Exploring Defect-Induced Emission in ZnAl₂O₄: An Exceptional Color-Tunable Phosphor Material with Diverse Lifetimes

Abstract

Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol-gel combustion route, which can be used both as a blue-emitting phosphor material and a white-emitting phosphor material, depending on the annealing temperature during synthesis. The material also has the potential to be used in optical thermometry. These fascinating color-tunable emission characteristics can be linked with the various defect centers present inside the matrix and their changes upon thermal annealing. Various defect centers, such as anionic vacancy, cationic vacancy, antisite defect, etc., create different electronic states inside the band gap, which are responsible for the multicolor emission. The color components are isolated from the complex emission spectra using time-resolved emission spectroscopy (TRES) study. Interestingly, the lifetime values of the various defect centers were found to change significantly from milliseconds to microseconds upon thermal annealing, which makes the phosphors more diverse (i.e., either long-persistent blue-emitting phosphors or short-persistent white-emitting phosphors). Fourier transform infrared (FTIR) and diffuse reflectance spectroscopy (DRS) confirmed the presence of antisite defect centers such as Al or Zn in the matrix. X-ray absorption fine structure (EXAFS) study showed that the spinel structure was more disordered in nature for low-temperature-annealed compounds. Electron paramagnetic resonance (EPR) and positron annihilation lifetime spectroscopy (PALS) studies were also carried out in order to characterize various anionic and cationic vacancies and their clusters present in the compounds. Antisite defect centers such as Al or Zn, which act as an electron or hole trap, were found to be responsible for the diverse lifetime behavior. To gain insight about the electronic states inside the band gap, density functional theory (DFT)-based calculations were performed for both pure and various vacancy-introduced spinel structures. Finally, based on the theoretical and experimental results, for the first time, a detailed investigation of various defect-induced emission behavior in ZA is presented, which also explains the mechanism of color tunability and dynamic lifetimes.

Investigating Cationic Distribution, Defect Structure and Phosphor Characteristics of Na2Sm2Ca6(PO4)6F2

“…Figure showed the room temperature recorded EPR spectrum of Na 2 Sm 2 Ca 6 (PO 4 ) 6 F 2 which showed a resonance signal for Lande g factor(g)= 2.046.Similar kind of resonance signal was also observed very recently in Apatite‐Type (Ba,Sr) 5 (PO4) 3 Br matrix and it was attributed to oxygen vacancy. We have also observed similar resonance signal for oxygen vacancy with g value close to the present samples . In a oxide based matrices the oxygen vacancies can exist in different charged states such as i) neutral i. e. V O 0 , which is formed when two electrons are trapped in doubly charged oxygen vacancy, ii) singly charged oxygen vacancy i. e. V O + .…”

“…These defect centers get stabilized through the redistribution of the electrons; such as one of the two electrons is transferred to a nearby P atom, resulting in the formation of negatively charged PO 3 2– reduced from PO 4 3– . In case of the +1 charge oxygen vacancy i. e. V O + , the removal of an electron results in a new empty state in the band gap of the material as observed in our earlier cases in other matrixes . Thus V O 0 acts as donor while V O + acts as the acceptor.…”

“…All the possible electronic transition from the VB to CB and CB to various defect states or via the defect states are shown here. Earlier also we have established several similar kind of possible mechanism for defects related emission …”

“…In order to evaluate the material performance on color‐luminescent emission we have calculated the CIE chromaticity coordinates for the present sample adopting standard procedures as that of earlier cases . The calculated values of x and y coordinates of the present system are 0.333 and 0.327 respectively and they are represented by asterisk point (*) in the CIE diagram shown in figure .…”

Defect states and kinetic parameter analysis of ZnAl2O4 nanocrystals by X-ray photoelectron spectroscopy and thermoluminescence

“…Metal oxides are drawing considerable attention owing to their non-toxic nature and tunable luminescence properties attributable to the defect states. Zinc aluminate (ZnAl 2 O 4 ) is a well known member of spinel family possessing versatile properties and magnificent applications [5][6][7][8][9][10] . It crystallizes in fcc crystal system with Fd3m space group.…”