Emitting-Color Tunable Phosphors Sr3GaO4F:Ce3+ at Ultraviolet

Sep 10, 2009 - Wanping Chen, Hongbin Liang*, Bing Han, Jiuping Zhong and Qiang Su ... Lixin Ning , Yongfeng Wang , Zongcui Wang , Wei Jin , Shizhong ...
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Emitting-Color Tunable Phosphors Sr3GaO4F:Ce3+ at Ultraviolet Light and Low-Voltage Electron Beam Excitation Wanping Chen, Hongbin Liang,* Bing Han, Jiuping Zhong, and Qiang Su MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen UniVersity, Guangzhou 510275, P. R. China ReceiVed: June 13, 2009; ReVised Manuscript ReceiVed: August 20, 2009

Sr3-2xCexNaxGaO4F phosphors were prepared through a high-temperature solid-state reaction method. Their photoluminescence and cathodoluminescence properties were investigated. Under ultraviolet light and lowvoltage electron beam excitation, the phosphors show a typical broad emission of Ce3+ and the luminescence color can be tuned in a large gamut from greenish-blue to yellowish-green by changing Ce3+ concentration in the range of 0.01-0.30 (x value). Their luminescence behaviors are discussed in terms of Ce3+ in two distinct sites. The lowest 5d absorption bands are respectively at ∼405 and ∼430 nm for Ce(1)3+ [i.e., Ce3+ in Sr(1)2+ site] and Ce(2)3+ [i.e., Ce3+ in Sr(2)2+ site] centers, while the emissions from relaxed 5d state to 2 F5/2 ground state are respectively at ∼456 nm for Ce(1)3+ and ∼567 nm for Ce(2)3+. 1. Introduction With the development of field-emission display (FED) technology, the phosphor requirements for FED application are gradually enhanced.1 The conventional cathode ray tube (CRT) phosphors do not meet the requirements for FED application, such as good stability and high cathodoluminescence efficiency under low excitation voltage (e5 kV).2 Especially, the conventional sulfide-based phosphors are easy to degrade in highenergy electron bombardment.3 Therefore, much attention has been attracted to explore novel phosphors for FED application. Recently, many papers on the cathodoluminescence properties of oxides and fluorides phosphors doped with rare earth ions were published.2-6 It is well-known that, as host materials, oxides and fluorides have their unique advantages. Oxides have chemical stability and withstand high-energy electron bombardment,5,6 though their high phonon frequency usually causes high nonradiative relaxation energy loss.7 In contrast, fluorides with wide band gap usually show low phonon frequency.2,7 Among phosphors doped with rare-earth ions, Ce3+ activated phosphors have been extensively applied in the field of lighting, display, and scintillation.8 In addition to the low cost, another advantage of Ce3+-doped phosphor is that their luminescence color can be tuned from red region to UV region9 because the 5d energetic position of Ce3+ is sensitive to the host lattice.10 For example, because of the difference of crystal field environment, Ce3+ exhibits red (peaking at ∼625 nm), yellow (peaking at ∼556 nm), and blue (peaking at ∼440 nm) emitting in compound CaSiN2,11 LaSr2AlO5,12 and CaLaGa3S6O,13 respectively. Therefore, Ce3+-doped phosphors with specific emittingcolor might be designed through selecting a compound with special crystal field environment. On other hand, the optical allowed 5d-4f transition makes Ce3+ has intensive emission with a short decay time (20-60 ns).8 Much attention has been paid to Ce3+-doped fluoride and oxide materials, but studies on compounds containing fluorine and oxygen ligand are rarely * Corresponding author. E-mail: [email protected]. Tel.: +8620-84111038. Fax: +86-20-84111038.

