An organic g-CNQDs-modified CVD monolayer TMDs photodetector and its preparation method

Abstract

The invention discloses an organic g-CNQDs modified CVD single-layer TMDs photoelectric detector, which comprises a CVD single-layer TMDs film and g-CNQDs covered on the surface of the TMDs film, wherein the single-layer TMDs film is a two-dimensional material and the g-CNQDs are a zero-dimensional material, and the two form a heterojunction to construct a photoelectric detector with a TMDs / g-CNQDs heterostructure; the invention also provides a method for preparing the g-CNQDs modified CVD single-layer TMDs photoelectric detector, which is prepared by sequentially performing cleaning, doping and drying steps, has a simple preparation method, is easy to control the process, is environmentally friendly and has excellent performance, and the g-CNQDs modified CVD single-layer TMDs photoelectric detector has not only a broadened spectral response range but also a greatly improved light response degree compared with the unmodified one.

Description

Technical Field

[0001] The invention belongs to the field of two-dimensional materials and devices, and relates to an organic g-CNQDs modified CVD single-layer TMDs photoelectric detector and a preparation method thereof. Background Art

[0002] The strong light absorption of TMDs, combined with their good mobility and the possibility of preparing ultra-thin crystalline films, has aroused great interest in this material for photodetection applications in the past decade. However, due to the intrinsic bandgap width of TMDs, the photoresponse of both single-layer and multi-layer TMDs is limited to the visible and near-infrared regions, and detection in a wider band cannot be achieved. Therefore, the development of broadband two-dimensional TMDs photodetectors is a research hotspot in this field.

[0003] Quantum dots (QDs) have the characteristics of spectral tunability and solution processability. The spectral absorption range of quantum dots can affect the spectral absorption range of TMDs photodetectors that form heterojunctions with quantum dots, thereby broadening the spectral absorption range of TMDs photodetectors. However, the current integration process of quantum dot enhanced photodetectors is complex, and quantum dots contain heavy metals such as Cd and Pb. Therefore, it is urgent to develop quantum dot materials with simple integration process, environmental friendliness and excellent performance. Summary of the invention

[0004] The present invention aims to provide an organic g-CNQDs modified CVD single-layer TMDs photoelectric detector, which includes a CVD single-layer TMDs film and g-CNQDs covering the surface of the TMDs film. The single-layer TMDs film is a two-dimensional material, and the organic g-CNQDs is a zero-dimensional material. The two form a heterojunction to construct a TMDs / g-CNQDs heterostructure photoelectric detector.

[0005] The present invention also provides a method for preparing the above-mentioned organic g-CNQDs modified CVD single-layer TMDs photoelectric detector, which is prepared by sequentially performing cleaning, doping and drying steps. The preparation method is simple, the process is easy to control, it is environmentally friendly and has excellent performance. Compared with the unmodified g-CNQDs modified CVD single-layer TMDs photoelectric detector, not only the spectral response range of the g-CNQDs modified CVD single-layer TMDs photoelectric detector is broadened, but also the light response degree is greatly improved.

[0006] The technical solution adopted by the present invention is as follows:

[0007] An organic g-CNQDs modified CVD single-layer TMDs photoelectric detector comprises a CVD single-layer TMDs film and g-CNQDs covering the surface of the TMDs film, wherein the single-layer TMDs film is a two-dimensional material and the organic g-CNQDs are a zero-dimensional material, and the two form a heterojunction to construct a TMDs / g-CNQDs heterostructure photoelectric detector.

[0008] The present invention also provides a method for preparing an organic g-CNQDs modified CVD monolayer TMDs photoelectric detector, which is carried out in the following steps in sequence:

[0009] (1) Cleaning

[0010] Fix the CVD single-layer TMDs photodetector on a spin coater, drop 0.2-1 mL of isopropanol on its surface, and spin coat at a speed of 800-1500 rpm for 5-10 seconds to obtain A;

[0011] This step is to clean the surface of the silicon oxide substrate and the TMDs film. The cleanliness of the substrate and film surface after cleaning has a certain influence on the uniformity of the doping process and the performance of the final product. Within the rotation speed and spin coating cleaning time of the present invention, the cleanliness of the substrate and film surface is guaranteed to prepare for subsequent doping, while also ensuring the integrity of the substrate and film surface.

[0012] (2) Doping

[0013] Take the g-CNQDs dispersion and drop it on the surface of A. After fully wetting, spin coat it at a speed of 800-1500 rpm for 30-60 seconds to make the g-CNQDs evenly dispersed on the surface of the TMDs film to obtain B.

