Dipole-driven ferroelectric assembly of styrene on au{111}

By studying styrene, a simple hydrocarbon with a weak dipole moment, we have investigated ferroelectric ordering and ferroelectric transitions at the single-molecule level. Low-temperature scanning tunneling microscopy imaging of individual styrene molecules reveals their internal structure and hence the orientation of their dipole moment parallel to the Au{111} surface. At near monolayer coverages, both local ferroelectric ordering of the molecules and long-range antiferroelectric ordering of the domains are observed, and the dipole-dipole interaction energies are quantified. A piezoelectric transition from ferroelectric to paraelectric ordering is observed upon further increase of the coverage. The effect of the STM tip in randomizing ordered domains is also discussed. This work demonstrates that important ferroelectric properties such as spontaneous polarization, long-range ordering, and piezoelectricity can be achieved in nanoscale domains of a weakly polar molecule on a metal surface.