Anti-counterfeiting labels of photonic crystals with versatile structural colors.

Labels with structural color based on photonic crystals (PCs) have drawn significant attention due to their unique color emission, offering promising solutions for anti-counterfeiting applications. However, to meet the demands of future high-security applications, conventional structural color labels require further improvement. This study introduces a novel approach to fabricate highly encrypted anti-counterfeiting labels by combining close-packed and non-close-packed monolayers of nanoparticles (NPs) onto adhesive surfaces. The photonic crystals, arranged in specific geometric shapes, exhibit overt-covert characteristics. The hidden label is only revealed under specific external triggers, such as attaching or removing a transparent cover film. The principle of color modulation of the photonic crystal is elucidated, highlighting the role of packing density and refractive index matching. Additionally, the scalability and cost-effectiveness of the fabrication process in this study are expected to facilitate future commercialization. Various anti-counterfeiting applications, including water-responsive labels and multi-layer authentication, are demonstrated also, which enables higher security levels and versatility of this study.