GB/T 40069-2021Nanotechnologies—Measurement of the number of layers of graphene-related two-dimensional (2D) materials—Raman spectroscopy method (English PDF)

3.1 Terms related to graphene-related two-dimensional materials

3.1.1 Graphene-related 2D material; GR2M

Carbon-based two-dimensional material with no more than 10 layers.

Note: Including graphene, double-layer graphene, few-layer graphene, graphene oxide, etc.

3.1.2 Graphene flake; graphene nanoplate; graphene nanoplatelet; GNP

Nanosheets composed of graphene layers.

Note: The common thickness is less than 3nm, and the lateral size range is about 100nm~100µm. [Source: GB/T 30544.13-2018, 3.1.2.11, with modification]

3.1.3 Number of layers

<two-dimensional material> The number of layers constituting the two-dimensional material.

3.2 Terms related to Raman spectroscopy

3.2.1 Raman spectrum

When a substance is irradiated with monochromatic radiant energy, the frequency-shifted spectrum is modulated due to inelastic scattering.

Note 1. Inelastic scattering refers to the rotation excitation, vibration excitation or phonon excitation of the medium.

Note 2. Modulation frequency shift refers to the energy loss or gain of monochromatic radiation photons. [Source: GB/T 33252-2016, 2.1]

3.2.2 Raman peak; Raman mode; Raman band

The Raman spectrum has a certain shape of peaks.

Note 1. Each scattering medium has its specific Raman peak.

Note 2. Including peak position, peak height, peak area, peak width and line type and other spectral characteristics, see Appendix C Figure C.1.

3.2.3 Peak height; peak maximum

The vertical distance between the highest point of the Raman peak and the baseline.

3.2.4 Peak area; peak intensity

The area enclosed by the Raman peak and the baseline.

3.2.5 Peak position

The wavenumber difference between the incident monochromatic light and the position of the highest point of the Raman peak.

Note: Also called Raman frequency shift, the unit is wave number (cm-1).

3.2.6 Peak width

The frequency shift difference between the two sides of the Raman peak at 1/2 peak height.

Note: Also known as Full Width at Half Maximum (FWHM).

3.2.7 G mode; G peak

A characteristic peak related to the stretching vibration between the nearest neighboring carbon atoms in the graphene layer.

Note 1. Generally located near 1582cm-1, has nothing to do with the number of graphene layers.

Note 2. G peak frequency shift will be affected by factors such as stress and carrier concentration.

3.2.8 D mode; D peak

The characteristic peaks located at 1300cm-1~1400cm-1 related to the structural defects in the edges of the graphene sheet and the layers.

Note 1. The D mode is a TO phonon mode near the K point of the Brillouin zone boundary by a disorderly activated graphene-related two-dimensional material. Peak height ratio of D mode to G mode The size can reflect the disorder degree of the graphene lattice structure to a certain extent.

Note 2. The D-mode frequency shift is related to the photon energy of the excitation light, and is generally linear, with a slope of 50cm-1/eV.

3.2.9 2D mode; 2D peak

The characteristic peaks of graphene flakes are located at 2600cm-1~2800cm-1.

Note 1. Also known as G'mode, its frequency is close to twice the frequency of D mode. The line type is related to the electronic band structure of graphene-related two-dimensional materials.

Note 2. The 2D mode frequency shift is related to the photon energy of the excitation light, and is generally linear, with a slope of about 100cm-1/eV.

5.1 Principle

The principle of Raman spectroscopy (A method) for linear measurement of graphene flakes based on 2D mode is as follows.

a) The 2D peak of the graphene sheet is derived from the double resonance Raman scattering process of TO phonons near the K point of the Brillouin zone boundary, so it is not The 2D model of the same number of graphene sheets shows a unique line shape. Single-layer graphene has linear properties near the Dirac point The 2D mold activated by the double resonance Raman scattering based on the band structure has a single Lorentz line and is not affected by the preparation method of the single-layer graphene and the substrate placed on it. With the change in the number of graphene flake layers, the line of the graphene flake 2D mold The type also changes significantly, and the line type is closely related to the wavelength of the excitation light.

b) For specific laser lines, such as 633nm laser, single-layer and 2~4 layer AB stacked graphene sheets have unique 2D mold line type, as shown in Figure 1. But these features are not obvious under 532nm laser excitation, so the number of layers of graphene flakes It can be judged by the line type of the 2D mode excited by the 633nm laser.

c) This method is suitable for single-layer graphene and graphene sheets with AB stacking and no more than 4 layers.

