Luminescence and long-lasting afterglow in Mn²⁺ and Eu³⁺ co-doped ZnO–GeO₂ glasses and glass ceramics prepared by sol–gel method

Abstract

To develop new fluorescent and afterglow materials, Mn²⁺ and Eu³⁺ co-doped ZnO–GeO₂ glasses and glass ceramics were prepared by a sol–gel method and their optical properties were investigated by measuring luminescence, excitation and afterglow spectra, and luminescence quantum yield (QY). Under UV irradiation at 254 nm, some glasses and all of the glass ceramics showed green luminescence peaking at 534 nm due to the ⁴T₁ → ⁶A₁ transition of tetrahedrally coordinated Mn²⁺ ions. The strongest luminescence was observed in a glass ceramic of 0.1MnO–0.3Eu₂O₃–25ZnO–75GeO₂ heat treated at 900 °C, with QY of 49.8%. All of the green-luminescent glasses and glass ceramics showed green afterglow, and the afterglow lasting for more than 60 min was obtained in a glass ceramic heat treated at 900 °C. It is considered that the Eu³⁺ ions may behave as electron trapping centers to be associated with the occurrence of the green afterglow due to the Mn²⁺ ions in the co-doped system.

1 Introduction

The research and development of phosphor materials are prosperous. Red (such as Y(P,V)O₄:Eu³⁺ and (Y,Ga)BO₃:Eu³⁺), green (such as Zn₂SiO₄:Mn²⁺ and (Y,Gd)BO₃:Tb³⁺), and blue (such as BaMgAl₁₀O₁₇:Eu²⁺ and CaMgSi₂O₆:Eu²⁺) inorganic phosphors are typical ones used for optical displays such as cathode-ray tubes (CRTs), liquid crystal displays (LCDs), and plasma display panels (PDPs). In recent years, new displays such as organic electroluminescence displays have begun to be used practically, but the demand for thermally-stable inorganic phosphors is still large. Moreover, there is great interest in rare earth (RE) activated oxide glasses in the field of waveguide photonics [1, 2], and the sol–gel method is suitable to obtain these new optically functional materials.

Transition metal and rare earth ions serve as luminescence centers in inorganic oxide compounds. The Mn²⁺ ion shows strong luminescence, the color of which can be changed over the range 490–750 nm depending on host materials [3, 4], and a number of Mn-containing phosphors have been investigated. A typical phosphor of this type is Zn₂SiO₄:Mn²⁺ which shows clear and high luminosity green luminescence under UV or electronic excitation, and it has been used for first stage type color displays of CRTs and PDPs [5]. The Eu³⁺ and Eu²⁺ ions have been used for luminescence centers of red and blue inorganic phosphors, respectively, as mentioned above. Thus, Mn²⁺, Eu³⁺, and Eu²⁺ ions will serve as good luminescence centers in doped systems.

Moreover, new optical characteristics are expected in the Mn and Eu ions co-doped system; for example, strong upconversion luminescence due to both Mn²⁺ and Eu³⁺ ions was observed in co-doped ZnS nanoparticles [6], and white light-emitting Mn²⁺ and Eu²⁺ co-doped Ba₃MgSi₂O₈ phosphors were synthesized through combustion process [7]. Long-lasting afterglow has also been reported in transition metal ions (like Mn²⁺)- and RE ions (such as Eu²⁺ and Dy³⁺)-doped systems [8, 9] and in an Mn and Eu co-doped system [10]. Almost all of those phosphors were heated or sintered under reducing atmospheres to reduce the selected rare earth ion (for example Eu³⁺ and Eu²⁺), but those with heat treatment in the air were only limited.

In the present study, we prepared ZnO–GeO₂ glasses and glass ceramics co-doped with Mn²⁺ and Eu³⁺ ions by a sol–gel method. Optical properties of these materials were investigated by measuring luminescence, excitation and afterglow spectra, and luminescence quantum yield (QY). Especially, green, long-lasting afterglow due to Mn²⁺ ions was observed in the co-doped system.

