Force microscopy contrasts due to adhesion force difference between organosilane self-assembled monolayers

Chemical and mechanical properties of organosilane self-assembled monolayers (SAMs) terminated with methyl (CH3) or fluoromethyl (CF3) groups have been studied using lateral force microscopy (LFM), force curve measurement and dynamic force microscopy (DFM). The CF3-terminated SAM had large adhesive interactions with a Si tip surface, that is, the intrinsic adhesion and the capillary force effect caused by adsorbed water, than the CH3-terminated SAM. The origin of these larger tip/sample interactions of the CF3-terminated SAM ascribed to its permanent dipole moment arisen from a strong electron negativity of fluorine atoms. Furthermore, microstructures consisting of CH3- and CF3-terminated SAMs were fabricated and imaged by LFM. The micropatterns were clearly imaged through a difference in lateral force (LF) between the SAMs. Such micropatterns could be also imaged by DFM through a difference in phase lag. When a cantilever of 20 N/m was used, the CF3-terminated SAM showed the larger phase lag than the CH3-terminated SAM due to the adhesion difference. However, when a stiffer cantilever of 40 N/m was used, the CF3-terminated SAM showed the smaller phase lag than CH3-terminated SAM. These experimental results show that the phase lag contrasts are not always the same, that is, the contrast depends on the spring constant of cantilevers.