1. Introduction

Recently, photoinduced electron-trapping materials, such as persistent phosphors, photostimulated phosphors and photochromic materials, have been attracting a great deal of interest. These materials have been studied for a variety of applications, for example luminous paints [1], imaging plates for X-ray detection [2], and optical memories [3,4]. The fundamental principles driving each of these phenomena are very similar to each other. In each, electrons are trapped into some defects and de-trapped by either light or heat.

Eu²⁺-doped compounds are good candidates for photoinduced electron-trapping materials. For example, MAl₂O₄:Eu²⁺ - Ln³⁺ (M = Ca, Sr, Ln = Nd³⁺, Dy³⁺), which was discovered by Matsuzawa et al. [5], is a series of bright and long-lasting persistently luminescent materials [6,7]. BaFX:Eu²⁺ (X = Cl, Br, I) is a famous photostimulated phosphor used for imaging plates [2,8]. BaMgSiO₄:Eu²⁺ was recently discovered as a pink photochromic material [9,10]. In these inorganic compounds, the Eu²⁺ ion has strong and broad absorption and luminescence bands due to the 4f⁶5d¹-4f⁷ transition in the UV and the visible range. In addition, because of its multi-valence nature, Eu²⁺ works as an electron supplier by changing its valence from Eu²⁺ to Eu³⁺ or (Eu²⁺ + h⁺) in these materials [10,11]. Therefore, certain defect centers in the Eu²⁺-doped compounds can capture the electron supplied from the excited 5d level of Eu²⁺ by UV and blue light through the conduction band or directly, even if the wavelength of the induced light is shorter than that of band gap absorption [12–14]. In many cases, the electrons trapped by defects are the cause of phenomena such as photochromism, persistent luminescence and photostimulated luminescence.

The Eu²⁺-Nd³⁺-codoped CaAl₂O₄ is known as the most famous blue persistent phosphor, showing persistent luminescence at 440 nm due to the Eu²⁺:4f⁶5d¹→4f⁷ transition. In this material, the Nd³⁺ ion as a co-dopant is considered to contribute to the formation of some electron traps. As a result, the persistent luminescence properties of Nd³⁺-codoped CaAl₂O₄:Eu²⁺ are significantly improved compared with those of Eu²⁺-singly doped CaAl₂O₄ [5,7]. Many studies on the properties and mechanisms of the blue persistent luminescence of Eu²⁺ have been carried out in the past [15,16]. However, there is only one report on the near-infrared persistent luminescence of Nd³⁺ in SrAl₂O₄:Eu²⁺-Dy³⁺-Nd³⁺ ceramics, even though the Nd³⁺ ion is an efficient near-infrared luminescence center at 0.90 μm, 1.06 μm and 1.3 μm [17]. We expected that CaAl₂O₄:Eu²⁺-Nd³⁺ would also possibly show the near-infrared persistent luminescence of Nd³⁺.

In this study, we carefully investigated the optical properties of CaAl₂O₄:Eu²⁺-Nd³⁺ and discovered that this material shows photochromism as well as near-infrared persistent luminescence. Therefore, we report the reflectance, photoluminescence (PL), persistent luminescence spectra and afterglow decay curves.

2. Experimental procedure

Polycrystalline Eu²⁺-Nd³⁺-codoped CaAl₂O₄ samples with compositions of (Ca_0.985Eu_0.005Nd_0.01)Al₂O₄, Eu²⁺-singly doped samples with (Ca_0.995Eu_0.005)Al₂O₄ and Nd³⁺-singly doped samples with (Ca_0.99Nd_0.01)Al₂O₄ were prepared by solid state reactions. The chemicals CaCO₃(99.9%), α-Al₂O₃ (99.99%), Eu₂O₃ (99.9%) and Nd₂O₃ (99.9%) were used as starting materials. The powders were mixed in an alumina mortar with ethanol and 1 wt% H₃BO₃ as a sintering additive. After drying, the mixed powders were calcined at 1200°C for 2 h. The calcined powders were pressed at 50 MPa into pellets of dimension 10 mm ϕ x 2 mm thick. The pellets were sintered at 1350°C for 2 h under a N₂(95%)-H₂(5%) atmosphere. The crystal phases of the samples were identified by an X-ray diffraction measurement system (Rigaku, Ultima IV). For the evaluation of photochromic phenomenon, reflectance spectra were measured after irradiating the sample with white light (including UV light) for various irradiation times. The measurement system consisted of a probe fiber, a CCD spectrometer (Ocean optics, USB 2000+) and a Xe lamp (Asahi Spectra Co., Ltd, MAX-302) emitting between 250 nm and 800 nm, which was used as a probe light and to induce photochromism. The photoluminescence (PL) spectra were measured with a 372 nm laser diode (LD) by combining a monochromator (Nikon, G250) and a Si photodiode (Electro-Optical System Inc., S-025-H). All the obtained PL spectra were calibrated by a standard halogen lamp (Labsphere, SCL-600). For the measurement of the afterglow spectrum and decay curve, the sample was excited by 330 nm light, which was obtained by combination of the Xe lamp and a band pass filter (330 nm) for 10 min before stopping the irradiation. Afterglow spectra were measured by the CCD spectrometer, and afterglow decay curves of Eu²⁺ and Nd³⁺ were detected by the Si photodiode with band pass filters of 440 nm and 870 nm, respectively. The afterglow intensity was converted to the radiance (mW/Sr/m²) by a CCD spectrometer (B&W Tek, Glacier X) absolutely calibrated by Konica-Minolta company.

