Controlling The Activator Site To Tune Europium Valence in

Article pubs.acs.org/cm

Controlling The Activator Site To Tune Europium Valence in Oxyfluoride Phosphors Kuan-Wei Huang,† Wei-Ting Chen,† Cheng-I Chu,† Shu-Fen Hu,‡ Hwo-Shuenn Sheu,§ Bing-Ming Cheng,§ Jin-Ming Chen,§ and Ru-Shi Liu*,† †

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan § National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan ‡

S Supporting Information *

ABSTRACT: A new Eu3+-activated oxyfluoride phosphor Ca12Al14O32F2:Eu3+ (CAOF:Eu3+) was synthesized by a solid state reaction. Commonly red line emission was detected in the range of 570−700 nm. To achieve the requirement of illumination, this study revealed a crystal chemistry approach to reduce Eu ions from 3+ to 2+ in the lattice. Replacing Al3+− F− by the appreciate dopant Si4+−O2− is adopted to enlarge the activator site that enables Eu3+ to be reduced. The crystallization of samples was examined by powder X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Photoluminescence results indicated that as-synthesized phosphors Ca12Al14‑zSizO32+zF2−z:Eu (z = 0−0.5, CASOF:Eu) display an intense blue emission peaking at 440 nm that was produced by 4f−5d transition of Eu2+, along with the intrinsic emission of Eu3+ under UV excitation. Moreover, the effect of Si4+−O2− substitution involved in the coordination environment of the activator site was investigated by further crystallographic data from Rietveld refinements. The 19F solid-state nuclear magnetic resonance (NMR) data were in agreement with refinement and photoluminescence results. Furthermore, the valence states of Eu in the samples were analyzed with the Xray absorption near edge structure (XANES). The quantity of substituted Si4+−O2− tunes chromaticity coordinates of Ca12Al14−zSizO32+zF2−z:Eu phosphors from (0.6101, 0.3513) for z = 0 to (0.1629, 0.0649) for z = 0.5, suggesting the potential for developing phosphors for white light emitting diodes (WLEDs). Using an activator that is valence tunable by controlling the size of the activator site represents a hitherto unreported structural motif for designing phosphors in phosphor converted light emitting diodes (pc-LEDs). KEYWORDS: phosphor, mixed valence, solid-state NMR, XANES, Rietveld refinement, crystal chemistry

■

INTRODUCTION Light-emitting diodes (LEDs) have received wide attention in the recent decade owing to their high brightness, long lifetime, material hardness, and environmental friendliness.1−4 The conventional means of generating white light in white LEDs is combining a phosphor layer with UV- or blue-LEDs that converts the initial radiation into a complementary color. Among all rare earth ions, Eu is the most commonly used activator because both Eu2+ and Eu3+ can function as an emission center in the host lattice. Since the line emissions via the 4f−4f parity-forbidden transition in Eu 3+ activated phosphor leads to a low color rendering index (CRI) and low efficiency, 4f−5d transitions in Eu2+, which produce intensely broad band photoluminescence, are more applicable for LED-pumped white light (i.e., Ca−α−SiAlON and Sr2SiO4:Eu2+).5,6 However, the coordination environment and crystal site size determine the valence state of activator ions and influence the photoluminescence properties of phosphors, explaining why dopant control of emission bands by modifying © 2012 American Chemical Society

the covalency and polarizability of activator−ligand bonds in phosphors has received considerable attention.7−10 The phenomenon was exhibited by incorporating Si4+−N3− in (Sr,Ba,Ca)Al2O4:Eu2+, subsequently leading a red shift in the 4f−5d emission owing to the lower electronegativity of N3− than O2−.7 Similarly, incorporating Si4+−N3− into Ce3+ doped garnet phosphors leads to the low energy Ce3+ emission band and is applicable in warm white LED.8 However, changing the valence state of activator in Eu3+-activated phosphors by modifying the coordination environment of an activator site has scarcely been investigated. Clearly, developing an approach to reduce Eu3+ is an alternative means of designing phosphors, which is of priority concern in phosphors-related research. The diverse particle size of different phosphors causes inhomogeneous suspension in epoxy resin, resulting in selfReceived: April 11, 2012 Revised: May 17, 2012 Published: May 21, 2012 2220

dx.doi.org/10.1021/cm3011327 | Chem. Mater. 2012, 24, 2220−2227

Chemistry of Materials

Article

X-ray absorption near edge structure (XANES) of Eu L3 edge was recorded with a wiggler beamline BL17C at NSRRC. Solid-state nuclear magnetic resonance (NMR) spectra were acquired on a 500 MHz Varian Unity Inova wide bore NMR spectrometer equipped with a 4 mm rotors. The Larmor frequencies for 19F and were 470.2 MHz. 19 F chemical shifts were externally referenced to tetramethylsilane (TMS) at 0.0 ppm.

