Open Access
Add to BibSonomyAbstract
ZnGa2O4:Cr3+ is an outstanding near-infrared (NIR) long-lasting phosphorescence (LLP) material with afterglow duration of more than 5h, which is potentially applicable in bioimaging. The well-studied antisite GaZn• and ZnGa′ defects are reported to serve as shallow traps responsible for the persistent luminescence. The less-studied but commonly available oxygen vacancy in this material may be associated with deep traps, which is barely investigated but of significance to influence the luminescence and LLP behavior. Moreover, persistent luminescence mechanisms associated with shallow and deep traps require identification of the carriers. This research attempts to reveal the detail of deep traps and the mechanism involved, with the assistance of photoluminescence (PL), thermoluminescence (TL), and alternating current (AC) impedance spectroscopy as well as density functional theory (DFT) calculations. Results show that the VO•• defects in the neighbor of Cr3+ could probably change the contents of antisite defects and result in variation of R/N2 ratio in PL and afterglow spectra. VO•• defects may also serve as the deep traps, which might agglomerate with Cr3+ and antisite defects to form cluster that is responsible for visible-light-stimulated LLP but with a negative effect. The research would be beneficial in understanding the common LLP phenomenon.
© 2017 Optical Society of America
1. Introduction
LLP is a phenomenon where the material can still emit light for hours after the exciting source (such as ultraviolet, X-ray, or gamma radiation) has been shut down [1–3]. It has drawn much attention of the public due to its application in decoration, safety guide and instrument display [4]. So far, researchers have discovered plenty of LLP materials that can emit visible light [5–7]. In the past several years, efforts have been put into the study of the NIR LLP nanoparticles with the concern of its advantages of high sensitivity in optical detection and complete avoidance of tissue autofluorescence in bioimaging applications [8–12]. In particular, trivalent chromium-doped zinc gallate (ZnGa2O4:Cr3+) has attracted serious attention owing to its bright NIR LLP irradiated by the ultraviolet (UV) light [13].
For the ZnGa2O4 (ZGO) system, the matrix itself shows sharp UV emission at 360 nm derived from the Ga-O charge transfer transition for distorted GaO69- polyhedra that contain the oxygen vacancies, accompanying the broad red emission of 680 nm that originated from the oxygen vacancies as well [14]. When the Cr3+ ions are induced to this system, it generally show multiple peaks at around 700 nm composed of stokes phonon side bands (PSB), anti-stokes PSB, R line and N2 line. Particularly, the N2 line is assigned to the luminescence of Cr3+ ions whose sites are distorted by antisite defects nearby [15–17]. And the antisite GaZn• and ZnGa′ defects can act as the shallow traps (below 60 °C in the TL spectrum) that cause the LLP in this system, which has been proved by Aurélie Bessière et al. [13,18] by using the techniques such as PL and LLP spectra [18], electron spin-resonance spectroscopy [19] and electron nuclear double resonance [20]. Further study like the research of structural disorder influence on spectroscopy strengthens the evidence linking antisite defects with LLP performance in a Cr3+ doped zinc gallate spinel [21–23]. In common, the Cr3+ ion is in the excited state under illumination, while Cr3+ returns to its ground state the released energy (electron or hole) is simultaneously trapped by the antisite defects in the vicinity of Cr3+ [24]. Subsequently, the reverse process occurs with thermal activation and the released energy is captured by Cr3+, which leads to the persistent luminescence of Cr3+. When stimulated by visible light with the result of population of Cr3+ 4T2 (4F) state well below ZnGa2O4 conduction band, LLP is contributed by entirely localized cluster of CrN2 and antisite defects with the driving force for charge separation of local electric field of the defects cluster [18,19]. However, there are still some remained questions in this system. First of all, the research of ZGO has shown that oxygen vacancies play a role in its luminescence properties that cannot be ignored [14,25]. It can be predicted that oxygen vacancies are still widely generated in addition to the antisite defects by the thermal evaporation of ZnO during synthesis of the ZGO:Cr system, which may also contribute to the LLP. Secondly, the charge carrier (electron or a hole) is not fully defined in this doping system.
