Origin and Luminescence of Anomalous Red-Emitting Center in

Aug 11, 2016 - The Ba9Lu2Si6O24:Eu2+ phosphor exhibits a temperature-sensitive luminescence property. At RT, it gives rise to a strong blue emission p...
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Origin and Luminescence of Anomalous Red-Emitting Center in Rhombohedral Ba9Lu2Si6O24:Eu2+ Blue Phosphor Yongfu Liu,*,† Changhua Zhang,†,‡ Zhixuan Cheng,‡ Zhi Zhou,§ Jun Jiang,*,† and Haochuan Jiang*,† †

Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Ningbo, 315201, China Department of Chemistry, College of Science, Shanghai University, Shanghai, 200444, China § College of Science, Hunan Agricultural University, No. 1 Nongda Road, Changsha, 410128, China ‡

S Supporting Information *

ABSTRACT: We obtain a blue phosphor, Ba9Lu2Si6O24:Eu2+ (BLS:Eu2+), which shows a strong emission peak at 460 nm and a weak tail from 460 to 750 nm. A 610 nm red emission is observed for the first time in this kind of rhombohedral structure material, which is much different from the same crystal structure of Ba9Sc2Si6O24:Eu2+ and Ba9Y2Si6O24:Eu2+. The luminescence properties and decays from 10 to 550 K are discussed. The new red emission arises from a trapped exciton state of Eu2+ at the Ba site with a larger coordination number (12-fold). It exhibits abnormal luminescence properties with a broad bandwidth and a large Stokes shift. Under the 400 nm excitation, the external quantum efficiency of BLS:Eu2+ is 45.4%, which is higher than the 35.7% for the commercial blue phosphor BAM:Eu2+. If the thermal stability of BLS:Eu2+ can be improved, it will show promising applications in efficient near-UV-based white LEDs. 460 nm blue emission of BLS:Eu2+ is much different from the 510 nm green emissions of Ba9Sc2Si6O24:Eu2+ (BSS:Eu2+) and Ba9Y2Si6O24:Eu2+ (BYS:Eu2+), which have the same rhombohedral structure as BLS.21,22 This difference was not investigated in BLS:Eu2+,Ce3+,Mn2+. In this paper, the luminescence properties of BLS:Eu2+ were detailed. An anomalous 610 nm red emission was observed for the first time in BLS:Eu2+. After investigating the luminescence and decay curves of BLS:Eu2+ from 10 to 550 K, we infer that the 610 nm red emission originates from an anomalous trapped exciton emission of Eu2+ at the Ba site that also has a normal df emission at 460 nm. Due to the abnormal red emission, BLS:Eu2+ exhibits a sensitive optical property to temperature.

1. INTRODUCTION Phosphor-converted white LEDs are widely applied in illumination and display fields due to their advantages of energy saving, high efficiency, long lifetime, etc.1,2 Phosphors play important roles in determining the performances of white LEDs, because they convert the light emitted from near-UV (NUV) or blue LEDs into white light.3−10 For the NUV-based white LEDs, blue phosphor is a critical item, as the blue light not only participates in the synthesis of white light but also can be used as an excitation light for green and/or red phosphors. Currently, the emission light of efficient NUV-LED chips is around 400 nm. Unfortunately, the main excitation region of most blue phosphors is in the range of 250−380 nm.11−16 Blue phosphors that can be effectively excited at 400 nm are quite rare.17,18 For example, the commercial blue phosphor BaMgAl10O17:Eu2+ (BAM:Eu2+) is effectively excited by 254 nm UV light, and the excitation intensity decreases dramatically around 400 nm, limiting its application in NUV-based white LEDs.19 In this work, we report a new Ba9Lu2Si6O24:Eu2+ (BLS:Eu2+) phosphor with a strong blue emission band at 460 nm and a very broad UV−NUV excitation band (250−430 nm). Its excitation intensity still remains high even at 400 nm. Under 400 nm excitation, the external quantum efficiency (QE) for BLS:Eu2+ reaches 45.4%, which is higher than 35.7% for BAM:Eu2+, indicating a probable application in NUV-based white LEDs. Actually, Eu2+ provides a good blue-emitting center in the BLS:Eu2+,Ce3+,Mn2+ phosphor that can generate white light under the excitation of the 395 nm NUV chip.20 However, the © 2016 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis. The samples were prepared by a high-temperature solid-state reaction with desired stoichiometric amounts of BaCO3 (AR), SiO2 (99.9%), Lu2O3 (99.99%), and Eu2O3 (99.99%). All of these raw materials were completely mixed and ground in an agate mortar and pestle for 40 min; then the mixtures were placed in an alumina crucible covered with a lid and sintered in a tube furnace under a reducing atmosphere with 95% N2 + 5% H2. The sintering temperature was kept at 1400 °C for 5 h. Subsequently, the finished samples were cooled and ground into fine powders. 2.2. Instrumental Methods. X-ray diffraction (XRD) data of all samples were collected using a Bruker D8 Advance diffractometer (Cu Kα radiation λ = 1.540 56 Å). X-ray photoelectron spectroscopy Received: May 16, 2016 Published: August 11, 2016 8628

