Patterning and pixelation of colloidal photonic crystals for addressable integrated photonics

Transcription

1 Journal of Materials Chemistry Dynamic Article Links C < Cite this: J. Mater. Chem., 2011, 21, Patterning and pixelation of colloidal photonic crystals for addressable integrated photonics Tao Ding, ab Liang Luo, c Hong Wang, d Li Chen, ab Kui Liang, a Koen Clays,* e Kai Song,* a Guoqiang Yang a and Chen-Ho Tung a Received 21st March 2011, Accepted 13th May 2011 DOI: /c1jm11194a PAPER Patterning colloidal photonic crystals are a first step to the realization of integrated photonic circuits. In this paper, we introduce a new way to pattern colloidal photonic crystals by applying ultrasonication to selectively remove certain parts of the colloidal photonic crystal film from the substrate. These patterns with hydrophobicity can further function as templates to direct the growth of another layer of colloidal photonic crystal in order to reach dual-patterned photonic crystals with distinct hydrophilic and hydrophobic domains. The dual-patterns are responsive to water vapor, which can reversibly switch the reflection color of the hydrophilic regions on and off many times, while the hydrophobic parts are almost unaffected. Introduction Due to the inherent limitations of electronics, photonics is expected to be the next enabling technology for information transfer, manipulation and storage in the very near future. Photons, as the alternative elementary information carriers, present a whole realm of advantages over electrons, such as: i) faster data transfer often in combination with wavelength division multiplexing schemes over the same optical fiber, a feature unique to photons; ii) faster processing speed often in combination with two-dimensional, or even three-dimensional, parallel schemes also unique to photons, as in spatial light modulators or volume holograms; iii) much less heat dissipation - hence a lower power consumption; iv) the possibility of an increase in the signal strength upon conversion of the electrical to optical to electrical signal; and v) no electromagnetic interference. On the global road map for photonics, the development of miniaturized and efficiently integrated optical circuits represents a milestone towards various components for the photonics a Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China. kai. song@iccas.ac.cn; Fax: ; Tel: b Graduate University of the Chinese Academy of Sciences, Beijing, China c State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China d Laboratory of Nano-Fabrication and Novel Devices Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences Beijing, China e Department of Chemistry, University of Leuven, Celestijnenlaan 200D, Leuven, Belgium. koen.clays@fys.kuleuven.be Electronic supplementary information (ESI) available: SEM and optical images of the patterned substrate and CPCs made of silica and polystyrene. The water contact angle measurement of the hydrophobic patterned CPCs. The video demonstrating the water switching optical property of the patterned CPCs. See DOI: /c1jm11194a industry. 1 Photonic crystals (PhCs), also known as photonic band gap (PBG) materials, hold a lot of promise to realize this aim due to their exciting and unique optical properties, such as inhibiting the transmission of light in selected directions or localizing light in the inner cavity for a particular spectral range (band gap) of photons, in analogy to the band gap for electrons in semiconductors. 2,3 Among numerous research studies on fabrication of photonic band gap materials, patterning colloidal photonic crystals can be regarded as a key step in the developing road from colloidal crystals as the original material to the final integrated optical circuits. To date, a wide range of approaches for patterning colloidal photonic crystals (CPCs) have been reported. 4 8 Early research in this direction was mainly focused on the direct deposition of CPCs on the patterned regions of a pre-treated template. The patterning effects of these pre-treated templates can be divided into two types: physical confinement 4 and chemical wettability. 