Modeling and Simulation of Phonon Transport at the Nanoscale for Optimum Thermal Management

Recent progress in nanostructured materials synthesis and device fabrication technology has brought the need for thermal analysis and power management to the forefront. In this thesis, we investigate the phonon transport in the nanoscale through theoretical modeling and simulation for the optimum thermal management.;Due to the superior electrical and thermal properties of graphene, the thermal transport of graphene/dielectric contact is first studied to provide better a understanding of phonon interactions and heat dissipation at nanoscale graphene interfaces. Using calculations from first principles and the Landauer approach for phonon transport, we calculated the Kapitza resistance in selected multilayer graphene/dielectric heterojunctions (hexagonal BN and wurtzite SiC) and demonstrate (i) the resistance variability (∼50--700x10 -10 m2K/W) induced by vertical coupling, dimensionality, and atomistic structure of the system and (ii) the ability of understanding the intensity of the thermal transmittance in terms of the phonon distribution at the interface.;As a natural extension, thermal properties in the metal/graphene (Gr) systems are analyzed by using an atomistic phonon transport model based on Landauer formalism and first-principles calculations. The specific structures under investigation include chemisorbed Ni(111)/Gr, physisorbed Cu(111)/Gr and Au(111)/Gr, as well as Pd(111)/Gr with intermediate characteristics. Calculated results illustrate a strong dependence of thermal transfer on the details of interfacial microstructures. In particular, it is shown that the chemisorbed case provides a generally smaller interfacial thermal resistance than the physisorbed one due to the stronger bonding. However, our calculation also indicates that the weakly chemisorbed interface of Pd/Gr may be an exception, with the largest thermal resistance among the considered. Further examination of the electrostatic potential and interatomic force constants reveals that the mixed bonding force between the Pd and C atoms results in incomplete hybridization of Pd and graphene orbital states at the junction, leading effectively to two phonon interfaces and a larger than expected thermal resistance. Comparison with available experimental data shows good agreement. The result clearly suggests the feasibility of phonon engineering for thermal property optimization at the interface.;Transition-metal dichalcogenides (TMDs) MX2 (M=Mo,W; X=S,Se), one of the beyond- graphene two-dimensional semiconductor materials, have emerged as promising candidates due to their distinctive electronic and optical properties. Unlike the zero-bandgap graphene, TMDs have intrinsic bandgaps in the range of 1.1--2.2eV, allowing low off-current for field effect transistors. Thermal transport properties at the metal/MoS2 interfaces are then analyzed by using the atomistic phonon transport model. The considered structures include chemisorbed Sc(0001)/MoS2 and Ru(0001)/MoS 2, physisorbed Au(111)/MoS2, as well as Pd(111)/MoS 2 with intermediate characteristics. Calculated results illustrate a distinctive dependence of thermal transfer on the details of interfacial microstructures. More specifically, the chemisorbed case with a stronger bonding exhibits a generally smaller interfacial thermal resistance than the physisorbed. Comparison between metal/MoS2 and metal/graphene systems suggests that metal/MoS 2 is significantly more resistive. Further examination of lattice dynamics identifies the presence of multiple distinct atomic planes and bonding patterns at the interface as the key origin of the observed large thermal resistance.;Finally, since the commensurate-incommensurate transitions are ubiquitous in the fabrication of the 2D material based devices, we have extended our investigation to the thermal/phonon transport across the misoriented 2D nanostructures. An analytical model that can incorporate the atomic level detail as well as being time-efficient is developed for the simulation of large misoriented systems. A decent match between the phonon dispersions of bulk AB stacking graphite obtained from the analytical model to the experimental data and DFT results verifies the accuracy of this model. The phonon dynamics of three turbostratic cases, theta=21.79°, theta=13.17° and theta=9.43°, are calculated to study the thermal transport properties of misorientated graphene systems. The number of phonon modes increases when the rotation angle reduces, which is the direct effect of the alteration of the Brillouin zone size and the modification of the interlayer interaction due to misorientation. The transport results indicate that the rotational degree of freedom can be explored to alter thermal conductivity in the cross-plane direction which could be an effective means of engineering high-performance thermoelectric materials.