Insights into the heterogeneous distribution of SERS effect in plasmonic hot spots between Au@SiO2 monolayer film and gold single crystal plates

Abstract

Plasmonic hot spots, capable of confining strong electromagnetic fields near metallic surfaces, are particularly essential to a variety of enhanced spectroscopic techniques. Understanding the electric field distributions in the hot spot plays a crucial role in controlling the fabrication of plasmonic nanostructures for a variety of plasmon-based applications. The investigation of plasmonic hot spots in metallic nanosystems has not been fully evaluated. Here, we develop a facile approach by experimental means for investigating the distribution of plasmonic hot spots in surface-enhanced Raman spectroscopy (SERS) based on the dual-probe strategy by coupling a p-mercaptobenzoic acid-embedded Au@SiO2 ((Au-pMBA)@SiO2) nanoparticle monolayer film with thiophenol-modified gold single crystal plates (TP-GSCPs). We demonstrated, for the first time, the heterogeneous distribution of SERS effect in the gap between Au@SiO2 monolayer film and GSCPs. As increasing the gap distance by changing the thickness of silica spacer, the SERS effect of the probe on the gold nanoparticles decayed with a slower rate than that of the other probe attached onto the GSCPs. It mainly originated from the difference of localized dielectric environments and curvatures. By deliberately controlling the silica shell thickness, the switchable plasmonic coupling effect can be achieved between "particle-particle" gap mode and "particle-surface" gap mode. The results reveal that the transfer effect is more evident for (Au-pMBA)@SiO2 films with thinner silica shell thickness and for 785 nm illumination as excitation wavelength than 633 nm. Moreover, the introduction of NaOH solution to (Au-pMBA)@SiO2 films leads to the transfer of the hot spots to the areas between neighboring nanoparticles again due to the removal and dissolution of silica shells. The understanding gained from our experimental observation provides keen insight into the plasmonic hot spots in coupled nanostructures, offering guidance for rational design of plasmonic substrates for ultrasensitive SERS detections.