COMMUNICATION DOI: 10.1002/asia.201400009
Cr3 + :SrGa12O19: A Broadband Near-Infrared Long-Persistent Phosphor Ju Xu, Daqin Chen,* Yunlong Yu, Wenjuan Zhu, Jiangchong Zhou, and Yuansheng Wang*[a]
Abstract: Cr3 + -doped SrGa12O19 is demonstrated to be a broadband near-infrared (650–950 nm) long-persistent phosphor whose luminescence can last for more than 2 h after ultraviolet irradiation is stopped. Detailed analysis of the photoluminescence and thermoluminescence spectra and of the persistent decay behavior of the Cr3 + -doped SrGa12O19 samples indicate that the persistent energy transfer from the SrGa12O19 host to the Cr3 + ions and the filling and release of electrons into and from the shallow and deep traps through the conduction band is responsible for the long-persistent phosphorescence.
emission (ca. 700 nm) due to the spin-forbidden 2E!4A2 transition, or a broadband emission (650–1600 nm) ascribed to the spin-allowed 4T2 !4A2 transition, which strongly depends on the crystal-field environment of the host.[6] Only a few Cr3 + NIR long-persistent phosphors have been reported, and these are mainly limited to gallate materials, such as Cr3 + :Zn3Ga2Ge2O10,[3] Cr3 + :LiGa5O8,[4b] Cr3 + :ZnGa2O4,[7] Cr3 + :MgGa2O4[8] and Cr3 + :La3Ga5GeO14,[9] because of the strong ability of Cr3 + to substitute for Ga3 + (with a similar ionic radius) in octahedral coordination. Notably, most of the reported Cr3 + -doped gallates have a narrow-band emission close to 700 nm since these gallates provide a strong crystal field environment for Cr3 + ions.[6, 10] Therefore, searching for novel hosts for Cr3 + activator and realizing tunable Cr3 + broadband long-persistent phosphorescence (LPP) are highly desired at present. Herein, we report a new NIR long-persistent Cr3 + :SrGa12O19 phosphor with an emission duration of more than 2 h and a broadband phosphorescence from 650 to 950 nm within the transparency window (700–1000 nm) of biological tissue.[8, 11] The incorporation of Cr3 + ions into Ga3 + octahedral sites of SrGa12O19 is evidenced by photoluminescence (PL) and photoluminescence excitation (PLE) spectra, luminescence lifetime measurements, and crystal field analysis. In contrast to the case of the NIR PL, which is achieved by 250–650 nm broadband excitation, the NIR LPP could be effectively realized only by UV illumination (250–400 nm). Finally, the NIR LPP mechanism of Cr3 + :SrGa12O19 is systematically discussed based on the LPP decay behaviors and the three-dimensional thermoluminescence (TL) spectrum. The Cr3 + x :SrGa12*(1x)O19 (Cr3 + x :SG1xO, x = 0, 0.0005, 0.001, 0.002, 0.005, 0.01, 0.02) phosphors were fabricated by a high-temperature solid-state reaction method. As demonstrated by XRD patterns of the as-prepared SGO and Cr3 + 0.02 :SG0.98O products shown in Figure 1, all the diffraction peaks are well indexed to those of the hexagonal SrGa12O19 phase (JCPDS No. 26-0983). The SrGa12O19 crystal exhibits a hexagonal structure with a space group of P63/mmc and lattice parameters of a = b = 5.7940, c = 22.8160 ;[12] it is composed of 4-, 5- and 6-fold coordinated gallium ions and 12-fold coordinated strontium ions (Figure 1 d). The doped Cr3 + ions prefer to occupy octahedral Ga3 + sites in the SGO host because of the similar ionic radii between Ga3 + (0.620 ) and Cr3 + (0.615 )[13] and the strong ligand-field stabilization energy of Cr3 + in 6-fold coordination.[14]
A long-persistent phosphor is a kind of optical material in which luminescence can last for several minutes to hours after stoppage of the excitation. In the past decade, visible persistent phosphors, including sulfides, aluminates, and silicates, have been well developed.[1] Among them, Eu2 + /Dy3 + :SrAl2O4 and Eu2 + /Nd3 + :CaAl2O4, with sufficiently strong and long-lasting (> 10 h) persistent luminescence under sunlight excitation, have already been commercialized and widely applied in various fields, such as security signs, emergency route signs, traffic signs, dials and displays, as well as in medical diagnostics.[2] In contrast, the research on near-infrared (NIR) long-persistent phosphors is far behind their visible counterparts, although there is a growing demand for such materials as taggants in night-vision surveillance[3] and as optical probes in in vivo bioimaging.[4] Recently, a persistent energy transfer (PET) mechanism has been proposed for NIR long-persistent phosphorescence in Eu2 + /Dy3 + /Er3 + :SrAl2O4[5] and Eu2 + /Dy3 + /Mn2 + :Ca0.2Zn0.9Mg0.9Si2O6 phosphors,[4a] where an NIR-emitting activator (Er3 + or Mn2 + ) is excited by the Eu2 + donors with visible long-persistent emission. However, the limited energy transfer efficiency and the inevitable visible persistent luminescence impede the further development of these phosphors. For NIR luminescence, Cr3 + is an ideal activator in hosts since its 3d3 electron configuration allows a narrow-band [a] J. Xu, Prof. D. Chen, Dr. Y. Yu, W. Zhu, J. Zhou, Prof. Y. Wang State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences Fuzhou, Fujian, 350002 (P. R. China) Fax: (+ 86) 591-83705402 E-mail: [email protected] [email protected]
