Cs3Bi2I9 film X-ray detector and preparation method thereof

Abstract

The invention discloses a cesium bismuth iodine film X-ray detector and a preparation method thereof, the detector comprises a substrate, a photosensitive layer and an electrode layer, the substrate is transparent glass, the photosensitive layer is a Cs3Bi2I9 film, and the electrode layer is a gold material. CsI and BiI3 are continuously and alternately deposited on a substrate according to a certain thickness ratio to form a multi-layer film with a certain thickness, the multi-layer film reacts after being annealed to generate a Cs3Bi2I9 thin film as a photosensitive layer, then a gold interdigital electrode is evaporated on the Cs3Bi2I9 photosensitive layer, and the X-ray detector with a planar MSM structure is prepared. The Cs3Bi2I9 thin film prepared by the method has relatively high phase purity, the thickness of the thin film can be deposited to 40 mu m, the prepared X-ray detector is simple in structure and low in cost, the sensitivity of the device can reach 145.8 mu CGy <-1 > cm <-2 >, and the Cs3Bi2I9 thin film has potential application value in the fields of medical imaging, safety detection, scientific research and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of X-ray detection, and in particular to a Cs3Bi2I9 thin film X-ray detector and a preparation method thereof. Background Art

[0002] X-ray detection technology is widely used in fields such as medical imaging, security inspection, and academic research. X-rays have the characteristics of high energy and short wavelength, and their strong penetrability requires high requirements for the material properties and thickness of the photosensitive layer of X-ray detectors. Therefore, the preparation of X-ray detectors faces higher technical challenges compared to other optical detectors.

[0003] Currently, the materials for industrial X-ray detectors are mainly silicon (Si), germanium (Ge), amorphous selenium (Se), cadmium zinc telluride (CdZnTe), etc. Si is difficult to absorb hard X-rays, Ge has harsh working environment requirements, amorphous selenium Se has low carrier mobility, and CdZnTe device structures are complex. Perovskite materials not only have excellent optoelectronic properties such as large optical absorption coefficients, high carrier lifetimes, and tunable direct bandgaps, but can also be prepared by traditional solution methods and vacuum evaporation methods; among them, all-inorganic perovskite cesium bismuth iodide (Cs3Bi2I9) has good stability compared to organic-inorganic hybrid perovskites (MAPbX3), and the lead-free group reduces the pollution of the material to the environment and the harm to the human body. In addition, its high atomic number and large resistivity make it a rational material for the photosensitive layer of X-ray detectors.

[0004] Currently, the preparation of perovskite thin film X-ray detectors still mainly uses solution methods. For relatively thick films, the evaporation of solvents easily forms pinholes, thereby reducing the film quality. For inorganic materials, it is difficult to prepare micron-scale films due to solubility limitations. The growth process of single-crystal perovskite thin films is complex, time-consuming, costly, and not conducive to large-area preparation; the vacuum evaporation method has a mature process and precise control of film thickness, and is a good method for preparing perovskite thin films with uniform coverage, high flatness, large area, high purity, and high quality. Moreover, compared with solution methods, there is less research on the preparation of perovskite thin films by the vacuum evaporation method. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above-mentioned deficiencies of the prior art, and provide a Cs3Bi2I9 thin film X-ray detector and a preparation method thereof. The prepared device has a simple structure, low cost, high sensitivity, and can work stably at room temperature environment, and has potential application value in fields such as medical imaging, security inspection, and nuclear power.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A Cs3Bi2I9 thin film X-ray detector, comprising a substrate, a photosensitive layer and an electrode layer. The substrate is a transparent glass, the photosensitive layer is a Cs3Bi2I9 thin film, and the electrode layer is made of gold material. The photosensitive layer is gradually deposited on the substrate by vacuum evaporation, and the electrode layer is deposited on the photosensitive layer.

[0008] The Cs3Bi2I9 thin film is prepared by annealing a multilayer film formed by alternately depositing CsI and BiI3 on the substrate.

[0009] In the multilayer film, the molar ratio of CsI to BiI3 is 3:2, and the mass ratio is 0.66:1.

[0010] During the alternating deposition of the CsI thin film and the BiI3 thin film, due to the uniformity of the thin film, it can be considered that there is a linear relationship between the deposition rate and the molar amount per unit time. Then there is:

[0011]

[0012] Among them, ρ1, M1, L1 and ρ2, M2, L2 respectively represent the density, molar mass and deposition thickness on the substrate of CsI and BiI3; the relative molecular weights and densities of CsI and BiI3 are 259.8, 4.51 g / cm -3 and 589.7, 5.78 g / cm -3 .

