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A general strategy to fabricate photonic crystal heterostructure with Programmed photonic stopband
Accepted Manuscript Regular Article A General Strategy to Fabricate Photonic Crystal Heterostructure with Programmed Photonic Stopband Lijing Zhang, Bofan Liu, Jie Wang, Shengyang Tao, Qingfeng Yan PII: DOI: Reference:
S0021-9797(17)30889-5 http://dx.doi.org/10.1016/j.jcis.2017.08.004 YJCIS 22651
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
30 June 2017 31 July 2017 1 August 2017
Please cite this article as: L. Zhang, B. Liu, J. Wang, S. Tao, Q. Yan, A General Strategy to Fabricate Photonic Crystal Heterostructure with Programmed Photonic Stopband, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.08.004
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A General Strategy to Fabricate Photonic Crystal Heterostructure with Programmed Photonic Stopband Lijing Zhang,a Bofan Liu,a Jie Wang,a Shengyang Tao,a,* Qingfeng Yanb,* a
Department of Chemistry, Dalian University of Technology, Dalian 116024, Liaoning, China.
b
Department of Chemistry, Tsinghua University, Beijing 100084, China.
Email Address： Lijing Zhang: [email protected] Bofan Liu: [email protected] Jie Wang: [email protected] Qingfeng Yan: [email protected] ∗ Corresponding author Shengyang Tao: Email address：[email protected]（S. Tao）Tel: 0411-84986035 Abstract In this paper, we present a general fabrication strategy to achieve the structure control and the flexible photonic stop band regulation of (2+1) D photonic crystal heterostructures (PCHs) by layer-by-layer depositing the annealed colloidal crystal monolayers of different sphere size. The optical properties of the resulting (2+1) D PCHs with different lattice constants were systematically studied and a universal photonic stopband variation rule was proposed, which makes it possible to program any kind of stopband structure as required, such as dual- or multi-stopbands PCH and ultra-wide stopband PCH. Furthermore, PCH with dual-stopbands overlapping the excitation wavelength (E) and emission wavelength(F) of Ru complex was fabricated by finely manipulating the spheres’ diameter of colloidal monolayers. And an additional 2-fold fluorescence enhancement in comparison to that on the single stopband sample was achieved. This strategy affords new opportunities for delicate engineering the photonic behaviour of PCH, and also is of great significance for the practical application based on their bandgap property. Keywords: photonic crystal heterostructure; photonic stopband; stopband regulation rule; multi-stopbands; ultra-wide stopband; fluorescence enhancement;
A General Strategy to Fabricate Photonic Crystal Heterostructure with Programmed Photonic Stopband Lijing Zhang,a Bofan Liu,a Jie Wang,a Shengyang Tao,a,* Qingfeng Yanb,* a
Department of Chemistry, Dalian University of Technology, Dalian 116024, Liaoning, China.
b
Department of Chemistry, Tsinghua University, Beijing 100084, China.
∗ Corresponding author: Email address：[email protected]（S. Tao） Abstract In this paper, we present a general fabrication strategy to achieve the structure control and the flexible photonic stop band regulation of (2+1) D photonic crystal heterostructures (PCHs) by layer-by-layer depositing the annealed colloidal crystal monolayers of different sphere size. The optical properties of the resulting (2+1) D PCHs with different lattice constants were systematically studied and a universal photonic stopband variation rule was proposed, which makes it possible to program any kind of stopband structure as required, such as dual- or multi-stopbands PCH and ultra-wide stopband PCH. Furthermore, PCH with dual-stopbands overlapping the excitation wavelength (E) and emission wavelength(F) of Ru complex was fabricated by finely manipulating the spheres’ diameter of colloidal monolayers. And an additional 2-fold fluorescence enhancement in comparison to that on the single stopband sample was achieved. This strategy affords new opportunities for delicate engineering the photonic behaviour of PCH, and also is of great significance for the practical application based on their bandgap property. Keywords: photonic crystal heterostructure; photonic stopband; stopband regulation rule; multi-stopbands; ultra-wide stopband; fluorescence enhancement;
1.
Introduction
Photonic crystals (PCs) are a kind of dielectric materials with artificial periodic structure,1,2 which display many properties analogous to semiconductors, including the appearance of photonic bandgaps (PBGs). The existence of PBGs enables PCs to manipulate, confine, and control light, and have been proposed for applications such as optical filters, switches, waveguides for optoelectronics, self-cleaning properties for optical devices, chemical and biological sensors and environmental monitoring.3-9 In recent years, photonic crystal heterostructures (PCHs) composed of two homogeneous PCs with different lattice constants have received extensive attention for their charming and plentiful photonic stopband features,10-12 which opens up the possibility of many novel applications, such as manufacturing integrated photonic crystal chips, multi-frequency optical Bragg filters, dualstopband fluorescence enhancement and broadband reflective mirror.13-16 What’s more, a significant amount of application are taking advantage of the stopband matching of PCHs. Therefore, it is very important and necessary to get a general method and rule of bandgap regulation. Considerable efforts have been made to the fabrication and photonic stopband modulation of PCHs. Wang et al.17 illustrated a room temperature floating self-assembled 3D colloidal crystal heterostructure with two apparently isolated stopbands, and showed that the two separate stopbands gradually got closer as the size difference of two homo crystals decreasing. Jiang et al. 18 adopted the multiple vertical deposition techniques to fabricate opaline colloidal crystals multilayer with approximately additive photonic bandgap properties, which can be tailored by manipulating secondary diffraction, film composition, and sphere size. Yan et al.19 fabricated opaline hetero photonic crystals by sequentially layer transfer of the floating opal films, and found that when the sphere size difference of the compositional colloidal crystal is large, the opaline heterostructure exhibits two apparent isolated stopbands, and when the sphere size difference is small, a wide stopband appears. Although the self-assembled 3D PCHs have presented some special bandgap structure, it is still challenging for delicate bandgap engineering due to the poor control in film thickness and single narrow-sharp bandgap structure. Compared with 3D PCs, (2+1) D PCs possess unique advantages in both structural tunability (control in single layer level) and optical property (PBG broadening and deepening, Fabry-Pérot oscillations).20-22 Binary Langmuir-Blodgett composite (2+1) D PCHs have also been studied by Reculusa, Masse and Ravaine et al.,23,24 which illustrated more complex bandgap information induced by the artificially defects. However, the structural imperfection greatly limits the systematic study on the photonic stopband regulation. To achieve fine regulation of the stopband position and structure is still a big challenge. In this paper, we presented the controllable fabrication of (2+1) D PCHs by layer-by-layer transfer technique, as schematically illustrated in Fig.1. The PS colloidal monolayers with high mechanical strength were created by a combination of the air/water interface self-assembly and the solvent vapour annealing treatment, which affords the PCH with enhanced crystalline integrity. Monolayers of different colours represent polystyrene colloids of different sizes. This layer-by-layer technique gives us opportunity to accurate control and design the structure and photonic stopband of the (2+1) D PCHs in single layer level. Then, we systematically studied the optical properties of the resulting (2+1) D PCHs with different lattice constants and summarised the stopbands variation rule in detail. Under the guidance of this rule, some PCHs with special bandgap structures were constructed as will, such as multi-bandgap and ultra-wide bandgap. In addition, we demonstrated the effect of the stacking order of different colloidal monolayers on the optical properties of heterostructures.
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Fig. 1 Schematic illustration of the fabrication process for a colloidal photonic crystal heterostructure A 3/B3. The different colors stand for PS spheres with different sphere sizes. 2.
