voltage, L is the interelectrode distance, and S is the electrode contact area.

The charge carrier mobility (μ), which quantifies the efficiency of photogenerated charge transport under an electric field, is derived from:

$$\mu=\frac{{V}_{{\mbox{dr}}}}{E}=\frac{{d}^{2}}{V\times {t}_{{\mbox{f}}}}$$
(2)

where Vdr is the drift velocity, E is the electric field (E = V / d), d is the electrode distance, V is the bias voltage and tf is the drift time.

The mobility-lifetime product (μτ), a critical figure of merit for charge collection efficiency (CCE), is quantified through the Hecht equation:

$$\eta=\frac{\mu \tau E}{d}(1-\exp (-\frac{d}{\mu \tau E}))$$
(3)

where E is the electric field intensity, d is the electrode distance, and μτ is the mobility-lifetime product of the detector.

X-ray response and stability testing

X-ray detection properties were evaluated using an Amptek Mini-X-ray tube with an Ag target. The 20 kVp operational voltage was selected based on 4MHB’s superior X-ray attenuation characteristics at this energy level, as evidenced by its photoelectric absorption dominance and thickness-dependent attenuation efficiency in the practical crystal growth range (0–6 mm) shown in Fig. S1c, d. Measurements were conducted at tube voltages of 20 kVp, with dose rates varied to assess S and response. Detection sensitivity (S), a key parameter for low-dose applications, was calculated using49:

$$S=\frac{\Delta J}{D}$$
(4)
$$\Delta J=\frac{{I}_{{\rm{ON}}}-{I}_{{\rm{OFF}}}}{s}$$
(5)

where S represents sensitivity, ION and IOFF are measured current of the detector with/without X-ray irradiation respectively, D is the dose rate of the incident X-ray beam, and s is the electrode area.

The SNR was calculated as:

$${SNR}=\frac{{I}_{{\mbox{signal}}}}{{I}_{{\mbox{noise}}}}=\frac{{\bar{I}}_{{\mbox{ON}}}-{\bar{I}}_{{\mbox{OFF}}}}{\sqrt{{\frac{1}{N}}{\sum }{\left({I}_{{{\rm{i}}}}-{\bar{I}}_{{\mbox{ON}}}\right)}^{2}}}$$
(6)

where Isignal is the signal current, Inoise is the noise current, ĪON is the average value of the photocurrent, and ĪOFF is the average value of the dark current.

Current drift was calculated using Supplementary Equation 1. Dark current stability was monitored over 2 h under a constant electric field, and cyclic stability was tested with 60 on-off cycles. Long-term stability was assessed after 42 days of storage under ambient conditions (25 ± 2 °C/1 atm). Continuous X-ray endurance assessment of 4MHB’s photocurrent cycling stability was performed under sustained irradiation (dose rate: 10.02 mGyair s−1) with cyclically programmed 1-h exposure periods.

Imaging experiments

The X-ray imaging capabilities of the 4MHB detector were evaluated using a custom-built system comprising an X-ray source, a 3D motion platform, and the detector. A line pair phantom was used to determine spatial resolution by counting distinguishable line pairs per millimeter, consisting of alternating leaded and non-leaded segments. To derive the MTF(ʋ), the measured photocurrent curve is first fitted with Boltzmann function to obtain the Edge Spread Function (ESF(x)). The Line Spread Function (LSF(x)) is then calculated from ESF(x) and fitted with Gaussian function. Finally, the MTF(ʋ) is derived by performing Fast Fourier Transform (FFT) on LSF(x):

$${{\mbox{LSF}}}(x)=\frac{d}{{dx}}{{\mbox{ESF}}}(x)$$
(7)
$$MTF(\upsilon )=\frac{1}{\sqrt{2\pi }}{\int }_{-\infty }^{\infty }LSE(x)\,{e}^{i2\pi \upsilon x}\,dx$$
(8)

where ʋ is spatial frequency, and x is distance.

During imaging, the motion platform provided precise control over object movement, while photocurrent variations were recorded at each scanning step. Practical imaging tests, such as imaging a nut and the plastic capsule with an inserted metal wire, validated the detector’s ability to resolve sub-millimeter structural details with high contrast.