Mn 2+ activated red phosphorescence in BaMg 2 Si 2 O 7 :Mn 2+,Eu 2+,Dy 3+ through persistent energy transfer

Transcription

1 JOURNAL OF APPLIED PHYSICS 101, Mn 2+ activated red phosphorescence in BaMg 2 Si 2 O 7 :Mn 2+,Eu 2+,Dy 3+ through persistent energy transfer Song Ye Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun , China and Graduate School of Chinese Academy of Sciences, Beijing , China Jiahua Zhang, a Xia Zhang, Shaozhe Lu, and Xinguang Ren Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun , China Xiao-jun Wang b Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun , China and Department of Physics, Georgia Southern University, Statesboro, Georgia Received 11 December 2006; accepted 23 January 2007; published online 30 March 2007 Based on persistent energy transfer, Mn 2+ activated red phosphorescence is achieved in BaMg 2 Si 2 O 7. Two types of Mn 2+ centers with emitting peaks at 620 and 675 nm, respectively, are observed and found to govern both the red luminescence and red phosphorescence spectral shapes. The spectral distribution of the red phosphorescence as a function of irradiation wavelengths and Mn 2+ concentrations is systematically studied in Mn 2+ and Dy 3+ doubly doped and Mn 2+,Eu 2+, and Dy 3+ tridoped samples, indicating the dominant role of persistent energy transfer on generation of the red phosphorescence. Moreover, it is found that the incorporation of Mn 2+ in BaMg 2 Si 2 O 7 :Eu 2+, Dy 3+ may prolong the phosphorescent persistence time 1 h compared with the Mn 2+ free materials American Institute of Physics. DOI: / I. INTRODUCTION a Author to whom correspondence should be addressed; electronic mail: zjiahau@public.cc.jl.cn b Author to whom correspondence should be addressed; electronic mail: xwang@georgiasouthern.edu Long persistent phosphors have been of great interest due to their applications on emergency route markings, dark version display devices, warning signboards, etc. Up to now, intense green and blue emitting persistent phosphors have been commercially available; 1 4 the desirable red emitting persistent phosphor, however, has not been achieved yet. Although some sulfides and oxysulfides exhibit good red phosphorescence, such as Y 2 O 2 S:Eu 3+, their poor chemical stabilities are unsatisfied in use. The achievement of oxide red persistent phosphors is thereby a challenging goal. It is generally accepted that phosphorescence requires the presence of carrier traps, which are associated with crystal defects or doped impurities in host. The traps can capture electrons and/or holes generated by photon irradiation and subsequently bleached thermally with the result of radiative emission of luminescence centers. 5,6 In fact, using such a general mechanism, however, is difficult to find chemically stable red persistent phosphor because no proper traps have been well known and selected to be introduced into some red phosphors to generate red long phosphorescence. In view of this difficulty, employing persistent energy transfer PET mechanism is proposed as a new way to design and obtain persistent phosphors with emitting color interested. 7 PET makes it possible to convert the blue or green phosphorescence of some commercial persistent phosphors into the red one if red emitting activators are additionally introduced in the phosphors. Based on PET, some persistent phosphors with green, yellow, and red emissions have been reported in rare earth and transition metal ion doped silicates and aluminates We have developed Sr 4 Al 14 O 25 :Eu 2+,Cr 3+ based on PET, 11 in which the Cr 3+ activated red phosphorescent emission line is located at 690 nm, belonging to deep red, to which human eyes are not well sensitive. Red emitting persistent phosphors are still required. As it is known, BaMg 2 Si 2 O 7 :Eu 2+,Dy 3+ demonstrates excellent purple 400 nm long persistent phosphorescence and highly chemical stabilities. On the other hand, the effective energy transfer from Eu 2+ to Mn 2+ has been observed in BaMg 2 Si 2 O 7, in which Mn 2+ exhibits red fluorescence when substituting for Mg 2+ site. 12,13 In our previous work, two types of Mn 2+ centers are observed in BaMg 2 Si 2 O 7, and the Mn 2+ concentration dependent energy transfer efficiency has been also systematically studied. 