Exploring the Spatial Distribution of Local Field Gradients by Tip-Enhanced Raman Mapping of a Single-Molecule Probe

Since tip-enhanced Raman spectroscopy (TERS) techniques have shown that the local electric field can be confined at the subnanometer scale, the strong inhomogeneity induced field-gradient effect and other high-order processes would show up and influence related spectral features. However, it is still challenging to precisely determine the spatial distribution of the local field gradient because of the complexity of high-order polarization processes. In this work, we propose using a single molecule as a local probe to evaluate the local electric field and its gradient distributions within a plasmonic nanocavity. Thanks to the high symmetry of the polarizabilities for the model molecule and the strong enhancement of the local plasmonic field along the vertical direction, the spatial distributions of the local electric field and the field gradient can be simultaneously deduced from the mapping images for different vibrational modes. The size of the probe molecule is found to determine the ability of resolving the field confinement, which can be improved to be as small as ∼0.1 nm once a small carbon monoxide molecule adsorbed on the metal surface is adopted as the probe. In order to demonstrate the general applicability of this method, we further designed different nanocavities with different protrusion geometries at the tip apex. The corresponding distributions of both the local electric field and the field gradient can always be captured through the TERS mapping images. The method we propose here is believed to be instructive not only for experimental investigations on the local field properties of the nanocavity but also for understanding related single-molecule nonlinear or high-order processes in plasmonic nanocavities.