Preparation And Light-Emitting Application Of Quantum Dots With Gradient Core-Shell Structure

Colloidal quantum dots?CQDs?have the advantages of wide and continuous excitation spectrum,tunable emission peak position,narrow half-peak width,high luminous efficiency,and good stability.As a result,they have been widely applied in the fields of lasing,light-emitting diodes?LEDs?,solar cells,biological labeling and so on.However,in terms of light-emitting devices,there are still many shortcomings for quantum dots.First of all,for the blue quantum dots,because the energy levels of the Zn Cd S core and the Zn S shell are close,the design of the alloyed layer will make it easier for electrons to be trapped by the surface defects of the shell,causing the loss of excitons in the quantum dots.In addition,due to the large difference in the reactivity of Zn²⁺and Cd²⁺precursors,it is difficult to precisely control the composition and structure of the intermediate alloyed monolayer,which causes the increase of defects.These problems lead to high amplification spontaneous emission?ASE?threshold of blue quantum dots.Secondly,the technology of cadmium-based quantum dots is more mature.However,the high toxicity of Cd greatly limits its commercial application.InP quantum dots have the potential to replace cadmium-based quantum dots due to their low toxicity and similar optical properties to Cd-based quantum dots.However,due to the serious lattice mismatch between the InP core and the shell,the green InP quantum dots have large surface tension,a number of defects,low photoluminescence quantum yield?PL QY?,and poor LED device performance.Therefore,the thesis of this paper focuses on the above two issues.First of all,we replaced the traditional trinoctylphosphine-sulfur?TOP-S?with 1-decanethiol?DCT?as the source of sulfur,and DCT?Lewis soft base?can balance the reactivity of Cd²⁺?Lewis soft acid?and Zn²⁺?borderline acid?as the ligand.Thus,the growth of the alloyed shell layer whose structure and composition can be precisely controlled is achieved.Subsequently,a thick Zn S shell was coated on the outside of the alloyed layer to suppress charge leakage.Finally,the gradient alloyed shell quantum dots which are of uniform monolayer composition and large size were obtained.The core-shell quantum dots can effectively release the surface tension of quantum dots,reduce defects of quantum dots,weaken non-radiative recombination processes such as Auger recombination.Under the excitation of nanosecond laser,the ASE threshold of core-shell quantum dots with gradient alloyed shell is as low as 28?J/cm²,which is about 27 and 9 times lower than those of single Zn S shell quantum dots and pure alloyed shell quantum dots,respectively.In particular,the ASE threshold of core-shell quantum dots with gradient alloyed shell has been the lowest value of the ASE based on blue quantum dots so far.Secondly,we used gallium acetylacetonate?Ga?acac?₃?as the source of gallium to generate In_1-xGa_xP core with gradient alloyed composition at high temperature through the activation of surface ligands by acetylacetonyl groups.And then the Zn Se shell layer and the Zn S shell layer were coated.The existence of the gradient alloyed core effectively solves the lattice mismatch between the traditional InP core and the Zn Se shell,which makes the photoluminescence quantum yield of the In_1-xGa_xP core-shell quantum dots as high as 82%and the maximum external quantum efficiency?EQE?of the LED device up to 3.1%.Compared with the traditional InP/Zn Se/Zn S core-shell quantum dots,they are increased by 25%and nearly doubled,respectively.In order to further improve the efficiency of charge injection,we performed ligand exchange for In_1-xGa_xP/Zn Se/Zn S core-shell quantum dots and replaced the long-chain hexadecanoic acid with short-chain hexanoic acid.In the end,the maximum brightness of the LED device is 2672 cd/m²,which is twice as much as before the ligand exchange.The maximum external quantum efficiency is also increased from 3.1%before the ligand exchange to 3.8%,which effectively improves the performance of the LED device.