Monitoring and manipulating single molecule rotors on the Bi(111) surface by the scanning tunneling microscopy

We have investigated rotation and manipulation of manganese phthalocyanine (MnPc) molecules on the semimetallic Bi(111) surface by low-temperature scanning tunneling microscopy (STM). At liquid-helium temperature, individual MnPc molecules adsorb flat on the Bi(111) surface but with a slight bend toward the substrate. The in-plane orientation of the MnPc molecule can be changed by 60 degrees by manipulation with the STM tip. At liquid-nitrogen temperature, the isolated MnPc molecules rotate around the central Mn ion because of the weak molecule-semimetal interaction. The MnPc rotor shows a flower-like feature with six lobes corresponding to the six-fold symmetry of Bi(111) lattice. Most importantly, the MnPc rotors can be started or stopped by controlling the intermolecular spacing with the STM tip. These results are important for understanding and controlling the performance of surface-mounted molecular rotors.