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Luminescence properties of a new green emitting long afterglow phosphor Ca14Zn6Ga10O35:Mn2+,Ge4+
Journal of Luminescence 206 (2019) 234–239
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Luminescence properties of a new green emitting long afterglow phosphor Ca14Zn6Ga10O35:Mn2+,Ge4+
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Bin Jianga, Fengfeng Chia, Lu Zhaoa, Xiantao Weib, Yonghu Chena, , Min Yina a
Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province 230026, PR China b Physics Experiment Teaching Center, School of Physical Sciences, University of Science and Technology of China, Hefei 230026, PR China
A B S T R A C T
A series of green emitting persistent luminescence phosphors Ca14Zn6Ga10O35:Mn2+,Ge4+ have been synthesized via a solid state reaction method at 1230 °C. The Xray diffraction spectra verified that all samples have a pure phase though the diffraction peaks exhibit a slight shift when the dopants added in. The green emission band peaking at 520 nm from Mn2+ was observed in the luminescence spectra of all the compounds with Mn2+ doping. The green afterglow can be perceived by naked eye in the dark environment for several minutes. With the Ge4+ co-doped, the quadrivalent Mn4+ existing in the Mn singly doped sample can be more sufficiently reduced to Mn2+ as indicated by the substantially subdued red emission from Mn4+ in the co-doped samples. From the photoluminescence spectra, 0.3% Mn2+ was determined to be the optimal Mn concentration for strongest emission. The decay curves of the Mn2+ emission from samples with different doping concentration were also recorded. It is found that the 0.5% Mn2+ sample yields the best afterglow performance in accordance with its stronger thermoluminescence intensity. The similar thermoluminescence curves of all samples suggests the similar distribution of traps occurring in the same host. Based on the experimental results, a model is proposed to discuss the persistent luminescence processes of the samples.
1. Introduction Luminescence from long afterglow phosphors, also called long persistent luminescence phosphors, can lasting for several minutes or even hours at room temperature after ceasing the ultraviolet (UV), visible or near infrared excitation [1–3]. Owing to this peculiarity, long afterglow phosphors have found wide applications in safety signage, decorations, optical memory, detection of high energy rays, night vision surveillance, drug carriers, and in vivo bio-imaging, and considerable research efforts have been invested in the development of new persistent phosphors in recent years [4–7]. Up to the present, many persistent phosphors with different emission colors have been developed, such as the green afterglow materials SrAl2O4:Eu2+,Dy3+ [8] and MgAl2O4:Mn2+ [9]; the blue emitting CaAl2O4:Eu2+,Nd3+ [10] and SrMgSi2O6:Eu2+,Dy3+ [11]; the red and near infrared materials Y2O2S:Eu3+,Mg2+,Ti4+ [12], CaS:Eu2+,Tm3+,Ce3+ [13], ZnGa2O4:Cr3+ [14], and Li5GaO5:Cr3+ [15]. Among them, the commercialized SrAl2O4:Eu2+,Dy3+ and CaAl2O4:Eu2+,Nd3+ phosphors have already been widely used in daily life due to their more than 20 h afterglow. Compared to the more successful rare earth ions (especially Eu2+) activated long afterglow phosphors, less well developed transition metal based persistent luminescent materials have drawn more and more attentions, particularly for Mn2+ (Mn4+) and Cr3+, because of their new color possibilities and different afterglow properties which ⁎
open new fields of application. In continuation of this effort, we will explore the afterglow property of a new Mn2+ based green phosphor in this work, and try to extend the database of the persistent luminescence materials. Mn2+ doped phosphors exhibit a broad emission band from green to red due to its parity forbidden d–d transition from the first excited state of 4T1(G) to the ground state of 6A1(S) which is strongly affected by the crystal field [16,17]. Generally, Mn2+ doped phosphors can produce green emission when Mn2+ found itself in the weak crystal field of a four-coordinated tetrahedral symmetry and yield red emission when Mn2+ is in strong crystal field of a six-coordinated octahedral symmetry [18,19]. Recently, many Mn2+ based persistent luminescence phosphors have been discovered, such as Zn2SiO4:Mn2+,Yb3+ [20] reported by Zou et al. which achieves a more than 30 h red afterglow before it drops below 0.32 mcd/m2. Other Mn2+ based afterglow phosphors previously reported include Li2(Zn,Mg)GeO4:Mn2+ [21,22], ZnGa2O4:Mn2+ [23], Zn2GeO4:Mn2+ [24], Li1.14Zn1.43SiO4:Mn2+ [25], LiGaSiO4:Mn2+ [26], NaCa2GeO4F:Mn2+,Yb3+ [27] and so on [28,29]. In the present work, cubic Ca14Zn6Ga10O35 (CZGO) has been chosen as host for Mn2+ doping because red long afterglow has been observed in the same host with Cr3+ doping [30], implying there are suitable traps in the host responsible for the persistent luminescence. On the other hand, Mn4+ doped Ca14Zn6Ga10O35 phosphor was reported to present a deep red emission which can be applied in white
Corresponding author. E-mail address: [email protected] (Y. Chen).
