Structural color: an emerging nanophotonic strategy for multicolor and functionalized applications

Overview of structural color applications. Multi-channels metasurfaces, transparency to structural color encryption, static PC encryption, responsive PC anti-counterfeiting, static structural color decoration, dynamic structural color display, drug detection, temperature indicator, solvent test, humidity measurement, pH gauge, hybrid sensing, metasurface high-resolution printing, PC low-cost printing, F-P high-brightness printing, plasmonic wide-gamut printing, two-photon lithography (TPL) 3D printing, standing wave lithography. Figure reproduced with permission from: ref.⁶⁴, American Chemical Society; ref.⁶⁵, AAAS; ref.⁶⁶, American Chemical Society; ref.⁵², John Wiley and Sons; ref.⁶⁷, American Chemical Society; ref.²¹, Springer Nature; ref.⁶⁸, John Wiley and Sons; ref.⁴⁹, American Chemical Society; ref.⁶⁹, Elsevier; ref.⁷⁰, Elsevier; ref.⁷¹, American Chemical Society; ref.⁷², Elsevier; ref.⁷³, American Chemical Society; ref.⁴², American Chemical Society; ref.⁷⁴, John Wiley and Sons; ref.⁷⁵, Springer Nature; ref.⁴⁵, Springer Nature; ref.³⁷, Springer Nature.

Mechanisms of structural color. (a) Schematic diagram of 1D PC with alternative high and low refractive index dielectric layer. n_H and n_L are the refractive indices of the high and low refractive index dielectric layer, and d_H and d_L are their corresponding thickness. (b) F-P cavity with a configuration that dielectric layer with thickness d is sandwiched between two metal layers. (c) 2D colloidal crystal with particle distance d. (d) 3D colloidal crystal that is surrounded by materials with refractive index contrast. (e) Schematic diagram of the metasurface.

Static PC encryption. (a) Schematic of the processes for the fabrication of photonic crystal composite film (PCCF) and schematic illustrations showing encryption and recognition of the paper-printed QR code and stamps encoded by PCCF. (b) Multiangle photochromism (indoors and outdoors) effect of the functional PC films with different patterns on the phone case. (c) The patterned photonic crystal film on a Korean banknote. (d) Anti-counterfeiting pattern displayed at different viewing angle on a traditional Chinese tea caddy. (e) A multi-industry applicable optical anti-counterfeiting system with higher security level and fast identification and decryption process of the PC. Figure reproduced with permission from: (a) ref.³⁰ , John Wiley and Sons; (b) ref.¹⁴⁸, American Chemical Society; (c) ref.¹⁴⁷, American Chemical Society; (d) ref.¹⁴⁹, John Wiley and Sons; (e) ref.¹⁵⁰, Elsevier.

Responsive PC anti-counterfeiting. (a) Schematic diagram of flexible 1D PC butterfly patterns in the process of blowing and drying. (b) The nanoscopic recovery and deformation process of the permanent 3D photonic crystal structure stimulated by drying the sample out of ethanol or applying an external contact pressure. (c) Schematic diagram of the information decoding process of thermosensitive structural colored labels. (d) Color switching of the bioinspired PC-PDMS kirigami under uniaxial tension and its programmable application. States 1–5 are the loading steps during which the gates lift and the color changes from red to blue. States 5–8 are the unloading process. The colors return to the same value as those during the loading process when the lifting angles of gate arrays return to the original state. (e) Magnetically responsive structural colors under different H. (f) Schematic illustration for the fabrication process of Fe₃O₄@PVP@PGDMA PNCs via a selective concentration polymerization of monomers in microheterogenous dimethyl sulfoxide–water (DMSO–H₂O) binary solvents and its anti-counterfeiting application. (g) Digital photographs of the prepared multiplexed patterned thermochromic photonic film soaked in water with different temperatures and viewed at different reflection angles. (h) Digital photographs of the printed 2D non-close-packed and 3D non-close-packed pyramids patterns with different colors and time-difference-printed tunable-multicolor patterns using inks with different structural colors. Figure reproduced with permission from: (a) ref.⁶⁶, American Chemical Society; (b) ref.⁵⁰, Springer Nature; (c) ref.¹⁵⁵, American Chemical Society; (d) ref.⁸⁴, John Wiley and Sons; (e) ref.⁵¹, John Wiley and Sons; (f) ref.⁵², John Wiley and Sons; (g) ref.⁷⁷, John Wiley and Sons; (h) ref.⁹⁴, Elsevier.