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value of the local crystal-field parameter Dq is obtained from the mean peak energy of the 4A2 !4T2 transition [Eq. (1)]:[14b] Dq ¼
Eð4 A2 ! 4 T2 Þ ¼ 0:206 eV 10
ð1Þ
Moreover, based on the mean peak energies of the 4A2 !4T2 and 4A2 !4T1 transitions, the Racah parameter B can be evaluated from Equation (2): Dq 15ðx 8Þ ¼ 2 B ðx 10xÞ
ð2Þ
where the parameter x is defined as in Equation (3): x¼
ð3Þ
The parameter B is calculated according to the experimental data, and the value of the Dq/B ratio is determined to be 2.51. The intermediate value of the parameter Dq/B for Cr3 + in the SGO host is consistent with the simultaneous presence of the broad and the narrow emission bands shown in Figure 2 b. Evidently, the RT decay curve of Cr3 + 0.01 :SG0.99O phosphor monitored at 750 nm emission (Figure 2 d) exhibits a double-exponential feature: the slow decay originates from Cr3 + spin-forbidden 2E!4A2 transition and the fast decay from Cr3 + spin-allowed 4T2 !4A2 transition. Besides the intense and broad NIR photoluminescence, the Cr3 + :SGO phosphors also exhibit NIR long-persistent phosphorescence after the removal of the excitation source. Figure 3 a shows the LPP decay of Cr3 + 0.001:SG0.999O phosphor monitored at 750 nm emission after irradiation by a 300 nm ultraviolet light for 10 min. The data were recorded as a function of LPP intensity versus time and the recording time lasted for 7200 s. The LPP intensity decreases quickly in the first several seconds and then decays very slowly. After 7200 s of persistent emission, the LPP intensity is still high, indicating that the NIR LPP of the present phosphor should last longer than 7200 s. As exhibited in the inset of Figure 3 a, the optimal Cr3 + content for LPP is determined to be 0.1 mol %, which is quite different to the value (1 mol %) for the photoluminescence in the Cr3 + :SGO phosphor. Although the NIR PL can be effectively induced by a wide range of excitation wavelength (250–650 nm, Figure 2 a), the situation for the NIR LPP should be different because of the different activation mechanisms. To understand the effectiveness of different excitation wavelengths (energies) for NIR LPP, the relationship between LPP intensity and excitation wavelength was studied. As exhibited in the inset of Figure 3 b, the LPP decay curves monitored at 750 nm emission under the excitation at different wavelengths between 250 nm to 650 nm with 10 nm step for 10 min were recorded. To avoid the influence of the early
Figure 1. XRD patterns of a) SGO and b) Cr3 + 0.02 :SG0.98O products and c) standard hexagonal SrGa12O19 crystal data (JCPDS No. 26-0983). d) Unit-cell structure of SGO; red, blue, and green spheres represent Ga3 + on tetrahedral, hexahedral, and octahedral sites, respectively.
The PLE spectrum of Cr3 + 0.01:SG0.99O (Figure 2 a) exhibits four broad excitation bands with maxima at 265, 320, 425, and 600 nm. The 265 nm excitation band is ascribed to the absorption of the SGO host,[15] and the 320, 425, and 600 nm bands are attributed to the 4A2 !4T1(4P), 4A2 !4T1(4F), and 4 A2 !4T2(4F) transitions of Cr3 + , respectively. The PL spectrum of Cr3 + 0.01:SG0.99O under 425 nm excitation at room temperature (RT) shows a broadband emission in the wavelength range of 650–950 nm originating from the Cr3 + 4T2 ! 4 A2 transition (Figure 2 b, black curve). The optimal Cr3 + content is determined to be 1 mol % (inset, Figure 2 b), and the luminescent quantum yield of the Cr3 + 0.01:SG0.99O sample is determined to be 67 %. Impressively, several narrow emission bands (assigned to Cr3 + 2E!4A2 transition) are superimposed on this broad emission band, and they are dominant in the low-temperature (77 K) PL spectrum, suggesting that the Cr3 + 2E level is close to the Cr3 + 4T2(4F) level, that is, Cr3 + ions locate in an intermediate crystalfield site of the SGO host. A Tanabe–Sugano diagram (Figure 2 c) is used to describe a complete level scheme for Cr3 + in the SGO host. The
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Eð4 A2 ! 4 T1 Þ Eð4 A2 ! 4 T2 Þ ¼ 4:0 Dq
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from which TL curve corresponding to 750 nm emission (Figure 3 d) is extracted. Obviously, the TL curve consists of two broad bands with maxima at 104 and 169 8C (Figure 3 d), which correspond to the shallow and deep traps, respectively. The depths of these two traps can be estimated utilizing the general peak-shape method by Equations (4)–(7):[17] E ¼ ct kT M 2 =tbt ð2kT M Þ
