Role of heat treatment on the structural and luminescence properties of Yb3+/Ln3+ (Ln = Tm, Ho and Er) co-doped LaF3 nanoparticles

Abstract

Hexagonal LaF₃:Yb³⁺/Ln³⁺ and tetragonal LaOF:Yb³⁺/Ln³⁺ (Ln = Ho, Tm, Er) have been successfully prepared via a two-step reaction, which includes a facile aqueous ligand free solution method and the following heat treatment of the as-prepared LaF₃ precursor. The phase formation evolution from LaF₃ to LaOF with different phase structures was characterized by X-ray diffraction (XRD), scanning electron microscopy, Fourier transform infrared, and Raman spectroscopy. At an annealing temperature of 500 °C pure hexagonal LaF₃:Yb³⁺/Ln³⁺ (Ln = Ho, Tm, Er) nanoparticles with an average size of 32 nm were obtained and they showed a strong visible upconversion and a modest infrared emission upon 976 nm laser excitation. Further, using an annealing temperature of 900 °C, tetragonal LaOF:Yb³⁺/Ln³⁺ (Ln = Ho, Tm, Er) nanoparticles with a size of around 44 nm were obtained (obtained from XRD) and an expressive enhancement in the emission of the VIS and near-infrared regions was observed. These results envision applications that require efficient emissions such as fluorescent and thermal images, and LaF₃ nanocrystals have recently been widely explored for applications in biological systems.