Fig. 1. (Color online) Schematics of the mechanisms of SERS. (a) With respect to EMs, when the incident laser is in resonance with the nanoparticle LSPR frequency, the incident laser excites electrons on the metal surface, leading to a polarization of charge and oscillating dipoles. As the frequencies of Raman scattering light are close to that of the incident laser, the resonance also increases the intensity of the Raman scattering light. (b) For CMs, electrons are transferred from the Fermi level of the substrate to the LUMO of the molecule, thereby forming a charge transfer intermediate. The energy transition ( ) between the Fermi level of the substrate and LUMO is much stronger than that ( ) between the highest occupied molecular orbit (HOMO) and LUMO, resulting in a higher Raman scattering cross-section.

Fig. 2. (Color online) Comparison of various 2D materials beyond graphene for SERS applications, including TMDs, BP, h-BN, MXenes, and their heterostructures.

Fig. 3. (Color online) SERS studies based on TMDs. (a) Schematic of measurement and enhanced Raman spectra of 4-mercaptopyridine on monolayer MoS₂. (b) Energy levels of the oxygen-incorporation MoS₂-R6G system. Here, and denote exciton transition and molecular transition, and denotes PICT. (c) Measured Raman spectra of R6G molecules on 1T MoSe₂, 1T MoS₂, 2H MoSe₂, and 2H MoS₂ substrates. (d) Measured Raman spectra of R6G on a 1T’-WTe₂ substrate, with and without the DBR. (a) is reprinted with permission from Ref. [50]. Copyright © 2016 American Chemical Society. (b) is reprinted with permission from Ref. [53]. Copyright © 2017, Nature Publishing Group. (c) is reprinted with permission from Ref. [57]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) is reprinted with permission from Ref. [ 58]. Copyright © 2018 American Chemical Society.

Fig. 4. (Color online) SERS studies based on h-BN, BP, and MXenes. (a) Preparation of SERS chips based on graphene, h-BN, and MoS₂. The layered 2D materials are shown in gray, while probe molecules are shown in red. (b) Raman spectra of CuPc molecules on SiO₂/Si (black line), MoS₂ (green line), h-BN (red line), and graphene (blue line) substrates. (c) Raman spectra of RhB molecules (~10^–8 M) on a BP substrate, showing different Raman peaks, which could be attributed to different vibrational transitions in the RhB molecules. (d) Schematic of Ti₂NT_x, etched and delaminated from Ti₂AlN, and employed as a SERS substrate. (a) and (b) are reprinted with permission from Ref. [68]. © 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (c) is reprinted with permission from Ref. [ 80]. Copyright © 2019 the Royal Society of Chemistry. (d) is reprinted with permission from Ref. [92]. Copyright © 2017 American Chemical Society.

Fig. 5. (Color online) SERS studies based on 2D heterostructures. (a) Schematic of Raman measurement of CuPc molecular coating on G/W/G/W chips. (b) Raman spectra of CuPc molecular coating on G/W/G/W and G/W chips, respectively. (c) Schematic of Raman enhancement mechanism of graphene/ReO_xS_y-MT chips. (d) Energy level diagrams and charge transfer in the R6G-W₁₈O₄₉/MoS₂ complex. (a) and (b) are reprinted with permission from Ref. [16]. Copyright © 2017 American Chemical Society. (c) is reprinted with permission from Ref. [98]. Copyright © 2020 American Chemical Society. (d) is reprinted with permission from Ref. [104]. Copyright © 2019 American Chemical Society.

Table 1. Comparison of SERS results based on different substrates.