# A Self-Color-Changing Film with Periodic Nanostructure for Anti-Counterfeit Application
## 1 Introduction  
Anti-counterfeiting technology (ACT) is a popular topic that has garnered a lot of attention in recent times [1, 2] Counterfeiting is responsible for global economic losses reaching USD 1.82T per year [3, 4, 5, 6]. The current trend of new anti-counterfeiting techniques gravitates toward those that are low-cost, easy-to-recognize, but challenging to imitate [6]. Techniques that can easily be recognized by the naked eye in a few seconds and do not require any special tool to analyze are in high demand [7, 8, 9, 6]. Among the reported ACTs [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32], only the security film has all of these features. The security film consists of a substrate and a coating agent. The substrate comprises a polymer, paper, glass, or fabric, and the coating agent can be fluorescent, graphene or metal nanoparticles. In 2021, [PERSON] et al. presented a new technique to prepare polyester fabric as a new anti-counterfeiting [33] material. Fluorescence was applied to polyester fabric by a microwave irradiation method. The anti-counterfeiting polyester fabric worked by absorbing the radiating UV to cause discoloration under UV. The efficiency was obtained from a reflectance and transmittance measurement. The results showed spontaneous discoloration behavior under UV that was attractive and easy to detect visually. However, the invention still relied on UV as a catalyst; therefore, it was not convenient to use in places where the UV was not readily available. Graphene, on the other hand, has good optical properties and can be designed to possess an unusual form of optical response by using light to induce free-electrons on the surface of nanoparticles with an electric field for oscillation [34, 35, 36, 37]. The oscillation gives the transmittance a convex characteristic, resembling a notch filter. The point where the lowest transmittance occurs is called the resonant frequency. This phenomenon is called localized surface plasmon resonance (LSPR) [38]. In addition to graphene, metal nanoparticles can also induce the LSPR. In 2020, [PERSON] et al. [38] introduced silver nanoparticles on a glass substrate as a color film using a laser printing technique. By sizing the appropriate silver particle dimension via adjustment to the laser intensity, [PERSON] et al. were able to create nanoparticle-based film in any color from the LSPR effect. Although graphene and nanoparticles have good light-absorbing properties, the behavior for displaying different colors can also be easily imitated by ink or other chemical absorbers that absorb light at the same wavelength. A more sensible alternative is to use light transmission and reflection instead of absorption. This mechanism is readily observable in a frequency-selective surface (FSS) structure. An FSS occurs when an array of metallic nanoparticles are laid together in a plane on an insulator substrate to create a resonant effect. An isolated nanoparticle forms only localized surface plasmon resonance, which is suitable for making light-absorbing materials. However, if a nanoparticle is placed in proximity to other nanoparticles in a proper arrangement, a mutual induction occurs between particles that results in the frequency-selective surface behavior of the film. FSS can transform from a convex transmittance curve (or a notch) to a low-pass or a high-pass filter. The intersection of transmittance and reflectance curves causes the color-changing phenomenon. Such a phenomenon would be difficult to imitate, which is the goal of our work. Our research focuses on designing an anti-counterfeiting film that changes color under visible light. It differs from that proposed by [PERSON], since ours does not require a UV excitation. Our technique also goes further than [PERSON] et al. or other graphene research. We not only make use of the LSPR, but also take advantage of the FSS structure in our design.