Patterning Colloidal Crystals and Nanostructure Arrays by Soft Lithography

Transcription

1 Patterning Colloidal Crystals and Nanostructure Arrays by Soft Lithography By Junhu Zhang and Bai Yang * As one of the most robust and versatile routes to fabricate ordered micro- and nanostructures, soft lithography has been extensively applied to pattern a variety of molecules, polymers, biomolecules, and nanomaterials. This paper provides an overview on recent developments employing soft lithography methods to pattern colloidal crystals and related nanostructure arrays. Lift-up soft lithography and modified microcontact printing methods are applied to fabricate patterned and non-close-packed colloidal crystals with controllable lattice spacing and lattice structure. Combining selective etching, imprinting, and micromolding methods, these colloidal crystal arrays can be employed as templates for fabrication of nanostructure arrays. Realization of all these processes is favored by the solvent swelling, elasticity, thermodecomposition, and thermoplastic characteristics of polymer materials. Applications of these colloidal crystals and nanostructure arrays have also been explored, such as biomimetic antireflective surfaces, superhydrophobic coatings, surfaceenhanced Raman spectroscopy substrates, and so on. 1. Introduction Patterned micro- and nanostructures have attracted great attention for a wide range of applications in microelectronics, optoelectronic devices, as well as biological and chemical sensors. [ 1 5 ] Many lithography methods for patterning surfaces in the micrometer and sub-micrometer ranges have been developed, including photolithography, electron beam lithography, X-ray lithography, dip-pen nanolithography, nanoimprinting, and so forth. [ 6 11 ] Each of these technologies has advantages and disadvantages regarding resolution, structural ordering, time and cost involved in constructing large-area surfaces. As an alternative non-photolithographic method, soft lithography has become one of the most robust and versatile routes to fabricate ordered microstructures. [ 12 ] The success of soft lithography relies on the use of a poly(dimethylsiloxane) (PDMS) elastomer as a stamp, mold, or mask, which ensures conformal contact between surfaces of PDMS and substrates of interest and easy release without destroying the formed microstructures. By flexibly applying [ ] Dr. J. Zhang, Prof. B. Yang State Key Lab of Supramolecular Structure and Materials College of Chemistry Jilin University Changchun , P. R. China byangchem@jlu.edu.cn DOI: /adfm different methods of soft lithography, a variety of materials could be selectively patterned through a PDMS stamp, such as organic molecules, proteins, nanoparticles, metals, polymers, as well as colloidal crystals. [ ] Colloidal crystals are ordered arrays of highly monodisperse silica or polymer microspheres, which represent a new class of advanced materials that have many potential applications in fields such as photonics, optics, and sensing. [ ] Selfassembly strategy is known as the most feasible route to fabricate two-dimensional (2D) or three-dimensional (3D) colloidal crystals. The self-assembly of colloidal microspheres can be achieved by solvent evaporation, gravity sedimentation, vertical deposition, electrophoresis, spin coating, crystallization in physically confined cells and other methods. [ ] Most colloidal crystals prepared by self-assembly strategy are entropy favorable face-centered cubic (fcc) and hexagonal close packing (hcp) stacking, however, specific microstructures, such as complex lattices and patterned structures, as well as designed defects are anticipated for their applications. [ ] To obtain these complex colloidal crystals, many lithography patterning methods have been employed to fabricate patterned surfaces with ordered micro- or nanostructures, which are effective templates for controlled assembly of colloidal microspheres. [ ] Soft lithography also provides a set of flexible methods for patterning colloidal crystals. Among them, micromolding in capillaries has been directly applied to pattern 2D and 3D colloidal crystals. [ 38 ] By soft lithography, solid surfaces could be patterned into nano- or micrometer-sized structures with different topological or chemical properties. These patterned surfaces could be subsequently used to direct the self-assembly of microspheres for creating colloidal crystals with defined crystalline orientations, shapes, and sizes. [ 39,40 ] In addition, preformed colloidal crystals can also be patterned by soft lithography through selectively removal or transfer printing of colloidal microsphere monolayers. [ ] There are many merits using colloidal microspheres as building blocks in constructing colloidal crystals, including multiple choices of chemical composition, tunable particle size and low cost. In addition, they are easy to be removed by solvent dissolution, chemical etching or thermal calcination, which enable them as templates for nanostructure arrays, macroporous materials or inverse opals. [ 44 ] Colloidal crystals have been 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3411

2 used as masks or templates for fabricating periodic nanostructures over large area, which are defined as colloidal lithography. Functional nanostructure arrays with size-dependent optical, magnetic, electrochemical, and catalytic properties have been fabricated by this strategy. [ ] In this paper, we introduce our research results on employing soft lithography methods to pattern colloidal crystals and related nanostructure arrays, along with recent important developments in the literature. Lift-up soft lithography and modified microcontact printing ( μ CP) method are applied to pattern and transfer self-assembled colloidal crystals by PDMS stamps. During the μ CP process, the PDMS elastomer stamps can be swelled or stretched, which result in non-close-packed (ncp) colloidal crystals with controllable lattice spacing and structure. Combining selective etching, imprinting, and micromolding methods, these colloidal crystal arrays can be employed as templates for fabrication of nanostructure arrays. Realization of all these processes is favored by the solvent swelling, elasticity, thermodecomposition and thermoplastic characteristics of polymer materials. Applications of these colloidal crystals and nanostructure arrays have also been explored, such as biomimetic antireflective surfaces, superhydrophobic coatings, surface-enhanced Raman scattering (SERS) substrates, and so on. 2. Patterning Colloidal Crystals by Lift-Up Soft Lithography Soft lithography has been used to modify solid surfaces with different properties, such as relief structures, charged nature, wettability, or current density, which help to direct colloidal microspheres to deposit on specific regions of surfaces. [ 40, ] Hammond and co-workers fabricated a patterned polyelectrolyte multilayer surface with different surface charge by soft lithography, and assembled two different sets of charged colloidal microspheres on the patterned surface through electrostatic interactions and supramolecular interactions respectively. [ 51 ] They also fabricated chemically modified nanostructured polymer templates, and combined physical and chemical patterning to achieve confined assemblies