Optical properties of Tm3+/Yb3+ co-doped XAl2O4 (X = Mg, Ca, Sr and Ba) phosphors: Effect of co-doping of alkaline earths on the radiative as well as non-radiative behaviours

Abstract

Tm3+-Yb3+ co-doped alkaline earths aluminate phosphors of the type XAl2O4 (X = Mg, Ca, Sr and Ba) are synthesized using solid state reaction method. The crystal structure and the phase of all these phosphor materials are confirmed by X-ray diffraction (XRD) measurements. The surface morphologies of all the samples are monitored by Scanning Electron Microscope (SEM) technique. The Ultraviolet (UV)-visible-near-IR region (NIR) absorption spectra show sharp f-f electronic transitions of Tm3+ and Yb3+ ions which are more prominent in CaAl2O4:Tm3+,Yb3+ phosphor. The upconversion (UC) emission spectra of the samples have been measured which give intense blue and NIR emissions on excitation with 980 nm radiation. It is observed that CaAl2O4:Tm3+,Yb3+ phosphor gives better UC emission in blue as well as NIR region than the others. We have also monitored the UC emission spectra of CaAl2O4:Tm3+, Yb3+ in the presence of other alkaline earths metals (i.e. Mg2+, Sr2+ and Ba2+ ions) as sensitizer. It is found that Mg2+ ion is a better enhancer for UC emission in CaAl2O4:Tm3+,Yb3+ phosphor (upto ten times) than the others. The spectral and color purity are found to be maximum for CaAl2O4:Tm3+,Yb3+,10Mg2+ phosphor. The laser induced optical heating is observed quantitatively in CaAl2O4:Tm3+,Yb3+,xMg2+ (x = 0, 5, 10 and 15 mol%) phosphors on excitation with 980 nm radiation. Thus, the maximum color and spectral purity for blue region in CaAl2O4:Tm3+,Yb3+,Mg2+ phosphor may be applicable as blue LED as well as the laser induced optical heating can be used for the application in cancer treatment based on opto-thermal therapy and as an optical heater.