- Email: [email protected]
Endowing Cr3+-doped non-gallate garnet phosphors with near-infrared long-persistent luminescence in weak fields
Optical Materials 96 (2019) 109322
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Endowing Cr3+-doped non-gallate garnet phosphors with near-infrared long-persistent luminescence in weak fields ⁎
Xinquan Zhou, Guifang Ju , Tiansong Dai, Yihua Hu
T
⁎⁎
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou, 510006, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Long persistent phosphor Luminescence Garnet Thermoluminescence
Recently, Cr3+-doped gallates near-infrared (NIR) long persistent phosphors (LPPs) have been developed. However, Cr3+-doped non-gallate NIR LPPs is little. Herein, we propose two alternative hosts (Na2CaSn2Ge3O12 and Na2CaTi2Ge3O12) for Cr3+-doped non-gallate garnet NIR LPPs. These phosphors were fabricated by a conventional solid-state method in air atmosphere. Predominant cubic phase of these phosphors were observed in XRD patterns. Cr3+ ions locate at weak field in the two hosts and emit a broadband emission due to 4T2→4A2 transition which were illustrated by Tanabe-Sugano matrix and spectroscopic data, respectively. Systematic investigations of the nature of energy traps were carried out by thermoluminescence measurement. Finally, the long persistent luminescence mechanism is also reported in this study.
1. Introduction Recently, trivalent chromium (Cr3+) activated near-infrared (NIR) long persistent phosphors (LPPs) have been developed based on their UV/visible-light-activating properties and emissions (typically near 700 nm) in the biological window [1,2]. The past few years have witnessed great progress in establish Cr3+ activated NIR LPPs as high signal-to-noise ratios and free from auto-fluorescence for real-time optical imaging in life sciences [3–5]. Cr3+ activated gallates system have been systematically studied since the Zn3Ga2Ge2O10 was used to fabricate NIR LPPs [6]. On the basis of the similar ionic radius of Cr3+ and Ga3+ ions and the excellent ability of Cr3+ ions to replace Ga3+ ions in a distorted octahedral coordination, gallates are the most attractive hosts for Cr3+-activated phosphors [7–9]. Until recently, Zn2SnO4:Cr3+ [9]; Zn1+xAl2-2xGexO4:Cr3+ [8] etc. were reported. It provided new methods for the research of Cr3+-doped no-gallate NIR LPPs. However, the further exploration of structural stable Cr3+-doped non-gallate NIR LPPs is still little. Garnets with a cubic A3B2C3O12 type structure form a wide range of inorganic compounds. Their unique structure has an effect on luminescence properties owing to the various cation sub-lattices. Based on it, Na2CaSn2Ge3O12 (NCSG) and Na2CaTi2Ge3O12 (NCTG) were selected by us as the Cr3+-doped LPPs host. Comparing to other Cr3+-doped garnet LPPs (e.g. Ca3Ga2Ge3O12, Gd3Ga5O12, Y/Gd3ScGa3O12, Y3Al2Ga3O12) [10–13], the two garnets are more element earth-
⁎
abundant and environmental friendliness. Herein, we propose a series of new Cr3+-doped non-gallate garnet NIR LPPs which may be beneficial to develop alternative hosts. Additionally, systematic investigations of the nature of energy traps and persistent luminescence (PersL) mechanisms were carried out. 2. Experimental and characterization 2.1. Experimental Phosphors with stoichiometric molar compositions of NCSG: m Cr3+, NCTG: n Cr3+, where m and n ranges from 0.5 to 2 mol% were synthesized using a solid-state method. Appropriate amounts of Na2CO3 (A.R.), CaCO3 (A.R.), GeO2 (A.R.), TiO2 (A.R.), SnO2 (A.R.) and Cr (NO3)3·9H2O (99.0%) were selected as raw materials and mixed in an agate mortar. The NCSG: m Cr3+ and NCTG: n Cr3+ were heated at 1200 °C and 950 °C for 8 h in a furnace and then cooled to room temperature in the furnace. 2.2. Characterization The prepared materials were analyzed by an X-ray diffractometer (D8 ADVANCE, Bruker, Germany) using a Cu Kα irradiation source with a power of 40 kV and current of 30 mA, and scanning steps of 0.02° from 10°–70°. The sample microstructures were investigated by means
Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Ju), [email protected] (Y. Hu).
⁎⁎
https://doi.org/10.1016/j.optmat.2019.109322 Received 12 June 2019; Received in revised form 21 July 2019; Accepted 13 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 96 (2019) 109322
X. Zhou, et al.
Fig. 1. The XRD patterns of NCSG: Cr3+ and NCTG: Cr3+ with the standard data given for comparison. In the right are schematics view of the crystallographic structure of the unit cell of NCSG and NCTG.
Fig. 2. SEM images of (a) NCSG: Cr3+ and (b) NCTG: Cr3+ phosphors.
3. Results and discussion
of a scanning electron microscope (SEM, SU8220, Hitach, Japan). Diffuse reflection spectra were obtained using a UV–visible spectrophotometer (EVOLUTION 220, Thermo Fisher Scientific, China) with BaSO4 as a standard reference. Room-temperature photoluminescence (PL) spectra, PL excitation (PLE) spectra, PersL emission spectra, and decay curves were measured using a high-resolution spectrofluorometer (FLS980, Edinburgh Instruments, UK) equipped with a 500 W Xe lamp as an excitation source and Hamamatsu R928P photomultiplier (250–850 nm) as a detector. PersL emission spectra were measured after using a UV lamp (30 W) excited. Thermoluminescence (TL) intensity versus wavelength and temperature were obtained using an automated TL/optically stimulated luminescence reader (Risϕ TL/OSL Da-20, DTU Nutech, Denmark) coupled with a spectrometer (QE65 Pro, Ocean Optics). All samples were first exposed to radiation from a low pressure mercury discharge lamp (ZW30Z18Y, 30 W, China) and then heated from room temperature to 400 °C at a rate of 2 °C/s. Prior to any spectral measurements, the samples were heat-treated in a muffle furnace at 500 °C for 30 min to completely empty their energy traps.
