Spectroscopic evidence for the origin of odd-even effects in self-assembled monolayers and effects of substrate roughness

Jiahao Chen; Jian Liu; Ian D Tevis; Richard S Andino; Christina M Miller; Lawrence D Ziegler; Xin Chen; Martin M Thuo

doi:10.1039/c6cp07580k

Spectroscopic evidence for the origin of odd-even effects in self-assembled monolayers and effects of substrate roughness

Phys Chem Chem Phys. 2017 Mar 8;19(10):6989-6995. doi: 10.1039/c6cp07580k.

Authors

Jiahao Chen¹, Jian Liu², Ian D Tevis³, Richard S Andino², Christina M Miller², Lawrence D Ziegler², Xin Chen², Martin M Thuo¹

Affiliations

¹ Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA. mthuo@iastate.edu and Microelectronic Research Centre, Iowa State University, Ames, IA 50011, USA.
² Department of Chemistry and the Photonics Centre, Boston University, Boston, MA 02215, USA.
³ Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA. mthuo@iastate.edu.

PMID: 28244512
DOI: 10.1039/c6cp07580k

Abstract

This paper reports the effects of substrate roughness on the odd-even effect in n-alkanethiolate self-assembled monolayers (SAMs) probed by vibrational sum frequency generation (SFG) spectroscopy. By fabricating SAMs on surfaces across the so-called odd-even limit, we demonstrate that differentiation of the vibrational frequencies of CH₃ from SAMs derived from alkyl thiols with either odd (SAM^O) or even (SAM^E) numbers of carbons depends on the roughness of the substrate on which they are formed. Odd-even oscillation in SFG susceptibility amplitudes was observed for spectra derived from SAM^E and SAM^O fabricated on flat surfaces (RMS roughness = 0.4 nm) but not on rougher surfaces (RMS roughness = 2.38 nm). In addition, we discovered that local chemical environments for the terminal CH₃ group have a chain-length dependence. There seems to be a transition at around C₁₃, beyond which SAMs become "solid-like".