Joshua Schrier , Haverford College
During this final period of the project, my students were able to determine that the additional thermal resistivity introduced by zinc-blende/wurtzite stacking faults in nanowires is smaller than the typical statistical error for a tractable molecular dynamics simulation. Based on this, we are able to conclude that the introduction of 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 reverse-non-equilibrium MD strategy for calculating the thermal conductivities of lithiated-porous graphene structures, followed by use VASP and BoltzTrap to compute the electronic terms in ZT. In the course of our studies on porous graphene structures for creating the chemical-equivalents of the thermoelectric effect (described in last year’s report), we have that many alkali metal and transition metal atoms selectively bind to the “rings” in 2D-PP, leaving the “pores” open for atom transition. These calculations also indicate that forming a disperse layer on the porous graphene surface is substantially more favorable than forming multi-atom clusters. Therefore, a small amount of, e.g., Li atoms deposited on the porous graphene surface will distribute themselves to form a single-atom monolayer. A second porous graphene layer placed on top will favor an aligned-pore geometry, rather than an offset Bernal-type stacking as in graphene. We hypothesize that these metaerals have the potential to be superb thermoelectrics for three reasons: (1) 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 0.050 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.
Finally, the pedagogical impacts of this project during this final period include: (i) Integrating molecular dynamics simulations into the Junior-level chemistry lab curriculum during Fall 2010; (ii) Teaching an advanced topics course on semiconductor nanowires during Spring 2011; (iii) Writing chapters on one-dimensional band theory of solids and molecular dynamics for a new undergraduate textbook on computational chemistry