| PbS Quantum dots(QDs)are direct band-gap semiconductor nanocrystals,which have the characteristics of high photoluminescence Quantum efficiency,high absorption coefficient in visible and near-infrared region,and easy size control,and have a wide range of applications in near-infrared light-emitting diodes and photoelectric detection,etc.There is still a long way to go before the commercial application of PbS QDs as a working material for photoelectric conversion.The improvement of stability and performance of PbS QDs has always been the focus of researchers.PbS QDs have small size effect and quantum limiting effect,and the study of regulating photoelectric properties such as light-induced absorption and luminescence with the change of QDs size is helpful to understand the physical mechanism of the interaction between light and PbS QDs.In this paper,we study the photoluminescence properties and optical nonlinear absorption of series of PbS QDs and core-shell PbS/Cd S QDs.The specific work is as follows:A series of oil-soluble PbS QDs of different sizes were prepared by thermal injection method,and the Cd S shell coating of 3.0 nm PbS QDs was realized by cation exchange method.Here,we selected the series of PbS QDs with the sizes of 3.1,4.1 and 4.8 nm and PbS/Cd S QDs with the size of 4.2 nm,and characterized and analyzed them by absorption spectrum,fluorescence spectrum and transmission electron microscopy(TEM).The change of band gap width with the size of PbS QDs is simulated theoretically,which is in accordance with the calculated band gap width of the absorption spectrum of QDs with the size change.The results of TEM measurement show the size of PbS/Cd S QDs.According to the experimental atomic ratio of Pb to Cd,the size of PbS core is about 2.9 nm,and the thickness of Cd S shell is about 0.65 nm.For these oil-soluble PbS QDs,the experiments of variable-temperature photoluminescence(PL)were carried out to investigate the thermal stability of QDs materials.The results show that the PL strength,the PL peak position and the half-height full width(FWHM)of PbS/Cd S QDs with different sizes vary with temperature.By fitting the results of PL strength varied with temperature,the exciton binding energy was obtained.The QDs size decreased from 4.8 nm to 3.1 nm,and the exciton binding energy increased from 39.8 me V to 55.5 me V.After Cd S coating,the exciton binding energy can reach 62.1 me V.The results show that the influence of temperature on the PL peak is mainly due to the electron-phonon interaction of the material,and the coating of Cd S can suppress the lattice thermal expansion effect of PbS quantum dots.It is found that the non-uniform broadening of the material and the exciton-longitudinal optical phonon are the main causes of the fluorescence spectrum broadening of PbS QDs.Optical nonlinear absorption measurements of a series of oil-soluble PbS QDs showthat PbS QDs possess anti-saturation absorption properties.The nanosecond Z-scan experiment results show that the nonlinear absorption coefficient of PbS QDs increases from 1.0×10-11 to 2.6×10-11 m/W with the decrease of QDs size,and the nonlinear absorption coefficient of Cd S-coated PbS QDs can reach 8.4×10-11 m/W.The results show that the nonlinear absorption coefficient of PbS QDs increases from 1.33×10-12m/W to 1.87×10-12 m/W with the decrease of QDs size,while the nonlinear absorption coefficient of Cd S-coated PbS QDs can reach 2.65×10-12 m/W.The results of Z-scan experiment show that the smaller the size of PbS QDs is,the larger the nonlinear absorption coefficient is,and the coating of Cd S can improve the nonlinear absorption capacity of PbS QDs. |
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