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Electronic structure and photoluminescence properties of Eu2+-activated Ca4Si2O7F2
Optical Materials 33 (2011) 1803–1807
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Electronic structure and photoluminescence properties of Eu2+-activated Ca4Si2O7F2 Yongchao Jia, Hui Qiao, Ning Guo, Yuhua Zheng, Mei Yang, Yeju Huang, Hongpeng You ⇑ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2011 Received in revised form 13 June 2011 Accepted 21 June 2011 Available online 23 July 2011 Keywords: Ca4Si2O7F2 Electronic structure Optical properties Inorganic phosphor
a b s t r a c t The blue-emitting phosphors Ca(4x)EuxSi2O7F2 (0 < x 6 0.05) have been prepared by solid-state reaction and the photoluminescence properties have been studied systematically. The electronic structure of calcium fluoride silicate Ca4Si2O7F2 was calculated using the CASTEP code. The calculation results of electronic structure show that Ca4Si2O7F2 has an indirect band gap with 5 eV. The top of the valence band is dominated by O 2p and Si 3p states, while the bottom of the conduction band is mainly composed of Ca 3d states. Under the 350 nm excitation, the obtained sample shows a broad emission band in the wavelength range of 400–500 nm with peaks of 413 nm and 460 nm from two different luminescence centers, respectively. The relative intensity of the two peaks changes with the alteration of the Eu2+ concentration. The strong excitation bands of the powder in the wavelength range of 200–420 nm are favorable properties for the application as lighting-emitting-diode conversion phosphor. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction White lighting emitting diodes (LEDs) have attracted increasing attention as a light source for the next-generation general illumination due to the high brightness, long lifetime, low power consumption, and environmentally friendly characteristics promised by solid-state lighting [1,2]. To generate white light from LEDs, the most common and simple method is to combine an InGaNbased blue diode with yellow phosphor material, namely YAG:Ce3+ (YAG denotes yttrium aluminum garnet). However, due to the deficiency in the red region of YAG:Ce3+ phosphor, this approach gives ‘‘cool’’-white light with correlated color temperature (CCT) greater than 4000 K and cannot fulfill the applications requiring high color rending properties, such as general illumination and medical lighting [3]. Hence, to realize warm white-lighting emitting diodes with CCT in the range of 2500 K–3200 K, the orange/red phosphors (e.g., CaAlSiN3:Eu2+, a-SrNCN:Eu2+, M2Si5N8:Eu2+, CaS:Eu2+) are needed [4–7]. But the problems about the complex preparation process of the nitride system and stability of the sulfide system should be solved before the ideas come true. Another possible approach to realize white emission is to use tricolor broadband-emitting phosphors with a shorter-wavelength InGaN LED [8]. In this way, the visible components of the white light are generated only by phosphors, exhibiting high color rending properties and low color point variation against the forward-bias currents [9]. With the development of the efficient LEDs that emit light in the n-UV range
⇑ Corresponding author. Tel.: +86 431 85262798. E-mail address: [email protected] (H. You). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.06.012
around 400 nm, much attention has been paid to the approach for meeting the requirement of various applications. The current blue phosphor material for solid-state lighting based on n-UV LED is mainly BaMgAl10O17:Eu2+ (BAM), but BAM shows a poor absorption band around 400 nm, not well suitable for InGaN chips. The synthesis of BAM is also a costly process which is usually at temperature as high as 1300–1600 °C for a long time. Accordingly, it is urgent to develop new blue phosphors that could be synthesized conveniently and effectively excited in the ultraviolet range. Halide-containing oxide-type hosts are good candidates as host structures due to several merits, such as low synthesis temperature, high chemical and physical stability [10–12]. There are some reports about the system used as the matrix for the inorganic phosphor. So as a member of the halide-containing oxide-type hosts, the calcium fluoride silicate (Ca4Si2O7F2) attracted our attention. The crystal structure of cuspidine (Ca4Si2O7F2) has been determined by Smirnova et al. (1955) and the schema of the structure was presented [13]. As far as we know, however, there is no report on the luminescent properties of rare earth ion-activated Ca4Si2O7F2. Eu2+ ion is one of the most important activators for LED application. Its emissions, arising from orbitally allowed d ? f electronic transitions, are wavelength tunable because they are sensitive to the crystal field splitting and nephelauxetic effects. The emission bands of Eu2+ activated phosphors can cover the whole visible range [14]. For the above reasons, in the present study, we firstly performed first-principles calculations to investigate the electronic structure of the undoped Ca4Si2O7F2 for understanding the crystal structure from theory aspect. Then we synthesized a series of Eu2+ doped Ca4Si2O7F2 compounds, studied and optimized the photoluminescence properties to value the
