Imaging nanoscopic elasticity of thin film materials by atomic force microscopy: Effects of force modulation frequency and amplitude

Octadecyltriethoxysilane (OTE) monolayers on mica are imaged using force mo dulation microscopy (FMM) and dynamic force modulation (DFM). Nanoscopic ar eas of mica are produced within OTE layers and serve as an internal standar d for contact stiffness measurements. The contact stiffness is systematical ly studied as a function of imaging medium and force modulation amplitude a nd frequency. Measurements taken in liquid media are found to reflect more accurately the viscoelastic properties of the sample, while imaging in air is perturbed by the capillary neck at contact. Increasing modulation amplit ude increases the overall signal in FMM. However, extremely large amplitude s diminish the contrast difference between OTE and mica because the sensing depth is much higher than the monolayer. The measured contact stiffness is found to depend sensitively upon the modulation frequency because of the p resence of several resonances within 10-50 kHz, which cause the image contr ast to vary or to flip. Collective motion of the molecules under contact is most likely responsible for these resonances. The observed amplitude and f requency dependence also allows active control of FMM image contrast.