Bottom-Up Photonic Crystal Approach with Top-Down Defect and

pubs.acs.org/Langmuir © 2009 American Chemical Society

Bottom-Up Photonic Crystal Approach with Top-Down Defect and Heterostructure Fine-Tuning Tao Ding,†,§ Kai Song,*,† Koen Clays,*,‡ and Chen-Ho Tung† †

Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, ‡Department of Chemistry, University of Leuven, Celestijnenlaan 200D, Leuven, BE-3001, Belgium, and § Graduate University of the Chinese Academy of Sciences, Beijing 100190, China Received September 8, 2009. Revised Manuscript Received November 3, 2009 We combine the most efficient (chemical) approach toward three-dimensional photonic crystals with the most convenient (physical) technique for creating non-close-packed crystalline structures. Self-assembly of colloidal particles in artificial opals is followed by a carefully tuned plasma etching treatment. By covering the resulting top layer of more open structure with original dense opal, embedded defect layers and heterostructures can be conveniently designed for advanced photonic band gap and band edge engineering.

1. Introduction Photonic crystals, or photonic band gap materials, are promising nanoscale materials for advanced photonic applications. The patterning of the dielectric properties of the material on the nanometer scale results in an optical band gap, very much as the periodic electric potential in semiconductor crystals leads to the band gap for electrons. This electronic band gap is exploited in electronic integrated circuits. The optical band gap holds promise for realizing optical integrated circuits.1,2 While one-dimensional (1D) and two-dimensional (2D) patterning of the optical properties can be conveniently realized by physical approaches such as lithography, the realization of threedimensional (3D) photonic crystals by such techniques relying on removing material from a homogeneous bulk remains challenging. Long-range interactions between colloidal particles in suspension leads in a natural way to self-assembly in a hexagonal close packing in 3D. Different variations of this approach have been explored, but the method of convective self-assembly of monodisperse submicrospheres appears to combine costeffectiveness with large-scale potential and short production time.3 The nature of the colloidal particles and the resulting crystal structure translate in low dielectric contrast with a high fill factor, ultimately leading to a pseudo band gap.4 For the realization of a full band gap in the optical regime, higher refractive index material has to be combined with a lower fill factor. Quite analogous to n- and p-impurity doping in semiconductor material leading to additionally allowed energy levels for electrons within the forbidden electronic band gap, the insertion of intentional defects in photonic band gap materials results in additionally allowed pass bands in the forbidden optical stop band, the photonic band gap, of photonic crystals.5 This domain of photonic band gap engineering relies on the intrinsic dependence of amplitude, width, and spectral position of the stop band on the *To whom correspondence should be addressed. E-mail: kai.song@iccas. ac.cn (K.S.); [email protected] (K.C.). (1) Yablonovith, E. Phys. Rev. Lett. 1987, 58, 2059. (2) John, S. Phys. Rev. Lett. 1987, 58, 2486. (3) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (4) L
opez, C. Adv. Mater. 2003, : : 15, 1679. (5) Pradhan, R. D.; Tarhan, I. I.; Watson, G. H. Phys. Rev. B 2006, 54, 13721.

