Advanced colloidal lithography: From patterning to applications

G Model

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NANTOD-691; No. of Pages 26

Nano Today xxx (2018) xxx–xxx

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Review

Advanced colloidal lithography: From patterning to applications Yandong Wang a , Mengyuan Zhang a , Yuekun Lai b,∗ , Lifeng Chi a,∗ a Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, PR China b College of Chemical Engineering, Fuzhou University, Fuzhou 350116, PR China

a r t i c l e

i n f o

Article history: Received 11 February 2018 Received in revised form 27 June 2018 Accepted 20 August 2018 Available online xxx Keywords: Colloidal lithography Colloidal crystals Functional nanostructures Antireflective LSPR SERS

a b s t r a c t This article presents a comprehensive review about the current research activities on colloidal lithography, a highly efficient technology for fabricating large-area patterned functional nanostructures. Three aspects are elaborated: i) self-assembly of monolayer of colloidal crystals (MCCs) and their modifications; ii) lithographic patterning methods, including deposition and patterning of functional materials and pattern transfer onto the underlying substrates; iii) promising applications, especially in the optical and photonic-related fields in the past several years. Finally, perspectives on the current challenges and future trends in this area are given. The present review intends to inspire more ingenious designs and exciting research in colloidal lithography for advanced nanofabrication. © 2018 Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Self-assembly of MCCs and their modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Self-assembly of MCCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Modifications of MCCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Morphological control of MCCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Functionalized decoration of MCCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 MCCs-assisted patterning of functional materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Physical processes assisted by MCCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Chemical processes assisted by MCCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 MCCs-assisted pattern transfer onto the underlying substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 MCCs-assisted dry etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 MCCs-assisted wet etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Applications of colloidal lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Biomimetic subwavelength structures for high-performance optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Noble metal nanostructures for plasmonic-related applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 LSPR-based sensors and absorbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 SERS substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Energy mediators: transfer absorbed photon energy to analyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Bioinspired colloid-based surfaces with special wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 New developments in other applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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∗ Corresponding authors. E-mail addresses: [email protected] (Y. Lai), [email protected] (L. Chi). https://doi.org/10.1016/j.nantod.2018.08.010 1748-0132/© 2018 Elsevier Ltd. All rights reserved.

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Introduction Surfaces with ordered nanostructures have attracted intensive attention due to their distinctive surface feature-dependent properties and promising applications in a variety of important technological fields [1]. Although nanostructures can be fabricated (in well equipped research laboratory) using a number of advanced lithographic techniques, such as photolithography [2], electron beam lithography [3], focused ion beam lithography [4], soft lithography [5] and nanoimprinting [6], further development of large-scale practical fabrications with less demands on instrumentation still requires great ingenuity. In this regard, lithographic method based on self-assembled templates from colloidal spheres provides an alternative and intriguing strategy for generating periodic nanostructures with the advantages of low cost, high throughput, easy controllability and no need of complex equipments [7–12]. With the development of colloidal science in the past decades [13], highly monodispersed colloidal spheres including polystyrene (PS), polymethyl methacrylate (PMMA) and silica with a narrow size distribution and good phase stability have been synthesized by suspension [14], emulsion [15,16], dispersion polymerization [17,18], and Stöber methods [19–21]. The diameter of colloidal spheres can be precisely modulated ranging from several micrometers down to tens of nanometers, which is comparable with or even better than the resolution limit of conventional lithographic methods. Such colloidal spheres can self-assemble into two-dimensional (2D) and three-dimensional (3D) colloidal crystals under appropriate conditions. In 1981, a lithographic method using self-assembled PS monolayer as a mask was firstly proposed by Fisher and Zingsheim for the fabrication of a Pt pattern [22]. After that, Deckman and co-workers successfully scaled the mask to a large area for patterning [23]. Since then, 2D colloidal crystals have drawn extensive attention due to the successful applications as versatile masks or templates in surface patterning. The entire or part of the fabrication process including self-assembly of monolayer of colloidal crystals (MCCs), their morphological control, and functionalized decoration is defined as colloidal lithography or nanosphere lithography. Surface features of ordered nanostructures based on colloidal lithography are first derived from the formation of MCCs, followed by modification and nanofabrication. As the prerequisites to colloidal lithography, the preparation and further modification of colloidal mask are very important for the desired structures and properties. There are two kinds of modification methods: morphological control and functionalized decoration. The precise morphological control of the MCCs features, such as the periodicity, arrangement, and the size and the shape of individual spheres, enables the flexible adjustability of the resultant nanostructures. Furthermore, by integrating the colloids with some special functional molecules, MCCs with special property that cannot be offered by common colloids can be obtained. Using MCCs and their derivatives as masks in the following deposition or assembly procedure, a wide variety of 2D arrays of nano-objects including triangular particle, dot, hole, disk, bowl, cup, hollow sphere/shell, ring and crescent have been obtained from the external materials. Additionally, 2D and 3D nanostructures such as cones, pyramids, pillars, holes, rings and crescents have also been fabricated on the underlying substrates by etching, imprinting or photolithography with MCCs or their derivatives as masks. Due to their well-controlled dimension, morphology, and organization, as well as the intrinsic property of the materials, these nanostructures have great potential in many important areas including antireflection, biological and chemical sensing, absorbers, surface enhanced Raman scattering (SERS), surface wettability, surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) and others.

This article will give a comprehensive on the current research activities based on colloidal lithography, from self-assembly of MCCs, their modifications and the subsequent nanofabrication, until the applications in several fields. Since synthesis of colloidal spheres including polymer and silica and their self-assembling samples have already been reviewed by a number of authors [8,9,11,24], we will not cover particle synthesis here. We elaborate on the fabrication of nanostructures from the perspectives of functional materials and the underlying substrates. Focused attention will be paid to summarize their wide variety of applications, especially in optical and photonic related topics including biomimetic antireflectance, localized surface plasmon resonance (LSPR)-based sensing, SERS, and SALDI-MS. The main text of this article is organized into six sections. After the introduction in this section, the next section will discuss the recent advances in colloidal assembly and modifications of colloidal masks including morphological control and functionalized decoration. The two sections after section 2 will illustrate the fabrication of nanostructures based on the introduced functional materials and the underlying substrates, respectively. Then, section 5 highlights a range of unique properties associated with the fabricated nanostructures, as well as their applications in various areas. In the last section, conclusions are drawn with perspectives of the future development in this area.

Self-assembly of MCCs and their modifications Self-assembly of MCCs Monodisperse spheres with nanometer and micrometer in size can spontaneously form hexagonal-close-packed (hcp) MCCs by drying the suspension that are thermodynamically stable if the free volume is restricted below a certain level [25,26]. Such colloidal crystals are useful, e.g., to provide the basis for antireflective and superdydrophobic surfaces or the combination of both. Several methods for the preparation of hcp MCCs have been proposed using drop-coating [27,28], dip-coating [29,30], spin-coating [31–33], electrophoretic deposition [34–36], and self-assembly at air-liquid interface [37,38]. Nagayama and co-workers firstly observed the solvent evaporation convective assembly phenomenon, based on which they developed the dip-coating procedure for the formation of 2D hcp MCCs [28,29]. However, high-quality hcp MCCs over a large area can only be obtained under carefully optimized conditions. Instead of dip-coating, self-assembly at air-water interface has been widely used as a facile and effective route towards monolayer selfassembly. The floating monolayer could also be transferred and stacked onto various types of substrates, regardless of the surface polarity, roughness, or curvature [39]. Stavroulakis and co-workers modified this method by simply inclining the substrate at the angle of 10◦ to the horizontal plane under the air-water interface, the quality of hcp MCCs was greatly improved (Fig. 1A) [40]. The problems of formation of concentric bilayer rings and triplets and dislocations, can be effectively solved by the improvement in the final crucial formation of the monolayer on the substrate. Additionally, ultrasonic annealing and barrier-sway with special frequency have been employed to increase the size of crystal domains according to the Ostwald ripening theory, as shown in Fig. 1B [41]. Recently, Ye and co-workers proposed a micro propulsive injection process to directly produce hcp PS MCCs with area up to square metres (Fig. 1C–E) [42]. The assembly process is based on the wellknown Langmuir-Blodgett method, where the injection needle tip is put on water surface to form a meniscus configuration, and the PS spheres are thus continuously added until the area fraction of PS monolayer approaches 100%.

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Fig. 1. New developments of the assembly of MCCs based on self-assembly at air-liquid interface. (A) Schematic depiction for the improvement of monolayer quality by inclining the substrate. Reproduced with permission [40]. Copyright 2009 Elsevier Ltd. (B) Schematic illustrations of the setup of Langmuir-Blodgett trough for ultrasonic irradiation and barrier-sway process to the particle monolayer. Reproduced with permission [41]. Copyright 2015 American Chemical Society. (C) Schematics of the micro propulsive injection systems. (D–E) Schematic and photograph of the dynamic and equilibrium behaviors near the interface between the injector tip and the water surface. (F) Photograph of as-deposited PS monolayer. The scale bar is 10 cm. (G–H) SEM images of PS monolayers with diameter of 300 and 2000 nm. The scale bar is 2 ␮m. Reproduced with permission [42]. Copyright 2015 American Chemical Society.

