Switching from electromagnetic to chemical mechanism in quantum plasmonic tip-induced graphene oxide enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is based on the ability of a surface substrate to increase Ra-man signals for sensing and imaging applications. The most widely used Au and Ag SERS substrates are primarily based on the electromagnetic mechanism (EM) with large enhancement factors (EFs), which are, however, limited by small gaps due to tunneling. Graphene has been explored as an alternative substrate for graphene-enhanced Raman scattering based on the chemical mechanism (CM). However, the limits of CM EFs in graphene-based substrates have not been well understood, especially as a function of tip-sample distance (TSD). Here we performed tip-enhanced Raman scattering of carbon nanotubes on Au and graphene-oxide (GO) hybrid substrates for different TSDs. We show evidence of quantum plasmonics with GO as a tunneling junction in an Au-GO-Au cavity with a 2-nm gap size. We demonstrate Raman signal enhancement by four orders of magnitude beyond the EM tunneling limit by switching to the CM regime for the resonant GO excitation at small TSD. Tip-induced GO-enhanced Raman scattering may be used to improve nanoimaging and biosensing on novel hybrid GO/Au substrates.