RESUMO
Tip-enhanced Raman spectroscopy (TERS) is particularly sensitive to analytes residing at plasmonic tip-sample nanojunctions, where the incident and scattered optical fields may be localized and optimally enhanced. However, the enhanced local electric fields in this so-called gap-mode TERS configuration are nominally orthogonal to the sample plane. As such, any given Raman active vibrational eigenstate needs to have projections (of its polarizability derivative tensor elements) along the sample normal to be detectable via TERS. The faint TERS signals observed from two prototypical systems, namely, pristine graphene and graphene oxide are a classical example of the aforementioned rather restrictive TERS selection rules in this context. In this study, we demonstrate that nanoindentation, herein achieved using pulsed-force lithography with a sharp single-crystal diamond atomic force microscope probe, may be used to locally enhance TERS signals from graphene and graphene oxide flakes on gold. Nanoindentation locally perturbs the otherwise flat graphene structure and introduces out-of-plane protrusions that generate enhanced TERS. Although our approach is nominally invasive, we illustrate that the introduced nanodefects are highly localized, as evidenced by TERS nanoscale chemical mapping. As such, the described protocol may be used to extend and generalize the applicability of TERS for the rapid identification of two-dimensional material systems on the nanoscale.