X-ray detector of covalent organic framework active layer based on metal ion anchoring and preparation method of X-ray detector

Abstract

The invention discloses an X-ray detector of a covalent organic framework active layer based on metal ion anchoring and a preparation method of the X-ray detector, and belongs to the technical field of radiation detection and sensing materials. A covalent organic framework active layer of the X-ray detector is COF366-M / CNTs, COF366 is a two-dimensional covalent organic framework which is formed by connecting a 5, 10, 15, 20-tetra (4-aminophenyl) porphyrin metal complex and terephthalaldehyde through an imine bond and has a specific square hole (sq) topology, and M is Cu or Co. According to the active layer material, metal ions (Cu < 2 + > or Co < 2 + >) are integrated to the center of a porphyrin unit and are polymerized with surrounding organic connectors (such as terephthalaldehyde) through imine covalent bonds, so that a rigid and conjugated two-dimensional extension framework is formed; the carbon nanotubes (CNTs) are uniformly dispersed in the composite active layer, so that an efficient conductive network is constructed, electron-hole separation is promoted, and the overall resistance of the material is reduced.

Description

Technical Field

[0001] This invention belongs to the field of radiation detection and sensing materials technology, specifically relating to a direct conversion X-ray detector, and particularly to an X-ray detector based on a metal ion-anchored covalent organic framework active layer and its preparation method. Background Technology

[0002] Since the discovery of X-rays, their applications in medical diagnosis, industrial non-destructive testing, and scientific research have become increasingly widespread. Direct conversion X-ray detection technology directly converts incident X-ray photons into charge carriers using semiconductor materials, offering advantages such as simplified structure, high photoelectric conversion efficiency, and avoidance of resolution loss due to visible light scattering, making it a key area of ​​development. However, existing mainstream direct conversion materials have many limitations: amorphous selenium (a-Se) and silicon have low attenuation coefficients for high-energy X-rays, limiting their sensitivity; single-crystal germanium and cadmium zinc telluride (CZT) are sensitive to defects during preparation, making it difficult to simultaneously achieve key indicators such as reproducibility, detection sensitivity, and detection limit, and they often contain toxic elements such as cadmium. While halide perovskite materials have attracted much attention in recent years due to their excellent performance, their soft lattice structure is prone to degradation or phase transition under humidity, light, or electric field stress, resulting in poor environmental stability, and many high-performance materials contain lead, posing environmental and health risks.

[0003] For direct-conversion X-ray detectors, a key stability challenge lies in the migration of metal ions under the operating electric field. Ion migration leads to dark current drift and defect accumulation, causing material performance degradation. Therefore, it is crucial to develop novel X-ray detection materials that combine high sensitivity, high stability, low detection limit, and environmental friendliness.

[0004] Covalent organic frameworks (COFs) are a class of crystalline porous materials composed of organic structural units linked by strong covalent bonds. They possess precisely designable molecular structures and excellent structural stability. Unlike perovskite or metal-organic frameworks, which are linked by dynamic ionic bonds, COF materials are entirely composed of covalent bonds. They do not contain migratable ions, and their three-dimensional framework provides clear pathways for charge carrier transport. This makes them an ideal platform for designing high-performance, highly stable X-ray detectors. Summary of the Invention

[0005] The purpose of this invention is to provide an X-ray detector based on a metal ion-anchored covalent organic framework active layer and its preparation method.

[0006] This invention aims to overcome the shortcomings of existing direct-conversion X-ray detectors in terms of stability (especially ion migration), environmental friendliness, and high performance, and provides a highly stable, low-dose X-ray detector based on a metal ion-anchored covalent organic framework active layer and its preparation method. This detector utilizes strong covalent bonds to firmly lock high atomic number metal ions (Cu, Co) to the porphyrin center of a two-dimensional COF, effectively enhancing X-ray attenuation while fundamentally suppressing ion migration, achieving a balance between high sensitivity, low detection limit, and exceptional stability.

[0007] This invention discloses an X-ray detector based on a metal ion-anchored covalent organic framework active layer and its preparation method. The covalent organic framework active layer is a metalloporphyrin-based two-dimensional covalent organic framework composite active layer COF366-M / carbon nanotubes (CNTs), wherein COF366 uniquely refers to a two-dimensional covalent organic framework with a specific square pore (sq) topology formed by linking 5,10,15,20-tetra(4-aminophenyl)porphyrin metal complex (M-TAPP) and terephthalaldehyde (TPA) through imine bonds, and M is Cu or Co. This active layer material utilizes metal ions (Cu... 2+ or Co 2+ The porphyrin unit is integrated into the center of the porphyrin unit and polymerized with the surrounding organic linkers (such as terephthalaldehyde) through imine covalent bonds to form a rigid, conjugated two-dimensional extended framework; carbon nanotubes (CNTs) are uniformly dispersed in the active layer of the COF366-M, with a mass percentage of 1~5% (preferably 2%), to construct an efficient conductive network, promote electron-hole separation, and reduce the overall resistance of the material.

[0008] The X-ray detector of the present invention has a sandwich structure, consisting of a top electrode, a COF366-M / CNTs composite active layer and a bottom electrode from top to bottom. The COF366-M / CNTs composite active layer is a wafer formed by a pressing method. A preferred device structure is Au / COF366-M / CNTs / BCP / Cr, where Au is the top electrode, BCP is the hole blocking layer and Cr is the bottom electrode.

