First-principles studies on the structure and electronic properties of two-dimensional semiconductors
Ever since the successful isolation of graphene in 2004, two-dimensional (2D) materials have attracted much attention because of their inhomogeneous electron distribution, optical, valley, and spin responses, and notable characteristics including ferroelectricity, magnetism, and superconductivity, w...
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| Other Authors: | |
| Format: | Thesis-Doctor of Philosophy |
| Language: | English |
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Nanyang Technological University
2025
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| Subjects: | |
| Online Access: | https://hdl.handle.net/10356/182726 |
| _version_ | 1826125543594000384 |
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| author | Tian, Yujia |
| author2 | Zhou Kun |
| author_facet | Zhou Kun Tian, Yujia |
| author_sort | Tian, Yujia |
| collection | NTU |
| description | Ever since the successful isolation of graphene in 2004, two-dimensional (2D) materials have attracted much attention because of their inhomogeneous electron distribution, optical, valley, and spin responses, and notable characteristics including ferroelectricity, magnetism, and superconductivity, which differ from those of the bulk. While graphene has been well studied with its promising properties, its lack of a band gap hinders its applications in electronic devices. Having small band gaps, 2D semiconductors can be exploited instead, and the significant induced effects of quantum confinement endow such materials with more possibilities for next-generation electronic applications. To fully utilise the potential of these materials in nanoelectronics applications, more thorough understanding on their electronic properties becomes necessary, which are highly dependent on the configuration and structure of the materials under various conditions.
Before the time-consuming experimental tests and full-scale deployment of 2D semiconductors, modelling and simulation of the material behaviour help to identify suitable material candidates and assess the effectiveness of potential property tuning strategies. To effectively capture the quantum effects and cater to the atomic scale of 2D semiconductors, first-principles calculations are commonly adopted. This approach has demonstrated its capability to serve as a robust tool to obtain the structure and electronic properties of 2D materials. Based on this modelling method, this thesis focuses on the material development process from a proof-of-concept model of a newly developed 2D semiconductor, antimonene, in its free-standing form to investigating the effects of contacting materials in the case of 2D tin monoxide (SnO). Defects, electric fields, and environmental gases are then added to the picture when modelling a wide-band-gap semiconductor, gallium nitride (GaN) in its 2D form. As more and more factors are considered, the model becomes closer to the real-world scenarios.
First, the effects of in-plane compression on the structure and electronic properties of antimonene are investigated. First-principles calculations are performed to compare the material behaviour under rippled and flat deformation modes. The structure and electronic parameters are obtained under uniaxial compression of up to 7.5% applied along the armchair and zigzag directions. The rippled structures exhibit highly stable properties, such as the work function and band gap, when they are fully relaxed, regardless of the compression level. These properties remain largely unchanged from those of the pristine structure. In contrast, significant alterations are observed in the flat structures. The mechanisms behind the different results are thoroughly explained through analysis of the density of states (DOS) and structural geometry. Additionally, the out-of-plane dipole moments of rippled antimonene are discussed, providing insights into their potential applications in sensors, actuators, and triboelectric nanogenerators, etc. This work provides understanding about the properties of antimonene as a 2D semiconductor and demonstrates the ability of the rippled form in property preserving under in-plane compression. Determining the optimal ripple amplitudes at which the electronic properties of antimonene can be restored to their pristine state will be crucial for guiding the rational design and development of antimonene-based devices.
Second, since a 2D material employed in electronic applications is usually in contact with other materials, the effects of the contacting materials are thoroughly examined and compared. The contact junctions formed between common metals (gold, nickel, platinum, or palladium) and 2D SnO are carefully modelled via the first-principles approach, and the charge transfer and bonding at the interface and the electronic behaviour of the material system are systematically evaluated. As states are induced in the band gap of 2D SnO by the contacting metal, its intrinsic semiconducting properties are hindered. The insertion of graphene at the contact interface is then proposed to alleviate the metal-induced gap states (MIGS). It has been demonstrated that the strong bonding formed between the metals and 2D SnO is the cause of the MIGS. In contrast, the graphene interlayer only forms weak van der Waals interaction with both the metal and 2D SnO, which minimises the perturbance to the band structure of the 2D semiconductor. The effects of out-of-plane compression are also analysed to assess the performance of the contact under mechanical deformation. While this example manifests external contact at the system level between metal pins and the native oxide formed on tin surface finish, it can represent internal contact between the metal electrode and channel material of a transistor.
