Lanthanide doping-facilitated growth of ultrasmall monodisperse Ba2LaF7 nanocrystals with excellent photoluminescence

► Monodisperse ultrasmall Ba2LnF7 (Ln = La, Ho, Er, Yb) nanocrystals were synthesized. ► Ln3+-doping acts as more than sensitizer or activator. ► Ln3+ with smaller atom radius can facilitate the growth of Ba2LaF7 nanocrystals. ► Intense upconversion emission can be observed from Ln3+-doped Ba2LaF7 nanocrystals. ► Ba2LaF7 nanocrystals present more intense upconversion than α-NaYF4.