Sum-Frequency Spectroscopy Amplified by Plasmonics: The Small Particle Case

Abstract

The infrared-visible Sum-Frequency Generation (SFG) spectroscopy response of a composite interface comprising molecules, spherical nanoparticles and a substrate is modelled in the dipolar approximation. The spheres modify the local electric fields felt by the molecules grafted on either the surfaces of the particles or the substrate below the spheres. In the case of plasmonic spheres, the excitation of their surface plasmons lead to amplifications of the molecular SFG signals at the incoming visible and SFG frequencies for both types of molecules. The spectral evolutions of these amplifications are described as a function of the natures of the metal, the molecules and the substrate; the chemical groups involved; the surface density of nanoparticles on the substrate and their surface coverage. The latter parameter is shown to be the main source of SFG from molecules adsorbed on these highly centrosymmetric objects. Models are compared to experimental data and excellent agreement is found for the amplification of the SFG vibrational signature of a grafting monolayer sandwiched between a silicon substrate and gold nanospheres.

The Prevailing Role of Hotspots in Plasmon-Enhanced Sum-Frequency Generation Spectroscopy

“…We have chosen the sample 2 for this study, which gives the highest amplitude for the surface plasmon resonance of isolated particles, without any measurable contribution from the aggregates. Figure S2: Examples of absorbance spectra of nanoparticle grafted silicon surfaces using 30 min (1,2,3) and 60 min (4,5,6) dipping times.…”

“…The coherent lineshapes for the three resonances show the reliability of the fits, as we expect similar evolutions for symmetric and antisymmetric stretches. 29 The intensities show a regular increase from blue to green, with the presence of two broad maxima around 580 and 660 nm. This fact proves that plasmonics influences indeed the molecular SFG response in the vicinity of the nanoparticles and produces a net enhancement of the SFG intensities.…”

“…29 This is also true for the molecules constituting the grafting monolayer sandwiched between the particles and the substrate. 29 Amplification of SFG spectroscopic signals has been shown on long deposited gold cylinders, 30 for which the inversion symmetry is greatly broken. In the case of spheres, no direct evidence of such enhancement has been provided and, for gold particles, the amplification factors are expected to be rather small.…”

“…E 0 is then expressed in the spherical coordinates (uθ,uφ,ur). The local field effects, which include here plasmonic amplification, are taken into account in the spherical frame through a matrix Λ (defined in Ref 5),. to obtain the local field E local eventually experienced by the molecule.…”

Two-Dimensional Layers of Colloidal CdTe Quantum Dots: Assembly, Optical Properties, and Vibroelectronic Coupling

“…To date, only metallic gold nanostructures have been studied, in particular from the point of view of their plasmonic properties. In the cases of gold nanoparticle monolayers on silicon substrates [7] or gold nano-pillars synthesized on metallic substrates (Au, Pt) [42], functionalized in both cases by alkanethiols, 2C-SFG demonstrates the existence of a coupling between gold plasmons and molecular vibrations: the vibrational response of organic molecules is in fact amplified by the excitation of the localized surface plasmon characteristic of these nanostructures [41,46,47]. Our work naturally falls within this latter category, and even attempts to broaden the framework of the study of nanostructure/molecule interactions by 2C-SFG spectroscopy in the case of semiconductor nanomaterials.…”