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Inorganic Chemistry (XPS) analysis was performed using an AXIS ULTRA DLD photoelectron spectrometer (Shimadzu) equipped with Al Kα X-rays (1486.69 eV). All the binding energies have been corrected by using the contamination carbon (C 1s, 284.8 eV) as a reference. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra at RT were obtained on a Hitachi F-4600 spectrometer. The PL spectra from 10 to 300 K were measured by a Horiba iR320 imaging spectrometer with a temperature-controlling system. The external QE and PL spectra from 300 to 550 K were collected on a quantum efficiency measurement system (Otsuka Photal Electronics QE-2100). The PLE and PL spectra at 77 K were obtained by a Horiba FluoroMax4 spectrometer. Decay curves at RT and 77 K were recorded on a Horiba Fluorolog (FL3-111) spectrometer equipped with a nanoLED excitation source. Photographs of the as-prepared sample under the 400 nm NUV light were taken by a Nikon D7100 digital camera.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure. BLS is a rhombohedral structure and belongs to the R3̅ (148) space group.23,24 The lattice parameters are a = b = 9.9905 Å, c = 22.1088 Å, α = β = 90°, γ = 120°, and V = 1911.04(7) Å3. The structure is depicted in Figure 1. The rhombohedra consist of corner-sharing SiO4−

Figure 2. XRD patterns for Eu2+-undoped (x = 0) and -doped BLS:xEu2+ (x = 0.01−0.15) in the range of 10−80° and highresolution patterns in the range of 29.5−31.5°.

