Abstract
Surface pyroelectricity and piezoelectricity induced by water incorporation during growth in α-glycine were investigated. Using the periodic temperature change technique, we have determined the thickness (~280 µm) of the near surface layer (NSL) and its pyroelectric coefficient (160 pC/(K × cm2) at 23 °C) independently. The thickness of NSL remains nearly constant till 60 °C and the pyroelectric effect vanishes abruptly by 70 °C. The piezoelectric effect, 0.1 pm/V at 23 °C measured with an interferometer, followed the same temperature dependence as the pyroelectric effect. Abrupt disappearance of both effects at 70 °C is irreversible and suggests that water incorporation to α-glycine forms a well defined near surface phase, which is different form α-glycine because it is polar but it too close to α-glycine to be distinguished by X-ray diffraction (XRD). The secondary pyroelectric effect was found to be <14% of the total, which is unexpectedly small for a material with a large thermal expansion coefficient. This implies that water incorporation infers minimal distortions in the host lattice. This finding suggests a path for the control of the piezoelectric and pyroelectric effects of the crystals using stereospecific incorporation of the guest molecules.
Highlights
The crystal surface symmetry and physical properties are very important in a wide variety of applications, for example: using a suitable substrate can control symmetry and epitaxial growth of thin films for various applications in microelectronics, microelectromechanical systems (MEMS), catalysis and more
It has been reported that some centrosymmetric crystals contain a near surface layer (NSL) with polar symmetry
The fact that the current-time response can be fitted with high degree of confidence to Equaion (3) at all temperatures implies (Figure 1a) that the polarization is constant within the surface layer. δs does not change noticeably with temperature upon heating to 60 ◦ C, above which the pyroelectric response rapidly drops and at 70 ◦ C vanishes completely, which implies that the surface layer is a well-defined structure
Summary
The crystal surface symmetry and physical properties are very important in a wide variety of applications, for example: using a suitable substrate can control symmetry and epitaxial growth of thin films for various applications in microelectronics, microelectromechanical systems (MEMS), catalysis and more. It has been reported that some centrosymmetric crystals contain a near surface layer (NSL) with polar symmetry These layers were detected by pyroelectric measurements [1,2,3] or suggested by theoretical calculations [4,5,6]. Piezoelectric properties of such layers are frequently detected by Piezoresponse Force Microscopy (PFM) that is, in particular, sensitive to surface piezoelectricity [7,8,9]. The most popular interpretation of the surface piezoelectricity was a polarization arising due to strain gradient at the surface, i.e., flexoelectric effect It can be caused, for example, by the PFM tip, that may cause highly inhomogeneous stress/strain just below the contact [10]. The main reason for the lack of surface piezoelectric measurements is that even an NSL with a large piezoelectric coefficient, Materials 2020, 13, 4663; doi:10.3390/ma13204663 www.mdpi.com/journal/materials
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