1. Introduction

As the hot spot in display area, field emission displays (FEDs) have gained a great interest and have been considered as one of the most promising technologies in the flat panel display market due to its attractive features [1–3]. Phosphors are efficient luminescence materials and irreplaceable components for the development of FEDs. However, FEDs must be operated at significantly lower excitation voltages and higher current densities than CRTs. Thus, phosphors for FEDs are required to have a higher efficiency at low voltages, higher resistance to current saturation, longer service time and equal or better chromaticity than CRT phosphors [4–6]. Although many efficient sulfide-based compounds have been explored as possible low-voltage phosphors, the volatility of sulfur has prohibited their use in the FEDs [7]. Oxide-based phosphors are more stable and environmentally-friendly in comparison with sulfides. Moreover, rare-earth and transitional-metal-doped, oxide-based phosphors for FEDs have been of great interest due to their excellent light output, color-rendering properties, and superior stability under electron bombardment [8–11]. On the other hand, colorimetry tells us that the colors observed by the human eye are those depicted in the whole 1931 Commission International del’Eclairge (CIE) chromaticity diagram. At the four-color system, if four points were selected as red, green, blue and yellow (RGBY), they would surround a larger color space than the three-color system. Thus, the four-color system (RGBY) display more natural color than three primary (RGB) system, more in line of its territory in person requirement. In addition, four-color system, has a higher “information density” (namely, the pixels per unit area) compared with three-color system. Accordingly, in order to enhance the display quality of full-color FEDs, it is necessary to exploit four-color phosphor (RGBY) systems. In other words, it is necessary to develop some yellow-emitting phosphors with color purity. However, so far rare reports are available for yellow-emitting phosphors for FEDs [12, 13]. Accordingly, we focus our attention on exploring yellow-emitting phosphors with high efficiency and good color purity for FEDs.

In recent years, M^IM^IIPO₄-type (M^I and M^II are monovalent alkali metal ions and divalent alkaline earth metal ions, respectively) orthophosphates that represent excellent thermal, hydrolytic stability and stabilization of ionic charge have testified their efficiency as luminescent hosts [14, 15]. In addition, as activator, the luminescence behavior of Mn²⁺ ions has been extensively studied by many researchers [15–17]. Its luminescence properties are strongly dependent on the crystal structure of host materials. So far, Mn²⁺-doped NaCaPO₄ phosphors have been reported by some researchers [17–19]. All these investigations mainly concentrate on the application in white LED and there is no available information on their application in the FEDs. Moreover, yellow-emitting Mn²⁺-doped NaCaPO₄ phosphor has not been reported. Herein, we employed the Pechini sol-gel process to prepare NaCaPO₄:Mn²⁺ phosphors with uniform size at a lower sintering temperature. Then these phosphors were deposited on indium tin oxide glass by a screen printing method to prepare the NaCaPO₄:Mn²⁺ phosphor screen. Their cathodoluminescence (CL) properties, including CL spectra, luminance, and emission intensity degradation behavior in a simulative FED environment have been investigated in detail. The results indicate that the as-prepared NaCaPO₄:Mn²⁺ phosphors have potential to apply in field emission display devices.

2. Experimental

Preparation. The NaCaPO₄: Mn²⁺ phosphor powders were synthesized by the Pechini sol-gel process. The doping concentrations of Mn²⁺ ions are 1–10 mol% of Ca²⁺ ions in NaCaPO₄ host. Typically, stoichiometric amounts of NaH₂PO₄·4H₂O, Ca(NO₃)₂·4H₂O and MnCl₂·4H₂O were dissolved in deionized water under stirring. Then citric acid and polyethylene glycol (PEG, molecular weight = 20000) were dissolved in the above solution (C_PEG = 0.01 M, citric acid: metal ion = 2: 1 in per mole). The resultant mixtures were stirred for 2 h and heated at 75 °C in a water bath until homogeneous gels formed. After being dried in an oven at 100 °C for 24 h, the gels were ground and prefired at 500 °C for 4 h in air. Then the samples were fully ground and calcined at 700 °C for 2 h in air to produce the final products. Subsequently, the phosphor powders were deposited onto the indium tin oxide (ITO) glass by a screen printing method following the same procedure for fabrication of an anode for FED devices. Then they were sintered at 450 °C for 1 h to burn out the residual organic binder. The thickness of printed phosphor layer is about 30 μm. All chemicals were analytical grade reagents (Beijing Fine Chemical Company of China) and used directly without further purification.

