Photoluminescence and cathodoluminescence properties of Na₂MgGeO₄:Mn²⁺ green phosphors

Abstract

Mn²⁺-doped Na₂MgGeO₄ green phosphors were prepared using a solid-state reaction method. X-ray diffraction, scanning electron microscopy, and photoluminescence (PL) and cathodoluminescence (CL) studies were utilized to characterize the prepared phosphors. Under UV radiation (297 nm), the Na₂MgGeO₄:0.03Mn²⁺ phosphor showed a strong green emission corresponding to the ⁴T₁(⁴G)–⁶A₁(⁶S) transition of the Mn²⁺ ions. A possible interaction mechanism was investigated. The CL spectra as a function of accelerating voltage and probe current were also measured. Under continuous low-voltage electron-beam excitation, the phosphor exhibited excellent degradation properties and good color stability. The results indicate that the Na₂MgGeO₄:0.03Mn²⁺ phosphor could be a suitable green phosphor candidate for FEDs.

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1. Introduction

Since Kenneth Shoulders proposed the idea of electron beam micro-devices based on field emissive arrays in the 1960s, rapid development of flat panel displays (FPDs) has occurred.^1,2 As one of the most promising candidates for next generation FPDs, field emission displays (FEDs) have attracted much attention due to their potential benefits such as distortion-free images, wide viewing angles and thin panel thicknesses, in addition to their self-emission properties, low power consumption and quick responses.^3,4 By employing similar operation principles to conventional cathode ray tubes, FEDs utilize cathode rays to excite phosphors on the anode. However FEDs have no deflection coils, so they can be very thin and should work with low voltage electrons (<5 kV).^3,5,6 As we know, a low voltage will decrease the CL intensity of a phosphor, and in order to solve this problem a high current density (10–100 μA cm⁻²) can be adopted.^7–9 Thus, it is important to develop FED phosphors with high luminescent efficiencies and excellent conductivities under low-voltage electrons and high current densities. The traditional sulfide-based phosphors such as Y₂O₂S:Eu, SrGa₂S₄:Eu, Zn(Cd)S:Cu,Al and ZnS:Ag,Cl have been developed as possible FED phosphors with high luminescence efficiencies and considerable conductivities.¹⁰ However, sulfide phosphors degrade significantly and release harmful gases under high-energy electron bombardment. This poisons the cathodes and subsequently shortens the device lifetime.^11,12 In order to improve the device performance and lifetime of the FEDs, it is necessary to find novel phosphors with high luminescence efficiencies, excellent color rendering properties and superior stabilities under electron bombardment. Oxide-based phosphors have attracted great interest due to their good chemical and thermal stabilities, which make them more environmentally friendly in comparison with sulfides. Moreover, they show high efficiencies, color rendering properties and good stabilities under low-voltage electron beam excitation.¹³ Germanates have all the advantages of oxide-based hosts and they also have potential applications in electronics, photodetection, electroluminescence and so on.^14–16 In recent years especially, the cathodoluminescence properties of germanates have attracted much attention. Lin et al. have investigated the red phosphor Ca₂GeO₄:Eu³⁺ and the green phosphors Li₂ZnGeO₄:Mn^2+ 17 and Zn₂GeO₄:Mn^2+ 18–20 for use in FEDs, thus germanates are possible candidates for FEDs. However, such compounds do not yet fully satisfy the requirements for the application, thus more investigation should be done. Morphology and size can influence the cathodoluminescence properties of phosphors.²¹ Regular morphologies, such as spherical morphology, are beneficial for giving higher packing densities. Furthermore, small particle sizes are advantageous for achieving higher resolution.^22,23 For phosphors, a suitable host should have a befitting luminescence center, thus a suitable doping-ion is important. The Mn²⁺ ion is a significant activator often used in FED phosphors. Mn²⁺ usually exhibits a broad band emission due to the ⁴T₁(⁴G)–⁶A₁(⁶S) transition. With the d electrons in the 3d shell, Mn²⁺ emission can be strongly affected by the crystal field, with the emission shifting from green to red.^24,25 Examples include the green phosphors Li₂ZnGeO₄:Mn²⁺,¹⁷ (Zn,Mg)₂GeO₄:Mn²⁺,¹⁸ Mg₂SnO₄:Ti⁴⁺,Mn²⁺,²⁶ the yellow phosphors NaCaPO₄:Mn^2+ 27 and Ca₂Gd₈(SiO₄)₆:Mn^2+ 28 and the red phosphors AlN:Mn²⁺,²⁹ Sr₃In(PO₄)₃:Mn²⁺,³⁰ Mg₂Y₈(SiO₄)₆O₂:Mn^2+ 31 and Ca₄Y₆(SiO₄)₆O:Mn²⁺.³²

