Low-Background Tip-Enhanced Raman Spectroscopy Enabled by a Plasmon Thin-Film Waveguide Probe

• Published in: Analytical chemistry (Washington) 2021-06, Vol.93 (21), p.7699-7706
• Main Authors: Zhang, Kaifeng, Bao, Yifan, Cao, Maofeng, Taniguchi, Shin-ichi, Watanabe, Masahiro, Kambayashi, Takuya, Okamoto, Toshihiro, Haraguchi, Masanobu, Wang, Xiang, Kobayashi, Kei, Yamada, Hirofumi, Ren, Bin, Tachizaki, Takehiro
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Diamonds; Far fields; Fluorescence; Gold; Illumination; Information processing; Near fields; Polaritons; Raman spectroscopy; Silver; Spatial discrimination; Spatial resolution; Spectroscopy; Spectrum analysis; Substrates; Technology assessment; Thin films; Waveguides
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/acs.analchem.1c00806

Tip-enhanced Raman spectroscopy (TERS) is a nano-optical approach to extract spatially resolved chemical information with nanometer precision. However, in the case of direct-illumination TERS, which is often employed in commercial TERS instruments, strong fluorescence or far-field Raman signals from the illuminated areas may be excited as a background. They may overwhelm the near-field TERS signal and dramatically decrease the near-field to far-field signal contrast of TERS spectra. It is still challenging for TERS to study the surface of fluorescent materials or a bulk sample that cannot be placed on an Au/Ag substrate. In this study, we developed an indirect-illumination TERS probe that allows a laser to be focused on a flat interface of a thin-film waveguide located far away from the region generating the TERS signal. Surface plasmon polaritons are generated stably on the waveguide and eventually accumulated at the tip apex, thereby producing a spatially and energetically confined hotspot to ensure stable and high-resolution TERS measurements with a low background. With this thin-film waveguide probe, TERS spectra with obvious contrast from a diamond plate can be acquired. Furthermore, the TERS technique based on this probe exhibits excellent TERS signal stability, a long lifetime, and good spatial resolution. This technique is expected to have commercial potential and enable further popularization and development of TERS technology as a powerful analytical method.