2 Experimental

Mn²⁺ and Eu³⁺ co-doped ZnO–GeO₂ glasses and glass ceramics were prepared by a sol–gel method. A starting solution consisted of tetraethoxygermanium (Ge(OC₂H₅)₄) as a precursor of germanium dioxide, (CH₃COO)₂Zn·2H₂O as a precursor of zinc oxide, C₂H₅OH and n-C₄H₉OH as solvents, H₂O, and CH₃COOH as the catalyst, with the molar ratio Ge(OC₂H₅)₄:(CH₃COO)₂Zn·2H₂O:C₂H₅OH:n-C₄H₉OH:H₂O:CH₃COOH = 1:0.33:10:10:1:0.1. To this solution, Mn(NO)₃·6H₂O and Eu(C₅H₇O₂)₃ as dopants were added in order to obtain oxides with final compositions of xMnO–yEu₂O₃–25ZnO–75GeO₂ (x = 0.1–2.0, y = 0.05–2.0). The resulting solution was continuously stirred in a closed Teflon container for 1.5 h at room temperature to prepare sol solution. To obtain dried gels, the sol solution was placed in a lid-free Teflon container and kept at 70 °C under drying atmosphere for 2 weeks, and then at 100 °C for 1 week. The dried gels were heat treated at 630 °C to obtain glasses, and at 700–1,000 °C to obtain glass ceramics. These heat-treatment processes were carried out under the air.

For luminescence spectral measurements, samples were powdered sufficiently using an agate mortar and a pestle. Luminescence, excitation, and afterglow spectra were recorded on a fluorescence spectrometer (Hitachi F-4500) with an Xe lamp as an excitation source; luminescence from the illuminated surface of the sample was collected at the right angle to the direction of the excitation light. Decay curves were recorded similarly, after 254-nm irradiation for 5 min with a handy UV lamp. QY values were obtained by an absolute photoluminescence quantum yield measurement system (Hamamatsu C9920-02) under excitation at 254 nm with an Xe lamp as an excitation source. X-Ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Rigaku RINT-2200) with CuKα radiation.

3 Results and discussion

All samples heat treated at 630 °C were amorphous (glasses). Figure 1 shows luminescence spectra of xMnO–yEu₂O₃–25ZnO–75GeO₂ glasses under UV excitation at 254 nm. A green luminescence spectrum peaking at 534 nm is due to the ⁴T₁ → ⁶A₁ transition of tetrahedrally coordinated Mn²⁺ ions [11–13]. On the other hand, red luminescence spectra peaking at 592, 612, 650, and 700 nm are due to the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄ transitions of Eu³⁺ ions, respectively, and these luminescence are quite weak compared with the green one. The strongest red luminescence was due to the hypersensitive transition ⁵D₀ → ⁷F₂ of Eu³⁺ ions [14–16]. According to the parity selective rule of the electronic transition, if the Eu³⁺ ion was in a non-central symmetric site, the ⁵D₀ → ⁷F₂ transition could be observed much stronger than that of the ⁵D₀ → ⁷F₁ transition [15]. Therefore, it is thought that Eu³⁺ ions occupied in a site with lower local site symmetry in the present glass samples. Neither green nor red luminescence was observed in the glasses with x ≥ 1.0. Green luminescence becomes dominant in the glasses with y ≤ 0.4, but red luminescence does in the glasses with y ≥ 0.5. Excitation spectra for 612-nm luminescence of Eu³⁺ are shown in Fig. 2. Absorption bands due to Eu³⁺ ions were observed in the sample with (x, y) = (0.1, 0.5) which showed red luminescence from Eu³⁺ ions as shown in Fig. 1. An Eu³⁺ absorption band at around 530 nm in Fig. 2 is located at almost the same wavelength as the green luminescence from Mn²⁺ in Fig. 1. Therefore, it is believed that the Eu³⁺ ions could absorb the emission energy from Mn²⁺ and red luminescence became dominant with increasing the Eu³⁺ concentration in the glass samples.

