Study On Stokes Fluorescence Properties Of Lead Sulfide Quantum Dots In Glasses

Lead chalcogenide(Pb S,Pb Se,etc.)quantum dots(QDs)have excellent optical and electronic properties,due to their small effective band gap energies and large exciton Bohr radii.Therefore,they have potential applications in optoelectronics,life science and information science field etc.IV-VI group of QDs have a large tunable range of effective bandgap energies,which can not only realize the absorption and radiation in near-and mid-infrared wavelength ranges,but also can be used as the light source and gain medium of NIR and MIR.Glass has good thermal stability and chemical stability,so it is one of the important matrix materials for the quantum dots.However,Pb S and Pb Se quantum dots doped glass still have many problems,such as the limited tunable band gap range,the difficult passivation of surface defects and the complicated change of Stokes shift.To solve these problems of quantum dots doped glasses,and based on the thermodynamics and kinetics of the crystallization in the glass,by controlling the over saturation solubility of the lead chalcogenide in the quantum dot doped glass materials,this work investigated the effect of sulfur element doping concentration,and S/Pb ratio on the crystallization behavior of Pb S quantum dot doped glass,the band gap of Pb S quantum dots and the fluorescence properties of Pb S quantum dots in glass.Silicate glass was prepared using the conventional melt-quenching method.Zn S and Pb O were used as precursors of Pb S,and glasses with different S/Pb ratio were prepared.Glass transition temperatures were obtained using simultaneous thermal analyzer.The Pb S QDs were formed upon thermal treatment.The microstructure and optical properties of Pb S quantum dot doped glasses were analyzed by X-ray diffraction,high resolution transmission electron microscopy,and absorption and fluorescence spectra.The effects of glass composition,doping concentration,S/Pb ratio on the band gap and fluorescence properties of the quantum dots were clarified.The experimental results are as follows:(1)Excess sulfur was added in the glass to increase the over saturation solubility of the sulfur,which further promoted the crystallization of Pb S quantum dots.Upon heat treatment,absorption and photoluminescence bands of Pb S QDs can cover the whole near-infrared range,even mid-infrared range.(2)Upon heat treatment,Pb S quantum dots was formed in silicate glasses with different S/Pb ratios,Diameter of Pb S QDs was turned from 3.5 to 13.3 nm.Absorption and photoluminescence wavelength of QDs was turned from 900 to2300 nm,which lay the foundation for the application in the mid-infrared band.(3)With the increasing of heat-treatment temperature,the absorption coefficient of Pb S QDs increased first and then decreased.The full width at half maximum(FWHM)of absorption spectra and photoluminescence spectra decreased first and then increased.When the thermal treatment temperatures were lower than530?,the photoluminescence bands were nearly Gaussian in shape.However,when the heat-treatment temperature increased to 530? or 540?,using two Gaussian functions,photoluminescence bands can be nicely simulated.These changes were most probably associated with the Ostwald ripening of the QDs when the heat-treatment temperatures were higher.The decrease in the number density of Pb S QD and the broaden in the size distribution occurred can be expected.(4)From the absorption spectra and emission spectra of Pb S quantum dot doped glasses with different S/Pb ratios,the Stokes shift was found for all the samples,and the maximum of Stokes shift is 206 me V.Stokes shift is due to surface defect of Pb S QDs,exchange interaction,intervalley interaction and energy transfer,and it is mainly caused by the surface energy levels related to surface defects.(5)In the experiment,the magnitude of Stokes shift decreased gradually with the increasing diameter of Pb S QDs,even close to zero at last.This is because small size QDs have large specific surface area thereby more surface defects.Studies had shown that exchange interaction and intervalley interaction increased with a decrease in Diameter,and the phonon vibration of smaller QDs consume more energy.