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.

中文翻译：

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在金属表面上的单个分子上测量的大逆压电效应

逆压电效应是由于施加的电场而在材料中产生机械应变的现象。在这项工作中，我们使用原子力显微镜 (AFM) 和总能量密度泛函理论 (DFT) 计算证明了 Ag(111) 表面上单个七螺旋烯衍生分子的逆压电效应。在很宽的偏置电压范围内获得的力-距离光谱揭示了分子和尖端之间建立接触的尖端-样品距离的线性变化。我们证明这种效应是由螺旋状多芳烃分子的弹簧状支架的偏置引起的变形引起的。我们将这种效应归因于分子螺旋的软振动模式与由分子-底物电荷转移引起的垂直电偶极子的耦合。此外，我们还对 Ag(111) 表面上更刚性的邻碳硼烷二硫醇分子进行了相同的光谱测量。在这种情况下，我们确定了较弱的线性机电响应，这巩固了螺旋支架对观察到的压电响应的重要性。