  ABC-stacked trilayer graphene has exhibited a variety of correlated phenomena
owing to its relatively flat bands and gate-tunable bandgap. However,
convenient methods are still lacking for identifying ABC graphene with
nanometer-scale resolution. Here we demonstrate that the scanning near-field
optical microscope (SNOM) working in ambient conditions can provide quick
recognition of ABC trilayer graphene with no ambiguity and excellent resolution
(~20 nm). The recognition is based on the difference in their near-field
infrared (IR) responses between the ABA and ABC trilayers. We show that in most
frequencies, the response of the ABC trilayer is weaker than the ABA trilayer.
However, near the graphene phonon frequency (~1585 cm-1), ABC's response
increases dramatically when gated and exhibits a narrow and sharp Fano-shape
resonant line, whereas the ABA trilayer is largely featherless. Consequently,
the IR contrast between ABC and ABA becomes reversed and can even be striking
(ABC/ABA~3) near the graphene phonon frequency. The observed near-field IR
features can serve as a golden rule to quickly distinguish ABA and ABC
trilayers with no ambiguity, which could largely advance the exploration of
correlation physics in ABC-stacked trilayer graphene.
