Research On The Properities Of Impurity Doping In NaYF₄:Yb³⁺,Er³⁺ Upconversion Materials

As for upconversion?UC? materials, their special UC luminescences are mostly associated with the abundant 4f energy level of itself. The UC emission process and the mechanism are usually related to the energy transformation of the ions besides each other in a host. The high efficiency upconversion emission mainly depends on the coordination of the hosts, sensitizers and activators. The impurity doping can tune the UC intensity of UC materials without change the local symmetric structure and hardly effect on the structure of the host.In this work, a facility hydrothermal method was prepared to synthesis hexagonal NaYF4:Yb³⁺,Er³⁺ UC materials, Al³⁺, Bi³⁺, Ca²⁺ and Mn²⁺ were introduced to investigate the different ions doping in the NaYF4:Yb³⁺,Er³⁺ hosts, respectively. Characterization methods as X-ray diffraction?XRD?, scanning electron microscope?SEM?, Fourier transform infrared?FT-IR? and Upconversion luminescence?FL? were used to investigate their structure, morphology, upconversion performance. This work mainly was foucused on the factor of the species and number of the impurity ions, the functional group on the surface and UC performance by impurity doping, tried to understand the various kinds of morphology of UC material by impurity doping and discussed the mechanism of UC efficiency enhancement.The results indicates that this kind of impurity ions had been promoted the UC emission of NaYF4:Yb³⁺,Er³⁺ in some degree. The visible light from the energy level transition?2H11/2, 4S3/2? ?4I15/2 and 4F9/2?4I15/2 were belong to the emission of Er³⁺ ions. Besides, the oleic acid on the surface lattice have taken effect on the intensity of red emission of Er³⁺ through the Fourier transformed infrared spectra. Nevertheless, to different various ions, the effection of impurity doping inNaYF4:Yb³⁺,Er³⁺ hosts are different.?1? Al³⁺ ions doping?0 10 mol%? have improved formatting pure hexagonal NaYF4:Yb³⁺,Er³⁺ UC materials, the pure hexagonal products was produced when the doping concentrantion ? 1 mol%. The morphology was changed from the mixture of nanoparticles with microrods to pure microrods. The best doping concentration of UC intensity was 7.5 mol% Al³⁺-sample, which enhanced 10.1 and 9.8 fold of green and red emission compared to Al³⁺-free samples, under the 980 nm excitation pump power of 120.8 mW, respectively. This was due to the radii of Al³⁺?r Al = 0.0535 nm? was smaller than that ofY³⁺?rY = 0.089 nm? which was be substituted that enhanced the 4f transition and improved the UC intensity.?2? In the situation of Bi³⁺ doping?0 10 mol%?, the purity of the hexagonal NaYF4:Yb³⁺,Er³⁺ UC materials was enhanced, the pure hexagonal products was produced when the doping concentrantion > 1 mol%. The morphology was changed from the mixture of nanoparticles with microrods to pure microrods. The best doping concentration of UC intensity was 4 mol% Bi³⁺-sample, which enhanced 4.9 and 5.5 time of green and red emission compared to Bi³⁺-free samples, under the 980 nm excitation pump power of 653.6 mW, respectively. This was because that the radii of Bi³⁺?rAl = 0.103 nm? was bigger than that ofY³⁺?rY = 0.089 nm? which was be substituted that enhanced the 4f transition and improved the UC intensity. However, with the doping concentration enhancing, the host expanding, the energy transfer center was far from Er³⁺, would took place of flourescence quenching.?3? When it's about the Ca²⁺ doping?0 35 mol%?, the pure hexagonal was produced at the doping concentration of Ca²⁺ ? 5 mol%, the morphology was drasticly changed while doping concentration was ? 15 mol%, produced the regular hexagonal prism micoparticles. The best doping concentration of UC intensity was 20 mol% Ca²⁺-sample, which enhanced 9.9 and 41.1 time of green and red emission compared to Ca²⁺-free samples, under the 980 nm excitation pump power of 653.6 mW, respectively. It can be contributed to that the radii of Ca²⁺?rAl = 0.103 nm? was bigger than that ofY³⁺?rY = 0.089 nm? which was be substituted that enhanced the 4f transition and improved the UC intensity. However, with the doping concentration enhancing, the flourescence quenching was not happen, which may be related to the vancancy hole in the host which caused by divalent ions that made the lattice more unstable.?4? As for the Mn²⁺ doping?0 30 mol%?, the NaYF4:Yb³⁺,Er³⁺ UC materials was pure hexagonal microcrystals at the doping concentration of 5 25 mol%, but changed to nanoparticles at 30 mol% doping sample. The best doping concentration of UC intensity was 20 mol% Mn²⁺-sample, which enhanced 5.6 and 3.0 time of green and red emission compared to Mn²⁺-free samples, under the 980 nm excitation pump power of 47.0 mW, respectively. This enhancement can be conluse with that the radii of Mn²⁺?rMn= 0.081 nm? was smaller than that ofY³⁺?rY = 0.089 nm? which was be substituted that enhanced the 4f transition and improved the UC intensity. But the UC intensity was decreased mostly can be connected with the phase transition with the doping concentration enchancing. This was because that the Mn²⁺ can take part in energy transformation and divalent ions that made the lattice more unstable in the host with more vancancy hole.At last, the future potential application was prospected through the aspects of the excitation pump power, the crystals size and the UC optical property of NaYF4:Yb³⁺,Er³⁺,M(M = Al³⁺, Bi³⁺, Ca²⁺ and Mn²⁺) in impurity doping.