(Buffer Layer Synthesis Step)

A buffer layer containing semiconductor nanocrystals containing group II-V elements as constituent elements is formed on the top of the semiconductor nanocrystal core by mixing and reacting the solution containing the semiconductor nanocrystal core obtained as described above with the solution in which the group II element precursor is dissolved and the solution in which the group V element precursor is dissolved. At this time, for the sake of simplification of production, the reaction solution after the core synthesis step described above is continuously used, and the solution in which the group II element precursor is dissolved is added to form a mixture, and to the mixture of the excess group III element precursor and the solution containing the group II element precursor, the group V element precursor is preferably reacted to grow the buffer layer. Further, for the purpose of suppressing aggregation after the synthesis of the buffer layer, it is desirable to add and dissolve the group II element precursor and the group V element precursor to the solution in which the ligand is dissolved, respectively.

The group V element precursor may be appropriately selected from the viewpoint of controlling the reactivity so that the desired particle size and particle size distribution can be obtained, as in the method shown in the core synthesis step. For example, tristrimethylsilylphosphine or trisdimethylaminophosphine may be selected. When using highly reactive tristrimethylsilylphosphine, it is advisable to dilute it with a solvent to control the reactivity and adjust the concentration appropriately for the reaction.

As examples of group II element precursors zinc fluoride, zinc chloride, zinc bromide, zinc iodide, zinc acetate, acetylacetonatozinc, zinc oxide, zinc carbonate, zinc carboxylate salt, dimethyl zinc, diethyl zinc, zinc nitrate, zinc sulfate or the like can be mentioned.

A plurality of methods for producing group II-V semiconductor nanocrystals have been published. A method of reacting zinc stearate with tristrimethylsilylphosphine, a method of reacting diethylzinc with tristrimethylsilylphosphine, a method of reacting multiple zinc precursors (diethylzinc and zinc carboxylate salt) with tristrimethylsilylphosphine, and a method of reacting diethylzinc with PH₃have been mentioned. Of these, the methods for synthesizing Zn₃P₂nanoparticles with observable luminescence are all methods using diethylzinc, and it is preferable to use diethylzinc as a group II element precursor. The method of reacting multiple zinc precursors (diethylzinc and zinc carboxylate salt) with tristrimethylsilylphosphine is particularly effective because it can be grown using the excess zinc carboxylate salt during later shell synthesis. However, as will be described later, when the group III element precursor remains in the solution after the core synthesis, the group III-V nanocrystals grow at the same time. Since a good buffer layer can be produced in the cases zinc carboxylate salt or zinc halide was used, these raw materials can also be suitably used, and the zinc precursor is not particularly limited. Further, the synthesis temperature and the holding time are not particularly limited because they can be appropriately adjusted so as to obtain desired characteristics.

The method for dissolving the solid group II element precursor raw material in the solvent is not particularly limited, and for example, a method for dissolving by heating to a temperature of 100° C. to 180° C. is desirable. In particular, it is preferable to reduce the pressure at this time because dissolved oxygen and water can be removed from the dissolved solution.

(Shell Synthesis Step)

A single or a plurality of semiconductor nanocrystal shells containing group II-VI elements as constituent elements is formed on the buffer layer by mixing and reacting the solution containing the semiconductor nanocrystal core in which a buffer layer containing semiconductor nanocrystals containing group II-V elements as constituent elements is formed with the solution in which the group II element precursor is dissolved and the solution in which the group VI element precursor is dissolved. At this time, in order to simplify the production, it is preferable to continue to use the reaction solution after the buffer layer synthesis step described above to grow the shell. The shell structure is preferably a shell structure containing ZnSe, ZnS or a mixed crystal thereof, and is not particularly limited, but it is preferable to use ZnS from the viewpoint of stability. For the purpose of suppressing aggregation after shell synthesis, it is desirable to add and dissolve group II element precursors and group VI element precursors, respectively, to the solution in which the ligand is dissolved. In the reaction, a group II element precursor solution is added to the reaction solution after the buffer layer synthesis step to prepare a mixed solution, and then the group VI element precursor solution is added under high temperature condition of 150° C. or higher and 350° C. or lower. Then, the II-VI group semiconductor nanocrystal shell can be synthesized.

