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Abstract The study reports novel photonic properties of Ti₃C₂T_xMXene flakes horizontally self‐assembled within cellulose nanofiber (CNF) matrix exhibiting unique bright multispectral colors combined with overall high transparency in the transmission regime. The intense reflection colors are reflected by individual flakes acting as effective micromirrors with shifts based on their subsurface positioning within the dielectric layers. Unique color appearances are controlled by an interplay of multiple bandgaps formed by constructive and destructive interferences at flake‐matrix interfaces. These colors manifest at the microscale under bright field optical microscopy, while the total physical film retains high transparency up to 85% and a typical greenish hue characteristic of the MXene content below 1% volume fraction. The diverse spectral appearance of 4 µm ultra‐thin films is ultimately controlled by the positioning of the horizontal flakes within the nanofiber matrix at diverse distances from the top surface. This work expands the understanding of thin films with assembled 2D materials within polymer matrix and their fundamental interactions creating new structural coloration functionalities with the potential for multispectral photonic applications such as camouflaging, photothermal treatment, and optical communication for flexible thin bio‐derived films. 

Abstract We demonstrate shear‐printed layered photonic films with vivid structural coloration from bio‐derived cellulose nanocrystals and highly aligned Ti₃C₂T_xMXene nanoflakes. These ultrathin films (700–1500 nm) show high light transmittance above 40% in the visible range. In reflectance mode, however, the films appear vividly colored and iridescent due to the multiple distinct photonic bandgaps in the visible and near‐infrared ranges, which are rarely observed in CNC composites. The structural coloration is controlled by the stacking of MXene nanoscale‐thin layers separated by the thicker cellulose nanocrystals matrix, as confirmed by photonic simulations. The unique combination of distinctly different optical appearances in transmittance and reflectance modes occurs in films printed with just a few layers. This is because of the molecularly smooth interfaces and the high refractive contrast between bio‐based and inorganic phases, which result in a concurrence of constructive and destructive interference. These lamellar biophotonic films open the possibilities for advanced radiative cooling, camouflaging, multifunctional capacitors, and optical filtration applications, while the cellulose nanocrystals matrix strengthens their flexibility, robustness, and facilitates sustainability.