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Materials Letters 62 (2008) 202 – 205 www.elsevier.com/locate/matlet
The relationship between photoluminescence quenching concentrations and excitation wavelengths in (Gd,Y)BO3:Tb Jiachi Zhang, Yuhua Wang ⁎, Zhiya Zhang, Zhilong Wang, Bin Liu Department of Materials Science, Lanzhou University, Lanzhou, 730000, PR China Received 7 October 2006; accepted 29 April 2007 Available online 8 May 2007
Abstract The quenching concentrations of 5D4–7F6 emission of Tb3+ in (Gd,Y)BO3:Tb under 130–290 nm excitation were systematically investigated. The results revealed that its quenching concentrations of luminescence excited at particular wavelengths are dependent on corresponding excitation bands. Resulting in a calculation of coupling interaction, it was found that the quenching concentrations at excitation regions due to electrostatic interaction are often small while those corresponding to exchange interaction are usually larger (N10%). Moreover, the quenching concentrations are also influenced significantly by luminescence sensitization of Gd3+ and Y3+ ions. Based on these results, a possible photoluminescence quenching mechanism was proposed. © 2007 Elsevier B.V. All rights reserved. Keywords: Quenching concentration; Excitation bands; (Gd,Y)BO3:Tb
1. Introduction The emission intensity of many phosphors as a function of activator concentration initially increases with activator concentration and then decreases, going through a maximum at some concentrations. The decrease in emission intensity at a high activator concentration is referred to as the phenomenon of “concentration quenching” [1,2] and this optimal activator concentration with the highest emission intensity is considered as “quenching concentration” of phosphors. A complete characterization of the luminescence quenching concentrations in inorganic phosphors is very important not only for technological design considerations but also for the basic understanding of the physical excitation processes involved. Over the last decades, the quenching behaviors in oxide systems had been widely investigated in the cathode-ray ultraviolet (CUV), X-ray ultraviolet (XUV) and ultraviolet (UV) excitation regions [1–6]. However, the majority only reported the quenching characteristics at one excitation wavelength without systematic investigation in an excitation region [2–10]. As a result, a point commonly overlooked in quenching
studies on phosphors is the relationship between quenching concentrations and excitation wavelengths. Moreover, the quenching behaviors in the vacuum ultraviolet (VUV) region are rarely investigated because of the lack of proper VUV light source. On the other hand, the (Gd,Y)BO3:Tb with a high absorption edge and shorter decay time in vacuum ultraviolet (VUV) region is a potential inorganic green phosphor for application in plasma display panels (PDPs), Hg-free lamps and back lighting fields [11–14]. Developing a model to research on quenching behaviors in (Gd,Y)BO3:Tb is significant to understand its photoluminescence properties, excitation pathways and quenching mechanisms, even developing some novel phosphors with efficient photoluminescence. It is our fundamental aim to clarify the relationship between quenching concentrations and excitation wavelengths of typical (Gd,Y)BO3:Tb phosphors. In this work, the characteristics of quenching concentration in (Gd,Y)BO3:Tb under 130–290 nm were investigated completely and a possible quenching mechanism was proposed according to these results. 2. Experimental section
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All (Gd,Y)BO3:Tb samples were prepared by solid-state method at 1100 °C for 3 h and were characterized as single
J. Zhang et al. / Materials Letters 62 (2008) 202–205
Table 1 showed the quenching concentrations of 5D4–7F6 transition of Tb in (Gd,Y)BO3:Tb samples excited at 130–290 nm. It was found that the quenching concentrations were correlated with the excitation bands of phosphors: (1) Under 265–285 nm excitation, the quenching concentration of Y1 − cTbcBO3 was about 10% which differed from the neighbor regions and this excitation region was generally ascribed to the lowest spinforbidden transition to 9D term of Tb3+[15–17]. This broad absorption band is also obvious in excitation spectrum of (Gd,Y)BO3:Tb as shown in Fig. 3. (2) Under 270–278 nm excitation in Y0.9 − cGd0.1TbcBO3, it increased from 10% to 12% by doping 10 mol% Gd3+ into YBO3:Tb. This excitation band could be attributed to 8S7/2–6IJ transition of Gd3+[11,16]. This band overlapped the lowest spin-forbidden transition to 9D term of Tb3+ so that it is difficult to distinguish in the excitation spectrum. (3) When excited at 230– 240 nm, the quenching concentrations of Y1 − cTbcBO3 varied to 6% and it was due to the lowest spin-allowed transition to 7D term of Tb3+[15–17]. The high excitation peak around 230–240 nm could also be observed in the excitation spectrum in Fig. 3. (4) In an interval of 144–186 nm excluding 150–156 nm, the quenching concentrations were 6% for Y1 − cTbcBO3 samples. This broad excitation band including 150–156 nm could be attributed to the borate host absorption band [18,19]. (5) In the region of 150–156 nm, the characteristic quenching concentrations were 10%, and it was due to the charge transfer (CT) transition from O2−:2P6 to Y3+:4P6(4d + 5s) . This ascription was also coincident with the identification of CT band of Y3+–O2− in some other systems confirmed in the excitation spectra [20–23]. The CT of Y3+–O2− transition overlapped the broad borate host absorption band (144–186 nm), so it was not obvious in the excitation spectrum (Fig. 3). (6) The quenching concentrations in 162–176 nm were 6% for Y1 − cBO3:Tbc but it increased to 10% for Y0.9 − cGd0.1BO3:Tbc. This excitation band could be contributed to the charge transfer (CT) transition of Gd3+–O2− [18,19]. Moreover, as calculated by Jφrgensen and Qiang Su [24,25], the CT transition of Gd3+–O2− in (Gd,Y)BO3 may be located at around 166 nm for most oxide systems. Similar to CT band of Y3+–O2−, it also overlapped the broad borate host absorption. These results indicated that the quenching concentration of (Gd,Y) BO3:Tb at a particular excitation wavelength was dependent on corresponding excitation bands in essence. In addition, it seemed that the “quenching concentrations” may serve as a method to identify these unclear excitation bands of phosphors in VUV region. 3+