of colloidal microspheres. [ 53 ] It was demonstrated that negatively charged colloidal microspheres selectively adsorbed in positively charged grooves or holes by one-to-one deposition, as well as controlled cluster size. Generally, most of these methods are based on the strategy of confined or directed self-assembly of microspheres, in which the patterns are fabricated before the assembly of colloidal crystals. Preformed self-assembled colloidal crystals can also be patterned by lift-up soft lithography. [ ] This strategy is based on the selective transfer of a single layer of close-packed microspheres from the colloidal crystal film to the surface of PDMS stamps with patterned features. [ 55 ] By this method, it is possible to realize fine control over the microstructures of colloidal crystal films. Figure 1a outlines the procedure used to pattern colloidal crystals using lift-up soft lithography. A PDMS stamp with patterned features was brought into conformal contact with the surface of the self-assembled colloidal crystal film under a certain pressure. After the sample was heated for a certain time, the PDMS stamp was peeled away. A Junhu Zhang received his Ph.D. degree in the field of polymer chemistry and physics in 2003 under the supervision of Prof. Bai Yang from Jilin University. He is currently an associate professor in College of Chemistry, Jilin University. His current research interests focus on biomimetic and smart polymer microstructures. Bai Yang is currently a professor of chemistry and the director of the State Key Lab of Supramolecular Structure and Materials in College of Chemistry, Jilin University. He received his Ph.D. in polymer physics and chemistry in 1991 under the supervision of Prof. Jiacong Shen at Jilin University. His research interests relate to the composite assembly of nanoparticles in polymers, fabrication of ordered microstructures, and high performance and functional polymer nanocomposite optical materials. single layer of close-packed microspheres was lifted up by the PDMS stamp, and the corresponding pattern was left on the crystal film surface. This method also offers an effective route for creating a 2D colloidal crystal film on PDMS stamp. A layer of hexagonally arrayed microspheres could be clearly observed on the protruding surface of PDMS stamp (Figure 1 b). At the same time, parallel lines of 2D colloidal crystal arrays were obtained by this method (Figure 1 c). The microstructures of the patterned crystal film can be further modified by employing another lift-up process. Ordered square arrays were fabricated by applying a second PDMS stamp with the same patterned feature to the primary patterned colloidal crystal film in a direction orthogonal to the initial stamp orientation (Figure 1 d and e). The structural ordering in micrometer scale was patterned by the PDMS stamp, whereas 2D ordered close-packed arrays resulted from the self-assembly of colloidal microspheres. The successful transfer of colloidal microspheres demonstrates that the interaction between the PDMS stamp surface and silica microspheres is greater than that between the silica microspheres and the substrate surface. The former can possibly be attributed to the existence of the unreacted highly mobile oligomers on the PDMS stamp surface, which provides a strong adhesion between the surface of PDMS stamp and the silica microspheres, whereas the latter is only related to the van der Waals interaction between the microspheres and the substrate or silica microspheres at neighboring layers wileyonlinelibrary.com 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Figure 2. a) SEM image of microhemisphere arrays (ommatidia) on mosquito eyes. Inset: hexagonally ncp nanonipples covering an ommatidia surface. b) Scheme for making the artifi cial compound-eye micro- and nanostructures. c,d) SEM images of hcp PDMS micro-hemispheres and silica microspheres mimicking the microstructures of mosquito compound eyes. Inset: a spherical water droplet on the artifi cial compound-eye surface. Reproduced with permission. [ 57 ] Copyright 2007, Wiley-VCH. Figure 1. a) Scheme for patterning colloidal crystals using lift-up soft lithography. b) SEM image of a single layer of close-packed microspheres on the protruding surface of a PDMS stamp. c) SEM image of parallel lines in the top layer of a 3D colloidal crystal produced using a lift-up process. d) SEM image of ordered square array produced using a twostep lift-up process. e) SEM image of ordered square array and square voids in the underlying layer produced using a two-step lift-up process. Reproduced with permission. [ 55 ] Copyright 2004, Wiley-VCH. This lift-up soft lithography method has been applied to pattern a monolayer of colloidal crystals on the surface of PDMS hemisphere arrays to mimic mosquito eyes, and to reveal their superhydrophobic antifogging strategy. [ 57 ] The compound eyes of mosquitoes possess ideal superhydrophobic properties. It is found that the mosquito eye is a compound structure composed of hundreds of microscale hemispheres (ommatidia) covered with numerous, fine, nanoscale nipples ( Figure 2a ). Artificial compound eyes were fabricated by using lift-up soft lithography to investigate the effects of micro- and nanostructures on the surface hydrophobicity (Figure 2 b). PDMS stamps patterned with micro-hemisphere arrays were brought into conformal contact with a monolayer of silica microspheres on silicon substrates, and after the PDMS stamps were carefully peeled from the substrate, we obtained compound-eye micro- and nanostructures (Figure 2 c and 2 d). Subsequently, the as-prepared artificial compound eyes were chemically modified with a selfassembled monolayer of fluoroalkylsilane molecules, which possess a water contact angle (CA) of 109 on a flat surface. The static water CA and tilted angle of the artificial compound eyes were ca. 155 and ca. 15 (inset of Figure 2 d), respectively, which indicated that the micro-hemisphere and microsphere structures could greatly enhance the water-resistant property of the surface. It is proved that the combination of nipples at the nanoscale and ommatidia at the microscale on the surface of mosquito eyes plays a crucial role in creating ideal antifogging properties via a superhydrophobic approach. By lift-up soft lithography, superhydrophobic PDMS films have also been fabricated by hierarchical structures of a single layer of close-packed microspheres and post-decoration with Ag nanoparticles. [ 58 ] PDMS films are flexible materials that could be easily rolled, stretched and folded. The silica spheres on the PDMS films were mechanically stable, because they sank into the polymeric film during the lift-up soft lithography process. Accordingly, these films could be easily transferred to other surfaces, acting as superhydrophobic coatings. Similar to lift-up soft lithography, nanoimprinting process have also been employed to transfer strongly attached microspheres from the substrates to polymer stamp, as well as to produce patterned 2D colloidal crystal arrays on the substrate. [ 59 ] 3. Microcontact Printing of Colloidal Crystals Based on the lift-up soft lithography method to fabricate ordered colloidal crystals on the protruding surface of PDMS stamp, the microspheres on PDMS stamp can serve as ink and be transferred onto the polymer-coated solid substrates by a modified μ CP technique. [ 60 ] A thin film of poly(vinyl alcohol) (PVA) was spin-coated on a planar substrate or dip-coated on the surface of a non-planar substrate. A PDMS stamp coated with 2D colloidal crystal film was brought into conformal contact with the PVA film. After the sample was heated for certain time, the PDMS stamp was carefully peeled away, leaving the 2D colloidal crystal on the substrate ( Figure 3a ). Using PDMS elastomer as stamp, μ CP provides a robust way for well-defined parallel lines of patterned 2D colloidal crystals on a polymer-coated planar substrate (Figure 3 b) or on the surface of a glass tube (Figure 3 c). Taking advantage of the flexibility of μ CP, this method is versatile for patterning colloidal crystals on non-planar surface and creating ordered heterogeneous colloidal crystal microstructures, such 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3413

