BERLIN, Sept. 6, 2019 — Tip-enhanced resonance Raman spectroscopy (TERRS) has been demonstrated by a research team at Fritz-Haber Institute. The results suggest that TERRS could offer a new approach for the atomic-scale optical characterization of local electronic states.

The researchers demonstrated TERRS on ultrathin zinc oxide layers epitaxially grown on a Ag(111) surface in which both physical and chemical enhancement mechanisms were operative. The underlying process was examined by modifying the localized surface plasmon resonance in a scanning tunneling microscope junction and recording different-thickness zinc oxide films that exhibited a slightly different electronic structure.

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Tip-enhanced resonance Raman scattering is measured by a silver tip fabricated by focused ion beam milling. Localized surface plasmon is excited by an excitation laser, which generates enhanced Raman scattering from ultrathin zinc oxide films grown on a single-crystal silver surface. Courtesy of Takashi Kumagai.

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In combination with scanning tunneling spectroscopy (STS), the research team demonstrated that the TERRS intensity was dependent on the local electronic resonance of the zinc oxide/Ag(111) interface. The team also showed that the spatial resolution of TERRS was dependent on the tip-surface distance and could reach nearly 1 nm in the tunneling regime. A comparison of STS and TERRS mapping showed a correlation between resonantly enhanced Raman scattering and the local electronic states at near-atomic resolution.

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Tip-enhanced resonance Raman spectrum of ultrathin ZnO films on a Ag(111) surface. (a) STM image of 2- and 3-monolayer ZnO films epitaxially grown on Ag(111) at 78 K. (b) Schematic of the ZnO film. (c) Tip-enhanced resonance Raman spectrum of the ZnO film. Courtesy of Takashi Kumagai.

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Resonance Raman spectroscopy is a powerful tool to analyze a specific chemical structure at a high sensitivity, but its spatial resolution has been restricted to a few hundred nm due to the diffraction limit. The Fritz-Haber team, led by Takashi Kumagai, was able to overcome this limitation by exploiting extreme field confinement at a metal tip apex through localized surface plasmon excitation and achieve 1-nm resolution with TERRS.

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Correlation between tip-enhanced Raman scattering and the local electronic structure of the ZnO film. (a-b) STM image and STS mapping of the ZnO film. (c) Tip-enhanced Raman spectra recorded at different sites over the ZnO film (red and blue) and the Ag surface (black). (d) Constant-current STS recorded at different sites over the ZnO film. (e-g) Line profile of STM height, STS intensity, and Raman intensity. The line is indicated in (a-b). (h) Tip-enhanced resonance Raman spectra recorded along the line in (a-b). Courtesy of Takashi Kumagai.

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Tip-enhanced resonance Raman spectroscopy could be used to investigate specific chemical structures at the nanoscale and at the single-molecule level, and to study local defects in low-dimensional materials and active sites of heterogeneous catalysis.

The research was published in Nano Letters (https://doi.org/10.1021/acs.nanolett.9b02345).

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