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## Creator

Huan Yang, Junqing Xiahou, Qi Zhu, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[Considerable improved near-infrared luminescence in ionic-free doped ZnAl2O4 by oxygen defects engineering](https://mdr.nims.go.jp/datasets/46dfce04-d192-42d4-92b7-332aea500447)

## Fulltext

Considerable improved near-infrared luminescence in ionic-free doped ZnAl2O4 by oxygen defects engineering1   Considerable improved near-infrared luminescence in ionic-free doped ZnAl2O4 by oxygen defects engineering  Huan Yang1, Junqing Xiahou1, Qi Zhu1*, and Ji-Guang Li2 1Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, PR China 2Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan           *Corresponding author Dr. Qi Zhu Tel: +86-24-8367-2700 E-mail: zhuq@smm.neu.edu.cn  mailto:zhuq@smm.neu.edu.cn2  Abstract: Recently, the phosphor for near-infrared LED is of importance, and a lot of the researches have been attracted on the synthesis and composition design. However, the large number of researches for near-infrared luminescent materials mainly focus on activator ions doped inorganic materials, including rare earth ions and transition metal ions. Here, an ionic-free doped NIR phosphor of ZnAl2O4 is proposed for NIR LED through oxygen defects engineering. The spinel-structured ZnAl2O4 phosphors with deficiency of zinc were synthesized by high temperature solid state reaction, which were characterized by a series of techniques, including XRD, UV-Vis, XPS, EPR, DFT calculation, PLE/PL spectroscopy, and temperature-dependent PL spectra analysis. The deficiency of zinc results in the appearance of zinc vacancy (VZn) and oxygen vacancy (VO). Most of the oxygen vacancy is VO+, along with a small content of VO++ in ZnAl2O4. Under the excitation of ultraviolet light at 302 nm, the phosphor outputs a broad near-infrared emission band with the maximum at 715 nm. Higher concentration of zinc deficiency leads to stronger near-infrared luminescence, because of the increased oxygen vacancy. The phosphors exhibited a good thermal stability, and they were successfully packaged on the 285-nm UV LED chip, which outputted bright and intense near-infrared light, indicating that the NIR phosphors synthesized in this work have the prospect application in NIR LED.  Key Words: ZnAl2O4; near-infrared; luminescence; oxygen defects; NIR LED          3  1. Introduction Light emitting diode (LED) is a kind of semiconductor electronic component that uses low-voltage direct current to generate light [1]. Compared with traditional light sources such as tungsten filament lamp and halogen lamp, LED has become a new lighting source because of its long service life, high conversion efficiency, and small volume, so it has been widely concerned [2]. In the early days, LED was generally used in display screen [3]. Recently, LED has been extended to backlight module, textile, lighting and other fields [1,4,5]. As a new emitting light source, near-infrared LED has a broad application prospect [2,6-8]. With the rapid development of science and technology, people's demand for LED light sources is not limited to the range of visible light, and began to explore new LED light sources beyond the range of visible light to the naked eye [9]. The near infrared LED is one of them. A large number of studies have shown that near-infrared LED lighting sources can be applied in many fields, such as plant lighting, bioengineering, medical imaging and so on [10-12], and have made indelible contributions to the development of human science and technology. Therefore, the phosphor for near-infrared LED is of importance, and a lot of the researches have been attracted on the synthesis and composition design. In recent years, a large number of near-infrared rare earth luminescent materials [13] have been synthesized, which can be roughly divided into two categories: rare earth ion organic compounds and rare earth ion doped inorganic materials. Trivalent lanthanide (Ln3+) ions, such as Nd3+, Yb3+, Pr3+, Sm3+, have been selected as activator ions for NIR emission [14-16]. However, the phosphors activated by Ln3+ ions usually have the problem of narrow spectrum and low absorptivity, because the odd and even selection of Ln3+ ions prohibits the 4f transition [17]. Moreover, the excessive use of rare earth