[0014] The spin coating speed and time in the doping process of the present invention have an important influence on the dispersion uniformity of g-CNQDs, and the uniformity directly affects the spectral response range and photoresponsivity of the final photodetector. When the spin coating speed is too high or the spin coating time is too long, it will cause g-CNQDs to be difficult to combine with the TMDs film and thus make it difficult for g-CNQDs to play a role, affecting the modification effect of the final detector. When the spin coating speed is too low or the spin coating time is too short, g-CNQDs are prone to agglomeration and cannot be evenly dispersed on the surface of the TMDs material. The agglomerated g-CNQDs will block light and reduce the photoresponsivity of the photodetector.

[0015] (3) Drying

[0016] B was placed in a vacuum oven and dried under a low vacuum environment to obtain a g-CNQDs-modified CVD monolayer TMDs photodetector.

[0017] As a limitation of the preparation method of the present invention:

[0018] (i) In step (2), the g-CNQDs dispersion is a dispersion formed by dispersing g-CNQDs in isopropanol, and the concentration of the g-CNQDs dispersion is 1 mg / 80 mL;

[0019] The concentration of the g-CNQDs dispersion of the present invention has an important influence on the performance of the photodetector after the final modification. When the solution concentration is less than 1 mg / 80 mL, the concentration is too small, and the g-CNQDs are too dispersed on the surface of the TMDs material after spin coating, and cannot play an obvious modification role; when the solution concentration is greater than 1 mg / 80 mL, the g-CNQDs are difficult to preserve and are prone to agglomeration, and cannot be evenly dispersed on the surface of the TMDs material. The agglomerated g-CNQDs will block light and reduce the light response of the photodetector.

[0020] (ii) In step (3), the vacuum degree is less than 100 Pa;

[0021] Heating and drying in a vacuum environment can prevent g-CNQDs from oxidative denaturation and failure.

[0022] (iii) In step (3), the drying temperature is 40-60°C and the drying time is 10-15min;

[0023] In this step, the g-CNQDs modification can be dried quickly at the drying temperature and time to form a heterogeneous structure of zero-dimensional quantum dots and a single-layer two-dimensional TMDs material. When the drying temperature is lower than 40°C, the drying is insufficient and the solvent (isopropanol) of the g-CNQDs dispersion will remain on the device surface in liquid form, which is easy to combine with dust in the air and affect the performance of the photodetector. When the drying temperature is higher than 60°C, carbonization or oxidation denaturation is likely to occur, resulting in failure.

[0024] As a further limitation of the present invention, in step (2), the particle size of g-CNQDs in the g-CNQDs dispersion is 5 to 10 nm;

[0025] Graphite phase carbon nitride (g-C3N4) is an organic planar two-dimensional sheet structure similar to graphene, and is also a non-metallic semiconductor with a good conjugated chemical bond structure. Unlike graphite phase carbon nitride (g-C3N4), the g-CNQDs used in the present invention is an organic quantum dot material, which is a zero-dimensional structure. Under the doping of 5-10nm particle size quantum dots, a heterostructure of zero-dimensional quantum dots and single-layer two-dimensional TMDs materials is constructed, which greatly broadens the spectral response range of single-layer two-dimensional TMDs materials and improves the photocurrent and photoresponsivity. This is mainly because under illumination conditions, after g-CNQDs and CVD single-layer TMDs absorb photons, excitons will be generated in the barrier regions on both sides of their interfaces. The excitons generated by illumination quickly dissociate into free carriers under the action of the built-in electric field of the interface, thereby increasing the photocurrent of the device.

[0026] The present invention also has a limitation that in step (3), when the CVD single-layer TMDs photodetector is a CVD single-layer MoS2 photodetector, the amount of g-CNQDs dispersion liquid used for spin coating is 100 μL.

[0027] The above-mentioned preparation methods as a whole influence each other, and only under the preparation conditions described in the present invention can the above-mentioned g-CNQDs modified CVD single-layer TMDs photoelectric detector be obtained.

[0028] After adopting the above technical solution, the beneficial effects achieved by the present invention are as follows:

[0029] (1) The present invention prepares a new type of composite photodetector by using g-CNQDs to modify a CVD single-layer TMDs photodetector. The preparation method is simple, the process is easy to control, it is environmentally friendly, and it is suitable for popularization and application.

