Silver nanoparticles, nanoneedles and nanorings: impact of electromagnetic near-field on surface-enhanced Raman scattering

Abstract

The dimension of plasmonic nanostructures does matter in localizing electromagnetic (EM) field and improving surface-enhanced Raman scattering (SERS) activity. Zero-dimensional (0D), one-dimensional (1D) and two-dimensional (2D) plasmonic nanostructures are promising candidates to validate SERS enhancement and the mechanisms thereof. In this work, silver (Ag) nanoparticles (NPs), nanoneedles (NNs) and nanorings (NRs) have been considered to demonstrate the impact of EM near-field distributions on SERS of the corresponding 0D (i.e. Ag-NPs), 1D (i.e. Ag-NNs) and 2D (i.e. Ag-NRs) nanostructures. Ag-NPs, Ag-NNs and Ag-NRs fabricated on zinc oxide (ZnO) ultrathin layer through sputtering technique have been characterized thoroughly by high-resolution field emission scanning electron microscopy (FESEM). FESEM micrographs confirmed a relatively narrow size distribution, 48.88 ± 8.32 nm, of Ag-NPs along with an estimated coverage density of ∼4 × 10¹⁰ cm⁻². In the case of 1D nanostructures, Ag-NNs were estimated to have a relatively broadened length distribution, 534.36 ± 85.61 nm, and relatively narrow base distribution, 77.39 ± 25.25 nm, along with an estimated coverage density of ∼5 × 10⁸ cm⁻². However, as for 2D nanostructures, the FESEM micrographs revealed that Ag-NRs consisted of Ag clusters of various shapes and sizes, instead of a perfect ring structure along with much lower coverage density, ∼8.05 × 10³ cm⁻². The same specimens were used in microscopic SERS measurements and SERS activities were evaluated for individual nanostructures using Rhodamine 6G as Raman-active dye. The SERS activity of Ag-NRs was found to be the highest with reference to those of Ag-NPs and Ag-NNs. The scenario was supported as well by EM near-field distributions extracted from finite difference time domain (FDTD) analysis. Three models were developed according to the FESEM micrographs of Ag-NPs, Ag-NNs and Ag-NRs nanostructures and FDTD analysis was carried out to understand EM near-field distributions for individual nanostructures. EM near-field distributions at different planes for individual models were extracted for s-, p- and 45° incident polarizations. Such a correlated investigation facilitated an understanding and correlation of the impact of EM near-field distributions on SERS of the corresponding 0D (i.e. Ag-NPs), 1D (i.e. Ag-NNs) and 2D (i.e. Ag-NRs) nanostructures.