Materials and methods
2.1 Reagents and materials. All chemicals, including styrene (St, ≥ 99.0 %), potassium persulfate (KPS, ≥ 99.5 %), ethanol (≥ 99.7 %), sulfuric acid (95 % - 98 %), hydrogen peroxide (≥ 30 %), and sodium dodecyl sulfate (SDS, 98 %), methylbenzene (C 7H8 ≥ 99.5%) and other chemicals were used as received without further purification from Sinopharm Chemical Reagent Co., Ltd. Deionized (DI) water (resistivity up to 18.2 MΩ•cm, Ultra Pure UV, China) was used in all experiments. The p-type 4-inch (100) silicon wafer with a resistivity of 1~10 Ω•cm was purchased from KYKY Technology Co., Ltd. The silicon wafer was single-side polished and cut into small pieces (1 cm × 1 cm) before use. 2.2 Synthesis of PS spheres and preparation of PS colloidal monolayers with high mechanical strength. Monodisperse PS colloidal spheres with diameters ranging from 100 to 600 nm were synthesized using the emulsifier-free emulsion polymerization method. 25 The as-synthesized PS spheres were purified and redispersed in DI water by at least four centrifugation/redispersion cycles before use. The large-area PS colloidal monolayers were fabricated by the air/water interface self-assembly method described in our previous paper.26 In short, the PS emulsion mixed with equal volume ethanol of about 200 μL was added drop wise on the surface of a glass slide, which was pre-placed in the center of a glass Petri dish, just flush with the air/water interface. The glass slide and Petri dish were pretreated by soaking in piranha solution (H 2SO4 and H2O2 in a 3:1 volume ratio) at room temperature for 2h. Once the emulsion contacted the surrounding water at the edge of the glass slide, PS spheres spread rapidly onto the water surface and assembled into monolayer colloidal arrays in several seconds. A drop of 1 wt% SDS solution was added to consolidate the colloidal arrays into a large-area monolayer. The as prepared floating PS colloidal monolayers were transferred to a sealed glass vessel (20.0 cm × 13.0 cm × 7.0 cm) filled with saturated toluene vapor for annealing treatment. The mechanical strength could be easily tuned by controlling the annealing time.26,27 In this way, the colloidal monolayers with high mechanical strength were fabricated. 2.3 Fabrication of (2+1) D colloidal PCHs with PS colloidal monolayers. The colloidal photonic crystal heterostructures were fabricated by a simple layer-by-layer transfer technique, as described in our previous paper. 28 First, two or more annealed PS colloidal monolayers of different sizes were prepared. A drop of 1 wt% SDS was added to these glass Petri dishes separately to push out some small cracked films and provided large-area annealed colloidal monolayer at the air/water interface. The silicon substrate was first inserted beneath the annealed colloidal monolayer (A) and then lifted off from the water surface as a whole and dried at room temperature (293 K), as shown in Fig. 1. The second layer of the monolayer crystal was transferred onto the first monolayer-coated substrate. After repeating this process for several times, the homogeneous PC of (A)m was prepared. Secondly, the annealed colloidal monolayer (B) was layer-by layer transferred onto the (A)m pre-deposited substrate. The (2+1) D colloidal PCHs described by listing the sphere size from bottom to top (A)m(B)n was fabricated. During the preparation process, the thickness of each crystalline sub-unit and the sphere size of A and B can be arbitrarily selected and easily tuned. 2.4 Characterization. Microstructures of the (2+1) D PCHs were observed by field emission-type scanning electron microscopy (FE-SEM) (Gemini LEO 1530) operated at 10 kV. The specimens were coated with gold prior to FE-SEM measurements. The optical reflection spectra of the colloidal crystal films were measured using a UV/visible/near IR spectrophotometer (Perkin Elmer Lambda 750). All measurements were made at normal incidence to the surface of colloidal crystal film-coated glass substrates. Photographs of the colloidal crystals and colloidal PCHs were taken with a Canon camera (70D). 3.
Results and Discussion
3.1 (2+1) D PCHs with enhanced crystalline integrity and controllable structure design. The crystalline quality is among the most important parameters in determining the performance of a photonic crystal in optical application. The top and cross-sectional view of annealed colloidal monolayer of 245 nm were shown in Fig. 2. The surface-contact ridge is evident, which is formed by coalescing neighbouring PS spheres together and results in colloidal monolayers with high mechanical strength. Fig. 3 illustrates the typical SEM images of the resulted photonic crystal heterostructures with flexible size chosen and controllable thickness. As can be seen, all the resultant heterostructures exhibit a highly ordered structure. Each homogeneous crystal film is 2D colloidal monolayers stacking along one -dimensional (1D) direction and with (001) planes oriented parallel to the surface of the substrate, which has been noted as (2+1) D colloidal
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crystals.20,29 Most importantly, we notice that even when stacking small spheres monolayer onto a monolayer of large spheres, the heterostructure still maintains good structural integrity. The small spheres don’t sag into the voids formed by the large spheres and distort the structure at the interface formed by the constituent monolayers of different sphere size. This is largely due to the selfsupporting ability of the colloidal monolayer, which benefits from the solvent vapour annealing treatment.27 The stacking number of the monolayers can be clearly counted from the SEM images which matches perfectly with the predefined number of transfer cycles.
Fig. 2 The top (a) and cross-sectional (b) view SEM image of 245 nm PS colloidal monolayer, showing the coalescence of PS spheres with each other.
Fig.3 Typical cross-sectional SEM images of (2+1) D PCHs with flexible size chosen and controllable thickness. (a) (180 nm)3/ (200 nm)3, (b) (245 nm)6/ (180 nm)6, (c) (245 nm)6/ (200 nm)6, (d) (317 nm)6/ (245 nm)6 3.2 Photonic stopband regulation of (2+1) D PCHs. Compared with the self-assembled face-centred-cubic (fcc) 3D colloidal crystal, (2+1) D colloidal PCs demonstrate unique optical properties, such as PBG broadening and deepening. The optical response of a (2+1) D PCH is closely related to the lattice constant of the two homogeneous photonic crystals. Fig. 4 gives the normal-incidence reflection spectra of the (2+1) D PCHs and their corresponding homo films. All the homo PCs used in this work are of 6 layers in thickness. We note that the spectra of the heterostructures have features corresponding to peaks in each of the homo crystals with size matching, which is basically consistent with the results of the literature.17,19,30 As shown in Fig. 4a, the heterostructure with small size difference of 245 nm/200 nm presents a broadened stopband from 460 nm to 740 nm, with four distinguished reflection peaks located at 482, 524, 572 and 635 nm, respectively. The reflection peaks at 524 and 635 nm seem to correspond to the stop bands of the homo colloidal crystals (508 nm for the PS 200 nm film and 626 nm for the PS 245 nm film, respectively). The peak located at 572 nm is believed to be the result of linear superposition of the two consecutive stopbands because they have a slight overlap due to the small size difference. The residual peak located at 482 nm is caused by the interaction between stop band of 200 nm PS film and the Fabry– Pérot fringes of 245 nm PS film. This is the unique phenomenon of (2+1) D PCs obtained by layer-by layer stacking method. When the sphere size difference of the compositional colloidal crystal gets larger, there is no overlap position between the two homo films. It can be seen that the heterostructure exhibits two apparent isolated stop bands centred at about 626 and 452 nm (see Fig. 4b), which is in agreement with the stop bands of the homo PS colloidal crystals. The minibands appeared within stop bands are also caused by the Fabry–Pérot vibration. 31
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Fig.4 The normal-incidence reflection spectra for the (2+1) D PCHs: (a) 245 nm/200 nm and (b) 245 nm/180 nm. For the purpose of comparison, the reflection spectra for the homo colloidal crystals (i.e., PS 180 nm film, PS 200 nm film, and PS 245 nm film) are also presented and shifted vertically for clarity. (c) The normal-incidence reflection spectra of series of homogeneous films showing the overlapping degree with the 245 nm film. (d) The evolution of the normal-incidence reflection spectra for the (2+1) D PCHs with different size matching. Thus, it can be concluded that the photonic stopband structure of the (2+1) D PCHs has a close correlation with the bandgap overlapping degree of compositional two homo crystals. Here, we fixed the sphere size of one homo crystal at 245 nm, and the other one with size changing from 180 nm to 387 nm. The bandgap overlapping degree between two homo crystals and the photonic stopbands evolution of their corresponding heterostuctures were systematically studied, and the resulted normalincidence reflection spectra are illustrated in Fig. 4c and 4d, respectively. By comparative analysis, we found that with decreasing sphere size difference of the constituting homo photonic crystals, the stopband overlapping degree increased gradually, and the photonic stopband changed from two isolate stopbands to one broadened stopband and finally merged into one enhanced resonance peak, as shown in Fig. 4d. This suggests one way to tailor the photonic stopband properties of the colloidal crystal heterostructure. To explore this discipline further, the conception of resolution (RAB) is introduced to represent the stopband overlapping degree of homo PCs of A and B, which is widely used in chromatographic analysis. We noted ΔλAB to represent the distance from λA to λB. The RAB can be defined as RAB = 2(λA -λB ) (WA +WB ) , where the WA and WB stand for the full width of λA and λB, respectively. The key parameters of stop bands of herterostruces and homo films are summarized in Table 1. The calculated value of ΔλAB and corresponding R AB are also listed. Table 1. The key parameters of stop bands for (2+1) D PC heterostructures and their constituting homo films.