4 Figure 3. a) Scheme for transfer of colloidal crystals by modifi ed μ CP. b) SEM image of parallel lines of the 2D colloidal crystal arrays made of 230 nm silica microspheres on a planar substrate. c) SEM image of parallel lines of the 2D colloidal crystal arrays on the surface of a glass tube with 3.7 mm radius of curvature. d) SEM image of the crossover in a heterogeneous colloidal crystal. The colloidal crystal was made of 200 nm PS microspheres and 230 nm silica microspheres. Reproduced with permission. [ 60 ] Copyright 2004, American Chemical Society. as heterogeneous structures of 230 nm silica microspheres and 200 nm polystyrene (PS) microspheres (Figure 3 d). The polymer thin films act as glue to provide an efficient interaction between the microsphere ink and substrate during the μ CP processes. During the μ CP process, the colloidal crystal arrays sank into the polymer film due to the softening of polymer film above its glass transition temperature ( T g ). Softening of polymer film also induces the increase of contact area and stronge adhesion between microspheres and the polymer film. The successful transfer of colloidal crystals from a PDMS stamp to a polymer film suggests that the interaction between microspheres and polymer film is larger than that between microspheres and the PDMS surface. In a similar water-mediated microcontact printing process, a water layer between 2D colloidal crystal and the substrate serves as an adhesion layer to enable the transfer of colloidal crystals from PDMS stamp. [ 61 ] During the evaporation of water, attractive capillary force gradually increases to pull the 2D colloidal crystals and the substrate into conformal contact, and the adhesive strength between the silica microspheres and the silicon substrate is significantly strengthened by the formation of covalent bonds between the two surfaces. Accurate control of the surface interaction during transfer printing has also been realized by Wolf and co-workers. [ 62,63 ] They developed a gravure printing process to fabricate patterned 2D colloidal crystals ( Figure 4a ). Through templatedirected self-assembly, colloidal microspheres were arranged in recessed patterns on PDMS stamp on printing plates with predefined features. These microspheres are subsequently printed onto planar substrates with high accuracy, even single-particle resolution (Figure 4 b d). A variety of particle arrangements including lines, arrays and bitmaps can be created (Figure 4 e), while preserving the catalytic and optical activity of the individual nanoparticles. This approach is compatible with different nanoparticles, including polymers, metals, semiconductors and Figure 4. a) Scheme for high-resolution particle printing. b) SEM image of lines from 60 nm Au particles directly printed onto Si wafers. c) SEM image of larger area of 200 nm wide lines from 60 nm Au particles. d) SEM overview of spaced array of 60 nm Au particles printed onto additional 30 nm PMMA adhesion layers. e) Detail (left eye) from a printed sun pattern (inset) composed of 60 nm Au particles with 280 nm pitch. Reproduced with permission. [ 63 ] Copyright 2007, Nature Publishing Group. oxides. A thin polymer film has also been proved to be useful for the transfer of colloidal crystals. Huskens and co-workers utilized supramolecular interactions to facilitate the transfer printing of patterned colloidal crystals. [ 64 ] Through convective self-assembly, colloidal crystals of β-cyclodextrin-modified PS microspheres formed in the grooves of patterned PDMS stamp templates. An adamantaneterminated dendrimer was subsequently infiltrated into the colloidal crystals, which provided multiple binding sites that connect neighboring microspheres by host-guest interactions of β -cyclodextrin and adamantane, and also bound the colloidal crystals to a β -cyclodextrin-functionalized substrate during the following transfer printing. Because of the strong supramolecular interactions, the mechanical stability of the colloidal crystals was drastically reinforced, allowing the formation of free-standing microstructures. Jeong and co-workers deposited colloidal microspheres in the buckling patterns of thin polymer film on PDMS stamp, and transferred them onto flat surfaces. [ 65 ] During the transfer, an embossing process is believed to reduce the adhesion between the microspheres and stamp, therefore, when the microspheres adhere to flat surface stronger than the stamp, the microspheres can be transferred by simply lifting off the substrate. μ CP provides a facile method to transfer patterned or monolayered colloidal arrays to desired substrates. Utilizing 3414 wileyonlinelibrary.com 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 this method, colloidal crystal monolayers were transferred onto the surface of organic light-emitting devices (OLEDs) to increase their light extraction. [ 66 ] Silica microsphere arrays were transferred onto PVA thin films that were spin-coated on the opposite side of the indium tin oxide (ITO) glass substrates. The ITO-coated glass substrates with the silica microsphere arrays were employed as electrodes to fabricate OLEDs. The glass substrate, PVA and silica microspheres compose a structure that has similar function to microlens arrays, which are able to increase the light extraction of OLEDs. Compared to those with flat ITO glass substrate, the luminance efficiencies of these OLEDs in the normal direction are increased by a factor of 1.6 and 1.3 with 400 nm and 1000 nm microspheres respectively, and the shapes of the emission spectra exhibit almost no angle dependence by using these arrays of microspheres. 4. Non-Close-Packed 2D Colloidal Crystals In comparison with close-packed colloidal crystals, ncp colloidal crystals possess a wide photonic band-gap, which makes them important for applications in photonic materials, including light-emitting diodes, all-optical chips and optical switches. The lattice structure and lattice spacing act on manipulating the photonic band-gaps of photonic crystals, which play significant roles in the realization of integrated optical circuit devices. In addition, ncp arrays are also pursued for applications in expanding the complexity of inverse opals fabrication due to their high void-filling fraction, as well as in making tunable superhydrophobic surface. Benefiting from the above reasons, various techniques, such as template-induced assembly, [ ] spin-coating, [ 69 ] etching, [ ] have been developed to fabricate 2D ncp colloidal crystals. Utilizing the solvent-swelling and mechanical deformation behaviors of PDMS stamps, the modified lift-up soft lithography and μ CP transfer technique were developed to fabricate 2D ncp colloidal crystals. [ 73 ] By lift-up soft lithography, 2D closepacked colloidal crystals were transferred onto the surface of PDMS stamp, which were subsequently stretched or swollen with organic solvents. The obtained 2D ncp arrays on the deformed PDMS stamps were then transferred onto substrates coated with thin PVA films by μcp technique (Figure 5a ). Since the deformation of PDMS stamp is isotropic during solvent swelling, the lattice spacing increased while the lattice structure was preserved. Because the swelling magnitude of PDMS film strongly depends on the polarity of solution, the lattice spacing could be controlled by tuning the composition of organic solvents. In case of anisotropic deformation by mechanical stretching, both the lattice spacing and the lattice structure changed simultaneously. After removing the PVA film by calcination, the ncp arrays on the substrate can be lifted up, deformed, and transferred again by another PDMS stamp, therefore, the lattice feature could be changed step by step. [ 74 ] The lattice spacing could be increased by multiple lift-up and transfer printing (Figure 5 b f). During all the processes, the microspheres remained homogeneously distributed with highly uniform separations, and the long-range ordering was well preserved after multiple swelling procedures. Figure 5. a) Scheme for fabrication of 2D ncp colloidal microsphere arrays. b) SEM image of the 2D hcp arrays of 560 nm silica microspheres assembled on a silicon wafer. c) SEM image of the hexagonal ncp arrays by swelling the PDMS fi lm with pure toluene. d) SEM image of the hexagonal ncp array obtained by employing two swelling cycles. e) SEM image employing three swelling cycles. f) SEM image employing four swelling cycles. Reproduced with permission. [ 74 ] Copyright 2010, American Chemical Society. Ncp arrays with new crystal lattices were designed and created by stretching the PDMS elastomer stamps. As is wellknown, there are five distinct Bravais lattices in two dimensions. Classified by two basis vectors and the angle between them, these five lattices are hexagonal, oblique, rectangular, square, and centered rectangular, which have different lattice symmetry and spatial parameters ( Figure 6 ). Combining isotropic solvent swelling and anisotropic mechanical stretching of PDMS stamps, not only the lattice spacing but also the lattice structure can be controlled by multiple transfer printing processes. Therefore, it is possible to transfer hcp colloidal arrays into full dimensional ncp ones in all five 2D Bravais lattices. This method affords opportunities and space for fabrication of novel and complex structures ncp colloidal crystal arrays through rational design. These as-prepared structures can be used as molds for colloidal lithography or prototype models for optical materials. 5. 3D Assembly of 2D Colloidal Crystals Based on the 2D ncp colloidal crystals and controllable in situ photopolymerization, A fast, reliable, and accurate procedure to create 3D colloidal crystals with tunable layer spaces was developed. [ 75 ] This particular approach uses polymerization of 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3415

6 Figure 6. Scheme illustration of fi ve distinct Bravais lattices and corresponding SEM images of the ncp colloidal crystals fabricated by combined use of isotropic solvent swelling and anisotropic mechanical stretching of the PDMS stamps. Reproduced with permission. [ 74 ] Copyright 2010, American Chemical Society. photoresist to assemble 2D ncp colloidal crystals through a layerby-layer photopolymerization process ( Figure 7a ). First, 2D ncp colloidal crystals were transferred onto a silicon wafer. Then, monomer of photoresist were spun onto the silicon wafer and polymerized by UV light, and the colloidal microspheres were transferred into the photoresist film. [ 76 ] The polymer film was then peeled off, turned over, and placed on another 2D ncp colloidal crystal covered with monomers, followed by exposing to UV light again under various mechanical pressures. Repeating these steps, 3D ncp colloidal crystals with desired layer numbers and layer spaces were obtained. From the cross-sectional image of several typical examples of 3D ncp colloidal crystals incorporated in photoresist films (Figure 7 b e), we can find that the average distance between particles remains constant during the processes. According to our previous work, the modulation of the sphere interstices in each layer depends on the process of modified μ CP, while the layer spacing of the 3D colloidal crystals is determined by the viscosity of the monomers and the mechanical pressure during in situ layer-by-layer photopolymerization. Further development of this approach allows control of the size and packing structure of microspheres in each layer, as well as fabrication of binary colloidal crystals over large area. [ 77 ] Due to the independence of the microsphere size and the packing structure of monolayers in each transfer step, this approach can diversify the structural complexity of colloidal crystals. The versatility of this method is favored by the integration of various microspheres or nanoparticles with different patterns and functions into a case due to the layer-by-layer process, which facilitate the exploration of 3D colloidal crystal structures and optical applications. Transfer printing method has also been employed by Ozin and co-workers to incorporate planar defect layers into 3D colloidal crystals. [ ] Polyelectrolyte multilayers on a flat PDMS stamp were transferred onto the surface of colloidal crystal, which were deposited by layer-by-layer self-assembly method. A second layer of colloidal crystals is then grown on top of this surface layer to form the embedded structural defect. Through this method, various functional materials can be incorporated or embedded in these defect layers, including organic, inorganic, or organometallic polyelectrolytes, charged quantum dots and dyes, colloids, or biological polymers, such as DNA and proteins. These planar defects could be used to modulate the photonic band-gap of colloidal crystals. The defect state can be tuned by varying the thickness and refractive index of the defect layer, and these composite structures can be used as chemical or biological sensors. 