[0009] The present invention discloses a method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer, comprising the following steps:

[0010] (1) Synthesis of COF366-M powder material

[0011] COF366-M powder material was synthesized using a solvothermal method: Equimolar amounts of terephthalaldehyde (TPA) and the 5,10,15,20-tetratetra(4-aminophenyl)porphyrin metal complex M-TAPP were thoroughly ground and mixed. The homogeneous solid raw material was then added to a pressure-resistant, sealed reaction vessel along with 1,2-dichlorobenzene, anhydrous ethanol, and a 5-8M aqueous solution of acetic acid. The specific proportions of TPA, M-TAPP, 1,2-dichlorobenzene, anhydrous ethanol, and the aqueous solution of acetic acid were as follows: The reaction mixture was prepared in the following proportions: 0.02-0.05 mol: 0.02-0.05 mol: 2-5 mL: 1-3 mL: 0.1-0.5 mL. The reaction system was then frozen in liquid nitrogen, evacuated, and flame-sealed before being placed in an oven at 110-130°C for 60-80 hours. After the reaction, the resulting solid product was filtered and washed continuously with tetrahydrofuran as solvent for 20-30 hours to remove unreacted monomers and solvent. Finally, the product was subjected to dynamic vacuum activation treatment at 110-130°C for 10-15 hours to obtain activated COF366-M powder with high crystallinity.

[0012] (2) CNTs composite and COF366-M / CNTs active layer molding

[0013] The COF366-M powder obtained in step (1) is thoroughly ground and mixed with multi-walled carbon nanotubes (CNTs) in a mortar to ensure that the CNTs are uniformly dispersed in the COF matrix, thus obtaining COF366-M / CNTs composite powder; the mass of COF366-M powder and multi-walled carbon nanotubes is calculated as 100%, wherein the mass percentage of multi-walled carbon nanotubes is 1~5%; then the COF366-M / CNTs composite powder is filled into a mold and held under a pressure of 0.8~1.2 tons for 8~15 minutes to prepare a dense and flat COF366-M / CNTs composite active laminate with a thickness of 2~3 mm;

[0014] (3) Electrode preparation and assembly

[0015] On both sides of the COF366-M / CNTs composite active laminate obtained in step (2), functional layers and electrodes are sequentially deposited by vacuum thermal evaporation to obtain the direct conversion X-ray detector. Preferably, an 8-15 nm thick hole blocking layer material, copper bath (BCP), is first deposited on one side of the composite active laminate as a functional layer and a 40-60 nm thick chromium (Cr) layer as a bottom electrode. Then, a 40-60 nm thick gold (Au) layer is deposited on the other side of the composite active laminate as a top electrode, finally obtaining a direct conversion X-ray detector with an Au / COF366-M / CNTs / BCP / Cr sandwich structure.

[0016] The process involves adding H2-TAPP and Cu(OAc)2H2O or Co(OAc)2·xH2O in a molar ratio of 1:3 to a mixed solvent of methanol, chloroform, and DMF under nitrogen protection. The mixture is then stirred at 70–90 °C under a nitrogen atmosphere for 20–30 hours. After cooling to room temperature, the mixture is washed several times with water, and the solvent is removed by rotary evaporation to obtain the 5,10,15,20-tetra(4-aminophenyl)porphyrin metal complex M-TAPP, i.e., 5,10,15,20-tetra(4-aminophenyl)porphyrin metal complex M-TAPP. 0-Tetra(p-aminophenyl)porphyrin copper(II) (Cu-TAPP) or 5,10,15,20-tetra(p-aminophenyl)porphyrin cobalt(II) (Co-TAPP) solid, wherein the ratio of H2-TAPP, Cu(OAc)2H2O or Co(OAc)2·xH2O, methanol, chloroform, and DMF is 0.2~0.4mmol: 1.0~1.5mmol: 15~30mL: 80~100mL: 20~40mL.

[0017] H2-TAPP (200 mg, 0.3 mmol) and Cu(OAc)2H2O (239.58 mg, 1.2 mmol) were added to a 250 mL three-necked round-bottom flask. Under nitrogen protection, a mixed solvent of methanol (20 mL), chloroform (90 mL), and DMF (30 mL) was added. The mixture was stirred at 80 °C under nitrogen atmosphere for 24 hours. After cooling to room temperature, the mixture was washed with water (3 × 100 mL) and the solvent was removed by rotary evaporation to obtain a dark purple solid, namely 5,10,15,20-tetra(p-aminophenyl)porphyrin copper(II) (Cu-TAPP, which is the key precursor raw material for the preparation of COF366-Cu in this invention), with a yield of about 80%.

[0018] The beneficial effects of this invention are as follows: it fundamentally solves the ion migration problem and exhibits excellent stability; the composite active layer is entirely composed of C, H, O, N, and environmentally friendly Cu or Co elements, and does not contain toxic heavy metals such as lead and cadmium, meeting the development requirements of green electronic devices; the composite active layer simultaneously possesses high resistivity and low trap density, overcoming the problem that traditional detection materials cannot simultaneously achieve high resistance and low defects; it exhibits excellent detection performance, with the introduction of high atomic number metals (Cu, Z=29; Co, Z=27) significantly improving X-ray attenuation capability (2.05 mm thick COF366-Cu attenuates 90% of X-rays at 49.1 keV); combined with the directional charge transport characteristics of COF ordered channels and the charge separation characteristics of CNTs, the device achieves high sensitivity (COF366-Cu reaches 11,784 μC·Gy). -1 ·cm -2 ) and low detection limit (minimum 39 nGy·s) -1 ). Attached Figure Description

[0019] Figure 1 This is a schematic diagram showing the synthesis route, structural model, and physical morphology of the metal ion-locked covalent organic framework material COF366-M (M=Cu, Co) described in this invention.

