Measurement of electrostatic tip–sample interactions by time-domain Kelvin probe force microscopy

Abstract

Kelvin probe force microscopy is a scanning probe technique used to quantify the local electrostatic potential of a surface. In common implementations, the bias voltage between the tip and the sample is modulated. The resulting electrostatic force or force gradient is detected via lock-in techniques and canceled by adjusting the dc component of the tip–sample bias. This allows for an electrostatic characterization and simultaneously minimizes the electrostatic influence onto the topography measurement. However, a static contribution due to the bias modulation itself remains uncompensated, which can induce topographic height errors. Here, we demonstrate an alternative approach to find the surface potential without lock-in detection. Our method operates directly on the frequency-shift signal measured in frequency-modulated atomic force microscopy and continuously estimates the electrostatic influence due to the applied voltage modulation. This results in a continuous measurement of the local surface potential, the capacitance gradient, and the frequency shift induced by surface topography. In contrast to conventional techniques, the detection of the topography-induced frequency shift enables the compensation of all electrostatic influences, including the component arising from the bias modulation. This constitutes an important improvement over conventional techniques and paves the way for more reliable and accurate measurements of electrostatics and topography.

A simple KPFM-based approach for electrostatic- free topographic measurements: the case of MoS2 on SiO2

“…Its implementation requires only using a few basic analog circuits to synchronously maintain the compensation bias and switch off the modulation voltage; it should also be easy to develop numeric counterparts directly integrated in last generation digital scanning probe microscope controllers. Like in the case of graphene [15], we have shown that conventional nc-AFM/KPFM measurements cannot assess correctly the stacking height of 2D TMD deposited on silicon-oxide substrates. This further highlights just how the variations of the tip-sample capacitance can affect the topographic measurement, despite the CPD compensation by the KPFM controller.…”

“…From what precedes, it is also clear that the question of electrostatic-related artefacts in dynamic AFM has long been overlooked by the AFM community. Only very recently, a few teams have rightly pointed out that electrostatic forces can misleadingly influence the stacking height measurement of 2D materials by AFM [14,15]. To address this problem, one may be tempted to apply an active KPFM loop to compensate the tip-surface potential difference.…”

“…In a recent work [15], Ritz and co-workers have developed a Kalman-filter based methodology that allows estimating the electrostatic influence due to the applied voltage modulation on the cantilever frequency shift. The imaging can then be performed on the basis of the sole topographyinduced part of the frequency shift.…”

Detection of a Chirality-Induced Spin Selective Quantum Capacitance in α-Helical Peptides

“…For the following experiments, open loop time-domain KPFM 44 is applied. In contrast to the FM-KPFM 19 configuration in Figure 1b…”

“…For the following experiments, open loop time-domain KPFM 44 is applied. In contrast to the FM-KPFM 19 configuration in Figure 1 b–d,f–h, where a phase-locked loop controls the cantilever phase response, the time-domain KPFM technique directly records the phase-time series.…”