Dark-field and SERS spectroscopic study on nanoparticle-molecule-thin film junction

Abstract

The localized surface plasmon resonance (LSPR) was largely investigated because it is the key factor of nanoparticle (NP) application. However, the role of molecules and quantum mechanical phenomenon such as tunneling effect was underrated in LSPR investigation. In this research, we observed the tunneling effect on LSPR by the dark-field spectroscopy. We used two systems to reduce the heterogeneity of LSPR originated from the metal structure: changing molecules in the junction by photoreduction and using well-defined single-crystalline junction. First, photoreduction of 4-nitrobenzenethiol (NBT) was used to change the molecules in the Ag nanoparticle (AgNP)-NBT-Au thin film (AuTF) junction. This allowed us to obtain the scattering spectrum of one junction with different molecules. Second, we synthesized atomically flat single-crystalline micron-size Au platelet to substitute the AuTF. The NPs were also substituted with well-defined nanocubes (NC) which have atomically flat single-crystalline surface. This allowed us to obtain the scattering spectrum with less heterogeneity and observe the tunneling effect on LSPR.

To obtain the scattering signal, we constructed a home-built confocal dark-field spectroscopy set-up. The collection efficiency was largely improved compared to the common dark-field spectroscopy or the reported confocal dark-field spectroscopy by removing the scattering signal from nearby junctions and collecting the signal in a high-NA region where the scattering signal radiates a lot. Also, using the correlated surface-enhanced Raman spectroscopy (SERS) set-up by the beam splitter, we can obtain the SERS spectrum of the targeted junction simultaneously.

Using photoreduction of NBT to 4-aminobenzenethiol (ABT), we can change the molecules in one junction without changing the metal structure. The blue-shift of LSPR was observed in most of the junctions. Because all other factors such as welding of nanoparticles, a difference of refractive index, or charging effect are negligible in this system, blue-shift of LSPR is originated from the tunneling coefficient change. Although the additional experiment is needed to observe exact LSPR shift distribution, this result shows the possibility of adjusting LSPR of metal nanostructure simply by changing the adsorbed molecules.

With the modified version of micron-size Au platelet synthesis reported previously, we synthesized wide (side length > 5 µm) and thin (thickness : ~ 30 nm) Au platelet. The surfactant was removed by solvent wash. The additional experiment with different molecules is needed to investigate the tunneling effect on LSPR in this system.