Amplitude dependence of image quality in atomically-resolved bimodal atomic microscopy

TL;DR

This study investigates how the combined oscillation amplitudes in bimodal AFM affect image quality, revealing an optimal amplitude range for high SNR in ambient conditions due to water layer effects.

Contribution

It demonstrates the correlation between sum amplitude and image quality in bimodal AFM and identifies the optimal amplitude range considering hydration layer influences.

Findings

Image quality peaks when sum amplitude is just below hydration layer thickness.

SNR drops significantly when sum amplitude exceeds approximately 200 pm.

Water layers in ambient conditions screen interactions, affecting imaging performance.

Abstract

In bimodal FM-AFM, two flexural modes are excited simultaneously. The total vertical oscillation deflection range of the tip is the sum of the peak-to-peak amplitudes of both flexural modes (sum amplitude). We show atomically resolved images of KBr(100) in ambient conditions in bimodal AFM that display a strong correlation between image quality and sum amplitude. When the sum amplitude becomes larger than about 200 pm, the signal-to-noise ratio (SNR) is drastically decreased. We propose this is caused by the temporary presence of one or more water layers in the tip-sample gap. These water layers screen the short range interaction and must be displaced with each oscillation cycle. Further decreasing the sum amplitude, however, causes a decrease in SNR. Therefore, the highest SNR in ambient conditions is achieved when the sum amplitude is slightly less than the thickness of the primary hydration layer.