Mechanisms of Upconversion Luminescence of Er 3+ -Doped NaYF 4 via 980 and 1530 nm Excitation. - Liu, Yu; Zhou, Ziwen; Zhang, Shaojian; Zhao, Enming; Ren, Jing; Liu, Lu; Zhang, Jianzhong (Nanomaterials (2079-4991), 2021)