[0020] This figure clearly illustrates the complete construction process from molecular monomers to crystalline materials. On the left are the chemical structural formulas of two organic precursor monomers: 5,10,15,20-tetratetra(4-aminophenyl)porphyrin metal complex (M-TAPP) and terephthalaldehyde (TPA), with reaction arrows pointing to the two-dimensional layered structure model in the center. This model visually represents a covalent organic framework with a highly regular square-pore (sq) topology formed by imine condensation, where metal ions (M) are stably integrated at the center of the porphyrin ring. The upper right side shows a photograph of the synthesized COF366-Cu / CNTs material powder, while the lower right side shows a dense, uniform square wafer obtained by pressing the powder under high pressure. This wafer, placed on a grid substrate, visually demonstrates the material's good processability, laying the foundation for the subsequent fabrication of a uniform and stable active layer for devices.

[0021] Figure 2 The X-ray powder diffraction (XRD) pattern of the COF366-M (M=Cu, Co) material prepared in this invention;

[0022] The horizontal axis of the figure represents the diffraction angle (2θ, in degrees), and the vertical axis represents the diffraction intensity. The figure shows two clear diffraction curves, corresponding to COF366-Cu (upper curve) and COF366-Co (lower curve), respectively. Both curves exhibit a very sharp and extremely high-intensity main diffraction peak in the low-angle region, corresponding to the characteristic period of the most important interlayer packing or pore direction in the crystal structure. In the higher diffraction angle region, a series of clearly distinguishable secondary diffraction peaks can be observed, whose peak positions and relative intensities are highly consistent with the diffraction patterns calculated based on the material theory crystal structure (a two-dimensional sq topology with P4mm symmetry). The appearance of these characteristic diffraction peaks, especially the presence of strong low-angle peaks, clearly demonstrates the successful synthesis of highly crystalline COF366-M (M=Cu, Co) material with a long-range ordered structure using the solvothermal method described in this invention. The XRD patterns of COF366-Cu and COF366-Co show largely consistent peak positions, indicating that the overall framework structure of the material is maintained after the metal center is replaced by Co²⁺, demonstrating the universality and structural controllability of the synthesis method. This characterization result provides crucial structural evidence that the material of this invention possesses well-defined nanopores and efficient charge transport pathways.

[0023] Figure 3The infrared spectrum (FT-IR) of the chemical structure of the COF366-M (M=Cu, Co) material prepared in this invention is shown.

[0024] This figure consists of two sub-figures side-by-side, corresponding to the copper-containing system and the cobalt-containing system, respectively. The horizontal axis of each sub-figure represents the wavenumber, ranging from approximately 4000 to 500 cm⁻¹. -1 The vertical axis represents transmittance. Each subplot contains three distinct spectral lines from top to bottom, representing the reaction precursor terephthalaldehyde (TPA), the metallized porphyrin monomer M-TAPP, and the final product COF366-M, respectively. In the left plot (COF366-Cu), the TPA line is at approximately 1687 cm⁻¹. -1 A distinct carbonyl (C=O) stretching vibration characteristic peak is visible at approximately 3446 cm⁻¹; the Cu-TAPP line is also visible at approximately 3446 cm⁻¹. -1 and 3357cm -1 The characteristic stretching vibration peak of amino (NH) appears at [location missing]. However, in the final COF366-Cu product spectrum, the carbonyl and amino peaks attributed to the precursor completely disappear, while [location missing] at approximately 1620 cm⁻¹ [value missing]. -1 A novel absorption peak appeared, attributed to the stretching vibration of the imine bond (C=N). A completely consistent chemical bond change pattern was observed in the right-hand figure (COF366-Co): the carbonyl characteristic peak of TPA (~1687 cm⁻¹). -1 ) and the amino characteristic peak of Co-TAPP (~3335cm) -1 and 3202cm -1 The 366-Co COF line disappears completely, replaced by a line at approximately 1618 cm⁻¹. -1 A strong characteristic absorption peak belonging to an imine bond (C=N) appeared. In COF366-Cu and COF366-Co, the aldehyde group of the precursor TPA and the amino group of the precursor M-TAPP both underwent imine condensation reactions, generating covalent imine bonds (-C=N-) that connect the framework. This is the most direct chemical bond formation evidence confirming the successful synthesis of the target two-dimensional covalent organic framework COF366-M (M=Cu, Co) in this invention, indicating that the material possesses the expected chemical structure.