https://doi.org/10.1016/j.jlumin.2018.10.048 Received 14 June 2018; Received in revised form 7 September 2018; Accepted 8 October 2018 Available online 16 October 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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LED [31,32]. In our work, an extraordinary phenomenon was observed where the oxidation of Mn2+ into Mn4+ in the synthesis process can be efficiently controlled with Ge4+ co-doping. Green emission from Mn2+ as well as green long afterglow were procured in CZGO:Mn2+,Ge4+ with UV excitation. The photoluminescence (PL) spectra indicate that the Mn2+ occupied the tetrahedral site favorable for the green emission of Mn2+. Meanwhile, an relatively weak blue afterglow originated from the CZGO host was also observed. Concentration dependence of the luminescence and afterglow was also investigated. Finally, a mechanism of persistent luminescence was suggested to clarify the emission and afterglow processes of the samples. 2. Experimental The Ca14Zn6Ga10O35:Mn2+,Ge4+ powders were synthesized by solid state reaction method. The starting materials CaCO3(99.99%), ZnO(99.99%), Ga2O3(99.999%), MnCO3(99.99%), and GeO2(99.999%) were weighted with stoichiometric molar ratio and then mixed thoroughly and fully ground. The homogeneous mixtures were put into a corundum crucible and calcined at 1230 °C for 4 h in an air atmosphere. Then the yellow-tinted as-grown products were annealed in N2/H2 (95%/5%) environment at 900 °C for 3 h to reduce the Mn4+ to Mn2+. After cooling down to room temperature, the obtained white samples were ground thoroughly for subsequent measurements. The crystal structures of the obtained samples were analyzed by a MXPAHF rotating anode X-ray diffractometer (Cu/Kα radiation) with a scanning step of 0.02° in the 2θ range from 10° to 70°. Both the photoluminescence emission (PL) and excitation (PLE) spectra of each sample were recorded by a HITACHI 850 fluorescence spectrometer, which utilized a 150 W Xe lamp as its excitation source. The long afterglow decay curves of all samples were measured by a HITACHI 850 fluorescence spectrometer. The persistent luminescence spectra were collected by using a charge coupled device (CCD) (Andor DU401ABVF). A 253.7 nm ultraviolet lamp was used as excitation source for measuring the long afterglow decay curves and persistent luminescence spectra of all samples. An FJ27A1 TL dosimeter was used to measure TL curves with a heating rate of 2 K/s in the range from 50 °C to 400 °C after the samples were exposed to radiation from a UV lamp for 2 min and then delayed in a dark room for 30 s.
Fig. 1. (a) XRD patterns of all obtained samples. (b) The as-synthesized CZGO host patterns and the standard JCPDS card (580031).
3. Results and discussion 3.1. Structure properties The XRD patterns shown in Fig. 1a reveal that all as-synthesized samples have a same kind of phase. All diffraction peaks are in good agreement with the standard card (JCPDS#580031) of the CZGO shown in 1a, though a slight shift of the diffraction peaks can be discerned in the doped samples. It means that all the samples synthesized at 1230 °C have a single phase, and Mn2+ doping does not change the phase of the CZGO host except a slight influence on the bond length of the crystal. The effective ion radius of Mn2+ with coordination number (CN) equal to 4 is 0.66 Å and the effective ion radius of Zn2+ with CN = 4 is 0.60 Å and of Ga3+ with CN = 4 is 0.47 Å [33]. Therefore the larger doping Mn2+ ions can lead to small crystal expansion. The Fig. 1b demonstrated the shift of the diffraction peaks distinctly. With the doping concentration raised, the peaks shift to lower 2θ values until the Mn2+ concentration arrived at 0.5%. When the concentration is larger than 0.5%, the peaks shift to greater 2θ values. It could be accounted by the Mn2+ occupying the Ca2+ site which has a larger ions radius (1.00 Å with CN = 6). The crystal structure of CZGO is shown in Fig. 2. The CZGO host belongs to the cubic crystal system with the space group F23 numbered 196. Zn2+ have five different sites in this host. Four of them are surrounded by four nearest O2- constituting a [ZnO4] tetrahedra of which the Mn2+ occupation will bring about green emission, and the fifth
Fig. 2. The crystal structure of CZGO host (a). The five Zn2+ site and four Ga3+ site was shown in (b) to (f).
Zn2+ consists a [ZnO6] octahedra with six nearest O2- surrounded, in which the Mn2+ can emit red emission. Among the four tetrahedra of [ZnO4], only one has a regular form, and all the others are distorted. The last four Zn2+ sites shown in Fig. 2c to f are mixed with possible Ga3+ occupation consisting an anti-sites unfortunately. The anti-sites mixing the Zn and Ga result in the difficulty of distinguishing which element Mn2+ actually substitutes. Namely, the doping Mn2+ replacing the Zn2+ is not an ensure thing though they have same chemical 235
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Fig. 3. PL spectra of CZGO:Mn2+ samples. (a) The normalized PL spectra of Mn single doped and Ge4+ co-doped samples (λex = 300 nm). (b) The normalized spectra of CZGO:0.5%Mn2+,5%Ge4+ under 250 nm (black line) and 300 nm (red line) excitation, and the normalized spectra of CZGO host (blue line) excited by 250 nm. (c) The concentration depended PL spectra of CZGO:x% Mn2+,5%Ge4+ (x = 0.1, 0.3, 0.5, 0.7) excited by 300 nm. (d) Variations of PL intensities of CZGO:x%Mn2+,5%Ge4+ with the Mn2+ concentrations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
valence. The anti-site defects also can play the role of carrier traps as reported by Bessière cited in reference [34], thus significantly influencing the afterglow of Mn2+. 3.2. Luminescent properties Upon 250 nm excitation, the CZGO host shows a wide emission band from 350 nm to 600 nm which overlaps with the Mn2+ emission and accounts for the broader spectral shape of Mn2+ doped sample under 250 nm excitation than that excited by 300 nm (as shown in Fig. 3b). The host emission is ascribed to the recombination of the holes and electrons captured in host vacancies. With the Mn2+ doped, a strong green emission band from 475 nm to 625 nm peaked at 520 nm emerged upon 250 nm excitation. Unexpectedly, not all doping Mn exist as divalent Mn2+ because of oxidation of divalent Mn2+ into quadrivalent Mn4+ in the synthesis process at air atmosphere and insufficient reduction in the annealing treatment, as clearly evidenced in the 250 nm excited emission spectra of Mn singly doped sample shown in Fig. 3a (black curve), where a longer wavelength band from 650 nm to 750 with five peaks ascribing to the transition 2Eg→2Ag of Mn4+ is distinctively alongside the green emission band of Mn2+. However, the weak host emissions around 470 nm can still be discerned in spite of the two different dopants Mn2+ and Mn4+, resulting in the shoulder structure on shorter wavelength side of the 520 nm green emission band of Mn2+. In the emission spectra of the Mn2+ and Ge4+ co-doped sample (Fig. 3a, red curve), the longer wavelength Mn4+ emission is substantially reduced, indicating an interesting effect of Ge4+ codoping on the reduction of Mn, perhaps related to the charge compensation function of Ge4+ for Mn2+ substitution of Ga3+. On the other hand, more cation vacancies due to the Ge4+ doping will probably prevent the oxidation of divalent Mn ions. When exposed to 300 nm excitation, the co-doped samples yield a strong green emission peaked at 520 nm from Mn2+. The normalized emission spectra shows that the emission band of host was weakened with the Ge4+ joined (Fig. 3b). The green emission band of Mn2+ is attributed to the spin forbidden d–d transition 4T1→6A1 of Mn2+ ions in tetrahedral coordination. When monitoring 520 nm emission, step-like shape of the excitation spectra were observed as displayed in Fig. 4. The stronger shorter wavelength band with wavelength < 250 nm can be attributed the host transition, implying an efficient energy transfer from the host to the Mn2+ ions. The weaker excitation band around 290 nm can be ascribed to the charge transfer band (CTB) of Mn2+—O2-. The inset image in Fig. 4 shows that the intensity of the absorption around 300 nm is correlated with the green emission intensities (Fig. 3c) for samples with different Mn2+ concentration. Fig. 3d depicts the variation of emission intensity with the Mn2+ concentration in the range from 0.1% to 0.7%. The intensity increases with Mn2+ concentration until 0.3% and then decrease dramatically, indicating the concentration quenching of Mn2+ occurred around 0.3% in CZGO. Green afterglow of all samples can be observed by the naked eye after irradiation of the UV lamp. Meanwhile there is a pallid and short blue afterglow originated from the CZGO host. The afterglow spectra of all samples were recorded by CCD at 2 min after irradiation by 253.7 nm UV lamp for 2 min and shown in Fig. 5. All the samples exhibited similar afterglow curves consisting of two bands, one is a weaker band nearby 450 nm related to the host emission and the other 236
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Fig. 4. Excitation spectra of CZGO:x%Mn2+,5%Ge4+ (x = 0.1, 0.3, 0.5, 0.7) monitoring at 520 nm. The inset shows the enlarged spectra in the wavelength range from 250 nm to 300 nm.
Fig. 6. The afterglow decay curves of all obtained samples. (a) The afterglow decay curves of single doped Mn2+ (not reduced completely) and Mn2+/Ge4+ co-doped samples. (b) The afterglow decay curves of CZGO:xMn2+,5%Ge4+, x = 0.1%, 0.3%, 0.5%, 0.7%. The inset is plotted with vertical log-scale.
Fig. 5. Afterglow spectra of all synthesized samples after irradiation by 254 nm UV lamp for 2 min. (a) The concentration depended afterglow spectra of Mn2+/ Ge4+ co-doped samples with Mn2+ concentration of 0.1%, 0.3%, 0.5%, 0.7%. (b) The spectra of Mn2+ single doped (not reduced completely) and Mn2+/ Ge4+ co-doped samples.
doped samples. The initial relative intensities of four samples increase in the order of Mn2+ concentration of 0.1% < 0.7% < 0.3% < 0.5%. Apparently, the decay curves consist of a rapid attenuation process at first and then a slow decay process. The inset figure of Fig. 6b discloses that the slow component is more prominent in the 0.3% and 0.7% samples than that of the 0.1% and 0.5% samples, indicating a longer duration could be achieved in the 0.3% and 0.7% samples.