absorption and a complex packaging process. A singlecomposition phosphor co-doped sensitizer and activator produces multiband emissions via an energy transfer mechanism that can alleviate the above limitations.11−18 Those phosphors are seriously limited in adjusting photoluminescence, and the energy is consumed during the transfer process. Therefore, an alternative means, mixed valence activated phosphor, has been reported, where the optical combination of different valences directly achieves white light.19,20 A notable example is LaAlO3:Eu, which exhibits white light emission by adopting the strategy of coexistence of Eu2+ and Eu3+.20 The packaging process may therefore be simplified, demonstrating its potential applications in LED industry. Unfortunately, the mixed valence of europium appears only in a few host lattices; fewer examples are suitable for phosphors.21,22 Developing an approach for tuning the valence of europium obviously makes designing mixed valence Eu activated phosphors more feasible. To develop mixed valence europium phosphors, inserting Li into EuIII0.33Zr2(PO4)3 would reduce the valence state of Eu from 3+ to 2+, as in a previous study.23−26 Mixed valence europium in Eu0.33Zr2(PO4)3 shows white light emission by mixing both Eu2+ (blue) and Eu3+ (red) emission bands.26 However, the approach is specific for a NaZr2(PO4)3-type structure, which can provide a vacant site for lithium occupation. This complex synthetic route and insertion process are difficult for practical LED-driven applications.23,24,26 In this work, we report an approach based on crystal chemistry that an appropriate dopant tunes the valence state of Eu via controlling the activator site in a novel phosphor Ca12Al14O32F2:Eu3+(CAOF:Eu3+), demonstrating the transform feasibility of Eu3+-activated phosphor into Eu2+ or mix valence Eu activated phosphor. We also point out how the dopant affects the crystal structure, photoluminescence, and valence state of Eu in phosphors. The proposed approach overcomes the limitation of Eu3+ activated phosphors, and the results of this study significantly contribute to future research in designing phosphors.

■

■

RESULTS AND DISCUSSION Figure 1a shows the results of Rietveld refinement for Ca12Al14O32F2:Eu3+ (CAOF:Eu) implemented with the crys-

EXPERIMENTAL SECTION

Synthesis. Ca12Al14−zSizO32+zF2−z:Eu (z = 0.1−0.5) powders were prepared by a solid-state reaction from CaCO3, Al2O3, SiO2, CaF2, and Eu2O3. For each compound, 1.2 g of starting materials were weighed out and mixed together in an agate mortar according to different values of z. The powder mixtures were then transferred to alumina crucibles, with subsequently firing at 1250 °C for 6 h in an electric tube furnace under a reducing atmosphere (N2/H2 = 95:5). After firing, the sample were gradually cooled to room temperature in the furnace and ground into powder form for subsequent analysis. Characterization. The crystal structure and phase purity of the assynthesized samples were studied by using high energy (λ = 0.774901 Å) XRD at a beamline BL01C2 of National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Structural refinements of X-ray diffractograms used the Rietveld method as implemented in a general structure analysis system (GSAS).28 The crystal structures were also examined by high resolution transmission electron microscopy (HRTEM, JEM-2000EX, operating at 200 kV). Photoluminescence (PL) and PL of excitation (PLE) spectra were recorded using a FluoroMax-3 spectrophotometer at room temperature. The vacuum ultraviolet (VUV) PL and PLE spectra were obtained using a beamline BL03A at NSRRC. The PLE spectra were obtained by scanning a 6 m cylindrical grating monochromator with a grating of 450 grooves/mm, which is capable of spanning wavelength range of 100−350 nm. A CaF2 plate was used as a filter to remove the high-order light from the synchrotron. Next, the PL spectra were evaluated in a photon-counting mode with a 0.32 m monochromator.

Figure 1. (a) Observed (crosses) and calculated (solid line) XRD patterns of the Rietveld refinement of Ca11.9Al14O32F2:Eu0.1. Black vertical lines represent the position of Bragg reflection. The difference profile is plotted on the same scale in the bottom. (b) Crystal structure of Ca11.9Al14O32F2 unit cell viewed in b-direction. (c) The coordination geometry of Ca2+ site in Ca11.9Al14O32F2.

tallographic information files identified by previous reports.29,30 The black crosses and red line depict the observed and calculated patterns, respectively; the as-obtained goodness of fit parameter χ2 = 2.89 and Rwp (10.3%) can ensure the sample phase purity. The compound exhibits a cubic crystal system with space group I4̅3d, and its cell parameter is a = b = c = 11.9937(5) Å, which matches the literature data (11.981 Å) reported by Qijun et al.30 Table 1 lists the crystallographic data of CAOF:Eu3+. Figure 1b presents the crystal structure of CAOF as viewed from [010]. Al3+ forms two kinds of 2221



Article

F2p → Eu4f were observed around 250 and 150 nm, respectively.36,37 Jørgensen has formulated an expression to estimate the CT band position:38

Table 1. Crystallographic Data of Ca11.9Al14O32F2:Eu0.1, As Determined by the Rietveld Refinement of Power XRD Data at Room Temperaturea atom

site

x

y

z

occu.