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refractive index and size of the colloidal microsphere. The relative size or refractive index of the particle in the defect layer then determines the characteristics of the pass band in the stop band. Apart from this band gap engineering relying on 2D defects, the realization of 1D line defects for optical waveguide and of 0D point defects for optical microcavities mostly uniquely derives from top-down approaches including confocal microscopy and direct laser writing.6 From the dependence of the pass band characteristics on refractive index and size of the particle in the defect (layer, line, or point), it follows that the natural way of introducing a defect is embedding particles of the same chemical material but with a different size or embedding the same size particles but from a different chemical nature. Both approaches result in an effective optical phase shift in the photonic crystal. Specifically for the realization of 2D planar defects (with the aim of band gap engineering to realize a specific optical filter function for light propagating perpendicular to the defect layer or for the insertion of a slab optical waveguide for light propagating parallel to the defect layer) surface-active techniques such as the Langmuir-Blodgett deposition technique,7 spin-coating,8 transfer printing,9 and chemical vapor deposition10 lend themselves naturally. This bottom-up approach toward 2D defect layers also allows the tunability of the defect mode by external stimuli and, (6) (a) Taton, T. A.; Norris, D. J. Nature 2002, 416, 685. (b) Vekris, E.; Kitaev, V.; von Freymann, G.; Perovic, D. D.; Aitchison, J. S.; Ozin, G. A. Adv. Mater. 2005, 17, 1269. (c) Jun, Y.; Leatherdale, C. A.; Norris, D. J. Adv. Mater. 2005, 17, 1908. (d) Yan, Q.; Zhou, Z.; Zhao, X. S.; Chua, S. J. Adv. Mater. 2005, 17, 1917. (e) Braun, P. V.; Rinne, S. A.; García-Santamaría, F. Adv. Mater. 2006, 18, 2665. (f) Yan, Q.; Wang, L.; Zhao, X. Adv. Mater. 2007, 17, 3695. (7) (a) Egen, M.; Voss, R.; Griesebock, B.; Zentel, R. Chem. Mater. 2003, 15, 3786. (b) Zhao, Y.; Wostyn, K.; de Schaetzen, G.; Clays, K.; Hellemans, L.; Persoons, A.; Szekeres, M.; Schoonheydt, R. A. Appl. Phys. Lett. 2003, 82, 3764. (c) Wostyn, K.; Zhao, Y.; de Schaetzen, G.; Hellemans, L.; Matsuda, N.; Clays, K.; Persoons, A. Lanmguir 2003, 19, 4465. (d) Mass
e, P.; Reculusa, S.; Clays, K.; Ravaine, S. Chem. Phys. Lett. 2006, 422, 251. (e) Mass
e, P.; Pouclet, G.; Ravaine, S. Adv. Mater. 2008, 20, 584. (8) (a) Wang, L.; Yan, Q.; Zhao, X. S. Langmuir 2006, 22, 3481. (b) Pozas, R.; Mihi, A.; Oca~na, M.; Míguez, H. Adv. Mater. 2006, 18, 1183. (9) T
etreault, N.; Arsenault, A. C.; Mihi, A.; Wong, S.; Kitaev, V.; Manners, I.; Miguez, H.; Ozin, G. A. Adv. Mater. 2005, 17, 1912. (10) (a) Palacios-Lid
on, E.; Galisteo-L
opez, J.; Ju
arez, B.; L
opez, C. Adv. Mater. 2004, 16, 341. (b) T
etreault, N.; Mihi, A.; Míguez, H.; Rodríguez, I.; Ozin, G. A. Adv. Mater. 2004, 16, 346.

Published on Web 11/17/2009

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Scheme 1. Schematic Illustration of the Procedures To Fabricate (a) Defect Layers and (b) Heterostructures in Colloidal Crystals via the Plasma Etching Technique

conversely, sensitivity to the environment, leading to photonic crystal sensors.11 However, these bottom-up approaches all require the deposition of a layer of different material. Either a specific different size of particle first has to be synthesized and then inserted or the identical size of particle is called for, yet from a different nature, but there is always an additional deposition step required because a different particle (different in nature or in size) will constitute the defect. This is where the combination of the bottom-up realization of the 3D photonic crystal with the top-down technique of removing material by etching can provide a distinctive advantage. Solutionphase etching with HF is restricted to silica (opals), very hard to control, and often harmful.12 Plasma etching is an efficient topdown technique, easy to operate and gentle toward material requirements, that has been widely used in surface function13 and patterning.14 It has been widely used to fabricate non-closepacked opals with reduced filling fraction to increase the fill factor in inverted opals.15 It has also been used to fabricate graded structures in non-close-packed opals. The nonuniform graded structure is believed to be the result of the etching rate being proportional to the exposed surface. Since the etching process itself exposes more material surface to the oxygen plasma, the etching rate decreases with increasing penetration depth for the plasma. It had been reported that the size of only the top four layers of particles was reduced after 9 min of plasma exposure, while the spheres in the lower layers kept their original size.16 We concluded that by reducing the plasma exposure time, the particle size of only the top one or two layers would be reduced, while the layers below would remain unaffected. We here demonstrate how the single convective self-assembly of colloidal particles of one size and nature can be used in (11) (a) Fleischhaker, F.; Arsenault, A. C.; Kitaev, V.; Peiris, F. C.; von Freymann, G.; Manners, I.; Zentel, R.; Ozin, G. A. J. Am. Chem. Soc. 2005, 127, 9318. (b) Fleischhaker, F.; Arsenault, A. C.; Wang, Z.; Kitaev, V.; Peiris, F. C.; Von Freymann, G.; Manners, I.; Zentel, R.; Ozin, G. A. Adv. Mater. 2005, 17, 2455. (c) Fleischhaker, F.; Arsenault, A. C.; Peiris, F. C.; Kitaev, V.; Manners, I.; Zentel, R.; Ozin, G. A. Adv. Mater. 2006, 18, 2387. (12) Fenollosa, R.; Meseguer, F. Adv. Mater. 2003, 15, 1282. (13) Riccardi, C.; Barni, R.; Selli, E.; Mazzone, G.; Massafra, M. R.; Marcandalli, B.; Poletti, G. Appl. Surf. Sci. 2003, 211, 386. (14) (a) Hua, F.; Shi, J.; Lvov, Y.; Cui, T. Nano Lett. 2002, 2, 1219. (b) Yang, S.; Yang, K.; Niu, L.; Nagarajan, R.; Bian, S.; Jain, A. K.; Kumar, J. Adv. Mater. 2004, 16, 693. (c) Choi, D.-G.; Yu, H. K.; Jang, S. G.; Yang, S.-M. J. Am. Chem. Soc. 2004, 126, 7019. (15) (a) Mı´ guez, H.; T
etreault, N.; Yang, S. M.; Kitaev, V.; Ozin, G. A. Adv. Mater. 2003, 15, 597. (b) Wang, L.; Yan, Q.; Zhao, X. J. Mater. Chem. 2006, 16, 4598. (16) Freyman, G. V.; John, S.; Kitaev, V.; Ozin, G. A. Adv. Mater. 2005, 17, 1273.

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Figure 1. SEM pictures of colloidal crystals after high-strength oxygen plasma etching: (A) top view of original slab after etching; (B) side view of original slab after etching; (C) top view of etched first crystal slab, after deposition of second crystal slab on top; (D) side view of etched first crystal slab, after deposition of second crystal slab on top. Inset: details of defect layer; scale bars: 2 μm. Langmuir 2010, 26(6), 4535–4539

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Figure 2. Optical reflection spectra of original colloidal crystal slab showing the forbidden stop band or band gap (dashed line) and of photonic crystal with embedded defect layer, showing the allowed pass band as a dip in the stop band (solid line).

conjunction with controlled top-down plasma etching to convert the top part of the close-packed photonic crystal into a less dense layer. The careful choice of the etching conditions allows to differentiate between the creation of a single defect layer or the introduction of a heterostructure: Treating the photonic crystal with strong oxygen plasma for a short period results in only the particles of the upper surface layer being etched, while leaving the underlying particles unchanged (Scheme 1a). If the etching conditions are milder, yet longer, a slightly graded but, more importantly, a non-close-packed photonic crystal will result (Scheme 1b). Only one more bottom-up deposition step is needed to arrive at an embedded defect layer (Scheme 1a, right) or heterostructure (Scheme 1b, right).

2. Experimental Section 2.1. Preparation of Embedded Defect or Heterostructure. Monodisperse submicrospheres of chemical composition poly(styrene-methyl methacrylate-acrylic acid) were kindly provided by Prof. Jingxia Wang (Institute of Chemistry, Chinese Academy of Sciences). The synthetic details have been reported in the literature.17 Convective self-assembly of these colloidal particles was induced at 65 °C from an aqueous suspension with volume fraction of 0.1%. The oxygen plasma etching was carried out in a vacuumed (80 mTorr) chamber prefilled with pure oxygen. The dry photonic crystal was subsequently subjected to oxygen plasma treatment (Harrick Plasma, PDC-32G) at high strength (18 W) for 1 min to induce the single layer defect. For the induction of the multiple layer non-close-packed structure, another dry photonic crystal is subjected to oxygen plasma treatment at medium strength (10 W) for an extended time of 5, 8, and 12 min. To cover these structures to arrive at an embedded defect layer or a heterostructure, a crystal slab similar to the original one is deposited on top of the defect layer or non-closepacked structure under identical conditions as for the first step. 2.2. Characterization of Structures. Scanning electron microscopy (SEM) images were taken with an accelerating voltage of 15 kV (JEOL S4800 or S4300). The diameter of the particles after different etching conditions and times was measured from the side views of the SEM images of the etched crystals. Optical reflection spectra were recorded from a typical sample area with a fiber spectrometer (Avantes AvaSpec-2048) with a light spot area of about 1 mm2.

3. Results and Discussion 3.1. Plasma-Etched 2D Defects in Self-Assembled 3D Photonic Crystals. Figure 1 demonstrates clearly that the suggested approach of post-treatment of self-assembled colloidal (17) Wang, J.; Hu, J.; Wen, Y.; Song, Y.; Jiang, L. Chem. Mater. 2006, 18, 4984.

