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Visualizing the valence states of europium ions in Eu-doped BaAl2O4 using X-ray nanoprobe mapping

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aDepartment of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan, and bNational Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
*Correspondence e-mail: bihsuan@nsrrc.org.tw

Edited by D. Bhattacharyya, Bhabha Atomic Research Centre, India (Received 26 September 2021; accepted 6 December 2021; online 18 January 2022)

This study develops and successfully demonstrates visualization methods for the characterization of europium (Eu)-doped BaAl2O4 phosphors using X-ray nanoprobe techniques. X-ray fluorescence (XRF) mapping not only gives information on the elemental distributions but also clearly reveals the valence state distributions of the Eu2+ and Eu3+ ions. The accuracy of the estimated valence state distributions was examined by performing X-ray absorption spectroscopy (XAS) across the Eu L3-edge (6.977 keV). The X-ray excited optical luminescence (XEOL) spectra exhibit different emission lines in the selected local areas. Their corresponding emission distributions can be obtained via XEOL mapping. The emission properties can be understood through correlation analysis. The results demonstrate that the main contribution to the luminescence intensity of the Eu-doped BaAl2O4 comes from the Eu2+ activator and the emission intensity will not be influenced by the concentration of Eu2+ or Eu3+ ions. It is anticipated that X-ray nanoprobes will open new avenues with significant characterization ability for unravelling the emission mechanisms of phosphor materials.

1. Introduction

Phosphor-converted white-light-emitting diodes (wLEDs) have been studied widely to develop more stable and efficient wLEDs (Xia et al., 2019[Xia, L., Yue, Y., Yang, X., Deng, Y., Li, C., Zhuang, Y., Wang, R., You, W. & Liang, T. (2019). J. Eur. Ceram. Soc. 39, 3848-3855.]). Investigating the emission mechanism of long-afterglow phosphors will help realize various applications in lighting. Alkaline earth aluminates are good candidates for host matrices for luminescent materials, and are widely used as hosts for rare earths. Persistently luminescent phosphors composed of alkaline earth aluminates can be represented by the general formula XAl2O4 (X = Mg2+, Ca2+, Sr2+ or Ba2+) (Lephoto et al., 2012[Lephoto, M. A., Ntwaeaborwa, O. M., Pitale, S. S., Swart, H. C., Botha, J. R. & Mothudi, B. M. (2012). Physica B, 407, 1603-1606.]). Recently, the widely used traditional host matrix BaAl2O4 has received extensive attention because of its unique properties, such as low cost, easy synthesis, high physical stability and better luminescence efficiency (Yin et al., 2020[Yin, X., Lin, H., Zhang, D., Hong, R., Tao, C. & Han, Z. (2020). Ceram. Int. 46, 3801-3810.]; Lephoto et al., 2012[Lephoto, M. A., Ntwaeaborwa, O. M., Pitale, S. S., Swart, H. C., Botha, J. R. & Mothudi, B. M. (2012). Physica B, 407, 1603-1606.]; Tian et al., 2021[Tian, Y., Chen, J., Yi, X., Zhao, D., Weng, Z., Tang, Y., Lin, H. & Zhou, S. (2021). J. Eur. Ceram. Soc. 41, 4343-4348.]; Rezende et al., 2012[Rezende, M. V. dos S., Montes, P. J. R. & Valerio, M. E. G. (2012). J. Lumin. 132, 1106-1111.], 2016[Rezende, M. V. dos S., Montes, P. J., Andrade, A. B., Macedo, Z. S. & Valerio, M. E. (2016). Phys. Chem. Chem. Phys. 18, 17646-17654.], 2015[Rezende, M. V. dos S., Valerio, M. E. G. & Jackson, R. A. (2015). Mater. Res. Bull. 61, 348-351.]).

