CN110408382B

CN110408382B - Core-shell semiconductor nanosheet, and preparation method and application thereof

Info

Publication number: CN110408382B
Application number: CN201910704614.0A
Authority: CN
Inventors: 彭笑刚; 王永红
Original assignee: Zhejiang University ZJU; Najing Technology Corp Ltd
Current assignee: Zhejiang University ZJU; Najing Technology Corp Ltd
Priority date: 2019-07-31
Filing date: 2019-07-31
Publication date: 2021-04-27
Anticipated expiration: 2039-07-31
Also published as: CN110408382A

Abstract

The invention provides a core-shell semiconductor nanosheet, a preparation method and an application thereof. The preparation method of the core-shell semiconductor nanosheet comprises the following steps: s1, preparing a solution containing wurtzite nanocrystals; s2, mixing a first cation precursor, a first anion precursor, a first ligand, a first solvent and a solution containing wurtzite nanocrystals, taking the wurtzite nanocrystals as cores, and carrying out heating reaction to carry out epitaxial growth on the surfaces of the cores to obtain a first product system containing the core-shell semiconductor nanosheets. In the preparation method, wurtzite nanocrystals are selected as the cores, and the surface crystal faces of the nanoparticles in the growth process are selectively passivated by the first ligand, so that different crystal faces are controlled to have different growth positions, the size of each dimension of the semiconductor nanosheet can be controlled, and an anisotropic structure can be epitaxially grown.

Description

Core-shell semiconductor nanosheet, and preparation method and application thereof

Technical Field

The invention relates to the field of synthesis of nano materials, in particular to a core-shell semiconductor nanosheet, a preparation method and application thereof.

Background

The semiconductor nanocrystal has quantum confinement effect in three dimensions, has the characteristics of continuous and adjustable fluorescence, wide absorption, narrow emission, high color purity, high quantum efficiency, good light stability and the like, and is widely applied to the fields of solar cells, illumination, display, laser, biological marking, imaging and the like. The two-dimensional semiconductor nano-sheet has a confinement effect in one direction, so that the two-dimensional semiconductor nano-sheet has luminous anisotropy, and has more obvious advantages in the application of light-emitting diodes, lasers and the like. At present, two-dimensional semiconductor nano sheets with a single structure and core-shell structures with the two-dimensional semiconductor nano sheets with the single structure as cores are successfully obtained. Although the two-dimensional semiconductor nanosheets with single structures have atomically flat surfaces, the optical properties such as fluorescence quantum yield and fluorescence stability are insufficient for practical application. The current research literature on two-dimensional core/shell structures is less, and the control of their optical properties is far from that of zero-dimensional core/shell structures and one-dimensional core/shell structures. On the other hand, the sizes of the C axis and the AB surface of the nanosheet cannot be flexibly adjusted in the prior art.

Disclosure of Invention

The invention mainly aims to provide a core-shell semiconductor nanosheet, a preparation method and an application thereof, and aims to solve the problems that the core-shell semiconductor nanosheet in the prior art is poor in optical property and cannot be effectively adjusted in size.

In order to achieve the above object, according to an aspect of the present invention, there is provided a method for preparing a core-shell semiconductor nanosheet, including: s1, preparing a solution containing wurtzite nanocrystals; and S2, mixing a first cation precursor, a first anion precursor, a first ligand, a first solvent and the solution containing the wurtzite nanocrystals, wherein the wurtzite nanocrystals are used as cores, and heating to react so as to perform shell epitaxial growth on the surfaces of the cores, thereby obtaining a first product system containing the core-shell semiconductor nanosheets.

Further, the wurtzite nanocrystal in the wurtzite nanocrystal-containing solution is CdSe, the cation in the first cation precursor is Cd, and the anion in the first anion precursor is S.

Further, the molar ratio of the first cation precursor to the first anion precursor is less than 1, preferably 1: 1.5-1: 2.

further, the first ligand is selected from one or more of a first fatty amine and a first fatty acid, and preferably, the molar ratio of the first ligand to the first cation precursor is less than 15.

Further, the first ligand is a first fatty acid, and the molar ratio of the first cation precursor to the first fatty acid is 1: 5-1: 10.

further, the first ligand is a first fatty amine and the first fatty acid, and the molar ratio of the first fatty amine, the first fatty acid, and the first cation precursor is 5:5: 1.

further, the reaction temperature in the above S2 is 220 to 280 ℃.

Further, the step S1 includes mixing a second cation precursor, a second anion precursor, a second ligand and a second solvent, heating and reacting the mixture to obtain a second product system containing wurtzite nanocrystals, separating and purifying the second product system to obtain the wurtzite nanocrystals, and dissolving the wurtzite nanocrystals in a third solvent to obtain the solution containing the wurtzite nanocrystals.

Further, the step S2 is further to separate and purify the first product system to obtain the core-shell semiconductor nanosheet.

Further, the preparation method further comprises the following steps: and S3, performing surface treatment on the core-shell semiconductor nanosheets to remove surface defects, and obtaining core-shell semiconductor nanosheets with optimized performance.

Further, S3 includes S3a, and the first surface-treated core-shell semiconductor nanosheet is obtained by mixing a second aliphatic amine, a fourth solvent, the core-shell semiconductor nanosheet, and the first anion precursor, holding for a certain time at 80 to 150 ℃, and purifying and separating.

