Enhancing selectively red spectral region by photonic crystals toward white light emission

Journal of Luminescence 177 (2016) 261–265

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Enhancing selectively red spectral region by photonic crystals toward white light emission Heng Li, Zhaohua Xu n, Woshan Pan Department of Material Technology, Jiangmen Polytechnic, Jiangmen, Guangdong 529090, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 28 October 2015 Received in revised form 18 April 2016 Accepted 2 May 2016 Available online 6 May 2016

The combination of red, green, blue (RGB) dyes and photonic crystals (PCs) is explored for white light application. The results show RGB emission is difficult to program by fluorescence resonance energy transfer, which is impossible to achieve the white light emission. When the PCs is used as an optical substrate, the green and blue spectral components obtain 2.7–3.5 times enhancement compared with the control sample due to large surface area of PCs. More importantly, the enhancement ratio of red spectral component is higher than that of the other color in the case of the stopband of PCs overlapping emission wavelength of Nile red, resulted from the enhanced light extraction of PCs, which makes up for the lack of red light, so that CIE coordinates are approaching to the white light region. The strategy will be of great guideline toward white light emission and play an important role on developing novel lighting devices. & 2016 Elsevier B.V. All rights reserved.

Keywords: Photonic crystals Light extraction Selective enhancement Red emission

1. Introduction Phosphor-converted white light-emitting diodes (LEDs) have received considerable attention because of their potential applications in solid-state lightings and display systems [1]. Three typical combinations exist for the formation of white LEDs: (i) red, green and blue LEDs; (ii) near-ultraviolet LED þred/green/blue fluorescence, and (iii) blue LED þyellow phosphorescence [2]. At present, the most popular commercial phosphors for white LEDs mainly rely on a combination of a blue light source and the YAG:Ce yellow phosphor. This type of LED shows a low color rendering index because of their weak emission in the red spectral region, which will deteriorate the illumination quality and environmental friendliness [3,4]. To overcome the problems, three approaches have been proposed in the literature. One is to develop quantum dots (QDs) considered as promising phosphor converters in white light fabrication due to their outstanding virtues, such as good optical stability, facile color tenability and solution processability, [5,6] but they are hazardous and expensive owing to their difficult synthesis method, and their long-term stability has not been verified [7]. The second method consists in mixing YAG:Ce phosphor with red or orange-emitting phosphors including oxide and sulfide phosphors [8,9]. Although this method has been widely adopted to achieve warm white LED, moisture instability of sulfide, n

Corresponding author. Tel.: þ 86 750 3725266. E-mail address: [email protected] (Z. Xu).

http://dx.doi.org/10.1016/j.jlumin.2016.05.009 0022-2313/& 2016 Elsevier B.V. All rights reserved.

and high thermal quenching at elevated temperatures of oxidebased phosphors are critical problems [10–12]. The third is fluorescence resonance energy transfer (FRET). With the help of partial FRET in a multicomponent donor-acceptor assembly one can be tune the emission property and generate white light emission [13– 18]. An efficient FRET from donor to acceptor needs to fulfill some criteria including, spectral overlap, distance between chromophore, and relative orientation between transition dipoles of the involved molecules. However, obtaining the appropriate spatial organization for efficient energy transfer is a key challenge; a structural matrix is required that furnishes both orientation and proximity between donor and acceptor molecules. Therefore, how to achieve white light with satisfactory luminous quality is still an important and challenging task. As we know, photonic crystals (PCs) have emerged as the most promising candidate in photonics applications owing to its unique properties [19–21]. Near the stopband of PCs, light propagates at reduced group velocity owing to resonant Bragg scattering, which can enhance optical gain leading to stimulated emission [22]. Amplified spontaneous emission has been observed when PCs are used as a matrix for emitters, such as dyes, polymers, semiconductors [23–25]. Importantly, PCs can act as photonic environment to modify the spectral properties of the FRET via stopband [26–29]. Furthermore, the PCs present large surface-to-volume ratio for the effective dispersion of phosphors, which can avoid the concentration quenching effect. Thus, PCs as the excellent optical substrate are a powerful tool to manipulate and improve

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luminescent signal. Herein, we suggest a simple fabrication method for emissive composites composed of PCs as an optical substrate and red, green, and blue (RGB) dyes as fluorophores at various ratios, which enable a facile white light emission via control of interactive energy transfer between dyes. Unlike the conventional use of physically dispersed color dopants, FRET between fluorophores is difficult to program. As the optical substrate, the PCs not only has large surface area, resulting in 2.7–3.5 times enhancement of the green and blue spectral components, but also possesses extraordinary light extraction ability, leading to selective enhancement of red spectral component, which makes up for the lack of red light, so that chromaticity parameters are approaching to the white light region.

2. Experimental 2.1. Materials Styrenes (St), methyl methyacrylate (MMA), arylic acid (AA) were purified by distillation under reduced pressure. The initiator of (NH4)2S2O4 (APS) was recrystallized three times. Milli-Q Water (18.2 MΩ/cm) was used for all experiments. All reagents and materials were purchased from Aldrich unless otherwise noted. 2.2. Fabrication of the crosslinked PCs film Monodisperse latex spheres of Poly(St-MMA-AA) were synthesized via our previous method [30]. The resulting latex spheres were used directly without purification. The polydispersity of the latex spheres was about 0.5%, which was detected by ZetaPALS BI90plus (Brookhaven Instrument). The PCs film was prepared on glass substrates by a vertical deposition method at constant temperature (80 °C) and humidity (80%). After the samples were dry, they were sintered at 85 °C for 30 min to increase the stability of the samples. Photo-crosslinkaged PCs film was fabricated by immersing arylamide solution for illumination of UV light [31]. 2.3. Preparation of RGB films based PCs The mixture of perylene, coumarin 6 and Nile red loaded PMMA films for tuning the white light were prepared by spincoating the chloroform solution of perylene, coumarin 6, Nile red and PMMA mixture (mPMMA: mdyes ¼ 50: 1) onto the crosslinkaged PCs film and glass substrates (as the control sample) at 1200 rpm for 20 s. Proportion of three dyes (perylene, coumarin 6 and Nile red) can be adjusted. 2.4. Characterization The scanning electron microscope (SEM) images were obtained with a field-emission SEM (JEOL JSM-4800, Japan), after sputtering the samples with a thin layer of gold. Atomic force microscopy (AFM) characterization was performed with an SPI 3800N multimode scanning probe microscope (Seiko Instruments). Topographical images were obtained in contact mode with a silicon cantilever having a nominal spring constant of 0.02 N/m and at a scan rate of 1.0 Hz. The Ultraviolet–visible (UV–vis) absorbance spectrum was obtained by an UV–vis spectrophotometer (UV2600, Japan). The fluorescence spectra were measured by a Hitachi F-4500 fluorescence spectrophotometer, samples were excited at 400 nm. The micro-reflectance spectra observation of the PCs were carried out by combining a reflected microscope (Olympus MX40, Japan) and a fiber optic UV–vis spectrometer (Ocean Optic HR 4000, USA). The illuminating light was focused onto the PC through an objective lens and the reflected light was collected by

the same lens and then transported to the spectrometer through the optic fiber. The reflectance spectra were recorded normal to the hkl¼ 111 planes of the PCs.

3. Results and discussion By tuning the overlap of the absorption and emission spectra, using appropriate materials act as both the converter and emitter simultaneously. Here, three types of commercially available lightemitting dye molecules: Nile red (R), coumarin 6 (G), and perylene (B) were prepared for red, green, and blue emissions, respectively [32]. Absorption and PL spectra of RGB dyes in chloroform solutions are illustrated in Fig. 1. The emission of B and the absorption of G and the emission of G and the absorption of R have good overlap, which means that energy transfer between donor (B or G) and acceptor (G or R) dyes will occur if they are in close proximity. The spectral overlap between the emission and absorption of chromophores indicates that photons emitted from the chromophore could definitely be absorbed and converted by the other. This chromophores energy transfer should be more important in the case of randomly mixed PMMA. We are able to control the mixing of the three RGB colors in order to tune the white light. Fig. 2 shows emission spectra of three dye containing varying concentrations of perylene, coumarin 6 and Nile red. The control of luminescence of multiple dyes in a single solution deposition process has proved elusive. It is clear that on gradual addition of R to the aqueous solution there is a gradual decrease of PL intensity of G indicating energy transfer from G to R. Nevertheless, it is to be noted that the intensity of emission peak of R starts to rise, then to reduce slightly, consistent with the formation of dye aggregates with low fluorescence quantum yield [33]. Similarly, no significant red part of PL is observed when B: G: R ratio is 3:3:3 because of the absence of energy transfer. Although the doping concentration of R dye is increased, the solid film shows a similar trend in the solution evolution of the photoluminescence, where it can be seen that the conversion to red light is fairly inefficient. In this spectral region, the eye sensitivity is also lowest, hence an enhancement in this region is required. Indeed, emission color tuning becomes difficult if RGB dyes are located altogether within the same matrix [15,34]. This is presumably due to scare chance of energy transfer resulting from the large separation between dyes, leading to the need for more population of red dye molecules. However, at low dye concentrations FRET will occur either with poor efficiency or not at all, and at higher concentrations the signal will be quenched as a result of aggregation. As shown in Fig. 2, the emission color is varied according to the RGB ratios, indicating the inadequacy for emission color tuning, even enhancing the dosage of red dyes. The results prove unsuitable for control of the desired emission due to extensive and undesirable Förster transfers between perylene, coumarin, and Nile red. In fact, it is reported that partial FRET between physically dispersed color dopants does not conform to a simple “linear” model and therefore is difficult to program. For a tricolor system, inadvertent FRET, for instance between green and red dyes, makes color tuning difficult [32]. Thus, we try to introduce PCs as the optical substrate to manipulate red emission. According to our previous work, the PCs film used as the substrate of optical devices was fabricated, which involved the selfassembly of latex spheres, the infiltration of monomers and photopolymerization. The step of photopolymerization can greatly improve solvent resistance of the PCs film. Fig. 3a shows the SEM image of poly(styrene-methyl methacrylate-acrylic acid) (P(StMMA-AA)) PCs film. The SEM image illustrates that the colloid spheres are in a face-centered cubic arrangement with a closepacked plane (111) oriented parallel to the substrate. From the

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Fig. 1. (a-d) Absorption and photoluminescence (PL) spectrum of perylene, coumarin 6 and Nile red solved in chloroform solutions, respectively. The insets of (a–c) show chemical structures of perylene, coumarin 6 and Nile red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (a, b) PL spectrum of solutions and films spin-cast from mixture of perylene (B), coumarin 6 (G) and Nile red (R) by varying concentrations ratios, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

insert in Fig. 3a, it can be seen that ordered periodicity of PCs film is well retained. Fig. 3b presents that the surface of the PCs film seemed to be overcasted, which implies that part of polyacrylamide (PAAm) is infiltrated among the interstice of latex spheres. As a result, a crosslinked polymer network is form. From the inset of Fig. 3b, the external surface of crosslinked PCs film has the convex-concave structure for the effective dispersion of phosphors. Clearly, the stopband position of PCs appears a slight

reduction after PCs being crosslinked in Fig. 3c, but it not impair the optic properties of the film. Furthermore, the PC film shows over 50% of reflectivity, which is good accordance with the ordered structure of PCs film. The period structure of PCs can lead to a dramatic modification of light propagation and the emission properties of active optical materials. In the following experiments, we select the matched PCs (λstopband ¼ 567 nm) as the substrate (in Fig. 3d), where the stopband just overlaps the

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Fig. 3. (a, b) Typical SEM images of the PCs and crosslinked PCs film, respectively. The inset of (a) shows the SEM image of a cross-section of the PCs. The inset of (b) shows the AFM image of crosslinked PCs film. (c) The reflection spectra of the PCs, black line indicates blank PCs, the red line is the crosslinked PCs. (d) the PL spectrum of the mixture (perylene, coumarin 6 and Nile red)-loaded PMMA film on the control (black line) and UV–vis reflection spectrum of the PCs (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. (a) Typical PL spectra of a mixture of blue, green and red dyes embedded in PMMA matrix on the control and on the PCs film under excitation wavelength of 400 nm, respectively. The inset of (a) shows the CIE color coordinates. (b) The relative intensity from left to right show blue, green and red colors on the control and on the PCs film under excitation wavelength of 400 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

maximum emission wavelength of Nile red in aim to enhance selectively red part. In order to evaluate the differences of emission on the control and PCs film, we acquired PL spectra. The photoexcitation was at

400 nm. Fig. 4a shows the emission spectra from a mixture of perylene, coumarin 6 and Nile red coated on the control and PCs film. Clearly, the emission intensity in red, blue and green components from the PCs is significantly stronger than that from

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control sample. Moreover, Commission International de l’Eclairage (CIE) coordinate from the control sample is located at (0.20, 0.23), close to blue-white region due to the redundant blue and insufficient red light. When the ternary dyes are coated on the PCs film, the CIE moves to (0.26, 0.33), which is close to the white light region. Additionally, the relative intensities of the red, green and blue spectral components from the control sample and PCs are exhibited in Fig. 4b, respectively. It can be seen that the enhancement ratios among red, green and blue spectral components are inconsistent. The green and blue spectral components has enhancement of 2.7–3.5 times relative to the control sample due to enrichment effect from large surface area of PC structure [35,36]. The PCs film possesses large surface-to-volume ratios for the effective dispersion of phosphors, which can avoid concentration quenching effect. More importantly, the emission intensity of red spectral component on the PCs is enhanced by a factor of 8.2, when comparing with that of the control sample. The enhancement ratio of red emission is higher than that of the other color, which makes up for the lack of red light, so that it is approaching to the white light region. It can be mainly attributed to the combination of large surface area and its optic properties of PCs, especially, the stopband of PCs overlapping the emission of the red spectral component. Here, PCs serve as the dielectric cavity and act as a local resonance mode for the emission propagation. Photons can couple to the overlapping local resonance mode and Bragg scatter out of the structure, thereby greatly reducing the amount of light trapped as guided modes. Moreover, high density of states near the stopband enhances the coupling of spontaneously emitted photons [37,38]. Therefore, enhancing selectively red spectral component by the PCs offers a feasible method toward white light emitting.

Technology of Guangdong Province, China (pdjh2016b0757), and the Training Programme Foundation for Outstanding Young Teachers in Higher Education Institutions of Guangdong Province, China (YQ2015225, YQ2015226).

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4. Conclusion Our novel and simple approach provides a facile route for the fabrication of white light. We have demonstrated that the PCs can be an efficient optical substrate for FRET. The PCs with a stopband centered at 567 nm to match emission wavelength of Nile red is designed in aim to enhance selectively red part. The results show the emission intensity of red spectral component on the PCs is enhanced by a factor of 8.2 resulted from extraordinary light harvesting of PCs, which makes up for the lack of red light, so that CIE coordinates are approaching to the white light region. The strategy not only can enhance selectively any weak part of phosphors without increasing the dose, but also would get rid of the synthesis of new materials, which has offered great potential to improve the emission characteristics of active optical materials, and played an important role on developing novel lighting devices.

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Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant no. 51302107), the Natural Science Foundation of Guangdong Province, China (Grant no. 2015A030313804), Training Special Funds of College Students' Innovation of Science and

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