1. Introduction

Field emission display (FED) is a self-emissive display, in which electrons emitted from field emitter arrays excite the phosphor to give off the light. FED has attracted much attention due to its advantages, such as self-emissive display, short response time, distortion-free, high brightness, wide viewing angle, low power consumption, good chromaticity and so on [1–5]. Development of new high performance phosphor is an important issue for the success of FED. Phosphors for FEDs are required to possess the following properties such as high efficiency under low voltages, high saturation current densities, long lifetime and equal or even better chromaticity than cathode ray tube (CRT) phosphors [6–9].

The color gamut is a vital index, which determines the colors the display can present. It depends on the location of the Commission International de l’Eclairage (CIE) coordinates of phosphors, which typically consists of triangle or quadrangle in the CIE diagram. To improve the display quality of full-color displays, such as FEDs as well as light emitting diode (LED) and so on, it is necessary to further enlarge the color gamut of phosphors. One method is to make the color of current commercial phosphors more saturated. Another approach is to develop novel materials, which have CIE coordinates locating outside the triangle of commercial FED tricolor phosphors. Cyan phosphor is considered to be the ideal candidate for this purpose, which can enlarge the color gamut of display and thus improve the display quality [10,11]. Besides, from the CIE chromaticity diagram, one may see clearly that color purity depends on the position of CIE coordinates. For similar colors, the closer to the spectral locus, the higher the color purity is.

As to the luminance of cyan phosphor, the required luminance is ought to be as high as possible, at least 300 cd/m² [12] for commercial use under FED’s operating condition. So far this critical parameter has not been reported yet. Sulfide-based phosphors, such as ZnS:Ag,Cl and SrGa₂S₄:Ce³⁺ [12], usually have higher luminance. However, sulfide-based phosphors are easy to decompose under high energy electron bombardment and will poison the cathode which limits their application in FED. On the contrary, oxide-based phosphor materials have much higher stability despite lower luminance [13–17]. Therefore, much attention has been paid to the oxide-based cyan phosphor materials recently.

So far, there are already some reports about novel cyan phosphors for FED application. Recently, one cyan phosphor: Mg₂SnO₄:Ti⁴⁺,Mn²⁺ has been reported by some of us for the purpose of enlarging the color gamut of FED display [10]. Its photoluminescence (PL) and cathodoluminescent (CL) spectra under low voltage electron beam excitation were measured and it was found that it can widen the color gamut by 27.9% [10]. As another wide band gap oxide phosphor material, Li₂CaSiO₄:Eu²⁺ (LCSO:Eu²⁺) phosphor shows good stability [18] and was ever studied for white LED application [19–24]. The luminescence of Eu²⁺ doped phosphor material is strongly dependent on the host lattice, namely the absorption and emission band from the 4f⁶5d→4f⁷ transition of Eu²⁺ ion can be tuned by the host matrix. It can emit light from UV to red region of the electromagnetic spectrum. The emission peak (480 nm) of LCSO:Eu²⁺ phosphor locates in the cyan region; therefore we think it could be a strong candidate of the phosphor for widening the color gamut of FED. In our previous study, we have already investigated properties such as concentration quenching effect, PL and CL properties of LCSO:Eu²⁺ phosphor powder under VUV (vacuum ultraviolet)-UV light and low-voltage cathode ray excitations [11].

To our best knowledge, no results have been reported on the performance of cyan phosphor film in a field emission device. It is known that the morphology of phosphor particle as well as fabrication procedure of phosphor screen may also affect the characteristics of the phosphors and sometimes the luminous efficiency may decrease dramatically [25–27]. Thus, it is important and necessary to study further the luminescent properties of these phosphor film. Moreover, there is still no report on how efficient the cyan phosphor materials, and on their purity. Such data are necessary for consideration for commercial use. In this study, a higher efficient, purer cyan-emitting LCSO:Eu²⁺ phosphor screen have been prepared and characterized. The CL properties, the luminance as a function of anode current densities under different anode voltages and degradation properties of LCSO:Eu²⁺ phosphor screen will be presented. Comparison of its performance with Mg₂SnO₄:Ti⁴⁺,Mn²⁺ cyan phosphor screen has also been carried out.