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potential as a blue-emitting phosphor for UV-converting white lighting-emitting diodes in the view of experiment. 2. Experiment procedures 2.1. Density functional theory (DFT) calculation All calculations were performed in the density functional theory (DFT) framework using the CASTEP (Cambridge serial total Energy package) module [15] of Materials Studio 4.0. The exchange– correlation effects were treated within the generalized gradient approximation (GGA) with the PBE functional [16]. Two steps were necessary to calculate the electronic band structure of Ca4Si2O7F2. The first step was to optimize the crystal structure using the crystallographic data reported in literature 13. The second step was to calculate the band structure and density of states of Ca4Si2O7F2 for the optimized structure. Lattice parameter and atomic coordinates were fixed at the values obtained by the crystal structure optimization process in the first step. For the two steps, the basic parameters were chosen as follows in setting up the CASTEP run: The 0 kinetic energy cutoff = 340 eV, k-point spacing = 0.05 Å A1, sets of k points = 4 2 2, self-consistent field tolerance thresholds = 1.0 105 eV/atom, and space representation = reciprocal. The reliability of the calculation was demonstrated by the result of the convergence test. 2.2. Synthesis and characterizations The phosphors with the composition of Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) were synthesized by a solid state reaction approach using CaF2 [analytic reagent (A.R.)], CaCO3 [analytic reagent (A.R.)], SiO2 [analytic reagent (A.R.)], Eu2O3 (99.99%) as the starting materials. The appropriate amounts of raw materials were weighed out and thoroughly mixed by grinding in an agate mortar, and subsequently the mixture was prefired at 873 K for 2 h. After slowly cooling down to room temperature, the prefired samples were thoroughly reground and then calcined at 1373 K for 3 h in the CO reducing atmosphere. X-ray powder diffraction measurements were performed on a D8 focus diffractometer (Bruker) at a scanning rate of 0.2°/min in the 2h range from 10° to 70°, with graphite-monochromatized Cu Ka radiation (k = 0.15405 nm) at 40 kV and 40 mA. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The diffuse-reflectance spectra were obtained by a SHIMADZU UV-3600 UV–vis-NiR spectrophotometer with the reflection of black felt (reflection 3%) and white BaSO4 (reflection 100%) in the wavelength region of 200–600 nm. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). Photoluminescence quantum yield (QY) was measured by absolute PL quantum yield measurement system C9920-02. All the measurements mentioned above were performed at room temperature. 3. Results and discussion 3.1. Electronic structure calculations The convergence test of the geometry optimization and energy calculation showed well, demonstrating our basic parameters were suitable. The lattice parameters obtained by the calculations are accordance with the experiments results, which are not indicated here.
Fig. 1 shows the band structure of Ca4Si2O7F2. It is seen that Ca4Si2O7F2 shows an indirect optical band gap. The gap between the lowest energy of the conduction band and highest energy of the valence band is about 5.0 eV. It can be concluded that Ca4Si2O7F2 belongs to the category of materials with large bandgaps, which is a favorable property for luminescence materials, helping to accommodate both the group and excited states of luminescent ions within the band gap [17]. Composition of the calculated energy bands can be resolved with the help of partial density of states (PDOS) and total density of states (DOS) diagram. Fig. 2 describes the total and partial density of states of Ca4Si2O7F2. These diagrams allow concluding that the conduction band in Ca4Si2O7F2 is about 2 eV wide and is mainly formed by the Ca 3d states, which are hybridized with Si 3s, 3p states and O 2p states. The valence band is wide – about 8 eV – and consists of two subbands, clearly seen in the band structure as well: the upper one (between 5 and 0 eV) is composed of the O 2s states, F 2p states and Si 3s, 3p states. The lower one is narrow (between 8 and 7 eV) and is mixed by the Si 3s, 3p states with a minor contribution coming from the O 2s, 2p states as well. Another band between 24 eV and 15 eV is created by O 2s states, F 2s states, Si 3s, 3p states and Ca 3p states. The lowest energy band is due to completely filled Ca 4s states. These preliminary results provide useful information of the host lattice which helps to understand the luminescence phenomenon. In addition, calculations on the rare-earth doped system have been considered. 