Graphene: An Introduction


As part of a joint collaboration, this article was written by Roni Peleg, Editor-in-Chief of Graphene-Info, the leading international graphene publication for over 9 years. It is visited by tens of thousands of professionals a month. Graphene-Info provide a multitude of services to the graphene market based on their extensive & up-to-date knowledge hub & close ties with industry leaders. 

Graphene is a 2D material made from carbon atoms that are arranged in a hexagonal pattern (a honeycomb crystal lattice). It is the basic building block of graphite, used in pencil tips, batteries and many other industrial applications.

 

graphene flake SEM image
SEM image of a single graphene flake.

 

Graphene is considered a “wonder material” since it has many remarkable properties: It is the thinnest material in the world (just one carbon atom thick), and also one of the strongest - much stronger than both diamond and steel of the same thickness. Graphene is also flexible and transparent, has an extraordinarily large surface area, and is an extremely stretchable crystal. Graphene is one of the best conductors of electricity ever found and is also an excellent thermal conductor. In addition, it has fascinating optical properties.

Scientists and researchers all over the world are vigorously working on finding ways to best use graphene, and while the industry is still growing, applications are starting to emerge. However, several issues must be addressed before graphene can live up to its potential and become the revolutionary material it is believed to be. Some of the issues include a lack of bandgap, cumbersome and expensive production and handling processes, among others.

Different Graphene Materials 

Mentions of graphene often refer to a graphene sheet—a single layer of carbon atoms in a perfect honeycomb lattice. The term graphene, however, is used to describe an entire family of materials that are different in structure and properties.

Monolayer graphene sheets, often made by vacuum processes like CVD (depositing gaseous reactants onto a substrate), are considered to be high-quality materials with electronic properties that are potentially valuable in a number of applications – like energy storage and generation, solar cells, and more. However, since the process is wasteful and requires expensive machinery, such materials are still expensive and are mainly used in research activities - although commercial applications are slowly appearing.

Stacking two sheets of graphene one on top of the other, for example, results in bilayer graphene. It turns out that this material differs from monolayer graphene in its electrical properties. Just like graphene, it has a zero bandgap, but a controllable bandgap can be introduced by applying an electric displacement field to the two layers. A bandgap can also be introduced by stacking the two layers in a specific arrangement. These materials are also mainly used today in various research activities.

Three or more sheets of graphene can be used to create materials - and these are referred to collectively as “few-layer graphene” (FLG). When you reach about thirty layers, the properties start to resemble graphite. The terms graphene, FLG, bi-layer graphene and graphite are not clearly defined in a standardized manner, which is one of the causes of confusion in the industry that is nowadays being addressed in various standardisation efforts.

Graphene Nanoribbons (GNRs)

GNRs are thin (under 50 nm) strips of graphene. These have interesting electronic properties, which depend on the width and edge type of the material (zigzag type or armchair type). In fact, GNRs can be metals, semiconductors, half metals, ferromagnets and antiferromagnets—depending on the width, shape, edge structures and chemical termination. Basically, GNRs are semiconductors with an energy gap that scales (inversely) with the width of the ribbon.

 

Armchair (L) and zigzag (R) graphene nanoribbons
An armchair graphene ribbon (left) and a zigzag ribbon (right).

 

GNRs have been the focus of intensive research due to their interesting electronic and spintronics features. GNRs have also been used to develop several transistor designs, DNA sequencing approaches, and more.

Producing GNRs with perfect edges (zigzag or armchair) is difficult. You can start with a graphene sheet and cut it into the desired shape. Another possible production method is “opening” (also called “unzipping”) carbon nanotubes (CNTs), which are rolled up sheets of graphene. Whether these methods are cost-effective and efficient ways to produce GNRs or CNTs is yet to be seen. GNRs today are still mainly used in small quantities for research activities.

Graphene Flakes / NanoPlatelets (GNFs / GNPs)

Producing and handling large graphene sheets is very difficult. Making tiny “flakes” of graphene, in powder or solution form, is much easier. These graphene flakes (GNFs) can retain some of graphene’s mechanical, thermal and electrical properties. It is possible to synthesise GNFs in different sizes and shapes (but not easy, however, to maintain consistency), which changes their properties—as different sized particles behave differently in a matrix. So, a triangle flake will behave differently than a round one, and you can also make them in different sizes.