Many researchers have extensively investigated methods to obtain perfect luminescent phosphors. For example, Tian et al. (2021[Tian, Y., Chen, J., Yi, X., Zhao, D., Weng, Z., Tang, Y., Lin, H. & Zhou, S. (2021). J. Eur. Ceram. Soc. 41, 4343-4348.]) reported a new BaAl2O4–YAG:Ce composite ceramic phosphor for high-efficiency wLEDs. Yin et al. (2020[Yin, X., Lin, H., Zhang, D., Hong, R., Tao, C. & Han, Z. (2020). Ceram. Int. 46, 3801-3810.]) reported a BaAl2O4:Eu2+–Al2O3 composite ceramic to enhance the luminescence output. Rafiaei et al. (2020[Rafiaei, S. M., Dini, G. & Bahrami, A. (2020). Ceram. Int. 46, 20243-20250.]) reported the synthesis, crystal structure and optical and adsorption properties of BaAl2O4:Eu2+, Eu2+/L3+ (L = Dy, Er, Sm, Gd, Nd, and Pr) phosphors. As BaAl2O4 doped with rare earths has significant application in luminescent phosphors, in this report we propose a powerful analysis method to help researchers improve their manufacturing methods.

Rare earth ions play the most important roles as activators in these phosphors. In particular, the valence states of the rare earth ions significantly affect the emission properties of the phosphors. In the case of a europium (Eu)-doped BaAl2O4 phosphor, a broad-band emission at ∼500 nm can be attributed to the Eu2+ emissions (4f5d → 4f transitions), and the narrow emission peaks at around 560–750 nm are associated with the Eu3+ emissions (5D07Fi, i = 0 to 4). X-ray nanoprobe (Sham, 2014[Sham, T. K. (2014). Adv. Mater. 26, 7896-7901.]; Martínez-Criado et al., 2014[Martínez-Criado, G., Segura-Ruiz, J., Alén, B., Eymery, J., Rogalev, A., Tucoulou, R. & Homs, A. (2014). Adv. Mater. 26, 7873-7879.]; Lin et al., 2020[Lin, B.-H., Wu, Y.-H., Li, X.-Y., Hsu, H.-C., Chiu, Y.-C., Lee, C.-Y., Chen, B.-Y., Yin, G.-C., Tseng, S.-C., Chang, S.-H., Tang, M.-T. & Hsieh, W.-F. (2020). J. Synchrotron Rad. 27, 217-221.]) techniques using a synchrotron source can be applied to characterize these phosphors (Huang et al., 2021[Huang, S.-C., Wu, Y.-H., Fu, S.-Y., Lee, C.-Y., Chen, B.-Y., Yin, G.-C., Chung, S.-L., Lin, B.-H. & Tang, M.-T. (2021). AIP Adv. 11, 055013.]). By exploiting the advantages of X-ray nanoprobes, including the continuously tunable X-ray energy (4–15 keV) and excellent spatial resolution of the nano-focused X-ray beam (<100 nm), we can easily and quickly investigate the valence states of the rare earth ions in the selected local area to unravel the emission mechanisms of phosphor materials. An X-ray nanoprobe can be used to perform X-ray absorption spectroscopy (XAS) across the L-edges of Eu (6.977 keV), Dy (7.790 keV), Er (8.358 keV), Sm (6.716 keV), Gd (7.243 keV), Nd (6.208 keV) and Pr (5.964 keV) to obtain information on the valence states.

In this study, we developed visualization methods for the characterization of the Eu-doped BaAl2O4 phosphor. Using an X-ray nanoprobe, X-ray fluorescence (XRF) and X-ray excited optical luminescence (XEOL) mapping can clearly reveal the distributions of the constituent elements, the valence states of the Eu2+ and Eu3+ ions, and the different emission wavelengths (λem). The accuracy of the estimated valence state distributions was examined using XAS spectra. As the X-ray nanoprobe can provide excellent spatial resolution, we selected different local areas with different valence states of the Eu2+ and Eu3+ ions to study their emission properties. The XEOL spectra consist of one broad intense peak at ∼500 nm and narrow emission peaks at around 560–750 nm in the local areas richer in Eu2+ and Eu3+, respectively. In addition, a weaker emission at ∼390 nm, which is related to the F colour centre of α-Al2O3, is observed.