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Figure 3. SEM top view pictures of colloidal crystals after medium-strength oxygen plasma etching as a function of etching time: (A) 0 min; (B) 5 min; (C) 8 min; (D) 12 min; scale bars: 2 μm. Inset of (A): enlargement of original particle, scale bar: 100 nm.

crystal results in a top layer of spheres with reduced size: Figure 1A shows an SEM top view of a colloidal crystal after DOI: 10.1021/la903371a

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Figure 4. Optical reflection spectra of original colloidal crystal slab and of this slab after different times of medium-strength oxygen plasma etching.

high strength plasma etching for 1 min. Figure 1B shows an SEM side view of the same crystal. The top view shows the top layer of particles with reduced size (380 nm, rather than the original 400 nm). The particles exhibit interconnects, ensuring the constant relative, epitaxial position. This is confirmed in Figure 1B, where it is also seen that the plasma etching has only affected the top two layers. The layers below still have the original 400 nm size. After depositing another layer of similar crystal slab on the plasmatreated colloidal crystal film, the resulted structure is a colloidal photonic crystal with an embedded planar defect layer. Figure 1C shows an SEM top view of the two colloidal crystal slabs with the plasma-etched defect layer in between. Figure 1D finally very clearly shows the embedded defect layer of 380 nm between two slabs of 400 nm particles. A defect layer in a photonic band gap material induces an allowed pass band in the forbidden stop band. The optical characterization of the plasma-etched defect in our colloidal photonic crystal is shown in Figure 2. The original opal slab has a strong reflection peak (pseudo band gap) at 920 nm, for light with normal incidence (normal to the (111) crystalline plane). Upon creating the defect layer and covering with a second crystal slab, the amplitude is reduced and a defect mode appears. Our combined approach of consecutive self-assembly of two colloidal particle slabs with plasma etching of the first slab only requires one size of particles of one nature. Moreover, it does not require an additional deposition step to insert the layer of particles of different size or nature. It does require an etching step, but its impact can be tuned to vary the particle size of the defect layer, thereby allowing the spectral tuning of the pass band from donor to acceptor mode. Finally, the interconnects between the smaller particles in the defect layer ensure the original crystal lattice parameters, allowing epitaxial deposition of the second crystal slab. This is a particular advantage that is highly searched for. It removes the unwanted crystalline defects from the intentional optical defects. The only other approach to realize this is the insertion of an optical defect layer of particles of exactly the same size, yet of a different nature (different refractive index). Apart from the difficulty to synthesize monodisperse and spherical submicroparticles of various nature, the requirement of an exactly identical size for submicrospheres of different nature is very demanding. Our plasma etching ensures this epitaxial growth and simultaneously allows for fine-tuning the spectral position of the pass band. 3.2. Non-Close-Packed Opals from Plasma-Etched Colloidal Crystals. Figure 3 shows the morphology changes in our colloidal photonic crystals as a function of increasing etching time for medium-strength etching. The original particle size of 400 nm (Figure 3A) is reduced to 387 nm (Figure 3B) after 5 min of 4538 DOI: 10.1021/la903371a

Figure 5. SEM images of photonic heterostructures made by depositing a photonic crystal slab on top of a first slap treated with mediumstrength etching: (A) top view, (B) side view for heterostructure after 8 min etching (hetero-S2); (C) top view, (D) side view for heterostructure after 12 min etching (hetero-S3). Scale bars in top views (A) and (C) are 5 μm; scale bars in side views (B) and (D) are 1 μm. Langmuir 2010, 26(6), 4535–4539

