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 An film), as well as to boost the detection sensitivity of IRSE through surface-enhanced infrared absorption (using discontinuous film of An 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 ALL substrates: an optically thick similar to1000 Angstrom continuous film, a continuous similar to230 Angstrom thick film, and a discontinuous similar to220 Angstrom thick film of ALL. 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 ALL. substrates are characterized by scanning electron microscopy. The IRSE results are discussed with use of currently known theoretical considerations for differential spectroscopy.