BiOCl single crystal, growth method thereof and application of BiOCl single crystal in X-ray detector

Abstract

The invention discloses a BiOCl single crystal and a growth method thereof and application of the BiOCl single crystal in an X-ray detector, the BiOCl single crystal is prepared by matching raw materials with different purities and a proper transport agent by adopting a vapor phase method, a millimeter-level large-size two-dimensional layered structure single crystal with optical transparency and small intrinsic polaron can be obtained, and the material is stable in performance, good in stability, good in stability and high in reliability. The repeatability is good. The obtained BiOCl single crystal has a wide band gap and high resistivity, so that an X-ray detector prepared from the BiOCl single crystal has low dark current and excellent stability and environmental durability, the crystal prepared from a high-purity raw material has the best quality, small polaron transition is used as a main mechanism for electrical transport, the detection performance is excellent, the carrier mobility lifetime product is up to 2.42 * 10 <-4 > cm < 2 >. V <-1 >, and the X-ray detector has good application prospects. The highest sensitivity reaches 3.53 mC.Gyair-1. Cm <-2 >, and the lowest detection dose is 6.0 nGyair.s <-1 >; the device is simple in structure, low in cost and suitable for large-scale production, can be compatible with an existing electronic packaging process, and can be widely applied to the fields of medical imaging, safety inspection, industrial detection and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to a detection device, in particular to a BiOCl single crystal and a growth method thereof and application in an X-ray detector. BACKGROUND

[0002] X-ray detectors have important applications in the fields of medical imaging, security inspection, industrial detection and scientific research, from promoting the development of physics and materials science, to industrial and security fields, to low-dose medical imaging escorting human health, and have a profound impact. Especially the low-dose medical imaging, in the process of disease diagnosis, reduces the radiation exposure of patients in the weak signal to obtain key detection data, and early detection and early treatment saves countless lives. At present, the commonly used X-ray detector materials mainly include traditional detection materials selenium (Se), silicon (Si), cadmium zinc telluride (CdZnTe) and lead-based halide perovskite. Although these materials have played a role in practical application, there are still obvious deficiencies, for example: the charge transport performance is insufficient, the amorphous selenium, silicon and other materials have strong carrier recombination effect, resulting in low charge collection efficiency and limited sensitivity; although some halide perovskite materials have high sensitivity, they have poor chemical stability and are easily affected by the environment and degrade, which is difficult to work for a long time; the heavy metal materials containing Pb, Hg and Cd have potential harm to the environment and human body, which does not meet the needs of green environmental protection and sustainable development; the detector process is complex and the cost is high, and the crystal growth process of some materials is complex and the device preparation is difficult, which limits its large-scale application. Therefore, it is an urgent technical problem to be solved in the current X-ray detection field to develop a non-toxic, stable, excellent charge transport performance and low dark current X-ray detection new material, and to support a high-quality crystal growth method and a low-cost device preparation process. SUMMARY

[0003] The purpose of the present application is to improve the detection sensitivity of the X-ray detector and reduce the minimum detection dose, and a BiOCl single crystal for preparing X-rays and a growth method thereof are provided. Another purpose of the present application is to provide an X-ray detector made of the above BiOCl single crystal, and to provide the application of the above detector in medical imaging, security inspection and industrial detection.

[0004] Technical scheme: The BiOCl single crystal provided by the present application is prepared by chemical vapor deposition, has a two-dimensional layered structure, a size of millimeter level, optical transparency and intrinsic small polaron transport structure, a band gap of 2.85eV-3.17eV, and a carrier mobility lifetime product of 1.10x10 -5 cm 2 ·V- 1 ~2.42x10 -4 cm2 ·V- 1 .

[0005] The application further provides a growth method of the BiOCl single crystal, comprising the following steps:

[0006] (1) mixing BiOCl powder with a transporting agent and placing in a vacuum environment;

[0007] (2) adjusting the temperature of a growth end and a raw material end by using a chemical vapor phase method, and performing heat preservation growth and cooling to obtain the BiOCl single crystal.