Characterization. The X-ray diffraction (XRD) patterns were performed on a D8 Focus diffractometer at a scanning rate of 10° min⁻¹ in the 2θ range from 10° to 80° with graphite-monochromatized Cu Kα radiation (λ = 0.15405 nm). The morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi). The photoluminescence (PL) measurements were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescent (CL) measurement was carried out in a vacuum chamber with the base pressure ≤ 2 × 10⁻⁴ Pa at RT, which is pumped by a rotary pump and turbomolecular pump. The phosphor screen is assembled with a gated carbon nanotube cold cathode to form a triode field emission display device. The samples were preheated at 100 °C for half an hour to remove the absorbed moisture before they were placed into the vacuum chamber. The CL spectra, Commission International de l’Eclairage (CIE) color coordinates, and the CL intensity were measured with an Ocean Optics QE65000 platinum spectrometer. A Konica-Minolta LS110 luminance meter recorded the luminescence, which faced directly to the front of phosphor screen. The voltage applied to the phosphor screen was changed from 1 to 7 kV, and the excitation anode current density was in the range from 0 to 150 μA/cm². All the measurements were performed at room temperature.

3. Results and discussion

The testing system for the CL properties with a triode-type configuration was assembled by the phosphor screen anode and the carbon nanotubes (CNTs) film cathode, as seen in Fig. 1 . The anode of phosphor screens coated on an indium tin oxide (ITO) glass plate (2 cm × 2 cm) by screen printing was separated from the CNTs film cathode from a distance about 1 cm. The CL measurement was carried out in a vacuum chamber with the base pressure ≤ 2 × 10⁻⁴ Pa at RT, which is pumped by a rotary pump and turbomolecular pump. The measured emission area was focused on the circular phosphor screen with the diameter of 1 cm.

 

Fig. 1 Schematic diagram of the CL-Property-Testing equipment for phosphor screen in an analogue FED device.

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Figure 2 shows the representative XRD pattern for NaCaPO₄: 3 mol% Mn²⁺ powder sample annealed at 700 °C. The diffraction peaks of as-prepared sample can be exactly assigned to pure orthorhombic phase of NaCaPO₄ [space group: Pnam (62)] according to JCPDS file 29-1193. No other phase can be detected, indicating that the Mn²⁺ ions were completely dissolved in the NaCaPO₄ host. In NaCaPO₄, the Na⁺ ions are found in a ten-fold coordination considering the distances between Na and O atoms < 3.3 Å, whereas the Ca²⁺ ions are eight-fold coordinated. The sizes of Na⁺ (1.38 Å, CN = 10) and Ca²⁺ (1.26 Å, CN = 8) are large enough for Mn²⁺ (1.10 Å, CN = 8) occuping at these sites [17]. However, in view of the similar coordinate environment and valence, the Mn²⁺ ions are prior to substitute of the Ca²⁺ ions in NaCaPO₄. In addition, the crystallization temperature of NaCaPO₄ via the current sol–gel process is much lower than that in the conventional solid-state reaction (T ≥ 900 °C) [20]. This is because the Mn²⁺ ions are homogeneously mixed at the molecular level and dispersed in the PEG precursor gel that promotes the formation of NaCaPO₄: Mn²⁺. The morphology of the NaCaPO₄: 3 mol% Mn²⁺ powder sample annealed at 700 °C is shown in the inset of Fig. 3 . The SEM image shows clearly that the slightly aggregated particles have approximately spherical shape and a narrow size range of 1.0-1.5 μm, which is propitious to produce a compact phosphor screen and thus to improve its CL properties. In addition, the section plane images of the NaCaPO₄: 3 mol% Mn²⁺ phosphor screen is presented in Fig. 3, which also confirms the formation of a dense phosphor lay on ITO glass and the thickness of printed phosphor layer is about 30 μm.

 

Fig. 2 The XRD patterns of representative NaCaPO₄: 3 mol% Mn²⁺ sample calcined at 700 °C for 2 h and standard data of NaCaPO₄ (JCPDS 29-1193).

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Fig. 3 The section-plane and frontispiece (inset) SEM images of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen.