ð4Þ
ct ¼ 1:51 þ 3:0ðm0:42Þ
ð5Þ
bt ¼ 1:58 þ 4:2ðm0:42Þ
ð6Þ
m ¼ ðT2 T M Þ=ðT M T 1 Þ
ð7Þ
where E is the activation energy (trap depth); m the geometrical shape factor; k the Boltzmann constant; t the low temperature half width (TMT1); ct and bt the con3+ 3+ Figure 2. a) PLE (lem = 750 nm) spectrum of Cr 0.01:SG0.99O. b) PL (lex = 425 nm) spectra of Cr 0.01:SG0.99O at stants related to the geometriRT (black curve) and 77 K (blue curve); left inset is the luminescent photograph of Cr3 + 0.01:SG0.99O under cal shape factor; T1, TM, and T2 300 nm light excitation; right inset shows the dependence of emission intensity of Cr3 + x :SG1xO on Cr3 + content. c) Tanabe–Sugano diagram for Cr3 + in SGO phosphor. d) PL decay curve of Cr3 + 0.01:SG0.99O monitored the temperature of half intensiat 750 nm emission. ty at the low-temperature side, the peak temperature, and the temperature of half intensity at fast decay on the analysis of LPP, the persistent emission inthe high-temperature side, respectively. For the shallow trap, tensities (I50s) recorded 50 s after irradiation ceased were T1 = 80 8C, T2 = 124 8C; for the deep trap, T1 = 142 8C, T2 = used as the references. Figure 3 b shows the persistent inten195 8C. As a result, the depths of the shallow and deep traps sity I50s of the Cr3 + 0.001:SG0.999O phosphor as a function of in SGO are evaluated to be 0.73 and 0.96 eV, respectively. Furthermore, it is found that the SGO host shows a selfthe excitation wavelength. The PL excitation spectrum is activated broadband luminescence in the range of 450– also presented in Figure 3 b for comparison. Evidently, the 650 nm with a maximum emission at 502 nm under 266 nm NIR persistent luminescence can be effectively achieved by excitation (Figure 4 a), which likely arises from electron– UV (250–400 nm) illumination, that is, by exciting electrons hole recombination between the defect states in SGO. After into the conduction band of SGO as well as the 4T1(4P) exciexcitation stops, the SGO host luminescence lasts for a long tation band of Cr3 + . time, that is, SGO exhibits visible long-persistent phosphorDefect centers play an essential role for LPP because escence. Interestingly, after doping Cr3 + into SGO, the host they can capture free carriers and then immobilize them for an appropriately long period of time. Notably, after UV irraemission almost disappears, and the LPP time is greatly diation, the body color of the Cr3 + 0.001:SG0.999O phosphor shortened (Figure 4 b). Considering the large spectral overlap between the host emission and the Cr3 + absorption origchanges from green to pink, and the UV-irradiation-induced coloration can be quickly bleached by heating at 400 8C. inating from 4A2 !4T1(4F) and 4A2 !4T2(4F) transitions (FigThis result indicates the formation of photochromic centers ure 4 a), this result indicates the existence of an effective probably induced by the trapping of photogenerated elecpersistent energy transfer from the host to the Cr3 + ions. trons by intrinsic defects (such as Schottky disorder, oxygen Based on the above results and discussions, we propose vacancy, and antisite defects)[7a, 16] in Cr3 + 0.001:SG0.999O. In a mechanism to account for the NIR LPP in the Cr3 + :SGO order to investigate the traps in SGO, the three-dimensional phosphors, as schematically demonstrated in Figure 5. Upon thermoluminescence (TL) spectrum of Cr3 + 0.001:SG0.999O was short-wavelength UV excitation (250–290 nm), the incident photons are absorbed by the SGO host, and the electrons recorded after UV (365 nm) radiation for 5 min (Figure 3 c),
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Figure 3. a) LPP decay curve of Cr3 + 0.001:SG0.999O phosphor monitored at 750 nm emission after irradiation at 300 nm for 10 min; inset shows the Cr3 + dependence of LPP decay for a series of Cr3 + x :SG1xO samples (x = 0.0005, 0.001, 0.002, 0.005, 0.01, 0.02). b) Phosphorescence intensity after 50 s (I50s) monitored at 750 nm emission as a function of excitation wavelength for the Cr3 + 0.001:SG0.999O phosphor (spheres). For comparison, the PL excitation spectrum of the Cr3 + 0.001:SG0.999O phosphor monitored at 750 nm emission (dotted line) is also provided. The inset shows the LPP decay curves of the Cr3 + 0.001:SG0.999O phosphor irradiated by 250–400 nm light for 10 min. c) Three-dimensional thermoluminescence spectrum of the Cr3 + 0.001:SG0.999O phosphor. d) Thermoluminescence curve of Cr3 + 0.001:SG0.999O corresponding to 750 nm emission after UV excitation for 10 min.
