Understanding the Role of Different Substrate Geometries for Achieving Optimum Tip-Enhanced Raman Scattering Sensitivity

Abstract

Tip-enhanced Raman spectroscopy (TERS) has experienced tremendous progress over the last two decades. Despite detecting single molecules and achieving sub-nanometer spatial resolution, attaining high TERS sensitivity is still a challenging task due to low reproducibility of tip fabrication, especially regarding very sharp tip apices. Here, we present an approach for achieving strong TERS sensitivity via a systematic study of the near-field enhancement properties in the so-called gap-mode TERS configurations using the combination of finite element method (FEM) simulations and TERS experiments. In the simulation study, a gold tip apex is fixed at 80 nm of diameter, and the substrate consists of 20 nm high gold nanodiscs with diameter varying from 5 nm to 120 nm placed on a flat extended gold substrate. The local electric field distributions are computed in the spectral range from 500 nm to 800 nm with the tip placed both at the center and the edge of the gold nanostructure. The model is then compared with the typical gap-mode TERS configuration, in which a tip of varying diameter from 2 nm to 160 nm is placed in the proximity of a gold thin film. Our simulations show that the tip-nanodisc combined system provides much improved TERS sensitivity compared to the conventional gap-mode TERS configuration. We find that for the same tip diameter, the spatial resolution achieved in the tip-nanodisc model is much better than that observed in the conventional gap-mode TERS, which requires a very sharp metal tip to achieve the same spatial resolution on an extended metal substrate. Finally, TERS experiments are conducted on gold nanodisc arrays using home-built gold tips to validate our simulation results. Our simulations provide a guide for designing and realization of both high-spatial resolution and strong TERS intensity in future TERS experiments.

Spatially-Localized Functionalization on Nanostructured Surfaces for Enhanced Plasmonic Sensing Efficacy

“…Lateral spatial resolution in TERS is defined as the full-width-at-half-maximum of the local field distribution around the tip in the plane of the surface: FWHM ≈ 0.87√Rd [ 33 ], where R is the tip radius and d is the height of the tip-to-surface gap. At R = 30 nm and d = 2 nm, FWHM resolution = 6.7 nm.…”

“…TERS instrument configurations and applications have recently been reviewed [ 26 , 27 ]. Field density and spatial distribution in the local neighborhood of nanoscale surface geometry features such as edges or surface heterogeneities have been precisely mapped with TERS [ 24 , 28 , 29 , 30 ], with studies focusing on the impact of tip size [ 31 ], commercial vs. custom-made tips [ 32 ], and positioning [ 33 , 34 , 35 ]. TERS is therefore ideally suited to characterize nanostructured SERS substrates to determine the optimal positioning of probe molecules on the sensor surface.…”

Tip-Enhanced Raman Spectroscopy of 2D Semiconductors

“…it utilizes the plasmonic enhancement around a sharp metallic tip. However, unlike SERS it employs a single SPM tip to collect local phonon information by taking advantage of the lateral resolution of SPM [37,38]. When excited by a suitable photon energy, the collective oscillation of the conduction band electrons in the sharp metallic tip amplifies and confines the optical field in the vicinity of the tip apex.…”

“…For a certain tip-substrate geometry both R and d are fixed at the smallest distance from the tip apex to the sample surface being d, which is along the center of the electric field lines. Therefore, the lateral offset of the electric field from the center can be written as [38].…”

“…The tip-substrate distance was kept at 1 nm.Figure(a) to (e) reproduced from ref [37]. with permission from the Royal Society of Chemistry and (f) and (g) from ref [38]…”