2. Experiment

The XAS, XRF and XEOL experiments were conducted on the Taiwan Photon Source (TPS) 23A X-ray nanoprobe beamline located at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The capabilities of the TPS 23A X-ray nanoprobe beamline have previously been described in detail by Lin et al. (2019[Lin, B.-H., Wu, Y.-H., Wu, T.-S., Wu, Y.-C., Li, X.-Y., Liu, W.-R., Tang, M.-T. & Hsieh, W.-F. (2019). Appl. Phys. Lett. 115, 171903.], 2020[Lin, B.-H., Wu, Y.-H., Li, X.-Y., Hsu, H.-C., Chiu, Y.-C., Lee, C.-Y., Chen, B.-Y., Yin, G.-C., Tseng, S.-C., Chang, S.-H., Tang, M.-T. & Hsieh, W.-F. (2020). J. Synchrotron Rad. 27, 217-221.]). This beamline can deliver an X-ray beam spot size of less than 60 nm. The test powder sample of Eu-doped BaAl2O4 (Ba0.97Eu0.03Al2O4) was doped with Eu2+ and Eu3+ ions and purchased from Dott Technology. The XAS and XRF spectra were measured using a silicon drift detector (SDD; Vortex-ME4, Hitachi). The XEOL spectra were collected using a multimode optical fibre (with a core diameter of 400 µm) attached to a spectrometer (iHR320, Horiba) with a deep thermoelectric cooling charge-coupled device (Syncerity BI UV-Vis) and a resolution of 2048 × 70 pixels. The XEOL mapping images were acquired through a photomultiplier tube, which is installed in another spectrometer (iHR550, Horiba).

3. Results

The mapping images used to visualize the Eu-doped BaAl2O4 phosphor, shown in Figs. 1[link]–3[link][link], have the same measured areas. Through XRF and XEOL mapping, detailed information about the measured area can be obtained. Fig. 1[link] shows the XRF and XAS analyses of the Eu-doped BaAl2O4 phosphor. As the X-ray energy was tuned at 6.985 keV, which is above the Ba L3-edge (5.247 keV), Al K-edge (1.560 keV) and Eu L3-edge (6.977 keV), the elemental distributions of Ba, Al and Eu were observed directly, as shown in Figs. 1[link](a), 1[link](b) and 1[link](c), respectively. The Eu L3-edge XAS spectra exhibit a strong white line caused by the electronic transitions from 2p3/2 to 5d. In particular, the resonances of the Eu2+ and Eu3+ ions are located at 6.975 and 6.983 keV, respectively.

[Figure 1]
Figure 1
XRF maps of Eu-doped BaAl2O4, showing the elemental distributions of (a) Ba, (b) Al and (c) Eu. (d) X-ray absorption spectrum of the location marked with the white dashed circle in panel (c). On the basis of features of the Eu2+ and Eu3+ ions, XAS can reveal the corresponding amounts of the Eu2+ and Eu3+ ions.
[Figure 2]
Figure 2
Estimation results for the distributions of (a) Eu2+ and (b) Eu3+ ions. Panels (c) and (d) show XAS spectra of the locations/areas marked with the red dashed circles in panels (a) and (b), respectively. The XAS spectra demonstrate the accuracy of the estimated results, implying that the valence states of the Eu ions can be easily and quickly visualized using XRF mapping.
[Figure 3]
Figure 3
XEOL mapping performed at emission wavelengths (λem) of approximately (a) 390, (b) 500, (c) 588 and (d) 698 nm. The emission distributions of the Eu-doped BaAl2O4 phosphor at different emission wavelengths (λem) are clearly visualized.

The core electrons in the Eu3+ ions have a larger binding energy than those in the Eu2+ ions, with the difference between the resonance energies equal to ∼8 eV (Korthout et al., 2013[Korthout, K., Parmentier, A. B., Smet, P. F. & Poelman, D. (2013). Phys. Chem. Chem. Phys. 15, 8678-8683.]). On the basis of features of the Eu2+ and Eu3+ ions, XAS can reveal the corresponding amounts of the Eu2+ and Eu3+ ions. Fig. 1[link](d) shows the XAS spectrum of the area marked with the white dashed circle in Fig. 1[link](c). The XAS spectrum of this local area shows that the fluorescence yield of the Eu2+ resonance (6.975 keV) is higher than that of the Eu3+ resonance (6.983 keV), which indicates that the highlighted local area is richer in Eu2+.

