CN111423869A

CN111423869A - Method for inhibiting surface defect state of quantum dot based on crystal face control

Info

Publication number: CN111423869A
Application number: CN202010203444.0A
Authority: CN
Inventors: 张建兵; 王康; 夏勇; 连霖源; 张道礼; 唐江
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2020-03-20
Filing date: 2020-03-20
Publication date: 2020-07-17

Abstract

The invention discloses a method for inhibiting surface defect states of quantum dots based on crystal face control, which belongs to the technical field of compound semiconductor nano material preparation, wherein a lead precursor solution is heated to 40-70 ℃, then a sulfur source or selenium source solution with the concentration of 0.6 mol/L-0.8 mol/L is rapidly injected, then the sulfur source or selenium source solution with the concentration of 0.1 mol/L-0.2 mol/L is slowly dripped, a ligand is added, then an anti-solvent is added until the solution becomes turbid, and then centrifugation is carried out, and precipitation is the lead sulfide quantum dots, lead selenide quantum dots, lead sulfide/lead selenide quantum dots or lead selenide/lead sulfide quantum core-shell dots with the body-core tetragonal lattice structure wrapped by the ligand, the amount of the lead precursor is larger than the sum of the amounts of the added sulfur source or selenium source, the whole reaction is carried out under protective gas, and the final product can effectively relieve the exposure condition of {100 crystal faces } so as to avoid invasion of oxygen and moisture, has few surface defect states and good stability in air.

Description

Method for inhibiting surface defect state of quantum dot based on crystal face control

Technical Field

The invention belongs to the technical field of compound semiconductor nano material preparation, and particularly relates to a method for inhibiting a quantum dot surface defect state based on crystal face control, in particular to a method for inhibiting a lead chalcogenide quantum dot surface defect state based on crystal face control.

Background

Quantum dots are semiconductor nanomaterials whose optical and electrical properties are determined by their size, shape and material composition. The size of the quantum dots in three dimensions is less than 100nm, less than the Bohr radius of excitons, and can be constrained in three spatial directions so as to be limited in a nanometer space. Internal electron and hole transport is limited, the electron mean free path is short, the electron locality and coherence are enhanced, so that a quantum confinement effect is generated, and the electron energy level near the fermi level is split from continuous states into discrete levels. The forbidden band width can be adjusted by changing the size of the particles. The method has wide application in photodiodes, photodetectors, lasers, transistors and solar cells.

With the intensive research of researchers on the synthesis of quantum dots, the subsequent surface treatment and the optimization of device structures, the quantum dots are more widely and deeply applied to the daily life of people. However, the quantum dots are extremely small in size, have extremely high specific surface area, and are prone to surface defect states. Deep defects assist carrier recombination, while shallow defects reduce carrier transport and form band-tail states that affect energy level matching. Therefore, the control of the surface defect state of the quantum dot is the core and key for realizing the improvement of the photoelectric performance of the quantum dot.

The surface of the lead chalcogenide quantum dot has {111} and {100} crystal planes. Wherein, the {111} crystal face is rich in lead, has good passivation effect with surface ligands and has wide coverage; and the {100} crystal planes are alternately arranged with equal proportion of lead and sulfur atoms, no redundant suspension bonds are used for effectively binding the surface ligand, so that the ligand is easy to fall off, and the surface of the quantum dot is easy to be influenced by the surrounding environment (such as water and oxygen in the air) so as to introduce defects into the surface of the quantum dot. In addition, the crystal face of the quantum dot {100} is easy to generate surface etching, agglomeration and fusion phenomena in the process of ligand exchange, and further more surface defects are introduced. In summary, effective control of the {100} crystal plane on the surface of the quantum dot is the key to inhibiting the formation of the surface defect state.