As a germanate, Na₂MgGeO₄ is isostructural to Na₂ZnGeO₄, which has the monoclinic space group P1n1. The Mn²⁺-doped Na₂MgGeO₄ phosphor has been reported by Asish Kumar Sharma,³³ who concentrated on its application in cold cathode fluorescent lamps and therefore no information on its application in FEDs can be found. Moreover, as the author paid more attention to the multi-objective genetic algorithm-assisted combinatorial method, there is little information on the luminescence except for the photoluminescence emission spectrum. Therefore, in this work, we employed a solid-state reaction to prepare Na₂MgGeO₄:Mn²⁺ phosphors, and then investigated the PL properties and CL properties in detail for fundamental research and their potential application in FED devices.

2. Experimental section

2.1 Preparation

Na₂Mg_1−xGeO₄:xMn²⁺ (0.005 ≤ x ≤ 0.06) powders were synthesized using a solid-state reaction method. The starting materials Na₂CO₃ (A.R.), 4MgCO₃·Mg(OH)₂·5H₂O (A.R.), GeO₂ (A.R.) and MnCO₃ (A.R.) were weighed stoichiometrically, ground thoroughly in an agate mortar and then sintered in an aluminium oxide (Al₂O₃) crucible at 950 °C for 6 h under a reductive atmosphere (N₂/H₂ = 40 : 5) inside a tube furnace:

5Na₂CO₃ + (1 − x)4MgCO₃·Mg(OH)₂·5H₂O + 5xMnCO₃ + 5GeO₂ → 5Na₂Mg(1 − x)GeO₄:xMn + (9 + x)CO₂ + 6(1 − x)H₂O

Finally, the as-synthesized samples were cooled down to room temperature and ground again for future measurement.

2.2 Characterization

The crystal structure was identified using a Bruker X-ray diffractometer (XRD, D2 PHASER X-ray diffractometer, Germany) with Ni-filtered Cu Kα radiation (λ = 1.54056 Å, scan rate = 15° min⁻¹, range from 10° to 80°). The morphologies were determined using field emission scanning electron microscopy (FESEM, Hitachi, S-4800). The photoluminescence (PL) and PL excitation (PLE) spectra of the samples were measured using an FLS-920T fluorescence spectrophotometer equipped with a 450 W xenon light source. The cathodoluminescence properties of the samples were obtained using a modified Mp-Micro-S instrument. All of the measurements were performed at room temperature.

3. Results and discussion

3.1 Phase identification and morphology

Fig. 1 shows a series of XRD patterns for the Na₂MgGeO₄:Mn²⁺ (0 ≤ x ≤ 0.06) phosphors with different doping concentrations (the XRD patterns of the other samples were similar), as well as the calculated XRD pattern according to the crystal structure parameters of Na₂MgGeO₄:Mn²⁺.³³ No detectable impurity phases were observed in the obtained samples. The XRD profiles are well fitted to the calculated XRD pattern, indicating that all of the samples were single phase and that the Mn²⁺ ions had been successfully incorporated into the Na₂MgGeO₄ host lattice without obviously changing the crystal structure. Moreover, it was found that the XRD diffraction angles of the studied Na₂MgGeO₄:Mn²⁺ samples gradually decreased with the increase in doping content of the Mn²⁺ ions due to the larger ionic radius of Mn²⁺ with respect to Mg²⁺. This indicates that Mn²⁺ ions had replaced Mg²⁺ lattice sites in the Na₂MgGeO₄ host.

Fig. 1 XRD patterns of Na₂MgGeO₄:0.01Mn²⁺, Na₂MgGeO₄:0.03Mn²⁺ and Na₂MgGeO₄:0.05Mn²⁺ (the calculated XRD data for Na₂MgGeO₄ is shown as a reference). The right part is the magnified area indicated by the red box and the inset is an SEM image of Na₂MgGeO₄:0.03Mn²⁺.