By comparing the parameters of stopbands for the two components and corresponding heterostructure, the stopband characteristics of the final heterostructure is related to the R AB value. We noted that there are two special situations as shown in Fig. 5, i.e. the two peaks coincide with half overlap (R AB = 0.5) and just do not overlap (RAB=1.0). By comprehensive analysis the data listed in Table 1, the optical properties of the heterostructure can be approximated from the spectra of the individual homo crystals according to a simple rule: 1) when RAB> 1, the two peaks of λA and λB do not overlap at all. The Bragg diffraction peak of heterostructure exhibits two separate peaks, and the peak’s position are basically the same with original λA and λB. It is equivalent
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to the simple addition of two stopbands of A and B, such as heterostructures of 245 nm/180 nm and 245 nm/387 nm. 2) when 0.5< RAB< 1, the overlapping degree of λA and λB is between half and just overlapping. The stopband of heterostructure presents a single and broad peak by stopband superposition. Several "passbands" also appear within the stop band due to the mutual coupling of the original bandgaps, such as 245 nm/200 nm and 245 nm/317 nm. 3) when RAB < 0.5, the overlapping degree of λA and λB is more than half peak width, and the interaction between A and B photonic crystals is very strong, resulting in the heterostructures with a coupled resonance strong peak. For example, the heterostructure of 245 nm/230 nm. Thus, the PBG variation rules of (2+1) D PC heterostructures were concluded, which provides a simple approach to achieve a desired stopband structures as will by stacking colloidal crystals with different stopband overlapping degree.
Fig. 5 Illustration for bandgap overlapping situation for (a) RAB=0.5 and (b) R AB=1. 3.3 (2+1) D PCHs with dual or multi-stopbands. Dual or multi-bandgap heterostructures exhibit more elaborate optical properties than their component homo crystals. They can simultaneously manipulate two or more optical signals and propose a wide range of potential applications in dual-mode band pass filter, multi-channel filters, multi-channel all-optical routing, multi-channel all-optical switch and other optical or optoelectronic devices. Based on the basic stopband rule, we have successfully prepared the (2+1) D PCHs with dual and multiphoton bandgap structures by finely selecting the colloidal size (RAB>1) and designing the stacking order of the homogeneous photonic crystals, as shown in Fig. 6. Fig. 6a shows the normal-incidence reflection spectra of a dual-stopbands photonic crystal heterostructure composed of a colloidal crystal of 200 nm on top of a colloidal crystal of 365 nm with a resolution value of 1.32. Fig. 6b shows a three-stopbands photonic crystal heterostructure (365 nm/245 nm/ 180 nm) with R 365nm/245nm=1.33 and R 245nm/180nm=1.16, respectively. They both exhibit apparent isolated stopbands, which are in agreement with the stopbands of the homo PS colloidal crystals, correspondingly. We also note that the band width is slightly broader which may be caused by the overlapping of the secondary diffraction peak (λs(365nm)) and Fabry–Pérot fringes. This suggests one way to finely tailor the photonic bandgap properties of the heterostructures. Of particular note is that the stopband property is slightly different when stacking in reversed order. From Fig. 6c, it is apparent that for 245 nm/180 nm the first stopband widens and displays two small passbands, while for the 180 nm/245 nm, the second stopband shows significant broadening and peak separation. We propose that these results arise from the fact that the illumination of the heterostructure from the side 180 nm or 245 nm. When light is irradiated from the surface of 180 nm, the primary reflection peak of 180 nm will interact with the secondary Bragg peak of 245 nm, and coupling to a broad peak. And when light is irradiated from the surface of 245 nm, the primary reflection peak of 245 nm will overlap with the Fabry–Pérot fringes of 180 nm. Similar behaviour has be explored by Pemble et al. in the previous work. 32
Fig. 6 The normal-incidence reflection spectra of (2+1) D PCHs with (a) two stopbands and (b) three stopbands. For the purpose of comparison, the reflection spectra for the homogeneous colloidal crystals (i.e., PS 180 nm film, 200 nm film, 245 nm film and 365 nm film) are also presented. (c) The normal-incidence reflection spectra of the heterostructure with different stacking order. 3.4 (2+1) D PCHs with ultra-wide stopband. Ultra-wide bandgap photonic crystals which can inhibit light propagation over a wide range of wavelength have been widely used in wide-spectrum solar cells, broadband filters and antireflective optical films. As discussed above, (2+1) D PCHs composed of two homo crystals with proper R AB value (0.5>RAB<1) exhibits a broadened stopband. Ultra-wide bandgap can be created by stacking three photonic crystals to form a double-heterojunction structure, which can achieve the purpose of further expanding the bandgap width and afford new opportunities for engineered photonic behavior. Fig. 7a demonstrates the normal-incidence reflection spectra of series of heterostructures with varied R AB value, and the number of layers of each homo photonic crystal is 6.
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It can be seen that the stopband of heterojunctions with R AB in the range of 0.5 to 1 show good bandgap superposition and widening feature, which further confirms the rules we concluded above. Finally, the homo crystals of 182 nm, 200 nm and 235 nm were chosen to build a double heterojunction structure. Fig. 7b shows the reflection spectra of this double heterojunction 235 nm/200 nm/182 nm, and the spectra of the homo colloidal crystals of 235 nm, 200 nm and 180 nm are also presented, respectively. It can be seen that the photonic bandgap width of this double heterostructure is broadened to about 200 nm, thus the light in the range of 450 ~ 650 nm could be inhibited effectively. Fig. 7c gives the optical photographs of the double heterojunction structure as well as its components. The color of the photonic crystal of 182 nm, 200 nm and 235 nm is blue-violet, yellow-green and yellow, respectively. Color superimposition is also observed when they are stacked together to fabricate heterostructures.
Fig. 7 (a) The normal-incidence reflection spectra of series of (2+1) D PCHs with different R AB value. (b) The reflection spectrum of the double heterojunction structure showing the ultra-wide bandgap as well as the spectra of the homo crystals. (c) Photographs of as-made homo crystals, heterostructures and double-heterojunction sample.
