## 3 Au TERS for Plasmon-Driven Synthesis Catalysis
Figure 3h shows the distance-dependent ratio of \(\frac{\partial E_{\beta\gamma}}{\partial r}/E_{\beta}\). When the measurement distance is greater than 5 nm in Figure 3h, the effect of the electric field gradient can be almost neglected, and thus the SERS spectra measured in the far field cannot observe the IR-active modes. Based on Figure 3, the reason for the difference between SERS and TERS in Figure 2 can be revealed. Figure 2: (**a**) Normal Raman scattering (NRS) of 2,4-DNBT power, (**b**) and (**c**) SERS and TER spectra of 2,4-DNBT [62]. Figure 3: **(a)** The diagram of TERS system, **(b)** the stimulated electric field intensity at the center, **(c)** the stimulated electric field gradient intensity along tip axis **(d)** the ratio between electric field gradient and electric field intensity, **(e)** the stimulated electric field intensity along the substrate, **(f)** the stimulated electric field gradient intensity along the substrate, **(g)** the ratio between electric field gradient and electric field and **(h)** the distance dependent ratio [62]. Comparing the TERS for 2,4-DNBT in Figure 4a with calculations for 2,2'DA-DMAB (Figure 4b,c) reveals that the peaks in the TERS spectrum can be approximately assigned to the Raman-active and IR-active modes. Moreover, the'mysterious' peaks at 1354 cm\({}^{-1}\) and 1370 cm\({}^{-1}\) in the TERS spectrum can be effectively interpreted with the Fermi resonance (FR). Further analysis is provided in the supporting information Reference [62]. In summary, TERS can be considered a powerful analytical method for observing Raman and IR spectra at the same time with stronger intensities and better peak classification. According to Figure 5, the ratio between the intensities of the IR-active and Raman-active peaks can be controlled by adjusting the tunneling current. The tunneling current determines the gap distance, i.e., the ratio can be controlled by the gap. When the gap distance decreases, the near-field gradient effect becomes stronger, leading to an increased ratio. In contrast, when the gap is sufficiently large, the IR-active peaks cannot be detected, similar to the SERS shown in Figure 3h. This conclusion is powerful evidence for the interpretation of the disappearance of IR-active modes in SERS. Figure 4: (a) HV-TERS of DMAB, (b) calculated Raman spectrum and (c) calculated IR spectra [62]. In previous SERS experiments, dimerization of p-aminothiophenol (PATP) to DMAB was an ideal candidate for revealing plasmon-driven catalytic reactions [16; 70; 71; 72; 73; 74; 75; 76]. Both theoretical simulations and experimental observations have been investigated in detail, while the dynamic process of the dimerization needs further investigation [77; 78; 79; 80]. Based on the TERS, the IR-active modes can be simultaneously obtained and can provide more convincing evidence [69; 81; 82; 83]. The TERS measurements for PATP on an Au film at different bias voltages in Figure 6 showed that the product was stable and there was little influence on the polarity of the bias voltage, indicating that the product should be structurally symmetric. Moreover, it is obvious that TERS can provide more information than SERS as more Raman peaks appear. Based on previous studies, DMAB can be generated from PATP on Au NPs, while the SERS for PATP on an Au film is similar to the NRS for PATP [71]. The difference in the probabilities for the chemical reaction on different substrates occurs because the plasmonic enhancement on bare Au film is weak. Figure 5: HV-TERS spectra of 2,2’DA-DMAB produced from 2,4-DNBT with different tunneling currents [62]. Comparing the experimental TERS spectrum with calculated Raman and IR spectra in Figure 7 shows that all of the peaks can be assigned and the IR-active modes are activated, owing to the electric field gradient effect (molecular quadrupole transitions). Figure 6: HV-TERS of PATP measured under 1 nA current and (**a**) +1 V, and (**b**) \(-\)1 V bias voltage at different positions, (**c**) SERS spectrum of DMAB in Au sol and NRS of PATP [68].