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etching. Interconnects between the smaller particles are clearly visible. Increasing the etching time to 8 min results in a complete removal of these connections and in the reduction of the size of the top layer particles to 286 nm. However, in the lower layers, we still observe interconnects and a larger particle size of around 375 nm. This had been rationalized in terms of the etching rate being proportional to the exposed surface.16 A further increase of the etching time to 12 min results in rather irregular shapes for the top layer of particles strongly reduced in size. However, the crystal lattice parameter of this top layer is highly conserved. The size for the underlying particles is further reduced to 360 nm. The optical reflection spectra (normal incidence, normal to the (111) plane) as a function of etching time are shown in Figure 4. The reflection peak at 920 nm is the original stop band without etching (see Figure 2). Upon increasing etching time, the fill factor is reduced (from 76.4% for the closest packing) and the reflection spectra blue shift to 903, 883, and 860 nm, after 5, 8, and 12 min of etching, respectively. This is in good agreement with the values (900, 881, and 858 nm, respectively) calculated from the Bragg relation and the measured particle sizes. There is also an increase of the amplitude of the reflection peak, which is in agreement with previous reports.12 3.3. Colloidal Crystal Heterostructures. The heterostructures were fabricated by depositing a second slab of colloidal crystal made of particles of the original 400 nm on top of the slab that was plasma-etched. This approach resulted in three different heterostructures (called hetero-S1, hetero-S2, and hetero-S3) for the three different etching times in section 3.2. SEM pictures of these heterostructures are shown in Figure 5. Figure 5A shows a top view and Figure 5B a side view of the heterostructure obtained after depositing 400 nm particles in a slab after etching for 8 min (reduced particle size 375 nm, hetero-S2). Figures 5C,D show the same for 12 min plasma etching (360 nm reduced particle size for the lower crystal slab, hetero-S3). Both top views clearly show a crystal slab of larger particles without etching effects on top of a crystal slab of particles with visual effects of etching (smaller and with interconnects). The side views (Figures 5B,D) show the interface region between the two crystal slabs, the lower layer clearly being the etched one. The optical reflection spectra for the heterostructures are shown in Figure 6. Comparing the experimental spectra and the simulated linear combination, it can be concluded that the defect lies in all three samples. The spectrum for hetero-S1 appears very similar to the spectrum shown in Figure 2 for a pass band in a stop band due to a defect layer. This is a consequence of the similarity in morphology resulting from 1 min high-strength etching or 5 min medium-strength etching (compare Figure 1A with Figure 3B). Therefore, hetero-S1 is much more an embedded defect layer structure than a true heterostructure. However, the spectra for the true heterostructures hetero-S2 and hetero-S3 are distinctively different from the spectrum for hetero-S1 and the defect layer structure. The most important difference is the total width of the optical band gap. For the heterostructures, this width is significantly larger. Second, the dip in the reflection is shifted to the blue for hetero-S3. Closer (18) (a) Jiang, P.; Ostojic, G. N.; Narat, R.; Mittleman, D. M.; Colvin, V. L. Adv. Mater. 2001, 13, 389. (b) Yan, Q.; Zhao, X.; Zhou, Z. J. Cryst. Growth 2006, 288, 205.

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Figure 6. Reflection spectra of different heterostructures (solid lines). Dashed lines illustrate the linear combination of the spectra of the etched bottom slab and the unetched top slab. According to the relative strengths of Bragg peaks as shown in Figure 4, the relative weight coefficients of the bottom slab to the top slab is set as 1.5.

inspection reveals that this is the simple consequence of the linear addition of the reflection spectra of the two slabs constituting the heterostructure.18 For hetero-S2, the reflection spectrum results from the band at 920 nm with the band at 870 nm, with a dip at 890 nm. For hetero-S3, the reflection spectrum results from the band at 920 nm with the band at 840 nm, with a dip at 865 nm. The same approach toward wide band gap engineering with a pass band has been used for spectral narrowing for reduced-threshold photonic crystal lasing.19

4. Conclusion It is possible to combine the advantages of bottom-up selfassembly of colloidal particles toward photonic crystals (inherently 3D, cost-effective, ease of operation, large scale) with the advantages of top-down etching (convenient, tunability) without being limited by the disadvantages of top-down approaches (difficult in 3D) to arrive at an approach toward embedded defect structures and heterostructures in photonic crystals. The approach not only eliminates the need for an additional defect layer deposition step but also eliminates the need for a specific defect layer particle of different size or different nature. A unique feature is that it inherently allows for epitaxial deposition of a second crystal slab for the embedding or creating of the heterostructures. SEM pictures and optical reflection spectra confirm the validity of the approach. Acknowledgment. We gratefully thank Prof. Jiang Zhao and Fei Wang for their help on oxygen plasma etching. We also thank Prof. Jingxia Wang and Dr. Jian Liu for their kindly providing monodisperse P(St-MMA-AA) colloidal particles. We acknowledge the financial support provided by the NSFC (No. 60877032) and the 973 program (Nos. 2007CB808004 and 2009CB930802). We also thank Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS. (19) Baert, K.; Song, K.; Vall
ee, R.; Van der Auweraer, M.; Clays, K. J. Appl. Phys. 2006, 100, 123112.

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