Atomic point contact significantly enhances Raman scattering

Researchers have discovered a huge enhancement of Raman scattering mediated by the formation of an atomic point contact between a plasmonic silver tip and a Si(111)-7×7 reconstructed surface. This state-of-the-art low-temperature tip-enhanced Raman spectroscopy (TERS) allows scientists to conduct atomic-scale vibrational spectroscopy. The developers suggest this technique could pave the way for exploring atomic-scale light-matter interactions, leading to a new discipline in light science and technology.

According to researchers, the rapid advancement of nanoscience and nanotechnology now requires atomic-scale optical spectroscopy to characterise its atomic-scale structures and defects in detail. TERS has demonstrated the exceptional sensitivity required to observe vibrational structures on the atomic scale. However, it relies on electromagnetic enhancement in plasmonic nanogaps, limiting the measurable samples. In a new paper, an international team headed by Takashi Kumagai at the Institute for Molecular Science, Japan, showed that atomic point contact (APC) formation between a plasmonic tip and the surface of a bulk Si sample can lead to a dramatic enhancement of Raman scattering and consequently the phonons of the reconstructed Si(111)-7 × 7 surface can be detected.

Figure 1: a) Illustration of the experiment. b) Scanning electron micrograph of a Ag tip (top) and scanning tunneling microscopy image of the Si(111)-7×7 surface. c) Atomic point contact Raman spectra [Credit: National Institutes of Natural Sciences (NINS)/ Institute for Molecular Science (IMS)].

The discovered enhancement mechanism of Raman scattering will open the possibility of atomic-scale ultrasensitive vibrational spectroscopy to investigate surface structures of semiconductors. Additionally, the researchers suggest it will expand the potential of atomic-scale vibration spectroscopy, which is applicable to non-plasmonic samples and many other materials.

The research team applied state-of-the-art low-temperature TERS, developed in collaboration with Fritz-Haber Institute, to obtain the vibration spectra from a silicon surface. TERS employs a strong light-matter interaction between a material and nanoscale light (localized surface plasmon resonance) generated at an atomically sharp metallic tip. The research team discovered that contact between the atomic point of a silver tip and a reconstructed Si(111)-7×7 surface leads to a huge enhancement of Raman scattering, allowing surface phonons to be detected. These disappear when  the tip is moved away from the surface and the atomic point contact is broken.

The research team further demonstrated that this atomic point contact Raman spectroscopy (APCRS) can resolve the atomic-scale structures of the silicon surface. Furthermore, the characteristic vibration modes can be observed selectively at the locally oxidised site, indicating the atomic-scale chemical sensitivity of atomic-point-contact Raman spectroscopy.

The paper was published in Nano Letters.