Note 1. The 2D mold line type of single-layer graphene (1LG) is a single Lorentz line type. The 2D peak of the 2~4 layer graphene sheet consists of multiple sub-peaks, and its linear Detailed features are marked with arrows, plus signs and asterisks.

Note 2. The 2D peak linear feature of the 2-layer graphene sheet (2LG) is that there is an obvious sub-peak on the left side (indicated by the arrow) in addition to the main peak.

Note 3. The 2D peak linear characteristics of the 3-layer graphene sheet (3LG) are. a) There are two sub-peaks on the left side of the main peak (shown by the arrow); b) The main peak in the middle has two sub-peaks (shown by the plus sign); the peak height of the sub-peak on the left (small frequency shift) is significantly higher than that of the sub-peak on the right (large frequency shift); c) There is a sub-peak on the right side of the main peak (shown by an asterisk).

Note 4. The 2D peak linear characteristics of the 4-layer graphene sheet (4LG) are. a) There are three sub-peaks (shown by arrows) on the left side of the main peak, of which the two sub-peaks near 2592cm-1 and 2623 cm-1 are more obvious; b) The main peak in the middle has two sub-peaks (shown by the plus sign), and their peak heights are close; c) There is a sub-peak on the right side of the main peak (shown by asterisks).

Note 5. With the increase of the number of layers, the number of sub-peaks on the left and right sides of the main peak of the 2D peak of the graphene sheet with 5 layers and above gradually increases, due to the overlap between the peaks. As a result, the spectral characteristics between multilayer graphene sheets with different layer thicknesses are no longer easy to distinguish. The 2D mold line type of graphite (HOPG) is characterized by a sharp A sharp main peak and a wider sub-peak on the left.

5.2 Apparatus

5.2.1 Use laser confocal Raman microscope as measuring instrument; use 633nm laser; laser confocal Raman microscope The wave number covered by a single array detector element should be better than 1.0cm-1, and the silicon material measured by the spectrometer is located at the 520cm-1 Raman mode. FWHM is not greater than 4.0cm-1; the lateral (XY) spatial resolution of the laser confocal Raman microscope should not be greater than 2µm.

5.2.2 Before measurement, the Raman spectrometer should be calibrated according to GB/T 33252, JJF1544 or related technical specifications.

5.3 Measurement procedure and determination of the number of layers

5.3.1 Use a microscope objective lens with a magnification of 100 times or 50 times; the laser power reaching the sample surface should be less than 0.5 mW. Avoid the sample being heated and damaged by the laser.

5.3.2 The selected spectral scan range should be greater than 2450cm-1~2800cm-1.

5.3.3 Use an optical microscope to analyze the image of the sample on the substrate, clarify the position of the graphene sheet, and determine the measurement area.

5.3.4 Choose an appropriate Raman spectrum acquisition time for the sample to be tested, and the 2D peak height should be counted above 5000.

5.3.5 In the area with the same chromaticity in the sample to be tested, select different locations to measure 3 sets of data and take the arithmetic average.

5.3.6 Obtain the Raman spectrum of the 2D peak of the sample to be tested, and compare it with Figure 1 to obtain the corresponding number of layers. If the measured spectrum is similar to that in Figure 1 If the main characteristics of the 2D mold line type of the 1LG~4LG samples do not match, the graphene sheet does not have an AB stacking method or the number of layers exceeds 4. Refer to Appendix E for characterization examples.

6.1 Principle

The principle of Raman spectroscopy (B method) for measuring the number of graphene sheets based on the peak height of the silicon Raman mode of the SiO2/Si substrate is as follows.

a) The Raman spectra of graphene sheets with different layers also show different characteristics in terms of the peak height of the characteristic Raman mode. Graphene flakes When placed or transferred to a SiO2/Si substrate, the laser and Raman modes are more likely to occur in the graphene sheet and its upper and lower media. The reflected and refracted beams will interfere with each other, making the Raman peak height of the silicon of the SiO2/Si substrate under the graphene sheet and the excitation light The wavelength, the type of substrate used and the characteristic thickness, the numerical aperture of the objective lens, and the number of graphene sheets are related. Laser excitation of a certain wavelength When the graphene sheet covered SiO2/Si substrate is located at 520.7 cm-1, the peak height of the silicon Raman characteristic peak is IG (Si), and The peak height of the silicon Raman characteristic peak of the substrate without graphene sheet coverage is I0 (Si), as shown in Figure 2. IG(Si) can be normalized with I0(Si). Use the transfer matrix method to calculate the numerical aperture of the substrate and the microscope objective with a specific SiO2 layer thickness Under a certain wavelength of laser excitation, the relationship between the ratio of IG(Si)/I0(Si) and the number of graphene sheets.

