Abstract
Surface-enhanced Raman scattering (SERS) and resonant Raman scattering are widely used techniques to enhance the Raman intensity of molecules and nanomaterials by several orders of magnitude. In SERS, typically, molecules are investigated and their intrinsic resonance is often ignored while discussing the plasmonic enhancement. Here, we study α-sexithiophenes encapsulated in carbon nanotubes placed in the center of a nanodimer. By dielectrophoretic deposition, we place the nanotubes precisely in the center of a plasmonic gold nanodimer and observe SERS enhancement from individual tube bundles. The encapsulated molecules are not subjected to chemical enhancement because of the protective character of the nanotube. Polarization-dependent Raman measurements confirm the alignment of the molecules within the carbon nanotubes (CNTs) and reveal the influence of the plasmonic near field on the molecule’s Raman intensity. We investigate the encapsulated molecules in small CNT bundles with and without plasmonic enhancement and determine the molecular and plasmonic resonance by tuning the excitation wavelength. We observe a strong red shift of the maximum Raman intensity under plasmonic enhancement toward the plasmon resonance.
Highlights
Raman scattering is a powerful chemically sensitive technique to analyze materials and detect the presence of target molecules
The molecular mode of α-6T on which we will focus in this work is located at ωα‐6T ≈ 1450 cm−1.27 The carbon nanotubes (CNTs) Raman modes, namely, the G, D, and radial breathing mode (RBM), are strong at this wavelength, while they are much weaker at an excitation wavelength of 638 nm, indicating enhancement by optical resonance
We did not observe any blinking in the Raman spectra of α-6T@CNT-Au, which is an indicator that no chemical enhancement is present
Summary
Raman scattering is a powerful chemically sensitive technique to analyze materials and detect the presence of target molecules. Another advantage is the alignment of some molecules, for example, rodlike molecules such as αsexithiophene (α-6T), inside of the CNT In this way, the orientation of the molecules may be determined from microscopy images.[25] Some of us showed in SERS experiments at fixed wavelengths that encapsulated α-6T do not experience chemical enhancement when placed inside a gold dimer, making them ideal model systems for studying the combination of plasmonic and intrinsic resonances.[26]. We chose these molecules because of their high Raman cross section and the alignment inside the CNTs.[27,28] Using the nanotubes as carriers, the encapsulated molecules were deposited on a substrate with gold nanodimers by dielectrophoretic deposition With this technique, we were able to precisely place small bundles of nanotubes into the dimer gap.
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