14 If the energy transfer from Eu 2+ to Mn 2+ continues during its persistence, the Mn 2+ activated red phosphorescence can be expected. In this work, we focus on Mn 2+ activated red phosphorescent properties in Mn 2+, Eu 2+, and Dy 3+ tridoped BaMg 2 Si 2 O 7 in comparison with Mn 2+ and Dy 3+ doubly doped BaMg 2 Si 2 O 7 and Eu 2+ and Dy 3+ doubly doped BaMg 2 Si 2 O 7 as a function of irradiation photon wavelengths and Mn 2+ concentrations. As reported in our previous work, 14 there exist two distinct Mn 2+ luminescent centers Mn 2+ I and Mn 2+ II when substituting for two nonidentical Mg 2+ sites, which emit at 620 and 675 nm, respectively /2007/101 6 /063545/6/$ , American Institute of Physics

2 Ye et al. J. Appl. Phys. 101, FIG. 1. X-ray powder diffraction patterns of A BaMg 2 Si 2 O 7 and B Ba Mg 1.85 Si 2 O 7 : 0.01 Eu 2+, 0.15 Mn 2+, Dy 3+. The effects of PET on red luminescence and phosphorescence of the two kinds of Mn 2+ centers are investigated. II. EXPERIMENT Samples of BaMg 2 Si 2 O 7 : Mn 2+, Eu 2+ ME, BaMg 2 Si 2 O 7 :Mn 2+,Dy 3+ MD, BaMg 2 Si 2 O 7 :Eu 2+,Dy 3+ ED, and BaMg 2 Si 2 O 7 :Mn 2+,Eu 2+,Dy 3+ MED are prepared by solid-state reaction. Eu 2+ and Dy 3+ are considered to replace Ba 2+ sites and Mn 2+ to replace Mg 2+ sites. BaCO 3, 4MgCO 3 Mg OH 2 5H 2 O, SiO 2, MnCO 3, Eu 2 O 3, and Dy 2 O 3 are employed as the raw materials all materials are in analytical grade. A proper amount of H 3 BO 3 about 5% is added as a flux. The raw materials are mixed homogeneously by the ball mill for 6 h, and then sintered at 1270 C for 4 h in weak reductive atmosphere 5%H 2 +95%N 2 mixed flowing gas. The crystalline structure of the samples is investigated by x-ray diffraction using a Siemens D-500 equipment with a Cu target radiation source. The x-ray diffraction pattern of pure BaMg 2 Si 2 O 7 is shown in Fig. 1 a. The lattice of BaMg 2 Si 2 O 7 crystallizes in orthorhombic structure with unit cells of a= Å, b= Å, and c = Å. The pattern of Ba Mg 1.85 Si 2 O 7 : 0.15 Mn 2+, 0.01 Eu 2+, Dy 3+ Fig. 1 b is also presented for comparison. Figure 1 b shows that all the diffraction peaks are assigned to BaMg 2 Si 2 O 7 phase and no characteristic peaks of the dopants have been observed. The result indicates that the pure phase of BaMg 2 Si 2 O 7 is formed in the doped samples to be investigated. The excitation, emission, and phosphorescence spectra are measured using HITACHI F-4500 fluorescence spectrophotometer. All the measurements are performed at room temperature. FIG. 2. Excitation em =400 nm and emission ex =300 nm spectra of Ba 0.99 Mg 2 Si 2 O 7 : 0.01 Eu 2+ dashed and excitation em =620 nm and emission ex =422 nm spectra of BaMg 1.99 Si 2 O 7 : 0.01 Mn 2+ solid. III. LUMINESCENCE A. Energy transfer between Eu 2+ and Mn 2+ in BaMg 2 Si 2 O 7 The excitation and emission spectra of Ba 0.99 Mg 2 Si 2 O 7 : 0.01 Eu 2+ and BaMg 1.99 Si 2 O 7 : 0.01 Mn 2+ are shown in Fig. 2. The Eu 2+ has a purple emission band at 400 nm corresponding to d-f transition with a broad excitation band within nm of UV range. The Mn 2+ with concentration of 0.01 in BaMg 2 Si 2 O 7 has a red emission band at 620 nm, originating from 4 T 1 4 G 6 A 1 6 S transition with some excitation bands in the purple range. The broad emission band of Eu 2+ ions overlaps with the excitation bands of the Mn 2+, making the energy transfer from Eu 2+ to Mn 2+ possible. The emission and excitation spectra for monitoring the red emission band of Mn 2+ in ME with various Eu 2+ concentrations 0.001, 0.005, 0.010, and are illustrated in Fig. 3. The excitation spectra of the red emission of Mn 2+ demonstrated in Fig. 3 are consistent with that of the purple FIG. 3. Excitation and emission spectra ex =300 nm of Ba 1 y Mg 2 x Si 2 O 7 : x Mn 2+, y Eu 2+ for different concentrations. x=0.05 and em =623 nm in A, x=0.20 and em =661 nm in B. y=0.001, 0.005, 0.010, and 0.015, respectively, for curves a, b, c, and d.

