  We report energy estimated to dissociate a C-C bond of a graphene sheet to
form nanoribbons of armchair and zigzag configurations using first principles
calculations. For the ground state energy calculations, the configurations
considered are with spin, and without spin polarization. It is observed that
the energy required to dissociate a C-C bond of a graphene sheet to form zigzag
configuration is higher than that of armchair configuration for both spin
polarized state, as well as non-spin polarized state. Therefore, formation of
smooth edged graphene nanoribbons along the crystallographic directions might
be engineered by a control over energy.
