The advent of nanoscience and nanotechnology has led to tremendous enthusiasm of researchers from different scientific disciplines such as physics, chemistry, and biology to engage with nanostructures with the intent of pursuing the innovative property derived from the nanometer dimension. In this context, fabrication of nanostructures accordingly becomes an increasing demand nowadays. Obviously, low-throughput and expensive maskless lithography is a less accessible choice for chemists, physicists, material scientists, and biologists. The success of extending mask-assisted lithography beyond microelectronics workshops is largely limited by the mask design and preparation. Recently a host of effort has been devoted to develop non-conventional lithographic techniques especially integrated with a bottom-up nanochemical procedure for surface patterning with low cost, flexible processing capability, and high throughput. However, most of the non-conventional lithographic techniques require an assistance of conventional lithographic techniques such as photolithography to design and make masks or masters. To develop ingenious, cheap, and non-lithographic ways to make masks or masters with high resolution (below 100 nm), a great deal of self-assembly nanostructures have been recruited for masking, including laterally structured Langmuir–Blodgett monolayers, liquid crystalline structures of surfactants, micro-phase separation structures of block copolymers, and self-assembly of proteins and nanoparticles.
Monodisperse colloidal particles with size ranging from tens of nanometers to tens of micrometers can be easily synthesized via wet chemistry ways such as emulsion polymerization and sol-gel synthesis. Due to the size and shape monodispersity, they can self-assemble into a two dimensional (2D) and three dimensional (3D) extended periodic array, usually referred to as colloidal crystal. Colloidal crystals are usually characterized by a brilliant iridescence arising from the Bragg reflection of light by their periodic structures. Despite the beauty, the iridescent color has recently inspired the explosive study of fabrication of 3D colloidal crystals or inverse opals – 3D inverted replication of the crystals – for pursuing a complete energy bandgap to manipulate electromagnetic waves, similar to that to do to electrons in semiconductors. Before being used as photonic materials, both the ordered arrays of solid particles and those of the interstices between the particles of colloidal crystals have already been used as masks or templates for surface patterning via for instance etching or deposition of materials. This bottom-up masking methodology has recently gained increasing attention for surface patterning due to the processing simplicity, the low cost, the flexibility of extending on various substrates with different surface chemistry and even curvatures, the ease of scaling down the feature size below 100 nm. In the present chapter, we refer to as various surface patterning processes based on use of colloidal crystals as masks as a whole as colloidal lithography (CL), overview the processing principles, and survey the recent advances.
2. Colloidal masks
The success of using colloidal crystals as masks for surface patterning is determined by the capability of directing self-assembly of colloidal particles and manipulating the crystal packing structures. Provided their size and shape are monodisperse, colloidal particles can be readily to self-assemble into long-range ordered arrays with a hexagonal packing, driven simply by entropic depletion and gravity. Up to date a variety of colloidal crystallization techniques – with and without the aid of templates – have successfully been developed to implement colloidal crystallization in a controlled fashion [1-3]. Due to enormous numbers of publications on colloidal crystallization and immense diversity of crystallization techniques reported thus far and especially by taking into account that colloidal lithography relies on masking of single layers or double layers of colloidal crystals, this section is centered mainly on techniques for 2D colloidal crystallization developed thus far.
2.1. Simple colloidal masks
Sedimentation is a natural way for colloidal crystallization. In dispersion colloidal particles tend to settle out of the fluid under gravity and to accumulate and precipitate on a wall, which can be described by Stokes’ law. This sedimentation process can be used to grow colloidal crystals with high quality, and the crystal thickness can be tuned by the particle concentration. However, the sedimentation time is always up to several hundreds of hours; time-consuming is a big drawback of this technique [4-6].
At the beginning of 1990’s Nagayama’s group has commenced a systematic study of sedimentation of colloidal particles in the presence of strongly attractive capillary forces . With the help of optical microscopy and using a Teflon ring to confine the dispersions of colloidal particles, they have directly observed the particle sedimentation dynamics on a solid substrate. Their observations suggest a two-stage mechanism for 2D colloidal crystallization: 1) nucleation and 2) crystal growth (Fig. 1) . Micheletto’s group has fabricated 2D colloidal crystals on a solid substrate via sedimentation by tilting the substrate about 9° and keeping the system temperature constantly using a Peltier cell .
