We present a study in which ferroelectric phase transition temperatures in epitaxial KxNa1−xNbO3 films are altered systematically by choosing different (110)-oriented rare-earth scandate substrates and by variation of the potassium to sodium ratio. Our results prove the capability to continuously shift the ferroelectric-to-ferroelectric transition from the monoclinic MC to orthorhombic c-phase by about 400 °C via the application of anisotropic compressive strain. The phase transition was investigated in detail by monitoring the temperature dependence of ferroelectric domain patterns using piezoresponse force microscopy and upon analyzing structural changes by means of high resolution X-ray diffraction including X-ray reciprocal space mapping. Moreover, the temperature evolution of the effective piezoelectric coefficient d33,f was determined using double beam laser interferometry, which exhibits a significant dependence on the particular ferroelectric phase.
We present a detailed analysis of the ferroelectric domain structure of KNaNbO thin films on (110) TbScO grown by metal-organic chemical vapor deposition. Upon piezoresponse force microscopy and nanofocus x-ray diffraction measurements we derive a domain model revealing monoclinic M domains. The complex domain pattern is formed out of four co-existing in-plane orientations of the shearing direction of the monoclinic unit cell resulting in four types of superdomains each being composed of well-ordered stripe domains. Finally, we present surface acoustic wave (SAW) experiments that exhibit extraordinary signal intensities given the low thickness of the tested film. Moreover, the SAW propagation is found to occur selectively along the identified shearing directions.
Thermal expansion coefficients of molecular solids are typically significantly larger than those of inorganic materials. Since they are furthermore highly anisotropic, the molecular arrangement and consequently the intermolecular orbital overlap strongly depend on temperature, hence also affecting the energetics of optoelectronic excitations and the efficiency of charge transfer processes. Here, we report on the precise determination of the anisotropic thermal expansion coefficients of the organic semiconductor pentacene in its solid state. We compare the thermal expansion coefficients of three different pentacene polymorphs and observe distinct differences between both pentacene bulk polymorphs and the interface-stabilized thin film phase. By comparing epitaxial films with films prepared on weakly interacting, amorphous substrates, we identify a notable influence of the substrate fixation on the thermal expansion in thin pentacene films. Furthermore, the results for pentacene are compared to the thermal expansion of perfluoropentacene, where an exceptionally large vertical thermal expansion coefficient is found in the substrate-mediated π-stacked polymorph. The present study underlines the importance of thermal expansion for the interpretation of temperature-dependent spectroscopic measurements and device characterizations since the notable changes in the unit cell geometries severely affect the intermolecular coupling and thus the excitonic energetics.
Ferroelectric phase transitions in multi-domain K 0.9 Na 0.1 NbO 3 epitaxial thin films To cite this article: Laura Bogula et al 2020 Nano Futures 4 035005 View the article online for updates and enhancements.
In this work, we demonstrate the electronic tunability of surface acoustic waves (SAWs) in epitaxially strained relaxor-type ferroelectric thin films. Epitaxial K0.7Na0.3NbO3 thin films of typically 30 nm in thickness are grown via pulsed laser deposition on (110)-oriented TbScO3. A partial plastic lattice relaxation of the epitaxial strain in these samples leads to a relaxor-type ferroelectricity of these films, which strongly affects the SAW properties. Without electronic bias, only tiny SAW signals of ∼0.2 dB can be detected at room temperature, which can be boosted up to ∼4 dB by a static voltage bias added to the high frequency driving current of the SAW transducers. Upon field cooling below the freezing temperature of polar nanoregions (PNRs), this strong SAW signal can be preserved and is even enhanced due to a release of the electronically fixed PNRs if the bias is removed. In contrast, at elevated temperatures, a reversible switching of the SAW signal is possible. The switching shows relaxation dynamics that are typical for relaxor ferroelectrics. The relaxation time τ decreases exponentially from several hours at freezing temperature to a few seconds (<5 s) at room temperature.
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