3.1. Structural and particle analysis The powder X-ray diffraction (XRD) patterns of NCSG and NCTG as well as the standard pattern ICSD-01547 (NCSG) and ICSD-01548 (NCTG) are shown in Fig. 1. The predominant phase of experimental sample is presented and doping Cr3+ does not make obvious changes in the host structure. The crystal structure of the unit cell of NCSG and NCTG are shown in the right of Fig. 1. Na2CaSn2Ge3O12 and Na2CaTi2Ge3O12 are first reported by Durif [14]. As depicted in Fig. 1, the Na/Ca atoms (CN = 8, rNa = 1.118 Å, rCa = 1.12 Å) occupy 24c sites in dodecahedral coordination, the Ge atoms (CN = 4, rGe = 0.53 Å) occupy 24d sites in [GeO4] tetrahedral coordination, the Sn/Ti atoms (CN = 6, rSn = 0.69 Å, rTi = 0.74 Å) occupy 16a sites in [SnO6]/[TiO6] octahedral coordination. In this structure, the dodecahedra, octahedra, and tetrahedra are interconnected with shared oxygen atoms at the corners of the polyhedral. Cr3+ ions as an impurity activator commonly occupies an octahedral site in an insulating (ionic) crystal [15]. Therefore, the Cr3+ ions (CN = 6, rCr = 0.62 Å) are 2
Optical Materials 96 (2019) 109322
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Fig. 3. (a) UV–vis diffuse reflectance spectrum of un-doped NCSG and NCTG. (b,c) Converted diffuse reflectance spectra of NCSG and NCTG via the Kubelka-Munk function F(R). The band gap energy for the sample was estimated from the intercept of a fitted straight line.
Fig. 4. (a) PLE and PL spectra of NCSG: Cr3+ and NCTG: Cr3+. (b) Decay cures of two samples(λex = 470 nm) monitoring Cr3+ emission recorded at room temperature. (c) Tanabe-Sugano diagram for 3 d3 ion in octahedral site symmetry with Dq/B (blue and red dashed vertical line) for Cr3+ in host. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3
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3.2. Photoluminescence and crystal field of Cr3+ in two garnets samples
Table 1 Estimated crystal field parameters of the Cr3+ singly doped NCTG and NCSG compared with Cr3+ singly doped Lu3Ga5O12 (LuGG) and Gd3Sc2Ga3O12 (GSGG). Sample
4
A2→4T1 (nm)
4
A2→4T1 (nm)
Dq (cm−1)
B (cm−1)
Dq/B
Ref.
NCSG:Cr NCTG:Cr LuGG:Cr GSGG:Cr
470 470 434 455
670 650 599 641
1493 1538 1669 1560
649 582 625 642
2.30 2.64 2.67 2.43
This work 10
Fig. 4a illustrates the excitation and emission spectra of NCSG: Cr3+ and NCTG: Cr3+ samples at room temperature. The PLE spectra of NCSG: Cr3+ and NCTG: Cr3+ monitored at 810 nm and 780 nm consists of four broad excitation bands respectively. In the PLE spectrum of NCSG: Cr3+, the excitation bands centered at 246 nm, 350 nm, 470 nm, 670 nm. In the PLE spectrum of NCTG: Cr3+, the excitation bands centered at 270 nm, 330 nm, 470 nm, 650 nm. The four excitation bands of two samples can be successively attributed to host absorption, 4 A2→4T1(te2), 4A2→4T1(t2e) and 4A2→4T2 transitions. Under 470 nm excitation, the two samples give NIR broadband emission peaking at 810 nm (NCSG: Cr3+) and 780 nm (NCTG: Cr3+) due to the 4T2→4A2 transition of Cr3+. The fluorescent decay curves of these samples which excite at 470 nm and monitor at Cr3+ emission peaks are also measured. There is an evident decrease from NCSG: Cr3+ to NCTG: Cr3+ that may be attributed to the different ions radius of Sn ions and Ti ions in the 16a sites. Based on the above data, Tanabe-Sugano matrix [17] (Fig. 4c) is used to describe the luminescent properties of Cr3+ formed by crystal field in these garnet samples. The parameters (crystal field strength Dq), (Racah parameter B and C) are evaluated by the following equations [15], and the results conclusion in Table 1.
