What molecular properties give rise to a strong piezoelectric response?

  • By Burcu Ozden
  • 22 November 2017

Materials mechanically deform under an applied electric field are known as piezoelectric materials.  Many biomaterials show piezoresponse as molecular property due to their high responsiveness. Because those biomaterials which has piezoelectric properties allow direct mechanical to electrical conversion, they find wide use in touch and force sensors, microscale actuators, and related components. Devices utilizing such components have applications in areas from consumer to medical to military. Despite widespread potential applications, limitations exist that hinder the advance of piezoelectric materials. For example, stability and strength of piezo coefficients in biomaterials, and device scalability is still an issue.  Furthermore, scientists still need to explain correlation between molecular properties and piezoelectric response.

To gain insight into the molecular basis for electromechanical response in organic scaffolds, Geoffrey R. Hutchison and his colleagues test the hypothesis of flexible response of local folding motifs to the applied field by examining the piezoresponse in a series of helical peptide-based oligomers. In this study, they systematically probe the interplay among peptide chemical structure, folding propensity, and piezoelectric properties, uncovering in the process new insights into the origin of peptide electromechanical response.

Three identical length but varying folding propensity of peptides and identical length but systematically altered helicity peptoids were designed to evaluate the effect of helicity, and thus the molecular basis of peptide piezoresponse. As a control group they have used dodecanethiol (DDT).  Circular dichroism (CD) measurements were performed to observe ellipticity. The CD spectrum of peptide showed  an increase in random coil character and support their design hypothesis regarding the relative helicity across the varying series. Qualitative features of the CD spectra of the peptoids were consistent with the expected lefthanded PPI helical fold. Polarization-modulated infrared reflection−absorption spectroscopy (PM-IRRAS) measurements were collected to gain information about folding by an analytical method. The observation from PM-IRRAS matches the trend observed by CD. Atomic force microscopy (AFM) and piezo-force microscopy (PFM) measurements were performed for the characterization of the surface.

They have used dual-AC resonance tracking piezoresponse force microscopy (DART-PFM) to determine the change in thickness (Δt) of each film over a series of applied voltages. The slope from a linear fit to a plot of Δt vs applied voltage provides a measure of the piezoelectric response along the polarization axis. They note that in general, across all monolayers the distribution of piezoresponse showed high positive skewness, which did not have a clear trend with voltage. On the other hand, for five out of six compounds the distributions yielded voltage-dependent increases in peak width and standard deviation. Comparing the two different backbone compositions, the average response from monolayers of peptides was significantly greater than that of monolayers made up of DDT or peptoids. On average, peptides yielded PFM response ∼41% larger than DDT and ∼59% larger than peptoids.X-ray photoelectron spectroscopy (XPS) measurements were collected to rule out the possibility that differences in piezoresponse observed resulted from differences in monolayer packing density rather than molecular structure. They didn’t observed significant variation among the packing densities to explain the observed differences in piezoresponse. Moreover, no significant trendwas apparent based on backbone composition or helicity in solution.

In summary, the results suggest that peptide-based materials exhibiting piezoresponse have regions of highly polar, flexible backbones. The results reported in their work demonstrate the promise of combining systematic synthesis PFM monolayer characterization, and computational design in peptides and related oligomers as a means to unlock new avenues to highly responsive piezomaterials.

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