Finally, the effects of defects, applied voltage, and environmental gases on the structure and electronic properties of 2D GaN are investigated. Upon the introduction of point vacancies, distinct magnetic responses and changes to the electronic band structure of the material are induced. The steady electronic performance of 2D GaN under a wide range of external electric fields is validated, with potential further enhancement observed in the presence of gallium vacancy defects. On the other hand, nitrogen vacancies allow 2D GaN to be magnetised at an electric field threshold of ~0.9 V/Å, which makes it serve as a switch. The capabilities of 2D GaN are placed in context by contrasting them with those of silicon carbide, another wide-band-gap semiconductor, in its 2D form. In practical terms, the gas sensing capability of 2D GaN is exemplified using nitrogen monoxide, with findings indicating its potential to be further modulated via the defects and electric fields. By examining the geometry, energetics, band structure, DOS, magnetic moment, and charge transfer of the relevant material systems using first-principles calculations, extensive analysis of the response mechanisms is presented, offering avenues to tailor the properties of 2D GaN for specialised uses.
This Ph.D. research successfully demonstrates the use of first-principles calculation to model a range of 2D semiconductors. The tuning of their structure and electronic properties via various static and dynamic strategies is presented. By incorporating more real-life factors, the obtained results are highly relevant for applications in different sectors. The established models and analysis approaches also provide solid guidance on exploration of other 2D semiconductors and their efficient property modulation. |
| first_indexed | 2025-03-09T14:03:49Z |
| format | Thesis-Doctor of Philosophy |
| id | ntu-10356/182726 |
| institution | Nanyang Technological University |
| language | English |
| last_indexed | 2025-03-09T14:03:49Z |
| publishDate | 2025 |
| publisher | Nanyang Technological University |
| record_format | dspace |
| spelling | ntu-10356/1827262025-03-04T02:57:33Z First-principles studies on the structure and electronic properties of two-dimensional semiconductors Tian, Yujia Zhou Kun School of Mechanical and Aerospace Engineering Infineon Technologies Asia Pacific Pte. Ltd. kzhou@ntu.edu.sg Engineering Computational materials science Two-dimensional materials Semiconductors Electronic properties Density functional theory Ever since the successful isolation of graphene in 2004, two-dimensional (2D) materials have attracted much attention because of their inhomogeneous electron distribution, optical, valley, and spin responses, and notable characteristics including ferroelectricity, magnetism, and superconductivity, which differ from those of the bulk. While graphene has been well studied with its promising properties, its lack of a band gap hinders its applications in electronic devices. Having small band gaps, 2D semiconductors can be exploited instead, and the significant induced effects of quantum confinement endow such materials with more possibilities for next-generation electronic applications. To fully utilise the potential of these materials in nanoelectronics applications, more thorough understanding on their electronic properties becomes necessary, which are highly dependent on the configuration and structure of the materials under various conditions. Before the time-consuming experimental tests and full-scale deployment of 2D semiconductors, modelling and simulation of the material behaviour help to identify suitable material candidates and assess the effectiveness of potential property tuning strategies. To effectively capture the quantum effects and cater to the atomic scale of 2D semiconductors, first-principles calculations are commonly adopted. This approach has demonstrated its capability to serve as a robust tool to obtain the structure and electronic properties of 2D materials. Based on this modelling method, this thesis focuses on the material development process from a proof-of-concept model of a newly developed 2D semiconductor, antimonene, in its free-standing form to investigating the effects of contacting materials in the case of 2D tin monoxide (SnO). Defects, electric fields, and environmental gases are then added to the picture when modelling a wide-band-gap semiconductor, gallium nitride (GaN) in its 2D form. As more and more factors are considered, the model becomes closer to the real-world scenarios. First, the effects of in-plane compression on the structure and electronic properties of antimonene are investigated. First-principles calculations are performed to compare the material behaviour under rippled and flat deformation modes. The structure and electronic parameters are obtained under uniaxial compression of up to 7.5% applied along the armchair and zigzag directions. The rippled structures exhibit highly stable properties, such as the work function and band gap, when they are fully relaxed, regardless of the compression level. These properties remain largely unchanged from those of the pristine structure. In contrast, significant alterations are observed in the flat structures. The mechanisms behind the different results are thoroughly explained through analysis of the density of states (DOS) and structural geometry. Additionally, the out-of-plane dipole moments of rippled antimonene are