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Abstract
The study of van der Waals (vdW) two-dimensional (2D) materials has garnered significant attention due to their unique optical, electronic, and mechanical properties, which are not present in their bulk counterparts. This dissertation explores the electronic and optical properties of these materials, focusing on alloyed and sharp interface heterostructures of transition metal dichalcogenides (TMDCs), and violet phosphorus (VP). The research focuses on the excitonic transitions and associated defect states within these materials by utilizing advanced photoluminescence (PL) spectroscopy and nano-imaging techniques.
A novel graded alloy of MoxW1-xS2, with a compositional gradient from a Mo-rich center to a W-rich periphery, is synthesized and characterized. Detailed PL spectroscopy, supported by ab initio calculations, reveals the presence of sulfur monovacancies (VS) and their spin-orbit interaction-induced doublet electronic states, leading to four spin-allowed optical transitions. Additionally, the creation and study of in-plane heterostructures of MoxW1-xS2 - WxMo1-xS2 provide insights into compositional gradients and their impact on excitonic properties.
The dissertation also addresses the photo- and thermal-degradation of VP, highlighting the dependency of degradation rates on light wavelength and exposure time. Low-temperature PL spectroscopy and Raman spectroscopy are employed to understand the effects of degradation on exciton emissions and phonon properties. The findings demonstrate how degradation influences exciton lifetime, formation, and stability.
The thesis explores an improved tip-based photoluminescence method, utilizing advanced demodulation techniques and a photomultiplier tube (PMT) detector to achieve higher spatial resolution by leveraging higher harmonics demodulation. PL methods using CCD detectors face limitations due to background noise. The new method enhances near-field signal detection and reduces background interference. Demonstrations on fluorescent particles show the superior performance of the PMT-based PL in both elastic and inelastic scattering imaging, representing an advance in nanoscale optical imaging and spectroscopy.
A novel graded alloy of MoxW1-xS2, with a compositional gradient from a Mo-rich center to a W-rich periphery, is synthesized and characterized. Detailed PL spectroscopy, supported by ab initio calculations, reveals the presence of sulfur monovacancies (VS) and their spin-orbit interaction-induced doublet electronic states, leading to four spin-allowed optical transitions. Additionally, the creation and study of in-plane heterostructures of MoxW1-xS2 - WxMo1-xS2 provide insights into compositional gradients and their impact on excitonic properties.
The dissertation also addresses the photo- and thermal-degradation of VP, highlighting the dependency of degradation rates on light wavelength and exposure time. Low-temperature PL spectroscopy and Raman spectroscopy are employed to understand the effects of degradation on exciton emissions and phonon properties. The findings demonstrate how degradation influences exciton lifetime, formation, and stability.
The thesis explores an improved tip-based photoluminescence method, utilizing advanced demodulation techniques and a photomultiplier tube (PMT) detector to achieve higher spatial resolution by leveraging higher harmonics demodulation. PL methods using CCD detectors face limitations due to background noise. The new method enhances near-field signal detection and reduces background interference. Demonstrations on fluorescent particles show the superior performance of the PMT-based PL in both elastic and inelastic scattering imaging, representing an advance in nanoscale optical imaging and spectroscopy.