[0013] In the multilayer film, the bottom layer is a CsI thin film, the top layer is a BiI3 thin film, and the middle is formed by alternately stacking BiI3 thin films and CsI thin films. The thickness ratios of the bottom CsI thin film to the top BiI3 thin film, and the middle CsI thin film to the BiI3 thin film are both 0.85:1.

[0014] In the multilayer film, the thickness of the bottom CsI thin film is 255 nm, and the thicknesses of the middle CsI thin film and the BiI3 thin film are 510 nm and 600 nm respectively, and the thickness of the top BiI3 thin film is 300 nm.

[0015] In the multilayer film, the total number of alternating deposition layers of CsI and BiI3 is 12 - 70.

[0016] The electrode layer is an interdigitated structure electrode, the interdigitated spacing is 60 μm, the deposition thickness is 100 nm, and the effective area of the device is 0.16 cm -2 .

[0017] A method for preparing a Cs3Bi2I9 thin film X-ray detector, comprising the following steps:

[0018] (a) The vacuum degree of the cavity is 4.5×10 -4Under the conditions of Pa and a substrate temperature of 100 °C, a multilayer film was formed by alternately depositing CsI thin films and BiI3 thin films on the substrate;

[0019] (b) The multilayer film was placed on a hot plate at 300 °C and annealed for 30 min to allow CsI and BiI3 to diffuse and react with each other to form a Cs3Bi2I9 thin film;

[0020] (c) An interdigital-shaped mask plate was closely attached to the Cs3Bi2I9 thin film, and gold was deposited by vacuum evaporation to form an electrode layer, thereby fabricating a planar MSM-structured Cs3Bi2I9 thin film X-ray detector.

[0021] The process for preparing the multilayer film in step (a) is as follows: First, the required CsI powder and BiI3 powder were respectively loaded into crucibles, compacted with a glass rod and made the surface flat, then the crucibles were transferred into a vacuum chamber to evacuate, the evaporation source baffle was closed and the two sources were slowly heated up. The BiI3 source was heated to 250 °C and the CsI source was heated to 450 °C, and maintained for 20 min to make the powder condense into a solid state. At the same time, the substrate temperature was raised to 100 °C and rotated slowly at 5 r / min; After that, the CsI source was heated to 600 °C and then the baffle was opened to deposit a 255 nm thick CsI thin film. After the CsI deposition was completed, the CsI source was cooled to 450 °C and the baffle was closed accordingly. At the same time, the BiI3 source was continuously heated to 350 °C and the baffle was opened to deposit a 600 nm thick BiI3 thin film. After the BiI3 thin film deposition was completed, the BiI3 source was cooled to 250 °C and the baffle was closed, and then a 510 nm thick CsI thin film was deposited. The 600 nm thick BiI3 thin film and the 510 nm thick CsI thin film were alternately deposited. After the deposition of the uppermost 510 nm thick CsI thin film, a 300 nm BiI3 thin film was deposited on it, and finally a multilayer film was formed.

[0022] The beneficial effects of the present invention are:

[0023] (1) The Cs3Bi2I9 thin film for the photosensitive layer of the X-ray detector was prepared by the continuous vacuum evaporation method provided by the present invention. By precisely controlling the thickness ratio of the CsI and BiI3 thin films, the Cs3Bi2I9 thin film has a high phase purity, good light absorption, and a thickness of up to 40 μm;

[0024] (2) The continuous vacuum evaporation method provided by the present invention can also control the thickness of the Cs3Bi2I9 thin film after annealing by adjusting the number of layers of the multilayer film, so that the thickness of the thin film deposited by the continuous vacuum evaporation method is accurately controllable;

[0025] (3) The Cs3Bi2I9 thin film X-ray detector provided by the present invention has a simple structure, low preparation cost, high sensitivity and stability at room temperature, and high application potential in the fields of medical treatment, security inspection and scientific research. Description of the Drawings

[0026] Figure 1 The following is the process flow chart for preparing the photosensitive layer of the Cs3Bi2I9 thin film X-ray detector prepared by the present invention.

[0027] Figure 2 The following is the structural schematic diagram of the Cs3Bi2I9 thin film X-ray detector prepared by the present invention.

[0028] Figure 3 The following is the XRD pattern of the photosensitive layer of the Cs3Bi2I9 thin film X-ray detector prepared in Example 1 of the present invention.