is a stronger band located around 520 nm ascribed to the Mn2+. In comparison with the Mn single doped sample, the co-doped samples produced a much weaker host afterglow and a relatively broader Mn2+ afterglow. The Ge4+ co-doping influence the emission band of Mn2+ as well as the afterglow. It is noticed that the afterglow spectra of all samples have a broader emission band of Mn2+ compared with the photoluminescence spectra. Interestingly, the emission of host shown in afterglow spectra had a stronger relative intensity. The reason may be that the energy stored in defects can release to the host vacancies more effectively after stop the excitation. The Fig. 5a depicts the concentration dependence of the afterglow spectra of the co-doped samples. The sample with the 0.5% Mn2+ concentration shows the strongest afterglow intensity. Fig. 6 presents the afterglow decay curves of the obtained samples after irradiation by a 253.7 nm UV lamp for 5 min. The data were recorded in the time range of 0–10 min with 5 s delay after ceasing the excitation. From the Fig. 6, the afterglow of Mn2+ still can be recorded after the removal of irradiation for 10 min. The Fig. 6a shows comparison of the decay curves of CZGO:0.5% Mn2+ (in which Mn could not reduced completely) and that of CZGO:0.5% Mn2+,5% Ge4+. Evidently, the latter exhibited a better afterglow properties. This can be accounted by the lower actual concentration of Mn2+ in the former. The Fig. 6b depicts the concentration depended decay curves of the co-
3.3. Thermoluminescence properties Thermoluminescence (TL) method is an appropriate tool to investigate the depth, types and density of traps which are most important factors bearing on the afterglow performance [35,36]. Therefore, TL curves of the obtained samples are recorded in Fig. 7 to provide relevant information about the traps existing in the CZGO host. Generally speaking, shallow traps are easily emptied by thermal energy, whereas deep traps can store captured charge carriers for a longer time at room temperature. The TL curves of all samples are dominated by one broad irregular band with peak intensity around 121 °C. When the temperature arrived at 350 °C, the intensity is closed to zero. From Fig. 7, a concentration depended TL curves are displayed. The 0.5% Mn2+ sample shows the strongest TL intensity in accordance with the trend observed in the afterglow spectra intensity (see Fig. 5). But different from Fig. 5, the 0.7% Mn2+ sample gives the weakest TL intensity. The reason may be found in the TL measurements processes. The TL measurements start only after waiting 30 s at room temperature and keeping at 45 °C for 10 s due to the time needed for the transfer of 237
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host prompts electrons in the conduction band and holes in the valence band which will be captured by the electron traps and hole traps respectively; the other is for the 300 nm excitation, where the Mn2+ is excited into higher energy states and transfer electrons into the traps via conduction band. With the phonon assistance, the carriers released from the traps via conduction band and valence band or due to tunneling effect, and recombine at Mn2+ sites, issuing the green afterglow. 4. Conclusions
Fig. 7. The concentration depended thermoluminescence CZGO:xMn2+,5%Ge4+, x = 0.1%, 0.3%, 0.5%, 0.7%.
curves
The CZGO:Mn2+ and CZGO:Mn2+,Ge4+ synthesized at 1230 °C show a green emission peaked at 520 nm with the 254 nm irradiation. The single doped sample not only shows the Mn2+ emission but also Mn4+ emission. With the Ge4+ co-doped, the Mn4+ reduced to Mn2+ more sufficiently. The single doped and co-doped samples show a similar green emission band located at 520 nm related to the Mn2+ emission. With the concentration of Mn2+ enhanced, the Mn2+ emission strengthen and then attenuate. The CZGO:0.3%Mn2+,Ge4+ sample shows the strongest emission and the CZGO:0.5%Mn2+,Ge4+ sample offers the strongest afterglow intensity. All the samples show similar afterglow luminescence consisting of the dominating green Mn2+ emission and a much weaker blue host emission. The CZGO host provides plentiful traps as revealed by TL curves. The 0.5% Mn2+ samples show the best TL intensity. The difference of the TL curves of the different samples can be accounted by the concentration influence on the carriers release efficiency. A novel green afterglow material CZGO:Mn2+,Ge4+ is corroborated and can be submitted to optimization and development in further studies.
of
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11574298 and 61635012), the National Key Research and Development Program of China (Grant No. 2016YFB0701001). References Fig. 8. The schematic diagram of the afterglow mechanism in CZGO:Mn2+.