reported,14 and there is no report on the cathodoluminescence properties of Ce3+-doped oxyfluorides up to now. Sr3GaO4F is an ordered oxyfluoride where there are two different Sr2+ lattice sites.15 The coordinate polyhedrons of Sr2+ contain both oxygen and fluorine ligand and then Sr3GaO4F is expected to combine the advantages of fluorides and oxides and to be a potential host material.16 In addition, when the rareearth ions are incorporated into this host, they may enter the two different lattice sites and exhibit different luminescence properties.8,10 In present work, the photoluminescence, cathodoluminescence, and luminescence decay properties of Ce3+doped Sr3GaO4F are investigated, and a color-tunable emission phenomenon are observed by changing the Ce3+ doping concentration. 2. Experimental Section All samples with nominal composition Sr3-2xCexNaxGaO4F (x ) 0.01, 0.03, 0.06, 0.10, 0.15, 0.20, and 0.30) were synthesized by a solid-state reaction method. Appropriate amounts of starting materials SrCO3 (analytical reagent, AR), NH4F (AR), Ga2O3 (99.99%), Na2CO3 (AR), and CeO2 (99.99%) were thoroughly mixed in an agate mortar and then heated at 1373 K for 12 h in a reducing atmosphere (5% H2-95% N2). Na+ ions provided by Na2CO3 were added as compensators for charge defects resulting from Sr2+ ions substituted by Ce3+ ions. The X-ray diffraction (XRD) analyses were carried out with a Rigaku D/max 2200vpc X-ray diffractometer. The photoluminescence spectra and luminescence decay curves of all samples were characterized by an Edinburgh FLS 920 combined fluorescence lifetime and steady-state spectrometer. The cathodoluminescence measurement were carried out in a vacuum chamber (10-4 Pa), where the phosphors were excited by an electron beam at a voltage range of 1-5 kV and different filament currents. The cathodoluminescence spectra were recorded by a spectrometer (Ocean Optics QEB0388) with a charge coupled device (CCD) camera through an optical fiber.

10.1021/jp905545f CCC: $40.75  2009 American Chemical Society Published on Web 09/10/2009

Emitting-Color Tunable Phosphors Sr3GaO4F:Ce3+

Figure 1. XRD patterns of Sr3-2xCexNaxGaO4F with x ) 0.01, 0.06, and 0.15 at RT. The unity cell of Sr3GaO4F and coordinate polyhedrons of Sr2+ are shown in the inset.

3. Results and Discussion 3.1. Phase Purity and Crystal Structure. The phase purity of all samples was characterized by XRD at room temperature (RT). Typical XRD patterns of Sr3-2xCexNaxGaO4F with x ) 0.01, 0.06, and 0.15 are displayed in Figure 1. All XRD patterns are in good agreement with the JCPD Standard Card No. 894484 of Sr3GaO4F, indicating that all samples are single phase and the crystal structure of Sr3-2xCexNaxGaO4F is identical to that of Sr3GaO4F. No second phase was found, showing that the Ce3+ ions were completely dissolved in the Sr3GaO4F host lattice by partial substitution for Sr2+ ions. The compound Sr3GaO4F, crystallizing in a tetragonal system with I4/mcm space group, has an ordered oxygen and fluorine layered structure (Figure 1a) where layers made of isolated GaO4 tetrahedra containing Sr2+ ion as intercalate are separated by Sr2F layers.15 There are two distinct Sr2+ lattice sites in Sr3GaO4F, denoted Sr(1)2+ and Sr(2)2+, respectively (Figure 1b). Sr2+ ions in Sr(1)2+ sites are coordinated by eight oxygen atoms and two fluorine atoms to form a bicapped square antiprism. Sr2+ ions in Sr(2)2+ sites are coordinated by six oxygen atoms and two fluorine atoms to form a distorted bicapped trigonal prism. Therefore, it is expected that the Ce3+ ions doped in this host will exhibit two kinds of luminescence properties. 3.2. Photoluminescence Properties at Low Temperature. Figure 2 presents the emission and excitation spectra of Sr2.98Ce0.01Na0.01GaO4F recorded at liquid helium temperature (LHT). The emission spectra (a) and (b) were measured under 405 and 430 nm excitation, respectively. The 405 and 430 nm correspond to the lowest 5d absorption peaks for Ce3+ resided in two different Sr2+ sites, denoted Ce(1)3+ and Ce(2)3+, respectively (see the discussion below). Upon 405 nm excitation, doublet emission bands (peaking at about 450 and 502 nm) were seen in curve a. To evaluate the emissions of Ce(1)3+ and Ce(2)3+ at this case, we fitted this curve with a sum of two Gaussian profiles with a maximum at about 456 and 499 nm, respectively. The energy separation between the two fitted peaks is ∼1890 cm-1. Clearly, the transitions from the relaxed lowest 5d state to ground states 2 F5/2,7/2 of identical Ce3+ center are responsible for these broad bands, and this Ce3+ center is denoted Ce(1)3+ center. In addition, a slight difference at wavelength range above 550 nm can be observed between the fitted curve and the experimental one, which may be due to the weak emission of Ce(2)3+ center

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Figure 2. (a, b) Emission and (c, d) excitation spectra of Sr2.98Ce0.01Na0.01GaO4F recorded at LHT.

