Chemical force microscopy (CFM) allows the high resolution detection of different functional groups on the surface of a sample. CFM, a recently developed AFM-technique, is based on the chemical modification of the AFM-tip surface [1,2]. By this means it is possible to adapt to different chemical spacies present on the surface. Besides the functional groups terminating the tip, also the solvent in which the measurement are carried out, strongly influences the character of the forces present between modified tip and sample. By the choice of the tip termination and the solvent, different interaction forces between uncharged surfaces can be measured. These forces include hydrophobic effect, hydrogen bonding and van der Waals forces. One of the aims of the presented study is to investigate these by measuring the adhesion forces quantitatively. In such a way, changing the solvent, it is possible to control the forces on which the detection by CFM is based.
Thiol compounds have been used to achieve self-assembly monolayers (SAMs) on gold coated cantilevers, resultingin the complete converge of the tip with functional groups. An advandage of thiols is beside the building up of a well defined monolayer, the aviability of a wide range of head groups. In the presented work examination with CH3-,CF3- and OH- terminated thiols have been carried out. By quantitave analysis of force vs. distance curves the adhesion forces between these functional groups have been determined in different solvents. The predominating forces depend strongly on the solvent. For example in water hydrophyic forces contribute almost competely to the adhesion forces, in the absence of electrostatic charges. It is demonstrated that this is a valid even in the case when only one of the involved surfaces is of hydrophobic nature. Instead for a non-polar solvent as hexadecane with low dielectric constant, hydrogen bonding between OH-terminatin areas is supported. The application of the above results is demonstrated by the detection of areas with different surface termination by chemical force microscopy. For this purpose samples were obtained by micro-contact printing of the above thiols on ultra-flat gold. The limitation of resolution achievable by CFM and the different possibilities of detection will be discussed.
References
1. C.D. Frisbie, L.F. Roznai, A. Noy, M.S. Wrighton, C.M. Lieber, Science 265, 2071, 1994.
2. S. Akari, D. Horn, H. Keller, W. Schrepp, Adv. Mat. 7, 549, 1995.