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Abstract
A series of luminescent composites was prepared by coating the surface of long-lasting phosphorescent SrAl2O4: Eu2+ ,Dy3+ (SAOED) compounds that emit yellow-greenish light, with various concentrations of red emitting coumarin (REC) fluorescent dye. The color of the luminescent SiO2/REC@SAOED composites, including the temporal hue shift in darkness, was characterized using photoluminescence (PL) emission spectra and spectroradiometric measurements. The PL emission spectra of the pAosphorescent/fluorescent composite contains a continuous band ranging from 450 to 700 nm with two emission peaks. The emission peak for the uncoated SAOED remained invariant, while the fluorescent emission peak exhibited a bathochromic shift from 607 to 618 nm when the concentration of REC in the composite was increased from 0.1 to 0.9 (wt%), indicating that the hue of the luminescent composite would gradually shift towards red with increasing concentrations of coumarin. Although the intensity of the fluorescent peak increased gradually with an increase in the concentration of REC, the ratio of the phosphorescent to fluorescent emission peak intensity also increased gradually over time. Spectroradiometric measurements of compounds showed that the hue of the emitted light from the composites exhibited a gradual hypsochromic (blue) shift over time in darkness, and the extent of the blue-shift decreased gradually with an increase in the concentration of REC.
© 2017 Optical Society of America
1. Introduction
SrAl2O4: Eu2+, Dy3+ (SAOED) is a yellowish-green light emitting long-lasting photo-luminescent compound that is primarily used for the manufacture of products that require luminescence [1–3]. Because of its high luminescence intensity, long luminescence duration, and recyclability, SAOED has also been added into certain polymer dopes to spin luminescent fibers [4-5]. Since the production of yellow-green light emitting polyester based luminous fibers [6], applications have included clothing, decorative textiles and stuffed toys [7]. However, the color of these luminescent fibers is limited to yellow-green thus limiting their potential application. Moreover, yellow-green cool-toned colored products, in contrast to yellow, orange and red warm-toned colors, are less popular for certain applications. In recent years researchers have attempted to produce luminescent fibers that emit long lasting warm-toned colors. Zhu et al. added red fluorescent pigments into the fiber dope to make luminous fibers that emitted red-shifted colors [8]. They noted that while both the emission wavelength and luminescent color of the fiber exhibited a bathochromic shift, the luminescence intensity decreased greatly compared to the original compound.
Over the years a series of warm-toned luminescent materials has been synthesized including the yellow light emitting Ca6BaP4O17:Eu2+, Ho3+ [9], orange-reddish light emitting SrSnO3:Sm3+ [10] and red light emitting ZnGa2O4:Cr3+ [11]. However, none of these compounds were found to be suitable for the production of luminous fibers because compared to the yellow-green emitting SAOED they show low luminescence intensity and much shorter luminescence duration.
Desired luminescent colors may be obtained by adjusting the proportion of emitted lights from appropriate amounts of suitable red, green and blue primaries. The colorimetric properties of luminescent SiO2/REC@SAOED composites were thus adjusted by combining the yellow-green light emitted from long lasting phosphorescent SAOED with light from red emitting coumarin (REC) fluorescent dye. The formation of these composites is based on coating the surface of SAOED with various amounts of REC and SiO2 [12-13]. In this composite system, the yellowish-green luminescent light from SAOED acts as the excitation light source for the REC resulting in the emission of red fluorescent light. As a consequence, the overall color of emitted light from the SiO2/REC@SAOED composite is composed of the additive mixture of the phosphorescent color of SAOED and the fluorescent color of REC. However, the phosphorescence of SAOED follows an exponential decay once the excitation source is removed, leading in turn to the simultaneous decay of the red fluorescent light emitted from REC. Due to the difference in the excitation as well as decay functions of the two light components in the composite system the overall color of the emitted light from the composite changes with time.
As a novel approach to production of color-tunable long-lasting luminescent material, SiO2/REC@SAOED phosphorescent composites show great potential, including applications in the production of luminous fibers. Characterization of the temporal hue shift of the composites is important since it can enable the generation of composites with desired initial color as well as tuning their ‘decay color’ after removing the excitation source. In this study the concentration of REC coating on SAOED was varied and the shift in the emitted color of the SiO2/REC@SAOED composite was investigated in darkness using the photoluminescence emission spectra and by measuring the radiometric attributes of the luminescent compounds.
