Fingerprinting the Hidden Facets of Plasmonic Nanocavities

Abstract

The optical properties of nanogap plasmonic cavities formed by a NanoParticle-on-Mirror (NPoM, or patch antenna) are determined here, across a wide range of geometric parameters including the nanoparticle diameter, gap refractive index, gap thickness, facet size and shape. Full understanding of the confined optical modes allows these nanocavities to be utilized in a wide range of experiments across many fields. We show that the gap thickness t and refractive index n are spectroscopically indistinguishable, accounted for by a single gap parameter G = n/t 0.47 . Simple tuning of mode resonant frequencies and strength is found for each quasi-normal mode, revealing a spectroscopic "fingerprint" for each facet shape, on both truncated spherical and rhombicuboctahedral nanoparticles. This is applied to determine the most likely nanoscale morphology of facets hidden below each NPoM in experiment, as well as to optimize the constructs for different applications. Simple scaling relations are demonstrated, and an online tool for general use is provided.

Quantitatively Revealing the Anomalous Enhancement in Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy Using Single-Nanoparticle Spectroscopy

“…The histogram (Figure 1d, gray) shows the resonance wavelength distribution of the antenna coupling mode which is mainly due to the size variation of the nanoparticles. 34 The small shoulder beside the transverse mode could be identified as the (20) mode that is sensitive to the size of the contacting area between the NP and Au film, 35 while the antenna coupling mode resonance position of single Au NP gap mode constructs (∼850 nm) exhibits a significant redshift compared with the single-SHIN counterparts (∼750 nm; see Figure S1c). Such a redshift of ∼100 nm is well matched with the predicted scattering spectra obtained by FDTD simulation (see Figure S2), which could be understood by the increase of the coupling strength of the two dipoles when the gap size largely decreases in the absence of the SiO 2 shell.…”

“…The residual variability in the correlation curve is likely to be induced by the contact surface diameter inconsistency underneath the NPs. 35 Unfortunately, the bottom contact surface diameter can not be easily quantified by SEM or any other techniques. One of the indirect solutions could be colocalization with an atomic force microscope (AFM) to measure the height of the NPs and calculate the contact surface diameter underneath, but the error could be large depending on the morphology of the NPs.…”

“…The results suggest that making the NPs more closely packed on the surface is important to achieve large signal enhancement in SHINERS measurements. The residual variability in the correlation curve is likely to be induced by the contact surface diameter inconsistency underneath the NPs . Unfortunately, the bottom contact surface diameter can not be easily quantified by SEM or any other techniques.…”

Boosting Optical Nanocavity Coupling by Retardation Matching to Dark Modes

“…The peak positions of these modes are in agreement with predictions (Figure c), except for n d = 1.49, where all peaks are blue-shifted (possibly due to coating morphology under AuNPs). We note that simulations here also do not capture variations in nanoparticle facet shape, which further breaks the degeneracy of ( l 1) modes. , While residual citrate molecules and a thin layer of water might remain coating the AuNPs (after thorough rinsing of the NPoMs), which may increase the refractive index, the effect on the modes is minimal because this coating would be of sub-nanometer.…”

“…We note that simulations here also do not capture variations in nanoparticle facet shape, which further breaks the degeneracy of (l1) modes. 30,33 While residual citrate molecules and a thin layer of water might remain coating the AuNPs (after thorough rinsing of the NPoMs), which may increase the refractive index, the effect on the modes is minimal because this coating would be of subnanometer.…”