Advanced EPR Studies in Materials Science.
From Ferroelectrics to Semiconductors
Dr. Emre Erdem
Albert-Ludwigs Universität Freiburg, Institut für Physikalische Chemie, Albertstrasse 21, 79104 Freiburg Germany
Ferroelectric materials offer a wide range of special physical properties such as high dielectric constant, spontaneous polarisation, pyroelectric and piezoelectric effects which can be applied in non-volatile memories, actuators, thin film capacitors, thermal sensors and transducers. At the nanometer scale physical and chemical properties are expected to differ from those of the bulk material. In particular, a critical particle size was predicted below which ferroelectricity does no longer occur. Electron paramagnetic resonance (EPR) is exceedingly powerful method for depicting the structural changes which is in causal connection with size effects and size-driven ferroelectric-to-paraelectric phase transitions in perovskite nanocrystals (BaTiO3 and PbTiO3 doped with paramagnetic ions). Size and interface effects cooperate in determining the ferroelectric, electronic and structural properties of the nanoparticles. This allowed for the first time to delineate a rather detailed idea of their core-shell structure of ferroelectric nanoparticle and the temperature- and size-driven phase transitions.
Understanding the role of defect centers (i.e. vacancies, interstitials, and antisites) and the incorporation of stable or meta-stable defects is a key tool toward controlling the electronic properties of ZnO. EPR is well suited for this task since it provides a direct method to monitor different paramagnetic states of vacancies and, thus, complements other experimental techniques such as photoluminescence. In this sense, EPR does not only work very well on the identification of defects but also one may obtain reliable correlation to the luminescence properties of the material. In order to characterize the ZnO defect structure, both light induced X band and high field EPR has been applied. To understand the behavior of defects in ZnO nano-particles under light, we imposed in-situ laser light with wavelengths of 445 nm and 532 nm on the samples during the X-band EPR measurements. This is crucially important since defect structures may show different properties under different wavelength. EPR measurement at 208 and 406 GHz allowed resolve small differences in the g-values.