[0030] (2) g-CNQDs modification can significantly improve the photosensitivity of CVD monolayer TMDs photodetectors and broaden the spectral response range. Compared with the unmodified CVD monolayer MoS2 detector, the photosensitivity of the organic g-CNQDs modified CVD monolayer MoS2 detector is increased by 70 times, and the spectral response range is broadened from 660nm to 900nm.

[0031] The invention is suitable for preparing a CVD single-layer TMDs photoelectric detector.

[0032] The specific implementation of the present invention will be further described in detail below with reference to the accompanying drawings.

[0033] Instruction Manual

[0034] Figure 1 TEM image of g-CNQDs in Example 1 of the present invention;

[0035] Figure 2 The optical microscope images of the CVD single-layer MoS2 on the sapphire growth substrate and the silicon oxide device substrate in Example 1 of the present invention, wherein: a-optical microscope image of the CVD single-layer MoS2; b-optical microscope image of the CVD single-layer MoS2 photodetector;

[0036] Figure 3 The Raman spectrum and photoluminescence analysis results of MoS2 grown in Example 1 of the present invention are shown in Figure 1, where: a-Raman spectrum; b-photoluminescence;

[0037] Figure 4 The effect of g-CNQDs modification on the photoelectric performance of CVD monolayer MoS2 photodetector in Example 1 of the present invention (optical power density is 10mW / cm 2 , wavelength is 385nm), where: aIt curve; b-light response curve; c-rise response time (t rise ) and falling response time (t fall ) curve; d-switching ratio curve;

[0038] Figure 5 The effect of the amount of g-CNQDs dripped in Example 1 and Example 7 of the present invention on the photocurrent and photoresponsivity of the CVD monolayer MoS2 photodetector, wherein: aI-λ curve; bR-λ curve; cIV curve; dRV curve;

[0039] Figure 6 The g-CNQDs droplet amount of the present invention has an influence on the R of the CVD single-layer MoS2 photodetector. CN / R, where: aR CN / R-λ curve (light power density is 10mW / cm 2 , V is 9V); bR CN / RV curve (light power density is 10mW / cm 2 , λ is 385nm);

[0040] Figure 7 The Raman spectrum and photoluminescence analysis results of WS2 grown in Example 2 of the present invention, wherein: a-Raman spectrum; b-photoluminescence;

[0041] Figure 8 The effect of g-CNQDs modification on the photoelectric performance of CVD monolayer WS2 photodetector in Example 2 of the present invention (light power density is 1100mW / cm 2 , wavelength is 405nm), where: aIt curve; b-light response curve; c-rise response time (t rise ) and falling response time (t fall ) curve; d-switching ratio curve;

[0042] Fig. 9 The energy band diagram before and after the formation of TDMs / g-CNQDs heterojunction;

[0043] Fig.10 It curve of the g-CNQDs photodetector in Example 9 of the present invention. DETAILED DESCRIPTION

[0044] In the following examples, the reagents described are all commercially available unless otherwise specified, and the following experimental methods and detection methods are all based on existing experimental methods and detection methods unless otherwise specified.

[0045] Example 1 Preparation method of organic g-CNQDs modified CVD single-layer MoS2 photodetector

[0046] The detector prepared in this embodiment includes a CVD single-layer TMDs film and g-CNQDs covering the surface of the TMDs film. The single-layer TMDs film is a two-dimensional material of MoS2, and g-CNQDs is a zero-dimensional material. The two form a heterojunction to construct a TMDs / g-CNQDs heterostructure photoelectric detector. The preparation method is carried out in the following steps in sequence:

[0047] (I) Preparation of CVD single-layer MoS2 photodetector

[0048] Ⅰ. Use an analytical balance to weigh 0.8g S powder and 0.002g MoO3 powder respectively and place them in two ceramic boats, and then place them in the center of the low-temperature zone and high-temperature zone of the CVD tube furnace respectively. The temperature zones are separated by insulation plugs; place the cleaned sapphire growth substrate 10cm downstream of the MoO3 powder and put on the insulation plug.

[0049] Ⅱ. Seal both sides of the quartz tube, turn on the vacuum pump to extract the air in the tube until the pressure in the tube drops to 8Pa, open the air inlet valve, and fill the quartz tube with high-purity argon gas to normal pressure. Repeat this process 5 times to expel the residual air in the tube. After the filling is completed, adjust the gas flow rate to 100sccm until the reaction is completed.

