# Functional Micro–Nano Structure with Variable Colour: Applications for Anti-Counterfeiting

Authors: Hailu Liu; Dong Xie; Huayan Shen; Fayong Li; Junjia Chen
Journal: Advances in Polymer Technology
 (2019)

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## Abstract

Colour patterns based on micro-nano structure have attracted enormous research interests due to unique optical switches and smart surface applications in photonic crystal, superhydrophobic surface modification, controlled adhesion, inkjet printing, biological detection, supramolecular self-assembly, anti-counterfeiting, optical device and other fields. In traditional methods, many patterns of micro-nano structure are derived from changes of refractive index or lattice parameters. Generally, the refractive index and lattice parameters of photonic crystals are processed by common solvents, salts or reactive monomers under specific electric, magnetic and stress conditions. This review focuses on the recent developments in the fabrication of micro-nano structures for patterns including styles, materials, methods and characteristics. It summarized the advantages and disadvantages of inkjet printing, angle-independent photonic crystal, self-assembled photonic crystals by magnetic field force, gravity, electric field, inverse opal photonic crystal, electron beam etching, ion beam etching, laser holographic lithography, imprinting technology and surface wrinkle technology, etc. This review will provide a summary on designing micro-nano patterns and details on patterns composed of photonic crystals by surface wrinkles technology and plasmonic micro-nano technology. In addition, colour patterns as switches are fabricated with good stability and reproducibility in anti-counterfeiting application. Finally, there will be a conclusion and an outlook on future perspectives.


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## Body

## 1. Introduction

The interaction of the micro/nano structure with incident light can produce a structural colour which is mainly consequent on multi-layer thin film interference, diffraction of surface or periodic structure, and wavelength selective scattering by sub-wavelength particles [1–5]. There are many natural examples of microstructure, such as peacock tails, chameleons, butterflies, pearls, the elytra of beetles and opal native to Australia [6–10]. Peacock tail feather has a central stem of barbs array with flat barbules which disperse the incident light and cause colouration. The barbules with different diameters of more than one hundred nanometers form a two-dimensional array, which are shown in different colours by varying the lattice constant and shifting the midgap frequency of the partial photonic bandgap [11]. For pearls with a closely packed multilayer structure, the colour have been ascribed to interference and/or diffraction of light within the binding regions of the aragonite tiles, and the rainbow-like diffraction colours of pearl has been demonstrated to depend on the width of the edge-band structure of organic matrix in aragonite [12]. The golden colour of elytra of beetles results from a multilayer interference involving a homogeneous melanoprotein layer and a melanoprotein nanoparticles-with-air-vids layer. Their scales can change colour from golden to red in wet state and back to golden in dry state [13]. Meanwhile, light scattering, dispersion, grating diffraction, etc. can also produce structural colour phenomenon. The blue sky is the most prominent representation of the structural colour of light scattering. When sunlight enters the atmosphere, light of shorter wavelengths such as blue, cyan, and purple is scattered by the action of fine dust in the air. The rainbow comes from the dispersion of light under the action of fine droplets.The animals control their colour with environmental change for camouflage, signal communication, conspecific recognition and reproductive behaviour [14, 15]. Researchers are inspired to discover new materials and structures to understand the mechanisms of stimulus response processes and structural colour formation. The artificial structural colour is produced by the interactions of natural light with microstructures, which is different from pigmentary colour produced by selective absorption of natural light. With its high brightness and saturation but no fading, iridescent phenomenon and polarization effect, structural colour is widely used in many fields such as display and imaging technology [16], printing and painting [17], textile industry [18], fluorescence manipulation [19], energy conversion [20, 21] information storage devices [22, 23], sensors [24–27], photonic devices [28] and anti-counterfeiting technology [29–32].In particular, fabricating micro-nano structure has become one of the most competitive and promising technologies in the world today. With different microstructures, the surface of the material can be characterized by superhydrophobicity, antireflection properties, drag reduction and light trapping in the fields of photonic crystal preparation [33], surface modification, adhesion regulation [34], chemical detection [35, 36], and optical devices [37–39]. Meanwhile, preparation of large-area micro-nano structure [40, 41], regulation of structural morphology and size [42–44], and the correlation between performance and structural parameters [45–47] have become hot topics in the research of new functional materials. At present, both the “top-down” micro-nano structure processing technology and the “bottom-up” micro-nano structure synthesis assembly technology have their own technical limitations in pattern construction [48, 49], so further research of the preparation method and mechanism of nanostructure arrays are needed, especially for security [50–52]. This review will focus on recent developments in fabrication method of micro-nano structures for patterns and summarizes the pattern construction in anti-counterfeiting by inkjet printing technology, ink responsiveness method, photomask technology, electromagnetic responsiveness method, stress responsiveness method, surface wrinkles technology, plasmonic micro-nano technology, and finally make predictions of future research.


## 2. Fabrication Methods of Micro–Nano Structures for Patterns

There are many methods for preparing micro-nano structures for patterns (see Table1), such as inkjet printing [53], angle-independent photonic crystal [54], self-assembled photonic crystals by magnetic field force [55], gravity [56], electric field [57], inverse opal photonic crystal [58, 59], electron beam etching [60], ion beam etching [61], laser holographic lithography [62], imprinting technology [63] and surface wrinkle technology [64], etc. In traditional micro-nano structure preparation methods, self-assembled monodisperse colloids are driven by noncovalent bond forces such as hydrogen bonding [65–67], π–π bonding [68], Van der Waals interactions [69], and metaloordination [70]. Usually monodisperse colloids are used when preparing photonic crystals by vertical deposition method, field force induced self-assembly method and inkjet direct writing technology combined with self-assembly. The inverse opal photonic crystal structure is similar to the opal structure which is obtained by introducing a high refractive index material into the voids of the opal photonic crystal and removing the template. Its preparation method includes sol-gel method [71–73], chemical vapor deposition [74], atomic layer deposition [75, 76], electrochemical deposition [77, 78], etc. As a micromachining technology, lithography has many advantages such as highly controllable and accurate graphics, simple design and process and easy industrialization. It has become the most mature and widely used in the field of micro-nano structure construction. Photolithography is a process in which a mask of a geometric pattern is applied to a photosensitive material (photoresist) on a substrate by ultraviolet light, but the lithography resolution is limited by the wavelength of the exposed ultraviolet light [79]. After that an electron beam etching technique (EBL) has emerged using an electron beam as a radiation source. The electron-sensitive adhesive coating prepared on the substrate is scanned and exposed to the focused electron beam, and then the patterned structure is obtained by using the developing solution, and the process of constructing the micro-nano structure is more flexible [60, 80]. All optical lithography and EBL can only be effectively exposed or written on the surface of soft materials (photoresists) and affected by the diffraction limit of the wave and the scattered electrons. Relatively, ion beam etching technology directly focuses the ion beam directly against the target with a higher energy density at a relatively short wavelength, and directly realizes the micro-nano array on the surface of the hard material [81]. Laser holographic lithography technique utilizes multiple beams of coherent light to converge in space, and its refractive index is periodically changed to form an ordered structure with periodic changes due to the different degree of sensitivity of the recording medium in the interference field [82, 83]. Imprinting technology is a simple and quick technique that uses a combination of physical pressure to precisely bond a rigid template to a substrate and mechanically modify the pattern on the template to the substrate. Its base material is typically thermoplastic with ultraviolet radiation (UV) [84, 85] or heat cure [86] or other methods of morphological changes [87]. The thermoplastic material can be embossed in its liquid state, and then its surface is fixed by ultraviolet light or heat. A relatively new method is the interface self-organizing technology, which utilizes the inherent Van der Waals force, polarity, steric hindrance and electrostatic force of the polymer film to produce yield deformation to relax the internal instability of the system and spontaneously form a micro-nano structure. The means including instability factors such as dewetting [88, 89], electrohydrodynamics [90, 91], phase separation [92], temperature [93], reaction-diffusion [94], strain [95–97] and swelling induced instability [98–101].Table 1
Fabrication of micro-nano structures for patterns.
Styles
Materials
Method
Characteristic
Inkjet printing
Photonic crystal latexes such as silica [102], polystyrene [103], and polymethyl methacrylate (PMMA) [104], and responsive functional monomers [105–109]
Combination of colloidal emulsion and self-assembly technology
By controlling the self-assembly conditions (substrate wettability [110, 111], substrate temperature [112], colloidal particle size [113], paper surface [114]), photonic crystal structure can be prepared simply, quickly, and at low cost
Angle-independent photonic crystal
Inorganic nanoparticles Silicon dioxide (SiO2) [115], titanium dioxide (TiO2) [116], copper oxide (CuO) [117], organic nanoparticles (polystyrene [118], polydopamine [119])
Spraying, atomizing, spin coating, dispensing
The self-assembly of biomimetic isotropic films are achieved with a characteristic length-scale comparable to the wavelength of visible light [120]
Self-assembled photonic crystals by magnetic field force
Magnetic particles and nanospheres (silica [121], polystyrene [122], carbon [123], polyvinylpyrrolidone [124], polyacrylamide [125])
Monodisperse microspheres self-assemble into ordered structures under the action of a magnetic field
The self-assembly process is reversible, and the band gap position can also be adjusted by controlling the strength of the magnetic field
Self-assembled photonic crystals by gravity
Monodisperse microspheres with higher density such as silica [126]
Colloidal particles are free to settle and self-assemble into an ordered structure in constant temperature, humidity and vapor pressure environment [127]
The photonic crystal obtained by the gravity is difficult to control and is prone to defects
Self-assembled photonic crystal by electric field
Charged monodisperse microspheres [128,129]
Monodisperse microspheres are deposited on the electrode by an electric field to form an ordered structure
Colloidal sedimentation rate can be controlled by applying a positive and negative electric field
Inverse opal photonic crystal
Template agent such as silica, polystyrene or polymethyl methacrylate microspheres, precursors such as metal salts, metal alkoxides, metal oxides, inorganic salt solutions and organic substances
Filling by melt method, chemical precipitation method, monomer polymerization, etc. Removing the template by melting, heating, etc. [130]
High refractive index precursor is filled into the voids of the crystal, and the corresponding infrastructure is obtained by removing the opal template [131]
Electron beam etching
PMMA (positive rubber) and SU-8 (negative rubber) materials [132]
Polymer in the region exposed to the electron beam will undergo a soluble change, and the micro-nano structure can be obtained by the developer
There is no need to contact the surface of the material, the deformation is small under stress, and the pollution is less, and the surface of the material is not oxidized
Ion beam etching
Metals and compounds, inorganic and organic, insulators and semiconductors [67,133]
The electrostatic beam is used to focus the ion beam emitted by the liquid metal ion source into a very small size ion beam stream, and the physical collision is used to cut and grind the base material to obtain a micro-nano structure
This method is not limited by etching materials, with good directionality and high resolution
Laser holographic lithography
SU-8, photoinitiator, photosensitive resin solution [83]
Single photon or two-photon polymerization with laser-assisted technology
The lattice symmetry, lattice period, band gap position, medium duty ratio, etc. control the structure
Imprinting technology
Photoresist resin combined with quartz (hard template), Polydimethylsiloxane (PDMS) and other imprint templates
Transfer the template pattern to the substrate by heat [86] or ultraviolet irradiation [84, 85]
The resolution is not affected by the most exposed wavelength, and is closely related to the original template resolution. It has high fidelity, high resolution, low cost and high yield
Surface wrinkle technology
PDMS [134], polystyrene [135] and shape memory materials [136–139] are used as base materials, organic polymer materials [140], metal materials [141], and inorganic materials [142] can be used as surface hard layer materials
Surface instability factors are induced by heat, force, electromagnetic field, humidity, phase separation, osmotic pressure, capillary force and polymerization shrinkage stress
The inherent Van der Waals force, polarity, steric hindrance and electrostatic force of the polymer film produce yield deformation to relax the internal instability of the system [143]