In this paper, the PL, TL and AC impedance spectroscopy as well as theoretical calculations of ZGO:Cr are investigated in detail to reveal the answers.
2. Experimental
The zinc gallate phosphors were synthesized by a solid-state reaction method. For simplicity, the ZGO:Cr is short for the molecule-designed Zn0.98(Ga0.99Cr0.01)2O3.98. The zinc element is designed to be deficient in order to induce the vacancy defects artificially to gain better LLP performance [13]. Typically, stoichiometric amounts of starting materials ZnO (99.99%), Ga2O3 (99.999%) and Cr2O3 (analytical reagent) were weighted and mixed thoroughly in an agate mortar. Then the mixture was fired at 1350 °C for 5 h. The partial as-synthesized samples were further annealed in 5%H2-95%N2 and pure O2 atmosphere, respectively, at 800 °C for 1 h. Sintered ceramics for the AC impedance spectroscopy measurement were prepared using the as-synthesised phosphors as precursors. The precursors were thoroughly mixed with polyvinyl alcohol (PVA) (5 wt%) binder solution and then pressed under 20 MPa pressure into pellets. Then the pellets were presintered at 600 °C for 2 h in air with a heating rate at approximate 1 °C/min to discharge rubber thoroughly and finally sintered at 1350 °C for 5 h. The sintered ceramic pellets were ~2 mm in thickness and ~12 mm in diameter.
The XRD data was collected by PANalytical X′pert Pro X-ray diffractometer with Cu anode target (Kα1 = 1.54059 Å). PL, photoluminescence excitation (PLE) spectra and the afterglow spectra over the spectral range of 200 to 900 nm and decay curves were recorded on a high resolution spectrofluorometer (Edingburgh Instruments FLS 920) equipped with a 450 W xenon lamp and a pulsed xenon flash lamp (μF900), respectively, as excitation sources. The TL curves were measured with a FJ427A1 thermoluminescent dosimeter (CNNC Beijing Nuclear Instrument Factory). Generally, the excitation source in TL measurement was a mercury lamp with an output power of 60 W for 5 minutes illumination (except otherwise stated), whose main emission wavelength is ~254 nm. Additionally, different modes aiming to detect different TL peaks associated with different traps were adopted, which will be detailedly described in the next section. All these measurements were performed at room temperature if no otherwise stated.
The AC impedance spectroscopy measurement was carried out with a Solartron 1260 frequency response analyzer over the 10−1-107 Hz frequency range at 700 °C. The measurement was conducted under an O2 partial pressure (pO2) range of 10−4~1 atm, which was monitored by the YSZ sensor close to the sample. The pO2 was controlled with O2 and N2 gas mixtures, for which the HORIBA mass flow controllers (S48 32/HMT) were used.
First principle calculations were conducted based on DFT using the Vienna ab initio simulation package (VASP) with the frozen-core projector-augment-wave (PAW) method [26,27]. The kinetic energy cutoff was set as 520 eV and a 5 × 5 × 5 k-point grid was used to sample the Brillouin zone. The generalized gradient approximation was employed and all structures were relaxed with the energy convergence criterion set to 1 × 10−4 eV ensuring that the maximum force on an atom at the end of a relaxation was <0.02 eV Å−1 [28].