U (Å2)

Ca1 Al1 Al2 O1 O2 F1 Eu1

24d 12a 16c 16c 48e 12b 24d

0.0975(3) 3/8 0.2312(3) 0.0630(4) 0.1917(0) 1/4 0.0975(3)

0 0 0.2312(3) 0.0630(4) 0.2844(8) 1/8 0

1/4 1/4 0.2312(3) 0.0630(4) 0.0989(0) 1/2 1/4

0.99 1.00 1.00 1.00 1.00 0.33 0.01

0.0397 0.0117 0.0283 0.0269 0.0232 0.0592 0.0397

σ = [χopt (X) − χuncorr (M)]30 × 103 cm−1

where σ is the energy of the CT band and χopt(X) is the optical electronegativity of the ligand ion, which is approximately the Pauling’s electronegativity. χuncorr(M) is the optical electronegativity of the central cation. With χopt(O) = 3.2 and the energy of O2p → Eu4f CT experimentally observed, the χuncorr(Eu) value is calculated to be 1.78, which is close to reported study.39 The F2p → Eu4f CT band wavelength in CAOF lattice can therefore be estimated as 151 nm. In Figure S1 (Supporting Information), we measured the VUV excitation spectrum (λem = 613 nm) from 300 to 125 nm, and a weakly broad band is observed around 125−150 nm, close to the estimated value. The intensity of the F2p → Eu4f CT band is weaker than O2p → Eu4f one because the coordinated F− ion is less than O2− in the Ca(Eu) site. Several weak peaks in the range of 350 to 500 nm are related to the 4f−4f transitions of Eu3+ ions as shown in Figure S2 (Supporting Information). The sharp emission lines within the red range under 236 nm excitation can be assigned to Judd-Ofelt transition (5D0 → 7FJ) of Eu3+ arising from electrical dipole (J = 2−4) and magnetic dipole transitions (J = ± 1) as depicted in Figure 2, indicating that an efficient phonon-assisted process leads to the relaxation from charge transition state to Eu3+ levels. Moreover, the 5D0 → 7F2 emission peak at 613 nm is the strongest one, indicating that Eu3+ which occupies a site without inversion symmetry correlates well with the coordination environment of Ca2+ in Figure 1c.40−42 However, Eu3+ activated phosphors are difficult to apply in pc-LED because line emission yields a rather low CRI and weak absorption in UV or blue-LED. Since Eu3+ cannot be reduced in CAOF:Eu structure under reducing atmosphere, we suggest that geometry is an important factor due to the following considerations. When the local structure of Eu3+ is considered, Eu2+ (7r = 1.2 Å) has a larger size than Ca2+ (7r = 1.06 Å). Moreover, the Ca2+ site is surrounded compactly by AlO4 tetrahedra on one side, leading to a distorted coordination environment of Ca2+ as shown in Figure 3. A previous study incorporated Si4+−N3− in structure, in which the red shift of 5d energy position of activators was attributed to a higher covalency and polarizability of activator− N3− bonds versus activator−O2− bonds. Although the replacement of Al3+ by Si4+ is normally attributed to charge compensation,7−10 the shrinkage of AlO4 tetrahedra by smaller Si4+ substitution has seldom been discussed. While attempting to expand the compact side of the Ca2+ site, this study introduced Si4+−O2− into the CAOF structure to replace Al3+− F− resulting in Ca12Al14−zSizO32+zF2−z:Eu (CASOF:Eu), and the substitution of Al3+ by Si4+ is herein assumed to shrink the AlO4 tetrahedra; because Si4+ has a smaller radius than Al3+ and O2−, replacing F− can achieve charge compensation in the whole structure. Figure 4 shows the synchrotron X-ray powder diffraction patterns of the Ca12Al14−zSizO32+zF2−z:Eu with increasing z, which matched with Joint Committee on Powder Diffraction Standards (JCPDS) card No. 00-070-1353. According to the XRD results, Ca 12 Al 14−z Si z O 32+z F 2−z (CASOF) solid solution is formed up to z = 0.5. The fine structure of CASOF:Eu (z = 0.5) is further examined by high resolution transmission electron microscopy (HRTEM) as shown in Figure 5. Figure 5b shows the related selected area

a

Space group: I4̅3d (No. 220), Z = 2, V = 1725.30(4) Å3, a = b = c = 11.9937(5) Å, Rp = 7.64%, Rwp = 10.27%, χ2 = 2.89.

tetrahedra and builds up to an AlO4 ring. Three-dimensional frameworks of the CAOF structure are further formed by sharing the O2− between AlO4 rings. Notably, the Ca2+ has one crystal site surrounded by AlO4 and coordinated by six O2− and one F−; all of the O2− is shared by AlO4 tetranedra. We can infer that Eu3+ replaces Ca2+ owing to the similar ionic radii between Ca2+ (7r = 1.06 Å) and Eu3+ (7r = 1.01 Å).27 We noted that a slight excess of positive charge is formed due to the substitution of Ca2+ by Eu3+. The structure offers many mechanisms to compensate this excess charge from Eu3+, the most possible being slight off-stoichiometry between F− and O2−.31 Moreover, according to the Kröger-Vink defect notation, cation vacancies and oxygen interstitials are also possibly formed to maintain electrical neutrality.32−35 Figure 2 describes the VUV excitation and emission spectra of CAOF:Eu3+ with the schematic energy state. The VUV excitation spectra monitored at 613 nm reveals a broad band in 210−270 nm range. We tentatively assigned this band to O2p → Eu4f CT band because the charge transfer (CT) band of O2p → Eu4f and

Figure 2. VUV excitation (λem = 613 nm, blue part) and emission (λex = 236 nm, red part) spectrum with schematic energy state of Eu3+. 2222



Article

Figure 5. (a) TEM image of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0.5), (b) corresponding SAED pattern along the [110] zone axis, and (c) HRTEM images of the selected area in (a).

