In Situ Determination of the Thermodynamic Surface Properties of Chemically Modified Surfaces on a Local Scale:  An Attempt with the Atomic Force Microscope

Abstract

We have monitored deflection-distance curves with an atomic force microscope (AFM) in contact mode, with a silicon nitride tip, on chemically modified silicon wafers, in the air. The wafers were modified on their surface by grafting self-assembled monolayers (SAMs) of different functional groups such as methyl, ester, amine, or methyl fluoride. A chemically modified surface with a functionalized hydroxyl group was also considered. Qualitative analysis allowed us to compare adhesive forces versus chemical features and surface energy. The systematic calibration procedure of the AFM measurements was performed to produce quantitative data. Our results show that the experimentally determined adhesive force or thermodynamic work of adhesion increases linearly with the total surface energy determined with contact angles measured with different liquids. The influence of capillary condensation of atmospheric water vapor at the tip-sample interface on the measured forces is discussed. Quantitative assessment values were used to determine in situ the SAM-tip thermodynamic work of adhesion on a local scale, which have been found to be in good agreement with quoted values. Finally, the determination of the surface energy of the silicon wafer deduced from the thermodynamic work of adhesion is also proposed and compared with the theoretical value.

Forces between Polymer Surfaces and Self-Assembled Monolayers

“…For the hydrophilic monolayers, the following order of surface free energy is found: MUA> HUT> AUT. The measured contact angles and surface free energies are in general agreement with related literature values (17)(18)(19)(20).…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Work of adhesion between a Si 3 N 4 AFM tip and methyl (CH 3 ), amine (NH 2 ), methyl fluoride (CF 3 ), and ester (CO(OCH 3 )) end-group SAMs, as well as hydroxyl (OH) functionalized surfaces showed quantitative agreement with contact angle measurements of the surface energies [371]. Comparison with the work of adhesion on soft polymer surfaces with same surface chemistry allowed separating chemical and mechanical contributions to the work of adhesion [372].…”

SFM study of the surface of halogen-bonded hybrid co-crystals containing long-chain perfluorocarbons

“…Concerning the large adhesion forces measured in type A curves (typically z100 nN; Fig. 3b), at first glance this could seem an unexpected result for surfaces given by PFCs and their derivatives 25,[29][30][31] being typically strongly hydrophobic. However, FS curves acquired on the pure perfluoroalkane 4 exhibit the same shapes and adhesions (not shown).…”