Nanometer-scale elasticity measurements on organic monolayers using scanning force microscopy

Local elastic compliance of organic monolayers (octadecyltriethoxysilane/mica and alkanethiol/gold) has been investigated with nanometer resolution by applying a force modulation technique to an atomic force microscope. Systematic measurements were taken as a function of modulation frequency and amplitude, as well as the local environment surrounding the surface. The topography and local elasticity of the monolayers are contrasted to the bare substrate created by the tip of the atomic force microscope at high imaging force. Under ambient laboratory conditions, the Young's modulus of mica calculated from the elasticity images is lower than the organic monolayer. Such an observation is not intuitive and can be explained by the thin film of water adsorbed on mica. Water adsorption can change the microscope tip surface interaction. As a result, mica appears as a softer surface than the organic layers. In addition, the elasticity is dramatically enhanced if the modulation frequency coincides with or is close to the natural resonance frequency of the tips of the atomic force microscope. Measurements taken under liquid provide more reproducible and accurate results because the resonance frequency is damped out and capillary interactions are avoided. The measured Young's modulus is also found to increase slightly with increasing modulation amplitude.