Observation of Franck-Condon Blockade in Single Molecules Gated by Local Electric Field

Electron transport through single molecules is greatly influenced by a discrete spectrum of vibrational modes in strong electron-vibron coupling regime. Theory predicts a current suppression at low biases known as Franck-Condon blockade. However, how Franck-Condon blockade emerges in in a real orbital-gated single molecule transistor is still elusive. In this study, by using a low-temperature scanning tunneling microscope, we report the real-space observation of Franck-Condon blockade in single molecules adsorbed on metal surfaces. The frontier molecular orbitals and charge state of the molecules are shown to be electrostatically gated by variation in the underlying surface potential and local chemical environment, allowing the observation of coulomb blockade as well as the Franck-Condon blockade. Moreover, strongly enhanced inelastic cotunneling is evidenced to dominate the electron transport in the Franck-Condon blockade regime, leading to unprecedented high-contrast single molecule vibrational spectroscopy and microscopy.