a1 Department of Materials Science and Engineering, National Tsing Hua University, HsinChu 300, Taiwan
a2 Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310012, China
a3 Building and Fire Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
a4 Department of Materials Science and Engineering, National Tsing Hua University, HsinChu 300, Taiwan
The relationship between atomic force microscopy probe-sample adhesion force and relative humidity (RH) at five different levels of surface free energy (γs) of an organic self-assembled monolayer (SAM) has been investigated. Different γs levels were achieved by exposing a patterned SiO2/CH3-terminated octyldimethylchlorosilane SAM sample to an ultraviolet (UV)/ozone atmosphere. A model consisting of the Laplace-Kelvin theory for capillary condensation for nanosized probe and probe-sample molecular interaction was derived to describe the adhesion force as a function of RH from 25 to 90% for different SAM γs values. The equations were solved analytically by using an equivalent curvature of the probe tip shape. Experimental results show that the adhesion force increases slightly with RH for nonpolar SAM. However, for polar SAM surfaces, it increases at first, reaches a maximum, and then decreases. Both the rate of increase and the maximum of the adhesion force with humidity are γs-dependent, which is in good agreement with theoretical prediction. The large rise in the adhesion force in this RH range is due to the capillary force.
(Received August 04 2009)
(Accepted November 30 2009)