Angle-dependent identification of twisted bilayer graphene using multi-spectral ellipsometric contrast microscopy

The electronic properties of bilayer graphene can be tuned by changing the relative twist angle between the two layers. Optical resonances can occur in twisted bilayer graphene due to van Hove singularities which have a twist angle dependent energy separation continuously tuneable from infrared to UV. This enhanced absorption has been demonstrated to be beneficial to the performance of photodetectors and other optoelectronic devices, motivating the development of fast, precise techniques to identify these regions.



Spectroscopic imaging ellipsometry has emerged as a tool to determine the optical constants of graphene and provide thickness information with single-atomic layer precision with a lateral resolution down to 1 μm. Alternatively, polariser settings can be optimised for material and thickness contrast and fixed during imaging in ellipsometric contrast microscopy (ECM). Here we extend the technique to multi-spectral ECM (MS-ECM) demonstrated on chemical vapour deposited graphene on Si/SiO2. We show MS-ECM provides layer number sensitivity and wavelength dependent contrast of optically resonant bilayer regions. We generate a detailed map of the twist angle variation of bilayer graphene which is used to build a spatially correlated multi-characterisation technique dataset.