  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.
