Strain tuning of thermal, electrical and optical properties of semiconductors
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The discovery of graphene by mechanical exfoliation has opened a new realm of research. Compared to traditional 3D crystal structures, 2D materials are characterized by strong in-plane covalent bond and weak interlayer van der Waals force, giving them unique 2D crystal structure. In the past ten years, various 2D materials have been explored with very different electronic properties, ranging from wide band-gap insulators to conductors. Owing to the different bond strength, 2D materials behave differently in their electrical, thermal properties along the cross-plane and in-plane direction. In addition, the in-plane electrical, optical and thermal properties are also found to be anisotropic for some particular 2D materials due to the asymmetric crystal structure. Both the in-plane and cross-plane anisotropic properties of 2D materials give rise to a possibility to design the micro/nano devices in various applications. The intrinsic properties of TMDs can be further adjusted by external factors, such as electrical fields, temperature, magnetic field, et al. Among all the external stimulations, strain has been shown an effective method to control the electronic, thermal, optical properties of semiconductors. With the discovery of 2D materials, the application of strain tuning has been growing since the reduced dimensional structures can sustain much larger strains than bulk crystals. In this dissertation, in-plane anisotropic nonlinear optical nonlinearity is studies with an Intensity-scan spectroscopy at ambient conditions. Then a diamond anvil cell (DAC) device is employed to generate large strain on MoS₂. With our home-built pico-second Transient Thermoreflectance technique, ~7x enhancement in cross-plane is observed due to the pressure/strain modified interlayer interaction. Moreover, photoluminescence and Raman spectroscopy are used to probe the impurity levels in BAs crystal. Pressure/Strain modified impurity level change will also have significant effect on this high thermal conductivity material. Lastly, a modified pico-second Transient Thermoreflectance system is developed to achieve simultaneous measurement on thermal conductivity and specific heat of materials.