2.1.2. Vertical deposition
When a supporting substrate is held vertically in a suspension of colloidal particles, moving the front of the suspension flow either by the solvent evaporation or by withdrawing the substrate out of the suspension can pin colloidal particles on the substrates – nucleation – and the convective transfer of the particles from the bulk phase to the drying front – crystallization (Fig. 2) . The thickness of colloidal crystals obtained via vertical deposition is dependent on the ratio of the thickness of the liquid films remaining of supporting substrates to the diameter of the colloidal particles . When the ratio is far larger than 1, 3D colloidal crystals are obtained with high quality; the crystal thickness can be tuned by the particle concentration . When the ratio is comparable to or smaller than 1, 2D colloidal crystals can be obtained . Vertical deposition may allow formation of large-area crack-free colloidal crystals provided the suspensions of colloidal particles wet well supporting substrates, there is no interaction between the particles and the substrates, the suspensions are sufficiently stable and the solvent evaporation is well controlled .
Dip-coating is a fast and dip-coater assisted variant of vertical deposition . Besides, a number of techniques have been developed to improve the efficiency and quality of colloidal crystallization via vertical deposition, such as variable-flow deposition , isothermal heating evaporation-induced self-assembly , two-substrate deposition , reduction of the humidity fluctuation , adjustment of the meniscus shape , temperature-induced convective flow  and vertical deposition with a tilted angle . The maximal size of colloidal particles used for vertical deposition is limited by the particles sedimentation of colloidal particles, for instance 400-500 nm for silica particles and 1 μm for polystyrene particles. To compete with sedimentation, Kitaev and Ozin have used low pressure to accelerate the solvent evaporation, and successful growth of large-area 2D binary colloidal crystals with the diameter ratios of the large particles to the small ones in the range of 0.175 to 0.225 (Fig. 3) .
Vertical deposition has recently been extended to stepwise growth of 2D colloidal crystals with large and small colloidal particles on a substrate [20, 21]. In their procedure, the 2D colloidal crystal of the larger particles firstly formed on the substrate is used to template the growth of the 2D colloidal crystal of the smaller particles. By deliberately tuning the concentration of the small particle suspension, binary colloidal crystals with the stoichiometric ratios of large to small particle sizes of 1:2, 1:3, 1:4, or 1:5 have been constructed [20, 21].
2.1.3. Spin coating
Spin coating was the first technique for growth of 2D colloidal crystal masks for colloidal lithography due to the fact that it allows easy and quick formation of 2D crystals over large area . The long range ordering degree of 2D colloidal crystals obtained via spin coating can be improved by increasing the wettability of the suspensions of colloidal particles on supporting substrates by for instance adding ethylene glycol into the suspension . However, the spin coating is a process far more complicated than it appears and the underlying mechanism remains in debate. Rehg and Higgins have conducted a theoretical analysis of the physics governing spin coating of a colloidal particle suspension on a planar substrate . Jiang and Mcfarland have succeeded in fabrication of wafer scale long-rang ordered and non-close-packed 2D and 3D colloidal crystals by spin coating of highly viscous triacrylate suspension of silica particles and subsequent polymerization of triacrylate, followed by partial removal of the polymer matrices [25, 26]. Wang and Möhwald have developed a stepwise spin coating protocol to consecutively deposit large and small colloidal particles into binary colloidal crystals, in which the interstitial arrays in the 2D colloidal crystal of the large particles are used to template the deposition of the small particles due to the spatial and depletion entrapment (Fig. 4) .
