Plasmonic enhancement of SERS measured on molecules in carbon nanotubes

Abstract

We isolated the plasmonic contribution to surface-enhanced Raman scattering (SERS) and found it to be much stronger than expected. Organic dyes encapsulated in single-walled carbon nanotubes are ideal probes for quantifying plasmonic enhancement in a Raman experiment. The molecules are chemically protected through the nanotube wall and spatially isolated from the metal, which prevents enhancement by chemical means and through surface roughness. The tubes carry molecules into SERS hotspots, thereby defining molecular position and making it accessible for structural characterization with atomic-force and electron microscopy. We measured a SERS enhancement factor of 10 on α-sexithiophene (6T) molecules in the gap of a plasmonic nanodimer. This is two orders of magnitude stronger than predicted by the electromagnetic enhancement theory (10). We discuss various phenomena that may explain the discrepancy (including hybridization, static and dynamic charge transfer, surface roughness, uncertainties in molecular position and orientation), but found all of them lacking in enhancement for our probe system. We suggest that plasmonic enhancement in SERS is, in fact, much stronger than currently anticipated. We discuss novel approaches for treating SERS quantum mechanically that appear promising for predicting correct enhancement factors. Our findings have important consequences on the understanding of SERS as well as for designing and optimizing plasmonic substrates.

Raman resonance profile of an individual confined long linear carbon chain

“…The Raman scattering intensity arises only from the projection of the incoming and scattered light onto the chain axis. This dependence is well known for one-dimensional systems like carbon nanotubes or J-aggregated molecules and originates from a depolarisation or antenna e↵ect [17,16,18]. It is also expected for the fully symmetric A 1g (⌃ + g ) Raman active C-mode of the chain, and confirmed here experimentally for the first time.…”

“…by the choice of surfactant in aqueous solutions or bundling effects [23,24,25]. The interior of a nanotube, however, is a highly unperturbed environment which e↵ectively protects encapsulated objects like fullerenes, dye molecules or an additional inner nanotube from external influences [26,27,28,18,29]. This shielding e↵ect is even more pronounced inside double-walled carbon nanotubes, where two concentric walls protect the confined LLCCs.…”

Electrodeposition of Prussian Blue/Carbon Nanotube Composites at a Liquid‑Liquid Interface

“…The enhancement could arise from adsorption of the aqueous [Fe (CN) 6 ] 4-ions on the surface of CNTs at the L/L interface, which could also decrease the hydration sphere of the complex. 70,71 In fact, a splitting of the E g mode of anhydrous K 4 [Fe(CN) 6 ] occurs, compared to the hydrated complex, that correlates with the C≡N band profile observed in Figure 5. 29,68 To evaluate PB formation over the CNT film, the laser was focused directly onto the L/L interface where the above-mentioned signals are present.…”

Photosensitive Field‐Effect Transistors Made from Semiconducting Carbon Nanotubes and Non‐Covalently Attached Gold Nanoparticles

“…In principle, top‐down and bottom‐up approaches of nanotechnology are conceivable. Top‐down approaches comprise the lithographic structuring of patterned metal structures, followed by a directed deposition of the CNTs for the transistor structures or vice‐versa. In this respect, integration of CNTs into nanoantenna arrays was reported to exhibit several fabrication difficulties that prevent the top‐down technology from suitable upscaling.…”