Study On Near-infaread Anti-stokes Fluorescence Properties Of Lead Sulfide Quantum Dots In Glasses

Anti-Stokes photoluminescence is the fluorescence where the emitted photons are higher in energy than the excitation photons,and it is a violation of Stokes law.The additional energy comes from the absorption of phonons so that it can achieve higher frequency emission while take out of the heat reservoir and cool the material.Due to the particular properties,ASPL has great potentials for applications in laser cooling and spectrum conversion.Quantum dots(QDs)have unique optical and electronic properties induced by quantum confinement effect,such as high fluorescence efficiency and tunable wavelength of photoluminescence,which have a great enhancement in anti-Stokes photoluminescence.Lead sulfide(PbS)have a large tunable range of effective bandgap energies and is easy to achieve anti-Stokes photoluminescence due to its small effective band gap energies and large exciton Bohr radii.Thus PbS QDs is a great selection of material to investigate anti-Stokes photoluminescence properties and mechanism which lay the foundation for the application of anti-Stokes photoluminescence of QDs.Synthesis of quantum dots in glasses is particularly attractive since it is inexpensive and chemically durable with high mechanical strength.Furthermore,the versatility of glassy hosts allows fabrication of devices in the bulk,planar or fiber forms.However,there has been not much researches on the ASPL of QDs doped glasses.Silicate glass containing ZnS and PbO,which were precursors of PbS QDs,was prepared by the conventional melt-quenching method.Glass transition temperature and crystallization temperature were obtained by DSC thermal analysis.Based on results of thermal analysis,PbS QDs were formed upon thermal treatment.The microstructure of PbS quantum dot doped glasses were analyzed by X-ray diffraction and high resolution transmission electron microscopy,and optical properties were measured by absorption and fluorescence spectra.The results are shown below:(1)A series of PbS QDs with different diameters were precipitated inside the silicate glasses.By controlling temperature and duration of heat treatment,diameter of PbS QDs was turned from 3.4 to 7.8 nm.Absorption wavelength of QDs was turned from 785 to 1734 nm,and photoluminescence wavelength was turned from 970 to 1765 nm.(2)Compared to the absorption bands,photoluminescence band of all the samples showed obvious Stokes shift.With the size of PbS QDs increased the Stokes shift gradually decreased.The maximum of Stokes shift is 301 meV,and such a big Stokes shift is mainly caused by surface trap states of PbS QDs,which are related to dangling bonds and surface defects.(3)Photoluminescence bands located at different wavelength when the samples excited with different energy.Upon excitation at the long wavelength side of the absorption bands,efficient anti-Stokes photoluminescence was observed.The anti-Stokes photoluminescence spectrum was significantly red shifted relative to the normal photoluminescence spectrum.This phenomenon indicated other defect states contributed to anti-Stokes photoluminescence.(4)The linear(3.9 nm-sized QDs)or sublinear(5.3 nm and 5.8 nm-sized QDs)dependence of the integral intensity of the ASPL showed the ASPL was a one-photon process.For 5.3 nm and 5.8 nm-sized PbS QDs,the integral intensity of ASPL band decreased with excitation power at high pump power.This phenomenon was due to the photo-induced photo-darkening.(5)The non-symmetry of the ASPL bands indicated that the ASPL was composed of two bands.Using dual Gaussian function simulation,one high energy band(P1)and one low energy band(P2)were obtained.P1 bands were related to the surface trap states,and P2 bands were related to the defects states.The energy difference between P1 and P2 bands decreased as the excitation power increased.For 5.8 nm-sized QDs,P1 and P2 bands overlapped with each other at 1400 mW or higher excitation power.(6)Based on the experiments results and previous researches,the optical properties and anti-Stokes photoluminescence of PbS QDs in glasses can be explained by a model including surface trap states due to the presence of dangling bonds and defects states induced by the defects located at the interface between the surface of QDs and the glass matrix.