Green/yellowish-white color-tunable up-conversion phosphor prepared from Er/Yb-polyoxoaluminoniobate complex

A new NaNbO3-based up-conversion phosphor doped with Yb3+, Er3+, and Al3+ was prepared by thermal decomposition of polyoxoaluminoniobate precursor, Na6H18[ErxYb6−x(H2O)6(OH)6O2Al2(Nb6O19)5]·nH2O (x = 1-6), at 700 °C for 2 h. A near infrared (980 nm) light excitation by a laser diode (LD) into the Er3+:4I11/2 ← 4I15/2 absorption band for Yb-free sample (x = 6) resulted in visual up-conversion emission consisting mainly of green 2H11/2,4S3/2 → 4I15/2 (520-550 nm) and red 4F9/2 → 4I15/2 (660 nm) bands of Er3+. In the presence of Yb3+ (x = 0.063-5), the overall emission was much enhanced as a result of effective absorption of photon by Yb3+:2F5/2 ← 2F7/2 followed by efficient energy transfer to Er3+. It was found that a ratio of green/red band intensities for the Er3+/Yb3+-codoped samples depends on an excitation pulse width: a CW or long 980 nm pulse (>2 ms) excitation causes yellowish-white emission, while a shorter pulse (<200 μs) does green emission. The color variation can be explained by much longer rise time of the red emission than that of the green one. The phenomenon would be potentially applied to a LD-excited color-tunable phosphor.