are promoted from the valence band to the conduction band of SGO. Subsequently, the excited free electrons are captured by the defects via nonradiative relaxation (process 1). After the irradiation is stopped, the recombination of electrons and holes released from the defect levels results in a broadband visible persistent luminescence (process 2). Because of the large spectral overlap between the host emission and the two absorption bands of Cr3 + , the energy of the host is slowly but persistently transferred to the Cr3 + ions via an effective nonradiative energy transfer (process 3), leading to the NIR LPP of Cr3 + .[4c] On the other hand, when long-wavelength UV light (290–400 nm) is used to irradiate the sample, the ground-state electrons of the Cr3 + ions are promoted to the 4T1(4P) level localized near the conduction band of the SGO host.[3, 4b] The excited electrons are subsequently captured by trap 1 and trap 2 through the conduction band (process 4). After a sufficient irradiation time, all of the traps are filled (with the deep trap being filled mainly by nonradiative relaxation from the shallow one). In the initial stage after irradiation stops, direct recombination between the conduction electrons released from the shallow trap (trap 1) and the Cr3 + ions (process 5) dominates the persistent luminescence and produces an intense
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NIR emission. After the depletion of the shallow trap, the electrons in the deep trap are gradually released to fill the shallow trap (process 6). Subsequently, process 5 and process 6 occur cyclically, finally resulting in the long-persistent phosphorescence of Cr3 + :SGO (process 7). In summary, we have fabricated a NIR long-persistent Cr3 + :SrGa12O19 phosphor that emits for more than 2 h. The doped Cr3 + ions prefer to occupy the octahedral sites of Ga3 + with intermediate crystalfield strength. The NIR LPP can be effectively realized by UV irradiation (250–400 nm) but is hardly achieved by visible-light irradiation (400– 650 nm). The realization of LPP in Cr3 + :SrGa12O19 is believed to arise from persistent energy transfer from the SrGa12O19 host to the Cr3 + ions and the filling and release of electrons into and from the shallow and deep traps, assisted by the conduction band.
Experimental Section Material preparation The Cr3 + x :SrGa12*(1x)O19 (Cr3 + x :SG1xO, x = 0, 0.0005, 0.001, 0.002, 0.005, 0.01, 0.02) phosphors were fabricated by a high-temperature solid-state reaction method. Stoichiometric amounts of SrCO3, Ga2O3, and Cr2O3 powders were mixed, ground, and pre-fired at 800 8C for 2 h in air. After the pre-fired material was finely ground again, the powder sample was sintered at 1350 8C for another 2 h in air. Characterization X-ray diffraction (XRD) patterns were recorded on a MiniFlex II powder diffractometer using Cu Ka radiation (l = 0.154 nm) at 30 kV and 15 mA. Photoluminescence excitation and photoluminescence spectra, luminescence decay curves, and long-persistent phosphorescence decay curves were measured on an Edinburgh Instruments FLS920 spectrofluorometer equipped with a standard R928P photomultiplier tube (250– 850 nm) and a liquid nitrogen-cooled R5509 near-infrared photomultiplier tube (800–1700 nm). Low-temperature PL and PLE spectra were measured using samples cooled in liquid nitrogen (77 K). Three-dimensional thermoluminescence spectra were recorded using a RISFDA-15B/C thermoluminescence/photoluminescence spectrometer. Immediately after 10 min exposure of the sample to a mercury lamp with maximum emission at 365 nm (Gp3Hg-2, Fei Ying Light Electrical Appliance Factory), thermoluminescence signals were recorded in the temperature range of 273–523 K and the heating rate was fixed at 5 K s1.
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h¼
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Lsample number of photons emitted ¼ number of photons absorbed Ereference Esample
ð8Þ
where h is the QY, Lsample the emission intensity, Ereference and Esample the intensities of the excitation light not absorbed by the reference and the sample, respectively. The difference in integrated areas between the sample and the reference represents the number of the absorbed photons. The photons emitted were determined by integrating the area of the emission band. The error associated with the QY measurement is 3 %.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51172231, 21271170, 11204301, 51202244, and 11304312), the Natural Science Foundation of Fujian for Distinguished Young Scholars (2012J06014) and the Major Sci & Tech Project of Fujian (2011HZ00012). The authors thank Prof. Jing Wang and Dr. Ye Li of Sun Yat-Sen University for the measurements of three-dimensional thermoluminescence spectra.
Keywords: doping · luminescence · near-infrared emission · solid-state structure
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Figure 4. a) PLE and PL spectra of the undoped SGO and Cr3 + 0.001 :SG0.999O samples; the inset shows the magnified PL spectra in the wavelength range 470–580 nm. b) LPP decay curve of the undoped SGO sample monitored at 502 nm emission; the inset shows LPP decay curves of the undoped SGO and Cr3 + 0.001:SG0.999O samples recorded in the initial 600 s (under 266 nm UV light excitation for 10 min and monitored at 502 nm emission).
