Asymmetric structural colors based on monodisperse single-crystal Cu₂O spheres

Structural colors have attracted broad attention owing to their anti-photobleaching capability and brilliant metallic color. In particular, the asymmetric structural colors generated by a simple material will have great practical significance in the fields of biomimetic materials, double-side display and anti-counterfeiting. The asymmetric optical effects were usually achieved by the plasmonic effects of Ag or Au nanocrystals. Here, for the first time, we realized the asymmetric structural colors based on the asymmetric scattering of Cu₂O single-crystal spheres. By spray-coating Cu₂O spheres on a glass slide, different structural colors were viewed from the Cu₂O film side and the glass slide side. The FDTD simulations confirmed that the asymmetric colors were ascribed to the inhomogeneous distribution of the electric field intensity. The film built by 200 nm Cu₂O spheres on a glass slide shows green and cyan structural colors from the front and back sides, respectively. The colors on both sides of the Cu₂O films were proved to be tuned by changing the diameters of the Cu₂O single-crystal spheres. Different substrates were used to examine the influence of substrates on the asymmetric colors. Finally, inspired by different brilliant colors from the front and back of natural creatures, the patterns of butterfly and petals were fabricated by Cu₂O spheres. Impressively, similar to nature, the patterns show completely different colors viewed from the front and back sides. The asymmetric structural colors of Cu₂O single-crystal spheres will open up new avenues to realize multi-mode color output and pave their applications in display, biomimetic materials and anti-counterfeiting materials.