3.2. Phase formation and structural characters Fig. 3 presents the XRD patterns of the Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) samples. All the diffraction peaks of the samples can be basically indexed to the standard data of Ca4Si2O7F2 (JCPDS Card No. 41-1474). No other phase is detected, indicating that the obtained samples are single phase and the Eu2+ ion has been successfully incorporated in the Ca4Si2O7F2 host lattices by replacing the Ca2+ due to their similar ionic radii and charge [18]. Ca4Si2O7F2 belongs to the monoclinic system with space group P121/c 1. The different crystallographic sites are available for the divalent Ca2+ ions as shown in Fig. 4 [13]. Through observing the projection of the crystal structure carefully, valuable information can be concluded intuitively and clearly. Ca crystallographic sites have directly coordinated with the O and F atoms in different O/F ratio. The electronegativity of O (3.44) is different from the F (3.98), which leads to the difference of the chemical bonds Ca–O and Ca–F. Therefore, it would be expected that these local structure characters would have great influences on the luminescence properties of Eu2+ when Eu2+ ions substitute Ca2+ ions. 3.3. Luminescence properties of Ca4Si2O7F2:Eu2+ The diffuse reflectance spectra of the host lattice and Eu2+ doped Ca4Si2O7F2 are shown in Fig. 5. The undoped sample shows a weak absorption band in the wavelength range of 200–300 nm, corresponding to the host absorption band. For the Eu2+ doped samples, two broad absorption bands can be seen from the diffuse reflectance spectra. One is in the wavelength range of 300–420 nm; the other is a short-wavelength absorption band in the wavelength range of 200–300 nm. The two absorption bands are attributed to the 4f ? 5d transition of the Eu2+ ions, which can be concluded from the high reflection in this region of the host and the changing intensity with the Eu2+ content of the activator doped samples. The absorption bands cover the ultraviolet region, indicating the samples meet the requirement of the UVLED conversion phosphor. Fig. 6 illustrates the PLE and PL spectra of Ca3.99Eu0.01Si2O7F2. Upon excitation of 350 nm, the sample exhibits intense blue luminescence with the asymmetric emission band peaking at 413
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Fig. 1. Calculated band structure of Ca4Si2O7F2. Left: overall view; right: enlarged view of the band gap with indication of its indirect character.
Fig. 2. Partial and total densities of states for Ca4Si2O7F2.
Fig. 4. Projection of the crystal structure of Ca4Si2O7F2.
Fig. 3. XRD patterns of Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05).
Fig. 5. Diffuse reflection spectra of Ca(4x)EuxSi2O7F2 (x = 0, 0.005, 0.01).
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Fig. 6. PLE and PL spectra of Ca3.99Eu0.01Si2O7F2 phosphor.
and 460 nm. The two emission band corresponds to the allowed 4f65d ? 4f7 electronic transitions of Eu2+ ions occupying two different Ca2+ sites, which can be presumed from the crystal structure of host matrix and the excitation spectra. As discussed above, the Ca4Si2O7F2 crystal structure has different Ca2+ crystallographic sites, a requisite condition for multi-luminescence centers. Monitored by the emission at 413 and 460 nm, the PLE spectra show the different profiles. Both PLE spectra consist of two broadbands from 200 to 300 nm and 300 to 420 nm. The bands in UV region between 200 and 420 nm are assigned to well-known transitions from the ground states 4f7 to crystal-field split 4f65d1 configuration, which are strongly affected by the environments of the Eu2+ ions. The different shapes of the PLE spectra in this region indicate that the emission bands peaking at 416 and 460 nm come from two luminescence centers with different characteristic crystal environments. In addition, it can be summarized that the emission band of the Eu2+ activated fluorine-containing silicate shifts the short wavelength compared with the silicate. The blue-shift of the emission band is attributed to the difference between the Ca–O and Ca–F chemical band. The crystal structure of the fluorine-containing silicate is much more ionic than the silicate lattice, which leads to the related luminescence properties. It is generally accepted that the concentration of activator plays an important role in the search of optimal composition of a phosphor. Appropriate content of the activator can achieve the suitable emission intensity and wavelength [19,20]. Therefore, the variations of PL intensity and shape with different Eu2+ concentrations for Ca(4x)EuxSi2O7F2 have been investigated in this paper. Fig. 7 shows the dependence of the emission spectra of Ca(4x)EuxSi2O7F2 on the concentration of Eu2+. The profile change in the emission spectra is attributed to the ratio of the 413–460 nm emission peak alteration with the Eu2+ concentration. The inset in Fig. 7 expresses the intensity of two emission peaks as a function of the Eu2+ content. The optimal doping concentrations were observed to be at 3 mol% and 1 mol% for the 413 and 460 nm emission peaks, respectively. Fig. 8 shows the PL decay curves of the Eu2+ ions in Ca3.99Eu0.01 Si2O7F2, which measured with excitation at 355 nm and monitored at 413 nm and 460 nm. The decay curves exhibit different profiles on the condition of the measurement, which confirms the above conclusion about the origin of the emission band. Table 1 lists the decay lifetime of Eu2+ ions in the Ca(4x)EuxSi2O7F2. One can see that the decay lifetime of Eu2+ ions at the high energy site generally decreases with increasing the doping content, which may re-
Fig. 7. Emission spectra of Ca4Si2O7F2:Eu2+ with various amounts of Eu2+ (Inset: the PL intensity of two emission peaks as a function of Eu2+ concentration).
