1. Introduction

Synthesis and architecture of nanostructured semiconductive crystal have been vigorously investigated due to its extensive application in photonics [1–5]. Particularly, aiming to produce advanced-photonic component, researchers have made a great effort to fabricate the nanocrystallized glass (NCG) consisting of semiconductive nanocrystals showing various functions, e.g., photoluminescence (PL) and photocatalysis [6,7].

Willemite-type wide-gap-semiconductive oxide, Zn₂GeO₄, has recently attracted much attention because of its various applicabilities and excellent properties: Since Sato et al. reported an excellent photocatalytic property of the Zn₂GeO₄ ceramics for water-splitting [8], different nanostructured Zn₂GeO₄ crystals have been synthesized, which enable to convert CO₂ into a hydrocarbon fuel and to apply an anode for Li-ion battery [9,10]. In addition, Mn²⁺-activated Zn₂GeO₄ thin film has been largely expected for its display-application since brilliant green electroluminescence was demonstrated [11,12]. Furthermore, Mn²⁺: Zn₂GeO₄ nanocrystal is a great candidate for spectral shifter for photovoltaic device (i.e., solar cell), which enable to convert ultraviolet (UV) light to visible light, to improve the conversion efficiency of the cell [13].

Recently, present authors’ group have fabricated transparent NCG with Zn₂GeO₄ in Li₂O-ZnO-GeO₂ system, and have demonstrated an bluish PL and anomalous long-lasting photoluminescence (LLP) in the NCG without any emissive dopants [13,14]. The LLP is attributed to presence of an excess of interstitial Zn-defects (Zn_i ^•), which are introduced during crystallization process, in the Zn₂GeO₄ nanocrystals [14]. Several transition-metal ions with non-fulfilled 3d-shell state possess various PL features based on d-d transition, depending on coordination environment. Particularly, Mn²⁺ (3d ⁵) is an emissive center able to occupy an equivalence Zn site because ionic radius of Mn²⁺ is close to that of Zn²⁺. Elucidation of the Mn²⁺-activation effect on PL properties in the precursor and resulting NCG is significant for sophisticated glass-ceramic application, such as photoemission and spectral-sifter devices. Therefore, the aim of this study is to fabricate the NCGs with willemite-type Zn₂GeO₄ activated by different Mn²⁺-concentrations, and to discuss the activation effects on their PL and LLP properties.

2. Experimental

We employed a zincogermanate glass, which reveals nanocrystallization of willemite-type Zn₂GeO₄ along with Li₂Ge₄O₉ phase, i.e., 15Li₂O-15ZnO-70GeO₂ glass, as a base glass [14,15]. The base glasses added by various amount of MnO, i.e., 15Li₂O-15ZnO-70GeO₂-xMnO (x = 0−1.0) were synthesized as the precursor by a conventional melt-quenching technique (melting condition: 1300°C for 30 min by Pt crucible in air) using commercial powders of reagent grade Li₂CO₃, ZnO, GeO₂, and MnO (purity: ≥ 99.9%). As a preliminary study, we checked the thermal property in the precursors with a differential thermal analysis at a heating rate of 10 K/min, and their glass-transition and crystallization-peak temperatures were situated within T _g = 476−483°C and T _p = 569−572°C, respectively, suggesting no significant effect of MnO-addition on crystallization behavior. Indeed, the precursors heat-treated at each T _p for 1 h showed the crystallization of Zn₂GeO₄ phase along with Li₂Ge₄O₉ phase, which was confirmed by means of a powder X-ray diffraction analysis, irrespective of the MnO-concentration (x), as previously reported [14,15]. The precursors were annealed at each T _g for 1 h to reduce internal stress and then cut into several pieces with an appropriate size. The pieces were heat-treated at the T _p for 1 h to obtain the NCG samples.

Field-emission transmission electron microscope (FE-TEM) was utilized for observation of nanometric texture in the NCG. Visual confirmation of PL and afterglow of the precursor and NCG samples were carried out using UV lamps with wavelength of 312 nm. PL and PL excitation (PLE) spectra were studied by a spectrofluorometer with a xenon lamp as an excitation source. Diffuse-reflectance spectra were measured using a UV-visible spectrometer with an attached integrating sphere. PL and LLP properties and quantum yield (QY) were examined referring to [14,16]. For the LLP spectral and decay curve measurements in this study, monitoring of the LLP intensity was started after exposing the sample to excitation source for 30 s. All of experimental measurements in this study were done at room temperature.