Additionally, the morphology of the Na₂MgGeO₄:0.03Mn²⁺ sample prepared using the solid-state reaction is presented in the inset of Fig. 1 (as a representative sample). The Na₂MgGeO₄:0.03Mn²⁺ sample consisted of spherical-like particles with sizes ranging from 1 to 5 μm, the surfaces of the particles were smooth, but they agglomerated with each other. The spherical-like particles achieved at a relatively low temperature of 950 °C can be attributed to the starting materials 4MgCO₃·Mg(OH)₂·5H₂O and NaCO₃, of which the fusion points are 350 °C and 851 °C respectively. They can play the role of flux, then melt and cover other components to form spherical-like particles, and also result in serious agglomeration.³⁴ The obtained spherical-like morphology and the particle sizes of this sample are beneficial for producing a compact phosphor screen and also for improving the CL properties.^35,36

Fig. 2 shows the Rietveld structural refinement of the powder diffraction pattern of Na₂MgGeO₄:0.03Mn³⁺. The structural parameters reported in ref. 33 were used as the initial parameters in the Rietveld analysis. The structural refinement of Na₂MgGeO₄:0.03Mn²⁺ indicates that it crystallized in a monoclinic crystal system with the space group P1n1 and the following unit cell parameters: a = 7.1232(12) Å, b = 5.5985(11) Å, c = 5.3255(9) Å, β = 89.954(9)°, V (cell volume) = 212.38(7) ų. The reliability parameters of the refinements are R_p = 13.879%, R_wp = 9.846%, and χ² = 2.88. Fig. 3 shows the crystal structure of Na₂MgGeO₄. It can be seen that the structural framework of the Na₂MgGeO₄:0.03Mn³⁺ lattice is built up by GeO₄ tetrahedra and MgO₄ tetrahedra which connect with each other through corner sharing. Na⁺ ions in this schematic occupy two positions: one forms an almost regular tetrahedron with four oxygen ions, and the other is surrounded by six oxygen ions.³⁷

Fig. 2 Observed (crossed) and calculated (red line) XRD patterns and the difference profile (blue line) of the Rietveld refinement of Na₂MgGeO₄:0.03Mn²⁺. Bragg reflection is shown as vertical bars. Inset shows the fit parameters.

Fig. 3 Crystal structure of Na₂MgGeO₄.

3.2 Photoluminescence properties

Fig. 4 shows the PLE and PL spectra of the Na₂MgGeO₄:0.03Mn²⁺ phosphor. Under 297 nm excitation, the as-prepared Na₂MgGeO₄:0.03Mn²⁺ phosphor shows a strong green emission band ranging from 475 nm to 550 nm centered at 521 nm, which can be ascribed to the ⁴T₁(⁴G)–⁶A₁(⁶S) transition of the Mn²⁺ ions.³⁸ When monitoring the emission at 521 nm, there is a broad strong excitation band ranging from 240 nm to 350 nm with a maximum at 297 nm and several weak sharp peaks at 363 nm, 384 nm, 431 nm and 451 nm, which were attributed to the transitions from ⁶A₁(⁶S) to ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)] and ⁴T₂(⁴G) respectively. As the d–d transitions are spin and parity forbidden, the excitation bands in the wavelength range of 350–500 nm were very weak, which correlates well with the emission spectrum (10× magnified emission spectrum). Meanwhile, the emission spectrum does not change under different wavelengths of excitation, thus indicating that there was only one luminescence center in this host, which agrees well with Mn²⁺ occupying the position of Mg²⁺.

Fig. 4 The PLE and PL spectra of Na₂MgGeO₄:0.03Mn²⁺. The magnified part reveals the excitation spectrum in the range of 350–500 nm.

The dependence of the PL intensity on the Mn²⁺ concentration was also investigated, as shown in Fig. 5. It can be seen that the PL intensity rapidly increased when the Mn²⁺ content increased from 0.005 to 0.03 and reached a maximum at about 0.03. Above this value, concentration quenching occured.

Fig. 5 PL spectra of Na₂MgGeO₄:xMn²⁺ samples (0 ≤ x ≤ 0.06). Inset: the dependence of log(I/x) on log(x).

Concentration quenching is associated with the critical energy transfer distance between the Mn²⁺ ions. The critical distance (R_c) of Mn²⁺ ions is determined by the following formula given by Blasse:³⁹

where V is the volume of the unit cell, χ_c is the critical concentration of Mn²⁺ ions and N is the number of cations substituted by Mn²⁺ ions in the unit cell. The values of N, χ_c and V for the Na₂MgGeO₄ host were 2, 0.03, and 212.38 ų, respectively. So the R_c value was approximately 21.70 Å. Moreover, a possible interaction mechanism was investigated because the critical distance was longer than 5 Å, which indicates that the interaction mechanism couldn’t result from an exchange interaction.⁴⁰ According to Dexter’s theory,⁴¹ it can be confirmed that concentration quenching is caused by multipolar–multipolar interactions. The interaction type can be determined by the following equation:
where I is the emission intensity, x is the concentration of activator and K and β are constants under the same excitation for a given matrix. The value of θ equals 6, 8, or 10 and represents the dipole–dipole, dipole–quadrupole or quadrupole–quadrupole interaction mechanisms, respectively. When x exceeds the critical concentration, a simple equation can be used:

For this part, the ratio of log(I/x) and log(x) equals the value of (−θ/3). According to the inset of Fig. 5, the ratio is −1.77, which means the value of θ is close to 6. Therefore, the concentration quenching of Mn²⁺ ions in the Na₂MgGeO₄ host should have originated from a dipole–dipole interaction.

