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## Creator
[Jumpei Ueda](https://orcid.org/0000-0002-7013-9708), Tomoaki Minowa, [Jian Xu](https://orcid.org/0000-0002-1040-5090), Shogo Tanaka, [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), [Takashi Takeda](https://orcid.org/0000-0003-2510-4562), [Setsuhisa Tanabe](https://orcid.org/0000-0002-7620-0119)
## Rights
This document is the Accepted Manuscript version of a Published Work that appeared in final form in Highly Thermal Stable Broadband Near-Infrared Luminescence in Ni2+-Doped LaAlO3 with Charge Compensator, copyright © 2023 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsaom.3c00041[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)
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[Highly Thermal Stable Broadband Near-Infrared Luminescence in Ni2+-Doped LaAlO3 with Charge Compensator](https://mdr.nims.go.jp/datasets/688bd408-12d4-465a-ad0a-d20c926c4280)
## Fulltext
Template for Electronic Submission to ACS Journals 1 Highly Thermal Stable Broadband Near-Infrared Luminescence in Ni2+-doped LaAlO3 with Charge Compensator Jumpei Ueda1,2*, Tomoaki Minowa1, Jian Xu1,3, Shogo Tanaka1, Takayuki Nakanishi4, Takashi Takeda4, Setsuhisa Tanabe1 1 Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan 2 Graduate School of Advanced Science and Technology, Japan Advanced Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan 3 International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba, 305-0044, Japan 4 Luminescent Materials Group, National Institute for Materials Science (NIMS), Tsukuba, 305-0044, Japan Keywords: NIR, Phosphor, Ni2+, Perovskite, Quenching 2 ABSTRACT Ni2+-doped LaAlO3 perovskite ceramic samples with a charge compensator (M4+) were prepared and their luminescent properties were investigated. The LaAlO3:Ni2+-M4+ (M = Sn, Hf, Ti, Zr) perovskites show a broad near-infrared (NIR) luminescence peaking at around 1070 nm with 150 nm FWHM due to the Ni2+:3T2-3A2 transition. The Sn4+-codoped one showed the highest NIR luminescence intensity. The valence state of major Ni ions is changed from Ni3+ to Ni2+ by co-doping with Sn4+ based on X-ray absorption spectroscopy. The LaAlO3:Ni2+-Sn4+ shows the shortest luminescence peak wavelength (λem = 1070nm) at 300 K due to the strong crystal field and also shows the highest quenching temperature, T50% of 608 K in the temperature dependence of PL intensity, due to the largest activation energy among the Ni2+-doped NIR phosphors ever reported. Based on the positive trend in the plot of T50% vs PL peak energy, it is concluded that the dominant quenching process in almost all Ni2+ phosphors is the thermally activated crossover process. It is also demonstrated that the luminescence peak wavelength is shifted from 1070 nm to 1235 nm continuously by substituting Ga ions for the Al site. 3 Introduction Near-infrared (NIR) spectroscopy is one of the important technologies for sensing and tracing chemical substances in non-destructive and non-contact manners, leading to significant applications such as pharmaceutical technology, medical diagnosis, food quality assessment, agriculture analysis, bio imaging and so on1-6. NIR phosphor-converting light emitting diode (pc-LEDs) have attracted much attention as a broad NIR light source instead of old fashioned tungsten lamps because of the high electric-light conversion efficiency, long lifetime, stable intensity, low cost and small device size7-10. In general, the NIR region is from 700 nm to 2500 nm; if a NIR pc-LED covers the whole NIR range, it becomes a comparable light source to the tungsten lamp. On the other hand, the NIR pc-LED with a selective wide wavelength range is also practically useful based on the biological windows of NIR-I, II, III (I: 650 ~ 950 nm, II: 1000 ~ 1350 nm, III: 1300 ~ 1700 nm) 11, 12 and the sensitivity curves of Si photodiodes and CCDs (190 ~ 1100 nm) and InGaAs photodiodes and camera (high sensitivity type: 900 ~ 1700 nm, long wavelength type: 900 ~ 2600 nm). For the NIR pc-LED in the NIR-I wavelength region, many Cr3+-doped NIR phosphors utilizing the 4T24A2 transition have been studied and commercialized successfully10. Although the luminescence wavelength of the Cr3+: 4T24A2 transition can be controlled by the crystal field splitting, almost all Cr3+-doped NIR phosphors show shorter peak wavelength than 1000 nm13. On the other hand, the Cr4+ ion in the tetrahedral site with the 3T2-3A2 transition, which is also sensitive to the crystal field, is one of the suitable candidate phosphors for luminescence above 1000 nm luminescence. The luminescence peak wavelength of Cr4+-doped phosphors has been reported in the range between 1150 nm and 1600 nm13, 14. Therefore, it is not easy to cover the luminescence wavelength from 1000 nm to 1150 nm only by the Cr3+ and Cr4+-based NIR 4 phosphors. In order to realize the luminescence in this region, a quite weak and strong crystal fields are necessary for the Cr3+ ion in the octahedral site and the Cr4+ ion in the tetrahedral site, respectively. In this study, we focus on the Ni2+ ion in the octahedral site. The Ni2+ ion has [Ar]3d8 electronic configuration and the energy levels follow the d8 Tanabe-Sugano diagram15, 16, which is the same energetic structure as Cr4+ ([Ar]3d2) in the tetrahedral site (Figure 1). Until now, Ni2+-doped NIR phosphors with 3T23A2 broad NIR luminescence have also been studied well for applications of NIR solid-state lasers and NIR LEDs in many host materials such as MgO17, 18, LiGa5O819, LaMgAl11O1920, Garnets21-24, perovskites25-31, spinel32-35, Zn(Al,Ga)O4 36-38 and so on 39, 40. The 3T2 energy level is very sensitive to the crystal field as shown in Figure 1. Therefore, the Ni2+: 3T2-3A2 luminescence wavelength can be controlled by tuning the crystal field. The peak wavelength of the reported Ni2+-doped phosphors listed above are located between 1100 nm and 2000 nm. In order to shift the 3T2 energy level to the higher energy (1000 nm to 1100 nm in wavelength), the AlO6 octahedral site with a relatively small size is focused because of the possibility of the high crystal field. Here, we selected the lanthanum aluminate perovskite (LaAlO3) host material. The general oxide perovskites are expressed by ABO3, in which A-site has the12-fold coordination and B-site is the octahedral site. Since the luminescence peak of YAlO3:Ni2+ was reported to be 1100 nm21, it is expected that the LaAlO3 also becomes a suitable host with a strong crystal field. However, the detailed spectroscopy of LaAlO3:Ni2+ has never been reported. Also, the Ni2+ ion stabilization in the LaAlO3 host is needed to be discussed because the Ni ion can take both 2+ and 3+ valence states. The Ni ion in the pure LaAlO3 host may prefer the Ni3+ state owing to the trivalent Al site. Actuality, the LaNiO3 perovskite with Ni3+ is known to exist.41, 42 In order to stabilize the Ni2+ ion in the LaAlO3, a charge 5 compensator must be adopted; the NiAl′ state should be compensated by any of 𝑀𝑀La・, 𝑀𝑀Al・ and VO・ defects (M is a tetravalent metal ion). Considering candidates of M4+ ions with proper ionic radius and the controllability of the defect concentration, a suitable M4+ ion in the Al site should be explored as a charge compensator. Here, several charge compensators are tested and the Ni valence states were investigated by XANES (X-ray Absorption Near Edge Structure) spectroscopy. The luminescence properties and its temperature dependence of LaAlO3:Ni2+ with a proper charge compensator are investigated in detail. Also, the tunability of luminescence wavelength is examined by changing Ga3+ substitution for the B-site Al3+ ion in LaAlO3. Figure 1. General Tanabe-Sugano diagram for the d8 electron configuration in the octahedral crystal field, C/B=4.71 and B=1030 cm-1. 6 Experimental Section Material preparation Ceramic samples with LaAl0.995Ni0.005O3, LaAl0.99Ni0.005M0.005O3 (M = Hf, Sn, Ti, Zr), LaAl0.995Ni0.005Sn0.01O3 and La(Al1-xGax)0.99Ni0.005Sn0.005O3 (x=0, 0.25, 0.5, 0.75, 1) compositions were prepared by solid-state reaction. Here, the lanthanum aluminate perovskite (LAP) samples are referred as LAP:Ni and LAP:Ni-M (M = Hf, Mn, Sn, Ti, Zr). La2O3 (99.99%, Furuuchi Chemical), Al2O3 (99.99%, Taimei Chemicals), NiO (99.99%, Sigma-ALDRICH), HfO2 (99.98%, Kojundo KOJUNDO CHEMICAL LABORATORY), SnO2 (99.99%, KOJUNDO CHEMICAL LABORATORY), TiO2 (99.99%, Mitsuwa Chemicals), ZrO2 (99.99%, Furuuchi Chemical) were used as starting chemicals. In order to remove moisture, the hygroscopic La2O3 chemical was heat-treated at 800℃ for 10 h and then kept in a glove box filled with Ar gas. The chemicals were mixed with alumina balls (φ 1mm, SSA-999W, Nikkato) and 20 mL ethanol in an alumina jar by a ball mill (Premium Line P-7, Fritsch) with the condition (400 rpm, 30 min×2 times). Drying the obtained slurry, the powder was heated at 1000 °C for 10 h. Pellets with 13 mm diameter were made and then sintered at 1500 ℃ for 10 h. Crystal structure and valence state The crystal structure of the obtained samples was investigated by an X-ray diffraction (XRD) apparatus (XRD-6000, Shimadzu). The XRD patterns were compared with reference XRD patterns which were simulated from the crystal structure data in ICSD (Inorganic Crystal 7 Structure Database). The valence state of Ni ion in the LaAlO3 host was checked by XANES (X-ray Absorption Near Edge Structure) spectroscopy in the beamline BL-9A of Photon Factory (KEK, Japan). The obtained XANES spectra were analyzed using Athena software package43. Optical Measurements The PL spectra in the NIR region were measured using a multi-channel NIR spectrometer (NIRQUEST, Ocean Optics) by 600 nm excitation light, which is generated by a Xe lamp (MAX302, Asahi Spectra) and two filters of a 600 nm band-pass filter with 40 nm FWHM (FBH600-40, Thorlabs) and an additional 900 nm short-pass filter (FESH900, Thorlabs) to exclude stray light in the NIR region. The PL spectra were calibrated by a standard halogen lamp (DH-2000-RECAL-EXT, Ocean Optics) into the photon flux spectrum. The PLE spectra were measured using a self-build setup; the monochromatic light is generated by two monochromators (SpectraPro-300i, Acton) and a halogen lamp (TS-428, Acton) in the range 500 and 1050nm and a Xe lamp (PE300BUV, Excelitas Cermax) in the range 250 nm and 500 nm. The luminescence was detected by an InGaAs photodiode with a 1050 nm long-pass filter (FELH1050, Thorlabs). The PLE spectra were calibrated by photon flux spectrum of excitation light which is measured using a standard Si photodiode (S1337-1010BQ, Bunkoukeiki). In order to control the temperature, a cryostat (CRT-A020-SE00, Ulvac Cryogenics) with vacuum in the range from 3.5K to 300K and a thermal stage (10035L, Japan High Tech) with N2 atmosphere in the range from 80 K to 800 K were used. The energy level calculation was executed using the same calculation method in the previous paper44. 8 The luminescence decay curves were measured by a fluorescence lifetime spectrometer (Quantaurus-Tau C11367-02, Hamamatsu Photonics). An optional Xe flash lamp with a 600 nm band-pass filter (FBH600-40, Thorlabs, Inc), a 550 nm colored glass filter and a 950 nm short-pass filter (FESH950, Thorlabs, Inc) was used as an excitation source. The luminescence wavelength was selected by a 900 nm long-pass filter and a built-in grating. In order to control the temperature, a cryostat (CRT-006-2000, Iwatani) in the range 20K and 300K and another cryostat (LT3 Helitran, Advanced Research System) in the range 80 K and 800 K were used. Internal quantum efficiency was measured by a NIR fluorescence spectrophotometer (FP-8750, JASCO Corporation) with a calibrated integrating sphere. Results and Discussion Crystal structure Figure 2 shows XRD patterns of the prepared LAP samples doped with Ni2+ and various co-dopants with the reference data of the LaAlO3 (ICSD: 191409). The XRD peaks of all the prepared LAP samples correspond to that of reference LaAlO3 perovskite (𝑅𝑅3�𝑐𝑐). There are not any other XRD peaks due to impurity phases in the LAP:0.5%Ni2+-0.5%M4+(M=Sn, Ti, Hf). For the LAP:0.5%Ni2+-0.5%Zr4+ and LAP:0.5%Ni2+-1%Sn4+, weak unknown XRD peaks at around 28° and 30° were observed. However, it does not affect the main spectroscopic discussion part because the LAP:0.5%Ni2+-0.5%Sn4+ without any impurity phases were used for the spectroscopic investigation. 9 Figure 2. XRD pattern of the LAP:Ni(0.5%)-M(0.5%) (M=Ti, Zr, Hf, Sn) and LAP:Ni(0.5%)-Sn(1%) with the reference data of the LaAlO3 (ICSD #191409). 10 Figure 3. PL spectra of the LAP:0.5%Ni and LAP:0.5%Ni-0.5%M (M =Sn, Hf, Ti, Zr) samples by 600 nm excitation at room temperature. Effect of charge compensator Figure 3 shows PL spectra in the NIR region of LAP:0.5%Ni and LAP:0.5%Ni-0.5%M (M =Sn, Hf, Ti, Zr) samples at room temperature. All the LAP:0.5%Ni-0.5%M (M =Sn, Hf, Ti, Zr) samples show strong luminescence peaking at around 1070 nm (Note that a small dip at 1145 nm is due to the error by a pixel of the NIR spectrometer.). The similar broad NIR luminescence around 1100 nm due to the 3T2g 3A2g transition of Ni2+ was reported in YAlO3 perovskite doped with Ni2+ ion.21 Thus, the observed NIR luminescence can be attributed to the Ni2+: 3T2g 11 3A2g transition. The small blue shift for the Ni2+ NIR luminescence band in the LaAlO3 host can be caused by the different crystal fields between YAlO3 and LaAlO3 perovskites. Compared with Ni singly doped sample, all the samples with tetravalent co-dopants show stronger PL intensity. The tetravalent metal ions (M4+) in Al3+ site can act as a charge compensator for the Ni2+ ion in the Al3+ site. The enhancement of the NIR luminescence intensity of co-doped samples can be caused by the stabilization of Ni2+ in the LaAlO3 perovskites. When the type of tetravalent metal ions is changed, the NIR luminescence becomes stronger in the order of Zr, Ti, Hf and Sn co-doped samples. In the series of LaAlO3:Ni-M samples, the Sn4+ is found to be the best co-dopant to enhance the Ni2+ NIR luminescence intensity. The ionic radii of Zr4+, Hf4+, Sn4+ and Ti4+ in the 6-fold coordination are 0.72, 0.71, 0.69 and 0.605 Å, which are larger by 35%, 33%, 29% and 13% than Al3+ (0.535Å). The PL intensity increases in the order of the Zr4+, Hf4+ and Sn4+ co-doped samples except the Ti4+ co-doped one, which indicates that the solubility of co-dopant ions into LaAlO3 can be related to the stabilization of Ni2+. For instance, the Zr4+ co-doped sample shows the small XRD peaks of an impurity phase, which implies the low solubility of Zr4+ and the weak effect for Ni2+ stabilization. On the other hand, it is still unknown why the PL intensity of the Ti4+ co-doped sample is weaker than that of the Sn4+ co-doped sample. There may be additional effect except the solubility. We also investigated the effect of the atmosphere for the sintering and post-annealing process. However, the sintering of LAP:Ni2+ under 95%N2-5%H2 and the post-annealing of LAP:0.5%Ni2+-0.5%Sn4+ under the different atmospheres (air, N2, 95%N2-5%H2) do not affect to the NIR luminescence intensity (See Fig. S1 in Supporting Information.). 