First-principles study of phonon transport in low-dimensional structures

Abstract

Efficient thermal transport in solids plays a key role in maintaining the efficiency of electronic devices while low thermal conductivity is preferred in other fields such as thermoelectrics. There is strong interest in nano-engineering materials to tune thermal transport properties. Since phonon is the major heat carrier in semiconductors and insulators, exploring the mechanisms of phonon transport and scattering with the shrinking size is of great importance for tuning thermal transport properties for targeted applications through nanoengineering. The first-principles method can be integrated with lattice dynamics to recapture the lattice thermal conductivity, offering high accuracy, transferability, and the ability to reveal the mode-wise phonon transport mechanism without introducing empirical parameters. In this thesis, the first-principles-based lattice dynamics approach with both the single-mode approximation and fully iterative solution is developed. To address the issues due to computational errors, such as the unphysical dispersions of low-dimensional structures near the Brillouin zone center, a series of corrections based on symmetry invariance have been adopted. To avoid the arbitrary Gaussian smearing factor widely used in conventional first-principles methods, a double integration method is developed with the construction of a hyperspace tetrahedron method. After careful verification, this first-principles approach is systematically applied to 2-D thin film, quasi-1-D complex materials and 1-D chain materials. With the explicit consideration of phonon depletion induced by phonon confinement and the corresponding variation in interphonon scatterings, the in-plane thermal conductivities of Si thin films with different thicknesses have been predicted from 80 K to 800 K and excellent agreements with experimental results are found. The validities of adopting the bulk phonon properties and gray approximation of surface specularity in thin film studies are clarified. It is found that in ultra-thin films the phonon depletion effects are largely offset by the reduction of interphonon scatterings. The mode-wise contributions to the thermal transport and isotope effects in Si films are also analyzed. In quasi-1-D structures such as crystalline polyethylene (PE) and complex metal organic framework (MOF) crystals, it is found that the complex atom arrangement and van der Waals (vdW) interactions help generate rattling-like behaviors of atoms with large vibrational amplitude that would block phonon transport. It is also found that the perfect alignment of PE chains in bulk PE crystals boosts the thermal conductivity by 2 orders than amorphous PE with the longitudinal modes dominating the thermal transport. As the dimensionality shrinks, for example in 1-D single-chain PE, a significant increase of axial thermal conductivity is observed compared with bulk PE, due to the diminished inter-chain vdW interactions. Different from many precedent studies, the thermal conductivity of single-chain PE is predicted to converge near room temperature. The convergence is attributed to the indirect thermal resistance contribution from large normal scatterings and anharmonic phonon scatterings due to the coupling between low-frequency bending and twisting phonon modes. It is also found that longitudinal phonon modes dominate the thermal transport in the single PE chain while transverse phonon branches with quadratic dispersion contribute minor to κ due to their vanishing group velocity and limited lifetimes in the long-wavelength limit. The cross-plane heat transport between graphene and substrates is also a great concern for efficient heat dissipation in graphene-based devices. To estimate the thermal boundary conductance (TBC) at the interface, a model based on the Green’s function is derived to account for different interfacial coupling strength, which is missing in most previous studies. It is found to predict TBC with higher accuracy and is able to explain the differences of TBC with different surface treatments. Therefore the phonon transport in nanostructures has been studied by solving the Boltzmann transport equation and the mechanisms of phonon transport are unveiled. The findings may facilitate the future development in nanoengineering to artificially tune the thermal transport for specific applications.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology