AVS 52nd International Symposium
    Nanometer-Scale Science and Technology Monday Sessions
       Session NS1-MoA

Paper NS1-MoA8
Understanding the Frictional Response of Organic Monolayer Coatings using Atomic Force Microscopy

Monday, October 31, 2005, 4:20 pm, Room 204

Session: Nanotribology
Presenter: E.E. Flater, University of Wisconsin-Madison
Authors: E.E. Flater, University of Wisconsin-Madison
R.W. Carpick, University of Wisconsin-Madison
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Motivated by the lack of fundamental understanding of friction, and that friction and wear are major limiting factors for surface micromachined devices, we use atomic force microscopy (AFM) to determine the nanoscale frictional properties of alkylsilane monolayers commonly used in these microscale devices to reduce adhesion and friction. Quantitative single asperity measurements of friction and contact stiffness are performed using SiO@sub 2@- and monolayer-terminated AFM tips on monolayer-terminated silicon. Using octadecyltrichrolosilane (OTS) and fluorinated monolayers (FOTAS), we observe that friction for these systems depends on the type of molecule and its packing properties. For the OTS monolayer there are two phases evident; the liquid condensed phase shows measurably lower friction at low loads than the liquid expanded phase, demonstrating that lower friction is associated with higher molecular packing density. However, the films exhibit the same frictional response at higher loads, indicating that compressed forms of both phases are tribologically equivalent. For FOTAS monolayer interfaces, fluorination increases friction at most loads, in agreement with measurements of micromachined interfaces. We find that frictional variation, as well as overall friction and adhesion for monolayer-coated tips are significantly reduced as compared with uncoated tips. Thus, the behavior of uncoated tips is highly dependent on the scanning history. This has wider importance for the acquisition of reproducible AFM nanotribology measurements in general, and strongly suggests that AFM researchers should strive to characterize and control the chemistry and structure of their tips to obtain meaningful measurements. We acknowledge Robert Ashurst for preparing the OTS samples, and Maarten de Boer and Alex Corwin for providing the FOTAS samples and microscale friction coefficients.