Electron field emission in nanostructures: A first-principles study
Driscoll, Joseph Andrew
This dissertation presents the study of electron field emission from nanostructures using a first-principles computational framework. Field emission is studied under various conditions such as laser illumination, spin polarization, and the presence of adsorbates. Several nanostructures are considered (carbon nanowires, graphene nanoribbons, and nanotubes of varying composition) which allow general conclusions to be made. The calculations are performed using a real-space, real-time implementation of time-dependent density functional theory. In addition to field emission results, we also present rigorous evaluations of basis sets and complex absorbing potentials, both of which are needed in order to allow these demanding calculations to be performed efficiently. The best basis choice (e.g., atomic orbitals or real-space grids) depends on the structure of the system being analyzed and the physical processes involved (e.g., laser illumination). For this reason, it was important to conduct thorough tests of basis set performance, in terms of accuracy and computational efficiency. In non-periodic systems, emitted electron density can experience non-physical reflections from boundaries of the calculation volume, leading to inaccuracies. To prevent this, we used complex absorbing potentials (CAPs) to absorb density before it could reach the boundaries. We evaluate CAPs using test cases relevant to field emission calculations. Our results show that the adsorbate atoms studied significantly increase the field emission current of carbon nanotubes. We also show that short laser pulses can cause nanostructures to emit electrons with very high spatial and time resolutions. The results predict that GaN, SiC, and Si nanotubes are particularly good field emitters. The highest-current nanotube, Si, is predicted to produce currents an order of magnitude higher than BN or C nanotubes. Carbon nanotubes with various adsorbates are shown to be able to emit spin-polarized current. These are the first first-principles calculations showing spin-polarized field emission for carbon nanotubes with iron adsorbates. Finally, our results suggest that graphene nanoribbons could be extremely good field emitters. The results and computational methods discussed provide a foundation for future theoretical work in nanoscale field emission.