Surface Enhanced Raman Scattering Revealed by Interfacial Charge-Transfer Transitions

Abstract

Surface enhanced Raman scattering (SERS) is a fingerprint spectral technique whose performance is highly dependent on the physicochemical properties of the substrate materials. In addition to the traditional plasmonic metal substrates that feature prominent electromagnetic enhancements, boosted SERS activities have been reported recently for various categories of non-metal materials, including graphene, MXenes, transition-metal chalcogens/oxides, and conjugated organic molecules. Although the structural compositions of these semiconducting substrates vary, chemical enhancements induced by interfacial charge transfer are often the major contributors to the overall SERS behavior, which is distinct from that of the traditional SERS based on plasmonic metals. Regarding charge-transfer-induced SERS enhancements, this short review introduces the basic concepts underlying the SERS enhancements, the most recent semiconducting substrates that use novel manipulation strategies, and the extended applications of these versatile substrates.

Keywords: SERS; charge transfer; chemical mechanism; defect engineering; semiconductor.

Graphical abstract

Figure 1

Schematic Illustrating the Charge-Transfer (CT) Induced SERS: Substrates and Applications

Figure 2

Electromagnetic and Chemical Mechanism for SERS Enhancement. (A) Electromagnetic enhancement in SERS based on plasmonic nanospheres. (B) Schematic illustration of a “hot spot” in the gap between adjacent particles and the corresponding change in SERS enhancement factor with relative positions. Comparison of the charge-transfer transitions in a metal-molecule system (C) and a semiconductor-molecule system (D).

Figure 3

Graphene-Based SERS Substrates. (A) Raman signals of Pc deposited on graphene. (B) Single-layer graphene substrate modulated with electrical field. (C) Graphene substrate by ozone treatment. (D) Graphene quantum dots (GQDs) as SERS substrate. Reprinted with permission from Ling et al. (Copyright 2010, American Chemical Society) (A), Xu et al. (Copyright 2011, American Chemical Society) (B), Huh et al. (Copyright 2011, American Chemical Society) (C), and Liu et al. (Copyright 2018, Nature Publishing Group) (D).

Figure 4

TMD and MXene Substrates. (A) Oxygen incorporation in MoS₂ substrate. (B) 1T′-WTe₂ substrate with distributed Brag reflector (DBR) as field enhancer. (C) Spray-coated Ti₃C₂T_x multi-layered structure as SERS substrate. (D) Al oxyanion functionalized Ti₃C₂T_x substrate. Reprinted with permission from Zheng et al. (Copyright 2017, Nature Publishing Group) (A), Tao et al. (Copyright 2018, American Chemical Society) (B), Sarycheva et al. (Copyright 2017, American Chemical Society) (C), and Li et al. (Copyright 2020, American Chemical Society) (D).

Figure 5

Semiconducting Oxides as SERS Substrates. (A) TiO₂ shell-based resonators. (B) Oxygen-defect engineering strategy for boosted SERS in tungsten oxide. (C) Electrochromic SERS substrate with high reproducibility. (D) Mechanism diagram of the “coupled resonance” strategy on the Ta₂O₅ substrate. Reprinted with permission from Alessandri (Copyright 2013, American Chemical Society) (A), Cong et al. (Copyright 2015, Nature Publishing Group) (B), Cong et al. (Copyright 2019, Nature Publishing Group) (C), and Yang et al. (Copyright 2019, John Wiley & Sons, Inc.) (D).

Figure 6

MOFs and Conjugated Molecules as SERS Substrates. (A) MOF substrates with high tailorability. (B) Nanostructured organic semiconductor films. Reprinted with permission from Sun et al. (Copyright 2019, American Chemical Society) (A) and Yilmaz et al. (Copyright 2017, Nature Publishing Group) (B).

Figure 7

Van der Waals Heterostructure and Metal-Semiconductor Heterostructure. (A) Heterostructure between WSe₂ monolayer and graphene. (B) Metal/W₁₈O₄₉ heterojunction for controllable charge transfer. (C) Charge transfer in TiO₂-Ag-MPY-FePc System. (D) Irreversible accumulated SERS based on Ag/TiO₂ hybrid. Reprinted with permission from Tan et al. (Copyright 2017, American Chemical Society) (A), Gu et al. (Copyright 2018, Royal Society of Chemistry) (B), Wang et al. (Copyright 2019, John Wiley & Sons, Inc.) (C), and Zhou et al. (Copyright 2020, Nature Publishing Group) (D).

Figure 8

Typical Applications of SERS (A) Au@ZIF-8 3D structure for cancer-related VOCs diagnostic. (B) Scheme of an “on-edge” junction under Raman setup. (C) SERS detection of the electrolyte solvation structure at solid-liquid interface. (D) SERS monitoring the dynamics of photoinduced surface oxygen vacancies in metal-oxide semiconductors. Reprinted with permission from Qiao et al. (Copyright 2018, John Wiley & Sons, Inc.) (A), Ioffe et al. (Copyright 2008, Nature Publishing Group) (B), Yang et al. (Copyright 2018, American Chemical Society) (C), and Glass et al. (Copyright 2019, John Wiley & Sons, Inc.) (D).