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Blue and red long lasting phosphorescence (LLP) in β-Zn3(PO4)2:Mn2+,Zr4+
Journal of Physics and Chemistry of Solids 66 (2005) 1171–1176 www.elsevier.com/locate/jpcs
Blue and red long lasting phosphorescence (LLP) in b-Zn3(PO4)2:Mn2C,Zr4C Jing Wanga, Qiang Sua,b,*,1, Shubin Wangb a
State Key Laboratory of Optoelectronic Materials and Technology, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People Republic of China b Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People Republic of China Received 10 March 2004; revised 28 January 2005; accepted 5 March 2005
Abstract Multi-color long lasting phosphorescent (LLP) phenomenon in b-Zn3(PO4)2:Mn2C,Zr4C was systematically investigated. It is found that the red (lEmZ616 nm) LLP performance of Mn2C such as brightness and duration is largely improved, and that the blue (lEmZ475 nm) LLP of Zr4C with lower intensity appears when Zr4C ions are co-doped into the matrix. The fluorescence, phosphorescence and thermoluminescence (TL) spectra show that Mn2C ion is solely expected as a luminescent center, while Zr4C ion not only acts as a luminescent center, but also induces an electron trap (TrapZr) associated with a TL peak at 344 K. The trap depth for TrapZr is 0.25 eV, while that for the intrinsic trap is 0.38 eV, associated with a dominant peak at 385 K for Zn3(PO4)2:Mn2C. The Zr4C-induced trap with suitable depth is responsible for the improvement of the red LLP of Mn2C ion and the appearance of the blue LLP of Zr4C ion. The LLP mechanism is also investigated. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; D. Defects; D. Luminescence
1. Introduction During the last decade, LLP materials have attracted much attention because they have large practical and potential applications in many fields, e.g. emergent lighting, electronic displays, the detection of high-energy rays such as UV, X-ray, b-ray, etc., and multi-dimensional optical memory and image storage [1–4]. To our knowledge, research interests worldwide have been mainly focused on rare earth ions, e.g. Ce3C, Pr3C, Sm3C, Eu3C, Tb3C, Dy3C, Tm3C and especially Eu2C [5–13]. The LLP materials activated by Mn2C ions remain to be explored.
* Corresponding author. Address: State Key Laboratory of Optoelectronic Materials and Technology, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People Republic of China. Tel.: C86 431 526 2208; fax: C86 431 569 8041. E-mail addresses: [email protected] (Q. Su), [email protected] (S. Wang). 1 Tel.: C86 20 8411 1038; fax: C86 20 8411 1038.
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.03.006
Mn2C ion should be expected as an efficient activator for LLP since it shows a high luminescence efficiency and saturated color, and could be excited by almost all the common methods: X-ray, electron bombardment, UVirradiation and electric field [14–16]. Mn2C ion is usually characterized by green or red emission. It consists of a 3d–3d broad band corresponding to the transition of Mn2C from the excited 4T1g state to the ground 6A1g state level. The emission color is strongly dependent on the coordination environment of Mn2C in the host lattice as determined by the coordination number (CN) and the strength of the ligand field. Mn2C ion emits green light when it is tetrahedrally coordinated (CNZ4), whereas it emits red light in octahedral configurations (CNZ6) [17]. The strength of the ligand field has also a great effect on the emission color: the stronger the ligand field, the longer the emission peak wavelength [18]. In 1999, Qiu et al. [19] reported the infrared femtosecond laser induced-red LLP phenomenon in Mn2C doped-sodium borate glass. In 2002, our group developed a green LLP zinc aluminosilicate ceramic doped with Mn2C and a red LLP zinc borosilicate glass activated by Mn2C[20,21]. Recently,
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Iwasaki et al. observed the red LLP phenomenon in MgGeO3:Mn2C,Yb3C, while Wang et al. found that the red LLP of MgSiO3:Mn2C could be obtained by co-doping with Eu2C and Dy3C [22,23]. However, the red LLP phenomena of Mn2C have not been reported under polycrystalline phosphate conditions. In the previous paper [24], we reported the effect of the host composition on the optical properties of Mn 2C in b-Zn3(PO4)2 and found that the intrinsic defect in the matrix could result in the appearance of the red LLP of Mn2C. This prompts us to investigate the role of foreign defects in the red LLP of Mn2C. In this paper, we report that the foreign defect induced by incorporating Zr4C ion into the host leads to not only an intensive red (lEmZ616 nm) LLP of Mn2C, but also a weak blue (lEmZ475 nm) LLP of Zr4C in b-Zn3(PO4)2.
