Near-infrared High Efficiency And Broadband Photoluminescence From ?-? Quantum Dots In Glasses

Quantum dots(QDs)have attract much attention due to their unique optical and electronic properties induced by the quantum confinement effect.?-? quantum dots,such as PbS and PbSe,have narrow band-gap energies and large exciton Bohr radii(0.41 e V/18 nm for PbS and 0.28 e V/46 nm for PbSe,respectively),and as a result,effective bandgap energies of lead chalcogenides QDs can be adjusted in a wide range,covering the most important telecommunication window,biological tissue semitransparency window as well as the atmosphere transmission window.Therefore,lead chalcogenide QDs have great potential applications in near-infrared and mid-infrared LED,bio-labeling,solar cell,optical amplifier and environmental monitoring.Glass has been used as host material for various luminescence centers,such as QDs,rare-earth ions and transition metal ions due to the good thermal,chemical and mechanical stabilities as well as the low cost and easy fabrication procedure.Embedding QDs into glasses can effectively keep the QDs from aggregation and maintain good thermal and chemical stabilities.Besides,QDs doped glasses can be easily made into fiber using for optical fiber amplifier.However,due to the presence of defects on the surface of QDs,the photoluminescence quantum efficiency of QD embedded in glass is generally low and photo-darkening and photo-brightening were generated.These surface defects trapped carriers and were difficult to be passivated like colloidal QDs because of the obstruction from glass matrix,which severely limited the application of QDs doped glass materials.One the one hand,investigating the mechanism of the surface trap reduction in photoluminescence intensities is conductive to understanding of improving the final photoluminescence efficiency.One the other hand,realization of the surface passivation of QDs in glasses can enhance the efficiency.Besides,broadband photoluminescence from PbSe QDs can cover the whole telecommunication window.Both of the high efficiency and broadband photoluminescence was important to promote the potential applications of ?-? QDs embedded glasses towards various optoelectronic devices.The main part of this dissertation is composed of six chapters.In chapter 1,the basic physical characteristics and optical properties of quantum dots were presented.Then the research progress of PbS and PbSe quantum dos with narrow band-gap was reviewed.Last,the growth dynamics of QDs in glass matrix and the methods of passivation of QDs in glass were discussed.In chapter 2,the preparation process of QDs doped-glasses and related characterization methods were illustrated.In chapter 3,the effect of excitation light condition on the photo-darkening and photo-brightening phenomenon from PbS QDs was discussed and the correlated fluorescence progress was shown in one schematic diagram.In chapter 4 and 5,two different surfaces passivation methods of PbS QDs in glass were discussed,respectively.And in chapter6,preparation of PbSe QDs doped glasses with broad-band photoluminescence was introduced.(1)PbS quantum dots(QDs)with mean radii of 3.7 nm to 9.0 nm are precipitated in silicate glasses.Upon above-band-gap excitation,photoluminescence from PbS QDs is strongly dependent on their size and excitation wavelength,exhibiting photo-darkening(PD)or photo-brightening(PB).Photoluminescence of PbS QDs exhibits strong darkening by short excitation wavelength but the darkening gradually mitigated as the excitation wavelength increased and even turns to be photo-brightening at room temperature.But PD and PB show a much more complicated variation tendency under same excitation condition when the size of QDs increased.The dependence of PD and PB on QDs'size and excitation wavelength indicates that electron/hole trap states of PbS QDs,defect states in surrounding glass matrix as well as on the interface between glass matrix and PbS QDs all have strong effects on photoluminescence properties of PbS QDs.And based on the above phenomena,a model for the photoluminescence process from PbS QDs in glass was established.(2)A series PbS QDs doped glasses with three different precursor S/Pb ratio(42.0,9.4,2.1)were prepared,and upon different heat-treatment conditions,luminescence was adjusted in the wavelength region of?984 nm to 2043?nm.Surface passivation was realized through increasing the S/Pb ratio in glasses,making sulfur enriched,and the PLQY reached as high as 46.9%.The external quantum efficiency(EQE)of 7.95%and 4.77%were achieved for 1380 nm and 1600 nm QD LEDs.Based on the ultrafast spectroscopic analysis,this kind of quantum yield enhancement of PbS quantum dots in glass was attributed to passivation of PbS QDs surface by excess sulfur bonding to unpaired dangling bonds of Pb ions on the surface of nanocrystals.(3)The surface passivation of PbS QDs with a low sulfur to lead ratio was realized by the introduction of 2%,3%and 4%mol calcium chloride into glass matrix.Upon different heat-treatment conditions,PbS QDs generated in all the three glass matrixes with different contents of chlorine and exhibited tunable photoluminescence from?1000 nm to?2100 nm.STEM-EDS mapping showed that the surface of PbS QDs were enriched in chlorine.So,the unpaired dangling bonds of Pb ions on the surface linked with chloride ions,and the quantum efficiency nearly doubled.The highest quantum efficiency was about 49.3%.(4)Upon thermal treatment,PbSe QDs were precipitated in the oxyfluoride glass-ceramics containing BaF₂ nanocrystals.By controlling the thermal treatment conditions,dual-band photoluminescence from PbSe QDs doped glass-ceramics was observed when the glasses were heat-treated at high-temperatures(560°C or 570°C)or underwent two step heat-treatment.With the crystallization of BaF₂,formation of two sets of PbSe QDs with different average sizes in the glass matrix was considered as the reason for the appearance dual-band photoluminescence.Large full width at half maxima of these dual-band photoluminescence(?500 nm)indicates that formation of lead chalcogenide quantum dots in oxyfluoride glasses with dual-band photoluminescence has potentials for broad-band infrared light sources related applications.