discussed, providing insights into their potential applications in sensors, actuators, and triboelectric nanogenerators, etc. This work provides understanding about the properties of antimonene as a 2D semiconductor and demonstrates the ability of the rippled form in property preserving under in-plane compression. Determining the optimal ripple amplitudes at which the electronic properties of antimonene can be restored to their pristine state will be crucial for guiding the rational design and development of antimonene-based devices. Second, since a 2D material employed in electronic applications is usually in contact with other materials, the effects of the contacting materials are thoroughly examined and compared. The contact junctions formed between common metals (gold, nickel, platinum, or palladium) and 2D SnO are carefully modelled via the first-principles approach, and the charge transfer and bonding at the interface and the electronic behaviour of the material system are systematically evaluated. As states are induced in the band gap of 2D SnO by the contacting metal, its intrinsic semiconducting properties are hindered. The insertion of graphene at the contact interface is then proposed to alleviate the metal-induced gap states (MIGS). It has been demonstrated that the strong bonding formed between the metals and 2D SnO is the cause of the MIGS. In contrast, the graphene interlayer only forms weak van der Waals interaction with both the metal and 2D SnO, which minimises the perturbance to the band structure of the 2D semiconductor. The effects of out-of-plane compression are also analysed to assess the performance of the contact under mechanical deformation. While this example manifests external contact at the system level between metal pins and the native oxide formed on tin surface finish, it can represent internal contact between the metal electrode and channel material of a transistor. Finally, the effects of defects, applied voltage, and environmental gases on the structure and electronic properties of 2D GaN are investigated. Upon the introduction of point vacancies, distinct magnetic responses and changes to the electronic band structure of the material are induced. The steady electronic performance of 2D GaN under a wide range of external electric fields is validated, with potential further enhancement observed in the presence of gallium vacancy defects. On the other hand, nitrogen vacancies allow 2D GaN to be magnetised at an electric field threshold of ~0.9 V/Å, which makes it serve as a switch. The capabilities of 2D GaN are placed in context by contrasting them with those of silicon carbide, another wide-band-gap semiconductor, in its 2D form. In practical terms, the gas sensing capability of 2D GaN is exemplified using nitrogen monoxide, with findings indicating its potential to be further modulated via the defects and electric fields. By examining the geometry, energetics, band structure, DOS, magnetic moment, and charge transfer of the relevant material systems using first-principles calculations, extensive analysis of the response mechanisms is presented, offering avenues to tailor the properties of 2D GaN for specialised uses. This Ph.D. research successfully demonstrates the use of first-principles calculation to model a range of 2D semiconductors. The tuning of their structure and electronic properties via various static and dynamic strategies is presented. By incorporating more real-life factors, the obtained results are highly relevant for applications in different sectors. The established models and analysis approaches also provide solid guidance on exploration of other 2D semiconductors and their efficient property modulation. Doctor of Philosophy 2025-02-19T10:41:15Z 2025-02-19T10:41:15Z 2024 Thesis-Doctor of Philosophy Tian, Y. (2024). First-principles studies on the structure and electronic properties of two-dimensional semiconductors. Doctoral thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/182726 https://hdl.handle.net/10356/182726 10.32657/10356/182726 en This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). application/pdf Nanyang Technological University |
| spellingShingle | Engineering Computational materials science Two-dimensional materials Semiconductors Electronic properties Density functional theory Tian, Yujia First-principles studies on the structure and electronic properties of two-dimensional semiconductors |
| title | First-principles studies on the structure and electronic properties of two-dimensional semiconductors |
| title_full | First-principles studies on the structure and electronic properties of two-dimensional semiconductors |
| title_fullStr | First-principles studies on the structure and electronic properties of two-dimensional semiconductors |
| title_full_unstemmed | First-principles studies on the structure and electronic properties of two-dimensional semiconductors |
| title_short | First-principles studies on the structure and electronic properties of two-dimensional semiconductors |
| title_sort | first principles studies on the structure and electronic properties of two dimensional semiconductors |
| topic | Engineering Computational materials science Two-dimensional materials Semiconductors Electronic properties Density functional theory |
| url | https://hdl.handle.net/10356/182726 |
| work_keys_str_mv | AT tianyujia firstprinciplesstudiesonthestructureandelectronicpropertiesoftwodimensionalsemiconductors |