This talk explores a new path forward toward the goal of probing single-molecule processes via scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS). At the beginning of this talk, I will show you the molecular resolution imaging with multiple vibration modes detection by a plasmonic STM probe. The probe plays a key role both in achieving atomic resolution and enabling single molecule chemical sensitivity through plasmonic enhancement.1 Besides chemical identification of these molecular adsorbates, intramolecular vibrational distribution can be used for unraveling the intricacies of adsorbate-adsorbate/ adsorbate−substrate interactions. At room temperature, the strong adsorbate-substrate interaction between the meso-tetrakis-(3,5-ditertiarybutylphenyl)-porphyrin (H2TBPP) and the underlying Cu(111) substrate leads to the formation of the bowl up/down conformations. Through simultaneous TERS and STM analysis on the neighboring conformational isomers, we have observed ~15 cm-1 spectral shift in one of the porphyrin-ring Raman modes and analyzed the origin of this shift using DFT calculations. Angstrom-scale spatial resolution in TERS has been achieved. 2 Then we reveal insights into the influence of intermolecular interactions on the vibrational spectra of a N-N’-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) self-assembled monolayer adsorbed on single-crystal Ag substrates. In particular, we have revealed the lifting of a vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule.3 This last topic illuminates a new two-dimensional molecular superstructure of subphthalocyanine (SubPc) was discovered to consist of two distinct molecular binding configurations, both of which interact relatively weakly with the underlying metallic substrate as revealed by high-signal-to-noise enhanced Raman spectra. Our results demonstrate the necessity of advanced Raman techniques such as TERS when precisely probing molecule-molecule and molecule-substrate interactions.4

Reference: [1] N. Jiang, E. T. Foley, J. M. Klingsporn, M. D. Sonntag, N. A. Valley, J. A. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne, Observation of Multiple Vibrational Modes in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Combined with Molecular-Resolution Scanning Tunneling Microscopy, Nano Lett., 12, 5061 (2012). [2] N. Chiang, X. Chen, G. Goubert, D. V. Chulhai, X. Chen, E. A. Pozzi, N. Jiang, M. C. Hersam, T. Seieman, L. Jensen, and R. P. Van Duyne, Conformational Contrast of Surface-Mediated Molecular Switches Yields Angstrom-Scale Spatial Resolution in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy, Nano Lett., 16, 7774 (2016). [3] N. Chiang, N. Jiang, L. R. Madison, E. A. Pozzi, M. R. Wasielewski, M. A. Ratner, M. C. Hersam, T. Seideman, G. C. Schatz, and R. P. Van Duyne, Probing Intermolecular Vibrational Symmetry Breaking in Self-Assembled Monolayers with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy, J. Am. Chem. Soc., 139, 18664 (2017). [4] P. Whiteman, J. Schultz, Z. Porach, H. Chen., and N. Jiang, Dual Binding Configurations of Subphthalocyanine on Ag(100) Substrate Characterized by Scanning Tunneling Microscopy, Tip-Enhanced Raman Spectroscopy and Density Functional Theory, J. Phys. Chem. C, 122, 5489 (2018).