). To determine which of these terminations is more likely to result from exfoliation, we calculated their surface energies (for details, see Theoretical Calculations in SI Section 2). Surface energies have been shown to correlate well with the work of adhesion, which is the energy per unit area required to cleave a crystal at a particular surface. This quantity is important for determining whether a non-van der Waals crystal, such as Mn3O4, can be mechanically exfoliated into few-layer structures. The surface energies of the two (101) terminations are shown in Figure for monolayer, bilayer, and bulk geometries of Mn3O4. The surface energy of two commonly used crystal substrates, (0001) ZnO and AlN, are included as references to show the feasibility for cleavage of Mn3O4. The octahedral surface termination has a lower energy for all thicknesses tested here with the bilayer having the lowest energy, which is about the same as (0001) ZnO by 1 meV/Å 2 . The lower surface energy of bilayer than the monolayer is because of a surface reconstruction that moves some of the Mn atoms in the interior to the surface (SI section 2, Figure S7). From these results, we conclude that the octahedral (101) surface termination is the basal plane termination in the exfoliated Mn3O4 membranes.