Modifications of MCCs Assembly of MCCs and their further modifications render colloidal lithography more attractive for creating novel nanostructures with desirable properties [43,44]. Modifications of the MCCs have been performed before, during and after the self-assembly process, and can be divided into two types to achieve full control of MCCs. One of them is through morphological control by changing the size and shape of individual spheres, the periodicity, density, and arrangement of the array, while the other controls chemical composition by integrating colloids with functionalized molecules. Morphological control of MCCs A most common example of the morphological control is the fabrication of non-close-packed (ncp) MCCs, which can be obtained by spin-coating of colloids with photo-polymerized monomer [32], electric-field-directed assembly [45] and plasma etching [46,47] before, during, and after the assembly process, respectively. Jiang and co-workers created 2D ncp MCCs under the pressure produced from spin-coating. The silica colloids were mixed with photopolymerizable monomers (Fig. 2 A) [32]. As illustrated in Fig. 2B, the density and spacing of colloidal mask could also be controlled by using heterogeneous colloids including silica and PS with different ratios prior to assembly, where silica and PS nanospheres were selected as masking and sacrificial materials, respectively

[48]. During the assembly process, more fascinating patterns of colloidal mask have been demonstrated with the assistance of solvents and templates. For example, Fig. 2C shows that an asymmetric and freestanding connected colloidal film of hemispheres can be created by adding special solvents such as tetraethyl orthosilicate (TEOS) and cyclohexane onto the 2D PS MCCs floating on water surface [49]. A template-directed assembly, which rubs the patterned substrate with dry silica colloids and is termed as dry manual assembly, has been proposed to rapidly yield perfect 2D arrays of silica colloids with controlled symmetry and orientation in the centimeter scale (Fig. 2D) [50]. Recently, Chen and co-workers exploited the Moiré nanosphere lithography, which incorporated in-plane rotation between neighboring monolayers by sequential layer stacking with the assistance of inclined container, to extend the patterning capability, as illustrated in Fig. 2E [51]. Usually, more diversified arrangements and modulations with delicate design are performed by different techniques after the assembly process, such as polydimethylsiloxane (PDMS) assisted transfer printing [53] and plasma etching treatment [44,46]. In particular, isotropic etching is used to control the size and interspace, and the anisotropic etching enables transformation from spheres to oblate ellipsoids or oblate smooth spheres. Notably, in addition to anisotropic etching, electron beam irradiation with subsequent thermal decomposition also enables the creation of ordered ncp star-like particle arrays (Fig. 2F) [52]. Moreover, the formation of PS MCCs dualistic con-

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Fig. 2. Morphological modulations of MCCs (A, B) before, (C–E) during and (F) after the assembly process. (A) SEM images of ncp MCCs obtained by spin-coating. The upper inset is a high-magnification image and the bottom one is a Fourier transform image, respectively. Reproduced with permission [32]. Copyright 2005 American Chemical Society. (B) Schematic diagram of heterogeneous nanosphere lithography and corresponding SEM images. Reproduced with permission [48]. Copyright 2013 The Royal Society of Chemistry. (C) Preparation of asymmetric free-standing photonic crystal arrays film: First, TEOS is added onto the PS MCCs floating on water surface and forms

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figuration [54] and PS nanoring arrays [55] can be achieved by combining argon plasma etching with heating treatment or sonication in organic solvent. Recently, Lu and co-workers demonstrated that PS fragments were produced from PS colloids and automatically deposited on the substrate to form an ultra-thin resist pattern by reactive ion etching (RIE) treatment [56]. The obtained ultra-thin resist pattern can serve as a mask for the following dry etching of underlying substrate and for wet electroless deposition of metallic nanoparticles. Functionalized decoration of MCCs In majority of cases, polymer or silica colloids without functionalization are directly assembled into monolayer for further processing in colloidal lithography. However, the incorporation of functional molecules or materials into colloids offers a novel possibility for additional properties, for example, embedding fluorescent dye molecules for photoswitchable fluorescence (Fig. 3A) [57] and helical stack of silver and silica layers for chiroptical responses (Fig. 3B) [58]. Recently, hollow silica nanosphere colloidal crystals, transformed from silica coated PS core-shell nanosphere by calcination, were obtained and used as masks to fabricate a bowl-shaped 2D and complicated 3D nanostructures via direct RIE process [59]. For the successive fabrication, silica colloids functionalized with azobenzene were assembled and anchored on the surface bonded with cucurbit[8]uril, by which the colloids were protected against falling off during the first-step assembly, while they could be easily removed from the surface on demand by UV irradiation (Fig. 3C) [60]. Additionally, colloids decorated with protein molecules can also be used to generate surface-imprinted polymers for selective protein recognition [61]. MCCs-assisted patterning of functional materials Based on the obtained MCCs, various functional materials have been introduced to fabricate ordered nanostructures with desired properties. Apart from the directly obtained MCCs, the primarily arrays of nano-objects such as dot, hole and free-standing net can also be used as templates for the subsequent process. Normally, physical deposition routes, including vapor deposition and catalytic growth techniques, and chemical deposition routes, including solution interface assembly and reaction techniques, have been used to fabricate nanostructures. Physical processes assisted by MCCs The physical deposition route refers to the deposition of materials performed in a vacuum system by techniques including thermal/E-beam evaporation deposition, atomic layer deposition, pulsed laser deposition and sputtering deposition, as well as chemical vapor deposition and molecular/chemical beam epitaxy. Usually, thermal evaporation deposition is suitable for metals due to their relatively low melting points compared with metal oxides. Various metallic nanostructures such as triangle nanoparticles and their overlapping, nanochains, heterogeneous binary arrays and nanorings can be obtained, whose surface features are tunable by controlling the size and shape of colloids and the processing parameters including evaporation direction angle and rotation angle

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[62–64]. The shape of metal nanoparticles has been further changed from triangular to spherical by laser irradiation [65,66] or annealing treatment [67,68]. Recently, as shown in Fig. 4A, concentric and nonconcentric nanorings and nanocrescents with controlled asymmetry were generated by introduction of in situ mask materials through the combination of MCCs mask and angle deposition [69]. With full utilization of the shadow areas, Whitesides and co-workers developed the shadow-sphere lithography to expand the variety and complexity of structures with the combination of intricate nanoscale structure, high areal density, and/or heterogeneous composition [70]. Multi-angled sequential deposition of one or more materials was performed through plasma-etched MCCs to achieve desired structures, which could be designed from a customized software, as illustrated in Fig. 4B. The obtained patterns depend on four independent parameters: the diameter and gap of spheres, the inclination and azimuthal angles of the source relative to the MCCs. Various kinds of nanoantennas such as connected npoles, angled resonators, chiral resonators, split dipoles, split-ring resonators and loop antennas, as well as multimaterials structures have been produced by designing the inclination and azimuthal angles and the deposition time. Electron-beam evaporation technique can deposit many kinds of materials including most metals, semiconductors and ceramics with a high deposition rate and at a low substrate temperature due to its high energy of electron beam [10]. In contrast to sight-line deposition from the targets to the substrate via thermal/E-beam evaporation deposition, it is difficult for sputtering/pulsed laser deposition to keep the directional deposition due to the collisions between the target particles and the background gas molecules in the chamber [71,72]. However, some ordered nanostructures such as columns and rods can also be obtained due to the shadow effect between neighboring colloidal spheres and multiple directional deposition [73,74]. For example, Gall and co-workers fabricated two component nanorod arrays using simultaneous opposite glancing-angle deposition on the MCCs substrate [74]. Besides, atomic layer deposition is a thin film fabrication technique, where the target materials can be uniformly deposited over the whole substrate surface [75]. Arrays of hollow sphere and nanobowl have been obtained using this technique [76,77]. Metallic nanoparticle arrays obtained from the techniques mentioned above can be used as second template or mask in the following process, such as chemical vapor deposition and molecular/chemical beam epitaxy, to create new periodic arrays of nanorod and nanotube [78–81]. By using the primary structures as sacrificial masks, such as Al nanohole arrays as reported by Sengupta and co-workers, arrays of protein nanodot with dot-size tunable independently of spacing were obtained [82]. Additionally, by taking holes with a suitable depth as shadow masks, a versatile methodology termed as hole-mask colloidal lithography has been developed to fabricate various nanostructures including disk, dimer, trimer and cone with tunable dimension and shape [83–87].

Chemical processes assisted by MCCs As for colloidal crystal-assisted chemical deposition routes, functional materials and components in the solution or other matrixes can diffuse around and even fill into the interstice of

a thin layer on top of the PS particle arrays. Then, the PS particles in TEOS swell and partially dissolve. Finally, the neighboring particles fuse as the TEOS evaporates. Reproduced with permission [49]. Copyright 2013 American Chemical Society. (D) SEM images of template-induced dry manual assembly of silica colloids. (a, b) Silica colloids fitted into silicon template and (c, d) silica colloids fitted into photoresist pattern while the pattern was removed. Reproduced with permission [50]. Copyright 2009 American Chemical Society. (E) Illustration of relative rotation between the bottom (black) and top (gray) monolayers of spheres, leading to the formation of various moiré patterns. Reproduced with permission [51]. Copyright 2015 American Chemical Society. (F) Schematic layout and corresponding SEM images of PS MCCs (a) after electron irradiation and (b, c, d) subsequent heating at 360 ◦ C for 1, 2 and 4 h, respectively. Reproduced with permission [52]. Copyright 2008 American Chemical Society.

