Electronic Structure and Photoluminescence Properties of Eu -Activated Ca4si2o7f2 ⇑ Yongchao Jia, Hui Qiao, Ning Guo, Yuhua Zheng, Mei Yang, Yeju Huang, Hongpeng You

Optical Materials 33 (2011) 1803–1807

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Optical Materials

journal homepage: www.elsevier.com/locate/optmat

2+ Electronic structure and photoluminescence properties of Eu -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 article info abstract

Article history: The blue-emitting phosphors Ca(4x)EuxSi2O7F2 (0 < x 6 0.05) have been prepared by solid-state reaction Received 9 April 2011 and the photoluminescence properties have been studied systematically. The electronic structure of cal- Received in revised form 13 June 2011 cium ﬂuoride silicate Ca4Si2O7F2 was calculated using the CASTEP code. The calculation results of elec- Accepted 21 June 2011 tronic structure show that Ca Si O F has an indirect band gap with 5 eV. The top of the valence band Available online 23 July 2011 4 2 7 2 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 Keywords: wavelength range of 400–500 nm with peaks of 413 nm and 460 nm from two different luminescence Ca Si O F 4 2 7 2 centers, respectively. The relative intensity of the two peaks changes with the alteration of the Eu2+ Electronic structure Optical properties concentration. The strong excitation bands of the powder in the wavelength range of 200–420 nm are Inorganic phosphor favorable properties for the application as lighting-emitting-diode conversion phosphor. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction around 400 nm, much attention has been paid to the approach for meeting the requirement of various applications. White lighting emitting diodes (LEDs) have attracted increasing The current blue phosphor material for solid-state lighting 2+ attention as a light source for the next-generation general illumi- based on n-UV LED is mainly BaMgAl10O17:Eu (BAM), but BAM nation due to the high brightness, long lifetime, low power con- shows a poor absorption band around 400 nm, not well suitable sumption, and environmentally friendly characteristics promised for InGaN chips. The synthesis of BAM is also a costly process by solid-state lighting [1,2]. To generate white light from LEDs, which is usually at temperature as high as 1300–1600 °C for a long the most common and simple method is to combine an InGaN- time. Accordingly, it is urgent to develop new blue phosphors that based blue diode with yellow phosphor material, namely YAG:Ce3+ could be synthesized conveniently and effectively excited in the (YAG denotes yttrium aluminum garnet). However, due to the defi- ultraviolet range. Halide-containing oxide-type hosts are good can- ciency in the red region of YAG:Ce3+ phosphor, this approach gives didates as host structures due to several merits, such as low syn- ‘‘cool’’-white light with correlated color temperature (CCT) greater thesis temperature, high chemical and physical stability [10–12]. than 4000 K and cannot fulfill the applications requiring high color There are some reports about the system used as the matrix for rending properties, such as general illumination and medical light- the inorganic phosphor. So as a member of the halide-containing ing [3]. Hence, to realize warm white-lighting emitting diodes with oxide-type hosts, the calcium fluoride silicate (Ca4Si2O7F2) CCT in the range of 2500 K–3200 K, the orange/red phosphors (e.g., attracted our attention. The crystal structure of cuspidine 2+ 2+ 2+ 2+ CaAlSiN3:Eu , a-SrNCN:Eu ,M2Si5N8:Eu , CaS:Eu ) are needed (Ca4Si2O7F2) has been determined by Smirnova et al. (1955) and [4–7]. But the problems about the complex preparation process the schema of the structure was presented [13]. As far as we know, of the nitride system and stability of the sulfide system should however, there is no report on the luminescent properties of rare 2+ be solved before the ideas come true. Another possible approach earth ion-activated Ca4Si2O7F2.Eu ion is one of the most impor- to realize white emission is to use tricolor broadband-emitting tant activators for LED application. Its emissions, arising from orb- phosphors with a shorter-wavelength InGaN LED [8]. In this way, itally allowed d ? f electronic transitions, are wavelength tunable the visible components of the white light are generated only by because they are sensitive to the crystal field splitting and nephel- phosphors, exhibiting high color rending properties and low color auxetic effects. The emission bands of Eu2+ activated phosphors point variation against the forward-bias currents [9]. With the can cover the whole visible range [14]. For the above reasons, in development of the efficient LEDs that emit light in the n-UV range 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 2+ ⇑ Corresponding author. Tel.: +86 431 85262798. we synthesized a series of Eu doped Ca4Si2O7F2 compounds, stud- E-mail address: [email protected] (H. You). ied and optimized the photoluminescence properties to value the