Fig. 8 The normal-incidence reflection spectra of the double heterojunction structure with different stacking order. The construction of ultra-wide stopband photonic crystal heterostructure is not only related to the size selection of each component colloidal spheres, but also has a great influence on the stacking order of the homo photonic crystals. Up to now, all the heterostructures demonstrated in this work are stacking along the direction of the vertical substrate with colloidal monolayer sphere size from large to small order. Fig. 8 shows the normal-incidence reflection spectra of the double heterojunctions with different stacking orders. It can be seen that the arrangement of the homo photonic crystals has a very large effect on the final stopband structure of the heterojunction. A number of very narrow "defects" occur in the range of 400 to 700 nm at heterojunctions of 200 nm/182 nm/235 nm, and the wavelengths of light can pass through photonic crystals with little or no loss, which can be used to produce high quality narrow-band filter or photonic crystal laser.33-35 The heterojunction of 182 nm/200 nm/235 nm presents a big “defect” at about 520 nm, and the stopband intensity is relatively low in the range of 400 ~ 500 nm. The specific reason needs further analysis. 3.5 Fluorescence enhancement by (2+1) D PCHs with dual stopbands.
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PCs can achieve fluorescence enhancement in two ways: 1) the bandgap position matching with the fluorophore excitation wavelength (E) for enhancing excitation; 2) the bandgap position matching with the emission wavelength (F) for enhanced light extraction. PCHs with dual bandgaps can simultaneously matching with the excitation wavelength and the emission wavelength of fluorescent medium, which may realize remarkable enhancement of fluorescence.36-38 The mechanism of luminescent enhancement has been widely studied in previous reports. Our developed layer-by-layer method and concluded PBG structure evolution rules make it possible to finely design the bandgap position to overlapping both E and F of different fluorescent medium.
Fig.9 (a) UV–visible absorption (red) and fluorescence (blue) spectra of the [Ru(dpp) 3]Cl2 in absolute ethanol solution under excitation wavelength of 450 nm. (b) The normal-incidence reflection spectra of (2+1) D PCH 245 nm/ 180 nm of different layers with two separate bandgaps, one centred at 450 nm and the other red edge covering 620 nm. The fluorescence spectra (c) and enhancement factor (d) of [Ru(dpp)3]Cl2 on heterostructure of 245 nm/ 180 nm with different layers and on the homogeneous photonic crystal films compared to that on glass. Fig. 9a presents the absorption and emission spectra of fluorescent medium [Ru(dpp) 3]Cl2, which shows the maximum absorption at 450 nm and emission at 620 nm under maximum excitation. According to the experienced result reported before, λ c ≈2.53D,28 then 180 nm and 245 nm was finally selected. The RAB≈ 1.16. Thus, a series of dual-bandgap heterostructures 245 nm/180 nm with different layers were fabricated, whose stopbands just respectively overlap the wavelength of E and F, as shown in Fig. 9b. It can be seen that the intensity of E-matching stopband increases with the increasing monolayers while the F-matching stopband experiences a trend of first increase and then decline. The centre positions of the dual stopbands did not change obviously but the minibands disappeared gradually, which is probably owing to more defects involved in many times stacking. Fig. 9c shows the fluorescence spectra of [Ru(dpp)3]Cl2 on PCH of 245 nm/ 180 nm with different layers as well as homogeneous PC films and glass substrate, and Fig. 9d gives the luminescent enhancement factor of different samples. Compared with the luminescence intensity of [Ru(dpp 3)]Cl2 on the glass , the E-matching and F-matching PC films can largely increase the FL intensity due to the modulation through the periodic dielectric constant. The (2+1) D PCH with E-F-matching dual-bandgap leads to an extra luminescence enhancement of nearly 2 times, and the enhancement factor was further improved with the layer number increase. Therefore, the dual-bandgap PCH has the strongest effect on enhancing the total fluorescence intensity and improving fluorescent behaviour in thicker PCH films. 4.
Conclusions
In summary, we have demonstrated a facile and efficient method to achieve the controllable fabrication and flexible photonic stopband design of (2+1) D PCH. Based on the accurate regulation of structure, the optical properties of the resulting (2+1) D PCHs were systematically studied, and a universal PBG variation rule was proposed, which makes it possible to construct any kind of photonic stopband structure as required. Dual- or multi- stopband PCH and width-extended stopband PCH were successfully fabricated. The dual-stopbands PCH renders an additional 2-fold enhancement to the single stopband PC. The method introduced here could be used as a general tool to tailor the photonic properties of photonic crystal films and may play important role in special kind of optical filters, reflectors, waveguides and signal enhancement substrate. Acknowledgements The authors would like to thank financial support from the National Science Foundation of China (No.51173097, 91333109, 21473019), the Financial Grant from the China Postdoctoral Science Foundation (No.2016M601302) and the Fundamental Research Funds for the Central Universities (DUT17LK36 and DUT16RC(3)069 ). References 1. E. Yablonovitch, Phys. Rev. Lett., 1987, 58, 2059. 2. S. John, Phys. Rev. Lett., 1987, 58, 2486.