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Fig. 3. Functionalization of colloids. (A) For photoswitchable colloids. (a) Schematic illustration for the synthesis of functional photoswitchable colloidal monolayer. (b) Fluorescence emission of the PS dispersion for reversible switching between the on and the off state induced by alternating irradiation with UV and VIS-light over five switching cycles. Reproduced with permission [57]. Copyright 2014 The Royal Society of Chemistry. (B) For chiroptical response. (a) Schematics of deposition processes and expected structures for left-handed and right-handed helically stacked plasmonic layers (HSPLs). (b) TEM images of RH-HSPLs on 500 nm nanospheres and the optical measurements. The black, red and green lines represent data collected from PS monolayer, left-handed and right-handed HSPLs, respectively. Reproduced with permission [58]. Copyright 2014 American Chemical Society. (C) For successive assembly. Stepwise preparation of nanopatterned dual polymer brushes via the reversible host–guest complexation of CB. Reproduced with permission [60]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA.

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Fig. 4. Physical deposition routes. (A) Introduction of in situ mask material for the fabrication of (a) concentric rings, (b) nonconcentric rings and (c) crescents by angled evaporation. Reproduced with permission [69]. Copyright 2013 American Chemical Society. (B) The developed shadow sphere lithography. (a) Schematic for multi-angled deposition. (b) SEM images of patterns including multidirectional and multimaterials deposition accessible with shadow sphere lithography. Reproduced with permission [70]. Copyright 2014 American Chemical Society.

the MCCs and the primary derivatives with the assistance of different forces such as gravitational, spreading coefficient, capillary, as well as surface tension gradient. Thus arrays of nano-objects including particle, hole, bowl, net, pillar/wire/rod and ring can be obtained after evaporation or reaction [88–91]. Deposition techniques such as electrochemical deposition or electroless plating can be used to deposit diverse metals and semiconductors [91–94]. For instance, PMMA hole arrays on ITO substrate obtained by nanoimprint lithography were used to electrochemically deposit single gold particle arrays, as shown in Fig. 5A. Additionally, a packed plasma etched PS monolayer was used to create arrays of nanoscale cylindrical electrode, which were then used for the fabrication of nanorings and concentric double rings by electrochemical deposition (Fig. 5B) [95]. Controllable self-assembly of monodisperse PbS nanostars with six symmetric horns into ncp MCCs was also achieved by evaporation-induced self-assembly assisted by MCCs template [96]. In addition to those structures prepared on substrates, free-standing patterned thin films such as pores/nets and bowls can also be obtained by transferring the colloidal MCCs to the air-solution interface [97,98]. By employing the bilayer nanonets as the lithographic mask for metal deposition, novel metal particle arrays with various patterns such as nanoleaf arrays in a triple-hexagonal order were fabricated. Additionally, Qi and co-workers recently reported a two-step successive

interfacial chemical deposition method for fabricating Ag2 S-Ag heterostructured nanobowl arrays consisting of Ag2 S nanonets lying on Ag nanobowl arrays (Fig. 5C) [99] and heterostructured TiO2 nanorods@nanobowl arrays consisting of rutile nanorods grown in the inner surface of nanobowls (Fig. 5D) [100]. MCCs-assisted pattern transfer onto the underlying substrates With the combination of colloidal lithography and nanofabrication techniques such as etching, 2D and 3D nanostructures have been created directly onto the underlying substrates. It has been demonstrated that nanowells and nanorings can be obtained on polymer and silicon substrates by imprinting with MCCs mold [94,101]. The surface features can be controlled by adjusting the experimental parameters including the pressure, temperature, heating time and thickness of polymer layer. Furthermore, large-area arrays of various functional materials including polymer, molecule, inorganic precursor and nanoparticle can be obtained by MCCs-assisted dewetting [102]. Recently, MCCs-assisted photolithography has also attracted much attention as a simple and convenient technique to fabricate 2D or 3D nanostructures, using the light scattering/diffraction through colloidal spheres into the photoresist layer [103–108]. As shown in Fig. 6A, by taking the light

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Fig. 5. Chemical deposition routes. (A) Single gold particle arrays with pattern directed electrochemical deposition. (a) AFM image and cross-section analysis of polymer resist pattern. SEM images of Au particle arrays (b) before and (c) after lift-off process. Reproduced with permission [93]. Copyright 2012 American Chemical Society. (B) Metallic nanoring arrays with pattern directed electrochemical deposition. SEM images of (a) cross section and (b) planar view of nanoholes with Au nanorings. Reproduced with permission [95]. Copyright 2013 American Chemical Society. (C) Schematic illustration of the fabrication of Ag2 S–Ag heterostructured nanobowl arrays. Reproduced with permission [99]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. (D) Schematic illustration of the preparation process of TiO2 nanorod@nanobowl arrays. Reproduced with permission [100]. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

scattering from a colloidal particle to the underlying photoresist, hollow 3D nanostructures were created. With careful control over thicknesses of the photoresist and spacer layers, 2D arrays of nanoobjects (hole, dot, anti-ring and dot in hole) and 3D alternating array layers comprising pillar and hole were conveniently fabricated by taking MCCs or their primary nanohole arrays as masks, as shown in Fig. 6B–D. MCCs-assisted etching, where colloidal MCCs or their derivatives are used as masks, is a common and straightforward method to pattern the underlying substrate into various nanostructures.

Generally, there are two etching approaches: dry etching and wet etching. The former, such as RIE, means that the materials are etched using plasma or vapor phase etchant, and the latter, such as metal-assisted chemical etching, refers to etching of materials performed in liquid chemical etchant. MCCs-assisted dry etching MCCs-assisted dry etching has been demonstrated as an effective strategy to create ordered nanostructures due to its advantages

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Fig. 6. MCCs-assisted photolithography. (A) Light scattering from a colloidal particle for the fabrication of hollow 3D nanostructures. Reproduced with permission [103]. Copyright 2013 American Chemical Society. (B) Fabrication of periodic 2D/3D nanostructures using colloidal phase mask. 2D (a) nanohole, (b) nanodot and (c) 3D alternating array layers comprising pillar and hole were obtained by varying the thickness of photoresist and PDMS spacer layer. Reproduced with permission [105]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA. (C) Fabrication of periodic 2D nanostructures using metal hole arrays mask. Different nanostructures such as (a) hole, (b) rod and (c) antiring were obtained by varying the thickness of spacer layer. Reproduced with permission [108]. Copyright 2017 The Royal Society of Chemistry. (D) Fabrication of periodic 2D nanostructures using PDMS hemisphere soft mask. Different nanostructures such as (a) dot in hole, (b) shallow hole, (c) dot and (d) deep hole were obtained by controlling the exposure & develop condition and the thickness of photoresist film, respectively. Reproduced with permission [104]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

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Fig. 7. Dry etching routes for shaping the underlying substrates. (A) Fabrication of Si (a) concentric double nanoring and (b) nanoring based on O2 RIE and Cl2 RIE process. Reproduced with permission [116]. Copyright 2015 IOP Publishing. (B) Fabrication of (a) concentric SiO2 pattern and (b) its reversal Au pattern based on O2 plasma etching and CF4 /O2 RIE process. Reproduced with permission [118]. Copyright 2015 American Chemical Society. (C) Fabrication of 3D photonic crystal structures based on sequential passivation RIE process. Reproduced with permission [119]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

of high selectivity, controllability and reproducibility [109]. The fabrication procedure usually involves the following three steps. First, MCCs are assembled on substrate surface. Then, a dry etching process with a gaseous mixture is performed to create arrays of nano-object, where the areas of substrate uncovered by colloidal spheres are etched and vice versa. Finally, the rest of the colloids, if they still exist on the substrate surface, will be removed by dissolution in solvents such as HF or toluene. Many kinds of nanostructures have been fabricated on various materials, such as polymer, silicon, silica, carbon material, metal and other semiconductors [109–115]. The surface features are tuned by changing the diameter and arrangement of colloidal spheres, as well as the etching conditions. To date, arrays of nano-objects containing pillar/wire, cone and hole have been fabricated by one-step MCCs-assisted RIE etching, while twice PS-sphere-masked etching steps and stepwise selective etching have also been proposed to achieve multiscale nanostructures [116,117]. Recently, Jiang and co-workers fabricated arrays of metallic and silicon concentric double nanorings by using asymmetric nickel nanorings as second etching mask (Fig. 7A) [116]. Similarly, Lee and co-workers reported the fabrication of SiO2 concentric rings and its reversal Au pattern based on O2 plasma etching and CF4 /O2 RIE process (Fig. 7B) [118]. In addition to 2D nanostructures, sequential passivation RIE has been proposed to transcribe 2D MCCs masks into well-ordered 3D photonic crystals by inserting regular size variations in the etch profile, as shown in Fig. 7C [119]. Apart from constructing nanostructures on the substrate, the obtained nano-objects after etching can also be lifted off and dispersed in suitable solvents with great control over the size and shape. For instance, functional units such as antiferromagnetic nanoparticles and nanoporous gold disks were synthesized by using colloidal spheres as etching masks and dispersed as suspensions with high stability [120–122].

