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.

Department of Inorganic Chemistry Faculty of Science Charles University Prague Hlavova 2030 8 12843 Prague 2 Czech Republic

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences Flemingovo nám 2 16610 Prague 6 Czech Republic

Institute of Physics of the Czech Academy of Sciences Cukrovarnická 10 18221 Prague 6 Czech Republic

Regional Center of Advanced Technologies and Materials Palacký University 77147 Olomouc Czech Republic

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