[0008] Further, the purity of the BiOCl crystal in the step (1) is greater than or equal to 99%, the transporting agent is I2 or H2O, the temperature of the growth end of the growth container in the step (2) is 600-700 DEG C, and the temperature of the raw material end is 700-800 DEG C. By controlling the purity of the raw material and selecting the appropriate transporting agent, the high-quality BiOCl single crystal can be grown at the appropriate growth temperature by using the chemical vapor deposition method, so that the internal defects of the crystal are effectively reduced. The single crystal utilizes the halogen anion Cl - Induces the formation of a transmission mode dominated by intrinsic small polarons, enhances the charge transport performance, so that the device has high carrier mobility-lifetime product, and the charge collection rate is significantly improved, thereby achieving breakthrough progress in reducing the detection dose and improving the sensitivity.

[0009] Further, the concentration of the transporting agent H2O in the growth container in the step (1) is 5.0*10 -4 g / cm 3 -10.0*10 -4 g / cm 3 , and the concentration of the transporting agent I2 in the growth container is 2.0*10 -3 g / cm 3 -4.0*10 -3 g / cm 3 Compared with I2, the use of H2O as the transporting agent can reduce the introduction of impurities and further avoid the defects of the BiOCl crystal; at the same time, H2O has adsorption, can selectively adsorb on the surface of the crystal on specific crystal faces, thereby improving the growth quality of the crystal and increasing the size of the crystal.

[0010] Further, the heat preservation growth in the step (2) is at least 7 days, and sufficient growth time further improves the growth quality and size of the crystal.

[0011] The application further provides an X-ray detector made of the BiOCl single crystal, comprising the BiOCl single crystal and an electrode.

[0012] Furthermore, the electrode is a gold electrode, which is deposited on the upper and lower surfaces or both sides of the same surface of the BiOCl single crystal by vapor deposition or sputtering.

[0013] This invention also provides applications of the above-mentioned X-ray detector in medical imaging, security inspection, and industrial testing.

[0014] Working principle: Layered bismuth halide contains Bi, which has low toxicity, excellent X-ray absorption, and a two-dimensional layered structure. Its ultra-wide bandgap effectively suppresses dark current. This invention uses Cl... - Using high-purity raw materials and suitable transport agents, BiOCl single crystals were prepared via a gas-phase chemical transport method, with the use of halide anions as the primary ion. In particular, the selection of H₂O as the transport agent significantly reduced the introduction of impurities. Furthermore, H₂O exhibits selective adsorption on the crystal surface, promoting directional growth of the crystal along specific crystal planes, resulting in high-quality BiOCl single crystals with millimeter-scale dimensions, optical transparency, and low defect concentration. This single crystal exhibits stable structure and restricted ion migration, significantly reducing the dark current of detection devices. Regarding charge transport, the halide anion Cl₂... - This method enhances electron-phonon coupling, promotes the formation of intrinsic small polarons, effectively extends carrier lifetime, and improves mobility. By improving crystal quality and reducing internal defects, this scheme achieves the transformation of extrinsic small polarons into near-intrinsic small polarons, thereby simultaneously increasing mobility and lifetime, and significantly increasing the carrier mobility-lifetime product (μτ). The X-ray detector fabricated using a high-quality BiOCl single crystal with low defect concentration exhibits higher sensitivity, a lower minimum detection dose, and improved sensitivity and dark current ratio (S / J). dark The gain is significantly improved, resulting in higher charge collection efficiency, higher signal-to-noise ratio, stable imaging capability, and improved overall operational stability and environmental adaptability.