12 Figure 4. XANES spectra near the Ni K-edge of the LAP:0.5%Ni samples with/without 0.5%Sn and the LaNiO3 and Ni metal references. In order to check the effect of the charge compensator for the Ni valence state, the Ni K-edge XANES spectra of the LAP:0.5%Ni and LAP:0.5%Ni-0.5%Sn samples were measured (Figure 4). As references, the XANES spectra of Ni metal and LaNiO3 were also investigated. The XANES edge of Ni can be shifted by not only the valence state but also the type of compounds. Because there is no good reference of Ni2+ in the perovskite structure, the Ni metal was selected as a reference for the low valence state of Ni. The Ni K-edge absorption peak in the perovskite samples is shifted to lower energy in the order of LaNiO3, LAP:0.5%Ni and LAP:0.5%Ni- 13 0.5%Sn. Because the Ni valence state is trivalent in the LaNiO3 perovskite, the XANES peak at 8349 eV is characteristic of Ni3+ in oxide perovskites. On the other hand, the XANES peak of Ni metal is located at the lowest energy. These results follow a general trend that the XANES peak shifts to lower energy with decreasing valence state. Therefore, LAP:0.5%Ni-0.5%Sn LAP are expected to take the lower valence state of Ni2+. This valence state was also confirmed by the observed NIR luminescence. Also, the XANES peak at 8345 eV of LAP: 0.5%Ni-0.5%Sn with the charge compensator is different from that at 8346 eV of LAP:Ni. These results indicate that LAP:Ni includes both Ni2+ and Ni3+ and LAP:0.5%Ni-0.5%Sn dominantly has Ni2+. In the LaAlO3 doped with NiO, which is the starting chemical, if Ni2+ is not oxidized and the charge neutrality is kept by the oxide vacancy, the defect reaction can be expressed by following equation. 𝐿𝐿𝐿𝐿2𝑂𝑂3 + 2𝑁𝑁𝑁𝑁𝑁𝑁𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝑂𝑂3�⎯⎯⎯� 2𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿× + 2𝑁𝑁𝑁𝑁𝐴𝐴𝐴𝐴, + 5𝑂𝑂𝑂𝑂× + 𝑉𝑉𝑂𝑂.. However, Ni2+ can be oxidized as following defect reaction because the sample was sintered under air. 𝐿𝐿𝐿𝐿2𝑂𝑂3 + 2𝑁𝑁𝑁𝑁𝑁𝑁 +12𝑂𝑂2𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝑂𝑂3�⎯⎯⎯� 2𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿× + 2𝑁𝑁𝑁𝑁𝐴𝐴𝐴𝐴× + 6𝑂𝑂𝑂𝑂× Based on the above both defect reactions, the LAP:Ni sample results in the mixed valence states of Ni2+ and Ni3+. On the other hand, by co-doping Sn4+, the Ni2+ ion was stabilized according to the following defect reaction. 𝐿𝐿𝐿𝐿2𝑂𝑂3 + 𝑁𝑁𝑁𝑁𝑁𝑁 + 𝑆𝑆𝑆𝑆𝑂𝑂2𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝑂𝑂3�⎯⎯⎯� 2𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿× + 𝑁𝑁𝑁𝑁𝐴𝐴𝐴𝐴, + 𝑆𝑆𝑆𝑆𝐴𝐴𝐴𝐴. + 6𝑂𝑂𝑂𝑂× 14 Based on the XANES and NIR PL results, it is clarified that the Sn4+ ion act as a charge compensator for the Ni2+ ion in the LaAlO3 host. Determination of Ni2+ energy levels based on low temperature spectroscopy In order to determine the detailed energy levels of Ni2+ in the LaAlO3 perovskite, PL and PLE spectra were measured at 3.6 K of the LAP:0.5%Ni-0.5%Sn sample that showed the strongest luminescence intensity (Figure 5). The LaAlO3 crystal structure belongs to 𝑅𝑅3�𝑐𝑐 space group and the Al site has S6 symmetry. Because the Ni2+ ion occupies the Al octahedral site, each energy level in the ideal octahedral site with Oh symmetry can be split into multiple levels. However, we first discuss the energy levels assuming the ideal Oh symmetry to compare the Tanabe-Sugano diagram. In the PL spectrum excited by 600 nm, several sharp PL lines due to the 3T2g3A2g were observed in the range from 1000 nm and 1200 nm. In the PLE spectrum of NIR luminescence, several PLE bands with sharp structures were observed at around 950, 750, 580, 420, 360 and 300 nm, which can be attributed to transitions from 3A2g ground state to 3T2g, 1Eg, 3T1g(3F), (1A1g, 1T2g), 3T1g(3P) and (1Eg, 1T2g), respectively. The energy level calculation also confirms these assignments as shown in green lines (spin allowed transition) and gray lines (spin forbidden transition) in Figure 5. The PLE band at around 270 nm can be attributed to the charge transfer band from O2- to Ni2+.45 The strongest PL line and PLE line are overlapped at 1012.5 nm (9877 cm-1), which can be assigned to be the zero phonon line (ZPL) of the 3T2g-3A2g transition. Also, the PLE band at around 580 nm due to the 3T1(3F)3A2 has sharp lines (see Fig.S2 in Supporting Information). Consequently, the ZPL of 3T1g(3F)-3A2g transition is 15 determined to be 626.6 nm (15960cm-1). Different from the 3T2g (t2g5eg3) and 3T1g (t2g5eg3), the 1Eg level has the same electronic configuration of t2g6eg2 as the ground state of 3A2g (t2g6eg2). In this case, the phonon sideband, PSB may not be observed because the vibrational interaction is weak due to the small configurational offset in a coordinate configuration diagram. Thus, the transition energy of 1Eg-3A2g is determined to be 749.5 nm (13340 cm-1) based on the centroid energy. The obtained transition energies enable to estimate the Racah parameters (B, C) and crystal field parameter (Dq) by following Eqs. 1~344. 𝐸𝐸 ( 𝑇𝑇2⬚3 ) = 10𝐷𝐷𝐷𝐷 (Eq. 1) 𝐸𝐸 ( 𝑇𝑇1⬚3 ( F⬚3 )) = 15𝐷𝐷𝐷𝐷 + 7.5𝐵𝐵 − 0. 5�100𝐷𝐷𝐷𝐷2 − 180𝐷𝐷𝐷𝐷𝐷𝐷 + 225𝐵𝐵2 (Eq. 2) 𝐸𝐸 ( 𝐸𝐸⬚⬚1 ) = 10𝐷𝐷𝐷𝐷 + 2𝐶𝐶 + 8.5𝐵𝐵 − 0.5�400𝐷𝐷𝐷𝐷2 + 40𝐷𝐷𝐷𝐷𝐷𝐷 + 49𝐵𝐵2 (Eq. 3) The calculated values of Dq, B, and C are 988, 846 and 3490 cm-1, and then the crystal field of 10Dq/B is located at 11.7. Compared to the reported 10Dq/B in various host compounds44, the LaAlO3 has a large value. It was reported that the MgAl2O4 shows the larger 10Dq/B (11.9)33. However, they used the peak energies instead of the ZPL energies, which may give the overestimation of Dq parameter. In MgAl2O4: Ni2+, the PLE peak due to the 3T2g3A2g is located at 970 nm, but the longer wavelength edge extends to around 1200 nm. In addition, the PL peak due to 3T2g3A2g is located at 1180 nm. Based on the comparison of 3T2g3A2 PLE band wavelengths in LaAlO3 and MgAl2O4, it is concluded that the LaAlO3 has the strongest crystal field for Ni2+ among the hosts ever reported. In order to construct the configurational coordinate diagram, the effective phonon energy (ℏ𝜔𝜔) and the Huang-Rhys parameter (S) were estimated. Since the PL peak energy (EPLpeak) at 300 K 16 and the ZPL energy (EZPL) due to the 3T2g3A2g transition are 9346 cm-1 and 9877 cm-1, respectively, the Stokes shift (SS) was estimated to be 1062 cm-1 by the following equation. 𝑆𝑆𝑆𝑆 = 𝐸𝐸𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 − 𝐸𝐸𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 = 2�𝐸𝐸𝑍𝑍𝑍𝑍𝑍𝑍 − 𝐸𝐸𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃� = 2𝑆𝑆ℏ𝜔𝜔 (Eq. 4) Based on the phonon sidebands (PSBs) in the PL spectrum of 3T2g3A2g, the average effective phonon energy was estimated to be 121 cm-1 (see Fig. S3 and Table S2 in Supporting Information). Also, the average effective phonon energy in the PSBs of the 3T1g3A2g PLE band was estimated to be 128 cm-1 (see Fig. S2 and Table S1 in Supporting Information), which is very similar to that of the 3T1g-3A2g transition. Consequently, the Hung-Rhys parameter was estimated to be 4.39 from Eq. 4. The constructed configuration coordinate diagram is shown in Figure 6. Here the configurational offset of the 1E excited state is assumed to be zero because of the same electronic configuration (t2g6eg2) as the 3A2 ground state. Also, the splitting of 3T2g by the crystal field is ignored in this diagram. 17 Figure 5. PL and PLE spectra of LAP:0.5%Ni-0.5%Sn at 3.6K with the calculated energy levels (gray lines and green lines). 18 Figure 6. Configuration coordinate diagram of LaAlO3:Ni2+. The green arrow and the dotted curve express the schematic description of tunneling process and anharmonicity of vibrational potential, respectively. Precisely, the Ni2+ ion experiences the S6 symmetry in LaAlO3. In this case, the 3T2g level in Oh symmetry is divided to 3A1g and 3Eg levels by the crystal field theory. In the PL and PLE spectra of 3T2g-3A2g transition, several sharp lines beside the ZPL at 1012.5 nm (9877 cm-1) originate from the PSBs. On the other hand, the strong PLE line at 992.5 nm (10076 cm-1) does not seem to be a PSB because the corresponding PSB was not observed in the PL spectrum (see Fig. S3 and Table S2, S3 in Supporting Information). Also, the calculated 3A1g (9982 cm-1) and 3Eg 19 (10099 cm-1) levels are similar to the observed two strong PLE lines. Thus, it is concluded that the observed sharp PLE lines at 1012.5 nm (9877 cm-1) and 992.5 nm (10076 cm-1) are attributed to the transition from 3A2g to 3A1g and 3Eg, which are split from 3T2g by reducing symmetry. Temperature-dependent spectroscopy Figure 7. (a) NIR PL spectra at various temperatures (4 K-750 K), (b) temperature dependence of NIR PL integrated intensity, (c) NIR fluorescence decay curves at various temperatures (50K- 20 600K) and (d) Temperature dependence of fluorescent lifetime(Ni2+:3T23A2) of the LAP:0.5%Ni-0.5%Sn sample. Figure 7a shows the NIR PL spectra from 4 K to 750 K. The PL spectral structure of Ni2+: 3T2 3A2 is dramatically changed by temperature. The