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The Atomic Force Microscopy Resource Library


Electric Force Microscopy and Kelvin Force Microscopy with Agilent MAC Mode III

Electric Force Microscopy (EFM) and Kelvin Force Microscopy (KFM)6 are atomic force microscopy (AFM) techniques in which a conductive AFM tip interacts with the sample according to the sample’s electrostatic characteristics. EFM is an imaging technique that maps, at each in-plane coordinate pair (X,Y), the out-of-plane (Z) gradient of the electric field emanating from the sample surface. KFM, also an imaging technique, maps the variation of the contact potential between the tip and the sample at each in-plane position. (KFM is also known as surface potential imaging.) Agilent’s MAC Mode III control electronics allows truly-simultaneous recording of topography and either EFM or KFM images in a single pass, that is, without the time-consuming process of having to scan twice (two-pass scanning), once for topography and once for the electrical image. MAC Mode III enables this by incorporating three independently-controlled lock-in amplifiers, one of which is dedicated to the electrical measurement at the same time that another one is used for topography imaging. This arrangement allows the user to choose the frequencies at which the two lock-in amplifiers operate independently from each other, increasing the operational freedom for electrical experiments.7 The simultaneous measurement scheme, obviating the need for two-pass scanning, also eliminates the adverse effects on the fidelity of the electrical image that come about due to the drift that the scanner may suffer during the second pass of a two-pass implementation.

6 Kelvin Force Microscopy is based on the original work of Sir William Thomson (later, Lord Kelvin), who demonstrated that when two dissimilar conductors are brought into contact, an electric potential develops across the junction. This potential is known as the contact potential difference (CPD), and is determined by work functions of the two materials, which in turn depends on the environment in which the materials are and the contact is made. But in the original Kelvin method and in Kelvin Force Microscopy, the CPD measurement is performed absent perpetual contact between the two conductors. See for example, the article by M. Nonnenmacher et. al., in Applied Physics Letters, Volume 58(25), pp. 2921-2923, June 24, 1991.

7 For example, the electric signal may be monitored at a frequency twice that of the mechanical signal that drives the AFM cantilever (typically at or near the cantilever’s fundamental resonance frequency), and this provides a measure of the gradient of the tip-sample capacitance (C) with z, or dC/dz.

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