Racemic Amino Acid Piezoelectric Transducer

Single crystal $L$-amino acids can exhibit technologically useful piezoelectric and nonlinear optical properties. Here we predict, using density functional theory, the piezoelectric charge and strain and voltage tensors of the racemic amino acid $DL$ alanine, and use the modeling data to guide the first macroscopic and nanoscopic piezoelectric measurements on $DL$-alanine single crystals and polycrystalline aggregates. We demonstrate voltage generation of up to 0.8 V from $DL$-alanine crystal films under simple manual compression, twice as high as other amino acid crystals. Our results suggest that net molecular chirality is not a prerequisite for piezoelectric behavior in organic crystals. The transducer presented herein demonstrates that $DL$-alanine crystals can be used in applications such as temperature and force measurement in biosensors, data storage in flexible electronic devices, and mechanical actuation in energy harvesters.

Figure 1

Relationship between longitudinal ($\sigma$) and shear ($\tau$) stress and corresponding piezoelectric polarization ($P$) acting along arbitrarily chosen mutually orthogonal 1-2-3 axes for the five piezoelectric strain constants of $DL$-alanine single crystals, $d_{31}$, $d_{32}$, $d_{33}$, $d_{24}$, and $d_{15}$. Gray shading indicates the corresponding surfaces on which the electrodes are contacted to measure the polarization.

Figure 2

(a) Crystal packing controls the orientation of alanine molecular dipoles (green arrows) in the unit cell, which determines the magnitude and direction of the corresponding piezoelectric responses. Molecules are shown with carbon atoms colored gray, hydrogen atoms in white, oxygen atoms in red, and nitrogen atoms in blue. (b) Comparison of the piezoelectric voltage constants of $L$ and $DL$ alanine, showing that both the longitudinal and shear response of the racemic mixture exceed that of the $L$ enantiomer.

Figure 3

(a) Open circuit voltage harvested from $DL$-alanine films under periodic manual compression. (b) Racemic amino acid films grown on copper electrodes for energy harvesting tests. (c) Manual compression of $DL$-alanine films between electrodes to generate piezoelectric voltage.