Currently, most researchers are focusing on the surface ligand treatment work after the synthesis of quantum dots, and neglect the effective control of the surface defect state from the source, namely the initial stage of the synthesis of the quantum dots. At present, all schemes reported in the literature produce lead chalcogenide quantum dots with a geometrical structure of truncated octahedron, the surface of the lead chalcogenide quantum dots possesses {111} crystal planes and {100} crystal planes simultaneously, and the {100} crystal planes on the surface of the lead chalcogenide quantum dots can cause the quantum dots to generate irreversible surface defect states. Therefore, in order to further improve the photoelectric performance of the quantum dots, a simple and efficient method must be developed from a synthesis source to solve the problem of defects of the quantum dots caused by insufficient {100} surface passivation.

Disclosure of Invention

The invention solves the problems that in the prior art, a ligand on the surface of a quantum dot is easy to fall off and is easily influenced by the surrounding environment, so that defects are introduced into the surface of the quantum dot, and provides a method for inhibiting the surface defect state of the quantum dot based on crystal face control, wherein under the conditions of low temperature and high monomer concentration, the quantum dot is dominated by a kinetic growth mechanism and follows the anisotropic growth principle, the obtained quantum dot with the size of 3nm is approximately regular octahedron in geometric structure, and the surface almost only contains {111} crystal faces. The synthesis method can effectively solve the defect problem caused by insufficient passivation of the {100} crystal face, and can effectively improve the photoelectric performance of the crystal.

According to the purpose of the invention, a method for inhibiting the surface defect state of a quantum dot based on crystal face control is provided, which comprises the following steps:

(1) dissolving a lead precursor to obtain a lead precursor solution, wherein the concentration of the lead precursor in the lead precursor solution is 0.125 mol/L-0.4 mol/L;

(2) heating the lead precursor solution obtained in the step (1) to 40-70 ℃, then injecting a sulfur source solution or a selenium source solution with the concentration of 0.6 mol/L-0.8 mol/L, wherein the injection speed of a sulfur source in the sulfur source solution or a selenium source in the selenium source solution is 1-6 mol/s, and reacting the lead precursor with the sulfur source or the lead precursor with the selenium source to obtain nucleated lead sulfide or lead selenide quantum dots, wherein the injection operation time is not more than 1 s;

(3) after the reaction in the step (2) is carried out for 15s-3min, dropwise adding a sulfur source solution or selenium source solution with the concentration of 0.1 mol/L-0.2 mol/L into the obtained reaction system, wherein the dropwise adding speed of the sulfur source in the sulfur source solution or the selenium source in the selenium source solution is 0.04 mmol/min-0.1 mmol/min, and simultaneously keeping heating to grow the lead sulfide quantum dots or the lead selenide quantum dots nucleated in the step (2) and increase the size;

(4) adding a ligand into the reaction system obtained in the step (3), then adding an anti-solvent until the solution becomes turbid, and centrifuging, wherein the precipitate is quantum dots with a body-centered tetragonal lattice structure wrapped by the ligand, and the quantum dots are lead sulfide quantum dots, lead selenide quantum dots, lead sulfide/lead selenide core-shell quantum dots or lead selenide/lead sulfide core-shell quantum dots; the lead sulfide/lead selenide core-shell quantum dot takes lead sulfide as a core and lead selenide as a shell, and the lead selenide/lead selenide core-shell quantum dot takes lead selenide as a core and lead sulfide as a shell;

the amount of the lead precursor substance in the step (1) is larger than the sum of the amounts of the sulfur source or selenium source substances added in the step (2) and the step (3);

the steps (1) to (3) are all carried out under protective gas.

Preferably, the lead precursor is lead chloride, lead oxide or lead acetate.

Preferably, the sulfur source is zinc sulfide quantum dots, sulfur powder, cadmium sulfide quantum dots or hexamethyldisilazane.

Preferably, the selenium source is a cadmium selenide quantum dot, a selenium powder or a zinc selenide quantum dot.

Preferably, the ligand is oleic acid or dodecyl mercaptan.

Preferably, the anti-solvent is at least one of ethanol, acetone, ethyl acetate, and the like.

Preferably, before adding the ligand in the step (4), a step of adding an organic solvent for dilution is further included, and the dilution is used for preventing the quantum dots from agglomerating during centrifugation.

