Improving the imaging speed in Atomic Force Microscopy (AFM) is of high interest due to its typically prolonged imaging duration. Conventionally, the line rate of the scan is fixed at a conservative value in order to ensure a safe tip-sample contact force even in the worst case of sample aspect ratio and linear scan speed. In this paper, an adaptive scan method is proposed to adapt the scan line rate based on the extremum-seeking control framework. A performance metric balancing both imaging speed and accuracy is proposed, and an extremum-seeking approach is designed to optimise the metric based on error feedback. Semi-global practical asymptotic stability (SGPAS) result is shown, and the proposed method is demonstrated via simulation.