Enhanced deep-ultraviolet upconversion emission of Gd³⁺ sensitized by Yb³⁺ and Ho³⁺ in β-NaLuF₄ microcrystals under 980 nm excitation

The shortest ever recorded and enhanced 204.5 nm ultraviolet (UV) upconversion (UC) emission from ⁶G_7/2 levels of Gd ³⁺ ions was observed in NaLuF₄:20%Yb³⁺, 2%Ho³⁺,20%Gd³⁺ microcrystals (MCs) under 980 nm near infrared (NIR) excitation for the first time. The ⁶G_J levels of Gd³⁺ ions can be efficiently populated by the energy transfer (ET) processes of Yb → Ho → Gd and Yb → Gd. The effect of the pH value on the morphology of the MCs was explored and the UC mechanisms in the tri-doped NaLuF₄ samples were analyzed in detail based on the emission spectra and the dependence of the luminescence intensity on the laser pumping power where a 6-photon UC process of the 204.5 nm emission is observed. The contrast spectra of UC luminescence suggest that β-NaLuF₄ MCs are a better UC host material than the β-NaYF₄ counterpart. Furthermore, this paper also provides an effective and facile approach to modulating the sizes of β-NaLuF₄ MCs by manipulating the pH values of the initial reaction solutions and the doping of Gd³⁺ ions.