## 3. Micro-Nano Structure of Photonic Crystal in Patterns Construction and Anti-Counterfeiting Application

The photonic crystal anti-counterfeiting technology is a method for intelligently adjusting the optical forbidden band structure under certain physical or chemical conditions to change the ordered structure of the reflected signal and the structural colour. The responsive photonic crystal could be combined with various substrates to prepare anti-counterfeiting materials. Conventional luminescent material has a fixed structural colour, while patterns of photonic crystals is invisible under normal conditions, and the invisible pattern can appear under certain physical or chemical effects, and the pattern can be hidden when the action is removed. Here, we introduce some pattern construction of photonic crystal by different means such as inkjet printing, ink responsiveness, photomask, electromagnetic responsiveness, stress responsiveness, and systematically summarize the application of photonic crystal patterning in the field of anti-counterfeiting.


### 3.1. Inkjet Printing Technology

Inkjet printing technology is a non-contact technology that developed in the 1970s. The required pattern or graphic is input into a computer through a scanner or a digital camera, then it was directly drawn by a computer, and the characteristic digital signal obtained after pattern or graphic conversion is transmitted to a printer as electronic signal, and a nozzle can control the ejection of ink droplets to create a corresponding image on a print carrier. Laser direct writing required a special high-cost device, and the traditional method of self-assembly photonic crystal is complicated, time-consuming and expensive. Direct preparation of patterned photonic crystals can be achieved by inkjet printing, which is simple and easy to operate by effectively controlling the wettability of the printing substrate and the composition of the monodisperse latex emulsion ink, efficient spread of the monodisperse latex droplets on the surface of the substrate and highly ordered three-dimensional assembly of the monodisperse latex particles ultimately. The whole or part of the pattern is hidden or displayed by changing external conditions such as angle, wettability, medium, light intensity, etc. (see Figure1).Figure 1
The inkjet printing of photonic crystals.Park et al. [144, 145] fabricated a photonic crystal structure composed of hemispherical colloidal assemblies by inkjet printing, and they used monolayer (SAM) of octadecyltrichlorosilane (OTS) modifying the silicon wafer to get a hydrophobic surface. Based on the nucleation and growth kinetics and the magnitude of the capillary stress, they proposed interrelationship of the crystal quality of the photonic crystal, the colloidal particle size, and the pore size which in turn influenced the evaporation-induced solvent flow through the pore and the magnitude of the capillary stress. They also used microreflectance spectroscopy and studied reflectance spectrum from an individual colloidal droplet. The wet conditions of the colloidal suspension have an important influence on the size, shape and self-assembled structure of the colloidal aggregates, which is determined from the intrinsic properties of the substrate, ink composition, and evaporation conditions. It is possible to control the printing of photonic crystals and patterns on different substrates by controlling the wettability of the substrate [146], the evaporation of the solvent [147–149], and the size of the photonic crystal [150], space and delay time of neighboring droplets [151–153], composition of inks [154].Wang et al. [155] studied response of photonic crystal microdots incorporating poly (N-isopropyl acrylamide) (PNIPAm) by inkjet printing, they designed a fastest response of 1.2 s to water vapor attributed to the hydrophobic transition of PNIPAm above its lower critical solution temperature (LCST) (see Figure 2(a1)), and patterned a macroscopic image of a Chinese dragon with reversible response by changing wetting states/adhesion properties of adsorbed water on the polymer segments (see Figure 2(a2)). When wet vapor approached the responsive dragon image, it resulted in a red shift from 480 to 580 nm within 6.2 s (see Figure 2(a3)), and with distinct colour alteration from transparent to bright green, and the corresponding recovery process was reversible when the water vapor was removed. Bai et al. [156] built a method of controlling the original colour and vapor-responsive colour shift by mesoporous colloidal nanoparticle ink. By designing inks of droplets of MSNs (mesoporous silica nanoparticles) or solid silica nanoparticles (SSNs), photonic crystal patterns with multicolour shifting properties were printed on both rigid and soft substrates, and by adjusting the size, type and mesopore proportion of nanoparticles, the wavelength of the responsive pattern was well controlled by flection peak position movements. Patterns printed on PDMS substrate usually causes a problem of angle dependence of photonic crystals. It could be solved by controlling the wettability of the substrate, and patterns presented angle-independent structural colour due to the high height/diameter rate of mesoporous colloidal photonic crystal (MCPC) domes. Even if the view angle was changed, the incident light reflected passed through their centers back, and the wavelengths were similar (see Figure 2(b)). This bio-inspired vapor-responsive colloidal photonic crystal pattern method was of great potential for anti-counterfeiting applications. Nam et al. [146] patterned photonic crystals (silica particles) with 500 nm diameter on various substrates with different wettabilities (glass slides, silicon wafers, polypropylene, and polydimethylsiloxane) by inkjet printing. They applied the mono-layered, self-assembled photonic crystal (SAPC) patterns technology to several items. They built anti-fake label patterns on a PDMS slab with a contact angle of 10° and a thickness of 200 nm, and moved it to a chemical bottle, a bare glass, a paper bill, and a credit card respectively (see Figure 2(c)). These anti-fake label patterns could be hidden or rendered in colour by controlling the light density and background colour. This technique solved the difficulty of anti-counterfeiting application caused by the instability of photonic crystals in the construction process by constructing mono-layered, self-assembled crystal on PDMS slab. Wu et al. [157] proposed a brilliant structure colour pattern by using monodisperse cadmium sulfide (CdS) spheres with different diameters on photonic papers with contact angle >50° via a common inkjet printer. Their technique took advantage of the angle dependence of photonic crystals, used the observation angle as a switch. When the angle of observation was changed, the colour of the pattern could be selectively hidden and rendered. They designed patterns of a red butterfly and a green butterfly in a yellow background with three different diameters of CdS spheres (270 nm, 290 nm and 335 nm). Due to the angle dependence of photonic crystals, at a particular angle, the butterfly patterns were hidden, and only the yellow background could be seen. When the viewing angle matched the incident angle, the green (270 nm CdS spheres) and red butterflies (335 nm CdS spheres) appeared in the yellow background (290 nm CdS spheres). To explain the mechanism, they studied the evolution of reflection spectra of two butterflies at different viewing angles, they found that when incidence angle was fixed at 10° and viewing angle was close to specular angle at 8–10°, the patterned colour was highest (see Figure 2(d)).Figure 2
(a1) Hydrophobic transition of the molecular conformation of PAAm-co-PNIPAm incorporated in the PC microdot above its LCST. (a2) Changes in reflectance spectra of the PC image with time. (a3) Colour photographs of dragon images taken at low (left) and high (right) concentration of wet air. Adapted from [155]. (b) Patterns printed on a modified PDMS film with different contact angle and vapor. Adapted from [156]. (c) Covert-overt transformation of SAPC patterns on a chemical bottle, a bare glass, a paper bill, and a credit card for anti-counterfeiting applications. Adapted from [146]. (d) Patterns were shown at viewing angle of 70° and 10º with colour-filled contour map of red and green regions. Adapted from [157].
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### 3.2. Ink Responsiveness