metals will cause great harm and irreversibility to the environment. To solve this problem, a large number of studies have been carried out on transition metal elements for near-infrared luminescent materials, among which Cr3+ ions as a good activator of near-infrared phosphors have attracted extensive attention [18,19]. It is reported that in many related studies of near-infrared phosphors with Cr3+ ions as the 4  light-emitting center, the interaction between defects and Cr3+ was mainly discussed [20,21]. The defects can not only affect the luminous intensity of phosphors, but also be used to adjust the emission wavelength of phosphors [22,23]. However, most of these phosphors with defects are long persistent luminescence materials, with a complex composition. Their luminescent properties are affected by many factors and are not easy to adjust, and their development in the field of real-time luminescence is limited. Therefore, developing the near-infrared phosphor with defect as the sole luminous center has become a matter of concern. Defects are often considered as a disadvantage of inorganic materials because they may significantly reduce their physical and chemical properties (optical, magnetic, electrical, etc.) [24-26]. In order to obtain higher quality luminescent materials, minimizing the number of internal and external defects has become the main goal. However, defects are not always harmful, and sometimes have a positive impact on material properties [27,28]. More and more evidences show that some special types of defects, such as vacancies and impurities, will also lead to different emission characteristics of phosphors in some cases [29,30].  Zinc aluminate (ZnAl2O4) is considered as a functional material in the main luminescence applications because of its wide band gap energy, high chemical and thermal stability, good cationic dispersion and unique crystal structure [31-33]. ZnAl2O4 is an aluminate with the spinel structure, belonging to Fd-3m space group, with cubic structure and wide band gap energy (3.8 eV) [34]. The chemical formula is generally expressed as AB2O4, where A represents divalent metal cations, occupying tetrahedral positions, and tetrahedral gaps are formed by four oxygen ions, B represents trivalent cations, occupying octahedral positions, and octahedral gaps are formed by six oxygen ions [35]. At present, most of the studies on ZnAl2O4 are focused on the role of activator ions such as Mn2+ and Cr3+ in ZnAl2O4, while rather few studies focus on the ZnAl2O4 matrix [36-39]. It is reported that ZnAl2O4 contains many types of defects, such as VO+, VO2+, VO0, VZn-, VZn2-, O2-, VAl3- [29]. The theoretical calculation of various defects in ZnAl2O4 shows that the formation probability of anti defects is the lowest [33], therefore, the anti site defects of AlZn will be ignored. However, through the first principle calculation, the researchers found that 5  the different types of defects in ZnAl2O4 would possible lead to the emission of light with different wavelengths, while the existence of oxygen vacancy defects may lead to the generation of near-infrared light. In this work, an ionic-free doped NIR phosphor of ZnAl2O4 is proposed for NIR LED through oxygen defects engineering. The spinel-structured ZnAl2O4 phosphors with deficiency of zinc were synthesized by high temperature solid state reaction, which were characterized by a series of techniques, including XRD, UV-Vis, XPS, EPR, DFT calculation, PLE/PL spectroscopy, and temperature-dependent PL spectra analysis. The outcomes in this work may pave a new way to promote the development of near-infrared luminescence and play a demonstration role on near-infrared phosphors. 2. Experiment section 2.1. Sample preparation ZnAl2O4 and Zn1-xAl2O4 (x=0-0.13) phosphors were synthesized by a high temperature solid-state reaction. Al2O3 (99.99%) and ZnO (99.99%) were used as raw materials. The sources were purchased from China pharmaceutical group chemical reagent Co. LTD (Shanghai, China). Based on the composition, the above materials were weighed stoichio-metrically and mixed homogeneously. Then, all of the raw materials were put into an agate mortar and then were ground for 30 min to make them fully mixed. Alumina crucible and alumina support were employed for high-temperature synthesis. After that, the samples were pre-fired at 1000 °C for 5 h and ground again for 30 min, then were sintered at different temperatures for 8 h in air. Finally, after cooling down to room temperature (RT) naturally, ZnAl2O4 and Zn1-xAl2O4 phosphors were obtained for further measurement. 