potential as a blue-emitting phosphor for UV-converting white Fig. 1 shows the band structure of Ca4Si2O7F2. It is seen that lighting-emitting diodes in the view of experiment. 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 2. Experiment procedures Ca4Si2O7F2 belongs to the category of materials with large band- gaps, which is a favorable property for luminescence materials, 2.1. Density functional theory (DFT) calculation helping to accommodate both the group and excited states of luminescent ions within the band gap [17]. Composition of the calcu- All calculations were performed in the density functional theory lated energy bands can be resolved with the help of partial (DFT) framework using the CASTEP (Cambridge serial total Energy density of states (PDOS) and total density of states (DOS) diagram. package) module [15] of Materials Studio 4.0. The exchange– Fig. 2 describes the total and partial density of states of Ca Si O F . correlation effects were treated within the generalized gradient 4 2 7 2 These diagrams allow concluding that the conduction band in Ca approximation (GGA) with the PBE functional [16]. Two steps were 4- Si2O7F2 is about 2 eV wide and is mainly formed by the Ca 3d necessary to calculate the electronic band structure of Ca4Si2O7F2. states, which are hybridized with Si 3s, 3p states and O 2p states. The first step was to optimize the crystal structure using the crys- The valence band is wide – about 8 eV – and consists of two sub- tallographic data reported in literature 13. The second step was to bands, clearly seen in the band structure as well: the upper one calculate the band structure and density of states of Ca4Si2O7F2 for (between 5 and 0 eV) is composed of the O 2s states, F 2p states the optimized structure. Lattice parameter and atomic coordinates and Si 3s, 3p states. The lower one is narrow (between 8 and were fixed at the values obtained by the crystal structure optimi- 7 eV) and is mixed by the Si 3s, 3p states with a minor contribu- zation process in the first step. For the two steps, the basic param- tion coming from the O 2s, 2p states as well. Another band be- eters were chosen as follows in setting up the CASTEP run: The 0 1 tween 24 eV and 15 eV is created by O 2s states, F 2s states, kinetic energy cutoff = 340 eV, k-point spacing = 0.05 ÅA , sets of Si 3s, 3p states and Ca 3p states. The lowest energy band is due k points = 4 2 2, self-consistent field tolerance thresh- 5 to completely filled Ca 4s states. These preliminary results provide olds = 1.0 10 eV/atom, and space representation = reciprocal. useful information of the host lattice which helps to understand The reliability of the calculation was demonstrated by the result the luminescence phenomenon. In addition, calculations on the of the convergence test. rare-earth doped system have been considered.

