Application of Raman Spectroscopy in Characterization of Graphene Structure
- Typical Raman spectral features
Fig.1 Typical Raman spectrum of monolayer graphene excited by 514.5nm laser
Figure 1 is a typical Raman image of single-layer graphene excited by a 514.5nm laser. Single-layer graphene has two typical Raman characteristic peaks, which are the G peak located near 1582cm-1 and the G' peak located at 2700cm-1, while for graphene samples containing defects or at the edge of graphene, There will also be a D peak around 1350cm-1 and a D' peak around 1620cm-1.
The G peak is the only first-order Raman scattering process in single-layer graphene.
The G' peak and the D peak are second-order double resonance Raman scattering processes. The G' peak is generated by two inter-valley inelastic scattering of two iTO optical phonons. While the D peak is related to the inter-valley scattering of an iTO phonon and a defect.
The D' peak is the double resonance process in the valley, and the two scattering processes are the inelastic valley scattering process of iLO phonons and the valley scattering process of defects.
Near the K point, the valence band and conduction band of graphene are mirror-symmetrical to the Fermi level. Electrons can not only scatter with phonons, but also scatter with holes, so there will be third-order resonance Raman scattering. process of generation.
- Raman Spectroscopy Determination of Graphene Layer Number
Fig.2 Raman spectra of graphene with 1~4 layers
It can be seen from the figure that the intensity of the G' peak of single-layer graphene is greater than that of the G peak, and it has a perfect single Lorentz peak type. High wavenumber shift (blue shift).
The intensity of the G peak also increases approximately linearly with the number of layers, which is due to the fact that more C atoms will be detected in multilayer graphene.
Therefore, the intensity of the G peak, the intensity ratio of the G peak to the G' peak, and the peak shape of the G' peak are often used as the basis for judging the number of graphene layers.
- Quantitative analysis of graphene defect types and defect densities
The relationship between ID/IG and LD under the action of three different laser energies
Graphene with defects will have a Raman D peak near 1350cm-1, so detecting the intensity of the D peak can make some quantitative analysis of the defect density.
The intensity ratio of the D peak to the G peak is often used as an important parameter to characterize the defect density in graphene. Assuming that the defect in graphene is a zero-dimensional point defect, and the average distance between two points is LD, the LD can be quantified by calculating the intensity ratio ID/IG of the D peak and the G peak of the Raman spectrum, thereby estimating Defect density in graphene. The relationship between them is ID/IG∝1/LD2.
LD/LD' is closely related to the type of graphene surface defects.
When the defect concentration is low, the intensities of D peak and D' peak both increase with the increase of defect density, which is proportional to the defect density.
When the defect concentration increases to a certain extent, the D peak intensity reaches the maximum and then begins to weaken, while the D' peak remains unchanged.
For example, for defects generated by sp3 hybrid orbitals, LD/LD' is the largest, about 13;
For vacancy-type defects, this ratio is about 7;
For graphene edge-type defects, this ratio is the smallest at about 3.5.
- Raman Spectroscopy Characterization of Doped Graphene
By applying a vertical electric field on the surface of graphene and observing in situ the process of its Raman spectrum changing with the magnitude and direction of the electric field, the graphene Raman spectrum under the electric field effect can be studied. Graphene Raman spectra show the following features in electric field-induced doping:
1) The peak position of the G peak increases with the increase of the absolute value of the Fermi level, and reaches saturation at high doping concentrations.
2) The half width of the G peak decreases with the increase of the absolute value of the Fermi level, and reaches saturation when the electron-phonon energy band is higher than the phonon energy.
3) The peak position of the G' peak increases with the increase of the absolute value of the Fermi energy level due to p-type doping, but decreases with the increase of the absolute value of the Fermi energy level due to n-type doping.
The relationship between G peak (left) and G’ peak (right) as a function of gate voltage regulation
The following quantitative relationship exists between the graphene G peak position and its Fermi level:
- Temperature dependence of graphene Raman spectrum:
Temperature dependence of the Raman G peak position of single-layer (a) and double-layer (b) graphene
When the external temperature of graphene changes, its G peak position will also change accordingly. It can be seen from the figure that within the measured temperature range, when the temperature increases, the Raman G peak of graphene shifts to a lower wave number, which is linearly related to the temperature, so the temperature dependence of the G peak shift can be expressed as
Although the shift of the G peak changed, its half width did not change over the measured temperature range.
6. The interlayer stacking method of graphene (ABA semi-metal, ABC semiconductor. G, G' peak position, half maximum width), stress effect (stretching → low wave number, compression → high wave number) will also be reflected in its The change of characteristic peaks of Raman spectrum.
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