6. Surface Patterning with Colloidal Crystal Templates Well-ordered monolayer or double-layer microsphere arrays can be used as either etching or deposition masks to pattern surface nanostructures. The so-called colloidal lithography is considered as a low cost, high-resolution, and relatively high throughput technique. [ ] Many ordered nanostructures, including arrays of nanoparticles, nanopores, nanopillars, and nanobowls have been successfully fabricated using this strategy, as have been reviewed by Yang and co-workers [ 47 ] and Cai and co-workers. [ 48 ] These nanostructure arrays have found applications in SERS, [ 81 ] superhydrophobic coatings, [ 82 ] biological surfaces, [ 49 ] and nanoplasmonics. [ 83 ] Recently, inspired by the nipple arrays in insects compound eyes and wings, antireflective structures with nipple-like or tapered profiles have been fabricated by colloidal lithography with colloidal microspheres as templates. [ ] These structures can suppress the reflection of light at the interface, because they provide a gradient in refractive index between air and substrate. [ 89 ] To avoid scattering from the optical interface, the structure dimension has to be smaller than the wavelength of the incident light. By reactive ion etching (RIE) of silicon substrate with 350 nm PS microspheres as masks, Chen and coworkers fabricated antireflective silicon nanocone arrays, [ 85 ] while Jiang and co-workers employed 360 nm ncp SiO 2 colloidal crystals. [ 86 ] Antireflective properties of the surfaces were measured, which matched with theoretical calculations. In order to improve the antireflective properties of the surfaces, they prepared high aspect ratio silicon (about 10) nanotip arrays by modified RIE process. [ 87 ] Cui and co-workers demonstrate 3416 wileyonlinelibrary.com 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

7 Figure 7. a) Scheme for fabricating 3D ncp colloidal crystals with tunable sphere interstices. b) Cross-sectional SEM image of ncp monolayer silica spheres incorporated in the photoresist fi lm. The inset shows the SEM image of 2D ncp silica spheres on the Si substrate. c) Cross-sectional SEM image of 3D colloidal crystals with layer spacing of 1.16 μm. d) Cross-sectional SEM image of 3D colloidal crystals with close-packing layers. e) Cross-sectional SEM image of 3D colloidal crystal with threelayer 451 nm and μ m silica spheres. Reproduced with permission. [ 75 ] Copyright 2008, The Royal Society of Chemistry. the fabrication of hydrogenated amorphous Si nanowires and nanocones, These nanostructured surfaces exhibited enhanced absorption due to superior antireflection properties over a large range of wavelengths and incident angles. [ 88 ] More recently, by deep RIE and diffusion doping, Yang and co-workers fabricate large-area silicon nanowire radial p-n junction photovoltaics with enhanced efficiency. [ 90 ] This colloidal lithography strategy is compatible with the soft lithography methods we have established on controlling the arrangement of colloidal spheres, and silicon cone arrays with tunable periodicities, different lattice structures, and various patterns can be prepared ( Figure 8a ). [ 91 ] With the increase of the etching duration, silica microsphere masks were etched away gradually in fluorine-based plasma, while the obtained structure underneath changed from cylinder to frustum of cone and then to cone in the end. By adjusting the diameters of the colloidal microsphere masks, silicon nanocones with tunable sizes can be fabricated (Figure 8 b and 8 c). The surface roughness of the obtained silicon nanocone arrays can be adjusted by controlling the etching duration, which is proved to be of importance in tailoring the wetting behavior of water droplets. These silicon nanocone arrays with different morphology can be used to mimic the surfaces of nature materials, such as the leaves of lotus and rice, the moth eye or wings of cicadas, with superhydrophobic or antireflective properties. [ 93 ] For example, silicon cone arrays with strips can be used as hydrophobic substrates with anisotropic dewetting properties just like the leaves of rice (Figure 8 d); biomimetic antireflective silicon hollow-tip arrays with water-repellent properties can be prepared using metal catalytic wet etching of silicon followed by a short time RIE process (Figure 8 e). [ 92 ] Based on colloidal lithography method, large-area silica nanocone arrays can be fabricated on planar fused silica substrates and planconvex lenses as high performance antireflective and antifogging surfaces ( Figure 9a and b). [ 94 ] These antireflective structures dramatically suppress the surface reflective loss from the ultraviolet to near-infrared regions (Figure 9 c). In order to further decrease the reflection and increase the transmission, both sides of the silica substrates are modified with the antireflective structures. To demonstrate the versatility of this method, we have also fabricated antireflective surfaces on the planconvex lens (Figure 9 b). More intense light reflection is seen from unstructured lens on the left, whereas the structured one on the right appears less bright than the left one. The antireflective properties are relative to the feature size of the nanostructures. By increasing the size of colloidal microspheres, high performance silica surfaces with improved transmission in near-infrared region can be prepared. [ 95 ] Besides, such surfaces exhibit high performance superhydrophilic and antifogging properties (Figure 9 d). Such surfaces are promising for fabrication of highly light transmissive, antireflective and antifogging optical materials to be used for optical devices, display, projection optics and laser applications. These biomimetic silica antireflective surfaces have been employed to increase the light extraction from white organic light-emitting devices. [ 96 ] Silica nanocone arrays were directly etched on the opposite side of the ITO coated fused silica substrates, which were used as electrodes in white OLEDs. The antireflective surfaces dramatically suppressed the reflection loss and increased the transmission of light over a large range of wavelength and a large field of view. Significant enhancement in light intensity is obtained for OLEDs with antireflective surfaces. Using such surfaces, the luminance efficiency of the device in the normal direction is increased by a factor of 1.4 compared to that of the device using flat silica substrate, and the enhancement are very large for large viewing angles. 7. Colloidal Microsphere Imprinting and Micromolding As ordered structures in nanoscale, 2D arrays of colloidal microspheres are ideal templates to fabricate ordered arrays of hollow spheres, nanovoids or nanowells. [ ] Cai and co-workers 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3417

8 Figure 8. a) Scheme for the fabrication of silica nanocone arrays using RIE with a monolayer of silica spheres as the mask. b) SEM image of silica nanocone arrays using colloidal spheres with a lattice spacing of 1.4 times of the diameter. c) Cross-sectional SEM images of silicon cones fabricated using silica spheres with diameters of 246 nm as mask. d) Obtained silica nanocone arrays with strip widths of 4 and 12 μ m. Insets show the optical images of water droplet taken perpendicular (left) and parallel (right) to the strip arrays. Reproduced with permission. [ 91 ] Copyright 2009, American Chemical Society. e) Tilt view SEM image of silicon hollow-tip arrays of 7.1 μ m in length. Reproduced with permission. [ 92 ] Copyright 2009, The Royal Society of Chemistry. reported the fabrication of nanopore arrays in the interstices between substrates and 2D colloidal crystals. [ 48, ] In their approach, 2D polymer colloidal crystals on substrates were used as templates for chemical deposition or electrodeposition. Thin films of silica, metal or metal oxide with nanopore arrays have been prepared by this approach, with different thickness, nanopore shape. Wang and