2.1.4. Colloidal crystallization at interface
Using the water/air interface as a platform for molecular self-assembly has been extensively studied. Langmuir-Blodgett (LB) technique has been proved as a powerful and versatile way to organize amphiphilic molecules (referring molecules that are hydrophobic on one end and hydrophilic on the other end) to macroscopic monolayer films at the water/air interface and transfer the films to solid substrates in a controlled manner . It is also well studied but less recognized that in a biphasic system, e.g. water/oil, colloidal particles behave rather similar to amphiphilic molecules; they thermo- dynamically prefer to attach to the interface . Due to this analogy, the water/air interface has been extended to support self-assembly of colloidal particles. Pieranski has conducted the first deliberate microscopic observation of 2D colloidal crystallization at the water/air interface and hypothesized the repulsive interaction between the dipoles of colloidal particles trapped at the interface due to the asymmetric charge distribution on the particle surface drives the particles to self-assemble into an ordered array (Fig. 5) . Park
Colloidal monolayers with high order and increased complexity beyond plain hexagonal packing geometries are useful for 2D templating of surface nanostructures and lithographic applications. Weiss and co-workers developed binary colloidal monolayers featuring a close-packed monolayer of large spheres with a superlattice of small particles in a single step using a Langmuir trough .
As compared with the water/air interface, the water/oil interface is a much better platform to trap colloidal particles due to the relatively low interfacial tension . Thus, water/oil interfaces have been used for growth of 2D colloidal crystals [37, 38], while transfer of the resulting 2D colloidal crystals to solid substrates remains problematic. Besides water/air interfaces, air/water/air interfaces have also been utilized for colloidal crystallization. Velikov and coworkers have studied of colloidal crystallization in thinning foam films . Using air/water/air interfaces for crystallization, Wang and co-workers have successfully obtained free-standing and crack-free colloidal crystal films with sizes over several square millimeters . Instead of water/air interface, Zental and co-workers have used the interface between melted germanium and air for colloidal crystallization and obtained crack-free colloidal crystals .
2.2. Complex colloidal masks
2.2.1. Deformed colloidal masks
In general, polymers undergo a second-order phase transition from hard glassy state to soft rubbery state above a glass transition temperature (Tg) due to the free-volume change between the polymer chains. Therefore, annealing slightly above Tg can cause deformation of spherical polymeric beads. It is demonstrated that microwave radiation can much more precisely control the deformation of spherical polymer particles by adjusting the microwave intensity than heating in oven .
Giersig and coworkers have recently developed a new annealing approach – using microwave pulse to heat polystyrene (PS) microspheres in a mixture of good and poor solvents for PS, which allows not only reduction of the sizes of the interstices of 2D PS colloidal crystals but also deformation of their geometry from triangular to rodlike, while preserving the interparticle spacing and packing order of the original crystals (Fig. 6) . Recently, Yang
2.2.2. Colloidal masks derived from modified colloidal particles
Many specific colloidal masks have been made using methods mentioned above, usually utilizing one or two kinds of spherical colloidal particles as building blocks. Colloidal particles with anisotropic interactions are expected to enable a wide range of materials with novel optical and mechanical properties. While the self-assembly of spherical particles into periodic structures is relatively robust and well-characterized, the phase space describing the self-assembly of anisotropic particles is vast and has been only partially explored. It includes phases that are impossible for spherical particles to form, including gyroids, simple cubic lattices, and plastic crystals.
Eric R. Dufresne and co-workers demonstrated the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick, which was shown in Fig. 7 .
3. Colloidal lithography
3.1. Controllable etching
When a 2D colloidal crystal is formed on a solid substrate, the interstices between the solid particles can used as masks for reactive ions to create patterned bumps or pores on the substrate. In the beginning of 1980’s Deckmann and Dunsmuir have pioneered the work of etching of a colloidal crystal into a textured surface using a reactive ion beam (RIE) . Since then, reactive ion etching (RIE) has been widely used to interdependently reduce the particle sizes and thus widen the interstitial space in 2D colloidal crystal masks and eventually turn close-packing structures of the crystals to non-close packing one (vide infra). RIE in 3D colloidal crystals is an anisotropic process as the upper layers act as shadow masks for etching the lower layer particles. This anisotropic RIE can turn spherical particles to non-spherical particles, and the particle shapes and the hierarchical nanostructures obtained so strongly depends on the stacking sequence of the colloidal crystals, the crystal orientation relative to the substrate, the number of colloidal layers, and the RIE conditions (Fig. 8) . Of most significance is that the anisotropic RIE paves a new way to machine the surfaces of colloidal particles. Such as nanopores arranged in threefold or fourfold symmetry, depending on the crystalline orientation of the original colloidal crystals, were machined on PS particles [50-52].