Preferably, the centrifugation rate of the step (4) is 7000r/min to 9000 r/min.

Preferably, in step (2), the duration of the injection operation is 1 s.

Preferably, in the step (3), the heating is kept to be always kept to ensure that the temperature meets 40-70 ℃;

or the heating is kept to be specifically divided into two stages, namely a first stage of heating to ensure that the temperature meets 40-70 ℃, and a second stage of heating to ensure that the temperature is higher than that of the first stage under the premise of not exceeding 120 ℃.

In general, compared with the prior art, the technical scheme of the invention can obtain the following gain effects:

(1) the invention provides a solution for realizing effective regulation and control of a crystal face of a quantum dot by regulating and controlling the balance between kinetics and thermodynamics in the growth process of the quantum dot so as to inhibit the surface defect state of the quantum dot. Under the conditions of low temperature and high monomer concentration, the quantum dots take a kinetic growth mechanism as a leading factor, the anisotropic growth principle is followed, the obtained quantum dot geometrical structure with the size of-3 nm is approximately regular octahedron, and the surface almost only contains {111} crystal faces; under the conditions of high temperature and low monomer concentration, the quantum dots are dominated by a thermodynamic growth mechanism, an isotropic growth principle is followed, the obtained quantum dot geometrical structure with the size of 3nm is approximately a truncated octahedron, and the surface of the quantum dot simultaneously has {111} and {100} crystal faces. The quantum dot obtained mainly by kinetic growth has an approximate octahedral geometric structure, the surface almost only contains {111} crystal planes, and the quantum dot has a better surface ligand passivation effect, so that the suppression of a surface defect state is facilitated, and the photoelectric property of the quantum dot is improved. Under the condition of low temperature and high concentration monomer, the growth mechanism of the quantum dots takes kinetic growth as the dominant factor, the geometric structure of the quantum dots is approximate regular octahedron, the surface of the quantum dots almost only contains {111}, the lattice structure of the quantum dots is a body-centered tetragonal (body-centered tetragonal) lattice structure, the diameter of the lattice structure is about 2-3 nm, and the absorption peak of a first exciton is less than 1100 nm.

(2) The method comprises the steps of adding a sulfur source or a selenium source, wherein the sulfur source or the selenium source is quickly injected firstly, then slowly dropwise adding the sulfur source or the selenium source, a part of the sulfur source or the selenium source which is injected firstly reacts with a lead precursor for nucleation, the number of quantum dot nuclei is quickly increased in the process, and the subsequently dropwise added part reacts on the surfaces of the nucleated quantum dots for continuous growth, so that the size of the quantum dots is increased.

(3) In the invention, the selected sulfur source is zinc sulfide quantum dots, thioacetamide or cadmium sulfide quantum dots, the stability in air is good, and the large-scale production can be realized, wherein the zinc sulfide quantum dots and the thioacetamide are clean and nontoxic.

(4) The crystal face control method provided by the invention can obtain the chalcogen lead quantum dots with more mass at one time; the method can prepare the chalcogen lead quantum dots with different sizes, has wide size span range, and can effectively meet the spectrum requirements from near infrared to intermediate infrared; the quantum dots prepared by the method have excellent size distribution and high synthesis quality.

(5) The crystal face control method provided by the invention is simple and flexible, can obtain different required sizes by adjusting the synthesis temperature and the proportion of the molar weight of the sulfur source or the selenium source in initial injection and subsequent dropwise addition, and is convenient and rapid.

(6) The final product obtained by the crystal face control method provided by the invention can effectively relieve the exposure condition of the {100} crystal face, avoid the invasion of oxygen and moisture, has few surface defect states and good stability in air.

Drawings

FIG. 1 is a schematic flow chart of a production process in examples of the present invention and comparative examples.

Fig. 2 is a schematic diagram of a quantum dot in which kinetic growth dominates thermodynamic growth.

