1. Introduction

Oxide phosphors have attracted great attention because of their potential applications in flat panel displays, electroluminescent devices, light-emitting diodes and solar energy converters [1–3]. They are preferred over sulfide phosphors when it comes to the chemical and thermal stabilities in the air and at high temperatures. Among many oxide phosphors, zinc gallogermanate (Zn₃Ga₂Ge₂O₁₀) is a native defect phosphor with some outstanding properties, such as bright luminescence and solubility of rare-earth or 3-d transition metals. A breakthrough was recently achieved by Pan et al, who reported a new type of Cr³⁺ doped zinc gallogermanate (Zn₃Ga₂Ge₂O₁₀:Cr³⁺) phosphor with super-long afterglow [4]. In the following work, Mathieu et al presented a detailed study of the structure of such kind of phosphor and found that the zinc gallogermanate as a solid solution of ZnGa₂O₄ and Zn₂GeO₄ can be easily obtained by annealing the relevant compounds in the air [5].

The easy preparation of zinc gallogermanate caught our attention to Mn²⁺-doped zinc gallogermanate phosphors which have been much less studied. Moreover, the native defects responsible for the luminescence of zinc gallogermanate have remained arguable. In this work, the structure and luminescence properties of Mn²⁺-doped zinc gallogermanate phosphors are studied, and the luminescence mechanism of zinc gallogermanate resulting from the native defects is discussed.

2. Experimental

Zinc gallogermanate of the composition Zn₃Ga₂Ge₂O₁₀: 0.5% Mn²⁺ (at. %) was synthesized by solid state reaction employing the high-purity compounds ZnCO₃ (99.9%), Ga₂O₃ (99.9%), GeO₂ (5N) and MnCO₃ (99.9%). The raw materials were weighed in stoichiometric ratio and mixed by grinding with ethanol in an agate mortar. The mixed powders were then sintered at 1200 °C for different durations of time. Following that, the furnace was turned off and the samples were cooled down to room temperature in the furnace.

X-ray diffraction (XRD) data were collected using a Bruker AXS D8 Advance diffractometer (Voltage 50 kV, current 40 mA, Cu Ka) with a step width of 0.02o. X-ray photoelectron spectroscopy (XPS) was carried out by using a Thermo Fisher Escalab 250Xi with a Al Kα source. All binding energies were referenced to the C 1s peak at 284.63 eV of the surface adventitious carbon and revised. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded by a Fluorolog-3-P UV-vis-NIR fluorescence spectrophotometer (Jobin Yvon, Longjumeau, French) with a Xenon lamp as the excitation source. The decay curves were performed by FLSP920 (Edinburgh Instruments). Electron paramagnetic resonance (EPR) spectra were recorded on an EXM-8/2.7 EPR spectrometer (Bruker BioSpin GmbH), operating in the X-band frequency (9.457 GHz) at 100 K. All measurements except for EPR were performed at room temperature.

3. Result and discussion

Zinc gallogermanate is a solid solution of cubic spinel ZnGa₂O₄ and phenacite Zn₂GeO₄ [6]. The similar structure of ZnGa₂O₄ and Zn₂GeO₄ endows the easy formation of the solid solution. In the solid solution, Ga³⁺ is substituted by Ge⁴⁺ and Zn²⁺. The formation of zinc gallogermanate was checked by XRD and XPS spectra shown below.

According to Fig. 1(a), Zn₃Ga₂Ge₂O₁₀ is indeed formed. The structures of the Mn²⁺-doped samples obtained by sintering for different times are almost the same as the non-doped sample. However, compared with the indexation of ZnGa₂O₄ and Zn₂GeO₄, all XRD peaks of zinc gallogermanate shift toward higher 2θ values because the ion radius of Ge⁴⁺ (0.053 nm) is smaller than that of Ga³⁺ (0.062 nm).A close check of the XRD peaks shows that the (311) diffraction peaks of the Mn²⁺-doped samples locate at lower angle compared with the non-doped sample, as seen in Fig. 1(b). It indicates that the lattice constant slightly increases with the Mn²⁺ doping [7]. However, the degree of the lattice expansion decreases with the increased sintering time until for 8 hours. The slight change of structure for different sintered hours is reflected in the emission intensity of the Mn²⁺-doped samples.

 

Fig. 1 a) XRD patterns of the non-doped (non) and Mn²⁺-doped samples sintered for different times. The non-doped zinc gallogermanate is sintered for 6 hours. The ZnGa₂O₄ and Zn₂GeO₄ indexation are indicated below; b) the (311) peak for the Mn²⁺-doped samples sintered for different times.