MCCs-assisted wet etching Compared with the dry etching route, the wet etching assisted by MCCs is more cost-effective. Silicon substrate, which is very important in semiconductor industry, is the most commonly employed material in the MCCs-assisted chemical etching. Zhu and co-workers proposed a metal-assisted catalytic etching method by combining colloidal lithography with catalytic etching technique [123–125]. By using this method, vertical and tilted silicon nanowire (SiNW) arrays have been obtained by choosing silicon wafer with different orientations. The diameter, length, center-tocenter distance and density of the nanowires can also be accurately controlled. Conventionally, the etchant solution contains deionized water, HF and H2 O2 , where the silicon under metal layer is oxidized by H2 O2 first and then dissolved by HF [126]. Recently, combining CF4 /O2 inductively coupled plasma-RIE with evaporation of thin Ti adhesive layer, as well as HF-rich etchant mixtures enabled fabrication of sparse and smooth arrays of SiNW with large pitch-to-diameter ratio (Fig. 8A) [46]. Furthermore, Lu and co-workers reported that the morphology of structure could be transferred from wire to cone in one step metal-assisted catalytic etching (Fig. 8B) [127]. The tilt angle of nanocones ranging from 69.2◦ to 88.6◦ was precisely tuned by varying the composition of the etchant solution (H2 O2 , HF, C2 H5 OH). Beyond building on Si substrates, a suspension of intact porous silicon nanodisks with uniform diameter and height was synthesized through metal-assisted catalytic etching and electro-polishing delamination [128]. Anisotropic KOH chemical etching is another common strategy to obtain Si pyramids or pyramidal pits, where the sidewalls are very smooth and the anisotropic angle is 54.7◦ [129]. The characteristic KOH etching angle could be explained in term of the large difference in the etching rate between the Si<100> and Si<111>

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Fig. 8. Wet etching routes for shaping the underlying substrates. (A) Fabrication of ultra-smooth SiNW arrays with high aspect ratio of pitch/diameter (ca. 10) with metalassisted catalytic etching by reducing the roughness of PS sphere and the thickness of metal layer, as well as increasing the HF content in etchant solution. Reproduced with permission [46]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA. (B) Fabrication of cone arrays with one-step metal-assisted catalytic etching by varying the composition of the etchant. Reproduced with permission [127]. Copyright 2017 The Royal Society of Chemistry. (C) Ultra-thin molecular resist pattern assisted KOH chemical etching. (a) AFM image of nanohole resist pattern and (b) the obtained Si “V” shape nanocavity. (c) AFM image of nanoring resist pattern and (d) the obtained Si “W” shape nanocavity. Reproduced with permission [132]. Copyright 2017 IOP Publishing.

surfaces. Due to the different densities of Si atoms on Si<100> and Si<111 > , the OH– etching rate of Si<111> is about 30 times slower than that of Si<100 > . Jiang and co-workers fabricated silicon pyramidal pits with nanoscale pits and high pit density (6 × 108 pits cm−2 ) by anisotropic KOH etching with chromium nanohole arrays acting as etching mask [130]. In addition to chromium hole mask, Lu and co-workers reported that pattern of octadecyltrichlorosilane (OTS) self-assembled monolayer was used as etching mask for the following KOH etching due to the difference of wettability between the regions covered with and without the OTS layer [131]. Recently, they further developed nanoholes and nanorings resist pattern as etching masks to create V and W shape cavity arrays, respectively, as shown in Fig. 8C [132]. The resist patterns were obtained by simply carrying out a RIE process on a MCCs masked Si substrate with and without pre-assembled OTS layer. The size, basal length

and depth of pyramids pits were easily controlled by changing the sphere size and the etching conditions.

Applications of colloidal lithography Thanks to the simplicity, low cost and high controllability, colloidal lithography has been extensively applied to fabricate periodic 2D or 3D nanostructures with length scale ranging from several micrometers to tens of nanometers. Due to the well-controlled dimension, morphology and arrangement, as well as the intrinsic property of the materials, these structures hold great promise for potential applications in many technological fields. In this part, we will summarize a range of applications based on colloidal lithography, with an emphasis on the optical and photonic related fields

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Fig. 9. Biomimetic antireflective coatings. (A) The eye of a Mourning Cloak butterfly on different length scales. Reproduced with permission [137]. Copyright 2016 Nature Publishing Group. (B) Pyramidal and honeycomb structures as antireflective coatings fabricated by PS hcp MCCs masked RIE etching. Reproduced with permission [146]. Copyright 2007 Optical Society of America. (C) Pillar arrays as antireflective coatings fabricated by silica ncp MCCs masked RIE etching. Reproduced with permission [147]. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA. (D) Silicon hollow-tip arrays with high aspect ratio as antireflective coatings obtained by combining metal-assisted catalytic etching with RIE etching. Reproduced with permission [149]. Copyright 2009 The Royal Society of Chemistry. (E) SiNC arrays as antireflective coatings obtained by one-step metal-assisted catalytic etching. Reproduced with permission [127]. Copyright 2017 The Royal Society of Chemistry.

including biomimetic antireflective coatings, LSPR-based sensing, perfect absorbers, SERS and SALDI-MS. Biomimetic subwavelength structures for high-performance optics Reduction of optical reflection and increase of transmission are crucial for the performance of optical and optoelectronic devices such as solar cells, displays and light sensors [133–136]. Fig. 9A shows the eye of a Mourning Cloak butterfly on different length scales, consisting of millions of nano-sized nipples arranged on the surface of the ommatidia in the insect compound eyes [137]. The nipples layer acts as antireflection coating, reducing reflection from the air-cornea interface about 1000-fold over the entire visible band [138]. Inspired by observations of the corneas of nocturnal insects [137,138], periodic subwavelength structures (SWS) such as nanocone arrays have been developed as transition layers to suppress reflection and increase the transmission. These SWS are also

commonly referred to as moth-eye structures due to the similar arrays of conical nipple which reduce reflection by effectively providing a gradient transition of the refractive index from air to the substrate. Many techniques based on top-down lithography, such as electron-beam lithography, laser interference lithography, and nanoimprint lithography have been used to generate antireflective SWS surfaces [139–142]. However, the practical applications of these techniques are restricted, e.g., the demand for expensive equipment, the difficulty of fabrication on non-planar surface or the time-consuming and complicated procedures [143]. Compared with these techniques, colloidal lithography is a simple and costeffective alternative way to create periodic SWS arrays on many materials. Crystalline silicon is a very important material for optical and optoelectronic devices. However, the high reflective index of Si results in the reflection of up to 40% of the incident light, which

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Fig. 10. Multiscale corrugated nanostructures for high-performance antireflective coating. The cartoon picture describes the fabrication procedure of multiscale corrugated nanostructures. SEM images of (A) the planar SiNC arrays and (B) the corrugated SiNC arrays, the scale bar is 1 ␮m. Reflection measurements of (C) specular, (D) hemispherical and (E, F) polarized angled reflection for the polished Si substrate, the planar SiNC arrays and the corrugated SiNC arrays. (G) Specular reflectance of the planar SiNW arrays and the corrugated SiNW arrays, whose SEM images are shown in (H) and (J) respectively. Reproduced with permission [117]. Copyright 2010 Springer International Publishing.

severely limits the performance of Si-based optical and optoelectronic devices [144]. By using hcp MCCs as etching masks, Chi and co-workers fabricated silicon nanocone (SiNC) arrays through RIE technique to reduce reflection from above 32% to below 8% in the range of 600 to 1300 nm [145]. Chen and co-workers fabricated optimized pyramidal and honeycomb-like structures with reflectance less than 1.5% (Fig. 9B) [146]. They calculated the reflectivity of structures with different morphologies using finite-difference time-domain algorithm and rigorous coupledwave analysis. Apart from taking hcp MCCs as etching masks, Jiang et al. reported a bioinspired templating technique using ncp silica nanospheres as etching masks to fabricate silicon pillar arrays with chlorine-RIE process, as shown in Fig. 9C [147]. The silicon pillar arrays with a high aspect ratio of ca. 10 can also be used as second-generation templates to achieve antireflective performance on glass by twice replications. More detailed regulation and fabrication of silicon SWS were realized, as reported by Yang and co-workers, SWS with different periods, different morphologies, and different distances between cones were prepared by RIE using 2D ncp and hcp silica MCCs as etching masks [148]. For an ideal case of biomimetic SWS antireflective surface, it should show tapered profile. At the same time, the period should be as fine as possible and the depth should be as great as possible, in order to give the widest possible bandwidth and almost omnidirectional antireflective properties. Fig. 9D depicts the fabrication of biomimetic silicon hollow-tip arrays using metal-assisted catalytic etching followed by a short time RIE process [149]. The long tip arrays (>3 ␮m) have an average reflectance as low as 1.3% in the 250–2500 nm range, and the best specular reflectance values are below 5% in the 2.5–15 ␮m range. Silicon post arrays with

high aspect ratio above 12 were easily obtained by metal-assisted catalytic etching. However, the tapered profile was required to further modulate via a short RIE process. Recently, Lu and coworkers demonstrated a facile method to precisely regulate the tilt angle of nanocones with metal-assisted catalytic etching in a one-step process (Fig. 9E) [127]. Taking Au nanohole arrays obtained from MCCs as templates, the tilt angle of SiNC arrays was tuned from 69.2◦ to 88.6◦ by varying the composition of the etchant (H2 O2 , HF, C2 H5 OH). The reflectivity of nanocones decreased along with the decrease of the tilt angle when the height of the nanocones remained the same. The average reflectivity was reduced to lower than 1.37% in the 250–1000 nm wavelength range for the nanocones with height of 2 ␮m and tilt angle of 83.0◦ . Although upright tapered structures with high aspect ratio can suppress reflections over a wide spectral bandwidth and angles of incidence, the fabrication of silicon cones with high aspect ratio always requires special etching technologies such as chlorinebased RIE [147] or electron cyclotron resonance plasma etching [144]. Instead of upright tapered structures with high aspect ratio, Chi and co-workers proposed twice MCCs-masked etching method to produce multiscale corrugated nanostructures for fabricating high-performance antireflection coatings [117]. As shown in Fig. 10A and B, the increase of the height of the transition layer, the disappearance of the grooves and the decrease of the center-tocenter distance between the neighboring cones resulted in much improved antireflective performance including lower reflectivity for the specular and hemisphere reflectance over broadband wavelengths and independence of light polarization. Additionally, it is well known that SiNW with a column-like structure shows poor antireflection properties because of the large refractive index dis-

