A first-principles study of low dimensional transition metal carbides and transition metal dichalcogenides
Open Access
- Author:
- Wang, Jiayang
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 17, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Mauricio Terrones, Outside Unit & Field Member
Ismaila Dabo, Major Field Member
Joshua Robinson, Major Field Member
Susan Sinnott, Chair & Dissertation Advisor - Keywords:
- 2D materials
transition metal carbides
transition metal dichalcogenides
DFT
superconductivity - Abstract:
- Two-dimensional (2D) materials consist of atomically thin layers of a material that exhibit properties different from their bulk counterparts. Since graphene's discovery, the interest in 2D materials has surged, leading to extensive research into various 2D systems and heterostructures for applications such as catalysis and quantum computing. By integrating computation, controlled synthesis, and advanced characterization, this synergistic approach establishes a tightly interconnected feedback loop that significantly enhances each aspect of the investigation. In this dissertation, density functional theory (DFT) calculations were used to compute geometric and energetic information of various 2D systems and related chemical properties such as defects, diffusion barriers, and surface effects. The established structure-property relationships provide valuable insights for analyzing experimental data or guiding further experimentation. The transition metal carbides possess intricate crystallization pathway, and the emergence of metastable phases pose challenges in synthesis with precise compositions and surfaces. In the case of tungsten carbide, both stable WC and meta-stable W2C can be synthesized on different experimental conditions. DFT calculations are performed to calculate the Gibbs formation energy of various WC and W2C phases based on Quasi harmonic approximation (QHA). The influence of surface effect was also discussed by calculating the stability of WC and W2C thin film through the chemical potential phase diagram. To investigate the sulfurization of tungsten carbide, the formation energy of the carbon vacancy and diffusion barrier of sulfur atoms into tungsten carbide were considered. These efforts presented the possible reasons that control the sulfurization of WC and W2C. various types of heterostructures between WC/WS2 and W2C/WS2 were built to investigate their interfacial binding energy and formation enthalpy. Second, novel TMC/TMD heterostructures are considered as Schottky barrier conjunction. This work has been published to physical review material: https://doi-org.ezaccess.libraries.psu.edu/10.1103/PhysRevMaterials.8.044004. Based on DFT with vdW corrections, geometric, energetic, and electronic properties of heterostructures between tungsten carbide and tungsten disulfide (diselenide) are calculated. The moiré pattern heterostructures are quantitatively proved to be energetically more favorable due to the large lattice mismatch between TMC and TMD. SBH were quantitatively obtained by projected band structures and partial density of states. The WC-c/WSe2 heterostructures possess the lowest SBH among the heterostructures considered. Our results thus provide the insights into such novel heterostructures and important for the designing and fabrication of novel electronic Schottky devices based on WS2 and WSe2. Third, investigation on topological materials with superconducting properties have received intensive interest because of the possibility of hosting Majorana fermion on their boundaries, which have been proposed for fault-tolerant topological computations or working as topological wire and circuit. Using Majorana fermion in these devices to encode the information prevent from local perturbation. Based on DFT, the topological band structure of molybdenum nitride with and without SOC are compared, confirming δ-MoN to be a type of novel semimetal. The superconductivity properties are also be investigated based on electron-phonon coupling and Allen-Dynes formular. Fourth, GaAs(111)B is a semiconductor substrate that is widely used in both research and commercial fields due to its low cost, established synthesis technology, and excellent properties for electronic device manufacturing. However, the GaAs suffered from the dangling bonds at surface that prevent if working as van der Waals substrate so that the surface passivation is required. DFT calculation predict energetically favorable surface configuration and coverage. Additionally, the GaSe are synthesized on GaAs substrate using MBE method while the meta-stable γ’-GaSe polytypes are dominant. Through DFT calculation, existence of Ga vacancies was considered as main reason. The γ’-GaSe have lower formation enthalpy than ε-GaSe. It is because with Ga vacancies, the top-layer and bottom-layer is energetically more favorable to from inversion symmetry structure, resulting octahedron with closer bond length to the equilibrium, so that centrosymmetric γ’-GaSe can be energetically favorable.