Discovered by Jacques and Pierre Curie in 1880, the piezoelectric effect was first observed when they applied pressure to a quartz crystal, which generated electric charges. This effect was called a direct piezoelectric effect. Later on, the converse piezoelectric effect was verified when they applied an electric field to quartz, producing deformations in the crystal.
Improving Piezoelectricity of Materials
One known technique for enhancing the piezoelectricity of a substance by reducing its dimensionality. For instance, researchers have been venturing into piezoelectricity in a single layer of molecules. Further reducing this two-dimensional array of molecules result in individual molecules manifesting electromechanical responses.
For the first time, the piezoelectric effect in a single molecule has been demonstrated by a team of researchers from the Czech Academy of Sciences and Palacký University Olomouc. The study published in the Journal of the American Chemical Society is the brainchild of a collaboration between chemists who produced the molecules and physicists who demonstrated experimentally the piezoelectricity using a scanning probe microscope.
This demonstration marks the beginning of a revolution in comprehending the electromechanical behavior of individual molecules, and subsequently provide a different approach in developing low-power logic switches, molecular motors, ultrasensitive biological sensors, and energy harvesting at the nanoscale.
“In a close collaboration with physicists, it was proved for the first time that a strong converse piezoelectric effect can be observed at individual molecules of the heptahelicene derivative, which is a screw-like carbon molecule resembling a spring,” said Ivo Starý, co-author and leader of the study and chemist at IOCB Prague.
Co-leader and physicist Pavel Jelínek describes the unprecedented results of the study: “The magnitude of the piezoelectric constant calculated from the experimental data is significantly higher than that one of known piezoelectric polymers and is comparable to the magnitudes measured at some inorganic materials such as zinc oxide. Moreover, we explained the origin of the single molecule piezoelectric effect by employing quantum mechanics calculations.”
How Was the Converse Piezoelectricity Demonstrated?
The effect was experimentally demonstrated in individual polar molecules grafted on a silver surface using an atomic force microscope (AFM).
Polar molecules have an intrinsically uneven distribution of charges due to its unsymmetrical molecular structure such that one end is positively charged while the other end is negatively charged. This effect is called dipole. An example of molecules exhibiting dipole is hydrochloric acid or HCl. Its hydrogen atom is the positive end, while its chlorine atom is the negative pole.
AFM is a type of scanning probe microscope that has a microscopic cantilever whose tip is in contact with a sample material’s surface. As the cantilever scans the sample’s surface, its tip and the sample’s surface just beneath it creates an atomic force and electric signal, resulting in a topographic profile of the sample’s surface.
The researchers were able to demonstrate the converse piezoelectric effect at the nanoscale by attaching a screw-like polar molecule on a silver pad. When a voltage bias is applied between the silver pad and AFM’s atomically sharp tip, which is positioned just above the studied molecule, the polar molecule stretches (b) or squeezes (a) depending on the strength and polarity of the applied voltage. These changes in molecule’s height were monitored as evidence of mechanical response of the individual molecules with an external electric field.