## 1 Introduction of Tip-Enhanced Raman Spectroscopy (TERS)  
Based on local surface plasmon resonance (LSPR), surface-enhanced Raman scattering (SERS) has been investigated intensively and widely applied since 1974 [1; 2; 3; 4]. The electric field enhancement of Raman signals can reach \(10^{8}\)-\(10^{11}\)[5; 6; 7], while chemical enhancement can only reach \(10^{2}\)-\(10^{3}\)[8; 9; 10]. SERS provides an opportunity to overcome the limitation of low Raman scattering cross-sections, and has been widely applied to the investigation of vibration and chemical properties of molecules. However, SERS can only detect the average of all the vibrations of randomly adsorbed molecules on account of its poor spatial resolution [11; 12; 13; 14]. Currently, surface plasmons have participated widely in plasmon-driven surface catalytic reactions [2; 15; 16; 17; 18; 19; 20; 21; 22; 23].  
To improve the spatial resolution and overcome the limitation of substrates, the use of tip-enhanced Raman spectroscopy (TERS) emerged in 1985, combining the advantages of SERS and scanning probe microscopy (SPM) techniques [24]. The first TERS results were reported independently by [PERSON], [PERSON], [PERSON] and [PERSON] in 2000 [25; 26; 27]. Since these first reports, TERS has attracted wide interest as a high-sensitivity optical analytical technique [28; 29]. TERS can achieve high spatial resolution beyond the diffraction limit of light and can provide chemical information without sacrificing the molecular resolution at very low temperatures [30]. The electromagnetic enhancement factor of Raman signals at the tip apex in TERS can reach \(10^{6}\). With this significant enhancement, the spatial resolution can be enhanced immensely with a sharp metal tip. The enhancement factor can still be maintained for detection of a single molecule (atomic-scale resolution of 1-10 nm) [31; 32; 33; 34].  
Another remarkable advance of TERS is that the IR-active and Raman-active modes of target molecules can be obtained simultaneously, whereas SERS can only provide information on the Raman-active modes. The distance between the tip and substrate can be controlled effectively by adjusting the tunneling currents and the ratio between the signal intensities of the Raman-active and IR-active modes can also be controlled. In summary, several unique advantages make TERS a promising technique for investigating samples with weak scattering [35; 36].  
Samples can be easily contaminated when exposed to air, which can produce an unexpected influence on the results [37; 38; 39]. As a result, it can be difficult to distinguish whether the Raman signal originates from the target molecule, contamination or decomposition of the samples. SPM allows investigations to be monitored in a vacuum, air or aqueous environments. Hence, an ultrahigh vacuum and low temperature environment is usually adopted to enhance the accuracy of TERS results, as these results can shift as a result of the differences in the local environment.  
This review focuses on the principles and application of TERS. The fundamental theory and setup of TERS will be presented in detail and current advanced reports on TERS, especially for plasmon-driven reactions, will be presented. SERS and TERS are of great advantage to the field of plasmon-driven surface catalytic reactions. Finally, some challenges and the outlook for the future will be discussed.