2.2. Fabrication of the LED device The LED device was prepared by coating the near infrared phosphor on LED chip. Firstly, the phosphor was mixed with epoxy resin and curing agent with a mass ratio of 1:5:1. After stirring for 15 minutes, the prepared mixture was uniformly coated on the 285 nm LED chip, dried in an oven at 60 oC for 60 minutes, and then put in an oven at 135 oC for 80 minutes. 6  2.3. Characterization Techniques X-ray diffractometry (XRD, Model SmartLab, Rigaku, Tokyo, Japan) was employed for phase identification, which was operated at 40kV/40mA using nickel-filtered Cu Kα radiation and the data was measured at a speed of 10.0° 2θ per minute, and the scanning range was 10°-70°. The XRD data for Rietveld refinement was gained through the step-scan mode, using a step interval of 0.02 and a counting time of 0.85 s per step. Rietveld refinement was performed using the TOPAS software. The X-ray photoelectron spectroscopy (XPS) data were obtained using X-ray photoelectron spectroscopy (Model Axis Supra, Kratos Analytical Ltd, Manchester, U.K.) under the monochromatic Al KαX-ray radiation of 1486.6 eV. The binding energies were calibrated by using C 1s (284.8 eV) of adventitious carbon as reference. The photoluminescence and persistent luminescence of Zn1-xAl2O4 were detected using a JY FL3-21 spectrophotometer (HORIBA), and using a slit width of 10 nm in the kinetics mode. Diffuse reflectance spectra of the samples were obtained using a UV-Vis-NIR spectrophotometer (UV-3600 Plus, Shimadzu, Kyoto). Electron paramagnetic resonance (EPR) spectra were obtained using an EPR spectrometer (JES-FA 200, JEOL, Kyoto), and the frequency was 9.063 GHz. 2.4. Theoretical calculations The first principle calculations are performed by using the Vienna ab initio simulation package [40,41]. The program has the projected enhancement wave pseudopotential [42] and the generalized gradient approximation of Perdew, Burke and Enzzerhof (PBE) exchange correlation functional [43], which is used to optimize the structure and obtain the free energy of all structures. The cutoff energy of the plane waves basis set is 500 eV and a Monkhorst-Pack mesh of 3×3×1 is used in K-sampling in the adsorption energy calculation and 7×7×1 is used in K-sampling in the density of state (DOS) calculation. In addition, van der Waals interactions corrected using DFT-D2 [44] are considered. The electronic self-consistent iteration is set to 10−5 eV, and the positions of all of the atoms are fully relaxed until the residual force on each atom is below 0.02 eV Å−1.  7  3. Results and discussion 3.1. Synthesis and crystal structure of Zn1-xAl2O4 (x=0-0.13)               Fig. 1. XRD patterns of (a) Zn1-xAl2O4 (x=0-0.13) calcined at 1500 oC and (b) ZnAl2O4 calcined at different temperatures. Fig. 1a shows the X-ray diffraction patterns (XRD) of Zn1-xAl2O4 (x=0-0.13) samples calcined at 1500 oC, while Fig. 1b shows the X-ray diffraction patterns (XRD) of ZnAl2O4 which sintered at different temperatures. The diffraction peaks saw in the XRD patterns of all Zn1-xAl2O4 and ZnAl2O4 samples have a similar structure and in good agreement with the standard patterns of cubic spinel structure ZnAl2O4 (JCPDS No. 05-0669). The diffraction peaks of each sample are similar and can be well matched to (220), (311), (400), (331), (422), (511), (440) diffractions of ZnAl2O4. In addition, no trace of impurities was detected in Zn1-xAl2O4 samples, through closely observation. The above results show that the zinc deficiency with the content of x=0-0.13 and the varied sintering temperatures do not significantly affect the crystal structure of ZnAl2O4. XPS survey spectra of Zn0.87Al2O4 sample is shown in Fig. S1, and no other elements are detected in addition to the original components and contaminating carbon. The C1s peak at 284.8 eV from carbon contamination was used as a reference. 