ZPL and PSBs broaden and become one broad luminescence band above around 250 K. The centroid wavelength is shifted to a longer wavelength with increasing temperature. At 300 K, the PL spectrum covers from around 950 nm to 1300 nm, and the centroid wavelength, peak wavelength and the FWHM (full width at half maximum) are 1100 nm, 1080 nm and 149 nm, respectively. These values are slightly different from that at ambient temperature. In addition, the thermal stability for NIR PL intensity is relatively high. Figure 7b shows the temperature dependence of the NIR PL integrated intensity. With increasing temperature, the intensity increases slightly up to 400 K and then decreases significantly. The quenching temperature (T50%), at which the intensity becomes half with respect to that at low temperature, is 608 K. Because of the increase of PL intensity in the range 4 K and 400 K, the quenching curve does not follow a simple single barrier quenching (Eq. 5) in the whole temperature range. 𝐼𝐼(𝑇𝑇) = 𝐼𝐼0(1 + 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶(−Δ𝐸𝐸 𝑘𝑘𝑘𝑘� ))� (Eq.5) Here I0 is the PL intensity at low temperature, C is the ratio of the attempt rate of the non-radiative process and the radiative decay rate, ∆E is the activation energy, k is Boltzmann constant and T is the temperature. By fitting the quenching curve above 400 K by the single barrier quenching (Figure 7b), the activation energy (∆E) was estimated to be 0.68 eV. 21 In order to investigate the temperature-dependent luminescent properties in detail, fluorescent decay curves of Ni2+:3T23A2 were measured at various temperatures (Figure 7c). In the semi-log plot of decay curves, the exponential profiles were observed within 100 ms at all temperatures. With increasing temperature, the fluorescent decay becomes shorter. The decay curves were fitted by the following equation I = I0*exp(-t/τ) to estimate the lifetime. Figure 7d shows the temperature dependence of lifetime. The lifetime at 20 K is 1.08 ms, which is similar to YAlO3:Ni2+ (1.02 ms) and LaGaO3:Ni2+ (1.12 ms)21. With increasing temperature, the lifetime decreases slightly up to 500 K. The temperature dependence of lifetime in this temperature range is opposite to that of PL intensity. Thus, the decrease in the lifetime is caused by the increase in the radiative decay rate. This is because the population at higher energy levels interacting with phonons is increased by temperature and the radiative rate of PSBs is larger than that of ZPL. In other words, the anti-thermal quenching observed in the temperature dependence of integrated PL intensity (Figure 7b) is caused by the enhancement of total radiative rate due to the participation of the phonon induced transitions with the high radiative transition probability. The lifetime above 500 K decreases significantly, and this tendency is the same as the temperature-dependent PL intensity. This decrease of the lifetime is caused by the increase of the non-radiative decay rate. By taking into account the phonon-induced transition rate46-48 and the nonradiative process, the temperature dependence of the lifetime was fitted by the following equation (Eq. 6). 𝜏𝜏 = 1Γ𝜈𝜈 coth�𝐸𝐸𝑝𝑝𝑝𝑝2𝑘𝑘𝑘𝑘� �+Γ0𝑒𝑒𝑒𝑒𝑒𝑒(−Δ𝐸𝐸 𝑘𝑘𝑘𝑘� ) (Eq. 6) Γ𝜈𝜈 is the radiative rate, Γ0 is the attempt rate of the non-radiative process and Epc is the energy of phonon coupling to the 3T2g-3A2g transition. The obtained Γ𝜈𝜈, Γ0, Epc and Δ𝐸𝐸 were estimated 9.3 22 ×102 s-1, 7.9×108 s-1, 0.059 eV and 0.68 eV, respectively. The obtained activation energy of the quenching process from the lifetime is the same as that from the PL intensity. Here the quenching temperature (T50%) is the temperature to satisfy that the radiative rate and nonradiative rate become equal, Γ𝜈𝜈 coth �𝐸𝐸𝑝𝑝𝑝𝑝 2𝑘𝑘𝑘𝑘� � = Γ0𝑒𝑒𝑒𝑒𝑒𝑒(−Δ𝐸𝐸 𝑘𝑘𝑘𝑘� ), and it was estimated to be 608 K. The quenching temperature estimated by the lifetime is also the same as that estimated by the PL intensity. The activation energy of 0.68 eV is smaller than the expected energy gap between the bottom of 3T2g potential curve and the cross point with 3A2g potential curve in the harmonic oscillation model (Figure 6). This can be related to several factors such as semi-quantitative estimation of energy gap based on the simple configurational coordinate diagram, tunneling process, anharmonicity of the vibrational potential and so on (Figure 6). In order to compare the thermal stability for the luminescence intensity of LaAlO3:Ni2+, the quenching temperature of Ni2+-doped phosphors are listed as shown in Table 1. The T50% of LaAlO3:Ni2+ is the highest among the Ni2+-doped phosphors ever reported. In order to investigate the quenching process, the luminescence peak wavenumber of Ni2+: 3T2-3A2 is collected (Table 1). If the quenching process is caused by thermally assisted crossover from the 3T2 to 3A2 level, the lower transition energy results in the smaller activation energy that causes severe quenching (Figure 6). Figure 8 shows the trend of T50% of Ni2+:3T2-3A2 luminescence as a function of luminescence peak wavenumber and the positive linear trend was observed. Therefore, the quenching process of Ni2+:3T2-3A2 luminescence in almost all hosts including LaAlO3 can be caused by the thermally assisted crossover. Because the LaAlO3:Ni2+ has the highest luminescence peak energy by the strong crystal field, the activation energy from 3T2 to 3A2 becomes large and the highest quenching temperature is realized. 