Imaging Mechanism and Quantification of Scanning Probe Microscopies on Ferroelectric Surfaces
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Imaging Mechanism and Quantification of Scanning Probe Microscopies on Ferroelectric Surfaces Sergei V. Kalinin and Dawn A. Bonnell Dept. Mat. Sci. Eng., University of Pennsylvania, 3231 Walnut St, Philadelphia, PA 19104 ABSTRACT In the last few years a wide spectrum of non-contact, intermittent contact and contact scanning probe microscopies have been applied to imaging ferroelectric surfaces. The imaging mechanism in non-contact SPM is ultimately related to the total charge distribution on the ferroelectric surface, including both polarization and screening charges. Contact voltage modulation (piezoresponse) force microscopy (PFM) is sensitive to both local polarization via electromechanical coupling and surface charge via capacitive interactions. In the present research we analyze the electrostatic and electromechanical contrast in PFM using analytical solutions for the electrostatic sphere-dielectric plane problem and for the piezoelectric indentation problem. The contribution of electrostatic forces to the image is estimated. Variable-temperature PRI imaging of domain structures in BaTiO3 is performed and the temperature dependence of the piezoresponse is compared with the Ginzburg - Devonshire theory. INTRODUCTION In the recent years, scanning probe microscopy (SPM) based techniques have been successfully employed in the characterization of ferroelectric surfaces on the micron and submicron level [1]. The primary SPM techniques used are variants of non-contact electrostatic SPM such as Electrostatic Force Microscopy (EFM), Scanning Surface Potential Microscopy (SSPM) and contact techniques such as Piezoresponse Force Microscopy (PFM). Both SSPM and PFM are based on voltage modulation, i.e. during imaging the piezoelectric actuator driving the cantilever is disengaged and an ac bias is applied directly to a conductive tip. In PFM the tip is brought into contact with the surface and the piezoelectric response of the surface is detected as the first harmonic component of the bias-induced tip deflection. In SSPM the tip is held at a fixed distance above the surface (typically 10-100 nm) and the first harmonic of the electrostatic force between the tip and the surface is nullified by adjusting the constant bias on the tip. A detailed analysis of EFM and SSPM imaging on ferroelectric surfaces is given by Kalinin and Bonnell [2]. Contrast formation mechanism in PFM is less understood [3-6]. Luo et al [7] found that the temperature dependence of piezoresponse contrast is similar to that of the spontaneous polarization. This behavior was attributed to a dominance of electrostatic interactions due to the presence of unscreened polarization bound charge as proposed by Hong et al [8], since electromechanical response based on the piezoelectric coefficient, d33, would diverge in the vicinity of the Curie temperature. This hypothesis is also reinforced by the observations of the response on nonpiezoelectric surfaces [9] and the existence of a nulling potential that allows implementation of nanopotentiometry techniques [10].
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