8  20 30 40 50 60 70 80 90 100 110 ObservedIntensity (a.u.)2-Theta (deg.)Temperature:1000 ℃(a) ZnAl2O4Rwp    11.09%Rp      7.22%χ2        1.44 Peak position Difference Calculated  20 30 40 50 60 70 80 90 100 110 ObservedIntensity (a.u.)2-Theta (deg.)Temperature:1500 ℃(b) ZnAl2O4Rw       9.75%Rp       6.34%χ2         1.34 Peak position Calculated Difference 20 30 40 50 60 70 80 90 100 110 ObservedIntensity (a.u.)2-Theta (deg.)Zn0.87Al2O4 Calculated Difference Peak positionTemperature:1500 ℃(c)Rwp    11.01%Rp       6.82%χ2         1.4820 30 40 50 60 70 80 90 100 110 ObservedIntensity (a.u.)2-Theta (deg.)(d) Calculated Difference Peak positionTemperature:1500 ℃Zn0.87Al2O3.87Rwp    11.71%Rp       7.67%χ2         1.57   Fig. 2. Rietveld refinement analysis of XRD patterns for x=0 sample calcined at (a) 1000 oC and (b) 1500 oC, and (c, d) x=0.1 sample calcined at 1500 oC. (e) is the crystal structure for ZnAl2O4. To further analyze the effect of zinc deficiency on the local structure of the spinels, the XRD Rietveld refinements for Zn1-xAl2O4 samples with different zinc deficiency concentrations and ZnAl2O4 sample calcined at different temperatures are performed using the TOPAS software. The refinement results are shown in Fig. 2(a-d). The calculation results are consistent with the experimental data. The lattice parameters and the values of Rwp, Rp, and χ2 are provided in the corresponding figures. The values of Rwp, Rp and χ2 are quite low, suggesting that the results are credible. The structure of ZnAl2O4 spinel and its corresponding atomic positions (Table S1) are shown in Fig. 9  2e. In ZnAl2O4 crystal, Zn is coordinated with four oxygen atoms to form tetrahedron, and Al is coordinated with six oxygen atoms to form octahedron. ZnO4 tetrahedron and AlO6 octahedron are connected to each other by sharing O vertices and are evenly distributed in the unit cells of face centered cubic structure. When zinc is deficient in ZnAl2O4, zinc vacancy will appear, accompanied by the generation of oxygen vacancy. It is found that the lattice of ZAO shrinks in presence of Zn deficiency (Table S2). 536 534 532 530 528 526O1:O2=0.1640O21500 ℃Binding Energy (eV) O1O1O21000 ℃O1:O2=0.1878O 1sIntensity (a.u.)(a)O1:O2=0.1808O1O2 1150 ℃536 534 532 530 528 526O1O2Intensity (a.u.)x=0O 1sO1:O2=0.1640(b)O1O2O1:O2=0.1721x=0.07O1:O2=0.1816x=0.13Binding Energy (eV)O1O2 Fig. 3. XPS spectra of O1s for (a) ZnAl2O4 samples calcined at different temperatures, and (b) Zn1-xAl2O4 (x=0-0.13) samples calcined at 1500 oC. In order to verify the existence of oxygen vacancy, the XPS spectra of O1s were detected (Fig. 3). It is found that there are two peaks in the spectra of ZnAl2O4 (with different sintering temperatures) and Zn1-xAl2O4 (x = 0, 0.07, 0.13). The former peak at 532.5 eV is attributed to adsorbed oxygen (O1) and the second peak at 531.3 eV is attributed to lattice oxygen (O2) [45,46]. The amount of adsorbed oxygen is closely related to the number of defects for oxygen vacancies. The number of oxygen vacancies can be determined indirectly by the amount of adsorbed oxygen. Therefore, the change of oxygen vacancy defect can be estimated through the peak intensity ratio of O1 and O2 corresponding the fitting peaks [30]. In Fig. 3a, with the increase of temperature, the ratio of O1 to O2 gradually decreases. Higher temperature makes the crystallinity be more perfect, and the intrinsic oxygen defects in the lattice decreases gradually. However, during the heating process, high temperature sintering will lead to a little loss of zinc, which leads to the increase of oxygen defects arising from charge balance. Because the zinc loss contributes to the appearance of oxygen defects 10  a little comparing with the decreased intrinsic oxygen defects, the total number of the oxygen defects decreases with the increase of temperature (Fig. 3a). However, for Zn1-xAl2O4 sintered at 1500 oC (x=0-0.13, Fig. 3b), the ratio of O1 to O2 gradually increases with the increase of zinc deficiency concentration. When zinc is deficient in zinc aluminate, zinc vacancies appear in the crystal structure, which give rise to the appearance of oxygen vacancies for the charge balance. More oxygen vacancies arising from the decreased zinc content (x value) lead to a larger ratio of O1 to O2. 