Breaking Raman Selection Rule at the Nanoscale in Tip-enhanced Raman Spectroscopy of Carbon Quantum Dot

Tip-enhanced Raman spectroscopy (TERS) is an advanced analytical technique that combines chemically sensitive Raman spectroscopy with a high spatial resolution of scanning probe microscopy. In TERS, strong coupling between light and surface plasmon resonance at a metal tip apex results in a very intense electromagnetic (EM) field confinement, significantly enhancing weak Raman signal. This strong confined field also has a high gradient, leading to the breakdown of Raman selection rules. This opens up new vibrational features and IR active modes become allowed in Raman scattering. In this work, we present a TERS investigation of carbon quantum dots (CDs). The TERS nanoimages reveal the spatial distribution of graphene crystallinity, carbon amorphous, and defects at the nanoscale. We observed the IR vibrational modes which are, in principle, not allowed in Raman spectroscopy. Particularly, at the carbon dot surface materials, water is capable of dissociation, forming functional groups, such as C—OH, C═O, and C—H. These vibrational modes are inactive in far-field Raman spectra but appear in TERS because of selection rule breakdown. By varying the polarization angle of light with respect to the tip axis, we redistribute the EM field confinement and consequently tune the field gradient. When the light polarization aligns parallel to the tip axis, the EM field is strongly confined to a small region beneath the tip apex with a high gradient, resulting in a significant enhancement of both IR-active and Raman modes.