Further, the S3 further includes S3b, the first surface-treated core-shell semiconductor nanosheet, the third aliphatic amine and the fifth solvent are mixed, heated to 150 to 250 ℃, added with trialkylphosphine for reaction for a certain time, cooled to 0 to 100 ℃, and added with the short-chain carboxylic acid precursor of the first cation and the fourth aliphatic amine to obtain a third product system containing the second surface-treated core-shell semiconductor nanosheet.

And further comprising S3c, and performing illumination treatment on the third product system to obtain a third surface-treated core-shell semiconductor nanosheet.

In the preparation method, the wurtzite nanocrystals are selected as the cores, and the first ligand is presumed to selectively passivate the crystal faces of the surfaces of the nanoparticles in the growth process, so that different crystal faces are controlled to have different growth positions, the size of each dimension of the semiconductor nanosheet can be controlled, and an anisotropic structure can be formed by epitaxial growth. On the other hand, the first cationic precursor also affects the growth rate, so the first ligand and the first cationic precursor jointly regulate the growth of the semiconductor nanosheets.

In addition, the core-shell semiconductor nanosheet with higher optical performance is obtained through further surface optimization treatment.

According to another aspect of the present application, there is provided a core-shell semiconductor nanosheet, wherein a core of the core-shell semiconductor nanosheet is made of a first semiconductor material, and the core is a wurtzite nanocrystal located at a central position of the core-shell semiconductor nanosheet; the shell is made of a second semiconductor material, and the core-shell semiconductor nanosheet has anisotropic luminescence.

Further, the core-shell semiconductor nanosheet is made of CdSe as the first semiconductor material, CdS as the second semiconductor material, the fluorescence quantum yield of the semiconductor nanosheet is greater than or equal to 90%, and the fluorescence attenuation curve of the core-shell semiconductor nanosheet is single-index and fluorescence non-flickering.

The core-shell semiconductor nanosheet with the structure has outstanding optical performance and can be used for practical application. According to a further aspect of the present invention, there is provided a device comprising a core-shell semiconductor nanosheet prepared by any one of the methods described above or any one of the core-shell semiconductor nanosheets described above.

According to a further aspect of the present invention, there is provided a composition comprising core-shell semiconductor nanoplatelets prepared by any of the above methods or any of the above core-shell semiconductor nanoplatelets.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:

fig. 1 shows Transmission Electron Microscopy (TEM) images of different orientations of semiconductor nanoplates according to example 2 of the present invention;

figure 2 shows high resolution TEM images of different crystallographic planes of semiconductor nanoplatelets according to example 2 of the present invention;

fig. 3 shows a TEM image of a comparative example 1 semiconductor nanoplate;

fig. 4 shows a TEM image of a semiconductor nanoplate of example 11;

FIG. 5 shows Selected Area Electron Diffraction (SAED) and X-ray diffraction (XRD) pattern tests performed on CdSe @ CdS semiconductor nanosheets of example 2;

figure 6 shows a TEM plot of the rotation angle from-30 ° to +30 ° of semiconductor nanoplates according to example 2 of the present invention;

fig. 7 shows TEM images of semiconductor nanoplates according to example 1 of the present invention, wherein the right image is a high resolution TEM image;

fig. 8 shows a TEM image of the semiconductor nanoplate of comparative example 2, wherein the right image is a high resolution TEM image;

fig. 9 shows TEM images of semiconductor nanosheets prepared in example 3, example 6, and comparative example 3, respectively, from left to right;

figure 10 shows TEM images of semiconductor nanoplates at different reaction times according to example 5 of the present invention;

fig. 11 shows the change of fluorescence and uv-vis absorption spectra during the growth of semiconductor nanoplates according to example 5 of the present invention;

figure 12 shows SAED plots of semiconductor nanoplates at different reaction times according to example 5 of the present invention;

figure 13 left image shows TEM variation of semiconductor nanoplates during the reaction of example 8; figure 13 right graph shows a plot of the variation in size and thickness of AB face diagonal of semiconductor nanoplates during the reaction of example 8;

figure 14a shows a TEM image of a semiconductor nanoplate of example 7;

fig. 14b shows a graph of the variation in size and thickness of AB face diagonal of the semiconductor nanoplates of example 7;

FIG. 15a shows plots of fluorescence efficiency change (PL QY) before and after surface treatment of an assembly of semiconductor nanoplatelets (6 nm in thickness and 10.6nm in lateral dimension) of example 5;

figure 15b shows a semiconductor nanoplate assembly fluorescence kinetic decay curve;

fig. 15c shows a comparison graph of the luminescence anisotropy property of semiconductor nanoplates and theoretical values;

figure 15d shows a single-particle semiconductor nanoplate fluorescence stability profile;

fig. 15e shows the bright state distribution ratio in the fluorescence stability test of 91 semiconductor nanosheets;

fig. 15f shows fluorescence peak position and fluorescence half-peak width distribution diagrams of 53 semiconductor nanosheets; and

fig. 16 shows TEM images of the nanoplatelets prepared in example 9, example 5, example 10, respectively, from left to right.

Detailed Description

It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.

As analyzed by the background art, the optical properties are poor in the prior art, and the size of the nanosheet cannot be effectively adjusted.