3 Ye et al. J. Appl. Phys. 101, FIG. 5. Red phosphorescence emission spectra of Ba Mg 2 x Si 2 O 7 : x Mn 2+, Dy 3+ x=0.05, 0.10, and 0.15, respectively, for 1, 2 and 3 with irradiation wavelengths of 210, 230, and 254 nm, respectively. The emission solid is fitted by two Gaussian functions dashed and normalized to unit. FIG. 4. Emission spectra of Ba 0.99 Mg 2 x Si 2 O 7 : x Mn 2+, 0.01 Eu 2+ x=0.01, 0.12, 0.21, 0.27, and 0.30, respectively, for a, b, c, d, and e under the excitation of 300 nm. The red emission of Mn 2+ solid is fitted by two Gaussian functions dashed. The peak wavelengths are 620 and 675 nm, and widths are 65.8 and 60.6 nm, respectively, for Mn 2+ I and Mn 2+ II. excitation of Eu 2+ presented in Fig. 2, indicating the performance of energy transfer from Eu 2+ to Mn 2+. Moreover, fixing Mn 2+ concentration while increasing Eu 2+ concentration greatly enhances the red emission of Mn 2+ under 300 nm excitation into the f-d absorption band of Eu 2+, further indicating the realization of the energy transfer. Meanwhile, one thing should be noted that the Mn 2+ emission peak is located at 623 nm Fig. 3 a at a low Mn 2+ concentration of 0.05, but moved to 661 nm Fig. 3 b at a higher concentration of The redshift with increasing Mn 2+ concentrations is analyzed in the following section. B. Redshift of Mn 2+ emission in BaMg 2 Si 2 O 7 with increasing Mn 2+ concentrations Figure 4 shows the emission spectra of Ba 1.99 Mg 2 x Si 2 O 7 : x Mn 2+, 0.01 Eu 2+ for various Mn 2+ concentrations x=0.01, 0.12, 0.21, 0.27, and 0.30 under 300 nm excitation. The red emission of Mn 2+ consists of two bands Mn 2+ I and Mn 2+ II, located at 620 and 675 nm, respectively. At a low Mn 2+ concentration of 0.01, the red emission is dominantly contributed by the 620 nm band, while at a high Mn 2+ concentration of 0.30, the 675 nm band dominates the emission. The red emission distribution of Mn 2+ can be fitted very well by two Gaussian functions, one of which is centered at 620 nm with a width of 65.8 nm for Mn 2+ I band and the other one at 675 nm with a width of 60.6 nm for Mn 2+ II band, respectively, as shown in Fig. 4 dashed. The Gaussian fitting results reveal that the emission intensity of Mn 2+ II increases rapidly relative to the Mn 2+ I with the increase of Mn 2+ concentration, therefore, leading to a redshift of the envelope of overall Mn 2+ emission bands. The Gaussian fitting and energy transfer analysis also demonstrates that when Mn 2+ concentration is over 0.15, the number of Mn 2+ I remains almost unchanged while that of Mn 2+ II increases rapidly. We therefore speculate that the Mn 2+ I center is preferentially formed in comparison with Mn 2+ II. In such a case, Mn 2+ I will reach a saturation when most of the Mg 2+ I sites are preferentially occupied by Mn 2+ at a critical Mn 2+ concentration of 0.15, beyond which the Mn 2+ II centers start to be mainly formed. 14 IV. PHOSPHORESCENCE It is experimentally observed that both MD and MED emit phosphorescence after irradiation with UV in the range of nm. The phosphorescence in MD consists of only a red component nm activated by Mn 2+, while the phosphorescence in MED consists of a purple component 400 nm activated by Eu 2+, and a red component activated by Mn 2+. The spectral distribution of the purple phosphorescence is unchanged with varying irradiation wavelengths and Mn 2+ concentrations, while that of the red one in either MD or MED changes with irradiation wavelengths and Mn 2+ concentrations. As the experimental phenomena, the phosphorescent properties of Mn 2+ I and Mn 2+ II are required to be understood individually to comprehend the properties of whole red phosphorescence. A. Different spectral properties of red phosphorescence in BaMg 2 Si 2 O 7 :Mn 2+,Eu 2+, Dy 3+ and BaMg 2 Si 2 O 7 :Mn 2+,Dy 3+ : Evidence of the role of PET Figure 5 shows the red phosphorescence spectra of Ba Mg 2 x Si 2 O 7 : x Mn 2+, Dy 3+ x=0.05, 0.10, and

