1.1. General background of white light-emitting diodes
In recent times, light-emitting diodes (LEDs) have been used all over the world, such as general lighting, automotive lighting, backlighting of displays, and street signals. In addition, white LEDs (WLEDs) that have grown rapidly have been a popular source of general illumination. WLEDs have attracted considerable attention for the lighting industry because of their numerous advantages, such as high brightness, high reliability, and low energy consumption . The color of WLEDs is evaluated by two parameters: the correlated color temperature (CCT) and the color-rendering index (CRI). WLEDs used for general lighting exhibit CCT of approximately 2700 K (as incandescent bulbs), 3000 K (as warm-fluorescent tubes), and 5000 K (as cold-fluorescent tubes), respectively. Cold-WLEDs (
Several approaches to fabricate WLEDs have been developed [2-3]. Commercially WLEDs generally use a GaN blue-emitting LED for stimulating yellow-emitting phosphor (YAG:Ce3+) to yield phosphor-converted WLEDs, which are the most efficient lights that provide high luminous efficiency and a low cost . Although phosphor-converted WLEDs have numerous advantages, for example low cost, and high phosphor conversion efficiency. However, several PC-WLEDs problems remain, including a high CCT and a low CRI [5-6]. All WLEDs have required achieving a CRI that exceeds 80, a standard that is used in general lighting applications. Therefore, two or more color phosphors are required when the CRI exceeds 90. That have been achieved by WLEDs with a larger weight percent (wt%) of red phosphor, or by multi-color LED combinations . Currently, all nitride- and borate-based red phosphors are typically expensive than yellow phosphors, a phenomenon that contributes to the high cost of fabrication
1.2. General background of photonic crystals
Photonic crystals (PhCs) were first proposed by E. Yablonovitch  and S. John  in 1987. They proposed a period arrangement of refractive index can possess the photonic bandgap (PBG) in the wavelength range. PBG is, photons can only travel across the material if they are localized into distinct energy states and obey strict rules relating to direction of travel, polarization state and wavelength. A wavelength range can also exist for which there are no allowed states for propagation. PhCs are regular arrays of materials with different refractive indexes that are classified into three main categories, according to the dimensionality of the stack, such as one- (1D), two- (2D), and three-dimensional (3D), as shown in Figure 1. PhCs nanostructures provide new ways to control photons. Recently, optical properties of the PhCs have been much study. 1D PhCs have been applied on many technology, such as Bragg reflectors of the optical feedback mechanism in distributed feedback lasers  and vertical cavity surface emitting lasers . In addition, 2D and 3D PhCs has been subject of much intensive research in areas related to LEDs [12-14], sensing , telecommunications , slow light , and quantum optics .
In this chapter, we focus on the PBG material of 3D colloidal PhCs (CPhCs), which is important because it exhibits a forbidden optical energy band . 3D CPhCs have been studied extensively because of the unique optical properties and applications . The refractive index of colloidal particles gives rise to PBG properties. Light energy in the PBG cannot propagate through materials, and the light is consequently reflected. The 3D CPhC structures, that based on the face-centred-cubic (fcc) opals and inverse opals, are interesting due to they can be produced using colloidal solutions.
1.3. Research niche
WLEDs have multiple advantages, such as their small size, conservation of energy, and long lifetime. WLEDs will comprehensive to replace conventional lighting sources within years. Commercial WLEDs are made by a GaN-based blue LED combination with a yellow-emitting YAG:Ce phosphor; the combination of blue and broadband yellow approximates white light. For residential lighting, many people prefer the
In this chapter, we first introduce the theoretical analysis and synthesis method of 3D CPhC structures in section 2. Then, in section 3, we explain the fabrication method of WLEDs and 3D CPhCs, where 3D CPhCs includes polystyrene (PS) nanospheres and silica (SiO2) nanospheres. Next, in section 4, we demonstrate the theoretically and experimentally for luminescence-spectrum modification of WLEDs by using 3D CPhCs. Finally, conclusions provide in section 5.
2. Fundamental and modelling of 3D colloidal photonic crystals
2.1. Photon band structure properties of 3D colloidal photonic crystal structures
Three-dimensional (3D) PhC, which can have the novel properties, is periodic along three different axes. For 3D PhCs, complete PBG are more rare. The PBG must smother the entire 3D Brillouin zone, not just any one plane or line. For example, Figure 2 shows the photonic band structure (PBS) for an fcc lattice of close-packed PS nanospheres (
In Figure 2, we showed that an fcc lattice of nanospheres does not have a complete PBG . Sanders found that precious opal mineraloids are formed of close-packed arrangements of submicron-diameter silica spheres in a silica–water matrix , with a relatively low dielectric contrast. Just as for the case of an fcc lattice of close-packed dielectric spheres (see Figure 2(b)), small gaps appear only at particular points in the band diagram. The wavevectors
2.2. Bragg’s law analysis methods for 3D colloidal photonic crystals
The 3D CPhCs are artificial opal structures of dielectric materials that exhibit unique photonic dispersion properties and that control light emission behaviours. The optimal design of 3D CPhC structures is strongly dependent on three parameters, for example lattice constant (
3. Fabrication method of white light-emitting diodes and 3D colloidal photonic crystals
3.1. White light-emitting diodes prepared
3.2. Polystyrene nanosphere of 3D colloidal PhCs prepared
PS nanospheres were manufactured as follows. The monodisperse polymer nanospheres were prepared using styrene (Acros Organics) as the monomer, sodium dodecyl sulphate (SDS; Acros Organics) as the emulsifier, and potassium persulphate (KPS; Acros Organics) as the initiator in emulsion polymerization . In this synthesis process, 15 mL of styrene, 100 mg of SDS, and 200 mL of deionized (DI) water were added to a 1000 mL three-necked flask, which was placed in a water bath at 70 °C in an atmosphere of nitrogen gas. Subsequently, 250 mg of KPS dissolved in 50 mL of DI water was added to the mixture while stirring at 300 rpm. After stirring for 24 h, monodispersed PS nanospheres with an average diameter of 270 nm were obtained. By varying the amount of styrene monomer, PS colloidal spheres with different sizes were synthesized with the same method. The 10-μL 3D PS CPhCs were deposited on the entire emission region of the
3.3. Silica nanosphere of 3D colloidal PhCs prepared
Silica nanospheres were synthesized using a modified Stober method  as follows. Monodispersed silica nanospheres were synthesized using ammonium hydroxide (NH4OH; SHOWA), the tetraethoxysilane (TEOS; Aldrich), and the DI water condensation was controlled in anhydrous ethanol solution [20-31]. In this synthesis process, 200 mL of anhydrous ethanol, 10 mL of NH4OH, and 50 mL of DI water were added to a 500 mL three-necked flask, which was placed in a water bath at 35 °C. Subsequently, 15 mL of TEOS was dropped to the mixture while stirring at 300 rpm. After stirring for 24 h, monodispersed silica nanospheres with an average diameter of 250 nm were obtained. By varying the amount of TEOS, silica colloidal nanospheres with different sizes were synthesized with the same method [32-33]. We prepared the latex (100.0 mg/mL) with silica nanosphere of four diameters (
4. Luminescence-spectrum modification of WLEDs by using 3D colloidal photonic crystals
4.1. Luminescence spectra modification of WLEDs by using 3D PS CPhCs
The luminescence spectra of the WLEDs with and without 3D PS CPhCs were measured by the integration sphere at a current of 120 mA, as shown in Figure 9. Due to the photonic stop-bands, and phosphor reabsorption and reemission affect the luminescence spectrum of WLEDs with CPhCs . The light emission of the WLEDs through the 3D PS CPhCs was propagated according to the PBSs of the 3D PS CPhCs. This study measured the angular-resolved transmission spectra to study the light emission distribution of the WLEDs with 3D PS CPhCs, which will be discussed in the next section.
In addition, we also measured the luminous flux, luminous efficiency, CRI, CCT, and the International Commission on Illumination (CIE) color chromaticity coordinates (
4.2. Photon band structure theoretical discussion of WLEDs containing 3D PS CPhCs
Several research groups have investigated the optical properties of 3D CPhCs [34-37]. They have indicated a photonic stop-band at the
The light emission distribution of WLEDs containing 3D PS CPhCs was measured using the angular-resolved transmission spectrum technique. The apparatus for the angular-resolved transmission spectra measurement was the same as in
4.3. Luminescence spectra modification of WLEDs by using 3D silica CPhCs
In this section, we prepared a silica nanosphere of diameter (
In this study, we also measured the optical characteristics of WLEDs using the integration sphere that was equipped with a radiometer and photometer. The luminous flux of the WLEDs,
4.4. Photon band structure theoretical discussion of WLEDs containing 3D silica CPhCs
The angular-resolved reflection spectra were measured to examine the light-reflection distribution of the 3D silica CPhCs; Figure 12 shows the PBS of the 3D silica CPhCs. The angular-resolved reflection measurement systems used in this study were identical to those described previously [32-33]. To confirm that the high-order band of PBS affected the luminescence spectrum of the WLED, we measured the transmittance spectrum of the 3D silica CPhC thin films by using a UV-Vis spectrophotometer (Figure 13). We used a transmittance measurement system equipped with a halogen lamp as the white-light source, an integration sphere, and a spectrometer with a charge-coupled device to measure the transmittance spectra through the 3D silica CPhC thin films. The 3D silica CPhC thin films exhibited a graduated transmission from the blue wavelength region to the red wavelength region. The transmittance of 3D silica CPhC thin films was similar to that of the graduated neutral density filter (GNDF). The 400–600-nm wavelength spanning from the blue region to the red region exhibited a low transmittance that was affected by the high-order band of PBS (Figure 13). In other words, when the WLED photons passed through the 3D silica CPhC thin films, one part was reflected and the other was transmitted. The reflected light of the WLEDs can be absorbed by the phosphor layer and increase the red-light emission [12-14]. In this section, we report that the WLED devices containing 3D silica CPhCs achieved a higher luminous flux than did the commercial
In conclusion, we designed and fabricated WLEDs containing 3D CPhCs that had luminescence modified that were according to the PBSs of 3D CPhCs. The
The authors gratefully acknowledge the financial support for this research by the National Science Council (NSC), and Ministry of Science and Technology (MOST) in Taiwan, under grant numbers NSC102-2221-E-035-046, NSC102-2622-E-035-030-CC2, MOST103-2221-E-035-029, and MOST103-2622-E-035-007-CC2.
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