A material whose properties are dominated by two-dimensional effects, the properties of the material on a two-dimensional scale are different from its properties on a larger scale. 

The scale of two-dimensional materials in the out-of-plane direction is much smaller than that in the in-plane direction. Epitaxial layer, which grows or deposits a single crystal layer with the same structure on a single crystal substrate, is called a single crystal epitaxial film. Atoms or molecules are arranged on the substrate crystal in a specific, repeated manner in three-dimensional space, which makes the crystal have some special physical and chemical properties. The orientation of the film layer is generally determined by the substrate material, and the orientation of the crystals in the upper layer is affected by the crystals in the lower layer. In other words, the arrangement of the crystals in the upper layer will be as consistent as possible with the arrangement of the crystals in the lower layer, which determines the arrangement of the crystals in a certain direction.

Homogeneous epitaxy: The film material and the substrate material have the same crystal structure or lattice parameters.

Heteroepitaxy: The crystal structure or lattice parameters of the film material and the substrate material have a large difference.

Structure type of film: