Study of SERS Chemical Enhancement Factors Using Buffer Layer Assisted Growth of Metal Nanoparticles on Self-Assembled Monolayers

Abstract

Highly controlled morphology Au nanoparticle films can be formed on the surfaces of self-assembled monolayers (SAMs) by vapor deposition at cryogenic temperatures (approximately 10 K) with intervening condensed Xe layers on the SAMs serving as a buffer to reduce the kinetic energy of the Au atoms impinging on the surface (buffer layer assisted growth or BLAG). Under these conditions pristine Au nanoparticles (AuNp) of a uniform shape and size were deposited onto two SAMs differing only by their terminal groups, 4-benzenedithiol (BDT) and 4-methylbenzenethiol (MBT), to form -S/Au and -CH(3)/Au interfaces with essentially identical AuNp overlayer morphologies. A surface enhanced Raman (SERS) enhancement factor ratio EF(BDT)/EF(MBT) of approximately 130 was observed uniformly across the surfaces (approximately <10% variation). Since equal electromagnetic contributions to the SERS enhancements are expected from the two identically structured Au overlayer films, the observed SERS intensity ratio accordingly reflects a pure chemical enhancement (CE) contribution arising from the -S/Au relative to the -CH(3)/Au interface and thereby provides the first quantitative experimental data for the magnitude of the SERS CE for well-defined Au-molecule contacts.

Quantitative SERS by hot spot normalization – surface enhanced Rayleigh band intensity as an alternative evaluation parameter for SERS substrate performance

“…We note that the chemical enhancement mechanism is not considered in this study since the electromagnetic enhancement mechanism is typically the dominant one for SERS. 26–29 $$\frac{I_{\text{italicRaman}}}{I_{\text{italicRayleigh}}}=\frac{E_s^{2}false({\omega }_{\mathit{\text{Raman}}}false)}{E_s^{2}false({\omega }_{\mathit{\text{Rayleigh}}}false)}=\frac{N_A{\alpha }_A^{2}false({\omega }_{\mathit{\text{Raman}}}false){\mathrm{I}}_{0}false({\omega }_{0}false)}{N_B{\alpha }_B^{2}false({\omega }_{\mathit{\text{Rayleigh}}}false){\mathrm{I}}_{0}false({\omega }_{\mathit{\text{Rayleigh}}}false)}$$…”

Toward Understanding the Influence of Intermolecular Interactions and Molecular Orientation on the Chemical Enhancement of SERS

“…2 While the plasmonic, electric field effect accounts for most of the enhancement and can be understood in terms of a simple surface selection rule, the chemical effect can greatly affect the relative enhancements of the different frequencies in the spectrum. As a result, the peaks in a SERS spectrum can be difficult to relate to the unperturbed Raman spectrum of the neat compound.…”

Raman spectral study of metal–cytosine complexes: A density functional theoretical (DFT) approach

“…Despite extensive experimental and theoretical efforts [10][11][12][13][14][15][16], its microscopic mechanism and relative importance are still not completely understood, and there are many questions that remain unanswered thus far, such as the precise geometries of the junctions and the molecular orientations [17].…”