[0050] III. First, set the high temperature zone to heat up from room temperature to 600°C at a heating rate of 20°C / min. At this time, the low temperature zone begins to heat up and heats up to 230°C at a heating rate of 20°C / min after 12 minutes. The high temperature zone heats up to 750°C at a heating rate of 5°C / min, keeps warm for 15 minutes, and then closes the gas valve after the furnace cools naturally to room temperature to prepare a single-layer triangular MoS2 uniformly dispersed on the growth substrate.

[0051] IV. Place the growth substrate with a single layer of MoS2 on a heating table and preheat it at 40°C for 3 minutes. Use wet transfer to drop PMMA solution on the surface of the growth substrate and use spin coating at a speed of 1500 rpm for 20 seconds.

[0052] V. Place the growth substrate on a heating table and heat it to solidify PMMA. The heating temperature is 120°C and the heating time is 15 minutes. After the heating is completed, place the growth substrate in deionized water and soak it for 5 minutes. Use tweezers to tear off the PMMA film from the substrate and stick it on PDMS. Then use acetone to remove the PMMA film to obtain a single layer of MoS2 attached to PDMS.

[0053] VI. Fix PDMS-MoS2 on the surface of a glass slide and invert it on the material displacement platform. Place the silicon oxide substrate on the device displacement platform. Use an optical microscope to adjust the material displacement platform and the device substrate displacement platform. Position and transfer the CVD single-layer MoS2 to the device substrate to prepare a CVD single-layer MoS2 photodetector.

[0054] (II) Preparation of organic g-CNQDs modified CVD monolayer MoS2 photodetector

[0055] (1) Cleaning

[0056] Fix the CVD single-layer MoS2 photodetector on a spin coater with tape, drip 0.2-1 mL of isopropanol onto its surface, and spin coat at 1000 rpm for 5 s to obtain A;

[0057] (2) Doping

[0058] 100 μL of g-CNQDs dispersion (g-CNQDs dispersion is a dispersion formed by dispersing g-CNQDs in isopropanol, with a concentration of 1 mg / 80 mL and a particle size of 5 to 10 nm) was added dropwise to the surface of A. After sufficient wetting, the surface was spin-coated at a speed of 1500 rpm for 45 s to uniformly disperse the g-CNQDs on the surface of the TMDs film to obtain B.

[0059] (3) Drying

[0060] B was placed in a vacuum oven and dried at 40°C for 12 min in a vacuum environment below 100 Pa to obtain an organic g-CNQDs modified CVD single-layer TMDs photodetector.

[0061] Figure 1 is a transmission electron microscope image of g-CNQDs in Example 1 of the present invention, Figure 2 This is an optical microscope image of the CVD single-layer MoS2 on a sapphire growth substrate and a silicon oxide device substrate in this embodiment. Figure 3The Raman spectrum and photoluminescence analysis results of MoS2 grown in this example are shown.

[0062] Figure 4 a is the It curve of CVD single-layer MoS2 photodetector under different g-CNQDs droplet amount (light power density is 10mW / cm 2 , wavelength is 385nm). Figure 4 It can be seen that the surface modification of g-CNQDs improves the photoresponsivity of the CVD monolayer MoS2 photodetector ( Figure 4 b), while slightly prolonging the device's response time ( Figure 4 c) Reduce the switching ratio of the device ( Figure 4 d). After g-CNQDs surface modification, the photoresponsivity of CVD monolayer MoS2 photodetector changes with the amount of g-CNQDs added and ( Figure 4 a) The photocurrent and dark current have the same pattern as the change with the amount of g-CNQDs added. In this embodiment, the g-CNQDs surface is modified at a dropwise spin coating amount of 100 μL. The photoresponsivity of the modified detector is increased by 70 times compared with the unmodified CVD single-layer MoS2 detector.

[0063] Example 2 Preparation method of organic g-CNQDs modified CVD single-layer WS2 photodetector

[0064] (I) Preparation of CVD single-layer WS2 photodetector

[0065] Ⅰ. Use an analytical balance to weigh 0.8g S powder and 0.02g WO3 powder respectively and place them in two ceramic boats, and then place them in the center of the low-temperature zone and high-temperature zone of the CVD tube furnace respectively. The temperature zones are separated by insulation plugs. Place the cleaned sapphire growth substrate 10cm downstream of WO3 and put on the insulation plug.

