## 3 Au TERS for Plasmon-Driven Synthesis Catalysis  
As previously mentioned, TERS is a promising candidate for investigating the internal mechanism and concepts of plasmon-driven surface catalytic reactions owing to its high sensitivity and spatial resolution [14; 17; 21]. Additionally, application in high vacuum environments can improve its potential for ultrasensitive spectral analyses on nanoscales. In addition to the enhanced intensity of the Raman signals, the IR-active modes and Fermi resonance can also be simultaneously obtained with high vacuum TERS (HV-TERS). Moreover, the enhancement factor can be modified by adjusting the gap distance. Here, the selective reduction of 2,4-DNBT to 2,2\({}^{\prime}\)DA-DMAB is used as an example to interpret the unique advantages of HV-TERS over SERS. Based on prior studies, it is revealed that different molecular positions (two nitro groups in different positions) can generate different chemical reactions. Assisted by surface plasmons (SPs), 2,4-DNBT underwent two different types of reductions, one nitro group of 2,4-DNBT was reduced to an amine, while the other group was reduced to an N=N bond as shown in Figure 2. Figure 2a shows the normal Raman scattering spectrum (NRS) of the 2,4-DNBT powder. Combined with the experimental and theoretical Raman spectra given in Ref [62], it can be determined that the peaks at 1306 cm\({}^{-1}\) and 1336 cm\({}^{-1}\) correspond to the NO\({}_{2}\) stretching modes of the two nitro groups, while the peak at 1524 cm\({}^{-1}\) is attributed to the simultaneous vibration of the nitro groups. Based on the SERS spectrum for 2,4-DNBT (Figure 2b), the disappearance of these Raman peaks related to nitro groups clearly indicates the existence of a surface catalytic reaction. Several new Raman peaks are strongly enhanced and the Raman peak at 1432 cm\({}^{-1}\) is attributed to the N=N bond. However, the dynamics of the process are hard to determine. The TERS spectrum in Figure 2c differs significantly from the NRS and SERS of 2,4-DNBT. In HV-TERS, the IR-active modes of the target molecule can be obtained simultaneously owing to the large electric field gradient effect [63; 64; 65; 66]. The Hamiltonian can be expressed as follows [67]:  
\[H=H_{0}+H_{1}+H_{2}=\bigg{(}\alpha_{\alpha\beta}E_{\beta}+\frac{1}{3}A_{\alpha, \beta\gamma}\frac{\partial E_{\beta\gamma}}{\partial r}\bigg{)}E_{\alpha}+ \frac{1}{3}\bigg{(}A_{\gamma,\alpha\beta}E_{\gamma}+C_{\alpha\beta,\gamma\delta }\frac{\partial E_{\gamma\delta}}{\partial r}\bigg{)}\frac{\partial E_{\alpha \beta}}{\partial r} \tag{5}\]  
where \(E_{\alpha}\) represents the external electric field and \(\frac{\partial E_{\alpha\beta}}{\partial r}\) is the external electric field gradient [67; 68; 69]. According to the calculation above, the reason the IR-active modes were obtained in TERS and not in SERS was due to their different types of measurement. TERS is a near-field measurement, in which the electric field gradient effect plays a crucial role. To analyze the electric field gradient effect, calculations based on the TERS system were extensively investigated (Figure 3a). The distribution in Figure 3b shows that the radius with the strongest electric field intensity is approximately 3.0 nm. Figure 3c shows that the strongest intensity is focused in the area of 1.0 nm \(<r<\) 4.0 nm Figure 3d shows the ratio of the electric field gradient to the electric field along the tip axis, which is only 1/50. Figure 3e shows the electric field intensity at the center along the substrate, which differs from Figure 3b where the strongest electric field intensity is focused in the region of 1.0 nm \(<r<\) 4.0 nm with the electric field intensity along the tip excluded. The electric field gradient along the substrate in Figure 3f is similar to that along the tip. According to the ratio in Figure 3g, the effective region is 1.0 nm \(<r<\) 4.0 nm, and the ratio can increase to 1.2 when the larger terms in Equation (1) are taken into account.