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the samples to the TL equipment. The energy stored at shallow traps will be released effectively in these delay times, much more so for the higher concentration sample. Interestingly, there is also a hump from 90 °C to 159 °C in the TL curves of the 0.5% and 0.3% samples, suggesting a suddenly increased rate of energy release when heated to 90 °C. The 0.1% sample show the broadest band among the four samples, which can be explained the tardy release in the low concentration sample. With a fast heating rate of 2 k/s, part of the energy released from shallow traps in the 0.1% sample will incorrectly registered at higher temperatures. The TL results indicate that the CZGO compounds have a wide distribution of traps favorable for the long duration of the afterglow emission. From the crystal structure, there exists many antisite defects between the Zn and Ga which can be regard as an effective trap [34]. The carriers stored in anti-sites can be transfer into the near Mn2+ sites and result in the afterglow. At the same time, the Ge4+ dopant can cause abundant cation vacancies because of its excess charge valences. The cation vacancies usually play the roles of traps. Those cation vacancies also can capture the carriers and generate the persistent luminescence. Due to those different traps, the TL curves showed a wide temperature range. Fig. 8 presents a mechanism diagram for the luminescence and afterglow processes of CZGO:Mn2+. The locations of the energy levels shown in pictures are schematically and not to scale accurately. The diagram illustrates two processes of the energy storage: one is for the 253.7 nm UV lamp excitation, where the inter band transition of the
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Luminescence properties of a new green emitting long afterglow phosphor Ca14Zn6Ga10O35:Mn2+,Ge4+
T
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Bin Jianga, Fengfeng Chia, Lu Zhaoa, Xiantao Weib, Yonghu Chena, , Min Yina a
Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province 230026, PR China b Physics Experiment Teaching Center, School of Physical Sciences, University of Science and Technology of China, Hefei 230026, PR China
A B S T R A C T
A series of green emitting persistent luminescence phosphors Ca14Zn6Ga10O35:Mn2+,Ge4+ have been synthesized via a solid state reaction method at 1230 °C. The Xray diffraction spectra verified that all samples have a pure phase though the diffraction peaks exhibit a slight shift when the dopants added in. The green emission band peaking at 520 nm from Mn2+ was observed in the luminescence spectra of all the compounds with Mn2+ doping. The green afterglow can be perceived by naked eye in the dark environment for several minutes. With the Ge4+ co-doped, the quadrivalent Mn4+ existing in the Mn singly doped sample can be more sufficiently reduced to Mn2+ as indicated by the substantially subdued red emission from Mn4+ in the co-doped samples. From the photoluminescence spectra, 0.3% Mn2+ was determined to be the optimal Mn concentration for strongest emission. The decay curves of the Mn2+ emission from samples with different doping concentration were also recorded. It is found that the 0.5% Mn2+ sample yields the best afterglow performance in accordance with its stronger thermoluminescence intensity. The similar thermoluminescence curves of all samples suggests the similar distribution of traps occurring in the same host. Based on the experimental results, a model is proposed to discuss the persistent luminescence processes of the samples.
1. Introduction Luminescence from long afterglow phosphors, also called long persistent luminescence phosphors, can lasting for several minutes or even hours at room temperature after ceasing the ultraviolet (UV), visible or near infrared excitation [1–3]. Owing to this peculiarity, long afterglow phosphors have found wide applications in safety signage, decorations, optical memory, detection of high energy rays, night vision surveillance, drug carriers, and in vivo bio-imaging, and considerable research efforts have been invested in the development of new persistent phosphors in recent years [4–7]. Up to the present, many persistent phosphors with different emission colors have been developed, such as the green afterglow materials SrAl2O4:Eu2+,Dy3+ [8] and MgAl2O4:Mn2+ [9]; the blue emitting CaAl2O4:Eu2+,Nd3+ [10] and SrMgSi2O6:Eu2+,Dy3+ [11]; the red and near infrared materials Y2O2S:Eu3+,Mg2+,Ti4+ [12], CaS:Eu2+,Tm3+,Ce3+ [13], ZnGa2O4:Cr3+ [14], and Li5GaO5:Cr3+ [15]. Among them, the commercialized SrAl2O4:Eu2+,Dy3+ and CaAl2O4:Eu2+,Nd3+ phosphors have already been widely used in daily life due to their more than 20 h afterglow. Compared to the more successful rare earth ions (especially Eu2+) activated long afterglow phosphors, less well developed transition metal based persistent luminescent materials have drawn more and more attentions, particularly for Mn2+ (Mn4+) and Cr3+, because of their new color possibilities and different afterglow properties which ⁎
open new fields of application. In continuation of this effort, we will explore the afterglow property of a new Mn2+ based green phosphor in this work, and try to extend the database of the persistent luminescence materials. Mn2+ doped phosphors exhibit a broad emission band from green to red due to its parity forbidden d–d transition from the first excited state of 4T1(G) to the ground state of 6A1(S) which is strongly affected by the crystal field [16,17]. Generally, Mn2+ doped phosphors can produce green emission when Mn2+ found itself in the weak crystal field of a four-coordinated tetrahedral symmetry and yield red emission when Mn2+ is in strong crystal field of a six-coordinated octahedral symmetry [18,19]. Recently, many Mn2+ based persistent luminescence phosphors have been discovered, such as Zn2SiO4:Mn2+,Yb3+ [20] reported by Zou et al. which achieves a more than 30 h red afterglow before it drops below 0.32 mcd/m2. Other Mn2+ based afterglow phosphors previously reported include Li2(Zn,Mg)GeO4:Mn2+ [21,22], ZnGa2O4:Mn2+ [23], Zn2GeO4:Mn2+ [24], Li1.14Zn1.43SiO4:Mn2+ [25], LiGaSiO4:Mn2+ [26], NaCa2GeO4F:Mn2+,Yb3+ [27] and so on [28,29]. In the present work, cubic Ca14Zn6Ga10O35 (CZGO) has been chosen as host for Mn2+ doping because red long afterglow has been observed in the same host with Cr3+ doping [30], implying there are suitable traps in the host responsible for the persistent luminescence. On the other hand, Mn4+ doped Ca14Zn6Ga10O35 phosphor was reported to present a deep red emission which can be applied in white
Corresponding author. E-mail address: [email protected] (Y. Chen).