2. Experimental

Preparation. LCSO:Eu²⁺ phosphor powder was synthesized by high temperature solid state reaction method [11]. The starting materials Li₂CO₃ (A.R.), CaCO₃ (A.R.), SiO₂ (A.R.), and Eu₂O₃ (99.99%) were weighed stoichiometrically and grounded thoroughly in an agate mortar and pestle. Then the mixture were heated at 1073 K for 6 h under a reductive atmosphere (N₂/H₂ = 3:1). The final products were cooled down to room temperature (RT) and grounded again. The optimum doping concentration of Eu²⁺ is about 0.9%, which substitutes for the Ca²⁺ due to similar ionic radii (Eu²⁺: 0.098 nm, and Ca²⁺: 0.100 nm).

Characterization. Phosphor screen was prepared on indium-tin-oxide (ITO) glass by screen printing method following the procedure for fabricating a FED phosphor screen. Phosphor slurry was screen printed onto ITO glass and sintered at 450 °C for 1 hour under ambient atmosphere to burn out the residual organic binder. The details of the process can be found in an early literature [25]. Multiple printing process was used to improve the packing densities of phosphor layer. The thickness of the printed phosphor layer was about 24 μm, which was measured by a surface profiler (Dektak 150).

The morphology of phosphor layer was observed by a scanning electron microscope (SEM) and the composition and crystal structure were measured using Rigaku D/max-IIIA x-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). The cathodoluminescent measurement of both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor screen was carried out in a vacuum chamber with a pressure below 2 × 10⁻⁴ Pa. The phosphor screen is assembled with a gated CNT cold cathode to form a triode field emission display device. The samples were heated at 100 °C for 30 min to remove the absorbed moisture before they were put into the vacuum chamber. The CL spectra and the CL intensities were measured with an spectrometer (Ocean Optics QEB1004 Platinum). A luminance meter (Konica-Minolta LS110) recorded the luminance transmitted through the phosphor layer on the ITO substrate, which faced directly to the front of phosphor screen. The voltage applied to the phosphor screen was changed from 1 kV to 7 kV, and the excitation anode current density was in the range from 0 mA/cm² to 4.5 mA/cm². Decay time curves as well as temperature dependence of emission spectra for LCSO:Eu²⁺ phosphor were acquired using a time-correlated single photon counting spectrometer with a 450 W nanosecond flash lamp (nF900; channels: 1000; pulse frequency: 40 kHz). While for Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor, a 60 W microsecond flash lamp was introduced (μF900; channels: 2000; pulse frequency: 50 Hz). We used a tungsten microprobe installed inside a SEM system to measure the electrical characteristics of the phosphor. The microprobe was pressed on the surface of a single phosphor particle. Positive and negative biases were applied to the microprobe and current-voltage (I-V) characteristics were recorded using a Keithley 6487 pico-ammeter. The I-V characteristics of these two phosphors were measured from three randomly chosen particles.

3. Results and discussion

We used the vacuum light emitting tube as a device for the following measurements. Its electron source used our patented design [28] and the performance of lighting emitting tubes using carbon nanotube as cold cathode was reported early [29]. Firstly, both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ cyan phosphor screens were prepared on ITO substrate by screen printing method. Then, CL properties of both phosphor screens were measured in a vacuum chamber (<2x10⁻⁴ Pa) with the bombardment of electron beams from the electron source using a gated carbon nanotube cold cathode.

Figure 1(a) shows the SEM image of cross-section review of LCSO:Eu²⁺ phosphor screen prepared on ITO glass. The SEM image shows clearly that the sample is composed of phosphor particles with size distribution ranging within 3 to 8 μm. The film looks compact, since there are few hollows between phosphor particles. The thickness is estimated to be about 24 μm, which is in accordance with the result from surface profiler.