successfully. It can also be seen from the XRD patterns in the range of 29.5−31.5° (2θ) that the diffraction peaks slightly shift to high-angle regions with increasing Eu2+ concentration from 0 to 15%. These phenomena suggest that the lattice shrinkage occurs due to the replacement of Ba2+ by the smaller Eu2+. 3.2. Luminescence Properties. Figure 3a shows the normalized PL and PLE spectra for BLS:3%Eu2+ at RT. All the PL spectra (λex = 342, 370, and 430 nm) exhibit an emission peak around 460 nm and a long tail ranging from 460 to 750 nm. The emission ratio around 510 nm increases when the excitation wavelength shifts from 342 nm to 430 nm, and a shoulder emerges for the end profile (λex = 430 nm). The PLE spectra monitored at the peak, the shoulder, and the tail (λem = 460, 510, and 610 nm, respectively) show a broad excitation band covering from 240 to 470 nm. The bandwidth tends to broaden and the ratio around 400 nm increases when the monitored emission wavelength increases from 460 nm to 510 nm or to 610 nm. Meanwhile, the profiles of the excitation bands are different from each other. Both the PL and PLE spectra change with a change in excitation and emission wavelengths. These phenomena indicate that more than one luminescence center exists in BLS:Eu2+. To clarify the luminescence centers, the PL and PLE spectra of BLS:3%Eu2+ at 77 K were measured. As shown in Figure 3b, all the PL spectra exhibit three distinguished emission peaks, at 460, 510, and 610 nm. The PLE spectra for the three peaks are also significantly different. Especially, the PLE spectrum for λem = 510 nm shows mainly a narrow peak at 430 nm and two secondary peaks at 370 and 280 nm at 77 K, which is much different from the broad excitation band recorded for the same wavelength at RT. These results indicate that there are three Eu2+ luminescence centers in BLS:Eu2+, and the three emission peaks should originate from the different Eu2+ centers. It is also noticed that the PLE spectrum monitored at 610 nm contains two sharp peaks, situated at 464 and 525 nm, at 77 K. These two excitation peaks correspond to the characteristic Eu3+ transitions from 7F0 to 5D2 and from 7FJ (J = 0 and 1) to 5 D1, respectively.25 However, the typical Eu3+ sharp emissions from 5D0 to 7F0 (580 nm) and to 7F2 (610 nm) are not observed. These phenomena mean that although Eu3+ still exists in BLS even though the sample is synthesized under a reducing condition, it should be in trace amounts and will not

Figure 1. Crystal structure of BLS viewed perpendicular to the b axis (a) and the c axis (b) and coordinated environment of Ba(1) (blue balls), Ba(2) (pink balls), Ba(3) (gold balls), and Lu (green balls) with oxygen atoms (c).

LuO6−SiO4 layers, forming a rigid three-dimensional framework. Lutecium provides one site coordinated by six oxygen atoms, forming a distorted LuO6 octahedron that has three shorter Lu−O bonds (2.162(0) Å) and three longer bonds (2.223(7) Å). Barium provides three independent sites, Ba(1), Ba(2), and Ba(3), coordinated by 12, 9, and 10 oxygen atoms, respectively. The polyhedra for the Ba sites are also distorted, and the average bond lengths for Ba(1)−O, Ba(2)−O, and Ba(3)−O are 3.085(3), 2.925(4), and 2.934(1) Å, respectively. Considering the ionic radius (r) at the same coordination environment, such as six-coordinated, Eu2+ (r = 1.17 Å) is larger than Lu3+ (r = 0.861 Å) but smaller than Ba2+ (r = 1.35 Å). The charge between Eu2+ and Lu3+ is also different. If Eu2+ is introduced in BLS, it will be easy to occupy the Ba2+ site rather than the Lu3+ site. Therefore, samples of the Eu2+-doped BLS were prepared based on the substitution of Eu2+ for Ba2+. Figure 2 exhibits the XRD patterns for BLS:xEu2+ (x = 0.01− 0.15). It is clear that all the Eu2+-doped samples form a single crystal phase as BLS (for x = 0), and no impurity phase is observed. These confirm that Eu2+ has entered the BLS host 8629

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Figure 3. PL and PLE spectra of BLS:3%Eu2+ at RT (a) and 77 K (b) under various excitation and emission wavelengths. Normalized PLE spectra recorded for the 460 and 610 nm emissions at 77 K (c) and digital images of the sample at RT (d) and 77 K (e) irradiated by 400 nm UV light.

