Green Zn₃Al₂Ge₂O₁₀: Mn²⁺ Phosphors: Solid-Phase Synthesis, Structure, and Luminescent Properties

3.1 The Crystal Structure

There is no standard PDF card for Zn₃Al₂Ge₂O₁₀ crystal, but the diffraction peaks of samples are in agreement with the standard pattern of Zn₂GeO₄ (PDF#85-0454) and ZnAl₂O₄ (PDF#74-1138), which means that Zn₃Al₂Ge₂O₁₀ is a solid solution of gahnite ZnAl₂O₄ and zinc germanium oxide Zn₂GeO₄ [8]. As shown in Figure 1, the XRD patterns of Zn₃Al₂Ge₂O₁₀, like ZnAl₂O₄ crystal, have spinel structure and belong to the Fd3m space group with a cubic symmetry [9]. Zn²⁺ ions occupy the cubic tetrahedral symmetry (T_d) sites, Al³⁺ ions occupy the trigonal octahedral symmetry (D_3d) sites. Considering that the radii of Ge⁴⁺ (r_Ge = 0.053 nm) and Al³⁺ (r_Al = 0.0535 nm) are very similar to each other, Ge⁴⁺ will partly substitute for Al³⁺ and occupy six oxygen octahedral coordination centres in ZnAl₂O₄, and then the Zn₃Al₂Ge₂O₁₀ crystal is formed. As mentioned above, there are two kinds of polyhedral coordination in Zn₃Al₂Ge₂O₁₀: octahedral coordination of Zn, Al, and Ge and tetrahedral coordination of Zn. The proportion of octahedron and tetrahedron is 2, which is similar to that of the structure of literature [10].

Figure 5:

Photoluminescence spectra of Zn₃Al₂Ge₂O₁₀: xMn²⁺ samples. EX denotes the excitation spectrum (monitored at λ_em = 537 nm), and EM is the emission spectrum (monitored at λ_ex = 334 nm), and the inset is the CIE chromaticity coordinates and green luminescence of Zn₃Al₂Ge₂O₁₀: 0.04Mn²⁺ phosphor.

Figure 6:

The thermal stability measurement of Zn₃Al₂Ge₂O₁₀: 0.04Mn²⁺ phosphor.

Figure 7:

Fluorescence decay time of 537-nm emission of Zn₃Al₂Ge₂O₁₀: 0.04Mn²⁺ under excitation at 334 nm.

3.2 Morphology and Composition

The HAADF image and element mappings of Zn, Al, Ge, O, and Mn are shown in Figure 2. The typical particle of Mn²⁺-doped Zn₃Al₂Ge₂O₁₀ phosphor is irregular in shape. The bright spots in different colours of the mappings correspond to the presence of the elements Zn, Al, Ge, O, and Mn, respectively, indicating that all elements are distributed uniformly throughout the whole area. Interestingly, Mn²⁺ ions are nearly homogenously incorporated into the Zn₃Al₂Ge₂O₁₀ matrix. The EDS of Zn₃Al₂Ge₂O₁₀: Mn²⁺ phosphors in Figure 2b also show that the signals of Zn, Al, Ge, O, and Mn are successfully detected. It should be noted that the very small amount of copper in EDS should be caused by the use of copper mesh as substrate in the measurement.

HRTEM images of Zn₃Al₂Ge₂O₁₀: Mn²⁺ phosphor are shown in Figure 3. The a-, b-, and c-spacings of 0.2692, 0.1917, and 0.2917 nm are obtained; such spacings correspond to (3¯ 2 1), (2¯ 4 1), and (1 2 0) planes, respectively. These are correspondent with the PDF card number 85-0454 (i.e. Zn₂GeO₄). The d-spacing of 0.2868 nm corresponds to (2 2 0) plane of the PDF card number 74-1138 (i.e. ZnAl₂O₄). These results further confirmed that Zn₃Al₂Ge₂O₁₀ is a solid solution of ZnAl₂O₄ and Zn₂GeO₄.

As shown in Figure 4, the XPS spectrum provides the chemical compositions of the sample, containing O, Zn, Al, Ge, and Mn, where they were expected to be. The Zn 2p1, Zn 2p3, Zn Auger1, Zn Auger2, Zn 3s, and Zn 3d are situated at about 1045, 1022, 495, 472, 140, and 9 eV, respectively; 89 and 74 eV are consistent with the binding energies of Al Auger and Al 2O3, respectively; 1250, 1218, 182, and 34 eV are, respectively, corresponding to the Ge 2p1, Ge 2p3, Ge 3s, and Ge 3d electron emitted energies. O Auger and O 1s are located at 978 and 531 eV, respectively. Mn 2p3 is situated at about 639 eV. All peaks of these elements are in good agreement with the standard corresponding central values of those intensity photopeaks. It should be pointed out that C 1s (285 eV) was detected that was ascribed to adventitious carbon arising from atmospheric exposure or incomplete precursor decomposition [11]. Combined with the XRD, EDS, and XPS spectra, it further proves that the Zn₃Al₂Ge₂O₁₀: Mn²⁺ samples have been successfully synthesised.

3.3 Photoluminescence Properties

A broad green emission band peaking at 537 nm is observed from the doped sample as shown in Figure 5. The green emission is so bright that it can be seen by naked eyes when the samples are excited by 334-nm UV lights (see the inset in Fig. 5). The 537-nm emission is attributed to the d-d transition [⁴T₁ (⁴G)-⁶A₁ (⁶S)] of the Mn²⁺ and shows the highest intensity for the doped content 4 mol% Mn²⁺ in Zn₃Al₂Ge₂O₁₀.

A broad and strong excitation band at 334 nm, which is the electronic transition between ⁶A₁ (⁶S) and ⁴E (⁴D) levels of Mn²⁺, can be observed in the Mn²⁺-doped samples. It is beneficial to harvest energy from the pump source because of the broadened and strengthened excitation band for the Mn²⁺ doping case. In addition, from the optical spectral data, the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of 4 mol% Mn²⁺ in Zn₃Al₂Ge₂O₁₀ are about (0.279, 0.625), which are corresponding to green emission. It should be noted that the relationship between the luminescence property and the crystal structure can be established according to the crystal-field theory. Generally, in the calculation, the number of the fitting parameters should be less than that of the experimental spectral data. However, in this case, the spectral data have only two values, i.e. 537 and 334 nm. The number of the fitting parameters is more than that of the experiment values. Thus, it is difficult to establish the relationship between the luminescence spectra and the crystal structure.

The thermal stability measurement of Zn₃Al₂Ge₂O₁₀: 0.04Mn²⁺ phosphor is shown in Figure 6. When the temperature is lower than 50 °C, the luminescence decreases slightly with the increase in temperature, but when the temperature is higher than 100 °C, the luminescence quenching is very obvious. Thus, the phosphor has excellent luminescent properties at room temperature.

The typical fluorescence decay pattern of 4 mol% Mn²⁺ in Zn₃Al₂Ge₂O₁₀ is shown in Figure 7. The decay curve is well fitted into a four-exponential function [12], [13], [14], [15]:

in which τ₁, τ₂, τ₃, and τ₄ are time constants, and A₁, A₂, A₃, and A₄ are coefficients. The average lifetime can be calculated by using the formula:

As can be seen, the average lifetime of 4 mol% Mn²⁺ in Zn₃Al₂Ge₂O₁₀ is 4.88 μs, which is caused by the nonradiative relaxation channels and reduction of the ⁴T₁ (⁴G) lifetime in the 4 mol% Mn²⁺ in Zn₃Al₂Ge₂O₁₀ phosphor. Generally, the fluorescence lifetime of the fluorescent substance is related to its lattice structure. Therefore, the information about the lattice structure can be achieved through the study of the fluorescence lifetime. From (1) and (2), because there are four time constants, fitting values indicate that Mn²⁺ ions may occupy four lattice positions in the host crystal, i.e. octahedral Zn, Al, and Ge, and tetrahedral Zn. In addition, the quantum efficiency (about 87 %) of the phosphor is obtained by FLS980.