Enhancing of the blue/NIR emission of novel BaLaAlO4:Yb3+(X mol%) Tm3+ (0.5 mol%) upconversion phosphors with the Yb3+ concentration (X=0.5 to 6)

We report the structural, morphological and luminescent properties of the BaLaAlO4:Yb3+,Tm3+ (BAL:Yb,Tm) phosphors synthesized by a combustion method. The concentration of Tm3+ was fixed at 0.5 mol%, while that of Yb3+ was varied from 0 to 7 mol%. The X-ray diffraction patterns demonstrated that the samples synthesized with the Yb3+ co-dopant had an orthorhombic phase. In addition, the scanning electron microscopy images revealed that the BAL:Yb,Tm phosphors are formed by a mixture of coalesced microparticles with irregular shape and their surface is decorated with smaller nanoparticles. The size of these nanoparticles increases from 67 to 838 nm after increasing the Yb3+ concentration from 0 to 7 mol%. The BAL:Yb,Tm phosphors produced blue, red and Near infrared emission bands centered at 478, 658 and 800 nm (under 980 nm excitation), respectively. Moreover, the CIE (International commission on illumination) coordinates were calculated for all the samples and realized that they are always in the blue region of the CIE map. This suggests that the increase of the Yb3+ concentration in the BAL:Yb,Tm phosphors mainly favors the blue emission at 478 nm, since the red emission was always weak and contributed at most with approximate to 26% of the overall emission. Thus, the results of this work indicate that the BAL:Yb,Tm phosphors could be good candidates for their use in biomarkers or lighting applications.