expected to enter the octahedral site of Sn4+/Ti4+ under the condition of matching atomic radius and geometrical lattice. When the Cr3+ ions are substituted for Sn4+ or Ti4+ ions, charge compensation is necessary because the charges are not equal. In fact, it is Cr2O3 replaces Sn2O4 or Ti2O4 resulting in missing an oxygen. Therefore, oxygen vacancies (positively charged) compensate for these substitution defects (negatively charged). It is a main charge compensation mechanism in this system. Based on the cation arrangements, non-equivalent substitutional defects (SnCr/TiCr) are also expected to be formed. Fig. 2 (a) and (b) shows the SEM photograph of NCSG: Cr3+ and NCTG: Cr3+ samples. The samples were dispersed in absolute ethyl alcohol and then scanning by SEM. Both NCSG: Cr3+ and NCTG: Cr3+ have dispersity in dispersed solution. The minimum particle size of NCSG: Cr3+ can be less than 200 nm which may have potential applications in autofluorescence-free in vivo bio-imaging. To evaluate the band gap, the experimental diffuse reflectance spectra of NCSG and NCTG phosphor were measured (Fig. 3a). The experimental results were converted using the Kubelka-Munk function as shown in Fig. 3b–c. The optical band gap can be calculated by the following equations [16]:
F(R) = (1−R)2 /2R
(1)
(αhν )2 = A (hν − Eg )
(2)
ΔE = E (4 A2 → 4T1 ) − (4 A2 → 4T2 )
(3)
10Dq = E (4 A2 → 4T2 )
(4)
B=
C≅
(ΔE2 − 10ΔEDq ) (15Δ E− 120Dq )
E(2E) 7.90B 1.8 ⎛ B2 ⎞ − + ⎜ ⎟ 3.05 3.05 3.08 ⎝ Dq ⎠
(5)
(6)
Note that the estimated value is the average since the local environment varies by many factors in a real crystal system [18]. The value of crystal field determines the emission band shape: the 2E→4A2 transition is dominant in the strong field owing to 2E level is lower than the 4T2 level, whereas the broadband 4T2→4A2 transition is the dominant transition in the weak field case [18]. The spectroscopic data and Tanabe-Sugano matrix given in Fig. 4c suggest Cr3+ ions locate at weak field in NCSG and NCTG hosts. Struve et al. [19] have investigated the basic spectroscopic properties of Cr3+-doped gallium garnets. They obtained relatively weak crystal fields by the incorporation of Ga3+ at
where R, α , h, ν , A, are the reflectivity, absorption coefficient, Planck constant and a constant respectively. By extrapolating the linear function to the energy axis, the Eg values of NCSG and NCTG are determined to be 3.44 eV.
Fig. 5. Room-temperature PersL decay curves of NCSG/NCTG: (0.005, 0.008, 0.01, 0.015, 0.02) Cr3+LPPs after excitation by a 30 W UV lamp for 5 min obtained after proper bleaching of the samples. The insets are persistent luminescence spectra of NCSG/NCTG: 0.008Cr3+ and its decay curve plotted as I−1 versus time. 4
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Fig. 6. (a) TL glow curves of NCSG/NCTG: 0.008 Cr3+ LPPs showing in wavelength-temperature contour plots with image profiles. The figure on the top and right show the temperature and wavelength image profiles respectively. (b)The bottoms are experimental and deconvoluted glow curves of NCSG/NCTG: 0.008 Cr3+ LPPs. The samples were irradiated by a UV lamp (30 W) for 5 min. The heating rate is 2 °C/s.
Fig. 4b.
Table 2 The estimated energy trap depths for NCSG: 0.008Cr3+ and NCTG: 0.008Cr3+ phosphors. Sample
peak
Tm(K)
E(eV)
NCSG
1 2 3 1 2 3
357.4 400.6 496.0 346.1 383.3 439.2
0.71 0.80 0.99 0.69 0.77 0.88
NCTG
3.3. Persistent luminescence properties The effect of the Cr3+ concentration of NCSG/NCTG on the PersL properties is investigated. Fig. 5 shows the room temperature PersL decay curves and PersL spectra of Cr3+-doped NCSG/NCTG with different concentrations. After an excitation of 254 nm, the NCSG: Cr3+ and NCTG: Cr3+ exhibit broad band PersL peaking at 790 nm and 760 nm respectively. With increasing the concentration of Cr3+ ions, the PersL properties increased and reached a maximum at 0.008 mol and then concentration quenching occurred. The decay curves of NCSG/NCTG: 0.008 Cr3+ are also plotted as I−1 (I = intensity) versus time (t) as the insets shown in Fig. 5. The I−1~t curve can be fitted well by a straight line indicating that tunneling-related process [20] probably occurs in the NCSG: Cr3+ and NCTG: Cr3+ LPPs.
24c sites, Ga3+, Sc3+ or Lu3+ at the 16a sites, and Gd3+ or La3+ at the 24c sites with their larger ionic radius compared to A13+ or Y3+, respectively. Considering the ionic radius that Ti4+ > Sn4+ in the 16a sites of the NCSG and NCTG garnet structure, it can be suggested that the crystal field strength of NCTG host is weaker than NCSG host which is in accordance with the performances in gallium garnets as shown in Table 1 [10]. This phenomenon is also confirmed by the declining fluorescence lifetime from NCSG: Cr3+ to NCTG: Cr3+ as shown in
3.4. Trap analysis Thermoluminescence (TL) is a common phenomenon in LPPs. By 5
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same time. Then these charge carriers trapped by energy traps through CB or tunneling process. After stop the UV irradiation, PersL is produced by the recombination of the charge carriers and holes. Note that both PL and PersL are caused by Cr3+ illustrated that Cr3+ not only act as emit center but also provide traps. 4. Conclusions In summary, we have successfully developed two Cr3+ singly doped non-gallate garnets (NCSG and NCTG) exhibiting Cr3+ PersL in NIR region. The spectroscopic data and Tanabe-Sugano matrix illustrate that Cr3+ ions locate at weak field in NCSG and NCTG hosts. The crystal field strength of NCTG is weaker than NCSG owing to the ionic radius that Ti4+ > Sn4+ in the 16a sites of these two hosts. PersL decay curves revealed that the optimum composition of phosphor was NCSG/NCTG: 0.008Cr3+. By analyzing the TL curves, it is shows that the different kinds of defects exist in these samples. The average depths of the energy trap were estimated as well. We also put forward a reasonable mechanism to illustrate the PersL formation. On the basis of the above results, this work brings new alternative hosts for composition flexible and structural stable Cr3+-doped non-gallate NIR LPPs and more experiments are expected carry out.
