Single-Molecule Surface-Enhanced Raman Spectroscopy: A Frequency Domain Existence Proof and Examination of the Role of the Electromagnetic Enhancement Mechanism

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This work is a research effort aimed at understanding the mechanisms of single-molecule surface-enhanced Raman spectroscopy (SMSERS). In the decade since its discovery in 1997, the role of resonance Raman (RR) enhancement, the origin of blinking, and the properties of the hot spot formed at the junction of two nanoparticles remained inconclusive. The lack of concrete mechamisms confounded progress of SMSERS towards application. Additionally, the existence of SMSERS remained in doubt. The work herein provides insight into these problems and places the understanding of SMSERS on a firmer basis. The role of RR was studied by performing wavelength-scanned surface-enhanced resonance Raman excitation spectroscopy of Ru[(bpy)3]2+ monolayers on nanosphere lithography-fabricated Ag nanotriangle arrays. Since the localized surface plasmon resonance (LSPR) of the Ag triangle array is well defined, the contributions from RR and surface effects can be decoupled. The results indicate that there is a multiplication of the two enhancement mechanisms. Confirmation of this mechanism was acheived by modeling with quasi-static electrodynamics. Wavelength-scanned excitation profiles of rhodamine 6G (R6G) were also performed ensemble-averaged and at the single-molecule level establishing the RR enhancement profile of this common SMSERS analyte. A frequency domain existence proof of SMSERS was performed by isotopically labeling R6G. A 1:1 mixture of the R6G isotopologues was adsorbed to aggregated, chemically synthesized Ag colloids at a 1:1 ratio. 50 active SMSERS junctions were analyzed and 46 sites had a spectral signature corresponding to a single isotopologue. Under ambient conditions and higher dosing (100 molecules per nanoparticle), blinking was observed. Single-molecule signatures were observed as the spectrum evolved in time and 2D cross-correlation revealed that molecules compete for access to the hot spot. Therefore, one mechanism of blinking in SMSERS is surface diffusion in proximity of the hot spot. The nature of the SMSERS hot spot was analyzed by correlating the LSPR scattering spectrum of a SMSERS active junction to its high resolution transmission electron microscopy image. Calculations performed by applying the discrete-dipole approximation revealed that the electromagnetic enhancement in the hot spots was on the order 10^9. When combined with the large RR cross-section of R6G, the overall cross-section observed in SMSERS (10^-15 cm^2) is attainable in the absence of chemical enhancement. Finally, the calculations reveal that the hot spot has large electromagnetic enhancement (> 10^7) over a large excitation bandwidth.

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  • 09/20/2018
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