influence the comprehensive analyses of the Eu2+ performance. The XPS of BLS:3%Eu2+ in Figure S1 could also evidence the existence of Eu3+ in a trace amount, as the photoelectric peak of Eu3+ almost cannot be detected. The crystal structure of BLS is the same as that of BSS and BYS. Eu2+ also occupies the Ba2+ site in BSS and BYS.21,22 Both BSS:Eu2+ and BYS:Eu2+ display a green emission around 505 nm at RT. The green emission can be identified only as two bands peaking at 460 and 510 nm at a low temperature.26,27 Although BLS has a similar crystal structure to that of BSS and BYS, at RT, BLS:Eu2+ mainly shows a blue emission at 460 nm differing from the 505 nm green emission in both BSS:Eu2+ and BYS:Eu2+. Furthermore, a new red emission band around 610 nm, which was not observed in BSS:Eu2+ and BYS:Eu2+, appears in BLS:Eu2+ at 77 K. In the BLS crystal structure, the average bond of the 9-fold Ba(2) site (2.925(4) Å) is similar to that of the 10-fold Ba(3) site (2.934(1) Å), leading to a similar crystal-field strength between Ba(2) and Ba(3), while the average bond of the 12-fold Ba(1) site (3.085(3) Å) is longer than that of Ba(2) and Ba(3). Thus, the crystal-field strength from Ba(1) is weaker than that from Ba(2) and Ba(3). As known, the 5d energy level of Eu2+ is strongly influenced by the crystal field environment. Therefore, the 460 nm higher energy emission band should arise from Eu2+ at the Ba(1) site with a weaker crystal-field strength, and the 510 nm lower energy emission band should arise from Eu2+ at the Ba(2) and Ba(3) sites with a stronger crystal-field strength. This attribution is

reasonable according to the similar phenomenon in BSS:Eu2+.26,27 So it is necessary for us to understand where and why the 610 red center arises. It is noticed that the bond lengths display the order of Ba(1)−O > Ba(3)−O > Ba(2)−O. In terms of the inverse relation between the crystal-field strength and the bond length, one could think that the 460, 510, and 610 nm centers may come from the Ba(1), Ba(3), and Ba(2) sites, respectively. The energy difference between the 610 and 510 nm centers is apparent, while the bond length difference between Ba(2)−O and Ba(3)−O is too close to be distinguished. Thus, the hypothesis that the 610 nm center comes from the Ba(2) and/ or the Ba(3) site is incorrect. If the 610 nm center is from a site with a strong crystal-field splitting, the 6-fold Lu3+ site should be considered because the Lu−O bond (2.192(9) Å) is much shorter than the Ba−O bonds. Actually, Lu3+ cannot be substituted by Eu2+, as discussed in Section 3.1. Moreover, there is no reference on the luminescence of Eu2+ at the lanthanide site.28 If Eu2+ occupies the Lu3+ site, the excitation and emission bands of Eu2+ can be predicted by the following empirical relationship between Eu2+ and Ce3+ at the same crystallographic site:29 E(7, 2 + , A) = 0.64E(1, 3 + , A) + 0.53 eV

(1)

where the variable A corresponds to the same host compound and the E(7, 2+) and E(1, 3+) are the 4f−5d transition energies of Eu2+ and Ce3+, respectively. Utilizing the 490 nm emission 8630

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Inorganic Chemistry and the 400 nm excitation of Ce3+ presenting at the Lu3+ site in BLS,23 the Eu2+ excitation and emission wavelengths are calculated to be about 493 and 577 nm, respectively. The estimated 577 nm emission band seems to be close to the experimental value of 610 nm, while the observed excitation band at 610 nm still remains in the range of 250−400 nm (Figure 3b) and does not red shift to the calculated position around 493 nm. Therefore, the hypothesis that the 610 nm red center is from Eu2+ at the Lu3+ site is also incorrect. For BYS, a minor impurity phase of Ba2SiO4 can be observed.30 This impurity phase is not observed in BLS, as discussed in Section 3.1. Furthermore, Eu2+ always exhibits a green emission around 511 nm in Ba2SiO4.31 Thus, the 610 nm center cannot come from the impurity even through Ba2SiO4 exists in BLS. To get insight into the origin of the 610 nm red center, the PLE spectra are considered carefully again. For the 460 and 610 nm centers, the PLE spectra at 77 K are normalized (Figure 3c). It is found that the two excitation bands have very similar shapes at both RT (Figure 3a) and 77 K (Figure 3c), indicating that the two emission bands could originate from the same Eu2+ center. There is only a small difference in the PLE bands for the 610 nm center at different temperatures. At RT, the PLE spectrum for the 610 nm center shows a sharp onset at 450 nm, while the onset broadens to 550 nm at 77 K. Meanwhile, the broad excitation bands of both the 510 and 610 nm centers overlap with the 460 nm emission band, causing energy transfers to Eu2+ ions emitting at 510 and 610 nm at RT (Figure 3a). The broadening excitation band of the 610 nm center even overlaps with the 510 nm emission band at 77 K (Figure 3b). Accordingly, the possible energy transfers among the 460, 510, and 610 nm centers are illustrated in Figure 4. That is to say, only part of the energy from the 460 nm center transfers to the 610 nm emission (process a) at RT, while energy from both the 460 and 510 nm centers transfers to the 610 nm emission at 77 K (processes d and e). Therefore, the 610 nm emission is strong and marked at 77 K, as represented in Figure 3b. This phenomenon is also