 

Fig. 1 (a) SEM image of cross-section view of Li₂CaSiO₄:Eu²⁺ phosphor layer prepared on ITO glass substrate. (b) Typical XRD pattern of Li₂CaSiO₄:Eu²⁺ phosphor layer.

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Figure 1(b) shows the structure and phase purity of the prepared LCSO:Eu²⁺ phosphor screen. The diffraction peaks match well with the standard data of Li₂CaSiO₄ (JCPDS 27-0290) with two main peaks. The doped Eu²⁺ does not change the XRD pattern. The prominent peak (112) locates at 37.58° belonging to tetragonal structure and the second peak (110) lies at 25.09 ^o. The FWHM of (112) and (110) peak are 0.176 and 0.146, respectively. The lattice constant (d value) of (112) and (110) in our experiment are calculated to be 2.39 Å and 3.55 Å, while the theoretical d-value of these two peaks are 2.40 Å and 3.58 Å, respectively. The lattice parameters are a = b = 5.047 Å, c = 6.486 Å, and the space group is $I\overline{4}2m$. The experimental results are almost identical with the theoretical results, and this implies that the degree of crystallization is good during the phosphor powder synthesis procedure under the circumstance of a relatively lower sintering temperature (800 °C) by solid state reaction method. Besides, a second phase locating at 29.503 ^o is observed, which we are not sure how exactly it comes from. But after comparing CL spectrum below (Fig. 2(a) ) with early result [11], we could conclude that this does not have any influence on the emission and therefore it could be neglectable.

 

Fig. 2 (a) Typical CL spectrum of Li₂CaSiO₄:Eu²⁺ phosphor screen measured under V_a = 7 kV and J_a = 50 μA/cm². Top left inset shows the luminescent image of the phosphor screen during CL measurement and top right inset shows the amplified CL spectrum from 550 nm to 780 nm. (b) Integrated CL intensity for Li₂CaSiO₄:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphors as a function of anode voltage; Top left inset shows the calculated electron penetration depth for both phosphors.

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The CL spectrum of LCSO:Eu²⁺ phosphor screen is shown in Fig. 2(a). It consists of one dominant emission band locating at 480 nm and four weaker emission bands (594 nm, 618 nm, 653 nm and 702 nm, see the inset of Fig. 2(a) at top left). The peak at 480 nm is attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺ with FWHM of only 37 nm, which is much narrower than most phosphors doped with Eu²⁺. Smaller FWHM is considered to be beneficial for obtaining higher luminous output due to more integral overlap with eye sensitivity curve [19,30]. The CIE coordinates of single emission peak is more saturated than that of another cyan phosphor, i.e. Mg₂SnO₄:Ti⁴⁺,Mn²⁺, whose spectrum consists of two emission peaks and has a wider FWHM. The four weaker emission peaks are considered to be due to the ⁵D₀→⁷F_J (J = 1,2,3,4) transitions of Eu³⁺. This might result from the residual composition Eu₂O₃ during the mass synthesis procedure and we think that this could be negligible because it possesses only about 2% intensity of the main emission peak as is shown in Fig. 2(a). The luminescent image during CL measurement was recorded and a typical one is shown in the inset (top right) of Fig. 2(a), from which one can observe it emits intense cyan light.

A schematic diagram of emission mechanism for LCSO:Eu²⁺ could be described below. As the primary electrons hit phosphor layer surface, they would induce ionization processes, which in turn generate highly energetic electrons. These energetic electrons can be further multiplied in number through collisions, creating secondary electrons. These secondary electrons can migrate in the solid with high kinetic energy and could excite Eu²⁺ from 4f ground state to 5d excited state. When it returns to 4f ground state by radiative transition, cyan emission locating at 480 nm would occur. At the same time, this process would be accompanied by ⁵D₀→⁷F_J (J = 1, 2, 3, 4) transitions of Eu³⁺ due to the residual matter during the synthesis process.