To investigate how Si4+−O2− substitution affects the CAOF:Eu3+ crystal structure, we performed Rietveld refinement with GSAS program to obtain more detailed information. Figure S3 (Supporting Information) plots the experimental, calculated, and difference results from the refinements of the CASOF samples, in which all of the observed peaks consist with the Bragg reflections that verify the formation of a single phase. During the refinement procedure, the occupancy parameters of all atoms are referenced by stoichiometry; in addition, the temperature factors are fixed for all substituted ions. Table 2 summarizes lattice parameters and reliability factors of CASOF:Eu (z = 0−0.5) samples that crystallized in a cubic structure with space group I4̅3d. A gradual change in the lattice parameter with increasing z indicates that CASOF:Eu (z = 0−0.5) solid solutions are formed. Although the radius of Si4+ (4r = 0.26 Å) smaller than Al3+ (4r = 0.39 Å) appears to shrink three-dimensional frameworks, according to Vegard’s rule,45 a larger O2− (2r = 1.35 Å) occupying the F− (2r = 1.285 Å) site causes a slight increase in the lattice as shown in Figure 6a. The unit cell expansion by Si4+−O2− replacing Al3+−F− is also observed by Im et al.46 To elucidate the site feature of Ca2+, the effect of Si4+−O2− incorporation involved in the Ca2+ site can be analyzed by the bond lengths of (Al,Si)−O and Ca−O. Figure 6b,c plots the average bond lengths of (Al,Si)−O and Ca−O as obtained by the refinement results. The (Al,Si)−O bond length decreases with increasing amount of Si4+−O2−. It can be assigned to the Si4+ replacement of Al3+. Consequently, the Ca−O bond length is further elongated due to shrinking of the (Al,Si)O4 tetrahedra when incorporating Si4+, thus loosening the crystal site of Ca2+. The Ca−(F,O) length is also elongated by incorporating Si4+−O2− as shown in Figure 6d. However, because the F− originally occupied in a cage-like site is surrounded by the AlO4 framework and coordinated to Ca2+,29 the incorporated O2− generates another Ca2+ site in a crystal, which is coordinated to seven O2− ions. Adding Si4+−O2− affects both the crystal structure and the photoluminescence properties. Figure 7 illustrates the indispensable effect of incorporating Si4+−O2− in the CASOF host lattice. Figure 7a displays the emission spectra of CASOF with z = 0.0−0.5 at room temperature under 254 nm excitation. Besides the intrinsic line emission of Eu3+ within the red range as discussed above, a surprising observation which is an

Figure 3. Coordination environment of the Ca2+ site in the Ca11.9Al14O32F2:Eu0.1 structure.

Figure 4. Powder XRD patterns of Ca12Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5), which are compared with JCPDs Card.

electron diffraction (SAED) pattern, in which diffraction spots correspond to the [110] zone axis of cubic structure with space group I4̅3d. This result reveals that the particle has a single crystal structure. Figure 5c displays the HRTEM image of the selected area. In the SAED pattern, the d-spacing can be calculated by the following equation:43,44 λ×L=d×R

where λ is the wavelength of TEM accelerating voltage; L is the camera length; and R is the measured distance of the spots. The d-spacing of indexed spots (22̅ 0) and (112̅ ) are calculated to be 4.35 Å and 4.91 Å, respectively, which are constituent with the measured distance in Figure 5c, and correlate well to the theoretical value of 4.24 Å for (2̅20) and 4.89 Å for (11̅2). 2223



Article

Table 2. Crystallographic Data and Reliability Factor of Ca12Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5) Phosphorsa a=b=c cell volume χ2 Rwp Rp a

z=0

z = 0.1

z = 0.2

z = 0.3

z = 0.4

z = 0.5

11.9937(2) 1725.30(4)

11.9944(1) 1725.58(4)

11.9965(1) 1726.51(4)

11.9967(2) 1726.61(5)

11.9973(2) 1726.84(5)

2.898 10.27 7.64

2.911 9.97% 7.35%

11.9978(2) 1727.05(4) Reliability Factor 3.176 10.02% 7.44%

3.199 9.57% 7.18%

3.59 10.21% 7.47%

4.295 11.19% 8.06%

Crystal system: cubic, space group: I4̅3d (No. 220).