3.3 CL properties of Na₂MgGeO₄:Mn²⁺

To explore its potential use as an FED material, the CL properties were investigated in detail. Fig. 6(a) shows the CL spectrum and PL spectrum of the Na₂MgGeO₄:0.03Mn²⁺ sample. Under low-voltage electron-beam excitation (90 mA, 3 kV), Na₂MgGeO₄:Mn²⁺ shows a broad emission ranging from 475 nm to 600 nm with a maximum at 521 nm, which is similar to its PL spectrum. The broad emission is ascribed to the ⁴T₁(⁴G)–⁶A₁(⁶S) transition of the Mn²⁺ions. For FEDs, the color purity of the phosphor is an important index for practical applications. The full width at half maximum (FWHM) of the CL spectrum is about 35 nm, which reveals high color purity and excellent chromaticity coordinate characteristics. In order to evaluate the color purity of the prepared phosphor, we compared the CIE chromaticity coordinates with those of ZnO:Zn²⁺ and Zn₂SiO₄:Mn²⁺, as shown in Fig. 6(b). A, B and C refers to ZnO:Zn²⁺, Zn₂SiO₄:Mn²⁺ and Na₂MgGeO₄:0.03Mn²⁺ respectively. ZnO:Zn²⁺ shows a whitish-green emission and Zn₂SiO₄:Mn²⁺ shows a highly pure green emission, while Na₂MgGeO₄:0.03Mn²⁺ has an even better green emission than them. The CIE color coordinates of Na₂MgGeO₄:Mn²⁺ are x = 0.172 and y = 0.745.

Fig. 6 (a) The CL spectrum and PL spectrum of the Na₂MgGeO₄:0.03Mn²⁺ sample; (b) the CIE chromaticity diagram for ZnO:Zn²⁺ (A), Zn₂SiO₄:Mn²⁺ (B) and Na₂MgGeO₄:0.03Mn²⁺ (C).

The CL emission spectra has been recorded as a function of the accelerating voltage and the filament current, as shown in Fig. 7. When the accelerating voltage was fixed at 3 kV, the CL intensity increased with an increase in the filament current from 60 mA to 100 mA, no saturation effect could be observed. As mentioned above, FEDs utilize low-voltage electrons (<5 kV) to suit the thinner panels, which will also reduce the CL intensity of the phosphors. Thus, in order to enhance the CL intensity of FEDs, a high current should be adopted. The CL intensity will increase with the increasing filament current. However, as the filament current exceeds the threshold, the CL intensity will no longer increase or may even decline, which is called a saturation effect.⁷ Wagner and co-workers have discussed the factors affecting the performance of low voltage CL phosphors. They have found that there are a lot factors influencing it such as electron penetration, diffusion length, surface recombination, activator decay rate and so on.⁴² The saturation effect is also an important factor which hinders the application of phosphors in FEDs, thus FED phosphors should have a high saturation current. As shown in Fig. 7, the CL intensity continuously increases with increasing beam current, indicating that the phosphor is resistant to current saturation, which is a benefit for FEDs.

Fig. 7 CL intensity of the Na₂MgGeO₄:0.03Mn²⁺ sample as a function of filament current (a) and accelerating voltage (b). The insets clearly show their relationship. All of the inset data were recorded at 521 nm.

Similarly, under a 90 mA filament current excitation, the CL intensity increased with increasing accelerating voltage from 2 to 4 kV (Fig. 7(b)). Incident electrons hitting the surface of the phosphors will produce kinds of secondary electrons. As most of the secondary electrons lack energy, they will be scattered. Thus the penetration of the secondary electrons is superficial and the CL intensity is reduced for surface defects. With increasing accelerating voltage, the incident electrons have higher energies, thus the increase in the CL brightness is attributed to the deeper penetration of electrons into the phosphor body. The deeper the electron penetration depth, the more plasma will be produced, which results in the excitation of more doped-ions and thus, an increase in the CL intensity.¹² The electron penetration depth can be estimated using the empirical formula:⁴³

L (Å) = 250(A/ρ)(E/Z^1/2)ⁿ

n = 1.2/(1 − 0.29 log Z)

According to the equation, A is the atomic or molecular weight of the material, Z is the atomic number or the number of electrons per molecule in the compounds, ρ is the bulk density and E is the accelerating voltage (kV). For Na₂MgGeO₄, Z = 98, A = 206.5, ρ = 3.23 g cm⁻³, the estimated electron penetration depths at 2, 3 and 4 kV were 17.02 nm, 53.84 nm, 121.90 nm, respectively. Furthermore, the CL spectra maxima adopt almost the same positions, with peak locations at 521 nm when changing the accelerating voltage and the filament current.