directly observed when the powder sample is immersed in liquid nitrogen, as the photographs show in Figure 3d and e. 3.3. Fluorescent Decays. Figure 5 shows the fluorescent decays for the three emission centers at different temperatures.

Figure 5. Decay curves and lifetimes (τ) of three different emission bands (λem = 460, 510, and 610 nm) of BLS:3%Eu2+ after the pulsed excitation (λex = 342 nm) at RT (a) and 77 K (b).

All the curves exhibit a single exponential mode at RT (Figure 5a). The lifetimes for both the 460 and 610 nm emission are around 0.6 μs, which are shorter than 0.672 μs for the 510 nm emission. The similarity of the decay curves and lifetimes for the 460 and 610 nm emission could further demonstrate that these two emission bands should originate from the same Eu2+ luminescent center. Two different excited states in thermal equilibrium will give rise to the same decay times for emissions from both states. The decay modes change remarkably at 77 K (Figure 5b). The curves for the 460 and 510 nm emission become multiexponential. This can be ascribed to the effective energy transfers from the 460 nm emission to the 510 and 610 nm emissions (processes c and d in Figure 4) and from the 510 nm emission to the 610 nm emission (process e in Figure 4), respectively. The energy from the 510 nm center transfers only to the 610 nm center, while the 460 nm center transfers its energy to both the 510 and 610 nm centers. Thus, the decay of the 460 nm emission is faster than that of the 510 nm emission, and the lifetime of the 460 nm (0.695 μs) emission is shorter than that of the 510 nm (0.736 μs) emission, as shown in Figure 5b at 77 K. However, the decay for the 610 nm emission still remains close to a single-exponential mode. Its lifetime is (1.127 μs) much longer than that for the 460 and 510 nm emission, which can be partially attributed to the effective energy transfers from both the 460 and 510 nm emissions to the 610 nm emission. Comparatively, all three lifetimes at 77 K are longer than that at RT, as shown in Figure 5, due to the

Figure 4. Energy transfer processes among three luminescence centers of 460, 510, and 610 nm at RT (a, b) and 77 K (c, d, e). The 460 and 610 nm emissions are from Eu2+ at the Ba(1) site, and the 510 nm emission is from Eu2+ at the Ba(2) and Ba(3) sites. The polyhedra correspond to Eu2+-occupied Ba(1) (blue ball), Ba(2) (pink ball), and Ba(3) (gold ball) sites, respectively. 8631

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relative integral intensities of the three different emission bands are plotted as a function of temperature in Figure 6. An

decrease of the nonradiative transition probability with decreasing temperature. Therefore, the prolonged lifetime for the 610 nm emission also can be partially attributed to the decline of the nonradiative transition probability at 77 K. Commonly, Eu2+ exhibits a normal df transition with a Stokes shift (ΔS) of ∼1350 cm−1 and a bandwidth (Γ) of ∼1600 cm−1.32 However, Eu2+ also exhibits an anomalous emission characterized by a larger ΔS (>4000 cm−1) and a broad Γ (>3000 cm−1) in some particular compounds. For the 460, 510, and 610 nm emission bands, the ΔS and Γ are calculated and listed in Table 1. The Stokes shift estimated here Table 1. Stokes Shifts (ΔS) and Emission Band Width (Γ) of the 460, 510, and 610 nm Emissions from BLS:Eu2+ and the Normal and Anomalous df Emissions of Eu2+ in Inorganic Compounds luminescence −1

ΔS (cm ) Γ (cm−1)

460 nm

510 nm

610 nm

normal28

anomalous28

2950 2200

3650 1800

8300 3650

∼1350 ∼1600

>4000 >3000

Figure 6. Temperature-dependent luminescence intensities of the three different emission bands (460, 510, and 610 nm) of BLS:3%Eu2+ under 342 nm excitation.

is the energy difference between the lowest 4f 65d1 excitation band and the maximum emission band. The three emission bands come from the transitions from excited states to 4f ground states of Eu2+; however, the ΔS and Γ for the three emissions are different. For the blue emission, ΔS is about 2950 cm−1 (the energy difference between the 405 and 460 nm emission). For the green emission, ΔS is about 3650 cm−1 (the energy difference between the 430 and 510 nm emission). Combining the band widths, the blue and green emissions should belong to normal df transitions of Eu2+. In terms of the luminescence and decay properties, it is confirmed that the 610 nm red emission is from the same Eu2+ center at the Ba(1) site originating the 460 nm emission as well. However, the 610 red emission energy is too low to accord with the weak crystal-field strength at the Ba(1) site. The ΔS for the red emission is large, ∼8300 cm−1 (the energy difference between the 405 and 610 nm emission), and the Γ is broad, ∼3650 cm−1. These values are in the range for an anomalous Eu2+ emission. To explain the abnormal observations for the 610 nm red emission, we assign it to the anomalous Eu2+-trapped exciton (ETE) emission. The ETE emissions are often observed when Eu2+ is located at a large cation site with a high coordination number.33−35 In this situation, Eu2+ automatically changes to Eu3+ by losing one electron (Eu2+ → Eu3+ + e−). The electron does not really leave Eu2+; it is stabilized in the vicinity by the neighboring cations forming a trapped exciton.34 Usually, the ETE state is lower than the normal 4f 65d 1 energy level of Eu2+.33 Thus, the ETE emission exhibits properties of a large Stokes shift and a low thermal stability, 36−40 such as in Sr 4 Al 14 O 25 :Eu 2+ , 33 BaF2:Eu2+,36 and CsMgPO4:Eu2+.40 In the present case for BLS:Eu2+, the coordination number for Ba(1) (12-fold) is higher than that for Ba(2) (9-fold) or Ba(3) (10-fold). Thus, it should be easy to understand that the ETE comes from the Ba(1) site. The Ba(1) site also gives rise to the 460 nm emission. So it is also easy to understand why the luminescence and decay properties of the ETE emission at 610 nm are similar to those of the 460 nm emission. To analyze the three emission bands quantitatively and illustrate the temperature-sensitive property of the ETE, the luminescence from 10 to 550 K was measured. The spectra were fitted using three Gaussion profiles based on Table 1. The