2.2. Synthesis and characterizations 3.2. Phase formation and structural characters

The phosphors with the composition of Ca(4x)EuxSi2O7F2 Fig. 3 presents the XRD patterns of the Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) were synthesized by a solid (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) samples. All the diffraction state reaction approach using CaF [analytic reagent (A.R.)], CaCO 2 3 peaks of the samples can be basically indexed to the standard data [analytic reagent (A.R.)], SiO2 [analytic reagent (A.R.)], Eu2O3 of Ca4Si2O7F2 (JCPDS Card No. 41-1474). No other phase is de- (99.99%) as the starting materials. The appropriate amounts of tected, indicating that the obtained samples are single phase and raw materials were weighed out and thoroughly mixed by grinding 2+ the Eu ion has been successfully incorporated in the Ca4Si2O7F2 in an agate mortar, and subsequently the mixture was prefired at 2+ host lattices by replacing the Ca due to their similar ionic radii 873 K for 2 h. After slowly cooling down to room temperature, and charge [18].Ca4Si2O7F2 belongs to the monoclinic system with the prefired samples were thoroughly reground and then calcined space group P121/c 1. The different crystallographic sites are avail- at 1373 K for 3 h in the CO reducing atmosphere. 2+ able for the divalent Ca ions as shown in Fig. 4 [13]. Through X-ray powder diffraction measurements were performed on a observing the projection of the crystal structure carefully, valuable D8 focus diffractometer (Bruker) at a scanning rate of 0.2 /min in ° information can be concluded intuitively and clearly. Ca crystallo- the 2h range from 10 to 70 , with graphite-monochromatized Cu ° ° graphic sites have directly coordinated with the O and F atoms in K radiation (k = 0.15405 nm) at 40 kV and 40 mA. The photolumi- a different O/F ratio. The electronegativity of O (3.44) is different nescence (PL) and photoluminescence excitation (PLE) spectra of from the F (3.98), which leads to the difference of the chemical the obtained powders were recorded with a Hitachi F-7000 spec- bonds Ca–O and Ca–F. Therefore, it would be expected that these trophotometer equipped with a 150 W xenon lamp as the excita- local structure characters would have great influences on the lumi- tion source. The diffuse-reflectance spectra were obtained by a 2+ 2+ 2+ nescence properties of Eu when Eu ions substitute Ca ions. SHIMADZU UV-3600 UV–vis-NiR spectrophotometer with the reflection of black felt (reflection 3%) and white BaSO4 (reflection 2+ 3.3. Luminescence properties of Ca4Si2O7F2:Eu 100%) in the wavelength region of 200–600 nm. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital 2+ The diffuse reflectance spectra of the host lattice and Eu oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, doped Ca4Si2O7F2 are shown in Fig. 5. The undoped sample shows gate = 50 ns) as the excitation source (Continuum Sunlite OPO). a weak absorption band in the wavelength range of 200–300 nm, Photoluminescence quantum yield (QY) was measured by absolute corresponding to the host absorption band. For the Eu2+ doped PL quantum yield measurement system C9920-02. All the mea- samples, two broad absorption bands can be seen from the diffuse surements mentioned above were performed at room temperature. reflectance spectra. One is in the wavelength range of 300–420 nm; the other is a short-wavelength absorption band in the wavelength 3. Results and discussion range of 200–300 nm. The two absorption bands are attributed to the 4f ? 5d transition of the Eu2+ ions, which can be concluded 3.1. Electronic structure calculations from the high reflection in this region of the host and the changing intensity with the Eu2+ content of the activator doped samples. The The convergence test of the geometry optimization and energy absorption bands cover the ultraviolet region, indicating the sam- calculation showed well, demonstrating our basic parameters were ples meet the requirement of the UVLED conversion phosphor. suitable. The lattice parameters obtained by the calculations are Fig. 6 illustrates the PLE and PL spectra of Ca3.99Eu0.01Si2O7F2. accordance with the experiments results, which are not indicated Upon excitation of 350 nm, the sample exhibits intense blue here. luminescence with the asymmetric emission band peaking at 413 Y. Jia et al. / Optical Materials 33 (2011) 1803–1807 1805

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). 1806 Y. Jia et al. / Optical Materials 33 (2011) 1803–1807

