Contrast Mechanism Maps for Piezoresponse Force Microscopy
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Piezoresponse force microscopy (PFM) is one of the most established techniques for the observation and local modification of ferroelectric domain structures on the submicron level. Both electrostatic and electromechanical interactions contribute at the tip-surface junction in a complex manner, which has resulted in multiple controversies in the interpretation of PFM. Here we analyze the influence of experimental conditions such as tip radius of curvature, indentation force, and cantilever stiffness on PFM image contrast. These results are used to construct contrast mechanism maps, which correlate the imaging conditions with the dominant contrast mechanisms. Conditions under which materials properties can be determined quantitatively are elucidated.
In recent years, piezoresponse force microscopy (PFM)1 has been successfully employed in the characterization and modification of ferroelectric surfaces on the micron and nanometer level. In PFM, a conductive tip is brought into contact with the surface, and an alternating current (ac) bias, Vtip ⳱ Vdc + Vaccos(t), is applied to the tip. The piezoelectric response of the underlying surface is detected as the first harmonic component A of the bias-induced tip deflection d ⳱ d0 + Acos(t + ). The phase yields information about the polarization direction below the tip. For domains with polarization vectors pointing downward, the application of a positive tip bias results in the expansion of the sample, and surface oscillations are in phase with the tip voltage, ⳱ 0. For domains with polarization vector pointing upward, ⳱ 180°. [Often the piezoresponse image is collected as x-Signal Acos().] The piezoresponse amplitude PR ⳱ A/Vac defines the local electromechanical activity of the surface and in the early treatments was assumed to be equal or proportional to the piezoelectric constant d33 of the material. Numerous observations of local domain dynamics as related to polarization switching, fatigue, phase transitions, etc., have been made.2–13 Especially of interest are spectroscopic variants of PFM, in which ramping of direct current (dc) voltage offset on the tip Vdc allows local hysteresis loops to be acquired. A determination of local ferroelectric properties, including hysteresis, stress, and size effects, requires quantitative interpretation of the PFM interactions. Both longrange electrostatic forces and the electromechanical response of the surface contribute to the PFM signal so that the experimentally measured piezoresponse is 936
http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 09 Feb 2015
A ⳱ Ael + Apiezo + Anl, where Ael is electrostatic contribution, Apiezo is electromechanical contribution and Anl is nonlocal contribution due to capacitive cantilever– surface interactions.12 Quantitative PFM imaging requires Apiezo to be maximized to achieve predominantly electromechanical contrast. Contrast in PFM strongly depends on the appropriate choice of the probe, particularly on the cantilever spring constant and tip materia
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