A self-driven photoelectric detector based on quantum dots and its preparation method

Abstract

The present invention belongs to the technical field of photoelectric detectors, and discloses a method for preparing a self-driven photoelectric detector based on quantum dots. First, a layer of ZnO is spin-coated on an ITO glass substrate, and then a CdSe / CdS core-shell quantum dot photosensitive layer is deposited by layer-by-layer spin coating and ligand exchange. Next, a layer of MoO3 is deposited by thermal evaporation, and finally a metal electrode layer is evaporated to obtain a self-driven photoelectric detector based on quantum dots. The present invention constructs a self-driven photoelectric detector based on quantum dots by adjusting the energy band structure of core-shell quantum dots, thereby optimizing and improving the photoelectric performance of the device.

Description

Technical Field

[0001] The invention relates to a self-driven photoelectric detector and a preparation method thereof, in particular to a preparation method of a self-driven photoelectric detector based on a CdSe / CdS core-shell quantum dot heterojunction. Background Art

[0002] With the development of Internet of Things technology, the requirements for sensors tend to be miniaturized, highly integrated, intelligent, and multifunctional. Photodetectors, as a type of sensor with broad prospects, are widely used in optical communications, environmental monitoring, and sensing technology. Traditional photodetectors usually need to work under external bias conditions, which greatly limits their application in the next generation of new sensing systems. In view of this, researchers have proposed a self-driven photodetector based on pn heterojunction or Schottky junction, which provides a solution strategy for the next generation of miniaturized and highly integrated optoelectronic devices. Its working principle is to use the built-in electric field of the heterojunction to effectively separate photogenerated carriers, convert the incident light into electrical signals without the need for external bias, and realize the ability of self-driven photodetection.

[0003] As a core component of photodetectors, photosensitive sensing materials have always been the focus of researchers. Among the many sensing materials, solution quantum dots have attracted widespread attention due to their low cost, simple synthesis method, potential large-scale preparation, and compatibility with flexible substrates. In particular, core-shell quantum dots are a core-shell heterojunction, which can adjust the physical properties of the material by adjusting the size of the core and shell, and then adjust the photoelectric properties of the device. Traditional core-shell quantum dots, such as CdSe / CdS, achieve physical property regulation of core-shell heterojunction quantum dots by fixing the particle size of the core CdSe and adjusting the size of the CdS shell. This type of core-shell quantum dot heterojunction has the following problems: the fixed core size and the shell adjustment cannot effectively adjust the band structure of the heterojunction, so the ability to improve the carrier transfer efficiency at the interface is limited; in addition, this adjustment method is not conducive to the light absorption of the heterojunction to a certain extent, thereby affecting the photoelectric properties of the material. Summary of the invention

[0004] The purpose of the present invention is to provide a solution for a self-driven photodetector based on quantum dots and a method for preparing the same, in particular, to provide a method for adjusting the heterojunction based on CdSe / CdS core-shell quantum dots, which is different from the traditional CdSe / CdS core-shell quantum dots. The present invention adjusts the size of the core layer CdSe by fixing the size of the shell layer CdS to achieve the regulation of the heterojunction band structure and the regulation of the photoelectric performance of the constructed device. The preparation method of the device is as follows: first, a layer of ZnO nanoparticle film is spin-coated on the cleaned ITO glass as an electron transport layer; then, a CdSe / CdS core-shell quantum dot film is deposited on the electron transport layer as a photosensitive active layer by layer spin coating and ligand exchange, and annealed at 60°C for 10 minutes; next, a layer of MoO3 is evaporated on the photosensitive active layer as a hole transport layer; finally, a layer of gold thin film electrode layer is evaporated on the hole transport layer using a metal mask. Among them, the ITO film and the gold film are used as the bottom electrode and top electrode layers of the device, respectively.

[0005] According to the first aspect of the present invention, the present invention provides a self-driven photodetector device, including a substrate, a sensing material, various functional layers and an electrode material. The self-driven photodetector includes, from bottom to top, a glass substrate, an ITO film layer, an electron transport layer, a CdSe / CdS core-shell quantum dot photosensitive material film layer, a MoO3 hole transport layer, and a metal electrode layer, wherein the ITO film layer is stacked on the surface of the glass substrate to form an ITO glass substrate, wherein the ITO film layer and the metal electrode layer serve as the bottom and top electrode layers of the device respectively;

[0006] The CdSe / CdS core-shell quantum dots can achieve band structure regulation by fixing the thickness of the CdS shell layer and adjusting the size of the CdSe core layer. Three different types of core-shell quantum dots are designed and prepared, namely type II heterojunction, quasi-type II heterojunction, and type I heterojunction. Three types of photodetectors are constructed to achieve regulation of the photoelectric performance of the device.

[0007] Preferably, the bottom electrode ITO thin film layer is stacked on a glass substrate, and the thickness of the ITO thin film layer is 10-500 nm.

[0008] Preferably, the electron transport layer comprises one of ZnO, TiO2, and SnO2; more preferably ZnO.

[0009] Preferably, the CdSe / CdS core-shell quantum dot photosensitive material film layer is stacked on the surface of the electron transport layer, and its structure is one of type II heterojunction, quasi-type II heterojunction, and type I heterojunction, and its thickness is 10 to 1000 nm.

[0010] Preferably, the thickness of the MoO3 hole transport layer is 1 to 50 nm.

[0011] Preferably, the thickness of the ITO thin film layer of the bottom electrode is 10-500 nm; the material of the top metal electrode is one of Au, Ag, Al, Cu or Pt, and the thickness is 10-1000 nm.

[0012] According to a second aspect of the present invention, a method for preparing a quantum dot-based self-driven photodetector is provided, comprising the following steps in sequence:

[0013] 1. Spin-coat a layer of ZnO nanoparticle film on the cleaned ITO glass as an electron transport layer;

[0014] Second, a CdSe / CdS core-shell quantum dot film was deposited on the electron transport layer as a photoactive layer by using a layer-by-layer spin coating and ligand exchange method, and annealed at 60°C for 10 min;

[0015] 3. Vapor deposit a layer of MoO3 on the photoactive layer as a hole transport layer;

[0016] 4. Using a metal mask to evaporate a metal electrode layer on the hole transport layer.

[0017] Preferably, the ITO glass may be FTO glass, or one of transparent flexible substrates such as ITO / PDMS, ITO / PET, FTO / PDMS, FTO / PET, etc.

[0018] Preferably, the CdSe quantum dots are prepared by the following method: CdSe quantum dots are synthesized by hot injection. First, 0.0488g of CdO is mixed with 1ml of oleic acid and 9mL of octadecene (ODE) solution in a three-necked flask and stirred thoroughly. Subsequently, nitrogen is passed through the mixed solution at room temperature for 30min, and the temperature is raised to 110°C and nitrogen is continued for 30min to exclude impurities such as water and oxygen in the mixed solution. Next, the temperature is raised to 220°C under nitrogen to obtain a colorless clear solution. When the reaction temperature drops to room temperature, 1g of trioctylphosphine oxide (TOPO) is added to the reaction solution, and the above-mentioned impurity removal step (nitrogen is passed through the mixed solution at room temperature for 30min, and the temperature is raised to 110°C and nitrogen is continued for 30min to exclude impurities such as water and oxygen in the mixed solution) is repeated to obtain a Cd ion solution. After the reaction temperature was raised to 300°C, 0.3158 g of selenium powder was dissolved in 4 mL of trioctyl phosphate (TOP), diluted with 1 mL of octadecene (ODE) and 3 mL of oleylamide (OLA), and then quickly injected into the above Cd ion solution. The size of CdSe quantum dots is well controlled by the reaction time. When the size of the quantum dots reaches the required requirements, the reaction is terminated by water cooling. The relationship between the size of CdSe quantum dots and the reaction time is shown in Figure 2As shown. Subsequently, the reaction solution was centrifuged twice using acetone as a solvent to purify the CdSe quantum dots. Finally, the purified CdSe quantum dots were dissolved in toluene for subsequent shell growth.

[0019] Preferably, the CdSe / CdS core-shell quantum dots are prepared by the following method: Before preparing the CdS shell layer, a precursor solution is prepared by the following method: at a temperature of 250°C, 4 mmol of CdO is dissolved in a mixed solution of 5.6 mL of oleic acid and 14.4 mL of octadecene (ODE) to obtain a 0.2 M cationic precursor (Cd-oleic acid) solution. At a temperature of 200°C, 4 mmol of sulfur is dissolved in a 20 ml octadecene (ODE) solution to obtain an anionic precursor (S-ODE) solution. The above precursor solutions are all prepared under nitrogen gas conditions.