The degradation properties of a phosphor are very important for FED applications. Electron-beam radiation reduces the CL intensity after irradiation for some time because it causes permanent damage to the surface of the phosphor. Moreover, the color stability also represents a challenge.²² Thus we also investigated the degradation behavior of the Na₂MgGeO₄:0.03Mn²⁺ sample under continuous low-voltage electron-beam excitation and this is illustrated in Fig. 8(a). The accelerating voltage was kept at 3.0 kV and the filament current was 90 mA. For comparison, the degradation properties of the commercial Zn₂SiO₄:Mn²⁺ were also measured. After continuous electron radiation for 1 h, the CL intensity of Na₂MgGeO₄:0.03Mn²⁺ remained at 95.2% and the CL intensity of Zn₂SiO₄:Mn²⁺ fell to 93.1% of the initial values. For most phosphors used in FEDs, their CL intensities decrease consecutively with the increase in bombardment time. This is due to the accumulation of carbon at the surface during electron bombardment. During continuous electron bombardment, graphitic carbon will accumulate on the surface of the phosphors and cause the well-known effect of carbon contamination, which exacerbates surface charging and thus lowers the CL intensity.⁴⁴ In this test, after continuous electron radiation for 1 h, Na₂MgGeO₄:0.03Mn²⁺ showed better degradation resistance properties than the commercial phosphor. Moreover, in this case an interesting phenomenon was worth noting, the degradation curve of the Na₂MgGeO₄:0.03Mn²⁺ phosphor did not fall consecutively but showed a slight increase after 10 minutes of bombardment. According to an investigation by Wu and co-workers, this abnormal degradation behavior could be caused by an increase in the specific surface area.⁴⁵ It is well known that the CL intensity will increase as the bombarded surface area is increased, and also that degradation behavior will occur after continuous electron radiation. When the effect of the specific surface area increase overcomes the degradation behavior, an increasing curve appears, and vice versa.

Fig. 8 (a) CL intensity of the Na₂MgGeO₄:0.03Mn²⁺ sample as a function of radiation time (accelerating voltage = 3.0 kV, filament current = 90 mA). The inset shows a comparison to the degradation properties of Zn₂SiO₄:Mn²⁺. (b) The CIE color coordinates of Na₂MgGeO₄:0.03Mn²⁺.

The CIE color coordinates of the Na₂MgGeO₄:0.03Mn²⁺ phosphor under continuous electron-beam radiation for different radiation times (min) were also measured to investigate the color stability, as presented in Fig. 8(b). The CIE values were near invariable under continuous electron radiation for 1 h. x and y remained at about 0.17 and 0.74, respectively. In summary, this short time experiment indicates that the stabilities of the CL intensity and CIE color coordinates of the Na₂MgGeO₄:0.03Mn²⁺ sample are good, which are potential advantages in FED applications.

4. Conclusion

In summary, Mn²⁺ doped Na₂MgGeO₄ phosphors with spherical-like morphology were prepared by a solid-state reaction. The PL and CL properties of the Na₂MgGeO₄:0.03Mn²⁺ sample were investigated in detail. Under UV or electron beam radiation, the phosphor showed a highly bright green emission, which was ascribed to the ⁴T₁(⁴G)–⁶A₁(⁶S) transition of the Mn²⁺ ions. The concentration quenching was investigated and a possible interaction mechanism between the Mn²⁺ ions in the Na₂MgGeO₄ host was identified as a dipole–dipole interaction under UV radiation. The CL intensity increased with the increase in accelerating voltage from 2 to 4 kV as a result of the deeper electron penetration depth. With the increase in filament current from 60 mA to 100 mA, no saturation trends were observed. Moreover, Na₂MgGeO₄:0.03Mn²⁺ exhibited a higher color purity and better degradation behavior than the Zn₂SiO₄:Mn²⁺ phosphor. After continuous electron radiation for 1 h, the CL intensity remained at 95.2% and the CIE coordinates were nearly invariable. These excellent CL properties indicate that Na₂MgGeO₄:0.03Mn²⁺ has great potential as a green phosphor for full color FEDs.

Acknowledgements

This work was supported by the National Natural Science Funds of China (Grant No. 51372105) and the Gansu Province Development and Reform Commission.

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