2. Experiment 2.1. Synthesis of b-Zn3(PO4)2:Mn2C,Zr4C Powder samples were synthesized by high temperature solid-state reactions. The raw materials were analytical reagents: ZnO, (NH4)2HPO4, ZrO2 and MnCO3. The concentrations of Mn2C and Zr4C ions were, respectively, adjusted to 5 and 3 mol% of the Zn ions in 3ZnO$P2O5. The mixtures of corresponding raw materials were thoroughly ground and were then fired at 500 8C for 3 h. After regrinding, they were sintered at 950 8C for 5 h under reducing atmosphere conditions. 2.2. Materials characterization The structure of all the synthesized-samples was analyzed by Rigaku D/max-IIB X-ray powder diffract˚ ) radiation and was ometer using Cu Ka1 (lZ1.5405 A coincident with b-Zn3(PO4)2 (JCPDS: 30-1489). Fluorescence experiments of all the samples were performed using a HITACHI F-4500 Spectrofluorometer equipped with a 150 W Xe lamp. The LLP emission spectra and decay curves were measured as follows: immediately after irradiation by UV lamp output peaking at 254 nm with a power of 4.07 mW cmK2 for 5 min, the signals were recorded by the HITACHI F-4500 photomultiplier. Thermoluminescence (TL) measurements were carried on a FJ-427A thermoluminescence-meter. Prior to the measurements, powder samples were pressed into pellets (5 mm diameter and 0.5 mm thickness), then sintered at 500 8C under reducing atmosphere conditions for thermal bleaching. They were finally exposed to UV lamp output peaking at 254 nm with a power of 4.07 mW cmK2 for 5 min. The heating rate was fixed at 2, 3, 4 and 5 K sK1 within the temperature range from 300 to 600 K. All measurements except those involving TL spectra were performed at room temperature.
3. Results and discussion 3.1. Fluorescence properties of Zn3(PO4)2:Mn2C0.05 and Zn3(PO4)2:Mn2C0.05, Zr4C0.03 The excitation and emission spectra of the synthesizedsamples are shown in Figs. 1 and 2, respectively. For Zn3(PO4)2:Mn2C, one broad band dominates the wavelength range from 220 to 250 nm when monitored via the emission at 616 nm. This band is assigned to the charge transfer state (CTS) of Mn2C–O2K [25] rather than the host absorption because the edge of the optical absorption band for Zn3(PO4)2 is situated at w6.9 eV (180 nm) beyond the detection of the spectrometer [26]. In the wavelength range from 300 to 580 nm, there are four groups of excitation bands with lower intensity affiliated with the 3d–3d forbidden transitions of Mn2C. For clarity, the intensity of these bands is magnified by a factor of four. The two weak bands at 300–340 nm derived from the 4Eg(4D) levels, and the other three bands in the region from 340 to 380 nm are ascribed to the splitting 4T2g(4D) levels. The sharp bands at the wavelength from 400 to 430 nm are assigned to the splitting 4Eg–4A1g(4G) levels while three broad bands resolved in the region from 430 to 580 nm are assigned as components of the 4T2g(4G) and 4T1g(4G) levels [27]. When Zr4C ions are co-doped, no significant changes in the excitation spectra of Mn2C ions are found in the range from 200 to 580 nm, except for the decrease in the optical intensity. Via sample excitation at 240 nm, one broad emission band predominant at 616 nm is observed for Zn3(PO4)2:Mn2C, whereas two broad emission bands, the one dominant at 475 nm and the other situated at 616 nm, are observed for Zn3(PO4)2:Mn2C, Zr4C. The emission band at 616 nm is derived from the 4T1g(4G)/6A1g(6S) transition of Mn2C. The emission at 475 nm is assigned to the charge
4C Fig. 1. Excitation spectra of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 (curve 1) and Zn3(PO4)2:Mn2C0.05 (curve 2) by monitoring the emission at 616 nm.
J. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1171–1176
4C Fig. 2. Emission spectra of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 (curve 1) and Zn3(PO4)2:Mn2C0.05 (curve 2) under the excitation at 240 nm (a) and 418 nm (b), respectively.
transfer luminescence of zirconate [28,29]. For further confirmation of this assignment, another sample doped only with Zr4C ions was prepared and investigated. The spectroscopic results also indicated that the emission at 475 nm was derived from Zr4C ions [30]. It is observed in Figs. 1 and 2 that the incorporation of Zr4C ions decreases the optical intensity of Mn2C ions. We suggest that two reasons may be responsible for this phenomenon. First, Zr4C ions competitively absorb the excitation energy at 220–250 nm and subsequently emit light at 475 nm. Hence, it is reasonable to observe that the excitation intensity of Mn2C at the wavelength range from 200 to 300 nm is lower for Zn3(PO4)2:Mn2C,Zr4C when monitoring the emission of Mn2C at 616 nm. The same reason holds for the emission intensity decrease for Zn3(PO4)2:Mn2C,Zr4C at 616 nm under 240 nm excitation conditions as shown in Figs. 1 and 2a. This mechanism cannot explain, however, the decrease in the excitation intensity at 300–600 nm, and that of the emission intensity of Mn2C ions at 616 nm responding to the excitation at 418 nm. In b-Zn3(PO4)2, the Zr4C ion presents no absorption between 300 and 600 nm [30]. However, Mn2C presents the characteristic 3d–3d optical transition bands within this region. Therefore, there should not exist the competitive absorption between Zr4C and Mn2C from 300 to 600 nm. We suggest that the aliovalent substitution of Zr4C ions induces foreign traps that can capture part of the excitation energy under the steady irradiation [31,32], resulting in the optical intensity decrease of Mn2C. The existence of Zr4C-induced traps is supported via our LLP and TL spectra.