Fig. 7. Schematic representations of the PersL mechanism in NCSG/NCTG: Cr3+ LPPs.
performing TL experiments, the trap depth and distribution can be estimated which is essential to understand the PersL process [21]. Theoretically, trap properties decide the PersL properties of LPPs since charge carriers (electrons, holes) have different distribution in different traps. On the one hand, the capabilities of different traps to capture carriers are discrepant. On the other hand, the trap depths determine the abilities of charge carriers to escape from traps. Fig. 6 evaluate the trap properties of NCSG/NCTG: 0.008Cr3+ by TL experiments. The broad TL peak and wide range temperature distribution which shown in image profiles reveals that Cr3+ ions act as charge carrier recombination centers and participated entirely in PersL. As shown in Fig. 6b, there are some differences between the NCSG: 0.008Cr3+ and NCTG: 0.008Cr3+ samples. The TL curve of NCSG: 0.008Cr3+ shows a broad band located at 369 K indicating the presence of other overlapped peaks. The TL curve of NCTG: 0.008Cr3+ has two bands located at 353 K and 440 K, which indicates the possibly of two kinds of defects in this sample. To explore more trap properties, the experimental data of TL curves are calculated by the integral method and then deconvoluted using commercially available ORIGIN 9.3 software as shown in Fig. 6b. Then the average depth of the energy trap was estimated using the following equation [22]:
E=
Tm 500
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No.21671045). References [1] Q. le Masne de Chermont, C. Chanéac, J. Seguin, F. Pellé, S. Maîtrejean, J.P. Jolivet, D. Gourier, M. Bessodes, D. Scherman, Proc. Natl. Acad. Sci. 104 (2007) 9266. [2] D.J. Naczynski, M.C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C.M. Roth, R.E. Riman, P.V. Moghe, Nat. Commun. 4 (2013) 2199. [3] R. Zou, J. Huang, J. Shi, L. Huang, X. Zhang, K.-L. Wong, H. Zhang, D. Jin, J. Wang, Q. Su, Nano Res 10 (2017) 2070–2082. [4] 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, C. Richard, Nat. Mater. 13 (2014) 418. [5] J. Liu, T. Lécuyer, J. Seguin, N. Mignet, D. Scherman, B. Viana, C. Richard, Adv. Drug Deliv. Rev. 138 (2018) 193–210. [6] Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 11 (2011) 58. [7] A. Abdukayum, J.-T. Chen, Q. Zhao, X.-P. Yan, J. Am. Chem. Soc. 135 (2013) 14125–14133. [8] Y. Zhang, R. Huang, H. Li, D. Hou, Z. Lin, J. Song, Y. Guo, H. Lin, C. Song, Z. Lin, J. Robertson, Acta Mater. 155 (2018) 214–221. [9] Y. Li, Y. Li, R. Chen, K. Sharafudeen, S. Zhou, M. Gecevicius, H. Wang, G. Dong, Y. Wu, X. Qin, J. Qiu, NPG Asia Mater. 7 (2015) e180. [10] J. Xu, J. Ueda, S. Tanabe, J. Am. Ceram. Soc. 100 (2017) 4033–4044. [11] J. Xu, D. Murata, J. Ueda, B. Viana, S. Tanabe, Inorg. Chem. 57 (2018) 5194–5203. [12] D. Chen, Y. Chen, H. Lu, Z. Ji, Inorg. Chem. 53 (2014) 8638–8645. [13] H. Lin, T. Yu, G. Bai, M.-K. Tsang, Q. Zhang, J. Hao, J. Mater. Chem. C 4 (2016) 3396–3402. [14] A. Durif, G. Maupin, Acta Crystallogr. 14 (1961) 440–441. [15] M. Casalboni, A. Luci, U.M. Grassano, B.V. Mill, A.A. Kaminskii, Phys. Rev. B 49 (1994) 3781–3790. [16] E.L. Simmons, Appl. Opt. 14 (1975) 1380–1386. [17] Y. Tanabe, S. Sugano, J. Phys. Soc. Jpn. 9 (1954) 766–779. [18] M. Grinberg, P.I. Macfarlane, B. Henderson, K. Holliday, Phys. Rev. B 52 (1995) 3917–3929. [19] B. Struve, G. Huber, Appl. Phys. B 36 (1985) 195–201. [20] J. Trojan-Piegza, J. Niittykoski, J. Hölsä, E. Zych, Chem. Mater. 20 (2008) 2252–2261. [21] Y. Li, M. Gecevicius, J. Qiu, Chem. Soc. Rev. 45 (2016) 2090–2136. [22] R. Chen, J. Appl. Phys. 40 (1969) 570–585.