3. Results and discussion
3.1 PL spectra of ZGO:Cr
The XRD patterns (not shown here) of all the samples are well in agreement with JCPDS 38-1240, suggesting that the prepared samples are single-phased. The PL and excitation spectra (not shown here) of the ZGO:Cr samples are analogous with those reported in previous research. The PL excitation spectra compose of 550 nm (4A2 (4F) → 4T2 (4F)), 400 nm (4A2 (4F) → 4T1 (4F)), and 290 nm (4A2 (4F) → 4T1 (4P)), which are attributed to the d-d absorption bands of Cr3+ [18]. As seen in Figs. 1(a)-1(c), the PL spectrum of ZGO:Cr is attributed to the 2E(2G) → 4A2(4F) transition of Cr3+ ions in an octahedral crystal field (Ga3+ site). The line at 688 nm is the so-called zero phonon R line. The R line emission is caused by the pure electronic transition, accompanied by their Stokes PSB peaking at 708, 715 nm and anti-Stokes PSB peaking at 663, 670 and 680 nm [13]. As it had been well studied by Kim et al. [14], the reduced ZGO host shows sharp UV emission from the Ga–O transition at distorted Oh sites that contain oxygen vacancies (VO••), accompanied by the red emission of 680 nm that originated from oxygen vacancies. Thus, the introduced Cr3+ ions may interact with oxygen vacancies. The N2 line at 695 nm is reported to be associated with Cr3+ whose coordinated environment is distorted by the first cationic neighbor of antisite defects (ZnGa′ and GaZn•) [15,29,30].
Fig. 1 (a-c) PL spectra of the ZGO:Cr annealed at different atmosphere and excited at different wavelength light; (d) the enlarged spectra of (a-c).
Apparently, the intensity of R line is comparable with that of N2 line (their emitter is marked as CrR and CrN2, respectively) when excited by 290 nm ultraviolet light (Fig. 1(a)) while the cases are quite different under the excitation of 400 nm (Fig. 1(b)) or 550 nm (Fig. 1(c)) visible light. This can be primarily explained by the different selection rules of the different Cr3+ ions for the 4F → 4P and 4F → 4F transitions [18]. If looking at these R line-normalized spectra in detail, the enlarged N2 lines in Fig. 1(d) show gradually decrease with the lowering oxygen partial pressure for 290 nm excitation, whereas they vary less with longer wavelength excitation. According to the defect equilibrium equation with the Kröger-Vink notation:
The oxidation atmosphere makes the reaction to the left and reduces oxygen vacancy defects, while the reduction atmosphere leads to a reverse result. The oxygen vacancy VO•• would inevitably change the equilibrium of antisite defects. It might attract the negative charged defects nearby like the ZnGa′ defects, resulting the agglomeration of CrN2, VO•• and ZnGa′ and the release of more non-distorted CrR ions. This is probably why the R line is more intense than N2 line with a more reductive atmosphere for 290 nm excitation, while the latter is likely compensated for 550 nm excitation owing to the different relaxation extent of the 4F → 4F transition selection rules for CrR and CrN2. Actually, the R/N2 ratio is larger for all the emission and afterglow spectra of all our samples upon all the wavelength excitation, compared to that in previous research [18], also probably attributed to the more concentrated oxygen vacancy.3.2 TL and afterglow spectra of ZGO:Cr
To further reveal the defects’ role in the PL, the TL is conducted and discussed in this section. Three kinds of modes are listed in Table 1 to conduct the TL measurement in order to verify the shallow and deep traps. In detail, mode-1 is to measure the samples stimulated by the sunlight directly. For mode-2 and mode-3, the samples are prior heated to 350 °C to completely de-trap all the carriers, and then the samples are exposed to 254 nm illumination from a mercury lamp at an output power of 60 W for 30 seconds or 5 minutes. Subsequently, in mode-2 the TL of samples are recorded with a total delay time of 1 minute after removal of the excitation source; in mode-3, the TL of samples are recorded after ceasing the excitation source followed by further heating at 100 °C for 30 minutes to de-trap the shallow carriers. Doing our best, the excitation time, sample amount, and distance between sample and excitation source are kept constantly in different modes.
Table 1. Three kinds of modes for TL measurements.