5 In the physical confinement method, submicrometer colloidal particles are driven into defining microchannels by capillary forces. The latter approach, chemical wettability, is typically done by controlling the crystallization of colloids on certain regions with selective wetting properties. Besides these template-based methods, ink-jet printing 6a and pintool plotter microcontact printing techniques 6b can also be applied to write CPC patterns directly onto a substrate. However, the methods mentioned above are only effective at producing patterned artificial opals under certain appropriate conditions, but fail under others. They have some disadvantages, such as being incapable of fabricating dual- and multi-patterns 4 6 and complex-shaped patterns, 4h,6,7a,b non-uniformity of the thickness and crystalline orientation of the patterns, 5a c,6 being time-consuming, 5d making patterns too big (millimetre-sized features) for integration 7c and requiring complicated fabricating J. Mater. Chem., 2011, 21, This journal is ª The Royal Society of Chemistry 2011

2 procedures. 7b,c,8 Therefore, it is highly desirable to develop a more robust, facile, scalable and cost-effective approach to fabricate complex integrated patterns of different kinds of CPCs, which is crucial for the integration of PhCs into optical chips. To answer these demands, herein, we present an approach of fabricating micropatterned CPCs by selectively removing the predetermined parts from the uniform and integral colloidal crystal film with ultrasonication. The micropatterned CPCs obtained by this method allow pixelation and can lead to addressing of pixels. As illustrated in Fig. 1, the process starts by spin-coating a layer of photoresist (AZ9918) on a substrate (like a silicon wafer). After being illuminated by deep ultraviolet (DUV) light, the masked regions of the photoresist were patterned on the wafer. The metals chromium and copper were sequentially sputter -coated onto the wafer. Afterwards, a thin colloidal crystal film was deposited onto the patterned silicon wafer by a vertical convective self-assembly method. The copper surface was pre-treated with oxygen plasma for 2 min to enhance the surface hydrophilicity for the growth of colloidal crystals. After the CPC film was deposited onto the copper surface, the whole structure was immersed in acetone, used as a solvent, with mild ultrasonication. As a result, the CPCs in the regions with the photoresist were easily peeled off from the silicon surface due to the ultrasonic cavitation effect (also known as ultrasonic-assisted lift-off ), 9 while the CPCs in the regions without photoresist patterns remained. In this way, patterning of CPCs was realized. The size of the CPCs patterns can be conveniently varied from millimetres to micrometres based on the dimensions of the mask, taking into account the optical diffraction limit. Furthermore, the pattern of the CPCs itself can serve as a physical and chemical template to realize dual-patterning and continuous pixelation through the deposition of another CPC on top of it. Experimental Fabrication of patterned substrate A clean glass or silicon substrate was spin-coated with a layer of photoresist (AZ9918) with a thickness of 2.3 mm. The pattern was transferred to the silicon via photolithography, followed by deposition of a 5 nm chromium layer and a 50 nm copper layer. Patterning CPCs Monodisperse silica microspheres were synthesized by the St ober method. 10 Polystyrene beads were synthesized following our previous report. 11 A thin CPC film was deposited onto the patterned silicon substrate via the convective self-assembly method. Typically, 3 ml of an aqueous suspension of silica spheres with a volume fraction of 0.1% was put into a beaker with a total volume of 5 ml. The patterned glass substrate was vertically immersed in the suspension. Afterwards, the beaker was placed in an oven preheated at 60 C for about 2 days to dry the suspension. The CPC film, along with the patterned substrate, was subsequently immersed in acetone, used as the solvent, for 1 h to totally dissolve the residual photoresist. Ultrasonication (250 W) was applied for no more than 2 s to completely peel off the unpatterned regions. This procedure has to be done with great care, since time and the power