1. Introduction

Recent disasters, e.g., great earthquakes or terroristic explosions, have raised requirements for emergency signs and illuminators that glow independently of electric power sources [1, 2]. Conventional ZnS-based phosphors, which have long been used on clockfaces, cannot be used for illumination, since they are insufficient in both brightness and afterglow duration. Recently, bright, long-duration phosphorescence has been realized with rare-earth doped strontium aluminum oxides [3]. These oxide phosphors shine brightly enough to allow one to recognize faces or to read words on paper [4]. However, their phosphorescence is restricted in the short wavelength range from violet to green. Blue or green illumination with these phosphors creates eerily uncomfortable environments in which one feels as if in a haunted house. If illumination with warm colors (orange or red) is provided, people who are trapped in a building or a vehicle will feel a sense of security. Further, if phosphors of various colors are available, one can improve visibility of emergency signs [5, 6], which currently exhibit only low-contrast images due to limited color combinations.

Until now, red phosphorescence has been realized with oxides that are doped with various metals, e.g., Eu, Mg, Ti, Tb, or Mn [7–10]. However, these red phosphors are inferior to the blue or green phosphors in both brightness and afterglow duration. In previous works, we demonstrated that weak phosphorescence could be converted efficiently to strong fluorescence of a different color by using the down-conversion characteristic of organic dyes [11, 12]. This phenomenon seems to be useable to synthesize full-color phosphors. In this work, we mixed red dyes with green or blue phosphors, and fabricated acrylic resins that could emit various colors extending over the entire visible spectral range.

2. Red phosphorescence

Figure 1(a) shows afterglow spectra of typical phosphors that are currently available for purchase [9, 13, 14]. The phosphor compositions were CaAl₂O₄:Eu,Nd (violet),Sr₄Al₁₄O₂₅:Eu,Dy (blue), SrAl₂O₄:Eu,Dy (green), and Y₂O₂S:Eu,Mg,Ti (red). These phosphor particles (average size: 10-20 μm) were dispersed in a resin at 5 wt. %. Each resin (thickness: 5 mm) was exposed to a fluorescent lamp for 5 min, and the phosphorescence was measured with a multichannel spectrometer (Soma, S-2300) ~1 s after the lamp had been turned off. The red phosphor exhibited several phosphorescence peaks including a pronounced peak at 630 nm wavelength. However, the total phosphorescence intensity of the red phosphor was lower than those of the other phosphors because of its narrow peak width. In addition, the red phosphorescence decayed very rapidly, and hence, the phosphorescence became almost invisible in ~30 min after the lamp had been turned off. By contrast, the green and blue phosphors continued to glow for a few hours. The violet phosphor looked faint, since the human eye became less sensitive at the short wavelength range.

 

Fig. 1. (a) Emission spectra of violet, blue, green, and red phosphors. (b) Absorption (thick curves, left axis) and emission spectra (thin curves, right axis) of organic dyes (rhodamine 6G,rhodamine B, and kiton).

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It is well known that full-color display is achievable by superimposition of red, green, and blue (RGB) light [15]. The key to development of full-color illumination is, therefore, to synthesize a red phosphor whose decay time is comparable to those of the green and blue phosphors. Since laser emitting dyes, e.g., rhodamine and kiton, have a high red-conversion efficiency, mixtures of the dyes and phosphors are expected to emit bright fluorescence as long as the green or blue phosphor glows. The thick curves in Fig. 1(b) show the absorption spectra of some typical red dyes, i.e., rhodamine 6G, rhodamine B, and kiton [16]. The absorption coefficient was evaluated from the difference in transmittances that were measured for methanol solutions of 0-0.1 mM (1×10^-4 mol/l). The absorption bands of these dyes coincided with the emission band of the green phosphor, and partly overlapped the emission band of the blue phosphor. When we mixed the green phosphor and these dyes, we could observe red or orange fluorescence, as shown by the thin curves in Fig. 1(b).

We used photocurable acrylate (Maruto, #4111) [17] as a matrix to suspend phosphor particles and dye molecules. First, a methanol solution of a dye was mixed with the raw solution of the acrylate at the volume ratio of 1:20. Dye concentration in the acrylate was adjusted between 0.025 and 5 mM by changing dye concentration in the methanol solution. Next, phosphor particles were dispersed in the mixed solution so that the concentration became 1-10 wt. %. Finally, the mixed solution was poured into a polypropylene mold (inner size: 30-mm diameter and 5-mm depth), and was exposed to a blue fluorescent lamp (Matsushita, FML27EB, emission band: 400-600 nm, peak wavelength: 450 nm) for solidification. Solidification was complete in 2-4 h depending on dye and phosphor concentrations.

