Figure 1.

(a) The formation energies as a function of atomic areal density. Four compact surfaces were calculated using DFT because of the size limit of DFT calculations. (b) The Wulff structure shows two major surfaces (0001) and $\{\overline{1}011\}$ and two minor surfaces $\{\overline{1}010\}$ and $\{\overline{1}\overline{1}10\}$.