FIG. 3 is a two-dimensional grazing incidence small angle scattering (GISAXS) of the present invention by a-3 nm PbS quantum dot (hereinafter collectively referred to as to-QDs) thin film dominated by the thermodynamic growth mechanism.

FIG. 4 is a two-dimensional grazing incidence small angle scattering (GISAXS) by a-3 nm PbS quantum dot (hereinafter collectively referred to as o-QDs) thin film dominated by a kinetic growth mechanism in the present invention.

FIG. 5 shows a superlattice structure formed by close-packed to-QDs, which is a face-centered cubic lattice structure.

FIG. 6 shows a superlattice structure formed by close-packing of o-QDs, which is a body-centered tetragonal lattice structure.

FIG. 7 shows the absorption spectra, photoluminescence spectra and fluorescence quantum yields of the to-QDs and o-QDs solutions of the present invention.

FIG. 8 shows the present invention in which the to-QDs and the o-QDs are linked via the same ligand (PbI)₂And PbBr₂) The transient fluorescence spectrum curve of the film prepared by the exchange strategy.

FIG. 9 shows the present invention in which the to-QDs and the o-QDs are linked via the same ligand (PbI)₂And PbBr₂) The film prepared by the exchange strategy has an energy curve of a band tail of an Ulbach band.

FIG. 10 shows the present invention in which the to-QDs are coupled via ligands (PbI)₂And PbBr₂) The exchange strategy is prepared into a transient absorption diagram of the film.

FIG. 11 shows the o-QDs of the present invention via ligand (PbI)₂And PbBr₂) The exchange strategy is prepared into a transient absorption diagram of the film.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

Example 1

(1) Weighing 8.82g of zinc stearate and 0.45g of thioacetamide in a two-mouth reaction bottle, weighing 100ml of ODE (1-octadecene), vacuumizing the mixture, heating to 140 ℃, and maintaining for 50 min;

(2) cooling the product to 50 ℃ in a water bath, adding 4ml of n-octylamine, adding ethanol after the solution is clarified, centrifuging, and cleaning twice to obtain ZnS quantum dots with a first excitation peak of 256 nm;

(3) respectively preparing the washed ZnS quantum dots into two solutions of 0.6 mol/L with higher concentration and 0.2 mol/L with lower concentration, wherein the solvents are ODE;

(4) weighing 2.919g of lead chloride in another two-mouth reaction bottle, weighing 35ml of oleylamine, vacuumizing the mixture, heating to 140 ℃, and maintaining for 30 min;

(5) cooling to 60 ℃, quickly injecting 5ml of ZnS quantum dot solution with the concentration of 0.6 mol/L (the time for quick injection is not more than 1s, certainly, the faster the injection is, the better the injection is), slowly dropwise adding the ZnS quantum dot solution with the concentration of 0.2 mol/L after 15s, wherein the speed is about 0.04 mmol/min;

(6) detecting size change by sampling, cooling in water bath after 40min, immediately injecting 50ml of n-hexane for dilution, and injecting 30ml of oleic acid as ligand when the temperature of the solution is reduced to below 40 ℃;

(7) adding a mixed solution of ethanol and acetone for centrifugation, and removing supernatant to obtain the 3nm PbS quantum dots.

FIG. 1 is a schematic flow diagram of a portion of the preparation process in the example of the invention on the left.

Example 2

(1) The preparation of ZnS quantum dots is the same as that of example 1, 3.336g of lead chloride is weighed in a two-mouth reaction bottle, 40ml of oleylamine is weighed, the mixture is heated to 140 ℃ after being vacuumized, and the temperature is maintained for 30 min;

(2) cooling to 40 ℃, quickly injecting 5ml of ZnS quantum dot solution with the concentration of 0.6 mol/L, slowly dropwise adding the ZnS quantum dot solution with the concentration of 0.2 mol/L after 15s, wherein the speed is about 0.04mmol/min, and starting timing;

(3) and (3) detecting the size change by sampling, wherein the heating temperature is up to 60 ℃ in 35min, the growth rate of the quantum dots can be properly accelerated (the temperature can be higher and cannot exceed 120 ℃ at most), the size change is continuously detected by sampling, 60ml of n-hexane is injected in 55min for dilution, and 35ml of oleic acid is injected as a ligand after the temperature is reduced to 40 ℃.

