Atomically-thin two-dimensional (2D) materials have attracted remarkable interest in a wide range of research and applications. However, one of the main challenges is how to visualise the extremely thin films and accurately identify its layer thickness. Due to the ultimately thin thickness and the low absorption of light, many 2D films, such as graphene, graphene oxide and hexagonal boron nitride (hBN) are nearly completely transparent on most surfaces. They are only visible when deposited on specific contrast-enhancing substrates. However, there is no universal substrates which can be used to visualise all 2D materials. A substrate often can only provide enhanced visibility for a specific category of 2D materials. For instance, the oxidised Si substrates can considerably enhance the optical contrast of graphene, but produce negligible enhancing effect on hBN. It is therefore desirable to develop a general theoretical guidance on how to design contrast-enhancing substrate for any given 2D materials.
Here we report a universal theoretical model which can be employed to design high-contrast substrates for any 2D materials. For a given thin film of known optical properties, the optical contrast is completely defined by the complex reflectivity of the underlying substrate. By engineering the optical properties of the underlying substrate, we fabricated a range of structures, significantly enhancing the contrast of graphene, graphene oxides and hBN. Monolayers of these transparent 2D films are readily visible (>10% contrast) on a range of substrates with metallic or dielectric materials as top surface layers. The measured optical contrasts excellently match theoretical calculations.