(this emission band is much obvious in curve b). When the excitation wavelength moves to 430 nm, a rather broad band was observed in the range of 470-700 nm (curve b). The main peak of this broad band is at about 517 nm and with an evident shoulder band at long-wavelength side. The broad emission derived from two sites was observed at the moment, containing Ce(1)3+ emission located at short-wavelength side and Ce(2)3+ emission at long-wavelength side. Curves c and d in Figure 2 show the excitation spectra under 460 and 560 nm emissions of Sr2.98Ce0.01Na0.01GaO4F, respectively. Both excitation curves keep similar profile in 275-420 nm range, but a shoulder band A occurs in curve d. Band A (∼430 nm) corresponds to the absorption from the ground state 2 F5/2 to the lowest 5d state of Ce(2)3+, while band B (∼405 nm) corresponds to that of Ce(1)3+. At present, the higher 5d excited states (bands C-E) cannot be unambiguously assigned because of the possible energy overlapping of 5d levels of two distinct Ce3+ centers in the host lattice. When we accept the lowest 5d absorption at ∼405 nm and the emission of relaxed 5d f 2F5/2 at ∼456 nm for Ce(1)3+ center, the Stokes shift of this center is estimated to be around 2762 cm-1. Simultaneously, it can be found from the curve d that the band B is with a high absorption intensity by monitoring Ce(2)3+ emission at 560 nm, indicating the Ce(2)3+ can be excited indirectly by 405 nm through an energy transfer (ET) process from Ce(1)3+ to Ce(2)3+. This standpoint is also supported by the spectral overlap of Ce(1)3+ emission to Ce(2)3+ absorption, which can be found from curves a and d. Then, if the ET from Ce(1)3+ to Ce(2)3+ is efficient, why the emission intensity from Ce(2)3+ center is so weaker to that from Ce(1)3+ center in Figure 2a? We think this may be an indication of low site occupancy of Ce3+ in Sr(2)2+ site for this diluted sample. The influence of the doping concentration on the spectroscopic characteristics will further confirm this viewpoint. To examine the influence of doping concentration and excitation wavelength on the emission spectra, the emission spectra of samples Sr3-2xCexNaxGaO4F with x ) 0.15 and 0.30 were measured respectively under 400, 430, and 460 nm excitation at LHT, as displayed in Figure 3. The curves in Figure 3a,b and Figure 2a give the emission spectra of samples Sr3-2xCexNaxGaO4F under about 400 nm excitation with different doping content x ) 0.01, 0.15, and 0.30 at LHT. Dominant Ce(1)3+ emission and very weak emission of Ce(2)3+ are observed for sample with x ) 0.01 (see Figure 2a). When the doping concentration increases to x )

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Figure 3. Influence of excitation wavelength and doping concentration on the emission spectra of samples Sr3-2xCexNaxGaO4F with x ) 0.15 and 0.30 at LHT.

