Giant Raman response obtained by forming atomic point contact

"New spectroscopic principle that goes beyond conventional wisdom"

An international research team led by Takashi Kumagai, an associate professor at the Center for Mesoscopic Sciences, IMS, has collaborated with the Fritz Haber Institute (Germany) to develop an atomic-scale polar differential optical method that applies the advanced measurement technology of probe-enhanced Raman spectroscopy. For the first time, they discovered that a giant Raman response was obtained when an atomic point contact was formed between a silver probe and a single-crystal silicon surface. As a cutting-edge research technique, spectroscopy, which examines the formation of matter using light, has reached the scale of atoms and molecules, which are the constituent units of matter.

According to Associate Professor Kumagai, "We are conducting research on the principle of light manipulation on such ultimate scales and also investigating new measurement methods that apply it. This time, we have been able to develop a micro-vibration spectroscopy technique that can distinguish atomic-level structures using silicon, which is a typical semiconductor material." The research team conducted a demonstration experiment to investigate the structure of the surface of silicon at an atomic level using atomic point contact Raman spectroscopy.

Measurements were made on the flat surface of single-crystal Si (111) (7 × 7 reconstructed surface) and on a structure known as the atomic step (small steps at the atomic level). A spectrum different from that of the flat surface was obtained. In addition, the silicon surface was exposed to a very small amount of oxygen gas, and measurements were also performed on this exposed surface with silicon oxide at a scale of several nanometers. As a result, we successfully detected the characteristic vibration mode derived from oxidation.

This result is expected to be useful for ultra-sensitive micro-vibration spectroscopy that can investigate the chemical structure of the surface of a material at an atomic scale. Associate Professor Kumagai said, "We used a technique called probe-enhanced Raman spectroscopy. A common viewpoint was that semiconductor materials, such as silicon, would not be able to obtain sufficient sensitivity. However, this result overturns this common belief and indicates the existence of an extremely strong interaction between light and matter owing to a special structure at the atomic level and establishes a new spectroscopic principle that utilizes it."

■ Probe: Very thin and pointed needle used in scanning tunneling electron microscopes, etc.
■ Raman response (scattering/spectroscopy):
This refers to either the phenomenon that occurs when light is applied to a substance and the scattered light is of a color different from that of the applied light or a measurement method based on this phenomenon. This was named after the Indian physicist Chandrasekhara Venkata Raman (who won the Nobel Prize in Physics in 1930).