We investigate the orientation switching of individual azobenzene molecules adsorbed on a Au(111) surface using a laser-assisted scanning tunneling microscope (STM). It is found that the rotational motion of the molecule can be regulated by both sample bias and laser wavelength. By measuring the switching rate and state occupation as a function of both bias voltage and photon energy, the threshold in sample bias and the minimal photon energy are derived. It has been revealed that the tip-induced local electrostatic potential remarkably contributes to the reduction in hopping barrier. We also find that the tunneling electrons and photons play distinct roles in controlling rotational dynamics of single azobenzene molecules on the surface, which are useful for understanding dynamic behaviors in similar molecular systems.

A controlled chemical reaction on a specific bond in a single molecule is an inevitable step toward atomic engineering and fabrication. Here, we explored the debromination of a single 9,10-dibromoanthracene (DBA) molecule on a surface as stimulated by the voltage pulse through the tip of a scanning tunneling microscope (STM). A voltage threshold of about 2.2 V is obtained, and the nature of single-electron process is revealed. The spatially resolved debromination yield is obtained as a function of the pulse magnitude, which presents strong asymmetry for the two C–Br bonds. The optimal stimulation parameters including the pulse magnitude and the tip locations are suggested. The distinct dynamics in dissociation of the two bonds are illustrated by their energy diagrams and recoil paths, as derived by the first-principles density functional theory (DFT) calculation. The influence of the local electric field due to the STM tip on the dissociation of the C–Br bond has also been discussed. The study presents detailed practice for the controlled debromination in a single DBA molecule, which may lead to automated atomic engineering and fabrication of artificial nanostructures in the future.