0.15, intensive emissions are found from both Ce(1)2+ and Ce(2)2+ sites (see Figure 3a). And when the doping concentration further increases, the emission of Ce(2)3+ is predominant (see Figure 3b). The Ce(2)3+ emission intensity increases with the increasing of doping concentration. Similar phenomena can be observed in Figure 3c,d. This may relate to three factors. First, the Ce3+ ions predominately occupy the Sr(1)2+ sites at low doping concentration, and the increase of Ce(2)3+ site occupancy with increasing doping concentration leads to the increase of Ce(2)3+ emission intensity. Second, the distance between adjacent Ce3+ ions decreases with the increasing of doping concentration, and then the probabilities of energy transfer from Ce(1)3+ to Ce(2)3+ rapidly increase, resulting in the increase of Ce(2)3+ emission and the decrease of Ce(1)3+ emission. Finally, the difference of chemical environment around the two Ce3+ centers may cause a different quench concentration for two luminescence center,8 and this will also show influence on their relative intensity. It can be expected that the emission from Ce(2)3+ centers will be stronger at 430 nm excitation than that at about 400 nm excitation for same doping samples, while emission from Ce(1)3+ centers will be weaker under 430 nm excitation than that under about 400 nm excitation. This was just case that we observed in Figure 3a,c (or Figure 3b,d). When doping content x ) 0.15 (or 0.30), Ce(2)3+ emission at 430 nm excitation is stronger than that at ∼400 nm excitation, whereas Ce(1)3+ emission under 430 nm excitation is weaker than that under ∼400 nm excitation. When we change the excitation wavelength to 460 nm, it is expected that only Ce(2)3+ centers are excited if we neglect the thermal-assisted excitation of Ce(1)3+ centers at this low temperature (LHT). We measured the emission spectra of samples Sr3-2xCexNaxGaO4F (x ) 0.15 and 0.30) upon 460 nm excitation, as shown in Figure 3e,f. It can be found that two normalized emission spectra are nearly same. This result supports above statement that emission is only from Ce(2)3+ centers under 460 nm excitation at LHT. When the emissions are from both centers, the sample with a definite doping concentration is expected to have different emission spectra at different wavelength excitation. As a comparison, it can be found from Figure 3a,c and Figure 3b,d, the emission spectra are always different for samples Sr3-2xCexNaxGaO4F with same doping concentration [x ) 0.15 (for curves a, c) or 0.30 (for curves b, d)] and different excitation wavelength (400 and 430

Chen et al. nm) because the emissions are from both centers in these cases. In terms of above consideration, we fitted the curve f with a sum of two Gaussian functions to estimate the possible doublet emission positions of Ce(2)3+ centers. This operation produces two rather broad bands peaking at ∼567 and ∼627 nm that may correspond to the 5d-4f (2F5/2,7/2) transitions of Ce(2)3+ in the host lattice. If the lowest 2F5/2 f 5d absorption and the relaxed 5d f 2F5/2 emission are respectively assumed to be ∼430 and ∼567 nm for Ce(2)3+ center, the Stokes shift of this center is estimated to be around 5620 cm-1. A larger Stokes shift of Ce(2)3+ center in comparison with that of Ce(1)3+ center (∼2762 cm-1) may imply a stronger electron-lattice interaction for Ce(2)3+ center. In most case, a larger Stokes shift means a stronger coupling of the 5d electron with the lattice phonons and hence results in a broader and worse resolution of emission band of Ce3+. The emission of Ce(1)3+ center extends ∼6.7 × 103 cm-1 (∼425-595 nm), and the doublet bands are wellresolved (see Figure 2a). For Ce(2)3+ center, the emission occurs in ∼475-775 nm; the band is worse resolved with width around ∼8.1 × 103 cm-1 (see Figure 3e,f). Now, we will assign the nature of Ce(1)3+ and Ce(2)3+ centers in Sr3GaO4F in terms of the crystal-field splitting (CFS) and the 5d centroid of 5d states. First, it is well-known that the 5d crystal field splitting (CFS) of Ce3+ in different host lattices is mainly decided by the shape and size of coordination polyhedron around Ce3+. This influence can be understood as following two aspects: If the bond length (site size) remains constant, the polyhedral shape exhibits intense influence on the CFS magnitude. On the other hand, the CFS increases with the decrease of Ce-ligand bond distance for an exact polyhedral shape.17 For Sr3GaO4F, Sr(2)2+ sites are 8-fold coordination with an average Sr2+-anion bond distance of ∼2.586 Å, while Sr(1)2+ sites are 10-fold coordination with an average Sr2+-anion bond distance of ∼2.858 Å. The polyhedral shapes for Sr(2)2+ sites and Sr(1)2+ sites are different, and the site sizes around Ce3+ doped in this host are also different. However, it is still possible to estimate roughly the magnitude of CFS for Ce3+ in these two lattice sites. Many relevant work17,18 shows that the magnitude of CFS for Ce3+ in complex oxides and fluorides is always larger in 8-fold coordination sites (17.0 × 103-19.6 × 103 cm-1) than in 10-fold coordination sites (11.9 × 103-12.6 × 103 cm-1). In terms of above data, the CFS magnitude of Ce(2)3+ is considered to be larger than that of Ce(1)3+. Second, the 5d centroid for Ce3+ in a specific host lattice can be understood according to the following semiempirical formula:18,19 N

εc ) 1.79 × 1013

∑ (R i)1

i

i Rsp

- 0.6∆R)6

(1)

where εc is the shift of the centroid energy in eV relative to the free ion value of 6.35 eV. Ri is the distance (pm) between Ce3+ and anion i in the undistorted lattice. ∆R is the radius difference between Ce3+ and the substituted cation (for present case are Sr(1)2+ and Sr(2)2+ ions), and 0.6∆R is a correction for lattice i (10-30 m-3) is the spectroscopic relaxation around Ce3+. Rsp polarizability of anion i, which is closely connected with the polarizability of the anion. The summation is over all N anions that coordinate Ce3+. The values of Rsp (in units of 10-30 m3) for oxygen and fluoride are against the inverse square of the weighted average of the electronegativity of the cation, and can be defined as follows, respectively.

Emitting-Color Tunable Phosphors Sr3GaO4F:Ce3+

for oxides:

for fluorides:

O Rsp ) 0.33 +

4.80 χav2

F Rsp ) 0.15 +

0.96 χav2

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(2)

(3)

where χav is the weighted average of the electronegativity of the cation. Using χav ) [6χ(Sr) + 3χ(Ga)]/9 ) 1.2 for Sr3GaO4F, O F ) 3.66 Å and Rsp ) 0.82 Å. For 10-coordinate we obtain Rsp 3+ 2+ Ce(1) in Sr(1) site because the Ce3+-anion distance (Ri) is identical 285.8 pm and ∆R is 11 pm (Ce3+ and Sr2+ radius is 125 and 136 pm, respectively). But for 8-coordinate Ce(2)3+ in Sr(2)2+ site, Ri is different, viz., two Sr-F distance is 252.3 pm, two Sr-O distance is 242.3 pm, and other four Sr-O distance is 269.9 pm, and ∆R is 12 pm (Ce3+ and Sr2+ radius is 114 and 126 pm, respectively). Inserting these data in eq 1, we obtain εc 1.17 and 1.71 eV for Ce(1)3+ and Ce(2)3+, respectively. The results show that the decreasing of Ce3+ 5d centroid is larger in Sr(2)2+ sites than that in Sr(1)2+ sites. Considering above two factors, the crystal-field splitting (CFS) and the 5d centroid energy of 5d states in a whole, it will find that the 5d centroid of Ce3+ is higher in Sr(1)2+ sites than that in Sr(2)2+ sites, and the CFS magnitude for Ce3+ in Sr(2)2+ sites is larger than that in Sr(1)2+ sites. Hence, the Ce(1)3+ centers are assigned to the Ce3+ ions residing in Sr(1)2+ sites, and the Ce(2)3+ centers are assigned to the Ce3+ ions residing in Sr(2)2+ sites. In detail, Ce(1)3+ sites are coordinated by eight oxygen atoms and two fluorine atoms to form a bicapped square antiprism, while Ce(2)3+ sites are coordinated by six oxygen atoms and two fluorine atoms to form a distorted bicapped trigonal prism. Figure 4 shows the normalized emission spectra of samples Sr3-2xCexNaxGaO4 upon 405 nm excitation with x ) 0.01, 0.03, 0.06, 0.10, 0.15, 0.20, and 0.30, respectively. In addition to the shape difference of the emission spectrum, it can be seen that the position of the emission peak shifts toward long-wavelength side with the increase of Ce3+ concentration (x value). The variation of position/shape of emission spectrum has been discussed before. The chromaticity coordinates of samples Sr3-2xCexNaxGaO4F with x ) 0.01, 0.03, 0.06, 0.10, 0.15, 0.20, and 0.30 upon 405 nm excitation are presented in the CIE chromaticity diagram (Figure 5). With increasing of Ce3+ concentration, the chromaticity coordinate gradually move from greenish-blue area, through bluish-green, and finally to yellowish-green area. The emitting is tunable in a large color gamut by adjusting the doping concentration of Ce3+ ions. Figure 6 presents three decay curves of Sr3-2xCexNaxGaO4F (with x ) 0.01, 0.10, and 0.30) upon 403 nm excitation and monitoring the emission of 460 nm at RT. The decay curves consist of both fast and slow components. For x ) 0.01, the decay curve contains an inconspicuous fast decay component at initial time (curve a). With increasing concentration of Ce3+, the fast decay component become conspicuous; it may be resulted from the energy transfer from Ce(1)3+ to Ce(2)3+. All decay curves were fitted with a double-exponential decay, and the concentration dependence of the lifetimes of the slow decay component is displayed in the inset of Figure 6. The luminescent lifetime of the slow decay component, corresponding to the decay of 5d excited state of Ce(1)3+, decrease gradually with increasing x value due to the existence of energy transfer and defect (see the inset of Figure 6). As an example, the lifetime values are 27.7, 16.4, and 12.0 ns for x ) 0.01, 0.10, and 0.30, respectively.

