Experiments were conducted using an overall gold coated contact mode cantilever (Cont-GB-C, BudgetSensors Innovative Solutions Bulgaria Ltd.) with a nominal first resonance frequency of (13 ± 4) kHz and a nominal spring constant between 0.04 N m -1 and 0.40 N m -1 . As source for photothermal excitation a mid-IR external cavity quantum cascade laser (EC-QCL) (MIRcat-QT, DRS Daylight Solutions Inc.) was used. All AFM-IR measurements were performed in resonance enhanced contact mode AFM-IR.