Discovery of low-dimensional functional materials from first-principles calculations
Open Access
- Author:
- Lu, Yanfu
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 04, 2020
- Committee Members:
- Susan B Sinnott, Dissertation Advisor/Co-Advisor
Susan B Sinnott, Committee Chair/Co-Chair
Ismaila Dabo, Committee Member
Roman Engel-Herbert, Committee Member
Mauricio Terrones, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- perovskite oxides
transition metal dichalcogenides
density functional theory
epitaxy
piezielectricity
ferroelectricity
strain engineering - Abstract:
- Through a combination of computation, controlled synthesis, and state-of-the-art characterization, the synergetic investigation creates a closely coupled feedback loop and guides each component for a substantial improvement. In the work described in this dissertation, density functional theory (DFT) calculations are used to compute the fundamental energetic information and related physical or chemical properties for crystal structures with different kinds of defects, such as interstitials, vacancies, homogenous or heterogeneous interfaces, and surfaces. The resulting structure-property relationships serve as guidance to, or assist in the analysis of, experimental data. The discovery of low dimensional materials in this dissertation includes two groups of materials, perovskite oxides and transition metal dichalcogenides. In the case of perovskite oxides, CaTiO3 thin films are grown on different perovskite oxide substrates under tensile or compressive epitaxial strains. Slight changes in octahedral tilts in CaTiO3 thin films through interfacial tilt control can dramatically influence the functional properties of ultrathin films. Based on DFT calculations, the reconstructed 3D atomic structure in ultrathin films clearly reveals the intertwining roles of tilt epitaxy, substrate strain, and substrate surface terminations. The analysis illustrates that the Ca cations and Ti cations play an important role to generate in-plane and out-of-plane polarization, respectively, when the CaTiO3 thin film is under epitaxial strains. In the case of transition metal dichalcogenides (TMDs), the work described in this dissertation is divided into three areas of focus. First, from recent synthesis and data mining efforts, 56 two-dimensional (2D) transition metal chalcogenides (TMCs) under epitaxial strain have been investigated through DFT calculations. The results indicate that the majority of these 2D TMCs can accommodate ±10% strain without being energetic unfavorable or breaking their crystal symmetry. The elastic and piezoelectric tensors indicate that 22 of 56 candidates are piezoelectric, and we derive their in-plane piezoelectric coefficient d11. The epitaxial strain is further predicted to enhance the d11 by over 100% at 10% tensile epitaxial strain for most of these piezoelectric 2D TMCs. These findings have implications for the use of high-performance 2D piezoelectric materials in devices. Second, carbon doped WS2 monolayers are synthesized by plasma-enhanced chemical vapor deposition. By performing DFT calculations at different doping concentrations, with different dopant groups, and at different dopant locations, 2.67 at% CH-groups in sulfur vacancies are found to be the most stable way to introduce carbon into WS2, and the optical bandgap is reduced from 1.98 eV to 1.83 eV. The resulting CH-WS2 monolayers show the emergence of a p-branch and gradually become entirely p-type in the device characterization, as the carbon doping level increases. This route can be used to dope other 2D-TMDs for more efficient electronic devices. Third, the sulfurization of thin (<50 nm) Mo2C systems using gaseous H2S is investigated. The molybdenum carbide goes through a phase transition from the α-phase Mo2C to the metastable γ-phase MoC and transforms the carbide into MoS2 layers. The DFT calculations reveal that the sharp vertical interface between γ-MoC and MoS2 is generated by either van der Waals interactions or epitaxial strain. The optical bandgap of MoS2 reduces dramatically depending on interfacial types and the number of layers in MoS2. The transformation from transition metal carbide to TMDs provides a new route to stabilize metastable phases, confine 2D layers by superconducting layers and control properties, such as superconductivity, magnetism, ferroelectricity, and piezoelectricity.