326 328 330 332 334 336 338 340Intensity (a.u.)Magnetic field (mT) 1000 ℃ 1150 ℃ 1500 ℃g=1.9548g=2.0010g=2.014950(a)326 328 330 332 334 336 338 340 342 344Intensity (a.u.)Magnetic field (mT) x=0 x=0.07 x=0.13 g=1.9548g=2.0149g=2.0010(b) 50 Fig. 4. EPR spectra for (a) ZnAl2O4 samples calcined at different temperatures and (b) Zn1-xAl2O4 (x=0-0.13) calcined at 1500 oC. The EPR spectra of ZnAl2O4 samples at different sintering temperatures and different zinc deficiency concentrations were measured to further clarify the number of oxygen vacancies (Fig. 4a and 4b). There are three different detection peaks in EPR spectra for all the samples, with the corresponding g values of 2.0149, 2.0010 and 1.9548, respectively. According to the reported works, the three values correspond to VZn (2.0149), VO++ (2.0010), VO+ (1.9548) defects respectively [47-50]. At different sintering temperatures, the EPR spectrum of ZnAl2O4 changes only in peak intensity, but not in the number of EPR signals in Fig. 4a, which indicates that there are three defects of VZn, VO++, and VO+ in ZnAl2O4, and the types of defects do not change with the change of sintering temperature. With the increase of sintering temperature from 1000 to 1150 oC, the signal intensity becomes stronger, which confirms that the number of oxygen vacancy is gradually increasing. However, further increasing the temperature from 1150 to 1500 oC, the signal intensity becomes weaker. As 11  mentioned in the previous text, higher temperature makes the intrinsic oxygen defects decrease. However, high temperature sintering will also lead to the loss of zinc, which leads to the increase of oxygen defects arising from charge balance. Therefore, the above phenomenon is the comprehensive result from the variation of intrinsic oxygen defects and the oxygen defects arising from charge balance. Under the calcination condition of 1500 oC, it is found that the lack of zinc in ZnAl2O4 greatly improves the EPR signal intensity in Fig. 4b, and the signal peaks corresponding to the three types of vacancy defects are sharper and more intense compared with that in Fig. 4a. The signal intensity increases with the increase of zinc deficiency concentration. The EPR signal corresponding to g=1.9548 is attributed to VO+, which increases significantly with the increase of zinc deficiency concentration, indicating the increased amount of VO+. It can be conferred from the above results that the rapidly increased VO+ takes the dominate role on the increased oxygen vacancies. 200 250 300 350 400 450 500Reflection (%)Wavelength (nm) x=0 x=0.03 x=0.07 x=0.13(a)4.0 4.5 5.0 5.5 hν (eV)x=04.95 eV[F(R∞)hν]24.94 eV x=0.034.93 eV x=0.074.90 eV  x=0.13(b) Fig. 5. (a) Reflectance spectra and (b) the band-gap energies for Zn1-xAl2O4 (x=0-0.13) samples. To estimate the optical band gap, the diffuse reflection (R∞) of the ZnAl2O4 host was converted to the Kubelka−Munk function F(R∞), and the formula is as follows [51]:  F(R∞)=S×(1- R∞)2/(2×R∞).................................................(1) As an approximate value, the diffusion coefficient S is independent on the wavelength. To reveal the possible influence of Eg, diffuse reflection spectra were recorded in Fig. 5a for the series of Zn1-xAl2O4 (x=0-0.13), which were further 12  analyzed in Fig. 5b using the following equation [51,52]:  (hν×F(R∞))2=A×(hν-Eg)......................................................(2) Where A is a constant and hν and Eg are the incident photon energy and the band-gap energy, respectively. Fig. 5b shows the values of the band gap energy for the samples with different zinc deficiency concentrations at 1500 oC, which can be read out from the intercept of the fitting straight lines. The results show that with the increase of zinc deficiency concentration (x value) from 0 to 0.13, the band gap energy decreases from 4.95 eV to 4.9 eV. The increase of zinc deficiency concentration will lead to more zinc vacancies and oxygen vacancies. The defect energy level of oxygen vacancy is below the bottom of conduction band, and the defect energy level of zinc vacancy is above the top of valence band. The defects of oxygen vacancies and zinc vacancies usually contribute to a little decrease of band gap energy [20].   -50510Energy (eV)G M K G3.75 eVZnAl2O4 host(b)   -10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.005101520Density (State/eV)Energy (eV) Total Zn Al OEF3.75 eV(c) -5-4-3-2-1012345Energy (eV)G M K GZn0.87Al2O4(e)-10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 10.0 12.5051015Density (State/eV)Energy (eV) Total Zn Al OEF3.75 eV1.98 eV(f) Fig. 6. Atomic distribution of the ZnAl2O4 host (a) and Zn0.87Al2O4 (d). Electronic band structure (b and e) and the corresponding density of states (c and f) for the ZnAl2O4 host (b and c) and Zn0.87Al2O4 (e and f). In order to reveal the role of defects in luminescence behavior and clarify the location of defect energy levels, ZnAl2O4 host and Zn0.87Al2O4 were calculated and analyzed by first principles. Fig. 6a shows the atomic distribution of ZnAl2O4. The 13  perfect ZnAl2O4 crystal has an indirect band gap (Fig. 6b) of about 3.75 eV, which is basically consistent with the reported ZnAl2O4 band gap (3.8 eV) [53]. This value is less than the experimental band gap (4.95 eV), because the calculated band structure is based on DFT method, which is insufficient description of exchange correlation. Then the density of states (DOS) of ZnAl2O4 is calculated, as shown in Fig. 6c. It is found that the energy level of oxygen vacancy defect exists below the bottom of conduction band, and which has also been proved by XPS (Fig. 3) and EPR (Fig. 4) analysis. The local atomic structure of Zn0.87Al2O4 is shown in Fig. 6d. With the loss of Zn atoms, oxygen vacancy VO is generated, the energy band structure and DOS of Zn0.87Al2O4 are shown in Figs. 6e and 6f, and the band gap is 3.75 eV, which is similar to the perfect ZnAl2O4. However, there are two different trap bands in the band gap, with different depths (relative to the minimum conduction band) at 0.53 eV and 2.51 eV. According to the theoretical calculation, the two traps should be attributed to VZn and VO respectively. The energy difference between the two defect levels is about 1.98 eV, which may possibly contribute to the NIR emission with the energy close to ~1.98 eV. Our previous work has found that defect energy levels could contribute to the near-infrared luminescence [30].  220 240 260 280 300 320 3400100000200000300000400000500000600000700000800000Intensity (a.u.)Wavelength (nm) x=0.13275 nm  302 nmλex=715 nm(a)650 675 700 725 750 775 80050000100000150000200000250000300000 302 nmIntensity (a.u.)Wavelength (nm) 275 nm715 nm(b) 14  45000 40000 35000 30000 Wavenumber (cm-1)x=0x=0.03  x=0.05  x=0.07  Intensity (a.u.)x=0.1 x=0.13  λex=715 nm(c)-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.141.31.41.51.61.71.81.9Intensity ratioThe x value λex=715 nm(d) Fig. 7. (a) PLE and (b) PL spectra for x=0.13 sample at room temperature, (c) Gaussian fitting spectra and (d) intensity ratio for the PLE spectra of Zn1-xAl2O4 (x=0-0.13) samples. 3.2. Photoluminescence, luminescence mechanism and application In order to analyze the luminescence characteristics, the photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the spinel phosphors were obtained. Fig. 7a shows the excitation spectrum (PLE) of Zn1-xAl2O4 sample with zinc deficiency concentration of x=0.13 calcined at 1500 oC. It can be seen that the PLE spectrum exhibits an obvious wide excitation band under 715 nm monitoring, which contains two excitation peaks at 275 nm and 302 nm. The peak at 275 nm is due to the transition of the electrons from valence band to the oxygen vacancy level, while the peak at 302nm is due to the transition of the electrons from the zinc vacancy level to the oxygen vacancy level. Under the excitation of ultraviolet light at 275 nm and 302 nm, Zn0.87Al2O4 both output a broad near-infrared emission band with the maximum at 715 nm in Fig. 7b. As can be seen in Fig. 7c, the PLE spectrum is fitted by Gaussian function. It is found that there are two excitation bands at 275 nm and 302 nm, and the intensity of 302 nm excitation band is higher. When the concentration of zinc deficiency increases gradually, the concentration of zinc vacancy also increases gradually, which results in more and more electrons being excited. When the samples are excited by 302 nm ultraviolet light, a larger number of electrons in the zinc vacancy level are excited to the oxygen vacancy level, which contributes to the stronger near-infrared luminescence. Therefore, the intensity ratio of I302 to I275 15  gradually increases with the increase of zinc deficiency concentration in Fig. 7d.  650 675 700 725 750 775 800050000100000150000200000250000300000Intensity (a.u.)Wavelength (nm) x=0 x=0.03 x=0.05 x=0.07 x=0.1 x=0.13λex=302 nm(a)-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14Peak Intensity (a.u.)