Scanning force microscopy study of patterned monolayers of alkanethiols on gold. Importance of tip-sample contact area in interpreting force modulation and friction force microscopy images

Several patterned monolayers of alkanethiols CH3(CH2)(n-1)SH on a polycrystalline Au substrate were prepared by using microcontact printing and solution deposition methods, and their surfaces were examined by IR spectroscopy, scanning force microscopy, lateral force microscopy (LFM), and force modulation microscopy(FMM). Our work Shows that LFM and FMM can detect differences in packing density of chemically identical molecules which are too small to be detected by IR, ellipsometry, and wetting measurements and suggests that the tip-sample contact area is an important parameter governing the contrasts of LFM and FMM images. Stiffness images obtained with FMM depend on changes in the Young's modulus of a sample surface as well as in the tip-sample contact area. As a result, a surface region of small modulus can have a large stiffness due to its large contact area.