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4C Fig. 3. LLP emission spectra of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 at different time after the removal of UV lamp peaking at 254 nm.
observed for Zn3(PO4)2:Mn2C,Zr4C. One predominates at 616 nm, whereas another is observed around 475 nm; both are coincident with PL emissions under steady excitation conditions as shown in Fig. 2. In addition, it is observed that both blue and red LLP emissions do not change over time. The above results indicate that the blue and red LLP emissions are, respectively, derived from Zr4C ions and from the 3d–3d transition of Mn2C(4T1g (4G)/6A1g(6S)). Without co-doping Zr4C ions, the phosphor Zn3(PO4)2:Mn2C also shows a comparatively weak red LLP. The red LLP of Zn3(PO4)2:Mn2C,Zr4C is visible for about 3 h in the limit of human eye light perception (0.32 mcd mK2). As the decay curves 1 and 2 in Fig. 4 show, the incorporation of Zr4C enhances the LLP performance of Mn2C ion such as brightness and duration. The results indicate that traps are induced by Zr4C ions, which play an important role under excitation conditions [24]. This accounts for why the optical
3.2. Phosphorescence properties of Zn3 ðPO4 Þ2 : 4C Mn2C 0:05 ; Zr 0:03 Fig. 3 shows LLP emission spectra within 600 s. Upon the removal of the excitation source, two LLP emissions are
4C Fig. 4. The decay curves of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 (curve 1) and Zn3 ðPO4 Þ2 : Mn2C (curve 2) by monitoring the emission at 616 nm. 0:05
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In Fig. 5, the first peak of curve 1 at 344 K is associated with the Zr4C-induced trap (TrapZr). The shoulder of curve 1 at 385 K is due to the intrinsic trap (TrapA) [24]. The nature of other peaks at 408, 450 and 500 K are not considered in detail since they do not influence on the red LLP performance of Mn2C. The trap depth can be estimated according to Eq. (1) developed by Hoogenstraaten [33] 2 Tm E E C ln ln (1) Z kTm ks b
4C Fig. 5. Normalized TL spectra of the samples Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 K1 (curve 1) and Zn3 ðPO4 Þ2 : Mn2C (curve 2) in the heating rate at 2 K s . 0:05
intensity of Mn2C for Zn3(PO4)2:Mn2C,Zr4C is lower, compared with that for Zn3(PO4)2:Mn2C. 3.3. Thermoluminescence (TL) properties of Zn3 ðPO4 Þ2 : 2C 4C Mn2C 0:05 and Zn3 ðPO4 Þ2 : Mn0:05 ; Zr 0:03 The glow curves of Zn3(PO4)2:Mn2C and Zn3(PO4)2:Mn2C, Zr4C derived from a heating rate of 2 K sK1 are shown in Fig. 5. For Zn3(PO4)2:Mn2C, three peaks are clearly observed. One peak predominates at 385 K, and the other two peaks exist at 450 and 500 K. For Zn3(PO4)2:Mn2C,Zr4C, one peak is significant at 344 K and two shoulders are observed at 385 and 408 K.
where Tm is the peak temperature corresponding to the maximum TL intensity, and b is the heating rate. E is the energy of an electron trap, while s is the escape frequency, and k is Boltzmann’s constant. From Eq. (1), the plot of lnðTm2 =bÞ versus 1/Tm is linear with a slope E/k and intercept ln(E/ks). Apart from being simple, the method for determining E and s has the advantage of being insensitive to re-trapping effects and thermal quenching. However, the method is only applicable to the prominent peaks, such as at 344 and 385 K. Fig. 6 illustrates the plots of lnðTm2 =bÞ versus 1/Tm. The results are summarized in Table 1. For the peak at 385 K, E and s are found to be 0.38 eV and 3.72!103 sK1, while for the peak at 344 K, E and s are equal to 0.25 eV and 0.31!103 sK1. The mean time (t) for an electron residing in the trap at room temperature can be estimated by Eq. (2) [34]: t Z sK1 expfE=kTg
(2)
Taking TZ293 K, one finds tAZ803 s for the peak at 385 K, and tZrZ74 s for the peak at 344 K. These results indicate the metastable nature at room temperature,
Fig. 6. The plot between lnðTm2 =bÞ against 1/Tm. (The filled square and triangle for experiment data; the solid and dot line for deconvoluted line.)
J. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1171–1176
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Table 1 Thermoluminescent data of the dominant peaks in Zn3 ðPO4 Þ2 : Mn2C 0:05 and 4C Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 Dominant peaks
Zn3(PO4)2:Mn2C (385 K)
Zn3(PO4)2:Mn2C,Zr4C (344 K)
E (eV) s (sK1) t (s)
0.38 3.72!103 803
0.25 0.31!103 74.25
which is also supported by the TL fading behaviors of Zn3(PO4) 2:Mn2C,Zr4C. Prior to measurements, the sample was exposed to a 254 nm-UV lamp with excitation duration about 5 min. Subsequently, the sample was kept in the dark for a period of time after the excitation source was removed away. The TL spectra of the pre-irradiated sample were measured under the same conditions. It is observed in Fig. 7 that the TL intensity of the peaks at 344 and 385 K decrease over time. The fading phenomenon is ascribed to the depopulation of electrons at TrapZr and TrapA due to thermal energy. It is reasonable to observe the red LLP of Mn2C and the blue LLP of Zr4C at room temperature. Furthermore, the 344 K peak decreases faster than the 385 K peak, which is coincident with the value of t(tAOtZr). The significant role of electron traps has been elucidated in the field of LLP [1,6,8,24,35,36]. Note that if the trap depth is too great, electrons are trapped irreversibly, while if the trap depth is too little, thermal depopulation takes place. Traps with suitable depth are therefore necessary for the long-lasting phosphorescence. In the present case, the red LLP performance of Mn2C is obviously enhanced by incorporating Zr4C ions into Zn3(PO4)2:Mn2C. And the Zr4C-induced trap (EZrZ0.25 eV) is shallower than the intrinsic trap (EAZ0.38 eV). Therefore, the trap depth for the former case is more suitable to the red LLP of Mn2C. For a heating rate of 2 K sK1, Tm is situated slightly above the room temperature, i.e. 323–383 K [1,6,12,24,37]. This also confirms our interpretation.
Fig. 8. The schematic diagram of the possible mechanism for multi-colour 4C LLP of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 .
A thermally assisted-tunnel model is proposed to account for multi-color LLP in Zn3(PO4)2:Mn2C,Zr4C [38,39]. Fig. 8 shows the schematic diagram of the proposed mechanism. Under UV excitation, electron–hole pairs are created, which involve the charge transfer reaction of Mn2C–O2K and Zr4C–O2K (step 1): 254 nm
/ ðMnC K OKÞ * Mn2C–O2K 254 nm
/ ðZr3C K OKÞ * Zr4C K O2K
(3) (4)
The electron–hole pairs dissociate. And some of the electrons relax to the lowest excited levels, from which electrons return to the ground state levels of Mn2C and Zr4C. This results in the characteristic fluorescence of Zr4C and Mn2C ions as shown in Fig. 2a (step 6). Other electrons are transferred through the tunnel state (step 2) rather than the conduction band since the incident light (254 nm, 5.2 eV) has insufficient energy to induce electrons into conduction band. The transferred electrons are then captured at TrapA and especially TrapZr (step 3) as the distribution of trapped-electrons presents in Fig. 5. The electrons captured at TrapA and especially TrapZr are thermally released into the tunnel state at room temperature (step 4) due to their meta-stability at room temperature as shown in Fig. 7 and Table 1. From the tunnel state, the thermally released-electrons transfer into the excited level of Mn2C and Zr4C (step 5), from which electrons relax and finally give a rise to the blue and red LLP as shown in Fig. 2b (step 6).
4. Conclusions
4C Fig. 7. The TL fading spectra of the sample Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 .
We have reported the multi-color LLP phenomenon observed for b-Zn3(PO4)2:Mn2C,Zr4C. The blue LLP with lower emission intensity appears near 475 nm, and the intensive red LLP predominates at 616 nm. These are, respectively, derived from Zr4C ions and from the 3d–3d transition of Mn2C(4T1g(4G)/6A1g(6S)). The red LLP of
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Zn3(PO4)2:Mn2C,Zr4C is visible for up to 3 h. The Zr4Cinduced traps with suitable trap depth (EZrZ0.25 eV) are responsible for the improvement of the red LLP performance of Mn2C such as brightness and duration, and appearance of the blue LLP of Zr4C.