4 Ye et al. J. Appl. Phys. 101, FIG. 6. Excitation spectra of Mn 2+ I em =620 nm and Mn 2+ II em =675 nm in Ba Mg 1.9 Si 2 O 7 : 0.10 Mn 2+, Dy after irradiated with various wavelengths 210, 230, and 254 nm for Mn 2+ concentrations of , , and In accordance with the photoluminescence spectra in Fig. 4, the red phosphorescence spectra also consists of two bands dashed originated from Mn 2+ I and Mn 2+ II, located at 620 and 675 nm, respectively. For each Mn 2+ concentration, the Mn 2+ I activated phosphorescence decreases relatively to the Mn 2+ II with increasing irradiation wavelengths, leading to a redshift of the phosphorescence. In order to understand the experimental phenomena, the excitation spectra of Mn 2+ I and Mn 2+ II are measured, as shown in Fig. 6. An excitation band appears in UV region for each Mn 2+ center, which is attributed to 6 A 1 6 S 4 T 1 4 F. 5 The excitation band for Mn 2+ I is located at shorter wavelength side 226 nm than that for Mn 2+ II 238 nm, indicating that longer wavelength excitation is ineffective to Mn 2+ I in comparison with Mn 2+ II. In general, phosphorescence is related to carrier traps, which can be filled by holes or electrons generated by photon irradiation, and subsequently release the carriers to radiatively recombine with activated centers. Dy 3+ acting as a hole trap may locate nearby Mn 2+ I to form trap I or locate nearby Mn 2+ II to form trap II. Due to short distance, excitation energies in Mn 2+ I and Mn 2+ II prefer to storing energies into trap I and trap II, respectively. In the same way, these localized trap I and trap II prefer transferring stored energies to Mn 2+ I and Mn 2+ II, respectively. According to the excitation spectra in Fig. 6, trap I is ineffectively filled comparing with trap II as irradiation wavelength is increased; the Mn 2+ I activated phosphorescence hence decreases relatively. It can be also seen in Fig. 5 that the phosphorescence bands shift to the red side with increasing Mn 2+ concentrations. This shift in phosphorescence is similar to that in photoluminescence, and well understood considering the growth of the number proportion of Mn 2+ II to Mn 2+ I centers with increasing Mn 2+ concentrations. Figure 7 shows the red phosphorescence spectra of Ba Mg 2 x Si 2 O 7 : x Mn 2+, 0.01 Eu 2+, Dy 3+ x=0.05, 0.10, and 0.15 after irradiation with various wavelengths 210, 230, 254, and 300 nm for Mn 2+ concentrations of , , and The red phosphorescence spectra FIG. 7. Red phosphorescence emission spectra of Ba Mg 2 x Si 2 O 7 : 0.01 Eu 2+, x Mn 2+, Dy 3+ x=0.05, 0.10, and 0.15, respectively, for 1, 2 and 3 with irradiation wavelengths of 210, 230, 254, and 300 nm, respectively. The emission solid is fitted by three Gaussian functions dashed and dotted and normalized to unit. consist of two Mn 2+ components dashed and a tail of the Eu 2+ component at higher energy side dotted. It is obviously demonstrated in Fig. 7 that the Mn 2+ I and Mn 2+ II activated phosphorescence in MED exhibits opposite behavior to MD system as irradiation wavelengths are changed. At each Mn 2+ concentration, the Mn 2+ I activated phosphorescence increases relatively to the Mn 2+ II with increasing irradiation wavelengths, leading to a blueshift of the whole red phosphorescence, exhibiting opposite result to Mn 2+ and Dy 3+ doubly doped system. To understand the origin, the role of Eu 2+ on PET must be considered. Considering the experimentally observed enhancement of luminescence and phosphorescence as Eu 2+ is doped, trap filling should perform dominantly through exciting Eu 2+ in MED, not Mn 2+ as in MD, and the red phosphorescence should be generated mainly through PET to convert the purple phosphorescence of Eu 2+. In such a case, trap filling through exciting Eu 2+ determines the relative intensity of Mn 2+ I and Mn 2+ II components in red phosphorescence.the number of the traps filled is proportional to the excitation efficiency of Eu 2+ and the capture efficiency of trap. From Fig. 3, the excitation spectra of Mn 2+ I and Mn 2+ II are identical, indicating unchanged relative excitation efficiency of them with changing irradiation energies. It is thus speculated that capture efficiency of trap I relative to trap II grows up with increasing irradiation wavelengths, so that the result represented in Fig. 7 is achieved. Mn 2+ activated red phosphorescent properties in MED in comparison with MD as a function of irradiation photon wavelengths and Mn 2+ concentrations are summarily illustrated in Fig. 8 for clear presentation, where the height of bars describes the phosphorescent intensity ratios of Mn 2+ I to Mn 2+ II. B. Prolonged persistence time of phosphorescence in BaMg 2 Si 2 O 7 :Mn 2+,Eu 2+,Dy 3+ compared with BaMg 2 Si 2 O 7 :Eu 2+,Dy 3+ In our previous work, we have developed a persistent phosphor based on PET: Sr 4 Al1 4 O 25 :Cr 3+,Eu 2+,Dy 3+,in which both the red of Cr 3+ and the blue of Eu 2+ phosphorescence, however, decay faster than that in Cr 3+ free sample. 11 This is attributed to formation of shallower traps by adding

