Crystal lattice site occupation-based optical properties in Co²⁺-doped ZGGO persistent luminescence nanoparticles

In this work, near-infrared (NIR) persistent luminescence nanoparticles Co²⁺ ions doped Zn_2(1-x)Co_2xGa₃Ge_0.75O₈ (ZGGO:xCo²⁺) were successfully synthesized via hydrothermal synthesis. The average particle size decreased from 65.9 to 52.3 nm with increasing Co²⁺ concentration (x = 0.001-0.05). Co²⁺ ions can substitute for Zn²⁺ at tetrahedral and antisite octahedral sites in the ZGGO host lattice, with corresponding formation energies (ΔE) values are - 4.60 and - 3.03 eV, respectively. The crystal field intensities for Co²⁺ in the tetrahedral and octahedral sites were relatively weak, with values of 0.45 and 1.08, respectively. All samples exhibit strong emission peaked at 685 nm and weak emission peaked at 825 nm. The strong peak was deconvoluted into two distinct peaks at 672 nm and 697 nm, which were attributed to the⁴A₂(⁴F) → ⁴T₁(⁴F) transition of Co²⁺ ions occupying octahedral sites, and the⁴T₁(⁴P) → ⁴A₂(⁴F) transition of Co²⁺ ions occupying tetrahedral sites, respectively. The NIR persistent luminescence of samples was longer than 2 min and primarily originates from octahedral Co²⁺ sites. The relevant defects, specifically Ga^o_Zn and Zn^'_Ga-Ga^o_Zn, act as the charge traps. The corresponding ΔE values are - 0.55 eV and 1.84 eV, respectively. These results substantially advance the fundamental understanding of the doping and luminescence mechanisms of Co²⁺ in spinel gallate-based persistent luminescence materials.