23 Here, the difference of Ni2+:3T23A2 PL peak wavenumber between LaAlO3 and YAlO3 is discussed. The lattice constant of YAlO3 is smaller than that of LaAlO3. In general, the small lattice constant leads the stronger crystal field. However, the crystal field of YAlO3:Ni2+ is weaker than that of LaAlO3:Ni2+ based on the Ni2+:3T23A2 PL position (Table 1). The deviation from the general trend of the crystal field vs the lattice constant is also observed in Cr3+-doped LaAlO3 (10Dq/B=29.7)49 and YAlO3 (10Dq/B=26.5)44. These deviations can be caused by the distortion of the site for transition metal ion. The change of crystal field by distortion is well known in Ce3+-doped garnets50, 51. Table 1. Quenching temperature (T50%) and peak wavenumber of Ni2+:3T2-3A2 luminescence Host T50% Luminescence peak wavenumber around 300 K Reference LaAlO3 608 K ~9346 cm-1 This work YAlO3 ~490 K ~9130 cm-1 21 LaGaO3 ~415 K ~8065 cm-1 21 LaMgAl11O19 ~315 K (~8770 cm-1)*1 20 SrTiO3 ~300 K ~8265 cm-1 29 CaTiO3 ~300 K ~7576 cm-1 29 BaTiO3 ~260 K ~6494 cm-1 29 MgTiO3 ~170 K ~5882 cm-1 29 MgO ~520 K ~7500 cm-1 18 Y3Al2Ga3O12 ~515 K ~6897 cm-1 24 MgF2 ~180 K (~6100 cm-1) *2 25, 52 KZnF3 ~205 K (~5875 cm-1) *3 25 MgCl2 ~220 K ~5300 cm-1 27 24 CsMgCl3 ~190 K ~5200 cm-1 27 CsCdCl3 ~18 0K ~4750 cm-1 27 CdCl2 ~150 K ~4250 cm-1 27 *1: Centroid energy at room temperature. *2: Peak energy at 195 K. *3: Centroid energy at 15 K. Figure 8. T50% of Ni2+:3T2-3A2 luminescence as a function of luminescence peak wavenumber. Improvement of luminescence quantum efficiency For the NIR LED applications, the luminescence quantum efficiency (QE) is very important. However, the QE of the NIR luminescence in the NIR-II region has been rarely reported due to 25 the limitation of the experimental setup. Here, the internal QE was investigated using the calibrated PL measurement system composed of an integrating sphere and an InGaAs PMT. It was discussed in Figure 3 that the luminescence intensity is increased by the Sn4+ charge compensator. Thus, the internal quantum efficiency of the LaAlO3:0.5%Ni samples with different Sn4+ concentrations (0, 0.5, 1%) was measured by 590 nm excitation (Table 2). As expected in the PL spectra with Sn4+ charge compensator (Figure 3), the internal QE is improved from 0.005% to 3.0 % by codoping 0.5% Sn4+ charge compensator. The internal QE reaches 20.7 % by increasing to 1% Sn4+ concentration. On the other hand, the LAP:0.5%Ni2+-2.5% Sn4+ shows weaker PL intensity compared with the 1% Sn4+ co-doped sample. The improvement of internal QE by the Sn4+ charge compensator up to 1% is due to the decrease of Ni3+, which brings the quenching processes such as energy transfer, re-absorption and intervalence charge transfer. If we prepare the high-quality LaAlO3 with only Ni2+ ions, the QE is expected to be more improved. Table 2. Internal quantum efficiency of LaAlO3:0.5%Ni-(0, 0.5, 1%)Sn4+. Excitation wavelength is 590 nm and the integration area is 900 - 1600nm. 0%Sn4+ 0.5%Sn4+ 1%Sn4+ Internal QE 0.005 3.0 % 20.7% Tuning luminescence wavelength 26 The advantage of the Ni2+:3T2-3A2 transition is the tunability of luminescence wavelength by the crystal field. The composition of oxide perovskites can be changed widely, and the solid solutions can be prepared, resulting in the continuous tuning of luminescence wavelength. Figure 9 shows the normalized PL spectra of LaAl1-xGaxO3 solid solutions doped with 0.5%Ni-0.5%Sn by 600 nm excitation at ambient temperature. The PL peak wavelength shifts to a longer wavelength with increasing Ga content. The peak wavelengths of LaAl1-xGaxO3:Ni2+ with x= 0, 0.25, 0.5, 0.75, 1 are determined to be 1070, 1108, 1150, 1190, and 1235 nm, respectively. This result shows that the peak wavelength can be controlled between 1070 nm and 1235 nm by tuning the Ga content in the LaAl1-xGaxO3 host. The crystal structure of LaAl1-xGaxO3 belongs to the 𝑅𝑅3�𝑐𝑐 (No. 167) trigonal space group except LaGaO3 (Pbnm, No. 62, orthorhombic)53, meaning that the longer wavelength shift up to x=0.75 is not related to the phase transition. The lattice constant of LaAl1-xGaxO12 (x≦0.9) increases monotonically with increasing Ga content53 because the ionic radius of Ga (0.62 