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The Mn²⁺ ions in solids generally show a broad band emission due to the ⁴T₁→⁶A₁ transition within the 3d [5] shell in which the electrons are strongly coupled to lattice vibration and affected by crystal field strength and site symmetry. The different crystal field strength on Mn²⁺ can tune the emission color point, which varies from green (strong crystal field) to orange/red (weak crystal field) [12], [15–17]. Under low-voltage electron beam excitation, the NaCaPO₄: Mn²⁺ phosphor screen give a bright yellow emission, as shown in the inset of Fig. 4(a) (V_a = 5 kV; J_a = 50 μA/cm²). The possible reason is that the substitution of Ca²⁺ makes Mn²⁺ ions locate in a high coordination environment (eight coordination) and strong crystal field, which results in a yellow emission. Figure 4(a) shows the CL spectra of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen under different voltage electron beam excitation (V_a = 1, 3, 5, 7 kV; J_a = 50 μA/cm²). The corresponding CL spectra all consist of a broad emission from 500 to 650 nm with a maximum at 560 nm, which are assigned to the ⁴T₁(4G)→⁶A₁(6S) of Mn²⁺. Obviously, the emission peaks of the studied phosphor screen are unchanged with the increase of excitation voltage (V_a), indicating a good stability under different voltages electron beam bombardment, which can be validated by its CIE chromaticity coordinates at different excitation voltages, for examples, (0.421, 0.545) for V_a = 1 kV; (0.431, 0.548) for V_a = 3 kV; (0.424, 0.551) for V_a = 5 kV; (0.428, 0.552) for V_a = 7 kV; as shown in Fig. 4(a). In addition, the Fig. 4(a) also reveals the dependence of the CL intensity of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen on the excitation voltage (V_a). When fixing J_a = 50 μA/cm², the CL intensity increases with raising the excitation voltage from 1.0 to 7.0 kV. Similarly, the CL intensity also increases with increasing J_a under a fixed excitation voltage, as shown in Fig. 4(b). There is no obvious saturation effect for the CL intensity of these samples with the increase of V_a and J_a. The increase in CL intensity with an increase in electron energy and filament current is attributed to the deeper penetration of the electrons into the phosphor body and the larger electron-beam current density. The electron penetration depth can be estimated using the empirical formula:$L\left[\AA \right]=250\left(\frac{A}{\rho }\right){\left(\frac{E}{\sqrt{Z}}\right)}^n,n=\frac{1.2}{1-0.29\lg Z}$ where A is the atomic or molecular weight of the material, ρ is the bulk density, Z is the atomic number or the number of electrons per molecule in the compounds, and E is the accelerating voltage (kV) [21]. For example, for NaCaPO₄: 3 mol% Mn²⁺, Z = 78, A = 158.5, ρ = 3.12 g/cm³, the estimated electron penetration depth at 3.0 kV is about 72 nm. For CL of NaCaPO₄: Mn²⁺ samples, the Mn²⁺ ions are excited by the plasma produced by the incident electrons. The deeper the electron penetration depth, the more plasma will be produced, which results in more activator ions being excited, and thus the CL intensity increases. In addition, Fig. 5 shows the CL intensity of Mn²⁺ as a function of their doping concentration (x) in NaCaPO₄: 3 mol% Mn²⁺ samples. At first, the CL intensity of Mn²⁺ increases with the increase of its concentration, reaching a maximum value at 3 mol% of Ca²⁺, then decreases with further increasing its concentration due to the concentration quenching effect. Thus, the optimum doping concentration for Mn²⁺ is 3 mol% of Ca²⁺ in the NaCaPO₄ host lattice. In general, the concentration quenching of luminescence is due to the energy migration among the activator ions at the high concentrations. In the energy migration process the excitation energy will be lost at a killer or quenching site, resulting in the decrease of luminescence intensity [22].

 

Fig. 4 (a) The CL spectra of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen under different voltage electron beam excitation (V_a = 1, 3, 5, 7 kV; J_a = 50 μA/cm²). (b) The CL intensity of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen as a function of J_a (μA/cm²).

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Fig. 5 The CL intensity of NaCaPO₄: Mn²⁺ samples as a function of Mn²⁺-doping concentration.

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Figure 6 shows the dependence of luminance on the anode current density and excitation voltage for NaCaPO₄: 3 mol% Mn²⁺ phosphor screen. As shown in Fig. 6, the luminance of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen increase with increasing the V_a and J_a and the luminance of 1510 cd/m² can be achieved when V_a = 7 kV and J_a = 130 μA/cm². However, the increscent rate of the luminance gradually decreases with the increase of J_a when fixing V_a. This result reveals that the luminance of the studied sample trend to saturation with a continuous increase of J_a at a fixed V_a. In addition, from Fig. 6, we also see that as the anode voltage increases, the luminance saturation current density becomes lower. This may be due to the charge build-up effect or the thermal quenching effect or the interaction of both effects on the surface of phosphor screen. When the anode voltage and current density increased, the power density on the surface of the layer increased, which might cause more charge accumulation or temperature increase, and thus decreased the luminance of phosphor layer.