Figure 2(b) shows CL intensity of LCSO:Eu²⁺ phosphors measured under different anode voltages when the anode current density is 50 μA/cm². The result from Mg₂SnO₄:Ti⁴⁺,Mn²⁺ is also shown for comparison. The CL intensities of both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphors increase with anode voltages. The CL integrated intensity of LCSO:Eu²⁺ is much larger than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ under same anode voltage. The ratio of CL integrated intensity between LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor screens increases with anode voltages. For instance, the ratios are 1.2, 1.9, 2.9, 3.6, 4.6 and 5.5 under the bombardment of electron beams of 2 keV, 3 keV, 4 keV, 5 keV, 6 keV and 7 keV, respectively. To find the physical origin, we make the following analysis.

We calculate the electron penetration depth (L(Å)) for both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphors using the following empirical Eq.:

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) [31]. The results are shown in the top left inset of Fig. 2(b). For LCSO:Eu²⁺, ρ = 2.90 g/cm³, Z = 72, A = 146.00, thus the estimated electron penetration depths at 1 kV, 3 kV, 5 kV and 7 kV are 4.8 nm, 84.2 nm, 318.0 nm, 763.0 nm, respectively. The electron penetration depths for Mg₂SnO₄:Ti⁴⁺,Mn²⁺ were also estimated and shown in the inset (top left) of Fig. 2(b). For Mg₂SnO₄:Ti⁴⁺,Mn²⁺, ρ = 4.77 g/cm³, Z = 106, A = 231.33, thus the estimated electron penetration depths at 1 keV, 3 keV, 5 keV and 7 keV are 1.4 nm, 33.6 nm, 148.5 nm, 395.0 nm, respectively. As the anode voltage increases, the penetrations of electron beam into both phosphor layers increase too, so more light would be emitted for both phosphor layers.

The results in the inset (top left) of Fig. 2(b) also show that the electron penetration depth of LCSO:Eu²⁺ is larger than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺. To improve the electron penetration depth, phosphor materials with lower ρ and Z are preferred [12]. The ρ and Z value for LCSO:Eu²⁺ are lower than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺. This is the reason to the larger electron penetration depth of LCSO:Eu²⁺ phosphor. Also, with the increase of anode voltage, the difference on electron penetration depth between LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ increases too. For example, differences on electron penetration depth are 3.4 nm, 50.6 nm, 169.5 nm and 368.0 nm when the anode voltage is 1 kV, 3 kV, 5 kV and 7 kV, respectively. The larger electron penetration depth is one reason for the better CL performance of LCSO:Eu²⁺.

Figure 3(a) shows the average luminance as a function of anode current densities for LCSO:Eu²⁺ phosphor screen under different anode voltages. The luminance of LCSO:Eu²⁺ phosphor screen increased with the anode current densities, luminance saturation phenomenon was observed only when anode current density went to as high as 2.8 mA/cm². The highest luminance could reach 12000 cd/m² under V_a = 7 kV and J_a = 2.8 mA/cm². The highest luminance at 5 kV is about 4800 cd/m² (3 mA/cm²), which shows LCSO:Eu²⁺ phosphor has good luminance even under relatively low excitation voltage. For comparison, we also measured the luminescent property of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor screen under the bombardment of electron beams of 7 keV, as shown in Fig. 3(b). The luminance of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor screen begins to saturate when the current density is above 1.25 mA/cm² under V_a = 7 kV. This indicates that LCSO:Eu²⁺ phosphor possesses much higher saturation current density than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor, that is 2.8 mA/cm² to 1.25 mA/cm² under V_a = 7 kV. Under the condition of V_a = 7 kV and J_a = 1.25 mA/cm², the luminance for Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor screen is only about 650 cd/m² while the luminance for LCSO:Eu²⁺ phosphor screen can be 9 times higher, i.e. 6000 cd/m². This means that LCSO:Eu²⁺ phosphor is more efficient than Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor.

 

Fig. 3 (a) Luminance of Li₂CaSiO₄:Eu²⁺ phosphor screen as a function of anode current density under different anode accelerating voltages. (b) Luminance of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor screen under 7 kV; (c) Decay time for Li₂CaSiO₄:Eu²⁺. (d) Decay time for Mg₂SnO₄:Ti⁴⁺,Mn²⁺.