Multi-modes metasurfaces for anti-counterfeiting. (a) Schemes of aperture geometry and arrangement and microscopic images with dual color information states “printed” with nanoscale resolution. (b) Schematic configuration of the proposed color filter where the incident white light is filtered into different colors depending on the polarization. (c) Bright field optical images of the “fish and bird” comprising nanorods under x- and y-polarized light. (d) Schematic illustration of the tri-functional metasurface integrating a color print, hologram, and luminescence image by controlling amplitude, phase, and luminescence properties. (e) Full color image printing with TiO₂ metasurfaces. (f) The optical microscope images of phoenix with different colors in the air and DMSO. Figure reproduced with permission from: (a) ref.³¹, American Chemical Society; (b) ref.⁹², Springer Nature; (c) ref.⁶⁴, American Chemical Society; (d) ref.¹³³, Elsevier; (e) ref.²⁰⁰, American Chemical Society; (f) ref.³⁶, Springer Nature.

Emerging anti-counterfeiting technology. (a) Schematic illustration of Ag nanostructures fabrication and its dark-field printing. (b) The process of structural-color printing with a single transparent polymer ink and the optical Janus property of coloration and transparency of the printed structural-color panel viewing from the bare unpatterned (blank) side. (c) Schematic of the fabrication procedure of the kirigami grating sheet and grating patterns with different azimuth angles (illustrated by code patterns) and the process of reading encrypted patterns by stretching. Figure reproduced with permission from: (a) ref.⁸⁹, AIP Publishing; (b) ref.⁶⁵, AAAS; (c) ref.⁹⁰, Elsevier.

Static structural color decoration. (a) Optical microscope image of demonstration of color printing of institutional logo of authors of this paper. (b) Color reproduction ability of the structural color metasurface after introducing height regulation into X–Y plane: a comparison between the original picture and as-fabricated structural color metasurface including its optical micrograph and a large area SEM image and its details of the micro-pixels with real three-dimensions structure regulation in X–Y–Z directions. (c) Photographs showcasing the fabricated structures alongside their target color, fabricated color, and the respective color difference, denoted as Exp. ΔE; the Bayesian optimization process is presented below. (d) Proposed structure involving an asymmetric F−P nanocavity based on Al−TiO₂−Pt and measured (solid lines) and simulated (dashed lines) reflection spectra and the corresponding colors. Figure reproduced with permission from: (a) ref.²²⁰, American Chemical Society; (b) ref.²²¹, John Wiley and Sons; (c) ref.⁸², Springer Nature; (d) ref.⁶⁷, American Chemical Society.

Dynamic structural color display. (a) Photographic images of two tri-layer films that were bonded together with their patterns facing forwards, illustrating how different patterns are revealed under outward and inward bending. (b) Active color changes according to the different fabrication methods and the applied electric potential. (c) Representation of the electrochemical cell fabricated for the electrical actuation of the active inverse opal and proof of full-color tuning by recorded spectra. (d) PC based display unit composed of 3 × 3-pixel cell array. (e) Photographs of the "tree" signage under different voltages. (f) Showing and hiding of the pattern in the 10th cycles. (g) Numerical indicator based on bistable electrically responsive photonic crystals. Figure reproduced with permission from: (a) ref.²²⁵, John Wiley and Sons; (b) ref.²²⁹, John Wiley and Sons; (c) ref.⁸⁰, John Wiley and Sons; (d) ref.⁴⁶, John Wiley and Sons; (e) ref.⁴⁷, Royal Society of Chemistry; (f) ref.⁴⁸, John Wiley and Sons; (g) ref.²¹, Springer Nature.

Drug detection. (a) Schematic diagram of the specificity of PDA-decorated PC barcodes for multiplex miRNA detection. (b) Schematic diagram of the AIEgens-integrated structural color barcode particles for multiplex detection with binary optical channels. (c) Schematic diagram of the fabrication of electrodeposition templates and plasmonic metasurfaces. (d) Schematic of scattering engineered metapixels in the dark-field for multiplexed nanospectroscopy based on PRET. Strong PRET occurs when the metapixels scattering peak matches the distinctive molecular absorption peaks. Figure reproduced with permission from: (a) ref.²⁴⁶, Elsevier; (b) ref.²⁴⁷, Elsevier; (c) ref.⁶⁸, John Wiley and Sons; (d) ref.²⁴⁵, John Wiley and Sons.

