Observation of classical and quantum coupling via tip-enhanced resonant Raman scattering.

Nano-scale optical analysis of several layers MoS2 is demonstrated with spatial resolution of 20 nm by tip-enhanced Raman spectroscopy (TERS). We analyze different optical signatures at the edges and near the center of the MoS2 flakes. With controlling the tip-sample distance with subnanometer precision by atomic force, we explore the unique distance dependence behavior of the tip enhancement. We observe the transition from the classical near-field coupling to the quantum coupling regime. At subnanometer length range, TERS enhancement is significantly reduced, due to tunneling quantum effects.