[0015] Beneficial Effects: Compared with existing technologies, this invention has the following significant advantages: 1. By using a gas-phase method with high-purity raw materials and matching transport agents, large-size, high-purity, and low-defect BiOCl single crystals are obtained, ensuring material stability and repeatability; 2. The wide bandgap and low dark current of the BiOCl single crystal material improve the operational stability of the X-ray detector; 3. The intrinsic small polarons within the BiOCl single crystal enhance charge transport, resulting in a high carrier mobility-lifetime product, and consequently, high charge collection efficiency and sensitivity. The X-ray detector achieves a maximum sensitivity of 3.53 mC·Gy at room temperature. air -1 ·cm -2 The minimum detectable dose is 6.0 nGy. air ·s -14. The device has a simple structure, low manufacturing cost, is suitable for mass production, and is compatible with existing electronic packaging processes; 5. It is widely used in medical imaging, security inspection, and industrial testing. Attached Figure Description

[0016] Figure 1 The diagram shows (a) of the vapor phase growth apparatus for BiOCl single crystals and (b) of the layered crystal structure of the present invention.

[0017] Figure 2 The XRD pattern and crystal photograph (a), EDS pattern (b), Raman pattern (c), absorption spectrum and Taue plot (d) of the BiOCl single crystal are the material characterization results of this invention.

[0018] Figure 3 The diagram shows the structure of the X-ray detector fabricated from BiOCl single crystal according to the present invention. (a) is the in-plane device structure, and (b) is the out-of-plane device structure.

[0019] Figure 4 The X-ray detectors prepared from BiOCl crystals grown in Examples 1-4 of this invention are compared in terms of detection performance. S1-S4 correspond to Examples 1-4, respectively. Example 1 shows the It diagram (a), sensitivity and minimum detection dose comparison (b), carrier lifetime comparison (c), and carrier mobility comparison (d) under different bias voltages.

[0020] Figure 5 The following are the performance characterization results of the X-ray detector prepared from the BiOCl crystal grown in Example 1 of this invention: (a) Sensitivity variation under different bias voltages; (b) Carrier mobility-lifetime product variation under different bias voltages; (c) Stability test; (d) Charge collection efficiency under gain effect; (e) Comparison of Examples 1-4 and commonly used detection materials in terms of minimum detection limit and S / J. dark The comparison. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to the embodiments and accompanying drawings. All reagents used are commercially available.

[0022] Example 1

[0023] like Figure 1 (b) shows a BiOCl single crystal grown by chemical vapor deposition, which has a two-dimensional layered structure, a size on the millimeter scale, optical transparency, and an intrinsic small polaron transport structure.

[0024] like Figure 1 As shown in (a), the preparation method of BiOCl single crystal includes the following steps:

[0025] (1) Mix 1.2g of 99.999% BiOCl powder with the transport agent H2O, ultrasonically clean with detergent and ethanol, and then clean and dry a quartz tube (2cm in diameter and 20cm in length) with deionized water. Load the mixed starting material into the quartz tube. The density of the transport agent in the quartz tube is 9.04 × 10⁻⁶. -4 g / cm 3 (The concentration of the transport agent can also be selected as 5.0 × 10⁻⁶) -4 g / cm 3 ~10.0×10 -4 g / cm 3 ), Evacuate to 10 -4 Pa is sealed using a gas torch;

[0026] (2) Place the growth container in a gas-phase dual-zone tube furnace and heat it at a rate of about 1.5℃ / min. Adjust the temperature of the growth end to 650℃ (or 600℃-700℃) and the temperature of the raw material end to 750℃ (or 700℃~800℃). Keep it warm for ten days (at least seven days). After cooling to room temperature, BiOCl single crystal is obtained.

[0027] Example 2

[0028] Unlike Example 1, the transporter is I, and its density inside the quartz tube is 1.965 × 10⁻⁶. -3 g / cm 3 (The concentration of the transport medium can also be selected as 2.0 × 10⁻⁶) -3 g / cm 3 ~4.0×10 -3 g / cm 3 ).

[0029] Example 3

[0030] Unlike Example 1, the purity of the BiOCl powder was 99%, and the amount used was 1.2g.

[0031] Example 4

[0032] Unlike Example 3, the transporter is I, and its density inside the quartz tube is 1.965 × 10⁻⁶. -3 g / cm 3 .

[0033] like Figure 1 (b) is a schematic diagram of the obtained BiOCl single crystal structure, which has a two-dimensional layered structure and is beneficial for carrier separation. The BiOCl single crystals prepared in the above examples were subjected to XRD patterns, EDS patterns, Raman patterns, optical absorption spectra, and Tauc plots.

[0034] like Figure 2As shown in (a), the XRD pattern shows that the prepared BiOCl single crystal has a highly preferred orientation and no obvious impurity peaks, proving that it has high purity and good crystallinity; the crystal photograph further shows that the grown BiOCl single crystal has a size in the millimeter range, with a length of about 3.5 mm and a thickness of about 0.2 mm, and has optical transparency. Figure 2 (b) shows the EDS spectrum, which indicates that the proportions of Bi, O, and Cl are close to the stoichiometric ratio, suggesting that the material has a uniform composition. Figure 2 (c) shows the Raman spectrum, where the characteristic peaks are consistent with the theoretical vibrational modes of the BiOCl crystal, proving the structural integrity. Figure 2 (d) shows the absorption spectrum and Tauc plot, with a band gap of approximately 3.17 eV.

[0035] like Figure 3 As shown in (a), the BiOCl single crystal prepared in Examples 1-4 was used to make an X-ray detector. Gold electrodes were deposited on both ends of the BiOCl crystal by vapor deposition or sputtering. The size of the BiOCl single crystal was 3.5 mm × 1.7 mm × 0.2 mm, and the size of the gold electrode was 1.0 mm × 0.5 mm. An Amptek Mini-X2 tungsten target X-ray tube was used as the X-ray source, with an energy of up to 50 keV and a peak intensity of 22 keV.

[0036] Performance tests were conducted on the BiOCl single-crystal X-ray detectors fabricated in Examples 1-4. The X-ray dose rate was adjusted by regulating the tube current and using aluminum and copper foil of different thicknesses as filters. The X-ray dose rate was calibrated using a Radcal Accu-Gold+10X6-180 ionization chamber dosimeter. Different bias voltages were applied, and the induced photocurrent was recorded using a Keithley 6517B. Uncollimated X-ray detectors were used. 241 The sample was irradiated with an Am@5.49MeV α particle source, and pulse height maps under different bias voltages were obtained. Simultaneously, a charge-sensitive preamplifier (ORTEC 570) was used to record pulse shape information, and the output pulses of the preamplifier were recorded using a high-speed waveform digital acquisition card.

[0037] like Figure 4 As shown, by comparing the X-ray detectors made from BiOCl single crystals prepared in Examples 1-4, the influence of crystal quality on device performance can be clearly seen. Figure 4 (a) is the It diagram under different bias voltages. The detector in Example 1 has a dark current of less than 33pA under a bias voltage of 10V, and it hardly changes with the increase of bias voltage, showing operational stability. Figure 4 (b) The detector performance gradually improves as crystal purity increases and defect concentration decreases. The device prepared in Example 1 exhibits high sensitivity and a low minimum detectable dose, with a sensitivity reaching 3.53 mC·Gy. air-1 ·cm -2 The lowest detectable dose is as low as 3.0 nGy. air ·s -1 In contrast, the sensitivity of Example 4 was only 0.053 mC·Gy. air -1 ·cm -2 The lowest detectable dose increased to 43.37 nGy. air ·s -1 .at the same time, Figure 4 (c) and (d) show the carrier lifetime and carrier mobility of Examples 1-4. In the high-quality crystal, a carrier migration mechanism dominated by small intrinsic polarons is formed, achieving a carrier lifetime of 230 μs, two orders of magnitude higher than that of conventional semiconductors. Furthermore, the anisotropy of the layered structure further enhances the intralayer mobility, reaching 1.05 cm⁻¹. 2 ·V- 1 ·s -1 The improved mobility and lifetime significantly enhanced μτ and sensitivity, with the sensitivity being approximately 440 μC·Gy at 15,000 V for amorphous selenium (α-Se). air -1 ·cm -2 8 times that of ).

