Fig. 4

a Schematic representation of (i) “structure-level” and (ii) “material-level” 3D bulk, 2D, 1D, and 0D perovskite materials, and (iii) is the corresponding density of states versus the energy Eg [64]. b Electric band structure of low-dimensional MHPs with different n values [65]. c Illustration of quantum- and dielectric-confinement effects in low-dimensional MHPs [66]. d Exciton binding energy and PL emission wavelength as a function of number of n layers in low-dimensional MHPs [67, 68]