Forests of silicon pillars with diameters of sub-500 nm and an aspect ratio of up to 10 were have been fabricated by firstly conduct O2 RIE to turn close-packed PS particle monolayers to non-close packed on and subsequently conduct a “Bosch” process to etch the supporting silicon wafers . Sow
3.2. Controllable deposition
3.2.1. Colloidal masks-assisted chemical deposition
Combining microcontact printing with colloidal crystal masking, Xia
The thiol molecules were released from the stamp to the silica particle during contact and subsequently transferred to the substrate along the surfaces of silica particles, leading to a self-assembled monolayer (SAM) circling the footprint of each silica particle. The area of the thiol SAM could expand laterally via reactive spreading as long as the thiols were continuously supplies. Upon removal of the stamp and lift-off of the beads, the ring pattern was developed by wet etching with aqueous Fe3+/thiourea using the patterned SAM as a resist . Of importance is that ESL allows generation of the concentric rings of different alkanethiol SAMs by successive printing different thiol inks, and the removal of silica particle templates and selective etching yield concentric gold rings and the width of the rings were determined by the printing period (Fig. 9, right panel) .
3.2.2. Colloidal masks-assisted physical deposition
In 1981 Fischer and Zingsheim have used 2D colloidal crystals as masks for contact imaging with visible light . A year later Deckman and Dunsmuir have demonstrated the feasibility of using 2D colloidal crystals as masks for both physical deposition of materials and in turn patterning the surfaces of supporting substrates . They have coined the term “Natural Lithography” to describe this process as “naturally” assembled single layers of latex particles were used as masks rather than lithographic masks. Later on they have expanded the capability of “Natural Lithography” and especially developed the RIE process for increasing the structural complexity of 2D colloidal crystal masks . Since then the group of Van Duyne has devoted numerous efforts to develop patterning techniques using colloidal crystals as masks for metallic vapor deposition [23, 64-66]. In the context of nanoscience, they changed the name “Natural Lithography” to “Nanosphere Lithography” (NSL). Most important is that they have intensively investigated the plasmon resonance properties of metallic patterns obtained via NSL and their correlation with the feature morphology with the intent of developing high sensitive biosensors based on surface enhanced Raman spectroscopy (SERS) .
In a NSL procedure, a 2D colloidal crystal is used as masks for material deposition. The materials for physical deposition can be freely chosen without any limitations; commonly used are various metals such as gold and silver. The projection of the interstices between ordered close-packed particles defines the shape of the nanodots deposited on substrates; the dots usually show a quasi-triangular shape and are arranged in a space group P6mm array due to the hexagonal packing of the colloidal crystal mask (Fig. 10a-c). Van Duyne
In a general NSL procedure, the substrate to be patterned is positioned normal to the direction of material deposition. The in-plane shape of the nanodots and the spacing of the nearest-neighboring dots derived from NSL are dictated by the projection of the interstices of single or double layers of colloidal crystals on substrates. They can be tuned by varying the projection geometry of the interstices on substrates by titling the masks with respect to the incidence of the vapor beam for instance. This has inspired development of angle-resolved NSL (AR-NSL), pioneered by the group of Van Duyne .
In a AR-NSL process, the incidence angle of the propagation vector of the material deposition beam with respect to the normal direction of the colloidal mask (θ) and/or the azimuth angle of the propagation vector with respect to the nearest neighboring particles in the colloidal masks (ϕ) – the mask registry with respect to the vector of the material deposition beam – have been employed to reduce the size of the nanodots obtained and, at the same time, elongate their triangular shape (Fig. 11). By rotating substrates, Giersig and coworkers have recently found that AR-NSL can generate much more complicate metallic nanostructures and they referred to this process as shadow NSL [43, 68, 69]. Zhang and Wang have recently demonstrated the feasibility of consecutively depositing two different metals, such as gold and silver, at two different incidence angles, to construct ordered binary arrays of gold and silver nanoparticles .Due to the rotation of the colloidal mask, shadow NSL relies in a process resolved by the azimuth angle (ϕ) of the incidence deposition beam rather than the incidence angle (θ).
