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By Egorov Artem
Graphene is a monolayer of carbon atoms organized in a honeycomb lattice that results in strong s-bonds in the plane and un-hybridized p-orbitals overlapped with nearby atoms to create p-bonds. While the s-bond provides most of the structural stability of graphene, the p-bond establishes electronic and optical qualities. The interaction of graphene with electromagnetic radiation is intriguing because of the two-dimensional confinement of electrons and the extraordinary band structure of graphene.
The band structure of graphene is made up of completely-filled conduction band and empty valence bands which intersect linearly at Dirac points. The optical absorption of pristine graphene is caused by the interband and intraband transitions.
For single-layer graphene, absorption of visible light is negligibly small and frequency independent. However, considering the extreme thinness of graphene, it's apparent that the light-matter interaction in graphene is extremely large. On extremely smooth substrates, graphene can be seen under white light illumination, despite being a single atom thick.
Tuning graphene
Absorption in the far infra-red area of the spectrum in graphene can be tuned over a wide terahertz frequency range by varying graphene nanostructures, as well as electrostatic gating.
Structural flaws in the lattice created during processing steps can have a big effect on graphene’s optical qualities. High-quality graphene can be created by numerous methods, such as mechanical exfoliation or chemical vapor deposition (CVD). Graphene from each technique has distinct optical qualities as a result of structural imperfections. For instance, CVD graphene is marked by small densities of sp3-defects and line defects at the grain boundaries. On the other hand, CVD graphene includes amorphous carbon and polymeric residue, as well as metal catalyst contaminants. These contaminants can lead to artefacts in the material optical response.
The optical qualities of graphene are distinctive due to the linear band structure, zero band-gap and strong interaction between Dirac Fermions and electromagnetic radiation. Graphene absorbs over a broad spectral range due to interband and intraband optical transitions. The absorption of graphene from visible to near infrared area of the spectrum is affected by interband transitions and frequency independent, as described by fine structure constant. In far infrared, graphene's optical response is a result of intraband transitions or free carrier absorption.
The far-infrared response can be adjusted to terahertz range by plasmonic excitations, which can be triggered by modifying graphene nanostructures. Photoluminescence in graphene can be achieved by producing an energy band-gap through a technique that involves dividing graphene into small bits, such that it creates structural flaws.
Optical applications of graphene
Graphene and its variants interact with electromagnetic radiation from the ultraviolet to the terahertz parts of the spectrum. Such broadband interaction with light and distinctive electronic qualities makes graphene a good prospect for photonic uses.
Graphene has high electrical conductivity and a massive transmission value, both of which indicate potential as a transparent conductor for photonic devices like flat panel video displays, solar panels and light-emitting devices. Indium tin oxide (ITO) and fluorine doped tin oxide (FTO) are the transparent electrodes used in commercial devices today. Graphene is lighter, more flexible, more chemically stable and lower cost than ITO or FTO.
There are also potential uses of graphene in photovoltaic devices, like a bendable transparent electrode or photosensitized material. In solid-state dye-sensitized solar cells (DSSCs), a TiO2 layer catches the photoexcited electrons from the dye molecules. Graphene materials are integrated in TiO2 scaffold to enhance the photocurrent density by enhanced electron transportation, dye adsorption and light scattering.
Graphene can also be used to make ultra-high bandwidth photodetectors that are responsive from the ultraviolet to the terahertz area of the spectrum. Photodetection is determined by the transformation of absorbed photons to electrical signal.
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