Graphene is a two-dimensional material composed of a single layer of carbon atoms arranged in a honeycomb lattice. For many years, it was considered a hypothetical structure that could not exist independently. However, in 2004, researchers Andre Geim and Konstantin Novoselov from the University of Manchester successfully isolated graphene from graphite in a laboratory, proving that it could indeed exist as a standalone material. Since then, research on graphene preparation and its applications has become a major focus in materials science. Graphene exhibits unique electronic properties, making it a promising candidate for next-generation electronic devices. It is regarded as one of the most likely materials to replace silicon due to its high electron mobility—over 15,000 cm²/V·s at room temperature. Unlike carbon nanotubes, graphene has a perfect hybrid structure, allowing electrons to move with minimal resistance, resulting in low heat generation and high thermal conductivity. This makes graphene-based electronics faster and more efficient than traditional silicon-based systems.
Graphene's potential extends beyond electronics. It can be used in advanced optical devices that operate across a wide range of frequencies, from terahertz to visible light, with fast response times, low energy consumption, and compact size. Researchers have also explored the use of terahertz spectroscopy to study ultrafast electron dynamics in graphene layers. Fabricating terahertz emitters and detectors on semiconductor chips offers a promising path toward miniaturizing terahertz systems and expanding their practical applications.
The unique electronic structure of graphene gives rise to fascinating quantum phenomena. Electrons in graphene behave like relativistic particles (Dirac fermions), moving at high speeds without mass. This behavior has brought relativistic quantum mechanics into the realm of everyday laboratory experiments. The quantum Hall effect observed in graphene at room temperature, along with its tunable band gap and high flexibility, has sparked significant scientific interest. By adjusting the carrier concentration, scientists can control the band gap between the valence and conduction bands, opening up new possibilities for electronic device design.
In terms of crystal structure, graphene consists of a single layer of sp²-hybridized carbon atoms arranged in a hexagonal lattice. When multiple graphene layers are stacked, they form graphite. If curled, they become carbon nanotubes, and if wrapped, they create fullerenes. Each graphene cell contains two carbon atoms, forming three sigma bonds and one delocalized π orbital. These π orbitals contribute to the formation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). At the K point of the Brillouin zone, the π and π* bands degenerate, resulting in a zero band gap and metallic behavior.
Defects in graphene, such as Stone-Wales defects, vacancy defects, and adsorbed atoms, significantly affect its electronic properties. First-principles calculations using density functional theory (DFT) have shown that these defects can introduce new energy bands, increase the band gap, and reduce the material’s metallic character. For example, a single vacancy defect can increase the band gap to 1.591 eV, while a double vacancy defect increases it to 1.207 eV. These findings highlight the importance of controlling defect structures to tailor graphene’s electronic properties for specific applications.
Graphene’s versatility makes it suitable for a wide range of applications. In the field of sensors, it shows high sensitivity to pH changes and can detect various gases and molecules through conductivity changes. As an electrode material, graphene offers excellent conductivity and stability, outperforming traditional metals in certain applications. In energy storage, it has potential for use in batteries, supercapacitors, and fuel cells due to its high surface area and electrical conductivity.
Beyond these areas, graphene composites have found applications in catalysis, biomedical devices, and structural materials. Its mechanical strength, flexibility, and chemical stability make it ideal for creating multifunctional materials. Whether in electronics, sensing, or energy systems, graphene continues to push the boundaries of what is possible in modern technology.
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