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.

The rational optimization of plasmonic property of metal nanocrystals by manipulating the structure and morphology is crucial for the plasmon-enhanced application and has always been an urgent issue. Herein, Au nanorods with tunable surface roughness are prepared by growing PbS, overgrowing Au, and dissolving PbS nanoparticles on the basis of smooth Au nanorods. The transverse plasmon resonance of Au nanorods is notably improved due to plasmon coupling between Au nanorods and the surface-modified Au nanoparticles, resulting in the strong and full-spectrum light absorption. Numerical simulations demonstrate that the surface-rough Au nanorods have abundant and full-surround hotspots coming from surface particle–particle plasmon coupling between ultrasmall nanogaps, sharp tips, and uneven areas on Au nanorods. With these characters, the surface-roughness-adjustable Au nanorods possess high tunability and enhancement of surface-enhanced Raman scattering (SERS) detection of Rhodamine B and significantly improved photothermal conversion efficiency. Au nanorods with the largest surface roughness have the highest Raman enhancement factor both at 532 and 785 nm laser excitation. Meanwhile, photothermal conversion experiments under near-infrared (808 nm) and simulated sunlight irradiation confirm that the Au nanorods with rough surface have prominent photothermal conversion efficiency and can be regarded as promising candidates for photothermal therapy and solar-driven water evaporation.