  The stacking order degree of freedom in trilayer graphene plays a critical
role in determining the existence of an electric field tunable band gap. We
present spatially-resolved tunneling spectroscopy measurements of dual gated
Bernal (ABA) and rhombohedral (ABC) stacked trilayer graphene devices. We
demonstrate that while ABA trilayer graphene remains metallic, ABC trilayer
graphene exhibits a widely tunable band gap as a function of electric field.
However, we find that charged impurities in the underlying substrate cause
substantial spatial fluctuation of the gap size. Our work elucidates the
microscopic behavior of trilayer graphene and its consequences for macroscopic
devices.