Figure 5. Schematic illustration of the NIR LPP mechanism of Cr3 + :SGO; the straight-line and dotted-line arrows represent optical transition and electron transfer processes, respectively, and the wavy arrows represent nonradiative relaxation. Quantum yield (QY) is defined as the ratio of emitted photons to absorbed photons and was measured by the FLS920 spectrofluorometer equipped with an integrating sphere. All spectroscopic data were corrected for the spectral responses of both the spectrofluorometer and the integrating sphere. The responses of the detecting systems (integrating sphere, monochromators and detectors) in photon flux were determined using a calibrated tungsten lamp. Based on this setup, internal QY is calculated by Equation (8):[18]
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Cr3 + :SrGa12O19: A Broadband Near-Infrared Long-Persistent Phosphor Ju Xu, Daqin Chen,* Yunlong Yu, Wenjuan Zhu, Jiangchong Zhou, and Yuansheng Wang*[a]
Abstract: Cr3 + -doped SrGa12O19 is demonstrated to be a broadband near-infrared (650–950 nm) long-persistent phosphor whose luminescence can last for more than 2 h after ultraviolet irradiation is stopped. Detailed analysis of the photoluminescence and thermoluminescence spectra and of the persistent decay behavior of the Cr3 + -doped SrGa12O19 samples indicate that the persistent energy transfer from the SrGa12O19 host to the Cr3 + ions and the filling and release of electrons into and from the shallow and deep traps through the conduction band is responsible for the long-persistent phosphorescence.
emission (ca. 700 nm) due to the spin-forbidden 2E!4A2 transition, or a broadband emission (650–1600 nm) ascribed to the spin-allowed 4T2 !4A2 transition, which strongly depends on the crystal-field environment of the host.[6] Only a few Cr3 + NIR long-persistent phosphors have been reported, and these are mainly limited to gallate materials, such as Cr3 + :Zn3Ga2Ge2O10,[3] Cr3 + :LiGa5O8,[4b] Cr3 + :ZnGa2O4,[7] Cr3 + :MgGa2O4[8] and Cr3 + :La3Ga5GeO14,[9] because of the strong ability of Cr3 + to substitute for Ga3 + (with a similar ionic radius) in octahedral coordination. Notably, most of the reported Cr3 + -doped gallates have a narrow-band emission close to 700 nm since these gallates provide a strong crystal field environment for Cr3 + ions.[6, 10] Therefore, searching for novel hosts for Cr3 + activator and realizing tunable Cr3 + broadband long-persistent phosphorescence (LPP) are highly desired at present. Herein, we report a new NIR long-persistent Cr3 + :SrGa12O19 phosphor with an emission duration of more than 2 h and a broadband phosphorescence from 650 to 950 nm within the transparency window (700–1000 nm) of biological tissue.[8, 11] The incorporation of Cr3 + ions into Ga3 + octahedral sites of SrGa12O19 is evidenced by photoluminescence (PL) and photoluminescence excitation (PLE) spectra, luminescence lifetime measurements, and crystal field analysis. In contrast to the case of the NIR PL, which is achieved by 250–650 nm broadband excitation, the NIR LPP could be effectively realized only by UV illumination (250–400 nm). Finally, the NIR LPP mechanism of Cr3 + :SrGa12O19 is systematically discussed based on the LPP decay behaviors and the three-dimensional thermoluminescence (TL) spectrum. The Cr3 + x :SrGa12*(1x)O19 (Cr3 + x :SG1xO, x = 0, 0.0005, 0.001, 0.002, 0.005, 0.01, 0.02) phosphors were fabricated by a high-temperature solid-state reaction method. As demonstrated by XRD patterns of the as-prepared SGO and Cr3 + 0.02 :SG0.98O products shown in Figure 1, all the diffraction peaks are well indexed to those of the hexagonal SrGa12O19 phase (JCPDS No. 26-0983). The SrGa12O19 crystal exhibits a hexagonal structure with a space group of P63/mmc and lattice parameters of a = b = 5.7940, c = 22.8160 ;[12] it is composed of 4-, 5- and 6-fold coordinated gallium ions and 12-fold coordinated strontium ions (Figure 1 d). The doped Cr3 + ions prefer to occupy octahedral Ga3 + sites in the SGO host because of the similar ionic radii between Ga3 + (0.620 ) and Cr3 + (0.615 )[13] and the strong ligand-field stabilization energy of Cr3 + in 6-fold coordination.[14]
A long-persistent phosphor is a kind of optical material in which luminescence can last for several minutes to hours after stoppage of the excitation. In the past decade, visible persistent phosphors, including sulfides, aluminates, and silicates, have been well developed.[1] Among them, Eu2 + /Dy3 + :SrAl2O4 and Eu2 + /Nd3 + :CaAl2O4, with sufficiently strong and long-lasting (> 10 h) persistent luminescence under sunlight excitation, have already been commercialized and widely applied in various fields, such as security signs, emergency route signs, traffic signs, dials and displays, as well as in medical diagnostics.[2] In contrast, the research on near-infrared (NIR) long-persistent phosphors is far behind their visible counterparts, although there is a growing demand for such materials as taggants in night-vision surveillance[3] and as optical probes in in vivo bioimaging.