Acknowledgements We are very grateful to State Key Project of Basic Research (G1998061312) of China for financial support. The authors are greatly indebted to the referees for a great deal of work with language and presentation.
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Blue and red long lasting phosphorescence (LLP) in b-Zn3(PO4)2:Mn2C,Zr4C Jing Wanga, Qiang Sua,b,*,1, Shubin Wangb a
State Key Laboratory of Optoelectronic Materials and Technology, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People Republic of China b Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People Republic of China Received 10 March 2004; revised 28 January 2005; accepted 5 March 2005
Abstract Multi-color long lasting phosphorescent (LLP) phenomenon in b-Zn3(PO4)2:Mn2C,Zr4C was systematically investigated. It is found that the red (lEmZ616 nm) LLP performance of Mn2C such as brightness and duration is largely improved, and that the blue (lEmZ475 nm) LLP of Zr4C with lower intensity appears when Zr4C ions are co-doped into the matrix. The fluorescence, phosphorescence and thermoluminescence (TL) spectra show that Mn2C ion is solely expected as a luminescent center, while Zr4C ion not only acts as a luminescent center, but also induces an electron trap (TrapZr) associated with a TL peak at 344 K. The trap depth for TrapZr is 0.25 eV, while that for the intrinsic trap is 0.38 eV, associated with a dominant peak at 385 K for Zn3(PO4)2:Mn2C. The Zr4C-induced trap with suitable depth is responsible for the improvement of the red LLP of Mn2C ion and the appearance of the blue LLP of Zr4C ion. The LLP mechanism is also investigated. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; D. Defects; D. Luminescence
1. Introduction During the last decade, LLP materials have attracted much attention because they have large practical and potential applications in many fields, e.g. emergent lighting, electronic displays, the detection of high-energy rays such as UV, X-ray, b-ray, etc., and multi-dimensional optical memory and image storage [1–4]. To our knowledge, research interests worldwide have been mainly focused on rare earth ions, e.g. Ce3C, Pr3C, Sm3C, Eu3C, Tb3C, Dy3C, Tm3C and especially Eu2C [5–13]. The LLP materials activated by Mn2C ions remain to be explored.
* Corresponding author. Address: State Key Laboratory of Optoelectronic Materials and Technology, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People Republic of China. Tel.: C86 431 526 2208; fax: C86 431 569 8041. E-mail addresses: [email protected] (Q. Su), [email protected] (S. Wang). 1 Tel.: C86 20 8411 1038; fax: C86 20 8411 1038.
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.03.006
Mn2C ion should be expected as an efficient activator for LLP since it shows a high luminescence efficiency and saturated color, and could be excited by almost all the common methods: X-ray, electron bombardment, UVirradiation and electric field [14–16]. Mn2C ion is usually characterized by green or red emission. It consists of a 3d–3d broad band corresponding to the transition of Mn2C from the excited 4T1g state to the ground 6A1g state level. The emission color is strongly dependent on the coordination environment of Mn2C in the host lattice as determined by the coordination number (CN) and the strength of the ligand field. Mn2C ion emits green light when it is tetrahedrally coordinated (CNZ4), whereas it emits red light in octahedral configurations (CNZ6) [17]. The strength of the ligand field has also a great effect on the emission color: the stronger the ligand field, the longer the emission peak wavelength [18]. In 1999, Qiu et al. [19] reported the infrared femtosecond laser induced-red LLP phenomenon in Mn2C doped-sodium borate glass. In 2002, our group developed a green LLP zinc aluminosilicate ceramic doped with Mn2C and a red LLP zinc borosilicate glass activated by Mn2C[20,21]. Recently,
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Iwasaki et al. observed the red LLP phenomenon in MgGeO3:Mn2C,Yb3C, while Wang et al. found that the red LLP of MgSiO3:Mn2C could be obtained by co-doping with Eu2C and Dy3C [22,23]. However, the red LLP phenomena of Mn2C have not been reported under polycrystalline phosphate conditions. In the previous paper [24], we reported the effect of the host composition on the optical properties of Mn 2C in b-Zn3(PO4)2 and found that the intrinsic defect in the matrix could result in the appearance of the red LLP of Mn2C. This prompts us to investigate the role of foreign defects in the red LLP of Mn2C. In this paper, we report that the foreign defect induced by incorporating Zr4C ion into the host leads to not only an intensive red (lEmZ616 nm) LLP of Mn2C, but also a weak blue (lEmZ475 nm) LLP of Zr4C in b-Zn3(PO4)2.