https://doi.org/10.1016/j.jlumin.2018.10.048 Received 14 June 2018; Received in revised form 7 September 2018; Accepted 8 October 2018 Available online 16 October 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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LED [31,32]. In our work, an extraordinary phenomenon was observed where the oxidation of Mn2+ into Mn4+ in the synthesis process can be efficiently controlled with Ge4+ co-doping. Green emission from Mn2+ as well as green long afterglow were procured in CZGO:Mn2+,Ge4+ with UV excitation. The photoluminescence (PL) spectra indicate that the Mn2+ occupied the tetrahedral site favorable for the green emission of Mn2+. Meanwhile, an relatively weak blue afterglow originated from the CZGO host was also observed. Concentration dependence of the luminescence and afterglow was also investigated. Finally, a mechanism of persistent luminescence was suggested to clarify the emission and afterglow processes of the samples. 2. Experimental The Ca14Zn6Ga10O35:Mn2+,Ge4+ powders were synthesized by solid state reaction method. The starting materials CaCO3(99.99%), ZnO(99.99%), Ga2O3(99.999%), MnCO3(99.99%), and GeO2(99.999%) were weighted with stoichiometric molar ratio and then mixed thoroughly and fully ground. The homogeneous mixtures were put into a corundum crucible and calcined at 1230 °C for 4 h in an air atmosphere. Then the yellow-tinted as-grown products were annealed in N2/H2 (95%/5%) environment at 900 °C for 3 h to reduce the Mn4+ to Mn2+. After cooling down to room temperature, the obtained white samples were ground thoroughly for subsequent measurements. The crystal structures of the obtained samples were analyzed by a MXPAHF rotating anode X-ray diffractometer (Cu/Kα radiation) with a scanning step of 0.02° in the 2θ range from 10° to 70°. Both the photoluminescence emission (PL) and excitation (PLE) spectra of each sample were recorded by a HITACHI 850 fluorescence spectrometer, which utilized a 150 W Xe lamp as its excitation source. The long afterglow decay curves of all samples were measured by a HITACHI 850 fluorescence spectrometer. The persistent luminescence spectra were collected by using a charge coupled device (CCD) (Andor DU401ABVF). A 253.7 nm ultraviolet lamp was used as excitation source for measuring the long afterglow decay curves and persistent luminescence spectra of all samples. An FJ27A1 TL dosimeter was used to measure TL curves with a heating rate of 2 K/s in the range from 50 °C to 400 °C after the samples were exposed to radiation from a UV lamp for 2 min and then delayed in a dark room for 30 s.
Fig. 1. (a) XRD patterns of all obtained samples. (b) The as-synthesized CZGO host patterns and the standard JCPDS card (580031).
3. Results and discussion 3.1. Structure properties The XRD patterns shown in Fig. 1a reveal that all as-synthesized samples have a same kind of phase. All diffraction peaks are in good agreement with the standard card (JCPDS#580031) of the CZGO shown in 1a, though a slight shift of the diffraction peaks can be discerned in the doped samples. It means that all the samples synthesized at 1230 °C have a single phase, and Mn2+ doping does not change the phase of the CZGO host except a slight influence on the bond length of the crystal. The effective ion radius of Mn2+ with coordination number (CN) equal to 4 is 0.66 Å and the effective ion radius of Zn2+ with CN = 4 is 0.60 Å and of Ga3+ with CN = 4 is 0.47 Å [33]. Therefore the larger doping Mn2+ ions can lead to small crystal expansion. The Fig. 1b demonstrated the shift of the diffraction peaks distinctly. With the doping concentration raised, the peaks shift to lower 2θ values until the Mn2+ concentration arrived at 0.5%. When the concentration is larger than 0.5%, the peaks shift to greater 2θ values. It could be accounted by the Mn2+ occupying the Ca2+ site which has a larger ions radius (1.00 Å with CN = 6). The crystal structure of CZGO is shown in Fig. 2. The CZGO host belongs to the cubic crystal system with the space group F23 numbered 196. Zn2+ have five different sites in this host. Four of them are surrounded by four nearest O2- constituting a [ZnO4] tetrahedra of which the Mn2+ occupation will bring about green emission, and the fifth
Fig. 2. The crystal structure of CZGO host (a). The five Zn2+ site and four Ga3+ site was shown in (b) to (f).
