Infrared Ellipsometry of Self-Assembled Octadecylmercaptan on Gold Films and Nanoislands:  Effects of Thickness and Morphology of the Gold Layer

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.