Graduate Thesis Or Dissertation
First-Principles Study of Phonon Transport in Two-Dimensional Materials and Phonon Transmission Across Magnesium Silicide/Magnesium Stannide Interfaces Public Deposited
Low-dimensional and nanostructured materials have been shown to be of exceptional electronic, optical and mechanical properties, with great potential for novel applications. Understanding the thermal transport properties of low-dimensional materials is essential for designing reliable devices with these novel materials. The objectives of this thesis are to develop numerical methods based on the first-principles calculations to predict the thermal transport properties of novel two-dimensional and nanostructured materials, to explore their unique phonon dynamics and to exploit them for thermal applications.
In the first part of this thesis, the first-principles based Boltzmann transport equation approach is developed. We apply this approach to predict a series of novel two-dimensional materials, including silicene and single-layer transition metal dichalcogenides (TMDs). Their thermal conductivities are found to be highly correlated to their crystal structures and atomic masses. Using the same approach, we also study the layer thickness-dependence of thermal conductivity of MoS2. Unlike conventional thin film materials, whose thermal conductivity is usually suppressed when the thickness decreases due to phonon-boundary scattering, the thermal conductivity of MoS2 decreases when increasing its thickness. It appears that both the phonon dispersion and the anharmonicity change with the thickness ofMoS2. To further reduce the thermal conductivity of single-layer MoS2 for potential thermoelectric applications, we study the thermal conductivity of Mo1-x WxS2 alloy embedded with WS2 nanodomains. The nanostructured two-dimensional alloy has a very low thermal conductivity, only one-tenth of MoS2, because both high-frequency and low-frequency phonons can be effectively scattered by atomic-difference and nanodomains, respectively.
In the second part of this thesis, the first-principles-based atomistic Green’s function approach is developed to study phonon transport across interfaces between dissimilar materials. When two dissimilar materials with different lattice constants are connected at an interface, the lattice near the interface is usually distorted. Such a lattice distortion can extend to several unit cells away from the interface. Using direct first-principles calculations to model thermal transport near the interfacial region becomes infeasible. To overcome such numerical challenges, a methodology is developed to extract second-order harmonic interatomic force constants based on higher-order force constant model, which is originated from virtual crystal approximation but considers the local force field difference. Phonon transmission across Mg2Si/Mg2Sn interfaces and Mg2Si/Mg2Si1-xSnx is studied. The interfacial thermal resistance across Mg2Si/Mg2Si1-xSnx interface is found to be weakly dependent on the composition of Sn when the composition of x is less than 40% but increases rapidly when it is larger than 40% due to the transition of high-frequency phonon density of states in Mg2Si1-xSnx alloys.
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