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Graphene is a nanomaterial that has a lot of different properties, a lot of potential for many scientific applications, and the potential to enter many commercial markets. So, it is of no surprise that much of the basic science, i.e. its properties, of graphene has already been deduced. When most people think of graphene, they often think about its flexibility, it’s high tensile strength, or its high electrical conductivity and charge carrier mobility, as these are the properties that are most often discussed. However, graphene does have many other notable properties, and in this article, we will look at one of them—it’s optical properties. It such be noted that this article centers around non-functionalized graphene (i.e. pure graphene), and not any of the derivatives such as graphene oxide and reduced graphene oxide.
While most people focus on the high-strength and high conductivity properties, graphene’s optical properties are worth a mention. It should be noted though, that the optical properties of graphene do vary by the layer number. For example, a single layer of graphene only absorbs 2.3% of light, so 97.7% of light passes through a single layer, with around 0.1% reflected from its initial trajectory. However, the more graphene layers there are stacked on top of each other, the greater the light absorption becomes and the lower the optical transparency becomes. But the relationship is linear, with each layer absorbing 2.3% of light; so, a graphene sample composed of 5 layers would have an absorption of 11.5 % and an optical transparency of around 88-88.5% (when you also take into account the small degree of light reflected).
But graphene also has many other specific properties when it interacts with electromagnetic radiation. For one, graphene possesses unique optical transitions and can absorb light over a wide range of frequencies. The ability for graphene to absorb radiation from many different regions in the electromagnetic spectrum is down to its band structure, lack of a band gap and the interaction between the electromagnetic radiation and the Dirac fermions in the graphene sheet. Each of these optical responses is different, with visible to near-infrared light causing intraband transitions, and the far-infrared absorption being possible through either intraband transitions or free carrier absorption mechanisms. Graphene can also adsorb this radiation independent of its frequency because it doesn’t have discrete energy band levels like most materials.
Graphene can also produce optical transitions in electric fields, and this is known as gate-dependent optical transitions. Under an applied electrical field, the low density of states near the Dirac point causes the Fermi level of the graphene to shift. It is a process often used in electronics to modulate the current, as a change in the Fermi level changes the conductivity, as well as for tuning the transmission of an optical source. These modulations are performed by the material absorbing electromagnetic radiation, which for graphene is infrared radiation, and the change in the Fermi level dictates how much radiation is absorbed by the graphene. For tuning the optical transmission of an electromagnetic radiation source, a greater degree of absorption results in a lower optical transmission, and vice versa. This is most relevant to single-layer graphene, as more than one layer can significantly alter the absorption properties and ability to tune the absorbance.
Graphene can also emit some form of photoluminescence. While its zero bandgap means that it can’t form relaxed states—which is the usual photoluminescence mechanism whereby an electron is excited to a higher energy band, before releasing a photon as the electron returns to its electronic ground state—but pristine graphene is known to emit light when it has been excited by a near-infrared laser. In this case, the mechanism of light emittance is due to high temperature of the femtosecond laser photons which hit the graphene sheet, as they are known to emit in the visible light spectrum. For non-pristine graphene, this photoluminescence is observed by creating a bandgap through either functionalization or through cutting the graphene into smaller sheets to form quantumly confined regions with defects. These quantumly confined regions then open the bandgap, which enables conventional photoluminescence mechanisms to occur.
Because graphene possesses some unique optical properties and can absorb a wide range of electromagnetic radiation, there are a lot of potential optical and photonics applications that graphene can be used in, from saturable absorbers to transparent conductors in photonic devices and high-bandwidth photodetectors.
Sources and Further Reading
- CheapTubes: https://www.cheaptubes.com/graphene-synthesis-properties-and-applications/
- “Graphene and Graphene Oxide: Synthesis, Properties, and Applications”- Ruoff R. S. et al, Advanced Materials, 2010, DOI: 10.1002/adma.201001068
- “Graphene: A review of optical properties and photonic applications”- Jaiswal M and Kavitha, Asian Journal of Physics, 2016
- “Optical properties of graphene”- Falkovsky L. A., Journal of Physics Conference Series, 2008