Raman Spectroscopy

Raman spectroscopy is a technique to obtain information on materials by spectroscopy of Raman scattered light. When a substance is irradiated with light, scattered light is generated. At the same time, the substance interacts with the light, which excites molecular vibrations unique to the substance. Therefore, in addition to the normal scattered light that has the same scattered light as the incident light, Raman scattered light whose energy is changed by the amount of molecular vibration is generated. By spectroscopy of this light, the molecular and crystal structures of materials can be evaluated. There are two types of Raman scattering light: Stokes light, which has a frequency lower than that of incident light, and anti-Stokes light, which has a frequency higher than that of incident light. In the analysis, the Stokes light, which has a stronger intensity, is mainly used. A schematic diagram of Raman scattering light and its energy level diagram are shown in Figure 1.

Figure 1. Schematic and energy level diagram of Raman scattered light

The advantages of Raman spectroscopy are as follows

• Non-destructive, non-contact analysis of materials
• Evaluation at a narrow point on the 0.1 µm scale is possible
• Can measure samples in various states, such as gas, liquid, and solid.

Evaluation of the number of layers of 2D semiconductor using Raman microspectroscopy

By combining Raman spectroscopy and micro-optics, the vibration of molecules in a focused spot can be evaluated. Each layer of TMDC is bound together by weak intermolecular forces. Therefore, the A1g phonon modes, which are vibrations perpendicular to the layers, are suppressed by the intermolecular forces when the bulk structure is taken. However, as the suppression decreases with decreasing layer number, the intermolecular forces between the layers become smaller and the vibrational energy of a particular phonon mode shifts. From the amount of the shift, the number of layers in the fabricated sample can be evaluated. Raman spectra show a peak shift to the low energy (low wavenumber) side; thin-layer samples of TMDC become smaller the thinner they are, with single layers being on the scale of a few µm. Raman spectroscopy can be used to evaluate the number of layers in such a fine sample. Figure 2 shows the A1g phonon modes and Raman spectra.5The black arrows in (a) and (b) indicate the direction of the phonons, and in (b) the phonons become stronger with a single layer. In (b), the phonon becomes stronger due to the monolayer, and the Raman peak shifts to the low energy (low wavenumber) side because the frequency of the molecular vibration increases thereby.

Figure 2. A1g phonon modes and Raman spectra

(a) A1g phonon mode of several layers

(b) Single-layer A1g phonon mode

(c) Raman spectrum of A1g phonon mode. (red: bulk, purple: single layer)