Bulk KAgSe crystallizes in a tetragonal layered structure stacked along the z direction, (26) which can be experimentally exfoliated to KAgSe QL. (27,28) Motivated by this progress, we systematically investigated all of the possible ABC (A = Na, K, and Rb; B = Cu, Ag, and Au; C = S, Se, and Te) ternary QLs, within the framework of the lattice of KAgSe QL. The structures of ABC QLs and the corresponding Brillouin zone are shown in Figure 1. Clearly, the ABC QLs host the square lattices (see Figure 1a) due to their tetragonal parent compounds. As shown in Figure 1b, A atoms and C atoms are separated by the central B atoms, thus forming the five-atomic-layer structure.