Optical Microscopic Characterization And Photophysical Process Of Upconversion Nanocrystals

The advent of upconversion nanocrystals promotes the development of optical mi-croscopy in the fields of biological imaging and super-resolution.The main step to accu-rately detect the luminescence signal is to characterize the upconversion nanocrystals with high quality optical microscopy.In this thesis,we used self-built inverted microscope and optical detector to characterize and evaluate the morphology and imaging quality of upcon-version nanocrystals at the nanoscale.At the same time,we revealed the photophysical laws of the luminescence intensity,radiation spectrum and fluorescence lifetime of upconversion nanocrystals by changing the excitation type,and achieving the analysis of the optical prop-erties of upconversion nanocrystals.In the study of optical microscopic imaging of upconversion nanocrystals,we prepared single particle upconversion nanocrystals with controllable distribution density by dilut-ing and spin coating the solution of upconversion nanocrystals.In the meantime,the self-built inverted microscope combined with wide-field illumination imaging mode is used to achieve high-quality imaging of single particle upconversion nanocrystals,and the confocal laser scanning imaging mode is used to scan the distribution of single particle upconver-sion nanocrystals,and use atomic force microscope to characterize the morphology of single particle upconversion nanocrystals.In the study of luminescence properties of upconversion nanocrystals,we detected the fluorescence signal of upconversion nanocrystals in laser confocal scanning mode,which revealed the law of the optical properties of upconversion nanocrystals changing with the excitation power density and dopant concentration.Based on the analysis of the photophys-ical model,it is found that for the same single particle upconversion nanocrystals,the in-crease of excitation light power density significantly increases the number of sensitizer ions in the excited state,the activator absorbs more energy through the energy transfer process,increases the number of ions in the excited state,and improves the luminescence intensity.Simultaneously,with the increase of activator doping concentration,the quantum yield and luminescence intensity of single particle upconversion nanocrystals decrease greatly due to the concentration quenching effect.The luminescence spectra show that the upconversion nanocrystals have multiple sharp emission peaks,and the emission range covers the range of ultraviolet to infrared light.The intensity of the characteristic peaks will be significantly affected by increasing the excitation power density.Meanwhile,the cross relaxation phe-nomenon between ions in the high doping concentration activator makes the energy con-sumption rapidly,which shows the decrease of the overall intensity of the spectral line.It is found that when the doping concentration of the activator is increased,the energy is dis-sipated rapidly due to the concentration quenching effect,which leads to the increase of the nonradiative rate and the decrease of the luminescence lifetime;When the powerdensity of the excitation light is increased,the luminescence components of the high excited state increase,and the proportion of the luminescence lifetime of the low excited state decreases.