[0038] Figure 5 To test the X-ray detection performance of the optimal sample (Example 1). Figure 5 (a) shows that the detector's sensitivity remains at a high level under different bias voltages, with the highest sensitivity reaching 3.53 mC·Gy. air -1 ·cm -2 . Figure 5 (b) The test results for the carrier mobility-lifetime product μτ are given, with a value reaching 2.42 × 10⁻⁶. -4 cm 2 ·V- 1 This indicates that near-intrinsic small polarons effectively enhance carrier transport performance. Figure 5 (c) The operational and environmental stability of detectors fabricated from high-quality BiOCl crystals was demonstrated. Figure 5 (d) With increasing electric field and decreasing dose rate, the charge collection efficiency with gain increased from 3202.40% to 12698.77%, which is significantly better than existing reported materials (such as Cs2AgBiBr6: 14–40%, MAPbBr3: 16.4%). Figure 5 (e) S / J of Examples 1–4 dark The comparison results show that as the crystal quality improves, the S / J ratio increases. darkThe increase was significant, with Example 1 showing the highest value, indicating that the device has the most prominent advantages in terms of sensitivity and dark current ratio, further demonstrating its ability to achieve stable imaging at low doses. This shows that the high-quality BiOCl single-crystal detector prepared in this invention has significant advantages in low-dose detection.

Claims

1. A BiOCl single crystal, characterized in that, The crystal is grown by chemical vapor deposition and has a two-dimensional layered structure, millimeter-scale dimensions, optical transparency, and intrinsic small polaron transport structure. It has a band gap of 2.85 eV to 3.17 eV and a carrier mobility-lifetime product of 1.10 × 10⁻⁶. -5 cm 2 ·V- 1 ~2.42×10 -4 cm 2 ·V- 1 .

2. A method for growing BiOCl single crystals according to claim 1, characterized in that, Includes the following steps: (1) Mix BiOCl powder with a transport agent and place it in a vacuum environment; (2) BiOCl single crystals were obtained by adjusting the temperature of the growth end and the raw material end using chemical vapor deposition and then growing and cooling.

3. The method for growing BiOCl single crystals according to claim 2, characterized in that, In step (1), the purity of the BiOCl crystal is greater than or equal to 99%.

4. The method for growing BiOCl single crystals according to claim 2, characterized in that, In step (1), the transport agent is I2 or H2O.

5. The method for growing BiOCl single crystals according to claim 4, characterized in that, In step (1), the concentration of the transport agent H2O in the vacuum environment is 5.0 × 10⁻⁶. -4 g / cm 3 ~10.0×10 -4 g / cm 3 .

6. The method for growing BiOCl single crystals according to claim 4, characterized in that, In step (1), the concentration of transport agent I2 in the growth vessel is 2.0 × 10⁻⁶. -3 g / cm 3 ~4.0×10 -3 g / cm 3 .

7. The method for growing BiOCl single crystals according to claim 1, characterized in that, In step (2), the growth end temperature of the growth container is 600℃~700℃, the raw material end temperature is 700℃~800℃, and the growth is kept warm for at least 7 days.

8. An X-ray detector made using the BiOCl single crystal as described in claim 1, characterized in that, This includes BiOCl single crystals and electrodes.

9. The X-ray detector according to claim 8, characterized in that, The electrodes are gold electrodes, deposited on the upper and lower surfaces or both sides of the same surface of the BiOCl single crystal by vapor deposition or sputtering.

10. The application of the X-ray detector of claim 8 in medical imaging, security inspection and industrial testing.

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