  Atomically thin graphite, known as graphene, has been a marvel in material
science because of its exceptional properties, novel physics and promising
applications. Atomically thin diamond, called diamane, has also attracted
considerable scientific interest due to its potential physical and mechanical
properties. However, until now there has been no reports of successful
synthesis of a free-standing pristine diamane film. Here, we report the
synthesis and electronic structure characterization of diamane. Electrical
measurements, x-ray diffraction and theoretical simulations reveal that
trilayer and thicker graphene transform to hexagonal diamane (h-diamane) when
compressed to above 20 GPa, which can be preserved down to few GPa. Raman
studies indicate that the sample quenched from high pressure and high
temperature also has a h-diamane structure, i.e., h-diamane is recovered back
to ambient conditions. Optical absorption and band structure calculations
reveal an indirect energy gap of 2.8 eV in the diamane film. Compared to
gapless graphene, diamane with sizable bandgap may open up new applications of
carbon semiconductors.
