Host dependent upconversion, color tunability and laser induced optical heating via NIR excitation in Ho3+, Yb3+ doped YXO4 (X = V, Nb, Ta) phosphor materials: Application as a security tool and optical heater

Abstract This work reports a comparative upconcversion (UC) emission study of the Ho3+, Yb3+co-doped in pure YVO4, YTaO4, YNbO4 and in their mixed (YTa1-xVxO4, YTa1-yNbyO4, YNb1-zVzO4, where x, y, z = 0, 0.25, 0.50 and 0.75, 1 mol) phosphor materials synthesized via high temperature solid state reaction method at 1473 K. The pure and the mixed phosphors have been structurally analysed through X-ray diffraction technique. The phonon frequencies and the surface morphologies are investigated through Fourier transform Infrared (FTIR) and scanning electron microscope (SEM) techniques for pure and mixed compositions. Under NIR (980 nm) excitation, YNbO 4:Ho3+,Yb3+ and YTaO4:Ho3+,Yb3+phosphors emit intense green alongwith weak red and very weak NIR UC emissions, whereas the YVO4:Ho3+,Yb3+ phosphor exhibits strong red emission with weak green and NIR UC emissions. The UC emissions in the three cases is explained using energy level diagram and the phonon frequency of the host matrix. The pure YNbO4 host shows comparatively efficient UC emission than the other two due to its large particle size and low phonon frequency. Therefore, the YNbO4:Ho3+,Yb3+ serves as an excellent infrared to green upconverter and useful in display devices. On the other hand, the color tunability from green to orange/red UC emissions is observed on varying the vanadate (V) concentration in the mixed compositions (in YTa 1-xVxO4 and YNb1-zVzO4). The variation of ‘V’ in YTa1-xVxO4:Ho3+,Yb3+ and in YNb1-zVzO4:Ho3+,Yb3+intensify the vibrational bands which promotes the non-radiative relaxation and hence tune the upconversion emission color as well as the intensity. The UC emission intensity in YTa1-xVxO4:Ho3+,Yb3+is maximum for x= 0.25. For higher concentrations of V, the green to orange/red color tunable emission is observed. Thus, the mixing of ‘V’ acts as sensitizer as well as modulater in YTa1-xVxO4:Ho3+,Yb3+ phosphor. The colortunable emission in mixed compositions with high color purity makes it promising for security and display applications. The mixing of different compositions of NbO4 and VO4 give different proportions of red and green emissions. On the other hand, mixing of NbO4 and TaO4 give intense green color. Such type of materials are also very useful in security ink applications. The laser induced optical heating is found maximum in case of pure YVO4:Ho3+,Yb3+ phosphor. The temperature of sample YVO4:Ho3+,Yb3+ increases upto 353 K at power density 107.59 W/cm2. Therefore, the YVO4:Ho3+,Yb3+ can be used as an optical heater in medicals such as physiotherapy, cancer and wound treatments etc.