  It was generally assumed that weak van der Waals interactions exist between
neighboring layers in the two-dimensional group-IV chalcogenides. Using PbSe as
a prototypal example, however, we find additional strong coupling between the
Pb-Pb layers, as evidenced by detailed analysis of the differential charge
density. The coupling resembles covalent-like bond and exhibits strong
harmonicity around the equilibrium distance, which can be fine tuned to
obviously reduce the phonon thermal conductivity but slightly change the
electronic transport of PbSe. As a consequence, a maximum ZT value of 2.5 can
be realized at 900 K for the p-type system. Our work offers an effective and
feasible design strategy to enhance the thermoelectric performance of similar
layered structures.
