Photoluminescence study in Ho/Tm/Yb/Li:Gd(MoO) nanophosphors for near white light emitting diode and security ink applications.

Ho/Yb/Tm/Li:Gd(MoO) nanophosphors successfully synthesised via solid state reaction method have been structurally and optically characterised. Under 980 nm diode laser excitation the nanophosphors emit intense blue, green, red and NIR emissions peaking at ∼476 nm, ∼543 nm, ∼646 nm and ∼798 nm corresponding to the G → H (Tm), F, S → I (Ho), F  → I (Ho) and H → H (Tm) transitions respectively. The upconversion emission intensity enhancement in the Ho-Yb-Tm-Li:Gd(MoO) nanophosphors for the green band is found to be ∼367, ∼50 and ∼9 times compared to the singly Ho doped, Ho-Yb co-doped and Ho-Yb-Tm tri-doped Gd(MoO) nanophosphors. The enhancement observed has been explained on the basis of energy transfer process and local field modifications around the rare earth ions. The energy transfer efficiency ∼5% is determined in the tridoped nanophosphors. The interaction involved between rare earth ions for energy transfer process is found to be dipole-dipole type. On changing the Tm ions concentration the colour emitted from the tridoped nanophosphors is tuned from near white to blue region. In the tridoped nanophosphors, on varying the pump power the colour tunability has been observed.