Although the elemental distribution of Eu can be obtained from Fig. 1[link](c), it cannot provide information on the positions of the Eu2+ and Eu3+ ions. Therefore, we developed a visualization method that directly images the valence states of the Eu ions. As the X-ray energy of the synchrotron source is continuously tunable, we selected three X-ray energies at 6.970, 6.975 and 6.983 keV for XRF mapping; these correspond to the background, Eu2+ resonance and Eu3+ resonance, respectively. Using the following equations, we subsequently obtained the distributions of the valence states of the Eu ions,

[\left [ {\matrix{ I_{\rm 6.975\,keV} \cr I_{\rm 6.983\, keV} } } \right ] = \left [ {\matrix{ I_{{\rm Eu}^{2+} ({\rm 6.975 \, keV})} & I_{{\rm Eu}^{3+} ({\rm 6.975 \, keV})} \cr I_{{\rm Eu}^{2+} ({\rm 6.983 \, keV})} & I_{{\rm Eu}^{3+} ({\rm 6.983 \, keV})} } } \right ] \left [ {\matrix{ x \cr y } } \right ] , \eqno(1)]

[x + y = 1 , \eqno(2)]

[I_{{\rm Eu}^{2+} ({\rm 6.975 \, keV})} / I_{{\rm Eu}^{2+} ({\rm 6.983 \, keV})} = C_1 , \eqno(3)]

[I_{{\rm Eu}^{3+} ({\rm 6.975 \, keV})} / I_{{\rm Eu}^{3+} ({\rm 6.983 \, keV})} = C_2 , \eqno(4)]

[I_{{\rm Eu}^{2+} ({\rm 6.983keV})} / I_{{\rm Eu}^{3+} ({\rm 6.983 \, keV})} = C_3 , \eqno(5)]

[{\rm Eu}^{2+} \ (\%) = {x \over {x + y}} , \eqno(6)]

[{\rm Eu}^{3+} \ (\%) = {y \over {x + y}} . \eqno(7)]

Assuming that the concentrations of the Eu2+ and Eu3+ ions are x% and y%, respectively, the fluorescence yields of I6.975 keV and I6.983 keV can be estimated using equation (1)[link]. The corresponding ratio parameters C1, C2 and C3 were determined from the XAS spectra of the reference materials for Eu2+ (EuS) and Eu3+ (Eu2O3) (Korthout et al., 2013[Korthout, K., Parmentier, A. B., Smet, P. F. & Poelman, D. (2013). Phys. Chem. Chem. Phys. 15, 8678-8683.]). In the report by Korthout et al., the Eu L3-edge XAS spectra of the EuS and Eu2O3 reference materials show that the values of [I_{{\rm Eu}^{2+} ({\rm 6.975\,keV})}], [I_{{\rm Eu}^{2+} ({\rm 6.983\,keV})}], [I_{{\rm Eu}^{3+} ({\rm 6.975\, keV})}] and [I_{{\rm Eu}^{3+} ({\rm 6.983\,keV})}] were 1.9, 1, 0.36 and 2.1, respectively. So, the ratio parameters C1, C2 and C3 can be calculated to be equal to 1.9, 0.17 and 0.476, respectively.

After estimating the above parameters, we obtained the distributions of the Eu2+ and Eu3+ ions using equations (6)[link] and (7)[link], as shown in Figs. 2[link](a) and 2[link](b), respectively. To verify the accuracy of the estimated results, we obtained the XAS spectra of the selected local areas to measure the valence states of the Eu ions. Fig. 2[link](c) shows the XAS spectrum of the area marked with the red dashed circle in Fig. 2[link](a). The XAS spectrum in Fig. 2[link](c) shows that the fluorescence yield of the Eu2+ resonance (6.975 keV) is higher than that of the Eu3+ resonance (6.983 keV), indicating that this local area is richer in Eu2+. According to our estimation, another local area marked with the red dashed circle in Fig. 2[link](b) is clearly richer in Eu3+. Fig. 2[link](d) illustrates the XAS spectrum of the local area marked in Fig. 2[link](b), which is consistent with the expected results. In Fig. 2[link](d), it can be observed that the fluorescence yield of the Eu2+ resonance (6.975 keV) is lower than that of the Eu3+ resonance (6.983 keV), indicating that this local area is richer in Eu3+.

The XAS spectra demonstrate the accuracy of the estimated results, implying that the valence states of the Eu ions can be easily and quickly visualized using XRF mapping. This model relies on the approximation that EuS and Eu2O3 are used as the reference for Eu2+ and Eu3+, respectively, in the composite. Since the difference between the resonance energies of Eu2+ and Eu3+ is ∼8 eV, the Eu L3-edge XAS can easily reveal the corresponding amounts of Eu2+ and Eu3+ ions. It may increase the discrepancy while the difference between the resonance energies is smaller.

