Potential Steps at C60–TiOPc–Ag(111) Interfaces: Ultrahigh-Vacuum–Noncontact Scanning Probe Metrology

Nanoscale structure–electric potential relations in films of the organic molecular semiconductors C60 and titanyl phthalocyanine (TiOPc) on Ag(111) have been measured under UHV conditions. Noncontact force methods were utilized to image domain structures and boundaries with molecular resolution, while simultaneously quantifying the local surface electric potential. Sensitivity and spatial resolution for the local potential measurement were first established on Ag(111) through direct observation of the electrical dipole and potential step, φstep = 10 ± 3 mV, of monatomic crystallographic steps. A local surface potential increase of 27 ± 11 mV occurs upon crossing the boundary between the neat Ag(111) surface and C60 islands. Potential steps in binary C60–TiOPc films, nanophase-separated into crystalline C60 and TiOPc domains, were then mapped quantitatively. The 207 ± 66 mV potential step across the C60-to-TiOPc domain boundary exhibits a 3.6 nm width that reflects the spatial resolution for electric potential across a material interface. The absence of potential asymmetry across this lateral interface sets the upper bound for the C60–TiOPc interface dipole moment as 0.012 e nm.