First-layer effect in graphene-enhanced Raman scattering.

Graphene as a substrate for enhancing Raman scattering, called graphene-enhanced Raman scattering (GERS), has been reported in previous work. Herein, it is found that the "first-layer effect", which is widely used to explain the chemical-enhanced mechanism in surface-enhanced Raman scattering (SERS), exists in the GERS system. The Langmuir-Blodgett (LB) technique is used to construct mono- and multilayer ordered aggregates of protoporphyrin IX (PPP). Raman spectra of PPP with different layer numbers of the LB film on graphene are collected. The Raman signal from the first monolayer LB film of PPP has a larger contribution to the Raman enhancement than that from subsequent monolayers. Meanwhile, the Raman enhancement is dependent on the molecular configuration in contact with graphene, in which the functional group of PPP in direct contact with graphene has a stronger enhancement than other groups. These results reveal that GERS is strongly dependent on the distance between graphene and the molecule, which is convincing evidence that the Raman enhancement effect based on graphene belongs to the chemical-enhanced mechanism. This discovery provides a convenient system for the study of the chemical-enhanced mechanism and will benefit further understanding of SERS.