Energy Conversion and Transfer in the Luminescence of CeSc[sub.3][sub.4]:Cr[sup.3+] Phosphor

Novel near-infrared (NIR) phosphors are in demand for light-emitting diode (LED) devices to extend their suitability for new applications and, in turn, support the sustainable and healthy development of the LED industry. The Cr[sup.3+] has been used as an activator in the development of new NIR phosphors. However, one main obstacle for the Cr[sup.3+]-activated phosphors is the low luminescence efficiency due to the spin-forbidden d-d transition of Cr[sup.3+]. The rare-earth (RE) huntite minerals that crystallize in the form of REM[sub.3](BO[sub.3])[sub.4] (M = Al, Sc, Cr, Fe, Ga) have a large family of members, including the rare-earth scandium borates of RESc[sub.3](BO[sub.3])[sub.4]. Interestingly, in our research, we found that the luminescence efficiency of Cr[sup.3+] in the CeSc[sub.3](BO[sub.3])[sub.4] host, whose quantum yield was measured at 56%, is several times higher than that in GdSc[sub.3](BO[sub.3])[sub.4], TbSc[sub.3](BO[sub.3])[sub.4], and LuSc[sub.3](BO[sub.3])[sub.4] hosts. Hereby, the energy conversion and transfer in the luminescence of CeSc[sub.3](BO[sub.3])[sub.4]:Cr[sup.3+] phosphor were examined. The Stokes shift of electron energy conversion within the Cr[sup.3+ 4]T[sub.2g] level for the emission at 818 nm and excitation at 625 nm in CeSc[sub.3](BO[sub.3])[sub.4] host was evaluated to be 3775.1 cm[sup.−1], and the super-large splitting energy of the [sup.2]F[sub.5/2] and [sup.2]F[sub.72] sub-states of the Ce[sup.3+] 4f[sup.1] state, about 3000 cm[sup.−1], was found in CeSc[sub.3](BO[sub.3])[sub.4] host. The typical electronic thermal vibration peaks were observed in the excitation spectra of CeSc[sub.3](BO[sub.3])[sub.4]:Cr[sup.3+]. On this basis, the smallest phonon energy, around 347.7 cm[sup.−1], of the CeSc[sub.3](BO[sub.3])[sub.4] host was estimated. Finally, the energy transfer that is responsible for the far higher photoluminescence of Cr[sup.3+] in CeSc[sub.3](BO[sub.3])[sub.4] than in other hosts was proven through the way of Ce[sup.3+] emission and Cr[sup.3+] reabsorption.