1. Key Laboratory of Luminescence and Optical Information (Beijing Jiaotong University), Ministry of Education, Beijing 100044, China
2. Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China
Abstract：In order to investigate the effect of pump power on the luminescence properties of upconversion materials doped with different concentrations of sensitizer ions, in this study, NaYF4∶Yb3+, Er3+ upconversion nanoparticles doped with different concentration of sensitizer Yb3+ were successfully synthesized by solvent-thermal method. The morphology and structure of prepared sample were charactered by XRD and TEM measurements. The results suggested that these samples were all hexagonal nanocrystals with good crystallinity. As the concentration of Yb3+ increased，the particle size increased slightly. At the same time, the photoluminescence properties of these prepared nanoparticle excited by 980 nm were studied in detail by collecting the pump power-dependence fluorescence spectrum. For all samples, the intensity of upconversion fluorescence increases with the enchancement of excitation irradiance which can be attributed to the fact that high pump power induced the higher absorption efficiency of nanoparticles. Besides, the red green ratio (RGR) can be tuned by adjusting the excitation irradiance too. And it’s worth noted that the tuning range of RGR depends on the doping ratio of sensitizer Yb3+ in NaYF4∶Yb3+, Er3+. In order to deeply understand the mechanism of upconversion luminescence, the possible electron energy transfer process was proposed. We assumed that the tuning range of RGR is related to the different average distance between rare earth ions and the comprehensive effect of the process of multi-phonon relaxation, cross-relaxation, and back energy transfer. The sample with low Yb3+doping concentration has a negligibly back energy transfer probability due to the fact that average distance between Yb3+ and Er3+ is long. The multiphonone relaxation and corss-relaxation are the main processes that convert a part of green emission into red emission. Following the enchancement of excitation irradiation, the benefit of the high excitation irradiance can relief this insufficient, and the red green ratio increases slightly. In heavily doping samples, the back energy transfer process between neighboring Yb3+ and Er3+ happened more probably and became the main factor for the nonradiative process. High-lying levels show a decreasing contribution, which leads to a increasing red green ratio followed the enhancement of pump power. The red green ratio increases with the increasing pump power due to different emphases of nonradiative processes in NaYF4∶Yb3+, Er3+nanoparticles doped with different concentrations of Yb3+. The luminescence properties of the prepared UCNPs not only allow us obtaining upconversion nanoparticles with better red emission performance, but also determine the doping ratio by measuring the red-green ratio of the material. All results indicated that the material is potentially to be a multifunctional photodynamic therapy nanoplatform used in bio-detection filed through further design and modification. The possible electron energy transfer process is proposed which is helpful in designing and optimizing the doping of rare earth ion-pair, and understand the mechanism of upconversion luminescence.
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