Although this model can provide the relative quantitative values of Eu2+ and Eu3+, the validity of the quantitative values of the valence states can be measured by XAS spectra. In the quantitative analysis reported by Yamamoto & Yukumoto (2018[Yamamoto, T. & Yukumoto, A. (2018). J. Anal. At. Spectrom. 33, 585-592.]), the oxidation state of Eu in phosphor samples was established using XAS to investigate the influence of the constituent elements, absorption edge and measurement mode on the evaluated oxidation states.

The variation in the valence states of the Eu ions tends to influence the emission properties. Because the Eu2+ and Eu3+ ions have different distributions, as shown in Fig. 2[link], we used XEOL mapping to study further the emission distribution of the main emission wavelength of the Eu-doped BaAl2O4 phosphor. Figs. 3[link](a), 3[link](b), 3[link](c) and 3[link](d) show the XEOL mapping at emission wavelengths (λem) of approximately 390, 500, 588 and 698 nm, respectively. The emission distributions of the Eu-doped BaAl2O4 phosphor at different emission wavelengths (λem) were clearly visualized. At λem = 500 nm, the well documented emission of the Eu2+ ions due to the 4f5d → 4f transitions was observed, along with emissions of the Eu3+ ions at λem = 588 and 698 nm due to transitions of the 5D07Fi (i = 0 to 4) states (Rezende et al., 2016[Rezende, M. V. dos S., Montes, P. J., Andrade, A. B., Macedo, Z. S. & Valerio, M. E. (2016). Phys. Chem. Chem. Phys. 18, 17646-17654.]). However, we suggest that λem = 390 nm is attributed to the F centre of α-Al2O3, which will be discussed later. Thus, XEOL mapping can be used to visualize clearly the emission behaviour of the Eu-doped BaAl2O4 phosphor. Compared with Figs. 3[link](a), 3[link](c) and 3[link](d), Fig. 3[link](b) shows a larger emission intensity and better emission uniformity. This result demonstrates that the main contribution to the luminescence intensity of Eu-doped BaAl2O4 comes from the Eu2+ activator.

To study further the emission properties of the Eu-doped BaAl2O4 phosphor in different local areas, we selected four such local areas, indicated by the white dashed circles in Figs. 3[link](a)–3[link](d), and plotted the corresponding XEOL spectra. The room-temperature XEOL spectra of the four local areas (P1–P4) for excitation across the Eu L3-edge (6.977 keV) are shown in Figs. 4[link](a)–4[link](d).

[Figure 4]
Figure 4
(a)–(d) Room-temperature XEOL spectra of the four local areas marked with the white dashed circles in Figs. 3[link](a)–3[link](d). The XEOL spectra corresponding to each of the four local areas were acquired at three X-ray energies: below the Eu L3-edge (6.960 keV), at the Eu2+ resonance (6.975 keV) and at the Eu3+ resonance (6.983 keV).

The XEOL spectra corresponding to each of the four local areas were acquired at three X-ray energies: below the Eu L3-edge (6.960 keV), at the Eu2+ resonance (6.975 keV) and at the Eu3+ resonance (6.983 keV). The XEOL spectra of P1–P4 exhibit one common broad intense peak at 500 nm, which can be attributed to the Eu2+ 4f5d → 4f transitions.

The above results not only reinforce the conclusion that the Eu2+ activator dominates the luminescence intensity in the Eu-doped BaAl2O4 phosphor, but also illustrate that the Eu2+ ions have unique local symmetry in the matrix. According to Rezende et al. (2011[Rezende, M. V. dos S., Valerio, M. E. G. & Jackson, R. A. (2011). Opt. Mater. 34, 109-118.], 2016[Rezende, M. V. dos S., Montes, P. J., Andrade, A. B., Macedo, Z. S. & Valerio, M. E. (2016). Phys. Chem. Chem. Phys. 18, 17646-17654.]), Eu2+ ions may be incorporated on more than one non-symmetric site, as substitution might occur at either the Al3+ or Ba2+ site, which results in more than one band in the emission spectra associated with the 4f5d → 4f transition. Another shoulder emission due to the doping of Eu2+ ions on the non-equivalent sites can be observed in the intense peak. In addition, the emission intensity of the single intense peak in P1–P4 decreased slightly as the X-ray energy was tuned across the Eu L3-edge. This behaviour suggests that the energy transfer to the Eu2+ 4f5d → 4f transitions is less efficient above than below the Eu L3-edge (Huang et al., 2021[Huang, S.-C., Wu, Y.-H., Fu, S.-Y., Lee, C.-Y., Chen, B.-Y., Yin, G.-C., Chung, S.-L., Lin, B.-H. & Tang, M.-T. (2021). AIP Adv. 11, 055013.]).

In addition to the single intense peak at ∼500 nm produced by the Eu2+ 4f5d → 4f transitions, the XEOL spectra of P1–P4 exhibit other weaker peaks. The XEOL spectra of P1 shown in Fig. 4[link](a) consist of a special weaker emission line at ∼390 nm. This emission may be attributed to the F-centre emission in α-Al2O3 (den Engelsen, Fern, Ireland & Silver, 2020[Engelsen, D. den, Fern, G. R., Ireland, T. G. & Silver, J. (2020). ECS J. Solid State Sci. Technol. 9, 026001.]). The F-centre is a type of colour centre that is associated with an oxygen vacancy with two electrons (Wang et al., 2013[Wang, Z., Li, C., Liu, L. & Sham, T.-K. (2013). J. Chem. Phys. 138, 084706.]; Ghamnia et al., 2003[Ghamnia, M., Jardin, C. & Bouslama, M. (2003). J. Electron Spectrosc. Relat. Phenom. 133, 55-63.]; Itou et al., 2009[Itou, M., Fujiwara, A. & Uchino, T. (2009). J. Phys. Chem. C, 113, 20949-20957.]). den Engelsen, Fern, Ireland, Yang & Silver (2020[Engelsen, D. den, Fern, G. R., Ireland, T. G., Yang, F. & Silver, J. (2020). Opt. Mater. Expr. 10, 1962-1980.]) also observed an F-centre emission in the Eu-doped BaAl2O4 phosphor via photoluminescence and cathodoluminescence.

The XEOL spectra of P2 shown in Fig. 4[link](b) consist of only one intense peak produced by the 4f5d → 4f transitions of Eu2+ and very weak peaks associated with the 5D07Fi (i = 0 to 4) transitions of the Eu3+ ions, showcasing the perfect luminescence intensity of the Eu-doped BaAl2O4 phosphor. Compared with P1 and P2, the XEOL spectra of local areas P3 and P4 clearly consist of emission peaks associated with the 5D07Fi (i = 0 to 4) transitions of the Eu3+ ions, as shown in Figs. 4[link](c) and 4[link](d), respectively. Clearly, the emission lines caused by the Eu3+ transitions in P3 and P4 are narrower than those in P1 and P2, indicating more crystal homogeneity in local areas P3 and P4 than in P1 and P2. The broad Eu3+ emission is a result of crystal inhomogeneity causing small distortions around the Eu ions (Rezende et al., 2016[Rezende, M. V. dos S., Montes, P. J., Andrade, A. B., Macedo, Z. S. & Valerio, M. E. (2016). Phys. Chem. Chem. Phys. 18, 17646-17654.]; Gasparotto et al., 2008[Gasparotto, G., Lima, S., Davolos, M. R., Varela, J. A., Longo, E. & Zaghete, M. (2008). J. Lumin. 128, 1606-1610.]).

To study the valence states of the Eu ions, the same four local areas P1–P4 were used to measure the Eu L3-edge XAS spectra, as shown in Figs. 5[link](a)–5[link](d). The coexistence of the two common valence states of Eu2+ and Eu3+ can be seen in the XAS spectra of P1–P4, represented by the two well resolved edge resonances. Although the Eu2+ and Eu3+ ions coexist, the corresponding concentrations of the Eu2+ and Eu3+ ions in these local areas can be determined from the fluorescence yield of the XAS spectra. The XAS spectrum of P1 shows that the fluorescence yield of the Eu2+ resonance is similar to that of the Eu3+ resonance, indicating similar concentrations of the Eu2+ and Eu3+ ions in local area P1. However, the XAS spectrum of P2 shows that the fluorescence yield of the Eu2+ resonance is higher than that of the Eu3+ resonance, indicating that local area P2 is richer in Eu2+. Compared with P1 and P2, the XAS spectra of P3 and P4 show the opposite behaviour, that is, the Eu3+ resonances have a higher fluorescence yield and therefore local areas P3 and P4 are richer in Eu3+.

[Figure 5]
Figure 5
(a)–(d) XAS spectra of local areas P1–P4 marked with the white dashed circles in Figs. 3[link](a)–3[link](d). The coexistence of the two common valence states of Eu2+ and Eu3+ can be seen in the XAS spectra of P1–P4, represented by the two well resolved edge resonances.

The results of the XAS spectra are consistent with those of the XEOL spectra, as shown in Fig. 5[link]. Because local area P2 is richer in Eu2+, the XEOL spectra of P2 consist of only one intense peak at ∼500 nm produced by the Eu2+ 4f5d → 4f transitions. As P3 and P4 are richer in Eu3+, their XEOL spectra consist of narrow emission lines around 580–700 nm that are produced by the 5D07Fi (i = 0 to 4) transitions of the Eu3+ ions.

Since we can obtain the distributions of the Eu2+ and Eu3+ ions from the results of Figs. 2[link](a) and 2[link](b), a pixel-by-pixel analysis can be conducted to determine the overall correlation between λem = 390, 500, 588 and 698 nm and Eu2+ and Eu3+ ions. For a given map, λem emission intensity and Eu ions were plotted against each other for each pixel in the map. Figs. 6[link](a)–6[link](d) and 6[link](e)–6[link](h) show the emission intensity of λem = 390, 500, 588 and 698 nm as a function of Eu2+ and Eu3+ ions, respectively. The correlations are consistent with the measured results of the XEOL and XAS spectra. The emission intensity of λem = 390 nm shown in Figs. 6[link](a) and 6[link](e) has largest emission intensity at around 50% Eu2+ or Eu3+ ions, suggesting that P1 has similar concentrations of Eu2+ and Eu3+ ions. Figs. 6[link](b) and 6[link](f) show that the emission intensity of λem = 500 nm is a zero correlation with Eu2+ or Eu3+ ions. This suggests that, regardless of whether local areas P1–P4 are richer or poorer in Eu2+ or Eu3+ ions, the single intense peak at ∼500 nm is still the main contribution to the luminescence intensity. The emission intensities of λem = 588 and 698 nm show negative correlation with the Eu2+ ions shown in Figs. 6[link](c) and 6[link](d), and positive correlation with the Eu3+ ions shown in Figs. 6[link](g) and 6[link](h). This result is also corroborated by the fact that the local areas P3 and P4 are richer in Eu3+. Thus, the emission mechanisms of Eu-doped BaAl2O4 phosphors can be further understood through such a correlation analysis.

[Figure 6]
Figure 6
Rows (a)–(d) and (e)–(h) show the emission intensities of λem = 390, 500, 588 and 698 nm as a function of Eu2+ and Eu3+ ions, respectively. A pixel-by-pixel analysis can be conducted to determine the overall correlation between the λem = 390, 500, 588 and 698 nm and Eu2+ and Eu3+ ions.

4. Conclusions

In this paper, we report powerful characterization capabilities for investigating the features of Eu-doped BaAl2O4 phosphor materials using an X-ray nanoprobe. XRF and XEOL mapping can provide clear visualization images containing detailed distribution information on Eu-doped BaAl2O4 phosphors, including the elements, the valence states of the Eu ions and the different emission wavelengths (λem). The accuracy of the estimated valence state distributions was examined by performing XAS across the Eu L3-edge (6.977 keV), and the corresponding concentrations of the Eu2+ and Eu3+ ions were obtained from the XAS spectra.

Exploiting the excellent spatial resolution of the X-ray nanoprobe, we selected four local areas with different valence states of the Eu2+ and Eu3+ ions to study their emission properties. The XEOL spectra consisted of one broad intense peak at ∼500 nm and narrow weaker emission peaks at around 560–750 nm in the local areas richer in Eu2+ and Eu3+, respectively. In addition, a weaker emission at ∼390 nm relating to the F colour centre of α-Al2O3 was also observed.

The XEOL spectra demonstrated that the main contribution to the luminescence intensity of Eu-doped BaAl2O4 comes from the Eu2+ activator and the emission intensity will not be influenced by the concentration of Eu2+ or Eu3+ ions.

We believe that X-ray nanoprobes will open new avenues with significant characterization ability for unravelling the emission mechanisms of phosphor materials.

Acknowledgements

We would like to thank the National Synchrotron Radiation Research Center (NSRRC) and the Ministry of Science and Technology of Taiwan for supporting this project.

Funding information

The following funding is acknowledged: Ministry of Science and Technology of Taiwan (grant No. MOST 110-2112-M-213-008).

References

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