[0029] Figure 4 The following is the ultraviolet-visible absorption spectrum of the photosensitive layer of the Cs3Bi2I9 thin film X-ray detector prepared in Example 1 of the present invention

[0030] Figure 5 The following is the scanning electron microscope image of the cross-section of Cs3Bi2I9 thin films with different thicknesses prepared in Examples 1-6 of the present invention.

[0031] Figure 6 The following is the current-time (I-t) curve of the Cs3Bi2I9 thin film X-ray detector prepared in Example 1 of the present invention under X-ray radiation with different dose rates. Detailed implementation manners

[0032] The present invention will be further described below in conjunction with the accompanying drawings and detailed implementation manners:

[0033] Example 1

[0034] As Figure 2 shown, the Cs3Bi2I9 thin film X-ray detector provided by the present invention includes a substrate 1, a photosensitive layer 2, and an electrode layer 3. The substrate 1 is glass, the photosensitive layer 2 is a Cs3Bi2I9 thin film, the electrode layer 3 is gold, the photosensitive layer 2 is gradually deposited on the substrate 1 by vacuum evaporation, and the electrode layer 3 is deposited on the photosensitive layer 2.

[0035] In this example, the Cs3Bi2I9 thin film X-ray detector is prepared by vacuum evaporation.

[0036] Step (A): First, clean the substrate 1 material. First, scrub the substrate 1 with deionized water containing Triton detergent, then rinse it with deionized water and ultrasonically clean it in acetone, absolute ethanol, and deionized water for 15 minutes in sequence. After blowing the deionized water on its surface with an N2 gas gun, put it into an ultraviolet O3 machine for 15 minutes to remove the residual organic matter on the surface, and finally quickly transfer it into the coating machine cavity to pump vacuum.

[0037] Step (B). Then, the photosensitive layer 2 is prepared by a continuous vacuum evaporation method. The preparation process flow of the photosensitive layer 2 is as Figure 1 shown. The evaporation materials are respectively selected as CsI powder with a purity of 99% and BiI3 powder with a purity of 99%.

[0038] The specific preparation steps of the photosensitive layer 2 are as follows:

[0039] In the first step, first load the required CsI powder and BiI3 powder into two crucibles on the left and right respectively. Then, use a glass rod to compact them and make their surfaces flat. Next, move the crucibles into the vacuum chamber to evacuate (the vacuum degree of the chamber is 4.5×10 -4 Pa). Close the evaporation source baffle and slowly heat the two sources. The BiI3 source is heated to 250 °C, and the CsI source is heated to 450 °C. Keep for 20 min to make the powder condense into a solid state. At the same time, the temperature of the substrate 1 is raised to 100 °C, and it rotates slowly at 5 r / min.

[0040] In the second step, under the conditions that the vacuum degree of the chamber is 4.5×10 -4 Pa and the temperature of the substrate (1) is 100 °C, film coating is carried out; as Figure 1 (a) shown, the CsI source continues to be heated to 600 °C and then the baffle is opened to deposit a 255-nm-thick CsI thin film. When the CsI deposition ends, the CsI source is cooled to 450 °C and the baffle is closed accordingly. At the same time, the BiI3 source continues to be heated to 350 °C and the baffle is opened, as Figure 1 (b), deposit a 600-nm-thick BiI3 thin film. When the BiI3 thin film deposition ends, the BiI3 source is cooled to 250 °C and the baffle is closed. Then deposit a 510-nm-thick CsI thin film, and continue to alternately deposit a 600-nm-thick BiI3 thin film and a 510-nm-thick CsI thin film. After depositing the topmost 510-nm-thick CsI thin film, deposit a 300-nm BiI3 thin film on it. A total of 52 layers are deposited in sequence and finally a multi-layer film 4 is formed, as Figure 1 (c) shown. During the thin film deposition process, the deposition rate of the film material atoms on the substrate 1 and the deposition rate on the quartz crystal oscillator can be converted by the quartz crystal oscillator factor. Therefore, as long as the reading of the film thickness gauge is observed, the deposition rate on the substrate can be obtained.

[0041] In the third step, place the prepared multi-layer film 4 (52 layers) on a hot plate and anneal it at 300 °C for 30 min to obtain a Cs3Bi2I9 thin film, as Figure 1 (d) shown.