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Fig. 11. The effect of variability in surface features on the antireflective performance. SEM images of SiNWs patterned by (A) E-beam lithography and (B) colloidal lithography. (C) The total reflectance and (D) specular reflectance of two kinds of SiNWs. (E) Schematic and (F) simulated spectrum of the SiNWs in theoretical analysis with diameter variability in all three diameters (top, main body and bottom). Reproduced with permission [150]. Copyright 2015 IOP Publishing. (G) The defined orientation map of nipple arrays on the ommatidium and its magnified image in (H). Reproduced with permission [137]. Copyright 2016 Nature Publishing Group.

continuity at the interface with air. As shown in Fig. 10G, however, the reflectance was reduced from above 6% at most wavelengths and above 10% at around 500 nm for the planar SiNW arrays to below 0.8% over the whole measured wavelength range for the corrugated SiNW arrays (the dotted line), and values lower than 0.17% were achieved from 400 to 700 nm. The tilt-view SEM images of planar and corrugated SiNW arrays are shown in Fig. 10H and J, respectively. Therefore, by employing two successive mask-etching method to obtain multiscale SWS for antireflective coatings, strict requirements such as sharp conical shape and high aspect ratio can be avoided. Small-sized features are also simply created using this technique. Recently, an interesting phenomenon has been found both experimentally and theoretically that the lattice defects, dimensional variability and positional randomization can strongly decrease the reflectivity of SiNW arrays [150]. Gegolides and coworkers fabricated perfectly ordered and quasi-ordered SiNW arrays using E-beam lithography and colloidal lithography, respectively, followed by cryogenic Si plasma etching (Fig. 11A, B). They found that the antireflective performance of the quasi-ordered SiNW arrays achieved by colloidal lithography was better than that of the perfectly ordered SiNW arrays from E-beam lithography. Experimentally measured reflectivity was found to be much lower than that predicted theoretically. Smyrnakis et al. [150] modeled the influence of disorder using the T-matrix approximation and showed that even small dimensional variation (10–20%) led to a dramatic reduction of the reflectance, which well explained the experimental results (Fig. 11F). It is worthy to note that such variability in surface feature is very similar to that of the nipple arrays on the ommatidium of insects, as shown in Fig. 11G and H [137]. In addition to silicon, SWS surfaces have also been fabricated on various materials such as GaSb [151], fused silica [113] and polymer [142]. Yang and co-workers fabricated double-sided paraboloid-

like arrays to further improve the transmittance of silica surface [113]. Lu and co-workers fabricated antireflective polymer coating via a one-step imprinting process using a 3D PDMS stamp that mimicked moth compound eyes [142]. The proposed biomimetic replication method is simple yet highly effective to achieve polymer coatings with excellent antireflective performance. Due to high optical performance of SWS structures (suppression of the reflection and increment of the absorption or transmission of light), they have many important applications such as solar cells [152] and light emitting diodes (LEDs) [153]. Cui and co-workers fabricated nanocone glass or quartz substrates first by hcp silica MCCs masked RIE, and then nanocone was used as platform to provide periodic nanoscale modulation for all layers in solar cells (Fig. 12A) [152]. The fabricated nanodome devices can absorb 94% of the light in wavelengths of 400–800 nm and possess a power efficiency of 5.9%, which is 25% higher than that of the flat film control. However, despite the increased absorption, the growth of thick amorphous silicon (a-Si) layer on textured surface always suffers from surface recombination and crack generation, leading to reduction of the collection efficiency of photocarriers. A dielectric embedded nanospheres layer was constructed on top of a planar ultrathin-film amorphous Si absorber to improve the absorption, and avoid the restrictions including surface recombination and crack generation (Fig. 12B) [154]. Recently, high-refractive-index TiO2 hexagonal nanostructures were fabricated as top coatings in hydrogenated amorphous Si absorb layers to increase the photocurrent [155]. The designed TiO2 nanostructures do not only suppress the reflected light at short wavelengths but also increase the optical path length of the longer wavelengths. Embedding nanostructures in LEDs have attracted considerable interest because they are able to improve the light output power efficiently [156]. An overview of recent advances in LEDs based on colloidal lithography has been reported by Yan and co-workers [153].

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Fig. 12. Improvement of light absorption for solar cells. (A) The growth of a-Si active layer on nanocone surface to obtain nanodome solar cells. Reproduced with permission [152]. Copyright 2010 American Chemical Society. (B) Dielectric structure of embedded nanospheres on top of ultrathin a-Si flat film to enhance the broadband light trapping. Reproduced with permission [154]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

Although tapered profile with high aspect ratio is an ideal unit for antireflection in principle, these structures are difficult to integrate with other functional applications due to their fixed and incompatible surface features. In the future, more attention should be concentrated on novel morphologies and paths to achieve high antireflective performance, which can avoid the strict requirements such as sharp conical shape and high aspect ratio. The antireflective property can then be more effectively integrated with other applications. Noble metal nanostructures for plasmonic-related applications Noble metal nanostructures have pronounced surface plasmon resonance (SPR) absorption, resulting from incident electromagnetic radiation exciting coherent oscillations of conduction electrons near a metal-dielectric interface [157,158]. Optical methods are enhanced because of the plasmonic properties of nanostructures, leading to LSPR sensing [159], SERS [160,161], enhancement of conventional SPR sensing [162], metal-enhanced fluorescence [163,164] and surface-enhanced infrared absorption spectroscopy [165,166]. LSPR occurs when the collective resonance is localized within the near surface region of the metal objects. The two consequences of exciting LSPR are (i) selective photon

absorption, which is sensitive to local refractive index change and (ii) generation of locally enhanced electromagnetic fields, which relies on the hybridization of plasmons. Generally, both are highly sensitive to the morphology and interparticle distance of noblemetal nanostructures and can be used for sensors and enhancement substrates for fluorescence and Raman spectroscopy. LSPR-based sensors and absorbers Van Duyne and co-workers pioneered the work of LSPR properties based on colloidal lithography, where the hexagonally ordered triangle nanoparticles were fabricated by metal thermal evaporation using hcp single layer or double layer MCCs as masks [7]. They systemically investigated the effect of surface features including particle size, shape, interparticle spacing, nanoparticle–substrate interaction and environment factors such as solvent, dielectric overlayers, and molecular adsorbates on LSPR absorption. Recently, Chen and co-workers demonstrated that the LSPR response of plasmonic nanoantennas was tuned by controlling the immersion depth of Au nanospheres in Si substrate due to the change of the surrounding environment, as shown in Fig. 13A and B. The photocurrent of plasmonic nanoantennas was significantly increased under laser illumination coincident to the wavelength of maximum LSPR scattering because of the near-filed enhancement, as

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Fig. 13. LSPR modulation through tuning the immersion depth of metallic nanostructures in silicon. (A) Schematics, SEM images and corresponding scattering spectra of Au nanocrystal/Si nanoantennas with varied degrees of immersion (DOI). (B) The simulated E-field intensity distribution cross sections of bare Si substrate and Au nanocrystal arrays/Si nanoantennas with varied DOI of 0, 0.5, and 1 under 532 nm (left column) and 655 nm (right column) laser illumination. (C) Photocurrent increment response with time at 10, 100, and 200 mV under 532 nm (left column) and 655 nm (right column) laser illumination. Reproduced with permission [167]. Copyright 2013 American Chemical Society.

demonstrated in Fig. 13C [167]. Due to the high sensitivity to the changes in refractive index near metal surface, plasmonic-based chemical and biological sensing have been realized without the need of fluorescent labels. Furthermore, the tighter evanescent field localization in nanoplasmonic sensors renders them less susceptible to environmental disturbing factors such as changes in buffer composition and temperature [168]. Usually, surfaces of metal nanostructures or thin film are activated by bonding receptors and employing special functional materials. For example, Van Duyne and co-workers demonstrated sensitive response to concanavalin A based on Ag nanoparticle arrays functionalized with mannose [169]. Pt nanodisk array were employed as an H2 detecting sensor due to the hydrogen-induced electronic changes associated with LSPR adsorption [170]. In the context of practical bioanalytical sensor applications, a layer designed to be inert to undesired biomolecular absorption should be coated to prevent false positive signals. Although thicker inert coating results in improved resistance against nonspecific adsorption, the sensitivity will be significantly reduced due to a greater distance from metal-liquid interface. To solve this problem, Höök and co-workers deposited a thin layer of alumina by atomic layer deposition on gold nanodisks and nanoholes to precisely determine the absolute bulk sensitivity and the reduction in sensitivity [171]. Adopting this strategy, the sensor can be optimized by selecting the inert coating with the most suitable thickness. Recently, Zhang and co-workers fabri-

cated a series of hollow metallic nanostructures including arrays of hollow cone, asymmetric half-cone and disk-in-volcano to explore their plasmonic properties and enhance the sensing performance, where the electric field was redistributed and enhanced under the interaction between the propagating surface plasmon polarition and LSPR [172–175]. Usually, periodic hcp MCCs have been used as masks for the following deposition or etching process in plasmonic related applications [176–178]. However, such arrangement exhibits grating effects, generally referred to as Raileigh anomalies. This will deteriorate the absorption of the optical elements especially at higher incident angles. Giessen and co-workers fabricated an angle-, and polarization-independent large-area Au-MgF2 perfect absorber by a combination of colloidal lithography and dry etching [179]. The PS colloidal spheres were dispersed on substrate in disordered arrangement, rendering the fabricated optical element with absorbance higher than 98% for incidence angles up to 50◦ and fully polarization independent. Dao and Chen demonstrated the fabrication of Al disks-Al2 O3 -Al trilayers as mid-infrared perfect absorber by using similar process while keeping the hcp arrangement [180,181]. These structures are able to act as thermal emitters and be used in surface-enhanced infrared absorption spectroscopy. Plasmonic metal nanostructures can also enhance the efficiency of photovoltaic devices due to an increased light absorption in the active layer, which is generally attributed to surface plasmon reso-

