“Our work reveals a critical fallacy of the Dirac cone approximation in the higher-energy thermionic emission in graphene, thus prompting future researchers to be more careful in choosing the right graphene model”Yee Sin Ang
A team from Singapore University of Technology and Design have provided a unique insight into thermionic emission in graphene. Thermionic emission is the boiling of thermally excited electrons from the surface of a material, and is a key physical process that allows the functions of a range of solid-state devices in electronics, optoelectronics and energy conversion, with thermionic emission in transistors, for instance, helping dictate the performance and energy efficiency of laptops and smartphones.
While thermionic emission in conventional 3D materials is well understood, its physics in 2D layered materials such as graphene is much less so, but will be crucial for the development of many future technologies. By examining graphene’s electronic properties, the researchers developed a new theoretical framework to accurately capture the thermionic emission physics in the material, a breakthrough that should prove useful for modeling a wide range of graphene-based devices.
Although the electronic properties of graphene are usually assessed by Dirac cone approximation, as described in Physical Review Applied [Ang et al. Phys. Rev. Appl. (2019) DOI: 10.1103/PhysRevApplied.12.014057] it was shown that when erroneously using this approach to model the conduction of electricity and heat energy from thermionic emission in graphene, the expected results can deviate by over 50% from the new model. The generalized thermionic emission model works for both low- and high-energy electrons, and can be generalized to other 2D materials, offering an improved theoretical approach for accurately analysing, modeling and designing graphene-based thermionic energy devices.
While for low-energy electrons, the Dirac cone approximation helps provide a simplified description of the electrons in graphene, for optoelectronic devices and energy converters, the thermionic emission involves electrons with much higher energy. Reliability of the model is therefore based on a more sophisticated theory that works to capture the electronic properties of graphene in the high-energy regime, circumventing these low-energy limitations. The new model allows a wide array of graphene-based devices operating at different temperatures and energy regimes to be universally described under a single framework.
As researcher Yee Sin Ang told Materials Today, “Our work reveals a critical fallacy of the Dirac cone approximation in the higher-energy thermionic emission in graphene, thus prompting future researchers to be more careful in choosing the right graphene model”. Further work into electron emission is needed to better understand how defects, impurity scattering and surface roughness in graphene can change thermionic emission behavior, and the team also hope to integrate the theory into a compact model or computer-aided design software to improve the design and optimization of graphene-based devices.
Physics of thermionic emission in graphene: (a) thermionic emission of low-energy electrons around the Dirac cone electronic band structure (left); low-energy thermionic emission typically occurs in electronic devices, such as a graphene Schottky diode (right); (b) thermionic emission high-energy electrons from the full electronic band structure of graphene (left). Such an effect typically occurs in optoelectronic devices, such as photodetectors and solar cells (right).