In an exemplary embodiment of the present application, there is provided a method for preparing a core-shell semiconductor nanosheet, including: s1, preparing a solution containing wurtzite nanocrystals; and S2, mixing a first cation precursor, a first anion precursor, a first ligand, a first solvent and the solution containing the wurtzite nanocrystals, wherein the wurtzite nanocrystals are used as cores, and heating to react so as to perform shell epitaxial growth on the surfaces of the cores, thereby obtaining a first product system containing the core-shell semiconductor nanosheets. In the preparation method, the wurtzite nanocrystals are selected as the cores, and the selective passivation of the crystal faces of the surfaces of the nanoparticles in the growth process is presumed by the first ligand, so that the selective passivation can be realized because the crystal faces comprise polar faces and non-polar faces, different crystal faces are controlled to have different growth positions, the dimensions of the semiconductor nanosheets in all dimensions are controlled, and the semiconductor nanosheets can be epitaxially grown into an anisotropic structure. On the other hand, the first cation precursor also affects the growth rate of the semiconductor nanosheets, and thus the first ligand and the first cation precursor together regulate the growth of the semiconductor nanosheets. The selection of the first cation precursor and the first anion precursor, the first ligand and the first solvent can be raw materials used for conventional semiconductor nanocrystal synthesis, and the raw material selection is carried out according to the specific material of the semiconductor nanosheet to be synthesized. The prior art can be referred to how to synthesize wurtzite nanocrystals.

In some embodiments, the wurtzite nanocrystal in the wurtzite nanocrystal-containing solution is CdSe, the cation in the first cation precursor is Cd, and the anion in the first anion precursor is S.

In some embodiments, targeted for synthesis of CdSe @ CdS semiconductor nanoplates, the molar ratio of the first cationic precursor to the first anionic precursor is less than 1, preferably 1: 1.5-1: 2. within the aforementioned preferred ranges, it is easier to synthesize anisotropic nanoplates at a molar ratio based on the molar ratio of cations to anions.

In some embodiments, the first ligand is selected from one or more of a first fatty amine and a first fatty acid, preferably, the molar ratio of the first ligand to the first cationic precursor is less than 15. Thermodynamically, wurtzite nanocrystals are more likely to produce elongated hexagonal structures, such as nanorods, and by selecting ligands suitable for synthesizing semiconductor nanoplatelets, such as one or more selected from the group consisting of a first fatty amine and a first fatty acid, such that the polar faces are passivated, thereby modulating the growth rate of the polar faces to be less than the growth rate of the non-polar faces, epitaxial growth will grow in the desired direction. When the first ligand is the first aliphatic amine, the above molar ratio is more than 15, it may be easy to generate a nanorod structure, and the first aliphatic amine is presumed to be mainly used for controlling the C-axis size (thickness). When the first ligand is the first fatty acid, the mole ratio is more than 15, the first fatty acid may selectively etch CdS self-nucleation, and the etched cadmium salt selectively passivates a polar surface, so that the epitaxial growth of a non-polar surface is promoted, and a large-size semiconductor nanosheet can be obtained. However, excessive amounts of the first fatty acid in the system can cause inter-particle ripening to occur, thereby reducing the size unity between the semiconductor nanosheets. Through experiments, it is speculated that the first fatty acid is mainly used to control the AB plane size (lateral size). It is to be construed that the first ligand is the starting material for the synthesis. When the first ligand is the first fatty acid, the first fatty acid serves as a ligand raw material, but the ligand of the semiconductor nanosheet is substantially the corresponding first fatty acid salt, not the first fatty acid.

In some embodiments, the first ligand is a first fatty acid, and the molar ratio of the first cation precursor to the first fatty acid is 1: 5-1: 10. the range of the molar ratio is beneficial to synthesizing the size of the conventional semiconductor nano-sheet, and the transverse size of the semiconductor nano-sheet can be regulated and controlled by regulating the proportion.

In some embodiments, the first ligand is a first fatty amine and the first fatty acid, the ratio of the first fatty amine: the first fatty acid: the molar ratio of the first cation precursor is 5:5: 1. the molar ratio range is favorable for synthesizing the size of the conventional semiconductor nanosheet, and the longitudinal and transverse sizes of the semiconductor nanosheet can be regulated and controlled by regulating the proportion.

In some embodiments, the number of C atoms of the first aliphatic amine is 8 to 18.

In some embodiments, the number of C atoms in the first fatty acid is 8 to 18.

In some embodiments, the reaction temperature in S2 is 220 ℃ to 280 ℃. The reaction temperature of S2 affects the size uniformity of the semiconductor nanoplatelets, and therefore growth in a suitable temperature range is required to improve size uniformity.

In some embodiments, in S2, the first cation precursor, the first anion precursor, the first ligand, and the first solvent are mixed in advance to form a mixed precursor, and then the mixed precursor is added in portions to the solution containing wurtzite nanocrystals to participate in the reaction, wherein the addition in portions can improve the size uniformity of the semiconductor nanosheets, and thus improve the optical properties. The speed of addition in portions (interval time and amount of addition per time) can be obtained from a groping experiment.

In some embodiments, S1 includes mixing and heating a second cation precursor, a second anion precursor, a second ligand, and a second solvent to obtain a second product system containing wurtzite nanocrystals, separating and purifying the second product system to obtain the wurtzite nanocrystals, and dissolving the wurtzite nanocrystals in a third solvent to obtain a solution containing the wurtzite nanocrystals.

In some embodiments, the S2 further comprises separating and purifying the first product system to obtain the core-shell semiconductor nanosheet. The method for separation and purification can refer to the existing nanocrystalline separation and purification technology, such as a solvent precipitation and redissolution method.

