I am studying atomic slabs which are periodic in 2 dimensions. Normally in the third dimension, I add a vacuum layer and use dipole correction to mimic proper boundary conditions. Recently I have discovered a property 'assume_isolated' with a parameter '2D' in Quantum Espresso implemented in the way described in Phys. Rev. B, 96(7), 75448 (2017) In the paper the authors apply this formalism to the graphene which is atomically thin. Is this formalism applicable for slabs with a thickness of about 3 nm? When I applied it I saw strange oscillations in the potential for the vacuum layer where I expected to see flat lines. It seems like as there are some atoms in the vacuum layer appeared (image charges?).
Below are two examples with a silicon slab.