[0066] Ⅱ. Seal both sides of the quartz tube, turn on the vacuum pump to extract the air in the tube until the pressure in the tube drops to 8Pa, open the air inlet valve, and fill the quartz tube with high-purity argon gas to normal pressure. Repeat this process 5 times to expel the residual air in the tube. After the filling is completed, adjust the gas flow rate to 100sccm until the reaction is completed.

[0067] III. First set the high temperature zone to heat up from room temperature to 900°C at a heating rate of 20°C / min. At this time, the low temperature zone begins to heat up and heats to 230°C at a heating rate of 20°C / min after 12 minutes. The high temperature zone heats up to 1050°C at a heating rate of 15°C / min, keeps warm for 15 minutes, and then closes the gas valve after the furnace cools naturally to room temperature to prepare a single-layer triangular WS2 uniformly dispersed on the growth substrate.

[0068] IV. Place the growth substrate with a single layer of WS2 on a heating table and preheat it at 40°C for 3 minutes; use wet transfer to drop the PMMA solution on the surface of the growth substrate, and use spin coating at a speed of 1500 rpm for 20 seconds.

[0069] V. Place the growth substrate on a heating table and heat it to solidify PMMA. The heating temperature is 120°C and the heating time is 15 minutes. After heating, soak the growth substrate in deionized water for 5 minutes. Use tweezers to tear off the PMMA film from the substrate and stick it on PDMS. Then use acetone to remove the PMMA film, and a single layer of WS2 attached to PDMS will be obtained.

[0070] VI. Fix the transferred PDMS-WS2 on the surface of the glass slide and invert it on the material displacement platform, place the silicon oxide substrate on the device displacement platform, use an optical microscope to adjust the material displacement platform and the device substrate displacement platform, position and transfer the CVD single-layer WS2 to the device substrate, and prepare a CVD single-layer WS2 photoelectric detector.

[0071] (II) Preparation of organic g-CNQDs modified CVD monolayer WS2 photodetector

[0072] (1) Cleaning

[0073] The CVD single-layer WS2 photodetector was fixed on a spin coater with tape, 0.2-1 mL of isopropanol was dripped onto its surface, and the detector was spin-coated at 800 rpm for 10 s to obtain A;

[0074] (2) Doping

[0075] Take 100 μL of g-CNQDs dispersion (g-CNQDs dispersion is a dispersion formed by dispersing g-CNQDs in isopropanol, with a concentration of 1 mg / 80 mL and a particle size of 5 to 10 nm) and drop it on the surface of A. After sufficient wetting, spin coat at 1200 rpm for 60 s to evenly disperse g-CNQDs on the surface of TMDs film to obtain B.

[0076] (3) Drying

[0077] B was placed in a vacuum oven and dried at 50°C for 10 min in a vacuum environment below 100 Pa to obtain an organic g-CNQDs modified CVD single-layer TMDs photodetector.

[0078] Figure 7 The Raman spectrum and photoluminescence analysis results of WS2 grown in Example 2 of the present invention.

[0079] Figure 8The effect of g-CNQDs modification on the photoelectric performance of CVD monolayer WS2 photodetector in Example 2 of the present invention (light power density is 1100mW / cm 2 , wavelength is 405nm), from Figure 8 a and Figure 8 b It can be seen that the surface modification of g-CNQDs improves the photoresponsivity of the CVD single-layer WS2 photodetector. When the amount of g-CNQDs dropped is less than 100 μL, the response time of the device (rise time t rise and the fall time t fall ) is less than or equal to about 0.05s; when the amount of g-CNQDs added is greater than 150μL, the rise time of the device does not change significantly, but the fall time gradually increases ( Figure 8 c). Surface modification of g-CNQDs improves the photoresponsivity of CVD monolayer WS2 photodetectors ( Figure 8 b), but reduces the switching ratio of the device ( Figure 8 d). After g-CNQDs surface modification, the photoresponsivity of the CVD single-layer WS2 photodetector increased by 54 times when the g-CNQDs droplet was 200 μL, but its switching ratio was low and the fall time was long. When the g-CNQDs droplet was 100 μL, the photoresponsivity increased by 26 times when there was no significant change in the response time and the switching ratio was high.

[0080] Example 3-6 Preparation method of organic g-CNQDs modified CVD single-layer TMDs photodetector

[0081] Example 3-6 prepares g-CNQDs modified CVD single-layer MoS2 photodetector. The preparation method is similar to that of Example 1. The only difference is that the corresponding technical parameters in the preparation process of g-CNQDs modified CVD single-layer MoS2 photodetector are different, as shown in the table below.

