Reverse microemulsion synthesis of mixed α and β phase NaYF4:Yb,Er nanoparticles: calcination induced phase formation, morphology, and upconversion emission

A novel “water-in-oil” type reverse microemulsion assisted synthesis detail on the formation of mixed cubic and hexagonal (α + β) phase NaYF4:Yb,Er nanoparticles and their upconversion emission properties are presented. The effect of surfactants, fluorine precursors on the crystallographic phase fraction, crystallite size of NaYF4:Yb,Er nanoparticles on red upconversion emission is discussed. The NaYF4:Yb,Er nanoparticles synthesized with CTAB, and oleic acid surfactants give larger crystallite size and moderate hexagonal/cubic phase fraction. It has resulted very intense upconversion red emission. The oleic-acid-free preparation of NaYF4:Yb,Er nanoparticles resulted highly-agglomerated nanoparticles and low crystallite size, which gives less-intense upconversion emission. The cubic and hexagonal phase fractions of NaYF4:Yb,Er depends on surfactants, microemulsion, molar concentrations of precursors, and post-calcination. All these factors influence the mondispersibility and upconversion red emission properties. The 980 nm laser pump power dependent upconversion emission studies have confirmed the typical two-photon behavior in α + β phase NaYF4:Yb,Er nanoparticles. Their decay life was also measured to correlate the upconversion red emission intensity. The effect of mixed α + β phase NaYF4:Yb,Er nanoparticles on the 1530 nm NIR emission is also presented.