Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.312
Publication date: 18th July 2019
We give an overview of advanced multifrequency techniques for atomic force microscopy (AFM). These techniques exploit the nonlinear tip-surface interaction to perform electrical measurements with high speed and low noise.
Intermodulation electrostatic force microscopy (ImEFM) [1] is an open-loop alternative to Kelvin-probe force microscopy (KPFM), where the surface potential of the surface is obtained from a measurement of four force components within the cantilever resonance frequency. This single-pass technique combines the low noise of AM-KPFM with the high spatial resolution of FM-KPFM, while maintaining the speed and ease of use of an open-loop technique. We have recently developed a time-resolved variant of ImEFM [2], where the intermodulation of the cantilever drive and a series of excitation pulses on the sample produce a force at multiple frequencies around resonance. By measuring these force components we demonstrate the reconstruction of dynamic processes in the material with time resolution of 30 nanoseconds.
Intermodulation conductive AFM (ImCFM) [3] is a technique to acquire the full current-voltage characteristic (IVC) at every pixel of an AFM image. The AFM is operated in contact mode with an AC bias applied to the sample, and the current flowing through the tip is measured. A compensation voltage is used to cancel the effect of the parasitic capacitance arising from the measurement setup, while the frequency-domain analysis allows for the complete separation of the remaining displacement contribution to the current that flows through the tip-sample junction. We demonstrate high-resolution electrical characterization at imaging speeds normal for contact mode, with a speedup of up to four orders of magnitude, compared to the traditional way of slowly ramping the bias at a grid of points. In addition to acquiring the IVC, the technique maps the voltage dependence of the tip-sample capacitance, allowing for the investigation of effects such as quantum capacitance in two-dimensional materials.
We give an introduction to the theoretical and technical foundations of these techniques, and show experimental results on a variety of energy materials.
The authors acknowledge financial support from the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation.
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