  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.