As the Group II element precursor, for example, zinc fluoride, zinc chloride, zinc bromide, zinc iodide, zinc acetate, acetylacetonatozinc, zinc oxide, zinc carbonate, zinc carboxylate salt, dimethyl zinc, diethyl zinc, zinc nitrate, zinc sulfate or the like can be mentioned as in the buffer layer synthesis step. Since high reactivity is not required in the synthesis of shells, zinc carboxylate salt, zinc acetate, and zinc halide can be preferably used because of their ease of handling and compatibility with solvents. The method for dissolving the solid group II element precursor raw material in the solvent is not particularly limited, and for example, a method for dissolving by heating to a temperature of 100° C. to 180° C. is desirable. In particular, it is preferable to reduce the pressure at this time because dissolved oxygen and water can be removed from the dissolved solution.

As examples of the group VI element precursor, sulfur, alkylthiol, trialkylphosphine sulfide, bistrialkylsilylsulfide, selenium, trialkylphosphine selenide, trialkenylphosphine selenide, and bistrialkylsilyl selenide or the like can be mentioned. Of these, as the sulfur source, an alkylthiol having a long-chain alkyl group such as dodecanethiol is preferable from the viewpoint of dispersion stability of the obtained core-shell particles. The method for dissolving the solid group VI element precursor raw material in the solvent is not particularly limited, and for example, a method for dissolving by heating to a temperature of 100° C. to 180° C. is desirable.

Further, since the quantum efficiency is further improved by increasing the number of layers of the shell structure, it is particularly preferable to synthesize a ZnSe shell, then a mixed crystal shell of ZnSe and ZnS, and finally a ZnS shell. The decrease in quantum efficiency due to lattice mismatch when the ZnS shell is formed on the ZnSe shell can be suppressed more effectively, and the stability of the quantum dots becomes higher due to the ZnS that is stable even in the atmosphere.

EXAMPLE

Hereinafter, the present invention will be described in detail with reference to examples, but this is not limited to the present invention.

Example 1

(Core Synthesis Step)

Into a flask, 0.23 g (0.9 mmol) of palmitic acid, 0.088 g (0.3 mmol) of indium acetate, and 10 mL of 1-octadecene were added, and the mixture was heated and stirred at 100° C. under reduced pressure to dissolve the raw materials and degassing was performed for 1 hour. Then, nitrogen was purged into the flask and heated to 300° C. When the temperature of the solution became stable, 0.75 mL (0.15 mmol) of the solution prepared by mixing tristrimethylsilylphosphine with trioctylphosphine to 0.2 M was added, and the mixture was reacted for 20 minutes. It was confirmed that the solution was colored red and core particles were formed.

(Buffer Layer Synthesis Step)

Next, 0.3 M of octadecene zinc stearate solution was prepared by adding 2.85 g (4.5 mmol) of zinc stearate and 15 mL of 1-octadecene into another flask, and being heated and stirred at 100° C. under reduced pressure, and degassing for 1 hour while being dissolved and 0.5 mL (0.15 mmol) of the solution was added to the reaction solution after the core synthesis, and the mixture was cooled to 100° C. Then, a solution was prepared by adding 0.15 mmol of diethylzinc, 0.25 mmol of tristrimethylsilylphosphine, and 2 mL of trioctylphosphine into a Schlenk tube in a glovebox, hermetically removed from the glovebox, and the whole amount was added to the reaction solution. Then, the temperature was raised to 300° C. for 1 hour, held for 10 minutes, and then cooled to 200° C.

(Shell Synthesis Step)

1.5 M of selenium trioctylphosphine solution was prepared by adding 0.474 g (6 mmol) of selenium and 4 mL of trioctylphosphine into another flask, being heated to 150° C. to dissolve, and 0.1 mL was added to the reaction solution after the buffer layer step being cooled to 200° C., then the temperature was raised to 250° C. for 30 minutes, the temperature was maintained for 10 minutes, and then the mixture was cooled to room temperature. 0.44 g (2.2 mmol) of zinc acetate was added, and the mixture was dissolved by heating and stirring at 100° C. under reduced pressure. The inside of the flask was purged with nitrogen again, the temperature was raised to 230° C., 0.98 mL (4 mmol) of 1-dodecanethiol was added, and the temperature was maintained for 1 hour. The obtained solution was cooled to room temperature, ethanol was added, and the mixture was centrifuged to precipitate nanoparticles and remove the supernatant. Hexane was further added to disperse, ethanol was added again to centrifuge, the supernatant was removed and redispersed in hexane to prepare a hexane solution of InP/InP:Zn₃P₂/ZnSe/ZnS core-shell type quantum dots.