In early times, photonic crystal paper is coated with common solvents, salts or reactive monomers to change refractive index or lattice parameters to obtain different patterns, so the durability, inks and the removing and precision of the ink are the most important parameters, A number of scientific research achievements have been accumulated. Colour or colourless photonic paper can be processed by ink of silicone fluid, metal ions, inorganic/organic acids, chloride salt, water, pH, its patterns usually are erased by volatile solvent, acid/alkali treatment, water, drying, ethanol solution (see Figure3).Figure 3
Photonic paper patterned by ink responsiveness.Fudouzi and Xia [158] used the swelling capacity of liquid to vary the stop bands of photonic paper which was colloidal crystal of polymer beads embedded in an elastomer matrix made of PDMS. When the lattice constant and the wavelength of Bragg-diffracted light were varied (see Figure 4(a)), the pattern of the polystyrene beads could be varied from violet to other colours. They patterned some letters with silicone fluid DMS-T05 as ink, 202 nm polystyrene in a PDMS matrix, but the pattern removal process (by immersing the paper in a volatile solvent) was inconvenient. Liu et al. [159] prepared novel poly (4-vinylpyridine) based inverse opals (PVPIO) photonic crystal, a collapse of the protonation/deprotonation processes controlled by metal ions, organic acids or inorganic acid followed by dehydration, the contacted region coated with inks could transfer from a collapsed disordered to an ordered inverse opal structure. The pattern method by controlling the structure of inverse opal from disordered to ordered was actually to use Bragg diffraction. This strategy was developed by Ge et al. [160] They fabricated photonic paper by magnetically tunable assembly techniques using Fe3O4@SiO2 colloids mixed inside a poly (ethylene glycol) diacrylate (PEGDA) matrix. They used a water-ethanol solution of a chloride salt as ink (see Figure 4(b)), the letters could be patterned due to the increased interparticle space during swelling accompanied by red-shifting. The pattern could be erased by distilled water to wash the salt. The template method was also used to fabricate polymer photonic crystal paper by filling the polymer into the photonic crystal and then etching the photonic crystal. Gu et al. [161] constructed poly(ethylene glycol) diacrylate (PEGDA) photonic crystal paper, they prepared different colour papers, and patterned letters by writing saturated ethanol solution of LiCl, these papers were of vivid colours, wide viewing angle, rewritability, and flexibility. Du et al. [162] fabricated photonic crystal paper by filling chitosan into the colloidal crystals, the structural colour could be adjusted by the particle size of photonic coatings, or written with aqueous solutions of certain pH. At first, photonic crystal paper was colourless and transparent (see Figure 4(c)). When they patterned letters with water on papers, the letters were purple, green and red, respectively. Chen et al. [163] prepared clickable colloidal photonic crystal of monodispersed silica particles modified with vinyl group by rapid and controllable post-patterning approach. β-mercaptoethanol, pentanethiol, octanethiol, N-dodecyl mercaptan, 1H, 1H, 2H, 2H-perfluorodecanethiol as reactive monomer were introduced to modify the vinyl modified photonic crystals. Chemical encoded photonic crystal patterns were achieved by the different ethanol concentration for full infiltration of different chemical group. The pattern letters were modified with β-mercaptoethanol (“S”), pentanethiol (“E”), N-dodecyl mercaptan (“U”) by different templates (see Figure 4(d)), the rest playground region was modified by 1H, 1H, 2H, 2H-perfluorodecanethiol. The letters were gradually revealed by controlling the concentration of ethanol solution.Figure 4
(a) Photonic papers were assembled from 202 nm PS beads in a PDMS matrix. Adapted from [158]. (b) Photonic papers had a uniform blue colour when observed with the naked eye and patterns were shown by the flash lamp illuminating as a point light source. Adapted from [160]. (c) Digital photo images of photonic coatings with various sizes from ca. 160 nm to ca. 225 nm on glass slides before writing (left), after writing with water (middle), and erasing by water evaporation (right). Adapted from [162]. (d) Photonic crystal film with differently patterned photomask and patterned in different concentration of ethanol aqueous solution. Adapted from [163].
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### 3.3. Photomask Technology

Photomasks are usually required when preparing patterns by means of crosslinking and modification, and the preparation process is usually divided into two steps. Firstly, the mask is used to control the degree of reaction of the ultraviolet curing system, then the Bragg diffraction is changed by flushing unreacted monomers, cross-linking treatment, hydrophobic treatment, etc. and the process is simple and controllable.Wang et al. [164] patterned organic/inorganic one-dimensional photonic crystals (1DPCs) hybrid by photolithography of thin films of poly methyl methacrylate-co-hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate (PMMA-co-PHEMA-co-PEGDMA) and titania nanoparticle sol. The 1DPCs was of uniform colour, thickness and minimal adventitious striations, cracks, and comets (see Figure 5(a)). A pattern “JLU” was prepared by controlling the lighting time. Increased irradiation time decreased the thickness of the top layer, the structure became more disordered with blue shift. The patterns could be reversible by organic solvents. Lee et al. [165] structured composite photonic films with photocurable resin of ethoxylated trimethylolpropane triacrylate and silica particles. Small “K” was patterned to film by photolithography of a photomask (see Figure 5(c)). When the film was placed on a Korean bank note, the film was difficult to discern for high transparency. It could be used for anti-counterfeiting or optical identification codes taking advantage of angle-independent structural colour. Zhang et al. [166] demonstrated an easy-to-perform and efficient method to structure two dimensional responsive photonic crystals (2-D RPC), this technology combined the colloid crystallization and solid-liquid-assisted assembly processes. The close-packed colloid crystals were formed by negatively-charged polystyrene suspensions grafted the sulfopropyl methacrylate potassium salt that were transferred into the same charged hydrophilic substrate. The anti-forgery label was constructed by sulfonated polystyrene powders, acrylamide (Am), N,N-methylenebisacrylamide (MBA) and diethoxyacetophenone (DEAP), after curing by UV radiation for 5 min, then a mask was used to pattern the film for 24 h curing (further immersed into an aqueous solution containing Am, MBA, AA and DEAP), this nonsynchronous swelling technology could control the emergence of coding message by water (see Figure 5(b)). Chen et al. [129] combined multicolour control under electric field with photomask and UV curing to achieve multicolour pattern printing (see Figure 5(d)). When the pre-assembled photonic crystals were subjected to slight pressure or interference from the outside, they would be unassembled immediately, the reflection intensity would be reduced to 0, and the metastable colloidal crystals remaining stationary would be reassembled and restored to the structural colour. The strong ultraviolet light selectively cured the photonic crystal structure. SiO2/ETPTA (trimethylolpropane ethoxylate triacrylate) liquid photonic crystal was used as E-ink for printing petals, roots, leaves and stamens at different volts by periodical photomask covering, electric field tuning, and UV curing.Figure 5
(a) Cross-linking SEM images, photographs, and their corresponding reflective spectra. Adapted from [164]. (b) Optical image of the RPC-based barcode film at the dried state and after smeared with deionized water. Adapted from [166]. (c) Photolithography with an amorphous silicon photomask on a glass wafer enabled the patterning of small K’s with different colours by angle and its application on a Korean bank note. Adapted from [165]. (d) Lithographical printing of multicolour flower by periodically photomask covering, electric field tuning, and UV curing. Adapted from [129].
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### 3.4. Electromagnetic Responsiveness