Fig. 8. Photoluminescence decay curves of Eu2+ in the Ca3.99Eu0.01Si2O7F2 (excited at 355 nm, monitored at 413 nm and 460 nm).
Table 1 Decay lifetime of the Eu2+ in the Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) under the monitoring condition at 413 nm(s413) and 460 nm(s460). Eu concentration (%)
s413 (ls)
s460 (ls)
0.005 0.01 0.02 0.03 0.04 0.05
1.05 1.01 0.55 0.71 0.40 0.33
0.58 0.51 0.47 0.41 0.39 0.31
sult from the energy transfer between the high and low energy site; while the decay lifetime of Eu2+ ion at the low energy site shows the similar variation, the change may be attribute to the integrated effect of the Eu2+ ion concentration, energy transfer and crystal defects. The phenomenon of energy transfer often occurs between different luminescent ions, such as Ce3+–Eu2+ [21], Eu2+–Mn2+ [22,23], Ce3+–Tb3+ [24], which is used to explain the interaction
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of the ion pairs. Energy transfer among the same luminescent ions that occupy different crystallography sites also can affect spectral position, and usually be utilized to analyze the red shift of the emission band with the concentration change of activator [9,19]. As a mean of quantitative analysis, it helps us to understand the luminescence phenomenon and design the new inorganic phosphor. As for the luminescent system where the same activator occupies different crystallography sites, some other factors should also be taken into account, including the occupancy site and the quenching concentration of the luminescence center [25–30]. The change in occupancy site with the concentration of the activators has been reported by some groups. It is an important factor to determine the intensity and wavelength of the emission band. Another important factor affecting the luminescence properties is quenching concentration of the luminescence center related to the crystal field surroundings. Compared with the red-shift of emission band with increasing activator content, the blue-shift has rarely been reported [31]. In our case, we presume that the blue-shift of the emission band results from the integrated effect of the energy transfer, occupancy site and characteristic quenching concentration of activator at the crystallography site. The energy transfer from the high energy site to low energy site contributes the enhancement of the emission band peaking at 460 nm, which can be judged from the overlap between the emission spectra of the high energy site and the excitation spectra of the low energy site (Fig. 3) and the lifetime measurement (Table 1). The latter factors may benefit the enhancement of the emission band peaking at 413 nm. The occupancy of the two crystallography sites may change with the Eu2+ concentration. At lower doping concentration, Eu2+ ions enter the low energy site preferentially; while at higher doping concentration, Eu2+ ions primarily occupy the high energy site. The quenching concentration of the Eu2+ at high energy site may be higher than that of the Eu2+ ions at the low energy site. The collaboration and competition of these effects give rise to the concentration-dependent luminescence behaviors. Upon excitation at a wavelength of 350 nm, the absolute quantum efficiency (QE) of composition-optimized Ca3.99Eu0.01Si2O7F2 are determined to be 16.8%, the observed low QE of the sample can be resulted from the presence of different sites and can be further improved by process optimization. From the corresponding PL spectra upon 350 nm excitation, the commission International de I’Eclairage chromaticity coordination of the phosphor Ca3.99Eu0.01Si2O7F2 have been calculated to be (0.157, 0.108), falls into the blue region. The above characteristic indexes of the sample show that the optimized Ca3.99Eu0.01Si2O7F2 phosphor has intense blue emission under the excitation of 350 nm, thus the as-prepared sample has a potential to be a blue-emitting phosphor for the n-UV excited solid state lighting.