Fig. 1

Luminescence spectra of xMnO–yEu₂O₃–25ZnO–75GeO₂ glasses heat treated at 630 °C under UV excitation at 254 nm

Fig. 2

Excitation spectra for 612-nm luminescence in xMnO–yEu₂O₃–25ZnO–75GeO₂ glasses heat treated at 630 °C

Figure 3 shows luminescence spectra of xMnO–yEu₂O₃–25ZnO–75GeO₂ glass ceramics under UV excitation at 254 nm. Results of XRD measurements show that these glass ceramics consist of rhombohedral Zn₂GeO₄ and hexagonal GeO₂ (Fig. 4). All of these glass ceramics heat treated at 800–1,000 °C showed strong green luminescence due to the ⁴T₁ → ⁶A₁ transition of the Mn²⁺ ion, regardless of the value of y. The glass ceramics heat treated at 800 °C show weak, red luminescence around 610 nm due to the Eu³⁺ ion, however this red luminescence was not observed in the spectra of glass ceramics heat treated at 900 and 1,000 °C. Moreover, unlike the glasses, the glass ceramics with x ≥ 1.0 showed green luminescence, but those intensities are weak, therefore the concentration quenching is believed to start to occur at x = 1.0. It is considered that Eu³⁺ ions existing in the grain boundary were aggregated to clusters in host lattice upon crystallization of host matrix, resulting in the concentration quenching.

Fig. 3

Luminescence spectra of xMnO–yEu₂O₃–25ZnO–75GeO₂ glass ceramics heat treated at various temperatures under UV excitation at 254 nm

Fig. 4

XRD pattern of 0.1MnO–0.1Eu₂O₃–25ZnO–75GeO₂ samples heat treated at various temperatures

Figure 5 shows excitation spectra for green luminescence in the xMnO–yEu₂O₃–25ZnO–75GeO₂ glass ceramics. In all samples, strong and broad excitation bands were observed at 250 and 310 nm. Undoped and Mn-doped GeO₂ show a broad absorption band around 250 nm, therefore, it is thought that the 250-nm excitation band in Fig. 5 is a host absorption band or a charge transfer (CT) band associated with Mn²⁺ [17–19]. The 310-nm broad absorption band was not observed in the excitation spectra of GeO₂ and MnO–ZnO–SiO₂ [19, 20]. Thus, it is considered that the 310-nm absorption band is due to the host material of Zn₂GeO₄ [13].

Fig. 5

Excitation spectra for green luminescence in the xMnO–yEu₂O₃–25ZnO–75GeO₂ glass ceramics heat treated at various temperatures

Strong luminescence was observed in the samples with x = 0.1, and their QY values are shown in Fig. 6. In the samples that showed red luminescence due to the Eu³⁺ ion, QY values were too weak to measure. Large QY values were obtained in the samples heat treated at 900 °C, with the maximum QY value of 49.8% in the glass ceramic with y = 0.3.

Fig. 6

Quantum yield value for 534-nm green luminescence in 0.1MnO–yEu₂O₃–25ZnO–75GeO₂ glasses and glass ceramics heat treated at various temperatures

The longest afterglow was observed in the glass ceramic heat treated at 900 °C, the spectra of which were measured at the average interval of 4 min up to 60 min after 254-nm UV irradiation for 5 min as shown in Fig. 7a. The afterglow could be observed by naked eyes for several minutes under irradiation by a commercially available fluorescent lamp. Such a long afterglow has not been observed in Eu³⁺-undoped MnO–ZnO–GeO₂ glass ceramics [13]. Figure 7b shows decay curves of the 534-nm afterglow for 0.1MnO–0.3Eu₂O₃–25ZnO–75GeO₂ glasses and glass ceramics with high QY values.

