Nonlinear optical studies of monomolecular films under pressure

Abstract

Nonlinear optical techniques (second-harmonic and sum-frequency generation) have been used to study the structure of organic molecules that are confined and compressed between a lens and a flat surface. The molecules studied include self-assembled monolayers of n-octadecyltriethoxysilane and Langmuir-Blodgett films of stearic acid, octadecylalcohol, octadecylamine, and a liquid-crystal molecule 4'-n-octyl-4-cyanobiphenyl (8CB). The contact area created by elastic deformation of the flat surface and lens under pressure was large enough to contain the entire laser beam (& 100 pm radius at = 10 MPa for R =15 cm). Under these conditions, the sum-frequency generation (from CH3 and OH stretch modes) and second-harmonic generation (8CB) signals were found to decrease by a factor between 100 and 1000 times the original signal. This indicates vanishing of the second-order monolayer susceptibility due to disorder of the head groups and/or flattening of the molecular axis so that they lie parallel to the surface. The phenomenon was reversible and the nonlinear signals recovered completely upon removal of the pressure.

Design of surface forces apparatus for tribology studies combined with nonlinear optical spectroscopy

“…In our laboratory, a primitive SFA, consisting of a quartz sphere and optical flat, has been used to study orientation, conformation, and molecular vibrations of a variety of Langmuir-Blodgett ͑L-B͒ and SAM films under pressure. 10 Under a pressure of 50 MPa, the SFG and SHG signals were found to decrease by a factor of 100-1000 times the original signal. This indicated a vanishing of the second-order monolayer susceptibility due to disorder of the head groups and/or flattening of the molecular axis so that they lie parallel to the surface.…”

“…7 It has recently been demonstrated that SHG and SFG are ideal tools to study buried interfaces. 9,10 These techniques can be readily adopted to study molecular films under static pressure to provide molecular orientation or conformational information. In our laboratory, a primitive SFA, consisting of a quartz sphere and optical flat, has been used to study orientation, conformation, and molecular vibrations of a variety of Langmuir-Blodgett ͑L-B͒ and SAM films under pressure.…”

Raman Scattering from Confined Liquid Films in the Sub-Nanometre Regime

“…The most likely origin of the Raman signal in all these systems is liquid trapped in surface defects that are not flattened by the application of pressure. Pits and scratches have been observed on optical surfaces used for SFG analysis of confined boundary lubricants [41]. Third, the Raman spectra of the trapped material did not differ from that of the bulk liquid; there was no confinementinduced ordering.…”

Effects of Self-Assembled Monolayer and PFPE Lubricant on Wear Characteristics of Flat Silicon Tips

“…It has been reported that SAM could be damaged at a pressure of \1 GPa at the nano-scale [46,47]. However, the critical pressure at which SAM is destroyed is difficult to assess since it varies according to the contact area and also because SAM layer includes defects or disorders that are hard to characterize [48][49][50]. When the contact area increases, the number of defects or disorders involved in the force interaction is likely to increase as well.…”