Effect of A-site cations on the broadband-sensitive upconversion of AZrO3:Er3+,Ni2+ (A = Ca, Sr, Ba) phosphors

Abstract

We investigated broadband-sensitive upconversion processes in a series of AZrO3 type perovskites codoped with Ni2+ and Er3+, especially giving focus on the effect of the A-site host cations viz. Ca, Sr, Ba. Absorption and Stokes emission of the Ni2+ changed remarkably according to the A-site cations making difference in the Ni2+ to Er3+ energy transfer efficiency. The energy transfer extent from the Ni2+ sensitizers to the Er3+ emitters and the back transfer from the Er3+ to the Ni2+ were studied to clarify the guide for efficient broadband-sensitive upconversion. The Ni2+ to Er3+ energy transfer efficiency and hence the Er3+ upconversion emission intensity was dependent on the extent of overlap between the Er3+ absorption and the Ni2+ emission bands. Larger the overlap, faster was the energy transfer from the Ni2+ to the Er3+, leading to more intense Er3+ upconversion emission. However, back energy transfer from the Er3+ to the Ni2+ due to significant overlap of the Er3+ emission band with the Ni2+ absorption band reduced the upconversion emission intensity. Another important factor is the upconversion efficiency of the Er3+ emitters themselves after the energy transfer from the Ni2+ sensitizers, which was significantly improved when the symmetry around the Er3+ was lowered. As a result of these combined effects, the CaZrO3 host exhibited the most intense Ni2+-sensitized upconversion emission compared to the Sr and Ba analogues. Thus, for the efficient broadband-sensitive upconversion to be realized, a low symmetry host to manifest efficient upconversion of the Er3+ emitters and controlled Ni2+ absorption and emission bands to suppress the back energy transfer while maintaining efficient energy transfer in the forward direction are essential.