Self-assembled monolayers (SAMs) have increasingly being explored as potential protective films in devices friction and adhesion. However, accurate determination of the nanomechanical properties of monolayer films is difficult and fraught with uncertainties, which make the data interpretation uncertain. This paper addresses this issue by utilizing Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy with Fourier Transformed infrared spectroscopy to determine the film order of a series of SAMs with different chain lengths in order to understand nanofriction measurements. A series of n-Alkyltrichlorosilanes self-assembled monolayer films with various chain lengths (C5 to C30) were prepared on silicon surfaces. Friction measurements were made using an Atomic Force Microscope (AFM). Results showed that C12 film exhibited an optimum friction value contrary to long established observations of lower friction associated with longer chain length in macro-test results. To explain these observations, we use x-ray absorption technique NEXAFS to quantitatively measure the surface molecular orientation (order) of these films. We observe that C12, C16, and C18 films highly ordered with a molecular orientation of the carbon backbone perpendicular to the surface. C5 and C30 films were less oriented and C10 film showed partial ordering. Complimentary FTIR results using peak positions of asymmetric CH2 and the symmetric CH2 stretches also confirmed that these films had different degrees of order. Our results show that the degree of ordering in the SAM films affects the frictional properties of these films.
Citation: Journal of Vacuum Science and Technology A
Pub Type: Journals
FTIR, nanofriction, NEXAFS, orientation, SAM