Fig. 7

a 534-nm afterglow spectra of 0.1MnO–0.3Eu₂O₃–25ZnO–75GeO₂ glass ceramics heat treated at 900 °C, after 254-nm UV irradiation for 5 min. b Decay curves of 534-nm afterglow of 0.1MnO–0.3Eu₂O₃–25ZnO–75GeO₂ heat treated at various temperatures, after 254-nm UV irradiation for 5 min

Thus, it is considered that Eu³⁺ ions may play some roles in trapping the excitation energy and transferring it to Mn²⁺ ions in MnO–Eu₂O₃–ZnO–GeO₂ glass ceramics in the present study. The energy transfers from Eu²⁺ or various vacancies to Mn²⁺ in long-lasting afterglow have been well known and possible mechanisms for these energy transfers reported [10, 12], however, we could not detect experimentally the existence of Eu²⁺ ions or vacancies in the present samples. Therefore, it is considered that Eu³⁺ ions in our samples, Mn²⁺ and Eu³⁺ co-doped ZnO–GeO₂ glasses and glass ceramics, may become electron trapping centers. Previous studies [8–10, 12] and the above results lead to a possible mechanism of long-lasting afterglow of Mn²⁺ in this study as shown in Eqs. 1–4.

$$ {\text{Mn}}^{2 + } + h\nu \,({\text{excitation}}) \to \left( {{\text{Mn}}^{2 + } } \right)^{ + } +\,e^{ - } $$

(1)

$$ {\text{Eu}}^{3 + } + e^{ - } \to \left( {{\text{Eu}}^{3 + } } \right)^{ - } $$

(2)

During UV irradiation (Eqs. 1 and 2), the photo-oxidization of Mn²⁺ would occur and an excited electron e ⁻ would be generated. These electrons would be trapped at Eu³⁺ and an Eu²⁺-liked energy trapping species, (Eu³⁺)⁻, would be produced.

$$ \left( {{\text{Eu}}^{3 + } } \right)^{ - } \to {\text{Eu}}^{3 + } + e^{ - } $$

(3)

$$ \left( {{\text{Mn}}^{2 + } } \right)^{ + } + e^{ - } \to {\text{Mn}}^{2 + } + h\nu^{\prime } \,({\text{afterglow}}) $$

(4)

After stopping UV irradiation (Eqs. 3 and 4), the trapped electron would be released from (Eu³⁺)⁻ with thermal excitation and recombine with (Mn²⁺)⁺, then green long-lasting afterglow from Mn²⁺ would be emitted.

4 Conclusions

ZnO–GeO₂ glasses and glass ceramics co-doped with Mn²⁺ and Eu³⁺, xMnO–yEu₂O₃–25ZnO–75GeO₂, were prepared by the sol–gel method. Under excitation at 254 nm, the glasses with y ≤ 0.4 showed green luminescence peaking at 534 nm due to the ⁴T₁ → ⁶A₁ transition of tetrahedrally coordinated Mn²⁺ ions, and those with y ≥ 0.4 showed red luminescence peaking at 592, 612, 650, and 700 nm due to the ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4) transitions of Eu³⁺ ions, regardless of the value of x. All of the glass ceramics show green luminescence; the strongest luminescence is observed in a 0.1MnO–0.3Eu₂O₃–25ZnO–75GeO₂ glass ceramic heat treated at 900 °C with luminescence quantum yield of 49.8%. The 534-nm green afterglow due to the Mn²⁺ ion was observed in all of the green-luminescent glasses and glass ceramics, while no afterglow was observed in the red-luminescent glasses. The long afterglow lasting for more than 60 min is obtained in a glass ceramic heat treated at 900 °C. In these glasses and glass ceramics, the Eu³⁺ ions may behave as electron trapping centers to be associated with the occurrence of the green afterglow.