Study On Synthesis And Optical Characteristics Of Cadmium Based Colloidal Quantum Dots

Colloidal quantum dots(QDs)have been widely applied in biomarkers,photovoltaic devices,lasers and light-emitting diodes.Because of their superb optical properties,such as broad absorption,narrow emission bandwidth,easy control of the emission wavelength,and superior luminescence efficiency,QDs are considered as one of the most promising emitters for next-generation displays and solid-state lighting.Among numerous QD materials,Cd-based and perovskite QDs show excellent optical properties.However,the highly complex synthetic process and poor stability of these materials limit the application in optoelectronic devices.In order to prepare Cd-based and perovskite QDs using simple synthetic process and owning high stability,the Cd-based and perovskite QDs were obtained by coating the shell and preparing alloy.The QLED was prepared by using the prepared QDs as the emitter layer.The main contents are as follows:(1)In order to reduce the toxicity of the precursor,Cd Se QDs were prepared by hot injection using cadmium acetate and selenium powder as precursors.The effects of reaction temperature(from 125? to 205?)and reaction time on the optical properties of Cd Se were studied in detail.At the same temperature,the longer the reaction time,the larger the Cd Se size,leading to the red shift of the luminescence wavelength.When the reaction time is the same,the emission color of Cd Se QDs is red-shifted obviously with the increase of reaction temperature.When the reaction time is ranged from 2 min to 10 min at 165 ?,the luminescence wavelength of Cd Se QDs can be adjusted from 476 nm to 550 nm,and the decay lifetime of Cd Se QDs can be extended from 16.89 ns to 21.60 ns.In addition,as the reaction time continues to extend to more than 10 minutes at 165 ?,Cd Se QDs with red emission can be obtained,and the emission wavelengths can cover the whole visible light region at165 ?.The 165 ? is the optimal synthesis temperature for Cd Se QDs.(2)In order to improve the optical properties and stability of Cd Se QDs,Cd Se/Cd S/Zn S and Cd Se/Zn S core/shell QDs were synthesized by hot injection method.The morphology,crystal structure and luminescent properties were studied.TEM images demonstrated that the size distribution of the QDs is uniform,and the average size is about 6-9 nm.The luminescent wavelength of Cd Se/Cd S/Zn S red quantum dot is 648nm,the luminescent wavelength of Cd Se/Zn S green quantum dot and blue quantum dot are 518 nm and 486 nm,the quantum yields are 63%,81%and46%,respectively,and the lowest half width of the luminescent spectrum is 37 nm.Based on the Cd Se/Cd S/Zn S red QDs,a red QLED was successfully fabricated with a structure of ITO/PEDOT/Poly-TPD/QDs/Zn O/Li F/Al.The maximum luminance of the device is 16 cd m^-2,and the maximum current efficiency is 0.002 cd A^-1.(3)To mitigate the lattice mismatch between core and inorganic shell interface,a simple one-step hot injection method was used to synthesize the versatile Cd_xZn_1-xS_ySe_1-y QDs with large-span self-regulating ability.By changing the ratio of Cd/Zn and Se/S in the precursor solution,the influence of this factor on the morphology,crystal structure and optical properties of the alloyed QDs were studied in detail.The results showed that the emission wavelength control span of the as-obtained gradient alloyed Cd_xZn_1-xS_ySe_1-y(x=0.12,0.17,0.23,0.33,y=0,0.64,1)QDs is widely close to 200 nm(445?643 nm)and the narrow full width at half-maximum surprisingly emerges down to 14 nm,evidently revealing that excellent self-regulating ability and superior monochromaticity.Using these different coloured QDs,the typical red,yellow and blue light emitting diode(LED)lamps were fabricated integrated with the ultraviolet chip(365nm),showing steady and bright emission.(4)CsPbI₃/Pb S QDs with core/shell structure were synthesized using hot injection method with Cs Pb I₃ QDs as the core and Pb S QDs as the shell.Small Pb S QDs can passivate the surface defects of Cs Pb I₃ QDs and improve the environmental stability and optical properties of Cs Pb I₃ QDs.The effects of the molar ratio of S:Pb on the Cs Pb I₃ QDs optical properties and stability were investigated systematically.The results showed that quantum yield of Cs Pb I₃/Pb S QDs is as high as 78%when the molar ratio of S:Pb is 0.1,and such Cs Pb I₃/Pb S QDs can maintain good luminescent performance after 7 days storage at room temperature.