co-workers fabricated ordered arrays of TiO 2 nanobowls by atomic layer deposition, which can be liftedoff and used as nanoscale masks. [ 103,104 ] The silica microspheres transferred on PDMS stamps are especially suitable for imprinting, since elastic PDMS stamps are generally used in imprinting based surface patterning techniques. When 2D colloidal crystals are employed as molds for imprinting, the ordered structure, as well as the spherical shape, will be transferred onto the polymer films. [ ] Yang and co-workers reported the imprinting and replica molding of patterned colloidal microsphere arrays. In a imprinting process, 2D colloidal crystals of silica microspheres on the surface of patterned PDMS stamps were brought into contact with polymer films under certain pressure and heating temperature. Alternatively, in a replica molding process, patterned colloidal stamps were brought into contact with the prepolymer, and were then cured with UV light. After removal of the silica microspheres, ordered arrays of nanowells were obtained on surface of the polymer films. [ 110 ] Combining the modified μ CP and colloidal microsphere imprinting method, ordered Agnanoparticle-doped polymer and gold composite voids were prepared. [ 111 ] Figure 10a outlines the procedure for preparing polymer nanovoid arrays doped with Ag nanoparticles. Ag nanoparticles were deposited by chemical reduction onto a single layer of close-packed silica microspheres (Figure 10 b), which were previously transferred onto the surface of a PDMS stamp by lift-up soft lithography. Then the PDMS stamp was brought into contact with a PVA film under a certain temperature and pressure, which transferred the microspheres into the PVA film on the substrate after peeling the PDMS stamp away. The Ag nanoparticle-coated sides were embedded into the polymer film and the uncoated sides were left outside (Figure 10 c). By subsequent etching of the silica microspheres, ordered Ag-nanoparticle-doped polymer voids are finally obtained (Figure 10 d). These silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids can also be used as templates to fabricate ordered Ag-nanoparticle-doped polymer and gold composite voids with different morphologies. PDMS stamps with hexagonal ncp arrays of hemispherical nanowells can be fabricated by casting PDMS prepolymer onto 2D colloidal crystals. The size and depths of the nanowells, as well as the lattice spacing, can be adjusted by the sphere interstices and surface chemical composition of the 2D colloidal crystals. [ ] These PDMS stamps with ordered nanowell nanostructures can be used as molds in subsequent micromolding processes. [ ] Using PDMS elastomers with hexagonal arrays of nanowells as template, close-packed or ncp hemisphere arrays have been imprinted on UV-curable polymers. Jiang and co-workers fabricated polymer-embedded ncp colloidal crystals, and transferred the surface features to PDMS stamps. After that, the PDMS stamps with ncp nanowell arrays were put on top of thin monomer films on substrates, followed by a curing process with UV light. [ 115 ] They demonstrated that the resulted polymer films with nipple arrays could be used as multifunctional optical coatings that mimic both unique functionalities of antireflective moth eyes and superhydrophobic cicada wings. [ 116 ] Besides nipple arrays, nanowell arrays with honeycomb structures have also been fabricated by micromolding with PDMS stamp, as shown by Park and co-workers [ 121 ] 3418 wileyonlinelibrary.com 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

9 Figure 9. a) Tilt-view SEM images of the fused silica antireflective surfaces. b) Comparison of the photographs of planar silica substrate with double-sided antireflective surfaces (top) and the planconvex lens with double-sided antireflective lens (bottom). c) Wavelength and angledependent specular reflection of the single-sided and double-sided antireflective surfaces. d) Photograph of a planar silica substrate (bottom) and double-sided antireflective surfaces (top) taken from a refrigerator to humid air. Reproduced with permission. [ 94 ] Copyright 2009, Wiley-VCH. Figure 10. a) Scheme for the preparation of Ag-nanoparticle-doped polymer voids. b) SEM image of silica microspheres unsymmetrically coated with Ag nanoparticles on a PDMS stamp. Inset is a magnifi ed SEM image. c) SEM image of the silica microspheres unsymmetrically coated with Ag nanoparticles that have been transferred onto the PVA spin-coated silicon wafer, the inset shows the morphological details. d) SEM image of the ordered Ag-nanoparticle-doped PVA and gold composite voids obtained by directly evaporating a layer of gold fi lm onto the Agnanoparticle-doped PVA voids. Reproduced with permission. [ 111 ] Copyright 2006, Wiley-VCH. Using PDMS nanowell arrays as molds, we demonstrate a modified micromolding method to fabricate morphology controlled elliptical hemisphere arrays ( Figure 11a ). [ 122,123 ] The PDMS nanowell arrays were stretched during the micromolding process, which changed the spherical nanowells into elliptical ones. During stretching of the PDMS molds, average length of the nanowells along the stretching direction increased, while the depth and width decreased compared with the original dimension, due to the shrinkage along the directions perpendicular to the stretching direction. These stretched PDMS nanowell mold was compressed onto a silicon substrate coated with a polymer film (e.g., PS) under a certain pressure and heated to a temperature over the T g of the polymer for a period of time. Because of the flow of polymer under heat and pressure, 2D polymer elliptical hemisphere arrays were obtained (Figure 11 b and c). By varying the stretching direction, stretching force, size of the colloidal spheres used and other experimental conditions in the fabrication process, we can control the shape, aspect ratio and size of the resulting microstructures. This method does not involve any costly micromanufacture technique and can be applied to many materials, such as oil soluble polymers, water soluble polymers, polymer/nanoparticle composites, and even TiO 2 sol gel systems (Figure 11 d). Since surface morphology can influence surface characteristics significantly, the asprepared elliptical hemisphere arrays exhibit anisotropic characters on the wetting behavior of water drops. [ 122 ] The static contact angles measured from the direction parallel to the long axis were larger than those measured from the perpendicular direction. The difference in the contact angles of two orthogonal directions is induced by the difference of energy barriers. Potential application of the elliptical hemisphere arrays is to provide a model for the fundamental research of anisotropic surfaces and a template or mask for the fabrication of anisotropic surface patterns for potential applications of shape-dependent optical and magnetic devices. Polymer nanowell arrays can be used as templates for physical deposition, chemical deposition or electrodeposition to fabricate ordered arrays of nanostructures with tunable morphologies. [ ] Polymer films with spherical nanowell arrays have been used as sacrificial templates to form bowl-like ZnO nanostructures ( Figure 12a ). [ 128,129 ] Thin films of zinc acetate were obtained within the 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3419