Colloidal particles dispersed in solution are usually disordered due to Brownian motion. However, when an electric field is applied to the colloidal solution, the charged colloidal particles will rapidly move toward the electrode opposite to its surface charge due to electrostatic attraction, and this attraction is sufficient to gather and assemble the colloidal particles to form a stable colloid. Similarly, as the external magnetic field increases or decreases, the lattice structure of the photonic crystal changes as the external magnetic field changes. Therefore, the use of electromagnetic fields to regulate photonic crystals to achieve patterning and anti-counterfeiting has become a very effective way.Ge et al. [167] constructed magnetically responsive photonic structures that could operate in nonaqueous solutions, the fundamental details of interparticle forces was very different from assembling highly charged superparamagnetic Fe3O4 colloidal nanocrystal clusters (CNCs) in aqueous solutions. Tunable photonic structures have been formed in alkanol solutions by assembling silica-coated surface modification Fe3O4-CNC colloids using magnetic fields. To further illustrate the applications of magnetic response, patterns were structured by background film and the letters using ethylene glycol (EG) /PDMS mixtures that included Fe3O4@SiO2 with different core diameters/shell thicknesses (see Figure 6(a)). The letters with small roughness at the edges could be seen clearly with striking blue letters on green background when a magnetic field was applied. Similarly, they also studied superparamagnetic Fe3O4 colloidal nanocrystal clusters (CNCs), ethanol and the photocurable resin as a three-phase material system [168]. Hu et al. [169] dispersed the carbon-coated superparamagnetic particles in a solution of ethylene glycol and polydimethylsiloxane precursors, and polymerized them to obtain photonic crystal paper. After polymerization, the ethylene glycol solution enveloped the magnetic particles to form a droplet in the PDMS matrix. In order to achieve the printing of the invisible pattern, they partially polymerized the photonic crystal paper under a photomask to form an invisible pattern. In the initial state, the background and the pattern were both brown (see Figure 6(b)). When a magnetic field was added, the unpolymerized pattern produced a colloidal crystal assembly under a magnetic field, and showed a structural colour. Under the same magnetic field, magnetic particles with three different diameters exhibited different structural colours after assembly. They also synthesised monodisperse carbon-capped superparamagnetic colloidal nanoparticles [170], anti-counterfeiting labels were prepared by background (filled with EG/PDMS precursor containing carbon black) and plastic templates (filled with the glycol solution containing carbon-capped SCNps). The carbon-capped SCNps in the glycol droplets could exhibit colours by a vertical magnetic field. Continually, this photonic anti-counterfeiting watermark technique provide double security information to identify the authenticity of banknotes [171]. Wang et al. [172] manipulated responsive free-writing based on magnetic response, and chalcogenides were designed by Cd2+ loaded polystyrene (PS), N-isopropylacrylamide (NIPAm) and Fe3O4 nanoparticles. The construction technology of Janus suprabeads (JSs) was achieved by synthesis of monodisperse functionalized PS-PMMA microspheres, CdS QD-loaded fluorescent microspheres. The suprabeads was structured by microfluidic device filled with discontinuous phases (QDs-loaded CPC latexes) and continuous phases (methylsilicone oil). Images of “JANUS” were prepared for responsive display. The display colour was reversible by temperature response and magnetic response, and chalcogenide could pattern images with both the electronic confinement and photon confinement response. Xuan and Ge [173] accomplished a novel photonic printing process by repeating the magnetic alignment, orientational tuning, and lithographical photopolymerization. To print labels composed of multiple orientations, the patterns printed by multiple photonic orientations (magnetic inks were first placed above a cubic magnet with a field angle of 90°, and the logo was patterned with UV polymerization under a photomask. Backgrounds were printed onto a cylindrical magnet with changeable colour as controlling the angle of incident light from angle 1–4°. This printing technique was also capable of fabricating colourful or invisible binary codes and recognized with transmission signals. Zhao et al. [174] constructed Janus particles with controllable and predictable shape by microfluidic directed self-assembly method. Subsequently, Liu et al. tried approaches to assemble two kinds of colloidal photonic crystals into different hemispheres of a single Janus supraball (PS/SiO2 and PS/Fe3O4CPC JSs and PS@SiO2 core–shell CPC supraballs) (see Figure 6(c)) [175]. Supraballs were prepared in a triphase microfluidic device that was made up of a polydimethylsiloxane capillary and a pair of paralleled inner needles (one was injected with mono-dispersed PS colloidal microspheres and the other injected with photopolymerizable monomer along with monodispersed SiO2 microspheres), and uniform biphasic droplets were formed by the outer PDMS capillary broking the inner solutions at the tip of the pair of needles. Patterns of PS/Fe3O4 JSs (J-1 and J-2) were fabricated using the magnetic field that could easily switch and rotate as well as exhibit different dual optical properties. Chen et al. [176] prepared large-area crack-free multifunctional photonic crystal film by grafting PS with carbosilane-thioether generation 3 vinyl-terminated (G3-Vi) dendrimers (see Figure 6(d)). Later-generation carbosilane-thioether of PS-co-G3Vi microspheres could enable the ingress of Ag ions into the interiors, and form Ag nanoparticles to improve the interaction among the microspheres, the crack-free CPC films technology was used to pattern photonic display devices as a switch with good stability and reproducibility under an electronic field.Figure 6
(a) Fabrication of a field-responsive PDMS composite embedded with droplets of EG solution of Fe3O4@SiO2 colloids and patterned letters on a flexible PDMS film magnetically induced colour change. Adapted from [167]. (b) Photographs of slides with printed-graphics designed on a computer and latent graphics on P-papers revealed by a magnetic field. Adapted from [169]. (c) Schematic representation of the triphase microfluidic device, the balance of the three interfacial tensions, and the formation of Janus and core –shell CPC supraballs, and their applications as switches and displays. Adapted from [175]. (d) PS-co-G3Vi/Ag microspheres fabricated crack-free CPC and switch of photonic display devices under an electronic field. Adapted from [176].
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### 3.5. Stress Responsiveness

The photonic crystal combines some elastomer materials to change the interface distances of the microspheres in the colloidal layer by stretching or compressing. Some scholars have reported that the movement of polymer molecular chains can be controlled by force to realize the change of photonic crystal anti-protein structure cycle from disordered to ordered.Ye et al. [177] prepared photonic crystal by fixing metastable SiO2 colloidal crystalline array in the matrix of EG and poly (ethylene glycol) methacrylate (PEGMA) through photo-polymerization (see Figure 7(a1)). Patterns were achieved by soaking the film with poly(ethylene glycol) diacrylate by a mask, and they were hard to be distinguished in relaxed condition as the unshielded region caused slight change, it could be instant by non-uniform deformation caused by the different elasticity between the cross-linked and uncrosslinked sections. Simultaneously, this strategy was extended to pattern on PDMS rubbers with deformation (see Figure 7(a2)) [178]. Sun et al. [179] prepared mechanochromic photonic-crystal fibers by electrophoretically depositing polymer microspheres. Firstly, elastic fiber was prepared by curing the PDMS precursor in a mold, then the arrays were wrapped onto the PDMS through a rotation translation strategy fibers with a high conductivity. Afterwards the PS microspheres were deposited onto the elastic fiber by a fast electrophoretic deposition process. Several colourful patterns that were fade-resistant, durable, sensitive and responsive to mechanical strain were prepared with different elastic photonic-crystal fibers. The patterns exhibited brilliant colour changes and maintained very high sensitivity, reversibility, and stability for thousands of cycles of stretching in the vertical, horizontal and bilateral direction. Schäfer et al. [180] reported a preparation of adjustable thermoresponsive spherical core–shell architectures with poly (diethylene glycol methylether methacrylate-co-ethyl acrylate) (PDEGMEMA-co-PEA) shells and hard polystyrene cores. This polymer opal ﬁlms behaved as photonic rubbers that could be tuned by stretching with quick recover. At first, there was little variation of the reﬂection spectra between irradiated and non-irradiated regions, while under the same stress, the irradiated and non-irradiated regions produced different strains so that strong contrast produced clear images [181]. Fang et al. [182, 183] reported shape memory polymers (SMPs) with instantaneous shape recovery triggered by a contact pressure. This process was controlled by unusual “cold” programming that the polymers deformed to a temporary shape at or below room temperature. Macroporous SMPs membranes were prepared by templates of silica microspheres and copolymers of ethoxylated (20) trimethylolpropane triacrylate (ETPTA, Tg ≈ 40°C) and polyethylene glycol (600) diacrylate (PEGDA, Tg ≈ 42°C). The membranes exhibited colours in water and became translucent with a pale and no ordering of the deformed macropores when dehydrated. When finger, a “light bulb” relief pattern and printed pairs of parallel lines were under a small contact pressure on the collapsed macroporous membranes, several patterns immediately appeared (see Figure 7(b1)). The raising up of the smoother line patterns were tested by Atomic Force Microscope (AFM) image, and the corresponding depth proﬁle. The paper elucidated this unusual shape-recovery mechanism due to an apparent adhesive pull-off force of the attractive Van Der Waals interactions and the capillary force of layer between the indenter tip and the SMP membrane (see Figure 7(b2)).Figure 7
(a1) Printing process of invisible photonic patterns shown by deformation, adapted from [177] and (a2) its mechanism. Adapted from [178]. (b1) Arbitrary photonic crystal patterns printed on the new SMP membranes and (b2) macropore recovery induced by pull-off forces. Adapted from [183].
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## 3.1. Inkjet Printing Technology

