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

Abstract

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.

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

“…Zhang et al presented the new design of low-background TERS using a plasmon thin-film waveguide. 79 In typical commercial TERS setups, an excitation laser is focused directly on the tip apex, causing strong fluorescence or far-field Raman signals from the illuminated areas to be excited as a background. They may significantly reduce the near-field to far-field signal contrast in TERS spectra.…”

“…6a. 79 As a result, a flat Si/Au Kretschmann configuration was created at the tip apex as a triangular thin-film waveguide (Fig. 6b).…”

“…6c). 79 In Fig. 6d, the red line represents the Raman spec- trum of the diamond sample induced by the very weak leaked light from the waveguide, while the black line represents the sum of the TERS and Raman signals excited by the weak leaked light.…”

“…The peak of the black spectrum is significantly higher than that of the red one, suggesting the achievement of non-gap TERS measurement by the indirect-illumination method. 79 A 2D TERS mapping was constructed using a Raman signal at ∼1600 cm −1 from the 4-PBT thin-film sample to determine the spatial resolution of the prepared tips. Fig.…”

“…This indirect-illumination setup should speed up the development of TERS instruments because it decreases background signals, solves the problem of nano-focusing at the tip apex, possibly reduces TERS measurement durations, and makes the device more user-friendly. 79 Indeed, because the production technique is not standardized in practice, different tips often exhibit a variety of plasmonic enhancements. Therefore, recycling the TERS active tips appears to be one of the most efficient ways to maintain the longevity of the tips.…”