Ca11.9Al13.5Si0.5O32.5F1.5:Eu0.1 under 334 nm excitation is plotted in Figure S5 (Supporting Information). In the excitation spectra in Figure 7c monitored at 613 nm, which is produced by 5D0 → 7F2 of Eu3+, a peak appears around 234 nm, which can be assigned to O2p → Eu4f charge transfer band. As mentioned above, a decreased intensity with an increasing z is also compatible with the results in Figure 7a. Since the replacement of F− by O2− implies the variation of first coordination layer of the activation site, the asymmetric emission spectra in Figure 7a can be deconvoluted into two Gaussian components at 440 and 473 nm, revealing that Eu2+ has two centers in the CASOF lattice. Features of the emission position are discussed via the change in the Eu2+−ligand covalency. Previous investigations have studied the configuration of Eu2+ with the anion polarizability and cation electronegativity in various hosts.47−50 Two factors influence the energy position of the 5d band: gravity shift and crystal field splitting. Gravity shift is associated with the nephelauxetic effect caused by the interaction between cation and electron cloudy of ligands. The [6O1F]Ca site is original coordinated with six oxygen atoms and one fluorine atom. Due to the replacement of F− by O2−, the first coordination layer of the [6O1F]Ca site is changed to a [7O]Ca site. The two distinct Eu2+ emissions are therefore observed. Because the incorporated ligand O2− with a lower electronegativity than that of F− would lower the energy of the 5d1 level, the shoulder emission band with a long wavelength at 473 nm can thus be assigned to the [7O]Eu2+ due to the gravity shift and crystal field splitting. The main factor for changing photoluminescence respects the first coordination layer activator, so the second layer of Al3+/Si4+ replacement with negligible influence in photoluminescence just changes the steric structure. We can also conclude that replacing Al3+ by Si4+ cannot influenced the first coordination sphere; it mainly changes the crystal structure as the results in XRD refinement. During valence transfer of Eu, the CIE coordinates upon 254 nm excitation of CASOF:Eu are regularly shifted from (0.6101, 0.3513) to (0.1629, 0.0649) in relation to an increasing value of z as depicted in Figure 7d, and the inset schematically depicts the proposed crystal variation and photographs of each composition irradiated under a 254 nm UV lamp. The chromaticity coordinates of CASOF:Eu (z = 0−0.5) are summarized in Table 3. Solid-state NMR measurement, which is atom specific and sensitive to the local order around the nucleus, can be performed to complete the crystal chemistry experiments beyond XRD analysis.51−54 Owing to the fact that F− is coordinated with Ca2+, this study performs 19F solid state NMR to further investigate how incorporating Si4+−O2− affects the Ca2+ site. Figure 8 describes the 19F NMR spectra, in which the peak area correlates well with the amount of F in study samples. Only one peak is obtained in CAOF (z = 0) at −120 ppm, which can be assigned to the ligand atom to Ca.55−57 However,

Figure 6. (a) Lattice constant of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 with different amounts of dopants (z = 0−0.5). The average bond length of (b) Al−O, (c) Ca−O, and (d) Ca−F of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5).

apperence of a broadband peak at 440 nm with increasing z, it can be assigned to 5d → 4f emission of Eu2+. Interestingly, the emission of Eu3+ within 570−700 nm and broadband at 440 nm disappear and emerge simultaneously. This result can be therefore attributed to the increase of z value, suggesting that Eu3+ is transformed to Eu2+ in the lattice. With regard to the tendency in refined bond length of Al−O, Ca−O, and Ca−F, the expanded site of Ca2+ could be demonstrated by substitution of Al3+−F− by Si4+−O2−. Eu3+ can therefore be reduced to Eu2+ in the CASOF lattice. The excitation spectra of Ca12Al14−zSizO32+zF2−z:Eu (z = 0−0.5) in Figure 7b by monitoring 440 nm reveals a broad peak from 250 to 410 nm, which can be ascribed to 4f−5d transition of Eu2+. The peak intensity gradually increases with increasing value of z, which correlates well with the peak intensity of Eu2+ in Figure 7a. The emission spectra upon excitation of 334 nm shows blue luminescence of the CASOF phosphors, and the intensity is correlated to the increasing level of Si4+−O2− incorporation, as shown in Figure S4 (Supporting Information). Interestingly, no red line luminescence is detected upon 334 nm excitation that is consonant with no absorption of Eu3+ in this range as shown in Figure S2 (Supporting Information). The Commission International de I’Eclairage (CIE) chromaticity coordinate of 2224



Article

Figure 7. (a) VUV emission spectra (λex = 254 nm), (b) PL excitation spectra (λem = 440 nm), (c) VUV excitation spectra (λem = 613 nm) of Ca 12 Al 14−z Si zO 32+z F 2−z :Eu 0.1 (z = 0−0.5), and (d) dependence of the CIE chromaticity coordinates on varying z value in Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5) upon 254 nm excitation. The inset shows the schematic variation of the activator site driven by Si4+− O2− incorporation and the irridated phosphor images of each composition under a 254 nm UV lamp.

