E-Thesis 689 views 277 downloads
First principle-based Research on properties of twisted bilayer of Graphene and GaN / XIANG CAI
Swansea University Author: XIANG CAI
DOI (Published version): 10.23889/SUthesis.58747
Abstract
Since quantum theory was founded in 1920s, it has developed quite rapidly and led people to a new field that totally different from the classical ones. As the size of an object decrease to a certain level, the classical physical law would be invalid and quantum mechanics would become the ruler of ob...
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Swansea
2021
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| Institution: | Swansea University |
| Degree level: | Doctoral |
| Degree name: | Ph.D |
| Supervisor: | Li, Lijie |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa58747 |
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| Abstract: |
Since quantum theory was founded in 1920s, it has developed quite rapidly and led people to a new field that totally different from the classical ones. As the size of an object decrease to a certain level, the classical physical law would be invalid and quantum mechanics would become the ruler of object behaviors. The property under quantum mechanics achieves functions that never been seen before. So, scientist would invent new materials and devices with properties that never found before. This is the reason that new materials and devices have been emerging due to vast developments in nanotechnologies. The project of my PHD research in four years is to research and analysis the property of nano semiconductor materials. In this thesis, there are mainly four parts. The first part is Introduction. In this part the basic concepts and related information would be listed. The second part is the first project that guide me to the further study. This project is set to simulate the electronic transport in quantum region based on the Schrodinger equation for random shaped energy barriers, which is very basic to quantum mechanics. The method of simulation is matlab coding. The simulation will help in understanding the concept of quantum field and applications in design and analysis of nanometer scale devices and systems. The third part is my first paper. In this paper I cooperate with my workmate Shuo Deng and we investigate the electron transport and thermoelectric property of twisted bilayer graphene nanoribbon junction (TBGNRJ) in 0o, 21.8o, 38.2o and 60o rotation angles by first principles calculation with Landauer-Buttiker and Boltzmann theories. It is found that TBGNRJs exhibit a strong reduction of thermal conductance compared with the single graphene nanoribbon (GNR) and negative differential resistance (NDR) in 21.8 o and 38.2 o rotation angles under ±0.2 V bias voltage. More importantly, three peak ZT values of 2.0, 2.7 and 6.1 can be achieved in the 21.8o rotation angle at 300K. The outstanding ZT values of TBGNRJs are interpreted as the combination of the reduced thermal conductivity and enhanced electrical conductivity at optimized angles. The fourth part is my second paper. In this paper I report electronic and optical properties of the GaN bilayer structures that are rotated in plane at several optimized rotation angles by using the density functional theory method. For the aim of maintaining the structural stability and using a small cell size, the twisting angles of the GaN bilayer structures are optimized to be 27.8°, 38.2° and 46.8° using the crystal matching theory. The last part is conclusion, possible further study and acknowledgement. |
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| College: |
Faculty of Science and Engineering |