Twisted graphene is an intriguing contender for applications in next-generation energy conversion and storage devices due to its intrinsic physical qualities and the high degree of tunability of its electronic properties.
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What is Twisted Graphene?
Twisted graphene, also known as twisted bilayer graphene, is a unique structure that may be created by stacking two layers of graphene at a tiny angle, often somewhere between one and two degrees. Because the hexagonal lattices of the two graphene layers are not entirely aligned, a new unit cell with a larger size and a moiré pattern is produced.
The resulting structure of twisted graphene is particularly fascinating from a scientific point of view due to the fact that it possesses a range of unusual physical and electrical properties that are not found in either single-layer graphene or graphite in its bulk form. These characteristics are the result of interlayer coupling that occurs between the two graphene layers, which is mostly determined by the twist angle of the graphene layers.
Twisted graphene has a flat electronic band structure at particular "magic" twist angles, such as 1.1 degrees and 1.8 degrees, where the energy levels of the electrons are spread out over a large range of momentum space, resulting in a near-zero density of states. This results in a variety of intriguing phenomena, such as Mott insulator behavior, superconductivity, and topological states.
Batteries and Supercapacitors
Energy is stored in batteries as chemical potential energy, which is transformed into electrical energy when needed. To enhance the amount of energy that can be stored, battery electrodes are often comprised of materials having a high surface area, such as activated carbon or metal oxides. Because of the moiré pattern generated by the two layers, twisted graphene has a huge surface area, which improves its ability to store energy. Furthermore, the flat electrical band structure at the magic angles could be used to increase battery and supercapacitor performance.
Supercapacitors, on the other hand, store energy in the form of electrical charges that can be released quickly. To maximize capacitance, supercapacitors' electrodes are often composed of materials with high electrical conductivity and surface area. Due to its unique electronic properties, twisted graphene has a high electrical conductivity, and its enormous surface area can also increase the material's capacitance.
For example, researchers from India's Jawaharlal Nehru Centre for Advanced Scientific Research reported using twisted multilayer graphene to create an ultrafast supercapacitor with an ultrahigh-frequency response in the order of 10,000 Hz, the highest reported for any supercapacitor to date.
In solar cells, sunlight is transformed into electrical energy by absorbing photons in a material having a bandgap that matches the energy of the photons. The absorbed energy excites the material, which can then be collected as an electrical current. A solar cell's performance is determined by various parameters, including the material's bandgap, electrical conductivity, and light absorption.
The moiré pattern of twisted graphene can behave as a photonic crystal, increasing light absorption and improving solar cell efficiency. Furthermore, the flat electronic band structure at the magic angles may result in better charge carrier mobility, critical for effective charge separation and collection in solar cells.
The photovoltage of a 10° twisted bilayer graphene photodetector demonstrates a sevenfold photovoltage improvement (700%) at the best incident angle, according to researchers from Nankai University in Tianjin.
Fuel cells are electrochemical devices that directly transform chemical energy into electrical energy. They are made up of an electrolyte as well as two electrodes: an anode and a cathode. The fuel is oxidized in the anode to produce electrons and protons. Protons move through the electrolyte to the cathode while electrons travel through an external circuit, providing an electrical current. Protons react with oxygen in the cathode to generate water, completing the electrochemical reaction.
Because of its unique electronic properties, twisted graphene has a high electrical conductivity, which can improve the performance of fuel cells. Furthermore, the huge surface area of twisted graphene can increase the surface area of the electrodes and enable reactant adsorption, enhancing fuel cell efficiency.
Twisted graphene could be used in water-splitting applications due to its large surface area. Twisted graphene's moiré pattern can also serve as a template for forming catalytic nanoparticles, increasing the efficiency of the water-splitting process.
Due to its unique electrical characteristics, twisted graphene could be a viable material for thermoelectric applications. The flat electronic band structure at the magic angles may result in a high Seebeck coefficient, which is necessary for converting heat to electricity.
Challenges and Opportunities
The longevity of energy storage can be improved by using twisted graphene due to its mechanical and chemical stability. Compared to other high-performance energy storage materials, the production cost of twisted graphene is significantly lower, making it a promising candidate for widespread use.
However, before twisted graphene is used in a practical energy storage system, numerous hurdles must be cleared. For instance, there is still a technical challenge in the mass production of high-quality twisted graphene. The complexity of the electrical environment generated by twisted graphene's moiré pattern adds another layer of difficulty to the challenge of developing an efficient energy storage device based on this material.
Overall, research on solar cells using twisted graphene is exciting and growing rapidly. To fully understand the potential of this material and to optimize its use in practical applications, more research and development is required.
References and Further Reading
Gupta, N, Mogera, et al. (2022) Ultrafast planar microsupercapacitor based on defect-free twisted multilayer graphene, Materials Research Bulletin. p. 111841 https://www.sciencedirect.com/science/article/abs/pii/S0025540822001143?via%3Dihub
Xin, Wei, et al. (2016) "Photovoltage Enhancement in Twisted‐Bilayer Graphene Using Surface Plasmon Resonance." Advanced Optical Materials. pp. 1703-1710. https://onlinelibrary.wiley.com/doi/10.1002/adom.201600278
Mogera, Umesha, and Giridhar U. Kulkarni. (2020) "A new twist in graphene research: Twisted graphene." Carbon. pp. 470-487. https://www.sciencedirect.com/science/article/abs/pii/S0008622319309625?via%3Dihub