Fig. 2(a) is the TL spectra of the ZGO:Cr under mode-1 (dark ball), mode-2 (red star) and mode-3 (down triangle). The dark ball curve is a broad band, which starts from about 80 °C, centers at 120 and 190 °C and ends up at about 300 °C. It shows that the sunlight can lead to the population of the carriers trapped by defects widely distributed inside the band. The red and blue star curve of mode-2 apparently exhibit one peak centers at 90 °C and a shoulder at ~190 °C, which clearly show that there are deep traps besides to the shallow trap of antisite defects. The red and blue triangle curve of mode-3 again confirms the deep traps, since the treatment of 100 °C annealing would empty the carriers initially captured by the shallow traps. The difference between the curves of mode-2 exposed for different time reveals the different charge populations with and without saturation. The TL curves in Figs. 2(b) and 2(c) show a shift to high temperature when stimulated by 550 nm light, compared to 290 nm excitation, again convincing the existence of deep traps. The difference of TL curves in Fig. 2(d) may also suggest the oxygen vacancies have some relation with the deep traps, since the reduction atmosphere would result in more intensive distribution of oxygen vacancies according to Eq. (1).
Fig. 2 (a) TL spectra of the ZGO:Cr recorded under different modes; normalized TL spectra of the ZGO:Cr annealed under (b) 5%H2/95%N2 and (c) O2, excited by 290 nm ultraviolet and 550 nm visible light, respectively; (d) comparison of the TL spectra of ZGO:Cr samples annealed under reduction and oxidation atmosphere, excited by 550 nm (inset is the normalized spectra).
To further reveal the different behaviors of shallow traps and deep traps, both the TL and afterglow spectra of as-synthesized ZGO:Cr sample illuminated by the monochromatic light of 290, 400 and 550 nm (the first one may correspond to both d-d absorption of Cr3+ and interband transition of the host, the other two correspond to different d-d absorption peaks) for 5 minutes are recorded and illustrated in Fig. 3(a). The sample was de-trapped at 350 °C before the illumination. All the TL spectra show a broad band covering from 50 °C to 300 °C with apparent peaks at 150 and 220 °C. Interestingly, the relative intensity ratio of the latter to the former is gradually enhanced for the excitation of longer wavelength light, which implies that the excitation of long wavelength 550 nm light (corresponding to the 4A2(4F) → 4T2(4F) transition of Cr3+) populates the deep traps more efficiently. Since the band gap of ZGO is about ~4.7 eV (260 nm) [31], such long wavelength light could not generate electron-hole via interband transition. Therefore, there may be some tunnelling effect or localized cluster model responsible for the deep traps to capture and release carriers, the mechanism of which is also reported in previous work [18,32,33].
Fig. 3 (a) TL spectra of as-synthesized ZGO:Cr illuminated by monochromatic light at 290, 400 and 550 nm for 5minutes, respectively, and the samples were de-trapped at 350 °C before the illumination; (b) the corresponding afterglow spectra.
The afterglow spectra in Fig. 3(b) clearly show that the intensity of N2 line is stronger than that of R line for all wavelength excitation. But notably, the R to N2 ratio in these afterglow spectra is larger than those previously reported [13,18]. And the afterglow intensities with 400 nm and 550 nm illumination are much weaker than the one with 290 nm illumination, with apparent noises for the former. It shows analogous behaviors with Fig. 3(b) for those samples annealed at different atmosphere (not shown here). This may suggest that the incorporation of VO•• into the agglomeration of CrN2 and ZnGa′ hamper the release of carriers at deep traps. The more apparent R line in the afterglow spectra may be contributed by more CrR ions in these systems, assisted by the band with shallow traps refilled from deep traps during the illumination [18]. It can also explain why there is TL intensity at low temperature even with the illumination of 550 nm long wavelength light.