[0082] Table 1 Technical Parameters

[0083]

[0084] Example 7 Study on the influence of the drop amount of organic g-CNQDs dispersion on the performance of modified CVD monolayer TMDs photodetector

[0085] The drop coating amount of the organic g-CNQDs dispersion has an important influence on the performance of the photodetector after the final modification. This example explores the influence of different drop coating amounts of the g-CNQDs dispersion on the performance of the modified detector.

[0086] Group A: A series of g-CNQDs modified CVD monolayer MoS2 photodetectors were prepared respectively. The preparation method was similar to that in Example 1, except that the drop coating amount of g-CNQDs dispersion was 0 μL, 50 μL, 100 μL, 150 μL, and 200 μL, respectively. After preparation, a series of performance tests were conducted on the modified CVD monolayer MoS2 photodetectors. The specific results are shown in Figure 4-Figure 6 .

[0087] Figure 4 a is the It curve of CVD single-layer MoS2 photodetector under different g-CNQDs droplet amount (light power density is 10mW / cm 2 , wavelength is 385nm). It can be seen from the figure that when the g-CNQDs dripping amount is 50μL, the photocurrent and dark current of the device are significantly increased; when the dripping amount is 100μL, the photocurrent and dark current of the device are slightly higher than when the dripping amount is 50μL; when the dripping amount is 150μL and 200μL, the photocurrent and dark current of the device are basically unchanged. Figure 4 b~ Figure 4 d are respectively based on Figure 4 The relationship between the photoresponsivity, response time and switching ratio calculated from the It curve in a and the amount of g-CNQDs added. It can be seen from the figure that the surface modification of g-CNQDs greatly improves the photoresponsivity of the CVD monolayer MoS2 photodetector ( Figure 4 b); while slightly prolonging the device's response time ( Figure 4 c) Reduce the switching ratio of the device ( Figure 4 d). After g-CNQDs surface modification, the photoresponsivity of CVD monolayer MoS2 photodetector changes with the amount of g-CNQDs added and ( Figure 4 a) The photocurrent and dark current show the same pattern as the changes in the amount of g-CNQDs added.

[0088] Figure 5 a and Figure 5 b shows the relationship between the I-λ and R-λ curves of the CVD single-layer MoS2 photodetector and the amount of g-CNQDs added (the optical power density is 10 mW / cm 2 , bias voltage is 9V). It can be seen from the figure that I and R first increase with the increase of g-CNQDs dripping amount. When the g-CNQDs dripping amount increases to 150μL, I and R no longer change with the increase of g-CNQDs dripping amount. When the g-CNQDs dripping amount increases to 50μL, the increase values ​​of I and R are basically constant within the wavelength range of 330nm and 760nm. After the wavelength of 760nm, the increase values ​​of I and R decrease with the increase of wavelength. Figure 5 c and Figure 5d shows the relationship between the IV and RV curves of the CVD single-layer MoS2 photodetector and the amount of g-CNQDs added (the optical power density is 10 mW / cm 2 , wavelength is 385nm). It can be seen from the figure that when the amount of g-CNQDs added does not exceed 200μL, I i and R increase with the increase of g-CNQDs dropwise addition amount.

[0089] For ease of comparison, Figure 6 The effect of g-CNQDs droplet amount on the photoresponsivity R of CVD monolayer MoS2 photodetector after g-CNQDs surface modification is shown. CN The ratio of the photoresponsivity R of the unmodified CVD monolayer MoS2 photodetector (i.e., R CN / R). Figure 6 It can be seen that R CN / R-λ curve (light power density is 10mW / cm 2 When the g-CNQDs were added in an amount of 50 μL, only a relatively large peak appeared near 330 nm. Figure 6 a; When the amount of g-CNQDs added reached 100 μL, R CN The / R-λ curve will have two relatively large peaks, one near 330nm and the other near 760nm. CN The values ​​of / R at the two peak wavelengths of 330nm and 760nm reached 68 and 70 respectively, and the R at the peak and valley wavelengths was CN / R is between 20 and 30, which indicates that the effect of g-CNQDs surface modified CVD monolayer MoS2 photodetector is not only related to the amount of g-CNQDs dropped, but also to the wavelength. Figure 6 b shows the effect of g-CNQDs addition amount on the R CN / RV curve. As can be seen from the figure, when the voltage is less than 1.2V, R CN / R decreases sharply as the voltage increases.

[0090] Group B: A series of g-CNQDs modified CVD single-layer WS2 photodetectors were prepared respectively. The preparation method was similar to that in Example 2, except that the drop coating amount of g-CNQDs dispersion was 0 μL, 50 μL, 100 μL, 150 μL, and 200 μL, respectively. After preparation, a series of performance tests were conducted on the modified CVD single-layer WS2 photodetectors. The specific results are shown in Figure 8 .

[0091] Figure 8a is the It curve of CVD single-layer WS2 photodetector under different g-CNQDs droplet amount (light power density is 1100mW / cm 2 , wavelength is 405nm). It can be seen from the figure that when the g-CNQDs dripping amount is 50μL, the photocurrent and dark current of the device are significantly increased; when the dripping amount is 100μL, the photocurrent and dark current of the device are slightly higher than when the dripping amount is 50μL; when the dripping amount is 150μL and 200μL, the photocurrent and dark current of the device are higher than when the dripping amount is 100μL, but compared with the dripping amount of 150μL and 200μL, the photocurrent is basically the same, but the dark current increases by about an order of magnitude. Figure 8 b~ Figure 8 d are respectively based on Figure 8 The relationship between the photoresponsivity, response time and switch ratio calculated by the It curve in a and the amount of g-CNQDs added is shown in the figure. When the amount of g-CNQDs added is less than 100 μL, the response time of the device (rise time t rise and the fall time t fall ) is less than or equal to about 0.05s; when the amount of g-CNQDs added is greater than 150μL, the rise time of the device does not change significantly, but the fall time gradually increases ( Figure 8 c). Surface modification of g-CNQDs improves the photoresponsivity of CVD monolayer WS2 photodetectors ( Figure 8 b), but reduces the switching ratio of the device ( Figure 8 d). After g-CNQDs surface modification, the photoresponsivity of the CVD single-layer WS2 photodetector increased by 54 times when the g-CNQDs droplet was 200 μL, but its switching ratio was low and the fall time was long. When the g-CNQDs droplet was 100 μL, the photoresponsivity increased by 26 times when there was no significant change in the response time and the switching ratio was high.

[0092] Example 8 Mechanism of the performance of g-CNQDs surface modified CVD monolayer TDMs photodetector changing with the type of TDMs

[0093] The g-CNQDs of the present invention have an effect on improving the photocurrent and photoresponsivity of CVD single-layer MoS2 and WS2 photodetectors, but the effect of g-CNQDs on the photocurrent and photoresponsivity of CVD single-layer MoS2 photodetectors is greater than that on WS2. When the droplet volume is 100 μL, the photocurrent and photoresponsivity of MoS2 photodetectors are increased by 72 times, while those of WS2 are only increased by 26 times. In order to explain the difference in the effects of g-CNQDs on the performance of CVD single-layer MoS2 and WS2 photodetectors, Fig. 9The energy band diagrams of single-layer MoS2, single-layer WS2 and single-layer g-C3N4 before and after forming heterojunctions are compared. g are electron affinity, work function and band gap respectively. The 9 horizontal solid lines in the figure are the Fermi surfaces of monolayer g-C3N4, monolayer MoS2 and monolayer WS2 respectively. Due to energy level matching, when g-C3N4 and MoS2 form a heterojunction, the Fermi surface of g-C3N4 moves downward, while the Fermi surface of MoS2 moves upward. The Fermi surface of g-C3N4 and MoS2 after forming a heterojunction is located in the middle of the Fermi surface of g-C3N4 and the Fermi surface of MoS2 ( Fig. 9 When g-C3N4 and WS2 form a heterojunction, the Fermi surface of g-C3N4 moves upward and the Fermi surface of WS2 moves downward. After g-C3N4 and WS2 form a heterojunction, its Fermi surface is also located in the middle of the Fermi surface of g-C3N4 and the Fermi surface of WS2 ( Fig. 9 right horizontal dashed line).

[0094] g-C3N4 is an n-type semiconductor, and oxygen-doped MoS2 grown by CVD is a p-type semiconductor. A space charge region is formed at the interface between the two. Under light conditions, after g-C3N4 and MoS2 absorb photons, excitons are generated in the barrier regions on both sides of their interface. The excitons generated by light quickly dissociate into free carriers under the action of the built-in electric field of the interface, thereby increasing the photocurrent of the device. Fig. 9 It can be seen that the conduction band energy difference and valence band energy difference of the g-CNQDs / WS2 heterojunction are much smaller than those of g-CNQDs / MoS2, so its built-in electric field is relatively weak and the excitons generated are relatively few. Therefore, the effect of g-CNQDs modification on the photocurrent and photoresponse of WS2 is weaker than that on MoS2.

[0095] Example 9 Preparation and performance of organic g-CNQDs photodetector

[0096] The present invention prepares a g-CNQDs photoelectric detector, and the specific preparation process is as follows:

[0097] (1) Filtration

[0098] Take 80 mL of g-CNQDs isopropanol dispersion with a mass fraction of 1 mg / 80 mL, filter it to form a thin film on the surface of a 25 mm × 0.2 μm microporous nylon filter membrane to obtain A;

[0099] (2) Drying

[0100] Place A in a vacuum oven and dry at 50°C for 10 min under a vacuum environment below 100 Pa to obtain B;

[0101] (3) Evaporation

[0102] B is fixed on the back of the hard mask template and placed in a high vacuum thermal evaporation coating machine at a temperature below 5×10 -4 Pa vacuum environment, 5nm chromium Gr and 50nm gold Au electrodes were evaporated successively at a rate to obtain an organic g-CNQDs photodetector.

[0103] After preparation, a series of performance tests were performed on the g-CNQDs photodetector. Fig.10 It curves of different g-CNQDs photodetectors (light power density is 1100mW / cm 2 , wavelength is 405nm). The data comparison between the untreated MoS2, WS2 photodetectors and the MoS2, WS2 photodetectors modified with g-CNQDs with a drop volume of 100μL is shown in Table 2.

[0104] It can be observed from Table 2 that, except for the response time, the performance of the pure organic g-CNQDs photodetector is much lower than that of the untreated MoS2 and WS2 photodetectors and the MoS2 and WS2 photodetectors modified with a drop volume of 100 μL.

[0105] Table 2 Performance comparison of different photodetectors

[0106]

[0107] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the scope of protection of the claims of the present invention.

Claims

1. A method for preparing an organic g-CNQDs modified CVD monolayer TMDs photodetector, characterized in that: The photodetector comprises a CVD single-layer TMDs film and g-CNQDs covering the surface of the TMDs film, wherein the single-layer TMDs film is a two-dimensional material, and the organic g-CNQDs is a zero-dimensional material, and the two form a heterojunction to construct a photodetector with a TMDs / g-CNQDs heterostructure; The method for preparing the organic g-CNQDs modified CVD single-layer TMDs photoelectric detector is carried out in the following steps in sequence: (1) Cleaning Fix the CVD single-layer TMDs photodetector on a spin coater, drop 0.2-1 mL of isopropanol on its surface, and spin coat at a speed of 800-1500 rpm for 5-10 s to obtain A; (2) Doping Take the g-CNQDs dispersion and drop it on the surface of A. After fully wetting, spin coat at a speed of 800-1500 rpm for 30-60 s to make the g-CNQDs evenly dispersed on the surface of the TMDs film to obtain B. (3) Drying B was placed in a vacuum oven and dried under a low vacuum environment to obtain a g-CNQDs-modified CVD monolayer TMDs photodetector.

2. The method for preparing an organic g-CNQDs modified CVD monolayer TMDs photodetector according to claim 1, characterized in that: In step (2), the g-CNQDs dispersion is a dispersion formed by dispersing g-CNQDs in isopropanol, and the concentration of the g-CNQDs dispersion is 1 mg / 80 mL.

3. The method for preparing an organic g-CNQDs modified CVD monolayer TMDs photodetector according to claim 2, characterized in that: In step (2), the particle size of g-CNQDs in the g-CNQDs dispersion is 5 to 10 nm.

4. The method for preparing an organic g-CNQDs modified CVD monolayer TMDs photodetector according to claim 1, characterized in that: In step (3), the vacuum degree is less than 100 Pa.

5. The method for preparing an organic g-CNQDs modified CVD monolayer TMDs photodetector according to claim 1, characterized in that: In step (3), the drying temperature is 40-60°C and the drying time is 10-15 min.

6. A method for preparing an organic g-CNQDs modified CVD monolayer TMDs photodetector according to any one of claims 1 to 5, characterized in that: In step (3), when the CVD single-layer TMDs photodetector is a CVD single-layer MoS2 photodetector, the amount of g-CNQDs dispersion liquid used for spin coating is 100 μL.

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