Inelastic Light Scattering and Lattice Dynamics of 2-Dimensional Polar Metals
Open Access
- Author:
- Wetherington, Maxwell Turner
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 23, 2021
- Committee Members:
- Mauricio Terrones, Major Field Member
Venkatraman Gopalan, Major Field Member
Sukwon Choi, Outside Unit & Field Member
Joshua Robinson, Chair & Dissertation Advisor
John Mauro, Program Head/Chair - Keywords:
- Raman Spectroscopy
Polarized Raman
2-Dimensional Materials
Polar Metals
Epitaxial Graphene
Silicon Carbide
Intercalation - Abstract:
- The future demand for high-speed data transmissions will likely be met in part by adopting photonic materials into current semiconductor and conductor industries. There is an opportunity for graphene + metal photonic material systems to revolutionize this industry through improved materials integration and loss reduction as compared to the current silicon photonic systems. Quantum confined 2D polar metals are realized through confinement heteroepitaxy (CHet) in epitaxial graphene. These non-centrosymmetric metals demonstrate extremely large second-order susceptibilities (χ(2)~ 10 nm/V) and support superconducting states at higher Tc than their stable bulk counterparts. The 2D polar metal is air-stabilized by conformal graphene, which also acts as a versatile interface enabling complex heterostructure development. In addition to passivating the metal layer, the graphene also provides direct optical access to the 2D polar metal. The focus of this work demonstrates that these intercalated 2D polar metals have a unique inelastic scattering response that is specific to the intercalated 2D metal element (Au, Ag, Bi, Cu, Ga, In, Pb), 2D metal alloy (InxGa1-x), and structure of the 2D metal. The origin of the detected scattering is a result of an inter-band transition in the visible spectrum, providing a strong resonance in the light scattering response. Temperature dependent Raman polarimetry provides evidence that the 2D-Ga is a disordered phase at room temperature with a gradual transition to a liquid at > 600 K and return to a disordered phase after cooling back to ambient without any evidence of oxidation or loss of metal intercalant. Additionally, the 2D-Ga transitions to a trigonal crystalline phase, identified via Raman polarimetry, after cooling below 285 K. This analysis confirms that this 2D-Ga system is a polar crystal at low temperature. It is possible that polar structure remains at room temperature, however the structure cannot be confirmed by the inelastic light scattering response because the disorder precludes assignment to a crystal symmetry.