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3. F. Chen, Laser Photonics Rev., 2012, 6, 622-640. 4. R. V. Nair and R. Vijaya, Prog. Quant. Electron., 2010, 34, 89-134. 5. J. Wang, Y. Zhang, S. Wang, Y. Song, L. Jiang, Acc. Chem. Res., 2011, 44, pp 405-415. 6. J. Hou, H. Zhang, Q. Yang, M. Li, Y. Song, L. Jiang, Angew. Chem. In. Ed. 2014, 53,5791-5795. 7. A. M. Cubillas, S. Unterkofler, T. G. Euser, B. J. Etzold, A. C. Jones, P. J. Sadler, P. Wasserscheid and P. S. J. Russell, Chem. Soc. Rev., 2013, 42, 8629-8648. 8. V. Rinnerbauer, S. Ndao, Y. X. Yeng, W. R. Chan, J. J. Senkevich, J. D. Joannopoulos, M. Soljačić and I. Celanovic, Energy Environ. Sci., 2012, 5, 8815-8823. 9. M. E. Calvo, S. Colodrero, N. Hidalgo, G. Lozano, C. López-López, O. Sánchez-Sobrado and H. Míguez, Energy Environ. Sci., 2011, 4, 4800-4812. 10. N. Stefanou, V. Yannopapas and A. Modinos, Comput. Phys. Commun., 1998, 113, 49-77. 11. B. Ding, M. Bardosova, I. Povey, M. E. Pemble and S. G. Romanov, Adv. Funct. Mater., 2010, 20, 853-860. 12. E. Istrate, M. Charbonneau-Lefort and E. H. Sargent, Phys. Rev. B, 2002, 66, 075121. 13. S. Romanov, H. Yates, M. Pemble and R. De La Rue, J. Phys- Condens. Mat., 2000, 12, 8221. 14. R. V. Nair, R. Vijaya, K. Kuroda and K. Sakoda, J. Appl. Phys., 2007, 102, 123106. 15. C. Zhang, F. Qiao, J. Wan and J. Zi, J. Appl. Phys., 2000, 87, 3174-3176. 16. H. Li, J. Wang, F. Liu, Y. Song and R. Wang, J. Colloid Interf. Sci., 2011, 356, 63-68. 17. A. J. Wang, S.-L. Chen and P. Dong, Mater. Chem. Phys., 2011, 128, 6-9. 18. P. Jiang, G. N. Ostojic, R. Narat, D. M. Mittleman and V. L. Colvin, Adv. Mater., 2001, 13, 389-393. 19. Q. Yan, L. K. Teh, Q. Shao, C. Wong and Y.-M. Chiang, Langmuir, 2008, 24, 1796-1800. 20. S. Romanov and M. Bardosova, Phys. Solid State, 2010, 52, 533-543. 21. M. E. Pemble, M. Bardosova, I. M. Povey, R. H. Tredgold and D. Whitehead, Physica B, 2007, 394, 233-237. 22. S. Romanov, M. Bardosova, D. Whitehead, I. Povey, M. Pemble and C. Sotomayor Torres, Appl. Phys. Lett., 2007, 90, 133101. 23. S. Reculusa, P. Massé and S. Ravaine, J. Colloid Interf. Sci., 2004, 279, 471-478. 24. P. Massé and S. Ravaine, Chem. Mater., 2005, 17, 4244-4249. 25. S. E. Shim, Y. J. Cha, J. M. Byun and S. Choe, J. Appl. Polym. Sci., 1999, 71, 2259-2269. 26. J. Yu, Q. Yan and D. Shen, ACS Appl. Mater. Inter., 2010, 2, 1922-1926. 27. L. Zhang, W. Wang, L. Zheng, X. Wang and Q. Yan, Langmuir, 2016, 32, 451-459. 28. L. Zhang, Z. Xiong, L. Shan, L. Zheng, T. Wei and Q. Yan, Small, 2015, 11, 4910-4921. 29. S. Romanov, M. Bardosova, M. Pemble and C. Sotomayor Torres, Appl. Phys. Lett., 2006, 89, 043105. 30. P. Massé, S. Reculusa and S. Ravaine, Colloid. Surface. A, 2006, 284, 229-233. 31. S. Reculusa and S. Ravaine, Chem. Mater., 2003, 15, 598-605. 32. M. Bardosova , M. E. Pemble , I. M. Povey , R. H. Tredgold , D. E. Whitehead , Appl. Phys. Lett. 2006 , 89 , 093116 . 33. M. Djavid, A. Ghaffari, F. Monifi and M. Abrishamian, J. Appl. Sci., 2008, 8, 1891-1897. 34. M. Lončar, A. Scherer and Y. Qiu, Appl. Phys. Lett., 2003, 82, 4648-4650. 35. S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi and M. Notomi, Nat. Photonics, 2010, 4, 648-654. 36. N. Ganesh, W. Zhang, P. C. Mathias, E. Soares, V. Malyarchuk, A. D. Smith and B. T. Cunningham, Nat. Nanotechnol, 2007, 2, 515520. 37. H. Li, J. Wang, H. Lin, L. Xu, W. Xu, R. Wang, Y. Song, D. Zhu, Adv. Mater. 2010, 22, 1237-1241. 38. P. Zhou, D. Zhou, L. Tao, Y. Zhu, W. Xu, S. Xu, S. Cui, L. Xu and H. Song, Light- Sci. Appl., 2014, 3, 209.
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Graphical abstract
S0021-9797(17)30889-5 http://dx.doi.org/10.1016/j.jcis.2017.08.004 YJCIS 22651
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
30 June 2017 31 July 2017 1 August 2017
Please cite this article as: L. Zhang, B. Liu, J. Wang, S. Tao, Q. Yan, A General Strategy to Fabricate Photonic Crystal Heterostructure with Programmed Photonic Stopband, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.08.004
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A General Strategy to Fabricate Photonic Crystal Heterostructure with Programmed Photonic Stopband Lijing Zhang,a Bofan Liu,a Jie Wang,a Shengyang Tao,a,* Qingfeng Yanb,* a
Department of Chemistry, Dalian University of Technology, Dalian 116024, Liaoning, China.
b
Department of Chemistry, Tsinghua University, Beijing 100084, China.
Email Address： Lijing Zhang: [email protected] Bofan Liu: [email protected] Jie Wang: [email protected] Qingfeng Yan: [email protected] ∗ Corresponding author Shengyang Tao: Email address：[email protected]（S. Tao）Tel: 0411-84986035 Abstract In this paper, we present a general fabrication strategy to achieve the structure control and the flexible photonic stop band regulation of (2+1) D photonic crystal heterostructures (PCHs) by layer-by-layer depositing the annealed colloidal crystal monolayers of different sphere size. The optical properties of the resulting (2+1) D PCHs with different lattice constants were systematically studied and a universal photonic stopband variation rule was proposed, which makes it possible to program any kind of stopband structure as required, such as dual- or multi-stopbands PCH and ultra-wide stopband PCH. Furthermore, PCH with dual-stopbands overlapping the excitation wavelength (E) and emission wavelength(F) of Ru complex was fabricated by finely manipulating the spheres’ diameter of colloidal monolayers. And an additional 2-fold fluorescence enhancement in comparison to that on the single stopband sample was achieved. This strategy affords new opportunities for delicate engineering the photonic behaviour of PCH, and also is of great significance for the practical application based on their bandgap property. Keywords: photonic crystal heterostructure; photonic stopband; stopband regulation rule; multi-stopbands; ultra-wide stopband; fluorescence enhancement;
A General Strategy to Fabricate Photonic Crystal Heterostructure with Programmed Photonic Stopband Lijing Zhang,a Bofan Liu,a Jie Wang,a Shengyang Tao,a,* Qingfeng Yanb,* a
Department of Chemistry, Dalian University of Technology, Dalian 116024, Liaoning, China.
b
Department of Chemistry, Tsinghua University, Beijing 100084, China.
∗ Corresponding author: Email address：[email protected]（S. Tao） Abstract In this paper, we present a general fabrication strategy to achieve the structure control and the flexible photonic stop band regulation of (2+1) D photonic crystal heterostructures (PCHs) by layer-by-layer depositing the annealed colloidal crystal monolayers of different sphere size. The optical properties of the resulting (2+1) D PCHs with different lattice constants were systematically studied and a universal photonic stopband variation rule was proposed, which makes it possible to program any kind of stopband structure as required, such as dual- or multi-stopbands PCH and ultra-wide stopband PCH. Furthermore, PCH with dual-stopbands overlapping the excitation wavelength (E) and emission wavelength(F) of Ru complex was fabricated by finely manipulating the spheres’ diameter of colloidal monolayers. And an additional 2-fold fluorescence enhancement in comparison to that on the single stopband sample was achieved. This strategy affords new opportunities for delicate engineering the photonic behaviour of PCH, and also is of great significance for the practical application based on their bandgap property. Keywords: photonic crystal heterostructure; photonic stopband; stopband regulation rule; multi-stopbands; ultra-wide stopband; fluorescence enhancement;
1.