Figure 2(a) shows photographs of resins emitting red or orange afterglow in darkness.One resin (the lower right sample in the photograph) contained the red phosphor of 5 wt. % [9], whose emission spectrum was shown in Fig. 1(a). The other resins contained the green phosphor of 5 wt. % [13] together with an organic dye, i.e., rhodamine 6G (0.5 mM), rhodamine B (1.5 mM), or kiton (1.5 mM) [16]. We used three different fluorescent lamps for excitation; i.e., the abovementioned blue lamp, a white lamp (Toshiba, N18, emission band: 400-700 nm, peak wavelength: 570 nm), and a day-light lamp (Toshiba, D-EDL-D65, emission band: 350-750 nm, peak wavelength: 500 nm). However, the lamp type did not affect the comparison of phosphors described below. The excitation time was fixed at 30 min, since the afterglow intensity increased little even if the excitation time extended to over 5 min [12]. These photographs were taken 2 s or 2 min after stop of excitation. The afterglow from the red phosphor decayed rapidly, and became almost invisible in 30 min. By contrast, the dye-doped resins continued to glow for a long time; i.e., we could recognize the afterglow in darkness for 2-3 h. The resin with rhodamine B shined most brightly of all samples.

 

Fig. 2. (a) Photographs of afterglowing resins. The lower right sample contained red phosphor (5 wt. %). The other samples contained green phosphor (5 wt. %) and an organic dye; i.e.,rhodamine 6G (0.5 mM), rhodamine B (1.5 mM), or kiton (1.5 mM). The samples were excited for 30 min in a dark room, and the photographs were taken 2 s or 2 min after stop of excitation.(b) Afterglow decay process of the samples shown in (a). The top curve shows data that were measured for the resin with green phosphor (5 wt. %). The horizontal axis shows time after stop of excitation. A blue phosphorescent lamp was used for excitation.

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We measured the decay process by placing the Si detector (detection area: 6-mm square) of an optical power meter (Ando, AQ-2125) at 5-mm distance from the sample surface. Figure 2(b) shows irradiance that was measured after the excitation lamp had been turned off. The decay curves did not follow the simple exponential function; i.e., the time constant τ (1/e decay time) decreased as time passed. The red phosphor (the lowest curve) exhibited a fast decay with τ≈2 min in the first ten minutes, but the decay time extended to ~30 min when 60 min passed. Being compared to the red phosphor, the dye-phosphor mixture exhibited a longer decay time, i.e., τ≈3 min in the first ten minutes and ~60 min after the 60-min passage. It was plausible that the afterglow of the mixtures decayed with the same time-constant as that of the green phosphor (the top curve), since the dyes emit fluorescence immediately after the phosphorescence emission; i.e., dye molecules emit fluorescence in ~10 ns (the lifetime of the excited state) after they are excited through phosphorescence absorption [11, 18]. When 90 min passed after stop of excitation, irradiance of the red phosphor decreased to 0.0012 nW/mm². By contrast, the dye-phosphor mixtures still emitted fluorescence with irradiances of 0.009 (rhodamine B), 0.005 (kiton), or 0.004 nW/mm² (rhodamine 6G) at that moment. These results indicated that the afterglow intensities of the dye-phosphor mixtures were three to seven times stronger than that of the red phosphor. Since irradiance of the green phosphor was 0.016 nW/mm² at the same moment (90 min), color-conversion efficiencies of these mixtures, i.e., irradiance ratio of the dye fluorescence to the green phosphorescence, were evaluated to be ~60%, ~30%, and ~25%, respectively.

3. Full-color emission

Being based on the results in Section 2, we fabricated a variety of resins that contained green or blue phosphors and organic dyes. Concentrations of the phosphors and dyes were varied systematically to examine change in emission colors. Figures 3(a) and 3(b) show photographs of samples that contain both green (5 wt. %) and blue phosphors (1, 5, and 10 wt. %). The samples also contained a dye (rhodamine 6G or kiton) of various concentrations. When the dye concentration was low (0.025 or 0.05 mM), dye molecules absorbed phosphorescence too little to affect the emission color. Consequently, colors between blue and green were visible depending on the ratio of the two phosphors (samples a, f, k, A, B, F, G, K, and L).

 

Fig. 3. (a), (b) Photographs of afterglowing resins that contain green and blue phosphors. The resins also contain rhodamine 6G (0.05-5 mM) or kiton (0.025-2.5 mM). The resins were excited by a fluorescent lamp for 5 min in a dark room, and these photographs were taken 1-2 s after stop of excitation. (c), (d) Emission spectra of samples a-e and L-O.