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Shorter decay time is much helpful to increase saturation current density of phosphor due to more activator recycling frequency according to some researchers’ results [12,32–34]. To find physical reasons responsible for higher saturation current density in LCSO:Eu²⁺ phosphor, we measured decay time of both phosphors. And we found that LCSO:Eu²⁺ phosphor possesses much shorter decay time than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor. The decay time of LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ is 429 ns and 6.54 μs, respectively as Fig. 3(c) and Fig. 3(d) show. Our results match well with the theoretical one. Thus, we could propose that high saturation current density in LCSO:Eu²⁺ is mainly due to the shorter decay time observed. Therefore, the LCSO:Eu²⁺ cyan phosphor is more efficient and more suitable for high energy device application.

The aging properties for both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ were also measured and the results are shown in Fig. 4 . The measurement was carried out under the condition of anode voltage of 7 kV and anode current density of 50 μA/cm². Figure 4 gives the normalized CL peak intensities of these two phosphor screens as a function of bombardment time. It could be seen that LCSO:Eu²⁺ has a slower decay rate than Mg₂SnO₄:Ti⁴⁺,Mn²⁺. After being bombarded by persistent electron beam for 2.5 hours, LCSO:Eu²⁺ could retain about 64.7% of the initial intensity, while Mg₂SnO₄:Ti⁴⁺,Mn²⁺ could only keep about 48.6% of the initial value. For contrast, the result of degradation for commercial blue phosphor ZnS: Ag, Cl (IRICO Group Corp.) was also presented in Fig. 4 which was measured under the same condition and 40% of the initial intensity was retained. The better degradation properties of LCSO:Eu²⁺ is apparently due to the stable nature of oxide host. Therefore a long lifetime is expected for LCSO:Eu²⁺ when applied in FED.

 

Fig. 4 Comparison of the degradation properties of Li₂CaSiO₄:Eu²⁺, Mg₂SnO₄:Ti⁴⁺,Mn²⁺ and ZnS: Ag, Cl phosphor screen under constant electron bombardment (V_a = 7 kV and J_a = 50 μA/cm²).

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To find the intrinsic reason responsible for better degradation property of LCSO:Eu²⁺ phosphor, we measured photoluminescence emission spectra for both phosphors under different temperature ranging from 25 °C to 200 °C which is close to temperature range in the application of FED device as well as LED device. Figure 5(a) and Fig. 5(b) show the emission spectra of both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphors under 288 nm and 244 nm excitation, respectively. One could see clearly that FWHM of both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphors increase with the temperature rising, which is attributed to the population of higher vibration levels, the density of phonons, and the probability of non-radiative transfer (energy migration to defects) brought by the increasing temperature [35,36].

 

Fig. 5 Comparison of the emission spectra for (a) Mg₂SnO₄:Ti⁴⁺,Mn²⁺ and (b) Li₂CaSiO₄:Eu²⁺ as well as (c) dependence of the integral emission intensity on temperature for both phosphors.

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Figure 5(c) represents the dependence of the integral emission intensity on temperature for both phosphor screens. The emission intensity of both LCSO:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor gradually decrease with the increase of temperature due to the thermal quenching effect as shown in Fig. 5(c). Furthermore, when the temperature rises up from 25 °C to 200 °C, Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor could only maintain about 35% of the initial value, while LCSO:Eu²⁺ phosphor could keep above 60% of the initial value. Namely, the former one suffers a more serious thermal quenching, and this could be the main reason for the better stability for LCSO:Eu²⁺ phosphor. In both FED and LED devices, thermal quenching is a vital factor to succeed commercial products with longer lifetime. So in this case, LCSO:Eu²⁺ phosphor is still more advantageous than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor in the application of FED and LED.