(b) λex=302 nmThe x value       650 675 700 725 750 775 8000250005000075000100000125000150000175000200000225000Intensity (a.u.)Wavelength (nm) 1000 ℃ 1150 ℃ 1500 ℃λex=302 nm(c) Fig. 8. (a) PL spectra and (b) relative NIR emission intensity for Zn1-xAl2O4 (x=0-0.13) calcined at 1500 oC and (c) PL spectra of ZnAl2O4 at different temperatures. (d) are the appearances of Zn1-xAl2O4 (x=0-0.13) samples excited at 302 nm at room temperature, which are observed by night version device. Fig. 8a shows the emission spectrum excited by ultraviolet light at the wavelength of 302 nm. The NIR emission intensity increases gradually with the increase of Zn deficiency concentration (Fig. 8b). In Fig. 8d, the near-infrared signals of Zn1-xAl2O4 samples taken in the night vision mode under the excitation of ultraviolet light at 302 nm are observed. It can be found that their signal intensities are consistent with the emission spectra (Fig. 8a), and the x=0.13 sample emits brightest near-infrared light in night vision mode. However, no obvious NIR emission and weak NIR emission are found for the ZnAl2O4 samples calcined at 1000 oC and 1150 oC in Fig. 8c, respectively. Increasing the temperature from 1150 oC to 1500 oC yielded a considerable enhancement of NIR emission. The intrinsic oxygen defects may not contribute to the NIR emission, but the oxygen vacancy arising from the zinc loss is 16  closely related to the NIR emission. Although the number of oxygen vacancy decreases at a higher temperature, the oxygen vacancy arising from the zinc loss increases. Therefore, strong near-infrared light is observed for the sample calcined at 1500 oC (Fig. 8d). The lifetimes of the samples at 715 nm are about 2.8 ns (Fig. S2), which indicates that the lifetimes are not significantly affected by Zn deficiency.  Fig. 9. Schematic representation of luminescence mechanism. The luminescence mechanism of ZnAl2O4 was studied in Fig. 9, and the energy level diagram of ZnAl2O4 with Zn deficiency was constructed. The defect energy level below the conduction band is attributed to VO defects, and the defect energy level above the valence band is attributed to VZn defects. Under the excitation at 275 nm, the electrons are excited from the valence band to the oxygen vacancy level. Because the zinc vacancy level is close to the Fermi level, which is full of electrons, under the excitation at 302 nm, the electrons are excited from the zinc vacancy level to the oxygen vacancy level. When the excited electrons are transferred from the oxygen vacancy level to the zinc vacancy level, broad near-infrared emission at 715 nm is found. 17  600 620 640 660 680 700 720 740 760 780 800020040060080010001200140016001800250 300 350 400 450 500 550 600Intensity (a.u.)Wavelength (nm) 298 K 323 K 348 K 373 K 398 K 423 K 448 K 473 K 498 K 523 K 548 K 573 Kλex=302 nm711 nm705 nm(a)Relativite Intensity (a.u.)Temperature (K) Zn0.87Al2O4             18 20 22 24 26 28 30 32 34 36 38-3.0-2.5-2.0-1.5-1.0-0.50.00.51.0 Zn0.87Al2O4 linear fittingln(I0/I1-1)1/KTEa=-0.22458±0.00969λex=302 nm(b)  Fig. 10. (a) Temperature-dependent luminescence spectra excited at 302 nm, (b) ln (I0/I1-1) versus 1/KT plot for the determination of activation energy and (c) the appearances of Zn0.87Al2O4 sample at different temperatures observed by night version device. Fig. 10a shows the PL spectrum of Zn0.87Al2O4 as a function of temperature under the excitation at 302 nm. When the temperature increases gradually, the molecular thermal motion increases gradually, and the crystal field intensity decreases, resulting in a large lattice shrinkage distortion and the decrease of Stokes displacement [54,55]. A blue shift of the emission peak from 711 nm to 705 nm was found by increasing the temperature from 298 K to 573 K, mainly due to the stokes displacement. The decrease of luminescence intensity with the increase of temperature is due to the enhancement of molecular thermal motion at high temperature, which intensifies the non radiative transition [55]. When the sample is excited by ultraviolet light, a large number of electrons are excited to the oxygen vacancy level. Then some electrons return from oxygen vacancy level to zinc vacancy level and contribute to the NIR emissions. However, some of the electrons are trapped by the oxygen vacancy. With the increase of heating temperature, although the luminescence intensity of the sample