  The screened Coulomb potential plays a crucial role in the binding energies
of excitons in a thin dielectric slab. The asymptotic behavior of this
potential is studied when the thickness of the slab is very small as compared
to the exciton Bohr radius. A regularized expression is given and the exact
effective 2D potential is derived. These expressions may be useful for the
computation of the exciton binding energy in 2D or quasi-2D materials.