[4] Recently, a persistent energy transfer (PET) mechanism has been proposed for NIR long-persistent phosphorescence in Eu2 + /Dy3 + /Er3 + :SrAl2O4[5] and Eu2 + /Dy3 + /Mn2 + :Ca0.2Zn0.9Mg0.9Si2O6 phosphors,[4a] where an NIR-emitting activator (Er3 + or Mn2 + ) is excited by the Eu2 + donors with visible long-persistent emission. However, the limited energy transfer efficiency and the inevitable visible persistent luminescence impede the further development of these phosphors. For NIR luminescence, Cr3 + is an ideal activator in hosts since its 3d3 electron configuration allows a narrow-band [a] J. Xu, Prof. D. Chen, Dr. Y. Yu, W. Zhu, J. Zhou, Prof. Y. Wang State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences Fuzhou, Fujian, 350002 (P. R. China) Fax: (+ 86) 591-83705402 E-mail: [email protected] [email protected]
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value of the local crystal-field parameter Dq is obtained from the mean peak energy of the 4A2 !4T2 transition [Eq. (1)]:[14b] Dq ¼
Eð4 A2 ! 4 T2 Þ ¼ 0:206 eV 10
ð1Þ
Moreover, based on the mean peak energies of the 4A2 !4T2 and 4A2 !4T1 transitions, the Racah parameter B can be evaluated from Equation (2): Dq 15ðx 8Þ ¼ 2 B ðx 10xÞ
ð2Þ
where the parameter x is defined as in Equation (3): x¼
ð3Þ
The parameter B is calculated according to the experimental data, and the value of the Dq/B ratio is determined to be 2.51. The intermediate value of the parameter Dq/B for Cr3 + in the SGO host is consistent with the simultaneous presence of the broad and the narrow emission bands shown in Figure 2 b. Evidently, the RT decay curve of Cr3 + 0.01 :SG0.99O phosphor monitored at 750 nm emission (Figure 2 d) exhibits a double-exponential feature: the slow decay originates from Cr3 + spin-forbidden 2E!4A2 transition and the fast decay from Cr3 + spin-allowed 4T2 !4A2 transition. Besides the intense and broad NIR photoluminescence, the Cr3 + :SGO phosphors also exhibit NIR long-persistent phosphorescence after the removal of the excitation source. Figure 3 a shows the LPP decay of Cr3 + 0.001:SG0.999O phosphor monitored at 750 nm emission after irradiation by a 300 nm ultraviolet light for 10 min. The data were recorded as a function of LPP intensity versus time and the recording time lasted for 7200 s. The LPP intensity decreases quickly in the first several seconds and then decays very slowly. After 7200 s of persistent emission, the LPP intensity is still high, indicating that the NIR LPP of the present phosphor should last longer than 7200 s. As exhibited in the inset of Figure 3 a, the optimal Cr3 + content for LPP is determined to be 0.1 mol %, which is quite different to the value (1 mol %) for the photoluminescence in the Cr3 + :SGO phosphor. Although the NIR PL can be effectively induced by a wide range of excitation wavelength (250–650 nm, Figure 2 a), the situation for the NIR LPP should be different because of the different activation mechanisms. To understand the effectiveness of different excitation wavelengths (energies) for NIR LPP, the relationship between LPP intensity and excitation wavelength was studied. As exhibited in the inset of Figure 3 b, the LPP decay curves monitored at 750 nm emission under the excitation at different wavelengths between 250 nm to 650 nm with 10 nm step for 10 min were recorded. To avoid the influence of the early
Figure 1. XRD patterns of a) SGO and b) Cr3 + 0.02 :SG0.98O products and c) standard hexagonal SrGa12O19 crystal data (JCPDS No. 26-0983). d) Unit-cell structure of SGO; red, blue, and green spheres represent Ga3 + on tetrahedral, hexahedral, and octahedral sites, respectively.
The PLE spectrum of Cr3 + 0.01:SG0.99O (Figure 2 a) exhibits four broad excitation bands with maxima at 265, 320, 425, and 600 nm. The 265 nm excitation band is ascribed to the absorption of the SGO host,[15] and the 320, 425, and 600 nm bands are attributed to the 4A2 !4T1(4P), 4A2 !4T1(4F), and 4 A2 !4T2(4F) transitions of Cr3 + , respectively. The PL spectrum of Cr3 + 0.01:SG0.99O under 425 nm excitation at room temperature (RT) shows a broadband emission in the wavelength range of 650–950 nm originating from the Cr3 + 4T2 ! 4 A2 transition (Figure 2 b, black curve). The optimal Cr3 + content is determined to be 1 mol % (inset, Figure 2 b), and the luminescent quantum yield of the Cr3 + 0.01:SG0.99O sample is determined to be 67 %. Impressively, several narrow emission bands (assigned to Cr3 + 2E!4A2 transition) are superimposed on this broad emission band, and they are dominant in the low-temperature (77 K) PL spectrum, suggesting that the Cr3 + 2E level is close to the Cr3 + 4T2(4F) level, that is, Cr3 + ions locate in an intermediate crystalfield site of the SGO host. A Tanabe–Sugano diagram (Figure 2 c) is used to describe a complete level scheme for Cr3 + in the SGO host. The
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Eð4 A2 ! 4 T1 Þ Eð4 A2 ! 4 T2 Þ ¼ 4:0 Dq