3. Results and discussion

In Fig. 1 , we show the result of fabrication of Mn²⁺-activated precursor glasses and resulting samples subjected to the heat-treatment. As described in experimental section, the MnO-added precursors crystallized the willemite-type Zn₂GeO₄ along with the Li₂Ge₄O₉ phase, maintaining optical transparency, irrespective of the x [Fig. 1(a)]. For example, the sample with x = 0.3 (as representative) exhibited densely-crystallized texture consisting of crystallites (~50-100 nm) [Fig. 1(b)]. Nanocrystallization of the Zn₂GeO₄ phase together with Li₂Ge₄O₉ phase was also confirmed by comparison of the electron diffraction pattern to the ICDD data of No. 11-0687 (Zn₂GeO₄; red rings) and No. 37-1363 (Li₂Ge₄O₉; blue rings). Although the precursors with x = 0.05−0.5 were colorless, those with higher x values (0.8 and 0.1) possessed a dull purple coloration, implying the presence of Mn³⁺ [17]. In addition, reflectance difference spectrum between the precursors of x = 0 (i.e., base glass) and that of x = 1.0 (i.e., ΔR) revealed broad bands around 3.5 eV and 2.5 eV [Fig. 1(c)]. The former and latter bands could be attributed to the ⁶A₁→⁴E, ⁴T₂ transitions of Mn²⁺ and the ⁵E→⁵T₂ transition of Mn³⁺, which is probably occurred by oxidation of Mn²⁺ of the added MnO, respectively [17,18].

 

Fig. 1 (a) Precursor (glass) samples and the samples subjected to heat-treatment at T _p for 1 h (NCG). (b) TEM image of the heat-treated sample with x = 0.3 and a typical selected area electron diffraction (ED) pattern. Blue and red circles correspond to ED rings of the Li₂Ge₄O₉ and Zn₂GeO₄ phases. White bar corresponds to 200 nm. (c) Reflectance spectra of the precursors with x = 0 and 1.0, and the difference spectrum, ΔR.

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In Fig. 2 , we show the contour plot of PL spectra in the NCGs. In case of the non-activated NCG (x = 0), an expanse of broad emission was observed in the energy range of ~2.2-3.0 eV and ~3.6-5.1 eV for the PL and PL excitation, respectively [Fig. 2(a)]. This vast and asymmetric emission is due to that the PL observed in the NCG with x = 0 is as a consequence of plural emissive bands/phases, i.e., 2.30 eV and 2.63 eV for the Zn₂GeO₄ phase, and 2.91 eV for Li₂Ge₄O₉ phase [14]. On the other hand, stronger/sharper emission band around 2.30 eV (~540 nm in visible green region) could be singly detected in the Mn²⁺-activated NCG (x = 0.3) [Fig. 2(b)]. The emission band was almost identical to that of Mn²⁺-activated Zn₂GeO₄ thin-film/nanoparticle so far [12,13]. In addition, dependence of the band position on the excitation energy could not be observed. These clearly indicate that the origin of green emission is extrinsic, i.e., ⁴T₁→⁶A₁ transition of the Mn²⁺ with 3d ⁵-configuration, leading to localized PL. Although this transition is originally a Laporte forbidden, the transition is possible if the Mn²⁺ occupies a tetrahedral site with T_d symmetry. Since the ionic radius of Zn²⁺ is as large as that of Mn²⁺ under tetrahedral coordination state [19], the green emission generated from the activated NCG suggests that the Mn²⁺ exclusively occupies a four-coordinated Zn site in the Zn₂GeO₄ structure, which consisting of tetrahedral ZnO₄ and GeO₄ units. Thus, we conclude that the willimite-type Zn₂GeO₄ nanocrystals in the NCG are activated by the Mn²⁺.

 

Fig. 2 Contour plots of PL in (a) non-activated (x = 0) and (b) Mn²⁺-activated NCGs (x = 0.3). The PL spectra, which were measured under excitation at the peak energy of PL excitation spectra (~4.27 eV and ~3.81 eV for the non-activated and activated NCGs, respectively), are also indicated as white curves. Red bands across the plots correspond to secondary wave of excitation.

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Figure 3 depicts the visual PL features in the precursor and NCG samples under UV-light irradiation at 312 nm. In the precursors, we can see a visible orange/red PL, which is also ascribed to the ⁴T₁→⁶A₁ transition [Fig. 3(a)], because the activated Mn²⁺ in the based glass is also situated in oxygen-coordinated environment [17]. After the nanocrystallization, the emission color totally converted into a vivid green in the whole samples. This can be interpreted as coordination/crystal field of the Zn sites in the Zn₂GeO₄ nanocrystal being stronger than that in the precursor (i.e., glassy sate), in accordance with Tanabe-Sugano diagram [20]. In addition, the maximum of QY in the precursor and NCG samples were estimated to be ~16% and ~19%, respectively (i.e., x = 0.4). As increasing the x, QY of both the precursor and NCG samples slightly increased up to x = 0.4, and subsequently decreased [Fig. 3(b)]. This may be due to the so-called concentration-quenching [21]. In addition, the quenching in the precursor glasses was more significant than that in the NCGs. Since clustering of emissive ions in glasses has fatal effect on their PL properties [22,23], it is deduced that the nanocrystallization suppresses the clustering of Mn²⁺ in the NCG. Furthermore, as seen in bottom of Fig. 3(a), in the Mn²⁺-activated NCGs (x = 0.05 and 0.1), green LLP could be confirmed for several minutes by naked eyes in dark after the UV-irradiation was stopped.