anticorrelation in intensity between the 460 and 610 nm emission can be observed below 300 K. However, the total intensities of the 460 and 610 nm bands remains almost constant at low temperatures. This demonstrates that the 460 and 610 nm emissions indeed come from the same Ba(1) site, and there is only a gradual transition from 610 to the 460 nm emission with increasing temperature. The 610 nm emission is dominant at low temperatures, so an orange color is observed (Figure 3e), and the color changes to blue as the temperature increases to RT (Figure 3d). Thermal quenching of all bands occurs above 300 K. It can also be seen that the intensity of the 510 nm emission increases between 10 and 250 K, together with the increase of the 460 nm emission. This phenomenon can be explained by more efficient energy transfer due to the transition from 610 to the 460 nm emission, which increases the spectral overlap for energy transfer. 3.4. QE and CIE. Currently, the light emission of efficient NUV chips is located around 400 nm. BAM:Eu2+ is one of commercial blue phosphors for NUV-based white LEDs. In fact, the excitation intensity of BAM:Eu2+ decreases sharply, and only 43% of its optimal peak value (λmax = 314 nm) remains at 400 nm, as shown in Figure S2. For comparison, the normalized PLE spectrum for BLS:Eu2+ is shown as well. Its intensity stays at 83% at 400 nm. This means BLS:Eu2+ could be better for the NUV chip than BAM:Eu2+. Under 400 nm excitation, the PL spectra of BLS:xEu2+ (x = 0.01−0.15) and BAM:Eu2+ are shown in Figure 7. BLS:xEu2+ exhibits a blue emission peak around 460 nm and a long tail for various x values. When x increases to 7%, the peak intensity reaches a maximum and is nearly the same as that of the BAM:Eu2+ blue phosphor (peaking at 456 nm, dashed line). When x exceeds 7%, the 460 nm emission peak intensity decreases, while the 510 nm emission intensity still increases, especially for x = 12% and 15%. This phenomenon can be attributed to the more effective energy transfers from 460 to 510 nm emission bands, as the distance between two adjacent Eu2+ ions decreases with Eu2+ content increasing, leading to an enlarged energy transfer probability from 460 to 510 nm emission. The inset in Figure 7 illustrates the integrated PL intensity for various x (dotted 8632

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Figure 7. PL spectra of BLS:xEu2+ (x = 0.01−0.15) and the commercial BAM:Eu2+ blue phosphor (dashed line) by using 400 nm excitation. The inset illustrates the integrated PL intensities for BLS:xEu2+ (dotted circles) and the commercial BAM:Eu2+ blue phosphor (dashed horizontal line). Figure 8. CIE color coordinates for the BLS:xEu2+ (x = 0.01−0.15) series (black dots) and BAM:Eu2+ (dot with red circle) at RT. The results are calculated using the emission spectra under 400 nm excitation.

circles). The total intensity increases from x = 1% to 7% and drops slightly after x = 7% due to the concentration quenching effect of Eu2+ ions. Except for x = 1%, BLS:Eu2+ exhibits a higher level than BAM:Eu2+ (dashed horizontal line). The external QEs and band widths for BLS:xEu2+ and BAM:Eu2+ under 400 nm excitation are listed in Table 2, as well as the internal QEs and absorbance in Table S1, and the change trends are depicted in Figure S3. The change trend of external QEs in Figure S3 is consistent with that of the integrated PL intensity in Figure 7. The external QE for BLS:xEu2+, which gives a maximum value of 45.4%, is higher than the 35.7% for BAM:Eu2+. This suggests that BLS:Eu2+ could be better than BAM:Eu2+ for the NUV chips. The emission bandwidth for BLS:xEu2+ is around 51 nm when x = 1−9%, which is close to the width for BAM:Eu2+ (52.9 nm), especially for the optimum concentration (x = 7%). When x increases to 12% and 15%, the width broadens obviously due to the enhanced emission ratio around 510 nm. The CIE chromaticity coordinates of BLS:xEu2+ (x = 0.01−0.15) and BAM:Eu2+ are calculated and illustrated in Figure 8. The CIE chromaticity coordinates of BLS:xEu2+ shift slightly from blue (0.202, 0.195) to the bluegreen region (0.213, 0.278) as the Eu2+ concentration increases from 1% to 15%, resulting from the enhanced green emission around 510 nm. This means BLS:Eu2+ shows a relative moderate blue color compared with the dark blue color located at (0.147, 0.069) for BAM:Eu2+. 3.5. Thermal Characteristics. The temperature-dependent luminescence properties for the optimum sample, BLS:7%Eu2+, is measured and shown in Figure 9. The external QE declines when the temperature rises from RT to 500 K, and only 18% is preserved at 150 °C, as shown in Figure 9a. The emission