corresponding to the transition to the unoccupied 5d states.58−61 The energy difference originates from the shielding of the nuclear potential through an additional 4f electron in Eu2+, subsequently lowering the binding energy of the respective core electrons.54 To further examine two valence states of Eu in the CASOF samples, XANES was performed near Eu L3 edge with BaMgAl10O17:Eu2+ and Eu2O3 as reference for Eu2+ and Eu3+, respectively.58 The normalized Eu L3 edge XANES spectra of the study CASOF reveal two peaks at 6975 and 6983 eV, which are attributed to the electron transition of 2p3/2 → 5d in Eu2+ and Eu3+, respectively. This finding suggests that two valence states of Eu coexist in the CASOF samples. The relative intensities of absorption by Eu2+ at 6975 eV and Eu3+ at 6983 eV systematically increase and decrease, which correlates with the amount of Si4+−O2− incorporated. Owing to that the sum of the two peaks are nearly constants, the area ratio can be treated as the ratio of the amount of Eu2+ and Eu3+, as shown in Figure 9b.62−64 The increasing ratio of Eu2+/Eu3+ and the emission intensity of Eu2+ display a similar trend for the increased level of Si4+−O2− incorporation, demonstrating that the luminescent enhancement results mainly from the increasing number of Eu2+. This observation also corresponds to our results in crystal structure studies and solid state NMR.

Table 3. CIE Chromaticity Coordinates of Ca12Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5) CIE chromaticity Si −O 4+

2−

con. (z)

0 0.1 0.2 0.3 0.4 0.5

x

y

0.6101 0.4920 0.2873 0.2024 0.1705 0.1629

0.3513 0.2954 0.1653 0.0929 0.0713 0.0649

a shoulder peak appears at −173 ppm in z = 0.1, 0.3, and 0.5. The NMR spectra are decovoluted into two peaks in the bottom of Figure 8, in which the peaks appearing at −120 ppm and −173 ppm decrease and increase in correlation with the level of Si4+−O2− incorporation and the peak area ratio of I(−173 ppm)/I(−120 ppm) correlated well with the level of Si4+−O2− incorporation as shown in Figure 8b. The peak at −173 ppm can be attributed to F−, which is coordinated with the loose Ca2+ site, capable of maintaining higher electron density at F− than the original site, resulting in an upfield in chemical shift.47 The results imply the average Ca−F bond is elongated by incorporating Si4+−O2, which is supported by Rietveld refinement, indicating the increased amount of loose Ca2+ site which is suitable for Eu2+ occupation. XANES spectroscopy can easily identify that two valence states of Eu2+(4f7) and Eu3+(4f6), due to the different threshold energies around 8 eV of their white light (WL) resonance,

■

CONCLUSIONS In summary, we have revealed that by appropriate dopant incorporation, the valence state of Eu3+ can be tuned to Eu2+ in phosphors due to the enlargement of the activator site. 2225



Article

Effectiveness of the proposed approach is demonstrated in the new phosphor Ca12Al14O32F2:Eu3+; Si4+−O2− are incorporated to substituted Al3+−F− to release the geometry restriction of the activator site. Combinatorial studies with synchrotron XRD refinement, XANES, HRTEM, and solid state NMR help us to understand how the dopant affects the crystal structure and photoluminescence. The average bond lengths of Al−O and Ca−O obtained in refinement are systematically shortened and elongated, respectively, indicating that the enlargement of activator site corresponds to the amount of Si4+−O2−. Incorporating Si4+−O2− in CAOF:Eu3+ phosphor leads to a rise in broadband emission at 440 nm that can be ascribed to the 4f−5d transition of Eu2+. The emission intensity of Eu2+ and Eu3+ increases and decreases systematically with the amount of dopant. XANES results further confirm that Eu3+ is transferred to Eu2+ by incorporating Si4+−O2−. The proposed approach is highly promising for applications involving Eu3+ phosphors. Overcoming the limitations of Eu3+ activated phosphor via valence transfer, the broadband feature and efficient radiation in Eu2+ based phosphor are more useful in illumination. The proposed approach is also characterized by the fact that only a single activator, Eu, generates the multiband, even a white light by optical combination of different valences of europium. This approach does not just limited in present study materials but could be general to other related Eu system. Thus, our approach may lead to opportunities for more successful development of phosphors in LED applications.

■

ASSOCIATED CONTENT

S Supporting Information *

19

Figure 8. (a) F solid state NMR spectra of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 with z = 0, 0.1, 0.3, 0.5. The bottom represents the deconvolution components used for the spectral analysis. (b) The ratio of the deconvoluted peak area at −173 ppm and −120 ppm in 19F NMR, which is a function of the amount of Si4+− O2− incorporation.

Figures S1 and S2 show the VUV excitation and photoluminescence excitation measurements of Ca11.9Al14O32F2:Eu0.1. Figure S3 shows Rietveld refinement of synchrotron PXRD data of CASOF samples (z = 0.1−0.5). Figures S4 shows the photoluminescence emission spectra of CASOF samples. Figure S5 plots the CIE chromaticity coordinate of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0.5) under 334 nm excitation. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract Nos. NSC 97-2113-M-002-012-MY3 and NSC 97-3114-M-002.