of ultrasonication critically determine the amount of material removed. Dual-patterning and pixelation of CPCs A thin CPC film was deposited onto the patterned glass substrate, as described previously. The dried glass substrate covered with CPC film was submitted to a vapor of octadecyltrichlorosilane (OTS, 95%, Alfa Aesar) at 120 C in a vacuum chamber for 2 h. Consequently, the whole silica CPC film turned hydrophobic with a water contact angle of 120 (see Fig. S4 ). Afterwards, the unpatterned regions of the hydrophobic CPCs were peeled off via ultrasonication, as mentioned above. Another layer of CPC film made of silica spheres of a different size was deposited in the same way, as mentioned before. Since the CPC films tend to grow in the hydrophilic region only, most of the CPCs were deposited in the unpatterned region with the bare glass surface. As a result, a dual-pattern of individual pixels of CCs made of silica with two different sizes was formed. Characterization The optical images of the patterns were captured with an Olympus microscope (MX40) equipped with a charge coupled device (CCD) detector connected to the computer. The reflection spectra were measured with a fiber spectrometer (AvaSpec-128, Avantes) averaged over a light spot of 1 mm 2. The optical response upon wetting and drying was checked by blowing water vapor over the surface of the dual-patterned CPC films. Contact angles were measured on an OCA20 (DataPhysics, Germany) contact angle system at ambient temperature. The patterned microstructures were characterized with scanning electronic microscopy (SEM, JEOL S4300 or S4800, Hitachi) at an accelerating voltage of 15 kv. Results and discussion Fabrication Fig. 1 wafer. A schematic diagram of patterning of the CPCs on a silicon As the first step, we fabricated masks with different sizes and shapes so that different patterns could be transferred onto the silicon wafer. After illumination by DUV, the unmasked regions were dissolved and patterns were formed on the wafer (see Fig. S1 ). Chromium was first coated onto the substrate, functioning as a sticky layer to stop the patterned regions from being peeled off from the substrate. The copper layer was coated because it can form microcavities on the silicon surface after the photoresist was dissolved, which is beneficial to peeling off the CPCs from the substrate during ultrasonication. This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21,

3 Ultrasonication is a widely used cleaning technique that is based on the principle of the cavitation effect. 9 The microscopic bubbles in the liquid medium implode or collapse under the pressure of agitation to produce shock waves, which impinge on the surface of the object through a scrubbing action and loosen particulate matter from that surface. Since the size of the bubbles is microscopic, they can penetrate the cracks and holes to loosen the contaminants and remove them. Here, the patterned substrate deposited with a slab of CPC film was immersed in acetone to form hollow cavities by dissolving the photoresist, as shown in Fig. 2. During the process of ultrasonication, the CPCs slab above the cavity was subjected to a cavitation force (F). The strength of the cavitation force (F) follows the equation: F ¼ P $ S (1) where P is the pressure induced by the shock waves, which is determined by the power of the applied ultrasonication, and S is the top surface area of the cavity or the bottom surface of the slab on which the cavitation force is applied. On the other hand, the slab that we want to peel off is also chelated by the surrounding CPCs, which prevents the CPCs from being peeled off from the substrate. This represents a templating force (f). This templating force is proportional to the thickness (h) of the CPC slab and the circumference (not indicated in Fig. 2) of the patterning unit (a single pixel) of the CPC slab. Only if the cavitation forces are larger than the templating forces can the CPC slab above a cavity be peeled off from the whole CPC. Consequently, a thick and narrow slab will be more difficult to peel off. Therefore, the thickness of the deposited CPCs, which can be controlled roughly by the volume fraction of the colloidal