Fig. 7. Emission spectra of Ca Si O F :Eu2+ with various amounts of Eu2+ (Inset: the Fig. 6. PLE and PL spectra of Ca Eu Si O F phosphor. 4 2 7 2 3.99 0.01 2 7 2 PL intensity of two emission peaks as a function of Eu2+ concentration). 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 2+ Ca4Si2O7F2 crystal structure has different Ca crystallographic sites, a requisite condition for multi-luminescence centers. Moni- tored 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 configura- tion, 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 fluo- 2+ rine-containing silicate is much more ionic than the silicate lattice, Fig. 8. Photoluminescence decay curves of Eu in the Ca3.99Eu0.01Si2O7F2 (excited which leads to the related luminescence properties. at 355 nm, monitored at 413 nm and 460 nm). It is generally accepted that the concentration of activator plays an important role in the search of optimal composition of a phos- Table 1 2+ phor. Appropriate content of the activator can achieve the suitable Decay lifetime of the Eu in the Ca(4x)EuxSi2O7F2 (x = 0.005, 0.01, emission intensity and wavelength [19,20]. Therefore, the varia- 0.02, 0.03, 0.04, and 0.05) under the monitoring condition at 2+ tions of PL intensity and shape with different Eu concentrations 413 nm(s413) and 460 nm(s460). for Ca(4x)EuxSi2O7F2 have been investigated in this paper. Fig. 7 Eu concentration (%) s413 (ls) s460 (ls) shows the dependence of the emission spectra of Ca Eu Si O F (4x) x 2 7 2 0.005 1.05 0.58 2+ on the concentration of Eu . The profile change in the emission 0.01 1.01 0.51 spectra is attributed to the ratio of the 413–460 nm emission peak 0.02 0.55 0.47 alteration with the Eu2+ concentration. The inset in Fig. 7 expresses 0.03 0.71 0.41 the intensity of two emission peaks as a function of the Eu2+ con- 0.04 0.40 0.39 0.05 0.33 0.31 tent. The optimal doping concentrations were observed to be at 3 mol% and 1 mol% for the 413 and 460 nm emission peaks, respectively. 2+ Fig. 8 shows the PL decay curves of the Eu ions in Ca3.99Eu0.01 sult from the energy transfer between the high and low energy 2+ Si2O7F2, which measured with excitation at 355 nm and monitored site; while the decay lifetime of Eu ion at the low energy site at 413 nm and 460 nm. The decay curves exhibit different profiles shows the similar variation, the change may be attribute to the on the condition of the measurement, which confirms the above integrated effect of the Eu2+ ion concentration, energy transfer conclusion about the origin of the emission band. Table 1 lists and crystal defects. 2+ the decay lifetime of Eu ions in the Ca(4x)EuxSi2O7F2. One can The phenomenon of energy transfer often occurs between dif- see that the decay lifetime of Eu2+ ions at the high energy site gen- ferent luminescent ions, such as Ce3+–Eu2+ [21],Eu2+–Mn2+ erally decreases with increasing the doping content, which may re- [22,23],Ce3+–Tb3+ [24], which is used to explain the interaction Y. Jia et al. / Optical Materials 33 (2011) 1803–1807 1807 of the ion pairs. Energy transfer among the same luminescent ions module of Materials Studio 4.0. The calculation results show that that occupy different crystallography sites also can affect spectral the Ca4Si2O7F2 has a proper band gap for using as a support of rare position, and usually be utilized to analyze the red shift of the earth activator. The structure, diffuse reflection spectra, photolu- emission band with the concentration change of activator [9,19]. minescence spectra and color-coordinate parameters of phosphor As a mean of quantitative analysis, it helps us to understand the were investigated as well. The diffuse-reflectance and PLE spectra luminescence phenomenon and design the new inorganic phos- show broadband absorption in the range of 200–420 nm, which phor. As for the luminescent system where the same activator match the emission of n-UV chip well. Under 350 nm excitation, occupies different crystallography sites, some other factors should the optimized phosphor, Ca3.99Eu0.01Si2O7F2, shows a intense blue also be taken into account, including the occupancy site and the light emitting with chromaticity coordination (0.157, 0.108). quenching concentration of the luminescence center [25–30]. The change in occupancy site with the concentration of the activators Acknowledgments has been reported by some groups. It is an important factor to determine the intensity and wavelength of the emission band. This work is financially supported by the National Natural Another important factor affecting the luminescence properties is Science Foundation of China (Grant No. 20771098) and the NSFC quenching concentration of the luminescence center related to Fund for Creative Research Group (Grant No. 20921002), and the the crystal field surroundings. Compared with the red-shift of National Basic Research Program of China (973 Program, Grant emission band with increasing activator content, the blue-shift No. 2007CB935502). has rarely been reported [31]. In our case, we presume that the blue-shift of the emission band results from the integrated effect References of the energy transfer, occupancy site and characteristic quenching concentration of activator at the crystallography site. The energy [1] E.F. Schubert, J.K. Kim, H. Luo, J.Q. Xi, Rep. Prog. Phys. 69 (2006) 3069. [2] H.A. Hoppe, Angew. Chem. Int. 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