Inkjet printing technology is a non-contact technology that developed in the 1970s. The required pattern or graphic is input into a computer through a scanner or a digital camera, then it was directly drawn by a computer, and the characteristic digital signal obtained after pattern or graphic conversion is transmitted to a printer as electronic signal, and a nozzle can control the ejection of ink droplets to create a corresponding image on a print carrier. Laser direct writing required a special high-cost device, and the traditional method of self-assembly photonic crystal is complicated, time-consuming and expensive. Direct preparation of patterned photonic crystals can be achieved by inkjet printing, which is simple and easy to operate by effectively controlling the wettability of the printing substrate and the composition of the monodisperse latex emulsion ink, efficient spread of the monodisperse latex droplets on the surface of the substrate and highly ordered three-dimensional assembly of the monodisperse latex particles ultimately. The whole or part of the pattern is hidden or displayed by changing external conditions such as angle, wettability, medium, light intensity, etc. (see Figure1).Figure 1
The inkjet printing of photonic crystals.Park et al. [144, 145] fabricated a photonic crystal structure composed of hemispherical colloidal assemblies by inkjet printing, and they used monolayer (SAM) of octadecyltrichlorosilane (OTS) modifying the silicon wafer to get a hydrophobic surface. Based on the nucleation and growth kinetics and the magnitude of the capillary stress, they proposed interrelationship of the crystal quality of the photonic crystal, the colloidal particle size, and the pore size which in turn influenced the evaporation-induced solvent flow through the pore and the magnitude of the capillary stress. They also used microreflectance spectroscopy and studied reflectance spectrum from an individual colloidal droplet. The wet conditions of the colloidal suspension have an important influence on the size, shape and self-assembled structure of the colloidal aggregates, which is determined from the intrinsic properties of the substrate, ink composition, and evaporation conditions. It is possible to control the printing of photonic crystals and patterns on different substrates by controlling the wettability of the substrate [146], the evaporation of the solvent [147–149], and the size of the photonic crystal [150], space and delay time of neighboring droplets [151–153], composition of inks [154].Wang et al. [155] studied response of photonic crystal microdots incorporating poly (N-isopropyl acrylamide) (PNIPAm) by inkjet printing, they designed a fastest response of 1.2 s to water vapor attributed to the hydrophobic transition of PNIPAm above its lower critical solution temperature (LCST) (see Figure 2(a1)), and patterned a macroscopic image of a Chinese dragon with reversible response by changing wetting states/adhesion properties of adsorbed water on the polymer segments (see Figure 2(a2)). When wet vapor approached the responsive dragon image, it resulted in a red shift from 480 to 580 nm within 6.2 s (see Figure 2(a3)), and with distinct colour alteration from transparent to bright green, and the corresponding recovery process was reversible when the water vapor was removed. Bai et al. [156] built a method of controlling the original colour and vapor-responsive colour shift by mesoporous colloidal nanoparticle ink. By designing inks of droplets of MSNs (mesoporous silica nanoparticles) or solid silica nanoparticles (SSNs), photonic crystal patterns with multicolour shifting properties were printed on both rigid and soft substrates, and by adjusting the size, type and mesopore proportion of nanoparticles, the wavelength of the responsive pattern was well controlled by flection peak position movements. Patterns printed on PDMS substrate usually causes a problem of angle dependence of photonic crystals. It could be solved by controlling the wettability of the substrate, and patterns presented angle-independent structural colour due to the high height/diameter rate of mesoporous colloidal photonic crystal (MCPC) domes. Even if the view angle was changed, the incident light reflected passed through their centers back, and the wavelengths were similar (see Figure 2(b)). This bio-inspired vapor-responsive colloidal photonic crystal pattern method was of great potential for anti-counterfeiting applications. Nam et al. [146] patterned photonic crystals (silica particles) with 500 nm diameter on various substrates with different wettabilities (glass slides, silicon wafers, polypropylene, and polydimethylsiloxane) by inkjet printing. They applied the mono-layered, self-assembled photonic crystal (SAPC) patterns technology to several items. They built anti-fake label patterns on a PDMS slab with a contact angle of 10° and a thickness of 200 nm, and moved it to a chemical bottle, a bare glass, a paper bill, and a credit card respectively (see Figure 2(c)). These anti-fake label patterns could be hidden or rendered in colour by controlling the light density and background colour. This technique solved the difficulty of anti-counterfeiting application caused by the instability of photonic crystals in the construction process by constructing mono-layered, self-assembled crystal on PDMS slab. Wu et al. [157] proposed a brilliant structure colour pattern by using monodisperse cadmium sulfide (CdS) spheres with different diameters on photonic papers with contact angle >50° via a common inkjet printer. Their technique took advantage of the angle dependence of photonic crystals, used the observation angle as a switch. When the angle of observation was changed, the colour of the pattern could be selectively hidden and rendered. They designed patterns of a red butterfly and a green butterfly in a yellow background with three different diameters of CdS spheres (270 nm, 290 nm and 335 nm). Due to the angle dependence of photonic crystals, at a particular angle, the butterfly patterns were hidden, and only the yellow background could be seen. When the viewing angle matched the incident angle, the green (270 nm CdS spheres) and red butterflies (335 nm CdS spheres) appeared in the yellow background (290 nm CdS spheres). To explain the mechanism, they studied the evolution of reflection spectra of two butterflies at different viewing angles, they found that when incidence angle was fixed at 10° and viewing angle was close to specular angle at 8–10°, the patterned colour was highest (see Figure 2(d)).Figure 2
(a1) Hydrophobic transition of the molecular conformation of PAAm-co-PNIPAm incorporated in the PC microdot above its LCST. (a2) Changes in reflectance spectra of the PC image with time. (a3) Colour photographs of dragon images taken at low (left) and high (right) concentration of wet air. Adapted from [155]. (b) Patterns printed on a modified PDMS film with different contact angle and vapor. Adapted from [156]. (c) Covert-overt transformation of SAPC patterns on a chemical bottle, a bare glass, a paper bill, and a credit card for anti-counterfeiting applications. Adapted from [146]. (d) Patterns were shown at viewing angle of 70° and 10º with colour-filled contour map of red and green regions. Adapted from [157].
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## 3.2. Ink Responsiveness

In early times, photonic crystal paper is coated with common solvents, salts or reactive monomers to change refractive index or lattice parameters to obtain different patterns, so the durability, inks and the removing and precision of the ink are the most important parameters, A number of scientific research achievements have been accumulated. Colour or colourless photonic paper can be processed by ink of silicone fluid, metal ions, inorganic/organic acids, chloride salt, water, pH, its patterns usually are erased by volatile solvent, acid/alkali treatment, water, drying, ethanol solution (see Figure3).Figure 3
Photonic paper patterned by ink responsiveness.Fudouzi and Xia [158] used the swelling capacity of liquid to vary the stop bands of photonic paper which was colloidal crystal of polymer beads embedded in an elastomer matrix made of PDMS. When the lattice constant and the wavelength of Bragg-diffracted light were varied (see Figure 4(a)), the pattern of the polystyrene beads could be varied from violet to other colours. They patterned some letters with silicone fluid DMS-T05 as ink, 202 nm polystyrene in a PDMS matrix, but the pattern removal process (by immersing the paper in a volatile solvent) was inconvenient. Liu et al. [159] prepared novel poly (4-vinylpyridine) based inverse opals (PVPIO) photonic crystal, a collapse of the protonation/deprotonation processes controlled by metal ions, organic acids or inorganic acid followed by dehydration, the contacted region coated with inks could transfer from a collapsed disordered to an ordered inverse opal structure. The pattern method by controlling the structure of inverse opal from disordered to ordered was actually to use Bragg diffraction. This strategy was developed by Ge et al. [160] They fabricated photonic paper by magnetically tunable assembly techniques using Fe3O4@SiO2 colloids mixed inside a poly (ethylene glycol) diacrylate (PEGDA) matrix. They used a water-ethanol solution of a chloride salt as ink (see Figure 4(b)), the letters could be patterned due to the increased interparticle space during swelling accompanied by red-shifting. The pattern could be erased by distilled water to wash the salt. The template method was also used to fabricate polymer photonic crystal paper by filling the polymer into the photonic crystal and then etching the photonic crystal. Gu et al. [161] constructed poly(ethylene glycol) diacrylate (PEGDA) photonic crystal paper, they prepared different colour papers, and patterned letters by writing saturated ethanol solution of LiCl, these papers were of vivid colours, wide viewing angle, rewritability, and flexibility. Du et al. [162] fabricated photonic crystal paper by filling chitosan into the colloidal crystals, the structural colour could be adjusted by the particle size of photonic coatings, or written with aqueous solutions of certain pH. At first, photonic crystal paper was colourless and transparent (see Figure 4(c)). When they patterned letters with water on papers, the letters were purple, green and red, respectively. Chen et al. [163] prepared clickable colloidal photonic crystal of monodispersed silica particles modified with vinyl group by rapid and controllable post-patterning approach. β-mercaptoethanol, pentanethiol, octanethiol, N-dodecyl mercaptan, 1H, 1H, 2H, 2H-perfluorodecanethiol as reactive monomer were introduced to modify the vinyl modified photonic crystals. Chemical encoded photonic crystal patterns were achieved by the different ethanol concentration for full infiltration of different chemical group. The pattern letters were modified with β-mercaptoethanol (“S”), pentanethiol (“E”), N-dodecyl mercaptan (“U”) by different templates (see Figure 4(d)), the rest playground region was modified by 1H, 1H, 2H, 2H-perfluorodecanethiol. The letters were gradually revealed by controlling the concentration of ethanol solution.Figure 4
(a) Photonic papers were assembled from 202 nm PS beads in a PDMS matrix. Adapted from [158]. (b) Photonic papers had a uniform blue colour when observed with the naked eye and patterns were shown by the flash lamp illuminating as a point light source. Adapted from [160]. (c) Digital photo images of photonic coatings with various sizes from ca. 160 nm to ca. 225 nm on glass slides before writing (left), after writing with water (middle), and erasing by water evaporation (right). Adapted from [162]. (d) Photonic crystal film with differently patterned photomask and patterned in different concentration of ethanol aqueous solution. Adapted from [163].
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## 3.3. Photomask Technology

Photomasks are usually required when preparing patterns by means of crosslinking and modification, and the preparation process is usually divided into two steps. Firstly, the mask is used to control the degree of reaction of the ultraviolet curing system, then the Bragg diffraction is changed by flushing unreacted monomers, cross-linking treatment, hydrophobic treatment, etc. and the process is simple and controllable.Wang et al. [164] patterned organic/inorganic one-dimensional photonic crystals (1DPCs) hybrid by photolithography of thin films of poly methyl methacrylate-co-hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate (PMMA-co-PHEMA-co-PEGDMA) and titania nanoparticle sol. The 1DPCs was of uniform colour, thickness and minimal adventitious striations, cracks, and comets (see Figure 5(a)). A pattern “JLU” was prepared by controlling the lighting time. Increased irradiation time decreased the thickness of the top layer, the structure became more disordered with blue shift. The patterns could be reversible by organic solvents. Lee et al. [165] structured composite photonic films with photocurable resin of ethoxylated trimethylolpropane triacrylate and silica particles. Small “K” was patterned to film by photolithography of a photomask (see Figure 5(c)). When the film was placed on a Korean bank note, the film was difficult to discern for high transparency. It could be used for anti-counterfeiting or optical identification codes taking advantage of angle-independent structural colour. Zhang et al. [166] demonstrated an easy-to-perform and efficient method to structure two dimensional responsive photonic crystals (2-D RPC), this technology combined the colloid crystallization and solid-liquid-assisted assembly processes. The close-packed colloid crystals were formed by negatively-charged polystyrene suspensions grafted the sulfopropyl methacrylate potassium salt that were transferred into the same charged hydrophilic substrate. The anti-forgery label was constructed by sulfonated polystyrene powders, acrylamide (Am), N,N-methylenebisacrylamide (MBA) and diethoxyacetophenone (DEAP), after curing by UV radiation for 5 min, then a mask was used to pattern the film for 24 h curing (further immersed into an aqueous solution containing Am, MBA, AA and DEAP), this nonsynchronous swelling technology could control the emergence of coding message by water (see Figure 5(b)). Chen et al. [129] combined multicolour control under electric field with photomask and UV curing to achieve multicolour pattern printing (see Figure 5(d)). When the pre-assembled photonic crystals were subjected to slight pressure or interference from the outside, they would be unassembled immediately, the reflection intensity would be reduced to 0, and the metastable colloidal crystals remaining stationary would be reassembled and restored to the structural colour. The strong ultraviolet light selectively cured the photonic crystal structure. SiO2/ETPTA (trimethylolpropane ethoxylate triacrylate) liquid photonic crystal was used as E-ink for printing petals, roots, leaves and stamens at different volts by periodical photomask covering, electric field tuning, and UV curing.Figure 5
(a) Cross-linking SEM images, photographs, and their corresponding reflective spectra. Adapted from [164]. (b) Optical image of the RPC-based barcode film at the dried state and after smeared with deionized water. Adapted from [166]. (c) Photolithography with an amorphous silicon photomask on a glass wafer enabled the patterning of small K’s with different colours by angle and its application on a Korean bank note. Adapted from [165]. (d) Lithographical printing of multicolour flower by periodically photomask covering, electric field tuning, and UV curing. Adapted from [129].
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## 3.4. Electromagnetic Responsiveness

Colloidal particles dispersed in solution are usually disordered due to Brownian motion. However, when an electric field is applied to the colloidal solution, the charged colloidal particles will rapidly move toward the electrode opposite to its surface charge due to electrostatic attraction, and this attraction is sufficient to gather and assemble the colloidal particles to form a stable colloid. Similarly, as the external magnetic field increases or decreases, the lattice structure of the photonic crystal changes as the external magnetic field changes. Therefore, the use of electromagnetic fields to regulate photonic crystals to achieve patterning and anti-counterfeiting has become a very effective way.Ge et al. [167] constructed magnetically responsive photonic structures that could operate in nonaqueous solutions, the fundamental details of interparticle forces was very different from assembling highly charged superparamagnetic Fe3O4 colloidal nanocrystal clusters (CNCs) in aqueous solutions. Tunable photonic structures have been formed in alkanol solutions by assembling silica-coated surface modification Fe3O4-CNC colloids using magnetic fields. To further illustrate the applications of magnetic response, patterns were structured by background film and the letters using ethylene glycol (EG) /PDMS mixtures that included Fe3O4@SiO2 with different core diameters/shell thicknesses (see Figure 6(a)). The letters with small roughness at the edges could be seen clearly with striking blue letters on green background when a magnetic field was applied. Similarly, they also studied superparamagnetic Fe3O4 colloidal nanocrystal clusters (CNCs), ethanol and the photocurable resin as a three-phase material system [168]. Hu et al. [169] dispersed the carbon-coated superparamagnetic particles in a solution of ethylene glycol and polydimethylsiloxane precursors, and polymerized them to obtain photonic crystal paper. After polymerization, the ethylene glycol solution enveloped the magnetic particles to form a droplet in the PDMS matrix. In order to achieve the printing of the invisible pattern, they partially polymerized the photonic crystal paper under a photomask to form an invisible pattern. In the initial state, the background and the pattern were both brown (see Figure 6(b)). When a magnetic field was added, the unpolymerized pattern produced a colloidal crystal assembly under a magnetic field, and showed a structural colour. Under the same magnetic field, magnetic particles with three different diameters exhibited different structural colours after assembly. They also synthesised monodisperse carbon-capped superparamagnetic colloidal nanoparticles [170], anti-counterfeiting labels were prepared by background (filled with EG/PDMS precursor containing carbon black) and plastic templates (filled with the glycol solution containing carbon-capped SCNps). The carbon-capped SCNps in the glycol droplets could exhibit colours by a vertical magnetic field. Continually, this photonic anti-counterfeiting watermark technique provide double security information to identify the authenticity of banknotes [171]. Wang et al. [172] manipulated responsive free-writing based on magnetic response, and chalcogenides were designed by Cd2+ loaded polystyrene (PS), N-isopropylacrylamide (NIPAm) and Fe3O4 nanoparticles. The construction technology of Janus suprabeads (JSs) was achieved by synthesis of monodisperse functionalized PS-PMMA microspheres, CdS QD-loaded fluorescent microspheres. The suprabeads was structured by microfluidic device filled with discontinuous phases (QDs-loaded CPC latexes) and continuous phases (methylsilicone oil). Images of “JANUS” were prepared for responsive display. The display colour was reversible by temperature response and magnetic response, and chalcogenide could pattern images with both the electronic confinement and photon confinement response. Xuan and Ge [173] accomplished a novel photonic printing process by repeating the magnetic alignment, orientational tuning, and lithographical photopolymerization. To print labels composed of multiple orientations, the patterns printed by multiple photonic orientations (magnetic inks were first placed above a cubic magnet with a field angle of 90°, and the logo was patterned with UV polymerization under a photomask. Backgrounds were printed onto a cylindrical magnet with changeable colour as controlling the angle of incident light from angle 1–4°. This printing technique was also capable of fabricating colourful or invisible binary codes and recognized with transmission signals. Zhao et al. [174] constructed Janus particles with controllable and predictable shape by microfluidic directed self-assembly method. Subsequently, Liu et al. tried approaches to assemble two kinds of colloidal photonic crystals into different hemispheres of a single Janus supraball (PS/SiO2 and PS/Fe3O4CPC JSs and PS@SiO2 core–shell CPC supraballs) (see Figure 6(c)) [175]. Supraballs were prepared in a triphase microfluidic device that was made up of a polydimethylsiloxane capillary and a pair of paralleled inner needles (one was injected with mono-dispersed PS colloidal microspheres and the other injected with photopolymerizable monomer along with monodispersed SiO2 microspheres), and uniform biphasic droplets were formed by the outer PDMS capillary broking the inner solutions at the tip of the pair of needles. Patterns of PS/Fe3O4 JSs (J-1 and J-2) were fabricated using the magnetic field that could easily switch and rotate as well as exhibit different dual optical properties. Chen et al. [176] prepared large-area crack-free multifunctional photonic crystal film by grafting PS with carbosilane-thioether generation 3 vinyl-terminated (G3-Vi) dendrimers (see Figure 6(d)). Later-generation carbosilane-thioether of PS-co-G3Vi microspheres could enable the ingress of Ag ions into the interiors, and form Ag nanoparticles to improve the interaction among the microspheres, the crack-free CPC films technology was used to pattern photonic display devices as a switch with good stability and reproducibility under an electronic field.Figure 6
(a) Fabrication of a field-responsive PDMS composite embedded with droplets of EG solution of Fe3O4@SiO2 colloids and patterned letters on a flexible PDMS film magnetically induced colour change. Adapted from [167]. (b) Photographs of slides with printed-graphics designed on a computer and latent graphics on P-papers revealed by a magnetic field. Adapted from [169]. (c) Schematic representation of the triphase microfluidic device, the balance of the three interfacial tensions, and the formation of Janus and core –shell CPC supraballs, and their applications as switches and displays. Adapted from [175]. (d) PS-co-G3Vi/Ag microspheres fabricated crack-free CPC and switch of photonic display devices under an electronic field. Adapted from [176].
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## 3.5. Stress Responsiveness