[0025] Figure 4 This is an Arrhenius curve of the ion migration activation energy of the material of the present invention;

[0026] This figure quantitatively characterizes the ability of two materials, COF366-Cu / CNTs (top) and COF366-Co / CNTs (bottom), to suppress ion migration by plotting the relationship between conductivity and the reciprocal of temperature (1000 / T). The horizontal axis in the figure represents 1000 / T (K). -1The range is from 2.4 to 3.6; the vertical axis is ln(σT), reflecting the logarithmic value of conductivity. Both curves show a good linear relationship in the high-temperature region (corresponding to the smaller value on the horizontal axis). By fitting this linear portion, the ion migration activation energy (Et) of COF366-Cu / CNTs and COF366-Co / CNTs was calculated. a The activation energies are 1.05 eV and 1.04 eV, respectively. These extremely high activation energies indicate that the metal ions (Cu)... 2 ⁺ or Co 2 (⁺) The metal ion is firmly locked within the porphyrin center and rigid covalent framework of the COF, requiring the overcoming of a significant energy barrier to migrate. This experimental data directly and quantitatively confirms the effectiveness of the "metal ion locking" design concept of this invention, explaining from a physical mechanism why the device can achieve ultra-low dark current drift and excellent operational stability, and is one of the key pieces of evidence supporting the core advantages of this invention.

[0027] Figure 5 This is a schematic diagram of the band structure arrangement of the X-ray detector described in this invention.

[0028] This figure clearly shows the relative energy levels of each functional layer in the device, with the vacuum energy level as a reference. From left to right: the gold (Au) bottom electrode, with a work function of approximately -5.1 eV; the valence band top (VBM) of the COF366-Cu / CNTs composite active layer at -5.1 eV and the conduction band bottom (CBM) at -2.4 eV, resulting in a calculated optical band gap of approximately 2.7 eV; the copper-containing hole-blocking layer (BCP), with its lowest unoccupied molecular orbital (LUMO) level at approximately -2.9 eV; and the chromium (Cr) top electrode, with a work function of approximately -6.4 eV. The arrows in the figure indicate the direction of increasing energy. The band structure diagram shows that the CBM (-2.4 eV) of COF366-Cu / CNTs is higher than the LUMO level (-2.9 eV) of BCP, while the LUMO of BCP is much higher than the work function of Cr. This energy level arrangement constitutes an efficient electron transport and extraction pathway. Simultaneously, the VBM (-5.1 eV) of COF366-Cu / CNTs matches well with the work function of the Au electrode, which is beneficial for hole collection. This optimized energy level matching is one of the key factors for achieving low dark current and high charge collection efficiency in the device.

[0029] Figure 6 This is a stability test graph showing the dark current density of the device of the present invention changing over time under a constant electric field;

[0030] This figure records the evolution of dark current density of COF366-Cu / CNTs and COF366-Co / CNTs-based devices under a continuous applied electric field over a period of 60 minutes. The vertical axis represents current density (nA·cm). -2The horizontal axis represents time (min). The two curves in the figure correspond to the two materials, and both are nearly parallel to the time axis, exhibiting extremely small fluctuations. By calculating the slope of the curves, the current drift (I_drift) of COF366-Cu / CNTs and COF366-Co / CNTs is as low as 1.07 × 10⁻⁶. -18 A·cm -2 ·V -1 ·s -1 and 3.12×10 -18 A·cm -2 ·V -1 ·s -1 This negligible current drift is a key indicator for evaluating the signal stability and reliability of direct-conversion X-ray detectors during long-term operation. The results in this figure are consistent with... Figure 1 The high activation energy data corroborate each other, intuitively demonstrating that the device based on ion-locked COF has exceptional electric field stability, fundamentally overcoming the performance degradation problem caused by ion migration in traditional perovskite and other materials, and ensuring the accuracy and repeatability of detector readings.

[0031] Figure 7 The current density-electric field relationship is shown as the effect of carbon nanotubes (CNTs) on the conductivity of COF366-Cu / CNTs composite material.

[0032] This figure compares the performance of COF366-Cu / CNTs devices with and without carbon nanotubes, revealing the crucial role of CNTs. The horizontal axis represents the electric field strength (V·cm). -1 The vertical axis represents the current density (nA·cm). -2 Both curves in the figure are straight lines passing through the origin, conforming to Ohm's law, and their reciprocal slope is the resistivity. Data shows that the resistivity (ρ) of the composite material with 2% CNTs by mass is 8.92 × 10⁻⁶. 11 The resistivity of COF material without CNTs is Ω·cm, while the resistivity of pure COF material without CNTs is as high as 9.17×10 Ω·cm. 12 The difference is more than an order of magnitude in Ω·cm. This comparative experiment clearly demonstrates that the introduction of CNTs constructs a highly efficient conductive network in the insulating / semiconductor framework of COF, significantly reducing the overall bulk resistance of the material and thus greatly improving the charge transport and extraction capabilities.

[0033] Figure 8 A double logarithmic coordinate plot of the trap state density of the material of the present invention determined by the space charge confinement current method;

[0034] This figure comprises two sub-figures, corresponding to Au / COF / Au symmetric structure devices based on COF366-Cu / CNTs (top) and COF366-Co / CNTs (bottom), respectively. The vertical axis of both sub-figures represents current density (nA·cm). -2 The graph uses a double logarithmic scale, with voltage (V) on the horizontal axis, to clearly illustrate different conduction mechanism regions. Each curve presents three typical regions: the linear ohmic region at low field, the trap-filled limit region (TFL Child) at medium field, and the trap-free space charge-limited current region at high voltage. The defect state (trap) density of the material can be calculated using the inflection point voltage (V_TFL) at the transition from the TFL region to the Child region. The calculation results show that the trap densities of COF366-Cu / CNTs and COF366-Co / CNTs are as low as 5.8 × 10⁻⁶. 8 cm - ³ and 5.2×10 8 cm - ³. The extremely low trap density indicates that the material has high-quality crystallinity and very few charge trapping centers. This ensures that photogenerated carriers can be efficiently collected and participate in conduction, rather than being trapped and recombinated by defects. This is an inherent structural advantage for achieving high charge collection efficiency (μτ) and low noise performance.

