Mechanistic Study of the Persistent Luminescence of CaAl₂O₄:Eu,Nd

Abstract

CaAl2O4:Eu is a persistent luminescence (PL) material in the blue light region with potentially wide commercial applications. With the doping of Nd, the decay time can be elongated to more than 19 h. These excellent properties are believed to be in close relation with the electronic structures of the dopants, the defects, and the host material. In this work, we attempt to achieve a better understanding of the PL mechanism of CaAl2O4:Eu based on first-principles calculations. The electronic structures of the host CaAl2O4, the luminescent center Eu, the O and Ca vacancies, and the dopant Nd are systematically studied. According to the calculations, the 4f and 5d levels of Eu are located within the band gap and slightly above the conduction band minimum (CBM), respectively. The electrons on the 4f levels can be excited into the 5d levels via ultraviolet radiation. The excited electrons on the 5d levels can move to the conduction bands and become free electron carriers. The electron carriers can be trapped for a short period by the empty defect levels below the CBM if they are very close to the defects and then return back into the conduction band. After the trap–release process, the electrons may re-enter the 5d levels of Eu and then move back to the 4f levels accompanied by light emission. The +2 charged-state O vacancies can serve as electron traps. The Ca vacancies cannot contribute to the PL property directly but can assist in stabilizing the +2 charged-state O vacancies. Nd dopants can serve as both electron donors and electron traps. These new insights into the electronic structures are useful for determining which materials may possess good PL properties, thereby motivating more experimental efforts in synthesizing improved PL materials.

Energy harvesting and conversion mechanisms for intrinsic upconverted mechano-persistent luminescence in CaZnOS

“…As we know, in luminescent materials, V O is usually a color center (or F-center). 34,39 In CaZnOS, V O can be formed with very low formation energy under the Zn-rich chemical limit. This F-center dominant fluorescence by V O with ÀU eff means that the emission is a two-electron process.…”

“…The exact inter-levels of activators and traps within the electronic structures suggested by theoretical study will provide a reference for the selection of such structures with optimal performance. 33,34,38,39 Therefore, it is necessary to open a preliminary discussion on the different native point defects in a potential UMPL to examine its energy conversion mechanism and to provide a useful reference for further experiments and various applications. 33,34,38,39 In this work, we discuss luminescence models covering the range from the typical oxygen vacancy (V O ) related F-center to other potential energy conversions through different optical transitions.…”

“…33,34,38,39 Therefore, it is necessary to open a preliminary discussion on the different native point defects in a potential UMPL to examine its energy conversion mechanism and to provide a useful reference for further experiments and various applications. 33,34,38,39 In this work, we discuss luminescence models covering the range from the typical oxygen vacancy (V O ) related F-center to other potential energy conversions through different optical transitions. Due to strong electron-phonon coupling, widened optical transitions result from the Stokes shift between the absorption and emission spectra, 40 and the analytical mechanism is still not clear by current theoretical methods.…”

Electronic, Optical, and Lattice Dynamical Properties of Tetracalcium Trialuminate (Ca4Al6O13)

“…Hussain [27] calculated the electronic structures of other common calcium aluminates, Ca 3 Al 2 O 6 , CaAl 2 O 4 , CaAl 4 O 7 , and CaAl 12 O 19 , by first principles based on the orthogonalized linear combination of atomic orbitals (OLCAO) method and found that their band gaps are 3.85, 4.16, 4.28, and 4.62 eV, respectively, which gradually increase with the increasing ratio of Al 2 O 3 . Qu et al [28] calculated the band structures of CaAl 2 O 4 using GGA–PBE, and the obtained band gap was 4.54 eV. The experimental band gap of CaAl 2 O 4 is likely between 5.8 and 6.7 eV.…”

Lu2CaMg2(Si1−xGex)3O12:Ce3+solid-solution phosphors: bandgap engineering for blue-light activated afterglow applicable to AC-LED

“…[6][7][8][9] Recently, this approach was revealed as applicable for phosphors (an insulator in nature) as well, endowing them with PersL properties, [10][11][12][13][14][15] which opens up a brand-new perspective for exploring novel phosphorescent materials that are highly desirable in security signs, night displays, optical storage media, etc. [16][17][18][19][20] PersL is an interesting phenomenon whereby luminescence from an active center still lasts for a long period after ceasing the excitation. 16 However, there is some complex and profound physics behind the PersL mechanism, including excitation, delocalization, migration, storage, release, and recombination of charge carriers, which casts a veil of mystery over it.…”