An Insight into the Various Defects-Induced Emission in MgAl₂O₄ and Their Tunability with Phase Behavior: Combined Experimental and Theoretical Approach

Abstract

The present work describes various defects-induced tunable emission behavior of MgAl2O4 compounds obtained after annealing at different temperatures through a sol–gel combustion route. Multiple defect centers, such as F, F2, F+, and F2 2+ and different shallow and deep defects were found to be present inside the band gap, as confirmed by the lifetime and time-resolved emission spectroscopy (TRES) studies. The tunable emission characteristic at different annealing temperatures could be linked with the phase behavior of the spinel. Excitation wavelength variation suggested that a photoconversion process of F to F+ centers was involved with λex = 250 nm, followed by a trapping–de-trapping mechanism of the released electrons within different trap states. An exchange mechanism of electrons in between conduction band and shallow states was also observed at room temperature, which was absent at low temperature, as indicated by the emission profile. These observations render it to be a potential optical-based thermal sensor material. DFT-based calculations were carried out for both pure and various oxygen-vacancy-introduced spinel phases in order to characterize the different defect states inside the band gap. Finally, on the basis of theoretical and experimental results, a model has been proposed to explain the mechanisms related to emission tunability.

On comparison of luminescence properties of La2Zr2O7 and La2Hf2O7 nanoparticles

“…In that context, density functional theory (DFT) studies stand out in deciphering the suitable model and mechanism involved in defect induced emission as reported for perovskite, pyrochlore, and spinel, etc. 28,29,32,33 Most optical and structural investigations of A 2 B 2 O 7 pyrochlores have been focused on the effect of substitutions in the A 3+ ions, especially in cases where the substitution is not charge compensated. Much less attention has been paid to the impact by B 4+ site substitution, which could have significant effects on the structure of pyrochlore hosts that consequently affect the spectral properties.…”

Probing crystal structure and site‐selective photo‐physical properties of various Eu3+‐doped niobates

“…To spectroscopically characterize and isolate these different Eu 3+ ions, we carried out the Time Resolved Emission Spectroscopy (TRES). The method applied to isolate the different emission spectrum of Eu 3+ ion at different environments is similar to that of our earlier reports . The PL signal from short‐lived and the long‐lived components can be isolated using TRES.…”

“…To further support our argument of Eu 3+ ion site occupancy, Judd‐Ofelt parameter are also calculated. The theoretical expression for various Judd‐Ofelt (JO) parameters such as Ω J (J = 2, 4), radiative emission rates (AR), nonradiative emission rates (ANR), overall quantum efficiency ( η ), radiative lifetime (τR), and nonradiative lifetimes (τNR) are given in our earlier reports . The JO parameter Ω 2, called as short range parameter is related to degree of covalency and polarizability of the Ln 3+ ion due to surrounding chemical environment.…”

“…The theoretical expression for various Judd-Ofelt (JO) parameters such as ΩJ (J = 2, 4), radiative emission rates (AR), nonradiative emission rates (ANR), overall quantum efficiency (η), radiative lifetime (τR), and nonradiative lifetimes (τNR) are given in our earlier reports. [34][35][36][37] The JO parameter Ω2, called as short range parameter is related to degree of covalency and polarizability of the Ln 3+ ion due to surrounding chemical environment. Ω4, which is called as long range parameters related to the bulk measurable such as viscosity and rigidity of the inorganic matrices.…”

“…The method applied to isolate the different emission spectrum of Eu 3+ ion at different environments is similar to that of our earlier reports. 36 The PL signal from short-lived and the long-lived components can be isolated using TRES. Thus the TRES at different delay time such as at 10 ms for BLN:Eu ( Figure 10A-b), 5 ms for BLCN:Eu ( Figure 10B-b), and at 4 ms for BLMN:Eu ( Figure 10C-b) are due to the long-lived Eu 3+ ion, since the short-lived components of the compounds have already been decayed considering their lifetime values.…”