Fig. 1. The emission spectra of YBO3:Tb under 147 nm and 254 nm excitation, respectively.
phase by Rigaku D/max-2000 powder X-ray diffraction. Their photoluminescence was recorded on FLS-920T and ARC model VM-504 vacuum monochromator. The VUV excitation spectra were corrected by dividing the excitation intensity of sodium salicylate. All the spectra were recorded at room temperature. 3. Results and discussion 3.1. Photoluminescence at 147 nm and 254 nm The 147 nm and 254 nm are the most typical excitation sources for application in VUVand UV region, respectively. As for typical examples, Fig. 1 showed the emission spectra of typical Y0.94Tb0.06BO3 samples under 147 nm and 254 nm excitation. As shown in Fig. 1, the emission intensity of samples excited at 147 nm is much higher than that at 254 nm. The main emission peaks at about 476, 543, 570 and 621 nm are due to the 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 transitions of Tb3+, respectively and the green 5D4–7F5 emission at 543 nm is dominant. 3.2. Quenching characteristics at 147 nm and 254 nm The photoluminescence characteristics of phosphors are generally correlated with their quenching behaviors. Fig. 2 exhibited the relationship between emission intensity of Tb3+(5D4–7F5 transition) and Tb3+-dopant concentration i.e. the quenching concentration curves of Y1 − cTbcBO3 samples excited at 147 nm and 254 nm, respectively. As shown in Fig. 2(a), excited by 147 nm, the emission intensity of Tb3+ first increases with the increase of Tb-doped concentration (C), reaching a maximum value at 6 mol%, and then decreases with increasing the concentration (C) due to the concentration quenching effect. Therefore, the quenching concentration for Tb3+ excited by 147 nm is 6 mol% in YBO3:Tb phosphor. Correspondingly, the quenching concentration under 254 nm excitation is about 12% as shown in Fig. 2(b). The results indicated that the quenching behaviors may be usually correlated with particular excitation wavelengths. 3.3. Quenching characteristics under 130–290 nm In order to clarify the relationship between quenching concentrations and excitation wavelengths, the quenching concentrations of (Gd, Y)BO3:Tb under 130–290 nm were investigated systematically.
Fig. 2. The quenching concentration curves of YBO3:Tb samples excited at 147 nm (a) and 254 nm (b), respectively.
J. Zhang et al. / Materials Letters 62 (2008) 202–205
Table 1 The quenching concentrations of 5D4–7F6 transition of Tb3+ in (Gd,Y)BO3:Tb samples when excited by 130–290 nm
3.4. The quenching mechanism (coupling interaction) under 130–290 nm It is known that the type of coupling interaction of activator-pairs is the emission quenching mechanism of phosphors in essence [1,2]. The coupling interaction of most phosphors at a particular excitation wavelength is due either to an electrostatic multipolar interaction or to a magnetic dipole interaction i.e. exchange coupling. As calculated by G. Blasse and B.C. Grabmaier , the critical distance between activators which would cause quenching effect is about 3 nm for electric multipolar interaction. If these quenching mechanisms of excitation transitions are due to exchange interaction, it restricts the value of Rc to some 0.5–0.8 nm. As a result, the quenching concentration for exchange interaction is often much larger than that for electric multipolar interaction and the type of coupling interaction could be calculated by its emission quenching curves [1,2]. As calculated by Dexter's formula and shown in Table 1, it was found that not only the quenching concentration but also the quenching mechanisms (coupling interaction) are dependent on excitation bands
under 130–290 nm. It is known that the type of coupling interaction determines the quenching concentration of phosphors. Our results indicated that the type of coupling interaction is also determined by particular excitation bands under 130–290 nm in essence. Therefore, it is understandable that, in different excitation bands, there are different coupling interactions for different excitation mechanisms and energy transfer processes. It is important to point out that, the quenching mechanism of (Gd,Y) BO3:Tb in 150–156 nm and 162–176 nm is an electrostatic interaction resulted in a calculation despite their higher quenching concentration (10%–12%). This is because of the influence of Y3+, Gd3+ ions as “sensitizers”. For example, in 162–176 nm, the Gd3+ would be strongly excited and transfer energy to Tb3+ for emission and the strongcoupling of Gd3+–Tb3+ pairs may decrease the interaction and energy transfer rate of Tb3+-pairs. As a result, the quenching concentrations are relatively larger in these “luminescence sensitization” regions (150– 156 nm, 162–176 nm and 270–278 nm). This discovery is very important and will help to develop some novel phosphors with both
J. Zhang et al. / Materials Letters 62 (2008) 202–205
Chinese Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20040730019). References           
Fig. 3. The excitation spectrum of YBO3:Tb samples monitored at 543 nm.
  
high emission intensity and large quenching concentration. However, the details of this influence process still need more investigation.
  
The photoluminescence quenching concentrations of (Gd,Y) BO3:Tb at particular excitation wavelengths are dependent on excitation bands in essence. The quenching concentrations of emissions due to electrostatic interaction are often small and those corresponding to exchange interaction are usually larger. In addition, the luminescence sensitization of Gd3+ and Y3+ ions would significantly influence the quenching behaviors. This dependence should be involved in the excitation mechanisms, types of coupling interaction and energy transfer pathways of (Gd,Y)BO3:Tb in particular excitation regions.
Acknowledgements This work was supported by Program for New Century Excellent Talents in University of China (NCET, 04-0978) and
      
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