Abstract
This paper presents a novel feedback based Scanning Probe Microscopy method which enables quantitative surface potential measurements without the need of the DC bias of Kelvin Probe Force Microscopy. In addition to the sinusoidal excitation signal at frequency ω, a sinusoidal signal with the frequency 2ω is applied to the conductive cantilever. By modulating the amplitude of the signal at 2ω, the resulting electric force component at the frequency ω can be nullified by a feedback controller. When the force and, hence, the cantilever oscillation is zero, the required amplitude represents the quantitative surface potential. Recording this amplitude while scanning over the sample allows to acquire a two dimensional map of the surface potential. The AC-KPFM method, shown analytically and with experimental results, keeps the compensation based principle of classical KPFM, resulting in quantitative measurements but without the need of a DC bias.
Highlights
Atomic force microscopes (AFM) [1] are important tools for topography and surface characterization on the molecular and atomic level
The AC-Kelvin Probe Force Microscopy (KPFM) method proposed in this paper shows a novel approach, which enables feedback based quantitative surface potential measurements without the need of a DC-bias and calibration
A custom made experimental setup which extends commercial AFM systems from classical KPFM to AC-KPFM is presented in Section 3, showing experimental results in Section 4, followed by a conclusion
Summary
Atomic force microscopes (AFM) [1] are important tools for topography and surface characterization on the molecular and atomic level. With KPFM as presented first in [5], it is possible to map the sample surface potential in a quantitative way with nanometer resolution In the following this method is referred to as classical KPFM. A possibility to overcome this limitation is to utilize “open-loop” or “Dual Harmonic” KPFM methods, which do not require DC-bias feedback [13,14,15,16] These methods apply a single sinusoidal excitation potential sin(yt) to the cantilever and observe the resultant amplitude of vibration at the frequency y and 2y. The advantage is that no DC-bias and no feedback controller is required They require correction factors for the used excitation amplitude as well as cantilever dynamic and readout sensitivity at the frequencies y and 2y. A custom made experimental setup which extends commercial AFM systems from classical KPFM to AC-KPFM is presented in Section 3, showing experimental results in Section 4, followed by a conclusion
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