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Fig. 14. Triangle metallic nanoparticle arrays and MFON for SERS based on colloidal lithography. (A) Auto-formed SERS substrate by trapping gold nanoparticles into the middle of nanobowtie structures via plasmonic trapping. Reproduced with permission [190]. Copyright 2016 American Chemical Society. (B) Measurement of the distribution of site enhancement on the Ag-FON SERS substrate. Reproduced with permission [192]. Copyright 2008 American Association for the Advancement of Science.

nances that lead to a strong near field enhancement as well as light scattering [182]. Colloidal lithography is suitable for placing metal nanostructures on the rear side of the organic solar cells, which can avoid the destructive Fano interference and the undesirable reflection or absorption of incident light before it reaches the active layer [183]. SERS substrates Metallic nanostructures with sharp tips or separated by nanogaps enable strong electromagnetic field confinement on the nanoscale for enhancing light-matter interaction, which are of great demand in SERS [184]. At present, SERS has been intensively explored as a powerful and ultrasensitive analytical technique for applications in biochemistry, chemical production and environmental monitoring. In general, an ideal SERS substrate should have uniform and plentiful electromagnetic hot sites to enhance the Raman intensities and minimize the deviations. Therefore, metallic ordered nanostructures [116] or core-shell nanoparticles with nanoscale defined features [185] should be employed. Many efforts have been devoted to the fabrication of SERS substrates based on colloidal lithography in recent years [186,187]. Van Duyne and coworkers first fabricated metal film on nanospheres (MFON) as SERS

substrates [188] and investigated the enhancement mechanism with Ag triangle nanoparticles [7,189] via colloidal lithography. Based on these triangle nanoparticle arrays, Choi and co-workers recently built hot spots by trapping gold nanoparticles onto gold triangle nanoparticle arrays in the desired position where the laser illuminated, as shown in Fig. 14A [190]. By this means, great enhancement was achieved owing to the narrower nanogaps and direct signal collection from hot spots as a result of the good alignment between hot spots and detection sites. In addition to triangle nanoparticles, Dlott and co-workers measured the distribution of Raman enhancements by taking MFON as a SERS substrate model via photochemical hole burning [191,192]. As concluded in the table of Fig. 14B, a very small percentage of molecules (0.0063%) in the hottest spots with enhancement factor larger than 109 have contributed 24% to the overall Raman intensity [192]. After this discovery, tremendous efforts have been devoted to the fabrication of SERS substrates with a focus on the improvement of intensity and density of hot spots [193–196]. Physical sputtering deposition has recently been used to create ordered metallic nanostructures with a tunable gap width ranging from several to hundreds of nanometers by means of its multiple direction deposition and shadow effect [194–196]. For example, Li

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Fig. 15. Fabrication for SERS substrates (A) with sub-10 nm nanogap and (B) without sub-10 nm nanogap based on colloidal lithography. In panel A, there are some SEM images. (a) Periodic spherical nanoparticle arrays. Reproduced with permission [194]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. (b) Annular cavity arrays. Reproduced with permission [195]. Copyright 2015 American Chemical Society. (c) Nanoring cavity arrays. Reproduced with permission [197]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. (d) Ag-nanorods bundle arrays. Reproduced with permission [198]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. In panel B, (a) SEM image of elevated bowtie arrays, the bottom schematic is the simulated electric field distribution. Reproduced with permission [199]. Copyright 2015 Springer International Publishing. (b) SEM image of gold nanoparticles conjugated SiNWs, the inner image is the simulated electric field distribution. Reproduced with permission [200]. Copyright 2017 American Chemical Society.

and co-workers fabricated periodic hexagonal spherical nanoparticle arrays with 10 nm gaps by MCCs induced solution dipping and further physical sputtering deposition (Fig. 15A-a) [194]. A low detection limit of 10−12 M rhodamine 6 G can be achieved after the modification of nanoparticles arrays with low surface free energy material. Wang and co-workers also demonstrated the fabrication of annular cavity arrays with a gap of 10 nm by employing a sol-gel co-assembly method, RIE and metal sputtering techniques (Fig. 15A-b) [195]. The obtained annular cavity arrays possess multiple SPR modes sweeping the entire visible and NIR region, and can be applied as SPR sensors and SERS substrates. As shown in Fig. 15Ac, similar arrays of ring-shape nanocavity with 10 nm gap width termed FON-gap structures are fabricated by combining colloidal

lithography with straightforward batch processing steps, namely, atomic layer deposition and ion milling [197]. The mean Raman intensity for the FON-gap structures was improved by about 70fold compared to the regular Ag-FON substrate. To pursue smaller nanogaps, capillary forces induced closure process is used to bring nanorods or nanowires in a proximity to each other between two adjacent rods/wires at the top ends. Meng and co-workers fabricated uniform Ag-nanorods bundle arrays with a small gap (about 2 nm) between the adjacent nanorods, which resulted in a large number of high-density hot spots in the whole chips, leading to a SERS enhancement factor up to 108 with good uniformity and reproducibility (Fig. 15A-d) [198]. However, for the extensive biosensing, it is very necessary to develop SERS substrates with

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Fig. 16. Schematic illustration of the one-step template transfer technique. Reproduced with permission [131]. Copyright 2015 Springer International Publishing.

large gap width, or even without nanogaps, that are able to load large biomacromolecules such as DNA, protein fibrils or their aggregates in the active regions while retaining excellent performance. Recently, Lu and co-workers created silver coated elevated bowtie nanoantenna arrays as SERS substrates that can achieve improved Raman enhancement without sub-10 nm nanogaps and facilitated a flexible control over the thickness of deposited silver layer due to the long-term effective “lightning rod” effect and resonance coupling in different morphologies (Fig. 15B-a) [199]. Furthermore, Yang and co-workers reported that hexagonal-packed 20 nm Au nanoparticle (AuNP) decorated SiNW arrays without nanogaps in the whole substrate exhibited stable and reproducible Raman signals with high sensitivity and small relative standard deviation (RSD) of 3.9–7.2% due to the coupling of AuNP-induced strong LSPR along the SiNWs, as well as the wide-range and zero-gap 3D enhanced electric field between SiNWs (Fig. 15B-b) [200]. In addition to using metal nanostructures solely, Xie and co-workers proposed a graphene-Au nanopyramid heterostructure platform serving as SERS substrate by transferring monolayer graphene on Au nanopyramids using PMMA as sacrificing layer [201]. The superimposition of monolayer graphene can not only locate the hot spots but also modify the surface chemistry to realize selective enhancement Raman yield. The binary structure boosts high-density and high-homogeneity hot spots that result in ultrahigh sensitivity. Apart from achieving uniformity over a single SERS chip, reducing the structure variations between various batches of substrates caused by the process fluctuations during the multiple fabrication steps remains a great challenge [131,202]. Lu and co-workers proposed a one-step template transfer method to improve the homogeneity and reproducibility across one substrate and among different batches of substrates [131]. As illustrated in Fig. 16, inverted pyramidal Si pit arrays were created as templates by etching the Si wafer with OTS pattern in the KOH etchant solution. Ag pyramid arrays were then obtained by thermal evaporation on the pyramidal Si pits and peeling off with a sticky tape, rather than being fabricated through the multiple steps. By this approach, the Ag pyramid arrays were created consistently, eliminating structural variations caused by the process fluctuations in other fabrication techniques. The close-packed Ag pyramid arrays exhibited excellent homogeneity and reproducibility with RSDs lower than 8.78% both across one substrate and among different batches of substrates, thus making this method an ideal candidate for fabricating inexpensive and reproducible SERS substrates. As a powerful analytical technique, it is desirable for SERS to achieve stable qualitative and quantitative measurements simul-

taneously. However, variations in enhancement over the entire substrate caused by array defects and signal degradation under the prolonged laser irradiation have severely hindered the quantitative progress. To overcome these limitations, SERS substrates with an inherent internal intensity standard have been typically employed for quantitative SERS measurements. Recently, employing Au-FON as a highly uniform SERS substrate and Raman signal from a silicon support as internal intensity standard, synthetic dyes in beverages have been detected quantitatively without any need of pretreatment [203]. SERS spectral analysis provided sufficient sensitivity (0.5–500 mg L−1 ) and was able to determine the dye concentrations in good agreement with those obtained by a standard HPLC technique. Similarly, Ag-FON was fabricated by the inclined vapor deposition as a SERS substrate, where the Raman peak of PS at 1004 cm−1 was used as an internal standard [204]. The SERS intensity ratio by normalizing the intensity of molecule peak from that of the internal standard peak was effective for revealing the absorption kinetics and for quantitative measurements. Although many efforts have been devoted to the fabrication of ordered metal nanostructures as SERS substrates with great success based on colloidal lithography, the SERS performance may be further improved by the following three routes: increase of the enhanced intensity, increase in the density of hot spots, and confinement of molecules in the hot region. Besides, more attention should be paid on the improvement of stability and performance of quantitative measurements in practical applications. Energy mediators: transfer absorbed photon energy to analyte Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a key method in the analysis of biological and synthetic macromolecules [205,206]. However, there are some limits associated with this method, including interference ions in the low mass region (m/z lower than 700 Da), the intolerance to salt, and random distribution of the hot spots due to the introduction of organic matrix in traditional MALDI method [207]. Therefore, SALDI-MS has been developed as an alternative technique, which offers several advantages such as the ability to detect small molecules, easy sample preparation, low-noise background and so on [206,208–210]. The nanostructure surfaces, just like matrixes used in MALDI, effectively transfer absorbed photon energy from a laser irradiation to the surrounding analyte molecules. So far, two types of substrates consisting of inorganic bulk materials/particles and structured surfaces have been evaluated as SALDI substrates with different degrees of success [112]. However,