Zn2+ consists a [ZnO6] octahedra with six nearest O2- surrounded, in which the Mn2+ can emit red emission. Among the four tetrahedra of [ZnO4], only one has a regular form, and all the others are distorted. The last four Zn2+ sites shown in Fig. 2c to f are mixed with possible Ga3+ occupation consisting an anti-sites unfortunately. The anti-sites mixing the Zn and Ga result in the difficulty of distinguishing which element Mn2+ actually substitutes. Namely, the doping Mn2+ replacing the Zn2+ is not an ensure thing though they have same chemical 235
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Fig. 3. PL spectra of CZGO:Mn2+ samples. (a) The normalized PL spectra of Mn single doped and Ge4+ co-doped samples (λex = 300 nm). (b) The normalized spectra of CZGO:0.5%Mn2+,5%Ge4+ under 250 nm (black line) and 300 nm (red line) excitation, and the normalized spectra of CZGO host (blue line) excited by 250 nm. (c) The concentration depended PL spectra of CZGO:x% Mn2+,5%Ge4+ (x = 0.1, 0.3, 0.5, 0.7) excited by 300 nm. (d) Variations of PL intensities of CZGO:x%Mn2+,5%Ge4+ with the Mn2+ concentrations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
valence. The anti-site defects also can play the role of carrier traps as reported by Bessière cited in reference [34], thus significantly influencing the afterglow of Mn2+. 3.2. Luminescent properties Upon 250 nm excitation, the CZGO host shows a wide emission band from 350 nm to 600 nm which overlaps with the Mn2+ emission and accounts for the broader spectral shape of Mn2+ doped sample under 250 nm excitation than that excited by 300 nm (as shown in Fig. 3b). The host emission is ascribed to the recombination of the holes and electrons captured in host vacancies. With the Mn2+ doped, a strong green emission band from 475 nm to 625 nm peaked at 520 nm emerged upon 250 nm excitation. Unexpectedly, not all doping Mn exist as divalent Mn2+ because of oxidation of divalent Mn2+ into quadrivalent Mn4+ in the synthesis process at air atmosphere and insufficient reduction in the annealing treatment, as clearly evidenced in the 250 nm excited emission spectra of Mn singly doped sample shown in Fig. 3a (black curve), where a longer wavelength band from 650 nm to 750 with five peaks ascribing to the transition 2Eg→2Ag of Mn4+ is distinctively alongside the green emission band of Mn2+. However, the weak host emissions around 470 nm can still be discerned in spite of the two different dopants Mn2+ and Mn4+, resulting in the shoulder structure on shorter wavelength side of the 520 nm green emission band of Mn2+. In the emission spectra of the Mn2+ and Ge4+ co-doped sample (Fig. 3a, red curve), the longer wavelength Mn4+ emission is substantially reduced, indicating an interesting effect of Ge4+ codoping on the reduction of Mn, perhaps related to the charge compensation function of Ge4+ for Mn2+ substitution of Ga3+. On the other hand, more cation vacancies due to the Ge4+ doping will probably prevent the oxidation of divalent Mn ions. When exposed to 300 nm excitation, the co-doped samples yield a strong green emission peaked at 520 nm from Mn2+. The normalized emission spectra shows that the emission band of host was weakened with the Ge4+ joined (Fig. 3b). The green emission band of Mn2+ is attributed to the spin forbidden d–d transition 4T1→6A1 of Mn2+ ions in tetrahedral coordination. When monitoring 520 nm emission, step-like shape of the excitation spectra were observed as displayed in Fig. 4. The stronger shorter wavelength band with wavelength < 250 nm can be attributed the host transition, implying an efficient energy transfer from the host to the Mn2+ ions. The weaker excitation band around 290 nm can be ascribed to the charge transfer band (CTB) of Mn2+—O2-. The inset image in Fig. 4 shows that the intensity of the absorption around 300 nm is correlated with the green emission intensities (Fig. 3c) for samples with different Mn2+ concentration. Fig. 3d depicts the variation of emission intensity with the Mn2+ concentration in the range from 0.1% to 0.7%. The intensity increases with Mn2+ concentration until 0.3% and then decrease dramatically, indicating the concentration quenching of Mn2+ occurred around 0.3% in CZGO. Green afterglow of all samples can be observed by the naked eye after irradiation of the UV lamp. Meanwhile there is a pallid and short blue afterglow originated from the CZGO host. The afterglow spectra of all samples were recorded by CCD at 2 min after irradiation by 253.7 nm UV lamp for 2 min and shown in Fig. 5. All the samples exhibited similar afterglow curves consisting of two bands, one is a weaker band nearby 450 nm related to the host emission and the other 236
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Fig. 4. Excitation spectra of CZGO:x%Mn2+,5%Ge4+ (x = 0.1, 0.3, 0.5, 0.7) monitoring at 520 nm. The inset shows the enlarged spectra in the wavelength range from 250 nm to 300 nm.
Fig. 6. The afterglow decay curves of all obtained samples. (a) The afterglow decay curves of single doped Mn2+ (not reduced completely) and Mn2+/Ge4+ co-doped samples. (b) The afterglow decay curves of CZGO:xMn2+,5%Ge4+, x = 0.1%, 0.3%, 0.5%, 0.7%. The inset is plotted with vertical log-scale.
Fig. 5. Afterglow spectra of all synthesized samples after irradiation by 254 nm UV lamp for 2 min. (a) The concentration depended afterglow spectra of Mn2+/ Ge4+ co-doped samples with Mn2+ concentration of 0.1%, 0.3%, 0.5%, 0.7%. (b) The spectra of Mn2+ single doped (not reduced completely) and Mn2+/ Ge4+ co-doped samples.
doped samples. The initial relative intensities of four samples increase in the order of Mn2+ concentration of 0.1% < 0.7% < 0.3% < 0.5%. Apparently, the decay curves consist of a rapid attenuation process at first and then a slow decay process. The inset figure of Fig. 6b discloses that the slow component is more prominent in the 0.3% and 0.7% samples than that of the 0.1% and 0.5% samples, indicating a longer duration could be achieved in the 0.3% and 0.7% samples.
