Abstract :
The effects of surface functionality and relative humidity (RH) on nanomechanical contact stiffness were investigated using atomic force
acoustic microscopy (AFAM), a contact scanned-probe microscopy (SPM) technique. Self-assembled monolayers (SAMs) with controlled surface
energy were studied systematically in a controlled-humidity chamber. AFAMamplitude images of a micropatterned, graded-surface-energy SAM
sample revealed that image contrast depended on both ambient humidity and surface energy. Quantitative AFAM point measurements indicated
that the contact stiffness remained roughly constant for the hydrophobic SAM but increased monotonically for the hydrophilic SAM. To correct for
this unphysical behavior, a viscoelastic damping term representing capillary forces between the tip and the SAM was added to the data analysis
model. The contact stiffness calculated with this revised model remained constant with RH, while the damping term increased strongly with RH for
the hydrophilic SAM. The observed behavior is consistent with previous studies of surface energy and RH behavior using AFM pull-off forces. Our
results show that surface and environmental conditions can influence accurate measurements of nanomechanical properties with SPM methods
such as AFAM.
