Sub-Nanometer Resolved Tip-Enhanced Raman Spectroscopy of a Single Molecule on the Si(111) Substrate

Abstract

The investigation of molecular adsorption structures on nonmetallic surfaces is important for understanding the functionalities of nanodevices and biomolecules. In contrast to extensive tip-enhanced Raman spectroscopy (TERS) studies on metallic substrates with strong plasmonic enhancement, the studies on nonmetallic substrates like silicon (Si) are relatively limited. It remains to be clarified whether it is possible to detect the TERS signal from a single molecule directly adsorbed on a nonmetallic surface and how precise its spatial resolution could be achieved. Here we use Si(111) as a prototype nonmetallic substrate without plasmonic responses (in the visible region) and investigate the TERS of single bis(phthalocyaninato) terbium(III) (TbPc2) molecules adsorbed on this surface. We can not only detect the TERS signal from a single TbPc2 molecule, but also specify distinct adsorption configurations according to different TERS spectral features. The spatial resolution of TERS is also found down to ∼0.6 nm, which is slightly poorer than but still comparable with the resolution for the TbPc2 molecule directly adsorbed on the silver substrate. By combining with theoretical simulations, we find that although a plasmonic substrate can induce a larger TERS enhancement than a dielectric substrate, the subnanometer spatial resolution of TERS is mainly determined by the atomistic protrusion at the apex of a plasmonic metal tip, regardless of whether the substrate is plasmonic or dielectric. These findings offer a promise for applying subnanometer resolved TERS to a wide range of systems that contain nonmetallic substrates, such as in the semiconductor industry, 2D materials, and biomedical science.