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from which TL curve corresponding to 750 nm emission (Figure 3 d) is extracted. Obviously, the TL curve consists of two broad bands with maxima at 104 and 169 8C (Figure 3 d), which correspond to the shallow and deep traps, respectively. The depths of these two traps can be estimated utilizing the general peak-shape method by Equations (4)–(7):[17] E ¼ ct kT M 2 =tbt ð2kT M Þ
ð4Þ
ct ¼ 1:51 þ 3:0ðm0:42Þ
ð5Þ
bt ¼ 1:58 þ 4:2ðm0:42Þ
ð6Þ
m ¼ ðT2 T M Þ=ðT M T 1 Þ
ð7Þ
where E is the activation energy (trap depth); m the geometrical shape factor; k the Boltzmann constant; t the low temperature half width (TMT1); ct and bt the con3+ 3+ Figure 2. a) PLE (lem = 750 nm) spectrum of Cr 0.01:SG0.99O. b) PL (lex = 425 nm) spectra of Cr 0.01:SG0.99O at stants related to the geometriRT (black curve) and 77 K (blue curve); left inset is the luminescent photograph of Cr3 + 0.01:SG0.99O under cal shape factor; T1, TM, and T2 300 nm light excitation; right inset shows the dependence of emission intensity of Cr3 + x :SG1xO on Cr3 + content. c) Tanabe–Sugano diagram for Cr3 + in SGO phosphor. d) PL decay curve of Cr3 + 0.01:SG0.99O monitored the temperature of half intensiat 750 nm emission. ty at the low-temperature side, the peak temperature, and the temperature of half intensity at fast decay on the analysis of LPP, the persistent emission inthe high-temperature side, respectively. For the shallow trap, tensities (I50s) recorded 50 s after irradiation ceased were T1 = 80 8C, T2 = 124 8C; for the deep trap, T1 = 142 8C, T2 = used as the references. Figure 3 b shows the persistent inten195 8C. As a result, the depths of the shallow and deep traps sity I50s of the Cr3 + 0.001:SG0.999O phosphor as a function of in SGO are evaluated to be 0.73 and 0.96 eV, respectively. Furthermore, it is found that the SGO host shows a selfthe excitation wavelength. The PL excitation spectrum is activated broadband luminescence in the range of 450– also presented in Figure 3 b for comparison. Evidently, the 650 nm with a maximum emission at 502 nm under 266 nm NIR persistent luminescence can be effectively achieved by excitation (Figure 4 a), which likely arises from electron– UV (250–400 nm) illumination, that is, by exciting electrons hole recombination between the defect states in SGO. After into the conduction band of SGO as well as the 4T1(4P) exciexcitation stops, the SGO host luminescence lasts for a long tation band of Cr3 + . time, that is, SGO exhibits visible long-persistent phosphorDefect centers play an essential role for LPP because escence. Interestingly, after doping Cr3 + into SGO, the host they can capture free carriers and then immobilize them for an appropriately long period of time. Notably, after UV irraemission almost disappears, and the LPP time is greatly diation, the body color of the Cr3 + 0.001:SG0.999O phosphor shortened (Figure 4 b). Considering the large spectral overlap between the host emission and the Cr3 + absorption origchanges from green to pink, and the UV-irradiation-induced coloration can be quickly bleached by heating at 400 8C. inating from 4A2 !4T1(4F) and 4A2 !4T2(4F) transitions (FigThis result indicates the formation of photochromic centers ure 4 a), this result indicates the existence of an effective probably induced by the trapping of photogenerated elecpersistent energy transfer from the host to the Cr3 + ions. trons by intrinsic defects (such as Schottky disorder, oxygen Based on the above results and discussions, we propose vacancy, and antisite defects)[7a, 16] in Cr3 + 0.001:SG0.999O. In a mechanism to account for the NIR LPP in the Cr3 + :SGO order to investigate the traps in SGO, the three-dimensional phosphors, as schematically demonstrated in Figure 5. Upon thermoluminescence (TL) spectrum of Cr3 + 0.001:SG0.999O was short-wavelength UV excitation (250–290 nm), the incident photons are absorbed by the SGO host, and the electrons recorded after UV (365 nm) radiation for 5 min (Figure 3 c),
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Figure 3. a) LPP decay curve of Cr3 + 0.001:SG0.999O phosphor monitored at 750 nm emission after irradiation at 300 nm for 10 min; inset shows the Cr3 + dependence of LPP decay for a series of Cr3 + x :SG1xO samples (x = 0.0005, 0.001, 0.002, 0.005, 0.01, 0.02). b) Phosphorescence intensity after 50 s (I50s) monitored at 750 nm emission as a function of excitation wavelength for the Cr3 + 0.001:SG0.999O phosphor (spheres). For comparison, the PL excitation spectrum of the Cr3 + 0.001:SG0.999O phosphor monitored at 750 nm emission (dotted line) is also provided. The inset shows the LPP decay curves of the Cr3 + 0.001:SG0.999O phosphor irradiated by 250–400 nm light for 10 min. c) Three-dimensional thermoluminescence spectrum of the Cr3 + 0.001:SG0.999O phosphor. d) Thermoluminescence curve of Cr3 + 0.001:SG0.999O corresponding to 750 nm emission after UV excitation for 10 min.