 

Fig. 6 The luminance of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen as a function of V_a (kV) and J_a (μA/cm²).

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The degradation property for phosphor is very important for FED application. Thus we investigated the degradation behavior of NaCaPO₄: Mn²⁺ samples under low voltage electron beam excitation. Figure 7 shows the decay behavior of the CL intensity of representative NaCaPO₄: 3 mol% Mn²⁺ phosphor screen under continuous V_a = 5.0 kV, J_a = 100 μA/cm2 electron beam bombardment. The peaks of CL spectra are almost the same as those before electron bombardment. However, the CL intensity of the studied sample monotonously decreases with prolonging the electron bombardment time. After the continuous electron radiation for 1 h, the luminance and CL intensities of the NaCaPO₄: 3 mol% Mn²⁺ phosphor screen still remain 86% of the initial value. This degradation of luminance and CL intensity may be due to the accumulation of carbon at the surface during electron bombardment [23]. Because the accretion of graphitic carbon during electron-beam exposure at high current densities is a well-known effect. This carbon contamination will prevent low-energy electrons from reaching the phosphor grains and also exacerbate surface charging, and thus lower the luminance and CL intensity. In addition, after stopping bombardment for a while the luminance and CL intensity could not restore to the initial value, indicating permanent damage to the phosphor occurs, which is another reason to result in the decrease of the CL intensity. On the other hand, the CIE color coordinates of the NaCaPO₄: 3 mol% Mn²⁺ sample under a continuous electron beam radiation with different radiation time (min) were measured to investigate the color stability, as presented in Fig. 8 (“+” for X and “×” for Y). The CIE values are nearly invariable under a continuous electron radiation for 1 h. X and Y keeps at about 0.43 and 0.55, respectively, corresponding to the point 2 in the yellow region of Fig. 8, which has a higher color purity than the commercial yellow-emitting phosphor (Zn, Cd)S:Ag⁺ (X = 0.49, Y = 0.48) [point 1, Fig. 8] [13]. More importantly, the CIE coordinate of NaCaPO₄: Mn²⁺ locate out of the typical FED phosphor color range enclosed by (0.66, 0.33) for red Y₂O₂S:Eu³⁺, (0.29, 0.61) for green ZnS:Cu,Au,Al and (0.14, 0.05) for blue ZnS:Ag, Al. It is clear that the ABC2 quadrangle has a higher area than ABC triangle and ABC1 quadrangle. Accordingly, if the yellow-emitting NaCaPO₄: 3 mol% Mn²⁺ were added as an additional phosphor of the typical tricolor FEDs phosphors (as mentioned above), the color range and chromaticity saturation would be greatly enhanced, which is favourable for improving the display quality of full-color FEDs.

 

Fig. 7 (a) CL intensity and (b) chromaticity coordinate decay of NaCaPO₄: 3 mol% Mn²⁺ phosphor screen with the electron beam bombardment time (min).

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Fig. 8 CIE chromaticity diagram of NaCaPO₄: 3 mol% Mn²⁺ and standard FED phosphors. A, B, C, 1 and 2 represent the CIE Chromaticity coordinates of Y₂O₂S:Eu³⁺ (P22R), ZnS:Ag:Cl (P22B), ZnS:Cu:Au:Al (P22G), (Zn, Cd)S:Ag⁺ (Product No.1010, Nichia Kagaku Kogyo Kabushiki, Japan) and NaCaPO₄: 3 mol% Mn²⁺ phosphors.

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4. Conclusions

In summary, NaCaPO₄: Mn²⁺ phosphors were prepared by the Pechini sol-gel method at a lower annealing temperature (700 °C). The as-prepared NaCaPO₄: Mn²⁺ powder samples have uniform size and shape, which is propitious to produce phosphor screen. The CL properties of NaCaPO₄: Mn²⁺ phosphor were investigated in an analogue FED device in detail. Under low voltage electron beam excitation, the NaCaPO₄: Mn²⁺ phosphor screen shows bright yellow emission (centering at 560 nm due to the ⁴T₁→⁶A₁ transition of Mn²⁺) with the CIE chromaticity coordinate (0.428, 0.552), which has a higher color purity than commercial yellow-emitting FED phosphor (Zn, CdS):Ag⁺. More importantly, the color gamut and chromaticity saturation were greatly enhanced when the yellow-emitting NaCaPO₄: Mn²⁺ was added as an additional phosphor of the typical tricolor FEDs phosphors. Therefore, the as-prepared NaCaPO₄: Mn²⁺ phosphors have potential to increase the display quality of full-color FEDs.