■

REFERENCES

(1) Nakamura, S.; Fasol, G. The Blue Laser Diode; Springer: Berlin, 1997. (2) Hashimoto, T.; Wu, F.; Speck, J. S.; Nakamura, S. Nat. Mater. 2007, 6, 568−571. (3) Pimputkar, S.; Speck, J. S.; DenBaars, S. P.; Nakamura, S. Nat. Photonics 2009, 3, 180−182. (4) Schubert, E. F.; Kim, J. K. Science 2005, 308, 1274−1278. (5) Xie, R. J.; Hirosaki, N.; Sakuma, K.; Yamamoto, Y.; Mitomo, M. Appl. Phys. Lett. 2004, 84, 5404−5406.

Figure 9. (a) Normalized Eu L 3 -edge XANES spectra of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5). (b) Dependence of I E u 2 + / E u 3 + on the amount of Si 4 + −O 2 − incorporation in Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5).

2226



Article

(42) Dai, Q.; Foley, M. E.; Breshike, C. J.; Lita, A.; Strouse, G. F. J. Am. Chem. Soc. 2011, 133, 15475−15486. (43) Pósfai, M.; Buseck, P. R.; Bazylinski, D. A.; Frankel, R. B. Science 1998, 280, 880−883. (44) Avivi, S.; Mastai, Y.; Gedanken, A. J. Am. Chem. Soc. 2000, 122, 4331−4334. (45) Ganguly, P.; Shah, N.; Phadke, M.; Ramaswamy, V.; Mulla, I. S. Phys. Rev. B 1993, 47, 991−995. (46) Im, W. B.; George, N.; Kurzman, J.; Brinkley, S.; Mikhailovsky, A.; Hu, J.; Chmelka, B. F.; DenBaars, S. P.; Seshadri, R. Adv. Mater. 2011, 23, 2300−2305. (47) Dorenbos, P. Chem. Mater. 2005, 17, 6452−6456. (48) Dorenbos, P. J. Alloys Compd. 2002, 341, 156−159. (49) Dorenbos, P. J. Phys.: Condens. Matter 2003, 15, 8417−8434. (50) Dorenbos, P. J. Lumin. 2003, 104, 239−260. (51) Taulelle, F.; Pruski, M.; Amoureux, J. P.; Lang, D.; Bailly, A.; Huguenard, C.; Haouas, M.; Gerardin, C.; Loiseau, T.; Ferey, G. J. Am. Chem. Soc. 1999, 121, 12148−2153. (52) Yan, Z.; Chen, B.; Huang, Y. Solid State Nucl. Magn. Reson. 2009, 35, 49−60. (53) Han, Z. X.; Picone, A. L.; Slawin, A. M. Z.; Seymour, V. R.; Ashbrook, S. E.; Zhou, W. Z.; Thompson, S. P.; Parker, J. E.; Wright, P. A. Chem. Mater. 2010, 22, 338−346. (54) Martineau, C.; Bouchevreau, B.; Tian, Z.; Lohmeier, S.-J.; Behrens, P.; Taulelle, F. Chem. Mater. 2011, 23, 4799−4809. (55) Mackenzie, K. J. D.; Smith, M. E. Multinuclear Solid-State NMR of Inorganic Solids; Pergamon: Amsterdam, 2002. (56) Kreinbrink, A. T.; Sazavsky, C. D.; Pyrz, J. W.; Nelson, D. G. A. J. Magn. Reson. 1990, 88, 267−276. (57) Harris, R. K.; Jackson, P. Chem. Rev. 1991, 91, 1427−1440. (58) Wortmann, G. Hyperfine Interact. 1989, 47−48, 179−202. (59) Wortmann, G.; Nowik, I.; Perscheid, B.; Kaindl, G.; Felner, I. Phys. Rev. B 1991, 43, 5261−5268. (60) Ravot, D.; Godard, C.; Achard, J. C.; Lagarde, P. In Valence Fluctuations in Solids; Falicov, L. M., Hanke, W., Maple, M. B., Eds.; North-Holland Publishing Company: Amsterdam, 1981; p 423. (61) Moreau, G.; Helm, L.; Purans, J.; Merbach, A. E. J. Phys. Chem. A 2002, 106, 3034−3043. (62) Kim, T. G.; Lee, H. S.; Lin, C. C.; Kim, T.; Liu, R. S.; Chan, T. S.; Im, S. J. Appl. Phys. Lett. 2010, 96, 061904−061906. (63) Bianconi, A.; Marcelli, A.; Dexpert, H.; Karnatak, R.; Kotani, A.; Jo, T.; Petiau, J. Phys. Rev. B 1987, 35, 806−812. (64) Sohn, K. S.; Kim, S. S.; Park, H. D. Appl. Phys. Lett. 2002, 81, 1759−1762.

