How to build 2D periodic two layer solid liquid structure

Dear All
I want to build a two dimensional periodic boundary condition structure with one layer of solid and one layer of liquid phase. Solid phase and liquid phase are stacked up on z direction, the 2D PBC is on x and y direction. I build a 2D solid phase by "cleaving surface". Then I don't know how to add the liquid phase while maintaining the 2D PBC.
Any suggestion would be appreciated.

Regards,
Jingjing