Selective Probing of Thin-Film Interfaces Using Internal Reflection Sum-Frequency Spectroscopy

Abstract

The study of interfacial properties of thin films such as polymers is an important area of surface science. The application of visible-infrared sum-frequency generation spectroscopy to such systems requires a careful interpretation of the results, as the electric field magnitude and phase at each interface must be determined in a manner that takes thin film interference effects into account. Several schemes have been proposed for handling the local field corrections, and these methods all have their origins in linear optics. We first provide an extensive discussion of the cases in which the film is sufficiently thick that multiple beam interference can be ignored, or sufficiently thin in which the relevant expressions collapse to simple forms. Then we illustrate a straightforward method that has a concise analytic solution in the case of a single thin film that exhibits interference effects. We demonstrate a visualization technique that allows the experimental geometry to be tuned to select the interface of interest, and rapidly switch between the interfaces when the film thickness is chosen to accommodate this.

Algae Adhesion onto Silicone is Sensitive to Environment-Induced Surface Restructuring

“…This break in inversion symmetry is what generates the SFG signal. Furthermore, the selection of the film thickness and angles of incidence restricts our probe region to the film–solution interface . A set of PDMS films was spin coated onto the surfaces of dove prisms and then subjected to the same soaking conditions (DI water and 0.9 M NaCl) as the films prepared on microscope slides.…”

“…A subset of the polymer films were soaked in either DI water (Milli-Q) or 5% wt/wt NaCl solution for 30 d. After removing the slides from the baths, the PDMS surface was rinsed with DI water and air dried for 20 min. The PDMS films for SFG experiments were spin coated at 4000 rpm for 3 min on the surface of IR-grade silica prisms using the same 10:1 ratio of base to curing agent but at a lower concentration (5% w/w PDMS in chloroform) to obtain a thickness of 370 ± 20 nm as measured by Raman spectroscopy …”

“…Vibrational SFG measurements were performed using a picosecond laser (Ekspla PL2241), scanning from 2820–3020 cm –1 using a parametric generator (Ekspla PG501) with an s-polarized visible beam (100 μJ per pulse, bandwidth < 1 cm –1 ) and a p-polarized IR beam (300 μJ per pulse, 5 cm –1 bandwidth) and selecting the s-polarized component of the resulting SFG beam. We have recently demonstrated that, by precisely controlling the film thickness and beam angles in an internal reflection geometry SFG experiment, one can selectively probe either the PDMS–glass substrate or the PDMS–environment (in this case, solution) interface . Incident angles of θ vis = 68° and θ IR = 78° were chosen for this purpose.…”

“…The PDMS films for SFG experiments were spin coated at 4000 rpm for 3 min on the surface of IR-grade silica prisms using the same 10:1 ratio of base to curing agent but at a lower concentration (5% w/w PDMS in chloroform) to obtain a thickness of 370 ± 20 nm as measured by Raman spectroscopy. 30 PDMS Surface Characterization. Polymer film thickness and RMS roughness values were characterized by profilometry (Bruker Dektak XT).…”

Investigating Thin Silicone Oil Films Using Four-Wave Mixing Spectroscopy and Sum Frequency Generation Vibrational Spectroscopy

“…There is a need to develop an ultrasensitive technique which could be used to study the distribution of ultrathin silicone oil layers on surfaces. In recent decades, second-order nonlinear optical spectroscopic techniques such as sum frequency generation (SFG) vibrational spectroscopy and second-harmonic generation (SHG) have been applied to study molecules (including proteins) on surfaces and at interfaces. − Such research demonstrated that second-order nonlinear optical spectroscopic techniques are very promising in detecting monolayer and even submonolayer molecules at interfaces in situ. − SFG has been applied to examine the molecular structures of surfaces and interfaces involving solid silicone and silicone oil to understand important interfacial properties such as adhesion, friction, antifouling, and fouling release. − In particular, SFG has been applied to study thin films of silicone on water surfaces . SFG provides chemical and orientational information on the molecular structure of chemical groups at interfaces.…”