suspension, should be limited to about a few layers above the cavities. It is also noteable that only ultrasonication with a carefully selected power and time can selectively peel off the CPC slab as designed. In other words, a delicate balance between keeping patterned CPC and peeling off unpatterned CPC is the key factor in our strategy. Otherwise, incomplete removal of the unpatterned region or destruction of the patterned region will occur (see Fig. S2c ). Resolution 290 nm silica spheres by the method of vertical convective selfassembly on a patterned silicon wafer for ultrasonication patterning. Fig. 3a is the optical image of the patterned CPCs after ultrasonication. The patterned stripes with an orange reflection are still covered with CPCs, while the blue background is the reflection color of the silicon wafer after removal of CPCs by ultrasonication. We can see that the unpatterned regions were totally peeled off when the gap between the patterns was larger than mm 2 (d l). Partial peeling off can be observed for a gap size of mm 2 and mm 2 in Fig. 1a. This result is consistent with the above discussion. The SEM images (top view, Fig. 3b, and side view, Fig. 3c) of the patterns further reveal detailed information about the patterned CPCs. The silica spheres with a diameter of 290 nm were closely packed in the fcc crystalline lattice. Some defects can be observed. Either the selfassembly or the sonication can introduce point defects. However, appropriate conditions for ultrasonication (250 W for 2 s) did not break the order of the crystalline lattice, as can be observed from the side view in Fig. 3c. Furthermore, the narrow and intense single reflection peak of the patterned CPCs in Fig. 3d also exemplifies the good crystallinity of the CPCs treated by ultrasonication. The good preservation of the crystalline lattice is probably caused by every sphere in the body of the CPCs being chelated by 12 spheres around it, which is a hard arrangement to break by weak ultrasonication. However, for the spheres in the top layer, each sphere is surrounded by only 9 spheres, which is relatively easier to disturb by ultrasonication. This might explain the small degree of disorder within the top layer (see Fig. 3c). Dual-patterning and optical switching Using this approach, we can get CPC patterns with different shapes and sizes (see Figs S2a c ). This approach can also be extended to pattern CPCs made of PS spheres, except that ethanol has to be used as the solvent during ultrasonication (see Fig. S3 ). Moreover, it provides a convenient method to fabricate CPCs with dual-patterns and pixels, as schematically depicted in Fig. 4a. First, a CPC film of silica spheres with a size of 245 nm (type A) was deposited onto the patterned glass substrate, followed by immersion in the vapor of To explore the highest resolution achievable by this ultrasonication patterning technique, we assembled CPCs made of Fig. 2 A scheme illustrating the essential microstructural features for the cavitation force F (surface area S) and templating force f (slab thickness h). Fig. 3 (a) An optical microscopy image of the patterned CPCs; (b) the corresponding SEM image with an insert of the magnified view of the pattern edge; (c) a side view of the pattern; (d) the reflection spectrum of the patterned CPCs. Scale bar in (a) is 50 mm, in (b) is 100 mm, in (c) is 5 mm, and in the insert of (b) is 2 mm J. Mater. Chem., 2011, 21, This journal is ª The Royal Society of Chemistry 2011

4 Fig. 4 (a) A scheme of the fabrication of the dual-patterned CPCs. SEM images of (b) the patterned and (c) the dual-patterned CPCs are magnified from the black-framed region of the insets, which are the corresponding optical microscope images. The scale bars in (b) and (c) are 1 mm and in the insets are 50 mm. octadecyltrichlorosilane (OTS) for 2h. The whole silica CPC film turned hydrophobic, with a water contact angle of 120 (see Fig. S4 ), while the hydrophilic glass substrate was protected by the photoresist film. Ultrasonication was applied to the hydrophobic CPC film to form patterned CPCs, as shown in Fig. 4b, in the inset of which the green structure of a patterned CPC (type A, 245 nm size) can be observed with the