Design, synthesis and mechanism of green afterglow phosphors Zn4B6O13:Mn2+ doped with Ln3+ (Ln = Sm, Yb, Eu) guided by VRBE

Abstract

In this work, the construction of host referred binding energy (HRBE) diagram and vacuum referred binding energy (VRBE) diagram is described in detail. The energy gap of Zn4B6O13 is calculated by diffuse reflectance spectroscopy and the energy level positions of Eu3+ in VRBE are determined by the charge transfer (CT) band of Zn4B6O13 doped Eu3+ to construct a complete lanthanide ions VRBE diagram. According to the constructed VRBE of Zn4B6O13 system, novel long-afterglow phosphors Zn4B6O13:Mn2+ co-doped with Ln3+ (Ln = Sm, Yb, Eu) is designed and synthesized successfully. The influence of B2O3 contents on the crystal structure and luminescence of the Mn2+ doped zinc borate phosphors were analyzed by XRD and emission spectra. Photoluminescence excitation and emission spectra, persistent luminescence spectra and afterglow decay curve indicate that the doping of lanthanide ions Sm3+ and Yb3+ prolonged the afterglow duration of green emission of Mn2+ effectively. The fitting results of persistent luminescence spectra is consistent with the fast decay process of afterglow decay curve. The energy level depths of Sm3+ and Yb3+ traps obtained by thermoluminescence analysis are 0.49 eV and 1.12 eV respectively, which are in good agreement with the results calculated by VRBE. The energy level position of Sm3+ and Yb3+ below the conduction band (CB) are suitable trap depth, which can store and release electrons continuously. However, that of Eu3+ is too deep to enhance the afterglow. According to the VRBE theory, the mechanism of the electron trap is satisfactorily explained, which is important to the subsequent research of system design and mechanism explanation of lanthanide ions doped long afterglow materials.