The elegant extension of AR-NSL is to stepwise conduct physical vapor deposition of identical or different materials at the different incidence angles. The group of Van Duyne has succeeded in growth of surface patterning features composed of two triangular nanodots either overlapped or separated by two deposition steps at θ = 0º and θ > 0º, respectively . Giersig
Prior to physical vapor deposition, colloidal crystal masks can undergo RIE to reduce the sizes of the particles and widen the interstitial spaces, thus increasing the dimension of triangular nanodots obtained via NSL. Increasing the RIE time can turn close-packed colloidal crystal masks to non-close packed ones, which leads to thin films with hexagonally arranged pores [71, 72]. Wang
Various colloidal spheres, organic and inorganic, can be produced that are exceedingly monodisperse in terms of size and shape. Nevertheless, their surfaces still remain chemically homogeneous or heterogeneous. Controlling the surface properties of colloidal particles is one of the oldest and, at the same time, the most vital topics in colloid science and physical chemistry. Patchy particles, i.e., particles with more than one patch or patches that are less than 50% of the total particle surface, should present the next generation of particles for assembly [77-79]. However, patterning the surface of colloidal particles with sizes of micrometers or submicrometers is a formidable challenge due to lack of the proper mask.
When 2D colloidal crystals are used as masks for physical vapor deposition, it is expected that only the upper surfaces of the colloidal particles, exposing directly to the vapor beam, will be coated with new materials, which leads to two spatially well-separated halves on the colloidal particles, coated and non-coated, with two distinct surface chemical functionalities [80, 81]. Such particles are usually referred to as Janus particles. By embedding a monomer of close-packed colloidal particles in a photoresist layer, Bao
3.2.3. Extension of colloidal lithography
One extension of NSL is to use the surface patterns obtained as templates to grow nanostructures of a variety of materials via bottom-up self-assembly. Mulvaney’s group has grown monolayer and multilayer films of semiconductor quantum dots on surface patterns derived from NSL, leading to nanostructured luminescent thin films [90, 91]. Valsesia
The second extension is to use NSL-derived surface patterns as etching masks to create surface topography. Chen
The third extension is to use NSL-derived surface patterns to template or catalyze the growth of other functional materials. Zhou
4.1. Optical properties
Surface patterns derived from CL, especially NSL, are usually made up of metals such as gold and silver. Noble metal nanostructure arrays have pronounced surface plasmon resonance, which results from incident electromagnetic radiation exciting coherent oscillations of conduction electrons near a metal-dielectric interface . Giessen and co-workers introduced an angle-controlled colloidal lithography as a fast and low-cost fabrication technique for large-area periodic plasmonic oligomers with complex shape and tunable geometry parameters, and investigated the optical properties and found highly modulated plasmon modes in oligomers with triangular building blocks. Fundamental modes, higher-order modes, as well as Fano resonances due to coupling between bright and dark modes within the same complex structure are present, depending on polarization and structure geometry. This process is well-suited for mass fabrication of novel large-area plasmonic sensing devices and nanoantennas (Fig. 15) .
One of the straightforward technical applications of CL is to use as highly sensitive biosensors relied on the localized-surface plasmon resonance (LSPR) of metallic nanostructures . The LSPR of metallic nanostructures composed of gold rings  and disks , obtained via NSL has been studied. It is found that the LSPR can be tuned by varying either the diameter of the disks at a constant disk height or the ring thickness. The shape-dependent red shift originates from the electromagnetic coupling between the inner and outer ring surfaces, which leads to energy shifts and splitting of degenerate modes . NSL has been also used to create nanocaps and nanocups; their LSPR behavior has been studied . Lee
Light trapping across a wide band of frequencies is important for applications such as solar cells and photo detectors. Yao Y. and Yao J.