[0020] Preparation of CdS shell: The prepared CdSe quantum dot toluene solution was mixed with 8mL of oleylamide (OLA) and 8mL of octadecene (ODE) in a 100ml three-necked flask and degassed at 120°C for 30min. After water, oxygen and toluene were completely removed, nitrogen was introduced into the reaction solution and the reaction temperature was raised to 240°C. Subsequently, the thickness of the CdS shell was controlled by the ion layer continuous adsorption reaction technology. Usually, the growth of a single layer of CdS is achieved by alternately injecting the precursor solution as follows: a certain amount of Cd-oleic acid (Cd-OA) precursor solution was added dropwise to the CdSe quantum dot solution and reacted for 2.5h, and an equal amount of S-octadecene (S-ODE) precursor solution was added dropwise to the mixed solution and reacted for 30min to obtain a single layer of CdS. The thickness of the CdS shell layer in the quantum dot heterojunction of the present invention is controlled to be 6 layers, the thickness of each single layer is 0.35nm, and the shell thickness is 2.1nm (shell diameter is 4.2nm). By changing the size of the core layer CdSe quantum dots to 2.8nm, 3.5nm and 4.8nm, three core-shell quantum dot heterojunctions with different energy band structures can be obtained. The concentrations of the Cd-OA precursor solution and the S-OED solution in the above reaction are both 0.2M.

[0021] The advantages of the technical solution provided by the present invention are:

[0022] 1. By fixing the thickness of the CdS shell and changing the size of the CdSe core, three different structural types of CdSe / CdS core-shell quantum dots were prepared, namely type II, quasi-type II and type I heterojunctions; the band structure regulation of the core-shell quantum dot heterojunction was achieved.

[0023] 2. Construct a self-driven photodetector based on CdSe / CdS core-shell quantum dots, realizing the regulation of the device's photoelectric performance and self-driven photodetection function. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the photodetector device structure.

[0025] Marked in the figure: 1-substrate, 2-conductive electrode layer, 3-electron transport layer, 4-photosensitive sensing material, 5-hole transport layer, 6-metal electrode layer

[0026] Figure 2 This is a graph showing the relationship between CdSe quantum dot size and reaction time.

[0027] Figure 3 The band structure diagrams of three different structural types of core-shell quantum dots.

[0028] Figure 4 This is a performance diagram of the device tested in Example 1.

[0029] Figure 5 This is a performance diagram of the device tested in Example 2.

[0030] Figure 6 This is a performance diagram of the device tested in Example 3.

[0031] Figure 7 It is a comparison chart of device performance of Example 1, Example 2 and Example 3. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with the embodiments, but are not intended to limit the present invention.

[0033] Example 1

[0034] CdSe / CdS core-shell quantum dots were prepared by the following method:

[0035] First, CdSe quantum dots were synthesized by hot injection. 0.0488g of CdO was mixed with 1ml of oleic acid and 9mL of octadecene (ODE) in a three-necked flask and stirred thoroughly. Subsequently, nitrogen was introduced into the mixed solution at room temperature for 30min, and the temperature was raised to 110℃ and nitrogen was continued to be introduced for 30min to remove impurities such as water and oxygen in the mixed solution. Next, the temperature was raised to 220℃ under nitrogen flow to obtain a colorless clear solution. When the reaction temperature dropped to room temperature, 1g of trioctylphosphine oxide (TOPO) was added to the reaction solution, and the above impurity removal steps were repeated to obtain a Cd ion solution. After the reaction temperature was raised to 300℃, 0.3158g of selenium powder was dissolved in 4mL of trioctyl phosphate (TOP), diluted with 1mL of ODE and 3mL of oleamide (OLA), and then quickly injected into the above Cd ion solution. The size of CdSe quantum dots is well controlled by the reaction time. When the size of the quantum dots reaches the required requirements, the reaction is terminated by water cooling. Subsequently, the reaction solution was centrifuged twice using acetone as solvent to purify the CdSe quantum dots. Finally, the purified CdSe quantum dots were dissolved in toluene for subsequent shell growth.

[0036] Before preparing the CdS shell, the precursor solution was prepared as follows: 4 mmol of CdO was dissolved in 5.6 mL of oleic acid and 14.4 mL of ODE at 250°C to obtain a 0.2 M cationic precursor (Cd-oleic acid) solution. 4 mmol of sulfur was dissolved in 20 mL of octadecene (ODE) solution at 200°C to obtain an anionic precursor (S-ODE) solution. The above precursor solutions were all prepared under nitrogen gas flow and the concentration was 0.2 M.