Figure 9. (a) Temperature-dependent external QE for BLS:7%Eu2+. The stars are experimental data, and the red line is the fitting curve. (b) Emission peak around 460 nm has almost no change from RT to 500 K, whereas the emission intensities clearly decrease with increasing temperature.

intensities in Figure 9b also decrease obviously with increasing temperature. This means that the thermal stability of BLS:Eu2+ still needs to be improved for use as white LEDs. However, the emission peak around 460 nm has almost no change as the

Table 2. External QEs and Emission Bandwidth (Γ) of BLS:xEu2+ (x = 0.01−0.15) and the Commercial Blue Phosphor BAM:Eu2+ under 400 nm Excitation x external QE (%) Γ (nm)

0.01

0.03

0.05

0.07

0.09

0.12

0.15

BAM:Eu2+

30.6 48

39.7 51

43.2 51

45.4 52

39.3 51

44.3 76

43.3 81

35.7 52.9

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (NSFC11404351, 51402317), Ningbo Municipal Natural Science Foundation (2014A610122), and Ningbo Science and Technology Innovation Team (2014B82004).

temperature varies, which means that the 460 nm luminescence center has an excellent color stability. To determine the activation energy for thermal quenching, the temperaturedependent external QE data are described by a modified Arrhenius equation as follows:41 I (T ) =

I0 1 + A e−ΔE / kBT

■

(2)

where I0 is the initial QE, IT is the QE at different temperatures T (K), A is a constant for a certain host, ΔE is the activation energy of thermal quenching, and kB is the Boltzmann constant (8.629 × 10−5 eV). The experimental data are well fitted by eq 2, as the red curve shows in Figure 8a. The ΔE is calculated to be 0.341 eV for BLS:Eu2+. Only 39.6% of the RT external QE is maintained at 150 °C. It seems that the thermal stability is not high for BLS:Eu2+, which could arise from the transition from the 460 to the 610 nm emission as the temperature increases, because the anomalous 610 nm emission in BLS:Eu2+ has an extremely sensitive thermal behavior. For the actual white LED application, therefore, the anomalous Eu2+ emission should be suppressed and the thermal stability of BLS:Eu2+ still needs to be enhanced.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01196. Listings of XPS data of BLS:3%Eu2+, normalized PLE spectra of BLS:Eu2+ and BAM:Eu2+, external QEs and emission bandwidth of BLS:xEu2+ (x = 0.01−0.15), internal QEs and absorbance of BLS:xEu2+ (x = 0.01− 0.15) and the commercial blue phosphor BAM:Eu2+ under 400 nm excitation (PDF)

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4. CONCLUSION The new phosphor BLS:Eu2+ shows a strong blue emission at 460 nm and a long tail from 460 to 750 nm at RT. Three emission bands, at 460, 510, and 610 nm, are observed at low temperatures. The 460 and 510 nm emissions originate from normal df transitions of Eu2+ at Ba(1) and Ba(2)/Ba(3) sites, respectively. The 610 nm emission is observed for the first time. On the basis of the luminescence properties at 10 to 550 K, the 610 nm center is attributed to an anomalous trapped exciton emission of Eu2+ at the Ba(1) site as well. It exhibits anomalous luminescence properties of a large Stokes shift, a broad bandwidth, and a low thermal quenching temperature. The energy transfer processes among the three emissions were discussed. The external QE for BLS:Eu2+ reaches 45.4% under 400 nm excitation, which is higher than the 35.7% for the commercial blue phosphor BAM:Eu2+ for NUV-based white LEDs. However, the thermal stability of BLS:Eu2+ still need to be improved for use in white LEDs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (Y. Liu): [email protected]. *E-mail (J. Jiang): [email protected]. *E-mail (H. Jiang): [email protected]. Notes

The authors declare no competing financial interest. 8634

DOI: 10.1021/acs.inorgchem.6b01196 Inorg. Chem. 2016, 55, 8628−8635

Article

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DOI: 10.1021/acs.inorgchem.6b01196 Inorg. Chem. 2016, 55, 8628−8635