1. Comprehensive Atomistic Modeling of Thermoelectric Semiconductor Nanowire Heterostructures
Joshua Schrier, Department of Chemistry, Haverford College, 370 Lancaster Avenue, Haverford, PA 19041
The additional thermal resistivity introduced by zinc-blende/wurtzite stacking faults in nanowires is smaller than the typical statistical error in a molecular dynamics simulation. Therefore, we conclude that introducing these types of defects into nanowires is not useful for increasing the thermoelectric figure of merit, ZT, which we hypothesized at the beginning of the project.
Moving forward, we intend to use the same methodologies developed in this project (reverse-non-equilibrium molecular strategy for calculating the thermal conductivities, followed by use VASP and BoltzTrap to compute the electronic terms in ZT), to study lithiated-porous graphene structures, which we hypothesize to have high-ZT due to:
Though graphene has a high in-plane thermal conductivity, the inter-layer thermal conductivity is substantially lower. Moreover, the partially ionic character of the benzene-ring-to-lithium bond should lead to a mismatch of the phonon density of states within and between the porous graphene sheet, which should lead to ultra-low thermal conductivities analogous to layered WSe2. (2) Though graphene is a semi-metal, 2D-PP is a large band-gap semiconductor. Our preliminary PBE calculations (which systematically underestimate the band gap) indicate that the bilayer-structure has a eV direct gap. This is important because the optimum band gap for a thermoelectric is 6-10 kBT, depending on the electron scattering mechanism. (3) Incorporating metal atoms between the porous graphene sheets will lead to sharp peaks in the electronic density of states from the Li atomic orbitals, enhancing the electronic contributions to ZT. While this is a very different molecular system than nanowires, the computational methodologies that we have learned in the course of the current project are immediately transferable to this new system, so we anticipate being able to make rapid progress in the future.
Low thermal conductivity, due to bond strength mismatch differences between intra-plane and inter-plane bonding (analogous to layered WSe2)
Narrow band gap semiconductor, with preliminary DFT calculations indicate eV direct gap (underestimated by DFT, optimum for thermoelectric is 6-10 kBT)
Sharp peaks in electronic density of states due to Li-benzene interactions