In this paper, using the difference in reactivity between the molar ratio of Cd/Zn and Se/S precursors, highly reproducible quaternary CdZnSeS quantum dots were synthesized by thermal injection. By changing the concentrations of Cd and Zn, Se and S, high photoluminescence in the range of 460~680nm was obtained, and electroluminescent devices were successfully prepared. The paper was published in the Journal of Luminescence, titled “Light-emitting diodes based on quaternary CdZnSeS quantum dots.”
Colloidal inorganic semiconductor quantum dots offer a wide range of opportunities for integration into optoelectronic devices. Quantum dots can be used in different applications such as solar cells, light-emitting diodes, photodetectors, and biosensors. Quantum dots have good optical properties such as color purity, light stability, high absorption cross-section, and band gaps that can be adjusted from ultraviolet to infrared light by controlling composition and size. In addition, the synthesis method of quantum dots is relatively simple and inexpensive. A range of technically important methods for the synthesis of semiconductors have been developed, and quantum dots can be customized to specific sizes, ranges and shapes.
Due to the quantum confinement effect, the electronic structure of semiconductor quantum dots is fundamentally different from the electronic structure of the corresponding bulk material.
Electron-hole recombination in direct bandgap semiconductor bulk crystals typically occurs by radiation to band transitions. In direct bandgap colloidal quantum dots, the transition to the band is almost monochromatic and has a large oscillator intensity due to quantization of the electronic state.
The synthesis of quantum dots is essential for the preparation of inorganic nano-heterostructures composed of binary , ternary, quaternary, or quaternary compound semiconductor or core/shell forming configurations. The spatial separation of electrons and holes reduces the exchange effect and the rate of radiation recombination. Therefore, designing the surface of quantum dots is the premise of device design, in which effective electroluminescence requires quantum dots with high photoluminescence quantum yield. Different research groups have reported on the synthesis and characterization of lineage quantum dots. Quaternary quantum dots are excellent candidates for improving the efficiency of light-emitting diodes. Fourth-generation quantum dots significantly improve luminescence by constricting core electrons and changing compounds that form shells, increasing the chance of recombination. Other important parameters are the color purity due to the narrow emission spectrum, as well as color adjustment by adjusting the particle size and composition. Recently, light-emitting diodes (LEDs) based on quantum dots (QDs) have been reported.
In 2013, the synthesis of CdSe/CdS core-shell quantum dots was reported, and a red light-emitting diode with a current efficiency of 19CdA−1 was prepared by using an inverted structure. Quantum dot light-emitting diodes (QD-LEDs) become relevant for applications that require high brightness and color purity. Quantum dot LEDs require the use of complex quantum dots such as CdSe/ZnS and CdSe/CdS, which are core/shell quantum dots. This type of quantum dot has a brightness of more than 10,000 cdm−2 and high color purity. Another type of quantum dot is quaternary quantum dots, such as InAlGaN quantum dots with high efficiency and luminescence in ultraviolet light.
Figure 1.a) Fluorescence spectra of CdZnSeS quantum dots with a CdZnSe molar ratio of 1:20 under 396 nm UV excitation. b) Absorption spectra of the corresponding CdZnSeS quantum dots. c) Photographs of colloidal CdZnSeS quantum dots of different composition and particle sizes, showing fluorescence without fluorescence (top) and with UV excitation (bottom). d) CIE chromaticity diagram showing the blue position.
Figure 2.a) Fluorescence spectra of CdZnSeS quantum dots with a molar ratio of 1:10 Se to S under 396 nm UV excitation. b) Absorption spectra of the corresponding CdZnSeS quantum dots. c) Photograph of colloidal CdZnSeS green-yellow emitting quantum dots of different composition and particle size, showing fluorescence with no (top) and with UV excitation (bottom). d) CIE chromaticity diagram showing color positions.
Figure 3.a) Fluorescence spectra of CdZnSeS quantum dots with a molar ratio of 1:4 Cd to Zn and a molar ratio of 1:10 Se to S under 396 nm UV excitation. B) Absorption spectra of the corresponding CdZnSeS quantum dots. C) Photographs of colloidal CdZnSeS red emitting quantum dots of different compositions and particle sizes, showing fluorescence without UV excitation (top) and fluorescence with UV excitation (bottom). D) CIE chromaticity diagram showing the red position.
Figure 4. (A) Electroluminescence intensity spectrum. (B) Current density-voltage and brightness-voltage curves. (C) Comparison of fluorescence spectra and electroluminescence spectra. (D) Photograph of LED devices using red-emitting CdZnSeS quaternary quantum dots.
In general, quaternary colloidal semiconductor quantum dots can be integrated into electronic and optoelectronic devices, and these materials have properties that traditional block semiconductors do not have. (Text: Aisin Kyaw Luo Xing)
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