  We perform first-principles structure optimization, phonon frequency and
finite temperature molecular dynamics calculations based on density functional
theory to study the interaction of chlorine atoms with graphene predicting the
existence of possible chlorinated graphene derivatives. The bonding of a single
chlorine atom is ionic through the transfer of charge from graphene to chlorine
adatom and induces negligible local distortion in the underlying planar
graphene. Different from hydrogen and fluorine adatoms, the migration of a
single chlorine adatom on the surface of perfect graphene takes place almost
without barrier. However, the decoration of one surface of graphene with Cl
adatoms leading to various conformations cannot sustain due to strong Cl-Cl
interaction resulting in the desorption through the formation of Cl$_2$
molecules. On the contrary, the fully chlorinated graphene, chlorographene CCl,
where single chlorine atoms are bonded alternatingly to each carbon atom from
different sides of graphene with $sp^3$-type covalent bonds, is buckled. We
found that this structure is stable and is a direct band gap semiconductor,
whose band gap can be tuned by applied uniform strain. Calculated phonon
dispersion relation and four Raman-active modes of chlorographene are
discussed.
