• Nandini Shiralkar

Properties of Graphene

To fully appreciate the growth of graphene in various engineering sectors, it is crucial to understand its properties. This article builds upon the knowledge of the previous article to explain the properties that advocate graphene’s use in various branches of engineering.


Mechanical properties

Due to Graphene’s strong covalent sp2 bonds, it has a tensile strength of more than 1000GPa – which is about 200 times more than the tensile strength of steel [1]. Tensile strength refers to the amount of tensile stress that graphene can withstand before failure. Furthermore, the density of graphene is about 0.001 g/m^2 , which makes it much lighter than steel. As graphene has a much higher Young’s Modulus than steel, it exhibits a higher resistance to deformation.

As shown in Figure 1, graphene’s Young’s Modulus decreases with an increase in the number of layers. This suggests that thicker graphene is less stiff.



Figure 1. The correlation between the number of graphene layers and their Young’s Modulus [2]

However, as Ritchie et. al. discovered [3], graphene is nowhere near as tough as steel. Toughness of a material refers to its ability to absorb energy without fracturing. This suggests that graphene cannot absorb high amounts of energy without breaking and therefore it is impractical to use it as a structural material. Despite this, it portrays promising properties that could help it revolutionize structural engineering on a nanoscale. As well as being used on bridges as sensors, it could be used to reinforce some structures [3]. Due to its low density, it has the potential to reduce the overall weight of structures whilst making materials stronger. This property could particularly be exploited to revolutionise aerospace engineering.



Electrical properties

Graphene has a long mean-free-path of the order of 65 x 10^-4 cm [4]. The mean free path is the average distance travelled by the electrons between successive impacts. This means that the electrons can travel greater distances between collisions and therefore graphene has a high electron mobility. This suggests that graphene has remarkable conductivity – a property highly valued in electronic circuits. This is because it can greatly increase the efficiency of the circuits by providing paths for the current with the least resistance. As such, it can help revolutionise electrical engineering by making electronics more sustainable.


Chemical properties

Graphene flakes with no defects are almost perfectly inert. Therefore, in order to exploit the chemical properties of graphene, it is often mixed with other elements to form compounds as this introduces irregularities. Graphene oxide – as shown in Figure 2 – is an example of such compounds. Furthermore, graphene sheets of less than 6000 atoms are thermodynamically unstable [5]. Graphene is also impermeable to any atoms or molecules in ambient conditions, with the exception of Hydrogen ions. This makes it highly suitable for use in water filters and hydrogen-based technologies [5].

Figure 2. The structure of Graphene oxide [6]

Another method to make graphene chemically reactive is by introducing structural defects in the flakes. Methods of introducing such defects include irradiation and chemical treatments [7]. Such defects also enable scientists to test how graphene will behave in the real world, imperfect scenarios.


Thermal properties

Graphene showcases exceptional thermal properties – with a thermal conductivity in the range 3000 – 5000 W/mK at room temperature [8]. Thermal conductivity is defined as the ability of a material to transmit heat. This is much higher in comparison to the thermal conductivities of any metals (e.g. copper has a conductivity of 385W/mK) [9].

Additionally, clamping graphene – as shown in Figure 3 – can enable scientists to reduce the heat flow from a hot system to a cold system. As a result, graphene can be exploited in thermal management technologies such as electronics.


Figure 3. The process of clamping graphene [10]

Within electronic circuits, heat often proves to be the most critical limiting factor as it hinders the growth in efficiency alongside a reduction in size of the circuit. Therefore, graphene could prove to be pivotal for such technologies.

Graphene LEDs are a prime example of practical application of its exceptional thermal conductivity. A graphene LED bulb is reported to be almost 10% more efficient than regular LED [11]. Furthermore, graphene bulbs are cheaper to manufacture and buy. This is because the exceptional thermal conductivity of graphene enables it to dissipate heat quickly and hence damages the LED less. As a result, the bulbs last longer and also appear brighter.

Graphene’s multifunctional properties – such as electrical properties combined with its thermal properties as demonstrated in the previous example – make it highly suitable for engineering applications. As well as fulfilling the primary purpose of its utility through a specific property, graphene has the potential to improve the overall sustainability and functionality of a product. Due to its relevance to various other applications, graphene’s thermal properties strongly advocate graphene’s utility in engineering.