Introduction
Photonic crystals (PCs) are a kind of dielectric materials with artificial periodic structure,1,2 which display many properties analogous to semiconductors, including the appearance of photonic bandgaps (PBGs). The existence of PBGs enables PCs to manipulate, confine, and control light, and have been proposed for applications such as optical filters, switches, waveguides for optoelectronics, self-cleaning properties for optical devices, chemical and biological sensors and environmental monitoring.3-9 In recent years, photonic crystal heterostructures (PCHs) composed of two homogeneous PCs with different lattice constants have received extensive attention for their charming and plentiful photonic stopband features,10-12 which opens up the possibility of many novel applications, such as manufacturing integrated photonic crystal chips, multi-frequency optical Bragg filters, dualstopband fluorescence enhancement and broadband reflective mirror.13-16 What’s more, a significant amount of application are taking advantage of the stopband matching of PCHs. Therefore, it is very important and necessary to get a general method and rule of bandgap regulation. Considerable efforts have been made to the fabrication and photonic stopband modulation of PCHs. Wang et al.17 illustrated a room temperature floating self-assembled 3D colloidal crystal heterostructure with two apparently isolated stopbands, and showed that the two separate stopbands gradually got closer as the size difference of two homo crystals decreasing. Jiang et al. 18 adopted the multiple vertical deposition techniques to fabricate opaline colloidal crystals multilayer with approximately additive photonic bandgap properties, which can be tailored by manipulating secondary diffraction, film composition, and sphere size. Yan et al.19 fabricated opaline hetero photonic crystals by sequentially layer transfer of the floating opal films, and found that when the sphere size difference of the compositional colloidal crystal is large, the opaline heterostructure exhibits two apparent isolated stopbands, and when the sphere size difference is small, a wide stopband appears. Although the self-assembled 3D PCHs have presented some special bandgap structure, it is still challenging for delicate bandgap engineering due to the poor control in film thickness and single narrow-sharp bandgap structure. Compared with 3D PCs, (2+1) D PCs possess unique advantages in both structural tunability (control in single layer level) and optical property (PBG broadening and deepening, Fabry-Pérot oscillations).20-22 Binary Langmuir-Blodgett composite (2+1) D PCHs have also been studied by Reculusa, Masse and Ravaine et al.,23,24 which illustrated more complex bandgap information induced by the artificially defects. However, the structural imperfection greatly limits the systematic study on the photonic stopband regulation. To achieve fine regulation of the stopband position and structure is still a big challenge. In this paper, we presented the controllable fabrication of (2+1) D PCHs by layer-by-layer transfer technique, as schematically illustrated in Fig.1. The PS colloidal monolayers with high mechanical strength were created by a combination of the air/water interface self-assembly and the solvent vapour annealing treatment, which affords the PCH with enhanced crystalline integrity. Monolayers of different colours represent polystyrene colloids of different sizes. This layer-by-layer technique gives us opportunity to accurate control and design the structure and photonic stopband of the (2+1) D PCHs in single layer level. Then, we systematically studied the optical properties of the resulting (2+1) D PCHs with different lattice constants and summarised the stopbands variation rule in detail. Under the guidance of this rule, some PCHs with special bandgap structures were constructed as will, such as multi-bandgap and ultra-wide bandgap. In addition, we demonstrated the effect of the stacking order of different colloidal monolayers on the optical properties of heterostructures.
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Fig. 1 Schematic illustration of the fabrication process for a colloidal photonic crystal heterostructure A 3/B3. The different colors stand for PS spheres with different sphere sizes. 2.
Materials and methods
2.1 Reagents and materials. All chemicals, including styrene (St, ≥ 99.0 %), potassium persulfate (KPS, ≥ 99.5 %), ethanol (≥ 99.7 %), sulfuric acid (95 % - 98 %), hydrogen peroxide (≥ 30 %), and sodium dodecyl sulfate (SDS, 98 %), methylbenzene (C 7H8 ≥ 99.5%) and other chemicals were used as received without further purification from Sinopharm Chemical Reagent Co., Ltd. Deionized (DI) water (resistivity up to 18.2 MΩ•cm, Ultra Pure UV, China) was used in all experiments. The p-type 4-inch (100) silicon wafer with a resistivity of 1~10 Ω•cm was purchased from KYKY Technology Co., Ltd. The silicon wafer was single-side polished and cut into small pieces (1 cm × 1 cm) before use. 2.2 Synthesis of PS spheres and preparation of PS colloidal monolayers with high mechanical strength. Monodisperse PS colloidal spheres with diameters ranging from 100 to 600 nm were synthesized using the emulsifier-free emulsion polymerization method. 25 The as-synthesized PS spheres were purified and redispersed in DI water by at least four centrifugation/redispersion cycles before use. The large-area PS colloidal monolayers were fabricated by the air/water interface self-assembly method described in our previous paper.26 In short, the PS emulsion mixed with equal volume ethanol of about 200 μL was added drop wise on the surface of a glass slide, which was pre-placed in the center of a glass Petri dish, just flush with the air/water interface. The glass slide and Petri dish were pretreated by soaking in piranha solution (H 2SO4 and H2O2 in a 3:1 volume ratio) at room temperature for 2h. Once the emulsion contacted the surrounding water at the edge of the glass slide, PS spheres spread rapidly onto the water surface and assembled into monolayer colloidal arrays in several seconds. A drop of 1 wt% SDS solution was added to consolidate the colloidal arrays into a large-area monolayer. The as prepared floating PS colloidal monolayers were transferred to a sealed glass vessel (20.0 cm × 13.0 cm × 7.0 cm) filled with saturated toluene vapor for annealing treatment. The mechanical strength could be easily tuned by controlling the annealing time.26,27 In this way, the colloidal monolayers with high mechanical strength were fabricated. 2.3 Fabrication of (2+1) D colloidal PCHs with PS colloidal monolayers. The colloidal photonic crystal heterostructures were fabricated by a simple layer-by-layer transfer technique, as described in our previous paper. 28 First, two or more annealed PS colloidal monolayers of different sizes were prepared. A drop of 1 wt% SDS was added to these glass Petri dishes separately to push out some small cracked films and provided large-area annealed colloidal monolayer at the air/water interface. The silicon substrate was first inserted beneath the annealed colloidal monolayer (A) and then lifted off from the water surface as a whole and dried at room temperature (293 K), as shown in Fig. 1. The second layer of the monolayer crystal was transferred onto the first monolayer-coated substrate. After repeating this process for several times, the homogeneous PC of (A)m was prepared. Secondly, the annealed colloidal monolayer (B) was layer-by layer transferred onto the (A)m pre-deposited substrate. The (2+1) D colloidal PCHs described by listing the sphere size from bottom to top (A)m(B)n was fabricated. During the preparation process, the thickness of each crystalline sub-unit and the sphere size of A and B can be arbitrarily selected and easily tuned. 2.4 Characterization. Microstructures of the (2+1) D PCHs were observed by field emission-type scanning electron microscopy (FE-SEM) (Gemini LEO 1530) operated at 10 kV. The specimens were coated with gold prior to FE-SEM measurements. The optical reflection spectra of the colloidal crystal films were measured using a UV/visible/near IR spectrophotometer (Perkin Elmer Lambda 750). All measurements were made at normal incidence to the surface of colloidal crystal film-coated glass substrates. Photographs of the colloidal crystals and colloidal PCHs were taken with a Canon camera (70D). 3.
Results and Discussion
3.1 (2+1) D PCHs with enhanced crystalline integrity and controllable structure design. The crystalline quality is among the most important parameters in determining the performance of a photonic crystal in optical application. The top and cross-sectional view of annealed colloidal monolayer of 245 nm were shown in Fig. 2. The surface-contact ridge is evident, which is formed by coalescing neighbouring PS spheres together and results in colloidal monolayers with high mechanical strength. Fig. 3 illustrates the typical SEM images of the resulted photonic crystal heterostructures with flexible size chosen and controllable thickness. As can be seen, all the resultant heterostructures exhibit a highly ordered structure. Each homogeneous crystal film is 2D colloidal monolayers stacking along one -dimensional (1D) direction and with (001) planes oriented parallel to the surface of the substrate, which has been noted as (2+1) D colloidal
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crystals.20,29 Most importantly, we notice that even when stacking small spheres monolayer onto a monolayer of large spheres, the heterostructure still maintains good structural integrity. The small spheres don’t sag into the voids formed by the large spheres and distort the structure at the interface formed by the constituent monolayers of different sphere size. This is largely due to the selfsupporting ability of the colloidal monolayer, which benefits from the solvent vapour annealing treatment.27 The stacking number of the monolayers can be clearly counted from the SEM images which matches perfectly with the predefined number of transfer cycles.