3. Results and discussion

All the obtained samples are almost colorless because of low dopant concentrations of rare earth ions. Figure 1 shows photographs of the CaAl₂O₄:Eu²⁺-Nd³⁺ sample under various conditions. The CaAl₂O₄:Eu²⁺-Nd³⁺ sample before UV irradiation was colorless (Fig. 1(a)), as were the CaAl₂O₄:Eu²⁺ and CaAl₂O₄:Nd³⁺ samples. After approximately 30 s of irradiation by UV light, the color of the CaAl₂O₄:Eu²⁺-Nd³⁺ sample was changed to purple (Fig. 1(b)) due to photochromism and blue persistent luminescence (Fig. 1(c)). Blue persistent luminescence was not observed with the naked eye under bright field 1 hour after stopping UV irradiation. However, photochromism was still observed 1 hour after stopping irradiation, and the color of the sample was changed to pink (Fig. 1(d)) because of photochromism.

 

Fig. 1 Photograph of CaAl₂O₄:Eu²⁺-Nd³⁺ (a) before UV irradiation, (b) after UV irradiation, (c) after UV irradiation under dark field, (d) 1 hour after UV irradiation.

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Figure 2 shows the reflectance spectra at various irradiation times using a probe light with a range of approximately 250 nm to 800 nm. Shortly after starting reflectance measurements (1 s), sharp absorption lines originating from the 4f-4f transitions of Nd³⁺ in the visible range and a broad absorption band originating from the 4f⁷→4f⁶5d¹ transition of Eu²⁺ in the UV range were observed. After continuing to irradiate the sample with the probe light (including UV light), an additional absorption band in the visible range appeared, the absorption intensity of which became much stronger with increasing irradiation time. Photochromism was not observed after irradiation with visible light longer than approximately 400 nm in the CaAl₂O₄:Eu²⁺-Nd³⁺ sample. These results indicate that the color center is created by UV light and that its density increases with irradiation time. In addition, the UV light inducing the photochromism corresponds to the absorption band of Eu²⁺ in the CaAl₂O₄ host. Therefore, the Eu²⁺ ion could be the electron supplier. To investigate the absorption band of the created color center, the difference spectrum was obtained by subtracting the absorption coefficient spectrum at 280 s after starting measurement from that at 1 s after start, as shown in Fig. 2. The absorption coefficient spectra were obtained by converting from the reflectance spectra with the Kubelka-Munk function. The absorption band of the color center was found to be broad and peaked at approximately 500 nm in the range between 450 nm and 800 nm. In addition, CaAl₂O₄:Eu²⁺ and CaAl₂O₄:Nd³⁺, the singly doped samples, did not show any coloration due to photochromism by UV irradiation. Therefore, the created color center can be ascribed to some electron-trapping states originating from a Nd³⁺ derived defect following photo-oxidation of Eu²⁺. To the best of our knowledge, this is the first report of photochromism in the CaAl₂O₄:Eu²⁺-Nd³⁺.

 

Fig. 2 Excitation time variation of the reflectance spectrum of CaAl₂O₄:Eu²⁺-Nd³⁺ (1, 5, 20, 60, 280 s) and the difference spectrum between reflectance at 280 s and 1 s.

