Mixed modifier effect in lithium manganese metaphosphate glasses on the emission of highly dispersed Mn2+ centers for red-LED

Abstract

Distribution of Mn2+ ions into lithium metaphosphate glass through modifier-site engineering is an effective approach to generate single-doped red phosphors for light emitting diodes (LED). This has been achieved through the systematic Li+-Mn2+ modifier mixing in xMnO‧(50-x)Li2O‧50P2O5 (0 ≤ x ≤ 50 mol%). In this regard, the mixed modifier effect on the structural and optical properties has been studied. While XRD demonstrates the amorphous structure of the prepared compositions, SAED reveals the formation of crystalline clusters in 50MnO‧50P2O5. XPS reveals that the ratio of BOs/NBOs (~0.5) is in agreement with that of theoretical stoichiometry. FTIR spectroscopy shows that the concentration of Q2 decreases, whereas Q1 increases. Also, it can be found that the density increases linearly with about 22% as the content of MnO increases. This is associated with a small decrease in molar volume (~3%). Overall, XPS, FTIR and density analyses indicate that Li+ and Mn2+ ions behave as network modifiers that resides in octahedral sites through the formation of LiO6 and MnO6, respectively. Indeed, there are strong absorption and excitation spectra due to the electronic transition in Mn2+. This results in an intensive orange-red emission around 608 nm (at x = 0.5) with a red-shift up to 684 nm (at x = 50) at excitation wavelength of 409 nm. This is ascribed to 4T1(G) → 6A1(S) electronic transition in the octahedrally coordinated Mn2+ ions with O2− anions. With increasing the MnO content, the emission is systematically intensified up to 10 mol% of MnO, and then quenched. This quenching is attributed to the increase in MnO6 groups, which cluster with each other. In these clusters, the electron transfer occurs between the neighboring Mn2+ ions. Furthermore, the independence of emission peak (shape and position) on the excitation wavelength demonstrates the presence of only high-dispersed Mn2+ activator center.