Piezoelectricity is one of the important functional properties inherent to many biomaterials. It stems from the non-centrosymmetric crystal structure of most biopolymers including proteins, polysaccharides, and lipids. Understanding the relationship between the generated electric field and applied mechanical stress has become the main motivation to studying piezoelectricity in biological systems and artificial biomaterials at the nanoscale. In this work, we present a review of the piezoelectric and ferroelectric properties of several molecular systems and nanomaterials revealed by Piezoresponse Force Microscopy (PFM) and compare the results with molecular modeling and computer simulations. Experimentally observed by PFM and calculated dielectric, piezoelectric, and ferroelectric properties of these materials are analyzed in the context of their possible role in functionality of biological systems. V C 2014 AIP Publishing LLC.
Glycine is the simplest amino acid and one of the basic and important elements in biology, as it serves as a building block for proteins. The interest in this material has recently arisen from its useful functional properties, such as its high value of nonlinear optical susceptibility and ferroelectricity. Three polymorphic forms with different physical properties are possible in glycine, the most useful β-polymorph being much less stable than the other two. In this work, we could grow stable microcrystals of β-glycine using a (111)Pt/SiO 2 /Si substrate as a template. The effects of the solution concentration and Pt-assisted nucleation on the crystal growth and phase evolution were evaluated using X-ray diffraction analysis and Raman spectroscopy. A second harmonic generation (SHG) method confirmed that the 2-fold symmetry is preserved in as-grown crystals, thus reflecting the expected P2 1 symmetry of the β-phase. Spontaneous polarization direction is found to be parallel to the monoclinic [010] axis and directed along the crystal length. These data are confirmed by computational molecular modeling. Optical measurements revealed also relatively high values of the nonlinear optical susceptibility (50% greater than in the z-cut quartz). The potential use of stable β-glycine crystals in nonlinear optical applications is discussed.
Glycine is a model crystal exhibiting three polymorphic phases and important functional properties such as piezoelectricity and ferroelectricity. We report here in situ observation of the irreversible transformation of the solutiongrown glycine crystals from a β phase into a γ phase. The slow transformation process was monitored by piezoresponse force microscopy at room temperature. The process of β to γ conversion was entirely controlled by the variation of relative humidity in the sample chamber. The results show that the rate of phase transformation in glycine is humidity dependent with a threshold of about 25% RH. It is demonstrated that the phase boundary is highly rugged and the transformation front propagates inhomogeneously along the polar axis of the β phase. The mechanism of the phase transformation is discussed.
Self-assembly of ferroelectric materials attracts significant interest because it offers a promising fabrication route to novel structures useful for microelectronic devices such as nonvolatile memories, integrated sensors/actuators, or energy harvesters. In this work, we demonstrate a novel approach for self-assembly of organic ferroelectrics (as exemplified by ferroelectric β-glycine) using evaporative dewetting, which allows forming quasi-regular arrays of nano- and microislands with preferred orientation of polarization axes. Surprisingly, self-assembled islands are crystallographically oriented in a radial direction from the center of organic "grains" formed during dewetting process. The kinetics of dewetting process follows the t law, which is responsible for the observed polygon shape of the grain boundaries and island coverage as a function of radial position. The polarization in ferroelectric islands of β-glycine is parallel to the substrate and switchable under a relatively small dc voltage applied by the conducting tip of piezoresponse force microscope. Significant size effect on polarization is observed and explained within the Landau-Ginzburg-Devonshire phenomenological formalism.
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