(6) Park, J. K.; Lim, M. A.; Kim, C. H.; Park, H. D.; Park, T. J.; Choi, S. Y. Appl. Phys. Lett. 2003, 82, 683−685. (7) Li, Y. Q.; de With, G.; Hintzen, H. T. J. Electrochem. Soc. 2006, 153, G278−G282. (8) Setlur, A. A.; Heward, W. J.; Hannah, M. E.; Happek, U. Chem. Mater. 2008, 20, 6277−6283. (9) Setlur, A. A.; Radkov, E. V.; Henderson, C. S.; Her, J.-H.; Srivastava, A. M.; Karkada, N.; Kishore, M. S.; Kumar, N. P.; Aesram, D.; Deshpande, A.; Kolodin, B.; Grigorov, L. S.; Happek, U. Chem. Mater. 2010, 22, 4076−4082. (10) Sun, W. Y.; Li, X. T.; Ma, L. T.; Yen, T. S. J. Solid State Chem. 1984, 51, 315−320. (11) Huang, C. H.; Liu, W. R.; Chen, T. M. J. Phys. Chem. C 2010, 114, 18698−18701. (12) Yang, W. J.; Luo, L.; Chen, T. M.; Wang, N.-S. Chem. Mater. 2005, 17, 3883−3888. (13) Huang, C. H.; Chen, T. M.; Liu, W. R.; Chiu, Y. C.; Yeh, Y. T.; Jang, S. M. ACS Appl. Mater. Interfaces 2010, 2, 259−264. (14) Kwon, K. H.; Im, W. B.; Jang, H. S.; Yoo, H. S.; Jeon, D. Y. Inorg. Chem. 2009, 48, 11525−11532. (15) Caldiño, U. G. J. Phys.: Condens. Matter 2003, 15, 3821−3830. (16) Huang, C. H.; Kuo, T. W.; Chen, T. M. ACS Appl. Mater. Interfaces 2010, 2, 1395−1399. (17) Huang, C. H.; Chen, T. M. J. Phys. Chem. C 2011, 115, 2349− 2355. (18) Duan, C.; Zhang, Z.; Rösler, S.; Rösler, S.; Delsing, A.; Zhao, J.; Hintzen, H. T. Chem. Mater. 2011, 23, 1851−1861. (19) Mao, Z. Y.; Wang, D. J.; Lu, Q. F.; Yu, W. H.; Yuan, Z. H. Chem. Commun. 2009, 3, 346−348. (20) Mao, Z. Y.; Wang, D. J. Inorg. Chem. 2010, 49, 4922−4927. (21) Gao, G.; Reibstein, S.; Peng, M.; Wondraczek, L. J. Mater. Chem. 2011, 21, 3156−3161. (22) Zeuner, M.; Pagano, S.; Matthes, P.; Bichler, D.; Johrendt, D.; Harmening, T.; Pöttgen, R.; Schnick, W. J. Am. Chem. Soc. 2009, 131, 11242−11248. (23) Varadaraju, U. V.; Thomas, K. A.; Sivasankar, B.; Subbarao, G. V. J. Chem. Soc., Chem. Commun. 1987, 11, 814−815. (24) Gopalakrishnan, J.; Rangan, K. K. Chem. Mater. 1992, 4, 745− 747. (25) Talbi, M. A.; Brochu, R.; Parent, C.; Rabardel, L.; Le Flem, G. J. Solid State Chem. 1994, 110, 350−355. (26) Saradhi, M. P.; Pralong, V.; Varadaraju, U. V.; Raveau, B. Chem. Mater. 2009, 21, 1793−1795. (27) Shannon, R. Acta Crystallogr., Sect. A 1976, 32 (5), 751−767. (28) Larson, C. Von Dreele, R. B. Generalized Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR 86-748, Los Alamos National Laboratory: Los Alamos, NM, 1994. (29) Williams, P. Acta Crystallogr., Sect. B 1973, 29, 1550−1551. (30) Qijun, Y.; Sugita, S.; Xiuji, F.; Jinxiao, M. Cem. Concr. Res. 1997, 27, 1439−1449. (31) Im, W. B.; Brinkley, S.; Hu, J.; Mikhailovsky, A.; DenBaars, S. P.; Seshadri, R. Chem. Mater. 2010, 22 (9), 2842−2849. (32) Li, Y. Q.; Hirosaki, N.; Xie, R. J.; Takeda, T.; Mitomo, M. Chem. Mater. 2008, 20, 6704−6714. (33) Trojan-Piegza, J.; Niittykoski, J.; Hölsä, J.; Zych, E. Chem. Mater. 2008, 20, 2252−2261. (34) Kröger, F. A.; Vink, H. J. Physica (Amsterdam) 1954, 20, 950− 964. (35) Jiao, W.; Wang, Y. J. Electrochem. Soc. 2009, 156, J117−J120. (36) Buijs, M.; Meyerink, A.; Blasse, G. J. Lumin. 1987, 37, 9−20. (37) Belsky, A. N.; Krupa, J. C. Displays 1999, 19, 185−196. (38) Resfeld,R.; Jφrgensen, C. K. Lasers and Excited States of Rare Earth; Springer: Berlin, 1977. (39) He, L.; Sun, W.; Ding, Y.; Wang, Y. Adv. Mater. Res. 2011, 311− 313, 1327−1331. (40) Sager, W. F.; Filipesc, N.; Serafin, F. A. J. Phys. Chem. 1965, 69, 1092−1100. (41) Filipescu, N.; Sager, W. F.; Serafin, F. A. J. Phys. Chem. 1964, 68, 3324−3346. 2227


Controlling The Activator Site To Tune Europium Valence in

Recommend Documents