10 circular nanobowls replicating the nanowell structures were obtained (Figure 12 b), while polygonal nanowell arrays were obtained in the case of PVA (Figure 12 c and 12 d). The resulting shape of the ZnO nanobowls also depends on the geometry lattice of the PVA nanowell arrays. Hexagonal, square and triangular ZnO nanobowls have been successfully prepared. The shape change from spherical to non-spherical was induced by a simultaneous thermal decomposition of the zinc acetate and the PVA nanowell templates. But PS has a higher decomposition temperature than zinc acetate, transformation of zinc acetate to zinc oxide happened before the thermal decomposition of PS, and no shape change was observed. This fabrication strategy can be applied to other functional metal and oxide materials, such as TiO 2, Fe2O 3, etc. Figure 11. a) Scheme for the fabrication of 2D elliptical hemisphere arrays using stretched PDMS nanowells as the molds. b) SEM image of hemispheres arrays without stretching. c) SEM image of hemispheres arrays with a stretching force of 0.9 N. d) SEM image of TiO 2 elliptical array fabricated through using the sol gel method. Reproduced with permission. [ 122 ] Copyright 2010, The Royal Society of Chemistry. spherical nanowell arrays on the polymer films by dip coating. Subsequent calcination removed the polymer templates and decomposed the zinc acetate into zinc oxide, and ZnO nanobowl structures were fabricated on the silicon substrate. It is found that thermal deformation behavior of the polymer nanowell templates was important to shape transition of the arrays, and polymer templates of different composition led to different structures. When using PS as the template material, Figure 12. a) Scheme for the fabrication of nanobowl structures using nanowell arrays as templates. b) SEM image of ZnO nanobowl arrays. c) SEM image of hexagonal ZnO nanobowls. d) SEM image of square ZnO nanobowls. Reproduced with permission. [ 128, 129 ] Copyright 2008, Elsevier. 8. Nanoring Arrays by Colloidal Crystal Templates With the assistance of colloidal crystals on substrates, nanoring arrays have been fabricated by controllable capillary infiltration and dewetting of desired species at the interstices of the colloidal crystals and substrates. [ ] Xia and coworkers combined μ CP technique with colloidal lithography, and transferred alkanethiols from a planar PDMS stamp to 2D colloidal microsphere arrays on Au or Ag surface. [ 130 ] Nanoring arrays of self-assembled monolayers of alkanethiols formed around the microspheres. The width of the rings could be conveniently controlled by the concentration of thiol in the ink and the spreading time. By removal of the template, the ring pattern can be developed on the metal films by wet etching. Garno and co-workers prepared octadecyltrichlorosilane nanoring arrays by vapor deposition of organosilanes selectively at the areas near the microsphere/substrate interface containing water residues, which could be controlled by the drying parameters. [ 131,132 ] By selection in designing the terminal groups of organosilanes, the nanostructured surface sites can be engineered for building more complex structures. Cremer and co-workers demonstrated the controlled formation of 2D periodic arrays of nanorings assembled from CdSe semiconductor nanoparticles using 2D colloidal crystals as templates ( Figure 13a ). This method involves the introduction of an aqueous solution containing both CdSe nanoparticles and PS microspheres onto the surface of a planar hydrophilic glass substrate. The nanoparticles were confined to the meniscus of the microspheres during evaporation, which drove ring assembly via capillary forces at the PS microsphere/glass substrate interface (Figure 13 b). The geometric parameters for nanoring formation could be controlled by tuning the size of the microspheres and the concentration of the nanoparticles employed. Hexagonal arrays of nanorings formed with thicknesses ranging from single dot necklaces to thick multilayer structures over surface areas of many square millimeters (Figure 13 c). [ 133 ] 3420 wileyonlinelibrary.com 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

11 Figure 13. a) Scheme for the fabrication of CdSe nanorings using microspheres as templates. b) AFM topographical image of CdSe nanorings on polymer-modifi ed glass substrates. c) AFM topographical 3D images of CdSe nanorings obtained from solutions using CdSe to PS microsphere ratios of A) 10000:1, B) 4000:1, C) 2000:1, D) 1000:1. E) Line profi les through the center of each ring. Reproduced with permission. [ 133 ] Copyright 2009 American Chemical Society. By similar methods, nanoring arrays of carbon nanotubes have also been fabricated. [ 135,136 ] We have fabricated nanostructure arrays with the assistance of colloidal crystals confined between two substrates by controllable capillary infiltration and dewetting of desired species at the interstices of the colloidal crystals and substrates. A solution containing the desired species such as PVA then wicks into the interstices of the colloidal crystals through capillary suction. During the ensuing impregnation and evaporation of the solution, physisorption and reaction between the desired species and dewetting of the solution containing the desired species occurs, which induced the formation of nanostructure arrays. [ 137 ] The hydrophilic substrate and microspheres allow a positive capillary action of the liquid. During the capillary infiltration of the aqueous PVA solution, capillary forces lead to the formation of micromenisci around the contact points of the microspheres and the substrate. The PVA solution film then becomes unstable as it is thinned and moves towards the microspheres due to the capillary force and the dewetting of the solution film, which results in the formation of the nanoring arrays. [ 138 ] In addition to nanoring arrays at low concentration of PVA, nanoporous arrays were fabricated at a higher concentration. The diameters and properties of the microspheres, the thickness of the colloidal crystals, the type and concentration of the infiltrating solution, as well as the treating process involved in this approach have been estimated to provide further information about the process and to tailor both the periodicity and morphology of the nanostructure arrays. During the formation of these nanostructure arrays, the colloidal crystals can be taken as elastomeric PDMS stamps in microcontact printing techniques. This strategy is suitable for nanopatterning self-assembled monolayers, polymers, and inorganic species on as surface due to their simplicity and feasibility. For example, 2D patterned TiO 2 film can be fabricated by a condensation reaction between titanium butoxide and surface silanols on a silicon wafer. Beside polymer solution, thin polymer films between colloidal crystals and substrates can also be patterned by colloidal crystal templates. At a temperature above the T g of desired polymer, the polymer behaved like liquid and flowed toward the microspheres due to capillary force under pressure. [ 139 ] The features of the polymer arrays can be changed by varying the thickness of the polymer films, the diameter of the building blocks, the heating temperature and the time of the imprinting