Toroidal dipole-modulated dipole-dipole double-resonance in colloidal gold rod-cup nanocrystals for improved SERS and second-harmonic generation

Hao-Sen Kang; Wen-Qin Zhao; Tao Zhou; Liang Ma; Da-Jie Yang; Xiang-Bai Chen; Si-Jing Ding; Qu-Quan Wang

doi:10.1007/s12274-022-4562-5

Toroidal dipole-modulated dipole-dipole double-resonance in colloidal gold rod-cup nanocrystals for improved SERS and second-harmonic generation

Nano Res. 2022;15(10):9461-9469. doi: 10.1007/s12274-022-4562-5. Epub 2022 Jul 6.

Authors

Hao-Sen Kang^#¹, Wen-Qin Zhao^#¹, Tao Zhou^#², Liang Ma¹, Da-Jie Yang³, Xiang-Bai Chen¹, Si-Jing Ding², Qu-Quan Wang⁴

Affiliations

¹ Hubei Key Laboratory of Optical Information and Pattern Recognition, Wuhan Institute of Technology, Wuhan, 430205 China.
² School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan, 430074 China.
³ Mathematics and Physics Department, North China Electric Power University, Beijing, 102206 China.
⁴ School of Science, Department of Physics, Southern University of Science and Technology, Shenzhen, 518055 China.

^# Contributed equally.

Abstract

Colloidal metal nanocrystals (NCs) show great potential in plasmon-enhanced spectroscopy owing to their attractive and structure-depended plasmonic properties. Herein, unique Au rod-cup NCs, where Au nanocups are embedded on the one or two ends of Au nanorods (NRs), are successfully prepared for the first time via a controllable wet-chemistry strategy. The Au rod-cup NCs possess multiple plasmon modes including transverse and longitudinal electric dipole (TED and LED), magnetic dipole (MD), and toroidal dipole (TD) modulated LED resonances, producing large extinction cross-section and huge near-field enhancements for plasmon-enhanced spectroscopy. Particularly, Au rod-cup NCs with two embedded cups show excellent surface-enhanced Raman spectroscopy (SERS) performance than Au NRs (75.6-fold enhancement excited at 633 nm) on detecting crystal violet owing to the strong electromagnetic hotspots synergistically induced by MD, LED, and TED-based plasmon coupling between Au cup and rod. Moreover, the strong TD-modulated dipole-dipole double-resonance and MD modes in Au rod-cup NCs bring a 37.3-fold enhancement of second-harmonic generation intensity compared with bare Au NRs, because they can efficiently harvest photoenergy at fundamental frequency and generate large near-field enhancements at second-harmonic wavelength. These findings provide a strategy for designing optical nanoantennas for plasmon-enhanced applications based on multiple plasmon modes.

Electronic supplementary material: Supplementary material (SEM image of Au rod-one-cup NCs; TEM image of Au/PbS hybrids; SEM image of Au rod-two-cup NCs; low-amplification SEM image of Au rod-two-cup NCs; experimental extinction and calculated electric field distributions of Au NR excited at different wavelengths; calculated absorption and scattering spectra of Au rod-one-cup NCs; schematic illustration of the cut plane and the corresponding magnetic field distribution under L3 excitation; Raman spectra of CV (10^-6 M) adsorbed on Au rod-cup NCs with different cup sizes; calculated magnetic field distribution of Au rodcup NCs excited at 532 and 633 nm; calculated electric field distributions of Au rod-one-cup NC excited at 600 nm along TE and LE; the models of Au rod-cup NCs used in the simulations) is available in the online version of this article at 10.1007/s12274-022-4562-5.

Keywords: near-field enhancement; plasmon; second-harmonic generation; surface-enhanced Raman spectroscopy (SERS); toroidal dipole.