Author ORCID Identifier:

https://orcid.org/0009-0003-6699-5522

Date of Graduation

5-2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Nakamura, Hiro

Committee Member

Churchhill, Hugh

Second Committee Member

Hu, Jin

Keywords

Biaxial tensile strain; Exciton energy; MoSe2 and MoS2; PVD; Transition Metal Dichalcogenides; Vibrational properties

Abstract

Mechanical strain represents a robust and non-invasive method for tuning the electronic and vibrational properties of two-dimensional transition-metal dichalcogenides (TMDs). This work examines homogeneous biaxial strain as a symmetry-preserving control parameter that governs excitonic and interlayer dynamics in atomically thin TMDs. In monolayer MoSe2, biaxial tensile strain is intrinsically generated during high-temperature physical vapor deposition due to thermal expansion mismatch with amorphous substrates. Optical spectroscopy demonstrates substantial and tunable excitonic energy shifts exceeding 100 meV per percent strain. Polarization-resolved second harmonic generation confirms the retention of lattice symmetry under biaxial deformation. The investigation is further extended to bilayer MoS2, where interlayer van der Waals coupling produces terahertz-frequency phonon modes. Low-frequency Raman measurements reveal a counterintuitive hardening of the interlayer breathing mode under tensile strain, which is attributed to Poisson-driven vertical contraction of the interlayer spacing. The analysis determines an effective out-of-plane Poisson ratio of 0.19–0.24 and a large Gruneisen parameter, indicating strong anharmonicity of interlayer interactions. Collectively, these findings establish biaxial strain as a unified platform for engineering excitonic and optomechanical responses in two-dimensional materials.

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