Probing the Vertical Resolving Ability of Tip-Enhanced Raman Spectroscopy

Recent developments in tip-enhanced Raman spectroscopy (TERS) have demonstrated its capabilities in lateral resolution down to Acirc;ngstro''m scale and chemical identification of adjacent molecules at the submolecular level. However, most studies are showcased only on the planar molecules lying flat on surfaces, and the resolving ability of TERS in the vertical direction still remains elusive, especially regarding its ability to resolve vertical heteromolecular structures. In this work, we artificially construct a bilayer-molecular system with one molecule at the tip apex and the other on the substrate to quantitatively investigate the vertical resolving ability of the TERS technique. The vertical detection depth is first measured using a pristine Ag tip by tracking the spectral evolution upon tip retraction away from a C-60 molecule on the substrate until the TERS signals become practically vanished, which yields a detection depth around 500 pm. Consistently, when the tip is decorated by a C-60 molecule at the tip apex, no TERS signals from the CuPc molecule on the substrate can be observed due to the large size of the decorated C-60 molecules of similar to 700 pm in diameter. However, when the Ag-tip apex is decorated with a single flat-lying CuPc molecule with a thickness of similar to 280 pm, the TERS signals of the C60 molecule on the substrate can be detected clearly, demonstrating the capability of TERS to resolve vertical heteromolecular structures. Furthermore, the presence of the molecule at the tip apex is found to influence the local plasmonic field and extend the vertical detection depth up to similar to 580 pm due to the hot-spot transfer effect. Our results not only provide a quantitative study on the vertical resolving ability of the TERS technique but also are instructive for potential applications in resolving subsurface chemical constituents and stereochemical structures.