Photon upconversion characteristics of intense green emitting BaYF₅:Yb³⁺,Er³⁺ nanoclusters prepared by reverse microemulsion

This work presents the first time synthesis of photon upconverting cubic BaYF₅:Yb³⁺,Er³⁺ nanoclusters by a novel reverse microemulsion method. Rietveld refinement analysis confirmed the cubic BaYF₅ and BaYF₅:Yb,Er. The spherical nanoclusters (∼35 nm) are formed by an ensemble of ∼10 nm individual nanocrystals. A plausible formation mechanism is explained based on HRSEM and HRTEM investigations. FTIR revealed the effect of CTAB and oleic acid surfactants on amine and carboxyl functionalization. The cubic BaYF₅:Yb,Er nanoclusters exhibited an intense green upconversion emission under 980 nm excitation. A three-fold upconversion green emission enhancement was achieved by increasing the calcination temperature from 200 °C to 400 °C. The energy transfer mechanism is described based on pump power dependent upconversion luminescence intensity. Furthermore, cytotoxicity effect of the different concentrations (2.5–300 μg/mL) of the BaYF₅:Yb,Er were investigated against human colon adenocarcinoma cell line (HT29 cells) for 24 h and 48 h treatment.