(4) Adding a mixed solution of ethanol and acetone for centrifugation, and removing supernatant to obtain the 3nmPbS quantum dots prepared at low temperature.

Example 3

(1) ZnS quantum dot preparation method is the same as example 1, weighing 2.502g of lead chloride in a two-mouth reaction bottle, weighing 30ml of oleylamine, vacuumizing the mixture, heating to 140 ℃, and maintaining for 30 min;

(2) cooling to 70 ℃, quickly injecting 5ml of ZnS quantum dot solution with the concentration of 0.6 mol/L, slowly dropwise adding the ZnS quantum dot solution with the concentration of 0.2 mol/L after 15s, wherein the speed is about 0.04 mmol/min;

(3) the size change is detected by sampling, water bath is carried out after 28min, 40ml of normal hexane is injected for dilution, and 20ml of oleic acid is injected as a ligand after the temperature is reduced to 40 ℃.

Comparative example 1

(1) Weighing 8.82g of zinc stearate and 0.45g of thioacetamide in a two-mouth reaction bottle, measuring 100ml of ODE, vacuumizing the mixture, heating to 140 ℃, and maintaining for 50 min;

(3) preparing the washed ZnS quantum dots into a solution with the concentration of 0.6 mol/L, wherein the solvent is ODE;

(4) weighing 1.668g of lead chloride in another two-mouth reaction bottle, weighing 20ml of oleylamine, vacuumizing the mixture, heating to 140 ℃, and maintaining for 30 min;

(5) cooling to 127 ℃, and injecting 5ml of prepared ZnS quantum dot solution;

(6) carrying out water bath after 90s, injecting 20ml of n-hexane for dilution after the temperature is reduced to 70 ℃, and injecting 16ml of oleic acid as a ligand after the temperature is reduced to 40 ℃;

(8) adding a mixed solution of ethanol and acetone for centrifugation, and removing supernatant to obtain the high-temperature prepared-3 nm PbS quantum dots.

FIG. 1 is a schematic flow chart of a part of the preparation method in the embodiment of the present invention on the right side.

Fig. 2 is a schematic diagram of a quantum dot in which kinetic growth dominates thermodynamic growth. Under the conditions of low temperature and high monomer concentration, the quantum dots take a kinetic growth mechanism as a leading factor, the anisotropic growth principle is followed, the obtained quantum dot geometrical structure with the size of-3 nm is approximately regular octahedron, and the surface almost only contains {111} crystal faces; under the conditions of high temperature and low monomer concentration, the quantum dots are dominated by a thermodynamic growth mechanism, an isotropic growth principle is followed, the obtained quantum dot geometrical structure with the size of 3nm is approximately a truncated octahedron, and the surface of the quantum dot simultaneously has {111} and {100} crystal faces. The quantum dot obtained mainly by kinetic growth has an approximate octahedral geometric structure, the surface almost only contains {111} crystal planes, and the quantum dot has a better surface ligand passivation effect, so that the suppression of a surface defect state is facilitated, and the photoelectric property of the quantum dot is improved.

Comparative example 1 had a poor size distribution and poor stability in air compared to example 1, and the performance was inferior to that of the examples when applied to devices.

The performance characteristics of the 3nmPBS quantum dots prepared in example 1 and comparative example 1 are as follows:

the two-dimensional grazing incidence small angle scattering (GISAXS) condition of the-3 nmPBS quantum dot (to-QDs) solution prepared under the high temperature condition is shown in FIG. 3.

The two-dimensional grazing incidence small angle scattering (GISAXS) condition of the PbS quantum dots (o-QDs) with the particle size of 3-nm prepared under the low temperature condition is shown in figure 4.

the superlattice structure formed by the close-packed to-QDs is a face-centered cubic lattice structure, as shown in fig. 5.