Optical properties

Graphene can absorb light over a wider range of frequencies as it has some unique optical transitions. This is because of its band structure and the electromagnetic radiation emitted by Dirac Fermions. Furthermore, graphene also demonstrates gate-dependent optical transitions. These are optical transitions in electric fields which are often used in modulating current in circuits. Optical transitions can be tuned to produce desirable outcomes: lower optical transitions correspond to a higher proportion of light being absorbed by graphene. However, the optical absorption capacity of graphene is linearly dependent on the number of sheets of graphene stacked together. For example, only 2.3% of light is absorbed by a single sheet of graphene; however, five layers would absorb about 11.5% of the light [12].

As demonstrated in Figure 4, graphene can exhibit different types of optical transitions. Figure 4a portrays the transition in pristine graphene. Pristine here refers to flawless graphene. Furthermore, Figure 4d demonstrates the intra-band transition in graphene.

Figure 4 outlines the possible optical transitions in graphene [13].

Analysis of graphene’s optical properties could reveal more about its electronic structure. Moreover, optical analysis of semi-conductors can reveal more about their states. Overall, graphene showcases some truly remarkable optical properties which could accelerate its journey towards revolutionizing electrical engineering. This demonstrates how graphene’s multi-functional properties in fact complement each other to make it suitable for a wider range of engineering applications.


Superconductivity

Twisted bilayer graphene behaves like a superconductor when squeezed together. A superconductor is a material that can conduct electricity with little or no resistance. Unlike most other materials – which demonstrate superconductivity near absolute zero temperature – graphene is a super conductor at room temperature. This eliminates the necessity for extensive cooling in order to carry out superconductivity research. The ‘magic angle’ between the bilayer sheets that provides superconductivity is 1.1degrees. Figure 5 illustrates the twisted bilayer graphene. This superconductor has various properties that are quite similar to those of cuprates, which are 2D materials with electrons moving within weakly coupled CuO2 layers [1].


Figure 5. Two layers of graphene twisted at 1.1o (illustrated right) show superconducting properties [14]



High surface area to volume ratio

The exposed surface of a material interacts with reactants such as gases, liquids, solids, photons, etc. Fully exposed graphene sheets have a theoretical surface area of 2,629 m^2 /g [15]. However, graphene is prone to layer stacking due to the van der Waals interactions between surfaces. This reduces the exposed surface area that can be exploited in reactions. Despite this, graphene has one of the highest surface-area to volume ratio, which makes it a very attractive material to be used in filter technologies.


Biocompatibility

Graphene is an excellent material to form composites due to the combination of its unique properties. Moreover, research has shown that graphene could provide a favourable environment for stem cell differentiation and accelerate healing processes [16]. Therefore, it is widely regarded to be pivotal in biomedical engineering – for example, it can be used in tissue regeneration and tissue engineering scaffolds [16].

Chemists from the Rice University have found a way to create a self-sterilizing laser-induced graphene (LIG) bacterial air filter [17]. This LIG filter can be installed in a standard vacuum air filtration system and it can capture various types of harmful microbes within the graphene, as shown in Figure 6. The pathogens cannot survive at temperatures of about 350 degrees Celsius. Therefore, the chemists used the conductive properties of graphene to raise its temperature within a few seconds. This also enabled the decomposition of the toxic by-products. The LIG filter could potentially replace the double filter required by US federal standards in hospital ventilation [17] and it could revolutionize air purification technologies in general.

Figure 6. An electron microscope image of Laser Induced Graphene sheets [17]

However, graphene is far from being used in wider biomedical engineering anytime soon. Although there has been immense research around the biocompatibility of graphene, it is not yet clear whether it could have long-term toxicity. Scientists are also trying to understand more about its biodegradable properties.


Graphene showcases impressive properties that could be exploited to enhance and innovate technologies. It has an interesting combination of high strength, high stiffness, biocompatibility, high surface area to volume ratio, unique optical transitions, low density and high thermal and electrical conductivity (including superconductivity). These properties justify why graphene is known as the “material of superlatives”. However, there are some inherent challenges which have hindered graphene’s commercialisation. Therefore, it is crucial to consider these extraordinary properties in the context of real world uses. Future articles will build upon this initial analysis of graphene’s properties but take a more pragmatic approach to explore the extent to which graphene could revolutionise engineering.


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