[0042] As Figure 3As shown, the prepared 30-μm Cs3Bi2I9 thin film was subjected to X-ray diffraction (XRD) testing. The characteristic peaks of the Cs3Bi2I9 thin film are located at 8.31°, 12.84°, 16.68°, 25.13°, 29.72°, 32.78°, and 37.47°, corresponding to the (002), (101), (004), (006), (204), and (211) crystal planes respectively. Comparing with the PDF#18-0364 standard card, no impurity peaks were observed in the figure, indicating that the prepared Cs3Bi2I9 thin film has high phase purity. As Figure 4 shown, the Cs3Bi2I9 thin film has obvious absorption above 500 nm, and the calculated optical band gap of 2.01 eV is close to the optical band gap of the Cs3Bi2I9 material. The number of alternating deposition layers of the CsI thin film and the BiI3 thin film is 52. After annealing, the cross-section of the Cs3Bi2I9 thin film was characterized by SEM. As Figure 5 (e) shown, the thickness of the Cs3Bi2I9 thin film is 30 μm, and the film thickness measured by cross-section SEM is in good agreement with the theoretically predicted height (the theoretically predicted thickness of the Cs3Bi2I9 thin film is 30 μm in sequence), which further indicates that the thickness of the Cs3Bi2I9 thin film prepared by continuous vacuum evaporation is accurately controllable.

[0043] Step (C): Press the interdigital-shaped mask closely against the Cs3Bi2I9 thin film, and deposit a 100-nm-thick gold wire with a purity of 99.99% on the Cs3Bi2I9 thin film by vacuum evaporation to form the electrode layer 3 with an interdigital structure, thereby preparing a planar MSM-structured Cs3Bi2I9 thin film X-ray detector.

[0044] For the Cs3Bi2I9 thin film X-ray detector under a 7V bias voltage, the current-time (I-t) curve is as Figure 6 shown, the dark current I dark is 3.30×10 -11 A. As the filament current of the X-ray tube increases from 10 mA to 100 mA in sequence, the corresponding X-ray radiation dose rate also increases from 9 μGys -1 to 87.6 μGys -1 , and the photocurrent increases significantly.

[0045] The sensitivity of the X-ray detector is calculated by the following formula:

[0046]

[0047] In the above formula, R s is the sensitivity (μCGy -1 cm -2 ), I light -I dark is the net photocurrent (A), and D is the X-ray dose rate (μGys-1 ), where A is the effective area of the detector (cm -2 ), the X-ray radiation dose rate is 9 μGys -1 and the detector sensitivity is 145.8 μCGy / cm -1 when the X-ray radiation dose rate is 9 μGys. As shown in Table 1, the sensitivity R of the device at different X-ray radiation dose rates under the same bias voltage was calculated. -2 s .

[0048] Table 1

[0049]

[0050] Examples 2 - 6

[0051] According to the method of Example 1, Cs3Bi2I9 thin film X-ray detectors were prepared respectively, where the deposition layers of the multi-layer film (4) were controlled to be 12, 22, 32, 42 and 70 layers in sequence.

[0052] After annealing, SEM characterization of the cross-section of Cs3Bi2I9 thin films with different thicknesses was carried out, as shown in: 12 layers ( Figure 5 (a)), 22 layers ( Figure 5 (b)), 32 layers ( Figure 5 (c)), 42 layers ( Figure 5 (d)), 70 layers ( Figure 5 (f)). The film thicknesses measured by cross-section SEM were approximately 6 μm, 12 μm, 18 μm, 24 μm and 40 μm in sequence, which were in good agreement with the theoretical prediction (theoretical predicted thicknesses of Cs3Bi2I9 thin films were 6 μm, 12 μm, 18 μm, 24 μm and 40 μm in sequence), further indicating that the thickness of Cs3Bi2I9 thin films prepared by continuous vacuum evaporation was accurately controllable. Figure 5

[0053] Since the multi-layer film 4 will have a slight expansion after annealing, the thickness of the Cs3Bi2I9 thin films prepared in Examples 1, 3 - 6 will be larger than that of the multi-layer film 4 before annealing. In addition, according to theoretical calculation, to finally obtain a 6-μm-thick Cs3Bi2I9 thin film, the number of layers of the multi-layer film 4 needs to be controlled to be 11 layers. However, if 11 layers are deposited, the top layer is a CsI thin film, and CsI is easily decomposed when exposed to water vapor and cannot protect the multi-layer film 4. Therefore, in Example 2, the number of film deposition layers was selected to be 12 layers, and the top layer was a BiI3 thin film. The thickness of the multi-layer film 4 before annealing after depositing 12 layers was 6.105 μm, and the thickness of the Cs3Bi2I9 thin film measured by cross-section SEM was approximately 6 μm, meeting the requirements.