The superlattice structure formed by close-packing of o-QDs is a body-centered tetragonal lattice structure, as shown in fig. 6.

FIGS. 3, 4, 5 and 6 indirectly illustrate that the to-QDs geometry is approximately truncated octahedral and the o-QDs geometry is approximately octahedral.

the absorption spectra, photoluminescence spectra and fluorescence quantum yields of the solutions to-QDs and o-QDs are shown in FIG. 7. It can be seen from the figure that the to-QDs have larger Stokes shift than the o-QDs and the latter has higher fluorescence quantum yield, which indicates that the o-QDs surface has fewer surface defect states and shows more excellent photoelectric performance because the o-QDs almost only contain the {111} crystal plane.

to-QDs and o-QDs are linked via the same ligand (PbI)₂And PbBr₂) The transient fluorescence spectrum curve of the exchange strategy after being prepared into a film is shown in FIG. 8. It can be seen from the figure that after the same ligand exchange strategy, the to-QDs have more interband defect states than the o-QDs thin film, and promote the nonradiative recombination of carriers, thereby influencing the average fluorescence lifetime of the carriers of the thin film.

to-QDs and o-QDs are linked via the same ligand (PbI)₂And PbBr₂) The film prepared by the exchange strategy showed a plot of the energy at the tail of the Ulbach band, as shown in FIG. 9. It can be seen from the figure that the o-QDs thin film has a shallower band tail state, which is beneficial to improving the photoelectric performance of the o-QDs thin film.

to-QDs Quantum dots Via ligands (PbI)₂And PbBr₂) The transient absorption profile of the exchange strategy after preparation into a thin film is shown in FIG. 10.

o-QDs quantum dots via the same ligand (PbI)₂And PbBr₂) The transient absorption profile of the exchange strategy after preparation into a thin film is shown in FIG. 11.

It can be seen from FIGS. 10 and 11 that the o-QDs films have shallower band tails, which are manifested in a smaller red-shift of the transient bleaching peak.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A method for inhibiting the surface defect state of a quantum dot based on crystal face control is characterized by comprising the following steps:

the steps (1) to (3) are all carried out under protective gas.

2. The method for suppressing the surface defect state of the quantum dot based on crystal plane control as claimed in claim 1, wherein the lead precursor is lead chloride, lead oxide or lead acetate.

3. The method for suppressing the surface defect state of the quantum dot based on crystal plane control as claimed in claim 1, wherein the sulfur source is zinc sulfide quantum dot, sulfur powder, cadmium sulfide quantum dot or hexamethyldisilathiane.

4. The method for suppressing the surface defect state of the quantum dot based on crystal plane control as claimed in claim 1, wherein the selenium source is a cadmium selenide quantum dot, a selenium powder or a zinc selenide quantum dot.

5. The method for suppressing the surface defect state of the quantum dot based on lattice control as claimed in claim 1, wherein the ligand is oleic acid or dodecyl mercaptan.

6. The method for suppressing the surface defect state of the quantum dot based on crystal plane control as claimed in claim 1, wherein the anti-solvent is at least one of ethanol, acetone, ethyl acetate and the like.

7. The method for suppressing the surface defect state of the quantum dot based on crystal plane control as claimed in claim 1, wherein before adding the ligand in the step (4), the method further comprises a step of adding an organic solvent for dilution, wherein the dilution is used for preventing the quantum dot from agglomerating during centrifugation.

8. The method for suppressing the surface defect state of the quantum dot based on the crystal plane control as claimed in claim 1, wherein the centrifugation rate in the step (4) is 7000r/min to 9000 r/min.

9. The method for suppressing the surface defect state of the quantum dot based on the crystal plane control as claimed in claim 1, wherein the duration of the implantation operation in the step (2) is 1 s.

10. The method for suppressing the surface defect state of the quantum dot based on crystal plane control as claimed in claim 1, wherein in the step (3), the heating is kept to be always kept to enable the temperature to meet 40-70 ℃;