Intense blue photoluminescence of the Tm3+/Yb3+ co-doped single-crystalline hexagonal phase NaYF4 nanorods

Research highlights: → Pure hexagonal phase NaYF4 nanocrystals with one-dimentiaonl rod-like shape present highly uniform structure and size. → The Tm3+/Yb3+ co-doped hexagonal phase NaYF4 nanorods emit intense eye-visible blue upconversion luminescence. → The Yb3+ ions concentrations have an important effect on the upconversion property. - Abstract: In this paper, Tm3+/Yb3+ co-doped NaYF4 single-crystalline nanorods were prepared via a rational hydrothermal method using oleic acid as a stabilizing agent. The microstructure analysis revealed that the as-prepared samples presented a pure hexagonal phase NaYF4 structure with high quality rod-like morphology by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). Under 980 nm excitation, the intense blue and weaker red upconversion emissions of the NaYF4: 1%Tm3+/x%Yb3+ (x = 2, 5, 10 and 15) samples centered at 450, 477, 649 and 690 nm were observed, which were attributed to the 1D2 → 3F4, 1G4 → 3H6, 1G4 → 3F4 and 3F3 → 3H6 transitions of Tm3+, respectively. Moreover, the photograph of the 1%Tm3+/10%Yb3+ co-doped NaYF4 nanorods exhibited a strong eye-visible blue emission. The upconversion luminescence mechanisms for the Tm3+/Yb3+ co-doped hexagonal phase NaYF4 nanorods were analyzed. In addition, the effect of Yb3+ ions concentrations on the photoluminescence properties of the Tm3+/Yb3+ co-doped hexagonal phase NaYF4 nano-rods were investigated in detail.