Many people will have heard of graphene, even if they don’t know exactly what it is. Graphyne, on the other hand, is not as well known. In this article, we look at what both of these materials are and what beneficial properties they exhibit. This article also looks at the difference between these interesting materials.
What is Graphene and what are its properties?
Graphene is an allotrope of carbon that exists as a two-dimensional sheet. One way to think of graphene is as a single layer of graphite, where all the carbons exist in a regular hexagonal array. Being composed of carbon makes graphene a non-metal, but it possesses semiconducting properties that means it often characterized as a quasi-metal. There are many forms of graphene available today, ranging from single layer, bi-layer and multi-layer graphene to graphene nanoplatelets (GNPs).
Graphene sheets are bound together by sp2 hybridized bonds and each carbon possesses one free electron. This free electron is a crucial component of graphene as it sits above the plane of the sheet in a p-orbital. This leads to each hexagon on the sheet possessing 2 delocalized pi-electrons which aids in many properties, especially electrical conductivity.
On the note of electrical conductivity, graphene possesses not only an excellent electrical conductivity but also a high charge carrier mobility. Graphene sheets also possess holes which allow phonons to travel through unimpeded, which gives rise to a high thermal conductivity.
Graphene doesn’t have an electronic band-gap because the valence and conduction bands are slightly overlapped, and the electrons act as massless relativistic particles. This is an important property as it is what enables graphene to have a high charge carrier mobility.
The electrons act as, what is known as, massless Dirac Fermions, i.e., quasi-particles. Graphene also exhibits a half-integer Quantum Hall Effect (QHE). The 2-dimensional nature of graphene means that the electrons become confined at discrete band levels (also known as Landau levels). Graphene is unique in that its charge carriers only half fill these levels and causes these levels to be quantized.
Graphene also possesses a range of other beneficial properties, including optical transparency, a high degree of flexibility and high mechanical/tensile strength (with a Young Modulus of up to 1.0 Tpa). Its vast range of properties has enabled graphene to be implemented in wide range of applications, including batteries, sensors, solar cells, conductive inks and even as a composite material in bicycle frames.
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What is Graphyne and what are its properties?
Graphyne is a theoretical material. It has not been produced and has only been studied through various theoretical means, i.e., computational studies. That aside, what could graphyne look like and what properties could it possess?
Structurally, graphyne is similar to graphene, but contains a mixture of sp2 and sp3 hybridized bonds. Whilst graphyne could potentially take a few forms, the structure of graphyne is composed of benzene rings and acetylene bonds.
These units on the graphyne sheet break up the hexagonal array showcased by graphene to form a ‘triangulated’ structure (although, they are not technically triangles, but are strained 6-member rings that resemble a closer appearance to a triangle than a hexagon). However, because the sp2 and sp3 bonds could be bound in various ways, the hybridization for each form of graphyne would be variable depending on the ratio of sp2 to sp3 bonds.
The exact properties are again not known. However, they can be theorized. There is another problem for quantifying the exact properties and that is due to the potential difference in structures. Each structure would most likely give different mechanical, optical and electronic properties depending on its internal make-up.
However, in general, researchers believe that graphyne should be able to conduct electrons, but only in one direction. It is expected to have similar electronic and semiconducting properties to graphene and could potentially be used in one-way electronics, such as transistors and nanoelectronics, but researchers have also stated that computer simulations alone cannot tell whether the theorized electronic properties will actually be present in a physical sample.
Where are the major differences?
Unlike graphyne, graphene exists today. This is a major point, as no amount of properties are going to beneficial if the material can’t be made. That being said, there is the potential for various similarities and differences between the two materials.
So where are they similar? Aside from being composed of carbon, both materials are theorized to possess Dirac cones in the Brillouin Zone. These Dirac cones are thought to be one of the main reasons why a sheet of graphene is highly conductive towards electricity. If graphyne was to possess Dirac cones, it may have a similar electrical conductivity to graphene and be feasible for electronic applications.
The high amount of phenyl rings, and therefore the number of delocalized electrons, could even produce a charge carrier mobility which is similar to what we see with graphene – the difference being that graphene can mobilize charge carriers in more than one direction.
Now onto the differences. If graphyne were to form, its hybridization and structure would be very different to graphene and this would lead to a difference in the exhibited properties. For one, graphyne would be more rigid – as it would possess a more sterically strained sheet than is found with graphene.
This would not only reduce the flexibility of graphyne against graphene, but this extra rigidity may cause it to be a more brittle material and therefore not as mechanically strong as graphene.
However, whilst theoretical calculations can give us an idea, we will not know the exact differences until a sheet of graphyne is physically created and isolated. It is thought that if it is created, that it will be stable. Many of the building blocks have already been synthesized, so we may know sooner than we think about how graphyne compares to graphene.
“Towards graphyne molecular electronics”- Li Z., et al, Nature Communications, 2015, DOI: 10.1038/ncomms7321
“Competition for Graphene: Graphynes with Direction-Dependent Dirac Cones”- Malko D., et al, Physical Review Letters, 2012, 10.1103/PhysRevLett.108.086804
Physics World: https://physicsworld.com/a/could-graphynes-be-better-than-graphene/