Abstract
1. Introduction
Since it was originally discovered in the 1970s[
Two-dimensional (2D) materials provide us with emerging opportunities in SERS, since the discovery of graphene in 2004[
In this review, we comprehensively summarize novel SERS techniques based on metal-free 2D materials beyond graphene, including TMDs, BP, h-BN, and MXenes. Firstly, we briefly introduce 2D materials’ physicochemical properties, before categorizing the cutting-edge progress of SERS studies based on these substrates. Due to this paper’s length limit, SERS studies of metal-2D-material hybrid substrates are not included here. Finally, we summarize the review, and discuss prospects in this area. We hope this review will serve as a useful reference for researchers in the fields of 2D materials, spectroscopy, and their applications in chemical and biological sensing.
2. Mechanisms of SERS
It is generally believed that the Raman enhancement in SERS originates primarily from two mechanisms: EMs and CMs. With respect to EMs, the Raman enhancement usually occurs around surface locations in noble metal nanoparticles, known as “hot spots”[
Figure 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 (
where
On the other hand, CMs are often attributed to the charge transfer effect, which is not yet clearly understood[
3. SERS based on 2D materials beyond graphene
In this section, we summarize SERS advances based on diverse 2D materials beyond graphene, as shown in Fig. 2. The section is divided into five parts, covering the development of SERS chips based on TMDs, h-BNs, BP, MXenes, and their heterostructures, respectively. In each part, we first briefly introduce the physiochemical properties of 2D materials for use in developing SERS, then discuss state-of-the-art experimental results.
Figure 2.(Color online) Comparison of various 2D materials beyond graphene for SERS applications, including TMDs, BP, h-BN, MXenes, and their heterostructures.
3.1. SERS based on TMDs
TMDs, consisting of covalently bonded X–M–X atoms (M = transition metals of groups IV–VI; X = chalcogen), where M is a transition metal atom (e.g., Mo or W), and X is a chalcogenide atom (e.g., S, Te or Se), have a series of superior properties, ideal for potential use in SERS chips[
Given these excellent properties, TMDs have been extensively studied for use in SERS chips[
Figure 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,
3.2. SERS based on h-BN
h-BN, sp2-hybridized 2D insulator is a structural analog of graphene, with sublattices being occupied by equal numbers of boron and nitrogen atoms, arranged alternately in a honeycomb configuration[
Based on these unique properties, h-BN may be a good alternative to graphene for the purpose of Raman enhancement. It has been proposed that the Raman enhancement mechanism for h-BN may stem from different factors. In 2014, Ling et al.[
Figure 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
3.3. SERS based on BP
Of all the 2D materials, BP demonstrates a series of unique characteristics for use in Raman enhancement. Firstly, BP, as a 2D layered material with anisotropy, can provide detailed information about the charge-transfer process as compared to the use of isotropic materials[
To explore the intrinsic SERS performance of BP[
3.4. SERS based on MXenes
MXenes share a general formula of Mn+1XnTx(n = 1–3), where M is an early transition metal, X is a carbon or nitrogen, and T is the surface termination (O, OH, F, or Cl)[
For their proof-of-the-concept, Ye et al.[
3.5. SERS based on 2D-material heterostructures
Recently, 2D van der Waals (vdW) heterostructures, which are assembled by stacking different 2D crystals on top of one another, have been shown to provide promising platforms for developing SERS chips, since they can take advantage of the merits of various SERS materials. For instance, GERS depends on the ground-state charge transfer at the interface, while the enhancement of the Raman scattering is subject to the DOS of graphene[
Owing to the merits of 2D heterostructures, many efforts have been made to develop 2D heterostructures for studying SERS[
Figure 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/ReOxSy-MT chips. (d) Energy level diagrams and charge transfer in the R6G-W18O49/MoS2 complex. (a) and (b) are reprinted with permission from Ref. [
4. Conclusion and perspective
In this paper, we have reviewed the recent advances in SERS chips based on 2D materials for chemical and biological sensing. Since many excellent review papers of GERS have been published in the past few years[
Compared with SERS based on metallic materials, 2D-material-based SERS chips are still in development. Firstly, in terms of mechanisms, Raman enhancement of 2D-material substrates mainly originates from CMs, specifically charge transfer resonance and dipole−dipole interaction, which varies based on different substrate materials and analytes. With the discovery of emerging low-dimensional materials, such as MnPS3–xSex[
Acknowledgements
This work was supported by the National Natural Science Foundation of China (61805175), Japan Society for the Promotion of Science (JP18K13798), China Postdoctoral Science Foundation (2020M670641).
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