3.3 DFT calculation results
According to the PL and TL spectra, it is infered that the oxygen vacancy VO•• may act as one of the deep traps in the ZGO:Cr system. As for the antisite defects, they served as the shallow traps have been well modelled and calculated, as depicted by Karen Hemelsoet et al. [34]. To illustrate the energy level of oxygen defects, first principle calculations based on DFT were conducted. The density of states (DOS) calculated using 2 × 1 × 1 supercell of ZGO, ZGO with one oxygen vacancy (ZGO- VO••) and ZGO with one zinc vacancy (ZGO- VZn′′) are given in Fig. 4. Compared with the DOS of the ZGO host, there is a new level slightly above the fermi level in the ZGO- VZn′′, which is introduced by the zinc vacancy. As for ZGO- VO••, it introduces a new level deep in the band gap due to the oxygen vacancy. These results suggest that VZn′′ could serve as shallow trap for holes, while VO•• could act as deep trap for electrons.
Fig. 4 Total DOS (TDOS), partial DOS (PDOS) for O and PDOS for Zn in (a) ZGO, (b) ZGO with an oxygen vacancy and (c) ZGO with a zinc vacancy.
To evaluate the interaction between the oxygen vacancy and Cr3+ ion, the formation energy of the models with different distance between the oxygen vacancy and the Cr3+ ion for the 2 × 1 × 1 supercell (denoted as M1 to M4, Fig. 5(a)) are calculated and the results are illustrated in Fig. 5(b). The formation energy E is defined as the energetic difference between the ZGO:Cr supercell and the isolated constituent atoms. The M1 model with the minimum Cr3+-VO•• distance has a lowest formation energy E than the other models with longer-distanced Cr3+-VO•• (M2-M4). We also see a shrunk cell comparing M1 with other models and initial models (not shown here), suggesting that the oxygen vacancy tends to locate in the site adjacent to the Cr3+ ion with attractive force. Then this positively charged oxygen vacancy may serve as the first neighbor anion defect around Cr3+ ion, probably gathering with the first neighbor cation defects (antisite defects). Of course, the contents of VO•• could be much less than that of antisite defects, which is responsible for the less influence on the LLP as shown above.
Fig. 5 (a) Four possible models of the ZGO with one oxygen vacancy and one Ga3+ substituted by Cr3+ ion in the 2 × 1 × 1 supercell. The green, blue and red balls denote the Ga, Cr and O ions, respectively. The Zn ions are not displayed herein and the oxygen vacancy has been marked by yellow circle; (b) the formation energy E of the series models with different distanced Cr3+-VO••. The distance is 2.0256 Å, 4.6956 Å, 8.8051 Å and 12.8049 Å for M1-M4 models, respectively.
3.4 Identification of the charge carriers
To figure out the type of carrier in the system, the AC impedance spectroscopy measurements under N2 atmosphere from 500 to 700 °C and atmosphere with a pO2 range of 10−4~1 atm at 700 °C were carried out. The grain conductivity σ is determined by the cell constant and the conductance obtained from the impedance spectra. The slope of log(σ) versus temperature in N2 atmosphere is 0.014 and that of log(σ) versus log(pO2) at 700 °C is −0.16, as shown in Figs. 6(a) and 6(b). It can be seen that temperature is dominant for determining the conductivity σ. While it declines with the increase of oxygen partial pressure, which can be understood by the Eq. (1) above. The equilibrium moves to the left side with increase of pO2, resulting in decrease of electron conductivity. Thus, it can be inferred, according to the conductive behavior [35,36], that it is the n-type conduction (electron carrier) in this doing system.
Fig. 6 (a) Temperature dependence of grain conductivity of ZGO:Cr under N2 atmosphere; (b) pO2-dependency of grain conductivity of ZGO:Cr at 700 °C.
