The UV spectroscopic results show that C8 HAT on Au exhibits the highest photoconversion after irradiation at 415 nm. When we further increase the linker length, it does not lead to an even higher conversion. As a cross validation, the photoisomerization of a C8 HAT SAM on Au was verified by TERS. TER spectra of a C8 HAT SAM on Au before and after irradiation at 415 nm are shown in Figure and, respectively. In the TER spectrum of a C8 HAT SAM, we observed similar spectral features as we found in the confocal Raman spectra, consistent with the predicted Raman spectrum of the Z isomer, except for the disappearance of the peak at 1680 cm -1 . This could result from the low signal-to-noise ratio of the spectrum, as the intensity of this peak is intrinsically weak and further reduced because of the surface selection rule of TERS , in which in-plane vibrations are cancelled by the induced mirror dipoles in the substrate. After irradiation at 415 nm, the peaks at 1430 and 1469 cm -1 almost disappear, while the intensity of the peak at 1073 cm -1 significantly grows, and simultaneously a new peak appears at 1572 cm - 1 . All these spectral differences indicate the occurrence of the Z-to-E isomerization. Furthermore, to quantify the isomerization ratio of C8 HAT after 415 nm light irradiation, we collected TERS maps of a C8 HAT SAM on Au before and after irradiation at 415 nm with a pixel size of ca. 30 nm. As shown in Figure and 5d, after irradiation with 415 nm light, we observed the same spectral changes in most of the TER spectra within the map as those observed in the single TER spectrum. This indicates that most of the Z-C8 HAT molecules have isomerized to the E form. As mentioned above, the peak at 1572 cm -1 is characteristic for the E isomer, such thus we could use this peak to calculate the conversion efficiency. As shown in the TERS map of the peak intensity at 1572 cm -1 (Figure ), this peak appears in 85% of the spectra. This value agrees with the conversion efficiency calculated from the UV-vis spectroscopic results of the C8 HAT SAM on Au. Apart from C8 HAT, we also collected TERS maps of C3 HAT/C11 HAT before and after irradiation at 415 nm (Figure and). In both cases, we observed spectral features similar to those found in the TERS maps of C8 HAT, indicating the occurrence of isomerization upon light irradiation. Furthermore, all TER spectra within a map were accumulated and averaged to obtain statistical information. Notably, we found that a small peak appears at 1709 cm -1 in the averaged TER spectra of C3 and C11 HAT SAMs on Au after irradiation at 415 nm, while the intensity of the peak at 1680 cm -1 decreases (Figure and). All these spectra changes confirm the isomerization.