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

Abstract

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.

Optical Characterization of Thiolate Self-Assembled Monolayers on Au(111)

“…47 We concentrated on optically thick (200 nm), flat samples, combining typical grain size on the scale of microns and homogeneity on the macroscopic scale. These two conditions were achieved using flame-annealed films deposited on glass slides coated with a Cr primer (Arrandee).…”

Studies on the electrochemical behavior of a cystine self-assembled monolayer modified electrode using ferrocyanide as a probe

“…Although a wide variety of substrates and functional groups are known to form SAM, thiol/disulfide monolayers on Au have received considerable attention in recent years [10][11][12][13]. The electron transfer at a thiol modified gold electrode can be influenced by many factors, such as alkyl chain length, active terminal groups and solution pH.…”

Infrared Spectroscopic Ellipsometry of n-Alkylthiol (C5-C18) Self-Assembled Monolayers on Gold

“…Fortunately, these problems can be recognized with spectroscopic ellipsometry and weaker molecular vibrations can be identified unambiguously with the Kramers−Kronig transformation (KKT). − In particular, the KKT presents specific pairs of vibrational features and makes it easier to assign the vibrational modes. Just recently, infrared spectroscopic ellipsometry (IRSE) has been used to characterize SAMs and ultrathin films on metals and semiconductor substrates. − In general, chemical bonds such as C−H have different orientations, which may be oblique to the substrate surface . IRSE measures the change of the amplitude and phase for p- and s-polarized light reflected from the samples, whereas in FT-IRRAS and ATR spectroscopy usually unpolarized light is used.…”