• Nandini Shiralkar

Challenges of using graphene

Although previous articles have highlighted the impressive multi-functional properties of graphene, its practical applications pose yet another challenge. The fundamental as well as niche challenges of using graphene question whether graphene can at all revolutionise engineering. In the previous article, some of the industry-specific exploitations of graphene have been discussed. This article builds upon some of those limitations to outline the inherent challenges that have questioned graphene’s potential to revolutionise engineering.

Zero band gap

Band gap represents the section between the conduction and the valence band of electrons. To be able to “switch off” a circuit, there must be a bandgap within the material which can be manipulated by controlling the energy of the electrons. However, graphene showcases a unique property – the Zero Band Gap as the Highest Occupied Molecular Orbital (HOMO) touches the Lowest Occupied Molecular Orbital (LOMO) at a single Dirac Point [1], as shown in Figure 1b. Therefore, it is difficult to stop the electrons from travelling to the HOMO from the LOMO. As a result, it is impossible to “switch off” graphene unless a band gap is engineered.

Figure 1b showcases the electronic dispersion of graphene [2]

This means that circuits using graphene in its purest form can never be switched off. As the ability to switch off circuits when they are not in use is highly valued in electronics, the Zero Band Gap has immensely hindered graphene’s exploitation in electrical engineering. This also restricts the exploitation of graphene’s optical properties. With a noticeable band gap, graphene would be able to demonstrate more optical transitions as electrons will be able to jump between energy levels. The Zero Band Gap affects a multitude of graphene’s properties, which makes it a significant challenge that has restricted graphene’s utility.

Transistors are critical electronic components primarily utilised as amplifiers or switches [3]. This means that they can be used in various circuits – such as those in phones and aircrafts – to control the basic functionality of the circuit. Silicon – the material currently used in most transistors – stocks are depleting at an alarming rate. This makes it crucial to actively search for alternative materials to be used in transistors. Graphene – due to its extremely high mobility of electrical carriers as discussed in an earlier article – is an ideal material to be used in transistors. Therefore, overcoming the barrier of the bandgap has attracted great scientific interest. Various methods that have been used to open the band gap of graphene include hydrogenation, electrically gated bilayer graphene, Stones-Wales defects, graphene-substrate interaction and absorption of molecules [4].

6.2 Production

Graphene was first isolated by the process of exfoliation, which involves the use of adhesive tape to simply peel it off from graphite. This method provides nearly defect-free graphene with the best physical properties for lab use; however, it is not sustainable for larger scale production [5]. Consequently, there has been immense research on the optimal methods of graphene production.

Chemical Vapour Deposition (CVD) – as outlined in Figure 2 – is currently the most popular method of isolating graphene. It involves the deposition of gaseous reactants (e.g. Methane) onto a substrate (e.g. Nickel) [6]. Although the gaseous by-products are usually very toxic, the reactant molecules are combined with the help of carrier gases to reduce the temperature necessary for the reaction – which improves its sustainability. When this mixture of gases comes into contact with the hot surface of the substrate, a material film is created. However, the separation of the graphene film from the substrate poses yet another challenge. Although this process is dependent on the type of the substrate used, it can introduce unwanted impurities in the graphene flakes. Furthermore, the relationship between graphene and the substrate is still not fully understood; for example, Nickel tends to dissolve carbon atoms during the cooling phase.

Figure 2. The mechanism for CVD graphene growth [7]

The benefits of using CVD include high purity and increased hardness over other coating methods. It strikes the perfect balance between the costs involved and the high purity of the films. Therefore, it is widely used in the semiconductors and optoelectronics industry.

Chemical Vapour Deposition is a very meticulous method which requires controls at every stage to monitor the quality of the graphene produced and to ensure that the reactions are efficient. Furthermore, creating a uniform layer of graphene on a substrate poses another challenge. The diffusion and convection (due to the high temperatures) of gases affect the volume occupied by the reactants in the chamber. As a result, the chemical reactions do not occur uniformly over the substrate as there might be depletion of reactants due to fluid dynamics principles. Despite these hurdles, CVD offers the most promising method to produce graphene flakes [8].

However, for graphene to transform from a research laboratory to more large-scale uses, it is important that the gap between perceived metrology and standardisation is addressed [9]. Metrology here refers to the study of making measurements in a scientific context [10]. It helps maintain measurement standards across the industry to ensure that the graphene produced is of the same quality regardless of the methods. This builds consumer confidence in the products and hence accelerates the utility of graphene in commercial contexts. Tackling this issue would accelerate graphene commercialisation as production will become more standardised. It would enable greater global collaboration to bridge the gap between the research laboratory and the real world.

Overall, the production of graphene for commercial use is perhaps the biggest challenge faced by the industry. Although production methods vary based on the intended use of the flakes (e.g. superconducting graphene must have an engineered band gap), much more focused research needs to be carried out to improve and standardise the quality of the flakes. Overcoming this obstacle will accelerate the graphene revolutionise. With larger quantities of graphene flakes available, the scientific community could also discover new graphene utilities that have not yet been considered.

From the very basics such as the production of suitable graphene to niche limitations such as its zero-band gap, there are many challenges which have questioned graphene’s potential to revolutionise engineering. However, cutting-edge research could help overcome these challenges by proposing pragmatic solutions. Although graphene might not penetrate the market imminently, it can be exploited in engineering with the help of some further research on how to overcome these challenges.

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