Highly efficient upconversion luminescence in hexagonal NaYF4:Yb3+, Er3+ nanocrystals synthesized by a novel reverse microemulsion method

A new reverse microemulsion system is proposed for the first time to synthesize NaYF4:Yb,Er nano crystals, which demonstrated high upconversion emission in 550 and 662 nm at 980 nm diode laser excitation. The reverse microemulsion (mu Es) system is comprised of CTAB and oleic acid as surfactant and 1-butanol co-surfactant and isooctane oil phase. The surfactant to water ratio is able to tune the microemulsion droplet size from 14 to 220 nm, which eventually controls the crystallinity and particulate morphology of NaYF4:Yb,Er. Also, the microemulsion precursor and calcination temperature plays certain role in transforming the cubic NaYF4:Yb,Er to highly luminescent hexagonal crystal structured upconversion material. Single phase hexagonal NaYF4:YbEr nanorod prepared by water-in-oil reverse microemulsion (mu Es) gives intense red upconversion emission. Both nanosphere and nanorod shaped NaYF4:Yb,Er was obtained, but nanorod morphology resulted an enhanced upconversion luminescence. The structural, morphological, thermal and optical luminescence properties of the NaYF4:Yb,Er nano particles are discussed in detail by employing powder X-ray diffraction, dynamic light scattering, high resolution electron microscopy, TGA-DTA, UV-DRS, FFIR and photoluminescence spectroscopy. Intense upconversion emission achieved in the microemulsion synthesized NaYF4:Yb3+,Er3+ nanocrystal can make it as useful optical phosphor for solar cell applications. (C) 2017 Elsevier B.V. All rights reserved.