| Rear earth upconversion luminescent phosphors have potential applications in solar cells,display and lighting,bioimaging,due to high optical stability,narrow emission band,long fluorescent lifetime,and low excitation energy.KLaF4 is considered as one of the most suitable host materials.In this thesis,the KLaF4:Yb3+/Er3+ nanocrystals doped with Al3+ or Na+ ions were synthesized by a hydrothermal method,and the properties of nanocrystals were investigated systematically.In addition,a sandwiched structure of Ag(Au)/ZnO/UC film were fabricated,and the effect of ZnO dielectric layer thickness on the UC luminescence properties were studied.The nanocrystals were characterized by an XRD,TEM,FT-IR,UV-Vis-NIR diffuse-reflectance spectra and upconversion spectra,respectively.The main results were obtained as follows:1.The influence of alkali ions tri-doping on the crystalline phase,morphology and FT-IR spectra in KLaF4:Yb3+/Er3+ nanocrystals was investigated.The results indicate that the doping of Na+ ions and Al3+ ions in nanocrystals has no influence on the crystalline phase of KLaF4:Yb3+/Er3+ nanocrystals,but result in the change of average particle size of nanocrystals.In addition,the bonding of the ligand molecules on the surface of samples were reduced.2.The upconversion emission intensities and lifetimes of the KLaF4:Er3+/Yb3+nanocrystals doped with Al3+ ions or Na+ ions were significantly increased compared to the undoped nanocrystals.For the nanocrystals doped with 15 at.%Na+ ions,the maximum red and green emission intensities were measured to be about 7.3 and 5.2 times,respectively,and the luminescence lifetime of the 4F9/2 state of the Er3+ was increased to 0.286 ms.However,The strongest UC emission intensity was observed in the nanocrystals doped with 10 at.%Al3+ ions,and the red and green emission intensities were enhanced by 7.3 and 5.9 times,respectively,compared to the Al3+-free sample.Moreover,the lifetimes of the 4F9/2 state and 4S3/2 state were increased from 0.413 ms and 0.104 ms to 0.477 ms and 0.136 ms,respectively.The enhancement of UC luminescence was mainly attributed to distortion of the local crystal field around Er3+ ions and reduction of the bonding of the ligand on the surface of nanocrystals.3.The studies of sandwich films indicate that the UC luminescence intensity can be tuned via controlling the thickness of ZnO film.The luminescence emission intensity of Ag/ZnO/UC structures was significantly improved,and the red intensities was enhanced by 18.6-fold for the sample with a 15 nm-thick ZnO film.In addition,the luminescence decay of the 4F9/2 state was fitted with a single exponential function,the highest fluorescence lifetime was obtained to be about 319 ?s.For a Au/ZnO/UC structure sample with a 10 nm-thick ZnO film,a maximum upconversion emission enhancements of 16-fold and 6-fold were observed for green and red emission,respectively.The decay times of 4S3/2 state and 4F9/2 state reached 170 ?s and 305 ?s,respectively.The luminescence enhancement for sandwich structure films may be attributed to the coupling between metal film and UC nanorod.In conclusion,KLaF4:Yb3+/Er3+ nanocrystals doped with alkali ions have been synthesized and the UC emission intensity was significantly enhanced,which may be helpful for preparing thin film solar cells with higher power conversion efficiency,therefore this work has a certain significance. |
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