During the last few years, there has been an explosion of interest and activity in the field of nanoscale vibrational spectroscopy. The goal is to confine and manipulate light on the sub-nanometer length scale using the properties of the collective electronic excitations in noble metal nanostructures, known as surface plasmons. An improved understanding of the interactions between adsorbed molecules and local environment under various experimental conditions is having a significant impact in a number of research areas including electrochemistry, surface science, catalysis for energy conversion and storage, low dimensional materials, biomedical diagnostics, and art conservation science. In the first part of this lecture, I will provide some background materials on the basic physical concepts underlying plasmonic enhanced Raman spectroscopy with an emphasis on localized surface plasmon resonance (LSPR), tip-enhanced Raman spectroscopy (TERS), and the high spatial resolution of TERS. In the second part of this lecture, I will focus in on two recent advances in ultrahigh vacuum (UHV) TERS which illustrate the power of this emerging technique. First, new insights into the nature of a dynamic phase boundary involved in the room temperature (RT), UHV-TERS of the Ag tip/ N-N'-bis(2,6-diisopropylphenyl)-1,7-(4'-t-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PPDI)/Ag(100) system will be described. We have unravelled the orientation of PPDI molecules at the dynamic molecular domain boundary with unprecedented 4 nm spatial resolution by UHV-TERS mapping. This topic illustrates that TERS provides access to molecular adsorption geometries in the region with little STM topographical information. Second, we have achieved unprecedented Angstrom-scale spatial resolution in TERS. At room temperature, the strong adsorbate-substrate interaction between the meso-tetrakis-(3,5-di-tertiarybutylphenyl)-porphyrin (H2TBPP) and the underlying Cu(111) substrate leads to the formation of the bowl up/down conformations. Though simultaneous UHV-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. This last topic demonstrates that TERS reveals more detailed information on the adsorbate-substrate interaction which is not easily accessible by other techniques.

Tip-enhanced Raman spectroscopy with Angstrom resolution

254th National Meeting (2017)

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