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Graphene displays a range of unique mechanical, electronic and optical properties as a result of its unique structure. Here, we explore how confinement in 2D dimensions results in the unique optical properties of graphene, and how changes in its structure can be used to tune its optical properties.
Graphene consists of a monolayer of carbon atoms organized in a honeycomb lattice. This 2D structure has strong s-bonds in the plane of the material with un-hybridized p-orbitals projecting out of the plane, this orbitals overlap with one another to form delocalized bonds. Whilst the s-bond provides most of graphene's structural stability, the p-bond determines its electronic and optical qualities.
The interaction of graphene with electromagnetic radiation is particularly intriguing because of the two-dimensional confinement of electrons within the structure, which means they display strange behavior, and the extraordinary continuous band structure of graphene.
The band structure of graphene is made up of a completely-filled conduction band and an empty valence bands which intersects linearly at a Dirac point. The optical absorption of pristine graphene is caused by the interband and intraband transitions. For single-layer graphene, absorption of visible light is appears to be negligibly small and it is frequency independent, as any transition within the visible range is possible. 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 only a single atom thick.
Tuning the Optical Properties of 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 using methods such as mechanical exfoliation or chemical vapor deposition (CVD).
Graphene from each technique has distinct optical qualities as a result of structural imperfections which are technique-specific. For instance, graphene produced via exfoliation 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 artifacts 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 the 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.
The Optical Applications of Graphene
Graphene and its variants interact with electromagnetic radiation from the ultraviolet to the terahertz parts of the spectrum. Graphne's broadband interaction with light and its distinctive electronic qualities make it a good candidate material for use in photonics.
Graphene has a high electrical conductivity and a massive transmission value, both of which indicate its potential as a transparent conductor for photonic devices such as 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.