Dimension is a knob to significantly alter various material properties. For a given material, we imagine a situation where the system becomes thinner and thinner, so that a bulk property is not anymore present. Instead, it gives rise to a new property unexpected from the bulk. The experimental techniques making a thin film of material has been remarkably developed these days from the first synthesis of graphene. Graphene is a truly two-dimensional material made one atomic- layer thin sheet of carbon atoms. These days many two-dimensional materials haven successfully synthesized, including silicene, bithmuthene, transition-metal dichalcogenides, two-dimensional halide perovskites. More than 800 materials are theoretically proposed to exist.