2. Materials and method
2.1 Preparation of samples
The SiO2/REC@SAOED phosphorescent composite was prepared by coating a layer of red emitting coumarin (REC) fluorescent dye together with SiO2 on the surface of yellow-green light emitting long lasting phosphor SrAl2O4: Eu2+, Dy3+ (SAOED). The synthesis of the REC and SAOED is described elsewhere [12–14]. The detailed preparation process of the SiO2/REC@SAOED was as follows.
First, a mixture of H2O2 and H2SO4 (VH2O2/VH2SO4 = 3/7) was used to acidify SAOED phosphor and increase the amount of available hydroxyl groups. Next, 3.47 g tetraethylorthosilicate (TEOS) was added into a mixture of ethanol and deionized water maintained at 60°C in a water bath and stirred for 30 min. The molar ratio of TEOS to ethanol and water was controlled at 1:15:35. Diluted H2SO4 was used to adjust the pH value of the TEOS/ethanol/water mixture to 2. Then 10 g of the already prepared SAOED phosphor and the desired amount of REC (wt REC% = 0.1, 0.3, 0.5, 0.7, 0.9) were added into the above mixture while stirring and the water bath was kept at 60°C. Finally, after the mixture of SAOED phosphors and REC swelled and showed a wet gel-like appearance stirring was stopped and the mixture was transferred to an oven at 60°C and dried for 12 h to obtain the final phosphorescent composite.
2.2 Characterization
The photoluminescence (PL) emission spectra of samples were recorded using a Fluorolog-3 spectrofluorometer (HORIBA Jobin Yvon Inc.). The SiO2/REC@SAOED phosphorescent composite samples were also placed inside a DigiEye illumination chamber and excited with the D75 daylight simulator light source for 5 min. The light source was then switched off and photographs or radiometric measurements of the luminescent composites were taken over the decay period. The colorimetric attributes of samples (including their luminance, and tristimulus X, Y, Z values) were obtained using a PR670 spectroradiometer. The radiometer was placed on the measurement port on top of the DigiEye chamber using the same geometry to that of the Nikon D90 camera, as shown in Fig. 1. During the measurement the camera/radiometer and all openings were shielded to minimize the effect of ambient lighting on dark condition measurements. All measurements were carried out at room temperature and all external lights were switched off.
Fig. 1 , The PR670 spectroradiometer (a) and Nikon D90 camera (b) mounted on the illumination chamber used for characterization of samples.
3. Results and discussion
3.1 PL emission spectra analysis
The emitted color of the SiO2/REC@SAOED composite is the result of the phosphorescent color of SAOED and fluorescent color of REC, which changes over time after removing the excitation source. The temporal shift in the color of the composite samples was therefore characterized. The PL emission spectra of samples were obtained over 800 seconds, with an interval of 100 seconds, by first exciting samples at 375 nm and then removing the source of excitation. Results are shown in Fig. 2a. Due to the 4f65d1 to 8S7/2 4f7 transition of Eu2+ situated at one of the two Sr2+ sites in the lattice of SAOED, the PL emission spectrum of the uncoated SAOED consists of a continuous broadband signature from 450 to 650 nm with a single emission peak at 525 nm, corresponding to its characteristic yellowish-green phosphorescent color [15]. Although the intensity of the spectra decreases gradually over time, the emission peak wavelength does not shift and thus the hue remains unchanged. In the case of the SiO2/REC coated samples the PL emission spectra cover a broader range from 450 to 700 nm, as shown in Fig. 2(b-f). The spectra of the coated composites contain two peaks, a strong peak at 525 nm corresponding to the original SAOED and a weaker peak around 607-618 nm. Increasing the concentration of SiO2/REC coating on the surface of SAOED increases the relative intensity of the second peak whilst decreasing that of the first peak. In addition, the second peak undergoes an apparent bathochromic (red) shift from 607 to 610, 613, 617 and 618 nm for SiO2/REC concentrations corresponding to 0.1, 0.3, 0.5, 0.7 and 0.9 wt%, respectively.
Fig. 2 , The temporal PL emission spectra of various SiO2/REC@SAOED composites over 800s; (a): uncoated SAOED, (b-f): wt REC% = 0.1, 0.3, 0.5, 0.7, and 0.9, respectively.
The perceived color of the emitted light from SiO2/REC@SAOED samples is the result of the additive mixing at 525 nm and 607-618 nm corresponding to the first and the second peaks of the emitted light. The ratio of the intensity of spectra for these peaks (Ina/Inb) can be used to characterize the color shift of composite samples over time. Results are shown in Fig. 3, where Ina/Inb gradually increases with time. Therefore, the emitted light from SAOED becomes an increasingly important component of the overall color of the composite after removing the source of excitation. In addition, the Ina/Inb is reduced with an increase in the concentration of REC also demonstrating the important role of phosphorescence at lower concentrations of REC and the increased role of fluorescence at higher concentrations of REC on the overall color of the emitted light. Overall, the color of the luminescent light is temporally variant and undergoes a gradual hypsochromic shift with time.
Fig. 3 , The ratio of PL peaks (Ina/Inb) at 525 and 607-618 nm for composites coated with various conc. of REC over 800s; (wt REC% = 0.1, 0.3, 0.5, 0.7, 0.9).
3.2 Analysis of colorimetric and radiometric attributes of composite samples
3.2.1 CIELAB color attributes
The colorimetric and radiometric attributes of SiO2/REC@SAOED composites were also characterized. Two kinds of analyses were performed. In the first approach, a D75 daylight simulator was used as the reference white after characterizing its spectral power distribution with a PR670 spectroradiometer. The tristimuls values of the light source were normalized and the colorimetric attributes of the reference white in the CIELAB space were obtained (X = 48, Y = 100, Z = 23.6). The L*a*b* values of composite compounds were then obtained based on normalizations against the measured reference white in the CIELAB space.
The temporal color shifts of the uncoated SAOED and SiO2/REC coated samples were then characterized in the CIELAB space as displayed in Fig. 4. Results in Fig. 4 (a) show that the L* drops continually over time, as expected due to the exponential decay of the luminescent light, and that the color of the uncoated SAOED is yellowish-green. After coating samples with SiO2/REC the color initially shifts towards red and then gradually towards blue as shown in Fig. 4 (b-f). Specifically, when the concentration of REC exceeds 0.5 wt%, the a* attribute of the sample is positive and then gradually moves towards the b* axis and finally becomes zero, indicating that its color is reddish-yellow first and then becomes yellower and finally fades away after luminescence decay. The reason, as indicated previously, is due to the luminescence mechanism of the composite which contains both phosphorescent as well as fluorescent components. Immediately after removal of the excitation source the yellow-green luminescence of SAOED is much stronger than the red fluorescence of REC and thus the emitted light appears as yellowish-green. However, over the luminescence decay period of SAOED, and especially at higher concentrations of REC coatings, increased fluorescence from REC leads to a shift in the perceived color towards red. This is shown more clearly in temporal variations of L*, a* and b* in Fig. 5 (a-c). The emitted light’s temporal hue shit occurs in two directions. First, a* increases gradually whilst b* decreases with a reduction in luminance. Next, a* decreases, while b* increases gradually with the continued reduction in luminance. Thus, the final color is a combination of a red-shift and a blue-shift. Moreover, the initial reduction in b* is larger than the increase in a*, over the measurement period of 800s, implying that the hypsochromic shift is stronger than the bathochromic shift thus the overall color of the luminescent composites shifts towards blue with the luminescence decay in a dark environment. However, with an increase in REC the decrease in b* becomes smaller with respect to the increase in the redness indicating that the composites overall blue-shift is gradually reduced. With further reduction in luminescence both b* and a* shift towards zero as the luminescence decay proceeds. Figure 5a also shows that the overall luminance of the composite samples decreases exponentially over time and with an increase in the concentration of the REC coating. The much stronger phosphorescent light from the SAOED is absorbed by, and excites, the REC coating to generate the associated fluorescent light. Thus, an increase in the amount of REC results in a reduction in the luminance of the SAOED in comparison to other coated compounds in similar decay periods, and the reduction in luminance is larger for higher concentrations of REC coating.
Fig. 4 , The L*a*b* of the SiO2/REC@SAOED samples coated with various concentrations of REC over 800s; (a): uncoated SAOED, (b): wt REC% = 0.1%, 0.3%, 0.5%, 0.7%, and 0.9% respectively.
Fig. 5 , Temporal variations of L*a*b* for SiO2/REC@SAOED samples coated with various concentrations of REC over 800s; (a) lightness, (b) yellowness-blueness and (c) redness-greenness for SAOED and wt REC% = 0.1, 0.3, 0.5, 0.7, and 0.9 composites respectively.
3.2.2 Relative CIEDE2000 color difference
The analysis of the L*a*b* values indicates that the composites’ emitted light exhibits an initial red-shift, followed by a blue-shift over time with respect to the D75 reference white source. In the second analysis, the initial luminescent color of the uncoated SAOED was used as the reference for calculation of relative CIE DE2000(1:1:1) values. Measurements were obtained at 20s intervals over a period of 800 seconds. The relative color difference values obtained in this manner provide a more comparable analysis of the color changing trends of the composites. Figure 6 illustrates DE00 values of SiO2/REC@SAOED samples with various concentrations of REC against SAOED immediately after removing the excitation source as the standard and over a period of 800 seconds.
Fig. 6 , The CIE DE2000 (1:1:1) values of the SiO2/REC@SAOED samples coated with various concentrations of REC with wt REC% = 0.1, 0.3, 0.5, 0.7, and 0.9 respectively against SAOED after removing the excitation source over 800s.
Compared with the uncoated SAOED after removing the excitation source, the colors of all samples change rapidly and to a large extent in the first 200 seconds. The magnitude of the difference in this period is approximately 15 DE00 units and adjacent measurements exhibit noticeable changes in 20s intervals. The second period of color change can be considered to be roughly from 200 to 400 seconds where changes of up to 10 DE00 units are noted. In this period the rate of color change is slower and some adjacent measurements indicate similar colors. The last period may be considered to be approximately from 400 to 800 seconds, which shows the smallest color differences against time for each of the samples. Here differences are roughly just noticeable or ≤1 DE00 unit. SAOED exhibited the largest color difference over time.
In order to further demonstrate the temporal color shift of the composites, images of luminescent SAOED samples coated with various concentrations of REC were captured in darkness over 800s, as displayed in Fig. 7. As shown, although the luminance of the uncoated SAOED decays gradually its phosphorescent hue remains roughly unchanged. After coating with various concentrations of REC, the overall hue of the composite samples changes from yellowish-green to reddish-yellow, and becomes redder with increasing concentration of REC. Furthermore, over time the composite samples undergo an apparent blue hue shift when compared to their initial hue immediately after removing the source of excitation (i.e. 0s). These results are consistent with the PL emission spectra as well as the measured radiometric and colorimetric attributes of samples. It should also be noted that the addition of REC produces a more neutral white luminescent color initially, but the decay results in a red tone as noted especially for samples in rows d-f in Fig. 7. The appearance of samples with 0.3% REC (row c) may be considered the most neutral, amongst all compositions, throughout the measurement period.
Fig. 7 , Images of composites coated with various concentrations of REC in darkness over 800s; (a): uncoated SAOED, (b-f): wt REC% = 0.1, 0.3, 0.5, 0.7, and 0.9 respectively.
4. Conclusions
The color characteristics, including the temporal hue shift, of a series of reddish-yellow long-lasting phosphorescent composites SiO2/REC@SAOED, were investigated using PL emission spectra, colorimetric and radiometric measurements including CIELAB and relative CIE DE2000 values. While the intensity of PL emission spectra of the uncoated SAOED decreased gradually over time, the wavelength of the emission peak remained at 525 nm. After coating the phosphorescent sample with different concentrations of red emitting coumarin (REC), the emission spectra of the composites showed an additional peak over 607-618 nm depending on the concentration of REC. The emission peak was stronger for higher concentrations of REC and shifted from 607 to 618 nm for REC concentrations ranging from 0.1 to 0.9 wt%. The ratio of the intensity of the first and the second emission peaks increased gradually over time. The colorimetric and radiometric measurement results of samples show that after coating SAOED with various concentrations of REC, the hue of the composite samples changes from yellowish-green to reddish-yellow, and the extent of the bathochromic shift increases with an increase in the concentration of REC. However, upon removal of the source of excitation the hue of the luminescent SiO2/REC@SAOED samples shows a gradual blue shift over time and the extent of the hypsochromic shift decreases gradually with an increase in concentration of REC. The extent of color variation for all samples was approximately between 12 and 25 DE00 units depending on the concentration of RED. The largest color differences were noted in the first 200 seconds after removing the excitation source and the rate of color change decreased from 200 to 400 seconds and was small in the last 400 seconds of measurement. The biggest calculated temporal color differences were noted for SAOED and the smallest for 0.9% REC. A visual and image based comparison of composites indicates that at 0.3% REC concentration, the emitted light has a relatively neutral white appearance throughout the decay period.
Funding
Ordinary University Graduate Students Academic Degree and Scientific Research Innovation Projects for Jiangsu Province, China (NO. KYLX16_0796).
Acknowledgements
The authors are grateful to Dr. Ahmed El-Shafei and Rui Su at North Carolina State University for their assistance in obtaining PL spectra of samples.
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