5 Ye et al. J. Appl. Phys. 101, FIG. 9. Time decay curves of Ba Mg 2 Si 2 O 7 : 0.01 Eu 2+,0.005Dy 3+ A and Ba Mg 1.9 Si 2 O 7 : 0.10 Mn 2+, 0.01 Eu 2+, Dy 3+ B after irradiation with 254 nm UV for 5 min, where a represents for 400 nm emission in B, b 620 nm emission in B, and c 400 nm emission in A. the red phosphorescence activated by Mn 2+ in MED is mostly contributed by the short distance persistent energy transfer from Eu 2+ II to Mn 2+, while the purple phosphorescence is dominantly contributed by the Eu 2+ I, from which PET to Mn 2+ is inefficient due to the long distance separation. As a result, the red phosphorescence governed by Eu 2+ II decays faster than the purple one governed by Eu 2+ I in EMD. FIG. 8. Phosphorescence emission intensity ratios of Mn 2+ I to Mn 2+ II for MD and MED with Mn 2+ concentrations of 0.05, 0.10, and 0.15, respectively. The irradiation wavelengths are 210, 230, 254, and 300 nm, respectively. Cr 3+ ions. In the present work, we find that in BaMg 2 Si 2 O 7 : Mn 2+,Eu 2+,Dy 3+, the incorporation of Mn 2+ results in a smaller decay rate of the phosphorescence than that in Mn 2+ free sample. Figure 9 shows the time decay curves of the phosphorescence in ED and MED. It is exhibited that both the purple 400 nm and the red 620 nm phosphorescence in MED decay slower than the purple phosphorescence in ED, indicating that codoping Mn 2+ creates deeper traps. Meanwhile, one can also find that the red phosphorescence of Mn 2+ decays faster than the purple one of Eu 2+ in MED. It is, therefore, speculated that the retrapping process must be taken into consideration for clearly explaining the experimental phenomenon. In MED, the Eu 2+ ions can be classified into two types, Eu 2+ I and Eu 2+ II, corresponding to the absence or presence of Mn 2+ nearby them, respectively. The holes released from the localized traps near Eu 2+ I will be either recombined with Eu 2+ I or retrapped by traps, while in the case of Eu 2+ II, the released holes will be recombined with both of Eu 2+ and Mn 2+, or retrapped. Therefore, the filled traps around Eu 2+ II exhaust more quickly than those around Eu 2+ I. Moreover, it is reasonably considered that V. CONCLUSION Red long persistent phosphors with highly chemical stabilities are hardly developed based on the conventional long persistent phosphorescence mechanisms. The idea of persistent energy transfer demonstrates a way to develop long persistent phosphors with almost any specified coloration. Under this idea, Mn 2+ activated red phosphorescence is achieved in BaMg 2 Si 2 O 7 :Eu 2+,Mn 2+,Dy 3+. Effective energy transfer from Eu 2+ to Mn 2+ is observed in both the photoluminescence and the phosphorescence. Two types of Mn 2+ centers with emitting peaks at 620 nm Mn 2+ I and 675 nm Mn 2+ II, respectively, are also observed. In MD, the energy storing is through the excitation on Mn 2+ I and Mn 2+ II, while in MED, the trap filling is performed mainly through exciting Eu 2+ and the red phosphorescence is dominantly performed through persistent energy transfer from Eu 2+. Moreover, the incorporation of Mn 2+ leads to the formation of deeper traps, and consequently prolongs the phosphorescent persistence time in MED compared with ED. In addition, the emission wavelength of the sensitizer Eu 2+ in BaMg 2 Si 2 O 7 is peaked at 400 nm, to which human eyes are not sensitive, and is superior to the similar long persistent phosphor materials, where the emission of the sensitizer is clearly visible and modifies the red phosphorescence. In our system, pure red phosphorescence is the only color viewed by the naked eyes.

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