Figure 4. Emission band position of Sr3-2xCexNaxGaO4F dependence on x value (x ) 0.01, 0.03, 0.06, 0.10, 0.15, 0.20, and 0.30) upon 405 nm excitation at RT.

Figure 5. Dependence of the chromaticity coordinates on Ce3+ concentration under 405 nm excitation.

3.3. Cathodoluminescence Properties. Under low-voltage electron beam excitation, all samples exhibit typical broadband emissions of Ce3+, but the emission spectra are different one another. This feature is also found in photoluminescence spectra. Like the photoluminescence, the cathodoluminescence color can also be tuned from greenish-blue to yellowish-green by controlling the doping concentration of Ce3+ ions. Figure 7 displays three typical cathodoluminescence spectra of Sr3-2xCexNaxGaO4F with x ) 0.03, 0.10, and 0.20 excited at 250 µA filament current and 4 kV accelerating voltage. With the increase of doping concentration, the emission maximum regularly move toward long-wavelength side. The phosphors Sr3-2xCexNaxGaO4F show greenish-blue (Figure 7a, x ) 0.03), green-blue (Figure 7b, x ) 0.10), and yellowish-green (Figure 7c, x ) 0.02) luminescence as to the naked eye, respectively. The relevant luminescence photographs are displayed in the insets of Figure 7.

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Chen et al.

Figure 6. Luminescence decay curves of Sr3-2xCexNaxGaO4F with x ) 0.01, 0.10, and 0.30 (λex ) 403 nm and λem ) 460 nm) at RT. The inset shows the dependence of decay time on Ce3+ concentration.

Figure 8. Dependences of cathodoluminescence intensity of Sr3.94Ce0.03Na0.03GaO4F on the (a) accelerating voltage and (b) filament current.

(about 2 kV).20 Under 2 kV voltage excitation, the cathodoluminescence intensity gradually increases with increasing filament current from 50 to 1800 µA (Figure 8b). These characteristics indicate that the phosphors Sr3-2xCexNaxGaO4F are resistant to the current saturation, which is of benefit to FED application. 4. Conclusions Figure 7. Cathodoluminescence spectra of Sr3-2xCexNaxGaO4F with x ) 0.03, 0.10, and 0.20 (250 µA and 4 kV excitation). The insets show the relevant luminescence photographs.

The dependences of the cathodoluminescence emission intensity of the Sr3-2xCexNaxGaO4F phosphors on the accelerating voltage and filament current are investigated. Figure 8a,b displays the typical cathodoluminescence emission intensity of Sr3.94Ce0.03Na0.03GaO4F phosphor as a function of accelerating voltages and filament current, respectively. It is known that an increasing current/voltage will directly lead to an increase in the amount of excited Ce3+ ions.2 When the filament current is fixed at 100 µA, with the increase of accelerating voltage from 1 to 5 kV, the cathodoluminescence intensity linearly increases in the range around and above 2.5 kV (Figure 8a). In general, B ) f(Ib)(V - V0)m is used to depict the dependence of cathodoluminescence intensity (B) on the electron beam current f(Ib) and voltage (V), where V0 is a “dead voltage” and 1 e m e 2 for most phosphors.4 On the basis of the equation, the crossing point of fitted line with the abscissa shows that the “dead voltage” of Sr3.94Ce0.03Na0.03GaO4F is about 2 kV (Figure 8a). The “dead voltage” was reported as being independence of the type of phosphors,20 and the “dead voltage” value of Sr3.94Ce0.03Na0.03GaO4F is in line with that of Zn2SiO4:Mn2+