[0037] Then, the ion layer continuous adsorption reaction technology is used to prepare and control the thickness of the CdS shell. The prepared CdSe quantum dot toluene solution is first mixed with 8mL of oleamide (OLA) and 8mL of octadecene (ODE) in a 100ml three-necked flask and degassed at 120°C for 30min. After water, oxygen and toluene are completely removed, nitrogen is introduced into the reaction solution and the reaction temperature is raised to 240°C. Usually, the growth of a single layer of CdS is achieved by alternately injecting the precursor solution as follows: a certain amount of Cd-oleic acid (Cd-OA) precursor solution is added dropwise to the CdSe quantum dot solution and reacted for 2.5h, and an equal amount of S-octadecene (S-ODE) precursor solution is added dropwise to the mixed solution for 30min of reaction to obtain a single layer of CdS.

[0038] In the synthesis process of CdSe / CdS core-shell quantum dots, by controlling the reaction conditions to fix the CdS shell thickness at about ~2.1nm (shell diameter ~4.2nm), and changing the size of the core layer CdSe quantum dots to 2.8nm, 3.5nm and 4.8nm, three different structural types of core-shell quantum dot heterojunctions can be obtained. When the size of the core layer CdSe quantum dots is 2.8nm, a type II heterojunction can be obtained; when the size of the core layer CdSe quantum dots is 3.5nm, a quasi-type II heterojunction can be obtained; when the size of the core layer CdSe quantum dots is 4.8nm, a type I heterojunction can be obtained. According to the UPS characterization method combined with the UV-visible absorption spectrum, the band structures of the three heterojunctions can be determined as follows: Figure 3 shown.

[0039] The preparation method of CdSe / CdS core-shell quantum dot type II heterojunction self-driven photodetector device is as follows: first, a layer of ZnO nanoparticle film is spin-coated on a cleaned ITO glass substrate as an electron transport layer; then, a CdSe / CdS core-shell quantum dot film with a type II heterojunction structure is deposited on the electron transport layer as a photosensitive active layer by layer spin coating and ligand exchange method, and annealed at 60°C for 10 minutes; next, a layer of MoO3 is evaporated on the photosensitive active layer as a hole transport layer; finally, a layer of gold film is evaporated on the hole transport layer as a metal electrode layer using a metal mask.

[0040] The time current response test results of the self-driven photodetector device of type II heterojunction quantum dots are shown in Figure 4 As shown. Under 405nm wavelength incident light, the optical power density is 94.93mW / cm 2 When the device is biased at 0V, the ratio of the light current to the dark current is greater than 10 5 After 6 cycles of optical switching experiments, the photocurrent and dark current of the device did not fluctuate or decay significantly, showing good electrical stability. Under 0V bias conditions, the maximum responsivity of the device was 10.64mA / W, and the detection rate was 2.63×10 10 Jones.

[0041] The above research results show that the photodetector based on type II heterojunction quantum dots has excellent photoelectric response performance and self-driven photodetection function under 0V bias conditions. The excellent photoelectric performance of the device depends on: the designed and prepared type II heterojunction CdSe / CdS core-shell quantum dot photosensitive material has excellent light absorption performance, good energy level matching, and a built-in electric field is generated at the heterojunction interface. Under the action of the built-in electric field, the photogenerated carriers generated in the quantum dots are quickly separated and transmitted at the type II heterojunction interface, thereby producing excellent photoelectric performance. In addition, the heterojunction photodetector with a vertical structure constructed by the present invention can effectively realize the self-driven photodetection of the device.

[0042] Example 2: Please refer to the synthesis method of CdSe / CdS core-shell quantum dots in Example 1. Control the thickness of the core layer CdSe to 3.5nm, and fix the thickness of the shell layer CdS to 6 monolayers to 2.1nm. The resulting CdSe / CdS core-shell structure is a quasi-type II heterojunction. Please refer to the preparation method of the photodetector device in Example 1 to construct a quasi-type II heterojunction photodetector based on CdSe / CdS quantum dots. The test conditions of the obtained device refer to Example 1, and the test results of the device photoelectric performance are shown as follows: Figure 5 shown.

[0043] Depend on Figure 5 The results of the study show that under the same incident laser wavelength and optical power density as in Example 1, the quasi-type II heterojunction photodetector based on CdSe / CdS core-shell quantum dots has no obvious fluctuation and attenuation of dark current and photocurrent under 0V bias after 6 cycles of optical switching cycles, showing good electrical stability. Further analysis shows that the device has a fast response speed to 405nm fluctuating visible light and a high light-dark current switching ratio, with a light-dark current ratio of 1.5×10 3 Compared with the type II heterojunction quantum dot device in Example 1, the bright current to dark current ratio of the quasi-type II heterojunction quantum dot device in Example 2 is reduced by about 2 orders of magnitude.