In some embodiments, the above preparation method further comprises: and S3, performing surface treatment on the core-shell semiconductor nanosheets to remove surface defects, and obtaining core-shell semiconductor nanosheets with optimized performance. Through optimization, the optical performance of the prokaryotic shell semiconductor nanosheet can be improved. The surface treatment may be a combination of one or more methods. The S3 may be repeated a plurality of times to perform the elimination of the surface defects.

In some embodiments, the S3 includes S3a, and the first surface-treated core-shell semiconductor nanosheet is obtained by mixing a second aliphatic amine, a fourth solvent, the core-shell semiconductor nanosheet, and the first anionic precursor, holding for a certain time at 80 ℃ to 150 ℃, and performing purification and separation. In some embodiments, this S3a can be repeated multiple times until the fluorescence intensity of the core-shell semiconductor nanoplatelets is zero. The second aliphatic amine replaces the original ligand of the core-shell semiconductor nanosheet, so that the surface of the semiconductor nanosheet is an anionic surface (namely a polar surface).

In some embodiments, the S3 further includes S3b, and the first surface-treated core-shell semiconductor nanosheet, the third aliphatic amine, and the fifth solvent are mixed, heated to 150 ℃ to 250 ℃, added with trialkylphosphine for reaction for a certain time, cooled to 0 ℃ to 100 ℃, and added with the short-chain carboxylic acid precursor of the first cation and the fourth aliphatic amine to obtain a third product system containing the second surface-treated core-shell semiconductor nanosheet. In some embodiments, the number of C atoms of the short chain carboxylic acid precursor is 2-8. The S3b further passivates the surface of the core-shell semiconductor nanosheet, so that the surface coordination is complete and the surface defects are eliminated. The first cation is a cation in the first cation precursor.

In some embodiments, the second aliphatic amine and the third aliphatic amine are each independently selected from one or more aliphatic amines having 8 to 18 carbon atoms. In some embodiments, when the second fatty amine and the third fatty amine each comprise a mixture of fatty amines with different chain lengths, the solubility of the semiconductor nanosheets can be increased according to the principle of entropic ligands.

In some embodiments, the first solvent, the second solvent, the third solvent, the fourth solvent, and the fifth solvent are each independently selected from non-coordinating organic solvents.

In some embodiments, further comprising S3c, the third product system described above is subjected to light irradiation to obtain a third surface-treated core-shell semiconductor nanoplate. The light treatment can remove surface defects caused by hydrogen sulfide.

According to another aspect of the present application, there is provided a core-shell semiconductor nanosheet, wherein a core of the core-shell semiconductor nanosheet is made of a first semiconductor material, and the core is a wurtzite nanocrystal located at a central position of the core-shell semiconductor nanosheet; the shell of the core-shell semiconductor nanosheet is made of a second semiconductor material, and the core-shell semiconductor nanosheet has anisotropic luminescence.

In some embodiments, the first semiconductor material is CdSe, the second semiconductor material is CdS, the fluorescence quantum yield of the semiconductor nanosheet is greater than or equal to 90%, and the fluorescence decay curve of the core-shell semiconductor nanosheet is single-exponential and is fluorescence non-blinking.

According to a further aspect of the present invention, there is provided a device comprising a core-shell semiconductor nanosheet prepared by any one of the methods described above or any one of the core-shell semiconductor nanosheets described above.

In some embodiments, the device is an optical device. The optical device may be an optical film.

In some embodiments, the device is a photovoltaic device. The photoelectric device can be a solar cell, an electroluminescent device, a single photon light source and the like.

In another exemplary embodiment of the present application, there is provided a composition comprising core-shell semiconductor nanoplatelets that are core-shell semiconductor nanoplatelets of any of the above.

The advantageous effects of the present application will be further described below with reference to examples and comparative examples.

The required chemical reagents are as follows: stearic acid (HSt, 90 +%), octanoic acid (Hoc, 90%), trioctylphosphine oxide (TOPO, 90%), octadecylamine (ODA, 90%), cadmium oxide (CdO, 99.998%), selenium powder (200mesh, 99.999%), 1-octadecene (ODE, 90%), n-octane (98% +) and oleic acid (HOl, 90%) were purchased from Alfa-Aesar, Tributylphosphine (TBP) was purchased from Acros. Sulfur powder (S, 99.98%), cadmium formate (Cd (fo))₂99.9%), octylamine (99%) and oleylamine (NH)₂Ol, 70%) was purchased from Aldrich. Cadmium acetate dihydrate (Cd (Ac)₂·2H₂O, 98.5%) was purchased from shanghai jinshan pavilion new chemical industry. All organic solvents used in the experiments were purchased from the national pharmaceutical group. All chemicals were used without purification.

Preparation of cadmium oleate precursor: oleic acid (20mmol) and tetramethylammonium hydroxide (20mmol) dissolved in 200mL methanol, Cd (Ac)₂·2H₂O (10mmol) was dissolved in 50mL of methanol and then stirredWhen the solution is dripped into the solution, white precipitate is generated. Cd (Ac)₂The solution was stirred for another 20 minutes after the addition. The precipitate was washed three times with methanol and dried under vacuum overnight for use.

Preparation of selenium precursor: selenium powder (10mmol) is weighed in a glove box and dissolved in 2.36g of TBP and 6.85g of ODE to obtain 0.1mol/L TBP-Se solution.

Preparation of S-ODE: sulfur powder (1mmol) was added to 10mL ODE and sonicated to give a clear solution.

Preparation of CdSe: spherical wurtzite CdSe semiconductor nanocrystals with a diameter of 3.2nm (first exciton absorption peak position at 560 nm). Cadmium oxide (0.0256g,0.0002mol), stearic acid (0.2277g,0.0008mol) and 2.5mL of ODE were added to a 25mL three-necked flask. After 10 minutes of aeration with stirring, the solution was heated to 270 ℃ to give a clear solution, which was then cooled to room temperature and 0.5g of trioctylphosphine oxide and 1.5g of octadecylamine were added. After blowing for 10 minutes under the condition of re-stirring, the temperature is raised to 290 ℃, 1mL of 0.1mol/L TBP-Se is rapidly injected, then the temperature is set to 250 ℃, and the reaction liquid is cooled to room temperature when the position of the first exciton absorption peak is moved to 560 nm. Extracting with hexane, chloroform, acetonitrile and methanol for 3 times, and purifying the CdSe semiconductor nanocrystal for later use.

Example 1:

preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.6mmol of S were dissolved in 1.5mmol of oleylamine and 1-octadecene to prepare a 3mL solution.

Preparing CdSe @ CdS semiconductor nanosheets: a hexane solution containing 150nmol of CdSe nanocrystals and 3mL of ODE were added to a 25mL three-necked flask. Blowing gas for 10 minutes under the stirring condition, then heating to 120 ℃ and blowing gas for 20 minutes to remove hexane, water and oxygen in the system. Then the temperature is raised to 250 ℃ at the speed of 18 ℃/min under the conditions of magnetic stirring and argon introduction. When the temperature was raised to 230 ℃, the prepared mixed precursor was added dropwise to a three-necked flask using an autosampler at a rate of 1.5 mL/hr. And purifying the semiconductor nanosheet obtained by the reaction with acetone, dissolving the purified semiconductor nanosheet in toluene, and precipitating the semiconductor nanosheet twice with toluene and methanol for later use.

Example 2

Before mixing for growthPreparation of the body: 0.3795g Cd (Ol)₂(0.5mmol) and 1mmol of S were dissolved in 2.5mmol of oleylamine, 2.5mmol of octanoic acid and 1-octadecene to prepare a 5mL solution. The rest is the same as example 1.

Example 3

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.6mmol of S were dissolved in 1.5mmol of oleic acid and 1-octadecene to prepare a 3mL solution. The rest is the same as example 1.

Example 4

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.6mmol of S were dissolved in 3mmol of oleic acid and 1-octadecene to prepare 3mL of a solution. The rest is the same as example 1.

Example 5

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.6mmol of S were dissolved in 1.5mmol of oleylamine, 1.5mmol of octanoic acid and 1-octadecene to prepare 3mL of a solution. The rest is the same as example 1.

Removing the surface defects of the obtained CdSe @ CdS semiconductor nanosheets: oleylamine (2mL) and 1-octadecene (1mL) were degassed with argon at 160 ℃ for 10 minutes then cooled to 110 ℃ to remove the purified CdSe @ CdS semiconductor nanosheets (1.0X 10)^-7mol) was added to the above solution, while adding 2mL of S-ODE (0.1 mol/L). After 20 minutes at 110 ℃, the nanoplatelets were purified with methanol. Repeating the steps until the fluorescence intensity of the nano-sheets is reduced to zero, and completely treating the nano-sheets. Then heating the nanosheet, oleylamine (1mL) and 1-octadecene (1mL) to 220 ℃, adding 0.2mL of tributylphosphine, cooling the reaction solution to 30 ℃ after 5 minutes, and adding 0.2mL of cadmium formate/octylamine solution (0.1mol/L) dropwise. Finally, the sample is irradiated by ultraviolet light for 20 minutes to eliminate the defect state.

Example 6

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.6mmol of S were dissolved in 3.6mmol of oleic acid and 1-octadecene to prepare a 3mL solution.

The rest is the same as example 1.

Example 7

Preparation of mixed precursors for growth: 0.34155g Cd (Ol)₂(0.45mmol) and 0.9mmol of S was dissolved in 4.5mmol of oleylamine, 2.25mmol of oleic acid and 1-octadecene to prepare 4.5mL of a solution.

The rest is the same as example 1.

Example 8

Preparation of mixed precursors for growth: 0.6148g Cd (Ol)₂(0.81mmol) and 1.62mmol of S were dissolved in 4.05mmol of oleylamine, 4.05mmol of octanoic acid and 1-octadecene to prepare 8.1mL of a solution. The rest is the same as example 1.

Removing the surface defects of the CdSe @ CdS semiconductor nanosheets obtained in example 8: oleylamine (2mL), octylamine (0.8mL) and 1-octadecene (1mL) were degassed with argon at 160 ℃ for 10 minutes then cooled to 110 ℃ to purify CdSe @ CdS semiconductor nanosheets (1.0X 10)^-7mol) was added to the above solution, while adding 2mL of S-ODE (0.1 mol/L). After 20 minutes at 110 ℃, the nanoplatelets were purified with methanol. Repeating the steps until the fluorescence intensity of the nano-sheets is reduced to zero, and completely treating the nano-sheets. Then heating the nanosheet, oleylamine (1mL) and 1-octadecene (1mL) to 220 ℃, adding 0.2mL of tributylphosphine, cooling the reaction solution to 30 ℃ after 5 minutes, and adding 0.2mL of cadmium formate/octylamine solution (0.1mol/L) dropwise. Finally, the sample is irradiated by ultraviolet light for 20 minutes to eliminate the defect state.

Example 9

The temperature was raised to 220 ℃ at a rate of 18 ℃/min, and the prepared mixed precursor was added dropwise to a three-necked flask using an autosampler at a rate of 1.5mL/hr while the temperature was raised to 220 ℃. The surface defect removal treatment was not performed, and the procedure was otherwise the same as in example 5.

Example 10

The temperature was raised to 280 ℃ at a rate of 18 ℃/min, and the prepared mixed precursor was added dropwise to a three-necked flask using an autosampler at a rate of 1.5mL/hr while the temperature was raised to 230 ℃. The surface defect removal treatment was not performed, and the procedure was otherwise the same as in example 5.

Example 11

Preparation of mixed precursors for growth: 0.2277g of Cd (Ol)2(0.3mmol) and 0.45mmol of S were dissolved in 1.5mmol of oleic acid and 1-octadecene to prepare 3mL of a solution. The rest is the same as example 1.

Comparative example 1

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.3mmol of S were dissolved in 1.5mmol of oleylamine, 1.5mmol of octanoic acid and 1-octadecene to prepare 3mL of a solution. The rest is the same as example 1.

Comparative example 2

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.6mmol of S were dissolved in 4.5mmol of amine and 1-octadecene to prepare a 3mL solution. The rest is the same as example 1.

Comparative example 3

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.3mmol of S were dissolved in 4.5mmol of oleic acid and 1-octadecene to prepare a 3mL solution. The rest is the same as example 1.

Comparative example 4

Preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.6mmol of S were dissolved in 1-octadecene to prepare 3mL of a solution. The rest is the same as example 1.

Comparative example 5

Preparation of CdSe @ CdS spherical core-shell nanocrystals: the difference from example 1 is the preparation of mixed precursors for growth: 0.2277g Cd (Ol)₂(0.3mmol) and 0.24mmol of S were dissolved in 3.6mmol of oleylamine and 1-octadecene to prepare a 3mL solution.

The reaction temperature is 270 ℃, and when the ultraviolet absorption peak of the monitored nanocrystal is close to the ultraviolet absorption peak of the nanosheet to be compared, the reaction is stopped by cooling. The rest is the same as example 1.

Testing the optical properties of the assembly at room temperature: the UV-visible absorption spectra were measured by Analytik Jena S600 UV-visible spectrometer. PL spectroscopic measurements were measured by an Edinburgh Instruments FLS920 spectrophotometer. Absolute PL QY was measured by an Ocean Optics FOIS-1 integrating sphere coupled QE65000 spectrophotometer. Fluorescence kinetic decay curves as a function of time were measured by coupling a single photon counting spectrometer (FLS920, Edinburgh Instrument) with a picosecond pulsed laser with an excitation wavelength of 405nm at a frequency of 0.2 MHz.

And (3) testing the luminous anisotropy of the sample: and dissolving the purified semiconductor nano-sheets in octane, and spin-coating the semiconductor nano-sheets on glass slides at the rotating speed of 800 rpm/min. The glass slide was mounted vertically on the sample holder and the fiber to the spectrometer was held horizontally on a movable turntable. To avoid interference from the excitation light, the laser light is excited at an angle of 30 degrees to the horizontal plane of the detector. The detection optical fiber receives the fluorescence signals at different angles on the same horizontal height.

Transmission electron microscope and SAED characterize the electron microscope equipment as Hitachi 7700 with 100kV working voltage. The high-resolution electron microscope equipment is JEM 2100F with 200kV working voltage.

Testing of individual nanosheets/nanocrystals: spectral information for individual nanosheets was obtained by a far-field epi-fluorescence transpose microscope system (Olympus IX 83) with 60 oil-immersed objectives with a numerical aperture of 1.49. The emission signal of each nanoplate was transmitted to an EMCCD camera (Andor iXon Ultra 897) for imaging. Fluorescence information was recorded by an Andor Kymera193i spectrometer. PL intensity traces of individual nanoplates Andor ixon Ultra 897 EMCCD. A CW laser (405nm, PicoQuant) was used as the excitation light source. The integration time for each data is 30 ms. The secondary photon correlation study was measured by the Hanbury Brown-Twiss method (with two avalanche photodiodes). The average photon number (< N >) is lower than 0.1 to ensure single photon excitation.

The experimental results are as follows:

the semiconductor nanosheet obtained in example 2 is analyzed by a transmission electron microscope, and as shown in fig. 1 and 2, the semiconductor nanosheet has a monodisperse size distribution in the thickness direction and the transverse direction, and the semiconductor nanosheet is approximately rectangular in shape in fig. 1 and approximately hexagonal in shape in fig. 2. Cd of comparative example 1: when S is 1:1, it can be seen from fig. 3 that the nanocrystals are more prone to epitaxial growth into an isotropic structure. Cd for example 11: when S is 1:1.5, it is apparent from fig. 4 that the morphology after epitaxial growth of the nanocrystals is no longer isotropic.

