Atomistic Modelling of Heat and Charge Transport Properties in Nanostructures

This thesis focus on the effect of nanostructuring, i.e., nanowires, grain disorder, grain size, interface and twin boundary, on thermal and electric transport properties. Firstly, we start from the development of algorithm in the frame of non-equilibrium molecular dynamics (NEMD) which is the mimic of the method applied to measure thermal conductivity in experiments. That means, the models in NEMD are actually nanostructures. Two new algorithms named time domain direct decomposed method (TDDDM) and frequency domain direct decomposed method (FDDDM) in the frame of NEMD are proposed successfully to extract the modal (frequency) thermal conductivity. Then, we extend the FDDDM to the interface problem. By using the FDDDM, we can fully consider the harmonic and anharmonic scattering channel at interface. Four different scattering modes at interface can be characterized by FDDDM as well. Then, we care about the thermal management through nanostructuring. Surface orientation, interface structures and diameter are used as factors to control the thermal conductivity of the nanostructures. Our results show that the thermal conductivity of Si NWs with different surface orientation can differ by as large as 2.7 ~ 4.2 times, which suggests a new route to boost the thermoelectric performance. Meanwhile, the thermal conductance is found to be enhanced by around 6 times via interface designing (mass graded interface). More interesting, the thermal conductivity of Si NWs with various diameters is studied. We report that the thermal conductivity (κ) of Si nanowires (NWs) does not diverge, but converges and increases steeply when NW diameter (D) becomes extremely small (dκ/dD < 0), a long debate of one-dimensional heat conduction in history. As a consequence, the thermal conductivity for Si NWs can be modulated in a quite large range by diameter. Finally, we studied how the thermoelectric performance is affected by nanostructuring. The grain boundary, twin boundary, anti-twin boundary and interfaces are chosen as elements to modulate the thermoelectric performance. Firstly, we found a strategy to decrease the thermal conductivity of Si NWs to a record value by polycrystallization, which means the figure of merit can be enhanced largely. Then, based on the results of first step, we designed a structure named superlattice nanocrystal heterostructures (SL-NC HSs) in which the thermal conductivity is found to be reduced by two orders of magnitude and reaches a value far below the amorphous limit of Si. Meanwhile, the power factor of such structures is found to be comparable to that of the bulk Si. As a consequence, the ZT of SL-NC HSs can reach 0.5 at room temperature. At last, we proposed a new strategy of incorporating nanotwinned structures to decouple the electronic and phononic transport. Combining the new concept of nanotwin with the previously widely used nanocrystalline approach, the power factor of the Si nanotwin-nanocrystalline heterostructures is enhanced by 120% compared to bulk crystalline Si, while the lattice thermal conductivity is reduced to a level well below the amorphous limit, yielding a theoretical limit of 0.52 and 0.9 for ZT coefficient at room temperature and 1100 K, respectively