[0044] The above research results show that the quasi-type II heterojunction photodetector based on CdSe / CdS quantum dots has good photoelectric response performance and also has self-driven photoelectric detection function under 0V bias conditions. However, the photoelectric response performance of the device in Example 2 is significantly lower than that of the device in Example 1. This is because: although the quasi-type II heterojunction CdSe / CdS core-shell quantum dot photosensitive material has good light absorption performance, its energy level matching is not as good as the band matching effect of the type II heterojunction in Example 1. Therefore, its built-in electric field is significantly lower than that of the type II heterojunction in the separation and transmission efficiency of photoelectric carriers, thereby affecting the photoelectric performance of the device.

[0045] Example 3: Please refer to the synthesis method of CdSe / CdS core-shell quantum dots in Example 1. Control the thickness of the core layer CdSe to 4.8nm, and fix the thickness of the shell layer CdS to 6 monolayers to 2.1nm. The resulting CdSe / CdS core-shell structure is an I-type heterojunction. Please refer to the preparation method of the photodetector device in Example 1 to construct an I-type heterojunction photodetector based on CdSe / CdS quantum dots. The test conditions of the obtained device refer to Example 1, and the test results of the photoelectric performance of the device are shown in Figure 1. Figure 6 shown.

[0046] Depend on Figure 6 The research results show that under the same conditions of incident laser wavelength and optical power density as in Example 1, the photoelectric response of the I-type heterojunction photodetector based on CdSe / CdS core-shell quantum dots is very weak under 0V bias, and there is no obvious self-driven photoelectric detection function. Compared with the II-type heterojunction quantum dot device in Example 1 and the quasi-II-type heterojunction device in Example 2, the light-dark current ratio of the I-type heterojunction quantum dot device in Example 3 is significantly reduced, by nearly 5 orders of magnitude and 3 orders of magnitude, respectively. The reason is that the CdSe / CdS quantum dot I-type heterojunction in Example 3 forms a quantum well at the interface, making it impossible to form an effective built-in electric field at the interface. Therefore, under 0V bias, the photogenerated carriers generated in the quantum dots cannot be effectively separated and transmitted at the heterojunction interface, resulting in the recombination of photogenerated electrons and photogenerated holes, which greatly affects the photoelectric performance of the device, and thus the self-driven photoelectric detection function cannot be achieved.

[0047] Comparative Example 1: Please refer to Example 1, Example 2 and Example 3 to prepare three different types of quantum dot heterojunction photodetectors: CdSe / CdS quantum dot II-type heterojunction photodetector, CdSe / CdS quantum dot quasi-II-type heterojunction photodetector, CdSe / CdS quantum dot I-type heterojunction photodetector. Control the test conditions, under the same incident light wavelength and light power density as Example 1, compare the photoelectric performance of the three different types of photodetector devices under 0V bias. The comparison results are as follows: Figure 7 shown.

[0048] Depend on Figure 7 The results show that the light-dark current density ratio of the type II heterojunction quantum dot device is the largest, with a ratio as high as 1.1×10 5 The ratio of the bright current to the dark current of the quasi-type II heterojunction device has decreased significantly, and its ratio is about 1.5×10 3; However, the light-dark current ratio of the I-type heterojunction quantum dot device is sharply reduced, and its ratio is only 1.3, and there is no obvious self-driven photoelectric detection function. Compared with the photoelectric performance of the devices in Example 2 and Example 3, the light-dark current ratio of the II-type heterojunction quantum dot photodetector under 0V bias in Example 1 increased by 2 orders of magnitude and 5 orders of magnitude, respectively. This excellent photoelectric performance is due to the fact that the designed and prepared II-type heterojunction CdSe / CdS core-shell quantum dot photosensitive material has excellent visible light absorption ability, and the II-type heterojunction has good energy level matching, and can generate a large built-in electric field at the heterojunction interface. Under the action of the built-in electric field, the photogenerated carriers generated in the quantum dots can be quickly separated and transmitted at the II-type heterojunction interface, thereby producing excellent photoelectric performance and self-driven photoelectric detection function.

