2-D materials have special lattice structures. Atoms in the same layer are usually bound by a covalent bond, while the force between layers is van der Waals coupling. They have super-clean surfaces without any dangling bonds. Thus, the design of heterojunctions is more flexible when 2-D materials are utilized to constitute heterojunctions. The heterojunctions formed by different 2-D materials have advantageous properties, including optimizing band alignment, bandgap, charge transfer and optical properties.