(7)
Tm (K) is the peak maximum temperature. E is the average energy trap depth. The estimated energy trap depths are shown in Table 2. It is believed that the charge carriers at deep traps going to upper shallow traps by thermal stimulation. 3.5. Possible mechanisms Fig. 7 shows schematic energy diagrams for demonstrating the PersL mechanism in NCSG/NCTG: Cr3+ LPPs. In present work, Cr3+ ions substitute Sn4+ or Ti4+ in distorted octahedral coordination in these two non-gallate garnets. Meanwhile, a variety of defects such as oxygen vacancies, antisite defects, substitutional defects (SnCr/TiCr) or others appears on these two hosts. Parts of these defects have been proposed as traps as shown in Fig. 7. Cr3+ is an activator itself as well as the charge carrier recombination centers. The PersL mechanism of these two LPPs can be explained as follows. The band gaps of un-doped NCSG and NCTG are 3.44eV, corresponding to the host absorption. The Cr3+ energy levels were placed in the valence band (VB) and conduction band (CB) for explanatory purposes because of the uncertain location of the Cr3+ levels relative to the VB and CB. Under UV irradiation, the electrons from the VB and ground state (4A2) of Cr3+ can be pumped into the CB and higher 4T1 level, respectively. Holes are generated at the 6
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Endowing Cr3+-doped non-gallate garnet phosphors with near-infrared long-persistent luminescence in weak fields ⁎
Xinquan Zhou, Guifang Ju , Tiansong Dai, Yihua Hu
T
⁎⁎
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou, 510006, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Long persistent phosphor Luminescence Garnet Thermoluminescence
Recently, Cr3+-doped gallates near-infrared (NIR) long persistent phosphors (LPPs) have been developed. However, Cr3+-doped non-gallate NIR LPPs is little. Herein, we propose two alternative hosts (Na2CaSn2Ge3O12 and Na2CaTi2Ge3O12) for Cr3+-doped non-gallate garnet NIR LPPs. These phosphors were fabricated by a conventional solid-state method in air atmosphere. Predominant cubic phase of these phosphors were observed in XRD patterns. Cr3+ ions locate at weak field in the two hosts and emit a broadband emission due to 4T2→4A2 transition which were illustrated by Tanabe-Sugano matrix and spectroscopic data, respectively. Systematic investigations of the nature of energy traps were carried out by thermoluminescence measurement. Finally, the long persistent luminescence mechanism is also reported in this study.
1. Introduction Recently, trivalent chromium (Cr3+) activated near-infrared (NIR) long persistent phosphors (LPPs) have been developed based on their UV/visible-light-activating properties and emissions (typically near 700 nm) in the biological window [1,2]. The past few years have witnessed great progress in establish Cr3+ activated NIR LPPs as high signal-to-noise ratios and free from auto-fluorescence for real-time optical imaging in life sciences [3–5]. Cr3+ activated gallates system have been systematically studied since the Zn3Ga2Ge2O10 was used to fabricate NIR LPPs [6]. On the basis of the similar ionic radius of Cr3+ and Ga3+ ions and the excellent ability of Cr3+ ions to replace Ga3+ ions in a distorted octahedral coordination, gallates are the most attractive hosts for Cr3+-activated phosphors [7–9]. Until recently, Zn2SnO4:Cr3+ [9]; Zn1+xAl2-2xGexO4:Cr3+ [8] etc. were reported. It provided new methods for the research of Cr3+-doped no-gallate NIR LPPs. However, the further exploration of structural stable Cr3+-doped non-gallate NIR LPPs is still little. Garnets with a cubic A3B2C3O12 type structure form a wide range of inorganic compounds. Their unique structure has an effect on luminescence properties owing to the various cation sub-lattices. Based on it, Na2CaSn2Ge3O12 (NCSG) and Na2CaTi2Ge3O12 (NCTG) were selected by us as the Cr3+-doped LPPs host. Comparing to other Cr3+-doped garnet LPPs (e.g. Ca3Ga2Ge3O12, Gd3Ga5O12, Y/Gd3ScGa3O12, Y3Al2Ga3O12) [10–13], the two garnets are more element earth-
⁎
abundant and environmental friendliness. Herein, we propose a series of new Cr3+-doped non-gallate garnet NIR LPPs which may be beneficial to develop alternative hosts. Additionally, systematic investigations of the nature of energy traps and persistent luminescence (PersL) mechanisms were carried out. 2. Experimental and characterization 2.1. Experimental Phosphors with stoichiometric molar compositions of NCSG: m Cr3+, NCTG: n Cr3+, where m and n ranges from 0.5 to 2 mol% were synthesized using a solid-state method. Appropriate amounts of Na2CO3 (A.R.), CaCO3 (A.R.), GeO2 (A.R.), TiO2 (A.R.), SnO2 (A.R.) and Cr (NO3)3·9H2O (99.0%) were selected as raw materials and mixed in an agate mortar. The NCSG: m Cr3+ and NCTG: n Cr3+ were heated at 1200 °C and 950 °C for 8 h in a furnace and then cooled to room temperature in the furnace. 2.2. Characterization The prepared materials were analyzed by an X-ray diffractometer (D8 ADVANCE, Bruker, Germany) using a Cu Kα irradiation source with a power of 40 kV and current of 30 mA, and scanning steps of 0.02° from 10°–70°. The sample microstructures were investigated by means
Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Ju), [email protected] (Y. Hu).
⁎⁎
https://doi.org/10.1016/j.optmat.2019.109322 Received 12 June 2019; Received in revised form 21 July 2019; Accepted 13 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 96 (2019) 109322
X. Zhou, et al.
Fig. 1. The XRD patterns of NCSG: Cr3+ and NCTG: Cr3+ with the standard data given for comparison. In the right are schematics view of the crystallographic structure of the unit cell of NCSG and NCTG.
Fig. 2. SEM images of (a) NCSG: Cr3+ and (b) NCTG: Cr3+ phosphors.
3. Results and discussion
of a scanning electron microscope (SEM, SU8220, Hitach, Japan). Diffuse reflection spectra were obtained using a UV–visible spectrophotometer (EVOLUTION 220, Thermo Fisher Scientific, China) with BaSO4 as a standard reference. Room-temperature photoluminescence (PL) spectra, PL excitation (PLE) spectra, PersL emission spectra, and decay curves were measured using a high-resolution spectrofluorometer (FLS980, Edinburgh Instruments, UK) equipped with a 500 W Xe lamp as an excitation source and Hamamatsu R928P photomultiplier (250–850 nm) as a detector. PersL emission spectra were measured after using a UV lamp (30 W) excited. Thermoluminescence (TL) intensity versus wavelength and temperature were obtained using an automated TL/optically stimulated luminescence reader (Risϕ TL/OSL Da-20, DTU Nutech, Denmark) coupled with a spectrometer (QE65 Pro, Ocean Optics). All samples were first exposed to radiation from a low pressure mercury discharge lamp (ZW30Z18Y, 30 W, China) and then heated from room temperature to 400 °C at a rate of 2 °C/s. Prior to any spectral measurements, the samples were heat-treated in a muffle furnace at 500 °C for 30 min to completely empty their energy traps.