decreases gradually, the increase of temperature promotes the electrons release from the oxygen vacancy trap to compensate the emission loss. Therefore, when the 18  temperature rises to 100 oC, the luminous intensity of Zn0.87Al2O4 still reaches 78.31%. After several times cycles of heating, the sample exhibited a good repeatability (Fig. S3). Combined with the corresponding fitting spectrum in Fig. 10b, it shows that the sample has good thermal stability, and the thermal activation energy is determined as Ea=-0.22458±0.00969 eV through the equation below:   0=1 exp( )AAEackT+ −  where A0 and A are the integral area of the emission peaks at room temperature (298 K) and experimental temperature T, respectively, k is the Boltzmann constant (8.62 × 10−5 eV), and c is a constant.     Fig. 11. (a) Schematic illustration for the near infrared LED and appearances of the LED lamp encapsulated by an ultraviolet LED chip and the Zn0.87Al2O4 phosphor (b) not powered observed by naked eye and (c) powered observed by naked eye. (d) photographs and relative intensity of the fabricated LED under various currents (120-300 mA) observed by night vision instrument.  Fig. 10c shows the near-infrared emission of Zn0.87Al2O4 at different temperatures 19  under the 302 nm real-time excitation. Through the observation of night vision instrument, there is not NIR afterglow in absence of UV light. However, the phosphor still exhibits intense NIR output with the temperature up to 125 oC. Further increasing the temperature to 175 oC resulted in the decrease of NIR signal. The results are consistent with the changes in temperature-dependent PL spectra.  Since the prepared phosphors can emit intense NIR emission under the UV light excitation, they are the candidate phosphors for NIR LED in Fig. 11a. As shown in Fig. 11, the LED lamp in Fig. 11b composed of Zn0.87Al2O4 near-infrared phosphor and UV LED chip (285 nm) is powered on through the electroluminescent device. When the current is powered on, obvious near-infrared light in Fig. 11c can be observed through the night vision instrument. Even at a low current of 25 mA, the LED can show bright near-infrared light, and the luminous intensity increases with the increase of the driving current (show in Fig. 11d, e). Therefore, the NIR phosphors synthesized in this work have the prospect application in NIR LED.  4. Conclusion A series of Zn1-xAl2O4 (x=0-0.13) phosphors were synthesized by high temperature solid state reaction, which were characterized by a series of techniques, including XRD, UV-Vis, XPS, EPR, DFT calculation, PLE/PL spectroscopy, and temperature-dependent PL spectra analysis. The deficiency of zinc does not change the crystal structure of ZnAl2O4, but it results in the appearance of zinc vacancy (VZn) and oxygen vacancy (VO). There are two kinds of oxygen vacancy, which are VO++ and VO+ in ZnAl2O4, with VO+ taking the dominate role. The defects of oxygen vacancies and zinc vacancies contribute to the decreased band gap. Under the excitation of ultraviolet light at 302 nm, the Zn1-xAl2O4 outputs a broad near-infrared emission band with the maximum at 715 nm, arising from the electron transition from oxygen vacancy energy level to zinc vacancy energy level. Higher concentration of zinc deficiency leads to stronger near-infrared luminescence. A blue shift of the emission peak from 711 nm to 705 nm was found by increasing the heating temperature from 20  298 K to 573 K, mainly due to stokes displacement. The sample exhibits a good thermal stability, because its luminous intensity remains 78.31% of that at room temperature with the heating temperature rising to 100 oC. Finally, the NIR phosphors have been successfully packaged on the UV LED chip (285 nm), which outputted bright and intense near-infrared light, indicating that the NIR phosphors synthesized in this work have the prospect application in NIR LED.  Acknowledgements This work was supported in part by the Natural Science Foundation of Liaoning Province (Grant 2020-MS-081), and National Natural Science Foundation of China (Grant 51302032).  References and Notes [1] J. Y. Shih, S. L. Lai, H. T. Cheng, Design and applications of LED textiles, Adv. Mater. Res. 821-822 (2013) 453-458. [2] T. Pulli, T. Dönsberg, T. Poikonen, F. Manoocheri, P. Kärhä, E. 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