Speaker
Description
Tip-enhanced Raman spectroscopy (TERS) is a promising technique to investigate chemical structures of individual molecules on surfaces. Irradiation of a plasmonic metal tip (such as Au and Ag) in a scanning tunneling microscopy (STM) by visible light leads to the surface plasmon excitation at the tip apex, yielding a localized electromagnetic field (EMF) that enhances Raman signals. The ultra-high vacuum and low temperature environment can stabilize a nanostructure at the tip apex, enabling high-sensitive STM-TERS measurements even with Ångström-scale resolution [1]. As the EMF will be further enhanced in a nanogap between a plasmonic metal surface and tip, most previous TERS studies have been performed on Ag and Au surfaces. However, utilizing this spectroscopic technique for chemical characterization of various practically important samples, such as molecule-semiconductor interfaces, metal chalcogenides, or functional two-dimensional materials, requires improved TERS sensitivity by designing and controlling the STM junction under light illumination.
We have demonstrated a novel way to drastically increase TERS intensities by formation of an atomic point contact (APC), where the Ag tip is approached to intentionally contact with the target atom or molecule on a surface [2-4]. For example, TERS peak intensities of vibrational modes of a single C60 molecule on Ag(111) are enhanced more than 10 times in an APC configuration relative to those recorded in the tunneling region [3]. This behavior cannot be explained by a classical EMF enhancement model for a Ag–Ag nanogap. We propose that chemical enhancement due to level shift caused by APC formation mediates the enhancement [3]. The versatility of the APC-enhanced Raman spectroscopy was confirmed by the observations in many systems: ZnO films on Ag(111) [2] and the Si(111)-7x7 surface [4], as well as C60 on non-plasmonic Cu(111), Pt(111) [3], and Si(111) surfaces. Furthermore, we observed a unique behavior upon APC formation for PTCDA/Si(111), where the Raman intensities were enhanced at a specific gap distance, but were subsequently attenuated at shorter distances, indicating tip-induced molecular switching. Such an ultrasensitive detection scheme may lead to a further understanding of light-matter interactions at the atomic scale and to the development of new techniques utilizing extremely localized light.
References
[1] Y. Zhang et al., Natl. Sci. Rev. 6, 1169 (2019).
[2] S. Liu et al., Nano Lett. 20, 5879 (2020).
[3] B. Cirera et al., Nano Lett. 22, 2170 (2022).
[4] S. Liu et al., Nano Lett. 21, 4057 (2021).
| Abstract Number (department-wise) | PC 14 |
|---|---|
| Department | PC (Wolf) |
