Contact-Free Polarity Characterization of Crystals by X-Ray Photoelectron Spectroscopy and Molecular Dynamics Simulations
2Institute of Chemistry and The Fritz Haber Research Center, The Hebrew University of Jerusalem, Jerusalem
3Department of Chemical Research Support, Weizmann Institute of Science, Rehovot
Pyroelectricity is a property of polar materials, encountering surface charge under temperature changes. This property was confined exclusively for the polar directions of the ten polar crystalline classes1. However, in contrast to the generally accepted symmetry restrictions, we found that non-polar crystals of amino acids exhibit surface pyroelectricity at specific crystal faces2-4.
Conventional pyroelectric measurements are frequently challenging, due to the typically rapid charge compensation by adsorbed moieties, as well as various difficulties arising as a result of contacts introduction. In particular, surface pyroelectric measurements are extremely sensitive to the above difficulties and, therefore, they require complementary measuring techniques. Here we exploit the recent chemically resolved electrical measurements5 based on x-ray photoelectron spectroscopy, to measure in a non-contact mode6 and, importantly, under ultra-high vacuum, the bulk and surface pyroelectricity of pure (non-polar) and L-threonine doped (polar) α-glycine crystals7.
Combined with atomic force microscopy studies, the pyroelectric measurements provide information on various types of crystal surface reconstructions. Molecular dynamics simulations provide the structure of the near surface hydrated glycine molecules of the crystal at the molecular level.
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