Fig. 2 The top (a) and cross-sectional (b) view SEM image of 245 nm PS colloidal monolayer, showing the coalescence of PS spheres with each other.
Fig.3 Typical cross-sectional SEM images of (2+1) D PCHs with flexible size chosen and controllable thickness. (a) (180 nm)3/ (200 nm)3, (b) (245 nm)6/ (180 nm)6, (c) (245 nm)6/ (200 nm)6, (d) (317 nm)6/ (245 nm)6 3.2 Photonic stopband regulation of (2+1) D PCHs. Compared with the self-assembled face-centred-cubic (fcc) 3D colloidal crystal, (2+1) D colloidal PCs demonstrate unique optical properties, such as PBG broadening and deepening. The optical response of a (2+1) D PCH is closely related to the lattice constant of the two homogeneous photonic crystals. Fig. 4 gives the normal-incidence reflection spectra of the (2+1) D PCHs and their corresponding homo films. All the homo PCs used in this work are of 6 layers in thickness. We note that the spectra of the heterostructures have features corresponding to peaks in each of the homo crystals with size matching, which is basically consistent with the results of the literature.17,19,30 As shown in Fig. 4a, the heterostructure with small size difference of 245 nm/200 nm presents a broadened stopband from 460 nm to 740 nm, with four distinguished reflection peaks located at 482, 524, 572 and 635 nm, respectively. The reflection peaks at 524 and 635 nm seem to correspond to the stop bands of the homo colloidal crystals (508 nm for the PS 200 nm film and 626 nm for the PS 245 nm film, respectively). The peak located at 572 nm is believed to be the result of linear superposition of the two consecutive stopbands because they have a slight overlap due to the small size difference. The residual peak located at 482 nm is caused by the interaction between stop band of 200 nm PS film and the Fabry– Pérot fringes of 245 nm PS film. This is the unique phenomenon of (2+1) D PCs obtained by layer-by layer stacking method. When the sphere size difference of the compositional colloidal crystal gets larger, there is no overlap position between the two homo films. It can be seen that the heterostructure exhibits two apparent isolated stop bands centred at about 626 and 452 nm (see Fig. 4b), which is in agreement with the stop bands of the homo PS colloidal crystals. The minibands appeared within stop bands are also caused by the Fabry–Pérot vibration. 31
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Fig.4 The normal-incidence reflection spectra for the (2+1) D PCHs: (a) 245 nm/200 nm and (b) 245 nm/180 nm. For the purpose of comparison, the reflection spectra for the homo colloidal crystals (i.e., PS 180 nm film, PS 200 nm film, and PS 245 nm film) are also presented and shifted vertically for clarity. (c) The normal-incidence reflection spectra of series of homogeneous films showing the overlapping degree with the 245 nm film. (d) The evolution of the normal-incidence reflection spectra for the (2+1) D PCHs with different size matching. Thus, it can be concluded that the photonic stopband structure of the (2+1) D PCHs has a close correlation with the bandgap overlapping degree of compositional two homo crystals. Here, we fixed the sphere size of one homo crystal at 245 nm, and the other one with size changing from 180 nm to 387 nm. The bandgap overlapping degree between two homo crystals and the photonic stopbands evolution of their corresponding heterostuctures were systematically studied, and the resulted normalincidence reflection spectra are illustrated in Fig. 4c and 4d, respectively. By comparative analysis, we found that with decreasing sphere size difference of the constituting homo photonic crystals, the stopband overlapping degree increased gradually, and the photonic stopband changed from two isolate stopbands to one broadened stopband and finally merged into one enhanced resonance peak, as shown in Fig. 4d. This suggests one way to tailor the photonic stopband properties of the colloidal crystal heterostructure. To explore this discipline further, the conception of resolution (RAB) is introduced to represent the stopband overlapping degree of homo PCs of A and B, which is widely used in chromatographic analysis. We noted ΔλAB to represent the distance from λA to λB. The RAB can be defined as RAB = 2(λA -λB ) (WA +WB ) , where the WA and WB stand for the full width of λA and λB, respectively. The key parameters of stop bands of herterostruces and homo films are summarized in Table 1. The calculated value of ΔλAB and corresponding R AB are also listed. Table 1. The key parameters of stop bands for (2+1) D PC heterostructures and their constituting homo films.
By comparing the parameters of stopbands for the two components and corresponding heterostructure, the stopband characteristics of the final heterostructure is related to the R AB value. We noted that there are two special situations as shown in Fig. 5, i.e. the two peaks coincide with half overlap (R AB = 0.5) and just do not overlap (RAB=1.0). By comprehensive analysis the data listed in Table 1, the optical properties of the heterostructure can be approximated from the spectra of the individual homo crystals according to a simple rule: 1) when RAB> 1, the two peaks of λA and λB do not overlap at all. The Bragg diffraction peak of heterostructure exhibits two separate peaks, and the peak’s position are basically the same with original λA and λB. It is equivalent
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to the simple addition of two stopbands of A and B, such as heterostructures of 245 nm/180 nm and 245 nm/387 nm. 2) when 0.5< RAB< 1, the overlapping degree of λA and λB is between half and just overlapping. The stopband of heterostructure presents a single and broad peak by stopband superposition. Several "passbands" also appear within the stop band due to the mutual coupling of the original bandgaps, such as 245 nm/200 nm and 245 nm/317 nm. 3) when RAB < 0.5, the overlapping degree of λA and λB is more than half peak width, and the interaction between A and B photonic crystals is very strong, resulting in the heterostructures with a coupled resonance strong peak. For example, the heterostructure of 245 nm/230 nm. Thus, the PBG variation rules of (2+1) D PC heterostructures were concluded, which provides a simple approach to achieve a desired stopband structures as will by stacking colloidal crystals with different stopband overlapping degree.
Fig. 5 Illustration for bandgap overlapping situation for (a) RAB=0.5 and (b) R AB=1. 3.3 (2+1) D PCHs with dual or multi-stopbands. Dual or multi-bandgap heterostructures exhibit more elaborate optical properties than their component homo crystals. They can simultaneously manipulate two or more optical signals and propose a wide range of potential applications in dual-mode band pass filter, multi-channel filters, multi-channel all-optical routing, multi-channel all-optical switch and other optical or optoelectronic devices. Based on the basic stopband rule, we have successfully prepared the (2+1) D PCHs with dual and multiphoton bandgap structures by finely selecting the colloidal size (RAB>1) and designing the stacking order of the homogeneous photonic crystals, as shown in Fig. 6. Fig. 6a shows the normal-incidence reflection spectra of a dual-stopbands photonic crystal heterostructure composed of a colloidal crystal of 200 nm on top of a colloidal crystal of 365 nm with a resolution value of 1.32. Fig. 6b shows a three-stopbands photonic crystal heterostructure (365 nm/245 nm/ 180 nm) with R 365nm/245nm=1.33 and R 245nm/180nm=1.16, respectively. They both exhibit apparent isolated stopbands, which are in agreement with the stopbands of the homo PS colloidal crystals, correspondingly. We also note that the band width is slightly broader which may be caused by the overlapping of the secondary diffraction peak (λs(365nm)) and Fabry–Pérot fringes. This suggests one way to finely tailor the photonic bandgap properties of the heterostructures. Of particular note is that the stopband property is slightly different when stacking in reversed order. From Fig. 6c, it is apparent that for 245 nm/180 nm the first stopband widens and displays two small passbands, while for the 180 nm/245 nm, the second stopband shows significant broadening and peak separation. We propose that these results arise from the fact that the illumination of the heterostructure from the side 180 nm or 245 nm. When light is irradiated from the surface of 180 nm, the primary reflection peak of 180 nm will interact with the secondary Bragg peak of 245 nm, and coupling to a broad peak. And when light is irradiated from the surface of 245 nm, the primary reflection peak of 245 nm will overlap with the Fabry–Pérot fringes of 180 nm. Similar behaviour has be explored by Pemble et al. in the previous work. 32
Fig. 6 The normal-incidence reflection spectra of (2+1) D PCHs with (a) two stopbands and (b) three stopbands. For the purpose of comparison, the reflection spectra for the homogeneous colloidal crystals (i.e., PS 180 nm film, 200 nm film, 245 nm film and 365 nm film) are also presented. (c) The normal-incidence reflection spectra of the heterostructure with different stacking order. 3.4 (2+1) D PCHs with ultra-wide stopband. Ultra-wide bandgap photonic crystals which can inhibit light propagation over a wide range of wavelength have been widely used in wide-spectrum solar cells, broadband filters and antireflective optical films. As discussed above, (2+1) D PCHs composed of two homo crystals with proper R AB value (0.5>RAB<1) exhibits a broadened stopband. Ultra-wide bandgap can be created by stacking three photonic crystals to form a double-heterojunction structure, which can achieve the purpose of further expanding the bandgap width and afford new opportunities for engineered photonic behavior. Fig. 7a demonstrates the normal-incidence reflection spectra of series of heterostructures with varied R AB value, and the number of layers of each homo photonic crystal is 6.
