Local Polarization, Charge Compensation, and Chemical Interactions on Ferroelectric Surfaces: a Route Toward New Nanostr
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Local Polarization, Charge Compensation, and Chemical Interactions on Ferroelectric Surfaces: a Route Toward New Nanostructures Dawn A. Bonnell and Sergei V. Kalinin Dept. Mat. Sci. Eng., University of Pennsylvania, 3231 Walnut St, Philadelphia, PA 19104 ABSTRACT The local potential at domains on ferroelectric surfaces results from the interplay between atomic polarization and screening charge. The presence of mobile charge affects surface domain configuration, switching behavior, and surface chemical reactions. By measuring the temperature and time dependence of surface potential and piezo response with scanning probe microscopies, thermodynamic parameters associated with charge screening are determined. This is illustrated for the case of BaTiO3 (100) in air, for which the charge compensation mechanism is surface adsorption and enthalpy and entropy of adsorption are determined. The local electrostatic fields in the vicinity of the domains have a dominant effect on chemical reactivity. Photoreduction of a large variety of metals can be localized to domains with the appropriate surface charge. It has been demonstrated that proximal probe tips can be used to switch polarization direction locally. Combining the ability to ‘write’ domains of local polarization with domain specific reactivity of metals, vapors of small molecules, and organic compounds leads to a new approach to fabricating complex nanostructures. INTRODUCTION Development of spontaneous polarization and related lattice distortion below the Curie temperature in a ferroelectric material results in the formation of regions of uniform polarization, i.e. ferroelectric domains. Polarization discontinuities in the vicinity of surfaces and interfaces result in polarization bound charge that significantly affects materials properties. Polarization charge can compensate Schottky barriers at the interfaces giving rise to Positive Temperature Coefficient of Resistance behavior in semiconducting BaTiO3. In the vicinity of surfaces, polarization charge results in domain specific adsorption of charged species from the ambience that effectively compensates the charge, i.e. extrinsic screening processes [1]. Screening charges thus affect the thermodynamic properties of a ferroelectric surface via a reduction in depolarization energy. Therefore, screening plays a significant role in the polarization reversal processes and, to a large extent, determines the stability of domain structures in ferroelectric materials. Another type of screening process involves band bending in the near surface region (intrinsic screening) with the formation of depletion and accumulation layers depending on the nature of majority charge carriers in the ferroelectric semiconductor. These domain specific space charge layers strongly influence local chemical activity of ferroelectric surfaces as discovered by Giocondi and Rohrer [2,3]. Studies of domain specific chemical phenomena are hindered by typical multidomain structure of the samples. Until recently, only limited progress has been achieved
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