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Fig. 17. Biomimetic SiNC arrays as SALDI substrate for detection of small biological molecules. (A) Schematic description for the fabrication of periodic SiNC arrays. (B) Mass spectra of (a) bradykinin, (b) arginine acids (Arg), and (c) temozolomide (TMZ) without interference measured with SiNC arrays. (C) Mass spectra of glucose with concentration of (a) 0.5 mM and (b) 10 mM in human urine which has been diluted to 20-fold. (c) Working curve for the quantification of glucose in the range of 0.5–10 mM in human urine. Reproduced with permission [112]. Copyright 2015 Springer International Publishing.

spot-to-spot variations in SALDI performance are inevitably caused by the random distribution of particles and structures [211]. To solve this problem, He and co-workers generated ordered nanocavity arrays as SALDI substrates by combining colloidal lithography with RIE technique [212]. They demonstrated that the desorption and ionization efficiency of the roughened cavity surface exhibited a nonmonotonic relationship to the increased total surface area. Although surface features such as in-plane width and outof-plane depth of the cavities are adjustable by varying etching time, the absorption and transfer efficiency of laser photon energy, which plays a paramount role in the laser desorption/ionization process, cannot be investigated in detail due to the difficulty in controllable photo-absorption and the complexity of desorption and ionization process. To overcome this difficulty, Lu and co-workers demonstrated a method to fabricate biomimetic SiNC arrays with the MCCs-masked RIE technique for suppressing the reflection (Fig. 17A) [112]. The obtained SiNC arrays were applied for improving the laser desorption and ionization performance due to the increased photo absorption. Furthermore, the gradual increase of the photo absorption was obtained by varying the surface features such as the height and period of the SiNC arrays independently, which made it possible to investigate the effect of the absorbed laser energy and its dissipation and/or relaxation. The role of the absorbed laser energy ranging from 54% to 97% and its distribution in the laser desorption and ionization process have been investigated in detail. As shown in Fig. 17B, the optimized SiNC arrays exhibited excellent performance in detecting peptide, amino acid, drug molecule, and carbohydrate with little or no interference in the low mass range. Quantitative analysis of glucose in human urine samples created a calibration curve with a good linear regression R2 value of 0.9992, as demonstrated in Fig. 17C. Moreover, the glucose in a real urine sample from a diabetic patient has been detected successfully with impressive intensity due to the low background

ions in the low mass range. This method shows great potential for applications in routine urine assay in clinical studies. Compared with MALDI-MS, SALDI-MS possesses many advantages in detection of small molecules, including low background noise and high desorption and ionization efficiency. More ordered structures based on colloidal lithography which can increase the photo-absorption and the laser desorption and ionization effectivity should be explored. The goals in the future work are to understand the role of structure in the laser desorption and ionization process unambiguously and fabricate novel nanostructures to improve the performances including limit of detection, qualitative and quantitative measurements of analytes over a broad range. Bioinspired colloid-based surfaces with special wettability Wettability is considered as one of the most fundamental properties to define the contact between liquid and solid surfaces, which has been playing a significant role in agriculture, industry and military fields [213–218]. The surface chemical compositions determine the surface free energy, and the surface topographic structures bring a large extent of air trapping in the trough area, which is essential for special wettability such as anisotropic dewetting [148] and antifogging [219,220] on functional surfaces. In nature, it has been frequently observed that the unique micro- or nano-structures of surfaces render them special wettability, e.g., the self-cleaning effect on the leaf of lotus [221] and directional water-repellency of butterfly wings [222]. Taking lotus leaf as an example, it has been demonstrated that the combination of microscale papillae and nanoscale branch-like nanostructures, along with the epicuticula wax, contributes to superhydrophobic and self-cleaning properties [221,223]. Jiang and co-workers reported the fabrication of arrays of artificial compound eyes which were composed of PDMS hemisphere with the

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Fig. 18. Superhydrophobic and oleophobic surfaces. (A) Biomimetic superhydrophobic surfaces with arrays of artificial compound eyes. Reproduced with permission [224]. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA. (B) Anisotropic wetting property of nanocone arrays with geometric gradient. Reproduced with permission [227]. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA. (C) Superamphiphobic polymeric pillars surface obtained by two-step successive anisotropic and isotropic plasma etching. Reproduced with permission [228]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. (D) Omniphobic Ni micronails surface obtained by colloidal occlusion template method. Reproduced with permission [229]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

diameter of 22 ␮m and covered with 100 nm silica nanospheres (Fig. 18A) [224]. After modification with monolayers of selfassembled fluoroalkylsilane, the average values of the static water contact angle (CA) and tilted angle were ca. 155◦ and ca. 15◦ , respectively. Similarly, Chi and co-workers fabricated the multiscale corrugated SiNC arrays by the successive RIE process with 1 ␮m and 580 nm diameter PS sphere monolayer as etching masks [117]. Modification of the surface of the corrugated SiNC arrays with the fluoroalkylsilane (heptadecafluoro-1,1,2,2tetrahydrodecyl)triethoxysilane produces coatings with a water CA exceeding 150◦ . This type of coating possesses excellent superhydrophobic properties and shows promise for the development of self-cleaning and antireflection coatings for silicon solar cells. They also fabricated flexible superhydrophobic films by directly thermally evaporating silver as nanoscale particles on sub-micro polymer hemispheres [225]. In addition to the biomimetic compound arrays, nanopillars with high aspect ratio were fabricated by Yang and co-workers, which exhibited superhydrophobic properties due to the high fraction of air trapped in the rough area between pillars [149]. Furthermore, the silicon nanotip arrays with stripes were fabricated and used as hydrophobic substrates with anisotropic dewetting just like the leaves of rice [148,226]. It was found that water droplets could transform from isotropic dewet-

ting to anisotropic dewetting by adjusting the strip width with and without silicon nanotips. More recently, as shown in Fig. 18B, they fabricated SiNC arrays with geometric gradient using inclined etching method. Because their CA, hysteresis and sliding angles were in gradient, the structured surfaces demonstrated site-specific wettability to form a composited wettability library with an anisotropic wetting distribution [227]. There are many superhydrophobic surfaces in living systems, by contrast, no oleophobic surfaces which can repel nonpolar liquids with a low surface energy are encountered. However, these kinds of omniphobic surfaces have attracted a lot of attention in recent years owing to their tremendous potential impact on industry. It has been demonstrated that omniphobic surfaces can be achieved using specially engineered topographic features with overhang or reentrant geometry [230,231]. For instance, a twostep etching process containing anisotropic and isotropic etching was performed to fabricate reentrant-shape PMMA pillars, which exhibited excellent superamphiphobic property and high tolerance to pinning pressure after coated with a perfluorinated monolayer (Fig. 18C) [228]. Apart from the topographic geometry, control over the density of nanostructures is very critical for their omniphobic property. Recently, a monolayer of PS nanospheres was spread on the surface of the anodic aluminum oxide membrane followed by

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Fig. 19. Applications of (A, B) arrays of 2D or 3D patterned colloidal crystals and (C–F) arrays of nano-objects. (A) Nanogenerator. Reproduced with permission [233]. Copyright 2015 The Royal Society of Chemistry. (B) Vapor sensor. Reproduced with permission [235]. Copyright 2014 American Chemical Society. (C) Molecular imprinting. Reproduced with permission [61]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. (D) Field effect transistor. Reproduced with permission [238]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. (E) Supercapacitor. Reproduced with permission [242]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA. (F) Resistance-type gas sensor. Reproduced with permission [243]. Copyright 2015 The Royal Society of Chemistry.

thermal annealing to regulate the available pores for electrochemical deposition of nickel micronails [229]. As shown in Fig. 18D, the micronails with the head size of 10 ␮m and densities ranging from 5 × 10−4 to 1 × 10−3 ␮m−2 exhibited omniphobic properties containing CAs of water exceeding 150◦ and CAs of hexadecane exceeding 90◦ . Additionally, by decorating polyethylene terephthalate nanocone arrays with poly (N-isopropyl acrylamide), Yang and co-workers reported a temperature controlled wettability switching surface between superoleophobicity and oleophilicity [232]. Although learning from nature gives us inspirations to fabricate multiscale arrays or pillars with high aspect ratio to obtain surfaces with special wettability, more effective structure topography, the unrevealed structure effect, directional or special wettability with high control and reversible/responsive switching should be further investigated in the future. New developments in other applications Arrays of 2D or 3D patterned colloidal crystals have important applications such as triboelectric nanogenerator [233] and vapor

sensors [234,235]. For instance, an arch-shaped triboelectric nanogenerator has been constructed by combining self-assembled PS MCCs with a poly(vinylidence fluoride) porous film as two friction layers, where the difference in the ability of trapping electrons between two friction layers leads to electron transfer when they are in contact with each other (Fig. 19A) [233]. Additionally, high resolution patterns of mesoporous silica nanoparticles has been created by inkjet printing as responsive photonic crystals, where the size and mesopores’ proportion of mesoporous nanoparticles determine the original color and vapor responsive color shift (Fig. 19B) [235]. Arrays of nano-objects obtained based on colloidal lithography can also be employed in other important applications, such as molecular imprinting [61], transfection [236], nanosucker [237] and electrodes [100,238–241] due to the increased surface area and uniformity in the geometry, as well as reproducibility in the fabrication. For example, as shown in Fig. 19C, surface-imprinted polymers for selective protein recognition have been generated by the electrosyntheis of poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate) (PEDOT/PSS) film using PS nanospheres

