LiYF4:Yb3+, Er3+ upconverting submicro-particles: synthesis and formation mechanism exploration

Abstract

In this work, submicro-composites of LiYF₄:Yb³⁺, Er³⁺ and reduced graphene oxide (r-GO) have been synthesized via a facile hydrothermal route in the presence of surfactant EDTA and r-GO. Subsequently, the LiYF₄:Yb³⁺, Er³⁺ submicro-composites are annealed in order to improve the crystallinity. The mechanism of growth evolution of LiYF₄:Yb³⁺, Er³⁺ submicro-crystals is investigated and a mechanism, based on preventive aggregation growth of YF₃ particles by r-GO and induced phase transformation from orthorhombic-phase YF₃ to tetragonal-phase LiYF₄ by an inward diffusion of quasi-steady state LiF species, is proposed. The upconversion luminescence is investigated as a function of the pump power density in a series of Yb³⁺, Er³⁺ co-doped fluoride submicro-crystals by excitation in the near-infrared region. The results show that the luminescence intensity can be effectively improved by improving the crystallinity and decomposition of r-GO via a calcination approach, while preserving the morphologies of the LiYF₄ submicro-crystals by preventing aggregation via r-GO. The present study demonstrates that the hydrothermal approach assisted by r-GO is important for the preparation of rare-earth-doped LiYF₄ upconverting submicro-materials as well as for the understanding of the mechanism of organic-template-directed crystallization.