are promoted from the valence band to the conduction band of SGO. Subsequently, the excited free electrons are captured by the defects via nonradiative relaxation (process 1). After the irradiation is stopped, the recombination of electrons and holes released from the defect levels results in a broadband visible persistent luminescence (process 2). Because of the large spectral overlap between the host emission and the two absorption bands of Cr3 + , the energy of the host is slowly but persistently transferred to the Cr3 + ions via an effective nonradiative energy transfer (process 3), leading to the NIR LPP of Cr3 + .[4c] On the other hand, when long-wavelength UV light (290–400 nm) is used to irradiate the sample, the ground-state electrons of the Cr3 + ions are promoted to the 4T1(4P) level localized near the conduction band of the SGO host.[3, 4b] The excited electrons are subsequently captured by trap 1 and trap 2 through the conduction band (process 4). After a sufficient irradiation time, all of the traps are filled (with the deep trap being filled mainly by nonradiative relaxation from the shallow one). In the initial stage after irradiation stops, direct recombination between the conduction electrons released from the shallow trap (trap 1) and the Cr3 + ions (process 5) dominates the persistent luminescence and produces an intense
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NIR emission. After the depletion of the shallow trap, the electrons in the deep trap are gradually released to fill the shallow trap (process 6). Subsequently, process 5 and process 6 occur cyclically, finally resulting in the long-persistent phosphorescence of Cr3 + :SGO (process 7). In summary, we have fabricated a NIR long-persistent Cr3 + :SrGa12O19 phosphor that emits for more than 2 h. The doped Cr3 + ions prefer to occupy the octahedral sites of Ga3 + with intermediate crystalfield strength. The NIR LPP can be effectively realized by UV irradiation (250–400 nm) but is hardly achieved by visible-light irradiation (400– 650 nm). The realization of LPP in Cr3 + :SrGa12O19 is believed to arise from persistent energy transfer from the SrGa12O19 host to the Cr3 + ions and the filling and release of electrons into and from the shallow and deep traps, assisted by the conduction band.
Experimental Section Material preparation The Cr3 + x :SrGa12*(1x)O19 (Cr3 + x :SG1xO, x = 0, 0.0005, 0.001, 0.002, 0.005, 0.01, 0.02) phosphors were fabricated by a high-temperature solid-state reaction method. Stoichiometric amounts of SrCO3, Ga2O3, and Cr2O3 powders were mixed, ground, and pre-fired at 800 8C for 2 h in air. After the pre-fired material was finely ground again, the powder sample was sintered at 1350 8C for another 2 h in air. Characterization X-ray diffraction (XRD) patterns were recorded on a MiniFlex II powder diffractometer using Cu Ka radiation (l = 0.154 nm) at 30 kV and 15 mA. Photoluminescence excitation and photoluminescence spectra, luminescence decay curves, and long-persistent phosphorescence decay curves were measured on an Edinburgh Instruments FLS920 spectrofluorometer equipped with a standard R928P photomultiplier tube (250– 850 nm) and a liquid nitrogen-cooled R5509 near-infrared photomultiplier tube (800–1700 nm). Low-temperature PL and PLE spectra were measured using samples cooled in liquid nitrogen (77 K). Three-dimensional thermoluminescence spectra were recorded using a RISFDA-15B/C thermoluminescence/photoluminescence spectrometer. Immediately after 10 min exposure of the sample to a mercury lamp with maximum emission at 365 nm (Gp3Hg-2, Fei Ying Light Electrical Appliance Factory), thermoluminescence signals were recorded in the temperature range of 273–523 K and the heating rate was fixed at 5 K s1.
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h¼
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Lsample number of photons emitted ¼ number of photons absorbed Ereference Esample
ð8Þ
where h is the QY, Lsample the emission intensity, Ereference and Esample the intensities of the excitation light not absorbed by the reference and the sample, respectively. The difference in integrated areas between the sample and the reference represents the number of the absorbed photons. The photons emitted were determined by integrating the area of the emission band. The error associated with the QY measurement is 3 %.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51172231, 21271170, 11204301, 51202244, and 11304312), the Natural Science Foundation of Fujian for Distinguished Young Scholars (2012J06014) and the Major Sci & Tech Project of Fujian (2011HZ00012). The authors thank Prof. Jing Wang and Dr. Ye Li of Sun Yat-Sen University for the measurements of three-dimensional thermoluminescence spectra.
Keywords: doping · luminescence · near-infrared emission · solid-state structure
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Figure 4. a) PLE and PL spectra of the undoped SGO and Cr3 + 0.001 :SG0.999O samples; the inset shows the magnified PL spectra in the wavelength range 470–580 nm. b) LPP decay curve of the undoped SGO sample monitored at 502 nm emission; the inset shows LPP decay curves of the undoped SGO and Cr3 + 0.001:SG0.999O samples recorded in the initial 600 s (under 266 nm UV light excitation for 10 min and monitored at 502 nm emission).
Figure 5. Schematic illustration of the NIR LPP mechanism of Cr3 + :SGO; the straight-line and dotted-line arrows represent optical transition and electron transfer processes, respectively, and the wavy arrows represent nonradiative relaxation. Quantum yield (QY) is defined as the ratio of emitted photons to absorbed photons and was measured by the FLS920 spectrofluorometer equipped with an integrating sphere. All spectroscopic data were corrected for the spectral responses of both the spectrofluorometer and the integrating sphere. The responses of the detecting systems (integrating sphere, monochromators and detectors) in photon flux were determined using a calibrated tungsten lamp. Based on this setup, internal QY is calculated by Equation (8):[18]
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