is a stronger band located around 520 nm ascribed to the Mn2+. In comparison with the Mn single doped sample, the co-doped samples produced a much weaker host afterglow and a relatively broader Mn2+ afterglow. The Ge4+ co-doping influence the emission band of Mn2+ as well as the afterglow. It is noticed that the afterglow spectra of all samples have a broader emission band of Mn2+ compared with the photoluminescence spectra. Interestingly, the emission of host shown in afterglow spectra had a stronger relative intensity. The reason may be that the energy stored in defects can release to the host vacancies more effectively after stop the excitation. The Fig. 5a depicts the concentration dependence of the afterglow spectra of the co-doped samples. The sample with the 0.5% Mn2+ concentration shows the strongest afterglow intensity. Fig. 6 presents the afterglow decay curves of the obtained samples after irradiation by a 253.7 nm UV lamp for 5 min. The data were recorded in the time range of 0–10 min with 5 s delay after ceasing the excitation. From the Fig. 6, the afterglow of Mn2+ still can be recorded after the removal of irradiation for 10 min. The Fig. 6a shows comparison of the decay curves of CZGO:0.5% Mn2+ (in which Mn could not reduced completely) and that of CZGO:0.5% Mn2+,5% Ge4+. Evidently, the latter exhibited a better afterglow properties. This can be accounted by the lower actual concentration of Mn2+ in the former. The Fig. 6b depicts the concentration depended decay curves of the co-
3.3. Thermoluminescence properties Thermoluminescence (TL) method is an appropriate tool to investigate the depth, types and density of traps which are most important factors bearing on the afterglow performance [35,36]. Therefore, TL curves of the obtained samples are recorded in Fig. 7 to provide relevant information about the traps existing in the CZGO host. Generally speaking, shallow traps are easily emptied by thermal energy, whereas deep traps can store captured charge carriers for a longer time at room temperature. The TL curves of all samples are dominated by one broad irregular band with peak intensity around 121 °C. When the temperature arrived at 350 °C, the intensity is closed to zero. From Fig. 7, a concentration depended TL curves are displayed. The 0.5% Mn2+ sample shows the strongest TL intensity in accordance with the trend observed in the afterglow spectra intensity (see Fig. 5). But different from Fig. 5, the 0.7% Mn2+ sample gives the weakest TL intensity. The reason may be found in the TL measurements processes. The TL measurements start only after waiting 30 s at room temperature and keeping at 45 °C for 10 s due to the time needed for the transfer of 237
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host prompts electrons in the conduction band and holes in the valence band which will be captured by the electron traps and hole traps respectively; the other is for the 300 nm excitation, where the Mn2+ is excited into higher energy states and transfer electrons into the traps via conduction band. With the phonon assistance, the carriers released from the traps via conduction band and valence band or due to tunneling effect, and recombine at Mn2+ sites, issuing the green afterglow. 4. Conclusions
Fig. 7. The concentration depended thermoluminescence CZGO:xMn2+,5%Ge4+, x = 0.1%, 0.3%, 0.5%, 0.7%.
curves
The CZGO:Mn2+ and CZGO:Mn2+,Ge4+ synthesized at 1230 °C show a green emission peaked at 520 nm with the 254 nm irradiation. The single doped sample not only shows the Mn2+ emission but also Mn4+ emission. With the Ge4+ co-doped, the Mn4+ reduced to Mn2+ more sufficiently. The single doped and co-doped samples show a similar green emission band located at 520 nm related to the Mn2+ emission. With the concentration of Mn2+ enhanced, the Mn2+ emission strengthen and then attenuate. The CZGO:0.3%Mn2+,Ge4+ sample shows the strongest emission and the CZGO:0.5%Mn2+,Ge4+ sample offers the strongest afterglow intensity. All the samples show similar afterglow luminescence consisting of the dominating green Mn2+ emission and a much weaker blue host emission. The CZGO host provides plentiful traps as revealed by TL curves. The 0.5% Mn2+ samples show the best TL intensity. The difference of the TL curves of the different samples can be accounted by the concentration influence on the carriers release efficiency. A novel green afterglow material CZGO:Mn2+,Ge4+ is corroborated and can be submitted to optimization and development in further studies.
of
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11574298 and 61635012), the National Key Research and Development Program of China (Grant No. 2016YFB0701001). References Fig. 8. The schematic diagram of the afterglow mechanism in CZGO:Mn2+.
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the samples to the TL equipment. The energy stored at shallow traps will be released effectively in these delay times, much more so for the higher concentration sample. Interestingly, there is also a hump from 90 °C to 159 °C in the TL curves of the 0.5% and 0.3% samples, suggesting a suddenly increased rate of energy release when heated to 90 °C. The 0.1% sample show the broadest band among the four samples, which can be explained the tardy release in the low concentration sample. With a fast heating rate of 2 k/s, part of the energy released from shallow traps in the 0.1% sample will incorrectly registered at higher temperatures. The TL results indicate that the CZGO compounds have a wide distribution of traps favorable for the long duration of the afterglow emission. From the crystal structure, there exists many antisite defects between the Zn and Ga which can be regard as an effective trap [34]. The carriers stored in anti-sites can be transfer into the near Mn2+ sites and result in the afterglow. At the same time, the Ge4+ dopant can cause abundant cation vacancies because of its excess charge valences. The cation vacancies usually play the roles of traps. Those cation vacancies also can capture the carriers and generate the persistent luminescence. Due to those different traps, the TL curves showed a wide temperature range. Fig. 8 presents a mechanism diagram for the luminescence and afterglow processes of CZGO:Mn2+. The locations of the energy levels shown in pictures are schematically and not to scale accurately. The diagram illustrates two processes of the energy storage: one is for the 253.7 nm UV lamp excitation, where the inter band transition of the
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