CC BY
10B-I-1
Surface-Enhanced Raman Scattering from Au Nanorods as a Function of their Aspect Ratio and Morphology: the Fourth-Power
Law Revisited
N. G. Khlebtsov
Institute of Biochemistry and Physiology of Plants and Microorganisms, Russian Academy of Sciences, Saratov, Russia E-mail: khlebtsov@ibppm.ru
The on-resonance excitation of plasmonic nanoparticles is assumed to be necessary for increasing the Raman signal intensity of nearby molecules [1]. Still, there remain some gaps in the current understanding of the interplay between the near-field (SERS) and the far-field optical response. Indeed, the SERS EF scales like the fourth power of the local field [2]. However, the existing experimental data for Au nanorods (AuNRs) do not confirm such theoretical predictions. Most previous studies have used samples of AuNRs with different diameters, particle concentrations, amounts of impurities, and probably some variations in the nanorod shape. Here, we discuss a reexamination study [3] using a well-defined experimental model. We used the controllable etching method to prepare a set of AuNR samples of equal number concentrations by keeping the AuNR width and shape morphology. At the same time, the plasmon resonance was incrementally decreased from 925 to 650 nm through the finely tuned aspect ratio. The AuNRs were functionalized with 1,4-nitrobenzenethiol (NBT), and SERS spectra of the colloids were measured under 785-nm laser excitation. The nanorod concentration was quantified by atomic absorption spectroscopy and spectrophotometry combined with TEM statistical data and T-matrix simulations. The number of adsorbed NBT molecules per one nanorod (~104) corresponded to the effective footprint ~0.55 nm2 and was close to the monolayer packing density with the topological polar surface area of NBT 0.468 nm2. SERS spectra of colloids and drop samples were measured under 785-nm laser excitation with a fiber-optic spectrometer and a Renishaw inVia Raman microscope. For PR wavelengths between 800 nm and 900 nm, both simulated and experimental EFs show minimal variations and are in reasonable agreement. However, for PR wavelengths between 650 nm and 785 nm, electromagnetic simulations still predict substantial variations in the SERS intensity within one order of magnitude, in stark contrast to the experimental EFs. By contrast to weak plasmonic dependence of SERS signals from the aspect ratio of AuNRs, minor variations in shape morphology lead to notable changes in SERS response. Specifically, when the initial AuNRs were further overgrown to have dumbbell morphology, their SERS intensity increased fivefold. Thus the rational design of the nanoparticle shape morphology is a more critical factor towards the highest SERS response compared to the tuning of the plasmonic peak for on-resonance excitation. This research was supported by the Russian Scientific Foundation (project no. 18-14-00016-n).
[1] N. Pazos-Perez, L. Guerrini, R. A. Alvarez-Puebla, Plasmon tunability of gold nanostars at the tip apexes, ACS Omega, 3, 17173-17179 (2018).
[2] E. C. Le Ru, P. G. Etchegoin. Principles of Surface Enhanced Raman Spectroscopy and Related Plasmonic Effects, Elsevier (2009)
[3] B. Khlebtsov, V. Khanadeev, A. Burov, E. Le Ru, N. Khlebtsov, Reexamination of surface-enhanced Raman scattering from gold nanorods as a function of aspect ratio and shape, J. Phys. Chem. C. 124, 10647-10658 (2020).
