Plasmonic Properties of Gold Nanoparticles Separated from a Gold Mirror by an Ultrathin Oxide

Abstract

That a nanoparticle (NP) (for example of gold) residing above a gold mirror is almost as effective a surface enhanced Raman scattering (SERS) substrate (when illuminated with light of the correct polarization and wavelength) as two closely coupled gold nanoparticles has been known for some time. The NP-overmirror (NPOM) configuration has the valuable advantage that it is amenable to top-down fabrication. We have fabricated a series of Au-NPOM substrates with varying but thin atomic layer-deposited oxide spacer and measured the SERS enhancement as a function of spacer thickness and angle of incidence (AOI). These were compared with high-quality finite-difference time-domain calculations, which reproduce the observed spacer thickness and AOI dependences faithfully. The SERS intensity is expected to be strongly affected by the AOI on account for the fact that the hot spot formed in the space between the NP and the mirror is most efficiently excited with an electromagnetic field component that is normal to the surface of the mirror. Intriguingly we find that the SERS intensity maximizes at ~60° and show that this is due to the coherent superposition of the incident and the reflected field components. The observed SERS intensity is also shown to be very sensitive to the dielectric constant of the oxide spacer layer with the most intense signals obtained when using a low dielectric constant oxide layer (SiO(2)).

Pulsed Molecular Optomechanics in Plasmonic Nanocavities: From Nonlinear Vibrational Instabilities to Bond-Breaking

“…A nanoparticle-on-mirror (NPoM) [35,36] configuration is adopted [ Fig. 1(c), inset] to provide robust reproducible plasmon enhancements on self-assembled molecular monolayers (SAMs) sandwiched inside the gap.…”

Coupling between plasmonic films and nanostructures: from basics to applications

“…Usually, Au NPs aggregates or dimmers are known to be excellent SERS substrates. Recently, Moskovits and coworkers evidenced the NP-overmirror (NPOM) -that is to say NPs on top of au film coated with a thin dielectric layer -allows SERS signal enhancement by a factor 10 even for aggregates [59]. Wang et al, confirmed these results by focusing -both numerically and experimentally -on SERS gap modes when molecules are located in-between au NP dimmers and Au film [60].…”

Effect of Foreign Molecules on the SERS of Probe Molecules Trapped in Gaps between Planar Ag and Nano-sized Ag Particles

“…This can be understood in terms of the electromagnetic "hot sites" formed at the gaps between two Au nanoparticles. 20,21 The Raman signal did not increase further, however, even after the adsorption of additional 1,4-PDI onto the vacant surfaces of the second group of Au nanoparticles. This is consistent with the observation of Chumanov et al that the additional exposure of the NAMF substrates to the same molecules did not yield more SERS signal, suggesting that the size of hot site is in fact very limited.…”