3.1. Structural and particle analysis The powder X-ray diffraction (XRD) patterns of NCSG and NCTG as well as the standard pattern ICSD-01547 (NCSG) and ICSD-01548 (NCTG) are shown in Fig. 1. The predominant phase of experimental sample is presented and doping Cr3+ does not make obvious changes in the host structure. The crystal structure of the unit cell of NCSG and NCTG are shown in the right of Fig. 1. Na2CaSn2Ge3O12 and Na2CaTi2Ge3O12 are first reported by Durif [14]. As depicted in Fig. 1, the Na/Ca atoms (CN = 8, rNa = 1.118 Å, rCa = 1.12 Å) occupy 24c sites in dodecahedral coordination, the Ge atoms (CN = 4, rGe = 0.53 Å) occupy 24d sites in [GeO4] tetrahedral coordination, the Sn/Ti atoms (CN = 6, rSn = 0.69 Å, rTi = 0.74 Å) occupy 16a sites in [SnO6]/[TiO6] octahedral coordination. In this structure, the dodecahedra, octahedra, and tetrahedra are interconnected with shared oxygen atoms at the corners of the polyhedral. Cr3+ ions as an impurity activator commonly occupies an octahedral site in an insulating (ionic) crystal [15]. Therefore, the Cr3+ ions (CN = 6, rCr = 0.62 Å) are 2
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Fig. 3. (a) UV–vis diffuse reflectance spectrum of un-doped NCSG and NCTG. (b,c) Converted diffuse reflectance spectra of NCSG and NCTG via the Kubelka-Munk function F(R). The band gap energy for the sample was estimated from the intercept of a fitted straight line.
Fig. 4. (a) PLE and PL spectra of NCSG: Cr3+ and NCTG: Cr3+. (b) Decay cures of two samples(λex = 470 nm) monitoring Cr3+ emission recorded at room temperature. (c) Tanabe-Sugano diagram for 3 d3 ion in octahedral site symmetry with Dq/B (blue and red dashed vertical line) for Cr3+ in host. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3
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3.2. Photoluminescence and crystal field of Cr3+ in two garnets samples
Table 1 Estimated crystal field parameters of the Cr3+ singly doped NCTG and NCSG compared with Cr3+ singly doped Lu3Ga5O12 (LuGG) and Gd3Sc2Ga3O12 (GSGG). Sample
4
A2→4T1 (nm)
4
A2→4T1 (nm)
Dq (cm−1)
B (cm−1)
Dq/B
Ref.
NCSG:Cr NCTG:Cr LuGG:Cr GSGG:Cr
470 470 434 455
670 650 599 641
1493 1538 1669 1560
649 582 625 642
2.30 2.64 2.67 2.43
This work 10
Fig. 4a illustrates the excitation and emission spectra of NCSG: Cr3+ and NCTG: Cr3+ samples at room temperature. The PLE spectra of NCSG: Cr3+ and NCTG: Cr3+ monitored at 810 nm and 780 nm consists of four broad excitation bands respectively. In the PLE spectrum of NCSG: Cr3+, the excitation bands centered at 246 nm, 350 nm, 470 nm, 670 nm. In the PLE spectrum of NCTG: Cr3+, the excitation bands centered at 270 nm, 330 nm, 470 nm, 650 nm. The four excitation bands of two samples can be successively attributed to host absorption, 4 A2→4T1(te2), 4A2→4T1(t2e) and 4A2→4T2 transitions. Under 470 nm excitation, the two samples give NIR broadband emission peaking at 810 nm (NCSG: Cr3+) and 780 nm (NCTG: Cr3+) due to the 4T2→4A2 transition of Cr3+. The fluorescent decay curves of these samples which excite at 470 nm and monitor at Cr3+ emission peaks are also measured. There is an evident decrease from NCSG: Cr3+ to NCTG: Cr3+ that may be attributed to the different ions radius of Sn ions and Ti ions in the 16a sites. Based on the above data, Tanabe-Sugano matrix [17] (Fig. 4c) is used to describe the luminescent properties of Cr3+ formed by crystal field in these garnet samples. The parameters (crystal field strength Dq), (Racah parameter B and C) are evaluated by the following equations [15], and the results conclusion in Table 1.
