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

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

Author information

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.

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 (SAMO) or even (SAME) 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 SAME and SAMO 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".

PMID:
28244512
DOI:
10.1039/c6cp07580k

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