2. Experiment 2.1. Synthesis of b-Zn3(PO4)2:Mn2C,Zr4C Powder samples were synthesized by high temperature solid-state reactions. The raw materials were analytical reagents: ZnO, (NH4)2HPO4, ZrO2 and MnCO3. The concentrations of Mn2C and Zr4C ions were, respectively, adjusted to 5 and 3 mol% of the Zn ions in 3ZnO$P2O5. The mixtures of corresponding raw materials were thoroughly ground and were then fired at 500 8C for 3 h. After regrinding, they were sintered at 950 8C for 5 h under reducing atmosphere conditions. 2.2. Materials characterization The structure of all the synthesized-samples was analyzed by Rigaku D/max-IIB X-ray powder diffract˚ ) radiation and was ometer using Cu Ka1 (lZ1.5405 A coincident with b-Zn3(PO4)2 (JCPDS: 30-1489). Fluorescence experiments of all the samples were performed using a HITACHI F-4500 Spectrofluorometer equipped with a 150 W Xe lamp. The LLP emission spectra and decay curves were measured as follows: immediately after irradiation by UV lamp output peaking at 254 nm with a power of 4.07 mW cmK2 for 5 min, the signals were recorded by the HITACHI F-4500 photomultiplier. Thermoluminescence (TL) measurements were carried on a FJ-427A thermoluminescence-meter. Prior to the measurements, powder samples were pressed into pellets (5 mm diameter and 0.5 mm thickness), then sintered at 500 8C under reducing atmosphere conditions for thermal bleaching. They were finally exposed to UV lamp output peaking at 254 nm with a power of 4.07 mW cmK2 for 5 min. The heating rate was fixed at 2, 3, 4 and 5 K sK1 within the temperature range from 300 to 600 K. All measurements except those involving TL spectra were performed at room temperature.
3. Results and discussion 3.1. Fluorescence properties of Zn3(PO4)2:Mn2C0.05 and Zn3(PO4)2:Mn2C0.05, Zr4C0.03 The excitation and emission spectra of the synthesizedsamples are shown in Figs. 1 and 2, respectively. For Zn3(PO4)2:Mn2C, one broad band dominates the wavelength range from 220 to 250 nm when monitored via the emission at 616 nm. This band is assigned to the charge transfer state (CTS) of Mn2C–O2K [25] rather than the host absorption because the edge of the optical absorption band for Zn3(PO4)2 is situated at w6.9 eV (180 nm) beyond the detection of the spectrometer [26]. In the wavelength range from 300 to 580 nm, there are four groups of excitation bands with lower intensity affiliated with the 3d–3d forbidden transitions of Mn2C. For clarity, the intensity of these bands is magnified by a factor of four. The two weak bands at 300–340 nm derived from the 4Eg(4D) levels, and the other three bands in the region from 340 to 380 nm are ascribed to the splitting 4T2g(4D) levels. The sharp bands at the wavelength from 400 to 430 nm are assigned to the splitting 4Eg–4A1g(4G) levels while three broad bands resolved in the region from 430 to 580 nm are assigned as components of the 4T2g(4G) and 4T1g(4G) levels [27]. When Zr4C ions are co-doped, no significant changes in the excitation spectra of Mn2C ions are found in the range from 200 to 580 nm, except for the decrease in the optical intensity. Via sample excitation at 240 nm, one broad emission band predominant at 616 nm is observed for Zn3(PO4)2:Mn2C, whereas two broad emission bands, the one dominant at 475 nm and the other situated at 616 nm, are observed for Zn3(PO4)2:Mn2C, Zr4C. The emission band at 616 nm is derived from the 4T1g(4G)/6A1g(6S) transition of Mn2C. The emission at 475 nm is assigned to the charge
4C Fig. 1. Excitation spectra of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 (curve 1) and Zn3(PO4)2:Mn2C0.05 (curve 2) by monitoring the emission at 616 nm.
J. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1171–1176
4C Fig. 2. Emission spectra of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 (curve 1) and Zn3(PO4)2:Mn2C0.05 (curve 2) under the excitation at 240 nm (a) and 418 nm (b), respectively.
transfer luminescence of zirconate [28,29]. For further confirmation of this assignment, another sample doped only with Zr4C ions was prepared and investigated. The spectroscopic results also indicated that the emission at 475 nm was derived from Zr4C ions [30]. It is observed in Figs. 1 and 2 that the incorporation of Zr4C ions decreases the optical intensity of Mn2C ions. We suggest that two reasons may be responsible for this phenomenon. First, Zr4C ions competitively absorb the excitation energy at 220–250 nm and subsequently emit light at 475 nm. Hence, it is reasonable to observe that the excitation intensity of Mn2C at the wavelength range from 200 to 300 nm is lower for Zn3(PO4)2:Mn2C,Zr4C when monitoring the emission of Mn2C at 616 nm. The same reason holds for the emission intensity decrease for Zn3(PO4)2:Mn2C,Zr4C at 616 nm under 240 nm excitation conditions as shown in Figs. 1 and 2a. This mechanism cannot explain, however, the decrease in the excitation intensity at 300–600 nm, and that of the emission intensity of Mn2C ions at 616 nm responding to the excitation at 418 nm. In b-Zn3(PO4)2, the Zr4C ion presents no absorption between 300 and 600 nm [30]. However, Mn2C presents the characteristic 3d–3d optical transition bands within this region. Therefore, there should not exist the competitive absorption between Zr4C and Mn2C from 300 to 600 nm. We suggest that the aliovalent substitution of Zr4C ions induces foreign traps that can capture part of the excitation energy under the steady irradiation [31,32], resulting in the optical intensity decrease of Mn2C. The existence of Zr4C-induced traps is supported via our LLP and TL spectra.