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As Fig. 3(a) shows, the blue or green color changed to yellow or orange with increase in concentration of rhodamine 6G. Figure 3(c) shows emission spectra of samples a-e. The spectrum of sample a was a superimposition of the two emission peaks that originated from the blue and green phosphors. This spectrum was modified in sample b, since rhodamine 6G absorbed phosphorescence below 550 nm wavelength and emitted fluorescence in the 550- 650 nm range, as shown in Fig. 1(b). Accordingly, sample b exhibited an yellow-green color. Sample c exhibited an yellow color, since phosphorescence below 550 nm was absorbed almost completely. As the dye concentration increased further, the emission peak shifted to a longer wavelength, and consequently, samples d and e exhibited an orange color.

Figure 3(d) shows emission spectra of resins that contained kiton (samples L-O). Sample M exhibited a bluish-white color, since its emission spectrum extended over the entire visible spectral region. Sample N exhibited a violet color that was created by superimposition of blue and red emissions. A red color was attained with sample O, in which phosphorescence below 580 nm disappeared due to absorption by kiton.

We fabricated and examined a variety of samples in this manner. Concentration of the organic dyes (rhodamine 6G, rhodamine B, and kiton) was varied between 0.025 and 5 mM. Concentration of the blue or green phosphors was varied between 0-10 wt. % (the total phosphor concentration: 0-20 wt. %). The violet phosphor, whose emission spectrum was shown in Fig. 1(a), was also used in some samples, but, as described below, no bright phosphorescence was attained with them. We selected 49 resins from the fabricated samples to create full-color illumination in darkness. Figure 4 shows photographs of the selected resins. These photographs were taken in (a) a bright room or (b) a dark room. The resins were arranged so that they constructed a hue circle in darkness. The three resins in the lower right corner, whose major component was the violet phosphors, were difficult to recognize in Fig. 4(b), although we could observe clear violet emission with the naked eye. This discrepancy seemed to be caused by the weak sensitivity of our digital camera (Canon, Power Shot G3) in the short wavelength range. One should note that the images in Fig. 4 are somewhat different from actual images that are observed with the naked eye, since the characteristics of cameras, displays, and printers affect the color quality.

 

Fig. 4. Photographs of resins that contain phosphors and dyes. Photographs were taken in (a) a bright room or (b) a dark room (~5 s after stop of excitation).

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As Fig. 4(b) shows, various colors from violet to red could be obtained by mixing the blue and green phosphors with one of the three organic dyes. There was a tendency that the dye concentration increased from the left samples to the right samples in the photograph. The ratio of the blue phosphor to the green phosphor increased from the upper samples to the lower samples.

4. Discussion

The green and blue phosphors have similar optical properties to each other, since they have a similar chemical composition, i.e., rare-earth doped strontium aluminum oxides (Section 2). However, the red phosphor is composed of different elements, and hence, its optical properties are different from those of the green and blue phosphors. As the top and bottom curves in Fig. 2(b) show, the irradiance ratio of the red phosphorescence to the green phosphorescence was ~1/5 at 10 min, and it decreased to ~1/13 at 90 min. If the irradiance ratio is constant independent of the time passage, brightness of various colors can be equalized by adjusting phosphor concentrations suitably. However, the color-balance cannot be maintained, if the irradiance ratio of different colors changes temporally due to difference in the decay time. Coincidence of the decay time is, therefore, an important issue in fabricating a color display. From this viewpoint, the dye-phosphor mixtures are more suitable than the conventional red phosphor.

In Fig. 2(b), we expressed the light intensity in irradiance (nW/mm²), since we measured it with an optical power meter. One might think that the irradiance can be converted to a luminance by using the emission spectra and the luminous efficiency spectrum (sensitivity spectrum of the human eye) [15]. As regards the phosphorescence, however, the intensity evaluation based on the eye sensitivity occasionally provides a misleading measure, since the sensitivity spectrum of the human eye changes depending on room brightness; e.g., the sensitivity peak shifts to a shorter wavelength in darkness (the Purkinje phenomenon). We are currently analyzing the phosphorescence spectra to evaluate the afterglow brightness by taking into account these complicated characteristics of the human eye [19].

As Fig. 4(a) shows, the resins with a large amount of dye assumed an orange or red color in a bright room, since the dye emitted strong fluorescence being excited by room illumination. Interestingly, some resins changed their color, when they were put in a dark room. The top three samples at the leftmost column, for example, exhibited an orange color in the bright room, but a green color in the dark room. The color of some other samples changed from red to blue (in the lower right portions of the photographs). This phenomenon can be used for fabricating a display that changes pictures depending on the room brightness [4, 20].

5. Conclusion

Bright red afterglow was attained by using mixtures of green phosphor and organic dyes. Optical power of the red emission was seven times higher than that of the conventional red phosphor, when 90 min passed after an excitation lamp had been turned off. Phosphorescent resins were fabricated by photocuring of acrylate in which phosphors and dyes were dispersed uniformly. Colorful emission from violet to red was attained with a group of resins that had suitable phosphor and dye concentrations. These resins will be useful for fabrication of fullcolor displays or illuminators that act independently of electric power sources in an emergency.