 

graphene flakes in dispersion on paper SEM image
Graphene flakes in dispersion, on paper.

 

Some GNFs are made from single-layer graphene flakes, and some are stacked graphene flakes (few-layer graphene flakes, in fact). GNFs can be made in different shapes, and this gives them a degree of engineering freedom you cannot achieve with larger graphene sheets. Graphene flakes are sometimes marketed as graphene nanoplatelets – or GNPs. A nanoplatelet is a small round disk-shaped particle (named after its plate-shape structure). In theory, graphene nanoplatelets are disk-shaped graphene sheets (or stacks of sheets) - so a GNP is a type of graphene flake. In practice, it is very difficult to create round-shaped graphene platelets—even if they are artificially synthesised. Virtually all GNPs are not technically disk-shaped, and should therefore be called GNFs.

GNFs are a relatively low-cost form of graphene, and found in growing use in various composite materials. Some applications have already been commercialised using GNFs - like various sports equipment.

Graphene Oxide (GO)

Graphite is a 3D material composed of many layers of graphene. Graphite oxide is a compound of graphite (carbon), hydrogen and oxygen. In graphite oxide, the carbon layers (the graphene sheets) are separated by oxygen molecules. When graphite oxide is placed in water, it is easily separated into graphene sheets – to get graphene oxide (GO) - single sheets of carbon, oxygen and hydrogen.

 

Graphene Oxide flakes.

 

Graphene oxide has properties quite different from those of graphene. For example, it is dispersible in water and other organic solvents (as well as in different matrices), whereas graphene is not. On the other hand, in terms of electrical conductivity, graphene oxide is much less conductive than graphene and is often described as an electrical insulator.

Graphene oxide’s unique properties make it suitable for different applications than graphene. It is heavily studied for uses that rely on its hydrophilic nature and lack of electrical conductivity, like water treatment membranes, medical applications and various composite materials. Use of graphene oxide however, is still mostly limited to R&D activity.

Reduced Graphene Oxide

Graphene oxide sheets can be reduced (which means removing the oxygen and hydrogen) to get regular graphene sheets (called reduced graphite oxide sheets, referred to as r-GO).

While this is a rather easy way to produce graphene sheets, those sheets usually contain many chemical and structural defects. The chosen process of reduction (and there are many methods, as well as the materials used) has a great impact on the quality of the resulting rGO, and some of these can sometimes be quite close in properties to pristine graphene.

Besides lower prices and relative ease of production, a major advantage of rGO is the ability to scale-up its production and make it in large quantities. High-quality rGO basically has properties similar to CVD graphene, so it is naturally suitable for similar applications. However, the advantages listed above (like lower price and scalability) often make it attractive when thinking of commercialisation, and so rGO is used heavily in development work.  

Graphene Quantum Dots (GQDs)

A quantum dot (QD) is a tiny semiconductor that has electronic properties between those of bulk semiconductors and of discrete molecules. QDs are being studied for several applications (including transistors, solar panels, LEDs and even quantum computing) and are recently being adopted in high-end LCD TVs.

 

Graphene quantum dot
A graphene quantum dot. (Source: Aalto University)

 

The size and shape of the quantum dot control its electronic characteristics. For example, if you use a QD to emit light in a LED-like application, the size determines the color (wavelength) of the light. The dot's size and bandgap are inversely related.

Graphene quantum dots (GQDs) are ultra-small graphene flakes, usually made by cutting GNRs into 100 nanometer-long pieces. Like GNRs, GQDs are semiconductors and have a bandgap. As in all quantum dots, the electronic properties are related to the size and shape of the dot.

Additional Graphene Materials

There are additional graphene intermediary materials (such as inks and coatings) and graphene-like materials. Reviewing the different materials makes it clear that choosing the right one for a specific project is a vital first step, as these have different properties and behaviors. Hopefully, graphene will continue its development and live up to its potential by being integrated into more commercial products and projects.

 



About the Author: This article was written by Roni Peleg, who is the Editor-in-Chief of Graphene-Info and Perovskite-Info. Her work experience spans technical translations, literary editing, content writing and market analysis. For the last 4 years, Roni has been focusing on graphene and perovskite technologies, gathering deep understanding of the technologies, markets and industries.