Temperature indicator. (a) Schematic illustration of the triggering agent melting-to-diffusing induced destruction of a P-TTI for indicating the time−temperature history of a vaccine. (b) Schematic diagram of the AIEgens-integrated structural color barcode particles for multiplex detection with binary optical channels. (c) Images of the structural color variation of the SCH with temperature increasing. (d) Schematic of the thermal response of the chromogenic material consists of SnO₂ inverse opal and thermochromic phase change system. Figure reproduced with permission from: (b) ref.⁴⁹, American Society of Chemistry. (c) ref.²⁵³, John Wiley and Sons. (d) ref.²⁵⁶, American Society of Chemistry.

Solvent test. (a) Schematic illustration of the self-supporting photonic composites with stimulus-responsive capability. (b) Digital photos of PC patterns and corresponding PC gel patterns in water, acetonitrile, and propanol. (c) Bright-field photographs for the sample in different solvents and color images of the logo of our university are composed of the TiO₂ metasurface. (d) Schematic diagram of femtosecond laser direct writing of micropillar arrays with different structural colors and optical micrographs of the dynamic color-switching of the micropillar array exposed to ethanol vapor, showing a “Tai Chi” pattern. Figure reproduced with permission from: (a) ref.⁶⁹, Elsevier; (b) ref.²⁶⁰, John Wiley and Sons; (c) ref.²⁶¹, American Society of Chemistry; (d) ref.⁹¹, American Society of Chemistry.

Humidity measurement. (a) Humidity responsiveness of the cholesteric liquid crystalline networks coating with a PKU logo as a permanent pattern and tree-like dynamic pattern. (b) Schematic diagram of 1D PC films built on the surface of artificial bowl array and partially enlarged details, angle-independent optical properties, flexibility and deformability, colorimetric sensing and display applications of the PC films on the bowl arrays. (c) Photographs of 1D PC showing the color transition during and after human blowing. (d) SEM images and angle-dependent optical microscopy images of periodic photonic structures were obtained through alternate fabrication of two types of square arrays, creating a checkerboard pattern. Figure reproduced with permission from: (a) ref.²⁶⁷, Elsevier; (b) ref.⁷⁰, Elsevier; (c) ref.²⁶⁸, Elsevier; (d) ref.²⁶⁹, Royal Society of Chemistry.

pH gauge. (a) Transmission dip shift of 2D PC- polyelectrolyte gels in response to different pH conditions. (b) Digital photos of the patterned P(Cys-co-Glu) films with different copolymerization ratios upon pH change. (c) Digital photographs of the leaf pattern in response to a solution with different pH values and patterned copolymer nanoparticles (up: letters; down: apple tree) reveal their encrypted color information that is controlled by the pH value of the surrounding solution. Figure reproduced with permission from: (a) ref.²⁷⁰, Royal Society of Chemistry; (b) ref.⁷¹, American Society of Chemistry; (c) ref.⁸¹, John Wiley and Sons.

Hybrid sensing. (a) Representative structural colors of a dual responsive pNIPAAmStMAA hydrogel film displayed at different temperatures and pH values. (b) Mechanism for the dually responsive P(NIPAAM-co-AAc)-PC. (c) Pattern display and color changing behavior of the film in response to the temperature and RH of the surroundings. (d) The programmable traffic lights are controlled by pressure, the intelligent traffic signal recognition/control system and the programmed movement of the intelligent vehicle. (e) Mechanism of the humidity and SO₂ responsiveness of the cholesteric liquid crystalline polymer network film. (f) Schematic illustration of the self-powered finger motion-sensing display based on an IHN-BCP film on ionic gel electrode and motion responsive SC change in the IHN-BCP layer. Chemical structures of PS-b-QP2VP, Li⁺TFSI^-, and PHEA-co-PEGDA are shown. (g) Schematic illustration of writing letters with distilled water on the photonic display tablets and relatively actual digital photographs and different stamp patterns on the hydrogel film and schematic illustration of the stress-induced pattern display process. Figure reproduced with permission from: (a) ref.²⁷⁷, American Society of Chemistry; (b) ref.²⁷⁸, Elsevier; (c) ref.⁷², Elsevier; (d) ref.²⁸¹, John Wiley and Sons; (e) ref.²⁸³, Elsevier; (f) ref.²⁸⁵, Elsevier; (g) ref.²⁸⁶, John Wiley and Sons.

