Effect of humidity on nanoscale adhesion on self-assembled thiol monolayers studied by dynamic force spectroscopy

The adhesion force between silicon nitride tips of an atomic force microscope and different self-assembled thiol monolayers (SAMs) was measured at different loading rates and humidity. SAMs were formed from HS(CH(2))(n)CH(3) with n = 6, 8, 9, 10, 15 and HS(CH(2))(n)OH with n = 6, 9, 11, 16. With a special setup, the loading rate could be increased to 10(7) nN s(-1). For the interaction with two-dimensional crystalline CH(3)-terminated SAMs (n > or = 8), two regimes can be distinguished. At loading rates below 10(4)-10(5) nN s(-1), the adhesion force increased proportional to the logarithm of the loading rate. Adhesion is most likely dominated by van der Waals attraction. At higher loading rates, the adhesion forces increased steeper with the logarithm of the loading rate. The specific process limiting separation is not yet identified. On OH-terminated SAMs, the adhesion force was approximately 6 times higher than on the CH(3)-terminated SAMs, even at low humidity. This can partially, but not fully, be explained by hydrogen bridges forming between the hydroxyl groups of the monolayer and silanol groups of the tip. For relative humidity above 10%, the capillary force further increased the adhesion force, which reached a maximum at values of relative humidity between 40% and 80%. Adhesion force versus loading rate (F(ad) versus r(F)) curves increased roughly linearly over the whole range of loading rates. The slope depended on the humidity, and it is correlated with the absolute strength of the capillary force.