Jing Tang1
Boston University1
Surface Enhanced Raman Spectroscopy (SERS), introduced in 1974 by Fleischmann et al., stands as a pivotal technique enabling the detection of molecules at an unprecedented level of sensitivity, reaching even the single-molecule scale. In its nascent stages, SERS relied upon the plasma generated by the metal nanoparticles. The resonance between the plasmonic electric magnetic (EM) field and the probe molecule effectively amplified Raman scattering signals. However, the advent of SERS on graphene, as initially reported by Ling et al. in 2009, marked a significant departure from the conventional EM theory because it lacks the capacity to generate EM fields within the visible light spectrum. Subsequently, the investigation of the mechanisms underlying SERS on 2D materials emerged as a prominent and popular topic. Graphene, as a 2D material, plays multifaceted roles in SERS, encompassing its use as a Raman probe, substrate, additive, and fundamental building block for flat surfaces.<br/>Among the burgeoning areas of interest, twisted moiré-patterned graphene has garnered particular attention owing to its intriguing properties, including superconductivity at the magic angle. Twisted graphene, characterized by varying twist angles, exhibits distinct electronic band structures, which, in turn, influence the coupling dynamics with probe molecules. By uniformly depositing the same quantity of probe molecules (e.g., Rhodamine B) onto diverse twisted graphene samples, a versatile platform is established for probing the intricacies of SERS mechanisms. Additionally, our investigation extends to single-wall carbon nanotube (SWCNT) substrates, where aligned and randomly oriented SWCNTs are employed. This study encompasses the comprehensive examination of polarized Raman spectra, photoluminescence (PL), and absorption characteristics across these systems. Our findings reveal the presence of band structure-dependent and angle-dependent interactions between the substrate and the target molecule, contributing significantly to the observed SERS effects.<br/>In light of our experimental results, we formulate a renormalized SERS mechanism theory, grounded in a normalized charge transfer model that accounts for the lattice symmetry, thus providing a robust explanation for 2D SERS phenomena. This in-depth analysis underscores the critical importance of elucidating the fundamental principles underpinning SERS, enriching our understanding of this remarkable spectroscopic technique. Our focused inquiry into the theoretical aspects of SERS offers valuable insights for researchers engaged in this dynamic field of study.