Control of red upconversion emission in Er3+–Yb3+– Fe3+ tri–doped biphasic calcium phosphate

Abstract

This paper reports a novel red upconversion (UC) emission in Er/Yb/Fe tri-doped biphasic calcium phosphate (BCP) synthesized using a solid-state reaction method. The crystal structure of BCP was analyzed using Rietveld refinement, which confirmed the hexagonal structure of hydroxyapatite and the rhombohedral structure of beta-tricalcium phosphate with good crystallinity. The results of Rietveld refinement, Fourier-transform infrared spectroscopy, and diffuse reflectance spectroscopy revealed that the presence of Fe3+ in the host lattices strongly influenced the crystal structure of the phosphors. The phosphors showed intense red UC emission with a maximum at 660 nm upon the near-infrared excitation of a diode laser at 975 nm. Significantly, the red emission intensities of the Fe3+-doped sample were 60-times greater than those of the undoped sample. The preferential enhancement of red UC emission was due to the energy transfer from the high-energy excited state |2F7/2, 4T1g > of the Yb3+-Fe3+ dimer to the 4F9/2 state of the Er3+ ions. The UC emission mechanism was primarily estimated by the dependence of the UC spectral intensities on the laser 975 nm excitation power. The BCP: Er/Yb/Fe phosphor generating strong red UC emission could be used to develop a highly effective bioimaging tool in biomedicine.