[0054] ​​​​​​​The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A Cs3Bi2I9 thin film X-ray detector, characterized in that: It includes a substrate (1), a photosensitive layer (2) and an electrode layer (3). The substrate (1) is transparent glass, the photosensitive layer (2) is a Cs3Bi2I9 thin film, the electrode layer (3) is made of gold material. The photosensitive layer (2) is gradually deposited on the substrate (1) by vacuum evaporation method, and the electrode layer (3) is deposited on the photosensitive layer (2).

2. The Cs3Bi2I9 thin film X-ray detector according to claim 1, wherein: The photosensitive layer (2) is prepared by annealing a multilayer film (4) formed by alternately depositing CsI and BiI3 on the substrate (1).

3. The Cs3Bi2I9 thin film X-ray detector according to claim 2, wherein: In the multilayer film (4), the molar ratio of CsI to BiI3 is 3:2, and the mass ratio is 0.66:

1.

4. The Cs3Bi2I9 thin film X-ray detector according to claim 2, wherein: In the multilayer film (4), the bottom layer is a CsI thin film, the top layer is a BiI3 thin film, and the middle is formed by alternately stacking BiI3 thin films and CsI thin films. The thickness ratios of the bottom CsI thin film to the top BiI3 thin film, and the middle CsI thin film to the BiI3 thin film are both 0.85:

1.

5. The Cs3Bi2I9 thin film X-ray detector according to claim 2, characterized in that: In the multilayer film (4), the thickness of the bottom CsI thin film is 255 nm, the thicknesses of the middle CsI thin film and BiI3 thin film are 510 nm and 600 nm respectively, and the thickness of the top BiI3 thin film is 300 nm.

6. The Cs3Bi2I9 thin film X-ray detector according to claim 2, characterized in that: In the multilayer film (4), the total number of alternating depositions of CsI and BiI3 is 12 - 70.

7. The Cs3Bi2I9 thin film X-ray detector according to claim 1, characterized in that: The electrode layer (3) is an interdigital electrode structure with an interdigital spacing of 60 μm, a deposited thickness of 100 nm, and an effective device area of 0.16 cm -2 .

8. A method for preparing a Cs3Bi2I9 thin film X-ray detector according to any one of claims 1-7, characterized in that: It includes the following steps: (a) Under the conditions that the cavity vacuum degree is 4.5×10 -4 Pa and the temperature of the substrate (1) is 100 °C, CsI thin films and BiI3 thin films are alternately deposited on the substrate (1) to form a multilayer film (4); (b) Place the multilayer film (4) on a hot plate at 300 °C and anneal for 30 min to allow CsI and BiI3 to diffuse and react with each other to form a Cs3Bi2I9 thin film; (c) Press a finger-shaped mask closely against the Cs3Bi2I9 thin film, and deposit gold by vacuum evaporation to form the electrode layer (3), thereby fabricating a planar MSM structure Cs3Bi2I9 thin film X-ray detector.

9. The preparation method of a Cs3Bi2I9 thin film X-ray detector according to claim 8, characterized in that: The specific steps for preparing the multilayer film (4) in step (a) are as follows: First, put the required CsI powder and BiI3 powder into crucibles respectively, compact them with a glass rod and make their surfaces flat. Then, move the crucibles into the vacuum chamber to pump out the vacuum, close the evaporation source baffle and slowly heat the two sources. The BiI3 source is heated to 250 °C, and the CsI source is heated to 450 °C and kept for 20 min to make the powder condense into a solid state. At the same time, the temperature of the substrate (1) is raised to 100 °C and rotated slowly at 5 r / min; After that, raise the temperature of the CsI source to 600 °C and open the baffle to deposit a 255-nm-thick CsI thin film. After the CsI deposition is completed, lower the temperature of the CsI source to 450 °C and close the baffle accordingly. At the same time, continue to raise the temperature of the BiI3 source to 350 °C and open the baffle to deposit a 600-nm-thick BiI3 thin film. After the BiI3 thin film deposition is completed, lower the temperature of the BiI3 source to 250 °C and close the baffle, then deposit a 510-nm-thick CsI thin film, and continue to alternately deposit a 600-nm-thick BiI3 thin film and a 510-nm-thick CsI thin film. After depositing the topmost 510-nm-thick CsI thin film, deposit a 300-nm BiI3 thin film on it to finally form the multilayer film (4).

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