Making Graphene Visible in Transparent Dielectric Substrates: Brewster Angle Microscopy

Romagnoli, P., Lopez-Cortes, D., Rosa, H.G., Souza, E.A.T., Viana-Gomes, J.C., Margulis, W. and de Matos, C.J.S.
Type of publication: 
Conference item

Typically, flakes of exfoliated graphene are detected by the optical contrast obtained on Si substrates containing a submicron layer of SiO2 [1-3]. This layer generates interference that is responsible for increasing the contrast, with values up to 12~14% being reported [1-2]. The detection of graphene on transparent dielectric substrates, such as microscope slides, is a greater challenge since the contrast between the regions with and without graphene is significantly lower, on the order of 7% [4]. An interferometrically increased contrast can be obtained by depositing a layer of polymethylmethacrylate (PMMA) onto the substrate before the graphene exfoliation. However, this invariably changes the substrate properties, which affects the characteristics of the exfoliated sample.

This work aims at improving the optical contrast of exfoliated graphene on transparent substrates without changing the substrate or the sample physical/chemical properties. This is obtained through optical imaging at the substrate’s Brewster angle. As the graphene changes the boundary conditions of the air-dielectric interface, cancellation of the p-polarized light ceases solely where graphene is present. This leads to an immense increase in contrast, since virtually no reflection is obtained where graphene is not present. For the experimental demonstration of the technique, broadband green light was collimated, linearly polarized and launched onto the sample at the substrate’s Brewster angle (~570); its reflection was collected and imaged on a camera. It was possible to observe a nearly quadratic growth of reflectance with the number of graphene layers. Also, we measured the contrast of a monolayer CVD graphene at normal incidence (using a standard optical microscope) to be 8.4%, while our Brewster angle setup yielded a contrast as high as 2900%. The method was then applied to identify few-layer flakes of different two-dimensional materials and on dielectric substrates.

San Jose, CA
United States
Published in: 
MRS Spring Meeting, 4 June 2015