Graphene is a well-known material to many people within and outside of the nanotechnology sector. However, many people still think of graphene as a high strength and electrically conductive material. Whilst this is true, and is graphene is widely used in those applications, it does possess many other properties that can be tailored for many applications. One such property is its excellent optical properties, which can be tweaked for many optical and optoelectronic applications. In this article, we look at graphene’s optical properties and how they can be changed.
Graphene, as many people will know by now, is a highly stable allotrope of carbon that arranges itself into hexagonal sheets. It is also a single layer of graphite, but only when it is in its monolayer form. Graphene exhibits many properties that have enhance a large number of applications, hence it has been gaining traction across many industries and academic sub-disciplines.
However, graphene is not a single entity. When most people talk about graphene, they refer to single layer CVD-grown graphene. In reality, there are many forms of graphene, from single-layer graphene, multi-layer graphene (up to three layers) and graphene nanoplatelets (3-10 layers).
Anything above 10 layers of graphene is then considered to be in the graphite spectrum. There are even functionalized forms of graphene, such as graphene oxide and reduced graphene oxide, that many people mistake as graphene, but these “graphene” sheets exhibit some drastically different properties to carbon-only graphene.
The Optical Properties of Graphene
Graphene exhibits some excellent optical properties, but again, these properties vary between the different types of graphene. Graphene, i.e. a pure CVD-grown monolayer of graphene, is a very transparent material, hence its used in optical devices. A single sheet of graphene will only absorb a maximum of 2.3% of incoming light (i.e. a 97.7% optical transmittance) with less than 0.1% reflectance.
Once you start stacking the graphene sheets on top of each to form multi-layer graphene, or even graphene nanoplatelets, the absorption of light increases linearly with the number of layers. This means that each layer of graphene adds an extra 2.3% light absorbance to the existing absorbance value, so, the greater the number of layers, the lower its optical transparency will be.
Tweaking the Optical Properties in Graphene
One of the most common ways of tuning any material is through doping with different atoms. Whilst it is a technique usually used to induce changes in the band gap of a material, the presence of other atoms can also change the optical properties of a material.
For graphene (unless it is functionalized with oxygen groups), any new atom that comes into contact will absorb to the surface, i.e. it becomes an adatom. The presence of adatoms of the surface of a graphene sheet can be used to change the optical absorbance of graphene as both the graphene sheet and adatom will absorb light.
Etching methods are another key way of tailoring the optical properties of a graphene sheet. One of the most common ways of etching graphene is to use ozone, which is also know as an ozone treatment.
Ozone etching induces oxidation on the surface of the graphene sheet, which in turn changes its optical properties due to the extra molecules at the surface. Ozone etching is a highly tunable technique because the optical transmittance can be tailored by simply changing the exposure time and temperature of the ozone treatment. Additionally, this method can be used to target specific areas of a graphene sheet, without changing other areas. This can lead to graphene sheets with optical differences, i.e. an optical contrast, within the same sheet.
On the other side of the graphene spectrum, there are ways to tune and tweak the optical properties of graphene oxide. A controlled deoxidation approach can be used to remove certain oxygen-rich functional groups from the surface of the graphene sheet. This then creates holes where the functional group was, which changes the optical gap of the graphene oxide sheet. The holes themselves can also be tweaked by selectively removing the carbonyl groups, rather than the epoxy or hydroxyl group, which then creates a larger hole and a greater reduction in the optical gap.
NGA Expo 2017: https://www.nationalgrapheneassociation.com/news/nga-graphene-innovation-summit-observers-view/
“Graphene and Graphene Oxide: Synthesis, Properties, and Applications”- Zhu W., et al, Advanced Materials, 2010, DOI: 10.1002/adma.201001068
“Modulating Optical Properties of Graphene Oxide: Role of Prominent Functional Groups”- Johari P. and Shenoy V. B., ACS Nano, 2011, DOI: 10.1021/nn202732t
“Optical properties of graphene”- Falkovsky L. A., Journal of Physics: Conference Series, 2008, DOI: 10.1088/1742-6596/129/1/012004
“Tuning the Electrical and Optical Properties of Graphene by Ozone Treatment for Patterning Monolithic Transparent Electrodes”- Yuan J., et al, ACS Nano, 2013, DOI: 10.1021/nn400682u