Opposite size dependences of the red/green upconversion intensity ratio in sub-20 nm Yb3+,Er3+-doped β-NaGdF4 nanophosphorsAiHua Li, Tengpeng Guan and Zhijun SunEPL, 106 4 (2014) 48001DOI: https://doi.org/10.1209/0295-5075/106/48001