b) Before the test, it is necessary to accurately measure the thickness of the SiO2 layer covering the surface of the SiO2/Si substrate. The numerical aperture of the mirror is calculated, and the relationship between IG(Si)/I0(Si) and the number of graphene sheets is calculated. Figure 3 shows the magnification of the microscope objective Is 50, the numerical aperture is 0.50, and the SiO2 thickness is 90nm, the relationship between IG(Si)/I0(Si) and the number of layers under 532nm excitation On the calculation results, the specific values are shown in Table 1. It can be seen that the ratio IG(Si)/I0(Si) and the number of graphene flake layers are monotonously changing.

This can measure the number of layers of graphene flake samples within 10 layers of AB stacking and ABC stacking on a specific substrate.

6.2 Apparatus

6.2.1 Use laser confocal Raman microscope as measuring instrument; use 532nm laser; single array detector element of spectrometer The covered wavenumber should be better than 1.0cm-1, and the FWHM of the silicon material in the 520cm-1 Raman mode measured by the spectrometer should not be greater than 4.0cm-1; the lateral (XY) spatial resolution of the Raman spectrometer should not be greater than 2µm.

6.2.2 Before measurement, the Raman spectrometer should be calibrated. The calibration requirements are the same as 5.2.2.

6.3 Measurement procedure and determination of the number of layers

6.3.1 Use a microscope objective lens with a magnification of 50 times and a numerical aperture of not greater than 0.55; the laser power reaching the sample surface should be less than 0.5mW, to avoid the sample being heated and damaged by the laser.

6.3.2 The selected spectral scan range should be greater than 450cm-1~600cm-1.

6.3.3 Use an optical microscope to analyze the image of the sample on the substrate, grasp the position of the graphene sheet, and determine the measurement area. Get to be tested Raman mode peak height of SiO2/Si substrate without graphene flake coverage near graphene flakes I0(Si). Raman mode peak height and test time excitation The light is very sensitive to the focus state of the sample, and it is necessary to accurately focus the SiO2/Si substrate first, so that the center of the laser spot is aligned with the bare area near the sample to be tested. On the exposed substrate that is not covered by obvious impurities, finely adjust the relative distance between the objective lens and the substrate (that is, fine-tune the focus) to obtain Si Raman mode The focus state when the peak height is maximum. Choose the appropriate Raman spectrum acquisition time, and the Si peak height should be counted above 5000. Use the Lorentz line The value of peak height I0 (Si) is obtained by type fitting.

6.3.4 Obtain the Raman mode peak height IG(Si) of the SiO2/Si substrate on the graphene sheet sample area to be tested. keep the focus when measuring I0(Si) The state remains unchanged, and the sample position is translated, so that the center of the laser spot is aligned with the graphene sheet sample area to be tested, and the acquisition time is the same as that in 6.3.3. The Raman mode peak height IG(Si) of the SiO2/Si substrate covered by the graphene flake sample. Use Lorentz linear fitting to get the peak height IG(Si) Numerical value.

6.3.5 Calculate the relative ratio of peak height IG(Si)/I0(Si).

6.3.6 Compare IG(Si)/I0(Si) with the theoretical calculation results in Table 1, and the corresponding number of layers will be rounded to the nearest integer. According to the party The method can determine the number of graphene sheets with 1 to 10 layers. Refer to Appendix F for measurement examples.

6.3.7 In the area with the same chromaticity in the sample to be tested, select different locations to measure 3 sets of data and take the arithmetic average.

6.3.8 For the case where the thickness of the substrate SiO2 is not 90nm±5nm, it is necessary to accurately measure the thickness of the SiO2 layer on the SiO2/Si substrate, and According to the thickness, laser wavelength and the numerical aperture of the objective lens, the relationship between IG(Si)/I0(Si) and the number of graphene sheets is calculated. in difference Under the conditions of numerical aperture and SiO2 layer thickness, the theoretical calculation ratio of IG(Si)/I0(Si) and the number of graphene sheets when excited by a 532nm laser See Table G.1 in Appendix G for the relationship.

6.3.9 The above method can be extended to the case of 633nm laser wavelength. About the use of 633nm laser to measure the number of graphene sheets For details, see Appendix H.