2D Material-Based Surface-Enhanced Raman Spectroscopy Platforms (Either Alone or in Nanocomposite Form)-From a Chemical Enhancement Perspective

Abstract

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopic technique with molecular fingerprinting capability and high sensitivity, even down to the single-molecule level. As it is 50 years since the observation of the phenomenon, it has now become an important task to discuss the challenges in this field and determine the areas of development. Electromagnetic enhancement has a mature theoretical explanation, while a chemical mechanism which involves more complex interactions has been difficult to elucidate until recently. This article focuses on the 2D material-based platforms where chemical enhancement (CE) is a significant contributor to SERS. In the context of a diverse range (transition metal dichalcogenides, MXenes, etc.) and categories (insulating, semiconducting, semimetallic, and metallic) of 2D materials, the review aims to realize the influence of various factors on SERS response such as substrates (layer thickness, structural phase, etc.), analytes (energy levels, molecular orientation, etc.), excitation wavelengths, molecular resonances, charge-transfer transitions, dipole interactions, etc. Some examples of special treatments or approaches have been outlined for overcoming well-known limitations of SERS and include how CE benefits from the defect-induced physicochemical changes to 2D materials mostly via the charge-transport ability or surface interaction efficiency. The review may help readers understand different phenomena involved in CE and broaden the substrate-designing approaches based on a diverse set of 2D materials.

Figure 1

Timeline of the crucial events contributed significantly to the advancement and realization of the SERS technique.

Figure 2

Schematic illustration of the EM and CE mechanism of SERS.

Figure 3

(A) Raman spectra of the CuPc (2 Å) molecule on the blank SiO₂/Si substrate (black line), graphene (blue line), h-BN (red line), and MoS₂ (green line) substrates. The numbers marked on the peaks are the peak frequencies of the Raman signals from the CuPc molecule. For all of the spectra, the baseline correction was removed to have a better comparison. (B) Optical image of a h-BN flake. Some h-BN flakes are marked by arrows or by a red dashed ring. (C) Raman mapping image for the CuPc vibrational mode at 1531 cm^–1 corresponding to (B). Reprinted with permission from ref (33). Copyright 2014 American Chemical Society.

Figure 4

(A) Schematic illustration of synthesizing the AuNPs@MoS₂ nanocomposite. TEM images of (B) MoS₂ and (C–G) AuNPs@MoS₂ nanocomposites. Reprinted with permission from ref (120). Copyright 2014 American Chemical Society.

Figure 5

(A) Schematic illustration of the CT process among S-g-C₃N₄, O₂, and Ag. The label δ denotes the negative charge of the Ag surface or S-g-C₃N₄. (B) TEM image of the S-g-C₃N₄/Ag hybrid. (A,B) Reprinted with permission from ref (43). Copyright 2016 the author(s).

Figure 6

(A) Raman profile of R6G (10^–6 M) on substrates deposited with an unincorporated MoS₂ sample, hydrothermally treated oxygen-substituted MoS₂ sample at 200 °C, partially oxidized sample at 300 °C for 40 min, completely oxidized MoO₃ sample, and bare SiO₂/Si. (B) Energy-level diagrams illustrating the electronic transitions. The calculated band structures of MoS₂ (a) and MoS_xO_y (b) taking the Fermi level as a reference. Schematic energy-level diagrams of R6G on (c) MoS_xO_y and (d) MoS₂ and MoO₃ with respect to the vacuum level. (C) Raman spectra of 10^–5, 10^–6, 10^–7, and 10^–8 M RhB on PdSe₂. (D) Energy band diagram showing the CT pathways in the RhB/PdSe₂ hybrid system. (A,B) Reprinted with permission from ref (138). Copyright 2017 the author(s). (C,D) Reprinted with permission from ref (141). Copyright 2023 the author(s).

Figure 7

(A) Graphical representation of in situ one-step solution processing synthesis of Ag, Au, and Pd@MXene (Ti₃C₂T_x) hybrids by soft-solution processing via a sonochemical approach. (B) Raman spectrum of Ti₃C₂T_x after soaking in MB dispersed in ethanol and subsequent drying. SERS spectra of MB with (b) Ag@, (c) Au@, and (d) Pd@MXene. Reprinted with permission from ref (169). Copyright 2016 the author(s).

Figure 8

(a,b) SEM images of a Ti₃AlC₂ bulk structure (a) and Ti₃C₂ MXene (b). (c,d) TEM images, HRTEM images, and the corresponding SAED patterns (inset in the HRTEM images) of Ti₃C₂ MXene (c) and Au–Ti₃C₂ (d). (e,f) SERS spectra of 4-MBA, MeB, and MV powder; SERS spectra of the mixed solution with 10^–5 M 4-MBA, MV, and MeB on Ti₃C₂ (e) and Au–Ti₃C₂ (f) substrates with different excitation lasers of 532, 633, and 785 nm. Reprinted with permission from ref (174). Copyright 2020 the author(s).

Figure 9

(a) SEM micrographs of the r-Ti₃C₂T_x powder. (b) Tapping mode AFM image of Ti₃C₂T_x nanosheets on SiO₂/Si. (c) XRD patterns of the Ti₃AlC₂ MAX phase (black), Ti₃C₂T_x (red), and r-Ti3C2Tx (blue). SERS spectra of the probe molecules: (d) crystal violet at 2 × 10^–6 M, (e) MB at 1 × 10^–6 M, and (f) rhodamine 6G (R6G) at 1 × 10^–7 M, respectively, collected on Ti₃C₂T_x/SiO₂/Si (black) and r-Ti₃C₂T_x/SiO₂/Si (red) substrates. Used with permission of The Royal Society of Chemistry, from ref (179); permission conveyed through Copyright Clearance Center, Inc.