Lay summary
Short summary: Methods for obtaining polar materials on the macroscopic scale are of central importance for various real world applications e.g. pyroelectric heat sensors, piezoelectric pressure sensors e.t.c. In this project, we present a new approach that takes advantage of charged surfaces under vacuum to grow unipolar polycrystalline materials of a polar texture. Background: Solid-state materials featuring uneven tensorial electrical properties (piezoelectric or pyroelectric) are commonly obtained by spontaneous nucleation of acentric or polar space groups or by electrical poling of a ceramic or glassy state. In the case of oxide ferroelectric materials, sintered ceramics are poled by lowering the temperature of the paraelectric phase to obtain a uniform polarization in the ferroelectric phase. For polymers, a similar procedure is applied by field-cooling through the glass transition temperature Tg. This project presents a new approach that takes advantage of intrinsically charged faces of polar crystalline organic materials when processed under high vacuum conditions. Under ambient conditions, materials belonging to polar space groups exhibit charge compensation of intrinsically charged (hkl) faces. Consequently, polar materials under normal conditions appear nonpolar unless a temperature or stress change is applied. In high vacuum however, charge compensation may become ineffective or even impossible so that nucleating and growing polar crystalline materials on charged electrodes may lead to unipolar polycrystalline materials forming a polar texture.Project goal: This aim of this project is thus to investigate four basic effects at moderately strong electric field on the deposition/poling of dipolar organic molecules in the high vacuum: (i) Field induced nucleation of unipolar growth of polar crystallites.(ii) Influence of torque imposed on crystallites growing under an angle, theta, to the electric field.(iii) Alignment of nuclei in a glass formed by dipolar molecules.(iv) Poling of an organic ferroelectric after depositing a polycrystalline layer.Characterization techniques: All kinds of deposits as well as transformations (e.g. glass to crystalline, paraelectric to ferroelectric) will be investigated by broadband dielectric spectroscopy (BDS), scanning pyroelectric microscopy (SPEM), second harmonic generation (SHG) microscopy and complementary chemical structural methods. These techniques including organic molecular beam deposition (OMBD) are well established in the group of the applicant.