  For graphene to be used in semiconductor applications, a wide energy gap of
at least 0.5 eV at the Dirac energy must be opened without the introduction of
atomic defects. However, such a wide energy gap has not been realized in
graphene, except in the cases of narrow, chemically terminated graphene
nanostructures with inevitable edge defects. Here, we demonstrated that a wide
energy gap of 0.74 eV, which is larger than that of germanium, could be opened
in uniform monolayer graphene without the introduction of atomic defects into
graphene. The wide energy gap was opened through the adsorption of
self-assembled twisted sodium nanostrips. Furthermore, the energy gap was
reversibly controllable through the alternate adsorption of sodium and oxygen.
The opening of such a wide energy gap with minimal degradation of mobility
could improve the applicability of graphene in semiconductor devices, which
would result in a major advancement in graphene technology.
