Color Contrast of Single-Layer Graphene under White Light Illumination Induced by Broadband Photon Management

Visualizing and manipulating the optical contrast of single-layer graphene (SLG) and other 2D materials has continuously been an interesting topic to understand fundamental light-matter interaction down to atomic thickness. Because the optical properties of SLG can be tuned by gating, demonstrating and manipulating the color contrast of SLG also has significant potential applications in ultrathin flexible color display. However, previous demonstrations of optical contrast of SLG are mostly limited to reflection intensity contrast under monochromatic illumination using the interference effect. The reported spectral contrast in SLG has mostly been narrow-band or at resonant wavelengths, and it required precise thickness control and/or nanolithography that are hardly scalable to large enough area for display applications. In this paper, we demonstrate novel color contrast optical visibility of SLG under white light using broadband photon management induced by nanoneedle-structured SnO (x ≤ 1) transparent conductive oxides (TCOs), which is scalable to large-area color display. The low-temperature fabricated, self-assembled, nanoneedle-structured SnO (x ≤ 1) thin films help to significantly increase the broadband optical absorption in SLG by enhancing the electromagnetic field and increasing the scattering efficiency at the SnO/SLG interface. With nanoneedle-structured SnO, the optical absorption in SLG on a fused quartz (SiO₂) substrate is drastically increased from ∼1.4 to >10% at λ = 560-990 nm (from yellow to near infrared spectral regimes), leading to a clear color contrast to the surrounding region without SLG. The self-assembly approach, rather than sophisticated and costly nanolithography, allows scalable fabrication of large area 2D photonic devices with a broadband and highly efficient photon management effect. Therefore, this approach can be further extended to color-tunable TCO/dielectric/SLG 2D photonic devices by adjusting the free carrier concentrations/Fermi levels in the TCO and SLG layers via gating-a stepping stone toward ultrathin flexible color display technologies utilizing 2D materials and nanostructured thin films.