Abstract
The converse piezoelectric effect is a phenomenon in which mechanical strain is generated in a material due to an applied electrical field. In this work, we demonstrate the Converse piezoelectric effect in single heptahelicene-derived molecules on the Ag(111) surface using atomic, force microscopy (AFM) and total energy density functional theory (DFT) calculations.
The force distance spectroscopy acquired over a wide range of bias voltages reveals a linear shift of the tip sample distance at which the contact between the molecule and tip apex is established. We demonstrate that this effect is caused by the bias-induced deformation of the spring-like scaffold of the helical polyaromatic molecules.
We attribute this effect to coupling of a soft vibrational mode of the molecular helix with a vertical electric dipole induced by molecule substrate charge transfer. In addition, we also performed the same spectroscopic measurements on a more rigid o-carborane dithiol molecule on the Ag(111) surface.
In this case, we identify a weaker linear electromechanical response, which underpins the importance of the helical scaffold on the observed piezoelectric response.