  The effect of hydrogenation on the topography and the electronic properties
of graphene and graphite surfaces are studied by scanning tunneling microscopy
and spectroscopy. The surfaces are chemically modified using Ar/H2 plasma.
Analyzing thousands of scanning tunneling spectroscopy measurements we
determine that the hydrogen chemisorption on the surface of graphite/graphene
opens on average an energy band gap of 0.4 eV around the Fermi level. We find
that although the plasma treatment modifies the surface topography in a
non-reversible way, the change in the electronic properties can be reversed by
a moderate thermal annealing and the samples can be hydrogenated again yielding
a similar, but slightly reduced, semiconducting behavior after the second
hydrogenation.