4. Conclusion
module of Materials Studio 4.0. The calculation results show that the Ca4Si2O7F2 has a proper band gap for using as a support of rare earth activator. The structure, diffuse reflection spectra, photoluminescence spectra and color-coordinate parameters of phosphor were investigated as well. The diffuse-reflectance and PLE spectra show broadband absorption in the range of 200–420 nm, which match the emission of n-UV chip well. Under 350 nm excitation, the optimized phosphor, Ca3.99Eu0.01Si2O7F2, shows a intense blue light emitting with chromaticity coordination (0.157, 0.108). Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 20771098) and the NSFC Fund for Creative Research Group (Grant No. 20921002), and the National Basic Research Program of China (973 Program, Grant No. 2007CB935502). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
In summary, a novel blue-emitting phosphor Ca4Si2O7F2:Eu2+ was synthesized by the conventional solid state reaction. Electronic structure of the host matrix was analyzed using CASTEP
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Electronic structure and photoluminescence properties of Eu2+-activated Ca4Si2O7F2 Yongchao Jia, Hui Qiao, Ning Guo, Yuhua Zheng, Mei Yang, Yeju Huang, Hongpeng You ⇑ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2011 Received in revised form 13 June 2011 Accepted 21 June 2011 Available online 23 July 2011 Keywords: Ca4Si2O7F2 Electronic structure Optical properties Inorganic phosphor
a b s t r a c t The blue-emitting phosphors Ca(4x)EuxSi2O7F2 (0 < x 6 0.05) have been prepared by solid-state reaction and the photoluminescence properties have been studied systematically. The electronic structure of calcium fluoride silicate Ca4Si2O7F2 was calculated using the CASTEP code. The calculation results of electronic structure show that Ca4Si2O7F2 has an indirect band gap with 5 eV. The top of the valence band is dominated by O 2p and Si 3p states, while the bottom of the conduction band is mainly composed of Ca 3d states. Under the 350 nm excitation, the obtained sample shows a broad emission band in the wavelength range of 400–500 nm with peaks of 413 nm and 460 nm from two different luminescence centers, respectively. The relative intensity of the two peaks changes with the alteration of the Eu2+ concentration. The strong excitation bands of the powder in the wavelength range of 200–420 nm are favorable properties for the application as lighting-emitting-diode conversion phosphor. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction White lighting emitting diodes (LEDs) have attracted increasing attention as a light source for the next-generation general illumination due to the high brightness, long lifetime, low power consumption, and environmentally friendly characteristics promised by solid-state lighting [1,2]. To generate white light from LEDs, the most common and simple method is to combine an InGaNbased blue diode with yellow phosphor material, namely YAG:Ce3+ (YAG denotes yttrium aluminum garnet). However, due to the deficiency in the red region of YAG:Ce3+ phosphor, this approach gives ‘‘cool’’-white light with correlated color temperature (CCT) greater than 4000 K and cannot fulfill the applications requiring high color rending properties, such as general illumination and medical lighting [3]. Hence, to realize warm white-lighting emitting diodes with CCT in the range of 2500 K–3200 K, the orange/red phosphors (e.g., CaAlSiN3:Eu2+, a-SrNCN:Eu2+, M2Si5N8:Eu2+, CaS:Eu2+) are needed [4–7]. But the problems about the complex preparation process of the nitride system and stability of the sulfide system should be solved before the ideas come true. Another possible approach to realize white emission is to use tricolor broadband-emitting phosphors with a shorter-wavelength InGaN LED [8]. In this way, the visible components of the white light are generated only by phosphors, exhibiting high color rending properties and low color point variation against the forward-bias currents [9]. With the development of the efficient LEDs that emit light in the n-UV range
⇑ Corresponding author. Tel.: +86 431 85262798. E-mail address: [email protected] (H. You). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.06.012
around 400 nm, much attention has been paid to the approach for meeting the requirement of various applications. The current blue phosphor material for solid-state lighting based on n-UV LED is mainly BaMgAl10O17:Eu2+ (BAM), but BAM shows a poor absorption band around 400 nm, not well suitable for InGaN chips. The synthesis of BAM is also a costly process which is usually at temperature as high as 1300–1600 °C for a long time. Accordingly, it is urgent to develop new blue phosphors that could be synthesized conveniently and effectively excited in the ultraviolet range. Halide-containing oxide-type hosts are good candidates as host structures due to several merits, such as low synthesis temperature, high chemical and physical stability [10–12]. There are some reports about the system used as the matrix for the inorganic phosphor. So as a member of the halide-containing oxide-type hosts, the calcium fluoride silicate (Ca4Si2O7F2) attracted our attention. The crystal structure of cuspidine (Ca4Si2O7F2) has been determined by Smirnova et al. (1955) and the schema of the structure was presented [13]. As far as we know, however, there is no report on the luminescent properties of rare earth ion-activated Ca4Si2O7F2. Eu2+ ion is one of the most important activators for LED application. Its emissions, arising from orbitally allowed d ? f electronic transitions, are wavelength tunable because they are sensitive to the crystal field splitting and nephelauxetic effects. The emission bands of Eu2+ activated phosphors can cover the whole visible range [14]. For the above reasons, in the present study, we firstly performed first-principles calculations to investigate the electronic structure of the undoped Ca4Si2O7F2 for understanding the crystal structure from theory aspect. Then we synthesized a series of Eu2+ doped Ca4Si2O7F2 compounds, studied and optimized the photoluminescence properties to value the
