Date of Award
Doctor of Philosophy (PhD)
Dr. Huijuan Zhao
Dr. Gang Li
Dr. Fadi Abdeljawad
Dr. Zhaoxu Meng
Due to their remarkable properties, transition metal dichalcogenides (TMDs) have received much scientific interest throughout the past decade. Two layers of chalcogen atoms (S, Se, Te) sandwich a layer of transition metal atoms (Mo, W, Ta) to form the three-atom thick unit cell in TMDs. The interaction between TMD "single layers" is mediated by neighboring chalcogen planes and bonded by Van der Waals forces. Due to this weak out-of-plane interaction, bulk samples can be thinned down to a single layer by exfoliation. Among the TMDs, Molybdenum Disulfide (MoS2) shows promise in the field of electronics, optics, and sensing applications.
Despite numerous experimental and computational studies, it is poorly understood how out-of-plane thermal properties behave, as they have a higher degree of freedom in the lattice. Recently, hybrid structures have been found to form at interface boundaries in small devices, but the mechanism behind them remains unclear. Additionally, the thermal and mechanical properties of MoS2 under the phase transition are poorly understood and reported. Here, we benchmarked the empirical potential and reported the layer-dependent out-of-plane thermal properties. From our approach to calculating the thermal properties, we have demonstrated that the out-of-plane thermal conductivity in MoS2 increases monotonically with the number of layers. We have filled the gap in understanding the contribution of stress on the quadrilateral phase (I4/mmm) transition from the stable 2H phase of MoS2 at room temperature and the contribution of local strain in the mechanism for single- and bilayer systems. This mechanism of the phase change in MoS2 will be helpful where a change in the device's mechanical properties can occur, e.g., during the removal of layers from the substrate due to the presence of external force or unwanted impurities (i.e., S line vacancies). The presence of intrinsic (edge, chirality) and extrinsic (system sizes) conditions in phase transitions is also presented in this work. We have also reported how the presence of sulfur line vacancies in single- and bilayer systems can affect phase transition while undergoing uniaxial tension. In this work, we have found that the metastable T" phase (formed from 2H in the transition process by sulfur plane gliding before transitioning into I4/mmm) is triggered by local tensile stress, while the local out-of-plane force triggers the I4/mmm phase. Additionally, we have also revealed that MoS2 has tunable elastocaloric properties, which can be utilized to cool nanodevices during the loading-unloading condition.
Rahman, Mahabubur, "Molecular Dynamics Simulation on Molybdenum Disulfide: Thermal-Mechanical Properties and Phase Transitions Under External Loading" (2023). All Dissertations. 3348.
Author ORCID Identifier
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