| Semiconductor quantum dots(QDs)have many excellent properties,such as quantized energy levels,size adjustability,etc.They are widely used in Light Emitting Diodes(LED),high efficiency solar cells,biofluorescent probes and sensors,and their photoelectric properties have been a research hotspot in the world.However,in practical applications,quantum dot devices will inevitably encounter strong electric/magnetic field,high/low temperature,high pressure and other external environmental conditions,which will affect their performance.Therefore,it is of great practical value to study the influence of environmental conditions on the PL characteristics of quantum dots.At present,the problem is that the research on the influence of external environment conditions on the Photoluminescence(PL)characteristics of semiconductor quantum dots needs to be further improved.For example,the influence mechanism of surface electron injection/ion adsorption on PL characteristics under electrochemical environment,PL quenching mechanism under high temperature,interaction between quantum dots under high pressure,and the influence mechanism of high photon energy excitation conditions on PL quantum yield,etc.In this thesis,CdSe/ZnS core-shell structure quantum dots are taken as the research object.By analyzing their absorption spectra,steady-state PL spectra and time-resolved PL spectra,the effects of different environmental conditions on the PL characteristics of quantum dots are studied.The influence of electrochemical environment on the PL properties of CdSe/ZnS quantum dots was studied.The steady-state PL spectra and time-resolved PL spectra were synchronously analyzed.It was found that the intrinsic and surface state PL of quantum dots exhibited different changing rules under different electrode potentials: the former showed red shift with the increase of negative electrode potential,while the latter showed blue shift with the increase of negative electrode potential.The results show that the red shift is due to the weakening of the quantum confinement effect due to the adsorption of cations,and the blue shift is due to the increase of the probability of surface state emission due to the injection of surface state electrons.In addition,the PL intensity of both intrinsic and surface states decreases significantly with the increase of negative electrode potential,which is due to the non-radiative recombination of excitons caused by space charge.The effects of high and low temperature on the PL properties of CdSe/ZnS quantum dots were studied.The analysis of steady-state and time-resolved PL spectra shows that the reversible PL quenching is due to the thermal escape of holes in the temperature range of 80-500 K.In the high temperature region of 320 K-500 K,there is an additional irreversible quenching pathway for PL,which is due to the formation of new surface defect states during heating,resulting in surface trapping,resulting in deep trap emission and non-radiative recombination process.In addition,in the temperature range of 80-500 K,with the increase of temperature,the PL peak position redshifts and the spectrum broadens.The red shift is due to the shrinkage of the bandgap of quantum dot material with the increase of temperature.The broadening of the spectrum is due to the coupling of excitons and acoustic/optical phonons.The effects of high pressure on the PL properties of CdSe/ZnS quantum dots were studied.The results of steady-state PL and time-resolved PL spectra show that the blue shift of PL emission wavelength gradually increases with the increase of pressure,but the blue shift rate of PL emission wavelength decreases in the high-pressure region.On the one hand,because the elastic modulus of the material increases gradually with the pressure,the rate of lattice compression decreases,so the blue shift rate decreases;on the other hand,the interaction between quantum dots under high pressure leads to the red shift of emission wavelength,which reduces the blue shift rate.In addition,in the high-pressure region,the interaction between quantum dots also reduces the overlap of electron hole wave functions in quantum dots,resulting in longer PL lifetime.The influence of excitation photon energy on the PL properties of CdSe/ZnS quantum dots was studied.The experimental data of absorption,PL and excitation spectra are comprehensively analyzed,and the decrease of relative PL quantum yield with increaseing excitation photon energy is obtained.Based on the experimental data and the theoretical simulation of phenomenological model,it is shown that the reduction of PL quantum yield depends on the competitive relationship between surface trapping and relaxation of hot electrons.The increase of hot electron trapping rate in higher excited states leads to the decrease of excitons involved in radiative recombination and the decrease of PL quantum yield.In addition,the theoretical simulation of the microscopic model shows that the depth of the surface trap has a great influence on the PL quantum yield.If the surface trap becomes depper,the electron trpping will be greater,which will more significantly reduce the PL quantum yield.In this thesis,the influence of different external environment conditions on the PL properties of CdSe/ZnS quantum dots was systematically studied,provides a new idea for exploring the physical mechanisms of exciton dynamics in quantum dots,and helps to solve the issues in the practical application environment of quantum dot devices. |
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