Antireflection Substrates for Determining the Number of Layers of Few-Layer Hexagonal Boron Nitride Films and for Visualizing Organic Monolayers

Visualization of layered materials with atomic-scale thickness by optical interference with a low-reflection substrate has been widely used to identify exfoliated thin flakes. However, the identification of optically transparent films with an atomic-scale thickness, such as hexagonal boron nitride (hBN), requires a substrate with a reflectance lower than a few percent. Although several types of multilayer antireflection (AR) substrates for identification have been developed, the lack of an established optical design limits the layer structure of AR coatings. In this study, the reflection circle diagram-based design of the AR coatings with a layer of arbitrary materials on top is proposed. The AR substrates with diverse materials on top would visualize the various organic monolayers formed on substrates with chemical bonding. As an experimental demonstration, Si substrates with an optically designed double-layer AR coating of SiNx and SiO2 were assessed by reflection spectroscopy and employed to visualize an exfoliated monolayer hBN film. As a result, the presence of the monolayer films on Si/SiNx (50 nm)/SiO2 (25 nm) was directly identified in the image photographed by a commercial camera without an optional optical filter and image processing. Additionally, the use of a narrow band-pass filter enhanced the optical contrast of the monolayer hBN up to 17% at 470 nm and enabled easy determination of the number of layers in few-layer hBN flakes.