Visibility of hexagonal boron nitride on transparent substrates.

The high transmittance and low reflectance of monolayer hexagonal boron nitride (hBN) lead to its invisibility under white-light, causing serious troubles in the search, transfer, and fabrication of 2D material devices. In this work, we demonstrate enhancing the contrast of hBN on a transparent substrate by simulation and experimental observation, where the highest contrast is obtained by using a polymer-based interfacial layer on a polydimethylsiloxane (PDMS) substrate. The simulation result reveals that the contrast under short wavelength light is higher than that under long wavelength. To confirm this, the red-green-blue components are extracted from the optical color image. The blue component image shows an hBN flake clearly on the substrate, while the hBN flake fades on the green and red components. Moreover, the contrast on transparent substrates have only positive value, while opaque substrates cause both negative and positive contrast depending on the thickness of the interfacial layer. Thus, the high contrast (∼4.5%) of hBN on the PDMS substrate enables us to observe mono- and few-layer hBN flakes under white-light illumination by an optical microscope.