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A three-necked, round-bottomed flask was used as the reaction vessel. A glass tube at the first flask-mouth to inject high-purity N2 into the reaction liquid, and the pressure in the three-necked flask was increased at the beginning of the reaction to prevent air entering. The gas was exhausted during the second flask-mouth. The glass tube was connected to the rubber tube and inserted into another beaker filled with silicone oil, to prevent the air from being sucked into the reaction flask while exhausting. It was equipped with a rubber plug and a glass tube for adding drugs at the last flask-mouth. The rubber plug was used to seal the glass tube after adding, and the NaHTe solution was extracted after completion of the reaction in the syringe. The specific operation was described as follows: first, connected the device and injected N2; second, added 30 ml deionized water and injected N2 into the water after 5 minutes; third, magnetic stirring for 10 minutes, the three-necked flask was in an oxygen-free environment; then added 2.5 g Te powder and 1.8 g NaBH4 through the stopper of the glass tube, and sealed the glass tube with a rubber plug quickly. To prevent air entering, a syringe was used and a long needle was inserted into a rubber plug at the third opening of the flask in the process of extracting the NaHTe solution after the reaction.
Using the device, the reaction at room temperature was achieved. The exhaust system was sufficient to handle hydrogen generated at the high speed, as well as the N2 provided cooling. In this paper, the new preparation method has successfully reduced the traditional harsh conditions of reaction from 10 hours at 5 ℃ to 1 hour at the room temperature and improved the quality of the NaHTe solution. The preparation quality and speed were increased, and Te source was the furthest avoided from being polluted by oxidation to the greatest extent.
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The synthetic procedure was instructed according to the method mentioned in Ref.[12] with minor modification Typically, took 500 mL of deionized water and put it into a 1000 mL three-necked flask, then added0.4565 g of CdCl2•2.5 H2O, 0.310 ml of TGA, passed in N2, and stirred. Adjusted the pH of the solution to 9 with 1 mol/L NaOH. Took 0.612 ml of the NaHTe solution which was prepared and added it into a three-necked flask, adjusted the heating temperature to 100 ℃, condensed and refluxed for 2 hours to complete the preparation of the CdTe core.
Repeated the above process twice and adjusted the pH value to 10 and 11 respectively. The condensation reflux was adjusted accordingly for 6 hours and 10 hours to prepare the CdTe core. The sample was observed under the 365 nm ultraviolet light and the color was shown in Tab.1.
Sample number Quantum dots Size/nm Core color Cladding time S1 CdTe/CdS-1 3.2 Orange 30 min S2 CdTe/CdS-2 3.3 Orange 1 h S3 CdTe/CdS-3 3.4 Orange 2 h S4 CdTe/CdS-4 3.0 Yellow 30 min S5 CdTe/CdS-5 3.1 Yellow 1 h S6 CdTe/CdS-6 2.6 Green 30 min S7 CdTe/CdS-7 2.7 Green 1 h S8 CdTe/CdS-8 2.8 Green 2 h S9 CdTe/ZnS-9 3.5 Orange 30 min S10 CdTe/ZnS-10 3.6 Orange 1 h Table 1. Ten kinds of the core-shell structure quantum dots
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Took 100 mL CdTe solution and put it into 250 mL three-necked flask, passed in the N2 and stirred. Weighed 0.0122 g thiourea in a three-necked flask and recovered the pH of the solution to 10 with 1 mol/L NaOH. Set the temperature for 90 ℃ and started the timer when the temperature reached 90 ℃. The reflux time was 30 min, 1 h and 2 h, respectively. After the reaction, the three-necked flask was cooled to room temperature quickly with cold water.
The obtained the CdTe/CdS quantum dots solution was put into the centrifuge tube, and the ratio of the CdTe/CdS quantum dots to the ethanol in the solution was 1∶2 to 1∶4 (volume ratio), and centrifuged in a high-speed centrifuge, with the rotation speed of 10000 r/min and the time was set for 10 minutes. After repeated centrifugation, the solids were dissolved in deionized water for testing.
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The CdTe/ZnS quantum dots were synthesized and improved according to the method of Li’s group[13]. To obtain these dots, a mixture of 80 ml CdTe solution, 0.230 g ZnSO4 and 0.148 ml TGA was loaded slowly into 20 ml Na2S•9H2O aqueous solution. Molar ratio of Zn to S was 1∶1, and the molar ratio of Cd to Zn was 1∶2. The reaction occurs when stirred and heated to 80 ℃. The size of the shell is controlled with the reaction time. Then, the CdTe/ZnS quantum dots were centrifuged and purified, finally re-dispersed into the deionized water.
To sum up, the parameters of the 10 water-soluble core-shell quantum dots are shown in Tab.1
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In order to explore the influence of shell thickness, core size and shell material of the semiconductor quantum dots on UV-VIS absorption spectrum, the eight kinds of CdTe/CdS and two kinds of CdTe/ZnS semiconductor quantum dots were measured (UV-2500 UV-vis spectrometer from Shimadsu, Japan), as shown in Fig.1 and Fig.2 respectively.
The fluorescence spectrum of the CdTe/CdS and the CdTe/ZnS quantum dots under the excitation wavelength of 400 nm were obtained using the LS55 fluorescence spectrometer from Perkin Elmer, as shown in Fig.3.
Preparation and fluorescence characteristics of CdTe/CdS and CdTe/ZnS core-shell semiconductor quantum dots
doi: 10.3788/IRLA20200287
- Received Date: 2020-08-04
- Rev Recd Date: 2020-10-15
- Publish Date: 2021-05-21
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Key words:
- CdTe/CdS /
- CdTe/ZnS /
- core-shell quantum dots /
- fluorescence characteristic
Abstract: Core-shell semiconductor quantum dots materials are being investigated due to their special performance in repairing surface defects for the single quantum dots and greatly improving the optical performance of quantum dots. Instead of a traditional small flask as a reaction vessel to prepare NaHTe, the preparation of the CdTe core using a three-necked flask was achieved. 10 CdTe/CdS and CdTe/ZnS core-shell semiconductor quantum dots with different core sizes, shell thicknesses and shell materials were synthesized. The UV–visible absorption and fluorescence spectrum of 10 kinds of core-shell semiconductor quantum dots materials were measured and analyzed. The absorption spectrum of quantum dots in the UV-visible band shows that with the increase of quantum dots size, the absorption peak is red-shifted. The CdTe/CdS quantum dots differ in fluorescence lifetime and intensity due to the conversion of different core and shell sizes of quantum dots between types I and II. When the shell thickness of CdTe/ZnS increases, the shell thickness of ZnS reduces the number of dangling bonds and defect states on the core surface, which increases the probability of electron-hole pairs recombination and causes the fluorescence peak red-shift.