Single-layer or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some future applications and remain in the focus of research. The most prominent single element material is graphene (C). Two-dimensional alloys (or surface alloys) are a single atomic layer of alloy that is incommensurate with the underlying substrate. A topological insulator is a material whose interior behaves as an electrical insulator while its surface behaves as an electrical conductor, i.e. electrons only move along the surface of the material.