“…Semiconductor crystals generally possess facet-dependent electrical conductivity properties, as revealed by conductivity measurements on single polyhedral Cu 2 O, Ag 2 O, TiO 2 , PbS, and Ag 3 PO 4 crystals. − Moreover, intrinsic Si, Ge, and GaAs wafers also exhibit electrical facet effects. − DFT calculations have revealed that the semiconductor facet effects should originate from the presence of an ultrathin surface layer with dissimilar band structures for different crystal faces. ,− DFT calculations further suggest that subtle variations in bond length, bond geometry, and frontier orbital electron energy distribution are present between metal-like and semiconducting surface planes of Si, Ge, and GaAs. − Thus, slight variations at the bond and orbital levels contribute to changes in the band structure of the surface layer, which produces the observed facet-dependent electrical conductivity behaviors of semiconductors. More recently, electrochemical impedance measurements on Si, Ge, and GaAs wafers have revealed that the presence of deep trap states located within the band gaps is linked to their facet-dependent electrical conductivity properties. − The idea of the ultrathin surface layer with facet-specific band structures can also explain the observations of facet effects in the photocatalytic activities and optical properties of semiconductor crystals. − Although several examples of semiconductors possessing strongly facet-dependent conductivity properties have been demonstrated, some unexpected phenomena can still emerge in other materials, as seen in the diverse interfacial plane-related photocatalytic outcomes of semiconductor heterostructures . Previously, size-tunable strontium titanate (SrTiO 3 ) cubes and {100} face-truncated rhombic dodecahedra with a perovskite crystal structure have been synthesized .…”