4. Conclusion
In summary, the influence of the oxygen vacancy on persistent luminescence in ZnGa2O4:Cr3+ material is discussed and its electron carriers are identified. The PL, TL and afterglow spectra of the samples annealed under different atmosphere using different wavelength light excitation show that the VO•• would probably change the contents of antisite defects and the CrR ions, agglomerate with CrN2 and antisite defects. The TL curves hint the existence of deep trap of VO••. The deep trap level of VO•• is further convinced by the theoretical calculation of ZGO- VO•• electronic structure. AC impedance spectroscopy result shows that the carrier in ZGO:Cr sample is the electron, which is likely responsible for the shallow-trap-related LLP. The agglomerated cluster of CrN2, VO•• and ZnGa′ likely trap and release charges in a localized manner when illuminated by visible light. This research gives an insight into the ZGO:Cr long persistent material with outstanding performance, and it would be helpful in understanding the mechanism of LLP phenomenon in other systems.
Funding
This work is financially joint supported by the NSFC (Grant Nos. 21101065, 51472088), the Fundamental Research Funds for the Central Universities (2015PT019, SCUT), Outstanding Young Teacher Training Program of Guangdong provincial Institute of higher education (Yq2013011) and Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306009).
References and links
1. J. Hölsä, “Persistent Luminescence Beats the Afterglow: 400 Years of Persistent Luminescence,” J. Electrochem. Soc. 18, 42–45 (2009).
2. Y. Li, M. Gecevicius, and J. Qiu, “Long persistent phosphors--from fundamentals to applications,” Chem. Soc. Rev. 45(8), 2090–2136 (2016). [CrossRef] [PubMed]
3. H. F. Brito, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, and L. C. V. Rodrigues, “Persistent luminescence mechanisms: human imagination at work,” Opt. Mater. Express 2(4), 371–381 (2012). [CrossRef]
4. T. Aitasalo, P. Dereń, J. Hölsä, H. Jungner, J.-C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, and W. Stręk, “Persistent luminescence phenomena in materials doped with rare earth ions,” J. Solid State Chem. 171(1-2), 114–122 (2003). [CrossRef]
5. P. Dorenbos, “Mechanism of persistent luminescence in Sr2MgSi2O7:Eu2+;Dy3+,” Phys. Status Solidi242(1), R7–R9 (2005) (b). [CrossRef]
6. X. Q. Xu, J. Ren, G. R. Chen, D. S. Kong, C. J. Gu, C. M. Chen, and L. R. Kong, “Bright green emission from the Mn2+-doped zinc gallogermanate phosphors,” Opt. Mater. Express 3(10), 1727–1732 (2013). [CrossRef]
7. L. C. V. Rodrigues, H. F. Brito, J. Hölsä, and M. Lastusaari, “Persistent luminescence behavior of materials doped with Eu2+ and Tb3+,” Opt. Mater. Express 2(4), 382–390 (2012). [CrossRef]
8. T. Maldiney, G. Sraiki, B. Viana, D. Gourier, C. Richard, D. Scherman, M. Bessodes, K. Van den Eeckhout, D. Poelman, and P. F. Smet, “In vivo optical imaging with rare earth doped Ca2Si5N8 persistent luminescence nanoparticles,” Opt. Mater. Express 2(3), 261–268 (2012). [CrossRef]
9. T. Maldiney, C. Richard, J. Seguin, N. Wattier, M. Bessodes, and D. Scherman, “Effect of Core Diameter, Surface Coating, and PEG Chain length on the Biodistribution of Persistent Luminescence Nanoparticles in Mice,” ACS Nano 5(2), 854–862 (2011). [CrossRef] [PubMed]
10. Z. Pan, Y.-Y. Lu, and F. Liu, “Sunlight-activated long-persistent luminescence in the near-infrared from Cr(3+)-doped zinc gallogermanates,” Nat. Mater. 11(1), 58–63 (2011). [CrossRef] [PubMed]
11. T. Maldiney, A. Bessière, J. Seguin, E. Teston, S. K. Sharma, B. Viana, A. J. J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, and C. Richard, “The in vivo activation of persistent nanophosphors for optical imaging of vascularization, tumours and grafted cells,” Nat. Mater. 13(4), 418–426 (2014). [CrossRef] [PubMed]
12. B. Viana, S. K. Sharma, D. Gourier, T. Maldiney, E. Teston, D. Scherman, and C. Richard, “Long term in vivo imaging with Cr3+ doped spinel nanoparticles exhibiting persistent luminescence,” J. Lumin. 170(Part 3), 879–887 (2016). [CrossRef]
13. A. Bessière, S. Jacquart, K. Priolkar, A. Lecointre, B. Viana, and D. Gourier, “ZnGa2O4:Cr3+: a new red long-lasting phosphor with high brightness,” Opt. Express 19(11), 10131–10137 (2011). [CrossRef] [PubMed]
14. J. S. Kim, H. I. Kang, W. N. Kim, J. I. Kim, J. C. Choi, H. L. Park, G. C. Kim, T. W. Kim, Y. H. Hwang, S. I. Mho, M.-C. Jung, and M. Han, “Color variation of ZnGa2O4 phosphor by reduction-oxidation processes,” Appl. Phys. Lett. 82(13), 2029–2031 (2003). [CrossRef]
15. W. Mikenda and A. Preisinger, “N-lines in the luminescence spectra of Cr3+-doped spinels (II) origins of N-lines,” J. Lumin. 26(1-2), 67–83 (1981). [CrossRef]
16. J. Derkosch and W. Mikenda, “N-lines in the luminescence spectra of Cr3+-doped spinels: IV. Excitation spectra,” J. Lumin. 28(4), 431–441 (1983). [CrossRef]
17. W. Nie, F. M. Michel-Calendini, C. Linares, G. Boulon, and C. Daul, “New results on optical properties and term-energy calculations in Cr3+-doped ZnAl2O4,” J. Lumin. 46(3), 177–190 (1990). [CrossRef]
18. A. Bessière, S. K. Sharma, N. Basavaraju, K. R. Priolkar, L. Binet, B. Viana, A. J. J. Bos, T. Maldiney, C. Richard, D. Scherman, and D. Gourier, “Storage of visible light for long-lasting phosphorescence in chromium-doped zinc gallate,” Chem. Mater. 26(3), 1365–1373 (2014). [CrossRef]
19. D. Gourier, A. Bessière, S. K. Sharma, L. Binet, B. Viana, N. Basavaraju, and K. R. Priolkar, “Origin of the visible light induced persistent luminescence of Cr3+-doped zinc gallate,” J. Phys. Chem. Solids 75(7), 826–837 (2014). [CrossRef]
20. L. Binet, S. K. Sharma, and D. Gourier, “Interaction of Cr3+ with valence and conduction bands in the long persistent phosphor ZnGa2O4:Cr3+, studied by ENDOR spectroscopy,” J. Phys. Condens. Matter 28(38), 385501 (2016). [CrossRef] [PubMed]
21. N. Basavaraju, K. R. Priolkar, D. Gourier, A. Bessière, and B. Viana, “Order and disorder around Cr3+ in chromium doped persistent luminescent AB2O4 spinels,” Phys. Chem. Chem. Phys. 17(16), 10993–10999 (2015). [CrossRef] [PubMed]