The photonic crystal combines some elastomer materials to change the interface distances of the microspheres in the colloidal layer by stretching or compressing. Some scholars have reported that the movement of polymer molecular chains can be controlled by force to realize the change of photonic crystal anti-protein structure cycle from disordered to ordered.Ye et al. [177] prepared photonic crystal by fixing metastable SiO2 colloidal crystalline array in the matrix of EG and poly (ethylene glycol) methacrylate (PEGMA) through photo-polymerization (see Figure 7(a1)). Patterns were achieved by soaking the film with poly(ethylene glycol) diacrylate by a mask, and they were hard to be distinguished in relaxed condition as the unshielded region caused slight change, it could be instant by non-uniform deformation caused by the different elasticity between the cross-linked and uncrosslinked sections. Simultaneously, this strategy was extended to pattern on PDMS rubbers with deformation (see Figure 7(a2)) [178]. Sun et al. [179] prepared mechanochromic photonic-crystal fibers by electrophoretically depositing polymer microspheres. Firstly, elastic fiber was prepared by curing the PDMS precursor in a mold, then the arrays were wrapped onto the PDMS through a rotation translation strategy fibers with a high conductivity. Afterwards the PS microspheres were deposited onto the elastic fiber by a fast electrophoretic deposition process. Several colourful patterns that were fade-resistant, durable, sensitive and responsive to mechanical strain were prepared with different elastic photonic-crystal fibers. The patterns exhibited brilliant colour changes and maintained very high sensitivity, reversibility, and stability for thousands of cycles of stretching in the vertical, horizontal and bilateral direction. Schäfer et al. [180] reported a preparation of adjustable thermoresponsive spherical core–shell architectures with poly (diethylene glycol methylether methacrylate-co-ethyl acrylate) (PDEGMEMA-co-PEA) shells and hard polystyrene cores. This polymer opal ﬁlms behaved as photonic rubbers that could be tuned by stretching with quick recover. At first, there was little variation of the reﬂection spectra between irradiated and non-irradiated regions, while under the same stress, the irradiated and non-irradiated regions produced different strains so that strong contrast produced clear images [181]. Fang et al. [182, 183] reported shape memory polymers (SMPs) with instantaneous shape recovery triggered by a contact pressure. This process was controlled by unusual “cold” programming that the polymers deformed to a temporary shape at or below room temperature. Macroporous SMPs membranes were prepared by templates of silica microspheres and copolymers of ethoxylated (20) trimethylolpropane triacrylate (ETPTA, Tg ≈ 40°C) and polyethylene glycol (600) diacrylate (PEGDA, Tg ≈ 42°C). The membranes exhibited colours in water and became translucent with a pale and no ordering of the deformed macropores when dehydrated. When finger, a “light bulb” relief pattern and printed pairs of parallel lines were under a small contact pressure on the collapsed macroporous membranes, several patterns immediately appeared (see Figure 7(b1)). The raising up of the smoother line patterns were tested by Atomic Force Microscope (AFM) image, and the corresponding depth proﬁle. The paper elucidated this unusual shape-recovery mechanism due to an apparent adhesive pull-off force of the attractive Van Der Waals interactions and the capillary force of layer between the indenter tip and the SMP membrane (see Figure 7(b2)).Figure 7
(a1) Printing process of invisible photonic patterns shown by deformation, adapted from [177] and (a2) its mechanism. Adapted from [178]. (b1) Arbitrary photonic crystal patterns printed on the new SMP membranes and (b2) macropore recovery induced by pull-off forces. Adapted from [183].
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## 4. Surface Wrinkles Technology in Patterns Construction and Anti-Counterfeiting Application

Micropatterns by molecular self-assembly and interface self-organization technology can be prepared in a large area, but the local surface is prone to defects, so it is still in the laboratory development stage. The interface self-organizing technology utilizes the inherent Van Der Waals force, polarity, steric hindrance and electrostatic force of the polymer film to produce yield deformation to relax the internal instability of the system to form micropatterns. The main response methods are heat, force and electromagnetic field, humidity, phase separation, osmotic pressure, capillary force and polymerization shrinkage stress, etc. Micropatterns can be obtained by controlling the induced surface instability factors. Material surface wrinkle technology of patterning micro-nano structure is a more conventional method. Polydimethylsiloxane elastomer is usually used as base material, also other hard film and soft substrate can achieve wrinkles by adjusting modulus of elasticity, coefficients of thermal expansion, and swelling abilities of the solvents. Typically, the shape memory material is one of the main ways of wrinkle formation by controlling the melting temperature of the crystalline phase adjusted within a certain temperature range.Zeng et al. [184] proposed a series of moisture-sensitive film-substrate bilayer devices that consisted of hydrophilic polyvinyl alcohol film onto hydrophobic PDMS. Three states (repeated reversible folds, disposable folds and permanent fold) were controlled by the thickness of the PVA film, PVA crosslinking degree/gradient, PDMS modulus, and PVA-PDMS interface (see Figure 8(a)). Letters “IMS” were patterned sustained for 30 s as moisture penetration to polymeric network and reduction modulus with a degree of hydrolysis of 88% PVA. These unique responsive dynamics also could motivate the invention of anti-counterfeit tabs, encryption devices, water indicators, light diffusors, and anti-glare films. Zong et al. [185] reported a simple method to dynamically tune and/or erase wrinkling patterns by photoisomerization on an azo-containing poly (disperse orange 3) (PDO3) film bonded to PDMS substrate. Reversible photoisomerization of the azobenzene moieties led to the release of the internal stress upon light irradiation (see Figure 8(c)). Patterns were obtained highly ordered by selective exposure. The unexposed part had a lower transmittance and stronger scattering, and it could be erased by blanket exposure. The writing/erasure velocity was determined by the light power density and the film thickness. Li et al. [186] fabricated near-infrared light-responsive dynamic wrinkles by bilayer systems (carbon nanotube and PDMS elastomer) and top stiff layers (functional polymers). The elastic CNT-PDMS substrate could be obtained with the reversible wrinkle patterns due to the high photon-to-thermal energy conversion efficiency and absorption by NIR irradiation. Switchable optical transparency in the wrinkled and wrinkle-free states were controlled by on/off cyclic NIR irradiation of 20 s and 30 s respectively. A positive letter “S” was obtained selectively by 365 nm UV light, then converted to a fully wrinkled surface. After that, a negative letter “T” was selectively erased by 254 nm UV light. Interestingly the information could temporarily be erased and swiftly restored to the original state. Xie et al. [187] demonstrated an approach to fabricate patterns by fluorescence and wrinkled topography controlling simultaneously by light. The bilayer system contained a top layer (anthracene (AN) and naphthalene diimide (NDI) moieties containing a copolymer, PAN-NDI-BA and a substrate of PDMS. Upon irradiation by 365 nm UV light, photodimerization leads to a cross-linked top layer with a higher modulus and a red to blue-green fluorescence change. A reversible dynamic dual-pattern with wrinkled topography and fluorescence was generated reversibly upon exposure to 254 nm UV light or 150°C (see Figure 8(b)). By masks of “stripe”, “annulus”, and “SJTU”, positive images with a green fluorescence patterned and erased followed by 365 nm UV light for 15 ,min and a thermal treatment at 70°C in red fluorescence background respectively.Figure 8
(a) Preparation approaches for the three moisture-responsive wrinkling devices with different responsive dynamics, and the hidden pattern “IMS” could be reversibly revealed by breathing toward the anti-counterfeit tab. Adapted from [184]. (b) Reversible dual-pattern with fluorescence and hierarchical wrinkles obtained through photomasks. Adapted from [187]. (c) Application of the optically erasable wrinkling system for the repetitive optical writing/erasure of information by multiple cycles of stimulus-induced wrinkling, selective exposure, and blanket exposure. Adapted from [185].
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## 5. Plasmonic Micro-Nano Technology in Pattern Construction and Anti-Counterfeiting Application