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potential as a blue-emitting phosphor for UV-converting white lighting-emitting diodes in the view of experiment. 2. Experiment procedures 2.1. Density functional theory (DFT) calculation All calculations were performed in the density functional theory (DFT) framework using the CASTEP (Cambridge serial total Energy package) module [15] of Materials Studio 4.0. The exchange– correlation effects were treated within the generalized gradient approximation (GGA) with the PBE functional [16]. Two steps were necessary to calculate the electronic band structure of Ca4Si2O7F2. The first step was to optimize the crystal structure using the crystallographic data reported in literature 13. The second step was to calculate the band structure and density of states of Ca4Si2O7F2 for the optimized structure. Lattice parameter and atomic coordinates were fixed at the values obtained by the crystal structure optimization process in the first step. For the two steps, the basic parameters were chosen as follows in setting up the CASTEP run: The 0 kinetic energy cutoff = 340 eV, k-point spacing = 0.05 Å A1, sets of k points = 4 2 2, self-consistent field tolerance thresholds = 1.0 105 eV/atom, and space representation = reciprocal. The reliability of the calculation was demonstrated by the result of the convergence test. 2.2. Synthesis and characterizations The phosphors with the composition of Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) were synthesized by a solid state reaction approach using CaF2 [analytic reagent (A.R.)], CaCO3 [analytic reagent (A.R.)], SiO2 [analytic reagent (A.R.)], Eu2O3 (99.99%) as the starting materials. The appropriate amounts of raw materials were weighed out and thoroughly mixed by grinding in an agate mortar, and subsequently the mixture was prefired at 873 K for 2 h. After slowly cooling down to room temperature, the prefired samples were thoroughly reground and then calcined at 1373 K for 3 h in the CO reducing atmosphere. X-ray powder diffraction measurements were performed on a D8 focus diffractometer (Bruker) at a scanning rate of 0.2°/min in the 2h range from 10° to 70°, with graphite-monochromatized Cu Ka radiation (k = 0.15405 nm) at 40 kV and 40 mA. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The diffuse-reflectance spectra were obtained by a SHIMADZU UV-3600 UV–vis-NiR spectrophotometer with the reflection of black felt (reflection 3%) and white BaSO4 (reflection 100%) in the wavelength region of 200–600 nm. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). Photoluminescence quantum yield (QY) was measured by absolute PL quantum yield measurement system C9920-02. All the measurements mentioned above were performed at room temperature. 3. Results and discussion 3.1. Electronic structure calculations The convergence test of the geometry optimization and energy calculation showed well, demonstrating our basic parameters were suitable. The lattice parameters obtained by the calculations are accordance with the experiments results, which are not indicated here.
Fig. 1 shows the band structure of Ca4Si2O7F2. It is seen that Ca4Si2O7F2 shows an indirect optical band gap. The gap between the lowest energy of the conduction band and highest energy of the valence band is about 5.0 eV. It can be concluded that Ca4Si2O7F2 belongs to the category of materials with large bandgaps, which is a favorable property for luminescence materials, helping to accommodate both the group and excited states of luminescent ions within the band gap [17]. Composition of the calculated energy bands can be resolved with the help of partial density of states (PDOS) and total density of states (DOS) diagram. Fig. 2 describes the total and partial density of states of Ca4Si2O7F2. These diagrams allow concluding that the conduction band in Ca4Si2O7F2 is about 2 eV wide and is mainly formed by the Ca 3d states, which are hybridized with Si 3s, 3p states and O 2p states. The valence band is wide – about 8 eV – and consists of two subbands, clearly seen in the band structure as well: the upper one (between 5 and 0 eV) is composed of the O 2s states, F 2p states and Si 3s, 3p states. The lower one is narrow (between 8 and 7 eV) and is mixed by the Si 3s, 3p states with a minor contribution coming from the O 2s, 2p states as well. Another band between 24 eV and 15 eV is created by O 2s states, F 2s states, Si 3s, 3p states and Ca 3p states. The lowest energy band is due to completely filled Ca 4s states. These preliminary results provide useful information of the host lattice which helps to understand the luminescence phenomenon. In addition, calculations on the rare-earth doped system have been considered. 