title: Importance of suppression of Yb(3+) de-excitation to upconversion enhancement in β-NaYF4: Yb(3+)/Er(3+)@β-NaYF4 sandwiched structure nanocrystals.
abstract: Nanosized Yb(3+) and Er(3+) co-doped β-NaYF4 cores coated with multiple β-NaYF4 shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb(3+) ions. We also explored the population of the Er(3+4)F9/2 level. The results reveal that energy transfer from the lower intermediate Er(3+4)I13/2 level is dominant for populating the Er(3+4)F9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er(3+4)S3/2 level for populating the Er(3+4)F9/2 level, which mainly comes from the cross relaxation energy transfer from Er(3+) ions to Yb(3+) ions followed by energy back transfer within the same Er(3+)-Yb(3+) pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er(3+)-Yb(3+) pairs. This population route is in good agreement with that in nanomaterials and bulk materials.
authors.affiliation: †State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.|‡Graduate School of Chinese Academy of Sciences, Beijing 100039, China.
authors.forename: Guotao
authors.initials: G
authors.lastname: Xiang
authors.affiliation: †State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.
authors.forename: Jiahua
authors.initials: J
authors.lastname: Zhang
authors.affiliation: †State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.
authors.forename: Zhendong
authors.initials: Z
authors.lastname: Hao
authors.affiliation: †State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.
authors.forename: Xia
authors.initials: X
authors.lastname: Zhang
authors.affiliation: †State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.
authors.forename: Guo-Hui
authors.initials: GH
authors.lastname: Pan
authors.affiliation: †State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.
authors.forename: Yongshi
authors.initials: Y
authors.lastname: Luo
authors.affiliation: §State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Lü
authors.affiliation: †State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.
authors.forename: Haifeng
authors.initials: H
authors.lastname: Zhao