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The composition of the zinc gallogermanate is checked by XPS as shown in Fig. 2. The Ge 2p_3/2 signal is detected to be at 1219.78 eV, which is smaller than 1219.8 eV for Zn₂GeO₄ and 1220.2 eV for GeO₂, indicating the formation of novel bond [6]. The signals of other elements (O 1s at 530.38 eV, Zn 2p_3/2 at 1021.08 eV, Ga 2p_3/2 at 1117.28 eV) are also different from those of ZnGa₂O₄ (Ga 2p_3/2 at 1117.37 eV, O 1s at 530.45 eV and Zn 2p_3/2 at 1021.28 eV). The energetic separation (ΔE) between the Zn 2p_3/2 and Ga 2p_3/2 peaks is a sensitive tool to confirm the formation of ZnGa₂O₄. In our work, the ΔE value is 96.2 eV, smaller than 96.7 eV for the mixture of ZnO and Ga₂O₃. It means the ZnGa₂O₄ formed, which is in accordance to the XRD data. Besides, the ΔE of zinc gallogermanate is bigger than that of ZnGa₂O₄, which further confirmed the formation of the solid solution [6].

 

Fig. 2 X-ray photoelectron spectroscopy (XPS) of zinc gallogermanate. a) Ge 2p; b) Zn 2p; c) Ga 2p; d) O1s

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A broad blue emission band peaking at 480 nm is observed from the non-doped sample as shown in Fig. 3. The emission band is ascribed to the native defects such as Zn_i, V_Ga, V_Ge and V_O (using Kröger–Vink notations) in Zn₃Ga₂Ge₂O₁₀ .The discussion on the specific defects responsible for the blue emission will be given below. In comparison, a rather narrow green emission band peaking at 520 nm is detected in the Mn²⁺-doped samples. The green emission is so bright that it can be seen by naked eyes when the phosphors are excited by UV lights of moderate powers (inset photo in Fig. 3). The 520 nm emission is attributed to the d-d transition (⁴T₁ (⁴G) →⁶A₁ (⁶S)) of the Mn²⁺ and shows the highest intensity in the sample sintered for 6h. This result is in accordance with the evolution of the (311) diffraction peak with the sintering time, demonstrating the effect of the lattice contraction on the PL properties of the Mn²⁺-doped samples. There are two kinds of coordination polyhedral in Zn₃Ga₂Ge₂O₁₀, tetrahedral coordination of Zn and octahedral coordination of Zn, Ga, and Ge. The proportion of tetrahedral and octahedral is 1/2 [5]. The green emission results from the Mn²⁺ occupying the tetrahedral sites of Zn because of the congruity of electronegativity and ionic radius between Mn²⁺ (0.66 Å in fourfold coordination) and Zn²⁺ (0.60 Å).

 

Fig. 3 Excitation (left) and emission (right) spectra of the non-doped (non) and Mn²⁺-doped samples sintered for different times (Indicating by the numbers). The top inset shows the Gaussian fits (dashed lines) to the excitation spectra of the Mn²⁺-doped sample. The right inset is the photograph image of the Mn²⁺-doped sample being excited by the 290 nm lights.

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The non-doped sample shows a narrow excitation band at 280 nm which is related to the band-to-band electronic transition, whereas a broad and strong excitation band at 290 nm can be observed in the Mn²⁺-doped samples, and the excitation band can be well fitted by two Gaussian peaks (top inset in Fig. 3). One of the peaks is from the band gap absorption alike in the non-doped sample; the other peak at 320 nm is the electronic transition between ⁶A₁ (⁶S) and ⁴T₁ (⁴G) levels of Mn²⁺ [8]. The broadened and strengthened excitation band with the Mn²⁺ doping is beneficial to the energy harvesting of the pump source.

Figure 4 presents the PL decay curves of the Mn²⁺ 520 nm emission and the 480 nm emission of the non-doped sample. The decay of Mn²⁺ can be well fitted by a second-order exponential decay model [9,10]:

 

Fig. 4 The decay curve of Zn₃Ga₂Ge₂O₁₀:0.5%Mn²⁺ monitored at 520 nm. The top inset show the non-doped sample monitored at 480 nm. The excitation wavelength is 290 nm. The red line is the fitted one.

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To probe into the luminescence centers responsible for the emissions of the non-doped and the Mn²⁺-doped samples, the EPR spectra at 100K are measured and shown in Fig. 5. One can clearly see that a series of paramagnetic centers form in the Zn₃Ga₂Ge₂O₁₀ viz., g-factor at 1.96 is assigned to V_O; g-factors at 1.9892, 2.0134 and 2.0310 are assigned to Zn_i, V_Zn and V_Ge, respectively [12]. The EPR result evidences that the blue emission of the non-doped sample is related to the native defects. However, none of the defects signals appear and the typical signals of Mn²⁺ are observed in the Mn²⁺-doped samples (Fig. 5 (b)). This result is consistent with the disappearance of the defect luminescence under 290 nm excitation in the Mn²⁺-doped samples.

 

Fig. 5 EPR (low temperature) spectra of the non-doped (a) and Mn²⁺-doped Zn₃Ga₂Ge₂O₁₀ (b) at 100K.

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4. Conclusion

The bright green emission is obtained from the Mn²⁺-doped zinc gallogermanate. The green emission results from the Mn²⁺ ions occupying the tetrahedral site of Zn²⁺ in the Zn₃Ga₂Ge₂O₁₀. The native defects contribute to the luminescence of the zinc gallogermanate. The present phosphor holds promising potential to be used in solid state displays.