Å in 6-fold coordination) is larger than that of Al (0.535 Å in 6-fold coordination)54. Because the size of the BO6 octahedron in ABO3 perovskites expands with increasing Ga content, the crystal field for Ni2+ in the octahedral site becomes weaker. In exchange for the longer wavelength shift by increasing Ga content, the quenching temperature is expected to be lower as shown in Figure 8. In actuality, the PL intensity shows decreasing tendency with increasing Ga content (see Fig. S4 in supporting information.). The internal QE of LaAl1-xGaxO3:0.5%Ni2+-0.5%Sn4+ with x=0.25, 0.5, 0.75, 1 are 4.0, 0.7, 1.0, 0.1 %, respectively. The decreasing tendency of the internal QE with increasing Ga concentration can be also affected by Ni2+/3+ ratio as shown in the Sn4+ concentration dependence of the internal QE in the LAP:Ni2+ samples (Table 2). In order to realize the longer wavelength NIR luminescence with high thermal stability in Ni2+-doped NIR phosphors, it is necessary to find a strategy to minimize 27 the configurational offset and increase the curvature of potential curves in the configurational coordinate diagram in addition to the crystal field theory. Figure 9. Normalized PL spectra of 0.5%Ni-0.5%Sn codoped solid solutions between LaAlO3 (LAP) and LaGaO3 (LGP). Application as NIR broadband light source The developed LaAlO3:0.5%Ni-0.5%Sn phosphors can be used as suitable NIR light sources. As a proof of concept, transmittance spectra of organic substances (isopropyl alcohol, ethanol, Toluene, Acetone) was tested using NIR light, which is generated by the 600 nm excitation light and the LaAlO3:0.5%Ni-0.5%Sn phosphor (Figure 10). In the transmittance spectra, several absorption peaks were observed at around 8500 cm-1. These absorption peaks are attributed to 28 the second overtone of C-H bond stretching55. Also, the second overtones of N-H bond stretching and O-H bond stretching are located in the range between 10500 cm-1 and 9100 cm-1 55. Thus, the NIR light source using the LaAlO3:0.5%Ni-0.5%Sn phosphor is suitable for sensing various organic substances. Figure 10. Transmittance spectra of organic substances (isopropyl alcohol, ethanol, toluene, acetone) using NIR broad light source using LaAlO3:0.5%Ni-0.5%Sn phosphors. Conclusions The series of lanthanum-based perovskite NIR phosphors doped with Ni2+ have been prepared. The Ni2+-doped LaAlO3 shows the broad NIR luminescence peaking at 1067 nm due to the 3T23A2 transition. The PL intensity is improved dramatically by co-doping with M4+ charge compensator (M =Sn, Hf, Ti, Zr). Among those samples, the Sn-codoped one shows the highest 29 PL intensity. Based on the XANES analysis, the valence state of Ni ion is changed by co-doping with Sn4+ from Ni3+ to Ni2+. The detailed Ni2+ energy levels are determined by low-temperature spectroscopy. The ZPLs of 3T2-3A2 and 3T1-3A2 are determined to be 9877 cm-1 and 15960 cm-1, respectively. The transition centroid energy of 1E-3A2 is 13340cm-1. Based on the obtained transition energies, the Racah parameters (B, C), crystal field parameter (Dq) and crystal field (10Dq/B) were estimated to be 987.7 cm-1, 845.9 cm-1, 3491cm-1, and 11.7, respectively. The estimated crystal field belongs to the highest class. The luminescence energy is the highest among the Ni2+-doped NIR phosphors ever reported. It is also found that the 3T2 excited level in the ideal Oh symmetry is split into two levels (3E and 3A1) due to the S6 symmetry in the LaAlO3 host based on the low-temperature spectroscopy. The temperature dependences of the 3T2-3A2 NIR luminescence intensity and the lifetime enables us to estimate that quenching temperatures (T50%) are 608 K and 527 K, respectively. The T50% of Ni2+-doped LaAlO3 is the highest value among the Ni2+-doped NIR phosphors ever reported. Based on the positive linear trend in the plot of T50% vs luminescence transition energy, the quenching process of almost all Ni2+ NIR phosphors including LaAlO3:Ni2+ is concluded to be the thermally assisted crossover. The internal quantum efficiency of LaAlO3:0.5%Ni2+-1%Sn4+ was found to be 20.7 %. By preparing the solid solutions of LaAl1-xGaxO3:Ni2+-Sn4+, the NIR luminescence wavelength is successfully shifted like 1070 nm (x=0), 1108 nm (x=0.25), 1150 nm (x=0.50), 1190 nm (x=0.75), 1235 nm (x=1.0). ASSOCIATED CONTENT 30 Supporting Information. The following files are available free of charge. PL and PLE spectra with high wavelength resolution, peak energies of PSBs, and comparison of PL intensity in solid solutions between LaAlO3 and LaGaO3. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: ueda-j@jaist.ac.jp Author Contributions JU and JX conceived the idea of the study. TM and S. Tanaka prepared the materials and investigated the optical properties. 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