3.2. Phase formation and structural characters Fig. 3 presents the XRD patterns of the Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) samples. All the diffraction peaks of the samples can be basically indexed to the standard data of Ca4Si2O7F2 (JCPDS Card No. 41-1474). No other phase is detected, indicating that the obtained samples are single phase and the Eu2+ ion has been successfully incorporated in the Ca4Si2O7F2 host lattices by replacing the Ca2+ due to their similar ionic radii and charge [18]. Ca4Si2O7F2 belongs to the monoclinic system with space group P121/c 1. The different crystallographic sites are available for the divalent Ca2+ ions as shown in Fig. 4 [13]. Through observing the projection of the crystal structure carefully, valuable information can be concluded intuitively and clearly. Ca crystallographic sites have directly coordinated with the O and F atoms in different O/F ratio. The electronegativity of O (3.44) is different from the F (3.98), which leads to the difference of the chemical bonds Ca–O and Ca–F. Therefore, it would be expected that these local structure characters would have great influences on the luminescence properties of Eu2+ when Eu2+ ions substitute Ca2+ ions. 3.3. Luminescence properties of Ca4Si2O7F2:Eu2+ The diffuse reflectance spectra of the host lattice and Eu2+ doped Ca4Si2O7F2 are shown in Fig. 5. The undoped sample shows a weak absorption band in the wavelength range of 200–300 nm, corresponding to the host absorption band. For the Eu2+ doped samples, two broad absorption bands can be seen from the diffuse reflectance spectra. One is in the wavelength range of 300–420 nm; the other is a short-wavelength absorption band in the wavelength range of 200–300 nm. The two absorption bands are attributed to the 4f ? 5d transition of the Eu2+ ions, which can be concluded from the high reflection in this region of the host and the changing intensity with the Eu2+ content of the activator doped samples. The absorption bands cover the ultraviolet region, indicating the samples meet the requirement of the UVLED conversion phosphor. Fig. 6 illustrates the PLE and PL spectra of Ca3.99Eu0.01Si2O7F2. Upon excitation of 350 nm, the sample exhibits intense blue luminescence with the asymmetric emission band peaking at 413
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Fig. 1. Calculated band structure of Ca4Si2O7F2. Left: overall view; right: enlarged view of the band gap with indication of its indirect character.
Fig. 2. Partial and total densities of states for Ca4Si2O7F2.
Fig. 4. Projection of the crystal structure of Ca4Si2O7F2.
Fig. 3. XRD patterns of Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05).
Fig. 5. Diffuse reflection spectra of Ca(4x)EuxSi2O7F2 (x = 0, 0.005, 0.01).
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Fig. 6. PLE and PL spectra of Ca3.99Eu0.01Si2O7F2 phosphor.
and 460 nm. The two emission band corresponds to the allowed 4f65d ? 4f7 electronic transitions of Eu2+ ions occupying two different Ca2+ sites, which can be presumed from the crystal structure of host matrix and the excitation spectra. As discussed above, the Ca4Si2O7F2 crystal structure has different Ca2+ crystallographic sites, a requisite condition for multi-luminescence centers. Monitored by the emission at 413 and 460 nm, the PLE spectra show the different profiles. Both PLE spectra consist of two broadbands from 200 to 300 nm and 300 to 420 nm. The bands in UV region between 200 and 420 nm are assigned to well-known transitions from the ground states 4f7 to crystal-field split 4f65d1 configuration, which are strongly affected by the environments of the Eu2+ ions. The different shapes of the PLE spectra in this region indicate that the emission bands peaking at 416 and 460 nm come from two luminescence centers with different characteristic crystal environments. In addition, it can be summarized that the emission band of the Eu2+ activated fluorine-containing silicate shifts the short wavelength compared with the silicate. The blue-shift of the emission band is attributed to the difference between the Ca–O and Ca–F chemical band. The crystal structure of the fluorine-containing silicate is much more ionic than the silicate lattice, which leads to the related luminescence properties. It is generally accepted that the concentration of activator plays an important role in the search of optimal composition of a phosphor. Appropriate content of the activator can achieve the suitable emission intensity and wavelength [19,20]. Therefore, the variations of PL intensity and shape with different Eu2+ concentrations for Ca(4x)EuxSi2O7F2 have been investigated in this paper. Fig. 7 shows the dependence of the emission spectra of Ca(4x)EuxSi2O7F2 on the concentration of Eu2+. The profile change in the emission spectra is attributed to the ratio of the 413–460 nm emission peak alteration with the Eu2+ concentration. The inset in Fig. 7 expresses the intensity of two emission peaks as a function of the Eu2+ content. The optimal doping concentrations were observed to be at 3 mol% and 1 mol% for the 413 and 460 nm emission peaks, respectively. Fig. 8 shows the PL decay curves of the Eu2+ ions in Ca3.99Eu0.01 Si2O7F2, which measured with excitation at 355 nm and monitored at 413 nm and 460 nm. The decay curves exhibit different profiles on the condition of the measurement, which confirms the above conclusion about the origin of the emission band. Table 1 lists the decay lifetime of Eu2+ ions in the Ca(4x)EuxSi2O7F2. One can see that the decay lifetime of Eu2+ ions at the high energy site generally decreases with increasing the doping content, which may re-
Fig. 7. Emission spectra of Ca4Si2O7F2:Eu2+ with various amounts of Eu2+ (Inset: the PL intensity of two emission peaks as a function of Eu2+ concentration).
Fig. 8. Photoluminescence decay curves of Eu2+ in the Ca3.99Eu0.01Si2O7F2 (excited at 355 nm, monitored at 413 nm and 460 nm).
Table 1 Decay lifetime of the Eu2+ in the Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) under the monitoring condition at 413 nm(s413) and 460 nm(s460). Eu concentration (%)
s413 (ls)
s460 (ls)
0.005 0.01 0.02 0.03 0.04 0.05
1.05 1.01 0.55 0.71 0.40 0.33
0.58 0.51 0.47 0.41 0.39 0.31
sult from the energy transfer between the high and low energy site; while the decay lifetime of Eu2+ ion at the low energy site shows the similar variation, the change may be attribute to the integrated effect of the Eu2+ ion concentration, energy transfer and crystal defects. The phenomenon of energy transfer often occurs between different luminescent ions, such as Ce3+–Eu2+ [21], Eu2+–Mn2+ [22,23], Ce3+–Tb3+ [24], which is used to explain the interaction
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of the ion pairs. Energy transfer among the same luminescent ions that occupy different crystallography sites also can affect spectral position, and usually be utilized to analyze the red shift of the emission band with the concentration change of activator [9,19]. As a mean of quantitative analysis, it helps us to understand the luminescence phenomenon and design the new inorganic phosphor. As for the luminescent system where the same activator occupies different crystallography sites, some other factors should also be taken into account, including the occupancy site and the quenching concentration of the luminescence center [25–30]. The change in occupancy site with the concentration of the activators has been reported by some groups. It is an important factor to determine the intensity and wavelength of the emission band. Another important factor affecting the luminescence properties is quenching concentration of the luminescence center related to the crystal field surroundings. Compared with the red-shift of emission band with increasing activator content, the blue-shift has rarely been reported [31]. In our case, we presume that the blue-shift of the emission band results from the integrated effect of the energy transfer, occupancy site and characteristic quenching concentration of activator at the crystallography site. The energy transfer from the high energy site to low energy site contributes the enhancement of the emission band peaking at 460 nm, which can be judged from the overlap between the emission spectra of the high energy site and the excitation spectra of the low energy site (Fig. 3) and the lifetime measurement (Table 1). The latter factors may benefit the enhancement of the emission band peaking at 413 nm. The occupancy of the two crystallography sites may change with the Eu2+ concentration. At lower doping concentration, Eu2+ ions enter the low energy site preferentially; while at higher doping concentration, Eu2+ ions primarily occupy the high energy site. The quenching concentration of the Eu2+ at high energy site may be higher than that of the Eu2+ ions at the low energy site. The collaboration and competition of these effects give rise to the concentration-dependent luminescence behaviors. Upon excitation at a wavelength of 350 nm, the absolute quantum efficiency (QE) of composition-optimized Ca3.99Eu0.01Si2O7F2 are determined to be 16.8%, the observed low QE of the sample can be resulted from the presence of different sites and can be further improved by process optimization. From the corresponding PL spectra upon 350 nm excitation, the commission International de I’Eclairage chromaticity coordination of the phosphor Ca3.99Eu0.01Si2O7F2 have been calculated to be (0.157, 0.108), falls into the blue region. The above characteristic indexes of the sample show that the optimized Ca3.99Eu0.01Si2O7F2 phosphor has intense blue emission under the excitation of 350 nm, thus the as-prepared sample has a potential to be a blue-emitting phosphor for the n-UV excited solid state lighting.
4. Conclusion
module of Materials Studio 4.0. The calculation results show that the Ca4Si2O7F2 has a proper band gap for using as a support of rare earth activator. The structure, diffuse reflection spectra, photoluminescence spectra and color-coordinate parameters of phosphor were investigated as well. The diffuse-reflectance and PLE spectra show broadband absorption in the range of 200–420 nm, which match the emission of n-UV chip well. Under 350 nm excitation, the optimized phosphor, Ca3.99Eu0.01Si2O7F2, shows a intense blue light emitting with chromaticity coordination (0.157, 0.108). Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 20771098) and the NSFC Fund for Creative Research Group (Grant No. 20921002), and the National Basic Research Program of China (973 Program, Grant No. 2007CB935502). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
In summary, a novel blue-emitting phosphor Ca4Si2O7F2:Eu2+ was synthesized by the conventional solid state reaction. Electronic structure of the host matrix was analyzed using CASTEP
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