Figure (a) shows a cross-sectional 2D Δƒ map obtained at the interface after spin-coating and settling for 29 h at room temperature. ∆ƒ-z curves obtained along line AB in (a) were shown in Figure (b). The curves acquired during the tip approach and retraction are shown in red and blue, respectively. Note that all the ∆ƒ-z curves in this paper were averaged over 25 adjacent curves to improve the signal-to-noise ratio and further smoothed with a moving average filter. In addition, the amplitude error of the sensor deflection was successfully kept to less than ±10 %, and no oscillatory error was observed in amplitude-z curves for all experiments.