The effect of pressure on graphene can be observed through changes in its Raman spectra, particularly in the G and 2D bands. These changes include alterations in the peak shapes, positions, and relative intensities, which are influenced by the number of graphene layers and the presence of defects.
Summary of the Answer: Pressure affects graphene by altering its Raman spectra, specifically the G and 2D bands. These changes are indicative of the number of layers and the presence of defects in the graphene structure.
Detailed Explanation:
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Raman Spectra Changes:
- G and 2D Bands: The G band and 2D band in the Raman spectra of graphene are sensitive to the number of layers. For single-layer graphene, the 2D band is usually more intense than the G band. However, as the number of layers increases, the intensity of the G band increases while the 2D band decreases. This is due to the resonant processes and the dispersive tendencies that are dependent on the Raman band position from the laser excitation.
- Peak Shifts and Broadening: For graphene with more than two layers, the 2D peak shifts to a higher wavenumber and the full-width at half-maximum (FWHM) broadens. This broadening is indicative of the layer thickness of the graphene sample.
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Effect of Defects:
- Structural Defects: Graphene contains various structural defects such as vacancies, wrinkles, functional groups, and contamination, which can be influenced by the growth conditions and the substrate. These defects can affect the properties and applications of graphene. For instance, epitaxially grown graphene on Cu(111) surfaces may exhibit more residual stresses and fewer wrinkles or folds.
- Defect Control: Controlling the defects in graphene is crucial for preparing high-quality films. The understanding of defect formation and control is still developing, but it is essential for optimizing the synthesis of graphene with a uniform number of layers and controlled stacking order.
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Implications for Synthesis and Characterization:
- Synthesis Challenges: The synthesis of multilayer graphene with a uniform number of layers and a controlled stacking order or torsion angle is challenging. Techniques such as chemical vapor deposition (CVD) are used, but the mechanisms and reproducibility of producing high-quality graphene are not yet fully understood.
- Characterization Techniques: Techniques such as Raman spectroscopy, X-ray spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) are used to examine and characterize graphene samples. These methods help in understanding the effects of pressure and other synthesis conditions on the properties of graphene.
In conclusion, pressure affects graphene through changes in its Raman spectra, which are influenced by the number of layers and the presence of defects. Understanding these effects is crucial for the controlled synthesis and application of graphene in various fields.
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