22. N. Basavaraju, K. R. Priolkar, D. Gourier, S. K. Sharma, A. Bessière, and B. Viana, “The importance of inversion disorder in the visible light induced persistent luminescence in Cr3+ doped AB2O4 (A = Zn or Mg and B = Ga or Al),” Phys. Chem. Chem. Phys. 17(3), 1790–1799 (2015). [CrossRef] [PubMed]
23. N. Basavaraju, K. R. Priolkar, A. Bessière, S. K. Sharma, D. Gourier, L. Binet, B. Viana, and S. Emura, “Controlling disorder in the ZnGa2O4:Cr3+ persistent phosphor by Mg2+ substitution,” Phys. Chem. Chem. Phys. 19(2), 1369–1377 (2017). [CrossRef] [PubMed]
24. M. Allix, S. Chenu, E. Véron, T. Poumeyrol, E. A. Kouadri-Boudjelthia, S. Alahrache, F. Porcher, D. Massiot, and F. Fayon, “Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa2O4,” Chem. Mater. 25(9), 1600–1606 (2013). [CrossRef]
25. J. S. Kim, H. L. Park, C. M. Chon, H. S. Moon, and T. W. Kim, “The origin of emission color of reduced and oxidized ZnGa2O4 phosphors,” Solid State Commun. 129(3), 163–167 (2004). [CrossRef]
26. G. Kresse and J. Hafner, “Ab initio molecular dynamics for liquid metals,” Phys. Rev. B Condens. Matter 47(1), 558–561 (1993). [CrossRef] [PubMed]
27. G. Kresse and D. Joubert, “From ultrasoft pseudopotentials to the projector augmented-wave method,” Phys. Rev. B 59(3), 1758–1775 (1999). [CrossRef]
28. J. P. Perdew, K. Burke, and M. Ernzerhof, “Generalized gradient approximation made simple,” Phys. Rev. Lett. 77(18), 3865–3868 (1996). [CrossRef] [PubMed]
29. W. Nie, F. M. Michel-Calendini, C. Linarès, G. Boulon, and C. Daul, “New results on optical properties and term-energy calculations in Cr3+-doped ZnAl2O4,” J. Lumin. 46(3), 177–190 (1990). [CrossRef]
30. W. W. Zhang, J. Y. Zhang, Z. Y. Chen, T. M. Wang, and S. K. Zheng, “Spectrum designation and effect of Al substitution on the luminescence of Cr3+ doped ZnGa2O4 nano-sized phosphors,” J. Lumin. 130(10), 1738–1743 (2010). [CrossRef]
31. Y. X. Zhuang, J. Ueda, S. Tanabe, and P. Dorenbos, “Band-gap variation and a self-redox effect induced by compositional deviation in ZnxGa2O3+x:Cr3+ persistent phosphors,” J. Mater. Chem. C Mater. Opt. Electron. Devices 2(28), 5502–5509 (2014). [CrossRef]
32. F. Liu, W. Yan, Y.-J. Chuang, Z. Zhen, J. Xie, and Z. Pan, “Photostimulated near-infrared persistent luminescence as a new optical read-out from Cr3+-doped LiGa5O8,” Sci. Rep. 3, 1554 (2013). [CrossRef] [PubMed]
33. D. Jia and W. M. Yen, “Trapping Mechanism Associated with Electron Delocalization and Tunneling of CaAl2O4:Ce3+,” A Persistent Phosphor,” J. Electrochem. Soc. 150(3), H61–H65 (2003). [CrossRef]
34. A. De Vos, K. Lejaeghere, D. E. P. Vanpoucke, J. J. Joos, P. F. Smet, and K. Hemelsoet, “First-principles study of antisite defect configurations in ZnGa2O4:Cr persistent phosphors,” Inorg. Chem. 55(5), 2402–2412 (2016). [CrossRef] [PubMed]
35. K. Fujii, Y. Esaki, K. Omoto, M. Yashima, A. Hoshikawa, T. Ishigaki, and J. R. Hester, “New Perovskite-Related Structure Family of Oxide-Ion Conducting Materials NdBaInO4,” Chem. Mater. 26(8), 2488–2491 (2014). [CrossRef]
36. X. Y. Yang, S. B. Liu, F. Q. Lu, J. G. Xu, and X. J. Kuang, “Acceptor Doping and Oxygen Vacancy Migration in Layered Perovskite NdBaInO4-Based Mixed Conductors,” J. Phys. Chem. C 120(12), 6416–6426 (2016). [CrossRef]
