La2O2SO4:RE/Yb new phosphors for near infrared to visible and near infrared upconversion luminescence (RE=Ho, Er, Tm)

The 3 new upconversion (UC) phosphors of La2O2SO4:RE/Yb (RE=Ho, Er, and Tm, respectively) were derived via facile dehydration of their layered hydroxide precursors that were hydrothermally synthesized at 100 degrees C. Rietveld XRD refinement found contracting cell dimension with decreasing RE3+ size, confirming the direct crystallization of solid solution. The Er3+ and Ho3+ activators both exhibited simultaneous green and red (dominant) emissions under 978-nm near-infrared (NIR) laser excitation (NIR-Vis UC). Particularly, Tm3+ produced a Gaussian-shaped pure NIR emission band at similar to 812nm via its (H4H6)-H-3-H-3 transition (NIR-NIR UC), which is highly desired for NIR biological application. Analysis of the excitation-power dependent UC properties manifested a 3-photon mechanism for the 3 phosphors, and the possible photon reactions leading to UC were illustrated.