Phosphors Sr3-2xCexNaxGaO4F (x ) 0.01, 0.03, 0.06, 0.10, 0.15, 0.20, and 0.30) were prepared using a high-temperature solid-state reaction technique. Photoluminescence studies show that the shape/position of emission spectra of phosphors may be controlled by tuning Ce3+ concentration or changing excitation wavelength and then result in a regular change of chromaticity coordinates. The luminescence color may be tuned from greenish-blue to bluish-green and finally to yellowish-green under 405 nm ultraviolet light excitation. It reveals that the Ce3+ ions enter two different sites in the host Sr3GaO4F. The emission bands peaking at ∼456 and ∼567 nm are respectively assigned from 10-coordinate Ce(1)3+ and 8-coordinate Ce(2)3+ site, and the lowest 5d states respectively locate ∼405 and ∼430 nm. Furthermore, the energy transfer from Ce(1)3+ to Ce(2)3+ centers is observed. Simultaneously, the tunable luminescence property is also observed under the excitation of low-voltage electron beam. The properties of the linearly increasing of emission intensity and tunable-emitting under the excitation of lowvoltage electron beam imply that Sr3-2xCexNaxGaO4F may be a potential FED phosphor. Acknowledgment. The work is financially supported by the National Basic Research Program of China (973 Program)

Emitting-Color Tunable Phosphors Sr3GaO4F:Ce3+ (Grant 2007CB935502), by the National Natural Science Foundation of China (Grants 20871121 and 20571088), and by the Science and Technology Project of Guangdong Province (Grant 2008A010500004). References and Notes (1) Talin, A.; Dean, K. A.; Jaskie, J. E. Solid-State Electron. 2001, 45, 963. (2) Wang, Z. L.; Chan, H. L. W.; Li, H. L.; Hao, J. H. Appl. Phys. Lett. 2008, 93, 141106. (3) Marsh, P. J.; Silver, J.; Vecht, A.; Newport, A. J. Lumin. 2002, 97, 229. (4) Hao, J. H.; Studeninkin, S. A.; Cocivera, M. J. Lumin. 2001, 93, 313. (5) Liu, X. M.; Pang, R.; Quan, Z. W.; Yang, J.; Lin, J. J. Electrochem. Soc. 2007, 154, 185. (6) Lin, X. M.; Zou, J. P.; Lin, J. J. Electrochem. Soc. 2009, 156, 43. (7) Yan, R. X.; Li, Y. D. AdV. Funct. Mater. 2005, 15, 763. (8) Lin, H. H.; Liang, H. B.; Han, B.; Zhong, J. P.; Su, Q. Phys. ReV. B 2007, 76, 35117.

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17199 (9) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer: Berlin, 1994. (10) Dorenbos, P. Phys. ReV. B 2001, 64, 125117. (11) Toquin, R. L.; Cheetham, A. K. Chem. Phys. Lett. 2006, 423, 352. (12) Im, W. B.; Kim, Y.; Fellows, N. N.; Masui, H.; Hirata, G. A.; DenBaars, S. P.; Seshadn, R. Appl. Phys. Lett. 2008, 93, 91905. (13) Yu, R. J.; Wang, J.; Zhang, M.; Zhang, J. H.; Yuan, H. B.; Su, Q. Chem. Phys. Lett. 2008, 453, 197. (14) Efryushina, N. P.; Dotsenko, V. P.; Berezovskaya, I. V.; Ryzhkov, M. V. Radiat. Meas. 2001, 33, 755. (15) Vogt, T.; Woodware, P. M.; Hunter, B. A.; Prodjosantoso, A. K.; Kennedy, B. J. J. Solid State Chem. 1999, 144, 228. (16) Tian, Z. F.; Liang, H. B.; Han, B.; Su, Q.; Tao, Y.; Zhang, G. B.; Fu, Y. B. J. Phys. Chem. C 2008, 112, 12524. (17) Li, L.; Liang, H. B.; Tian, Z. F.; Lin, H. H.; Su, Q.; Zhang, G. B. J. Phys. Chem. C 2008, 112, 13763. (18) Dorenbos, P. Phys. ReV. B 2000, 62, 15640. (19) Dorenbos, P. J. Lumin. 2003, 105, 117. (20) Ozawa, L.; Itoh, M. Chem. ReV. 2003, 103, 3835.

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