The wettability of solid surfaces is a significant property depending on both chemical compositions and the surface structure. A great number of ordered arrays generated through simple or modified colloidal lithography could induce different wettabilities. Recently, Koshizaki and co-workers fabricated vertically ordered Co3O4 hierarchical nanorod arrays using pulsed laser deposition (PLD) onto colloidal crystal masks followed by an annealing process, and the as-prepared Co3O4 nanorod arrays demonstrated stable superhydrophilicity without UV irradiation even after half a year owing to the improved roughness of the hierarchical structure and the abundant OH− groups induced by the PLD and annealing processes.
4.3. Other applications
Besides the exploitation of CL and the patterns obtained thereof in LSPR-assisted sensing, the magnetic properties of CL-derived nanostructures gain increasing attention. In general, nanoscale magnetic materials often exhibit superparamagnetic behavior. Moreover, an ordered nanostructure of magnetic materials is required for investigation of the mesoscopic effects induced by the confinement of magnetic materials in nanoscale domains . Since magnetic properties are strongly dependent on the domain size and the distance between domains, Weekes
What’s more, a core question in materials science is how to encode non-trivial organized structures within simple building blocks. A recent report from this laboratory described methods for functionalizing latex spheres to make them hydrophobic at their poles, leading to the directed self-assembly of a kagome lattice pattern in which each sphere was coordinated with four neighbors, two at each pole. Granick and co-workers developed methods for functionalizing micrometer-sized colloidal spheres with three or more zones of chemical functionality through colloidal lithography, literally combining double-sided angle-resolved physical deposition and controllable chemical etching. These synthesis methods allowed targeting of various lattice structures whose bonding between neighboring particles in liquid suspension was visualized in situ by optical microscopy [115,116].
The recent development of CL, especially the integration of etching the colloidal mask, altering the incidence angle, and stepwise and regularly changing the mask registry, leads to a powerful nanochemical patterning tool with low cost in capital and operation, high throughput, and ease to be adopted on various planar and curved surfaces and even on microparticles. Different from conventional mask-assisted lithographic processes in which the mask design and production usually remain a challenge for scaling down the feature size and diversifying the feature shape, CL embodies a simple way for masking – self-assembly of monodisperse microspheres on a targeted substrate. The feature size can easily shrink below 100 nm by reducing the diameter of the microspheres used according to the simple correlation between the interstice size and the sphere diameter. The feature shape can be easily diversified by the crystalline structure of a colloidal crystal mask, the time of anisotropic etching of the mask, the incidence angle of vapor beam and the mask registry (the azimuth angle of vapor beam). Currently, CL allows fabrication of very complicated 2D and 3D nanostructured features, such as multiplex nanostructures with a clear-cut lateral and vertical heterogeneity. A number of new nanostructures are hard to be implemented, or cannot be in some cases, by conventional lithographic techniques. As such, CL provides a nanochemical and complementary tool of conventional and fully top-down lithographic techniques, and thus holds immense promise in surface patterning.
However, CL is still in a very early stage of development. Despite the great progress in colloidal crystallization it still remains a formidable challenge to create a defect-free single crystal with a defined crystalline face. The presence of defects dramatically reduces the patterning precision of CL. For instance, the random orientation of polycrystalline crystalline domains in a colloidal mask is a disaster for collimating the mask registry. In this aspect, template-assisted epitaxy for colloidal crystallization is promising as it allows growth of colloidal crystals with defined packing structure and orientation. Since a patterned substrate is necessitated for the colloidal epitaxy, its applicability for patterning is limited. How to transfer a colloidal crystal derived from this colloidal epitaxy onto different substrates without deterioration of the crystal quality should be an ensuing task for CL. Besides, fabrication of large area monolayer of periodically close-packed microspheres with sizes smaller than 100 nm remains highly challenging, which brings a technical problem to reduce the feature size below 10 nm via CL. In a CL patterning process, furthermore, the feature size and the interspace size between the features cannot be separately manipulate, as they both are directly proportional with the sphere size in a colloidal mask, which largely limit the patterning capability of CL.
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