Comparative Example 1

The hexane solution of InP/ZnSe/ZnS core-shell type quantum dots was used as Comparative Example 1 in the same manner as in Example 1 except that the buffer layer synthesis step was not performed.

Example 2

(Core Synthesis Step)

Into a flask, 0.23 g (0.9 mmol) of palmitic acid, 0.088 g (0.3 mmol) of indium acetate, and 10 mL of 1-octadecene were added, and the mixture was heated and stirred at 100° C. under reduced pressure to dissolve the raw materials and degassing was performed for 1 hour. Then, nitrogen was purged into the flask and heated to 300° C. When the temperature of the solution became stable, 0.75 mL (0.15 mmol) of the solution prepared by mixing tristrimethylsilylphosphine with trioctylphosphine to 0.2 M was added and reacted for 20 minutes. It was confirmed that the solution was colored red and core particles were formed. The reaction solution was cooled to 100° C.

(Buffer Layer Synthesis Step)

Next, 0.02 g (0.15 mmol) of zinc chloride and 4 mL of oleylamine were added to another flask, and the mixture was heated and stirred at 100° C. under reduced pressure and degassed for 1 hour while being dissolved. The whole amount was added to the reaction solution cooled to 100° C. Then, the mixture was heated to 180° C., 0.25 mmol of trisdiethylaminophosphine was added and held for 10 minutes, and then the temperature was raised to 300° C. for 1 hour.

(Shell Synthesis Step)

Next, 0.3 M of octadecene zinc stearate solution was prepared by adding 2.85 g (4.5 mmol) of zinc stearate and 15 mL of 1-octadecene into another flask, and the mixture being heated and stirred at 100° C. under reduced pressure, and being degassed for 1 hour while being dissolved, then, 0.5 mL (0.15 mmol) of this solution was added to the reaction solution after the synthesis of the buffer layer, and the mixture was cooled to 200° C.

1.5 M of selenium trioctylphosphine solution was prepared by adding 0.474 g (6 mmol) of selenium and 4 mL of trioctylphosphine into another flask, being heated to 150° C. to dissolve, and 0.1 mL this solution was added to the reaction solution after the buffer layer step being cooled to 200° C., then the temperature was raised to 250° C. for 30 minutes, the temperature was maintained for 10 minutes, and then the mixture was cooled to room temperature. 0.44 g (2.2 mmol) of zinc acetate was added, and the mixture was dissolved by heating and stirring at 100° C. under reduced pressure. The inside of the flask was purged with nitrogen again, the temperature was raised to 230° C., 0.98 mL (4 mmol) of 1-dodecanethiol was added, and the temperature was maintained for 1 hour. The obtained solution was cooled to room temperature, ethanol was added, and the mixture was centrifuged to precipitate nanoparticles and remove the supernatant. Hexane was further added to disperse, ethanol was added again to centrifuge, the supernatant was removed and redispersed in hexane to prepare a hexane solution of InP/InP:Zn₃P₂/ZnSe/ZnS core-shell type quantum dots.

[Evaluation]

(Measurement of Average Particle Size)

To measure the average particle size, 20 particles were directly observed with a transmission electron microscope (TEM), the diameter of a circle having the same area as the projected area of the particles was calculated, and the average value of them was used.

(Elemental Analysis)

After the core synthesis, the buffer layer synthesis, and the shell synthesis, samples were taken respectively, ethanol was added to precipitate particles, and hexane was added to redispersed to prepare the sample solutions for each step. Elemental analysis was performed by energy dispersive X-ray spectroscopy analysis (EDX), and the ratios of In, P, Zn, Se, and S elements were calculated.

(Emission Wavelength, Emission Half-Value Width, Emission Efficiency Measurement)

In Examples and Comparative Examples, as evaluation of fluorescence luminous characteristics of quantum dots, the emission wavelength, fluorescence emission half-value width, and fluorescence luminous efficiency (internal quantum efficiency) at an excitation wavelength of 450 nm were measured using a quantum efficiency measurement system (QE-2100) manufactured by Otsuka Electronics Co., Ltd. The measurement results of Examples 1 and 2 and Comparative Example 1 are summarized in Table 1 below.

TABLE 1

Buffer layer
Shell

synthesis
synthesis

Core synthesis step
step
step