Many studies proved that the coherent oscillation of conduction electrons in plasmonic nanostructures with incident light could enhance the electromagnetic field strength on their surfaces and increases scattering [188–190]. Surface-enhanced Raman scattering (SERS) is a detection method to incorporate into security labels as its signal was strongly dependent on the incident field polarization and wavelength of plasmonic nanostructures. Some researchers used surface plasmon resonance to study the structural colour to form a clear and identifiable pattern identification for hiding or reading encrypted information at higher anti-counterfeiting level.Cui et al. [191] prepared a proof-of-concept novel plasmonic SERS security label with Ag nanowire structures. They designed full encrypted molecular images that was authenticated by polarized SERS imaging in both x- and y-polarizations. The vertical and horizontal nanowires exhibited strong SERS intensity correspondingly in an alternating fashion under the x-polarization and y-polarization respectively (see Figure 9(a)). Similarly, the upright “A” and the inverted “A” written by horizontal and vertical nanowires respectively could be clearly read-out or invisibly controlled, also an alphabet “A” fabricated using 50% horizontal nanowires and 50% vertical nanowires could hide visible molecular information of only half of the “A” under x- or y-polarization. Based on the above mentioned design, the alphabetical “CBC” and “NTU” could be clearly distinguished by x- and y-polarizations, respectively. Subsequently, their teams developed a multiplex plasmonic anti-counterfeiting platform with superior nanometer scale spectral and spatial resolution to increase the complexity of these plasmonic security labels. Sandwich nanowire structures functionalized with 4-methylbenzenethiol (4-MBT) and rhodamine B isothiocyanate (RhBITC) could generate unique security features when individual vibrational modes were selected. For a single sandwich structure with only 4-MBT, the SERS image of a panda was shown visibly with vibrational modes at 1078 cm−1 while invisibly at 1200 cm−1. For a sandwich structure with 4-MBT and RhBITC, the image of a panda could be shown with vibrational modes at 1078 cm−1 and 1200 cm−1 respectively (see Figure 9(b)) [192]. After that, their team studied the spatially selective encapsulation of dye molecules with Ag nanopillar array to fabricated molecular information which was revealed using fluorescence, SERS, and their signal intensities. They fabricated a “yin-yang” pattern with rhodamine 6G (R6G) and eosin Y and dimethyl sulfoxide (EY) encapsulated nanopillars. Bright-field microscopic nanopillars and fluorescence images of patterns with the R6G half and EY half were shown, the image could not fully reveal the “yin-yang” due to the higher fluorescence quantum yield of R6G. However, the SERS image of left- and right-half of “yin-yang” were selectively exhibited. The technology of multiple-layer platform with enhanced information security made anti-counterfeiting more practical in the near future [193]. Li et al. [194] fabricated flexible patterned plasmonic metafilms by bottom-up self-assembly of PS and top-down laser engraving of PS beads with a Langmuir-Blodgett technique. The patterned PS beads loosely bounded to the glass/silica surface could be completely transferred to the tape, then silver was deposited onto the tape to a get ﬂexible plasmonic meta-structures, then a multidimensional SERS barcode was fabricated (see Figure 9(c)). Firstly, the NCs super-lattice sheet was fabricated on glass, then it was patterned into a barcode by an adhesive stencil tape to remove the excessive sheet on the substrate and leave the remaining pattern barcodes. The obtained barcode with an encoding text of Zhejiang University was readable by a photo. Ag NCs were mostly assembled in face-to-face orientations with a strong LSPR due to the dipole-dipole coupling between the neighboring NCs. The patterns shown in the Raman maps perfectly matched the optical image of the NCs barcode. Huge coding capacity could be achieved by adjusting barcodes and choices of Raman dyes. The simple, low-cost, and efﬁcient approach to fabricate a ﬂexible plasmonic material displayed tunable plasmonic properties and excellent ﬂexibility for anti-counterfeiting applications [195]. Kang et al. [196] fabricated multiwavelength thermoplasmonic images by inkjet printing nanoparticle inks of gold nanorod (GNR) and gold nanosphere (GNS), the thermoplasmonic metal nanoparticles could generate localized heat to enhanced light absorption at a particular wavelength by localized surface plasmons, so patterns that are difficult to distinguish with the naked eye could be visible by thermal imaging infrared cameras. This method could operate instantaneously and for an unlimited period of time, also it was fully reversible. Park et al. [197] prepared a secure label with a combination of two codes which was composed of AgNPs formed on the adhesive scotch tape and the UCNCs on the Ag film, respectively, and then the decryption code with MUM structure was assembled by overlapping the tape and the Ag on the mask aligner. The code of “KIST” could not be visually or microscopically identified, however, it could be seen with green luminescence by irradiation of NIR light. Bakan et al. [198] constructed invisible patterns by ultrathin dielectrics which was transparent in the visible and exhibited strong infrared absorption in the spectral range of thermal cameras (see Figure 9(d)). To expand the possibilities of applications, they used bendable substrates of Al foils from rigid Si wafers. Al foils laminated with polyethylene film and paper were studied in the maximum SiO2 thickness that was imperceptible to the naked eye. Alternative mirror layers showed not only a security feature but also angle-independent absorbance/emissivity of the surfaces. Conductive oxides fluorine doped tin oxide (FTO) coated glasses were used as another alternative layer, however, the contrast between the SiO2 patterns and the background was weaker due to the ultrathin SiO2(≈5 nm) and greater absorbance/emissivity of FTO, it could only be observed at higher temperatures.Figure 9
(a) (i) SEM images, (ii and iii)x- and y-polarized 2D SERS imaging of different molecularly encrypted nanostructures formed by horizontal and vertical Ag nanowires. Adapted from [191] and (b) homogeneous plasmonic anti-counterfeiting security labels. Adapted from [192]. (c) Schematic illustration and optical image of the fabrication of the SERS barcode and its application. Adapted from [194]. (d) Photograph and thermal image of alternative mirror layers. Adapted from [198].
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## 6. Summary and Outlook

In this review, we have considered various published literature on fabrication of micro-nano structures for patterns including styles, materials, method. For better understanding the difference of fabrication methods of micro-nano structures, Table1 summarized inkjet printing, angle-independent photonic crystal, self-assembled photonic crystals by magnetic field force, gravity, electric field, inverse opal photonic crystal, electron beam etching, ion beam etching, laser holographic lithography, imprinting technology and surface wrinkle technology. In addition, we have reviewed studies aimed at photonic papers with micro-nano structure patterns of photonic crystal by inkjet printing technology, ink responsiveness, photomask technology, electromagnetic responsiveness, stress responsiveness and a summary of figures have been adapted from published literature, which can be manipulated by common solvents, salts or reactive monomers, electric and magnetic field and stress. Finally, we have briefly introduced the surface wrinkle anti-counterfeiting technology and the plasmonic micro-nano anti-counterfeiting technology. These methods have simple steps and can be prepared in a large area, but are still in the laboratory development stage. We put forward some suggestions. Future research may pattern based on the design of micro-nano structure and optical properties, explore the intrinsic connection and law between optical micro-patterns and colours, and adjust the interface strain state of polymer surface topography or apply stress to achieve nano-scale manipulation of light. In particular, micro-nano structure, surface chemistry, interfacial adhesion, and hydrophobicity may be used as optical switches to study patterning and intelligent anti-counterfeiting.



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*Source: 6519018-2019-12-08.xml*