 

Fig. 3 (a) Photographs of precursor and NCG samples under UV-irradiation, and of the NCGs after the UV-exposure was stopped (10 s later). (b) QY of the precursor and NCG samples. The QYs were measured under excitation at the peak energies of PL excitation spectra, which were situated within ~3.76-4.00 eV. As representative, chromaticity coordinates of the orange and green emission in the sample with x = 0.3 are (0.545, 0.386) and (0.351, 0.609), respectively.

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Figure 4 depicts the LLP features of the Mn²⁺-activated NCGs. In previous study, non-activated NCG possesses a LLP band consisting of broad bands at ~2.3 eV and ~2.6 eV, which are attributed to the intrinsic defects of Zn₂GeO₄ nanocrystals [13]. With respect to the Mn²⁺-activated NCG (x = 0.05, as representative), a sharp LLP band at ~2.3 eV (green solid curve) could be obtained by the deconvolution using Gaussian functions, in addition to broad bands at ~2.3 eV and ~2.6 eV (dashed curves) [Fig. 4(a)]. Particularly, the sharp band at ~2.3 eV could be superimposed on the PL band shown in Fig. 2(b). In PL phenomena based on the donor-acceptors recombination, the decay curves are expressed by the equation I(t) ∝ (1 + at)^{− n}, where a and n are parameters, and particularly the n value depends on recombination process [24]. The equation is also applicable to the LLP phenomenon [16,25]. In the both non-activated and Mn²⁺-activated NCGs, the decay curves for the LLP intensities, which were monitored at ~2.3 eV, were well expressed by this equation, and the slopes (or n value) of the activated NCG agreed with that of non-activated NCG (n ~0.6) [Fig. 4(b)]. According to Takahashi et al., the Zn_i ^• of 15Li₂O-15ZnO-70GeO₂ glass-ceramics (i.e., non-activated sample), which is produced through nanocrystallization of Zn₂GeO₄ in the glassy phase, acts as an electron trap for the LLP [14,26]. Thus, taking the LLP features concerning the emission and excitation into account, we strongly suggest that the ⁴T₁→⁶A₁ transition and Zn_i ^• are responsible for the emissive and electron-trapping centers, respectively, in the green LLP of the Mn²⁺-activated NCG. The green LLP phenomenon in Mn²⁺-activated Zn₂GeO₄ nanocrystals is peculiar to the glass-ceramic route since there is no report about such LLP in the activated Zn₂GeO₄ synthesized through different route, i.e, solid-state reaction and spattering techniques [11–13,27].

 

Fig. 4 LLP properties in the Mn²⁺-activated NCG: (a) LLP emission (closed circles) and excitation (open circles) spectra in the NCG with x = 0.05. LLP excitation spectrum of the non-activated NCG (triangles) is also included (cited from [13]). Emission and excitation photon energies for the LLP observations were ~2.31 eV and ~4.13 eV, respectively. These spectra correspond to the LLP after 10 s. (b) Decay curves of LLP in the NCGs with x = 0, 0.05, 0.3, and 1.0. Monitored photon energy was ~2.31 eV. Solid lines correspond to the fitting curves by the equation (see text). Afterglow lifetimes (time at which the initial LLP intensity becomes 1/10) were ~0.3 s, ~0.08 s, ~10⁻² s, and ~10⁻² s for the x = 0, 0.05, 0.3, and 1.0, respectively.

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In terms of the LLP excitation spectrum, the excitation band was deconvoluted into large band at ~4.1 eV and medium band at ~4.6 eV. he former band could be superimposed on the LLP excitation band of the non-activated NCG (plotted by triangles, cited from [14], indicating that origin of the latter band is extrinsic. For instance, Takeshita et al. attributed a PL excitation band within ~4.59−5.39 eV to the transition related to the charge-transfer to Mn²⁺ [13]. In addition, the green-LLP performance considerably declined as increasing the x [Fig. 4(a)], implying that the introduction of excess of the MnO leads to decrease the number of trapping-site or to incapacitate the Zn_i ^• for electron-trapping. At this moment, we tentatively assume that the decline of green-LLP is associated with presence of the Mn³⁺, which is increased with the MnO-addition in the precursor glass [Figs. 1(a) and 1(c)].

4. Summary

We presented the green-emissive Mn²⁺-activated NCGs with willemite-type semiconductive Zn₂GeO₄. The NCGs showed green-emission based on d-d transition of the Mn²⁺, which is incorporated at Zn sites of Zn₂GeO₄ nanocrystals, at room temperature. In addition, the NCGs with low Mn-concentration revealed the visible green LLP. To date, several phosphors have been considered to apply them to spectral shifter material [28–30]. We think that following features are required for the advanced spectral shifter phosphor, i.e., i) sun-energy preservation based on LLP process for night generation and ii) rare-earth (RE) free from a view point of RE-resource conservation. Since the NCG in this study actually meets the i) and ii), it also has a potential for the energy/environmental applications.