| Metasurface high-resolution printing. (a) Optical images of characters “NANO” created by gradually varying the size and period of Si₃N₄ color pixels and optical images of red, green, and blue Si₃N₄ metasurfaces of different areas: the lateral size changes from 25 to 2.5 μm. (b) Detail taken from the painting “Improvisation No. 9” by Wassily Kandinsky (Staatsgalerie Stuttgart). The top left depicts the original artwork while the lower left shows an optical microscope image of the colour-printed image. In the SEM image on the right, one can clearly identify the image as the pixel size is unchanged. In order to gain access to the full colour space, the diameter as well as the depth of the Mie voids has been varied, which is particularly well visible in the tilted SEM image. Figure reproduced with permission from: (a) ref.²⁹⁹, American Society of Chemistry. (b) ref.³⁵, Springer Nature.

PC low-cost printing. (a) Photographs of the brilliant noniridescent structural colors fabricated by screen printing on various substrates. (b) Schematic for 3D printing of the photonic granular hydrogel ink. (c) Digital photo of spreading and assembling of the color paste on the pattern layer under an external shear-induced force. (d) Schematic illustration of inkjet-printed melanin NP photonic microdomes. (e) Diffraction images obtained from a grating of pitch 1900 nm by white incident light of the given orientation. (f) The combination pattern obtained by overlapping three films templated from silica microspheres with three sizes. Figure reproduced with permission from: (a) ref.¹⁴³, Elsevier; (b) ref.³⁰¹, John Wiley and Sons; (c) ref.⁴², American Society of Chemistry; (d) ref.²¹⁴, American Society of Chemistry; (e) ref.⁴³, American Society of Chemistry; (f) ref.³⁰², John Wiley and Sons.

F-P high-brightness printing. (a) Color images printed on stainless steel substrates (3 × 3 cm² in size) along with a structure schematic. An Au layer was selectively deposited using a shadow mask to different thicknesses onto a 50 nm thick Si₃N₄ film. (b) Full-color reproduction of van Gogh's “Still Life: Vase with Twelve Sunflowers” using the obtained palette and the comparison before and after Ni deposition. (c) A cartoon character “Stitch” (d = 75 nm) and a symbol of Arizona “Cactus” (d = 150 nm) with various colors. (d) Schematics of the FP-type hybrid metasurface with Au-TiO₂-Al coatings. The polarized in-resonance laser pulses interact strongly with the optical cavity, making the metasurface extremely absorbing across the illuminated area, which creates ripples and modifies the optical cavity to an off-resonance state. Figure reproduced with permission from: (a) ref.¹⁸, John Wiley and Sons; (b) ref.⁷⁴, John Wiley and Sons; (c) ref.³⁰⁵, Springer Nature; (d) ref.¹⁹, American Society of Chemistry.

Plasmonic wide-gamut printing. (a) Color images of Hong Kong bauhinia flowers printed on various substrates. (b) The Au NDs can be thermally reshaped into nanospheres under single-pulse laser exposure with sufficient pulse energy. (c) Schematic of the single color plasmonic pixel consisting of a lattice of silver nanorods on a glass substrate. White light illumination polarized along the long axis of the nanorods results in distinct colors observed in reflection. The length L and width W of the nanorods set the local surface plasmon resonance; the periodicity along the x direction, P_x, sets the lattice coupling, and the periodicity along the y direction, P_y, sets the color luminance. (d) The fabricated hybrid structure of “Peony Flower” based on plasmonic systems. Figure reproduced with permission from: (a) ref.²⁹⁰, American Society of Chemistry; (b) ref.⁹⁵, John Wiley and Sons; (c) ref.³⁰⁸, American Society of Chemistry; (d) ref.⁷⁵, American Society of Chemistry.

Other typical full-color printings. (a) Schematic of the continuous DLP 3D printing apparatus for fabricating 3D Lego brick structure with volumetric color property. (b) Schematics illustrating the 3D printing of colloidal inks into objects with isotropic structural color. Coloration is generated by photonic colloidal glasses obtained upon complete drying of the as-printed objects. (c) Miniaturized 3D Merlions with monochromatic structural colors printed by TPL. (d) Scheme of surface coloring by ultrafast laser. Figure reproduced with permission from: (a) ref.³⁰⁹, Springer Nature; (b) ref.⁴³, Springer Nature; (c) ref.⁴⁴, American Society of Chemistry; (d) ref.³⁸, Springer Nature.