Large Converse Piezoelectric Effect Measured on a Single Molecule on a Metallic Surface

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.