dark background of the bare glass substrate. Fig. 4b also illustrates the well-preserved order of the silica CPCs at the sharp edge of the patterns. Another CPC (type B, 280 nm size) of silica was deposited onto the patterned CPCs substrate. Since the CPC films tend to grow in the hydrophilic region of the glass substrate, most of the CPCs type B were deposited in the unpatterned region with the bare glass surface instead of covering the hydrophobic CPCs type A patterned substrate. As a result, the glass substrate ended up patterned by two kinds of CPC, as is clearly demonstrated in the optical image in the inset of Fig. 4c. The green pixels are the previously patterned CPC (type A) and the orange region is the newly deposited CPC (type B). Detailed information on the dualpatterned CPCs at the heterojunction region was obtained by SEM, shown in Fig. 4c. The line defect ( crack ) between two adjacent colloidal crystals of different types is attributed to shrinkage during the drying process after the deposition of CPC type B. Clearly, this patterning technique, as demonstrated above, provides a facile route to integrate two different types of CPCs on one chip, which is promising for the realization of CPC Fig. 6 Reversible optical switching of type B CPCs wetted and dried by water vapor for ten cycles, showing the reflection percentage of 34% at 600 nm and 16% at 645 nm. heterojunctions, pixels and optically active elements for alloptical computations. 12 At the same time, the dual-patterned CPCs fabricated by this method exhibit clearly distinctive hydrophilic and hydrophobic microdomains with different wetting properties, leading to their optical switching property, as shown in Fig. 5. The reflection spectra were taken over a 1 x 1 mm region containing both CPC type A and CPC type B. Optical images are shown in the insets of Fig. 5. Initially, before water vapor wetting, two Bragg peaks can be clearly observed, which were caused by the reflection from both crystal spacings. The green (535 nm) color in the reflection microscope image corresponds to CPC type A and the orange (600 nm) finds its origins in CPC type B (see Fig. 5a). After a flow of water vapor was directed to the surface of the dual-patterned CPCs, water microdroplets condensed and quickly infiltrated into the voids of the hydrophilic type B CPCs. Because of the good refractive index match between silica and water, the reflection peak for CPC type B quickly diminished and, ultimately, only a very weak reflection was detected at 645 nm, as shown in Fig. 5b. On the other hand, water vapor that condensed on the hydrophobic type A CPCs could only accumulate to form water droplets on the hydrophobic surface and could not infiltrate into the voids (see the inset of Fig. 5b). Consequently, for CPC type A, the green color was preserved. As the flow of water vapor was stopped and the water in the sample was allowed to evaporate, the reflection spectra and the structural colors of the dual-patterned CPCs Fig. 5 Optical reflection spectra of the dual-patterned CPCs (a) before and (b) after a flow of water vapor was projected onto the surface of the dualpatterned CPCs and (c) after the dual-patterned CPCs were dried. The insets are the optical microscope images with scale bars of 50 mm. This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21,

5 returned to their initial state (see Fig. 5c). A detailed video clip of the whole water vapor switching process can be found in the ESI. To investigate the stability and reversibility of this optical switching scheme with a water vapor stimulus, ten cycles were carried out with good reproducibility, as indicated in Fig. 6. The water vapor flow was switched on and off and the reflection of the CPCs type B at 600 nm wavelength switched from 34% in the dry on (or 1 ) state to 16% at 645 nm in the wet off (or 0 ) state reversibly for at least 10 cycles and with high fidelity. In contrast, the change in the reflection intensity of the hydrophobic type A CPCs was less than 3% (not shown). The switching speed was determined by the rate of switching of the water vapor flow and water evaporation, which took less than 10 s (see the video clip in ESI ). Conclusions In summary, we