Spectroscopy Study Of Colloidal CdSe Quantum Dots

In the past decades,quantum dots have become a hot topic of research due to their peculiar optoelectronic properties,such as fluorescence spectra adjustable with size,narrow and symmetrical emission spectrum,broad and continuous absorption spectrum,good optical stability,high luminescence efficiency,and large Stokes shift.Studying the spectra of quantum dots,such as size and excitation power dependence of photoluminescence spectra,can help us understand the properties of semiconductor nanomaterials and further improve the quality of quantum dots materials.Deep understandings of quantum dots spectra are important to the exploration of carrier dynamics in quantum dots,which can further help us understand the basic physics of zero-dimensional structures and provide theoretical support to the improvement of photoluminescence efficiency and performance of optoelectronic devices.There have been many efforts devoted to the study of spectra of quantum dots,however,the carrier recombination processes are so complicated that detailed investigations are still needed.In this thesis,based on previous studies,colloidal CdSe quantum dots of different particle sizes are investigated.Photoluminescence spectra are analyzed systematically,and the recombination processes of carriers are summarized.The main research contents of this thesis include:(1)Observation of the steady-state and transient spectra of photoluminescence from colloidal CdSe quantum dots of different sizes,and discussion of the effect of particle size on the luminescence properties.In this thesis,colloidal CdSe quantum dots of five different particle sizes were successfully prepared by the thermal injection method.At room temperature,they have different photoluminescence wavelengths under the same light excitation condition,and the spectra are narrow and symmetric(FWHM?30nm),indicating that the particle size distributions of the colloidal CdSe quantum dots are relatively uniform and are well dispersed.The photoluminescence spectrum of colloidal CdSe quantum dots shows blue shifts as the size of the quantum dots decreases,which is a relatively common phenomenon caused by the size effect of quantum dots.When the size of the quantum dots is reduced,the space for the carrier(electron-hole pair)of the quantum dots is more restricted,which causes the energy band to move,that is,the bottom of the conduction band moves upward,and the top of the valence band moves downward.Therefore,the energy gap increases,and results in an increase in the energy of photons emitted by the exciton radiation recombination.The fluorescence lifetimes were measured at the wavelengths of emission peaks.The corresponding experimental results show that the fluorescence lifetime increases with the increase of the size of quantum dots,which is due to the smaller exciton oscillator intensity of the larger size quantum dots.(2)Investigation of steady-state and transient photoluminescence of CdSe quantum dots under different excitation powers,and discussion of the influence of excitation power on the integrated intensity and fluorescence lifetime of fluorescence spectra.At room temperature,the integrated photoluminescence intensity under different excitation powers increases linearly with the power,and only one fluorescence peak appears.It shows that the luminescence of colloidal CdSe quantum dots is single exciton luminescence,which belongs to intrinsic luminescence.The fluorescence lifetime of colloidal CdSe quantum dots decreases with the increase of excitation power.This is because the concentration of photogenerated carriers increases and the number of excitons in the whole system increases,which increases the exciton recombination rate and the luminescence intensity.This article aims to study the spectra of colloidal CdSe quantum dots,that is,the size dependence and power dependence of quantum dots spectra.Exploring the carrier dynamics process of quantum dots can help us understand the internal characteristics of quantum dots,provide data support for the preparation of high quality quantum dots,and further improve the efficiency of optoelectronic devices.