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4C Fig. 3. LLP emission spectra of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 at different time after the removal of UV lamp peaking at 254 nm.
observed for Zn3(PO4)2:Mn2C,Zr4C. One predominates at 616 nm, whereas another is observed around 475 nm; both are coincident with PL emissions under steady excitation conditions as shown in Fig. 2. In addition, it is observed that both blue and red LLP emissions do not change over time. The above results indicate that the blue and red LLP emissions are, respectively, derived from Zr4C ions and from the 3d–3d transition of Mn2C(4T1g (4G)/6A1g(6S)). Without co-doping Zr4C ions, the phosphor Zn3(PO4)2:Mn2C also shows a comparatively weak red LLP. The red LLP of Zn3(PO4)2:Mn2C,Zr4C is visible for about 3 h in the limit of human eye light perception (0.32 mcd mK2). As the decay curves 1 and 2 in Fig. 4 show, the incorporation of Zr4C enhances the LLP performance of Mn2C ion such as brightness and duration. The results indicate that traps are induced by Zr4C ions, which play an important role under excitation conditions [24]. This accounts for why the optical
3.2. Phosphorescence properties of Zn3 ðPO4 Þ2 : 4C Mn2C 0:05 ; Zr 0:03 Fig. 3 shows LLP emission spectra within 600 s. Upon the removal of the excitation source, two LLP emissions are
4C Fig. 4. The decay curves of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 (curve 1) and Zn3 ðPO4 Þ2 : Mn2C (curve 2) by monitoring the emission at 616 nm. 0:05
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In Fig. 5, the first peak of curve 1 at 344 K is associated with the Zr4C-induced trap (TrapZr). The shoulder of curve 1 at 385 K is due to the intrinsic trap (TrapA) [24]. The nature of other peaks at 408, 450 and 500 K are not considered in detail since they do not influence on the red LLP performance of Mn2C. The trap depth can be estimated according to Eq. (1) developed by Hoogenstraaten [33] 2 Tm E E C ln ln (1) Z kTm ks b
4C Fig. 5. Normalized TL spectra of the samples Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 K1 (curve 1) and Zn3 ðPO4 Þ2 : Mn2C (curve 2) in the heating rate at 2 K s . 0:05
intensity of Mn2C for Zn3(PO4)2:Mn2C,Zr4C is lower, compared with that for Zn3(PO4)2:Mn2C. 3.3. Thermoluminescence (TL) properties of Zn3 ðPO4 Þ2 : 2C 4C Mn2C 0:05 and Zn3 ðPO4 Þ2 : Mn0:05 ; Zr 0:03 The glow curves of Zn3(PO4)2:Mn2C and Zn3(PO4)2:Mn2C, Zr4C derived from a heating rate of 2 K sK1 are shown in Fig. 5. For Zn3(PO4)2:Mn2C, three peaks are clearly observed. One peak predominates at 385 K, and the other two peaks exist at 450 and 500 K. For Zn3(PO4)2:Mn2C,Zr4C, one peak is significant at 344 K and two shoulders are observed at 385 and 408 K.
where Tm is the peak temperature corresponding to the maximum TL intensity, and b is the heating rate. E is the energy of an electron trap, while s is the escape frequency, and k is Boltzmann’s constant. From Eq. (1), the plot of lnðTm2 =bÞ versus 1/Tm is linear with a slope E/k and intercept ln(E/ks). Apart from being simple, the method for determining E and s has the advantage of being insensitive to re-trapping effects and thermal quenching. However, the method is only applicable to the prominent peaks, such as at 344 and 385 K. Fig. 6 illustrates the plots of lnðTm2 =bÞ versus 1/Tm. The results are summarized in Table 1. For the peak at 385 K, E and s are found to be 0.38 eV and 3.72!103 sK1, while for the peak at 344 K, E and s are equal to 0.25 eV and 0.31!103 sK1. The mean time (t) for an electron residing in the trap at room temperature can be estimated by Eq. (2) [34]: t Z sK1 expfE=kTg
(2)
Taking TZ293 K, one finds tAZ803 s for the peak at 385 K, and tZrZ74 s for the peak at 344 K. These results indicate the metastable nature at room temperature,
Fig. 6. The plot between lnðTm2 =bÞ against 1/Tm. (The filled square and triangle for experiment data; the solid and dot line for deconvoluted line.)
