Amplitude Modulation (AM)-AFM Spectroscopy

Dynamic force spectroscopy measurements were performed using commercial Asylum Research Cypher ES equipped with photothermal excitation. The experiments were performed in liquid in a closed cell that allows exchange of liquid and control of the temperature (which was kept around 28 °C). The cantilever was immersed in a droplet of liquid (0.2 mL) that was sandwiched between the substrate and the top of the cell. Fluid was exchanged using glass syringes connected to the tubing. The drop volume was replaced at least 25 times by the new fluid and was left to equilibrate for 30 min before starting the measurements. Force spectroscopy was performed in the amplitude modulation (AM) mode.⁴⁵⁻⁴⁷ During this procedure, the amplitude and phase of the cantilever deflection were tracked as a function of z-piezo distance. For the measurement of the tip–sample interactions, the cantilever was driven at a fixed frequency (ω ≈ 0.97ω₀, where ω₀ ≈ 450 kHz) by an intensity-modulated blue laser diode that was focused on the gold-coated topside of the cantilever close to its base. AFM measurements were performed with an oscillation amplitude of A = 0.2–0.4 nm. For each amplitude– and phase–distance curve, the cantilever was ramped over 5 nm. For each fluid composition, we typically measured 200 approach curves. A threshold value was used to limit the approach of the tip and sample. This set point was set such that a linear part in the deflection signal was observed due to the tip contacting the surface. The recorded curves were aligned on the first oscillation found in the force gradient profile and were set at 0.25 nm, where the maximum of the first oscillation is found. This sets the 0, where we find the linear region in the deflection data indicating the tip–substrate contact.