The CdSe @ CdS semiconductor nanosheets prepared in example 2 are subjected to selective electron diffraction (SAED) and X-ray diffraction (XRD) spectrum tests and are compared with bulk wurtzite CdSe and CdS standard spectra, and it can be seen from FIG. 5 that apart from the difference of relative intensities of peaks of SAED and XRD, the peak positions of XRD spectra (curves) are basically consistent with bulk wurtzite CdS spectra (upper vertical line group), and the change of peak intensity is presumed to be due to the change of the number of various crystal faces in a two-dimensional structure. For example, the (110) crystal plane is parallel to the thickness direction, and the number of the crystal planes is the largest, so the strength is the strongest. When the semiconductor nano-sheet with the C axis vertical to the substrate is subjected to SAED test, as shown in FIG. 5, three diffraction peaks (100), (110) and (200) can be obtained, and analysis can find that the three diffraction peaks are all related to a crystal plane (C axis direction) vertical to an AB plane. This also demonstrates that the semiconductor nanoplatelets can be aligned on the substrate in an ordered orientation. On the other hand, the results of the corner electron microscopy (see fig. 6) also demonstrate that the obtained semiconductor nanosheets are anisotropic in structure: the projection of the semiconductor nanoplatelets is greatly changed during the sample rotation through 60 °.

In order to exclude the influence of fatty acid, the influence of lipid to amine on the growth of the shell layer was investigated under the condition that the reaction system has no fatty acid. FIG. 7 is the result of electron microscopy of example 1, with the TEM image showing still a plate-like shape. In comparative example 2, it can be seen from fig. 8 that when the molar ratio of the fatty amine to the cadmium salt is 15, the nanosheets grow epitaxially into nanorod-like structures, indicating that the nanosheets tend to grow more in the C-axis direction when the fatty amine is high. This shows that by adjusting the molar ratio of fatty amine to cadmium salt, nanostructures with different C-axis sizes can be obtained.

In order to eliminate the influence of the aliphatic amine, the influence of the aliphatic acid-proof on the growth of the shell layer is researched under the condition that the reaction system is free of the aliphatic amine. In comparative example 4, when no free fatty acid was added to the precursor, significant self-nucleation occurred and was not characterized. In example 3, when the molar ratio of fatty acid to cadmium salt is equal to 5, self-nucleation is suppressed to some extent, but the nanoplatelets have a poor size distribution and the nanoplatelets undergo interparticle ripening (fig. 9, left panel). In example 6, self-nucleation disappeared when the fatty acid to cadmium salt molar ratio was increased to 12: 1. Now SAED and XRD testing of the semiconductor nanoplatelets can confirm that they are two-dimensional structures, however their size is significantly larger (see figure 9). In comparative example 3, when the fatty acid was increased 15 times, a product having a very large size was obtained and monodispersity was not good (see right panel of fig. 9). At this time, it is believed that the fatty acid can selectively etch CdS self-nucleation and etch away the nanosheet surface to obtain cadmium salt which selectively passivates the polar face, thereby promoting the epitaxial growth of the non-polar face, but excessive fatty acid in the system can cause inter-particle ripening.

The reaction process of example 5 was monitored in order to gain more insight into the process of anisotropic epitaxial growth. As the precursor was added, the nanoplatelets increased in size and gradually appeared as hexagons (see fig. 10), and the uv-visible absorption and fluorescence peaks were increasingly red-shifted due to delocalization of the excitons. Figure 11 shows the uv-visible absorption versus fluorescence peak change during nanoplate growth. From the SAED map of fig. 12, it can be seen that the orientation becomes more and more evident during the epitaxial growth of the nanosheets, and the diffraction peak position shifts from bulk wurtzite CdSe to bulk wurtzite CdS. It is stated that the epitaxial growth is anisotropic at the very beginning and can be aligned in exactly the same orientation on the ultra-thin carbon film at least 120 min. Dotted arrows indicate the moving directions of the lattice planes of the semiconductor nanosheet diffraction peaks (100), (110), and (200).

Fig. 13, left image, shows TEM image during the reaction of example 8, same TEM image contains both nanoplatelets oriented perpendicular to the substrate and parallel to the substrate. FIG. 13 right shows the graph of the variation of the size and thickness of the AB face diagonal line during the reaction of example 8, and it can be seen that the C-axis (thickness) is not changed and maintained at 6 nm. The AB face diagonal is larger, indicating an increase in lateral dimension.

The TEM images and size distributions of the nanosheets obtained in example 7 are shown in figure 14. Comparing the right graph of fig. 13 with fig. 14b, it can be known that the samples reacted for 120min in example 7 and example 5 have the same transverse dimension, which indicates that the thickness of the nanosheet can be adjusted by adjusting the amount of fatty amine, and the right graph of fig. 13 is 6nm, and fig. 14b is 8 nm.

Optical properties were investigated using semiconductor nanoplatelets (from example 5) having an AB-plane diameter of 10.6nm and a C-axis thickness of 6nm as an example. Because the epitaxial growth is carried out in the conditions that Cd is 1: 2, so that the initial fluorescence quantum yield QY of the nanoplatelet synthesis is only about 20% (see dashed line in fig. 15a), the nanoplatelets having been effectively surface treatedQY can reach 95% (see figure 15a), and the fluorescence attenuation curve is close to single-channel attenuation, and the single exponential fitting degree is chi_R ²At 1.02, the lifetime measured for the nanoplatelets in hexane as solvent was 27ns (see fig. 15 b). In general, anisotropic structures are generally accompanied by the property of polarized luminescence, so that after purified nanosheets are dissolved in octane and then spin-coated on a glass plate, the polarized luminescence property of the nanosheets after being made into a film is detected, and surprisingly, the degree of polarization of the nanosheets is very close to the theoretical value (see fig. 15c), which is probably not separated from the high QY property of the nanosheets and the unique structure. On the other hand, the nanosheet is found to have very good fluorescence stability, almost no scintillation phenomenon (see fig. 15d), and the ratio of 91 nanosheets found to be in a "bright state" is counted to be as high as 98.3% (see fig. 15e), which is comparable to the zero-dimensional CdSe/CdS nanocrystal phase comprising 8-10 monolayers of CdS in comparative example 5, but it is worth mentioning that the nanosheet only comprises 4-5 monolayers in the C-axis direction, and the good fluorescence stability is presumably related to a flat AB surface. PL spectra of 53 nanosheets were detected with an average half-width of 60meV (see fig. 15 f).

