AVS 52nd International Symposium
    Surface Science Tuesday Sessions
       Session SS2-TuA

Paper SS2-TuA6
Correlation of Frictional and Thermal Properties with Molecular Chain Order in Self-Assembled Monolayers of Organosilanes on Silicon

Tuesday, November 1, 2005, 3:40 pm, Room 203

Session: Tribology, Adhesion and Friction
Presenter: S. Sambasivan, National Institute of Standards and Technology
Authors: S. Sambasivan, National Institute of Standards and Technology
S. Hsieh, National Institute of Standrads and Technology
D.A. Fischer, National Institute of Standards and Technology
S. Hsu, National Institute of Standards and Technology
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n-Alkyltricholorosilanes films with different chain lengths (Cn films where n=5-30) were characterized by Near edge x-ray absorption fine structure (NEXAFS), FTIR and AFM. The information afforded from these complementary techniques has provided the unique opportunity to interpret the frictional properties of the monolayer films in relation to the molecular assembly and chain lengths. In this study, we use the non-destructive x-ray absorption technique NEXAFS to quantitatively measure the surface (~6nm) molecular orientation of SAMs (self-assembled monolayers). We observe that the chain lengths having 12, 16 and 18 carbon atoms are highly oriented with a preferential molecular orientation of the polymeric C-C chains perpendicular to the surface. C5 and C30 SAMs did not exhibit preferential orientation of the alkyl chain and C10 showed partial ordering. Complimentary FTIR studies were done to estimate order qualitatively by peak positions of asymmetric CH@sub 2@ and the symmetric CH@sub 2@ stretches. The molecular order information from FTIR followed similar trends as determined by NEXAFS. The frictional properties of the organic monolayers were determined through the simultaneous measurement of normal (load) and lateral (friction) interfacial forces with AFM. Friction measurements on different chain lengths follows similar trends with surface order calculated from NEXAFS. In-situ heating and NEXAFS experiments on C16 films reveal that C16 film were well ordered up to 200°C and heating beyond that temperature it transitions from partially ordered to completely disordered/destroyed at 350°C. AFM images show a significant change in surface morphology after annealing. Friction force measurements indicated an increase after heating to 350°C, values approaching close to the friction forces from bare Si wafer. Our results show that the degree of ordering in the SAM governs the friction properties of the film.