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Table 1 Thermoluminescent data of the dominant peaks in Zn3 ðPO4 Þ2 : Mn2C 0:05 and 4C Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 Dominant peaks
Zn3(PO4)2:Mn2C (385 K)
Zn3(PO4)2:Mn2C,Zr4C (344 K)
E (eV) s (sK1) t (s)
0.38 3.72!103 803
0.25 0.31!103 74.25
which is also supported by the TL fading behaviors of Zn3(PO4) 2:Mn2C,Zr4C. Prior to measurements, the sample was exposed to a 254 nm-UV lamp with excitation duration about 5 min. Subsequently, the sample was kept in the dark for a period of time after the excitation source was removed away. The TL spectra of the pre-irradiated sample were measured under the same conditions. It is observed in Fig. 7 that the TL intensity of the peaks at 344 and 385 K decrease over time. The fading phenomenon is ascribed to the depopulation of electrons at TrapZr and TrapA due to thermal energy. It is reasonable to observe the red LLP of Mn2C and the blue LLP of Zr4C at room temperature. Furthermore, the 344 K peak decreases faster than the 385 K peak, which is coincident with the value of t(tAOtZr). The significant role of electron traps has been elucidated in the field of LLP [1,6,8,24,35,36]. Note that if the trap depth is too great, electrons are trapped irreversibly, while if the trap depth is too little, thermal depopulation takes place. Traps with suitable depth are therefore necessary for the long-lasting phosphorescence. In the present case, the red LLP performance of Mn2C is obviously enhanced by incorporating Zr4C ions into Zn3(PO4)2:Mn2C. And the Zr4C-induced trap (EZrZ0.25 eV) is shallower than the intrinsic trap (EAZ0.38 eV). Therefore, the trap depth for the former case is more suitable to the red LLP of Mn2C. For a heating rate of 2 K sK1, Tm is situated slightly above the room temperature, i.e. 323–383 K [1,6,12,24,37]. This also confirms our interpretation.
Fig. 8. The schematic diagram of the possible mechanism for multi-colour 4C LLP of Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 .
A thermally assisted-tunnel model is proposed to account for multi-color LLP in Zn3(PO4)2:Mn2C,Zr4C [38,39]. Fig. 8 shows the schematic diagram of the proposed mechanism. Under UV excitation, electron–hole pairs are created, which involve the charge transfer reaction of Mn2C–O2K and Zr4C–O2K (step 1): 254 nm
/ ðMnC K OKÞ * Mn2C–O2K 254 nm
/ ðZr3C K OKÞ * Zr4C K O2K
(3) (4)
The electron–hole pairs dissociate. And some of the electrons relax to the lowest excited levels, from which electrons return to the ground state levels of Mn2C and Zr4C. This results in the characteristic fluorescence of Zr4C and Mn2C ions as shown in Fig. 2a (step 6). Other electrons are transferred through the tunnel state (step 2) rather than the conduction band since the incident light (254 nm, 5.2 eV) has insufficient energy to induce electrons into conduction band. The transferred electrons are then captured at TrapA and especially TrapZr (step 3) as the distribution of trapped-electrons presents in Fig. 5. The electrons captured at TrapA and especially TrapZr are thermally released into the tunnel state at room temperature (step 4) due to their meta-stability at room temperature as shown in Fig. 7 and Table 1. From the tunnel state, the thermally released-electrons transfer into the excited level of Mn2C and Zr4C (step 5), from which electrons relax and finally give a rise to the blue and red LLP as shown in Fig. 2b (step 6).
4. Conclusions
4C Fig. 7. The TL fading spectra of the sample Zn3 ðPO4 Þ2 : Mn2C 0:05 ; Zr0:03 .
We have reported the multi-color LLP phenomenon observed for b-Zn3(PO4)2:Mn2C,Zr4C. The blue LLP with lower emission intensity appears near 475 nm, and the intensive red LLP predominates at 616 nm. These are, respectively, derived from Zr4C ions and from the 3d–3d transition of Mn2C(4T1g(4G)/6A1g(6S)). The red LLP of
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Zn3(PO4)2:Mn2C,Zr4C is visible for up to 3 h. The Zr4Cinduced traps with suitable trap depth (EZrZ0.25 eV) are responsible for the improvement of the red LLP performance of Mn2C such as brightness and duration, and appearance of the blue LLP of Zr4C.
Acknowledgements We are very grateful to State Key Project of Basic Research (G1998061312) of China for financial support. The authors are greatly indebted to the referees for a great deal of work with language and presentation.
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