Thermal stability behavior of 1H, 1H,2H,2H-perfluorooctyl trichlorosilane self-assembled on aluminum substrates is characterized using a grazing-angle Fourier transform infrared spectrometer, Fourier transform-Raman spectroscopy, and contact angle measurements. The self-assembled monolayer (SAM) is heated quasi-statically from room temperature to above 633 K with a heating rate of 1 K/s. Variations in peak frequencies, integrated areas of intensities of symmetric and antisymmetric CF2 stretches, and the relative tilt angle of the SAM are reported. We find that the conformational order in the SAM is not disrupted because of thermal cycling when the peak temperature is below 423 K. When the peak temperature is between 423 and 603 K, the cycling results in only a partial retention of the original order. When the peak temperature is above 603 K, the process is completely irreversible. Surface-enhanced Raman spectroscopy and contact angle measurements support these observations. We confirm these trends for a longer chain (1H,1H,2H,2H-perfluordecyl trichlorosilane) molecule of the same family using the same techniques. We discuss the possible reasons for the changes in light of the tilting-untilting and uncoiling-coiling of the helical silane monolayer.

We have measured the frictional responses of two silane molecules (Octadecyltrichlorosilane; OTS and 1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane; FOTS) self-assembled on an aluminium substrate in their non heat-treated and heat-treated states. We varied the contact length scale by performing the experiments in atomic force microscope, nanotribometer and macro scale pinon-disc machine. We have found that conformational disorder and molecular stiffness have major influences on friction in strongly distinguishing the response of OTS from that of the FOTS molecules. The frictional identity between the two molecules before and after heat treatment, though somewhat contrary to that suggested from purely surface energy considerations, is found to hold over six orders of contact length scales, ten orders of normal load and in three vastly different contact conditions; (a) the probe penetrates about 25% of the molecular backbone of a SAM; no substrate deformation, (b) the contact of the probe with the SAM is conformal and the substrate is plastically deformed and (c) the molecule is dispersed in 5% (v/v) concentration in n-hexadecane to lubricate a pin-on-disc contact.

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We use a displacement-controlled dual double-cantilever-based surface-force-type apparatus to dynamically probe perfluorooctyl trichlorosilane monolayers self-assembled on aluminum and silicon substrates of 1 nm and 0.7 nm rms ͑root mean square͒ roughness, respectively, using a 1.12 mm diameter ruby sphere of 0.25 nm rms roughness. We record stiffness and damping constant as a function of compression load and deconvolute the elastic modulus using contact mechanical formulations. When mechanical intervention is limited to the terminal end of the molecule there is a strong viscous response and a low level of elastic response in consonance with the ability of the molecule to generate conformational defects freely. When the intervention penetrates into the molecular backbone the damping disappears dramatically and the molecule registers at a contact mean pressure of about 0.2 GPa a monotonic and steep rise in elastic resistance in response to further intervention by the probe. We offer a physical explanation of the process and describe this change as due to a phase transition from a liquidlike to a solidlike state as indicated by a large increase in relaxation time constant.

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