Figure 4

(a) Exciton binding energy and (b) static screening as a function of the band gap. In panel (a) we see that the binding energy of the inter-layer exciton (black circles) is almost independent of the gap size, dramatically violating the universal law that all other materials follow. The two lines are best fits to the inter-layer exciton and all other materials. In panel (b) we display static screening a diverse set of two and three dimensional materials have been studied. 3D materials in the order of increasing band gaps: Ge, Si, GaAs, AlAs, ZnTe, SiC, ZnSe, SrTiO₃, BaTiO₃, ZnS, Diamond, BN, AlN, NaH, Kr, LiF, Ar, Ne. 2D materials: mono- and bi- layers of MoS₂, MoSe₂, WS₂ and WSe₂ in zero field. Evidently, there is a very well followed universal linear law for the dependence of static screening on gap size.