Nanofabrication of electronic devices has reached the scale of a single nanometre. The rapid advancement of nanoscience and nanotechnology now requires atomic-scale optical spectroscopy in order to characterise atomic-level structures that will affect the properties and functions of electronic devices. An international team, headed by Takashi Kumagai at the Institute for Molecular Science, 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 was achieved by means of state-of-the-art low-temperature tip-enhanced Raman spectroscopy, which allows atomic-scale vibrational spectroscopy to be conducted.
This newly discovered enhancement mechanism of Raman scattering will open up the possibility of atomic-scale ultrasensitive vibrational spectroscopy to investigate surface structures of semiconductors.
Nanofabrication of electronic devices has reached the scale of a single nanometre. The rapid advancement of nanoscience and nanotechnology now requires atomic-scale optical spectroscopy in order to characterise atomic-level structures that will affect the properties and functions of electronic devices. An international team, headed by Takashi Kumagai at the Institute for Molecular Science, 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 was achieved by means of state-of-the-art low-temperature tip-enhanced Raman spectroscopy, which allows atomic-scale vibrational spectroscopy to be conducted.
The research team applied state-of-the-art low-temperature tip-enhanced Raman spectroscopy, developed in collaboration with the Fritz-Haber Institute, to obtain the vibrational spectra from a silicon surface. Tip-enhanced Raman spectroscopy employs a strong light–matter interaction between a material and nanoscale light (localised surface plasmon resonance) generated at an atomically sharp metallic tip. The research team discovered that an atomic point contact formation of a silver tip and a reconstructed Si(111)-7 × 7 surface leads to a huge enhancement of Raman scattering. Figure 1a illustrates the experiment. A sharp silver tip fabricated by focused ion beam (Figure 1b, top) is moved toward the silicon surface (Figure 1b, bottom), while monitoring the Raman spectra from the junction. Figure 1c displays the waterfall plot of the obtained Raman spectra, where the horizontal axis is the Raman shift and the colour scale the Raman intensity. When the tip is in the tunnelling regime, only the optical phonon mode of the bulk silicon is observed at 520 cm–1. However, when the atomic point contact between the tip and the surface, the strong Raman scattering from the surface phonon modes suddenly appears. These modes disappear again 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. As shown in Figure 2, the Raman spectrum is different when it is recorded at an atomic step of the surface. Furthermore, the characteristic vibrational modes can be observed selectively at the locally oxidised site (Figure 3), indicating the atomic-scale chemical sensitivity of atomic-point-contact Raman spectroscopy.
Figure 2. Atomic point contact Raman spectra obtained at an atomic step and a flat surface of the Si(111)-7 × 7 surface. Credit: NINS/IMS
Figure 3. Atomic point contact Raman spectra obtained at a partially oxidised area over the Si(111)-7 × 7 surface (indicated by the arrow in the STM image). Credit: NINS/IMS
It was previously thought that a plasmonic nanogap is necessary to obtain ultrahigh sensitivity in tip-enhanced Raman spectroscopy, which typically requires a metal substrate. This imposed a severe limitation on measurable samples. The discovery of the huge Raman enhancement upon the atomic point contact formation will expand the potential of atomic-scale vibrational spectroscopy, which is applicable to non-plasmonic samples, and the exceptional chemical sensitivity will be obtained for many other materials. In addition, the results also suggest that atomic scale structures may play an indispensable role in metal–semiconductor hybrid nanosystems to affect their optoelectronic properties.
The research team applied state-of-the-art low-temperature tip-enhanced Raman spectroscopy, developed in collaboration with the Fritz-Haber Institute, to obtain the vibrational spectra from a silicon surface. Tip-enhanced Raman spectroscopy employs a strong light–matter interaction between a material and nanoscale light (localised surface plasmon resonance) generated at an atomically sharp metallic tip. The research team discovered that an atomic point contact formation of a silver tip and a reconstructed Si(111)-7 × 7 surface leads to a huge enhancement of Raman scattering. Figure 1a illustrates the experiment. A sharp silver tip fabricated by focused ion beam (Figure 1b, top) is moved toward the silicon surface (Figure 1b, bottom), while monitoring the Raman spectra from the junction. Figure 1c displays the waterfall plot of the obtained Raman spectra, where the horizontal axis is the Raman shift and the colour scale the Raman intensity. When the tip is in the tunnelling regime, only the optical phonon mode of the bulk silicon is observed at 520 cm–1. However, when the atomic point contact between the tip and the surface, the strong Raman scattering from the surface phonon modes suddenly appears. These modes disappear again 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. As shown in Figure 2, the Raman spectrum is different when it is recorded at an atomic step of the surface. Furthermore, the characteristic vibrational modes can be observed selectively at the locally oxidised site (Figure 3), indicating the atomic-scale chemical sensitivity of atomic-point-contact Raman spectroscopy.
It was previously thought that a plasmonic nanogap is necessary to obtain ultrahigh sensitivity in tip-enhanced Raman spectroscopy, which typically requires a metal substrate. This imposed a severe limitation on measurable samples. The discovery of the huge Raman enhancement upon the atomic point contact formation will expand the potential of atomic-scale vibrational spectroscopy, which is applicable to non-plasmonic samples, and the exceptional chemical sensitivity will be obtained for many other materials. In addition, the results also suggest that atomic scale structures may play an indispensable role in metal–semiconductor hybrid nanosystems to affect their optoelectronic properties.
