NANO LETTERS
Chain-Length-Dependent Vibrational Resonances in Alkanethiol Self-Assembled Monolayers Observed on Plasmonic Nanoparticle Substrates
2006 Vol. 6, No. 11 2617-2621
Carly S. Levin,†,| Benjamin G. Janesko,†,| Rizia Bardhan,† Gustavo E. Scuseria,† Jeffrey D. Hartgerink,†,‡ and Naomi J. Halas*,†,§ Department of Chemistry and the Laboratory for Nanophotonics, Department of Electrical and Computer Engineering, and Department of Bioengineering, Rice UniVersity, 6100 Main Street, Houston, Texas 77005 Received September 28, 2006
ABSTRACT Alkanethiol self-assembled monolayers (SAMs) on gold exhibit a series of sharp resonances in their surface-enhanced Raman spectrum that depend dramatically on carbon chain length. This unusual behavior suggests a coupling of the gold−sulfur bond stretch with the longitudinal acoustic, “accordion”, vibrations of the molecular alkane chain. A simple model of a one-dimensional chain attached to a surface quantitatively reproduces these previously unreported experimental observations in this important nanomaterial system.
Alkanethiols on gold surfaces are the prototypical selfassembled monolayer (SAM) system,1 utilized extensively in nanoscale science and applications. The structure and energetics of the gold-sulfur interaction has been studied extensively, both experimentally2-4 and theoretically.5-10 Surface-enhanced Raman spectroscopy (SERS) is an excellent method for probing this interaction because of the strong field enhancements that can be excited at the moleculesubstrate interface. However, despite extensive work on the SERS spectra of alkanethiol SAMs,11 there have been relatively few analyses in the low-frequency region (
