CHEM-SWITCH

The fundamental property of ferroelectric (FE) materials is their electrically switchable spontaneous polarization below the Curie temperature. However, the direction of the polarization in FE thin films is not only the result of a simple control through external electric field since it usually results from the minimization of the electrostatic energy (driven by an equilibrium between short and long-range forces) in the whole sample. In a FE, the presence of unscreened polarization charge creates an internal electric field, called the depolarizing field, which can partially or wholly cancel the FE polarization inside the material. This is particularly important in view of potential applications of FE thin films. The polarization charge at the surface can be screened through a variety of mechanisms including extrinsic screening by adsorbate species, intrinsic screening by defects or free charge carriers in for example adjacent electrodes, surface and near surface structural changes (rumpling, relaxation and reconstruction) and by domain ordering which reduces the energy of the system by screening the depolarizing field through ordering of the FE domains with anti-parallel polarization. Surface charge compensation can involve extra or missing ions arising from interaction with the environment rather than electrons. The chemical composition of the atmosphere (the oxygen chemical potential, for example) can induce preferential orientation of the polarization. Conversely, the polarization can influence surface chemical reactions (the adsorption of various molecules depends on the polarization direction). It has been suggested that oxygen vacancies stabilize negative polarization, i.e. polarization pointing inwards. The topology of the surface FE order is therefore a complex interaction of the chemical and electronic environment. X-ray diffraction (XRD) has demonstrated reversible domain growth or continuous polarization switching by oxygen partial pressure, pO2, rather than by an applied electric field. However, there is little experimental evidence of the changes in the electronic structure concomitant with switching. Moreover, the surface stoichiometry and phase may differ considerably from the bulk. The CHEM-SWITCH consortium brings expertise in diverse photoelectron spectroscopy techniques with energy, wave-vector and spatial resolution in order to unravel the electronic and chemical structure of the FE topology and the mechanisms responsible for chemically induced switching. The interactions of both ionic and free charge with the polarization will be studied for two prototypical FE materials: BaTiO3 (BTO) and PbTiO3 (PTO). These will also provide a comparison between ABO3 perovskite structures in which the FE is due to B and A cation displacements, respectively. Surface composition by high resolution XPS and band structure determination by ARUPS will be compared with theory. Structural determination will include electron diffraction (LEED, RHEED) and X-ray photoelectron diffraction (XPD), while the surface morphology, FE topology and chemistry will be assessed by scanning probe microscopy (SPM) techniques and by low energy electron microscopy (LEEM) and photoelectron emission microscopy (real and reciprocal space PEEM). The project will advance understanding of the electronic, structural, and chemical origins of polarization switching. It will explore the chemical potential-temperature polarization phase diagrams through the use of atomic and molecular oxygen and vicinal surfaces. Finally, it will furnish an understanding of the switching chemistry vital to a wide range of applications such as ferroelectric enhanced catalysis and photolysis, chemical sensing and oxide based electronics.