Tip-enhanced Raman spectroscopy of strained semiconductor nanostructures
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Raman spectroscopy can serve as a powerful tool to probe the vibrational modes of solid state materials. By taking advantage of the enhanced electric fields caused by the surface-enhanced plasmon resonance of a noble metal coated atomic force microscopy tip, tip-enhanced Raman spectroscopy can dramatically increase local signal intensity and measurement spatial resolution. In this dissertation, work is presented on conventional and tip-enhanced Raman measurements of various semiconductor nanostructures with a specific focus on analyzing strain and strain related properties in these material systems. We use tip-enhanced Raman to study Ge-Si₀.₅Ge₀.₅ core-shell nanowires where we observe two distinct Ge-Ge mode Raman peaks that are affected by strain in the core-shell structure. Tip-enhanced measurements show dramatically increased sensitivity to the modes at the interface between the core and shell and a shift in position of this mode due to plasmonic field localization at the tip apex and the resulting change in phonon self-energy caused by increased coupling between phonons and intervalence-band carrier transitions. We also use tip-enhanced Raman spectroscopy to characterize unstrained and strained MoS₂ and show spatial resolution of approximately 100 nm in the measurements. The strain dependence of the second order Raman modes in MoS₂ reveals changes in the electronic band structure in strained MoS₂ that are manifested through changes in the Raman peak positions and peak area ratios, which are corroborated through density functional theory calculations. Finally, we use conventional Raman spectroscopy to probe uniaxially strained monolayer and three-layer WSe₂. Using mechanical modeling of strain in atomically thin WSe₂ on a stretched elastic substrate, we confirm complete transfer of strain from the substrate to the WSe₂ flakes and analyze the evolution of the Raman spectra with applied uniaxial strain above 1 percent. These studies enable us to experimentally determine the strain induced Raman shift for various Raman modes and to calculate the Grüneisen parameter and strain deformation potential for the first order in-plane Raman mode, with experimental values confirmed with theoretical values calculated using density functional theory.