Reaching sub-nanoscale resolution with visible light to encounter surface structure has experienced significant advancement in nano-optics. As such, conventional spectroscopic techniques are limited by the optical diffraction limit to about half wavelength that offers about 200 nm × 200 nm microscopic zone for working in the visible light range. In that context, tip-enhanced Raman spectroscopy (TERS) emerges as an advanced analytical technique, where the plasmonically active probe is not only used to detect the tunneling current but also to interrogate the local chemical environment of surface adsorbed molecules with angstrom scale precision. In TERS, the incident laser irradiation is focused on the tip apex which excites localized surface plasmon (LSP) polaritons and causes the electromagnetic enhancement at the tip-sample junction. Here, we report various applications of scanning tunneling microscopy (STM)-based TERS towards single-molecule chemical analysis, surface-adsorbed configurations analysis, and on-surface chemical reaction at sub-nanoscale spatial resolution. Furthermore, the newly discovered tow-dimensional material i.e., borophene structure has been functionalized both physically and chemically by organic molecules and oxygen respectively. The sub-nanometer scale interfacial strain of borophene due the molecules has been accurately determined using TERS. Again, the chemical bond between oxygen and borophene can be tracked at the atomic resolution. Additionally, the highly energetic plasmons generated at the tip apex can also be utilized for site-selective activation inside a single molecule which could prove extremely useful in solar-to-chemical energy conversion-related problems.

Sub-nanoscale chemical analysis with nano-confined localized surface plasmons

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