[0049] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A self-driven photodetector device, which comprises, from bottom to top, a glass substrate, an ITO film layer, an electron transport layer of ZnO nanoparticles, a CdSe / CdS core-shell quantum dot photosensitive material film layer, a MoO3 hole transport layer, and a metal electrode layer, wherein the ITO film layer is stacked on the surface of the glass substrate to form an ITO glass substrate, wherein the ITO film layer and the metal electrode layer serve as the bottom and top electrode layers of the device, respectively; The CdSe / CdS core-shell quantum dots are obtained by fixing the thickness of the CdS shell layer and adjusting the size of the CdSe core layer to obtain CdSe / CdS core-shell quantum dots with type II heterojunction; The CdSe / CdS core-shell quantum dots are type II heterojunctions, which are prepared by fixing the thickness of the CdS shell layer to 2.1 nm and changing the size of the CdSe core layer to 2.8 nm.

2. The self-driven photodetector device according to claim 1, characterized in that: The ITO thin film layer is stacked on a glass substrate, and the thickness of the ITO thin film layer is 10-500 nm.

3. The self-driven photodetector device according to claim 1, characterized in that: The CdSe / CdS core-shell quantum dot photosensitive material film layer is stacked on the surface of the electron transport layer, and its thickness is 10-1000 nm.

4. The self-driven photodetector device according to claim 1, characterized in that: The thickness of the MoO3 hole transport layer is 1-50 nm.

5. The self-driven photodetector device according to claim 1, characterized in that: The thickness of the ITO thin film layer of the bottom electrode is 10-500 nm; the material of the top metal electrode is one of Au, Ag, Al, Cu or Pt, and the thickness is 10-1000 nm.

6. A method for preparing the photodetector device according to claim 1, comprising the steps of:

1. Spin-coat a layer of ZnO nanoparticle film on the cleaned ITO glass as an electron transport layer; Second, a CdSe / CdS core-shell quantum dot film was deposited on the electron transport layer as a photoactive layer by using a layer-by-layer spin coating and ligand exchange method, and annealed at 60°C for 10 min; 3. Vapor deposit a layer of MoO3 on the photoactive layer as a hole transport layer; 4. Using a metal mask to evaporate a metal electrode layer on the hole transport layer.

7. The method according to claim 6, characterized in that: The CdSe / CdS core-shell quantum dots are prepared by the following method: (1) CdSe quantum dots are prepared by the following method: CdSe quantum dots are synthesized by hot injection method; CdO is mixed with oleic acid and octadecene, and the mixture is stirred thoroughly to obtain a mixed solution; nitrogen is introduced into the mixed solution at room temperature, and the temperature is increased to continue to introduce nitrogen to remove water and oxygen impurities in the mixed solution; the temperature is increased under the condition of introducing nitrogen to obtain a colorless clear solution; when the reaction temperature drops to room temperature, trioctylphosphine oxide is added to the reaction solution, and the above impurity removal steps are repeated to obtain a Cd ion solution; After the reaction temperature is raised to 300°C, the selenium powder is dissolved in trioctyl phosphate, diluted with ODE and oleamide, and then quickly injected into the above Cd ion solution; the size of the CdSe quantum dots is controlled by the reaction time, and when the size of the quantum dots reaches the required requirement, the reaction is terminated by water cooling; acetone is used as a solvent, and the reaction solution is centrifuged to purify the CdSe quantum dots; the purified CdSe quantum dots are dissolved in toluene for subsequent shell growth; (2) CdSe / CdS core-shell quantum dots were prepared by the following method: Before preparing the CdS shell, the precursor solution was prepared as follows: CdO was dissolved in a mixture of oleic acid and ODE to obtain a 0.2 M cationic precursor Cd-oleic acid solution; Dissolving sulfur into the ODE solution to obtain an anion precursor S-ODE solution; Preparation of CdS shell: using ion layer continuous adsorption reaction technology to control the thickness of CdS shell, the prepared CdSe quantum dot toluene solution is mixed with oleamide and octadecene, and degassed; after water, oxygen and toluene are completely removed, nitrogen is introduced into the reaction solution and the reaction temperature is increased; the growth of a single layer of CdS is achieved by alternately injecting precursor solutions as follows: a certain amount of Cd-oleic acid solution is added drop by drop into the CdSe quantum dot solution and reacted, and an equal amount of S-ODE precursor solution is added drop by drop into the mixed solution to react, thereby obtaining a single layer of CdS; the thickness of the CdS shell in the quantum dot heterojunction is controlled to be 6 layers, each single layer is 0.35 nm thick, the shell thickness is 2.1 nm, and the shell diameter is 4.2 nm.

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