Inkjet Printing Based Mono-layered Photonic Crystal Patterning for Anti-counterfeiting Structural Colors

Photonic crystal structures can be created to manipulate electromagnetic waves so that many studies have focused on designing photonic band-gaps for various applications including sensors, LEDs, lasers, and optical fibers. Here, we show that mono-layered, self-assembled photonic crystals (SAPCs) fabricated by using an inkjet printer exhibit extremely weak structural colors and multiple colorful holograms so that they can be utilized in anti-counterfeit measures. We demonstrate that SAPC patterns on a white background are covert under daylight, such that pattern detection can be avoided, but they become overt in a simple manner under strong illumination with smartphone flash light and/or on a black background, showing remarkable potential for anti-counterfeit techniques. Besides, we demonstrate that SAPCs yield different RGB histograms that depend on viewing angles and pattern densities, thus enhancing their cryptographic capabilities. Hence, the structural colorations designed by inkjet printers would not only produce optical holograms for the simple authentication of many items and products but also enable a high-secure anti-counterfeit technique.


Supplementary Note
Behaviour of a particle suspension droplet on the substrate Fig. S1 shows the basic mechanism for producing photonic crystal patterns on a hydrophilic or hydrophobic substrate using an inkjet printing process. Particles with a diameter of 500 nm in a droplet are self-assembled on various substrates and then form photonic crystals during an evaporation process. First, we observed photonic crystal self-assembly on a hydrophilic glass substrate with a contact angle of 10° ( Fig. S1(a)). Particles were suspended in pure water, and the final densities were 10% (w/v) and 20% (w/v), respectively. Both of the densities produced a multilayered, coffee-ring shape pattern as shown in Fig. S1(d). Since a droplet spreads widely (high wettability) evaporation occurs more actively at the edge than at the centre. Temperature differences cause not only unbalanced surface tensions along the water-air interface, but also fluid flow within the droplet, as illustrated with arrows. The induced fluid flow replenishes fluid at the edge so that the particles are also transported to the edge 1 . This is called Marangoni flow, which produces a coffeering-shaped pattern in which several layers form along the perimeter.
Second, particle suspensions were blended with formamide (FA) of which final concentration was 20% v/v. However, the final densities of particles remained unchanged as used before (10% w/v and 20% w/v). In this case, the low density suspension (10% w/v) formed a mono-layered, coffeering-shaped pattern because of the low number density of particles in a droplet (not shown), while the highly concentrated suspension (20% w/v) produced a flat, mono-layered pattern ( Fig. S1(b)).
The high particle density was theoretically predicted to produce a flat, mono-layered photonic crystal structure on a hydrophilic substrate via the calculation of the particle number that could fully fill a circle, as shown in Fig. S1(e). The ideal concentration of the solvent (FA) was experimentally determined. We note that the solvent concentration (water/FA mixture) plays a more crucial role than the number density of particles in forming a flat, mono-layered self-assembly when we control the evaporation rate. This is because FA has a higher boiling point than water while the former has lower surface tensions than the latter as reported in other literature 2,3 .
Third, the high density suspension (20% w/v) was injected onto a hydrophobic polydimethylsiloxane (PDMS) substrate as shown in Fig. S1(c), and we observed that the particles formed a dome-shaped, multi-layered nanostructure (Fig. S1(f)). Interestingly, the size of the nanostructure, but not the shape of it, was much affected by the particle density and the solvent concentration. The hydrophobic surface appears to make the contact area of a droplet shrink, unlike the hydrophilic surface, and the shape of the droplet semi-spherical. Since evaporation takes place more vigorously at the centre than at the outer edge, the self-assembly of the particles becomes domeshaped, which is distinctly different from the hydrophilic surface 4 .       Figure S11. a)-c) Three SAPC patterns were produced as identically as possible by using the exactly same printing conditions such as the same bitmap image file, inkjet printer, and particle ink but resulted in microscopically (red rectangles) and nanoscopically (green rectangles) different patterns.  Figure S12. a) Three colorful images were obtained from the three SAPC patterns produced in Figure  S11 separately by using the exactly same imaging conditions. And then, the RGB histograms of the images were analyzed and then drawn, showing slightly different profiles of pixel intensities. Even though the patterns were not identical at the microscale and nanoscale, the RGB histograms are almost identical. Therefore, the SAPC patterns can be optically decrypted in a simple manner using a light source and a light detector located at PL(3 cm, 30°, -40°) and PC(40 cm, 90°, -60°), respectively. b) The RGB histograms of the #1 SAPC pattern were analyzed from the colorful images taken at three different camera positions (i.e. PC(40 cm, 90°, -58° ~ -60°) as shown in the insets. The profile shift turned out to be very sensitive to the viewing angles, demonstrating the feasibility for a high-secure anti-counterfeit system. The light source was fixed at PL(3 cm, 30°, -40°).

Supplementary Movies
Movie 1. Covert-overt transformation of mono-layered self-assembled photonic crystal patterns under four different experimental conditions such as weak light illumination on a black background, strong light illumination on a black background, strong light illumination without a background, and strong light illumination on a white background.

Movie 2.
Mono-layered self-assembled photonic crystal patterns exhibit colourful structural coloration that depends on the viewing angle.

Movie 3.
Practical anti-counterfeit applications of mono-layered self-assembled photonic crystal patterns.