[0035] Figure 9 This is a current response diagram of the detector of the present invention under extremely low dose rate X-ray irradiation, used to determine its detection limit;

[0036] The upper and lower parts of the figure respectively show the COF366-Cu / CNTs and COF366-Co / CNTs detectors at 100 V·mm⁻¹. -1 The response to an extremely weak X-ray signal from a periodic switch under an electric field. The horizontal axis represents time (s), and the vertical axis represents current density (nA·cm). -2 When the X-ray is "ON", the current rises rapidly to a stable plateau value; when the X-ray is "OFF", the current falls rapidly back to the dark current baseline. For COF366-Co / CNTs devices, at 65 nGy·s -1 At extremely low dose rates, a clear and discernible signal above the noise level can be generated (steady-state photocurrent 0.91 nA·cm). -2 For COF366-Cu / CNTs devices, the temperature is 39 nGy·s. -1 Achieved 1.16 nA·cm at a lower baseline dose rate. -2 The response signal. This figure visually demonstrates the device's ability to detect extremely weak X-ray signals. By extending this type of signal-to-noise ratio analysis to different dose rates and calculating according to standard formulas, the results can be obtained. Figure 7The quantitative noise equivalent dose (NED) and detection limit (LoD) shown in this figure are direct experimental evidence for determining the detector’s ultra-high sensitivity and extremely low detection limit.

[0037] Figure 10 This is a key performance graph showing the carrier mobility-lifetime product of the detector in this invention and its relationship with the electric field;

[0038] This figure uses a dual Y-axis format, with the left vertical axis representing the photocurrent density (nA·cm). -2 The right vertical axis represents the product of carrier mobility-lifetime product and electric field (μτE, unit cm), and the horizontal axis represents the applied electric field (V·mm). -1 The two rising curves in the figure represent COF366-Cu / CNTs and COF366-Co / CNTs devices, respectively. By analyzing the relationship between photocurrent and electric field and dose rate, and fitting based on the Hecht equation, the carrier mobility-lifetime product (μτ) can be extracted. Figure 8 As shown, the μτ values ​​of COF366-Cu / CNTs and COF366-Co / CNTs are as high as 2.76 × 10⁻⁶. -4 cm 2 ·V -1 and 1.09×10 -4 cm 2 ·V -1 At 500V·mm -1 Under the electric field, their μτE products reach 1.38 cm and 0.55 cm, respectively. μτ and μτE are core parameters for measuring the charge collection efficiency of semiconductor detectors; larger values ​​indicate a longer drift distance for charge carriers before recombination, resulting in more efficient collection. The results in this figure demonstrate that the material of this invention possesses excellent charge transport and collection performance, which is one of the fundamental physical reasons for its ultra-high sensitivity.

[0039] Figure 11 This is a graph showing the relationship between the noise performance of the detector of this invention and the calculated equivalent noise dose;

[0040] This figure depicts the total noise power spectral density of the detector (N). T 2 Unit fA 2 ·Hz -1 ) with incident X-ray dose rate (nGy·s) -1 The curves representing the changes in COF366-Cu / CNTs and COF366-Co / CNTs are used to evaluate the device's ultimate detection capability. The figure includes two curves: COF366-Cu / CNTs and COF366-Co / CNTs. Both curves represent the dose-rate-independent plateau at low dose rates (primarily determined by the device's own 1 / f noise and thermal noise, denoted as N). D 2The dose rate consists of a linear rise at high dose rates (primarily determined by X-ray photon shot noise). The dose rate value corresponding to the intersection of the two curves (i.e., the dose rate at which the noise power equals the shot noise power) is calculated as the noise equivalent dose (NED). As shown in the figure, the NEDs of COF366-Cu / CNTs and COF366-Co are as low as 314 pGy and 691 pGy, respectively. These extremely low NED values ​​indicate that the electrical noise introduced by the detector itself is extremely low, thus enabling the resolution of extremely weak X-ray signal variations. This is why the device can achieve NEDs as low as tens of nGy·s. -1 The direct result of the detection limit (LoD) demonstrates its potential application in low-dose medical imaging and environmental monitoring.

[0041] Figure 12 This is a fatigue test diagram showing the long-term operational stability of the detector of this invention;

[0042] This figure records the X-ray output of a COF366-Cu / CNTs-based X-ray detector at a speed of up to 9.9 mGy·s⁻¹. -1 The current response after undergoing a periodic X-ray "ON-OFF" cycle test for over 16,000 seconds (approximately 4.5 hours) at a dose rate. The vertical axis represents the current density (μA·cm). -2 The horizontal axis represents time (s). Throughout the entire test, the device absorbed a total X-ray dose of up to 148 Gy. As shown in the figure, in each cycle, the device's photocurrent response (peak value in the "ON" state) and dark current baseline (valence value in the "OFF" state) remained highly stable, without any significant attenuation or drift. This result strongly demonstrates the exceptional radiation resistance and operational stability of the ion-locked COF-based detector. Even under extreme test conditions of high dose rates and long durations, its performance remains reliable, meeting the stringent requirements for detector durability and consistency in practical medical diagnostics and industrial testing, which is one of the outstanding advantages of this invention.

[0043] Figure 13 This is a schematic diagram of the self-built X-ray scanning imaging system used by the present invention to verify the imaging capability of the detector;

[0044] This diagram clearly illustrates the main components and workflow of the imaging system from top to bottom: At the top is the X-ray source, used to generate a cone-shaped X-ray beam; in the middle is the object to be imaged, placed on a self-made translation stage that can move precisely along the X and Y axes to achieve point-by-point scanning of the entire object; below is the COF366-M-based X-ray detector device prepared in this invention, used to receive the X-rays after they penetrate the object and convert them into electrical signals; at the very bottom is the source measure unit, using a Keithley-2400 source meter manufactured by Keithley Instruments, responsible for providing bias voltage to the detector and acquiring current signals; finally, the signals are transmitted to a computer for data processing and image reconstruction. This diagram clarifies the position and role of the detector in the imaging system, illustrating how its integration with the mechanical scanning and data acquisition system extends point detection capabilities to two-dimensional imaging functionality, providing... Figure 10 The actual imaging results provide background on the system architecture.

[0045] Figure 14 This is a demonstration image showing the actual X-ray imaging results of a hidden object using the COF366-Cu detector of this invention;

[0046] This image uses three sub-images to visually illustrate the imaging process and effect: the top sub-image is an optical photograph of a transparent glass bottle containing a metal bolt; the middle sub-image shows the glass bottle completely wrapped with opaque black tape, at which point the internal object is no longer visible to the naked eye; the bottom sub-image shows the process of imaging... Figure 11 The X-ray image obtained by the imaging system shown is clearly visible in the X-ray image below, despite being encased in opaque material. The outline of the glass bottle and the overall shape of the metal bolts inside are clearly discernible, with a high signal-to-noise ratio and uniform background. This demonstration experiment undeniably proves that the COF366-Cu / CNTs detector prepared in this invention not only possesses excellent point detection performance (high sensitivity, low detection limit), but also translates this performance into high-quality two-dimensional imaging capabilities. It exhibits high spatial resolution and excellent density contrast recognition ability, fully demonstrating its specific application value in fields such as medical radiological imaging, security inspection, and industrial non-destructive testing. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Unless otherwise specified, all raw materials and reagents involved are commercially available products conventional in the art.

[0048] Example 1: Fabrication and performance testing of COF366-Cu / CNTs-based X-ray detector

[0049] (a) H2-TAPP (200 mg, 0.3 mmol) and Cu(OAc)2H2O (239.58 mg, 1.2 mmol) were added to a 250 mL three-necked round-bottom flask. Under nitrogen protection, a mixed solvent of methanol (20 mL), chloroform (90 mL) and DMF (30 mL) was added. The mixture was stirred at 80 °C under nitrogen atmosphere for 24 hours. After cooling to room temperature, the mixture was washed with water (3 × 100 mL) and the solvent was removed by rotary evaporation to obtain a dark purple solid, which is 5,10,15,20-tetra(p-aminophenyl)porphyrin copper(II) (Cu-TAPP, which is the key precursor raw material for the preparation of COF366-Cu in this invention), with a yield of about 80%.

[0050] (b) Cu-TAPP (18 mg, 0.02 mmol) and terephthalaldehyde (10 mg, 0.02 mmol) were ground and mixed, and then loaded into a Pylex glass tube along with 1,2-dichlorobenzene (1 mL), anhydrous ethanol (1 mL), and 6M acetic acid aqueous solution (0.2 mL). The glass tube was rapidly frozen in a liquid nitrogen bath, evacuated, and then sealed, with a total length of approximately 15 cm. The reaction was carried out in a 120 °C oven for 72 hours. After filtration, the reaction product was washed with tetrahydrofuran in a Soxhlet extractor for 24 hours. Finally, it was activated at 120 °C under dynamic vacuum for approximately 12 hours to obtain activated COF366-Cu powder with a yield of approximately 70%.

[0051] (c) The COF366-Cu powder obtained in step (b) is ground and mixed with multi-walled carbon nanotubes in a mortar at a mass ratio of 98:2 (i.e., the mass percentage of multi-walled carbon nanotubes in the mixture of COF366-Cu powder and multi-walled carbon nanotubes is 2%) to obtain COF366-Cu / CNTs composite powder.

[0052] (d) Take an appropriate amount of the composite powder obtained in step (c), place it in a square mold with a side length of 4 mm, and press it under a pressure of 1 ton for 10 minutes to form a dense COF366-Cu / CNTs composite active laminate with a thickness of 2.05 mm.

[0053] (e) On one side of the COF366-Cu / CNTs composite active laminate obtained in step (d), a 10 nm thick BCP layer and a 50 nm thick Cr layer are sequentially deposited by vacuum thermal evaporation as the bottom electrode; on the other side of the COF366-Cu / CNTs composite active laminate, a 50 nm thick Au layer is deposited as the top electrode, thus completing the fabrication of the COF366-Cu / CNTs (Au / COF366-Cu / CNTs / BCP / Cr) based X-ray detector;

[0054] (f) Device performance was tested using a 50 keV X-ray source. The detection limit was 65 nGy·s. -1 The dark current drift is ~1.05×10⁻⁶. -18 A·cm -2 ·V -1 ·s -1 The sensitivity was measured to be 11,784 μC·Gy under an electric field of 500 V / mm. -1 ·cm -2 After absorbing a cumulative X-ray dose of 148 Gy, the photocurrent response showed no attenuation.

