Evolution and Future of Raman Spectroscopy: Tip-Enhanced Raman Spectroscopy

Spectroscopy has been widely used in various fields such as physics, chemistry, and biology. Spectroscopy can be divided into several kinds according to the mechanisms of the interaction between electromagnetic radiation and matter. These kinds of spectroscopy may be subdivided into fluorescence, absorption, and scattering phenomena. Fluorescence spectroscopy detects the re-emitted light from the absorbed light energy, absorption spectroscopy measures the absorption of the light energy in the matter, and scattering spectroscopy is an optical technique that observes the scattered light when the light is irradiated into the matter. 

In particular, among the scattering spectroscopy techniques, Raman spectroscopy is known as a nondestructive optical technique that analyzes the physical and chemical properties of matter by using Raman scattered light. When light is irradiated into a substance, the light can be scattered due to the interaction of the light and the substance. When the scattered light retains the energy of the incident light, the scattering process indicates Rayleigh scattering. The scattering mechanism, which either obtains or loses the energy through either emitting or absorbing the phonon by lattice vibrations, is called Raman scattering. In particular, when an atom or molecule absorbs energy and the scattered phonon has less energy than the incident photon, this is called Stokes-Raman scattering, and this Stokes-Raman scattering has been commonly analyzed in Raman spectroscopy. In general, through Raman scattering analysis, the energy and symmetry of phonons are revealed and the information on the crystal structure of the substance can be attained. Although Raman spectroscopy has the great advantages of label-free detection and being a non-destructive technique, it is difficult to obtain the Raman scattering signal because of the small scattering cross-section in Raman spectroscopy.

To solve this problem, surface-enhanced Raman spectroscopy (SERS) was introduced to enhance the Raman signals. In SERS, the Raman scattering signals from the matter in the vicinity of the metal nanoparticles can be enhanced and measured by using the induced electric field due to localized surface plasmon resonance (LSPR), which indicates an interaction between an electromagnetic wave and the nanoparticles. Even though the limitations of conventional Raman spectroscopy have been overcome through SERS, it is technically difficult to obtain a Raman scattering spectrum or image that exhibits a high spatial resolution beyond the diffraction limit. Just as SERS can address limitations of traditional Raman spectroscopy, the limitations of SERS can be resolved through a state-of-the-art spectroscopic technique called tip-enhanced Raman spectroscopy (TERS).

TERS is an advanced spectroscopic technique capable of investigating the Raman scattering features at the scale of a few nanometers and is a system that combines scanning probe microscopy (SPM) for surface analysis of matter and confocal Raman spectroscopy. TERS uses the mechanisms of LSPR and the lightning rod effect with a metallic nanotip for SPM measurements, instead of the metal nanoparticles of SERS. Through the lightning rod effect, the charges in the metallic nanotip can be accumulated at the tip apex and these charges introduce a strong electric field at the apex. Moreover, together with the lightning rod effect, the LSPR, which can be generated by laser irradiation with a specific wavelength, also induces the strong electric field at the tip apex. Using the induced electric field at the apex, the intensity of Raman scattering in TERS can also be enhanced when compared to conventional Raman spectroscopy. Also, because the enhanced Raman signals are obtained from the sharp apex of the nanotip, it is possible to observe the Raman characteristics of a localized nanometer-scale region of matter and measure the nanoRaman scattering spectrum or image by using a combined SPM technique. Furthermore, TERS can overcome the limitations of SPM and electron microscopy. To analyze the properties of nanomaterials or the nano-scaled area of bulk materials, SPM, such as atomic force microscopy (AFM), scanning tunneling microscopy (STM); and electron microscopies such as transmission electron microscopy (TEM), and scanning electron microscopy (SEM), have been advanced. These analytical techniques facilitate observation of the physical properties, including various defects and crystal structures, of the sample at the sub-nanometer level. However, the optical and chemical properties cannot be measured using those surface-analysis techniques. Therefore, TERS has been developed to overcome those existing limitations.

Given that TERS has the big advantages of SPM and confocal Raman spectroscopy, it can be usefully utilized to investigate broadly based defects that affect the intrinsic properties of materials. Recent research has reported that TERS discovers the new properties of defects in two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs). Park and associates revealed the new structural disorders in large-area graphene using multispectral TERS imaging, showing that the grain boundaries have the structures of twisted bilayer (1). Furthermore, Lee and associates reported the new defect-related Raman vibrational mode of monolayer WS₂ among TMDs, unveiled by TERS imaging with a high spatial resolution (2). Additionally, Huang and associates showed the different Raman scattering properties depending on types of edge defects in mechanically exfoliated MoS₂ and also found a new edge-induced Raman peak generated by double resonance Raman scattering (3).

TERS, as a quantitative and qualitative analysis technique, enable us to acquire the phonon information of materials and to determine the types and amounts of defects through the Raman scattering process. As a result, TERS can play a crucial role in the field of defect evaluation for many materials and will be useful and helpful for analyzing a single molecule up to bulk materials.

References

1. K.-D. Park, M.B. Raschke, J.M. Atkin, Y.H. Lee, and M.S. Jeong, Adv. Mater. 29, 1603601 (2017).
2. C. Lee, B.G. Jeong, S.J. Yun, Y.H. Lee, S.M. Lee, and M.S. Jeong, ACS Nano 12(10), 9982–9990 (2018).
3. T.-X. Huang, X. Cong, S.-S. Wu, K. -Q. Lin, X. Yao, Y. -H. He, J.-B. Wu, Y.-F. Bao, S.C. Huang, X. Wang, P.-H. Tan, and B. Ren, Nat. Commun.10(1), 5544 (2019).

Chanwoo Lee is a postdoctoral researcher and Mun Seok Jeong is an associate professor, both in the Department of Energy Science at Sungkyunkwan University (SKKU), in Suwon, Korea. Direct correspondence to mjeong@skku.edu