expected to enter the octahedral site of Sn4+/Ti4+ under the condition of matching atomic radius and geometrical lattice. When the Cr3+ ions are substituted for Sn4+ or Ti4+ ions, charge compensation is necessary because the charges are not equal. In fact, it is Cr2O3 replaces Sn2O4 or Ti2O4 resulting in missing an oxygen. Therefore, oxygen vacancies (positively charged) compensate for these substitution defects (negatively charged). It is a main charge compensation mechanism in this system. Based on the cation arrangements, non-equivalent substitutional defects (SnCr/TiCr) are also expected to be formed. Fig. 2 (a) and (b) shows the SEM photograph of NCSG: Cr3+ and NCTG: Cr3+ samples. The samples were dispersed in absolute ethyl alcohol and then scanning by SEM. Both NCSG: Cr3+ and NCTG: Cr3+ have dispersity in dispersed solution. The minimum particle size of NCSG: Cr3+ can be less than 200 nm which may have potential applications in autofluorescence-free in vivo bio-imaging. To evaluate the band gap, the experimental diffuse reflectance spectra of NCSG and NCTG phosphor were measured (Fig. 3a). The experimental results were converted using the Kubelka-Munk function as shown in Fig. 3b–c. The optical band gap can be calculated by the following equations [16]:
F(R) = (1−R)2 /2R
(1)
(αhν )2 = A (hν − Eg )
(2)
ΔE = E (4 A2 → 4T1 ) − (4 A2 → 4T2 )
(3)
10Dq = E (4 A2 → 4T2 )
(4)
B=
C≅
(ΔE2 − 10ΔEDq ) (15Δ E− 120Dq )
E(2E) 7.90B 1.8 ⎛ B2 ⎞ − + ⎜ ⎟ 3.05 3.05 3.08 ⎝ Dq ⎠
(5)
(6)
Note that the estimated value is the average since the local environment varies by many factors in a real crystal system [18]. The value of crystal field determines the emission band shape: the 2E→4A2 transition is dominant in the strong field owing to 2E level is lower than the 4T2 level, whereas the broadband 4T2→4A2 transition is the dominant transition in the weak field case [18]. The spectroscopic data and Tanabe-Sugano matrix given in Fig. 4c suggest Cr3+ ions locate at weak field in NCSG and NCTG hosts. Struve et al. [19] have investigated the basic spectroscopic properties of Cr3+-doped gallium garnets. They obtained relatively weak crystal fields by the incorporation of Ga3+ at
where R, α , h, ν , A, are the reflectivity, absorption coefficient, Planck constant and a constant respectively. By extrapolating the linear function to the energy axis, the Eg values of NCSG and NCTG are determined to be 3.44 eV.
Fig. 5. Room-temperature PersL decay curves of NCSG/NCTG: (0.005, 0.008, 0.01, 0.015, 0.02) Cr3+LPPs after excitation by a 30 W UV lamp for 5 min obtained after proper bleaching of the samples. The insets are persistent luminescence spectra of NCSG/NCTG: 0.008Cr3+ and its decay curve plotted as I−1 versus time. 4
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Fig. 6. (a) TL glow curves of NCSG/NCTG: 0.008 Cr3+ LPPs showing in wavelength-temperature contour plots with image profiles. The figure on the top and right show the temperature and wavelength image profiles respectively. (b)The bottoms are experimental and deconvoluted glow curves of NCSG/NCTG: 0.008 Cr3+ LPPs. The samples were irradiated by a UV lamp (30 W) for 5 min. The heating rate is 2 °C/s.
Fig. 4b.
Table 2 The estimated energy trap depths for NCSG: 0.008Cr3+ and NCTG: 0.008Cr3+ phosphors. Sample
peak
Tm(K)
E(eV)
NCSG
1 2 3 1 2 3
357.4 400.6 496.0 346.1 383.3 439.2
0.71 0.80 0.99 0.69 0.77 0.88
NCTG
3.3. Persistent luminescence properties The effect of the Cr3+ concentration of NCSG/NCTG on the PersL properties is investigated. Fig. 5 shows the room temperature PersL decay curves and PersL spectra of Cr3+-doped NCSG/NCTG with different concentrations. After an excitation of 254 nm, the NCSG: Cr3+ and NCTG: Cr3+ exhibit broad band PersL peaking at 790 nm and 760 nm respectively. With increasing the concentration of Cr3+ ions, the PersL properties increased and reached a maximum at 0.008 mol and then concentration quenching occurred. The decay curves of NCSG/NCTG: 0.008 Cr3+ are also plotted as I−1 (I = intensity) versus time (t) as the insets shown in Fig. 5. The I−1~t curve can be fitted well by a straight line indicating that tunneling-related process [20] probably occurs in the NCSG: Cr3+ and NCTG: Cr3+ LPPs.
24c sites, Ga3+, Sc3+ or Lu3+ at the 16a sites, and Gd3+ or La3+ at the 24c sites with their larger ionic radius compared to A13+ or Y3+, respectively. Considering the ionic radius that Ti4+ > Sn4+ in the 16a sites of the NCSG and NCTG garnet structure, it can be suggested that the crystal field strength of NCTG host is weaker than NCSG host which is in accordance with the performances in gallium garnets as shown in Table 1 [10]. This phenomenon is also confirmed by the declining fluorescence lifetime from NCSG: Cr3+ to NCTG: Cr3+ as shown in
3.4. Trap analysis Thermoluminescence (TL) is a common phenomenon in LPPs. By 5
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same time. Then these charge carriers trapped by energy traps through CB or tunneling process. After stop the UV irradiation, PersL is produced by the recombination of the charge carriers and holes. Note that both PL and PersL are caused by Cr3+ illustrated that Cr3+ not only act as emit center but also provide traps. 4. Conclusions In summary, we have successfully developed two Cr3+ singly doped non-gallate garnets (NCSG and NCTG) exhibiting Cr3+ PersL in NIR region. The spectroscopic data and Tanabe-Sugano matrix illustrate that Cr3+ ions locate at weak field in NCSG and NCTG hosts. The crystal field strength of NCTG is weaker than NCSG owing to the ionic radius that Ti4+ > Sn4+ in the 16a sites of these two hosts. PersL decay curves revealed that the optimum composition of phosphor was NCSG/NCTG: 0.008Cr3+. By analyzing the TL curves, it is shows that the different kinds of defects exist in these samples. The average depths of the energy trap were estimated as well. We also put forward a reasonable mechanism to illustrate the PersL formation. On the basis of the above results, this work brings new alternative hosts for composition flexible and structural stable Cr3+-doped non-gallate NIR LPPs and more experiments are expected carry out.