[0055] Example 2: Fabrication and Performance of COF366-Co / CNTs-based X-ray Detector

[0056] (a) H2-TAPP (200 mg, 0.3 mmol) and Co(OAc)2·xH2O (239.58 mg, 1.2 mmol; are the chemical formulas accurate? Are the amounts of raw materials accurate?) were added to a 250 mL three-necked round-bottom flask. Under nitrogen protection, a mixed solvent of methanol (20 mL), chloroform (90 mL), and DMF (30 mL) was added. The mixture was stirred at 80 °C under a nitrogen atmosphere for 24 hours. After cooling to room temperature, the mixture was washed with water (3 × 100 mL), and the solvent was removed by rotary evaporation to obtain a dark green solid, which is 5,10,15,20-tetra(p-aminophenyl)porphyrin cobalt(II) (Co-TAPP, which is the key precursor raw material for the preparation of COF366-Co in this invention), with a yield of about 80%.

[0057] (b) Co-TAPP (18 mg, 0.02 mmol) and terephthalaldehyde (10 mg, 0.02 mmol) were ground and mixed, and then loaded into a Pylex glass tube along with 1,2-dichlorobenzene (1 mL), anhydrous ethanol (1 mL), and 6M acetic acid aqueous solution (0.2 mL). The glass tube was rapidly frozen in a liquid nitrogen bath, vacuumed, and then sealed, with a total length of approximately 15 cm. The reaction was carried out in an oven at 120 °C for 72 hours. After filtration, the reaction product was washed with tetrahydrofuran in a Soxhlet extractor for 24 hours; finally, it was activated at 120 °C under dynamic vacuum for approximately 12 hours to obtain activated COF366-Co powder with a yield of approximately 70%.

[0058] (c) The COF366-Co powder obtained in step (b) is ground and mixed with multi-walled carbon nanotubes in a mortar at a mass ratio of 98:2 (i.e., the mass percentage of multi-walled carbon nanotubes in the mixture of COF366-Co powder and multi-walled carbon nanotubes is 2%) to obtain COF366-Co / CNTs composite powder.

[0059] (d) Take an appropriate amount of the composite powder obtained in step (c), place it in a square mold with a side length of 4 mm, and press it under a pressure of 1 ton for 10 minutes to form a dense COF366-Co / CNTs composite active laminate with a thickness of 2.05 mm.

[0060] (e) On one side of the COF366-Co / CNTs composite active laminate obtained in step (d), a 10 nm thick BCP layer and a 50 nm thick Cr layer are sequentially deposited by vacuum thermal evaporation as the bottom electrode; on the other side of the COF366-Co / CNTs composite active laminate, a 50 nm thick Au layer is deposited as the top electrode, thus completing the fabrication of the COF366-Co / CNTs (Au / COF366-Co / CNTs / BCP / Cr) based X-ray detector;

[0061] (f) Under the same test conditions, the COF366-Co based X-ray detector had a sensitivity of 7,953 μC·Gy at an electric field of 500 V / mm. -1 ·cm -2 The detection limit is 39 nGy·s -1 The dark current drift is ~3.12×10⁻⁶. -18 A·cm -2 ·V -1 ·s -1 .

[0062] Comparative Example 1: Performance of pure COF366-Cu (CNT-free) devices

[0063] (a) Without adding CNTs, the pure COF366-M powder obtained in step (b) of Example 1 is directly pressed into tablets and assembled into COF366-Cu (Au / COF366-Cu / BCP / Cr) based X-ray detectors with the same structure.

[0064] (b) Tests show that the resistivity of the CNTs X-ray detector is 9.17 × 10⁻⁶. 12 Ω·cm), compared to composite devices containing 2% NTs (8.92×10 Ω·cm). 11 Its charge collection capability is an order of magnitude higher than that of the composite device described in this invention (Ω·cm), and its charge collection capability is significantly lower than that of the composite device described in this invention.

[0065] The test results from Examples 1 and 2 show that the X-ray detector based on metal ion-locked COF and composite CNTs prepared in this invention achieves excellent levels in all key performance indicators. The COF366-Cu / CNTs device exhibits a high performance of 11,784 μC·Gy. -1 ·cm -2 Sensitivity and 65 nGy·s -1The detection limit was lower than that of COF366-Co / CNTs devices, which also achieved a detection limit of 7,953 μC·Gy. -1 ·cm -2 Sensitivity and 39 nGy·s -1 Both devices exhibit detection limits far exceeding those of commercial a-Se detectors. More significantly, they demonstrate exceptional operational stability, with dark current drift as low as ~10⁻¹⁰. -18 A·cm -2 ·V -1 ·s -1 The COF366-Cu / CNTs device exhibits performance degradation after being subjected to a cumulative radiation dose of up to 148 Gy, demonstrating its extremely strong radiation resistance and long-term operational reliability.