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Figure 3 shows the PL spectrum excited by the 372 nm LD and the persistent luminescence spectrum after irradiation with 330 nm light for 10 min of CaAl₂O₄:Eu²⁺-Nd³⁺. In the PL spectrum, a broad PL band peaked at 440 nm in the visible range and sharp PL lines at approximately 880 nm and 1060 nm were observed. The visible PL band originates from the Eu²⁺: 4f⁶5d¹ → 4f⁷ while the near-infrared PL lines originate from the ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I_11/2 transitions of Nd³⁺. By considering the absorption spectra of Eu²⁺ and Nd³⁺ as shown in Fig. 2 and Fig. 3 (blue line), it can be inferred that the 372 nm light can excite only the 5d level of Eu²⁺, but not the 4f levels of Nd³⁺ directly. In addition, there is a spectral overlap between the Eu²⁺ PL spectrum and the Nd³⁺ absorption spectrum. Therefore, the Nd³⁺ near-infrared luminescence after 372 nm excitation can be caused by the energy transfer between Eu²⁺ and Nd³⁺. However, the PL intensity ratio (Eu²⁺/Nd³⁺) by 372nm exciataion was changed by irradiation time of excitation light; the PL intensity ratio depends on the trapped electron density. From the photoluminescence excitation spectrum measurement, it was found that Nd³⁺ also shows photostimulated luminescence by 372nm light. Therefore, Nd³⁺ luminescence was caused not only by the direct energy transfer but also by energy transfer from the electron traps, which results in photostimulated luminescence and persistent luminescence. The Eu²⁺ luminescence intensity also increased with irradiation time, i.e., with increasing trapped electron density, because the electron transfer from the excited 5d level to the traps is not likely to occur due to saturation of electron trapping state. For the persistent luminescence measurement, the persistent luminescence of Nd³⁺ in the near-infrared region was observed after UV excitation, in addition to blue persistent luminescence of Eu²⁺. While the blue persistent luminescence in the CaAl₂O₄:Eu²⁺-Nd³⁺ phosphor is reported by many researchers, the Nd³⁺ persistent luminescence has never been reported. Compared with the intensity ratio Nd³⁺/Eu²⁺ of persistent luminescence, the intensity of the luminescence of Nd³⁺ is much higher in the PL. This is because the Nd³⁺ luminescence by 372nm excitation included contribution of photostimulated luminescence.

 

Fig. 3 Photoluminescence by 370 nm excitation and persistent luminescence spectra 5 s after stopping 5 min of 330 nm excitation.

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Figure 4 shows the afterglow decay curves for the persistent luminescence of Eu²⁺ and Nd³⁺ after 10 min of excitation by 330 nm light. The decay profiles of Eu²⁺ and Nd³⁺ are quite similar. The persistent intensity ratio (Nd³⁺/Eu²⁺) is almost constant, approximately 0.6 ~0.8% at any time. Therefore, the persistent luminescence of both Eu²⁺ and Nd³⁺ originates from the common electron trap and electron transfer process. These results also support the near-infrared persistent luminescence of Nd³⁺ by energy transfer from Eu²⁺ to Nd³⁺.

 

Fig. 4 Afterglow curves for the persistent luminescence of Eu²⁺ and Nd³⁺ in CaAl₂O₄:Eu²⁺ -Nd³⁺ after 10 min of excitation by 330 nm light.

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Considering the results for photochromism and persistent luminescence, the electron transfer process can be assumed. Under UV-irradiation, the Eu²⁺ ion is photo-oxidized into Eu³⁺ or (Eu²⁺ + h⁺) and the electron can be trapped by some defects derived from Nd-codoping. The resulting electron-trapping states can have various traps with different enegy depths. The duration times of photochromism and persistent luminescence were different at room temperature, as shown in Fig. 1. Therefore, the electron traps that contribute to the photochromism may not be identical to those that contribute to the persistent luminescence. Some of the electron traps can absorb green light, as shown in Fig. 2, resulting in the pink photochromism. Additionally, some of the traps can contribute to persistent luminescence by being de-trapped via heat and recombined with the photo-oxidized Eu³⁺.

In summary, we reported results for photochromism and near-infrared persistent luminescence in the Eu²⁺-Nd³⁺-codoped CaAl₂O₄ ceramics. Although there are only a few reports of oxide materials showing photochromism, these materials are expected to be useful for the application of rewritable copy papers [9,18,19]. In addition, the research on near-infrared persistent luminescence has only just begun and has potential for many applications, such as bio-imaging [20], solar light storage and night-vision-luminous paint for security [21]. Further research of the electron transfer mechanism and improvement of the near-infrared persistent luminescence is in progress.

4. Conclusion

We discovered photochromism and near-infrared persistent luminescence of the Nd³⁺: ⁴F_3/2→⁴I_J/2 transition in the CaAl₂O₄:Eu²⁺-Nd³⁺. The sample’s color changed from white to pink after stopping UV irradiation due to the absorption of the color center. The persistent luminescence of Nd³⁺ is caused by the energy transfer from Eu²⁺ to Nd³⁺.