Fig. 7. Schematic representations of the PersL mechanism in NCSG/NCTG: Cr3+ LPPs.
performing TL experiments, the trap depth and distribution can be estimated which is essential to understand the PersL process [21]. Theoretically, trap properties decide the PersL properties of LPPs since charge carriers (electrons, holes) have different distribution in different traps. On the one hand, the capabilities of different traps to capture carriers are discrepant. On the other hand, the trap depths determine the abilities of charge carriers to escape from traps. Fig. 6 evaluate the trap properties of NCSG/NCTG: 0.008Cr3+ by TL experiments. The broad TL peak and wide range temperature distribution which shown in image profiles reveals that Cr3+ ions act as charge carrier recombination centers and participated entirely in PersL. As shown in Fig. 6b, there are some differences between the NCSG: 0.008Cr3+ and NCTG: 0.008Cr3+ samples. The TL curve of NCSG: 0.008Cr3+ shows a broad band located at 369 K indicating the presence of other overlapped peaks. The TL curve of NCTG: 0.008Cr3+ has two bands located at 353 K and 440 K, which indicates the possibly of two kinds of defects in this sample. To explore more trap properties, the experimental data of TL curves are calculated by the integral method and then deconvoluted using commercially available ORIGIN 9.3 software as shown in Fig. 6b. Then the average depth of the energy trap was estimated using the following equation [22]:
E=
Tm 500
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No.21671045). References [1] Q. le Masne de Chermont, C. Chanéac, J. Seguin, F. Pellé, S. Maîtrejean, J.P. Jolivet, D. Gourier, M. Bessodes, D. Scherman, Proc. Natl. Acad. Sci. 104 (2007) 9266. [2] D.J. Naczynski, M.C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C.M. Roth, R.E. Riman, P.V. Moghe, Nat. Commun. 4 (2013) 2199. [3] R. Zou, J. Huang, J. Shi, L. Huang, X. Zhang, K.-L. Wong, H. Zhang, D. Jin, J. Wang, Q. Su, Nano Res 10 (2017) 2070–2082. [4] 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, C. Richard, Nat. Mater. 13 (2014) 418. [5] J. Liu, T. Lécuyer, J. Seguin, N. Mignet, D. Scherman, B. Viana, C. Richard, Adv. Drug Deliv. Rev. 138 (2018) 193–210. [6] Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 11 (2011) 58. [7] A. Abdukayum, J.-T. Chen, Q. Zhao, X.-P. Yan, J. Am. Chem. Soc. 135 (2013) 14125–14133. [8] Y. Zhang, R. Huang, H. Li, D. Hou, Z. Lin, J. Song, Y. Guo, H. Lin, C. Song, Z. Lin, J. Robertson, Acta Mater. 155 (2018) 214–221. [9] Y. Li, Y. Li, R. Chen, K. Sharafudeen, S. Zhou, M. Gecevicius, H. Wang, G. Dong, Y. Wu, X. Qin, J. Qiu, NPG Asia Mater. 7 (2015) e180. [10] J. Xu, J. Ueda, S. Tanabe, J. Am. Ceram. Soc. 100 (2017) 4033–4044. [11] J. Xu, D. Murata, J. Ueda, B. Viana, S. Tanabe, Inorg. Chem. 57 (2018) 5194–5203. [12] D. Chen, Y. Chen, H. Lu, Z. Ji, Inorg. Chem. 53 (2014) 8638–8645. [13] H. Lin, T. Yu, G. Bai, M.-K. Tsang, Q. Zhang, J. Hao, J. Mater. Chem. C 4 (2016) 3396–3402. [14] A. Durif, G. Maupin, Acta Crystallogr. 14 (1961) 440–441. [15] M. Casalboni, A. Luci, U.M. Grassano, B.V. Mill, A.A. Kaminskii, Phys. Rev. B 49 (1994) 3781–3790. [16] E.L. Simmons, Appl. Opt. 14 (1975) 1380–1386. [17] Y. Tanabe, S. Sugano, J. Phys. Soc. Jpn. 9 (1954) 766–779. [18] M. Grinberg, P.I. Macfarlane, B. Henderson, K. Holliday, Phys. Rev. B 52 (1995) 3917–3929. [19] B. Struve, G. Huber, Appl. Phys. B 36 (1985) 195–201. [20] J. Trojan-Piegza, J. Niittykoski, J. Hölsä, E. Zych, Chem. Mater. 20 (2008) 2252–2261. [21] Y. Li, M. Gecevicius, J. Qiu, Chem. Soc. Rev. 45 (2016) 2090–2136. [22] R. Chen, J. Appl. Phys. 40 (1969) 570–585.
(7)
Tm (K) is the peak maximum temperature. E is the average energy trap depth. The estimated energy trap depths are shown in Table 2. It is believed that the charge carriers at deep traps going to upper shallow traps by thermal stimulation. 3.5. Possible mechanisms Fig. 7 shows schematic energy diagrams for demonstrating the PersL mechanism in NCSG/NCTG: Cr3+ LPPs. In present work, Cr3+ ions substitute Sn4+ or Ti4+ in distorted octahedral coordination in these two non-gallate garnets. Meanwhile, a variety of defects such as oxygen vacancies, antisite defects, substitutional defects (SnCr/TiCr) or others appears on these two hosts. Parts of these defects have been proposed as traps as shown in Fig. 7. Cr3+ is an activator itself as well as the charge carrier recombination centers. The PersL mechanism of these two LPPs can be explained as follows. The band gaps of un-doped NCSG and NCTG are 3.44eV, corresponding to the host absorption. The Cr3+ energy levels were placed in the valence band (VB) and conduction band (CB) for explanatory purposes because of the uncertain location of the Cr3+ levels relative to the VB and CB. Under UV irradiation, the electrons from the VB and ground state (4A2) of Cr3+ can be pumped into the CB and higher 4T1 level, respectively. Holes are generated at the 6