process. [ 140 ] When PS films with a thickness of 80 nm were deposited onto the substrate, PS nanopore arrays were obtained, while polymer nanoring arrays were obtained when the thickness of the polymer films was reduced to about 15 nm. Polymer nanoring arrays result from the fact that the thinner polymer films became unstable as they were heated and put under pressure at temperatures above their T g, and they flow towards the microspheres due to capillarity and the dewetting or rupture of the polymer films. This approach utilizes two substrates in the formation of colloidal crystals to make the colloidal crystals more stable against heat and/or pressure, and to control the flow of polymer at a temperature above its T g. Moreover, it opens up the choice of substrates with polymer films or hierarchical patterns when the second substrate is patterned. Hierarchical nanostructure arrays could be fabricated via using a patterned PDMS as the second substrate in the formation of the colloidal crystals. The surface morphologies of hierarchical pattern and separated islands clearly demonstrate that the hierarchical arrays resulted from the combination of patterned PDMS stamps and the colloidal crystal templates. The morphology and packing order of nanostructure arrays are determined by the colloidal microspheres. By pressing the polymer colloidal crystal chips at a temperature slightly lower than the T g of the polymer, the microspheres were transformed from spherical to non-spherical, and formed a flat contact to the substrate. Using these non-spherical colloidal crystal chips as templates, hexagonal networks can be fabricated on the substrates. [ 141 ] This strategy is not only suitable for preparing nanostructure arrays of polymers, but also effective for composite systems, which can be realized by adding functional materials into the original polymer films, including functional molecules, inorganic precursors, nanoparticles, etc. [ 142 ] By mixing Fe 3 O 4 nanoparticles with PS in a toluene solution, we prepared nanoparticle/ps composite nanoring arrays ( Figure 14a ). Magnetic force microscopy (MFM) image of the composite nanoring arrays (Figure 14 b and c) confirms the magnetism of the nanorings. This kind of nanoparticle/ps composite nanoring array can be released from the supporting substrate, forming freestanding nanoring structures (Figure 14 d). From an enlarged image of the nanoring wall (Figure 14 e), we can clearly distinguish the nanoparticles dispersed in a polymer ring. The polymer-based nanoring arrays could be further employed as templates to create corresponding features on the underlying 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3421

12 by spheres, there still remains a challenge, which is to fabricate complex non-spherical arrays, which are more attractive in applications such as photonic crystals than their spherical counterparts due to their lower symmetries. Furthermore, strategies for controllable fabrication of asymmetric nanostructure arrays with predefined morphology are urgently expected in potential applications of shape-dependent optical and magnetic devices. Acknowledgements The fi nancial supports from National Natural Science Foundation of China (Grant No , ) and the Special Founds for Major State Basic Research Projects (No. 2007CB936402) are greatly acknowledged. Received: April 23, 2010 Published online: September 3, 2010 Figure 14. a) Scheme for fabrication of polymer based nanoring arrays. b) AFM image of Fe 3 O 4 nanoparticle/ps composite nanoring arrays forming from 720 nm silica beads. c) MFM image of nanoring arrays fabricated by RIE for 25 s. The image was taken by setting the tip-lift height as 60 nm. d) TEM image of free-standing nanorings obtained by etching the wafers coated with the nanoparticle/ps composite nanoring arrays. e) Enlarged TEM image focusing on the wall of a composite nanoring. Reproduced with permission. [ 142 ] Copyright 2008 Wiley-VCH. supporting substrates, such as Si nanoring arrays, Au nanoring arrays and metal/polymer Janus nanoring structures. Many of these nanoring structures could also be released from the substrates, resulting in freestanding composite nanorings with good stability in solution, which might be further used as building blocks in self-assembly, magnetic scaffolds for tissue engineering, and ultrasensitive detectors in bio- and chemical systems. 9. Conclusions Soft lithography and related methods have been extensively utilized in the construction and assembly of colloidal crystals, as well as in the fabrication of nanostructure arrays using colloidal crystals as templates. These methods are facile, low-cost, and effective compared with traditional lithography-based methods. Combination of colloidal self-assembly with nanofabrication techniques has greatly enriched the complexity and diversity of the nanostructure arrays. Currently, much attention is paid to the design and functionality of these ordered nanostructure arrays, which may find pronounced applications in nanophotonics, nanoelectronics, energy conversion, and biology. In addition, since self-assembled colloidal crystals are usually built up [1 ] K. A. Arpin, A. Mihi, H. T. Johnson, A. J. Baca, J. A. Rogers, J. A. Lewis, P. V. Braun, Adv. Mater. 2010, 22, [2 ] R. Gordon, D. Sinton, K. L. Kavanagh, A. G. Brolo, Acc. Chem. Res. 2008, 41, [3 ] A. N. Shipway, E. Katz, I. Willner, ChemPhysChem 2000, 1, 18. [4 ] M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, P. D. Yang, Science 2001, 292, [5 ] Z. H. Nie, E. Kumacheva, Nat. Mater. 2008, 7, 277. [6 ] G. M. Wallraff, W. D. Hinsberg, Chem. Rev. 1999, 99, [7 ] R. D. Piner, J. Zhu, F. Xu, S. H. Hong, C. A. Mirkin, Science 1999, 283, 661. [8 ] S. Y. Chou, P. R. Krauss, P. J. Renstrom, Science 1996, 272, 85. [9 ] B. D. Gates, Q. B. Xu, M. Stewart, D. Ryan, C. G. Willson, G. M. Whitesides, Chem. Rev. 2005, 105, [10 ] S. Kramer, R. R. Fuierer, C. B. Gorman, Chem. Rev. 2003, 103, [11 ] M. Geissler, Y. Xia, Adv. Mater. 2004, 16, [12 ] Y. N. Xia, G. M. Whitesides, Angew. Chem. Int. Ed. 1998, 37, 550. [13 ] J. A. Rogers, R. G. Nuzzo, Mater. Today 2005, 2, 50. [14 ] D. B. Weibel, W. R. DiLuzio, G. M. Whitesides, Nat. Rev. Microbiol. 2007, 5, 209. [15 ] K. Kumar, G. M. Whitesides, Appl. Phys. Lett. 1993, 63, [16 ] J. Park, P. T. Hammond, Adv. Mater. 2004, 16, 520. [ 17 ] Q. Guo, X. Teng, S. Rahman, H. Yang, J. Am. Chem. Soc. 2003, 125, 630. [18 ] Y. L. Loo, R. L. Willett, K. W. Baldwin, J. A. Rogers, J. Am. Chem. Soc. 2002, 124, [19 ] Y. N. Xia, B. Gates, Y. D. Yin, Y. Lu, Adv. Mater. 2000, 12, 693. [20 ] C. López, Adv. Mater. 2003, 15, [21 ] F. Marlow, Muldarisnur, P. Sharifi, R. Brinkmann, C. Mendive, Angew. Chem. Int. Ed. 2009, 48, [22 ] D. J. Norris, E. G. Arlinghaus, L. Meng, R. Heiny, L. E. Scriven, Adv. Mater. 2004, 16, [23 ] S. Acharya, J. P. Hill, K. Ariga, Adv. Mater. 2009, 21, [24 ] P. Jiang, J. F. Bertone, K. S. Hwang, V. L. Colvin, Chem. Mater. 1999, 11, [25 ] S. Wong, V. Kitaev, G. A. Ozin, J. Am. Chem. Soc. 2003, 125, [26 ] J. H. Zhang, Z. Q. Sun, B. Yang, Curr. Opin. Colloid Interface Sci. 2009, 14, 103. [27 ] G. A. Ozin, S. M. Yang, Adv. Funct. Mater. 2001, 11, 95. [28 ] Y. N. Xia, Y. D. Yin, Y. Lu, J. McLellan, Adv. Funct. Mater. 2003, 13, wileyonlinelibrary.com 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim