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Peng, Jie
Chung, Peter W
Transition metal dichalcogenides (TMDCs) have attracted extensive interests due to outstanding electronic, optical, and mechanical properties, thus are highly promising in nanoelectronic device applications. However, comprehensive understanding of phonon mediated thermal transport in TMDCs is still lacking despite the important roles they play in determining the device performance. The topics requiring further explorations include the full Brillouin zone (BZ) phonons, temperature dependence of thermal properties, and structural-thermal relations of TMDCs. In determining above phonon transport characteristics, the anharmonic effect plays a central role. In this thesis, we present studies on the phonon properties of two TMDC materials, namely MoS2 and HfS2. In the first study, effect of folding on the electronic and phonon transport properties of single-layer MoS2 are investigated. The atomic structure, ground state electronic, and phonon transport properties of folded SLMoS2 as a function of wrapping length are determined. The folded structure is found to be largely insensitive to the wrapping length. The electronic band gap varies significantly as a function of the wrapping length, while the phonon properties are insensitive to the wrapping length. The possibility of modulating the gap values while keeping the thermal properties unchanged opens up new exciting avenues for further applications of MoS2. In the second study, we show that anharmonic phonon scattering in HfS2 leads to a structural phase transition. For the first time, we discover the 3R phase above 300 K. In experiments, we observe a change in the first-order temperature coefficients of A1g and Eg mode frequencies, and lattice parameters a and c at room temperature. Moreover, an anomalous phonon stiffening of A1g mode below 300 K is also observed. The first-principle simulations find a phase transition at 300 K which is characterized by a change in the stacking order from AAA to ABC. The simulations are validated by good agreements with experimental measurements on all the above temperature coefficients. By comparing DFT calculations under harmonic and anharmonic phonon approximation, we attribute the phase change to be due to phonon anharmonicity. The anomalous A1g phonon stiffening is due to decrease of the intralayer thickness of the HfS2 trailayer, as temperature increases.