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This project was funded starting 01 May 2009. During Summer 2009, two students, Anna Brockway '12 and Alex Vargo '12 began work on calculating the lattice thermal conductivity of nanowires with defects. (Brockway was funded by an internal fellowship for freshman students, and Vargo was funded by the Howard Hughes Medical Institute program.) They have: (1) Learned molecular dynamics methodology by writing their own Lennard-Jones simulation and reproducing experimental phase diagrams; (2) Validated a potential against experimental bulk moduli and lattice constants; (3) Written programs to generate nanowire input coordinates of arbitrary size and shape; (4) Learned to use the LAMMPS molecular dynamics program sufficiently to equilibrate their nanowires; (5) Determined supercell sizes than minimize the CPU time spent on long-range electrostatic calculation (which dominate computational cost) without changing the energy. They are continuing to work on this project during the academic year, specifically by creating nanowires with the four elementary stacking fault types for wurtzite and computing thermal conductivity using the Muller-Plathe reverse-non-Equilibrium molecular dynamics methodology.

We have settled on this because it: (1) Operates in the NVE ensemble, thus eliminating thermostating errors; (2) Allows for directional determinations of thermal conductivity (difficult in Green-Kubo approach); and is (3) Implemented in LAMMPS, and therefore can be performed on supercomputer resources. My work over the summer during the month funded by PRF focused on: (1) establishing this research effort with the two students; (2) installing the necessary simulation and visualization software; (3) Implementing a toy model for calculation of the electronic properties relevant to thermoelectrics. In particular, I have been doing this for graphene ribbons, since this is easy to debug (two-dimensions only) and the quantum chemistry calculations can be modeled using simple Huckel or PPP pi-electron theories. In addition, I plan to use this toy model in my spring Physical Chemistry class to demonstrate quantum transport theory. This will provide a quick baseline to test our nanowire calculations.

In September 2009, both students presented their work at a Howard Hughes Medical Institute-funded poster session for student research at area colleges. In addition, I briefly spoke about this work for a colleague's Intro Physics course this semester.