have demonstrated a scalable, low cost and facile approach to fabricate patterned CPCs based on ultrasonication. The cavitation force was utilized as the main force to peel off the CPCs in the unpatterned regions of the substrate. The shape and size of the patterned CPCs can be changed simply by changing the pre-patterned substrate. Moreover, CPCs made of latex and silica spheres can both be patterned on a silicon or glass substrate by this method. The resolution of the patterns can be better than ten micrometres under the current conditions. Based on this technique, we can fabricate dual-patterned CPCs with distinctive hydrophilic and hydrophobic domains, which are to attributed the dual-patterned CPCs with a prominent optical switching property in response to water vapor. These miniaturized CPC patterns may also function as small pixels in color displays, which can be tuned by electric stimuli. 13,14 Moreover, this transference of circuitry design and engineering from electronics to the photonics platform, after enhancement of the colloidal crystallinity and material aspects of colloidal photonic crystals in the future, may open up a method for manufacturing large-scale integrated optical circuits for all-optical computing. 15 Acknowledgements We acknowledge the financial support provided by the NSFC (No ) and the 973 program (Nos. 2007CB808004, 2009CB930802). T. Ding gives special thanks for the financial support of the CAS Special Grant for Postgraduate Research, Innovation and Practice. We also thank the Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS. Notes and references 1 R. Kirchain and L. Kimerling, Nat. Photonics, 2007, 1, E. Yablonovitch, Phys. Rev. Lett., 1987, 58, S. John, Phys. Rev. Lett., 1987, 58, (a) A. van Blaaderen, R. Ruel and P. Wiltzius, Nature, 1997, 385, 321; (b) S. M. Yang and G. A. Ozin, Chem. Commun., 2000, 2507; (c) H. Mıguez, S. M. Yang and G. A. Ozin, Appl. Phys. Lett., 2002, 81, 2493; (d) P. Ferrand, M. Egen, B. Griesebock, J. Ahopelto, M. M uller, R. Zentel, S. G. Romanov and C. M. Sotomayor Torres, Appl. Phys. Lett., 2002, 81, 2689; (e) C. Jin, M. A. Mclachlan, D. W. McComb, R. M. De La Rue and N. P. Johnson, Nano Lett., 2005, 5, 2646; (f) S.-K. Lee, G.-R. Yi and S.-M. Yang, Lab Chip, 2006, 6, 1171; (g) J. Ye, R. Zentel, S. Arpiainen, J. Ahopelto, F. Jonsson, S. G. Romanov and C. M. Sotomayor Torres, Langmuir, 2006, 22, 7378; (h) Y. Yamauchi, J. Imasu, Y. Kuroda, K. Kuroda and Y. Sakka, J. Mater. Chem., 2009, 19, 1964; (i) S. Arpiainen, F. Jonsson, J. R. Dekker, G. Kocher, W. Khunsin, C. M. S. Torres and J. Ahopelto, Adv. Funct. Mater., 2009, 19, (a) C. A. Fustin, G. Glasser, H. W. Spiess and U. Jonas, Adv. Mater., 2003, 15, 1025; (b) C. A. Fustin, G. Glasser, H. W. Spiess and U. Jonas, Langmuir, 2004, 20, 9114; (c) F. Q. Fan and K. J. Stebe, Langmuir, 2004, 20, 3062; (d) A. M. Brozell, M. A. Muha and A. N. Parikh, Langmuir, 2005, 21, (a) J. Park, J. Moon, H. Shin, D. Wang and M. Park, J. Colloid Interface Sci., 2006, 298, 713; (b) K. Burkert, T. Neumann, J. Wang, U. Jonas, W. Knoll and H. Ottleben, Langmuir, 2007, 23, (a) H.-L. Li and F. Marlow, Chem. Mater., 2005, 17, 3809; (b) S. M. Yang, H. Mıguez and G. A. Ozin, Adv. Funct. Mater., 2002, 12, 425; (c) Z. Z. Gu, A. Fujishima and O. Sato, Angew. Chem., Int. Ed., 2002, 41, H. Kim, J. Ge, J. Kim, S. Choi, H. Lee, H. Lee, W. Park, Y. Yin and S. Kwon, Nat. Photonics, 2009, 3, F. J. Fuchs, Ultrasonic Cleaning: Fundamental Theory and Application. Precision Cleaning 95 Proceedings, Precision cleaning 0 95 conference: May 15 17, Rosemont Convention Center, Rosemont, IL, 1995, W. St ober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, Z.-F. Liu, T. Ding, G. Zhang, K. Song, K. Clays and C.-H. Tung, Langmuir, 2008, 24, A. Arsenault, S. Fournier-Bidoz, B. Hatton, H. Mıguez, N. Tetreault, E. Vekris, S. Wong, S. M. Yang, V. Kitaev and G. A. Ozin, J. Mater. Chem., 2004, 14, D. Puzzo, A. C. Arsenault, I. Manners and G. A. Ozin, Nat. Photonics, 2007, 1, D. Puzzo, A. C. Arsenault, I. Manners and G. A. Ozin, Angew. Chem., Int. Ed., 2009, 48, J. D. Joannopoulos, P. R. Vileneuve and S. Fan, Nature, 1997, 386, J. Mater. Chem., 2011, 21, This journal is ª The Royal Society of Chemistry 2011