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 micropattems 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. (C) 2002 Elsevier Science B.V. All rights reserved.