Two-dimensional silica (2D silica) is a layered polymorph of silicon dioxide. Two varieties of 2D silica, both of hexagonal crystal symmetry, have been grown so far on various metal substrates. One is based on SiO 4 tetrahedra, which are covalently bonded to the substrate. The second comprises graphene -like fully saturated sheets, which interact with the substrate via weak van der Waals bonds. One sheet of the second 2D silica variety is also called hexagonal bilayer silica (HBS); it can have either ordered or disordered (amorphous) structure.