Tunable fluorescence emission of molecules with controllable positions within the metallic nanogap between gold nanorods and a gold film†
Metallic nanostructures can strongly influence the fluorescence emission process as they profoundly alter the field and local density of states at the nanoscale. So far, a number of studies use nanostructures with metallic nanogaps to control and enhance the fluorescence emission. Metallic nanoparticles on mirror (NPoM) nanostructures have drawn intense research interest due to their ease of fabrication and ability to form metallic nanogaps down to subnanometers. However, the emitters are usually randomly located in the whole gap region which prohibits a precise control of the fluorescence emission and a quantitative analysis. This study reports an NPoM nanostructure composed of gold nanorods (NRs) and a gold film with double polymethyl methacrylate (PMMA) layers as the spacer that sandwiches the molecules. With a two-step spin coating method, the thickness of each PMMA layer can be precisely controlled by varying the spin coating parameters. Therefore, the vertical position of the emitters in the nanogap can be adjusted as expected. This double layer structure results in a tunable and enhanced fluorescence emission. The experimental results show that the enhancement of the fluorescence intensity and the spontaneous decay rate is sensitive to the vertical position of the emitters and the thickness of the PMMA spacer, and unveil the complete evolution from fluorescence enhancement to quenching as the emitter approaches either the gold NR or the gold film, which are well predicted by the theoretical simulation results. The proposed results may have promising applications associated with the modification of spontaneous emission in plasmonic nanogap structures.