Infrared spectroscopic ellipsometry (IRSE) is a powerful optical probe of various chemical and physical properties of molecules adsorbed onto solid surfaces. In particular, IRSE can be useful for detecting adsorption-induced changes in the IR spectra of self-assembled monolayers (SAMs), and unlike traditional IR absorption spectroscopies, IRSE provides useful information about the phase of the reflected radiation from the SAMs. However, in the standard IRSE experimental geometry using flat substrates for SAMs, the detectable signal containing these phase data is considerably weaker than the corresponding absorbance data. In our present work, we demonstrate that enhancing the local optical fields at the sample surface through the use of a disordered Au nanoisland substrate can substantially increase both these absorbance and phase signals. We also demonstrate how this surface-enhanced infrared spectroscopic ellipsometry (SEIRSE) can be utilized for straightforward analysis of absorption peak widths, as well as to obtain information about the orientation of the terminal methyl on adsorbed SAMs. As model SAMs for this study, we use undecanethiol (UDT) containing 10 CH(2) units and a terminal CH(3) group, as well as dodecanethiol (DDT) containing 11 CH(2) units with its terminal CH(3) at a different orientation than UDT. We show that surface-enhanced IRSE is sensitive to subtle vibrational signatures of the differently oriented terminal methyls of these two homologous alkanethiol SAMs.

Infrared spectroscopic ellipsometry (IRSE) of organic self-assembled monolayers (SAMs) commonly uses the external reflection geometry and a three-phase system (bulk substrate, SAM, and ambient medium). In the present work, we study a four-phase system for IRSE, where a gold substrate film is sandwiched between a CaF2 prism and a SAM of octadecylmercaptan (ODM). This sample configuration can be employed for internal reflection IRSE (using a continuous Au film), as well as to boost the detection sensitivity of IRSE through surface-enhanced infrared absorption (using discontinuous film of Au nanoislands). We study how the thickness and morphology of the Au layer in the four-phase structure affect the IRSE results for ODM by using three Au substrates:  an optically thick ∼1000 Å continuous film, a continuous ∼230 Å thick film, and a discontinuous ∼220 Å thick film of Au nanoislands. The spectral features of ODM in the last case are different form those of the first two, and are associated with surface-enhanced IRSE. Surface morphologies of the Au substrates are characterized by scanning electron microscopy. The IRSE results are discussed with use of currently known theoretical considerations for differential spectroscopy.

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