Example 9, example 5 and example 10 are nano-sheets prepared at different temperatures, and a TEM image corresponding to each other from left to right is shown in fig. 16.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A preparation method of a core-shell semiconductor nanosheet is characterized by comprising the following steps:

s1, preparing a solution containing wurtzite nanocrystals;

s2, mixing a first cation precursor, a first anion precursor, a first ligand, a first solvent and the solution containing the wurtzite nanocrystal, wherein the wurtzite nanocrystal is used as a core, heating and reacting to perform shell epitaxial growth on the surface of the core to obtain a first product system containing the core-shell semiconductor nanosheet,

the wurtzite nanocrystal in the wurtzite nanocrystal-containing solution is CdSe, the cation in the first cation precursor is Cd, the anion in the first anion precursor is S,

the first ligand is selected from one or more of a first fatty amine and a first fatty acid, the molar ratio of the first ligand to the first cation precursor is less than 15,

the molar ratio of the first cation precursor to the first anion precursor is less than 1,

the number of C atoms of the first fatty amine is 8-18, and the number of C atoms of the first fatty acid is 8-18.

2. The method of claim 1, wherein the molar ratio of the first cation precursor to the first anion precursor is 1: 1.5-1: 2.

3. the method of claim 1, wherein the first ligand is the first fatty acid, and the molar ratio of the first cation precursor to the first fatty acid is 1: 5-1: 10.

4. the method according to claim 1, wherein the first ligand is the first fatty amine and the first fatty acid, and the molar ratio of the first fatty amine, the first fatty acid, and the first cation precursor is 5:5: 1.

5. The method of claim 1, wherein the reaction temperature in the S2 is 220-280 ℃.

6. The method according to claim 1, wherein the step S1 includes mixing a second cation precursor, a second anion precursor, a second ligand and a second solvent, heating and reacting to obtain a second product system containing wurtzite nanocrystals, separating and purifying the second product system to obtain the wurtzite nanocrystals, and dissolving the wurtzite nanocrystals in a third solvent to obtain the solution containing the wurtzite nanocrystals.

7. The preparation method according to claim 1, wherein the S2 further comprises separating and purifying the first product system to obtain the core-shell semiconductor nanosheet.

8. The method of manufacturing according to claim 7, further comprising: s3, performing surface treatment on the core-shell semiconductor nanosheets to remove surface defects to obtain core-shell semiconductor nanosheets with optimized performance, wherein S3 comprises S3a, mixing second fatty amine, a fourth solvent, the core-shell semiconductor nanosheets and the first anion precursor, keeping the mixture at 80-150 ℃ for a certain time, performing purification and separation to obtain first surface-treated core-shell semiconductor nanosheets, S3 further comprises S3b, mixing the first surface-treated core-shell semiconductor nanosheets, third fatty amine and a fifth solvent, heating to 150-250 ℃, adding trialkylphosphine for reacting for a certain time, cooling to 0-100 ℃, adding a first cationic short-chain carboxylic acid precursor and fourth fatty amine to obtain a third product system containing the second surface-treated core-shell semiconductor nanosheets, and S3 further comprises S3c, and performing illumination treatment on the third product system, and obtaining the third surface-treated core-shell semiconductor nanosheet.

9. A core-shell semiconductor nanosheet, characterized in that the core of the core-shell semiconductor nanosheet is made of the first semiconductor material, and the core is a wurtzite nanocrystal located at the center of the core-shell semiconductor nanosheet; the shell is made of a second semiconductor material, the core-shell semiconductor nanosheet has luminescence anisotropy,

the core-shell semiconductor nanosheet is subjected to surface treatment to remove surface defects, and the surface treatment comprises the following steps: mixing second fatty amine, a fourth solvent, the core-shell semiconductor nanosheet and the first anion precursor, keeping the mixture at 80-150 ℃ for a certain time, purifying and separating to obtain a first surface-treated core-shell semiconductor nanosheet, mixing the first surface-treated core-shell semiconductor nanosheet, a third fatty amine and a fifth solvent, heating to 150-250 ℃, adding trialkylphosphine for reacting for a certain time, cooling to 0-100 ℃, adding a first cationic short-chain carboxylic acid precursor and a fourth fatty amine to obtain a third product system containing the second surface-treated core-shell semiconductor nanosheet, carrying out illumination treatment on the third product system to obtain a third surface-treated core-shell semiconductor nanosheet, wherein the first semiconductor core-shell material is CdSe, the second semiconductor material is CdS, and the fluorescence quantum yield of the semiconductor nanosheet is greater than or equal to 90%, the fluorescence attenuation curve of the nuclear shell semiconductor nanosheet is of a single index and is fluorescence non-flashing.

10. A device comprising core shell semiconductor nanoplatelets prepared by the method of claim 8 or core shell semiconductor nanoplatelets of claim 9.

11. A composition comprising core shell semiconductor nanoplatelets prepared according to the method of claim 8 or core shell semiconductor nanoplatelets according to claim 9.