Twisted bi-layer graphene: microscopic rainbows

TL;DR

This study demonstrates how the color variations in twisted bi-layer graphene, observed through optical microscopy, are linked to specific rotational angles, enabling simple identification of bilayer regions based on their optical properties.

Contribution

The paper correlates optical colorations of tBLG with specific rotational angles, providing a straightforward optical method for identifying bilayer graphene.

Findings

Colorations correspond to specific rotational angles.

Optical contrast is angle-dependent due to electronic properties.

Color-based identification of bilayer graphene is feasible.

Abstract

Twisted bi-layer graphene (tBLG) has recently attracted interest due to the peculiar electrical properties that arise from its random rotational configurations. Our experiments on CVD-grown graphene from Cu foil and transferred onto Si substrates, with an oxide layer of 100 nm, reveal naturally-produced bi-layer graphene patches which present different colorations when shined with white light. In particular yellow-, pink- and blue- colored areas are evidenced. Combining optical microscopy, Raman spectroscopy and transmission electron microscopy we have been able to assign these colorations to ranges of rotational angles between the two graphene layers. Optical contrast simulations have been carried out, proving that the observation of the different colorations is due to the angle-dependent electronic properties of tBLG combined with the reflection that results from the layered structure tBLG / 100 nm-thick SiO2 / Si. Our results could lead the way to an easy selective identification of bi-layer graphene merely through the observation on an optical microscope.