[0066] By comparing with the comparative example (pure COF366-Cu, without added CNTs), the superiority of the design concept of this invention is clearly verified. Although the comparative device is based on the same COF material, its resistivity is as high as 9.17 × 10⁻⁶ due to the lack of an efficient charge transport network constructed with CNTs. 12 The charge collection efficiency (Ω·cm) is an order of magnitude higher than that of the composite device in Example 1, which directly leads to a significant reduction in its charge collection efficiency and overall photoelectric response. This comparative result strongly demonstrates that the introduction of CNTs is crucial for overcoming the bottleneck of the relatively low charge mobility of pure COF materials, thereby achieving ultra-high sensitivity. Meanwhile, the extremely low current drift and ultra-high stability exhibited by the device in the examples are directly attributed to the effective locking of metal ions by the rigid covalent framework of COF, fundamentally suppressing harmful ion migration. Therefore, the comparison between the examples and the comparative examples fully demonstrates that the present invention, through the synergistic strategy of locking metal ions with a covalent framework and enhancing charge transport with carbon nanotubes, successfully unifies the traditionally difficult-to-achieve performance characteristics of high sensitivity, low detection limit, high stability, and environmental friendliness, providing an innovative material and device solution for next-generation low-dose, high-performance radiation detection.

[0067] Therefore, the core advantage of the metal-ion-locked covalent organic framework (COF)-based X-ray detector proposed in this invention lies in its successful unification of high performance and high stability through synergistic innovation in materials and structure. Furthermore, its environmentally friendly composition avoids the risks associated with toxic elements such as lead and cadmium. This work opens up a completely new materials platform for next-generation low-dose, highly stable, and green direct X-ray detection technology.

Claims

1. A method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer, comprising the following steps: (1) Synthesis of COF366-M powder material Equimolar amounts of terephthalaldehyde and the 5,10,15,20-tetra(4-aminophenyl)porphyrin metal complex COF366-M were thoroughly ground and mixed. The homogeneous solid raw material was then added to a pressure-resistant, sealed reaction vessel along with 1,2-dichlorobenzene, anhydrous ethanol, and a 5-8M aqueous solution of acetic acid. The reaction system was then subjected to liquid nitrogen freezing, vacuuming, and flame sealing, followed by constant temperature reaction at 110-130°C for 60-80 hours. After the reaction, the resulting solid product was filtered and continuously washed with tetrahydrofuran as solvent for 20-30 hours to remove unreacted monomers and solvent. Finally, the product was subjected to dynamic vacuum activation treatment at 110-130°C for 10-15 hours to obtain activated COF366-M powder with high crystallinity; M is Cu or Co. (2) CNTs composite and COF366-M / CNTs active layer molding The COF366-M powder obtained in step (1) is thoroughly ground and mixed with multi-walled carbon nanotubes (CNTs) in a mortar to ensure that the CNTs are uniformly dispersed in the COF matrix, thus obtaining COF366-M / CNTs composite powder; then the COF366-M / CNTs composite powder is pressed at a pressure of 0.8~1.2 tons for 8~15 minutes to prepare a dense and flat COF366-M / CNTs composite active laminate. (3) Electrode preparation and assembly On both sides of the COF366-M / CNTs composite active laminate obtained in step (2), functional layers and electrodes are deposited sequentially by vacuum thermal evaporation to obtain an X-ray detector based on a covalent organic framework active layer anchored by metal ions.

2. The method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer as described in claim 1, characterized in that: In step (1), H2-TAPP and Cu(OAc)2H2O or Co(OAc)2·xH2O in a molar ratio of 1:3 are added to a mixed solvent of methanol, chloroform and DMF under nitrogen protection; then the reaction is stirred at 70~90℃ under nitrogen atmosphere for 20~30 hours; after cooling to room temperature, the mixture is washed with water several times and the solvent is removed by rotary evaporation to obtain 5,10,15,20-tetra(4-aminophenyl)porphyrin metal complex COF366-M.

3. The method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer as described in claim 2, characterized in that: The dosage ratio of H2-TAPP, Cu(OAc)2H2O or Co(OAc)2·xH2O, methanol, chloroform, and DMF is 0.2~0.4mmol: 1.0~1.5mmol: 15~30mL: 80~100mL: 20~40mL.

4. The method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer as described in claim 1, characterized in that: In step (1), the ratio of the amounts of terephthalaldehyde, 5,10,15,20-tetra(4-aminophenyl)porphyrin metal complex, 1,2-dichlorobenzene, anhydrous ethanol, and aqueous acetic acid is 0.02~0.05 mol: 0.02~0.05 mol: 2~5 mL: 1~3 mL: 0.1~0.5 mL.

5. The method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer as described in claim 1, characterized in that: In step (2), the mass of COF366-M powder and multi-walled carbon nanotubes is calculated as 100%, wherein the mass percentage of multi-walled carbon nanotubes is 1~5%.

6. The method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer as described in claim 5, characterized in that: The mass percentage of multi-walled carbon nanotubes is 2%.

7. The method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer as described in claim 1, characterized in that: In step (2), the thickness of the COF366-M / CNTs composite active laminate is 2~3 mm.

8. The method for fabricating an X-ray detector based on a metal ion-anchored covalent organic framework active layer as described in claim 1, characterized in that: In step (3), firstly, a hole blocking layer material BCP with a thickness of 8~15nm is deposited as a functional layer and a Cr with a thickness of 40~60nm is deposited as a bottom electrode on one side of the composite active laminate; then, Au with a thickness of 40~60nm is deposited as a top electrode on the other side of the composite active laminate, to obtain a direct conversion X-ray detector with a sandwich structure of Au / COF366-M / CNTs / BCP / Cr.

9. An X-ray detector based on a metal ion-anchored covalent organic framework active layer, characterized in that: It is prepared by the preparation method described in any one of claims 1 to 8.

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