Coexistence of Two Electronic Nano-Phases on a CH₃NH₃PbI_3-xCl_x Surface Observed in STM Measurements

Scanning tunneling microscopy is utilized to investigate the local density of states of a CH₃NH₃PbI_3-xCl_x perovskite in cross-sectional geometry. Two electronic phases, 10-20 nm in size, with different electronic properties inside the CH₃NH₃PbI_3-xCl_x perovskite layer are observed by the dI/dV mapping and point spectra. A power law dependence of the dI/dV point spectra is revealed. In addition, the distinct electronic phases are found to have preferential orientations close to the normal direction of the film surface. Density functional theory calculations indicate that the observed electronic phases are associated with local deviation of I/Cl ratio, rather than different orientations of the electric dipole moments in the ferroelectric phases. By comparing the calculated results with experimental data we conclude that phase A (lower contrast in dI/dV mapping at -2.0 V bias) contains a lower I/Cl ratio than that in phase B (higher contrast in dI/dV).