Besides, we carried out electrical conductivity measurement on these two phosphors to further understand the reason of higher saturation current density and better degradation property observed from LCSO:Eu²⁺ phosphor screen. The inset of Fig. 6(a) shows the set-up of using a microprobe to measure the electrical properties of a phosphor particle. From the I-V characteristics in Fig. 6, we observed that the resistance of LCSO:Eu²⁺ is much lower than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺. For example, when the voltage reached 300 V, the current was about 2 pA for LCSO:Eu²⁺ phosphor as shown in Fig. 6(a), while it was only about 1 pA for Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor as Fig. 6(b) shows. From the linear part of the I-V curve, we calculated the effective resistance to be 1.5 × 10⁸ MΩ for LCSO:Eu²⁺ and 3 × 10⁸ MΩ for Mg₂SnO₄:Ti⁴⁺,Mn²⁺. Namely, the resistance of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ is higher than that of LCSO:Eu²⁺. Thus, when the electrons bombard the phosphor, the charge-up phenomenon is more liable to happen for the phosphor with higher resistance. Therefore, we would expect high saturation current density from phosphor with high conductivity. This is in accord with the results we observed from LCSO:Eu²⁺ phosphor.

 

Fig. 6 I-V characteristics measured from (a) Li₂CaSiO₄:Eu²⁺ and (b) Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor particles using a microprobe. Bottom right inset shows one SEM picture of the microprobe on a Li₂CaSiO₄:Eu²⁺ phosphor particle.

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The CIE coordinates of LCSO:Eu²⁺ phosphor are calculated to be x = 0.108, y = 0.200. The CIE coordinates are shown in Fig. 7(a) , which locates outside the color gamut triangle of CRT (Y₂O₃:Eu, ZnS:Cu,Al, ZnS:Ag,Cl) and FED tricolor (Y₂O₃:Eu, Y₂SiO₅:Tb, Y₂SiO₅:Ce) phosphors. The area of color gamut after the introduction of LCSO:Eu²⁺ phosphor is calculated to be 17.6% larger than that of CRT tricolor and 30.1% larger than that of FED tricolor [12]. While for Mg₂SnO₄:Ti⁴⁺,Mn²⁺ cyan phosphor, the CIE coordinates are x = 0.146, y = 0.287. The increase of the area of color gamut is about 14.3% larger than that of CRT tricolor and 27.9% larger than that of FED tricolor [12]. Besides, from Fig. 7(b), we could see clearly that CIE coordinates of LCSO:Eu²⁺ phosphor is much closer to the spectral locus than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor, therefore, we could expect higher color purity in LCSO:Eu²⁺ phosphor. It is obvious that LCSO:Eu²⁺ phosphor is so far the best cyan phosphor for achieving the goal of widening color gamut of FED display.

 

Fig. 7 (a) CIE coordinates of Li₂CaSiO₄:Eu²⁺ phosphor in a CIE 1931 chromaticity diagram together with CIE coordinates of CRT and FED tricolor phosphors. (b) CIE coordinates comparison in FED tricolor phosphors for both Li₂CaSiO₄:Eu²⁺ and Mg₂SnO₄:Ti⁴⁺,Mn²⁺ phosphor.

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4. Conclusion

The performance of LCSO:Eu²⁺ cyan-emitting phosphor was characterized under conditions similar to FED device. High luminance (about 12000 cd/m² under V_a = 7 kV and J_a = 2.8 mA/cm²) was obtained. Higher saturation current density for LCSO:Eu²⁺ phosphor is obtained due to its shorter decay time than that of Mg₂SnO₄:Ti⁴⁺,Mn²⁺. Better luminescent performance as well as better stability property from LCSO:Eu²⁺ phosphor was observed compared with Mg₂SnO₄:Ti⁴⁺,Mn²⁺. One main reason for better stability property of LCSO:Eu²⁺ phosphor is due to less thermal quenching according to our experimental result. Another one is its higher conductivity. CIE coordinates of LCSO:Eu²⁺ phosphor show that it has purer color and that it can widen the color gamut of FED display. Due to its excellent characteristics, LCSO:Eu²⁺ phosphor is a most promising candidate for four-color FEDs.