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conjugated with protein as template, followed by the removal of PS nano-spheres. The binding capacity of such surface-imprinted polymer films is ca. 6.5 times higher than that of films imprinted with unmodified beads [61]. A transfection platform can be designed based on vertical-aligned SiNWs, whose surface features including diameter, height and density can be modulated precisely [236]. Nano-electrodes obtained by colloidal lithography have been employed in several areas such as biocurrent mapping [244], field effect transistors [238], supercapacitors [242] and organic lightemitting diodes [245]. A low cost H2 sensor with a Pd-decorated Si nanomesh structure has been developed using colloidal lithography, instead of the expensive high-resolution lithography process [246]. On the basis of light-to-heat conversion, gold nanohole arrays have been simultaneously used as absorbers and electrodes for creating ionic thermoelectric device [247]. By combining colloidal lithography with vaccum evaporation technique, a vertical-type transistor has been employed to enhance the performance of organic transistor due to the decreasing length and the increasing cross-sectional area through which the charge carriers travel [248]. Fuchs and co-workers fabricated nanopore electrode by combining colloidal lithography with shadow-mask thermal evaporation in bottom-gate, bottom-contact configuration organic field effect transistors (Fig. 19D) [238]. The introduction of this nanoporestructured electrode facilitates the formation of pentacene layer with small grain boundaries at the electrode interface, and induces the growth of pentacene, thus reducing the contact resistance and accordingly improving the mobility of charge carriers in the devices. Similarly, Au@MnO2 core-shell nanomesh structure on polyethylene terephthalate substrate can be employed as a highefficient electrode to fabricate transparent flexible supercapacitors, which exhibit excellent flexibility and reasonable cycling stability (Fig. 19E) [242]. 3D hexagonal Si-SnO2 core-shell nanorod arrays have been utilized as anode materials in on chip micro-lithiumbatteries to improve the mechanical stability and cycleability due to the larger buffer spaces and surface areas, as well as surface conductivity [249]. In addition to the fabrication of nanostructured electrodes, nanohole arrays of metal oxides have been fabricated as active layers and then integrated into sensing transducers to enhance the performance of vapor and glucose sensing (Fig. 19F) [243,250]. Similarly, nanocavity structure obtained from colloidal lithography has been employed as a template to fabricate a freestanding nanostructured electrolyte membrance decorated with platinum electrode/catalyst. The resulting solid oxide fuel cell achieved a power density of 1.34 W/cm2 at low operating temperature (500 ◦ ) by increasing the area and decreasing the thickness of the electrolyte membrane [251]. Conclusions and outlook Colloidal lithography is a simple and efficient surface patterning method. The process consists of an upstream preparation and modification of colloids as well as a downstream lithographic fabrication of nanostructures, with feature sizes ranging from several micrometers down to tens of nanometers with high controllability. So far, various patterns of colloidal spheres including Janus particles, hcp and ncp single layers, double layers, free-standing films and template-induced arrangements have been constructed. Decorating colloids with functional molecules or materials for novel properties is also ongoing. By using MCCs as masks or templates for the subsequent nanofabrication processes, various nanostructures such as triangle particles, crescents, disks, bowls, nets, cavities, holes, pillars and cones are fabricated on different types of materials including metals, polymers, and semiconductors. More precise control over the nano-objects or even delicately designed structures is also under way by newly developed techniques such as

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shadow sphere lithography and MCCs-assisted photolithography. It has been demonstrated that the obtained nanostructures have great applications in many important areas such as biomimetic antireflection, solar energy harvesting, LSPR-based sensing, perfect absorber, SERS, mass spectrometry and many others. More importantly, the performance can be further improved by taking advantage of high controllability of colloidal lithography over dimension, morphology and composition. In spite of the great extent of success in fabrication and applications of nanostructures, there are some issues in colloidal lithography remaining to be addressed in the future research. First, the density of defects must be further reduced to an acceptable level required by specific types of applications, or even ideally, largely defect-free. This is particularly important for colloids with diameter lower than 100 nm, where there is a noticeable weakness of colloidal lithography compared with conventional lithography techniques. Second, the diversity and functionalization of colloids materials must be further expanded to include major class of functional materials. These improvements will certainly provide new types of functionalities that failed to be offered by common colloids. Third, regardless of the abundant patterns/nanostructures that have been achieved by colloidal lithography, fabricating arbitrary patterns/nanostructures remains challenging due to the intrinsic limitations of colloidal materials and their arrangements. Thus, efforts should be devoted to exploration of more novel and integrated structures by combining colloidal lithography with other delicate methods including the combination of “top-down” patterning and “bottom-up” self-assembly. In addition, it is very desirable to investigate the role of nanostructures in the practical applications and identify areas for further improvement. With all the reviewed features and promising properties the technology is capable of, we expect colloidal lithography will bring new capabilities and promising applications in the near future. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21501127 and 21527805), China Postdoctoral Science Foundation (Grant No. 2016M591908) and the funds from the project of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] Y. Lei, S.K. Yang, M.H. Wu, G. Wilde, Chem. Soc. Rev. 40 (2011) 1247–1258. [2] M.D. Levenson, N. Viswanathan, R.A. Simpson, IEEE T. Electron Dev. 29 (1982) 1828–1836. [3] C. Vieu, F. Carcenac, A. Pepin, Y. Chen, M. Mejias, A. Lebib, L. Manin-Ferlazzo, L. Couraud, H. Launois, Appl. Surf. Sci. 164 (2000) 111–117. [4] J. Melngailis, J. Vac. Sci. Technol. B 5 (1987) 469–495. [5] Y. Xia, J.A. Rogers, K.E. Paul, G.M. Whitesides, Chem. Rev. 99 (1999) 1823–1848. [6] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Science 272 (1996) 85–87. [7] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 5599–5611. [8] J.H. Zhang, Y.F. Li, X.M. Zhang, B. Yang, Adv. Mater. 22 (2010) 4249–4269. [9] S.M. Yang, S.G. Jang, D.G. Choi, S. Kim, H.K. Yu, Small 2 (2006) 458–475. [10] Y. Li, G.T. Duan, G.Q. Liu, W.P. Cai, Chem. Soc. Rev. 42 (2013) 3614–3627. [11] X.Z. Ye, L.M. Qi, Nano Today 6 (2011) 608–631. [12] B. Ai, H. Möhwald, D. Wang, G. Zhang,Adv. Mater. Interfaces 4 (2017), 1600271. [13] R. Arshady, Colloid Polym. Sci. 270 (1992) 717–732. [14] P.J. Dowding, B. Vincent, Colloids Surf. Physicochem. Eng. Aspects 161 (2000) 259–269. [15] D. Zou, V. Derlich, K. Gandhi, M. Park, L. Sun, D. Kriz, Y. Lee, G. Kim, J. Aklonis, R. Salovey, J. Pol. Sci., Part A: Pol. Chem. 28 (1990) 1909–1921. [16] D. Zou, S. Ma, R. Guan, M. Park, L. Sun, J. Aklonis, R. Salovey, J. Pol. Sci., Part A: Pol. Chem. 30 (1992) 137–144. [17] C.K. Ober, K.P. Lok, M.L. Hair, J. Pol. Sci., Part C: Pol. Lett. 23 (1985) 103–108. [18] S. Shen, E. Sudol, M. El-Aasser, J. Pol. Sci., Part A: Pol. Chem. 32 (1994) 1087–1100. [19] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62–69. [20] E. Matijevic, Langmuir 10 (1994) 8–16.

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Mengyuan Zhang joined Prof. Chi’s group in Soochow University after received her bachelor degree in 2016. Her research interests focus on fabrication and application of high performance SERS substrates.

Yuekun Lai received his PhD degree from the Department of Chemistry, Xiamen University. During 2009–2011, he worked as a research fellow at Nanyang Technological University, Singapore. In July 2011, he moved to Muenster University with Prof. Harald Fuchs and Prof. Lifeng Chi. During 2013–2018, he was a Professor at the National Engineering Laboratory for Modern Silk, and School of Textile and Clothing Engineering in Soochow University. Currently, he is a “Minjiang Scholar” chair Professor at the College of Chemical Engineering at Fuzhou University. His research interests are focused on TiO2 nanostructures, bio-inspired intelligent surfaces with special wettability, energy & environmental materials. Lifeng Chi pursued her ph.D. degree at the University of Göttingen, Germany in 1989, and finished her Habilitation at the University of Münster, Germany in 2000. She became a Professor in Physics, University Münster in 2004. She joined FUNSOM at Soochow University as a chair Professor in 2012. Her key research is centered around supramolecular chemistry on surfaces, working intensively in the three important branches of this research area – molecular assembly and reactions, molecular patterning, and structured functional surfaces.

Please cite this article in press as: Y. Wang, et al., Advanced colloidal lithography: From patterning to applications, Nano Today (2018), https://doi.org/10.1016/j.nantod.2018.08.010