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It can be seen that the stopband of heterojunctions with R AB in the range of 0.5 to 1 show good bandgap superposition and widening feature, which further confirms the rules we concluded above. Finally, the homo crystals of 182 nm, 200 nm and 235 nm were chosen to build a double heterojunction structure. Fig. 7b shows the reflection spectra of this double heterojunction 235 nm/200 nm/182 nm, and the spectra of the homo colloidal crystals of 235 nm, 200 nm and 180 nm are also presented, respectively. It can be seen that the photonic bandgap width of this double heterostructure is broadened to about 200 nm, thus the light in the range of 450 ~ 650 nm could be inhibited effectively. Fig. 7c gives the optical photographs of the double heterojunction structure as well as its components. The color of the photonic crystal of 182 nm, 200 nm and 235 nm is blue-violet, yellow-green and yellow, respectively. Color superimposition is also observed when they are stacked together to fabricate heterostructures.
Fig. 7 (a) The normal-incidence reflection spectra of series of (2+1) D PCHs with different R AB value. (b) The reflection spectrum of the double heterojunction structure showing the ultra-wide bandgap as well as the spectra of the homo crystals. (c) Photographs of as-made homo crystals, heterostructures and double-heterojunction sample.
Fig. 8 The normal-incidence reflection spectra of the double heterojunction structure with different stacking order. The construction of ultra-wide stopband photonic crystal heterostructure is not only related to the size selection of each component colloidal spheres, but also has a great influence on the stacking order of the homo photonic crystals. Up to now, all the heterostructures demonstrated in this work are stacking along the direction of the vertical substrate with colloidal monolayer sphere size from large to small order. Fig. 8 shows the normal-incidence reflection spectra of the double heterojunctions with different stacking orders. It can be seen that the arrangement of the homo photonic crystals has a very large effect on the final stopband structure of the heterojunction. A number of very narrow "defects" occur in the range of 400 to 700 nm at heterojunctions of 200 nm/182 nm/235 nm, and the wavelengths of light can pass through photonic crystals with little or no loss, which can be used to produce high quality narrow-band filter or photonic crystal laser.33-35 The heterojunction of 182 nm/200 nm/235 nm presents a big “defect” at about 520 nm, and the stopband intensity is relatively low in the range of 400 ~ 500 nm. The specific reason needs further analysis. 3.5 Fluorescence enhancement by (2+1) D PCHs with dual stopbands.
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PCs can achieve fluorescence enhancement in two ways: 1) the bandgap position matching with the fluorophore excitation wavelength (E) for enhancing excitation; 2) the bandgap position matching with the emission wavelength (F) for enhanced light extraction. PCHs with dual bandgaps can simultaneously matching with the excitation wavelength and the emission wavelength of fluorescent medium, which may realize remarkable enhancement of fluorescence.36-38 The mechanism of luminescent enhancement has been widely studied in previous reports. Our developed layer-by-layer method and concluded PBG structure evolution rules make it possible to finely design the bandgap position to overlapping both E and F of different fluorescent medium.
Fig.9 (a) UV–visible absorption (red) and fluorescence (blue) spectra of the [Ru(dpp) 3]Cl2 in absolute ethanol solution under excitation wavelength of 450 nm. (b) The normal-incidence reflection spectra of (2+1) D PCH 245 nm/ 180 nm of different layers with two separate bandgaps, one centred at 450 nm and the other red edge covering 620 nm. The fluorescence spectra (c) and enhancement factor (d) of [Ru(dpp)3]Cl2 on heterostructure of 245 nm/ 180 nm with different layers and on the homogeneous photonic crystal films compared to that on glass. Fig. 9a presents the absorption and emission spectra of fluorescent medium [Ru(dpp) 3]Cl2, which shows the maximum absorption at 450 nm and emission at 620 nm under maximum excitation. According to the experienced result reported before, λ c ≈2.53D,28 then 180 nm and 245 nm was finally selected. The RAB≈ 1.16. Thus, a series of dual-bandgap heterostructures 245 nm/180 nm with different layers were fabricated, whose stopbands just respectively overlap the wavelength of E and F, as shown in Fig. 9b. It can be seen that the intensity of E-matching stopband increases with the increasing monolayers while the F-matching stopband experiences a trend of first increase and then decline. The centre positions of the dual stopbands did not change obviously but the minibands disappeared gradually, which is probably owing to more defects involved in many times stacking. Fig. 9c shows the fluorescence spectra of [Ru(dpp)3]Cl2 on PCH of 245 nm/ 180 nm with different layers as well as homogeneous PC films and glass substrate, and Fig. 9d gives the luminescent enhancement factor of different samples. Compared with the luminescence intensity of [Ru(dpp 3)]Cl2 on the glass , the E-matching and F-matching PC films can largely increase the FL intensity due to the modulation through the periodic dielectric constant. The (2+1) D PCH with E-F-matching dual-bandgap leads to an extra luminescence enhancement of nearly 2 times, and the enhancement factor was further improved with the layer number increase. Therefore, the dual-bandgap PCH has the strongest effect on enhancing the total fluorescence intensity and improving fluorescent behaviour in thicker PCH films. 4.
Conclusions
In summary, we have demonstrated a facile and efficient method to achieve the controllable fabrication and flexible photonic stopband design of (2+1) D PCH. Based on the accurate regulation of structure, the optical properties of the resulting (2+1) D PCHs were systematically studied, and a universal PBG variation rule was proposed, which makes it possible to construct any kind of photonic stopband structure as required. Dual- or multi- stopband PCH and width-extended stopband PCH were successfully fabricated. The dual-stopbands PCH renders an additional 2-fold enhancement to the single stopband PC. The method introduced here could be used as a general tool to tailor the photonic properties of photonic crystal films and may play important role in special kind of optical filters, reflectors, waveguides and signal enhancement substrate. Acknowledgements The authors would like to thank financial support from the National Science Foundation of China (No.51173097, 91333109, 21473019), the Financial Grant from the China Postdoctoral Science Foundation (No.2016M601302) and the Fundamental Research Funds for the Central Universities (DUT17LK36 and DUT16RC(3)069 ). References 1. E. Yablonovitch, Phys. Rev. Lett., 1987, 58, 2059. 2. S. John, Phys. Rev. Lett., 1987, 58, 2486.
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